Chapter 3

Sintering and Microstructure of Ceramics

3.1. Sintering and microstructure of ceramics

We saw in Chapter 1 that sintering is at the heart of ceramic processes. However, as sintering takes place only in the last of the three main stages of the process (powders o forming o heat treatments), one might be surprised to see that the place devoted to it in written works is much greater than that devoted to powder preparation and forming stages. This is perhaps because sintering involves scientific considerations more directly, whereas the other two stages often stress more technical observations M in the best possible meaning of the term, but with manufacturing secrets and industrial property aspects that are not compatible with the dissemination of knowledge. However, there is more: being the last of the three stages M even though it may be followed by various finishing treatments (rectification, decoration, deposit of surfacing coatings, etc.) M sintering often reveals defects caused during the preceding stages, which are generally optimized with respect to sintering, which perfects them M for example, the granularity of the powders directly impacts on the densification and grain growth, so therefore the success of the powder treatment is validated by the performances of the sintered part.

Sintering allows the consolidation M the non-cohesive granular medium becomes a cohesive material M whilst organizing the microstructure (size and shape of the grains, rate and nature of the porosity, etc.). However, the microstructure determines to a large extent the performances of the material: all the more reason why sintering

Chapter written by Philippe BOCH and Anne LERICHE. 56 Ceramic Materials deserves a thorough attention, and the reason for which this chapter interlaces OsinteringP and OmicrostructuresP. We will now describe the overall landscape and the various chapters in this volume will present, on a case-by-case basis, the specificities of the sintering of the materials they deal with.

Sintering is the basic technique for the processing of ceramics, but other materials can also use it: metals, carbides bound by a metallic phase and other cermets, as well as natural materials, primarily snow and ice.

Among the reference works on sintering, we recommend above all [BER 93] and [GER 96]; the latter refers to more than 6,000 articles and deals with both ceramics and metals. We also recommend [LEE 94], which discusses ceramic microstructures and [RIN 96], which focuses on powders.

3.2. Thermodynamics and kinetics: experimental aspects of sintering

3.2.1. Thermodynamics of sintering

Sintering is the consolidation, under the effect of temperature, of a powdery agglomerate, a non-cohesive granular material (often called compact, even though its porosity is typically 40% and therefore its compactness is only 60%), with the particles of the starting powder OweldingP with one another to create a mechanically cohesive solid, generally a polycrystal.

The surface of a solid has a surplus energy (energy per unit area: JSV , where S is for OsolidP and V is for OvaporP) due to the fact that the atoms here do not have the normal environment of the solid which would minimize the free enthalpy. In a polycrystal, the grains are separated by grain boundaries whose surplus energy (denoted JSS , or JGB , where SS is for Osolid-solidP and GB for Ograin boundaryP) is due to the structural disorder of the boundary. In general, JSS < JSV , so a powder lowers its energy when it is sintered to yield a polycrystal: the thermodynamic engine of sintering is the reduction of systemRs interfacial energies.

Mechanical energy is the reduction of the systemRs free enthalpy:

'GT = 'GVOL + 'GGB + 'GS where 'GT is the total variation of G and where VOL , GB and S correspond to the variation of the terms associated respectively with the volume, the grain boundaries and the surface. Sintering and Microstructure of Ceramics 57

Starting particles Sintering without densification

Sintering with densification and shrinkage

Figure 3.1. Sintering of four powder particles. In general, we want sintering to be OdensifyingP, in which case the reduction of porosity implies a shrinkage: L final = L 0 M 'L. Some mechanisms are non-densifying and allow only grain growth. This diagram shows a two-dimensional system but the powder is a three-dimensional system. We could consider an octahedral configuration where the interstice between the four particles is closed below and above by a fifth and a sixth particle [KIN 76]

The interfacial energy has the form G = JA, where J is the specific interface energy and A its surface area. The lowering of energy can therefore be achieved in three ways: i) by reducing the value of J, ii) by reducing the interface area A, and iii) by combining these effects. The replacement of the solid-vapor surfaces by grain boundaries decreases J, when JSS is lower than JSV . The reduction of A is achieved by grain growth: for example, the coalescence of n small spheres with surface s and volume v results in a large sphere with volume V = nv but with surface S < ns (this coalescence can be easily observed in water-oil emulsions). In fact, the term sintering includes four phenomena, which take place simultaneously and often compete with each other: M consolidation: development of necks that OweldP the particles to one another; M densification: reduction of the porosity, therefore overall contraction of the part (sintering shrinkage); M grain coarsening: coarsening of the particles and the grains; M physicochemical reactions: in the powder, then in the material under consolidation. 58 Ceramic Materials

3.2.2. Matter transport

Sintering is possible only if the atoms can diffuse to form the necks that weld the particles with one another. The transport of matter can occur in vapor phase, in a liquid, by diffusion in a crystal, or through the viscous flow of a glass. Most mechanisms are activated thermally because the action of temperature is necessary to overcome the potential barrier between the initial state of higher energy (compacted powder) and the final state of lower energy (consolidated material). Atomic diffusion in ceramics is sufficiently rapid only at temperatures higher than 0.6-0.8 T F, where T F is the melting point (in K). For alumina, for example, which melts at around 2,320 K the sintering temperature chosen is generally around 1,900 K.

3.2.3. Experimental aspects of sintering

The parameters available to us to regulate sintering and control the development of the microstructure are primarily the composition of the starting system and the sintering conditions: M composition of the system: i) chemical composition of the starting powders, ii) size and shape of the particles, and iii) compactness rate of the pressed powder; M sintering conditions: i) treatment temperature, ii) treatment duration, iii) treatment atmosphere and, as the case may be, iv) pressure during the heat treatment (for pressure sintering).

Pressureless sintering and pressure sintering In general, sintering is achieved solely by heat treatment at high temperature, but in difficult cases it can be assisted by the application of an external pressure: M pressureless sintering: no external pressure during the heat treatment; M pressure sintering (under uniaxial load or isostatic pressure): application of an external pressure during the heat treatment.

Pressure sintering requires a pressure device that withstands the high sintering temperatures, which is in fact a complex and expensive technique and therefore reserved for specific cases.

Sintering with or without liquid phase Sintering excludes a complete melting of the material and can therefore occur without any liquid phase. However, it can be facilitated by the presence of a liquid phase, in a more or less abundant quantity. We can thus distinguish solid phase sintering on the one hand and sintering where a liquid phase is present; the latter Sintering and Microstructure of Ceramics 59 case can be either liquid phase sintering or vitrification, depending on the quantity of liquid (see Figure 3.2): M for solid phase sintering, the quantity of liquid is zero or is at least too low to be detected. Consolidation and elimination of the porosity require a disruption of the granular architecture: after the sintering, the grains of the polycrystal are generally much larger than the particles of the starting powder and their morphologies are also different. Solid phase sintering requires very fine particles (micrometric) and high treatment temperatures; it is reserved for demanding uses, for example, transparent alumina for public lamps; M for liquid phase sintering, the quantity of liquid formed is too low (a few vol.%) to fill the inter-particle porosities. However, the liquid contributes to the movements of matter, particularly thanks to phenomena of dissolution followed by reprecipitation. The partial dissolution of the particles modifies their morphology and can lead to the development of new phases. A number of technical ceramics (refractory materials, alumina for insulators, BaTiO3-based dielectrics) are sintered in liquid phase; M lastly, for vitrification, there is an abundant liquid phase (for example, 20 vol.%), resulting from the melting of some of the starting components or from products of the reaction between these components. This liquid fills the spaces between the non-molten particles and consolidation occurs primarily by the penetration of the liquid into the interstices due to capillary forces, then solidification during cooling, to give crystallized phases or amorphous glass. This type of sintering is the rule for silicate ceramics, for example, porcelains. However, the quantity of liquid must not be excessive, and its viscosity must not be too low, otherwise the object would collapse under its own weight and would lose the shape given to it.

Sintering with and without reaction We can speak of reactive sintering for traditional ceramics, where the starting raw materials are mixtures of crushed minerals that react with one another during sintering. The presence of a liquid phase often favors the chemical reactions between the liquid and the solid grains. However, for solid phase sintering, reactive sintering is generally avoided: either we have the powders of the desired compound already, or sintering is preceded by calcination, i.e. a high temperature treatment of the starting raw materials to allow their reaction towards the desired compound, followed by the crushing of this compound to obtain the powders that will be sintered: M non-reactive sintering: an example is that of alumina, because the powders of this compound are available on the market;

M calcination and then sintering: an example is barium titanate (BaTiO 3). BaTiO 3 powders are expensive and some industrialists prefer to start with a less expensive 60 Ceramic Materials

mixture of barium carbonate BaCO 3 and titanium oxide TiO 2, the mixture being initially calcined by a high temperature treatment to form BaTiO 3, which is then crushed to give the powder that will be used for sintering;

M reactive sintering: an example is that of silicon nitride (Si 3N4), for which one of the preparation methods consists of treating silicon powders in an atmosphere of nitrogen and hydrogen, so that the reaction that forms the nitride (3Si + 2N 2 o Si 3N4) is concomitant with its sintering (see Chapter 7). This technique (RBSN = reaction bonded silicon nitride) makes it possible to circumvent the difficulties of the direct sintering of Si 3N4 and offers the advantage of minimizing dimensional variations, but the disadvantage of yielding a porous material (P > 10%). Mullite and zirconia mullite can also be prepared by reactive sintering [BOC 87 and 90].

Figure 3.2. Top, vitrification: the liquid phase is abundant enough to fill the interstices between the particles; in the middle, liquid phase sintering: the liquid is not sufficient to fill the interstices; bottom, solid phase sintering: organization and shape of the particles are extremely modified. This diagram does not show the grain coarsening: in fact, the grains of the sintered material are appreciably coarser than the starting particles [BRO 911]

Densification: sintering shrinkage The starting compact has a porous volume (P) of about 40% of the total volume. However, for most applications, we want relatively non-porous, even dense, ceramics (P | 0%). In the absence of reactions leading to an increase in the specific volume, densification must be accompanied by an overall contraction of the part: characterized by linear withdrawal (dl/l 0), this contraction usually exceeds 10%. The control of the shrinkage is of vital importance for the industrialist: on the one hand, the shrinkage should not result in distortions of the shape and on the other hand, it must yield final dimensions as close as possible to the desired dimensions. In fact, an excessive shrinkage would make the part too small, which cannot be corrected, Sintering and Microstructure of Ceramics 61 and an insufficient shrinkage would make the part too large; in this case machining for achieving the desired dimension must be done by rectification, often by means of diamond grinding wheel M a finishing treatment all the more expensive as the volume of matter to be abraded is large. It is difficult to control shrinkage with a relative accuracy higher than 0.5%.

Because of the phenomenon of shrinkage, dilatometry tests are widely used for the in situ follow-up of sintering: starting with the OgreenP compact to arrive at the fired product, a heating at constant speed typically comprises three stages: i) thermal expansion, accompanied by a vaporization of the starting water and a pyrolysis of the organic binders introduced to support the pressing of the powder; ii) a marked contraction, due to particle rearrangement, the development of sintering necks and granular changes; iii) a resumption of the thermal expansion of the sintered product.

Many studies have sought to correlate the kinetics of shrinkage and the growth of inter-particle necks [BER 93, KUC 49].

Porosity is open as long as it is inter-connected and communicating: the material is then permeable to fluids. Porosity is closed when it is not inter-connected: even if it is not yet dense, the material can then be impermeable. The porosity level corresponding to the transformation of open pores to closed pores is about P | 10%.

Sintering generally occurs in the absence of external pressure applied during the heat treatments (pressureless sintering); the particles of the starting powders weld with one another to form a polycrystalline material, possibly with vitreous phases; the presence or absence of a liquid phase is important. Finally, the term sintering covers four phenomena: i) consolidation, ii) densification, iii) grain coarsening and iv) physicochemical reactions. The beginning of the densification is the usual sign for the beginning of the sintering, frequently followed by dilatometry experiments.

3.3. Interface effects

From a macroscopic point of view, the driving force behind the sintering of a powder to form a polycrystalline material is the reduction of energy resulting from the reduction of solid-vapor surfaces in favor of the grain boundaries. The necessary condition for sintering is therefore that the grain boundary energy ( JGB ) is low compared to the energy ( JSV ) of the solid-vapor surfaces. But this condition is not always achieved, as shown by silicon carbide (SiC) or silicon nitride (Si 3N4): materials where the JGB /JSV ratio is too high to allow easy sintering. The solution for sintering such materials can be i) the use of sintering additives chosen to increase 62 Ceramic Materials

JSV or to decrease JGB or ii) the use of pressure sintering, which provides external work: dW = M P external dV.

From a microscopic point of view, it is the differential pressure on either side of an interface that causes the matter transport making sintering possible. This pressure depends on the curvature of the surface.

Interface energy

The increase in energy ( J) at the level of the interfaces, due to the fact that the atoms do not have their normal environment, is always very insignificant: J is typically a fraction of Joules per m -2 . Substances added in very small quantities can have a marked effect M this is also the case with liquids, as shown in the use of surface active agents in washing powders and detergents. The surfaces of the particles and the grain boundaries of sintered materials are frequently covered by adsorbed species, segregations or precipitations, which means that interfacial energies are in general modified by these extrinsic effects. We can give the example of silicon-based non-oxide ceramics (SiC or Si 3N4), whose particles are covered with an oxidized skin M silica. As the specific surface of a powder grows as the inverse of the squared linear dimensions of the grain, the interfacial effects are marked in fine powders, which is generally the case with ceramic powders M the diameter of the particles measures typically from a fraction of a micrometer to a few micrometers, which corresponds to specific surfaces in the order of a few m 2g-1 .

The role of the curvature in the energy of an interface can be illustrated by considering a bubble blown in a soapy liquid using a straw. If we disregard the differences in density, and consequently the effects of gravity, the only obstacle for the expansion of the blown bubble under the pressure P is the increase of the energy at the interface. For a spherical bubble, the equilibrium radius r is the one for which the expansion work is equal to this increase in energy [KIN 76]:

'Pdv = JdA dv = 4 Sr2dr dA = 8 Srdr

§ 8ʌrdr · 'P = JdA/dv = J ¨ ¸ = 2 J/r © 4Sr2dr ¹

We note that the difference in pressure 'P is proportional to the interfacial energy (here J = JLV , liquid-vapor interface) and inversely proportional to the radius of curvature.

Sintering and Microstructure of Ceramics 63

For a non-spherical surface with main radii of curvature r) and r)):

1 1 'P = J r'  r '' [3.1]

Likewise, the rise h of a liquid in a capillary of radius r is such that:

'P = 2 Jcos T/r = Ugh where U is the density of the liquid and T the liquid-solid wetting angle.

The relation: J = (r Ugh)/(2cos T) is used to assess J by measuring T.

These capillary effects contribute to the vitrification of silicate ceramics, because the viscous liquid formed by the molten components infiltrates into the interstices between the non-molten particles.

The difference in pressure through a curved surface implies an increase in vapor pressure and also in solubility, at a point of high curvature:

:' P = RT ln(P/P 0) = V J(1/r) + 1/r))) where : is the molar volume, P the vapor pressure above the curved surface, and P 0 the pressure above a plane surface. Thus:

ln(P/P 0) = ( :J /RT)(1/r) + 1/r))) = (M J/URT)(1/r) + 1/r))) [3.2] where R is the ideal gas constant, T the temperature, M the molar mass, and U the density. Equation [3.2] is the Thomson-Kelvin equation.

For a spherical surface, this relation can be seen when we consider the transfer of a mole of the compound as a result of the vapor pressure on the surface, the work provided being equal to the product of the specific energy and the variation of surface area:

RTlnP/P 0 = JdA = J 8 Srdr

As the variation of volume is dv = 4 Sr2dr, the variation in radius for the transfer of a mole is dr = :/(4 Sr2), consequently:

lnP/P 0 = ( :J /RT)(2/r), which is the above result when r) = r)) = r

The sign convention is to consider that the radii of curvature r) and r)) to be positive for convex surfaces and negative for concave surfaces. Equation [3.1] 64 Ceramic Materials shows that 'P = 0 for a plane surface ( r) and r)) = f). A bump tends to level itself and a hole to fill itself. We can mention as an example the progressive restoration of the flatness in a skating rink surface striped by the skates, after the skaters leave the rink (it is primarily surface diffusion that makes ice get back a smooth surface).

The concept of pressure on a surface is a macroscopic concept. At the microscopic scale, atomic diffusion in a crystallized phase occurs primarily due to the movements of the vacancies. However, the equilibrium concentration of the vacancies is less under a convex surface than under a plane surface, and it is higher under a concave surface than under a plane surface. Thus, the vacancies migrate from the high concentration areas to the low concentration areas, an action which implies a movement contrary to that of the atoms. The effects are all the more obvious according to how marked the curvature (1/ r) is and therefore the smaller particles are: sintering is facilitated by the use of fine powders (diameters of about a micrometer). However, the pressure variations and the energies brought into play by the interfacial effects still remain very low.

-2 EXAMPLE 1.M for spherical alumina particles ( JSV Al 2O3 |1Jm ), the surplus pressure associated with particles with a diameter of 1 micrometer is 0.2%.

EXAMPLE 2.M at what size must an Al 2O3 monocrystal be crushed to increase its energy from 500 kJmole -1 (500 kJ.mole -1 is a typical value of the energies brought -2 into play in chemical reactions involving metallic oxides)? If JSV Al 2O3 = 1 J m 3 -3 and U0Al 2O3 = 4.10 kg m , the answer is: at a size less than that of the crystal cell!

EXAMPLE 3.M what is the energy variation when 1 kg of SiO 2 powder composed of beads of 2 3m diameter sinters to give a dense sphere and without internal -2 3 -3 interfaces? If JSV SiO 2 | 0.3 J m and U0SiO 2 = 2.2. 10 kg m , the answer is: 20 kJ only.

The development of a sintering neck is illustrated by the simple model of two isodiametric spherical particles (see Figure 3.3). The connection between the two particles is a neck in the shape of a horse saddle, with r) < 0 depending on the concavity (in the plane of the figure) and r)) > 0 depending on the convexity (in the plane tangent to the two spheres, perpendicular to the figure). The neck is a very curved area, which constitutes a source of matter towards which the atoms coming from the surface (the sintering is then non-densifying) or the volume (the sintering is then densifying) migrate. The movements of matter result in the progressive coarsening of the sintering neck and the consolidation of the material.

Sintering and Microstructure of Ceramics 65

Surface diffusion Liquid Radius of the sintering x neck

Figure 3.3. At the beginning of the sintering, the consolidation is done by evaporation of the surfaces and condensation on the neck (on the left); this mechanism is not densifying. If there is a liquid (on the right) the capillary pressure helps the penetration of the liquid in the interstice and the dissolution-reprecipitation effects contribute to the matter transport

3.4. Matter transport

Even if the thermodynamic condition of sintering is met (A SV JSV > A GB JGB ), for the process to occur, its speed must be sufficient. However, the matter transport in a solid is very slow compared to a liquid or a gas. This matter transport can come from an overall movement (viscous flow of vitreous phases or plastic deformation of a crystal), the repetition of unit processes on an atomic scale (atomic diffusion in a crystal), from transport in vapor phase (evaporation then condensation) or in liquid phase (dissolution then reprecipitation). Speed is significant only if the temperature is sufficiently high. The diffusion (D) in a crystal or the inverse of the viscosity ( K) of glass vary as exp(E/RT), where E is the apparent activation energy of the process. The usual values of E are a few hundred kilojoules per mole. The normal sintering temperatures are about 0.6 to 0.8 T F, where T F is the melting point of the solid in question.

The matter movement takes place from the high energy areas towards the low energy areas M primarily, the sintering neck between the particles. We must distinguish two cases depending on the location of the source of matter: M when the source of matter is the surface, the mechanism is non-densifying, which means that the spheres take an ellipsoidal form, without their centers approaching one another. There is no macroscopic shrinkage and the porosity of the granular compact is not reduced significantly. The decrease in interfacial energy primarily comes from the grain coarsening; M when the source of matter is inside the grains (near the boundaries, or near defects such as dislocations), the mechanism is densifying: there is shrinkage and reduction in porosity (see Table 3.1 and Figure 3.4).

66 Ceramic Materials

For solid phase sintering, there are four ways of diffusion: i) surface diffusion, ii) volume diffusion (often called lattice diffusion), iii) vapor phase transport (evaporation-condensation), and iv) grain boundary diffusion: the boundaries are very disturbed areas, which allow Odiffusion short-circuitsP. For liquid phase sintering, we must add dissolution-reprecipitation effects or a vitreous flow. Finally, for pressure sintering the pressure exerted allows the plastic deformation of the crystallized phases and the viscous flow of the amorphous phases.

Path in Diffusion path Source Shaft of matter Result obtained Figure 3.4 of matter 1 Surface diffusion Surface Sintering neck Grain coarsening 2 Volume diffusion Surface Sintering neck Grain coarsening 3 Evaporation- Surface Sintering neck Grain coarsening condensation 4 Grain boundary Grain boundaries Sintering neck Densifying diffusion sintering 5 Volume diffusion Grain boundaries Sintering neck Densifying sintering 6 Volume diffusion Defects, like Sintering neck Densifying dislocations sintering

Table 3.1. Matter transport during a solid phase sintering [ASH 75]

3.4.1 . Viscous flow of vitreous phases

The difference in pressure on either side of a curved interface causes a stress V (a stress has the dimension of a pressure) which causes a viscous flow H of the glass. The flow rate d H/dt is proportional to the stress and inversely proportional to the viscosity K: d H/dt proportional to V/K.

In general, viscosity decreases exponentially when the temperature increases:

K = KO exp(Q/RT) o d H/dt proportional to V/Kexp(Q/RT) where Q is the apparent activation energy of the process. Sintering and Microstructure of Ceramics 67

Figure 3.4. Matter transport during a solid phase sintering; mechanisms 1, 2 and 3 are non- densifying; mechanisms 4, 5 and 6 are densifying; A schematizes a dislocation [ASH 75]

3.4.2. Atomic diffusion in crystallized phases

FickRs first law

J = MD( Gc/ Gx) for a unidirectional diffusion along x [3.3]

J is the flow of atoms passing through a unit surface, per time unit, D the diffusion coefficient of the species that diffuses and c its concentration.

FickRs second law

(Gc/ Gt) = D( G2c/ Gx2) [3.4]

Nernst-EinsteinRs equation The OforceP that acts on the atom that diffuses is the opposite of the chemical potential gradient. The mobility of the atom i is B i, the quotient of the speed of the atom by the driving force:

MB i = v i /[(1/N)d3 i/dx] [3.5]

Ji = M(1/N)(d3 i/x)B ici where N is the Avogadro number and 3i the chemical potential of the species i. 68 Ceramic Materials

Considering the activity equal to the unit:

d3 i = RTd(lnc i) [3.6]

Substituting [3.6] in [3.5] and comparing with [3.3], we obtain:

Ji = M(RT/N)B i (dc i/dx)

Di = kTB i, where k is the Boltzmann constant [3.7]

Therefore, the diffusion coefficient is proportional to the atomic mobility.

Besides, d3/dx is proportional to the pressure gradient dP/dx:

J | (D/kT)(dP/dx) [3.8]

The difference in pressure between the two sides of an interface causes a matter flow that is proportional to the pressure difference and to the diffusion coefficient of the mobile species [PHI 85].

NOTE.M D = D 0exp(MQ/RT), so despite the presence of the term kT in the denominator of [3.8], it is the exponential of the numerator that is more important: an increase of T results in a rapid increase of J.

2 -1 NOTE.M the volume diffusion coefficient D V is expressed in m s (or, often, in cm 2s-1 ). As regards grain boundary diffusion (or surface diffusion), it is usual to consider the thickness of the grain boundary e GB (or the thickness of the superficial area e S), so that the diffusion term is written as D GB eGB (or D SeS), the coefficients -1 -1 DGB and D S being then expressed in m s (or in cm s ).

3.4.3. Grain size distribution: scale effects

An essential objective in controlling the microstructure of a sintered material is to be able to control densification and grain growth separately. In a ceramic filter, for example, we want to preserve a notable porosity, with pores of sizes calibrated with respect to the medium to be filtered. In an optical porthole, on the contrary, we want the sintering to be accompanied by a complete densification (zero residual porosity) because the presence of residual pores would result in the diffusion of the light. However, we saw that certain matter transport mechanisms are non-densifying (like surface diffusion), while others are densifying (like grain boundary diffusion): the objective is to play on the sintering parameters in order to favor a particular mechanism. The size of the powder particles is one of the parameters at our disposal. Sintering and Microstructure of Ceramics 69

Although the powders in general consist of particles of irregular size and shape, the simplistic approach that considers isodiametric spherical particles makes a useful semi-quantitative analysis possible.

The laws of scale [HER 50] specify the manner in which a phenomenon associated with a OclusterP of particles must be transposed in the case of a homothetic cluster p times larger. For isodiametric spheres, the laws of scale relate to the radius of the spheres (r). Thus, the time taken to obtain a certain degree of progress in a process depends on granulometry, according to a law of scale that varies with the process brought into play. If t1 is the time that corresponds to the small cluster and t2 the time that corresponds to the large cluster, then n n t1/t2 = ( r1/r2) = (1/p) , where the value of n depends on the process. We are interested here in matter transport processes that ensure sintering. We will deal with only two cases (flow of a vitreous phase and diffusion-reprecipitation in a liquid) and will give the results for the other mechanisms.

Viscous flow of a vitreous phase The flow rate d H/dt is inversely proportional to the viscosity K and proportional to the stress, whose form (see equation [3.1]) is J/u, where u is the radius of the sintering neck. The duration Gt of the transport of a given quantity of matter is inversely proportional to the speed, therefore:

Gtviscosity f 1/(d H/dt) f Ku/ J

The radius of the neck u, is in a certain ratio k with the radius of the particles: u = kr . For a system that grows homothetically, particles p times coarser imply neck radii p times larger. Therefore, for this system that is p times larger:

Gtviscosity f p Ku/ J f p Kkr/ J [3.9]

This result shows that the duration is proportional to the size r of the particles and therefore that the sintering time necessary to obtain a certain degree of consolidation varies inversely to the size of the particles; for example, dividing the size of the particles by ten reduces the duration of the sintering in the same ratio.

Dissolution-reprecipitation in a liquid We suppose that the spherical particles are covered with a thin film of liquid, with a thickness of e L. The matter flow is:

J f (M D Liquid / kT)( GP/L) where D Liquid = transport coefficient in the liquid. 70 Ceramic Materials

The area of the section through which the flow of diffusion passes is A f e Lr, but the pressure at the points of contact between the particles is J/u, a term that is proportional to J/r, therefore:

GP f J/u f J/r

The volume of matter that must diffuse to make a given level of densification possible is proportional to the cube of the linear dimensions of the system and therefore proportional to r3. The time necessary to reach this level of densification is therefore:

Gtliquidphase f (displaced volume )/(speed diff x volume atom )

3 3 2 f r /(JA :) f r /[(D liquid /kT)( J/r )e LR:]

4 4 Gtliquidphase f [r kT][D liquid eLJ: ], therefore Gtliquidphase proportional to r [3.10]

The duration is proportional to the power of four of the size of the particles. Dividing the size of the particles by 10 helps, this time, to gain a factor of 10,000 in the sintering time.

Through a similar reasoning, we can show that the grain boundary diffusion and surface diffusion make the duration vary to the power of four of the size of the particles, the volume diffusion to the power of three, and evaporation-condensation to the power of two.

In short, liquid phase diffusion, surface diffusion and grain boundary diffusion (R -4 law in the three cases) are more sensitive to the reduction in size of the particles than to volume diffusion (R -3 ), evaporation condensation (R -2 ), and finally viscous flow (R -1 ).

3.5. Solid phase sintering

3.5.1. The three stages of sintering

Solid phase sintering refers to the case where no liquid phase has been identified (but observations through electronic microscopy in transmission sometimes show the presence of a very small quantity of liquid phase, for example due to a Sintering and Microstructure of Ceramics 71 segregation of the impurities along the grain boundaries). Solid phase sintering takes place in three successive stages: M initial stage: the particle system is similar to a set of spheres in contact, between which the sintering necks develop. If X is the radius of the neck and R the radius of the particles, the growth of the ratio X/R in time t, for an isothermal sintering, takes the form: (X/R) n = Bt/D m, where B is a characteristic parameter of the material and the exponents n and m vary according to the process brought into play. For example, n = 2 and m = 1 for viscous flow; n = 5 and m = 3 for volume diffusion; n = 6 and m = 4 for grain boundary diffusion; M intermediate stage: the system is schematized by a stacking of polyhedric grains intertwined at their common faces, with pores that form a canal system along the edges common to three grains, connected at the quadruple points (see Figure 3.5). The porosity is open. This diagram is valid as long as the densification does not exceed | 90-92%, a threshold beyond which the interconnection of the porosity disappears; M final stage: the porosity is closed; only isolated pores remain, often located at the quadruple points between the grains (Otriple pointsP on a two-dimensional section) but which can be trapped in intragranular position.

Figure 3.5. Diagram of the porosity in the form of inter-connected canals along the edges of a polyhedron with 14 faces, typical of the intermediate stage of sintering [GER 96]

72 Ceramic Materials

3.5.2. Grain growth

As the energy of the interfaces has the form JA, where J is the specific energy of the interface and A is the surface area of the interface, the systemRs energy can be reduced using two borderline cases: M pure densification: the particles preserve their original size, but the solid-gas interfaces ( JSG ) are replaced by grain boundaries ( JSS ), with a change in the shape of the particles; M coalescence and pure grain growth: the particles preserve their original form, but they change in size by coalescence, thus reducing the surface areas.

Pure densification has never been observed: there is always a grain growth. Owing to the difference in pressure ( 'P | J/r), the atoms diffuse from the high pressure area towards the low pressure area. In addition, a curved boundary blocked at its ends tends to reduce its length while evolving to a line segment. Because of these two causes, the boundary moves towards its center of curvature. By considering (in two dimensions) triple points with angles of 120 , the grains with less than six sides have their boundaries with the concave side turned towards the inside: the evolution towards the center of curvature makes these small grains disappear. A contrary evolution affects the grains with many sides: the small grains disappear in favor of the coarser grains, which grow (see Figure 3.6).

Figure 3.6. The pressure on the curved interfaces is such that the boundaries move towards their center of curvature: the small convex grains (less than 6 sides) disappear while the coarse concave grains (more than 6 sides) grow to the detriment of the neighboring grains; the grains with rectilinear boundaries have an appreciably hexagonal form [KIN 76]

In normal grain growth, the average grain size increases regularly, without marked modification of the relative distribution of the size; the microstructure expands homothetically. This type of grain growth is the one observed in a successful sintering. Sintering and Microstructure of Ceramics 73

Secondary recrystallization (or abnormal growth, or discontinuous grain growth) makes a few grains grow rapidly, to the detriment of the more moderately sized grains. The final microstructure is very heterogenous, with coexistence of very coarse grains and very small grains. This type of microstructure rarely leads to favorable properties and therefore is generally avoided.

In addition to the possibility of being homogenous or, on the contrary, heterogenous, the microstructure can be more or less isotropic. For the simple case of a mono-phased polycrystal, we can distinguish four cases: M equiaxed microstructure and random crystalline orientation of the grains (no orientation texture): the material is isotropic by effect of average; M equiaxed microstructure, but orientation texture: the matter loses its average isotropy and the overall anisotropy is all the more marked that the crystal in question is more anisotropic for the property concerned; M oriented microstructure, but no orientation texture: anisotropy; M oriented microstructure and orientation texture: maximum anisotropy. The polycrystal then offers properties close to those of the monocrystal M except for the intrinsic effect of the grain boundaries. We can cite as an example the case of graphite fibers (Ocarbon fibersP) used for the mechanical reinforcement of composites.

The majority of ceramics are multiphased materials that comprise both crystallized and vitreous phases. Porcelain thus consists of silicate glass OreinforcedP by acicular crystals of crystallized mullite, but we can also observe millimetric crystal agglomerates with a very porous microstructure (iron and steel refractory materials), or fine grained polycrystals (< 10 3m) without vitreous phases and with very low porosity (hip prosthesis in alumina or zirconia). It should be reiterated that, in addition to the chemical nature of the compound(s) in question, it is the microstructure of the material (size and shape of the grains, rate and type of porosity, distribution of the phases) that controls the properties.

3.5.3. Competition between consolidation and grain growth

Densification M and therefore the elimination of the pores M occurs effectively only if the pores remain located on the grain boundaries (intergranular position), because then the matter movements can take advantage of the grain boundary diffusion. However, a too rapid grain growth M and therefore a migration of the boundaries M leads to a separation of the pores and the boundaries: the pores are then trapped in the intragranular position, where they are difficult to eliminate, because only volume diffusion remains active. If the objective is to sinter a material to its 74 Ceramic Materials ultimate density, and therefore eliminate all the pores, the growth of the grains must be limited.

In addition to its role in the coupling between densification and grain growth, the size of the grains ( )) of the sintered ceramics is, together with the porosity, the essential microstructural parameter. We can give five examples: M the brittle fracture of the ceramics is controlled by the size of the microscopic -1/2 cracks, because the mechanical strength Vf is proportional to K cac , where K c is the toughness and a c the length of the critical microscopic crack. However, a c is of -1/2 the same order as the size of the grain. This means that Vf varies typically by ) : ceramics with high mechanical strength (machine parts, cutting tools, hip prostheses, etc.) must be very fine-grained; M the particle composites based on partially stabilized zirconia use mechanical reinforcement mechanisms that depend on the size of the zirconia inclusions: if they are too small (< 0.3 3m) they remain tetragonal and if they are too large (1 3m) they destabilize towards the monoclinical form, with swelling and thus micro-cracking of the surrounding matrix. The optimal effect is achieved for particles of intermediate size, metastable tetragonal, which are transformed from tetragonal to monoclinical in the stress field of a crack that propagates itself; M the high temperature creep of refractory materials is often due to diffusion mechanisms: volume diffusion leads to the Nabarro-Herring creep and grain boundary diffusion to the Coble creep, with creep rates in )-2 and )-3 , respectively: refractory materials must therefore be coarse-grained in order to slow down the creep; M ferroelectric or ferrimagnetic ceramics have performances sensitive to the size of the domains (size that interacts with the grain size) and to the migration of the walls (which is hampered by the grain boundaries): very fine grains are monodomain, and from them we have ferroelectric ceramics with very high dielectric constant or OhardP ferrimagnetic ceramics with very high coercitive field strength; M finally, the transport properties (electric conduction or thermal conduction) are sensitive to the intergranular barriers due to the structural disorder of the grain boundaries or to the presence of secondary phases that are segregated there: coarse grains mean fewer grain boundaries and therefore fewer barriers.

3.5.4. Normal grain growth

In a fine-grained polycrystal heated to a sufficient temperature, the size of the grains grows and correlatively the number of grains decreases. The driving energy is the one that corresponds to the disappearance of the grain boundaries (it is of the order of a fraction of J.m -2 ). Sintering and Microstructure of Ceramics 75

The grain growth rate is proportional to the rate of migration of the boundary; this rate ( v) can be written as the product of the mobility of the grain boundary (M GB ) and a driving force (F):

v | d )/dt with: v = M GB F

The driving force is due to the pressure difference caused by the curvature of the boundary:

'P = JGB (1/ r) + 1/ r)))

JGB is the energy of the grain boundary and r) and r)) are the curvature radii at the point in question.

When the grain growth is normal, the distribution of the grain sizes remains significantly unchanged, with a homothetic growth. Consequently:

(1/r) + 1/r))) | 1/ K ) where K is a constant.

Pure monophased material A simple reasoning based on a two-dimensional microstructure (section of a polycrystal), where the equilibrium configuration of a Otriple pointP corresponds to angles of 120 , is that grains with less than six sides are limited by convex boundaries and therefore tend to decrease, whereas those with more than six sides are limited by concave boundaries and therefore tend to grow (see Figure 3.6). If the curvature radius of a grain is proportional to its diameter, the driving force and the growth rate are inversely proportional to its size:

d)/dt = Cte/ ) hence ) | t 1/2 [3.11]

The grain size must increase by the square root of the time.

Among the simplistic assumptions that have been made, we note that only the curvature of the boundary has been considered and not the crystalline anisotropy.

Obstacles to grain growth When we express experimental results of grain growth in the form of a graph ln ) = f(lnt), we obtain a straight line whose slope is, in general, less than the exponent 1/2 predicted by the parabolic law. This means that the growth is slowed down by various obstacles. Based on the interaction between a mobile grain boundary and an obstacle, we can distinguish three main cases: i) impurities in solid solution or liquid phase wetting the boundaries, ii) immobile obstacles, which block 76 Ceramic Materials any movement of the boundary, and iii) mobile obstacles capable of migrating with the boundary.

The impurities in solid solution can slow down the movement of the boundaries because they prefer to lodge themselves close to the grain boundary and therefore the boundary can migrate either by carrying these impurities along M which slows down the movement M or by leaving them in the intragranular position M which puts them in an energetically less favorable position than before the migration of the boundary. The growth law is thus modified because of the presence of these impurities and we get:

d)/dt | 1/ )2 ) | K t 1/3 [3.12]

The growth law ) | t 1/3 (impure phase) is more frequently observed than the law ) | t 1/2 (pure phase).

The presence of a liquid phase that wets the boundaries tends to reduce the grain growth, by reducing the driving energy and increasing the diffusion path, since there now is a double interface. It is true that diffusion in a liquid is fast; however, the dissolution-diffusion-reprecipitation process is generally slower than the simple jump through a grain boundary. Thus, the presence of a small quantity of a molten silicate phase limits the grain growth of the sintered alumina with liquid phase. On the other hand, the presence of a liquid phase can favor chemical reactions of type A + B o C and therefore allow the growth of the grains C to the detriment of the grains A and B. This type of growth often leads to secondary recrystallization (exaggerated growth). The growth law is ) | t 1/3 , as for an impure phase.

The immobile obstacles, such as precipitates and inclusions, OpinP the boundaries, reducing their energy by a quantity equal to the product of the specific pinning energy and the surface area of the inclusion. To be OundraggedP, the boundary must be subjected to a tearing force. As long as the migration driving force of the boundary, due to the effects of curvature, does not exceed this tearing force, the boundary remains pinned and the grain size is stable. For grains anchored by inclusions, the growth can occur only if: M the inclusions coalesce by diffusion, to give less numerous but more voluminous inclusions (Ostwald ripening). If the coalescence takes place by volume diffusion, the radius of the inclusion ( r) increases as r 3 | t, which again yields a grain growth obeying a law ) | t 1/3 ; M the inclusions disappear by dissolution in the matrix: ) | t; M secondary recrystallization occurs: this is the end of normal growth. Sintering and Microstructure of Ceramics 77

The mobile obstacles are essentially pores. If vP and vGB are the speeds of the pore and the boundary, M P and M GB are the mobilities, and F P and F GB are the corresponding OforcesP, we have vP = M PFP and vGB = M GB FGB . The pore separates itself from the boundary if vGB > vP.

The force on the boundary F GB has two components, one due to the curvature (FR GB ) and the other due to the pinning effect by the pores, which equals NF P, if there are N pores. The condition for non-separation is therefore:

vP = M PFP = vGB = M GB (FR GB M N F P) vGB = F GB (M P MGB )/(N M GB + M P)

M if NM GB >> M P, then vGB = F GB (M P/N): the rate of migration of the boundaries is controlled by the characteristics of the pores;

M if NM GB << M P, then vGB = F GB (M GB ): the rate of migration of the boundaries is controlled by the characteristics of the boundaries themselves.

Different mechanisms lead to different laws of type ) | t1/n . The values of the exponent n depend on the mechanism and the diffusion path that control the process. For example, for control by the pores: n = 4 for surface diffusion, n = 2 for volume diffusion, and n = 3 for vapor phase diffusion; for control by the boundaries: n = 2 for a pure phase and n = 3 for the coalescence of a second phase by volume diffusion. The experimental studies of the grain growth consist of: i) quantifying the grain size ), ii) determining the exponent n of the growth law ) | t1/n , and iii) determining the apparent activation energy E of the process. The results are semi- quantitative, because of two difficulties: i) inaccuracy of the measures of the grain size and ii) simultaneous occurrence of several processes M with different values of n and E. The law of normal grain growth that is most frequently observed is the law ) | t1/3 .

3.5.5. Abnormal grain growth

Some grains develop in an exaggerated manner, the process occurring when a grain reaches a significant size with a shape limited by many concave sides: there is then a rapid growth of the coarse grain, to the detriment of fine convex grains that border it (see Figure 3.6). When the grain reaches this critical size )C, much higher than the average size of the other grains in the matrix )average , the concave curvature is determined by the size of the small grains and is therefore proportional to 1/ )average . Hence, this apparent paradox that the use of a very fine starting powder can sometimes increase the risk of secondary recrystallization, because the presence of a few particles of size much higher than )average , is more probable there than in coarser powders where )average is higher.

78 Ceramic Materials

In some sintered materials, we observe very coarse grains with straight sides, whose growth cannot be explained by the surface tension on the curved boundaries. These are often materials whose grain boundary energy is very anisotropic where the growth favors the low energy facets (see Figure 3.7). This effect is observed in many rocks. They can also be materials where the impurities lead to the appearance of a small quantity of intergranular phase between the coarse grain and the matrix, which favor the growth M but a larger quantity of liquid phase would make the penetration in all the boundaries possible, limiting both normal and exaggerated growth.

Abnormal grain growth generally obeys a law ) | t, whereas normal growth leads to laws ) | t1/3 or ) | t1/2 : the abnormal growth must be fought from the beginning, because, once started, its kinetics is rapid.

Figure 3.7. Abnormal grain growth in In 2O3 sintered at high temperature (1,500 C for 50 h). Some grains have grown exaggeratedly in a fine-grain matrix [NAD 97]

3.6. Sintering with liquid phase: vitrification

3.6.1. Parameters of the liquid phase

In general, the presence of a liquid phase facilitates sintering. Vitrification is the rule for silicate ceramics where the reactions between the starting components form compounds melting at a rather low temperature, with the development of an abundant quantity of viscous liquid. Various technical ceramics, most metals and cermets are all sintered in the presence of a liquid phase. It is rare that sintering with liquid phase does not imply any chemical reactions, but in the simple case where these reactions do not have a marked influence, surface effects are predominant. The main parameters are therefore: i) quantity of liquid phase, ii) its viscosity, iii) its Sintering and Microstructure of Ceramics 79 wettability with respect to the solid, and iv) the respective solubilities of the solid in the liquid and the liquid in the solid: M quantity of liquid: as the compact stacking of isodiametric spheres leaves a porosity of approximately 26%; this value is the order of magnitude of the volume of liquid phase necessary to fill all the interstices and allow the rearrangement of the grains observed at the beginning of the vitrification. However, the presence of a small quantity of liquid (a few volumes percent) does not make it possible to fill the interstices; M viscosity of the liquid: this decreases rapidly when the temperature increases (typically according to the Arrhenius law). Pure silica melts only at a very high temperature to produce a very viscous liquid. The presence of alkalines and alkaline earths quickly decreases the softening temperature and the viscosity of the liquid. The viscosity of the liquid should be neither too low M because then the sintered part becomes deformed in an unacceptable way M nor too high M because then the viscous flow is too limited, making grain rearrangement difficult; M wettability: wettability is quantifiable by the experiment of the liquid drop placed on a solid, because the equilibrium shape of the drop minimizes the interfacial energies. If JLV is the liquid-vapor energy, JSV the solid-vapor energy and JSL the solid-liquid energy, the angle of contact ( T) is such that (see Figure 3.8):

JLV cos T = JSV M JSL [3.13]

When JSL is high, the drop minimizes its interface with the solid, hence a high value of T: T > 90 corresponds to non-wetting (depression of the liquid in a capillary). On the contrary, when JSL << JSV , the liquid spreads on the surface of the solid: T < 90 corresponds to wetting (rise of the liquid in a capillary); and for T = 0, the wetting is perfect.

In a granular solid that contains a liquid, the respective values of JSL and JGB (grain boundary energy) determine the value of the dihedral angle 4:

2JSL cos 4/2 = JGB [3.14]

Figure 3.9 shows the penetration of the liquid between the particles of a granular solid according to the value of 4. For low 4 (0 to 30 ), the liquid wets the boundaries; when 4 continues to grow, the occurrence of the liquid phase becomes less marked and for a high value of 4 ( 4 > 120 ), the liquid tends to form pockets located at the Otriple pointsP M on a two-dimensional view, but at the Oquadruple pointsP in three-dimensional space. Based on mutual solubilities we can distinguish four cases (see Table 3.2). 80 Ceramic Materials

Figure 3.8. Drop placed on a liquid; the value of T characterizes the wettability: wetting on the left; non-wetting on the right

Figure 3.9. Penetration of the liquid between the grains depending on the value of 4 [GER 96]

Low solubility of the solid High solubility of the solid in the liquid in the liquid Low solubility of the liquid Low assistance High assistance in the solid to densification to densification High solubility of the Swelling, transitory liquid Swelling, and/or liquid in the solid densification

Table 3.2. Effects of mutual solubilities on sintering [GER 96]

3.6.2. The stages in liquid phase sintering

The shrinkage curve recorded during an isothermal treatment of liquid phase sintering shows three stages: M viscous flow and grain rearrangement: when the liquid is formed, the limiting process consists of a viscous flow, which allows the rearrangement of the grains. Sintering and Microstructure of Ceramics 81

The liquid dissolves the surface asperities and also dissolves the small particles. The granular rearrangement is limited to the liquid phase sintering itself, but it can be enough to allow complete densification if the liquid phase is in sufficient quantity, as is the case in the vitrification of silicate ceramics; M solution-reprecipitation: the solubility of the solid in the liquid increases at the inter-particle points of contact. The transfer of matter followed by reprecipitation in the low energy areas results in densification; M development of the solid skeleton: the liquid phase is eliminated gradually by the formation of new crystals or solid solutions; we tend to approach the case of solid phase sintering and the last stage of the elimination of porosity is similar to the one observed in this case.

The disintegration of the particles attacked by the liquid results in the Ostwald ripening (coalescence of small particles to give a larger particle) and changes in the shape of the particles, with flattening of the areas of contact. As the anisotropy of crystalline growth is less hampered when a crystal grows in a liquid than when it remains in contact with solid obstacles, we sometimes observe grains whose morphology reflects these anisotropy effects: for instance, they are elongated and faceted.

The role of chemical reactions is still significant, because they bring into play energies much higher than the interfacial ones and frequently the reactions between liquid and solid result in the formation of new phases. We can thus distinguish three cases: M weak reaction between liquid and solid: the liquid has the primary role, after cooling, of forming the matrix in which the grains that have not reacted have been glued. This is the case of abrasive materials where the grains (silicon carbide SiC or alumina Al 2O3) are bound by a solidified vitreous phase; M reaction between liquid and solid, solid with congruent melting: there is no appearance of new solid phases but modification of the existing ones. This is the case for silicate ceramics made of quartz sand (SiO2) and clay (whose primary mineral is kaolinite, written as (Al 2O3.2SiO 2.2H 2O), fired at rather low temperatures. The high viscosity of the silicate liquid prevents the system from reaching the equilibrium; in particular, glass of the eutectic composition does not decompose into mullite plus cristobalite, as suggested by the equilibrium diagram. Only the finest particles react; the coarsest do not dissolve. The coarse quartz grains, for example, hardly react with clay M but firing transforms them, almost completely, into cristobalite (a high temperature variety of crystallized silica); M reaction between liquid and solid, solid with incongruent melting: an example is that of the system containing quartz (SiO 2) + kaolinite (Al 2O3-2SiO 2-2H 2O) + potassic feldspar (6SiO 2-K 2O-Al 2O3), which is the basic system of porcelains.

82 Ceramic Materials

At about T = 1,150 C, the feldspar melts to give leucite (4SiO 2.Al 2O3.K 2O) and a vitreous phase (with a composition close to 9SiO 2. Al 2O3.K 2O). Leucite dissolves gradually into glass to produce a flow that is very viscous until it melts at about 1,530 C: at 1,300 C, the viscosity is equal to 10 6 poises and it decreases only slowly with temperature: at 1,400 C it is still 5.10 5 poises. Potassic feldspar is a flux (a component that, by reaction with the other components, gives rise to a phase with low melting point) which produces a liquid whose viscosity does not vary too quickly with the temperature, and which therefore does not require a very strict control of this temperature: the firing range is broad. On the contrary, certain fluxes (for example, calcic phases) have a sudden effect because they create phases with too low viscosity.

3.7. Sintering additives: sintering maps

The spectacular effect of the addition of a few hundred ppm of magnesia on the sintering of alumina is the best example of the role of sintering additives. These additives help to control the microstructure of the sintered materials; they can be classified under two categories: M additives that react with the basic compound to give a liquid phase, for example by the appearance of an eutectic at a melting point less than the sintering temperature. We then go from the case of solid phase sintering to liquid phase sintering M even if the liquid is very insignificant. Silicon nitride Si 3N4 ceramics are an example of where some sintering additives are selected to react with the silica layer (SiO 2) that covers the nitride grains, in order to produce a eutectic. Thus, magnesia MgO reacts with SiO 2 to form the enstatite MgSiO 3, from which we have a liquid phase at about 1,550 C. The liquid film wets the grain boundaries and shapes of the pockets at the triple points; M additives that do not lead to the formation of a liquid phase and which consequently enable the sintering to take place in solid phase. This is the case of the doping of Al 2O3 with a few hundred ppm of MgO, because the lowest temperature at which a liquid can appear in the Al 2O3-MgO system exceeds the sintering temperature (which, for alumina, does not go beyond 1,700 C).

The explanation of the role of this second category of additives is primarily phenomenological. It considers the respective values of the diffusion coefficients and the mobility of the boundaries:

M D L characterizes volume diffusion (L = lattice), D b grain boundary diffusion and D S surface diffusion; M M b characterizes the mobility of the grain boundaries.

The sintering maps [HAR 84] place the diameter of the grain (G) on the ordinate and densification ( U = d/d 0) on the abscissa (see Figure 3.10). The two extreme cases Sintering and Microstructure of Ceramics 83 would be i) a grain coarsening without densification (vertical trajectory) and ii) a densification with unchanged grain size (horizontal trajectory).

Experimentally, we always observe an intermediate trajectory between these two extremes because the densification is inevitably accompanied by grain growth.

In order to densify the material to 100%, the key point is to prevent the pores and the boundaries from separating because then, as we already said, the residual pores are trapped in the intragranular position, where it is practically impossible to eliminate them. The trajectory G = f( U) must therefore be as flat as possible and must, in particular, go below the lowest point of the pore-boundary separation area (in the figure: the point ordinate G* abscissa U*). Densification cannot reach 100% if the trajectory cuts this separation area. Various ratios characterize the relationship between Ocontribution of the diffusion to densificationP and Ocontribution of the diffusion to grain coarseningP, with the first term in the numerator and the second term in the denominator. For example, D L/D L means: Odensification controlled by volume diffusionP and Ograin coarsening controlled by volume diffusionP, whereas Db/D S means Odensification by boundary diffusionP and Ograin coarsening controlled by surface diffusionP (see Figure 3.11). The possible effect of an additive can be seen from the following observations:

M an increase in D L flattens the trajectory without affecting the separation area: this increase of D L is favorable to the densification; M a decrease in M b increases G* and therefore shifts the separation area towards the top and slightly flattens the trajectory: this decrease in M b also has a favorable effect on the densification;

M a decrease in D S flattens the trajectory (which is favorable), but decreases G* and therefore shifts the separation area to the bottom (which is unfavorable). All in all this decrease in surface diffusion M which as we said earlier leads to a non- densifying sintering M would not have a significantly useful (or harmful) effect.

The use of these sintering maps to explain the effectiveness of MgO as a sintering additive for Al 2O3 suggests that MgO increases D L (first favorable effect) and especially decreases M b (second favorable effect). This phenomenological explanation does not, however, provide information on the mechanisms brought into play and in particular it does not give the reason for which MgO reduces the mobility of the boundaries. An explanation [BAE 94] would be that the traces of impurities (SiO 2 and CaO), which continue to exist even in so-called high purity alumina powders, are located along the grain boundaries, to form at the sintering temperature a thin liquid film which promotes the grain growth M Osolid phase sinteringP then becoming a sintering controlled by a very insignificant liquid phase. The influence of MgO would then be Oto purifyP the grain boundaries while reacting with SiO 2 or CaO. 84 Ceramic Materials

Pore-boundary separation Grain size Thickness Grain size trajectory

Density Density Figure 3.10. Sintering map showing the grain size depending on the densification [HAR 84]. On the left: principle of the map; on the right: for complete densification to be possible, the sintering trajectory must not cut the hatched pore-boundary separation area

Figure 3.11. Role of a sintering additive [HAR 84]. On the left, the effect of the doping agent is to multiply D L by 10: the influence is favorable by the flattening of the trajectory. On the right, the effect of the doping agent is to divide M b by 10: the influence is doubly favorable by the raising of the separation area and flatness of the trajectory Sintering and Microstructure of Ceramics 85

The doping of Al 2O3 by MgO has been transposed to various ceramic systems, for which we have determined which sintering additives limit the grain growth and make a densification close to 100% possible [NAD 97]. These studies provide answers on a case-by-case basis and there is still no general theory for the selection of the optimal additive.

The choice of the sintering temperature also plays on the relative values of the diffusion coefficients and therefore favors a densifying or a non-densifying mechanism. For example, surface diffusion has an apparent activation energy generally less than the volume diffusion. The chronothermic effect (Oa long duration heat treatment at lower temperature is equivalent to a short duration heat treatment at higher temperatureP) therefore offers broader possibilities than those offered by the Arrhenius law with a single activation energy: low temperature sintering primarily bringing into play surface diffusion (non-densifying mechanism), and high temperature sintering volume diffusion or the grain boundary diffusion (densifying mechanisms). A high temperature treatment favors, all things being equal, high densification.

3.8. Pressure sintering and hot isostatic pressing

3.8.1. Applying a pressure during sintering

In most cases, ceramics are sintered by pressureless sintering and it is only for very special applications that we use Opressure sinteringP or Ohot pressingP, which consists of applying a pressure during the heat treatment itself. The characteristic of pressure sintering is that the pressures brought into play M which are usually about 10 to 70 MPa, but can exceed 100 MPa M have considerable effects compared to capillary actions, thus offering four advantages: i) thickening of materials whose interfacial energy balances are unfavorable; ii) rapid densification at appreciably lower temperatures (several hundred degrees sometimes) than those demanded by pressureless sintering; iii) possibility of reaching the theoretical density (zero porosity); iv) possibility of limiting the grain growth.

Furthermore, it can be possible to obtain the sintered part with its exact dimensions ( net shape ), without the need for a machine finishing in applications that require high dimensional accuracy. The other side of the coin is the technical complexity of the process and the high costs incurred, as well as the limitations on the geometry of the parts, which can only have simple forms and a rather reduced size. We must have pressurization devices manufactured in materials that resist the temperatures required by sintering M and even if these temperatures are lower 86 Ceramic Materials compared to those required by pressureless sintering, they are still high M and the chemical reactions between these materials and the environment (for example, oxidation of refractory metals), like the reactions between the mould and the ceramic powder, must be limited. One last difficulty: if the manufacture of parts with simple geometry (pellets) can be done in a piston + cylinder mould (Ouniaxial pressure pressingP), obtaining more complex shapes, in particular undercut parts, cannot be done by pressure sintering. We must then apply the technique of hot isostatic pressing or OHIPP, where the pressure is not transmitted by a piston but by a gas, hence the hydrostaticity (isostaticity) of the efforts, in analogy with Ocold isostatic pressingP described in Chapter 5, but where the pressure transmitting fluid is a liquid and not a gas.

3.8.2. Pressure sintering

Graphite is the most used material for the manufacture of the mould and the piston of uniaxial pressure sintering equipments, because of its exceptional refractarity, with this originality that the mechanical strength grows when the temperature rises (until beyond 2,000 C), also taking into account its easy machinability and the generally limited speed of the reactions with the ceramic powders M often protected by a fine boron nitride deposit. But the oxidation ability of the graphite requires a reducing or neutral processing atmosphere, which is appropriate for non-oxides (primarily carbides, like HPSC, and nitrides, like HPSN; see Chapter 7), but can lead to oxygen under-stoichiometry for those oxides that are reduced easily. Refractory metals (Mo or W) and ceramics (Al 2O3 or SiC) have also been used for the piston-cylinder couple of the mould.

The powders to be sintered are generally very fine (< 1 3m) and it is not always necessary for them to contain additives required by pressureless sintering (for example, MgO for the sintering of Al 2O3). The justifiable applications of pressure sintering are, for example, cutting tools (ceramics or cermets) or optical parts, with the essential objectives of achieving a 100% densification and/or very fine grains M but the microstructure and the crystallographic texture can present anisotropy effects because of the uniaxiality of the pressing. Alumina for cutting tools, carbides (B 4C, for instance) or cermets are examples of materials that can benefit from pressure sintering and HIP (see further down); the same is true for metallic OsuperalloysP used in the hot parts of turbojets. High temperature composite materials are another example where the application of a pressure during heat treatments can be necessary to allow the impregnation of the fibrous wicks and favor the densification. Functional ceramics (BaTiO 3 or, especially, magnetic ferrites) can gain from very fine grains and the absence of residual porosity made possible by pressure sintering. As optical transparency is no doubt the property that is most quickly degraded by the presence of pores, even in extremely small numbers, perfectly transparent Sintering and Microstructure of Ceramics 87

polycrystalline ceramics (MgAl 2O4, Al 2O3, Y 2O3, etc.) are examples of materials that benefit from the use of pressure sintering. As regards the mechanisms, pressure sintering implies: i) rearrangement of the particles, ii) lattice diffusion, iii) grain boundary diffusion, and finally iv) plastic deformation and a viscous flow. Pressureless sintering involves much less the effects i) and iv) and, as for the effects ii) and iii), the high level of the mechanical stresses (often close to and even exceeding the stresses caused by the normal operation of a part, for example a refractory part in a high temperature facility) brings them close to creep effects. This can be diffusion creep (Nabarro-Herring creep due to intragranular diffusion, Coble creep due to the grain boundary diffusion) or creep due to the movement of dislocations.

The creep equation, modified for pressure sintering, can be written as:

(1/ U)(d U/dt) = (CD)/(kT )m) [ Vn + 2 J/r] [3.15] where U is the density, C a constant, D the coefficient that controls the diffusion process, k the Boltzmann constant and T the temperature, ) the average grain size, V the pressure applied on the particles, J the surface energy and r the radius of the pores. The exponents m and n characterize respectively the role of the grain size and that of the pressure applied. Table 3.3 recapitulates the relevant parameters (see Chapter 8).

Mechanism Grain size Stress exponent, n Coefficient exponent, m of diffusion, D

Nabarro-Herring 2 1 Volume diff. D V

Coble 3 1 Boundary diff. D J

Intergranular sliding 1 1 or 2 D J, D V

Interface reactions 1 2 D J, D V

Plastic flow 0 t 3 D V

Table 3.3. Mechanisms of pressure sintering [HAR 91]

In most cases, the use of fine grained ceramics on the one hand, and the high level of plastic flow required by iono-covalent crystals on the other, are such that the diffusion terms (Nabarro-Herring or Coble) override the plastic flow. Grain boundary diffusion dominates over volume diffusion all the more when the grains are finer and the temperature lower, because the volume exponent of the former is 3 whereas that of the latter is only 2, and the activation enthalpy of D J is in general lower than that of D V. 88 Ceramic Materials

Boundary sliding is necessary in order to accommodate the variations in shape caused by the diffusion creep, which implies that the mechanisms must act sequentially and therefore that the overall kinetics is controlled by the slowest mechanism. Nonetheless, when the mechanisms can act concurrently (as is the case with diffusion creep and plastic flow), it is the fastest process that controls the overall kinetics.

An illustration [TAI 98] of pressure sintering (1 hour at 1,360 C, p = 20 MPa, graphite matrix, antiadhesive layer of BN, in vacuum) is obtaining particle composites 10%Al 2O2-80%WC-10%Co with a mechanical strength of 1,250 MPa: pressureless sintering would not allow the densification of this type of material, whose microstructure exhibits an inter-connected matrix of WC, with precipitates of Al 2O3 and Co 3W3C (see Figure 3.12).

Figure 3.12. 10% Al 2O3-80% WC-10% Co composite, sintered under pressure [TAI 98]

3.8.3. Hot isostatic pressing (HIP)

Whereas for cold isostatic pressing (CIC M see Chapter 5), the pressurization fluid is a liquid, it is a gas (in general argon, but reactive atmospheres are also used, for example oxygen) that provides the pressurization in HIP. This technique was invented by the Battelle institute (USA) in the 1950s. We can imagine the risks of destructive explosions (use of a compressible fluid instead of an incompressible fluid) and the difficulties in ensuring air-tightness as well as the problems of pollution and control of thermal transfers: under a pressure of 1,000 atmospheres, a gas like argon has a density higher than that of liquid water at 20 C!

The two main methods involving HIP are direct consolidation by HIP, and HIP perfecting a pressureless sintering having preceded it (see Figure 3.13). Sintering and Microstructure of Ceramics 89

Powder preparation Forming Powder preparation Forming Wrapped in a HIP post-sintering glass envelope Sintering

Envelope elimination HIP treatment

Figure 3.13. Direct HIP (on the left) and post-sintering HIP (on the right) [DAV 91]

Consolidation by HIP When HIP is used directly to consolidate a powder, the OcompactP must be encapsulated in an envelope in a form homothetic to that of the part to be obtained, with vacuum evacuation of gases, followed by sealing of the envelope. Soft or stainless steels can be used as envelope materials for relatively low temperature treatments (1,100M1,200 C), whereas it is necessary to use refractory metals (Ta, Mo) for higher temperatures treatments. As the risks of distortion become higher when the overall pressing increases, we gain from a powder pressed at a high rate and homogenously (by CIC primarily). An alternative is to carry out a Opre- sinteringP providing sufficient cohesion to the part to make its handling possible, and then to coat it powdered glass which, at sufficient temperature, will become viscous enough to coat the piece with an impermeable layer. This will make it possible for HIP to take place without the pressurized gas being able to penetrate the open porosity.

HIP as post-sintering operation This involves sintering the part until the inter-connected open porosity is eliminated (which requires a densification of about 95%) and then subjecting this part to a secondary HIP treatment. The greatest advantage is avoiding the need for an envelope (cost, complexity, restrictions on the possible forms, necessity to clean the end product to eliminate the envelope). It is furthermore possible, for manufacturers who do not have an HIP equipment, to sub-contract this stage to a specialized partner. There are HIP chambers whose size is more than one meter, which makes it possible to treat large parts or a great number of small parts.

90 Ceramic Materials

The densification of metallic powders (Opowder metallurgyP) involves HIP much more frequently than the densification of ceramic powders: a search on the Web shows that most sites dealing with HIP relate to metallic products (the term taken in its largest sense and including cermets).

3.8.4. Densification/conformity of shapes in HIP

Densification The densification of the parts by HIP implies primarily three phenomena: i) fragmentation of the particles and rearrangement, ii) deformation of the inter- particle areas of contact and iii) elimination of the pores. The first process is transitory and hardly contributes to the overall densification, at least if the initial forming (for example, by CIC) has been correctly carried out. The second process brings into play effects of plastic deformation by movement of dislocations and diffusion phenomena that are similar to those indicated in the case of uniaxial pressure sintering. Lastly, by considering the final reduction of porosity, we can write phenomenologically:

(1/ U)(d U/dt) i = B ifi ( U) [3.16] where U is the relative density, B i constant kinetics (implying the terms relating to the material and those relating to the characteristics of the HIP process) and f i(U) a geometrical function that depends only on the relative density. Each process i is described by specific expressions for B i and f i [LI 87]. For example:

3 1/2 Ki = 270 GDjg :P/kTR and f i ( U) = (1- U) if U > 90% [3.17] for grain boundary diffusion (Coble), if G is Othe thicknessP of the boundary, D jg the corresponding diffusion coefficient, : the volume of the atom that diffuses and R the radius of the grain assumed to be spherical, k, T and P having their usual meaning.

Ashby et al. [LI 87] have developed the approach of OHIP mapsP, where, for a material under given conditions, the areas in two-dimensional space (relative density depending on the pressure), in which the predominant phenomenon that controls the densification has been identified, are traced (in particular the grain size and temperature). These maps make the pendant of the Ocreep mapsP and Odeformation mapsP also credited to Ashby et al. (see Figure 7.2 in Chapter 7). The principle of these maps is certainly attractive, but their applicability requires three conditions: i) having a sufficient number of experimental data, ii) establishing, for each of these data, the nature of the predominant mechanism, and lastly iii) verifying the Sintering and Microstructure of Ceramics 91 similarity of the treated cases (for example, the fact that the powders used contain the same impurities as the powders used for tracing the maps). The application of the maps is therefore qualitative more than quantitative. Let us use an example to illustrate this comment: when we compare the case of a metal with that of a ceramic, we observe that the mobility of dislocations in the former material is much higher than it is in the latter. This means that the relationship between the effect of an increase in temperature and that of an increase in pressure is higher for ceramics than it is for metal, which suggests different managements of the parameters T and p for the two categories of material.

Conformity of the shapes The key point for HIP, which is an expensive treatment and therefore dedicated to high added value products, is to obtain parts whose final dimensions are as close as possible to the desired dimensions. However, this conformity of dimensions requires a perfect control of the shrinkage: it must occur particularly in a homothetical way, starting from the shape of the raw part until the consolidated and stripped part. However, this Ohomothetic shrinkageP is affected by various causes, including the envelope effect (in the case where there is not post-densification HIP) and the manner in which the consolidation front develops.

As regards the envelope effect: even if the OcompactP is overheated perfectly homogenously throughout the HIP cycle, the various areas of the part do not offer the same resistance to the effects of isostatic pressure. Geometrical compatibilities require that the volume deformations should be accompanied by shearing strains, a requirement which introduces distortions. For the simple example of a cylindrical part (see Figure 3.14), the presence of the envelope causes a distortion of the OcornersP. The numerical calculation methods like finite elements are used widely for the study of such distortions in order to eliminate them by redrawing the envelope [NCE 00].

As regards densification: this progresses from outside the part towards the core, causing the formation of a consolidated crust whose thermal conduction is higher than that of the core that is not yet consolidated. The heat fluxes thus provoked lead to heterogenities in temperature, which lead to the accentuation of the shell effect of the crust with respect to the core. The effect is all the more marked the bulkier part is.

As an extension of pressureless sintering HIP confirms that a major concern for the production of ceramic parts M OtraditionalP ceramics as well as OtechnicalP ceramic M is the maintenance of the shape and dimensions of the parts. As we said previously: the ceramist works on the product at the same time as he works on the material and therefore his efforts must be devoted to both sides of the problem. 92 Ceramic Materials

Figure 3.14. HIP: at the top, distortion due to envelope effects; at the bottom, example of an iterative approach to determine the shape of the envelope, which allows the correction of the distortions [NCE 00]

3.9. Bibliography

[ASH 75] ASHBY M.F., OA first report on sintering diagramsP, Acta Metall. , 22, p. 275, 1975. [BAE 94] BAE S.I. and BAIK S., OCritical concentration of MgO for the prevention of abnormal grain growth in aluminaP, J. Am. Ceram. Soc. , 77 (101), p. 2499, 1994. [BER 93] BERNACHE-ASSOLLANT D. (ed.), Chimie-physique du frittage , Herm s, 1993. [BOC 87] BOCH P. and GIRY J.P., OPreparation of zirconia-mullite ceramics by reaction sinteringP, High Technology Ceramics , Materials Science Monographs 38, Elsevier, 1987. [BOC 90] BOCH P., CHARTIER T. and RODRIGO, OHigh purity mullite by reaction sinteringP, Mullite and Mullite Matrix Composites , Ceramic Transactions, Vol. 6, The Am. Ceramic Society, p. 353, 1990. Sintering and Microstructure of Ceramics 93

[DAV 91] DAVIS R.F., OHot isostatic pressingP, in Brook R.J. (ed.), Concise Encyclopedia of Advanced Ceramic Materials , Pergamon Press, p. 210, 1991. [GER 96] GERMAN R.M., Sintering Theory and Practice , J. Wiley, 1996. [HAR 84] HARMER M.P., OUse of solid-solution additives in ceramic processingP, Advances in Ceramics , Am. Ceram. Soc., Vol. 10, p. 679, 1984. [HAR 91] HARMER P.P., OHot pressing: technology and theoryP, in Brook R.J. (ed.), Concise Encyclopedia of Advanced Ceramic Materials , Pergamon Press, p. 222, 1991. [HER 50] HERRING C., ODiffusional viscosity of a polycrystalline solidP, J. Appl. Phys. , 21(5), p. 437-445, 1950. [KIN 76] KINGERY W.D., BOWEN H.K. and UHLMANN D.R., Introduction to Ceramics , 2nd edition, John Wiley and Sons, 1976. [KUC 49] KUCZYNSKI G.C., OSelf-distribution in sintering of metallic particlesP, Trans. AIME , 185, p. 169, 1949. [LEE 94] LEE W.E. and RAINFORTH W.M., Ceramic Microstructures , Chapman & Hall, 1994. [LI 87] LI W.B., ASHBY M.F., EASTERLING K.E., OOn densifiaction and shape-change during hot isostatic pressingP, Acta Metallurgica , 35, p. 2831-2842, 1987. [NAD 97a] NADAUD N., KIM D.Y. and BOCH P., OTitania as Sintering Additive in Indium Oxide CeramicsP, J. Am. Ceram. Soc ., 80(5), p. 1208-1212, 1997. [NAD 97b] NADAUD N., ORelations entre frittage et propri t s de mat riaux  base dRoxyde dRindium dop  lR tain (ITO)P, Thesis, Paris-6 University, 1997. [NCE 00] National Center for Excellence in Metalworking Technology, CTC, 100 CTC Drive, Johnstown, Pa, USA. [PHI 85] PHILIBERT J., Diffusion et transport de mati re dans les oxydes , Editions de Physique, 1985. [RIN 96] RING T.A., Fundamentals of Ceramic Powder Processing and Synthesis , Academic Press, 1996.

[TAI 88] TAI W.T. and WATANABE T., OFabrication and mechanical properties of Al 2O3 M WCMCo composites by vacuum hot pressingP, J. Am. Ceram. Soc ., 81(6), p. 1673-1676, 1998. This page intentionally left blank Chapter 4 1

Silicate Ceramics

4.1. Introduction

Silicate ceramics are generally alumino-silicate based materials obtained from natural raw materials. They exhibit a set of fundamental properties, such as chemical inertia, thermal stability and mechanical strength, which explain why they are widely used in construction products (sanitary articles, floor and wall tiles, bricks, tiles) and domestic articles (crockery, decorative objects, pottery). They are often complex materials, whose usage properties depend at least as much on microstructure and aesthetics as on composition. Silicate products with an exclusively technical application (refractory materials, insulators or certain dental implants) will not be explicitly discussed in this chapter.

To distinguish silicate from technical ceramics, it is useful to qualify these products as traditional ceramics. This term refers to the centuries-old tradition that still strongly influences the classification of this type of materials and the vocabulary attached to them. However, it does not reflect the considerable evolution of a sector of activity where progress relates more to the manufacturing technologies (raw material mixtures, drying, sintering, etc.) than to the products themselves.

These products of terra cotta, earthenware, sandstone, porcelain or vitreous china are generally widely marketed materials. They represent a predominant share in the total sales turnover of the ceramic industry. In 1994, the fields of roof tiles and bricks, wall and floor tiles, crockery and ornamentation, and sanitary products

Chapter written by Jean-Pierre BONNET and Jean-Marie GAILLARD. 96 Ceramic Materials accounted for, respectively, 28, 14, 13 and 13% of the turnover of the French ceramic industry (technical + refractory + traditional) [LEC 96].

4.2. General information

Silicate ceramics can be formed in various ways: by casting in a mould aqueous suspension called slip, by extrusion or jiggering of a plastic paste, or by unidirectional or isostatic pressing of slightly wet aggregates. The quantity of water contained in the sample therefore depends on the method of forming. Generally, water is eliminated during a specific drying treatment. The raw part is transformed into ceramic by sintering, also called firing, carried out under suitable conditions of temperature, heating rate and atmosphere. Depending on the application considered, this ceramic, also called shard, can be dense or porous, white or colored.

Clay is the basic raw material for these products. Mixed with water, it can form a plastic paste similar to the one used by the potter on his wheel. Although easy to form, this paste often exhibits insufficient mechanical strength to enable handling without damaging the preform. Owing to the clay colloidal nature, a relatively pure paste is low in solid matter. It thus shrinks significantly during drying and sintering, which makes it difficult to control the shape and dimensions of the final piece. To limit all these effects, non-plastic products known as tempers can be added to the paste. They then form an inert and rigid skeleton that enhances the mechanical strength of the preform, favors the elimination of water during the drying stage and limits sintering shrinkage. Among the commonly used tempers, we can mention sand, feldspars, certain lime-rich compounds or grog (a paste sintered and ground beforehand).

Given the complexity of the composition of argillaceous raw materials, the appearance of a viscous liquid during firing can be observed. The addition of fluxes to the starting mixture amplifies this phenomenon. These compounds, which also behave as tempers, generally contain alkaline ions (Na, K, sometimes Li). The examination of the phase diagram of Al 2O3-SiO 2-K 2O represented in Figure 4.1 highlights the role of flux of a potassium feldspar (orthoclase with the composition K2O,A 2O3,6SiO 2) with respect to the deshydroxylation product of kaolinite, whose composition in equivalent oxides Al 2O3,2SiO 2 is symbolized by point MK. At equilibrium, the addition of a small quantity of feldspar leads to decrease the solidus temperature from 1,590 to 985 C. Under certain temperature and composition conditions, iron oxides and a few calcium-rich compounds, such as chalk, can also contribute to the formation of a liquid phase. In the presence of a sufficient quantity of molten matter, the heat treatment can be pursued until the almost complete disappearance of the porosity. The shards thus obtained are rich in vitreous phase and exhibit good mechanical strength. The quantity of liquid formed during partial Silicate Ceramics 97 vitrification must remain sufficiently low or its viscosity must be high enough so that the piece does not become deformed under its own weight.

Figure 4.1. Phase diagram Al 2O3-SiO 2-K 2O [LEV 69]

Some products are covered with a vitreous enamel film intended to modify the appearance of the ceramic and/or to waterproof it. This layer can be deposited on an engobe whose role is to mask the color of the shard and/or to facilitate the adhesion of the enamel. Depending on the case, the enameling operation is carried out on a green support, on a partially fired part during a so-called bisque firing (maximum temperature lower than that of enamel firing) or on a biscuit (completely fired shard at a temperature higher than that of enamel firing). The low temperature enamel intended for the protection of porous ceramics, such as earthenware and potteries, is also called glaze. Transparent glaze is the name used to denote enamel obtained by melting at the sintering temperature of the porcelain shard or the underlying stoneware. The enamel coloring is obtained using metallic oxides. 98 Ceramic Materials

4.3. The main raw materials

4.3.1. Introduction

Each mineral raw material has a specific influence on the rheology of the paste, the development of the microstructure, the phases formation during the heat treatment and the properties of the finished product. The manufacture of all silicate ceramics requires such a large number of raw materials, which cannot be discussed here. Only those most commonly used, i.e. clays, feldspars and silica, will therefore be described.

4.3.2. Clays

4.3.2.1. Common characteristics Clays are hydrated silico-aluminous minerals whose structure is made up of a stacking of two types of layers containing, respectively, aluminum in an octahedral environment and silicon in tetrahedral coordination. Their large specific surface (10 to 100 m 2g-1 ), their plate-like structure and the physicochemical nature of their surface enable clays to form, with water, colloidal suspensions and plastic pastes. This characteristic is largely used during the manufacture of silicate ceramics insofar as it makes it possible to prepare homogenous and stable suspensions, suitable for casting, pastes easy to manipulate and green parts with good mechanical strength. By extension, the term clay is often used to denote all raw materials with proven plastic properties containing at least one argillaceous mineral. The impurities present in these natural products contribute to a large extent to the coloring of the shard.

4.3.2.2. Classification All clays do not exhibit the same aptitude towards manipulation and behavior during firing. Ceramists distinguish vitrifying plastic clays, refractory plastic clays, refractory clays and red clays.

Vitrifying plastic clays, generally colored, are used for the remarkable plasticity of their paste. They are made up of very fine clay particles, organic matter, iron and titanium oxides, illite (formula Si 4x Al x)(Al,Fe) 2O10 (OH) 2Kx(H 2O) n) and micaceous and/or feldspathic impurities. These clays are also characterized by a high free silica content; sand can represent up to 35% of the dry matter weight. The product called Oball clayP is widely used for its plasticity and its particularly low mica content. Although it contains the same argillaceous mineral as kaolin, this clay has much higher plasticity because of the much smaller size of the kaolinite particles [CAR 98].

Refractory plastic clays are rich in montmorillonite (formula (Si 4-x Al x)(Al x- 2+ vRx)O 10 (OH) 2M2v (H 2O) n with R = Mg, Fe and M = K, Na), kaolinite or halloysite (Si 2Al 2O5(OH) 4(H 2O) 2). Silicate Ceramics 99

Refractory clays are used in high temperature processes. Their composition is rich in alumina. Kaolins are the most refractory among these clays. Always purified, they contain little quartz, generally less than 2% alkaline oxides in combined form and a small quantity of mica. Their plasticity is ensured by kaolinite and, if necessary, a little smectite or halloysite [CAR 98]. Very low in coloring element, they are particularly suited for the preparation of products in white shard.

Red clays used for the manufacture of terra cotta products are actually natural mixtures with a complex composition. They generally contain kaolinite, illite and/or other clays rich in alkaline, sand, mica (formula Si3Al 3O10 (OH) 2), goethite (FeO(OH)) and/or hematite (Fe 2O3), organic matter and, very often, calcium compounds. The latter, just like the micas and the other alkaline-rich compounds, help lower the firing temperature of the shard.

4.3.3. Kaolinite

4.3.3.1. Structure of kaolinite

Kaolinite, Si 2Al 2O5 (OH) 4 or Al 2O3,2SiO 2,2H 2O, is the most common among the argillaceous minerals used in ceramics. A projection of its crystalline structure is 2- represented in Figure 4.2. It consists of an alternate stacking of [Si 2O5] and 2+ [Al 2(OH) 4] layers, which confer to it a lamellate character favorable to the development of plates. The degree of crystallinity of the kaolinite present in clays is highly variable. It depends largely on the genesis conditions and the content of impurities introduced into the crystalline lattice.

Figure 4.2. Projected representation of the structure of kaolinite 100 Ceramic Materials

4.3.3.2. Evolution of the nature of phases during heat treatment

Figure 4.3. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of two kaolinites with different degrees of crystallinity Silicate Ceramics 101

During the heat treatment, kaolinite undergoes a whole series of transformations. The variations of exchanged heat and the corresponding mass changes are indicated in Figure 4.3. The departure of water, which occurs from 450 C onwards, is a very endothermic phenomenon. The amorphous metakaolin, Al2O3, 2SiO 2 then formed, exhibits a structural organization directly derived from that of kaolinite. The exothermic transformation observed between 960 and 990 C is a structural reorganization of the amorphous metakaolin, sometimes associated with the formation of phases of spinel structure like Al 8(Al 13,33 ฀2,67 )O 32 ( variety of Al 2O3) or Si 8 (Al 10,67 ฀5,33 )O 32 . In these formulae, ฀ represents a cation vacancy. Between 1,000 and 1,100 C (often around 1,075 C), these phases are transformed into mullite stoichiometry ranging between 3Al 2O3,2SiO 2 and 2Al 2O3,SiO 2. During this reaction, amorphous silica is released. The surplus amorphous silica starts to crystallize in the form of cristobalite from 1,200 C onwards. It should be noted that the impurities present, the degree of crystallinity (see Figure 4.3) and the speed of heating influence each of these transformations.

4.3.4. Feldspars

Four feldspathic minerals are likely to enter the composition of silicate ceramic pastes. They are:

orthoclase, a mineral rich in potassium with the composition K 2O,Al 2O3,6SiO 2; albite, a mineral rich in sodium with the composition Na 2O,Al 2O3,6SiO 2; anorthite, a mineral rich in calcium with the composition CaO,Al 2O3,2SiO 2; petalite, a mineral rich in lithium with the composition Li 2O,Al 2O3,8SiO 2.

Orthoclase and albite, which form eutectics with silica, respectively, at 990 (see Figure 4.1) and 1,050 C, are widely used as flux. Anorthite is rather regarded as a substitute to chalk. The use of petalite, especially owing to its negative expansion coefficient, is marginal [MAN 94].

Potassic feldspar is particularly appreciated by ceramists because its reaction with silica leads to the formation of a liquid whose relatively high viscosity decreases slightly when the temperature increases. This behavior is considered as a guarantee against the excessive deformation of the pieces during the heat treatment.

Natural feldspars used for the preparation of ceramics are mineral mixtures. Thus, the commercial potassium products can contain between 2.5 and 3.5% of albite mass, whereas anorthite and a small quantity of orthoclase, between 0.5 and 3.2%, are often present in the available sodium feldspars [MAN 94]. They can also be incorporated into the paste in the form of feldspathic sand. When these natural products are heated, mixed and homogenous feldspar is formed. This compound, 102 Ceramic Materials called sanidine, occurs at a temperature which varies according to the sodium/potassium ratio and ranges between 700 and 1,000 C. Then, in the presence of silica, the formation of the liquid takes place. Above 1,200 C, mullite is formed in the still solid part of the feldspar grains.

A rather recent trend among stonewares and porcelain manufacturers consists of replacing feldspars by nepheline syenite with average composition (Na,K) 2O,Al 2O3,2SiO 2. This rock, made up of nephelite (composition: K2O,3Na 2O,4Al 2O3,9SiO 2) and a mixture of potassium and sodium feldspars, is a powerful flux which makes it possible to decrease the sintering temperature of ceramics and increase the alkaline content of the vitreous phases [CAR 98].

4.3.5. Silica

Silica, SiO 2, is a polymorphic raw material found in nature in an amorphous (opal, pebbles) or crystallized form (quartz, cristobalite and tridymite). Sand contains between 95 and 100% of quartz mass. It is the most frequently used temper in the ceramic industry. To contribute significantly to the mechanical strength of the raw parts, it must consist of much coarser particles than those of clay. In the modern manufacturing processes of stonewares and porcelains, it is customary to use relatively fine sand grains (20 to 60 Pm).

Volumeexpansion (%)

Figure 4.4. Influence of temperature on the expansion of the various forms of silica Silicate Ceramics 103

When a ceramic is fired, the sand can react, particularly with the fluxes. This reaction is seldom complete. The transformation of residual quartz into cristobalite can then start from 1,200 C onwards. It is favored by the rise in temperature, the use of fine grained sand, the presence of certain impurities and a reducing atmosphere [JOU 90].

The form in which silica is found determines the thermal properties of silicate ceramics. Thus, quartz and cristobalite do not have the same influence on the expansion of the shard (see Figure 4.4). Quartz can also cause a deterioration of the mechanical properties of the finished product owing to the abrupt variation in dimensions ( 'L/L # M0.35%) associated, at 573 C, with the reversible transformation quartz E o quartz D. As the crystal of cristobalite formed from the flux are usually small, the transition cristobalite E o cristobalite D, which occurs at about 220 C (see Figure 4.4), often causes less damage to the finished product. It can even contribute to the shard/enamel fit by compressing it after cooling at room temperature.

4.4. Enamel and decorations

4.4.1. Nature of enamel

The enamel layer deposited on the shard generally has a thickness ranging between 0.15 and 0.5 mm. Its purpose is to mask the porosity and/or the color of the shard, to make the surface of the piece smooth and brilliant and to improve the chemical resistance of the ceramic. This layer, transparent or opaque, white or colored, is obtained from a silica-rich ceramic composition capable of developing glass during the heat treatment. The composition of the enamel also contains many other constituents, in particular alkaline and alkaline-earth oxides. They help to adjust the melting point, the thermal expansion coefficient, the surface tension and the viscosity to the enameling conditions, and ensure the wetting and adhesion of the enamel on the shard. The enamels used for sheet enameling have many common points with those described here [STE 81].

The properties of the enamel are often analyzed by considering that it is made up of a combination of acid oxides responsible for the vitreous structure (mainly SiO 2 and B 2O3), amphoteric oxides (Al 2O3) and basic oxides (K 2O, Na 2O, CaO, MgO, PbO). It should be noted that the role of flux, traditionally reserved for basic oxides, is now increasingly played by acid or amphoteric oxides, such as B 2O3 or Bi 2O3.

Enamel is obtained from a mixture of raw materials mineral and/or frits. The raw materials used are mainly feldspars, kaolin, quartz and chalk or dolomite. Frits are close mixtures of components prepared by melting several compounds at high temperature (T > 1,400 C). After quenching in air or water, the product, markedly 104 Ceramic Materials vitreous in character, is ground. Combined in this manner, water soluble salts or volatile oxides can be used without harm in the composition of enamel. We can distinguish raw enamels formed only from natural raw materials and sintered enamels. The latter are particularly suitable for low temperature applications that require flux bases, richer in basic elements, which are non-existent in nature. The role of frits in the composition of the enamel is all the more important as the firing temperature reduces and the heat treatment is shortened. Frits are widely used for the enameling of the tiles in the fast sintering process [ENR 95].

It is customary to classify the various types of enamel, based on the nature of the flux used. Thus, we can distinguish lead enamels (PbO rich enamels), boron oxide enamels, alkaline enamels, alkaline-earth enamels, zinc enamels and bismuth oxide enamels [STE 85]. Lead enamels, historically the oldest, were the most commonly used for a long time. Because of the toxicity of lead, their future hinges on the evolution of legislation relating to the leaching of this element. They tend to be replaced by enamels containing a very small quantity of bismuth oxide (< 5% mass) and, especially by alkaline borosilicate products.

4.4.2 . Enamel/shard combination

For the enamel to remain strongly attached to the shard, the interdiffusion must be effective and the thermal expansion coefficients of these two parts of the ceramic must be compatible. When the expansion coefficient of the shard is lower than that of the enamel, the latter is subjected to tension stresses when the piece cools. The stresses generated at the interface can cause the formation of cracks in the enamel. This flaw, also called crazing, is all the more important the more significant the difference between the expansion coefficients and the higher the modulus of elasticity of the enamel. To avoid this, it is customary to try stabilizing in the support, high expansion coefficient phases. On the other hand, when the shrinkage of the shard on cooling is the highest, the enamel is placed under compression and its mechanical strength is thereby reinforced. This positive effect occurs only if the difference between the expansion coefficients is sufficiently low to prevent the enamel from falling apart due to compression and from flaking off.

4.4.3. Optical properties of enamel

When the vitrification is complete, i.e. after the various components have melted completely, the enamel is generally brilliant, smooth and transparent. In most cases, opaque enamel is desired. This is achieved by favoring the formation of crystallized, vitreous or gas inclusions with an index of refraction different from that of the vitreous matrix. The differences between the indexes of refraction, the size and the Silicate Ceramics 105 form of the inclusions are then decisive parameters. The formation of crystallites can be favored by the presence in the enamel of mineralizers such as ZrO 2 and SnO 2 Vitreous inclusions occur when a decomposition takes place during the total fusion, as in the case of compositions like SiO 2-B 2O3-MO (M = Pb, Ca, Zn, Mg).

A mat appearance and opacity owing to the diffusion of light on asperities can be observed when the surface of the enamel is slightly rough. This phenomenon occurs when the melting is incomplete or when the viscosity of the formed liquid is high. It can be favored by increasing the contents of SiO 2, Al 2O3, CaO and ZnO.

4.4.4. Decorations

The pigments used for the production of decorations generally consist of colored frits or stain mixtures crystallized in a vitreous silico-aluminous phase. The main products used as coloring are oxides of antimony, chromium, copper, cobalt, iron, manganese, nickel, praseodymium, selenium, titanium, uranium, and vanadium [HAB 85].

In order to be applied on the parts, the ground pigments are mixed with liquid organic substances (for example, turpentine oil) which facilitate their adhesion. The nature of the process of decorating the enamel depends on the desired quality and the complexity of the decoration. The decalcomania technique is the most efficient, insofar as a very complex decoration, involving up to 20 colors, can be carried out by serigraphy. Processes making it possible to print directly on the enamel (direct transfer through a membrane) or decorate it without firing in the kiln (lazer sintering) can also be used.

An additional firing is generally necessary to fix the decorations. Depending on the application envisaged for the piece, it can be carried out below 800 C (low fire firing) or at round 1,200 C (high fire firing). Low fire firing makes it possible to obtain a very broad pallet of colors; high fire firing is especially used to fix decorations likely to change in a highly aggressive environment, a dishwasher for instance. In view of the interactions between phases existing at high temperature, the pallet of colors is therefore considerably reduced.

4.5. The products

4.5.1. Classification

Based on the criteria taking mainly into account open porosity and/or the coloring of the shard, it is customary to distinguish, among silicate ceramics, terra cotta 106 Ceramic Materials products, earthenware, stoneware, vitreous china and porcelains. The materials treated at higher temperatures or in the presence of a large quantity of flux are generally the least porous. Whiteness is primarily the result of the use of raw materials free from iron and titanium or containing only small contents of transition metals.

The representation given in Figure 4.5 helps locate each of these families. Terra cotta products and earthenwares are characterized by a porous shard. The strong coloring in the mass of the terra cotta products has given them the name Ored productsP. These porous ceramics can be used just as they are (bricks and tiles) or be covered with enamel (earthenwares). Among the dense products, stonewares shard is more colored than porcelain shard. Vitreous china forms an intermediate group between these two families. Many products are on the border between two of these groups; their name, which very often differs from one country to another, depends on the custom and the envisaged application.

Figure 4.5. Representation of the various traditional ceramic families

The nature of the raw materials used for the manufacture and the chemical and mineralogical compositions of the shards can also be used as additional criteria for classification.

4.5.2. Terra cotta products

We are referring here to potteries or construction products such as roof tiles, bricks, flues, drainage pipes or some floor tiles. Terra cotta products were obtained a long time ago by modeling, drying and firing common clays. Nowadays, the compositions are more complex; they combine clays and additives, such as coloring, Silicate Ceramics 107 tempers or agents which make it possible to improve the manufacturing behavior or the final characteristics. The raw materials added to water form a plastic paste whose rheology must be adapted to the shaping process (extrusion possibly completed by pressing). The raw parts are dried in a ventilated cell or a tunnel dryer. The temperature at the end of firing usually ranges between 900 and 1,160 C.

Terra cotta products are porous and mechanically resistant. They are marketed raw, enameled or covered with a glaze realized at low temperature, between 600 and 900 C, called varnish. They are appreciated for their esthetic quality, their stability through time and their hygrothermic and acoustic properties. They represent a highly automated industrial sector which is the scene of continual technological developments.

The coloring of terra cotta shards can vary from yellowish white to dark brown. The variety in the tonality of the tiles present on roofs illustrates the extent of the pallet available. For roof tile manufacturers, the mastery over colors represents a commercial stake, insofar as they often constitute a regional specificity or a decorative element. The coloring of the shard depends on the bonding of iron ions with inhibitors, such as the calcium ions or with additional coloring (titanium and manganese oxides). The crystallized phases that are formed during the firing of a terra cotta product can be described using a ternary system defined by the major oxides Al 2O3, SiO 2 and CaO. This includes primarily wollastonite (CaO, SiO 2), gehlenite (2CaO, Al 2O3, SiO 2) and anorthite (CaO, Al 2O3, 2SiO 2). A high temperature heat treatment favors the formation of anorthite to the detriment of the other two phases. The Fe 3+ ions dissolved in the anorthite confer a yellow coloring 2+ to it. Hematite, Fe 2O3, is brown-red. The presence of compounds containing Fe ions favors bluish or greenish tonalities. The final coloring of the shard is a combination of these three effects. In an oxidizing medium, a strong concentration of iron leads to the formation of a significant quantity of hematite and a brown red shard. The abundant presence of CaO in the starting mixture is favorable to the formation of anorthite and thus to the evolution of coloring towards yellow. A treatment at an excessively high temperature or the use of an atmosphere that is too low in oxygen can involve the formation of Fe 2+ ions and a green or black coloring of the shard. Based on these considerations and experimental observations, the following rules may be laid down:

 when the Al 2O3/Fe 2O3 mass ratio is lower than 3, the shard is red;  when the Al 2O3/Fe 2O3 mass ratio ranges between 3 and 5, the shard is pink;  when the Fe 2O3/CaO mass ratio is less than 0.5, a suitable heat treatment (high temperature and a sufficiently oxygen-rich atmosphere) yields a yellow shard;

 when the CaO/Al 2O3 mass ratio is close to 1, the color of the shard is particularly dependent on all the parameters likely to affect the formation of anorthite. It is also 108 Ceramic Materials significantly influenced by the other impurities present; MnO produces, for instance, black reflections.

As an atmosphere that is excessively oxidizing is detrimental to the formation of anorthite, the stacking density of the parts and the temperature of the final stage of firing can assume considerable importance, particularly in the last two cases. Thus, for a given composition, iron can be in the form of hematite at 1,000 C (pink coloring), dissolved in anorthite at 1,050 C (yellow coloring) and partially reduced at 1,100 C (coloring turning to green). The tile color therefore depends on the composition of the raw materials and the firing conditions (temperature, atmosphere and setting load of the kiln). Today it is often modified by using mineral coloring deposited, sometimes directly on the surface of the raw parts (colored engobe).

4.5.3. Earthenwares

4.5.3.1. General characteristics of earthenwares We call earthenware the ceramic products made up of a porous shard covered with a glaze. This enamel makes it possible to mask the appearance of the shard and to remedy the high permeability due to the existence of an open porosity ranging between 5 and 20%. Although present in the form of objects of imagination and crockery, earthenwares are especially used as wall tiles.

These products are prepared from one or more clays to which quartz, chalk, feldspar or ground glass are added. Earthenwares are primarily shaped by slip casting, jiggering of plastic paste and atomized powder pressing. After drying, the raw product is subjected to a heat treatment called biscuiting, carried out at a temperature ranging between approximately 900 and 1,230 C. The deformation and the shrinkage of the shard during this stage are limited because of the refractory nature of the raw materials used. The porous biscuit obtained is then enameled during an enamel firing carried out at a temperature lower than or sometimes equal to that of biscuiting. The third firing, at a lower temperature, is necessary to fix some decorations deposited on the glaze, in particular those containing gold or platinum and those known as Olow fireP decorations.

4.5.3.2. Common earthenwares Common earthenwares are found especially in old products. Their production is very limited nowadays. They are primarily glazed potteries and stanniferous earthenwares. Silicate Ceramics 109

4.5.3.2.1. Potteries glazed with low melting temperature argillaceous paste Potteries glazed with fusible argillaceous paste are very close to terra cotta products. Just like them, they are obtained from common argillaceous soils, relatively fusible, and naturally containing a certain quantity of sand. Although their sintering, carried out between 900 and 1,060 C, takes place in the presence of a significant quantity of liquid, their shard is still porous.

These products, used in construction (enameled bricks and tiles), for domestic uses and as crockery (jugs, pots, etc.), are generally covered with an engobe whose pores are finer than those of the shard. This engobe constitutes a smooth and regular surfacing intended to mask the coloring of the shard and to be used as decoration base.

4.5.3.2.2. Stanniferous earthenwares with low melting temperature argilo-calcareous paste To produce certain decorative objects, it is customary to use an argilo-calcareous paste obtained by mixing argillaceous marls, limestone and often sand. Magnesium carbonate (MgCO 3) and dolomite ((Ca,Mg)CO 3) can also be used. The biscuits are sintered at a temperature between 900 and 1,060 C. They are generally covered with a glaze opacified by tin dioxide, which explains the name stanniferous earthenwares.

4.5.3.3. Fine earthenwares Fine earthenwares are characterized by a white or very lightly colored shard, a thin and regular texture, high mechanical strength and the brightness and durability of their glaze. They are widely used as decorative objects and crockery, fields where the quality of their enamel is highly appreciated.

The argillaceous component of the paste consists of a mixture of kaolin and clays. The kaolin increases the refractory character of the paste, whereas the plastic clays contribute to the mechanical strength of the raw parts. Although the selected clays are very poor in colorings, the presence of very small quantities of impurities in them, such as Fe 2O3 and TiO 2, can be sufficient to slightly color the shard. The kaolin/clay ratio must therefore be adjusted in order to obtain an acceptable compromise between the whiteness of the biscuit and the resistance of the raw parts. Kaolin generally represents between 25 and 50% of the mass of all the argillaceous raw materials.

Grog, silica in its various forms and chalk (case of calcareous earthenware) can be used as tempering raw materials. The overall content of quartz, primarily introduced with the clays in the form of sand, is generally very high in fine earthenware pastes (30 to 40% of the mass). The biscuit is fired in an oxidizing atmosphere, at a temperature ranging between 950 and 1,150 C. The presence of a 110 Ceramic Materials large quantity of residual quartz increases the shrinkage of the shard on cooling, thus reinforcing the mechanical strength of the enamel.

4.5.3.4. Feldspathic earthenwares Feldspathic earthenwares are obtained by firing, at a temperature between 1,140 and 1,230 C, a mixture containing, for example, kaolin (40 to 70% of the mass), quartz (25 to 58%) and feldspar (3 to 14%). This latter component favors the formation of a viscous liquid during the high temperature treatment. After cooling, the biscuit has an increased solidity by virtue due to a significant quantity of vitreous phase formed during the solidification of the liquid. The porosity, 10 to 15%, is generally lower than the one observed in other earthenwares. The enamel is fired at a temperature between 1,000 and 1,140 C. At these high temperatures, it is possible to obtain glazes that cannot be easily scratched by steel. The increase in the silica content in the paste, the use of finer silica favorable to the formation of cristobalite or the reduction in the porosity of the shard contribute to the improvement of the shard/enamel combination.

Owing to their very high solidity, their particularly scratch-resistant enamel and their low open porosity, feldspathic earthenwares are particularly suitable for applications in the tiling field.

4.5.4. Stonewares

4.5.4.1. General characteristics of stonewares Stonewares have a vitrified, opaque, colored and practically impermeable shard (0 to 3% open porosity). They are obtained from a mixture of vitrifying plastic clays and flux, sometimes supplemented by sand or grog. They are formed by extrusion (pipes, bricks, etc.) or by granulated powder pressing (tiles, slabs, etc.). The firing temperature generally ranges between 1,120 and 1,300 C and it forms a critical parameter. In fact, sintering at an insufficient temperature (non-firing) results in the persistence of a significant open porosity and a treatment at too high a temperature leads to the deformation of the pieces because of the excessively large quantity and the low viscosity of liquid formed. If usage requires it, stonewares can be enameled. A salt glaze during firing can also be carried out (traditional salt-glazed stonewares).

Stonewares are known for their unchangeability, excellent mechanical performances and resistance to erosion and chemical agents. Silicate Ceramics 111

4.5.4.2. Natural stonewares Natural stonewares are obtained from natural vitrifying clays, i.e. capable of forming a significant quantity of liquid at high temperature. They are used just as they are or are modified only by adding a kaolinitic refractory clay. Fe 2O3 can represent up to 3% of the mass of the composition of these raw materials.

Irrespective of the firing atmosphere, mullite occurs in an acicular form between 1,000 and 1,100 C and continues to be formed up to 1,200 C. During this treatment, the viscous liquid dissolves the finest quartz grains. The solidification of this liquid on cooling leads to the formation of a significant quantity of vitreous phase. In an oxidizing atmosphere, the color of the shard can vary from ivory to dark brown. This coloring depends, in this case again, on the iron content and the nature of the other impurities present in the clays. Thus, titanium dioxide tends to color the shard of natural stonewares light yellow, whereas manganese oxide favors the development of darker colors. To avoid the appearance of blisters due to the presence of sulphates in the clays, the firing of natural stonewares must often be carried out in a reducing atmosphere. The Fe 3+ ions are then reduced above 570 C. The ferrous oxide formed confers on the shard a grayish color and acts as a very active flux. As this action compounds that of the alkaline derivatives, a high iron content can lead to a marked softening and the deformation of the pieces during firing.

The production of natural stonewares is primarily traditional, insofar as the clays necessary for the manufacture of this type of product are seldom available in large quantities and their firing is often difficult.

4.5.4.3. Compound or fine-grained stonewares Fine-grained stonewares are different from natural stonewares because the grains of flux are no longer contained in the clay, but added in the form of feldspars. They are obtained from clay very poor in coloring, kaolin, ball clay and a mixture of orthoclase and albite. Colorings are sometimes added to the paste to develop a particular color in the mass of the product. Fine-grained stonewares are used as crockery, walls or floor tiles, antacid tiles and sanitary pipes.

During firing, the deshydroxylation of clays occurs from 450 C onwards. Shortly before 1,000 C, orthoclase starts to react with silica and the liquid occurs. The mullite formation begins between 1,000 and 1,100 C. In this temperature range, certain micaceous phases contained in the clays can start to react with the products of the decomposition of metakaolin. The interaction of albite with silica begins from 1,140 C. The maximum firing temperature ranges between 1,250 and 1,280 C, so the amorphous silica derived from the kaolinite and undissolved in the liquid can be transformed into cristobalite. The degree of crystallization of SiO 2 in the end 112 Ceramic Materials product depends on the thermal past of the stonewares, the nature of the flux and the mineralizing impurities.

4.5.4.4. Porcelain stonewares Porcelain stonewares are characterized by an open porosity of less than 0.5%. This characteristic gives them remarkable mechanical properties and an excellent resistance to frost and corrosive agents. These products have experienced a rapid development as floor tiles. The world production rocketed from a few million m 2 in the 1980s to more than 150 million in 1997. This growth is linked to the use of new processes of grinding (wet process), forming (more powerful presses), sintering (fast mono-layer sintering roller-hearth kilns) and decoration (polishing, simultaneous pressing of several layers of enamel powder). These new technologies have reduced production costs and have considerably improved productsR esthetics.

Figure 4.6. Ranges of chemical composition of various types of stonewares tiles (mass %)

The paste used to manufacture stonewares tiles generally consists of a mixture of plastic clays, kaolin, feldspathic sand, sodium or potassium feldspar and small quantities of talc, dolomite and/or chlorite. The overall chemical composition of the Silicate Ceramics 113 shard is generally less pure than that of fine-grained stonewares tiles, and it is also richer in Al 2O3 (see Figure 4.6).

Expressed in mass % of oxide, it generally corresponds to 66 to 69% of SiO 2, 20 to 23% of Al 2O3, 0.5 to 5% of MgO, 1.2 to 1.8% of CaO, 2.5 to 3.6% of Na 2O, 1.7 to 2.8% of K 2O, 0.7 to 1.3% of Fe 2O3, 0.4 to 0.9% of TiO 2 and 0 to 2% ZrO 2 [DON 99].

The maximum temperature of the heat treatment generally ranges between 1,120 and 1,200 C. The evolution of the phases during this firing is very close to the one described in the case of fine-grained stonewares. The quantity of mullite formed represents only 50% of what is expected for this type of composition. The porcelain stonewares shards are mainly made up of mullite, amorphous phase and quartz. Their composition, after cooling, belongs to the field represented on Figure 4.7. These shards are free from open porosity and exhibit between 7 and 13% closed porosity [DON 99]. This microstructure confers on these materials a high YoungRs and rupture modulus (about 75 GPa and 85 MPa respectively) compatible with their use as floor tiles. These moduli increase with the Al 2O3 content, the quantity of mullite formed and the compactness of the shard (reduction in closed porosity).

Figure 4.7. Representation of the composition range of porcelain stonewares shards in the ternary mullite-quartz-vitreous phase diagram (mass %) 114 Ceramic Materials

4.5.4.5. Grogged stonewares Grogged stonewares are obtained from a paste made up of vitrifying clay sometimes rich in kaolinite or quartz, a small proportion of flux and a large quantity of grog and/or ground shard (40 to 60% of the mass). It is generally shaped by casting a slip in porous plaster moulds. The use of large-sized grog grains, up to 0.8 mm, increases the permeability of the rigid skeleton and the speed of drying. It reduces the capillary forces responsible for the formation of cracks and then slits.

The dry products are engobed and then enameled before firing. The engobe and the enamel are deposited successively by dipping or pulverization. The operation must be carried out in several layers in order to obtain a sufficient thickness and avoid excessively rewetting the dry part. After a final drying, it is fired in single thermal cycle (process known as single firing). The rigid lattice made up of grog grains does not allow sufficient shrinkage to eliminate all porosity during sintering. After a treatment between 1,250 and 1,280 C, open porosity remains considerable (8 to 15%). The presence of an opaque engobe masks the coloring of the shard, levels out the surface imperfections and facilitates the fixing of the enamel.

Grogged stonewares, easier to dry than most other products, are particularly suited for the manufacture of bulky and robust products. They are widely used in sanitary plumbing (sinks, shower basins, etc.).

4.5.5. Porcelains

4.5.5.1. General characteristics Thanks to the purity of the raw materials used, porcelain shards are white and translucent beneath the low thickness. They do not have open porosity (< 0.5%), but are likely to exhibit some large closed pores (air holes). Their fracture are brilliant and have a vitreous appearance. After enameling, the surface of the pieces is remarkably smooth and brilliant.

When porcelain is fired, a liquid phase surrounds the solid grains and dissolves the finest of them (< 15 Pm). During this stage, known as Opasty fusionP, the viscosity is sufficiently high for the deformation of the pieces to remain within acceptable limits. The solidification of the liquid on cooling leads to the formation of a large quantity of vitreous phase.

The manufacturing processes are changing constantly (see section 5.6.2). Thus, when the geometry of the parts allows it, pressure casting and shaping by isostatic pressing gradually replace jiggering and casting in plaster molds. Fast firing techniques are increasingly used for enamel and decorations. They improve the Silicate Ceramics 115 quality of the parts by limiting the risks of deformations [SLA 96]. A tendency has also been observed to decrease the sintering temperature [LEP 98].

4.5.5.2. Hard-paste porcelains Hard-paste porcelains are obtained from mixtures made up almost exclusively of kaolin, quartz and feldspars. A little chalk ( | 2% of the mass) can be added to favor the formation of the viscous liquid. This mixture is very similar to the one used to prepare fine earthenware. It differs from it only because of the almost exclusive use of kaolin as clay and the proportions of the various components.

Sintering is carried out at a temperature ranging between 1,350 and 1,430 C. The use of a reducing atmosphere, by favoring the reduction of the Fe 3+ ions to Fe 2+ , guarantees being able to obtain a white shard (possibility of bluish reflections) [SLA 96]. In a twice-firing process, this treatment is preceded by a bisque firing, carried out in oxidizing atmosphere at a temperature ranging between 900 and 1,050 C. The product then has a sufficient rigidity and the high open porosity necessary for its enameling. The development of passage kilns with several heating zones, each with its own atmosphere, has made it possible to prepare certain types of hard-paste porcelains in a single firing. To prevent carbon monoxide, present in the reducing atmosphere, from depositing carbon (Boudouard equilibrium) in the still porous paste, the heating zone ensuring the rise in temperature up to about 1,050 C is traversed by an oxidizing atmosphere. The reducing atmosphere therefore circulates only in the sintering zone.

Because of the high compactness of the shard after firing, enameling is done at the same time as sintering. The enamel is then a glaze with a feldspathic and calcium composition. The high firing temperature and the reducing atmosphere diminish the pallet of possible colors during this treatment to green, blue and brown only. Most decorations are therefore painted or deposited on enamel by decalcomania and then fired between 800 and 900M950 C.

Hard-paste porcelains are particularly used in the field of crockery (Limoges porcelain) and as technical ceramics (insulators).

4.5.5.3. Soft-paste porcelains Soft-paste porcelains differ from the above porcelains by their greater translucidity and lower sintering temperature. Chinese porcelains and porcelains for dental implants belong to this category. English porcelains, known as bone china, constitute a particular class. 116 Ceramic Materials

Fine bone china These are the most expensive porcelains on the market, particularly appreciated for their esthetics. They owe their name to the bone ash added to the mixture of raw materials. The composition of a paste is typically: 37 to 50 % of the mass of bone ash, 22 to 32% potassium feldspar, 22 to 41% kaolin and 0 to 4% quartz. Hydroxyapatite, Ca 5(PO 4)3OH, present in the bone ash contributes to the formation of a low viscosity liquid, whose quantity increases very rapidly when the solidus temperature is reached. Above 1,200 C, this liquid dissolves the free quartz gradually. At the maximum firing temperature of the biscuit, between 1,250 and 1,280 C, the material is made up of a paste that contains only calcium phosphate, Ca 3(PO 4)2 and liquid. Sintering shrinkage, highly dependent on the quantity and viscosity of liquid formed, is much more sensitive to the heat treatment conditions than in the case of hard-paste porcelains and vitreous china (see Figure 4.8). Mastery over the dimensions and deformation of the pieces requires the temperature conditions and sintering time to be strictly controlled. Deformation can be controlled by placing the raw products in a bed of alumina powder [SLA 93]. As anorthite is formed on cooling, the shard of a bone china porcelain is primarily made up of calcium phosphate (35 to 45% of the mass), vitreous phase (27 to 30%) and anorthite (25 to 30%).

The fixing of enamel presents difficulties inherent to the absence of porosity of the shard. Its firing is carried out at high temperature, ranging between 1,120 and 1,160 C. In order to avoid the appearance of efflorescence due to the decomposition of the enamel, this second firing must be done in a strictly oxidizing atmosphere. The brilliance of the enamel obtained is highly dependent on the lead oxide content.

Figure 4.8. Temperature influence on the shrinkage speed of a hard-paste porcelain, a bone china and a vitreous china Silicate Ceramics 117

4.5.5.4. Aluminous porcelains The composition of the paste of aluminous porcelains can contain up to 50% of Al 2O3 mass, about half of which can be introduced in the form of calcined alumina or corundum. The fluxes used are based on alkaline-earth oxides (CaO, MgO and/or BaO); lithium ions are sometimes added in the form of spodumene (Li 2O,Al 2O3,4SiO 2). Sintering and enameling are carried out in single firing at a temperature ranging between 1,280 and 1,320 C.

There are also products called extra-aluminous porcelains whose Al 2O3 content can range between 50 and 95% of the mass. In order to facilitate their forming, 3 to 5% of very pure plastic clays and/or organic binders are added to the paste. When enameling is necessary, the raw parts are generally covered by dipping in the enamel suspension. After sintering at a temperature between 1,430 and 1,600 C the shard exhibits very little vitreous phase.

These porcelains are used for their electrical characteristics (insulators, disconnecting switches, spark plugs, dielectrics for high voltage) and their hardness (cutting tools, wire guides, grinding ball). These are technical ceramics whose dimensions can be subject to considerable constraints because of low tolerances (machining after firing) or the large size of the piece. Consequently, the conditions imposed for producing the 4 m height and 1 m diameter aluminous porcelain insulators used for very high voltage electricity transmission have had to be mastered.

4.5.6. Vitreous china

The term Ovitreous chinaP denotes dense products obtained from pastes close to those used to manufacture feldspathic earthenwares. The feldspar content of these pastes is increased in order to produce, during the firing, a sufficient quantity of liquid to eliminate open porosity (< 0.5%). Used more particularly to manufacture sanitary articles and very robust crockery (wash basin, crockery for communities), vitreous materials are in the middle between white paste stonewares and porcelains.

These products are formed by jiggering, casting or isostatic pressing. A good mastery of the raw materials and shaping process makes it possible to obtain raw pieces with a mechanical strength sufficient to withstand the application of an enamel paste. Sanitary products are generally vitrified and enameled in a single treatment, carried out in oxidizing atmosphere at a temperature between 1,200 and 1,280 C. A twice-firing treatment is usually used for crockery. The first firing is thus carried out between 900 and 950 C. The elimination of open porosity and the formation of the enamel occur during the second firing at a temperature ranging 118 Ceramic Materials between 1,200 and 1,250 C. The enamel thus obtained, generally opacified, can allow a large variety of decorations.

4.6. Evolution of processes: the example of crockery

4.6.1. Introduction

Although at the origin of most of the advanced processes of manufacturing massive technical ceramics (casting, extrusion, injection, pressing, etc.), the silicate ceramic industry has for a long time remained aloof from technological progress, basing its development on tradition and empiricism. A modernization was launched after 1960. The globalization of the market and the resulting increasingly fierce competition gave a momentum to the change of mindsets and methods. The development of new technologies (atomization/drying, isostatic pressing, microwave drying, etc.) and the improvement of refractory materials and kilns furnaces (roller- hearth kilns, fibrous cellular kilns with thermal inertia allowing short thermal cycle, high temperature kilns, etc.) increased the rates of output, improved quality, diversified the products and reduced the costs. Today, it is an industrial sector under constant change where progress continues as much in the mastery of processes as in the development of new products.

The changes affecting, on a global scale, the processes of crockery manufacture well illustrate the innovative state of mind that drives traditional ceramic producers.

4.6.2. The case of crockery

For a long time, crockery was shaped by modeling a plastic paste prepared from a suspension of raw material particles. In the variant of this process used today, one of the first stages consists of eliminating a part of the water from the slip by filtration in a filter/press device. The cakes obtained are extruded in the form of clots. The use of vacuum chambers eliminates the trapped air bubbles, thus improving the plasticity of the pastes. The deaerated clots are cut to a suitable length and then modeled into the desired form. The pieces obtained still contain a large quantity of water, typically between 12 and 18% of the mass. This water is eliminated during a drying treatment that generates mechanical stresses, because of the inevitable humidity gradients. This method of forming crockery pieces presents disadvantages due to, amongst other things, the inhomogenities existing in the paste and the duration of the drying stage. As a consequence, it is gradually being replaced by more sophisticated and especially better mastered processes.

Silicate Ceramics 119

The use of atomization/drying techniques represents a breakthrough. It is thus possible to have spherical granules with a well defined size, made up of a very homogenous mixture of raw materials. These granules contain about 3% water mass and sometimes the binders necessary for their cohesion. They have given rise to the development of the shaping process of certain parts by isostatic pressing. This process uses the fact that the granules of the raw material mixture can behave like a very viscous liquid under strong pressure and recover their stiffness when the stress is relieved. In a sufficiently flexible mould (polymer-based diaphragms) and surrounded by a pressurized liquid, the compression is uniform in all the directions. The isotropic distribution of the residual stresses and porosity in the raw parts, therefore, helps control sintering shrinkage. In the case of forming dishes or plates, the pressure is applied only on two sides (quasi-isostatic pressing). Compared to jiggering, this technique presents, among others, the advantage of significantly decreasing the risk of deformation of the plates. The raw parts are then ready for enameling and firing, without prior stage of drying. The conditions to be met in order to obtain a good reproducibility of the parts are largely described in [OBA 82]. This process is particularly suited for mass productions and relatively flat parts. For more complex forms, pressure casting in porous resin molds is increasingly used [GAI 99, SLA 96].

The processes using a wet method for forming parts are still widespread. The duration of the drying proves to be an obstacle for the development of production lines continuously functioning. The recent progress made in the field of infrared and microwave drying should allow a rapid evolution. Indeed, it has been shown that it is possible to dry a cup containing more than 22% mass of water in less than 30 minutes [CUB 93].

New grinding technologies have modified the methods of preparing coloring and the enamel. The use of fine powders considerably improves the covering effect and the brilliance of the enamel, as well as the quality and reproducibility of the decorations.

All these processes, the availability of new types of kilns and a better understanding of the phenomena have made it possible to reduce by up to 5 to 10 times the duration of the production cycle of certain types of crockery while guaranteeing better reproducibility of the parts.

4.7. Conclusion

Silicate ceramics are very old materials that are very widely used in society thanks to their undeniable assets, namely their high mechanical strength, excellent chemical inertia and esthetic qualities. A common feature is that they are obtained 120 Ceramic Materials from a mixture of natural raw materials containing at least one argillaceous mineral, silica and, very often, alkaline oxides in a combined form. The finished products differ primarily in porosity, color, the quantity of vitreous phase, the nature of the crystallized phases (mullite, anorthite, wollastonite, gehlenite or tricalcium phosphate), the allotropic variety of silica (quartz or cristobalite) and the presence of enamel.

Apart from esthetics, which increasingly calls upon designers, the main characteristics of silicate ceramics available today are not different from those of the best performing products marketed 50 years ago. It is however an industrial sector undergoing rapid technological development, where efforts are made to reduce costs, improve the reproducibility of the pieces, adapt to an evolving demand and especially resist to the competition from products containing glass, concrete, metals and polymers. The progress made relies on the development of new equipment, the use of mineral or organic additives and a better knowledge of the parameters governing the rheological behavior and thermal evolution of ceramic pastes. In one decade, this approach has made it possible to reduce the scrap rate of pieces from 40% to a few percent.

Currently, the defects responsible for the downgrading of certain parts are primarily spots due to impurities contained in the raw materials. High magnetic field purification helps reduce by two the number of these scraps. Non-magnetic coloring impurities still cause uncertainties in quality. A few ppm of certain elements are enough to make 10% of the parts non-compliant with the standard. To achieve the zero defect objective, great effort must be made, in order to master the purification and the selection of raw materials and the constancy of their properties.

4.8. Bibliography

[CAR 98] CARTY W.M. and SENAPATI U., OPorcelain-raw materials, processing, phase evolution, and mechanical behaviorP, Journal of American Ceramic Society , vol. 81, p. 3-20, 1998. [CUB 93] CUBBON R.C.P., OReview of new process technology in tableware manufacture. Part I-shaping processesP, Interceram , vol. 42, p. 379-381, 1993. [CUB 94] CUBBON R.C.P., OReview of new process technology in tableware manufacture. Part II-glazing and decorationP, Interceram , vol. 43, p. 143-145, 1994. [DON 99] DONDI M., ERCOLANI G., MELANDRI C., MINGAZZINI C. and MARSIGLI M., OThe chemical composition of porcelain stonewares tiles and its influence on microstructural and mechanical propertiesP, Interceram , vol. 48, p. 75-83, 1999. Silicate Ceramics 121

[ENR 95] ENRIQUE J.E., AMOROS J.L. and MORENO A., OEvolution of ceramic tiles glazesP, Proceeding of Fourth Euro Ceramics , Gruppo Editoriale Faenza, Editions S.p.A. (Italy), p. 121-134, 1995. [GAI 99] GAILLARD J.M., OLe coulage sous pression. Pr sentation de lR tat de lRart de cette technologieP, Actes des journ es technologiques sur le coulage sous pression , Limousins Technologie editions, Limoges (France), p. 1-9, 1999. [HAB 85] HABER R.A., WILFINGER K.R. and MCCAULEY R.A., Stains and coloring agents , Ceramic Monographs, Handbook of Ceramics, Verlag Schmid GmbH, Freiburg (Germany), 1985. [JOU 90] JOUENNE C.A., Trait de c ramiques et mat riaux min raux , Septima editions, Paris, 1990. [LEC 96] LECAT D., Le secteur de lRindustrie c ramique en France , ANVAR editions, Limoges (France), p. 1-9, 1996. [LEP 98] LEPKOVA D. and PAVLOVA L., OLow-temperature porcelain synthesis and propertiesP, Interceram , vol. 47, p. 369-372, 1998. [LEV 69] LEVIN E.M., ROBBINS C.R. and MCMURDIE H.F., Phase diagrams for ceramists , American Ceramic Society, Colombus (USA), 1969. [MAN 94] MANDT P., OThe importance of feldspar as a flux in the ceramics industryP, Interceram , vol. 43, p. 21-23, 1994. [OBA 82] ORBARTO L.C., OIsostatic dry pressure of flatwareP, Technological innovations in whitewares , Editions Alfred University Press, Alfred (USA), p. 195-202, 1982. [SLA 93] SLADEK R., OBone china M a high quality soft porcelain as an enrichment of modern table artP, Interceram , vol. 42, p. 100-102, 1993. [SLA 96] SLADEK R., OWhitewares technology at the turn of the millenniumP, Interceram , vol. 45, p. 71-74 and 147-152, 1996. [STE 81] STEGMAIER W., Enamel and enamelling , Ceramic Monographs, Handbook of Ceramics, Verlag Schmid GmbH, Freiburg (Germany), 1981. [STE 85] STEFANOV S., Ceramic glazes and frits , Ceramic Monographs, Handbook of Ceramics, Verlag Schmid GmbH, Freiburg (Germany), 1985. Chapter 6 1

Alumina, Mullite and Spinel, Zirconia

6.1. Alumina, silica and mullite, magnesia and spinel, zirconia

In Chapter 1, we recalled the abundance of silicates on the EarthRs crust and indicated that they were the basic components of non-metallic inorganic materials: glasses, cements and ceramics. We restricted ourselves to ceramics and said that almost all traditional ceramics are silicate materials. This is not necessarily the case with technical ceramics. For which it is alumina that constitutes the most OusefulP compound and the most widespread. We can say that alumina is for a ceramist what iron is for a metallurgist and, to pursue the analogy further, that the alumina-silica diagram is as important for ceramics as the iron-carbon diagram is for metals.

Underlining the importance of the Al 2O3-SiO 2 diagram does not imply that technical ceramics are assimilated to silicate ceramics. This is first of all because it is high purity alumina, free from silica, which is the first of technical ceramics [JAC 94] and then because high purity silica products do not have very many uses as such apart from refractory applications. In the crystallized state, and if we exclude the considerable field of the piezoelectricity of quartz, silica is first and foremost a raw material. In the vitreous state, all the wonderful products that are derived from silica fall under glass industries, and not under ceramic ones.

The examination of the Al 2O3-SiO 2 diagram highlights that the two components combine to form a single crystallized compound: mullite, whose stoichiometry corresponds to 3Al 2O3 2SiO 2. In fact, mullite crystallizations develop in silicate

Chapter written by Philippe BOCH and Thierry CHARTIER. 200 Ceramic Materials ceramics. The usage spectrum of mullite is limited, even if this material is appreciated for refractory applications and, while we cannot consider alumina and mullite on equal footing, we cannot speak of the former while ignoring the latter. This chapter, primarily devoted to alumina therefore also treats mullite, though more briefly.

The Al 2O3-MgO diagram is also a useful diagram for ceramists: magnesia MgO is an essential compound in refractory industries and spinel Al2O3-MgO plays in the Al 2O3-MgO binary a role similar to the one played by mullite in the Al 2O3-SiO 2 binary. We will therefore say a few words about magnesia and spinel.

Finally, zirconia exhibits very original electric and mechanical properties. In this chapter we will discuss only mechanical properties, as electric properties are covered in Chapter 11.

6.2. Alumina

Alumina is produced primarily from bauxite rocks treated through the Bayer process (see section 6.2.2). Aluminum production consumes about 85% of the bauxite used and the non-metallurgical usages use up the remaining 15%: 10% in the form of alumina and 5% in the form of calcined bauxite, which is not transformed into alumina. The non-metallurgical applications of alumina in 1998 accounted for about five million tons [BAC 99]. The refractory industry is the largest consumer of alumina, followed by abrasive industries, technical porcelains (including spark plug bodies), ceramics for mechanical and electronic use, and chemistry (catalyses in particular). The prices vary in a very broad range: currently, the difference is of two orders of magnitude between metallurgical alumina (about X200 per ton) and high purity alumina, which is used for the preparation of sapphire monocrystals (about X20,000 per ton).

6.2.1. Alpha alumina

Aluminum oxide, or alumina, crystallizes in the corundum structure (mineralogical term) to form sapphire monocrystals. We are referring here to white sapphire, like the one used for the manufacture of scratchproof OglassesP for watches, whereas in gemmology sapphire is the blue sapphire, which is alumina with a little Ti 4+ ; ruby being alumina colored in red by Cr 3+ . This is the alpha variety (Al 2O3-D), which is the stable variety, but we will see that the preparation of alumina involves various metastable varieties: hydrated aluminas and transition aluminas. Alumina, Mullite and Spinel, Zirconia 201

The corundum structure belongs to the rhombohedric system, space group R 3 c, the lattice parameters at room temperature being, in hexagonal axes, a = 0.4759 nm and c = 1.299 nm, with Z = 6. The crystal can be described as a compact hexagonal stacking of O 2- anions, in which the two-thirds of the octahedral interstices are occupied by Al 3+ cations (see Figure 6.1) [KIN 76 and 83, KRO 57]. We note that this reference to anions and cations insists rather misleadingly on the ionicity of the bond: it is considered that the alumina bond is ionic for two-thirds and covalent for the remaining third part. The considerable amount of formation enthalpy (| 1,600 kJ mole -1 ), which explains the absence of natural deposits of metal aluminum, makes alumina one of the most tightly bonded compounds, resulting in very high hardness (9 in the Mohs hardness scale, which goes from 1 (talc) to 10 (diamond), and where quartz is classified at 7) and high melting (2,050 C) and boiling temperatures (3,500 C). The density of sapphire is very close to 4 g.cm -3 , i.e. a little more than half the density of iron.

Figure 6.1. Structure of D alumina. On the left, seen in the base plan: the large circles represent the anions, the small full circles the cations, the small empty circles the vacant octahedral interstices. On the right, the view of the cation sublattice: the full circles represent the full cations and the circles the vacant sites [KRO 57]

The strong iono-covalent bonds and the rhomboedric structure combine to restrict the dislocation movement [HEU 84]: the sapphire monocrystal remains brittle until very high temperatures and the basal slip (0001) < 11 2 0 > is only possible beyond 900 C. The plasticity of the polycrystal comes primarily from the 202 Ceramic Materials effects of intergranular diffusion or flow of vitreous phases. These are mechanisms that depend on the purity of the material and whose occurrence temperature thresholds are lower than 800 C for impure aluminas and higher than 1,100 C for aluminas with purity exceeding 99%.

Alumina is one of the best electrical insulators, hence its dielectric applications. Electric conduction depends primarily on impurities which act as acceptors (for example, Mg, Fe, Co, V or Ni) or as donors (for example, H, Ti, Si, Zr or Y), the balance between electronic and ionic contributions depending on the temperature and the partial pressure of oxygen [KRO 84]. Among the impurities, sodium deserves a particular mention because the synthesis of alumina by the Bayer process brings into play sodic mediums; sodium aluminate (Na2O11Al 2O3), also called beta alumina, is an ionic conductor with very high conductivity, which is why this compound is envisaged for solid electrolyte applications [WES 90].

With an optical gap of about 9.9 eV, pure alumina is transparent in the visible and colorless domain. Like the electrical properties, the colored effects depend primarily on the impurities. Sapphire and ruby have been the subject of numerous studies [BUR 84] and it is esthetically and scientifically interesting to mention the alexandrite effect (from the name of a chrysoberyl (Al 2BeO 4) containing a little Cr 3+ ) which is green in daylight and red if illuminated by an incandescent lamp [NAS 83].

6.2.2. Alumina and its numerous varieties: the Bayer process

D alumina is the stable form of aluminum oxide, but there are several metastable varieties and numerous hydrated phases [CAS 90, DOR 84, GIT 70]. For these hydrated forms, there are in nature three trihydroxydes ( bayerite D-Al(OH) 3, nordstrandite E-Al(OH) 3 and hydrargillite or gibbsite J-Al(OH) 3) and two oxyhydroxydes ( diaspore D-AlOOH and boehmite J-AlOOH). By adopting the notation of equivalent oxides mentioned in Chapter 1, the trihydroxydes correspond to Al 2O33H 2O and the oxyhydroxydes to Al 2O3.H 2O.

Bauxite rocks contain 40 to 60% of equivalent alumina in the form of hydrargillite, boehmite, diaspore and silico-aluminous minerals, 10 to 20% iron oxide and approximately 5% quartz sand and various impurities. The Bayer process consists of attacking the rock crushed by a hot caustic detergent (NaOH solution at a temperature of 150M160 C, under a pressure of 0.5 MPa) in order to dissolve alumina in the form of aluminate and to precipitate the impurities (Fe 2O3, SiO 2, TiO 2). These red deposit are eliminated by filtration and, after cooling, the hydrated sodium aluminate solution NaAl(OH) 4 is seeded with hydrargillite germs which leads to a massive precipitation of this latter phase. Alumina, Mullite and Spinel, Zirconia 203

Heat treatments of Bayer hydrargillite initially cause dehydration, then a recrystallization to many types of transition aluminas (khi, kappa, rho, eta, theta, delta, gamma, etc.) that can be classified under the collective name of gamma aluminas. The distinctions within transition aluminas are based on macroscopic criteria (particularly the specific surface of the products) and crystallographic criteria (particularly the presence of numerous defects: point defects as well as extended flaws).

The lionRs share of the Bayer production is used for the production of metal aluminum by electrolysis of the molten mixture of alumina plus cryolite. Apart from metallurgical alumina, hydrated aluminas are used in paper mills, water treatment, chemical industry, and act for instance as soft abrasives in toothpastes. Alumina gels among other applications are used as precursors for the formation of abrasive grains and in the pharmaceutical industry where they are used as gastric treatment. Thanks to their considerable specific surface area (up to 1,000 m 2.g -1 ) and their surface reactivity, transition aluminas are among the basic catalysis products, but for ceramic applications, which are our concern here, alumina is always in its alpha form. However, the morphology of the crystals is not the same. The three principal categories of products are: i) calcined aluminas obtained by heat treatment of transition aluminas at temperatures of about 1,100M1,250 C and then milling; ii) tabular aluminas prepared by sintering at very high temperature (1,950 C) to obtain pellets which aggregate grains in the form of plates with large lateral size (50 to 300 3m) but low thickness, from which the name OtabularP stands for; iii) molten aluminas or corundum which result from a fusion in electric furnace (temperature of around 2,100 C).

Purities and grain sizes vary considerably from one product to another, as is shown in the catalogues of alumina producers [ALC 99 or PEC 99]. Typically, sintered ceramics require fine (grain size of about one micrometer) and OreactiveP powders, like calcined alumina at rather low temperatures. An increase in the calcination temperature augments the size of the crystals, which disadvantages sintering. Refractory materials require coarse grains (several hundred micrometers) to limit creep and to reduce reactions with the environment, and therefore prefer tabular alumina. Finally, abrasives use electromelt grains, although the search for enhanced performances is fuelling the development of sol-gel abrasive grains [SCH 91].

The reactivity of alumina powders that must be used for the sintering of ceramics has not been fully understood, especially since it is covered by the shroud of the industrial secrecy in powder preparation treatments. The challenge is to obtain a densified ceramic very close to the theoretical density and with a well-controlled 204 Ceramic Materials microstructure, but sintered at a temperature as low as possible: high temperatures are expensive, in energy consumption as well as in investment costs.

The first way to decrease the sintering temperature is to use liquid phase sintering by additives that produce eutectics at low melting temperatures: we thus use aluminas with purity of 80 to 98% approximately, in particular 96% pure aluminas, the remaining 4% corresponding to additions of SiO 2, CaO or MgO. The Al 2O3-MgO-SiO 2 system contains a eutectic that melts at 1,355 C and the Al 2O3- CaO-SiO 2 system a eutectic that melts at 1,170 C. Low sintering temperatures made possible by the vitreous flow have the disadvantage of a microstructure of the polycrystal where the alumina grains are embedded in an amorphous phase, which considerably degrades performances at high temperatures (creep). However, this is not necessarily a drawback for certain electric applications; there is even the advantage that the OanchoringP of a metallized layer on a ceramic substrate can be favored by the emergence of these intergranular segregations on the surface.

However, the most demanding applications M refractory materials (creep strength), mechanics (resistance to corrosion under stress and other effects of delayed failure), optics (perfect transparency and absence of coloring) or electronics (high frequency uses) M prohibit the presence of parasitic phases, therefore the purity of the alumina must be high (in general higher than 99.7%), but with the need for to add a few hundreds ppm of additives in order to limit grain growth that hampers sintering (see section 6.4). We must, however, stress that knowing only the purity, the nature of the impurities and the grain size distribution expressed in equivalent diameter of particles assumed to be appreciably spherical is not enough to explain the OreactivityP of a powder that allows sintering at around 1,500 C. This is a low temperature for a compound where the atomic diffusion is slow and whose fusion occurs beyond 2,000 C, whereas another seemingly equivalent powder can require a treatment at 1,600 C. The necessary condition is to use fine (micronic) and well deagglomerated particles, but we cannot be sure that this is a sufficient condition: grain size distribution, particularly the proportion of the fine ones, as well as surface flaws also have an impact. However, these parameters depend on the calcination and milling stages, milling being a process whose scale effects are not clearly established. This explains the difficulty of a laboratory study, from a few grams of powders, of the attrition mechanisms involved in large industrial grinders.

For specific uses, particularly for the preparation of ultrapure powders used for the synthesis of sapphire monocrystals, the Bayer process makes room for other methods, including ammonium alum method (dissolution of Bayer hydrargillite in excess sulphuric acid, neutralization in ammonia in NH 4Al(SO 4)2.12H 2O form, calcination at about 1,000 C and then milling) or the alcoholate method (based on an aluminum isopropylate). Alumina, Mullite and Spinel, Zirconia 205

In addition to the use of the term OreactivityP according to the meaning given by ceramists (it then means Opossible sintering at low temperatureP), we must define the chemical reactivity of alumina. Transition aluminas (Al 2O3-J), with metastable structure and very high specific surface area, are soluble in aqueous, acid or basic solutions, alumina being thus regarded as an amphoteric oxide. Alpha alumina is much less reactive than gamma alumina, but it dissolves heated in alkaline media or eutectics with low melting points like those of the CaO-SiO 2 system. Highly reactive metals (calcium or magnesium) can reduce alumina beyond | 900 C.

Table 6.1 illustrates the main ceramic applications of alumina powders, shows the desired properties and indicates the recommended powders.

6.3. Alumina ceramics

This book does not cover non-ceramic applications of alumina (filler in paper or in polymers, gel, catalyses, etc.). The applications where alumina grains are used primarily will be only touched upon (abrasives) or commented on elsewhere in the book (refractory materials). This chapter therefore primarily considers alumina ceramics for structural and functional uses.

6.3.1. Structural applications of alumina

Alumina ceramics owe to the stability of this oxide and its strong atomic bonds their mechanical and thermal performances which imply M but do not necessarily combined in a single type of microstructure M high hardness, high modulii of elasticity, satisfactory mechanical strength, wear resistance and good tribological properties and refractarity. Giving accurate values would be useless because mechanical properties are sensitive properties which vary with the microstructure, and the hot properties depend highly on the temperature and chemical reactions when the environment is aggressive. To illustrate this point, a dense fine-grained alumina ceramic has a YoungRs modulus of | 400 GPa (twice the modulus of steel), a PoissonRs ratio of | 0.25, a Vickers hardness of 20 GPa and a mechanical bending strength VF of | 300 to 500 MPa M the mechanical strength being a value that expresses a particular sensitivity to measurement conditions (see Chapter 8). For high temperature applications, in the absence of corrosion, sintered alumina pieces allow long duration uses, at temperatures that can exceed 1,600 C, but at stress levels that should not exceed a few MPa.

206 Ceramic Materials

Applications
Main desired characteristics
Sodium content/reactivity

High voltage
Mechanical and electric strength,
Normal/ insulators resistance to arc effects, purity, resistance to thermal shocks

Spark plugs
Mechanical and electric strength,
Normal to low/ controlled shrinkage, durability, green workability, resistance to thermal shocks

Ceramics for
Resistance to high temperatures, high
Low/ electronics electric resistance, thermal conductivity, ease of metallization

Tiles, enamels
Mechanical and chemical properties,
Intermediate/ controlled fusion and viscosity, adjustment of surface effects

Metallized substrates
Good thermal conductibility, ease of
Low/reactive metallization

Catalysis support
Preparation of cordierite, thermal
Normal/ conductivity, surface control

Ceramic filters for
Surface control, good thermal
Normal/ molten metal conductivity, resistance to thermal shocks

Crockery, sanitary
Mechanical and chemical resistance,
Normal/ whiteness

Ceramics for
Mechanical resistance, thermal
Normal mechanics conductivity, resistance to corrosion, to weak/reactive abrasion and thermal shocks

Tribology
Uniform microstructure, compromise
Normal/ between hardness and toughness

Table 6.1. Recommendations for choosing alumina powders [ALC 99]

Toughness at 20 C is equal to | 3.5 MPa m 1/2 , which is a respectable amount for a ceramic, but does not make alumina a tough material, especially when compared to metals. Ductility is modest. The coefficient of thermal expansion -6 -1 between 20 C and 1,000 C ( D20-1,000 ) is 8.5 10 K . This relatively high thermal expansion for a ceramic combines with a high YoungRs modulus in order to lower the first parameter of resistance to thermal shocks (R | VF/E D): alumina ceramics are not very resistant to thermal shocks. The expansion anisotropy is low (9.10 -6 K -1 Alumina, Mullite and Spinel, Zirconia 207 parallel to the ternary axis of the crystal and 8.3. 10 -6 K -1 perpendicular to this axis) and therefore the residual stresses at the grain boundaries are reduced. In order to complete the comment on a good resistance at high temperatures under weak mechanical load, it must be noted that we observe a poor creep resistance under substantial load. Alumina ceramics are best suited for mechanical uses (excluding shocks) or refractory uses, but they are not thermomechanical materials, if this word means combination of mechanical and thermal performances.

Mechanical applications Used in the grain state M as opposed to sintered ceramics M alumina is the most widely used abrasive besides silicon carbide. The performances of an abrasive depend on a complex set of qualities, where hardness is not the only parameter in question. In addition to tribochemical effects (which, for example, prohibit the use of diamond for the grinding of metals capable of forming carbides, such as ferrous alloys), the toughness and the morphology of the degradation facets of the abrasive grain also come into play: wear should not blunt it but maintain cutting sides. Friction terms or thermal conductivity must also be considered. The industrial products are very varied [NOR 99] and range from free abrasives (powders or pastes) to agglomerated abrasives (grinding stones), passing through abrasives on flexible supports (Oemery paperP). White and brown corundum are the usual products, but increased performances are obtained with globular alumina, alumina- zirconia with high toughness, or alumina obtained by precursor methods [SCH 91].

Tabular alumina grains constitute the filler of many refractories, whether they are fashioned as bricks or not fashioned: castable refractory concretes whose binder is cement, itself with high alumina content [BOC 99].

The tablets of alumina cutting tools, if necessary filled with titanium carbide to increase thermal conductivity or with zirconia to increase toughness, certainly do not have a market as broad as WC-Co cemented carbides. However, they can make impressive cutting speeds possible and they are adapted to the machining of hard metals or metals coated with oxidized layers. As for tribological applications, they are at the heart of the structural uses of alumina. We are referring to thread-guides for textile industry, spray nozzles, rollers and other supports for the paper industry, extrusion dies and seals between parts which turn or slide on one another. We can mention automobile water pump seals, but especially mixing tap seals because this application is the most developed M at least in Europe and in Japan because, curiously, the North Americans continue to prefer the old technique of flexible seals, fibers or elastomers. In our OmodernP taps, the lever displaces two parts of polished alumina bored with ports, which help regulate hot and cold water flows (see Figure 6.2). The surface quality of the discs is decisive for the sealing of these taps, whose lifespan can exceed 10 years. 208 Ceramic Materials

Figure 6.2. In a mixing tap, hot and cold water flows are regulated by rotating two alumina discs bored with ports [SAI 99]

The uses of alumina as a biomaterial (hip prostheses or dental implants; see Chapter 12) also highlight the tribological performances of alumina ceramics.

Finally, we must mention shieldings (bulletproof jackets, protective coatings of helicopters or tanks), which must stop high speed projectiles ( | 1,000 m s -1 ) or the dart of melting metal at very high speed ( | 10,000 m s -1 ) from hollow charges. The mechanisms of shock resistance have not been clearly established, but they are primarily due to the fact that the shock wave is propagated at a speed higher (| 10,000 m s -1 ) than the speed of growth of cracks ( | 3,000 m s -1 ). Therefore, the brittle fracture cannot follow the displacement of the shock wave and the OresistanceP of ceramic then tends towards its theoretical value (about one-tenth of YoungRs modulus). We need hard products, with high modulus and high mechanical strength under compression. Even if it is outclassed by ceramics like boron carbide and if light shieldings require polymers like Kevlar , alumina is still a good shielding material. The data on these uses are classified and are therefore difficult to collect.

We may lastly underline that thanks to the availability of alumina powder on the market with varied characteristics and the relative ease of sintering, alumina constitutes the first choice for ceramics with mechanical usages. It is only subsequently that insufficient performances for a given application compel us to consider another oxide, even a non-oxide, whose preparation and use have the annoying tendency of reminding us that we live in an oxidizing atmosphere!

Refractory applications The performances of cutting tools, thread guides and bearings, tape cartridges, prostheses or shieldings require dense and fine-grained microstructures. On the other Alumina, Mullite and Spinel, Zirconia 209 hand, the high temperature applications of alumina demand a porous and coarse- grained material, except for the case where a need for sealing justifies the choice of dense materials. Also resistance to thermal shocks is better for a porous material and creep strength is improved by coarse grains, but it is affected by porosity, and therefore we have to find a compromise.

In addition to the iron and steel industry, the industries that use large quantities of refractories are glass, cement and incineration: a chapter is devoted to these uses. We must also mention lining and conveying equipment for furnaces (for the firing of ceramics, in particular), crucibles, protections of thermocouples, etc. The insulating tubes of laboratory electric furnaces illustrate such uses. Placed in horizontal position and thus subjected to bending creep, the alumina tubes should not be used beyond 1,400 C; placed in a vertical position and therefore subjected to traction or compression, their use can exceed 1,700 C. Fibrous refractories often require alumina fibers and thermo-structural composites can also use Al 2O3 fibers (but OthermalP fibers and OmechanicalP fibers are quite different, as are glass fibers for insulation and glass fibers for reinforcement). Metal-ceramic composites continue to develop: the dissemination in a metal matrix of short fibers or ceramic particles, alumina in particular, widens the field of application of the metal matrices to higher temperatures M but at the price of a loss of toughness. Radomes , for example, prepared by fusion of alumina powder and spraying using a plasma torch on a preform, are an exotic application that relies on the thermal performances and abrasion resistance of alumina.

In short, alumina is one of the basic compounds for the refractorist, in addition to other oxides (for example, MgO in the iron and steel industry and OAZSP: Al 2O3- ZrO 2-SiO 2 in glassmaking) and a few non-oxides (primarily carbons and graphite and silicon carbides).

6.3.2. Functional applications of alumina

6.3.2.1. Electricity and electronics Alumina ceramics are widely used by the electric and electronic industries.

Spark plugs for automobiles illustrate the oldest application using materials on the borderline between aluminous ceramics and alumina, the compositions containing here | 94% Al 2O3. We also find a number of insulation products, including those requiring tight ceramic-metal sealings.

For electronics, the main product is the insulating substrate on which conducting or resistive, even capacitive or inductive, circuits are deposited (see Chapter 11). The advantage of alumina is its very high resistivity (it may be recalled that for this 210 Ceramic Materials it must be very pure, or at least free from impurities like sodium or transition metals), but also its mechanical properties (hardness, mechanical strength), because it can never be stressed enough that functional materials must, in addition to the specificities demanded by their function, have a sufficient level of mechanical performance: spectacles with broken glasses cannot correct vision! Another advantage of alumina is that it lends itself to metallization by various techniques which require not only chemical compatibility, but also excellent surface quality. The top-of-the-range consists of very pure aluminas ( | 99.7%) sintered close to the theoretical density, whereas the less demanding applications call for aluminas containing doping agents, therefore sintered in liquid phase. The primary mode of production of substrates is tape casting [BOC 88] (see Chapter 3) where a slip of alumina powder and polymeric binders are cast on a plane surface, which is reminiscent of paper techniques. The three limitations of alumina for the manufacture of dielectric substrates are: i) a modest thermal conductivity ( | 30 W m -1 K -1 ), which does not facilitate the dissipation of the heat produced by the Joule effect; ii) a coefficient of expansion double that of silicon, which generates stresses at the substrate (OchipP interfaces); iii) a marked permittivity ( H | 10), which induces capacitive couplings and decreases the transit times.

Aluminum nitride AlN is one of the competitors for the manufacture of high technology substrates, where it brings improvements with respect to the three drawbacks mentioned above, but its manufacture is expensive and delicate.

6.3.2.2. Optics The remarkable combination of optical and mechanical performances explains the uses of monocrystals (sapphire, possibly ruby) [CAS 90, GIT 70]. The main manufacturing method is the Verneuil method , invented at the end of the 19 th century. It consists of melting the alumina powder in the flame of an oxyhydrogen blowpipe, then solidifying the liquid by making it drip on a monocrystal which turns around its axis; the stalagmite thus formed will grow several centimeters per hour. The monocrystals (called balls , although their form is elongated) can be large (diameter expressed in centimeters and length in tens of centimeters). The other crystallogenesis processes ( Czochralski method or more rarely Bridgman method ) are more expensive, but yield crystals containing less growth defects. We can now produce monocrystals in the desired form [CEA 99] and it is also possible to manufacture fibers of approximately 100 3m diameter [LAB 80]. Jewelry (jewels, scratch-proof glass for watches), lazer matrices, substrates for certain electronic circuits and waveguides are the primary applications of sapphire monocrystals, as well as abrasion-resistant optical windows: for military use, but also barcode readers in supermarkets. Alumina, Mullite and Spinel, Zirconia 211

It is, however, at the polycrystal state that alumina has found its most widespread use: the internal, transparent tube of sodium vapor lamps used for street lighting. The difficulties which had to be overcome and the methods adopted to succeed have made this application the academic case for the sintering of ceramics. Whatever the compound considered, the vantage point for approaching its sintering is initially to try to transpose the case of alumina, or at least take advantage of the information that it has provided. We will therefore detail this historical example [COB 61].

6.4. Sintering of dense alumina

The discharge lamps used for street lighting use various atmospheres (mercury, xenon, argon, sodium, or various halides including tin and sodium). High pressure sodium vapor lamps are the most widespread. Their principle [RHO 91] is to start the discharge in a rare gas (xenon or neon) in order to overheat and vaporize a mercury-sodium amalgam, and then to allow the establishment of the discharge in the mercury vapor, then followed by sodium vapor. Plasma, at a temperature of 3,700 C, emits a radiation centered on the wavelength of 589 nm characteristic of the OdP transition of sodium. The sodium pressure can reach half the atmospheric pressure; the partial pressure of oxygen must be low to avoid the oxidation of the components; the temperature of the envelope that contains the plasma is about 1,200 C.

The material constituting the envelope must be colorless and transparent at the wavelengths considered, it must resist the chemical aggressions due to plasma and, lastly, it must be able to tolerate this temperature of 1,200 C for a very long lifespan (more than 20,000 hours). The performance of the most refractory glasses M like silica glass of halogen lamps M does not satisfy these specifications, so the only solution is to use a sintered ceramic: alumina has therefore become indispensable.

The optical applications of ceramics demand a material that is transparent and not just translucent. Any inclusion, even a transparent one, but with an optical index different from that of the matrix, causes a scattering of the light M an effect illustrated by depolished glass. Owing to the variation between the optical index of gases inside a pore (n | 1) and that of alumina (n | 1.76), a porous alumina ceramic is not transparent. The effect of porosity is extremely marked and a porosity of 0.3% is enough to reduce transparency by 90% compared to a dense body [KIN 76]: the manufacture of alumina envelopes for sodium vapor lamps consequently requires that the densification practically reaches 100%.

To obtain a perfectly dense material is the major difficulty in the sintering of ceramics. As indicated in Chapter 1, it is generally easy to eliminate opened porosity, whose canal morphology allows various matter transport mechanisms 212 Ceramic Materials

(surface diffusion, grain boundary diffusion, etc.). When porosity becomes closed (towards U/U0 | 92M94%), intergranular pores are relatively easy to resorb thanks to the grain boundary diffusion, but intragranular pores are practically impossible to eliminate, because on the one hand they can now benefit only from volume diffusion, and on the other hand the gas that they contain can have trouble migrating through solid.

To be transparent, alumina must be very pure or, at least, free from impurities that are sources of coloring, as is the case with transition metal oxides (iron, titanium, etc.). However, experience shows that the sintering of very pure alumina (99.7%) does not prevent granular growth: large-sized grains (see Figure 6.3, the micrography on the left where the measuring bar is equal to 30 3m) trap pores in intragranular position and the ceramic is not transparent. It is to Coble [COB 61] that we must give the credit of showing that the addition to alumina of about 500 ppm MgO limits granular growth considerably, which prevents the trapping of intragranular pores and yields a transparent polycrystal (see Figure 6.3, the middle view where the measuring bar is equal to 10 3m, and the right-hand side view almost on the same scale).

Figure 6.3. Microstructures of sintered aluminas: on the left OpureP Al 2O3; in the middle and right, Al 2O3 doped with | 500 ppm MgO [RHO 91]

The exact role played by MgO in Al 2O3 has given rise to numerous and sometimes contradictory explanations, but it has been conceded since 1985 that MgO acts at two levels: by increasing surface diffusion M which contributes to pore mobility M and by decreasing grain boundary mobility [BEN 85]. More recent studies stress more the reduction of boundary mobility than the increase in mobility, and they show that MgO could also OpurifyP the grain boundaries where silicate segregations exist, because the behavior of Overy pureP aluminas (> 99.97%) is quite different from that of OultrapureP aluminas (> 99.995%) [BAE 94]. We refer the reader to Chapter 3, which gives further information on this question.

Alumina, Mullite and Spinel, Zirconia 213

The success of the sintering of dense alumina has been and continues to be a model for OoptimizingP the sintering of other ceramics. The use of sintering additives has become a common practice, even if all ceramics do not benefit from this to the same degree and a dose of empiricism remains as regards the choice of the best additive: in fact, there is still no irrefutable explanation justifying why it is the magnesia M and not another oxide M which is most effective sintering additive for alumina.

6.5. Point defects and diffusion in alumina

The sintering of ceramics, like their creep and their reactivity, depends primarily on the diffusion processes, these processes themselves depending on the structure of the compounds, the defects that they contain and the microstructure.

The many studies devoted to the defects found in alumina and diffusion mechanisms indisputably highlight the fact that the OcleanestP works have been carried out on high purity sapphire monocrystals, but they are not easily usable for sintered ceramics M polycrystalline and seldom free from involuntary impurities or voluntary doping agents. To understand the behavior of ceramics, experiments on diffusion creep are often more useful [LAN 91]. The Nabarro-Herring creep, controlled by volume diffusion [HER 50], can determine the performances of coarse-grained materials, for example refractory products, but it is especially the Coble creep, controlled by grain boundary diffusion [COB 63], that elucidates the properties of sintered ceramics (Chapter 3 shows the creep maps).

For experiments conducted at temperatures ranging between 1,150 and 1,550 C, the typical temperatures of creep/sintering problems, the coefficients of volume expansion deduced from the creep of polycrystalline alumina show that the process is controlled by the diffusion of aluminum in the lattice [GOR 84]. Grain boundary diffusion also seems to be controlled by the cation. The diffusion of oxygen in the ## x lattice implies oxygen vacancies V O and boundary diffusion neutral interstitials O i [KRO 84]. The grain boundary diffusion of oxygen seems very fast. Most doping agents increase cation mobility (in volume and at the boundaries), with particular mention of Mn, Ti and Fe. On the contrary, MgO does not seem to modify the diffusion kinetics significantly.

6.6. Al 2O3-SiO 2 system mullite

Argillaceous minerals of the sillimanite group were used as early as in the first era of ceramics, but it is mullite 3Al 2O3.SiO 2 that is the only stable crystallized compound in the binary phase diagram of the Al 2O3-SiO 2. However, mullite is very 214 Ceramic Materials rare in the state of natural ore, except in some places such as the Scottish island of Mull, which explains its name.

The phase diagram Al 2O3-SiO 2 has fuelled numerous debates. For Aramaki and Roy [ARA 62], mullite exhibits a narrow field of solubility close to 3Al 2O3.SiO 2 (Figure 6.4): this mullite is called 3:2, because of the alumina/silica ratio. It exhibits congruent melting, i.e. it melts to directly yield a liquid whose composition remains unchanged when the temperature continues to rise. This diagram accounted for most of the experiments, but did not explain certain anomalies and Aksay and Pask [AKS 75] modified it (Figure 6.5) by showing that mullite exhibits incongruent melting at | 1,830 C, with peritectic reaction Al 2O3-D plus liquid, which forms this mullite with 52.3% in mass of Al 2O3. However, the stable diagram often gives room to metastable extensions. For instance, a monocrystal that has grown in the absence of alumina germs does not have an alumina/silica ratio of 3:2 but 2:1, i.e. 77.2% in mass of Al 2O3, and the solid solution can extend up to the composition 3:1, with 83.6% of Al 2O3. A later fine-tuning of the diagram [KLU 90] confirmed the incongruent melting and the metastable extensions, but by modifying some of the limits. It is now admitted that when alumina germs are present, the peritectic reaction occurs and that there is incongruent melting, but for a homogeneous liquid free from these germs, the melting is congruent.

Mullite crystallizes in the orthorhombic system, space group Pbam. Its complex structure accommodates stoichiometry variations (from 3Al 2O3.SiO 2 to 3Al 2O3.iO 2) thanks to the presence of oxygen vacancies [EPI 91].

Figure 6.4. Al 2O3-SiO 2 diagram [ARA 62] Alumina, Mullite and Spinel, Zirconia 215

In comparison with alumina, mullite is slightly lighter ( U | 3.2 g cm -3 ) and has lower values of hardness (H V | 14 GPa), YoungRs modulus (E | 250 GPa), 1/2 mechanical bending strength ( VF | 250 MPa) and toughness (K c | 2.5 MPa m ). -6 -1 However, its thermal expansion is also less ( D20-1,000 C | 6 10 K ), which improves resistance to thermal shocks, and its mechanical strength drops much less quickly when the temperature increases than in the case of alumina: at 1,300 C, most mullite ceramics have a mechanical strength close to the one at room temperature, and some mullites with vitreous segregations even have a peak mechanical strength at about 1,300 C. These characteristics make mullite a material of choice for refractory applications, particularly when it is necessary to consider attacks by silica or silicates, because then an alumina refractory would react to form mullite.

Figure 6.5. Al 2O3-SiO 2 diagram: the dotted lines show metastable extensions [AKS 75]

The manufacture of sintered mullite is not easy. On the one hand, mullite powders are not easily available. Furthermore, the diffusion phenomena are slow in this compound, a characteristic which justifies the good mechanical hot properties M because less diffusion means less creep M but also more difficult sintering. The 216 Ceramic Materials reactive sintering [BOC 87, 90, GIR 87, ROD 85] of a mixture of alumina and silica powders can be adopted with the use of a mixture of zircon (ZrSiO 4) and alumina powders making the preparation of a mullite toughened by zirconia dispersions (ZrO 2) possible. The difficulty is to avoid the presence of silica segregations along the grain boundaries, which would degrade creep resistance. The peak of mechanical strength (a variable measured by applying a load for a short period of time, while creep implies longer periods) at 1,300 C is due to such segregations, here beneficial, because they release the stress concentrations and thus attenuate the manifestations of brittleness.

Mullites with high mechanical performances have hardly made any progress over the last ten years [SOM 90]. The efforts that their development would require have perhaps been overshadowed by the domination of alumina.

6.7. Al 2O3-MgO system: magnesia and spinel

6.7.1. Magnesia

Alkaline-earth elements form oxides with the general formula MO with high melting temperature ( | 2,800 C for MgO, | 2,580 C for CaO, | 2,430 C for SrO) [KIN 84, BRE 91]. Owing to reactivity with water and propensity to carbonation, the most common minerals that contain these elements are calcite CaCO 3 for calcium, magnesite MgCO 3 and its hydrated varieties for magnesium, and dolomite (CaMg)CO 3 which combines both cations. Sea water also contains an appreciable proportion of magnesium ( | 1.3 kg m -3 , in the form of chlorides and sulphates) and hydrated magnesia Mg(OH) 2 can be extracted by seawater treatment.

The archetype of oxides with ionic bond, magnesia MgO crystallizes in a NaCl- like structure (space group Fm 3 m). The simplicity of this structure, the very low difference from stoichiometry and the possibility of having of very pure monocrystals and perfectly densified polycrystals make MgO a Omodel materialP to understand the properties of oxides, such as the dislocation movement. The plasticity of MgO is remarkable, at least for a ceramic compound: if, below 350 C, slips occurs only according along (110) < 110 >, at high temperature ( | 1,700 C) five independent slip systems are activated and rupture is no longer brittle, because the von Mises criterion is met.

The majority of magnesia produced is used for refractories in the iron and steel industry (see Chapter 10), where the basic oxide properties of the material are necessary (steel-works use the BOF method = basic oxygen furnace). Magnesia withstands the very high temperatures of converters (1,700 C), where it can dissolve several times its weight of iron oxide without melting and it effectively resists Alumina, Mullite and Spinel, Zirconia 217 sealing and the attack of slag. The high reactivity of magnesia with water is a disadvantage for its use, but this reactivity decreases if we decrease the specific surface area of Chamottes by a high temperature treatment, which also makes impurities, including silicate impurities, form a less reactive skin on the grain surface. This is called dead-burnt magnesia. Calcined dolomite, which is an almost 50:50 mixture of CaO + MgO (in moles), is less sensitive to hydration, but it is carbon products that ensure, in most cases, the protection of magnesia and also less wettability with respect to the attacking agents, such as slag. Magnesia-carbon refractories (about 30% carbon, in various forms) are the reference refractories in steel-works.

6.7.2. Spinel

Spinel MgAl 2O4 has given its name to a crystalline structure adopted by several 2+ 3+ 2+ 2+ mineral phases. Based on the notation Mg Al 2 O4, Mg , Mg can be replaced by other divalent cations like Fe 2+ , Mn 2+ or Zn 2+ , and Al 3+ can be replaced by other 3+ 3+ trivalent cations like Fe or Cr . The spinel structure AB 2X4 (space group Fd3m) is made up of a face-centered cubic stacking of X 2- ions (O 2- in the case of 2- MgAl 2O4) containing eight structural units per array, i.e. a total of 32 X (eight units multiplied by four nodes per CFC array). The cations A and B are placed in the interstitial sites. There are 64 tetrahedral interstices of which eight (Wyckoff 8a positions) are occupied and 32 octahedral interstices of which 16 (Wyckoff 16d positions) are occupied. Based on the occupancy of the interstitial sites, we can distinguish two cases: 3+ M in a normal spinel, like MgAl 2O4, the trivalent Al ions are in octahedral sites and the bivalent Mg 2+ in tetrahedral sites; 2+ 3+ 3+ M in an inverse spinel, like Fe 3O4 (written as Fe Fe 2 O4), the trivalent Fe ions are divided half/half between octahedral sites and tetrahedral sites, and the bivalent Fe 2+ are in octahedral sites.

These two cases are borderline cases, but there can be various deviations from this distribution. Statistical spinel corresponds to complete disorder where trivalent and bivalent cations are distributed randomly in the A and B sites. Besides II-III spinels that we have just considered, the spinel structure can adapt to cations with other nominal oxidation states: II-IV spinels like Mg 2SiO 4, I-VI spinels like Na 2WO 4 and I-III spinels like K 2Zn(CN) 4. Gamma alumina (see section 6.2.2) has a lacunar spinel structure: the aluminum ions are all trivalent, but electric balance is respected by considering that a bivalent entity M 2+ corresponds to 2/3Al 3+ plus x 1/3V Al (by using the Krger notation [KR 74, WES 90] of point defects, where x VAL indicates a constitutional vacancy, whose effective charge compared to the normal occupation of the site is zero). 218 Ceramic Materials

Figure 6.6. MgO-Al 2O3 diagram

The magnetic properties of magnetite Fe 3O4 (also written as FeOFe 2O3) have been known since antiquity. This iron ferrite (different from OferriteP, the centered cubic form of iron) crystallizes into inverse spinel. There is an antiferromagnetic order between octahedral and tetrahedral sites, therefore the magnetic moments of Fe 3+ ions cancel each other out and overall magnetization is due only to the sole contribution of Fe 2+ ions: this is called ferrimagnetism. The magnetic properties of ceramics are the foundation for the tape recording industry (see Chapter 11).

The balance diagram of Al 2O3-MgO (see Figure 6.6) shows that, at high temperatures, spinel is not stoichiometric, but allows a broad solid solution domain, in particular on the alumina-rich side. The fusion of MgAl 2O4 occurs at | 2,105 C, i.e. more than 50 C above the fusion of alumina. In addition to its refractarity, the interest of spinel as refractory chamotte is due to its possible formation by in situ reaction, for example, in concretes made up of magnesia grains bound by an aluminous cement [CHA 98].

Dense sintered spinel is transparent in a broad domain of wavelengths, which explains its use in the manufacture of optical windows (military applications in particular) [BOC 91]. Alumina, Mullite and Spinel, Zirconia 219

6.8. Zirconia

The most striking characteristic of zirconia is that it exhibits phase transitions that were regarded for a long time as drawbacks for applications, but which, since 1975, have proved to be a rich source of possibilities [SOM 88].

6.8.1. Polymorphism of zirconia

Zirconium dioxide (or zirconia: ZrO 2) is found in natural state in the form of baddeleyite (primarily in South Africa), but is more frequently prepared from zirconium silicate sands (zircon: ZrSiO 4) by high temperature heat treatments, accompanied by chemical treatments, which eliminate the siliceous fraction from the zircon.

Zirconia is an oxide with very high melting temperature (T | 2,880 C), which solidifies in cubic phase (ZrO 2-c, group space Fm 3 m), then transforms (T < | 2,370 C) to tetragonal phase ( ZrO 2-t, P4 2/nmc) and finally, below | 1,170 C, becomes monoclinical (ZrO 2-m, P2 1/c). This last transition t om is accompanied by considerable dimensional variations (shear strains of | 0.16 and increase in volume of | 4%), which largely exceed the maximum stress limit, resulting in a fragmentation of the material. A zirconia part sintered at temperatures that the refractarity of this oxide requires M let us say sintered at about 1,500 C M breaks up and is destroyed during cooling, during the t om transition. This means that OpureP zirconia can be used only in powder form (for example, as starting product for the manufacture of ceramic enamels), and therefore for uses that do not require consolidation into a massive part. To produce zirconia sintered pieces, ZrO 2 must be combined with other oxides known as OstabilizersP (M xOy= primarily CaO, MgO or Y 2O3): the ZrO 2-M xOy phase diagram is then modified favorably, which helps preserve (at the stable state or metastable state) a Ostabilized zirconiaP, free from transitions in the entire useful temperature range M in practice from the sintering temperature to room temperature. In the ZrO 2-CaO diagram, for example, it is observed that for 20 mol% CaO, the material remains in cubic phase from room temperature to practically the melting temperature.

Among the uses of stabilized zirconia, denoted OSZP, we can mention four main fields: M the production of monocrystals for jewelry, because the optical properties of zirconia are not very different from those of a diamond, at an incomparably lower cost; M the manufacture of crucibles and other refractory parts, because of the high melting temperature and good resistance to corrosive mediums, including molten glass (refractories of A-Z-S system: Al 2O3-ZrO 2-SiO 2); 220 Ceramic Materials

M the manufacture of thermal barriers, for example deposited by plasma spraying using plasma torches for the internal protection of the combustion chamber of jet engines, because the thermal conductivity of zirconia is one of lowest ever known among non-metallic inorganic solids (k | 1 W .m-1. K-1 , i.e. 30 times lower than alumina); M the manufacture of ionic conductors. Although this last aspect is detailed in Chapter 11, we must mention it here because it is related to the same mechanism of OstabilizationP. ZrO 2-c has a fluorite structure (like, for example, UO 2), with the zirconium atoms at the nodes of a face-centered cubic lattice and the oxygen atoms occupying all the tetrahedral interstices of this lattice (four nodes for a CFC array and eight tetrahedral sites per array give the stoichiometry ZrO 2). The unit is very compact and we can say that the transitions that occur when we cool ZrO 2-c come from the need to decompress the structure. However, the introduction of bivalent (CaO and MgO) or trivalent (Y 2O3) metal oxides that are used as stabilizers requires, for the charges to be balanced, that the substitution of Zr 4+ by M 2+ or M 3+ is compensated by the presence either of interstitial cations or anionic vacancies. It is the second case that occurs here: a usual SZ, 20% CaO-80% ZrO 2 (in moles), therefore contains a 20% deficit of oxygen atoms: Zr1-x Ca xO2-x VOx , in Krger-Vink notation, where V O represents oxygen vacancies and x is equal to 0.2. The anionic sublattice is thus decompressed and this considerable concentration in vacancies M more than ten orders of magnitude higher than the normal concentration of defects in thermodynamic equilibrium M allows a considerable mobility of residual oxygen ions, since we know that the diffusion proceeds primarily by a vacancy mechanism, like a puzzle. By virtue of these constitutional oxygen vacancies, the stabilized zirconia offers properties of ionic conduction that allow its application as solid electrolyte, particularly in oxygen sensors and in solid oxide fuel cells [SIN 00].

6.8.2. Ceramic steel?

SZ has rather modest mechanical properties, significantly less remarkable compared to alumina which, associated with higher density, higher thermal expansion (consequently greater sensitivity to thermal shocks) and markedly increased costs explain why these stabilized zirconias a priori do not have a mechanical application.

It is partially stabilized zirconias (PSZ) that have justified the resounding article (OCeramic steel?P) published in 1975 by Garvie et al . [GAR 75]. The title suggests that a ceramic can exhibit the high mechanical performances associated with steel, but also that toughening mechanisms recall those used by steel manufacturers. The tom transformation of zirconia is a martensitic transformation, in analogy with the transformation used to obtain martensite in tempered steels, and the role of microstructural parameters in ZrO 2 is similar to what is observed in metals. Alumina, Mullite and Spinel, Zirconia 221

Figure 6.7. ZrO 2-MgO and ZrO 2-Y 2O3 diagrams; the hatched zones show the compositions normally chosen for ceramics with TT effect [HAN 00]

The mechanical properties of zirconias with high mechanical performances M of which there are multiple varieties M constitute one of the themes that have inspired the greatest number of publications in the field of ceramics. [HAN 00] constitutes an excellent study on the subject.

Let us start by analyzing what it is about, by reasoning at room temperature. A non-stabilized zirconia (Z) is subjected, during its cooling from its processing temperature, to destructive phase transition t om: its mechanical performances are therefore almost zero. A stabilized zirconia (SZ) is free from these transitions; it remains in general in cubic phase: its mechanical performances are modest. Lastly a partially stabilized zirconia (PSZ) can be made up of a matrix rich in stabilizers, therefore in the form of SZ, within which there is a dispersion of small precipitates of zirconia poor in stabilizers, which should be in monoclinic form but can subsist in a metastable state in tetragonal form. The propagation of a microscopic crack relaxes the stresses applied by the matrix on these precipitates, which enables them to switch to the monoclinic, stable state. Hence, local swelling (increase in volume of | 4% associated with the t om transformation) which OclampsP the crack and stops its propagation: we can thus considerably increase mechanical strength and toughness. However simplistic this explanation might be, it highlights the two essential points: i) the t om transformation, previously considered as a drawback, can become an advantage; ii) the control of microstructural characteristics (for instance, grain size) is essential. 222 Ceramic Materials

Figure 6.7 shows the ZrO 2-CaO and ZrO 2-Y 2O3 phase diagrams, where the main PSZ materials are located.

6.8.3. Transformation toughening

In transformation toughening (TT), toughening indicates the increase in toughness as well as in mechanical strength. We have the following acronyms: ZTC (zirconia toughened ceramics), Ca-PSZ or Mg-PSZ (partially stabilized zirconia containing | 8 mol% CaO or | 9 mol% MgO), TZP (tetragonal zirconia polycrystals, Y-TZP containing typically 2-3 mol% Y2O3), etc. There are indeed various categories of materials depending on the nature of the stabilizer, its concentration, microstructure and various associated phases: ZTA (zirconia toughened alumina) is for example alumina toughened by a zirconia dispersion, prepared in such a manner that TT mechanisms are operational.

The application of TT mechanisms to manufacture TTCs (transformation toughened ceramics) still requires that we retain a fraction of metastable ZrO 2-t, capable of being transformed to ZrO 2-m, at the usage temperature (close to room temperature), under the effect of the applied stresses; recent works have shown that in addition to elongation and variation of volume, these stresses imply significant shearing.

As in steels, martensitic transformation t om is an instantaneous transformation, displacive in nature, which develops when temperature decreases. In pure ZrO 2, cooling transformation starts at about 950 C (point known as M S) and reversible heating transformation occurs beyond 1,150 C (A s). We can summarize the crystallographic aspects by saying that the structures OtP and OmP derive from the fluorine structure OcP by various distortions, the most important of which is the one associated with the t om transition, with a shearing of | 9 parallel to the base plan of the array OtP to lead to an angle of the monoclinic cell E | 81 .

We have said that there are multiple categories of TTCs [CLA 85]. To limit ourselves to the three primary categories: i) in Mg-PSZ, the toughening is due to a dispersion of lenticular ZrO 2-t precipitates, ii) in TZP (Y-TZP or Ce-TZP), the material has very fine, monophase ZrO2-t grains and iii) in DZC (dispersed zirconia ceramics, like ZTA), the ZrO 2-t particles are dispersed within an alumina matrix (see Figure 6.8).

Alumina, Mullite and Spinel, Zirconia 223

Figure 6.8. Typical microstructures of three materials toughened by zirconia: (a) Mg-PSZ, (b) Y-TZP, (c) ZTA. The transformable precipitates in Mg-PSZ are lenticular in form; TZP is monophased, with very fine grains [HAN 00]

Phenomenologically, we can write the toughness of a multiphase brittle material as:

K1C = K 0 + 'KC [6.1]

where K 0 is the toughness of the matrix and 'KC the contribution of specific toughening mechanisms.

For TT, the transformation of a metastable particle OtP into a stable particle OmP in the stress field associated with the propagation of a potentially dangerous crack originates increased toughness, because on the one hand the increase in volume due to the transformation places the crack front under compression and on the other hand the strain energy associated with the shearing components contributes to an increase in the rupture energy. Lastly, the expansion of the transformed particle can lead to local microcracking, hence the beneficial effects of crack bifurcation. The interaction of these effects results in three main mechanisms: i) 'KCT (transformation toughening), ii) 'KCM (transformation-induced microcrack toughening) and iii) 'KCD (crack deflection toughening).

The essential term is 'KCT , which can be written [HAN 00]:

1/2 (1- Q)'KCT = KE*e TVfh [6.2] 224 Ceramic Materials where K is a characteristic geometrical factor of the crack front and the stress field, E* the effective YoungRs modulus of the material used as matrix e T the expansion strain, V f the volume fraction of the transformed particles, h the half height of the transformed zone, and Q the PoissonRs ratio. The elasticity characteristics of the matrix play an important role. For example, an alumina matrix (E* | 380 GPa, Q | 0.2) places more stress on transformable particles than a zirconia matrix (E* | 210 GPa, Q | 0.3), causing a reduction in the thickness of the transformed zone h and the transformed volume Vf.

These effects result in microplasticity phenomena, which explain a non-linear stress-strain behavior in the zones close to the transformation (for example, close to the edges of a microhardness indentation).

Figure 6.9. Mechanical strength/toughness relation for various materials toughened by ZrO 2. The maximum of the curve corresponds to the transition from a rupture controlled by the size of the cracks to a rupture controlled by the triggering threshold of the t om transformation [SWA 85]

We cannot cumulate maximum increase in toughness and maximum increase in mechanical strength and we must choose between increasing one or the other. Figure . 1/2 6.9 shows this compromise. For materials with K 1C < 8 MPa m , it is the size of the critical crack that limits the mechanical strength Vf (according to the usual law . -1/2 Vf | K 1C aC , where a C is the equivalent size of the critical crack). For . 1/2 K1C > 8 MPa m , it is the level of stress inducing the transformation that limits Vf: Alumina, Mullite and Spinel, Zirconia 225 the transformation consumes, at low stress levels, the entire metastable fraction of zirconia, after which the rupture is controlled again by the size of the microcracks.

An interesting effect made possible by the toughening with the help of transformable zirconia is the effect known as OR curveP, thanks to which non-linear phenomena close to the crack front lead to a growth in toughness K 1C when the crack propagates itself (Figure 6.10). In the absence of such an effect, the starting of a crack is generally catastrophic when the stresses that act on it reach the critical point. In the presence of an R curve effect, on the contrary, the development of damaged zones (process zone and shielding zone) can increase the toughness sufficiently for the crack to become OsubcriticalP; it is only a later increase in stress that would allow the continuation of its propagation.

Figure 6.10. Toughening depending on the propagation of the crack (OR curveP effect). This effect is pronounced in Mg-PSZ, Ce-TZPand Si 3N4; less pronounced in SiC; non-existent in Y-TZP, spinel MgAl 2O4 and graphite [HAN 00]

6.8.4. ZrO 2 based materials: treatments and microstructures

The management of the toughening is delicate: excessively stabilized, a zirconia exhibits mechanical properties that are hardly improved; insufficiently stabilized, it can be subjected to destructive phases transformation. Furthermore, we must negotiate between the reinforcement of toughness and the reinforcement of mechanical strength. Other factors also intervene, like resistance to temperature 226 Ceramic Materials

effects or destabilization and economic givens (for instance, Y 2O3 is a definitely more expensive stabilizer than MgO).

The stabilization of ZrO 2-t can be achieved by: i) additions of bivalent or trivalent metals, which introduce oxygen vacancies: Ca 2+ , Mg 2+ , Gd 3+ , Fe 3+ , Ga 3+ and Y 3+ and ii) trivalent cations but of size higher or lower than that of Zr 4+ Ti 4+ , Ge 4+ and Ce 4+ .

PSZ materials are bi-phased, generally in CaO-ZrO 2 or Mg-ZrO 2 diagrams. Based on this last case (see Figure 6.7), we choose compositions with | 9 mol% MgO, cooled quickly, which leads to a dispersion of lenticular precipitates of very small-sized ZrO 2-t (30M60 nm), therefore too stable (their temperature M S is below room temperature). A subsequent treatment for the enlargement of precipitates (Figure 6.8) must then be carried out, at about 1,400 C, i.e. above the eutectoid transformation temperature. We can also proceed by adjusting the post-sintering cooling speed to a sufficiently low value to allow an enlargement of the precipitates, which avoids the subsequent heat treatment.

TZP materials generally use Y 2O3 or CeO 2 as stabilizing oxides. For Y-TZP (see Figure 6.7), the compositions vary from 1.75 to 3.5 mol% (3; 5 to 8.7% in mass). A sintering performed at about 1,500 C can then yield a monophase polycrystal (100% tetragonal) made of very fine equiaxed grains (0.5M2 3m) (see Figure 6.8). Y-TZP materials exhibit the highest mechanical performances, but they are degraded when maintained in wet heat; from this point of view, Ce-TZPs exhibit much better properties. Figure 6.9 shows that Y-TZPs are unequalled as regards mechanical . 1/2 strength, but that Ce-TZPs have a much higher toughness (K CT Maxi | 10 MPa m for Y-TZP and 17 MPa.m 1/2 for Ce-TZP).

As regards non-zirconia matrices toughened by zirconia, the most widespread materials are based on Al 2O3 (ZTA). When the zirconia fraction is not stabilized, its expansion causes a microcracking of the matrix (microcrack toughening); when this is a TZP fraction, it allows a TT mechanism. Cutting tools use these ZTA materials. Non-oxide materials (including SiC or Si 3N4) have been the subject of studies, but have not yet experienced any industrial development. Lastly, reasoning on a large scale M i.e. beyond particulate composites that we have been discussing in order to speak about composite structures M we can use TT effects in, for example, laminates where y% (M aOb)(100-y)% ZrO 2 alternate layers and z% (M aOb) (100-z)% ZrO 2, MaOb being for example Al 2O3 [BES 87].

To conclude on zirconias with mechanical uses, we must underline that TT mechanisms cease to operate at temperatures too close to the M S point: TTCs therefore do not compete with non-oxide ceramics like SiC or Si 3N4, which have been developed to offer high mechanical strength or creep resistance at high Alumina, Mullite and Spinel, Zirconia 227 temperatures. Even at modest temperatures (T | 200 C) the effectiveness of transformation toughening is reduced on the one hand, and TTCs can suffer from harmful microstructure deteriorations on the other hand: it is only around room temperature that these Oceramic steelsP are at their best. Their uses are primarily intended for things that cut, rub or wear M because the tribological performances are very satisfactory M for instance, knife blades or scissors, scrapers, linings, valve seats or guides (automobile), wear parts (paper industry), extrusion nozzles or die (metallurgy), etc.

6.9. Other oxides

Alumina, materials of the binary systems Al 2O3-SiO 2 and Al 2O3-MgO, and zirconia do not exhaust the list of oxides with ceramic interest [RYS 85]; far from it.

We may repeat that SiO 2 and the compositions in the ternary system SiO 2-CaO- Na 2O are the basic ingredients in glass industries and that the quaternary system CaO-SiO 2-Al 2O3-Fe 2O3 contains the compositions of cement clinkers . Chapter 11 devoted to ceramics for electronics, considers oxides with magnetic performances (for example, ferrites with spinel structure) and also barium titanate BaTiO 3 and zinc oxide ZnO, not to speak of beryllium oxide BeO or superconductive cuprates of which YBa 2Cu 3O7 is the archetype.

Chapter 12 underlines, once again, the merits of alumina, as a material with low biological reactivity and shows the possibilities of calcium phosphates, with high biological reactivity. Chapter 11 focuses on uranium and plutonium oxides.

Among the oxides with industrial interest that this book, on account of its constraints, has been forced to omit, or to which it has devoted only very little place, we can mention TiO 2, chromium oxides and oxides of other transition metals, tin oxide and indium oxide, yttrium oxide Y 2O3 and rare earth oxides, highly refractory oxides like ThO 2 or HfO 2, and the various oxides developed in the form of monocrystals, for jewelry or professional uses (for example, radiation detectors). Either these oxides have very specialized uses (for example, ultra-refractories like HfO 2), or their main use falls outside the scope of ceramic industries (for example, TiO 2, which is used primarily as a pigment).

It may be recalled that the Journal of the European Ceramic Society and Journal of the American Ceramic Society are the most valuable sources for information on ceramics not dealt with in this book. 228 Ceramic Materials

6.10. Bibliography

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[ARA 62] ARAMAKI S. and ROY R., ORevised phase diagram for the system SiO 2-Al 2O3P, J. Am. Ceram. Soc. , 45, p. 229-242, 1962. [BAC 98] OAnnual Mineral ReviewP, Am. Ceram. Bull. , 78, 8, p. 118-121, 1999. [BAE 94] BAE S.I. and BAIK S., OCritical concentration of MgO for the prevention of abnormal grain growth in aluminaP, J. Am. Ceram. Soc. , 77(101), p. 2499, 1994. [BEN 85] BENNISON S.J. and HARMER M.P., OGrain growth kinetics for alumina in the absence of a liquid phaseP, J. Am. Ceram. Soc. , 68, C22-4, 1985.

[BES 87] BESSON J.L., BOCH P. and CHARTIER T., OAl 2O3-ZrO 2 layer compositesP, High Technology Ceramics , Materials Science Monographs 38, Elsevier, 1987. [BOC 87] BOCH P. and GIRY J.P., OPreparation of zirconia-mullite ceramics by reaction sinteringP, High Technology Ceramics , Materials Science Monographs 38, Elsevier Edit. 1987. [BOC 88] BOCH P. and CHARTIER T., OUnderstanding and improvement of ceramic processes: the example of tape castingP, Ceramic Developments, Materials Science Forum , Trans. Tech. Publ., p. 813, 1988. [BOC 90] BOCH P., CHARTIER T. and RODRIGO, OHigh purity mullite by reaction sinteringP, Mullite and Mullite Matrix Composites, Ceramic Transactions, Vol. 6, The Am. Ceramic Society, p. 353, 1990. [BOC 91] BOCH P. and RAYNAL P., OFabrication of transparent magnesium aluminium oxide (MgAl2O4) ceramicsP, Mat. Sc. Monographs 66D , P. Vincenzini Edit., Elsevier Publ., p. 2343, 1991. [BOC 99] BOCH P., LEQUEUX, MASSE S. and NONNET E., OHigh-performance concretes are exotic materialsP, Z. Mettallk ., 90, p. 12, 1-5, 1999. [BRE 91] BRETT N.H., OMagnesium and alkaline-earth oxidesP, p. 283-286, in [BRO 91]. [BRO 91] BROOK R.J. (ed.), Concise Encyclopedia of Advanced Ceramic Materials , Pergamon Press, 1991. [BUR 84] BURNS R.G. and BURNS V.M., OOptical and Mssbauer spectra of transition- metal-doped corundum and periclaseP, p. 46-61, in [KIN 84]. [CAS 90] CASTEL A., Les alumines et leurs applications , Nathan, 1990. [CEA 99] THEODORE F., ODes saphirs haute technologieP, CEA Technologies , 45, p. 2, 1999. [CER 99] Ceramics Monographs, Handbook of Ceramics , updated by Interceram , Verlag Schmidt (since 1982). Alumina, Mullite and Spinel, Zirconia 229

[CHA 98] CHAN C.F. and KO Y.C, OEffect of CaO content on the hot strength of aluminaM spinel castables in the temperature range of 1,000 to 1,500 CP, J. Am. Ceram. Soc. , 81(11), p. 2957-2960, 1998. [CLA 85] CLAUSSEN N., RUHLE M. and HEUER A.H., Advances in Ceramics, vol. 12, Science and Technology of Zirconia II , Am. Ceram. Soc., 1985. [COB 61] COBLE R.L., OSintering crystalline solids: I Intermediate and final stage diffusion models; II. Experimental test of diffusion models in powder compactsP, J. Appl. Phys., 32, p. 787-799, 1961 . [COB 63] COBLE R.L., OModel for boundary diffusion controlled creep in polycrystalline materialsP, J. Appl. Phys. , 34(6), p. 1679-1682, 1963. [DR 84] DORRE E. and HUBNER H., Alumina , Springer-Verlag, 1984. [EPI 91] EPICIER T., OBenefits of high-resolution electron microscopy for the structural characterization of mullitesP, J. Am. Ceram. Soc. , 74(10), p. 2359-2366, 1991. [GAR 75] GARVIE R.C., HANNINCK R.H.J and PASCOE R.T., OCeramic Steel?P, Nature , 258, p. 703-704, 1975. [GER 96] GERMAN R.M., Sintering Theory and Practice , John Wiley, 1996. [GIR 87] GIRY J.P., Etude du frittage-r action alumine-zircon: pr paration et propri t s des c ramiques zircone-mullite, Thesis, University of Limoges, February 1987. [GIT 70] GITZEN W.H., OAlumina as a ceramic materialP, The Am. Ceram. Soc. , 1970. [GOR 84] GORDON R.S., OUnderstanding defect structure and mass transport in polycrystalline Al 2O3 and MgO via the study of diffusional creepP, p. 418-437, in [KIN 84]. [HAN 00] HANNINCK R.H.J., KELLY P.M. and MUDDLE B.C., OTransformation toughening in zirconia-containing ceramicsP, J. Am. Ceram. Soc. , 83(3), p. 461-486, 2000. [HAR 84] HARMER M.P., OUse of solid-solution additives in ceramic processingP, Advances in Ceramics, Vol. 10, p. 679, Am. Ceram. Soc., 1984. [HER 50] HERRING C., ODiffusional viscosity of a polycrystalline solidP , J. Appl. Phys. , 21(5), p. 437-445, 1950.

[HEU 84] HEUER A.H. and CASTAING J., ODislocations in D-Al 2O3P, p. 238-257, in [KIN 84]. [JAC 94] Special series on alumina, J. Am. Ceram. Soc. , 77(2), 1994. [KIN 76] KINGERY W.D., BOWEN H.K. and UHLMANN D.R., Introduction to Ceramics , 2nd edition, John Wiley and Sons, 1976.

[KIN 84] KINGERY W.D. (ed.), OStructure and properties of MgO and Al 2O3 CeramicsP, The Am. Ceram. Soc. , 1984. [KLU 90] KLUG F.L., PROCHAZKA S. and DOREMUS R.H., OAlumina-silica phase diagram in the mullite regionP, p. 750-759, in [SOM 90]. [KRO 74] KROGER F.A., The Chemistry of Imperfect Crystals , North Holland, 1974. 230 Ceramic Materials

[KRO 84] KROGER F.A., OElectrical properties of D-Al 2O3P, p. 1-15, in [KIN 84]. [KRO 84] KROGER F.A., OExperimental and calculated values of defect parameters and the defect structure of D-Al 2O3P, p. 100-118, in [KIN 84]. [KRO 57] KRONBERG M.L., OPlastic deformation of single crystals of sapphire: basal slip and twinningP, Acta Metallurgica , 5, p. 508-527, 1957. [LAB 80] LABELLE H.E., OEFG, the invention and application to sapphire growthP, J. Cryst. Growth , 50, p. 8-17, 1980. [LAN 91] LANGDON T.J., OCreepP, p. 92-96, in [BRO 91]. [LEE 94] LEE W.E. and RAINFORTH W.M., Ceramic Microstructures , Chapman & Hall, 1984. [MOR 85] MORRELL R., Handbook of properties of technical & engineering ceramics, Part 1, An introduction for the engineer and designer , Her MajestyRs Stationery Office, 1985. [MOR 85] MORRELL R., Handbook of properties of technical & engineering ceramics, Part 2, Data Reviews , Her MajestyRs Stationery Office, 1987. [NAS 83] NASSAU K., The Physics and Chemistry of Color , J. Wiley, 1983. [NOR 99] NORTONMSAINT-GOBAIN, website: http://www.nortadvceramics. thomasregister.com/olc/nortadvceramics/home.htm. [PEC 99] Catalogue des alumines, P chiney, 1999. [RHO 91] RHODES W.H. and WEI G.C., OLamp envelopesP, p. 273-76, in [BRO 91]. [RIN 96] RING T., Fundamentals of Ceramic Powder Processing and Synthesis , Academic Press, 1996. [ROD 85] RODRIGO P.D.D. and BOCH P., OHigh purity mullite ceramics by reaction sinteringP, Int. J. High Technology Ceramics , 1, p. 3-30, 1985. [RYS 85] RYSHKEWITCH E. and RICHERSON D.W., Oxide Ceramics , Academic Press, 1985. [SCH 91] SCHWABEL M.G., OAlumina abrasive grains produced by sol-gel technologyP, Am. Ceram. Bull ., 70, 10, p. 1596-1598, 1991. [SIN 00] SINGHAL S.C., OScience and technology of solid-oxide fuel cellsP, MRS Bull ., 25, 3, p. 16-21, 2000. [SOM 88] SOMIYA S., YAMAMOTO N. and YANAGIDA H., OAdvances in ceramicsP, vol. 24A and B, Science and Technology of Zirconia III, The Am. Ceram. Soc. , 1988. [SOM 90] SOMIYA S. (ed.), OMullite and mullite matrix compositesP, Ceram. Trans. 6, The Am. Ceram. Soc. , 1990. [SWA 85] SWAIN M.V., OInelastic deformation of pg-psz and its significance for strength- toughness relationship of zirconia-toughened ceramicsP, Acta Metall. , 33, 11, p. 2083-2091, 1985. [WES 90] WEST A.R., Solid State Chemistry and its Applications , J. Wiley, 1990. 21

Glass and Glass-Ceramics

CHAPTER PREVIEW The structure of glass, particularly silica glass, was introduced at the end of Chapter 7. In this chapter we discuss the different types of glass and some of their various applications. Then, in Chapter 26, we will concentrate on processing glass and the wide variations in compositions. Only two chapters have been devoted to glass due to space limitations; do not take this as a reflection on its importance. Glass is arguably our most important material. It is used for windows, containers, lenses, optical fibers, insulators, glazing and enameling, surgical knives, spectacular art, and road signs! Glass is usually recyclable and environmentally friendly. The Egyptians were great glassworkers, but they were not the first. Glass was actually used much earlier and obsidian, a natural black volcanic glass, was important during Paleolithic times. We can say that glass has played a major role in shaping our civilization. However, there is some difficulty in defining a glass. We will discuss why the words glassy, vitreous, and amorphous are all used to describe glass and try to answer the question “what is glass”? Two thoughts should be kept in mind while studying this chapter:

᭿ The assertion by Sturkey: “glass at high temperatures is a chemical solution.” ᭿ If glass is a supercooled liquid, it is not a solid so it is not a ceramic (but it is).

The mechanical, optical, and electrical properties of glasses are discussed in detail, along with other ceramics, in those topical chapters.

21.1 DEFINITIONS Alternative definition 1: Glass is a solid material that does not show long-range order . The classic definition of glass is based on the historical method of formation: this is a very unusual way of defin- “No long-range order” means not longer than, say, one or ing any material. The result is that glass is now defined in two or three times the basic building block of the glass. several different ways. This definition is consistent with experimental observa- tions [X-ray diffraction (XRD), transmission electron The classic definition: Glass is a supercooled liquid . microscopy (TEM), etc.] but it is clearly a little arbitrary since it depends on the size of the building block.

The problem with this definition is that in some cases a Alternative definition 2: Glass is a liquid that has lost its ability particular glass can be prepared that never has been in the to fl ow . liquid state. This definition is consistent, but broader, than the one The American Society for Testing and Materials (ASTM) defi - given by ASTM and uses a mechanical property to describe nition: An inorganic product of fusion that has cooled to a rigid glass. It is actually close to the more modern physicist’s condition without crystallizing . view of glass. The main glasses we will discuss are the network This essentially says the same thing as the classic defini- oxide glasses, specifically the silicates. Then our defini- tion but excludes polymer glass. It is clearly not ideal to tion of such a glass is rely on the method of production to define a class of mate- rials. We would not consider doing this for crystalline Alternative definition 3: A solid assembly of vertex-sharing materials. tetrahedra lacking long-range order .

21.1 Definitions ...... 379 We are concerned only with ceramic glass, but you should V Liquid know that there are metallic glasses and polymer glasses. Glass Liquid transition How metallic glasses are formed provides a clue to why range they exist: they have complicated compositions to “frus- trate” crystallization when they are quenched quickly f Fast from the melt (a process known as splat quenching). For cooling f the same reason, glycerin can be a glass below −90°C. s With this understanding in mind, we can discuss the basic s Slow features of glasses. cooling Crystallize Structure Glasses are essentially noncrystalline (or amorphous) solids often obtained by freezing supercooled liquids. Long-range order (LRO) in the atomic arrangement does Crystal not exist over distances greater than, say, 1 nm. The regular arrangement resulting from the distribution over long dis- T T T gs gf m tances of a repeating atomic arrangement (unit cell), which T is characteristic of a crystal, is missing. There is often evi- FIGURE 21.1 Plot of volume versus temperature for a liquid that dence of a short-range order (SRO) in glasses, which cor- forms a glass on cooling and one that forms a crystalline solid.

responds to the atomic arrangement in the immediate The glass transition temperature, Tg, depends on the cooling rate

vicinity of any selected atom. Numerous attempts have and is not fi xed like Tm. been made to explain the formation or the nonfor- TERMINOLOGY from a supercooled liquid mation of glasses. We have to a glass. Below T the two basic approaches: The words “vitreous,” “amorphous” and “glassy” are not g actually synonymous although they tend to be used glass structure does not relax very fast because it is ᭿ interchangeably anyway. Consider the structure now a solid. In the region ᭿ Consider the kinetics Vitreous: from the Latin word for glass ( vitrum ) Amorphous: means having no definite form (strictly of Tg the viscosity is about of crystallization 13 speaking, shapeless from the Greek amorphos) . Now 10 dPa-s. The expansion the lack of ‘form’ implies ‘not crystalline.’ Liquids coefficient for the glassy In the first case, we examine state is usually about the the geometry of the con- are generally without form, so a ‘solid liquid’ is amorphous. same as that for the crys- stituent entities that make talline solid. If slower up the glass, the nature of cooling rates are used so the interatomic bonds, or the strength of the bonds. In the that the time for the structure to relax is increased, the second, we consider how the liquid transitions to a solid as supercooled liquid persists to a lower temperature, and the the temperature drops below the melting point. resulting glass may have a higher density as shown graphi- cally in Figure 21.1. Glass Transition Temperature The physics of glass examines the concept of fragil- ity; this is actually a property of glass-forming liquids We consider a plot of specific volume as a function of above Tg and is a measure of the strength of the intera- temperature as shown in Figure 21.1. This is a form of a tomic bonding. We will talk about water dissolving glass time–temperature–transformation (TTT) diagram for a or glass containers for water; what is less well know is glass. On cooling the liquid from a high temperature, two that you can freeze water into a glassy state by quench- phenomena may occur at the point of solidification, Tm. ing it into liquid ethane. It is a fragile glass, but it is thought that most of the water in the universe exists in ᭿ If the liquid crystallizes there is a discontinuous change this state! in V and a discontinuity in the rate of cooling (associ- ated with the heat of crystallization). ᭿ If no crystallization occurs the liquid passes into a 21.2 HISTORY supercooled state and V decreases at about the same

rate as above Tm. In Chapter 2 we gave a brief history of glass. We will expand on that discussion here. Glass, like flint, is inti- At the glass transition temperature, Tg, the slope of the mately connected with human history because of the use curve decreases to become close to that of the crystalline of obsidian, which is a natural glass. Nobody knows for solid. This break in the cooling curve marks the passage sure when the first glass objects were made. The oldest

380 ...... Glass and Glass-Ceramics finds date back to ∼7000 bce , or possibly even earlier. Methods of manufacturing glass for its own sake, and not just as a glaze for pots, had already been discovered in Mesopotamia by approximately 4500 bce . The use of glass as the glaze in pottery dates back even earlier. Around 3000 bce Egyptian glassmakers systemati- cally began making pieces of jewelry and small vessels from glass. Glass had both a functional and a decorative role. Pieces of glass jewelry have been found on excavated Egyptian mummies; an example is the turquoise blue glass fi gure shown in Figure 21.2. By about 1500 bce Egyptian glassmakers during the reign of Touthmosis III had devel- oped a technique to make usable hollowware. A most FIGURE 21.3 Core-formed bottle shaped like a bulti -fi sh, 1390– striking example is the core-formed bottle in the shape of 1336 BCE . This example is unusual because it is in polychrome a bulti -fish shown in Figure 21.3. This vessel was made glass. The length is ∼14.5 cm. between 1352 and 1336 bce and was believed to be used to hold scented oil (based on the narrowness of the neck). The wave pattern is very typical of core-formed objects rested them on lumps on nitrum from their cargo. When these and was made by drawing a sharp object along the sof- became heated and were completely mingled with the sand on tened glass. the beach a strange liquid flowed in streams; and this, it is said, The Roman author Pliny the Elder (23–79 ce ) explains was the origin of glass. the invention of glass in his encyclopedia Naturalis Historia : Nitrum is a naturally occurring soda, an important ingredient in both ancient and modern glasses. The ashes There is a story that once a ship belonging to some traders in of plants also provided the glassmaker with a rich source nitrum put in here [the coast of modern Lebanon] and that they of sodium. The plants saltwort and glasswort (known as scattered along the shore to prepare a meal. Since, however, no halophytes) were both used to supply sodium. stones for supporting their cauldrons were forthcoming, they Aside : Gerard’s Herbal (1633) says that ‘saltwort was called Kali by the Arabians’: hence the word alkali and the ashes are called soda .

One of the most common methods used to form glass is glassblowing. Although this technique was developed over two thousand years ago, the glassblowing pipe has not changed much over time. The main development that has been made in glassblowing is the automated blowing processes that are used to produce glass containers and light bulbs and the technique of blowing the glass inside a mold. Most of the important milestones in the history of glass, particularly in the twentieth century, are associated with developments in manufacturing technology. These developments have led to the low-cost production of com- mercial glasses, for example, window glass, and the use of glass in new applications, such as optical fibers. Many of the topics summarized here have been redis- covered many times. Now that we have the tools, we can see that nature often preempted humans.

Ancient History 50,000 bce to 1000 bce . Glass is used in potter’s colored glazes. 7000 bce to 1500 bce . Ancient glass artifacts (possibly as bce FIGURE 21.2 Glass head of a pharaoh (believed to be Amenophis early as 10,000 ). II) as sphinx, 1400–1390 BCE . It was made by lost-wax casting and 2600 bce . Earliest actual dated glass. is ∼3.2 cm high. 1500 bce . Egyptians are manufacturing glass articles.

21.2 History ...... 381 1200 bce . Earliest glass molding. 1882) and named after the town in upstate New York 100 bce . Blown glass is invented with the glassblowing where it is still located. Houghton already owned the pipe (Romans in Syria). Brooklyn Flint Glass Works, but moved to Corning c100 bce . In Alexandria the introduction of manganese because real estate was cheaper there. oxide into the glass composition together with improve- 1881. Thomas Edison brought out his first incandescent ments in glass-melting furnaces resulted in the first electric lamps using glass bulbs made by the Corning successful production of colorless glass. Glass Works. 450 ce . Stained glass was used. 1884. Otto Schott (1851–1935), Ernst Abbe (1840–1905), Carl Zeiss, and Roderick Zeiss established the Glastechnisches Laboratorium Schott und Genossen, Beginning to Engineer Glass which later became the Jenaer Glaswerk Schott und 1200s. In Germany a new process was developed to make Gen and in 1952 the Schott Glaswerke. This company mirrors. The back of a piece of flat glass was coated is now the leading European glass company. with a lead-antimony layer to produce quality (“sil- 1893. The Enterprise Glass Company in the United States vered”) mirrors. The mirror format remains essen- developed a press-and-blow mold that led to the wide- tially unchanged today. spread production of wide-mouth containers. 1268. Eye glasses described by Bacon. These had convex 1903. The automatic bottle-blowing machine invented by lenses for the correction of near sightedness. the American Michael Owens (1859–1923) began pro- 1291. Murano, a small island near Venice, became a glass duction. A machine for drawing large cylinders of center. Glass workers on Murano were generally not glass that were then flattened into window glass was allowed to leave the island. developed by another American, John Lubbers. c1590. The first telescope lenses were made in Italy and 1913. Emile Fourcault, a Belgian, developed a flat-glass later, in 1604, in the Netherlands. machine for commercial operation. 1609. Glass was made in Jamestown, Virginia. 1917. Edward Danner at the Libbey Glass Company intro- 1612. Publication of the textbook L’Arte Vetraria by duced an automatic method for tube making. The Antonio Neri in Pisa. This was the first systematic company remains in operation today and is the largest account of the preparation of the raw materials for manufacturer of glass dinnerware in the United glassmaking. States. 1676. George Ravenscroft, an English glassmaker, devel- 1926. The Corning ribbon machine for high-speed auto- oped lead-crystal glass (also known as flint glass). The matic production of glass light bulbs was developed. addition of lead oxide to the glass formula yielded a glass of high brilliance and a pure ring. It is not crystal, Present Technology but it does contain a lot of lead and it is heavy. 1688. Bernard Perrot, a glassmaker in France under Louis 1957. Corning introduced the Pyroceram ® brand of XIV, invented the plate-pouring process. This process glass-ceramics. allowed mirrors with a large surface area to be pro- 1959. Sir Alastair Pilkington’s float process for producing duced. Examples are the magnificent wall of mirrors flat glass worked. in the Galerie des Glaces at Versailles. 1960. Glass-ceramics were patented by S. Donald Stookey Late 1700s. Joseph von Fraunhofer (1787–1826), a German of Corning Glass Works. mirror maker and student of glassmaking technology, 1966. Optical fibers were developed. produced optical quality glasses for telescopes and 1975. Glass recycling became accepted/required. microscopes. (Fraunhofer diffraction and the Fraun- 1980. An acid-leaching process was introduced for pro- hofer Institutes are named in his honor.) ducing 99.6–99.9% silica fibers that resist devitrifica- tion up to 1370°C. These fibers were used as insulation for the space shuttle. Modern Times 1991. Schott produced an 8.2-m-diameter telescope blank 1857. William Clark of Pittsburgh patented a sheet-drawing from a glass-ceramic ZERODUR ®, which had been process: a plate glass. introduced in 1968. 1861. A British patent was granted to C.W. Siemens and 1997. Corning produces glass for the Subaru Telescope F. Siemens. Their patent included a discussion of the mirror. Weighing 27 t and more than 26 feet across it application of the principle of regeneration to glass is one of the largest pieces of glass ever made. melting. Regenerative heating is still used in glass melting furnaces today. New and improved methods for processing glass are 1865. A U.S. patent was issued for a press-and-blow being developed. One of the main thrust areas for these process. activities is our concern for the environment. Reducing 1875. Corning Glass Works was incorporated. The energy costs and reducing polluting emissions are impor- company was founded by Amory Houghton Sr. (1812– tant in the modern glass industry.

382 ...... Glass and Glass-Ceramics 21.3 VISCOSITY, h the glass has a viscosity similar to that of honey at room temperature. For a typical soda-lime-silicate flat-glass Viscosity is a key property of glass. We need to know the composition this viscosity is achieved in the temperature viscosity of glass at different temperatures so that it can range 1015–1045°C. The other key reference value to be formed, shaped, and annealed. The concept of viscous remember is that a solid has a viscosity of >10 15 dPa·s. flow was described (and illustrated) in Chapter 17 because The viscosity of glass varies dramatically with tem- this is the process by which permanent deformation occurs perature as shown in Figure 21.4a for various silicates. in glasses. Viscosity is a mechanical property. Consider The fictive temperature, Tf, is the temperature at which the the tangential force, F, required to slide two parallel plates liquid structure is frozen into the glassy state and is defined a distance d apart past one another when they are sepa- by the crossing of the extrapolated curves from high and rated by a layer of viscosity, η: low temperatures on the η against T plot. The fictive tem- perature like Tg is related to structural transformations in η = ( Fd )/( Av ) (21.1) glass; Tg is slightly lower. Figure 21.4b shows the The common area is A and temperature dependence VISCOSITY AND POISE the velocity of the planes of viscosity for the main If a force of 1 dyn is required to move an area of 1 cm 2 relative to one another is v. glass-forming oxides as a of liquid or gas relative to a second layer 1 cm away at Essentially, then, viscosity function of temperature. a speed of 1 cm s −1, then the viscosity is one P (poise). is a measure of the liquid’s You will notice that from response to a shearing. the slope of these lines we 1 P 1 dPa·s Liquids have viscosities = can obtain the activation measured in centipoises energy for viscous flow, Ev (cP) and gases have viscosities measured in micropoises (see Section 17.13). Table 21.3 gives some values for η and (µP). In the SI system we would say that liquids have some measured crystallization velocities, n. The latter viscosities in millipascal-seconds (mPa·s) while gas vis- term refers to the rate of movement of the solid/liquid cosities are tenths of micropascal-seconds ( µPa·s) (so we interface. The very low n for SiO 2 is indicative of its excel- stick with the poise). lent glass-forming ability: it is very difficult to crystallize Table 21.1 lists some of the viscosity values that are a solidifying melt of SiO 2. important for glass processing. The values given in Table There are several methods for measuring viscosity; 21.2 are used to define certain characteristics of a glass which is used depends on the expected value of the (again with an emphasis on processing). Many of the viscosity. values listed in this table of viscosities are “standards.” For example, ASTM C338-93(2003) is the “Standard Test Mergules viscometer ≤10 7 dPa·s Method” for determining the softening point of glass. Fiber elongation ≤10 7 to 10 9 dPa·s Beam bending ≤10 9 to 10 14 dPa·s Determine the softening point of a glass by determining the temperature at which a round fiber of the glass, nominally The schematics in Figure 21.5 illustrate the first two 0.65 mm in diameter and 235 mm long with specified tolerances, approaches to measuring η. The viscometer is used for elongates under its own weight at a rate of 1 mm/min when the upper 100 mm of its length is heated in a specified furnace at the rate of 5°C/min. TABLE 21.2 Viscosity “Milestones” The viscosities of some common liquids are given in Viscosity Example Table 21.2 for comparison. Notice that at the working point 10 −2 dPa·s Water at 20°C 10 0 dPa·s Light machine oil 10 1 dPa·s Heavy machine oil 10 2 dPa·s Olive oil at 20°C TABLE 21.1 Viscosity Values for Glass Processing 10 4 dPa·s Runny honey at 20°C; some measure it to be 2 Viscosity Example 10 dP-s 10 4 dPa·s Glass at its working point; begin working at 103 10 1.5 –10 −2.5 dPa·s ASTM melting 10 7.6 dPa·s Shortening point (deforms under its own weight; 10 3.7 –10 3.8 dPa·s Casting plate glass softening at 10 7.7 )a 10 3.8 dPa·s Seal glass to metal 10 8 dPa·s Upper limit for low viscosity 10 5.3 dPa·s Begin updrawing or downdrawing 10 13 dPa·s Annealing point (ASTM) 6 13.4 10 dPa·s Sinter glass powder to produce a porous body 10 dPa·s Glass at Tg 10 8 dPa·s Sinter glass powder to produce a solid body 10 14.6 dPa·s The strain point of glass (ASTM) 10 11.3 –10 11.7 dPa·s Glass deforms under gravity >10 15 dPa·s Solid 10 12.7 –10 12.8 dPa·s Practical annealing range (stress relief in 10 16 dPa·s Upper limit for measuring viscosity seconds) a The shortening point is also called the softening point.

21.3 Viscosity, η ...... 383 T, °C logη 2500 2000 1500 1000 800 600 500 17 12 log η 15

13 SiO2 + 0.2% H2O

11 8 Alumina-silicate SiO GeO2 glass 8409 9 2

P2O5 7 Borosilicate glass B O 4 5 2 3 3 4 810,000/T 12 Lead borate solder glass Soda-lime glass (B) 0 FIGURE 21.4 (a) Viscosity as a function of temperature ( T) for several 0 400 800 1200 1600 silicate glasses: units of η are dPa · s. (b) Viscosity as a function of T ( °C) temperature (1/ T) for the main glass-forming oxides. Notice the effect that a (A) small amount of water has on the viscosity of silica glass.

TABLE 21.3 Crystallization Velocities and Viscosities of Glass-Forming Liquids

Glass T m (°C) v max (cm/s) Temperature for v max (°C) Log h at T m (dPa·s)

−7 Vitreous SiO 2 1734 2.2 × 10 1674 7.36 −6 Vitreous GeO 2 1116 4.2 × 10 1020 5.5 −7 P2O5 580 1.5 × 10 561 6.7 −4 Na 2O·2SiO 2 878 1.5 × 10 762 3.8 −4 K2O·2SiO 2 1040 3.6 × 10 930 −3 BaO·2B 2O3 910 4.3 × 10 849 1.7 −4 PbO·2B 2O5 774 1.9 × 10 705 1.0

Viscometer

Viscometer Furnace Spindle support Glass fiber

Crucible Furnace Heating Molten elements glass Control thermocouple

Pedestal LVDT assembly Sample thermocouple LVDT signal

Weight FIGURE 21.5 Schematic illustration of instruments used to measure viscosity: (a) a viscometer; (b) the fi ber elongation (A) (B) method.

384 ...... Glass and Glass-Ceramics low viscosities and the method is similar to the concentric removing the surface flaws (acid polishing or applying a cylinder viscometer used to determine the sol-gel transi- protective coating). Prince Rupert’s drops are an inter- tion that we describe in Chapter 22. The spindle is rotated esting and entertaining illustration of the effect of resid- at an angular velocity, ω, and the resistance to its motion ual stress on the mechanical properties of glass. Small is measured. gobs of molten glass are dropped into cold water to form For higher viscosities the fiber elongation method is tadpole-shaped drops. The surface cools much more used. A load is attached to a glass fiber, which can be rapidly than the center creating internal residual stresses heated to a range of temperatures. The strain rate of the and a very high surface tension. The solidified drops can fi ber as it elongates is then measured. be hit with a hammer without breaking. But if the tail is broken off the entire drop shatters into powder.

21.4 GLASS: A SUMMARY OF ITS PROPERTIES, OR NOT Some Electrical Properties of Glass Glass usually has a high electrical resistivity because of We can summarize what we know about glass—and prob- the large band gap energy (see Chapter 30). In cases in ably be wrong. which they are conductive the charge is carried by ions, with alkali ions (e.g., Na +) being the fastest. Thus conduc- Glass is inert . This depends on the environment. It is tivity increases significantly as T is increased and it is nearly true if the glass is a silicate and essential if it different for silicate glasses, borate glasses, and phosphate is going to contain radioactive waste, but not all glasses glasses because the glass network is different. The mixed are inert. Bioglass ® is designed not to be inert. alkali effect is an interesting phenomenon that occurs in Glass is homogeneous. This depends on how the glass was glasses that contain more than one different alkali ion. formed and its composition. We can process glass to The resulting conductivity has been found experimentally make it inhomogeneous. to be signifi cantly lower than would be expected from Glass can be reshaped . This is generally true and is the simply adding their individual conductivities. This has reason why glass is so recyclable. Some glasses are applications in, e.g., high-wattage lamps. designed so that they can be modified by light, by dif- The dielectric constant of glass is quite high but not fusion, by irradiation, etc. high enough for some advanced memory applications such Glass has a small expansion coeffi cient . This is usually as dynamic random access memory DRAMS (see Chapter true, but not all glass is Pyrex tm . 31). The capacitance is a measure of the amount of charge Glass is transparent . This is essential for optical fibers, stored and is related to the thickness of the dielectric. As but we can make it translucent or opaque (see Figure the layer gets thinner the capacitance increases, but elec-

21.2). Most early glasses were not very transparent trical breakdown can occur. SiO 2 glass has a high dielec- because they contained impurities and inclusions. tric strength but not as high as some polymer dielectrics Glass is cheap . This is true for window glass since the such as phenolic resin. invention of the float-glass process. Thin films may be expensive. Some glass is colored red by doping it with Some Optical Properties of Glass Au. Some vases can cost >$50 k. Glass is a bulk material . This is true unless we grow it as Transmission in the ultraviolet (UV), visible, and infrared a thin film or it is present as an intergranular film (IR) depends on several factors including Rayleigh scat- (IGF) or pocket in, or on, a crystalline ceramic. tering, which is determined by impurities. The IR edge and the UV edge are the values where transmission of So the lesson is to beware of your preconceived ideas these frequencies cut off. A UV edge blocker removes the when thinking about glass. UV while a UV edge transmitter allows it through. The mechanical, optical, thermal, and electrical prop- Refraction depends on the refractive index and on dis- erties of glasses are discussed in detail, along with other persion. Reflection, which occurs at the surface, can be ceramics, in those topical chapters. Here we just give some internal. Because the optical properties of glass are so things to think about in relation to glass. important much of Chapter 32 is devoted to this topic.

Some Mechanical Properties of Glass Some Thermal Properties of Glass The theoretical strength of silicate glass is around 10 GPa Expansion coefficients of glass are generally smaller than (using the criterion given in Section 18.2), but this is for metals. But often we want to make the following con- usually much reduced by the presence of surface flaws nections to metals: (cracks and seeds). Glasses are elastic but break in ten- sion. They can be strengthened by creating a compres- Glass-to-metal seals: an obvious important technological sive surface layer (by ion exchange or tempering) or by process

21.4 Glass: A Summary of Its Properties, or Not ...... 385 SiO 2: metal/insulator junctions for the electronics industry Graded seals: for example, a graded seal structure can be constructed by joining a series of glass pieces, each of which has a slightly higher thermal coefficient of expansion ( α)

Thermal conductivity is ∼1% of that of a metal. The implications and applications of this fact are obvious.

21.5 DEFECTS IN GLASS

The idea is that although glass does not have a crystalline matrix, it can still contain point defects, precipitates, undergo segregation, and contain internal interfaces. Glass can be used to trap radioactive elements as point defects or as a “second phase.” The future value of this capability depends in part on how fast components can diffuse FIGURE 21.6 Region of liquid–liquid immiscibility for SiO 2–Li 2O. through glass. This applies to whether the radioactive Notice that these occur only in the silica-rich end of the phase material is diffusing out or other components are diffusing diagram. in (perhaps to leach out the trapped material).

21.6 HETEROGENEOUS GLASS segregation may be energetically less favorable than crys- tallization, but it is easier to accomplish because it requires Just because glass is a “supercooled” liquid does not mean only the segregation, not the correct rearrangement of the it must be homogeneous. Certain glasses can separate into atoms. As a general rule for silica, immiscibility is two phases, which need not be a crystallization process. increased by the addition of TiO 2, but decreased by the When these two phases are both glassy, there may either addition of Al 2O3. be no barrier to the separation (a spinodal decomposition) The Vycor process described in Chapter 8 uses the or, as in the case of liquid/liquid phase separation, there principle of phase separation. The resulting glass is 96% may be a nucleation step. In either case, diffusion is SiO 2 and 4% pores and is used as a filter and a bioceramic important. where porosity is important. It can be densified (after The principle of immiscibility in glass is very impor- shaping) to allow processing of a pure SiO 2 shape at a tant to today’s technology. For example, immiscibility lower temperature than for pure quartz glass. plays a role in forming glass-ceramics, making Vycor® and opal glass, and in the precipitation in glass. Many of the binary and ternary oxides with silica as a component 21.7 YTTRIUM–ALUMINUM GLASS show miscibility gaps. A miscibility gap is a region in the phase diagram in which a liquid separates into two liquids Yttrium–aluminum (YA) glasses can be formed in the of different composition (see Section 8.11). The following composition range ∼59.8–75.6 mol% Al O . With 59.8– are examples of systems exhibiting this effect: 2 3 69.0 mol% Al 2O3, a two-phase glass forms with droplets of one phase in the other. The glass can spontaneously SiO –Al O SiO –BaO SiO –MgO 2 2 3 2 2 crystallize to form YAG or a mixture of Al O and YAlO Na O–B O –SiO Na O–CaO–SiO 2 3 3 2 2 3 2 2 2 (YAP; P = perovskite). These YA glasses show a phenom- enon known as polyamorphism, meaning that they exist Figure 21.6 shows the SiO –Li O phase diagram. In 2 2 with different amorphous structures. the low-temperature silica-rich corner of the diagram one liquid phase separates into two chemically distinct, different liquid phases below the immiscibility dome. The dashed line represents the estimated region of 21.8 COLORING GLASS immiscibility. The difficulty in making these measure- ments is that phase separation occurs at a lower tem- Although many applications for glass require a colorless perature where the kinetics are slower. There is an product, for other applications colored glass is needed. interesting comparison with crystallization. Phase Windows in a church do not look as impressive when all

386 ...... Glass and Glass-Ceramics the glass is colorless. Glass is often colored by adding In a CdS-doped glass, adding more Se can result in transition-metal oxides or oxides of the rare-earth ele- “Selenium Ruby.” The details of all these colorings will ments to the glass batch. Table 21.4 lists the colors pro- depend on just what glass batch is used and the firing duced by some of the common glass colorants. We will conditions. look at how these additives actually result in the formation Corning makes microbarcodes (i.e., very small bar- of color in Chapter 32, but at this stage you should already codes) by doping glass with rare earths (REs); the REs know why glass bottles are often green. Bright yellow, have particularly narrow emission bands. Of 13 RE ions orange, and red colors are produced by the precipitation tested, four (Dy, Tm, Ce, and Tb) can be excited with UV of colloids of the precious metals. Au produces a ruby red radiation used in fluorescence microscopy but do not coloration, but it is not cheap. CdS produces a yellow interfere with other fluorescent labels. These microbar- coloration, but when it is used in conjunction with Se it codes can be used for biological applications since they produces an intense ruby red color. are not toxic; tags using quantum dots may be less benign. The questions are These bar codes can even label genes. The REs can be used together to give more color combinations. ᭿ How does coloring “work”? Special colored glasses include the following: ᭿ What causes the colors? Ruby and cranberry glass . Ruby glass is produced by ᭿ Is it the same as for crystals? adding Au to a lead glass with Sn present. Cranberry glass, first reported in 1685, is paler (usually a delicate Glass is intentionally colored by adding dopants (we pink) because it contains less gold. The secret of making are creating point defects in the glass). The color of the red glass was lost for many centuries and rediscovered glass depends on the dopant and its state of oxidation. The during the seventeenth century. explanation is the same as for coloring crystals, but because Vaseline glass or uranium glass . Uranium produces a the glass structure does not have LRO the absorption deep red when used in high-Pb glass. There are other spectra can be broader. uranium-containing glasses: the so-called “uranium Combinations of dopants can decolorize, mask, or depression-ware” glass (also called Vaseline glass), which modify the effect. For example, we can compensate for has a green color. True “depression ware” is actually the coloring effect of Fe by adding Cr; if too much Cr 3+ is greener than Vaseline glass because it contains both iron added, Cr 2O3 can precipitate out. When the glass is blown, and uranium oxides. What is special is that the glass these platelets of Cr 2O3 can align to give “chromium aven- actually fluoresces when illuminated with UV radiation turine.” Cu was used to produce Egyptian Blue glass. Co (Vaseline ware more strongly because of the higher con- is present in some twelfth-century stained glass and, of centration of uranium). Since 1940 or so, only depleted course, was used in the glazes on Chinese porcelains in uranium has been used as a dopant and that is quite plenti- the Tang and Ming dynasties; the color it produces is ful, but for the previous 100 years, natural uranium was known as cobalt blue. used. Figure 21.7 shows an example of Vaseline ware.

TABLE 21.4 Colors Produced by the Inclusion of Different Ions in a Glass Copper Cu 2+ Light blue (red ruby glass for Cu nanoparticles) Cu + Green and blue (includes turquoise blue) Chromium Cr 3+ Green Cr 6+ Yellow Cr 3+ + Sn 4+ Emerald green Manganese Mn 3+ Violet (present in some Egyptian glasses) Mn 2+ Weak yellow/brown (orange/green fl uorescence) Iron Fe 3+ Yellowish-brown or yellow-green Fe 2+ Bluish-green FeS Dark amber Cobalt Co 2+ Intense blue (especially if K + is present); in borates and borosilicates, pink Co 3+ Green Nickel Ni 2+ Grayish-brown, yellow, green, blue to violet, depending on glass Vanadium V 3+ Green in silicates; brown in borates Titanium Ti 3+ Violet (melting under reducing conditions) Neodymium Nd 3+ Reddish-violet Praseodymium Pr 3+ Light green Cerium Ce 3+ Green Ce 4+ Yellow Uranium U Yellow (known as“Vaseline glass”) Gold Au Ruby (ruby gold, Au nanoparticles)

21.8 Coloring Glass ...... 387 21.10 PRECIPITATES IN GLASS

Precipitation in glass is generally inevitable given time. The question is only how long it will take to occur, espe- cially if nucleation is homogeneous (i.e., no seeds are present). We may introduce seeds to produce particular effects. Nucleation of crystals in glass follows the classic theory. We will examine the topic more in Chapter 26 and see there that precipitates can also cause the coloring of glass.

21.11 CRYSTALLIZING GLASS FIGURE 21.7 Example of Vaseline ware produced by coloring with uranium. We will address processing these materials in Chapter 26. Here we will explain what a glass-ceramic is and relate it to other two-phase ceramics. We can crystallize a droplet of glass, as shown in Figure 21.9, or a complete film, as 21.9 GLASS LASER shown in Figure 21.10. We can also crystallize a bulk object almost completely to produce a glass-ceramic. The Rare-earth elements (e.g., Nd) are used to dope glass to basic idea is that there is sometimes a great advantage in create lasers and other optical devices (Figure 21.8). The processing a ceramic as a glass but producing the finished Nd-doped glass laser works like the ruby laser, although object as a polycrystalline body. there are some differences that relate to the glass. The Opal glass has a milky (opalescent) appearance caused 1 1 laser rod is about –4 to –2 inch diameter and is usually by the formation of small crystallites. Even window pumped by a helical lamp to give a discharge length longer glass crystallizes; given time it devitrifies to form than from a linear lamp. The energy is stored in a capaci- devitrite. tor (e.g., 500 mF) with a charge voltage of ∼4 kV giving a The visible light microscopy (VLM) image shown in pulse duration of ∼0.8 ms. The Nd:glass laser gives effi - Figure 21.11 illustrates the growth of perfectly symmetric ciencies up to 2%, which is four times that of the ruby individual crystals inside a glass matrix. The crystals and laser, and the Nd:glass rod can be made even larger. the matrix are all transparent, so the full shape shown here Because the thermal conductivity of the glass is lower it schematically can be appreciated. The SEM images in requires more time to cool between firings. Figures 21.12 and 21.13 show that the glass and crystal

Glass

Crystal 2 µm FIGURE 21.8 Examples of phosphate laser glass. Individual laser rods vary from 0.6 to 1.2 mm in diameter. FIGURE 21.9 An example of crystallization in a glass droplet.

388 ...... Glass and Glass-Ceramics FIGURE 21.12 SEM image of crystallized glass. The glass is a

FIGURE 21.10 Spherulitic crystallization of an amorphous SiO 2 binary Li–Be fl uoride opal glass 40 mol% LiF, 60 mol% BeF 2 fi lm on SiC. The spherulites are crystobalite. containing small amounts of Ag and Ce (0.001 and 0.01 mol%,

respectively). The glassy droplets (BeF 2 rich) are surrounded by a crystallized matrix.

(A)

(A)

o1

h h o

o1 o1 h (B) FIGURE 21.13 SEM images of (a) Na–Be and (b) K–Be fl uoride glass 15 mol% KF (or NaF), 85 mol% BeF . This glass is cloudy: if (B) 2 the droplets were smaller the glass would be clear. Replacing K by FIGURE 21.11 VLM image showing crystallization in a glass. Rb actually reduced the droplet size to 10–20 nm.

21.11 Crystallizing Glass ...... 389 bined with a liquid to allow smooth application. The deco- rated pot is then typically coated with a clear glaze before it is fired. In the majolica technique, the pot is coated with an opaque white glaze first, then decorated, and then fired so that the colors bond to the white coating forming an in-glaze (rather than under) decoration. Glaze crawling . The glaze separates from the underly- ing pot—it dewets the pot surfaces during firing. Crackle glazes . If the coefficient of thermal expansion, α, of the glaze is greater than that of the underlying ceramic, the glaze may fracture as it cools; this crackling can easily be achieved using higher concentrations of Na or K in the glaze. In most technological applications this is not desirable. Some glazes, however, are designed to have a pattern of hair-like cracks for an artistic effect; these are then known as crackle glazes. Fast cooling pro- duces a finer pattern of crazes. Celadon, tenmoku, raku, and copper glazes are par- FIGURE 21.14 TEM image showing crystallization on a nanometer ticular glazes found in the ceramics art world. scale in an LAS glass-ceramic. Celadon glazes (first produced 3500 years ago) vary from light blue to yellow green and can be quite dark. The

color is produced by iron (between 0.5 and 3.0 wt% Fe 2O3 added to the glaze). The appear very different even FLUX AND MODIFIERS glazed pot is then fired a on a fractured surface. The flux in a glaze is a modifier in glass. The clear glaze second time at about Breaking a sample can is glass: i.e., silica and alumina with added modifiers. 1300°C. An example of quickly reveal a “grain” The clear glaze may contain lead but Pb should not be Korean celadon pottery is size down to ∼0.1 µm. To used for modern food containers. Strong colored glazes the small water container, probe the structure on a also often contain heavy metals. 23 cm tall, given to former near-atomic level with high U.S. President Harry spatial resolution, TEM Truman in 1946 by the can be used as shown in Figure 21.14, but its use has been government of Korea. It is now valued at $3 million. limited, in part because of the difficulties of preparing the Tenmoku glaze (from the Sung Dynasty) is dark brown

TEM sample: the thickness of the sample tends to be or even black; 5–8 wt% Fe 2O3 is included in the glaze to greater than the dimensions of the crystalline phase. produce the effect. An interesting variation of the tenmoku glaze is the oil-spot tenmoku where bubbles begin to form in the glaze as it starts to melt; if the potter catches them 21.12 GLASS AS GLAZE AND ENAMEL just in time, they leave spots all over the surface. If a tenmoku glaze is fired in reducing conditions, the Fe2O3 Glazes are everywhere, just as glass is. In this section we is partially reduced to FeO, which acts as a modifier summarize the topic of complete books, namely the glazes instead of an intermediate in glass terms. Hence this glaze on pottery and enamels. Glazing uses the viscous proper- behaves differently under oxidizing and reducing condi- ties of glass to form a (usually) smooth continuous layer tions, and the color will change. Copper glazes may use on a ceramic substrate (a pot); enameling does the same 0.5 wt% CuCO 3 as the Cu source, but it breaks down to thing for a metal substrate. One thing that you will notice give CuO during firing, and this reacts with CO in the is that in general when it comes to ceramics, potters did furnace to give particles of Cu in the glaze. These parti- it first. The science of ceramics is often still unraveling cles of Cu provide the red color. just what they did. (In materials science, ceramists usually Raku glazes often appear metallic as if produced did it first.) by coating with a Ti metal. One modern method of raku First we summarize some terms in pottery. (The glazing involves firing the pots in the usual way, plunging processing of pottery was summarized in Chapter 2.) them into a reducing environment (like sawdust), and then Underglaze . When a pot [white bisqueware (also called quenching them before they can oxidize. These glazes are biscuit) is ideal] is decorated the first coating is the under- often the exception to the rule in that they can change with glaze. This is essentially a paint layer made by mixing time. This is simply because they oxidize when treated as oxide, carbonate, sulfate, etc., an opacifying agent to make other pots. The glaze is thus not as inert as others and is it opaque, and a flux to make it adhere better to the pot. used only for decoration. (However, see the historical note The mixture is calcined, ground, and then usually com- on Chojiro Rakuyaki.)

390 ...... Glass and Glass-Ceramics Crystalline glazes . These are decorative glazes but are directly related to the formation of technologically impor- tant glass-ceramics. The crystals form by slowly cooling the glaze to allow a few large crystals to grow. The growth is interesting since the glaze is typically only ∼0.5 mm thick and the crystals must therefore form as platelets. A seed of TiO 2 is usually used to nucleate the crystal in a low-viscosity glaze giving what is termed “rutile break- up,” which is actually the formation of PbTiO 3. The chem- istry of the glaze is thus important, with SiO 2 and Al 2O3 being low and PbO between 8 and 10 wt%. The growing crystal tends to incorporate Fe from the glaze, but can also preferentially exclude other dopants. Modern potters tend to use Zn as the modifier and produce willemite crystals. [Willemite is a somewhat rare zinc mineral (except as kidney stones), but is abundant at Franklin, NJ.] The technique is tricky because the addi- tion of large amounts of Zn (a network modifier) to the glaze causes its viscosity to remain low even at low tem- peratures, so that it tends to run off the pot! The crystals appear to grow out from a seed as in the spherulitic growth seen by VLM in Figure 21.15. Each spherulite is actually FIGURE 21.16 Example of glaze color produced by nanoparticles. a mass of radiating crystals that is similarly aligned with respect to the center of the spherulite. Opaque glazes . If crys- tals are added to the molten SPHERULITES are produced by forming glaze it can be made Dana described spherulites in obsidian in 1863; these very small crystals (e.g., opaque. SnO 2 was for long are the snowfl akes in snowfl ake obsidian. In 1879 Rutley wollastonite for a “lime the standard, but zircon is noted that artificial glass may develop a spherulitic matt” and willemite, much cheaper; ZnO 2 is structure. Zn 2SiO 4, for a “zinc matt”) used to make zircon glazes across the surface of the white. TiO 2 is used less glaze. The wollastonite because larger rutile crystals are a golden color and thus can be formed by adding calcite (also known as whiting) make the glaze yellow. We can also make the glaze opaque to an SiO 2-based glaze. An alternative is to add so much by forming crystals (e.g., wollastonite; CaSiO 3) using a crystalline material to the glaze that it remains unchanged suitable thermal treatment, or by trapping gas (F 2 or air), by the firing. A satin or vellum glaze, with smaller crystal or by causing a liquid/liquid phase separation. Matt glazes sizes, might contain 18% SnO 2 or ZnO and 4% TiO 2 in a high-lead glaze fired at 1000°C. Color in glass and glaze is the subject of much active research, with the realization that some colors are pro- duced by nanoparticles in the glass/glaze as for the luster- glazed plate in Figure 21.16. In general Ag and Au nanoparticles produce the gold color and Cu nanoparticles produce the red color; in the case of Cu especially, the ions may be reduced to the metal during processing. Explanations for the color of glazes are actually more complicated than for glass because the glaze is supported by a substrate and does not have to be transparent, so it can be a thin film or a multilayer and fired in a reducing or oxidizing environment. So this is a very large topic condensed into a paragraph! Cipriano Piccolpasso described luster preparation in 1557. Who says the use of nanomaterials is new! Colored glazes must use stable ceramic pigments if the color is to be consistent over repeated batches (e.g., for industrial production of sanitaryware, tiles, FIGURE 21.15 Illustration of spherulitic crystallization in a glass. etc.). Cheaper metal oxide colorants can be used when

21.12 Glass as Glaze and Enamel ...... 391 variability is acceptable or desirable as in the pottery effect can actually be greater than for what we think of as crafts. Of course, many glaze colorants are the same as stronger acids (sulfuric, nitric, and hydrochloric acids we use for coloring glass (see Table 21.4). Co 3O4 is a readily attack metals and skin). The Ca, Mg, and Al ions black powder but <1% gives a glaze a deep blue color, usually increase the chemical durability of a glass, but although it is usually added as the carbonate. Since Co 2+ they will react with these “food” acids. The tannic acid is present when it dissolves, it changes the viscosity of the present in red wine and tea can have a similar effect. Thus glaze. Cr 2O3 (2–3% can be added, but only 1.5% will Pb can be released from glass when the glass is in contact dissolve in the glaze) is intriguing because you expect with acid (even fruit juice). This means that we should not green but can produce red, yellow, pink, or brown. The use Pb in glazes either; however, this has often been done red can occur if Pb is present in the original glaze; if Zn because such glazes can be so brightly colored. is present, the glaze becomes brown unless Pb is also Silicate glass is strongly corroded by HF. The process present when the glaze becomes yellow. MnO 2 (added as of “frosting” glass light bulbs was carried out for many the carbonate) gives a brown glaze but can produce red, years by blowing HF vapor into the glass envelope and purple, or even black; the color depends in part on how then evacuating it after a short period. much Na is present in the initial glaze. CuO is equally Glass dissolves in water, particularly at elevated tem- interesting: 1–2% added to an Na-rich glaze gives tur- perature and pressure: we use this fact to grow all the quoise whereas up to a 3% addition produces a clear quartz crystals used by industry. Dishwashers make glass green/blue. If even more CuO is added the glaze can have dull. Roman glass (Figure 21.17) is iridescent because the a metallic appearance like pewter. If the glaze (0.3–2% glass has reacted with acid in the soil. (The iridescence CuO) is fired in a reducing atmosphere, the classic copper was not present in Roman times.) The corrosion products red is formed. This color is caused by the presence of form several distinct layers and, hence, generate the inter- colloidal Cu. If you see the bright yellow glaze, this might ference known as iridescence. It can easily be duplicated be the CdS/CdSe yellow (also produces orange and red) as shown by Tiffany and others. glaze. If Pb is present, then PbS can form, which makes Not all glass is attacked as readily. As described in the glaze black. Zircon is used in industry to help stabi- Chapter 8, we leach one component of a phase-separated lize these Cd-based colors. In fact, (V,Zr)SiO 4 (vanadium glass during the preparation of Vycor but leave the other zircon blue) and (Pr,Zr)SiO 4 (praseodymium zircon intact. yellow) are most important in the whitewares industry. It is possible to minimize these reactions to some Uranium is added to glazes but tends to produce a dark extent by adding inhibitors, such as Zr or Be, to the glass. brown rather than the pale yellow found in Vaseline glass; This question of reactivity is closely related to the phe- it can be yellow or bright red/orange, but this depends on nomenon of ion exchange. the glaze composition. Salt glaze . The pot is reacted with salt in the furnace while at temperature. In practice, the potter actually throws salt over the pot when it is in the kiln. The tech- nique was used by early potters in Iran and by the English in the 1700s. You will see many examples in Germany where a blue coloring is often produced using metal oxides. The salt reacts with the clay forming a glass layer on the surface; essentially the process is high-temperature soda corrosion of the fired clay. The term enamel usually implies a glaze applied to a metal, but it can be a glaze applied on top of a glaze. The market for enamel is large varying from toilet fixtures (whitewares) to jewelry. Enamel is the ever-lasting paint with the organic component replaced by glass.

21.13 CORROSION OF GLASS AND GLAZE

We think of glass as being inert. Citric acid and acetic acid (present in lemon and vinegar, respectively) can chelate with metal ions present in a glass and form water- soluble complexes. (A chelate is a complex compound with a central metal atom attached to a large molecule, a ligand, in a ring or cyclic structure like the claw of a crab.) The FIGURE 21.17 Example of iridescence in Roman glassware.

392 ...... Glass and Glass-Ceramics + H can replace alkali ions when glass is weathered. TABLE 21.5 Structural Elements in Glasses K+ can replace Na + and Na + can replace Li + when we want Silicates SiO 4 to strengthen the surface of glass. Borates BO + + + 3 Ag and Cu can replace Na to “stain” glass. Phosphates PO 4 Fluorides F A special feature here is that we have point defects Chalogenides S (and large defects) in glass just as we do in crystals. Our challenge is to understand what determines the properties (e.g., diffusion) of such defects when we do not have a reference lattice. Lead Glass Generally the composition will be a lead-alkali silicate

21.14 TYPES OF CERAMIC GLASSES glass SiO 2–PbO–R 2O, so the PbO replaces the CaO in soda-lime glass. These glasses have a high resistivity, a Not all glass is based on the silica tetrahedron. The large α, a low softening temperature, and a long working structural units are summarized in Table 21.5 and some range. The reason that Pb glass has been used to make representation glass compositions are given in Table so-called lead crystal glass is that it has a high refractive 21.6. index. Besides being used in art objects, it is used for lamp tubing, TVs (the “bulbs”), and thermometer tubing. In a traditional English lead crystal the concentration of PbO Silicate Glass (Soda-Lime Glass) will be at least 30%: an EU directive required that glass This is based on SiO 2–Na 2O–CaO (usually containing must contain ≥24% to be considered lead crystal. Then MgO and Al 2O3). It is relatively inexpensive and durable the EU had to exempt crystal glass from recycling laws! and is widely used in the building and packaging indus- Lead glass used for radiation shielding may contain as tries. It’s α is not negligible and it is not a good insulator. much as 65% PbO. Applications include TV tubes, The main uses are in sheet glass, bottles, tableware, and although Ba glass may be used in the face or panel of the in the light industry for envelopes (bulbs). The alkaline TV. (The electrons hitting the TV screen can create X-rays aluminosilicate glasses (SiO 2–Al 2O3–RO, where R is the that the glass must then absorb.) Lead-borate glasses can alkali) have low αs, are durable, and are better electrical be used as glass solder—they contain little SiO 2 or Al 2O3 insulators. They also have a high strain point. Uses include and are quite inert. combustion tubing, envelopes for halogen lamps, and sub- Flint glass is a high-dispersion, lead-alkali silicate strates in the electronics industry. glass originally made by melting flint rock, which is a

TABLE 21.6 Approximate Composition (wt%) of Some Commercial Glasses

Glass SiO 2 Al 2O3 Fe 2O3 CaO MgO BaO Na 2O K 2O SO 3 F 2 ZnO PbO B 2O3 Se CdO CuO

Container fl int 72.7 2.0 0.06 10.4 0.5 13.6 0.4 0.3 0.2 Container amber 72.5 2.0 0.1 10.2 0.6 14.4 0.2 S-0.02 0.2 Container fl int 71.2 2.1 0.05 6.3 3.9 0.5 15.1 0.4 0.3 0.1 Container fl int 70.4 1.4 0.06 10.8 2.7 0.7 13.1 0.6 0.2 0.1 Window green 71.7 0.2 0.1 9.6 4.4 13.1 0.4 Window 72.0 1.3 8.2 3.5 14.3 0.3 0.3 Plate 71.6 1.0 9.8 4.3 13.3 0.2 Opal jar 71.2 7.3 4.8 12.2 2.0 4.2 Opal illumination 59.0 8.9 4.6 2.0 7.5 5.0 12.0 3.0 Ruby selenium 67.2 1.8 0.03 1.9 0.4 14.6 1.2 S-0.1 0.4 11.2 0.7 0.3 0.4 Ruby 72.0 2.0 0.04 9.0 16.6 0.2 Trace 0.05 Borosilicate 76.2 3.7 0.8 5.4 0.4 13.5 Borosilicate 74.3 5.6 0.9 2.2 6.6 0.4 10.0 Borosilicate 81.0 2.5 4.5 12.0 Fiber glass 54.5 14.5 0.4 15.9 4.4 0.5 0.3 10.0 Lead tableware 66.0 0.9 0.7 0.5 6.0 9.5 15.5 0.6 Lead technical 56.3 1.3 4.7 7.2 29.5 0.6 Lamp bulb 72.9 2.2 4.7 3.6 16.3 0.2 0.2 0.2 Heat absorbing 70.7 4.3 0.8 9.4 3.7 0.9 9.8 0.7 Trace 0.5

21.14 Types of Ceramic Glasses ...... 393 adding fluorine ions to change the composition of the silica; this process allows transmission of wavelengths down to 157 nm. We have already discussed Vycor, which can be pure (porous) silica after we remove the second phase.

Phosphate Glass Phosphate glasses are important because they are semi- conducting; one application is in the manufacture of elec- tron multipliers (hence amplifiers) using Er doping (with

Er 2O3). The cations here are usually V and P, but Oak Ridge National Laboratory (ORNL) developed a lead indium phosphate glass that has a high index of refraction, a low melting temperature, and is transparent over a wide range of wavelengths. Since it can also dissolve significant concentrations of rare-earth elements (it was designed to FIGURE 21.18 Crown glass with bull’s eye. be a container for radioactive waste), it is being explored for new optically active devices (e.g., fiberoptic amplifiers

and lasers). Nd-doped (using Nd 2O3) phosphate glasses are being used in solid-state lasers (1.054 µm wavelength). particularly pure form of silica. Note that this rock is now The typical composition is 60P 2O5–10Al 2O3–30M 2O (or calcined and is still used extensively in the pottery MO); the Nd concentrations is ∼0.2–2.0 mol%. Calcium industry. phosphate glass will be discussed more extensively in Crown glass based on soda-lime glass, has quite a low Chapter 35. dispersion. It is still made by initially blowing the glass, flattening it, and transferring it to the pontil (a solid iron Chalcogenide Glass rod rather than the blow pipe) where it is spun until it is in the form of a disc that could be 1.5 m in diameter Based on As, Se, and Te, these glasses are IR transparent. (Figure 21.18). The disk shows concentric ripples from the They are nonoxide semiconductors and are used in special spinning and has a bull’s eye at the center of the crown. electronic devices and lenses. The devices use the abrupt The disk can be very smooth having been fl ame annealed change in electrical conductivity that occurs when a criti- without mechanical polishing. Historically, windowpanes cal voltage is exceeded. The applications have to be special could be cut around the bull’s eye or could contain it. because these glasses are not durable and have low soften- ing temperatures. Borate Glass (Borosilicate Glass) Fluoride Glass The alkali borosilicates, SiO 2–B 2O3–R 2O (R is the alkali), are special for their low α. They are durable and have In general, halide glasses are based on BeF 2 and ZnCl 2 useful electrical properties. The cookware material and are used in optical waveguides (OWG) where the cost Pyrex TM is a borosilicate. Borosilicate glass is widely used can be justified. in the chemical processing industry. Some borate glasses melt at very low temperatures ( ∼500 ± 50°C), so they can be used to join together other glasses. Zinc-borosilicate 21.15 NATURAL GLASS glass, known as passivation glass, contains no alkalis, so it can be used for Si electronics components. It surprises some people that not only does glass occur in nature, but it is relatively common. Tektites are formed within the impact craters of meteors. Moldavite is a green Fused Silica glass from Moldavia in the Czech Republic; Libyan Desert

Being essentially pure SiO 2, this is the silicate glass for Glass formed the same way but is yellow. Fulgarites are high-temperature applications. It has a near zero α [known fragile tubes of glass that can be formed when lightning as ultralow expansion (ULE) silica]. This is used for tele- strikes a sandy soil. Obsidian is the glass formed in scope mirrors and substrates. ULE ® glass containing 7% volcano flows. The usual black color is due to impurities;

TiO 2 is being used for photolithography masks [for extreme green and red obsidian also occur. Obsidian was used to ultraviolet lithography (EUVL) at a wavelength of 13.4 nm, make tools during the Paleolithic period. The advantages extreme UV]. Another silica glass that can be used for of using glass for scalpels have only recently been redis- 157–nm lithography was made by removing the water and covered: the cut made by a glass knife is particularly even

394 ...... Glass and Glass-Ceramics (B)

(A)

(C) FIGURE 21.19 SEM images of diatoms. so it heals fast. It has been proposed that the Aztec civili- has a skeleton that looks like a mesh of glassy silica fibers zation may not have developed metallurgy because it was (Figure 21.20). Each fiber actually consists of coaxial so adept at using obsidian and there are many sources of cylindrical layers with different optical properties. It is obsidian in the volcanic mountain ranges of Peru and reminiscent of the cladding used today on commercial Ecuador in particular. optical fibers, but nature did it eons earlier. The optical Pumice is another glass formed in volcanic eruptions. properties of the natural fibers are not as good as those of It can be very porous if it contains a high concentration human-made fibers but they are more resistant to break- of gas. Pumice is thus the porous form of obsidian. ing. Note that these layers are deposited at ambient water Trinitite is not really a natural glass but one that we temperatures! might say forms unintentionally. This glass has been found at the Trinity site where the nuclear bomb was exploded in New Mexico. Diatoms are included in this topic because they are both interesting and surprising. Not all living things on earth are based on carbon. Diatoms are small aquatic microorganisms, or one-celled plants, that live by ingest- ing silica that is dissolved in water; we usually think of seawater, but it can be a freshwater lake. The diatoms then use the silica to form and grow a pair of shells as illus- trated in Figure 21.19. The two shells resemble a pillbox. The shells come in many varieties—there are thousands of species of diatoms. When the microorganisms die, their siliceous skeletons have formed layers up to 3000 feet thick: they are not rare! The result is that there are regions in which deposits of silica have built up to form what is known as diatomite or diatomaceous earth. The compara- ble process for carbon-based creatures would be the formation of limestone and chalk. Diatoms do contain chlorophyll, so they are plant colored while alive. The Venus Flower Basket (Euplectella) is a sponge that lives in the deepest parts of the oceans in the tropics. It FIGURE 21.20 Sea sponge.

21.15 Natural Glass ...... 395 21.16 THE PHYSICS OF GLASS

We will now discuss glass from the physicist’s point of logη view. We have left this topic for last so as either to confirm Pa.s your excitement or not completely put you off the subject. 7 The idea is that glass is a condensed phase just as liquids and crystals are. The atomic interactions can be described by a potential energy function, φ. 3 If we describe the energy of a glass plotted against the coordinates of the glass, we would find a multidimensio- Strong nal energy hypersurface, which is multiply dented with -1 structured valleys. The glass structure corresponds to one Fragile of those valleys, but there may be another valley or a minimum not far away on the surface. Then we have the 0 0.4 0.8 Tg/T concept of polyamorphism, in which the glass can have FIGURE 21.21 Viscosity versus temperature for strong and fragile several distinct amorphous structures. (The comparison liquids. to crystalline materials is instructive!) The experimental observation is that the viscosity

of some glasses decreases suddenly above Tg in a non- fast relaxation processes (known as β processes) and low- Arrhenius way. It is as if the structure of the glass col- frequency processes, which contribute to the dynamic lapsed because it was fragile (the term fragile refers only structure factor. In the glass’s vibrational spectra detected to the liquid, not the glass). Hence fragility is a property by Raman or neutrons, these features are known as the

of some glass-forming liquids above Tg although we talk boson peak. The boson peak is large for strong glass about fragile and strong glasses. If we plot log η versus formers; low-frequency excitations in a glass suggest Tg/T (the reduced Tg as shown in Figure 21.21), then the intermediate-range order: so more order indicates a curve would be straight if it was for Arrhenius-like behav- stronger glass.

ior. For SiO 2 and other highly polymerized network glasses The molar heat capacity, cP, is a well-defined quantity. (strong glass formers), it is nearly straight. If the bonds At very low temperatures ( T < 1 K: a temperature familiar are not directional, the plot deviates signifi cantly from to physicists but less so to ceramists), glasses show a

Arrhenius behavior; this is a fragile glass former. Two linear term in cP due to an anharmonic contribution. At approaches have been used to explain this behavior: T∼5–10 K, an excess vibrational (harmonic) contribution causes a bump in cP. The excess vibrational contribution ᭿ Free volume to cP at this bump (call it cP–cD) can be plotted against the ᭿ Confi gurational entropy fragility of the glass. The resulting correlation suggests that the excess vibration and the fragility have a related

Each connects η to the macroscopic quantities of either origin. So SiO 2, a particularly strong glass, has a large volume or entropy. A newer approach considers factors cP/cD, whereas a fragile glass like CKN [Ca 0.4 K0.6 (NO 3)1.4 ] affecting the kinetics of the transformation. There are has a small cP/cD.

CHAPTER SUMMARY This chapter has been placed after powders but before processing because we are still empha- sizing the material. Because glass is an extremely important material the history of this topic is particularly rich. Remember that glazes on pots and enamels on metals are essentially two variations on a single theme—protecting other materials by coating them with glass. The variety of glasses is large and we have touched on only a small range here. Remember that historically, silica-based glass was for a long time synonymous with the word glass. So, it has dominated our thinking about glass. New glasses are being developed that may contain no Si and the properties may be very different; there is not just one material called glass any more than there is one material called crystal. Glass will crystallize given time; although glass- ceramics were the new materials of the 1960s, they were already old friends to the potter. The basic science of glass is more difficult than for crystals because we have no “frame of refer- ence.” However, there are point defects in glass (remember the origins of color). Glass has both internal and external interfaces and does have special structural features. Of its many important properties, transparency and viscosity must be the most important, although for many applica- tions the small, or controllable, expansion coefficient is the key to the value of glass.

396 ...... Glass and Glass-Ceramics As we explained from the beginning, glass appears throughout our discussion of ceramics. The processing of glass is treated in Chapter 26, mechanical properties are discussed in Chapter 18, and Bioglass is discussed in Chapter 35. The reason we treat glass separately is partly his- torical and partly because its behavior is often so different from crystalline ceramics, which emphasizes the point we made in Chapters 5 through 7, bonding and structure determine the properties and thus the applications.

PEOPLE IN HISTORY Bacon, Roger, a Franciscan Friar, described reading glasses made using two lenses in 1268. Salvino D’Armate of Pisa is sometimes credited with the invention in 1284. Cassius, Andreas (1685) in the book “ De Auro ” describes how to produce this ruby red color, which thus became known as “Purple of Cassius.” La Farge, John (patent No. 224,831; February 24, 1880); a “Colored-Glass Window,” the original patent on opal glass, was followed shortly by Louis Comfort Tiffany’s patent (No. 237,417; February 8, 1881) with the same title, “Colored-Glass Window.” Lipperhey, Hans was a lens grinder in the Netherlands; he applied for a patent for the telescope in 1608. Pascal, Blaise (1623–1662) was born in Clermont-Ferrand, France and died in Paris. The SI unit of pressure (stress) is named after him. He argued against Descartes in favor of the existence of vacuum. Perrot, Bernard (1619–1709) was a well-known early French glassmaker. Poiseuille, Jean Louis Marie (1799–1869) was the French physician after whom we name the Poise. Prince Rupert of Bavaria (1619–1682) was the grandson of James I of England and nephew of Charles II. He introduced his drops to England in the 1640s, where they became party pieces in the court of Charles II. The famous diarist Samuel Pepys wrote about them in his diary on January 13, 1662. Rakuyaki, Chojiro (died 1859) was the first member of the family to begin the tradition of raku. Their home is now an exquisite museum illustrating tea bowls made by 15 generations. van Leeuwenhoek, Anton (1632–1723) was born in Delft, Holland and worked as a cloth merchant; he devised a simple microscope that succeeded so well because he was a skilled lens grinder. The microscope itself was invented in the 1500s and was used by Robert Hooke. Warren, Bertram Eugene (1902–1991; at M.I.T. 1930–1976) is known for his textbook on X-ray diffraction and his studies of the structure of glass and carbon black. These began small-angle scattering research into nonperiodic and nearly periodic structures.

THE HISTORY OF GLASS Allen, D.(1998) Roman Glass in Britain , Shire Pub. Ltd., Bucks, UK. Boyd, D.C. and Thompson, D.A.(1980) Glass , 3rd edition (Kirk-Othmer: Encyclopedia of Chemical Technol- ogy , Vol. 11, p. 807). Bray, C.(2001) Dictionary of Glass Materials and Techniques , University of Pennsylvania Press, Philadelphia, PA. Douglas, R.W. and Frank, S.(1972) A History of Glass Making , Foulis & Co, London, UK. A very readable history of glassmaking with some super illustrations and photographs. Newby, M.S.(2000) Glass of Four Millennia , Ashmoleum Museum, Oxford, UK. Stern, E.M.(2001) Roman, Byzantine and Early Medieval Glass 10 BCE–700 CE , H. Cantz Publishers, Ost- fildern-Ruit, Germany. Stookey, S. Donald (2000) Explorations in Glass , American Ceramic Society, Westerville, OH. About 70 pages of essential reading. Zerwick, C.(1990) A Short History of Glass , H.N. Abrams Inc., New York.

JOURNALS J. Non-Cryst. Solids; J. Chem. Phys.; J. Appl. Phys.; J. Mater. Sci .

GENERAL REFERENCES Bach, H. and others (1998–) The Schott Series on Glass and Glass Ceramics , Springer, Berlin. Superb series from specialists at one of the leading glass companies. Bailey, M. (2004) Oriental Glazes , A & C Black, London. One of the Ceramic Handbooks series of texts aimed at the practicing potter. Brow, R.K. (2000) “Review: The structure of simple phosphate glasses,” J. Non-Cryst. Solids 263 and 264 , 1. Creber, D. (2005) Crystalline Glazes , A&C Black, London. One of the Ceramic Handbooks series. Davies, J. (1972) A Glaze of Color , Watson-Guptill Pubs, New York. Very practical insights for the potter.

Chapter Summary ...... 397 Doremus, R.H. (1994) Glass Science, 2nd edition, John Wiley & Sons Inc., New York. An essential text if you study glass. The discussion of definitions is very clear. Höland, W. and Beall, G. (2002) Glass-Ceramic Technology , American Ceramic Society, Westerville OH. The book on glass-ceramics. Dr. George Beall has the greatest number of patents (100 in 2004) granted to a single individual in Corning’s history. Ilsley, P. (1999) Macro-Crystalline Glazes: The Challenge of Crystals, The Crowood Press, Ramsbury, Wilts, UK. Beautiful illustrations from an experimentalist. Morey, G.W. (1954) The Properties of Glass , 2nd edition, Reinhold Publishing Co., New York. Includes a useful discussion of viscosity. Paul, A. (1982) Chemistry of Glasses , Chapman & Hall, London, UK. Pfaender, H.G. (1982) The Schott Guide to Glass , Chapman & Hall, London, UK. A small, enjoyable text with color illustrations. Rawson, H. (1967) Inorganic Glass-Forming Systems , Academic Press, New York. Another of the standards on glass. Shimbo, F. (2003) Crystal Glazes , 2nd edition, Digital Fire Co., Medicine Hat, Alberta, Canada. Stoemer, E.F. and Smol. J.P. (1999) The Diatoms , Cambridge University Press, Cambridge, UK. Concerned primarily with applications of these diatomaceous materials. Very comprehensive. Sturkey, S.D. (2000) Explorations in Glass , American Ceramics Society, Westerville, OH. A “must-read” for anyone interested in glass. Particularly nice discussion on opal glass. Taylor, J.R. and Bull, A.C. (1986) Ceramic Glaze Technology , Pergamon Press, New York. An excellent resource on glazes. Wiggington, M. (1996) Glass in Architecture , Phaidon Press, London. Zarzycki, J. (1991) Glasses and the Vitreous State , Cambridge University Press, Cambridge, UK.

SPECIFIC REFERENCES Aizenberg, J., Weaver, J.C., Thanawala, M.S., Sundar, V.C., Morse, D.E., and Fratzl, P. (2005) “Skeleton of Euplectella sp.: Structural hierarchy from the nanoscale to the macroscale,” Science 309 , 275. Angell, C.A. (1985) in Strong and Fragile Glass Formers in Relaxation in Complex Systems , edited by K.I. Ngai and G.B. Wright, National Technical Information Service, U.S. Department of Commerce, Springfield, VA, 3. Angell, C.A. (1995) “Formation of glasses from liquids and biopolymers,” Science 267 , 1924. A particularly important review. Angell, C.A. (2002) “Liquid fragility and the glass transition in water and aqueous solutions,” Chem. Rev. 102 , 2627. Much more relevant than it might appear. Bondioli, F., Manfredini, T., Siligardi, C., and Ferrari, A.M. (2004) “A new glass-ceramic pigment,” J. Eur. Ceram. Soc . 24 , 3593.

Kim, S.S. and Sanders, T.H., Jr. (2000) “Calculation of subliquidus miscibility gaps in the Li 2O-B 2O3-SiO 2 system,” Ceram. Int. 26 , 769. Knowles, K.M. and Freeman, F.S.H.B. (2004) “Microscopy and microanalysis of crystalline Glazes,” J. Microsc. 215 , 257. Pye, L.D., Montenero, A., and Joseph, I. (2005 ) Properties of Glass-Forming Melts , CRC Press, Boca Raton, FL. A collection of chapters on current aspects of molten glass. Rössler, E. and Sokolov, A.P. (1996) “The dynamics of strong and fragile glass formers,” Chem. Geol. 128 , 143. Strahan, D. (2001) “Uranium in glass, glazes and enamels: History, identification and handling,” Studies Conservation 46 , 181. Tangeman, J.A., Phillips, B.L., Nordine, P.C., and Weber, J.K.R. (2004) “Thermodynamics and structure of single- and two-phase yttria-alumina glasses,” J. Phys. Chem. B 108 , 10663. Vogel, W. (1971) Structure and Crystallization of Glasses , The Leipzig Ed., Pergamon Press, Oxford, UK.

Zhu, D., Ray, C.S., Zhou, W., and Day, D.E. (2003) “Glass transition and fragility of Na 2O–TeO 2 glasses,” J. Non-Cryst. Sol. 319 , 247.

WWW www.bell-labs.com Bell Labs www.corning.com The site for the Corning Glass Company www.cmog.org The Corning Museum of Glass www.glass.org The site for the NGA (National Glass Association)

398 ...... Glass and Glass-Ceramics www.pilkington.com The site for Pilkington Glass, a key developer of glass based in the UK www.schottglass.com The site for Schott Glass with descriptions of new glass developments www.focusmm.com/pasabahce/co_hi.htm Describes the history of the wonderful Pasabahce glass of Turkey www.doge.it/murano/muranoi.htm The history of Murano glass www.ortonceramic.com A source for testing equipment www.britglass.org.uk The site for the British Glass Manufacturers’ Confederation www.jlsloan.com/lct1.htm Julie L. Sloan’s site describing the rivalry between La Farge and Tiffany in developing opal glass

EXERCISES 21.1 What causes refraction in glass? 21.2 Why is smoky quartz smoky? 21.3 If you increase the wavelength, how does the refractive index change? 21.4 What is dispersion and why does glass cause it? 21.5 If Pb were added to a typical lead crystal glass, what weight percent would be added? What atomic percent of Pb would the glass then contain? What is actually added in industrial practice and will this practice con- tinue in the future?

21.6 If you are given crystalline SiO 2, quartz glass, silica gel, and a sample of liquid SiO 2, how would you analyze the bonding of the Si in each case? Would you detect a difference?

21.7 How would you expect the properties of GeO 2 glass to differ from those of SiO 2 glass? Be as quantitative as possible. 21.8 We can make glass based on B and on P. What will the bonding characteristics of these two glasses be? Suggest three modifiers for each glass. Compare the densities you expect for these glasses. 21.9 Libyan Desert glass was produced naturally. Is pressure or temperature the more important factor? Explain your reasoning as quantitatively as possible.

21.10 Na is a network modifier for SiO 2 glass. How would Li and K compare to Na in this role? Similarly Ca is present in soda-lime glass; if the Ca were replaced by an equal atomic percent of Mg or Ba, how would the properties of the glass change?

Chapter Summary ...... 399 26

Processing Glass and Glass-Ceramics

CHAPTER PREVIEW Glass has changed the world more than any other material. In Chapter 21 we described some of the different types of glass and their properties. In this chapter we will look at the main methods used to fabricate glass products. In terms of the volume of glass that is produced each year, glass processing could be said to be the most important processing method for ceramics. The largest segments of the market are

᭿ Flat glass for windows ᭿ Containers (including bottles, jars, and tableware)

These products are formed using essentially two methods [one is recent (in glass terms) the other has its roots in antiquity]:

᭿ The float glass process ᭿ Blowing

We will also look at some processes that are used to modify glass for specific uses such as the application of thin-film coatings for solar radiation control and tempering and laminating for safety glass. In the last section of this chapter we will examine how glass-ceramics are produced. This chapter does not cover the processing of some special glass products. For example, the processing of optical fibers is described in Chapter 32, but we introduce the important ideas here. Optical fibers must meet stringent quality requirements. We must understand these requirements to understand why the elaborate processing methods are necessary. This chapter covers three closely related topics:

᭿ Processing as dictated by applications ᭿ Shaping ᭿ Treating

26.1 THE MARKET FOR GLASS AND Hollow glass includes most of the container glass and table- GLASS PRODUCTS ware we use (i.e., consumer glassware). Table 26.1 lists examples of the applications for these product categories. More than half of the total worldwide ceramics market is glass products, accounting for over $50 billion/year. Figure 26.1 shows the distribution of glass sales. The 26.2 PROCESSING BULK GLASSES market for manufactured glass products emphasizes three main types of glass: Glass production starts with a mixture of raw materials, which for glass manufacture often contain a high propor- ᭿ Hollow glass (bottles, drinking glasses, lamp bulbs, tion of naturally occurring minerals (for example, sand glass containers) 35% and limestone). However, some industrial chemicals such ᭿ Flat glass (mirrors, windows) 30% as sodium carbonate (Na 2CO 3) and alumina (Al 2O3) are ᭿ Fiberglass (includes glass fiber) 17% also used. The mixture containing the raw materials in the

26.2 Processing Bulk Glasses ...... 463 processes occurring during melting, and fining (see Section 26.3). Lighting 18% Containers 12% We must also consider the possibility of oxidation and/or reduction of the glass, homogenization processes, and defect in the glass.

Fiberglass 17% By far the greatest amount of flat glass is soda-lime Flat glass 32% silicate glass. You must be familiar with the terminology used in the glassmaking industry: soda is sodium oxide

TV tubes, (Na 2O) and lime is calcium oxide (CaO), so soda-lime sili- CRTs 9% cate glass consists mainly of Na 2O, CaO, and SiO 2. Glass manufacturers often use the common names for these oxides, which are not always the generally recognized sci- Consumer Other 1% glassware 5% entific name. If you are unsure of the composition of a Technical/laboratory 1% particular mineral then you need to look at its formula. FIGURE 26.1 Percent distribution of glass sales; total: $48,260 All manufacturers of flat glass use basically the same million. formula, but they never actually use the compounds Na 2O or CaO. Values of the components are usually given in weight percent (wt%). Typical values are 72 wt% SiO , TABLE 26.1 Glass Product Types and Applications 2 14 wt% Na 2O, wt% CaO, 4 wt% MgO, and 1 wt% Al 2O3. Glass product type Applications The molecular formula for a glass of this composition can Flat glass Automotive: cars and trucks be calculated as follows. Architectural: commercial buildings, storefronts Step 1 : Divide the wt% of each component in the batch by Residential: windows, doors, sunrooms, its molecular weight. skylights Patterned glass: shower doors, privacy Molecular weight wt%/molecular glass Oxide wt (g) (g/mol) weight (mol) Ratio Blanks for microscopes and telescopes

Containers/tableware Beverage SiO 2 72 60.1 1.20 120

Liquor, beer, wine Na 2O 14 62.0 0.23 23 Food CaO 9 56.1 0.16 16 Pharmaceutical, drugs MgO 4 40.3 0.10 10

Glasses, plates, cups, bowls, serving dishes Al 2O3 1 102.1 0.01 1 Fiberglass and glass Wool: insulation, fi lters fi ber Textile: plastic or rubber tire reinforcements, Step 2 : Divide each ratio by the smallest ratio in column fabrics, roof shingle and roll goods 4. This gives the number in column 5. reinforcement Optical communications Step 3 : Write out the molecular formula for the glass. Specialty glass Artware, stained glass, lead and lead The molecular formula of our flat glass is then crystal, lighting Al 2O3·10MgO·16CaO·23Na 2O·120SiO 2. TV picture tubes and fl at-panel displays, ovenware and stovetop To make the batch it would not be economical to use Ophthalmics, aviation, tubing, foamed glass, marbles only synthesized or pure ingredients. For most glasses a large proportion of the batch is made up of naturally occurring minerals that appropriate amounts is FLAT GLASS RAW MATERIALS have been through a bene- known as the batch . The ficiation process. Table ᭿ Limestone, CaCO batch contains a mixture 3 26.3 (top) shows the typical ᭿ Silica sand, SiO of glass formers, modifi - 2 batch constituents used to ᭿ Soda ash, Na CO ers, and intermediates; the 2 3 make the Al O ·10MgO·16 ᭿ Alumina hydrate, Al O ·3H O 2 3 amount of each component 2 3 2 CaO·23Na O ·120SiO ᭿ Burnt dolomite, CaO·MgO 2 2 depends on the application glass. To determine how of the final glass product. much of the raw material In Table 26.2 we link the we need to add to the batch to obtain the desired amount different ways of giving a batch composition and the of oxide in the final glass, we need to know the fraction sources of some of the raw materials. of that oxide in the raw material. For the principal raw materials used in glass making this fraction is given in Batch melting depends on the source of energy, the refrac- column 5 in Table 26.2. For example, during melting lime- tory used to contain the glass, details of the batch, stone will decompose as follows:

464 ...... Processing Glass and Glass-Ceramics TABLE 26.2 Principal Raw Materials Used in Glassmaking Material Alternative name Theoretical formula Oxides Fraction Batch Purpose

Alumina Calcined alumina Al 2O3 Al 2O3 1.000 1.000

Aluminum hyd. Hydrated alumina Al 2O3 · 3H 2O Al 2O3 0.654 1.531

Aplite (typical — — Al 2O3 0.240 4.167 Source of Al 2O3

composition) — — Na 2(K 2)O 0.100 —

— — SiO 2 0.600 — — — CaO 0.060 —

Feldspar Microcline(composition K 2O · Al 2O3 · 6SiO 2 Al 2O3 0.180 5.556 Source of Al 2O3

of commercialspar) — K 2(Na 2)O 0.130 —

SiO 2 0.680 —

Nepheline syenite — — Al 2O3 0.250 4.000 Source of Al 2O3

(typical composition) — — Na 2(K 2)O 0.150 —

— — SiO 2 0.600 —

Calumite Calcium-aluminum 2CaO · MgO · 2SiO 2 SiO 2 0.380

silicate 2CaO · Al 2O3 · SiO 2 Al 2O3 0.117

2(CaO · SiO 2) CaO 0.400 MgO 0.080

Kyanite (90% — Al 2O3 · SiO 2 Al 2O3 0.567 1.763

concentrate) — — SiO 2 0.433 —

Kaolin China clay Al 2O3 · 2SiO 2 · 2H 2O Al 2O3 0.395 2.57

— SiO 2 0.465 —

Cryolite Kryolith Na 3AlF 6 — — — Flux and opacifi er in opal glasses

Antimony oxide — Sb 2O3 Sb 2O3 1.000 1.000

Arsenious oxide White arsenic As 2O5 As 2O5 1.160 0.860 Fining and decolorizing

Barium carbonate — BaCO 3 BaO 0.777 1.288 Source of BaO Barium oxide Baryta BaO BaO 1.000 1.000

Barium sulfate Barytes BaSO 4 BaO 0.657 1.523 Flux and fi ning

Boric acid Boracic acid B 2O3 · 3H 2O B 2O3 0.563 1.776 Source of B 2O3

Borax — Na 2O · 2B 2O3 · 10H 2O B 2O3 0.365 2.738 Source of B 2O3

— Na 2O 0.163 6.135

Anhydrous borax (“Pyrobor”) Na 2O · 2B 2O3 B 2O3 0.692 1.445 Source of B 2O3

— — Na 2O 0.308 3.245 Lime, burnt Quick lime CaO CaO 1.000 1.000

Lime, hydrated Calcium hydrate CaO · H 2O CaO 0.757 1.322

Limestone Calcium carbonate CaCO 3 CaO 0.560 1.786 Source of CaO

Calcium carbonate Whiting CaCO 3 CaO 0.560 1.786 Source of CaO Lime, dolomitic Burnt dolomite CaO · MgO CaO 0.582 1.720 Source of CaO — — MgO 0.418 2.390 and MgO

Dolomite Raw limestone CaO · MgO · 2CO 2 CaO 0.304 3.290 Source of CaO (dolomitic) MgO 0.218 4.580 and MgO

Lime, hydrated, dolomitic Finishing lime CaO · MgO · 2H 2O CaO 0.423 2.363 Source of CaO — MgO 0.304 3.290 and MgO Litharge Lead oxide, yellow PbO PbO 1.000 1.000

Red lead Minium Pb 3O4 PbO 0.977 1.024 Source of PbO

Bone ash Calcium 3CaO· 2P 2O5 + xCaCO 2 CaO 0.372 2.700

phosphate P 2O5 0.628 1.592

Iron oxide, red Rouge Fe 2O3 Fe 2O3 1.000 1.000 Color

Potassium hydroxide Caustic potash KOH K 2O 0.838 1.194 Source of K 2O

Potassium nitrate Saltpeter KNO 3 K 2O 0.465 2.151 Source of K 2O

Potassium carbonate. Calcined potash K 2CO 3 K 2O 0.681 1.469 Source of K 2O 3 Glassmaker’s potash Potassium carbonate, K 2CO 3 ·−2 H2O K 2O 0.570 1.754 Source of K 2O hydrated

Sand Glass sand, quartz SiO 2 SiO 2 1.000 1.000

Soda ash Sodium carbonate coml. Na 2CO 3 Na 2O 0.585 1.709 Source of Na 2O

Sodium nitrate Saltpeter chili NaNO 2 Na 2O 0.365 2.741 Oxidizing and fi ning

Salt cake Sodium sulfate Na 2SO 4 Na 2O 0.437 2.290 Oxidizing and fi ning Zinc oxide — ZnO ZnO 1.000 1.000

26.2 Processing Bulk Glasses ...... 465 TABLE 26.3 Batch Composition and Sources in the United States Oxides Fraction Weight of oxide Weight ofraw material Raw material supplied of oxide required (g) in batch (g)

Limestone CaO 0.560 90 61

Silica sand SiO 2 1.000 720 720

Soda ash Na 2O 0.585 140 239

Alumina hydrate Al 2O3 0.654 10 15 Burnt dolomite CaO 0.582 90 96 MgO 0.418 40

Source U.S.A. location

Silica sand Bank sand Jersey shore Sandstone Allegheny mountains Soda ash Trona deposits Wyoming Limestone Dolomite All over, e.g., Alabama Feldspars In pegmatites North Carolina

B2O3 Borax California

CaCO 3 (s) → CaO (s) + CO 2 (g) (26.1) are 100 m long and 13 m wide.) The furnace is constructed of alumina/zirconia refractories that can withstand the The fraction of CaO (molecular weight: 56.1 g/mol) in high temperature and corrosive environment of the melt

CaCO 3 (molecular weight: 100.1 g/mol) is 0.560, or 56%. (see Chapter 9). The life of a tank furnace is about 8 Hence, for each kilogram of limestone added to the batch years. the melt will contain only 0.56 kg of CaO. So using the raw With reference to Figure 26.3: The batch is loaded into materials listed above, and by bearing in mind the fraction the hopper (1). In industry, the batch-loading compartment of each oxide provided by each mineral, we can determine is referred to as the “doghouse.” In region (3) the batch the batch composition for a 1 kg melt of flat glass. These begins to melt. The surface temperature of the melt peaks are the numbers given in Table 26.3 (top). Note that the in region (4). During the melting process it is important lime comes from both burnt dolomite and limestone. A to control the homogeneity of the mixture and the oxida- batch weighing 2 t can be measured to an accuracy of tion state of the components. Region (5), the throat, divides 0.1%. the furnace into two parts—the melting end and the con- Table 26.3 (bottom) shows where we might actually ditioning, or working, region to the right. In the working have to go to obtain these raw materials. [If we are making region the melt is cooled down to about 1300°C and the bottles and jars the batch will likely contain cullet, recy- glass viscosity increases. The glass travels through narrow cled glass; see Section 37.7.] tunnels called the forehearth (7), where the temperature After the batch has been thoroughly mixed, it is melted is further lowered to 1000–1100°C, and on to the forming at high temperature to form a homogeneous liquid melt. For small-scale melting of glass the batch will be placed in a crucible. For glass-melting experiments in the labora- tory it is common to use Pt, or a Pt alloy such as Pt-5% Au or Pt-20% Rh crucibles. These crucibles are very expensive, but they can with- stand high temperatures, they do not contaminate the glass melt, and they can be easily cleaned and reused. Other crucible materials, usually used for larger batch sizes, include silica, alumina, and mullite. Large-scale industrial production of glass is carried out in continuous furnaces called tanks. Glass-melting tanks are the second largest industrial furnaces, the largest being the blast furnaces used in making iron and steel. Figure 26.2 shows the inside of a glass-melting tank and Figure 26.3 shows a schematic of a glass-melting tank for producing glass containers. The tanks are typically 10– 40 m long, 3–6 m wide, and 1–1.5 m deep and contain over 2 kt of molten glass. (The largest glass-melting furnaces FIGURE 26.2 Inside a typical glass melting tank.

466 ...... Processing Glass and Glass-Ceramics Side View 1700°C 1600°C Temperature distribution 1500°C 1 8 2 9 9 9 9 7 3 4 Tank 5 6

Feeder for automated processing Top View Regenerative chambers 7 Manual processing 9 9 9 9 7 Tank furnace

3 4 5 6 7

7 9 9 9 9

Regenerative 7 chambers

FIGURE 26.3 Schematics of a glass melting tank for producing containers.

machines. Depending upon the requirements of the working point is 10 4 dPa-s (see Chapter 21). The regenera- forming machines, the glass may be delivered in the form tive chambers, region (9), are used to preheat the fuel of a continuous stream (as shown in Figure 26.4) or in gases and air to increase the combustion efficiency of the discrete amounts, called gobs. The viscosity of glass at the furnace.

26.3 BUBBLES

The temperature required to form a glass melt varies with the composition of the batch. Typically melting tempera- tures are in the range 1300–1600°C. At this stage of the process the melt may contain many gas bubbles, mainly

CO 2 and SO 2, from the dissociation of carbonates and sulfates. Bubbles also come from reactions between the glass melt and the refractories. Gas bubbles (which are known as seeds or blisters if the size is >0.5 mm) are usually undesirable in the final product because they affect its appearance. Bubbles are eliminated during melting by a process known as fining. Fining can be achieved by increasing the temperature of the molten glass by about 150°C to reduce its viscosity. From Stokes’ law (or by comparing the rise of bubbles in FIGURE 26.4 Pouring molten glass into a mold. Cola versus liquid soap) we know that the drift velocity

2 6 . 3 Bubbles ...... 467 ᭿ Rolling ᭿ Drawing ᭿ The float process

The float process is so important to the glass industry that we will discuss it separately in the next section. In this section we will discuss the formation of flat glass by rolling and drawing . Figure 26.6 shows an example of a method for producing flat glass by rolling. The molten glass flows from the tank furnace over a refractory lip and between a set of water-cooled rollers where it solidifies into a continuous ribbon. The glass ribbon is transported over rollers into a tunnel-like annealing furnace (called a lehr in the glassmaking industry). Inside the lehr, the glass is first reheated to between 600 and 800°C. On the way through the lehr the temperature decreases slowly in a carefully controlled manner to minimize the develop- ment of internal stresses within the glass. The glass thickness is controlled by a combination of factors: FIGURE 26.5 Example of a bubble deliberately blown in glass. ᭿ The rotational speed of the rollers ᭿ Their spacing ᭿ The glass pull rate of the bubbles toward the DEFECTS IN GLASS MELTS Typical pull rates are surface of the melt is Stones: opaque particles of rock or batch material 0.5–5 m/min producing increased as the viscosity embedded in the glass sheets 3–15 mm thick and of the melt is decreased. Cords: thin string of inhomogeneity up to 3.6 m wide. Fining is also tradition- Seed or blister: elongated bubble ( ∼0.5 mm) A drawing process can ally achieved by adding also be used to produce flat fining agents, such as glass. A solid metal plate

Na 2SO 4 or NaCl, to the melt at the end of the melting is dipped into a bath of molten glass and then slowly process. The fining agent decomposes as shown in Eq. withdrawn from the melt. This process would present no 26.2 to produce a large quantity of gas bubbles, which problems if we were interested in producing a glass rod coalesce with existing bubbles, increasing their volume (Figure 26.7 ). Producing a planar sheet is problematic and hence taking them to the surface faster. because the sheet would neck down to a narrow ribbon. This difficulty is overcome by cooling the sheet as it is Na 2SO 4 (s) → Na 2O (l) + SO 2 (g) + 1/2O 2 (g) (26.2) drawn; it is passed between two coolers as shown in Figure

Arsenic oxide, As 2O3, is another fining agent (giving a mixed-valence effect), but it is not so widely used today because it is toxic. The reaction involving arsenic oxide is α a little more complicated than that shown for Na 2SO 4, but the effect is the same. Fining will occur toward the end of region (4) in Figure 26.3. Modern systems will also use Furnace mechanical bubblers. Inclined rollers In some studio glass and tableware, bubbles are intro- duced intentionally as illustrated in Figure 26.5. Glass

26.4 FLAT GLASS

Flat glass is not necessarily flat but is generally flatish! Casting Inclined table Historically, glass windows were not made in the way they Furnace rollers are now. There are three basic methods for producing flat glass: FIGURE 26.6 Continuous casting of fl at glass.

468 ...... Processing Glass and Glass-Ceramics Cooled glass Cooled Cooled Cooler Cooler glass glass

Pulling meniscus Bath

Bath of molten glass De´biteuse (A) (B) (C) FIGURE 26.7 Principle of drawing from molten glass: (a) a circular rod; (b) problem of pulling a planar sheet; (c) use of a débiteuse and cooling to allow the formation of a sheet of constant width.

26.7c. These coolers solidify the glass and produce a sheet In the Libbey–Owens or Colburn–Libbey–Owens of fixed width. (We will see a similar process used in process the drawn glass sheet is bent through 90°, ∼1 m crystal growth in Section 29.6.) above the surface of the bath, on a polished chrome-nickel There are several variations on the drawing process alloy roll; there is no debiteuse. The glass then travels into and these are illustrated schematically in Figure 26.8, a horizontal lehr for annealing. which gives a different view of Figure 26.7. By the mid 1900s these three processes were In the Fourcault process the glass is drawn through a responsible for the world’s entire production of flat glass, slot in a clay block (called the debiteuse or draw bar). The with 72% being made by the Fourcault process, 20% glass sheet is pulled upward through a system of steel by the Colburn–Libbey–Owens process, and 8% by the rollers about 7 m high. The sheet at the mouth of the deb- Pittsburgh process. These processes have now been iteuse is cooled at its edges in order to retain the width. replaced almost entirely by the float process. As the glass travels upward it is annealed in a vertical lehr. Plate glass is flat glass that has been ground and pol- After annealing and cooling the glass can then be cut into ished to produce two perfectly plane and parallel faces sections. with a high quality optical finish. The surface of a flat In the Pittsburgh process, the draw bar is completely glass sheet is flattened by grinding it between two cast- submerged where it lowers the temperature of the glass iron wheels with sand abrasive and water lubricant. As the below the meniscus. This modification allows increased grinding progresses, the particle size of the abrasive is drawing rates. decreased producing a very fine satin surface. The opera- tion is completed by polishing the sheet with a suspension (A) (B) of iron oxide on felt pads; this abrasive is known as rouge because of its color (see Chapter 36). The processing of Fourcoult Pittsburgh plate glass can be entirely mechanized with continuous simultaneous grinding and polishing on both faces. However, the float-glass process has almost entirely ended this older method of manufacturing plate glass. The quality of float-glass may not be quite as high as that of plate glass, but the cost is considerably lower because of the elimination of the mechanical grinding and polishing operations.

26.5 FLOAT-GLASS

The float process was developed in 1959 by Pilkington in the UK and revolutionized the flat-glass industry. It has (C) D been hailed as one of the major inventions on the twentieth century. Worldwide, there are only ∼170 float-glass plants, Libbey-Owens but these have a combined output of 3000 miles of flat glass, 4–8 feet wide, each day. Annually these plants produce the equivalent of a ribbon of glass over one FIGURE 26.8 Drawing processes for window glass. (a) Fourcault, million miles long. Over 90% of the world’s window glass (b) Pittsburgh, and (c) Libbey-Ownes. is produced using the float process.

26.5 Float-Glass ...... 469 Heating zone Flame polish zone

Cooling zone Molten glass Burners

Annealing lehr Cutting

Melting tank Rollers Metal bath FIGURE 26.9 Schematic diagram of the fl oat glass process.

A schematic of the float process is shown in Figure and the maintenance of constant surface tension, which 26.9. Molten glass is fed from the furnace between two controls the thickness of the sheet. The atmosphere is rollers onto a bath containing molten tin at about 1000°C approximately 90% N and 10% H. The hydrogen ensures (the melting temperature of tin is 232°C). The tin bath is that there is an oxygen-free environment: molten tin is 4–8 m wide and up to 60 m long. Equilibrium between easily oxidized. The glass can, itself, be affected adversely gravitational forces and surface tension produces a sheet by the presence of oxygen, which can make the surface of uniform thickness. The equilibrium thickness, he, can appear hazy. Purity and reliability are therefore critical be calculated using an equation derived by Langmuir: factors in float-glass manufacture. Approximately 5 m3 of hydrogen is required for each ton of glass produced. 1/ 2 (γGv+ γ Gt + γ tv ) 2 ρ t  he =   (26.3)  gρG( ρ t− ρ G )  26.6 GLASSBLOWING There are three interfacial energy ( γ) terms between the different phases glass (G), air (v), and tin (t); g is The classic glassblowing pipes are made from iron tubes the acceleration due to gravity, which has a value of about 100–150 cm in length with an opening about 1 cm in 2 9.806 m/s , ρG and ρt are the density of the glass and tin, diameter. At one end the tube had a mouthpiece and at the respectively. other end it has a button- In practice, it is possi- like extension. The glass- ble to produce glass thick- GLASS THICKNESS maker gathers a gob of It is instructive to estimate the value of he. A reasonable nesses between about 2 mm 2 molten glass on one end and 20 mm using the float value for the glass/Sn interfacial energy is ∼1 J/m , the then blows through the surface free energy of molten tin is 0.68 J/m 2, the density process, depending upon 3 mouthpiece to form a the glass viscosity and the of Sn is 7.5 g/cm , the surface free energy of a molten hollow shape as demon- soda-lime-silicate glass is 0.35 J/m 2, and the density for drawing speed. At the 3 strated in Figure 26.10. exit, the temperature of a silicate glass is 2.5 g/cm , hence he is ∼9 mm. This This process was important the glass is decreased to calculated value is actually slightly higher than the historically because by ∼600°C, at which point actual equilibrium thickness, ∼7 mm. Remember that we blowing glass with a pipe it the tin is still fluid, but are using only an estimate for the energy of the glass/Sn was possible not only to the glass can be removed interface. produce simple round in a “rigid” condition. The shapes but also other thin- glass ribbon leaves the bath and enters the lehr for anneal- walled objects. By blowing the glass into a wooden mold it ing. The glass leaves the lehr at ∼200°C, is cooled to room became possible to produce items with a standard reproduc- temperature, and is cut to size. The main advantage of the ible shape. The glassblowing pipe was also the first step in float process is the production of planar glass sheets with making flat glass. The glass was blown into a large cylindri- a high optical quality—the planarity approaches that of cal body, cut along its length, and “ironed” flat while it was plate glass without the need for polishing. Moreover, the still hot and soft. This is known as muffle glass and is still output rate is 5–10 times higher than the drawing rate for used today to make traditional stained glass. window glass. About 25 t of finished glass can be pro- Today the glassworker’s blowpipe is very similar to the duced per hour by the float process. original design and works in the same way. The original On the negative side, the equipment requires careful blowing technique is used now mainly for specialty glass control of the atmosphere above the bath, which must be applications such as glass ornaments and studio-art neutral or slightly reducing to avoid oxidation of the bath glass.

470 ...... Processing Glass and Glass-Ceramics To produce hollow glassware for bottles and jars the original blowing technique has been mechanized. The process starts by forming a blank or preform (called a parison) by blowing into or pressing a gob of glass as illustrated in Figure 26.11. (The hole into the furnace is called the parison hole; traditionally, the glassblower would gather a glob, or gob, of glass on the blowpipe and roll it to make the parison.) After the initial shaping process, the parison is blown by compressed air to the final shape. With today’s automated techniques, from parison to final product can take less than 10 seconds.

᭿ The blow-and-blow process is used to make narrow- neck containers (Figure 26.11a). ᭿ The press-and-blow process is used to make wide- mouth jars (Figure 26.11b).

Figure 26.12 shows the ribbon machine developed in 1926 by Corning for high-speed production of light bulbs. In this process a stream of molten glass is made into a ribbon by passing it through a pair of rollers. The ribbon

FIGURE 26.10 Demonstration of blowing glass into a mold. Note the worker’s bench, the annealing burners, and the box of frit.

(A)

(B)

FIGURE 26.11 Automatic fabrication of hollow ware: (a) formation of the blank by blowing followed by blowing in a mold (blow and blow); (b) formation of the blank by pressing followed by blowing (press and blow).

Blow heads Glass ribbon Blow box Blow box Stripper

Water-cooled Crack-off rollers bar Blow heads Rotating Glass bulbs paste molds Ware Rotating conveyor Mold-closing cam Mold-opening cam paste molds FIGURE 26.12 The Corning ribbon machine for glass lamp bulbs.

2 6.6 Glassblowing ...... 471 passes under a series of blowheads that are attached to a Absorption glass Normal glass rotating turret (the blowhead turret). Air is puffed into the Incident glass and the glass begins to take the form of a blown light shell. The glass shell is then enveloped by a mold mounted Direct Long wave transmission on a rotating turret below the blowhead turret. The shells radiation & convection are blown to their final form while in the mold. The fin- Absorption ished shells are cracked away from the ribbon, collected, and carried through the annealer. More than 50 light bulb shells (in single molds) can be made per second in one Secondary emission machine!

Gold coating Incident 26.7 COATING GLASS light Direct ion transmission lect Coated glass also has a long history. Stained glass used in Ref church windows has long used flashed glass, which is clear Absorption glass with a coating of a colored glass. Today we use many other types of coatings, but flashed glass is still used in Secondary the glass studio (see Section 26.14). emission Normal window glass transmits between 75% and Long wave radiation & 90% of the incident radiation. We can apply thin coatings convection to the surface to alter the radiation and heat-transmission Interference coatings characteristics. In architectural applications, coatings on Incident glass are used to reduce the amount of energy needed to light Direct heat buildings in the winter and cool them in the summer. ion transmission lect One way to do this is by allowing the transmission Ref of visible light while reducing the transmission of infrared (IR) radiation (heat). Such glass would reduce the amount of energy required to air condition a room in the summer. One conventional way to reduce the transmission of heat is to add a component to the glass batch that strongly absorbs radiation in the IR region. When FeO is added to FIGURE 26.13 Illustration of approaches for solar protective the glass formulation, the product strongly absorbs radia- double glazing with absorbing and refl ecting panes. tion in the 0.7–2.5 µm range. Since the absorption sharply increases above 0.6 µm, the transmitted visible light does have a noticeably green tint. But this can be corrected to tivity. The metal layer is deposited using physical vapor a certain extent by adding other components (commonly deposition (PVD) techniques such as evaporation and Se and Co). A 6-mm-thick glass window that contains sputtering (see Chapter 28). Metal coatings can also be FeO may absorb ∼50% of the total incident radiation but applied using dipping and spraying techniques, followed only ∼25% of the radiation in the visible part of the elec- by firing to densify the coating (see Chapter 27). tromagnetic spectrum. Single-layer and multilayer coatings of oxides are also Thin films are also used to control the light- used to enhance reflectivity in the near IR. These coatings transmission characteristics of window glass by reflecting work because of optical interference effects and usually a large amount of IR radiation. have a thickness about one-quarter of the wavelength of Such films allow a higher retention of heat in a room the radiation such that the primary wave reflected off the during the winter, thus reducing the energy costs associ- first interface is 180° out of phase with the secondary wave ated with heating. Normal flat glass can be coated with reflected from the second interface. The result is destruc- metallic or nonmetallic layers to maintain a high degree tive interference of the two waves. Thin films of TiO2, of visible light transmis- Ti 3N4, CrN x, and Zn 2SnO 4 sion combined with a con- are all used commercially siderable amount of heat MULTILAYER COATING for this application. reflection (40–60%). Thin An example of the effectiveness of a multilayer coating, Figure 26.13 shows a (10–20 nm) metal films of which works on the interference principle, is the Sb2S3/ schematic diagram sum- Cu, Ag, and Au are all CaF 2 superlattice. For 11 alternating layers of Sb 2S3 marizing the principles used commercially; each (n = 2.70) and CaF 2 ( n = 1.28), the reflectance is 0.9999 behind solar radiation gives a very high reflec- at λ = 1 µm. control panels. For solar-

472 ...... Processing Glass and Glass-Ceramics protective double glazing, a coated glass pane is combined that used to make crackle glass: the internal stress at the with an uncoated pane. surface is different from that in the bulk. Radiation can also be modified using electrochromism. The other form of safety glass is laminated glass, Electrochromism is the production of color by applying which consists of two or more glass sheets (usually float an electrical field. Electrochromic compounds such as glass) joined with a layer of an elastomeric polymer such

WO 3 are coated on the glass using a variety of thin film as poly (vinyl butyral) (PVB). The glass layers are bonded techniques. The point defect reaction is reversible. to the polymer by a combination of pressure and heating. When laminated glass is broken, the broken pieces of glass are stuck to the polymer and the broken sheet remains WO + xM+ + xe − = M WO (26.4) 3 x 3 transparent. Car windshields are laminated glass in which the two bonded sheets are both tempered glass. If a stone When a direct current (dc) voltage is applied, the optical breaks the windshield when the car is moving, the polymer absorption characteristics of the compound change. The sheet holds the fragments of glass together so that they do gain/loss of color occurs by the inclusion/elimination of not cause injury to the occupants of the car. The side and the M + ion. In this case, the M + ion can be H +, Li +, Na +, rear windows of an automobile are routinely made from or Ag +. The absorption characteristics of electrochromic tempered glass. materials can thus be tailored for specific applications The front and rear windows of an automobile are often simply by adjusting the applied voltage. curved. The flat glass is shaped before the tempering process because the tempered condition would disappear at the bending temperature. One of the advantages of 26.8 SAFETY GLASS laminated glass over tempered glass is that laminated glass can be processed further; for example, it can be cut and drilled. Bulletproof glass is thick laminated glass Flat glass often breaks due to impact or to the application (25–60 mm thick) consisting of at least four layers of glass of a relatively low applied pressure. At most temperatures, laminated with an elastomeric polymer. the failure occurs in a brittle manner and results in the production of a number of sharp fragments, which, of course, can cause serious injury. Special treatment of the glass can make it less susceptible to breaking and can thus 26.9 FOAM GLASS reduce the chance of injury. Hence the United States and many other countries require the use of safety glass in Because we do not usually want glass to contain large buildings and vehicles. There are two forms of safety numbers of bubbles, we reduce the number of bubbles glass: in the glass melt using the fining process. The opposite approach is used to make foam glass: additional gases or ᭿ Tempered gas-bearing materials are added to the melt producing a ᭿ Laminated large quantity of bubbles. One of the major applications of foam glass is in the manufacture of insulating panels for buildings. Insulating Tempered glass (or toughened glass as it is also often panels made from foam glass can be light, rigid, and good called) is made by quickly heating flat glass thermal insulators. As usual for porous materials, the to about 150°C above T . (Remember: the viscosity at the g thermal property is due to T is ∼10 13 dPa·s.) For a g the low heat conductivity of soda-lime silicate glass CRACKLE GLASS the bubbles (i.e., pores). 150 + T is 525–545°C. The g Crackle glass has been produced for its artistic appear- glass is then blasted with ance for over 150 years (perhaps since the sixteenth cold air. The outside of the ᭿ Foam glass is the porous century in Venice). The glass is heated to ∼1000°C and glass cools much more ceramic of the glass then plunged into water causing cracks to form across quickly than the inside. world. the surface; if reheated and blown further the cracks Hence the inside is still ᭿ Bubbles may be inten- heal on the interior; a similar effect is produced in contracting after the tionally included in art glazes. surface has already solidi- glass. fied. The result is that the outer surface layer is subject to compression and the inside layer is subject to tension. This tempering process changes 26.10 SEALING GLASS the behavior of the glass when it breaks. For example, when a sheet of tempered glass breaks, the fragments are Special glasses have been developed that are used as the small and almost regularly shaped with no sharp edges. “glue” usually between another glass and a metal. The The principle used in the tempering process is the same as main challenge is engineering α while controlling

26.10 Sealing Glass ...... 473 the chemistry and keeping the sealing temperature 26.12 PHOTOCHROMIC GLASS reasonable. PHOTOSENSITIVE AND PHOTOCHROMIC Photosensitive glass is the Borosilicate (alkaline Photosensitive: the glass is sensitive to light. glass analog of photo- earth/alumina) glass is Photochromic: the color is changed by exposure to graphic film where Au or used to seal glass to W light. Ag atoms (as halide parti- (lighting applications). cles) are affected by the Borosilicate (Na) glass is action of light. Such glass used to seal glass to Mo. is used in printing and image reproduction. Heat treating Borosilicate (K) glass, with a high concentration of B after exposure to light can cause a permanent change in the (20% B 2O3), is used to bond to Kovar (Westinghouse’s glass. Photochromic glass (discovered at Corning in the trade name for the Fe–Ni–Co magnetic alloy that has 1960s), darkens when exposed to light but then returns to α close to that of the glass). its original clear state when the light is removed; hence it Pb glass is used if electrical insulation of a joint is is used in sunglasses or other ophthalmic lenses. The light

critical. affects small ( ∼5 nm or a little larger) AgCl xBr 1−x crystals in the glass. To manufacture photochromic glass the glass must be heated to over 1400°C, shaped, and then annealed 26.11 ENAMEL again at 550–700°C to allow the small halide crystals to form in the initially homogeneous glass. Large flat glasses Ceramic enamel compositions are used for a wide variety are not generally affordable, but the new approach is to use of applications, including decorative coatings for glass- sol-gel processing (Chapter 22) to form a photochromic ware and china. In industry they are used in coating baths, layer on conventional preformed glass sheets. stovetops, etc. They are used whenever a metal requires a The principle is that the near-UV radiation generates coating that is more durable than paint. It is also used to electron-hole pairs in the halide particles. The electrons form colored borders around glass sheets used as automo- are trapped by the interstitial Ag ions producing Ag col- tive windshields. The reason for the colored borders is to loids on the surface of the halide particle. The activation enhance the appearance and to decrease the degradation energy for this process is only 0.06 eV so the Ag ion moves of the underlying adhesives by ultraviolet (UV) radiation. easily in the halide; in addition, a small amount of Cu The enamel compositions consist of a glass frit (a pow- increases the effect by at least an order of magnitude. This dered glass), a colorant, and an organic vehicle. The is the same process used in photography and is explained mixture is applied to the required part and then fired to by the same Gurney–Mott theory. The metal particles burn off the organic vehicle and to fuse the enamel to the formed in the process are 1–5 nm in size, but because the surface of the substrate. halide must be >5 nm to show the effect the second anneal- In the art world, enameling is carried out using small ing T is critical. furnaces and grams of powdered glass; in industry, square meters of surface can be enameled uniformly. The prin- ciple is the same, only the scale differs. A large piece like 26.13 CERAMMING: CHANGING GLASS a cast-iron bath would be shot-blasted to clean it, coated TO GLASS-CERAMICS with a ground coat that will bond to the Fe, dried, and heated to nearly 1000°C. It is then sprinkled with dry Worldwide sales of glass-ceramic products exceed $500 enamel frit while still hot and brought back to tempera- million a year. The highest volume is in cookware and ture. Five such coatings of enamel would be routine. tableware consumer items (such as plates and bowls) and The composition of the enamel must be chosen so that domestic ovens (stovetops and stove windows). In this it is in compression after cooling. The composition section we will look at how these commercially important would be modified on curved regions to maintain this ceramics are processed. compression. Glass-ceramics are formed by controlled crystalliza- Color is produced in ways similar to other glass. (We tion of a glass. They consist of a high density (maybe discussed color in glass in Section 21.8.) As recently as >95 vol%) of small crystals in a glass matrix. The impor- 1999, the French Consumer Safety Committee considered tant feature of the processing of glass-ceramics is that the the safety of the use of UO 2 dye to make enamels for the crystallization must be controlled. As usual, crystalliza- preparation of jewelry and enameled tiles. Old yellow tion occurs in two stages: enamels contained up to 9% UO 2 and frits using nonde- pleted U are certainly more “radioactive” than those of ᭿ First the crystals are nucleated. today. The committee noted that having 50% Pb in the frit ᭿ Then the crystals grow. might help absorb the radiation, but this increased the tox- icity of the frit powder! Safety in enameling is therefore The rate at which these two processes occur is a function always a concern because the technique uses powders. of temperature. We can control the nucleation process by

474 ...... Processing Glass and Glass-Ceramics adding a nucleating agent ION EXCHANGE β-quartz. The β-quartz (typically either TiO 2 or Glass can be strengthened by ion exchange in which crystals are <0.1 µm in ZrO 2) to the glass batch. small ions in the surface layer are replaced by larger diameter. Crystal sizes Initially the glass batch ions, which put the surface into compression. below the wavelength of is heated to form a homoge- + + + + visible light ( λ = 0.7– Step 1 : Rapid exchange of Na or K with H or H 3O neous melt. The shape from solution: 0.4 µm), a good match in of the desired object is refractive index between formed from the glass at the glass and the crystal Si–O–Na + + H + + OH − → Si–OH + + Na + ( solution ) + OH − the working point by the phase, coupled with a low usual processes such as This step is usually controlled by diffusion and exhibits birefringence result in high pressing, blowing, rolling, t−1/2 dependence. transparency. Table 26.4 or casting. Remember, the Step 2 : Loss of soluble silica in the form of Si(OH) , lists the compositions and viscosity of the glass is 4 applications of some com- 4 resulting from breaking of Si–O–Si bonds and the ∼10 dPa-s at the working formation of Si–OH (silanols) at the glass solution mercial transparent LAS point (like honey). After interface: glass-ceramics based on annealing to eliminate β-quartz. internal stresses, the glass Si–O–Si + H O → Si–OH + OH–Si object then undergoes a 2 Application 1 : Machin- thermal treatment that con- This stage is usually reaction controlled and exhibits a able glass-ceramics verts it into a glass-ceramic. t1.0 dependence. are derived from the The first part of the heat Step 3 : Condensation and repolymerization of an K 2 O–MgO–Al 2 O 3 – treatment is nucleation. The SiO -rich layer on the surface depleted of alkalis and SiO 2 system contain- optimum nucleation tem- 2 ing some fluorine. In alkaline-earth cations. ® perature generally corre- Step 4 : Migration of Ca 2+ and PO 3− groups to the Macor the crystalline sponds to a glass viscosity 4 phase is potassium flu- 11 12 surface through the SiO 2-rich layer forming a CaO– of 10 –10 dPa·s. During orophlogopite [KMg 3 P2O5-rich film on top of the SiO 2-rich layer, followed this step, which may last for by growth of the amorphous CaO–P O -rich film by (AlSi 3O10 F2)]. Phlogo- several hours, an extremely 2 5 pite is a mica mineral 12 15 3 incorporation of soluble calcium and phosphates from high density (10 –10 /cm ) solution. and the plate-like of nuclei forms. Following Step 5 : Crystallization of the amorphous CaO–P O mica crystals are ran- T 2 5 nucleation, is increased film by incorporation of OH −, CO 2−, or F − anions from domly oriented in the to allow growth to occur. 3 glass phase as shown in solution to form a mixed hydroxyl, carbonate, fluorapa- ® The crystal-growth step, tite layer. Figure 18.23. Macor like nucleation, may take can be machined to several hours. The optimum precise tolerances temperature for crystal growth is selected to allow for the (±0.01 mm) and into intricate shapes using conventional maximum development of the crystalline phase without steel tools: they can be drilled, cut, or turned on a lathe. viscous deformation of the object. Figure 26.14 shows the complete processing cycle for a glass-ceramic. The glass-ceramic process uses the same rapid form- ing techniques employed in glassworking, permitting the economical production of objects with complex shapes or thin walls that are difficult or impossible to produce with other, more traditional, ceramic-forming T( °C) Melting η techniques. 1600 Working (dPa.s) The most important and useful glass-ceramic system Shaping point 4 is the Li 2O–Al 2O3–SiO 2 (LAS) family that was first devel- 1250 ~10 oped at the Corning Company by Stookey. One commer- Growth ® cial LAS glass-ceramic is Pyroceram , which has 900 β-spodumene (LiAl[Si O ]; α = ∼ 10 −6/°C) as the major 2 6 Nucleation ~107.6 crystalline phase. The low α, together with excellent 800 high temperature stability and insensitivity to thermal Softening shock, means that these materials find applications as point cookware, benchtops, hot-plate tops, ball bearings, and 650 T ~1013.4 matrices for fiber-reinforced composites. g If the crystallization temperature is kept ≤900°C the t crystalline phase that is formed in LAS glass-ceramics is FIGURE 26.14 Processing cycle for a glass-ceramic.

26.13 Ceramming: Changing Glass to Glass-Ceramics ...... 475 TABLE 26.4 Base Compositions and Applications of Transparent Glass-Ceramics Based on Quartz Solid Solutions Composition, wt% Commercial

Material SiO 2 Al 2O3 MgO Na 2O K 2O ZnO Fe 2O3 Li 2O BaO P 2O5 F TiO 2 ZrO 2 As 2O3 application

Vision 68.8 19.2 1.8 0.2 0.1 1.0 0.1 2.7 0.8 — — 2.7 1.8 0.8 Transparent cookware Zerodur 55.5 25.3 1.0 0.5 — 1.4 0.03 3.7 — 7.9 — 2.3 1.9 0.5 Telescope mirrors Ceran 63.4 22.7 u 0.7 u 1.3 u 3.3 2.2 u u 2.7 1.5 u Black infrared transmission cooktop Narumi 65.1 22.6 0.5 0.6 0.3 — 0.03 4.2 — 1.2 0.1 2.0 2.3 1.1 Rangetops; stove windows u = unknown

Application 2 : Another commercial fluoromica glass- 4. Reexpose the glass to UV (no mask) and anneal; all ceramic called Dicor ® has been developed for dental is then polycrystalline, e.g., with 0.001% Au, 0.001–

restorations. Dicor has better chemical durability and 0.03% AgCl, or 0.001–1.00% Cu 2O. translucency than Macor. It is based on the tetrasilicic Before leaving the topic we should mention that this is mica, KMg Si O F , which forms fine-grained 2.5 4 10 2 not the only way to prepare glass-ceramics. Another group (∼1 µm) anisotropic fl akes. Dicor dental restorations of these materials used for dentistry go by the label In- are very similar to natural teeth both in hardness and Ceram ®. The material is prepared as a porous crystalline appearance. They are easy to cast using conventional ceramic (alumina, spinel, and zirconia have all been used) dental laboratory methods and offer significant advan- and then infiltrated with an La-rich glass. The resulting tages over traditional metal–ceramic systems. glass-infiltrated ceramic can be quite translucent, can be Application 3 : Calcium phosphate, Ca (PO ) , glasses can 3 4 2 colored to match other teeth, and is mechanically tough be made into glass-ceramics to form a material resem- (for a ceramic). It can be coated with porcelains for bling the mineral part of bone. Since bone is porous, improved appearance. the first step is to produce a foam glass. This is achieved by decomposing carbonate in the glass melt. The foam glass simultaneously undergoes a controlled crystalli- 26.14 GLASS FOR ART AND SCULPTURE zation, transforming it into a porous glass-ceramic. The dimensions of the interconnections between the Today, the glass art world is very active with more abstract pores must be sufficient to allow the ingrowth of living pieces by Chihuly and others being hung in many public bone tissue, which ensures a permanent joint with the buildings (e.g., the Victoria and Albert Museum in London; surface of the prosthesis. Figure 26.16). Exhibitions travel round the United States and the world. The field is actually much wider than you In principle, any glass can be converted to a glass- might imagine. ceramic by finding a suitable nucleation agent and the The glass harp produces the “sound of the universe” appropriate heat treatment. In practice, a number of tech- according to Goethe. The musician Gluck gave an entire nical factors (among which is the production of the base concert using the glass harp in London in 1746. Sound is glass) limit the choice. We can use the phenomenon of photosensitivity to Mask make a machinable glass! For example, we could use CorningWare (Li 2O·Al 2O3·SiO 2) as the starting material Au+ and then add <1 wt% Cu, Ag, or Au. The idea behind the technique is similar to the photochromic glass, but we UV Au ° then leach out the Li 2SiO 3 crystals to leave a high density of small voids in the glass. Au+ 1. Irradiate the glass with UV (Figure 26.15). 2. Anneal Au 0 agglomerates (small particles); nuclei for

crystalline ceramic Li 2SiO 3 (2SiO 2·Li 2O). Au ° 3. Place the glass in dilute HF to dissolve the Li2SiO 3 (dissolution is 10 × faster than for the glass). This Au+ method can produce densities of 3.6 × 10 4 holes/square inch. FIGURE 26.15 Process for making porous glass.

476 ...... Processing Glass and Glass-Ceramics FIGURE 26.17 Millefi oré paperweight showing the different glass canes used.

FIGURE 26.16 Chihuly installation at the Victoria and Albert Museum in London. Art glass, usually in the form of fi gures and vases, became popular in the 1800s with Lalique, Gallé, Daum Frères, Tiffany, and others made when a wine glass MARBLES as illustrated in Figure “sings”: run your finger Marble King in Virginia makes 10 6 marbles every day. 26.18. around the rim. The sound Between 1886 and is thus produced by making 1936, the father and son the glass vibrate rather than by making the air in the team of Leopold and Rudolph Blaschka produced the container vibrate (like blowing over a bottle). Benjamin Harvard glass flowers (shown in Figure 26.19) for use in Franklin made a glass armonica (Mozart wrote music for teaching botany (there are other collections, but this is the the armonica): he placed glasses inside one another (not best known). Tiffany produced similar specimens for the touching) and rotated them on a spindle to make playing easy! Galileo actually explained the resonance phenome- non in 1638. The tone will depend on the mechanical (elastic) properties of the glass. Murano, a small island near the city of Venice, was the center of glass making for many years. The location was chosen to stop glassworkers from moving away from the vicinity of Venice. It is still famous for its art glass and the glass companies provide free boat taxis from the mainland to encourage customers. The millefiore insets seen in paperweights and glass sculptures, and illustrated in Figure 26.17, are usually associated with Murano. Actually, the technique had already been used in the first century ce . Paté de verre is an art glass made by sintering pieces of glass, including powder, to make objects with distributions of color that would not be possible if the glass were melted. Essentially, the millefiore glass of Murano is made by this technique but now with larger “particles.” It is now being used by industry for the same purpose. The classical sinter- ing technique for processing ceramic powders is thus being used to produce unique glass objects. FIGURE 26.18 Small vase by Daum Frères.

26.14 Glass for Art and Sculpture ...... 477 developed in the early twelfth century stained glass has been made using several different techniques. Pot-metal glass is bulk glass that has been colored by adding metal oxides in the usual way (Cu for red, Fe for green, Co for blue, etc.). Since the glass must be thick enough for the window, light transmission was often reduced (making the church dark). Flash glass is produced by dipping a gob of clear glass into a pot of colored glass. When the glass is then blown, the two layers are thin. The colored layer remains only a fraction of the total thickness. If the glass were uniformly colored but not flashed, it would transmit too little light (old church windows make for dark interiors); if the glass were uniformly colored but thin so as not to decrease the light it would have insufficient mechanical strength. The layer can be abraded to produce variations in color on the one piece of glass. Details in the window images can be drawn on the glass using an iron oxide pigment. The drawing is then fired to incorporate it into the glass.

FIGURE 26.19 Example of a Harvard fl ower. 26.15 GLASS FOR SCIENCE AND ENGINEERING art market. The idea is that the color of glass does not Lenses have been made using glass since microscopes and change with time and the flowers are three-dimensional. telescopes were first invented. When visible light is used, Actually, the Blaschkas used wire, paint (especially in the this is still generally the case. For transmitted light, the early years), varnish, and glue to enhance the models challenge is usually to minimize absorption; some tele- at the time. Thus they will not be quite as inert to the scopes, such as that on Mount Palomar, use a very wide atmosphere as hoped, but they are superb examples of coated glass reflector. The critical factor is now not trans- lampworking (heating a soda-rich glass and shaping it by mission but thermal expansion. pulling and pinching with various tools including Stepper lenses are used in producing very large-scale tweezers). integration (VLSI) chips using photolithography. The In some countries, the best-known form of art glass is stepper lens is the opposite of an enlarging lens—it shrinks the stained-glass window (Figure 26.20). Since it was first the circuit pattern and prints it onto the semiconductor surface. (The term stepper is used to denote the step-and- repeat exposure system.) In 1997 the line width was 0.25 µm; it is now around 60 nm. The challenge then is to focus light, which has a wavelength much larger than the required resolution. The Hale Telescope is a 200-inch telescope that was built in ∼1949 using a Pyrex blank manufactured by Corning Glass works. Schott Glass has since produced a telescope with a dish that is 8.2 m wide; it is made from Zerodur ®, Schott’s glass ceramic, using a centrifugal casting process. An interesting link between art and science is the proposal by David Hockney that some of the masters used mirrors and lenses to help compose their paintings. Glass lenses range in size from the end of a fiber used in near-field scanning optical microscopy (NSOM) (see Chapter 10) to the Fresnel plate that was used in lighthouses. Glass is one of our most important building materials. In 1851, the windows in the Crystal Palace used 300,000 panes, which accounted for 33% of England’s annual pro- duction of glass. Biosphere 2 (built in the late 1980s) relies FIGURE 26.20 Example of drawing in stained glass. entirely on glass to separate the interior from Biosphere 1.

478 ...... Processing Glass and Glass-Ceramics Glass fibers are used FORMING MICROSPHERES Traffic signs and road in textiles for clothing. 1. Glass melting marking tape contain The spacesuits used on a. Select chemically pure raw materials (oxides) that do spheres that are ∼200 µm the Apollo lunar missions not contain any impurities that would form undesirable in diameter and act as were made from glass fiber radioisotopes during neutron irradiation. microlenses. industrial fabrics. b. Mix the raw materials. In the Ballotini fl ame Glass microspheres are c. Melt the raw materials to form homogeneous glass. drawing process, glass used, for example, in traffic 2. Spheroidization (microsphere formation): powder is injected into a signs and drug delivery. We a. Crush the glass to particles of the desired size. fl ame that draws it up a will describe radiotherapy b. Inject particles into a gas-oxygen fl ame to melt each tower until it melts and glasses used in the treat- particle and form a solid glass sphere (fl ame spray forms spheres (1–60 µm ment of unoperable cancers powder). diameter) dropping out- in more detail in Chapter c. Collect microspheres in a suitable container. side the fl ame and cooling 35. The key process is 3. Sizing: screen or separate microspheres into desired size under gravity. Filaments forming uniform glass range. used to make the biprism spheres. We willalso discuss for electron holography Bioglass in Chapter 35 but are made in the laboratory not ceramic cremation tiles for after-life applications. using a variation on the same principle.

CHAPTER SUMMARY The production of glass is a multibillion dollar industry. The widespread use of glass products has gone hand-in-hand with developments in glass processing. Arguably the most significant recent development was the float glass process, which allowed high-volume production of very high-quality glass sheets for architectural and transportation applications. Our city skylines certainly are influenced by the use of glass. More recent developments have led to improve- ments in the energy efficiency of buildings by controlling the transmission and absorption properties of glass. This is part of the “green” revolution in architecture. Glass processing, which is an industry and art, has its own language. It is necessary to learn that language: in particular, frit, cullet, lehr, fining, gob, and ribbon. The processing terms are from engineering: fl oating, rolling, drawing, tempering, laminating, and ceramming.

PEOPLE IN HISTORY Abbe, Ernst (1840–1905) was a professor of optics who worked with Schott, the glassmaker, and Zeiss, the lensmaker, in Jena. Burne-Jones, Sir Edward Coley (1833–1898) designed stained glass working with William Morris. Gallé, Émile (1846–1904) was a pioneer in the Art Nouveau movement; he used wheel cutting, acid etching, casing (i.e., layers of various glass), including metallic foils and air bubbles. He called his experiments “marquetry of glass.” When you see heavy pieces of deeply colored, nearly opaque glasses, which use several layers of glass and are then carved or etched to form plant motifs, you think of Gallé. Lalique, René (1860–1945) was one of the creators of Art Nouveau glass. Etched clear glass is a particular trademark. Littleton, Harvey K. (born 1922 in Corning, New York) was the University of Wisconsin professor who introduced Chihuly, Lipofsky, and others to glass and began the American Glass Movement. Owens, Michael (1859–1923) invented the automatic bottling machine. Schott, Otto (1851–1935) see Abbe. Stookey, S. Donald (another exception to the rule) is one of the greatest scientific creators from Corning Glass. Trancrede de Dolomieu, Guy S. (1750–1801) was a French professor of mineralogy. The mineral dolomite is named after him. Zeiss, Carl Friedrich (1816–1888) gave his name to the optics company that was, for many years, based in Jena.

GENERAL REFERENCES Bach, H. and Krause, D. (Eds.) (1999) Analysis of the Composition and Structure of Glass and Glass Ceram- ics , Schott Series on Glass and Glass Ceramics, Springer, Berlin. Bach, H. and Krause, D. (Eds.) (1997) Thin Films on Glass , Schott Series on Glass and Glass Ceramics, Springer, Berlin.

Chapter Summary ...... 479 Bach, H. and Neuroth, N. (1995) The Properties of Optical Glass , Schott Series on Glass and Glass Ceramics, Springer, Berlin. Bach, H., Baudke, F.G.K., and Krause, D. (2000) Electrochemistry of Glass and Glass Melts , Schott Series on Glass and Glass Ceramics, Springer, Berlin. Beall, G.H. (1992) “Design and properties of glass-ceramics,” Annu. Rev. Mater. Sci . 22 , 91. For more infor- mation about the different types of glass-ceramic written by the expert. Ellis, W.S. (1998). “The story of the substance that changed the world,” Glass . Avon, New York. Includes more on the glass harp and marbles. McMillan, P.W. (1979) Glass-Ceramics , 2nd edition, Academic Press, London. The standard glass-ceramics text. Pfaender, H.G. (1983) Schott Guide to Glass , Van Nostrand Reinhold, New York. A concise overview of glass-forming processes. Schultes, E.S. and Davis, W.A. (1982) The Glass Flowers at Harvard , Botanical Museum of Harvard University, Cambridge, MA. Tooley, F.V. (1983) The Handbook of Glass Manufacture , 3rd edition, Volumes I and II, Ashlee Publishing Co., New York. A comprehensive manual for industrial glass formation. Designed primarily for the person in industry. Waseda, Y. and Toguri, J.M. (1998) The Structure and Properties of Oxide Melts , World Scientific Pub. Co., Singapore. A compact book concerned primarily with silicate melts. Varshneya, A.K. (1994) Fundamentals of Inorganic Glasses , Academic Press, Boston. One of the most com- prehensive books on glasses. Chapter 20 is on the fundamental aspects of glassmaking. Zarzycki, J. (1991) Glasses and the Vitreous State , Cambridge University Press, Cambridge. This book was originally published in French as Les Verres et l’Etat Vitreux in 1982. The English edition does include an updated bibliography and some additional information not in the original version.

SPECIFIC REFERENCES Langmuir, I. (1933) “Oil lenses on water and the nature of monomolecular expanded films,” J. Chem. Phys . 1, 756. McNally, R.S. and Buschini, N. (1993) “The Harvard glass flowers: Materials and techniques,” JAIC 32 , 231. Miller, J. (2004) 20 th -Century Glass , Dorling Kindersley Ltd, London. Among the best.

EXERCISES 26.1 Describe how cords and stones could be removed. 26.2 How much glass in your area is recycled as cullet? 26.3 Why did ancient glass batches contain so much alkali content? 26.4 Why is tin used in the float glass process? 26.5 We process the glass to remove bubbles and thus avoid stresses in the glass. Is this statement true or false? Explain your answer. 26.6 On ceramming a glass, we produce a density of 10 15 /cm 3 nuclei. How far apart are the nuclei and how does this affect the resulting grain size? 26.7 Why has the art of glass produced so much new interest? Is it new? Discuss. 26.8 Figure 26.1 shows the distribution of glass sales. Using the internet or otherwise, compare these with the fi gures for Al, steel, and Cu. Discuss your results. 26.9 Estimate the cost of an automobile’s windows assuming they are made from tempered glass. 26.10 What are the compositions of some of the common sealing glasses?

480 ...... Processing Glass and Glass-Ceramics 35

Ceramics in Biology and Medicine

CHAPTER PREVIEW Bioceramics are ceramics used for the repair and reconstruction of human body parts. There are many applications for bioceramics; currently the most important is in implants such as alumina hip prostheses. Alumina is classified as an inert bioceramic because it has very low reactivity in the body. However, bioactive materials have the ability to bond directly with bone. The advantages are

᭿ Earlier stabilization of the implant ᭿ Longer functional life

Bioactive ceramics are relatively weak compared with common implant metals and high strength ceramics such as alumina and zirconia. As a result they are often used as coatings, relying on the mechanical strength and toughness of the substrate. An important bioactive ceramic is hydroxyapatite (HA). Natural bone is a composite in which an assembly of HA particles is reinforced by organic collagen fibers. Hydroxyapatite-reinforced polyethylene com- posites have been developed in an attempt to replicate the mechanical behavior of bone. A major problem with this topic stems from the realization that you cannot replace bone if you do not understand why bone has such incredible mechanical properties. So if you work in this field you must learn about biology.

35.1 WHAT ARE BIOCERAMICS? field of bioceramics is relatively new; it did not exist until the 1970s. However, many bioceramics are not new mate- A comprehensive definition of a biomaterial was provided rials. One of the most important is Al 2O3, which we first at the National Institutes of Health (NIH) Consensus encountered as a constituent of many traditional ceramic Development Conference on the Clinical Applications of products. Bioceramics are typically classified into sub- Biomaterials in the United States: groups based upon their chemical reactivity in the body. The subgroups are listed A biomaterial is any sub- in Table 35.1 and their rel- stance, other than a drug, or PROSTHESIS ative reactivities are com- combination of substances, A prosthesis is an artificial replacement for a part of the pared in Figure 35.1. synthetic or natural in origin, body. If a nearly inert mate- which can be used for any rial is implanted into the period of time, as a whole or body it initiates a protective response that leads to en- as a part of a system which treats, augments, or replaces any tissue, organ, or function of the body. capsulation by a nonadherent fibrous coating about 1 µm thick. Over time this leads to complete isolation of This definition was significantly simplified in 1986 the implant. A similar response occurs when metals at the European Society for Biomaterials Consensus and polymers are implanted. In the case of bioactive Conference: ceramics a bond forms across the implant–tissue interface that mimics the bodies natural repair process. Bioactive Biomaterial—a non-viable material used in a medical device ceramics such as HA can be used in bulk form or as part intended to interact with biological systems. of a composite or as a coating. Resorbable bioceramics, such as tricalcium phosphate (TCP), actually dissolve A bioceramic is defined as a ceramic used as a bioma- in the body and are replaced by the surrounding tissue. terial (which is great if you know what a ceramic is). The It is an important requirement, of course, that the

35.1 What are Bioceramics? ...... 635 TABLE 35.1 Classification Scheme for Bioceramics Skull: repair Nearly inert bioceramics Eye: replace lens Middle ear: repair orbit Examples : Al 2O3, low-temperature isotropic (LTI) carbon; ultra replace bones LTI carbon; vitreous carbon; ZrO 2 Simulate Jawbone: repair Tissue attachment : Mechanical hearing Preserve teeth Bioactive ceramics Repair mastoid bone Examples : HA; bioactive glasses; bioctive glass-ceramics Replace teeth: implants Tissue attachment : Interfacial bonding Resorbable bioceramics Anchor tooth implants: expand jawbone Examples : Tricalcium phosphate (TCP); calcium sulfate; trisodium phosphate Tissue attachment : Replacement Long bones: Composites replace segments Examples : HA/autogenous bone; surface-active glass ceramics/ poly(methyl methacrylate) (PMMA); surface-active glass/ Heart valves: metal fi bers; polylactic acid (PLA)/carbon fi bers; PLA/HA; use artificial PLA/calcium/phosphorus-based glass fi bers Tissue attachment : Depends on materials Vertebrae: replace & repair with spacers

Iliac crest repair after bone removal dissolution products are not toxic. As in the case of HA, TCP is often used as a coating rather than in bulk form. Fill bone It is also used in powder form, e.g., for filling space in space bone. Knee replace Figure 35.2 shows a number of clinical uses of Finger bioceramics. The uses go from head to toe and include joints repairs to bones, joints, and teeth. These repairs become Hip necessary when the existing part becomes diseased, replace totally or damaged, or just simply wears out. There are many other Tendons use revision surgery & ligaments: applications of bioceramics including pyrolytic carbon replace coatings for heart valves and special radioactive glass formulations for the treatment of certain tumors. We will describe these latter two applications toward the end of Foot joints: this chapter. In the next section we will look at the advan- repair tages and disadvantages of ceramics as biomaterials as compared to the use of polymers and metals. We note that nanomaterials show interesting possibilities for such appli- cations, but may pose as many health problems in other situations. FIGURE 35.2 Illustration of the head-to-toe clinical uses for bioceramics.

35.2 ADVANTAGES AND Relative Reactivity DISADVANTAGES OF CERAMICS Resorbable (e.g., TCP) In the selection of a material for a particular application we always have a choice. Materials selection is a critical Surface Reactive part of any component design process and is especially (e.g., bioGlass) important for implants and other medical devices. Nearly inert The three main classes of material from which we can (e.g., alumina) select for a load-bearing application are metals, polymers, and ceramics. Table 35.2 is a comparative list of the sig- 2 4 5 nificant physical properties of different biomaterials from 10 -1 1 10 10 10 3 10 10 t (days) each of the three classical material classes. Table 35.3 com- FIGURE 35.1 Relative reactivity of the different classes of pares the behavior of these different classes relevant to bioceramic. TCP is tricalcium phosphate. their potential use as implants.

636 ...... Ceramics in Biology and Medicine 35.2 35.2 v aes Diadantg o Cerami s ic m a r e C of s tage n dva isa D d n a s tage n dva A

TABLE 35.2 Significant Physical Properties of Different Biomaterials Fracture Compressive surface

Density UTS strength K 1c Hardness α energy Poisson’s k Material (g cm −3) (MPa) (MPa) E (GPa) (MPa m 1/2 ) (Knoop) (ppm/°C) (J/m 2) ratio (W m −1 K −1)

HA 3.1 40–300 300–900 80–120 0.6–1.0 400–4,500 11 2.3–20 0.28 TCP 3.14 40–120 450–650 90–120 1.20 14–15 6.3–8.1 Bioglasses 1.8–2.9 20–350 800–1200 40–140 ∼2 4,000– 5,000 0–14 14–50 0.21–0.24 1.5–3.6 A-W glass 3.07 215 1080 118 2 ceramic

SiO 2 glass 2.2 70–120 ∼70 0.7–0.8 7,000– 7,500 0.6 3.5–4.6 0.17 1.5

Al 2O3 3.85–3.99 270–500 3000–5000 380–410 3–6 15,000– 20,000 6–9 7.6–30 0.27 30 PSZ 5.6–5.89 500–650 1850 195–210 5–8 ∼17,000 9.8 160–350 0.27 4.11

...... Si 3N4 3.18 600–850 500–2500 300–320 3.5–8.0 ∼22,000 3.2 20–100 0.27 10–25 SiC 3.10–3.21 250–600 ∼650 350–450 3–6 ∼27,000 4.3–5.5 22–40 0.24 100–150 Graphite 1.5–2.25 5.6–25 35–80 3.5–12 1.9–3.5 1–3 ∼500 0.3 120–180 LTI-ULTI 1.5–2.2 200–700 330–360 25–40 1–10 0.3 2.5–420 Carbon fi ber 1.5–1.8 400–5000 330–360 200–700 Glassy carbon 1.4–1.6 150–250 ∼690 25–40 8,200 2.2–3.2 PE 0.9–1.0 0.5–65 0.1–1.0 0.4–4.0 170 11–22 500–8,000 0.4 0.3–0.5 PMMA 1.2 60–70 ∼80 3.5 1.5 160 5–8.1 300–400 0.20 Ti 4.52 345 250–600 117 60 1,800– 2,600 8.7–10.1 ∼15,000 0.31 Ti/Al/V alloys 4.4 780–1050 450–1850 110 40–70 3,200– 3,600 8.7–9.8 ∼10,000 0.34 Ti/Al/Nb/Ta 4.4–4.8 840–1010 105 50–80 ∼17,000 0.32 alloys Vitallium- 7.8–8.2 400–1030 480–600 230 120–160 3,000 15.6–17.0 ∼5,000 0.30

...... stellite alloys (Co–Cr–Mn) Low C steel 7.8–8.2 540–4000 1000– 4000 200 55–95 1,200– 9,000 16.0–19.0 ∼50,000 0.20–0.33 46 Fe–Cr–Ni alloys 637 TABLE 35.3 General Comparison of Materials for Implants Most bioceramic implants are in contact with bone. Bone is a living material composed of cells and a blood Material class Advantages Disadvantages supply encased in a strong composite structure. The com- Polymers Resiliant Weak posite consists of collagen, which is flexible and very Tough Low E tough, and crystals of an apatite of calcium and phosphate, Easy to fabricate Not usually bioactive resembling calcium hydroxyapatite; we will proceed as if Low density Not resorbable it is HA. It is the HA component that gives bone its hard- Metals Strong Can corrode in a Wear resistant physiological ness. The acicular apatite crystals are 20–40 nm in length Tough environment and 1.5–3 nm wide in the collagen fiber matrix. Two of the Easy to fabricate High E various types of bone that are of most concern in the use High density of bioceramics are Not usually bioactive Not resorbable ᭿ Ceramics Biocompatible Low tensile strength Cancellous (spongy bone) Wear resistant Diffi cult to fabricate ᭿ Cortical (compact bone) Certain compositions Low toughness lightweight Not resiliant Cancellous bone is less dense than cortical bone. Every bone of the skeleton has a dense outer layer of compact bone covering the spongy bone, which is in the form of a honeycomb of small needle-like or flat pieces called tra- beculae. Figure 35.3 is a schematic showing a longitudinal The main advantage of ceramics over other implant section of a long bone. The open spaces between the tra- materials is their biocompatibility: some are inert in the beculae are filled with red or yellow bone marrow in physiological environment while others have a controlled living bones. Because of its lower density, cancellous bone reaction in the body. The main disadvantages of most has a lower E and higher strain-to-failure ratio than corti- bioceramics are cal bone, as shown in Table 35.4. Both types of bone have higher E than soft connective tissues, such as tendons and ᭿ Low toughness (which can affect reliability) ligaments. The difference in E between the various types ᭿ High E (which can lead to stress shielding, see Section of connective tissues ensures a smooth gradient in mechan- 35.3) ical stress across a bone, between bones, and between muscles and bones. One of the main ways of increasing the toughness of ceramics is to form a composite. The ceramic may be the reinforcement phase, the matrix, or both. An example of a polymer–matrix composite (PMC) reinforced with a bio- ceramic is polyethylene (PE) reinforced with HA particles. The toughness of the composite is greater than that of HA E and, as we will see in Section 35.8, is more closely Compact matched to that of bone. Bioceramics are also used as bone coatings on metal substrates. An example is a bioactive glass coating on stainless steel, which utilizes the strength and toughness of steel and the surface-active properties of the glass.

Compact bone 35.3 CERAMIC IMPLANTS AND THE Spongy STRUCTURE OF BONE bone

The requirements for a ceramic implant depend on what

its role in the body will be. For example, the requirements Spongy for a total hip prosthesis (THP) will be different from bone those for a middle ear implant. However, there are two basic criteria:

1. The ceramic should be compatible with the physiologi- cal environment. 2. Its mechanical properties should match those of the FIGURE 35.3 Longitudinal section showing the structure of tissue being replaced. long bone.

638 ...... Ceramics in Biology and Medicine TABLE 35.4 Mechanical Properties of Skeletal Tissues Cortical Cancellous Articular Property bone bone cartilage Tendon

Compressive 100–230 2–12 strength (MPa) Flexural 50–150 10–20 10–40 80–120 strength (MPa) Strain to 1–3% 5–7% 15–50% 10% failure E (GPa) 7–30 0.5–0.05 0.001–0.01 1 1/2 K1c (MPa m ) 2–12 Compressive 20–60 stiffness (N/mm) Compressive 4–15 FIGURE 35.5 Medical grade alumina used as femoral balls in creep THP. The schematic shows how the femoral ball fi ts into the modulus socket. (MPa) Tensile 50–225 stiffness (MPa) 35.4 ALUMINA AND ZIRCONIA

The mechanical properties of the implant are clearly Al 2O3 and ZrO 2 are two nearly inert bioceramics. They very important. Figure 35.4 compares E of various implant undergo little or no chemical change during long-term materials to that of cortical and cancellous bone. exposure to body fluids. High-density, high-purity (>99.5%) alumina is used in a number of implants, par- ticularly as load-bearing ᭿ E of cortical bone is hip prostheses and dental 10–50 times less than STRESS SHIELDING 6 that of Al O . E implants. By 2006, >10 2 3 This occurs when a high- implant material carries hip prostheses used an ᭿ E of cancellous bone is nearly all the applied load. several hundred times alumina ball for the less than that of Al O . femoral-head component. 2 3 Figure 35.5 shows three femoral components of THP with If the implant has a much higher E than the bone it is alumina balls. The U.S. Food and Drug Administration replacing then a problem called stress shielding can occur. (FDA) approved the use of alumina in this type of applica- Stress shielding weakens bone in the region at which the tion in 1982. applied load is lowest or is in compression. (Bone must be Although some alumina dental implants are made loaded in tension to remain healthy.) Bone that is unloaded from single crystals, most alumina implants are very fine- or is loaded in compression will undergo a biological grained polycrystalline Al 2O3. These are usually made change that leads to resorption. Eliminating stress shield- by pressing followed by sintering at temperatures in the ing, by reducing E, is one of the primary motivations for range of 1600–1800°C. A small amount of MgO ( <0.5%) the development of bioceramic composites. is added, which acts as a grain growth inhibitor and allows a high-density product to be achieved by sintering without

400 Bioinert implants Bioactive implants Bone Bio- E composites

(GPa) Bone cement 3 O ®

200 2 Al 2 ® Bioactive composites ZrO Co-Cr Alloys Co-Cr 45S5 Bioglass Cortical bone 316L Steel 316L HA PMMA Cancelous bone A/W Glass-Ceramic A/W Ti Alloys Ti 0 Ceravita FIGURE 35.4 Young’s modulus ( E) for various implants compared with bone.

35.4 Alumina and Zirconia ...... 639 A similar transformation has been observed to occur TABLE 35.5 Physical Characteristics of Al 2O3 Bioceramics in aqueous environments. Commercially available high The wear resistance of zirconia is inferior to that of alumina ceramic alumina. In ceramic/ceramic combinations the wear Property implants ISO Standard 6474 rate of zirconia can be signifi cantly higher than that of alumina. In combination with ultrahigh-molecular- Alumina content (wt%) >99.7 ≥99.51 weight polyethylene (UHMWPE) excessive wear of SiO 2 + Na 2O (wt%) <0.02 <0.01 Density (g/cm 3) 3.98 ≥3.94 the polymer occurs. Average grain size ( µm) 3.6 <4.5 Zirconia may contain low concentrations of long half- Hardness (Vickers, HV) 2400 >2000 life radioactive elements such as Th and U, which are Bending strength (MPa, 595 >450 difficult and expensive to separate out. The main after testing in Ringer’s concern is that they emit α-particles (He nuclei) that solution) can destroy both soft and hard tissue. Although the activity is small, there are questions concerning the long-term effect of α radiation emission from zirconia pressure. Table 35.5 lists the physical characteristics ceramics. of Al 2O3 bioceramics. The International Standards Orga- nization (ISO) requirements are also given. The most current ISO standard relating to alumina for implants is 35.5 BIOACTIVE GLASSES ISO 6474: 1994, Implants for Surgery—Ceramic Materi- als Based on High Purity Alumina . The ISO is the inter- Bioactive materials form an interfacial bond with sur- national agency specializing in standards at the highest rounding tissue. Hench and Andersson (1993) give the level. Individual countries also have their own standards following definition: organizations. An important require- A bioactive material is one ment for any implant mate- BLOMEDICAL APPLICATIONS OF Al 2O3 that elicits a specifi c biologi- rial is that it should outlast There are many other applications of alumina as an cal response at the inter- the patient. Because of implant material including knee prostheses, ankle joints, face of the material, which the probabilistic nature of elbows, shoulders, wrists, and fingers. results in the formation of a failure in ceramics it is not bond between tissues and the material. possible to provide specific and definite lifetime predic- tions for each individual implant. This was the approach The first and most thoroughly studied bioactive glass that we discussed in Section 16.14. [See the discussion of is known as Bioglass ® 45S5 and was developed at the Figure 16.27, a diagram showing applied stress versus University of Florida. Bioglass ® 45S5 is a multicomponent probability of time to failure (SPT) for medical-grade oxide glass with the following composition (in wt%): 45% alumina.] It shows, as you might expect, that increased SiO , 24.5% Na O, 24.4% CaO, and 6% P O . loads and longer times increase the probability of failure. 2 2 2 5 The majority of bioactive glasses are based on the Results from aging and fatigue studies show that it is same four components and all current bioactive glasses essential that Al O implants be produced with the highest 2 3 are silicates. However, the structure of Bioglass ® 45S5 is possible standards of quality assurance, especially if different from that of the silicate glasses we described they are to be used as orthopedic prostheses in younger in Chapter 7. Bioactive glasses have a random, two- patients. dimensional sheet-like structure with a low density. This Although alumina ceramics combine excellent bio- is a result of the relatively low SiO content. (You can compatibility and outstanding wear resistance they have 2 compare the bioglass composition with that of other sili- only moderate fl exural strength and low toughness. This cates given in Table 21.6.) Bioglass is mechanically weak limits the diameter of most alumina femoral head pros- and has low fracture toughness. Both these properties are theses to 32 mm. Zirconia ceramics have higher fracture related to the glass structure. toughness, higher fl exural strength, and lower E than Bioactive glasses can readily be made using the alumina ceramics as shown in Table 35.2. However, there processes developed for other silicate glasses. The con- are some concerns with ZrO : 2 stituent oxides, or compounds that can be decomposed to There is a slight decrease in fl exural strength and tough- oxides, are mixed in the right proportions and melted at ness of zirconia ceramics exposed to bodily fluids. high temperatures to produce a homogeneous melt. On The reason is associ- cooling a glass is produced. ated with the martens- Because bioactive glasses itic transformation A MOIETY are going to be used inside from the tetragonal to A moiety is a group of atoms that forms a distinct part the body it is necessary to the monoclinic phase. of a large molecule. use high-purity starting

640 ...... Ceramics in Biology and Medicine materials and often the melting is performed in Pt or Pt tooth is removed. Bioactive glass implants have also been alloy crucibles to minimize contamination. used to repair the bone that supports the eye (the orbital Bioactive glasses have certain properties that are rele- socket). vant to their application in the body. In powder form, bioactive glasses have been used in the treatment of periodontal disease and for the treatment Advantages : There is a relatively rapid surface reaction that of patients with paralysis of one of the vocal cords. When leads to fast tissue bonding. There are five reaction mixed with autologous bone they have been used in maxil- stages on the glass side of the interface. The reaction lofacial reconstruction (i.e., mixed with natural bone to rates and mechanisms at each of the five stages have rebuild a jaw). been determined by Fourier transform infrared (FTIR) spectroscopy. Bonding to tissue requires a further series of reactions that are at present not as well defined. But 35.6 BIOACTIVE GLASS-CERAMICS the bonding process starts when biological moieties are

adsorbed onto the SiO 2–hydroxycarboapatite layer. In We know that glass-ceramics are produced by ceramming addition, E is in the range 30–35 GPa, close to that of a glass (see Chapter 26): converting it to a largely crystal- cortical bone (see Figure 35.4). line form by heat treatment. Several glass-ceramic com- Disadvantages : They are mechanically weak. Tensile positions are bioactive. Their behavior in the body is very bending strengths are typically 40–60 MPa. In addi- similar to that of the bioactive glasses, i.e., they form a tion, the fracture toughness is low. strong interfacial bond with tissue.

As a result of this combination of properties bioactive ᭿ Cerabone ® A-W is a glass-ceramic containing oxyfluo- glasses are not found in load-bearing applications, rather roapatite (A) and wollastonite (W). they are used as coatings on metals, in low-loaded or ᭿ Ceravital ® is primarily now used in middle ear compressively loaded devices, in the form of powders, and operations. in composites. The first successful use of Bioglass ® 45S5 ᭿ Bioverit I ® is a class of bioactive machinable glass. was as a replacement for the ossicles (tiny bones) in the middle ear. The position of these bones (the malleus, incus Cerabone ® A-W glass-ceramic is produced by crystal- and stapes) is illustrated in Figure 35.6. lization of a glass of the following composition (in

Cone-shaped plugs of bioactive glasses have been used wt%): 4.6 MgO, 44.7 CaO, 34.0 SiO 2, 6.2 P 2O5, and in oral surgery to fill the defect in the jaw created when a 0.5 CaF 2. The crystalline phases are oxyfluoroapatite

FIGURE 35.6 (a) The middle ear cavity and the auditory ossicles. (b) Ear implants.

35.6 Bioactive Glass-Ceramics ...... 641 2+ TABLE 35.6 Composition Range in wt% of Bioverit ® Glass lel to the c axis. Six of the 10 Ca ions in the unit cell are Ceramics associated with the hydroxyls in these particular columns. One group of three Ca 2+ ions describing a triangle, sur- Composition Composition Composition Constituent range 1 2 rounding the OH group, is located at z = 0.25 and the other set of three is located at z = 0.75. The six phosphate SiO 29.5–50 30.5 38.7 3− 2 (PO 4) tetrahedra are in a helical arrangement from levels MgO 6–28 14.8 27.7 z = 0.25 to z = 0.75. The network of (PO )3− groups pro- CaO 13–28 14.4 10.4 4 vides the skeletal framework that gives the apatite struc- Na 2O/K 2O 5.5–9.5 2.3/5.8 0/6.8

Al 2O3 0–19.5 15.9 1.4 ture its stability. (It is complicated but certainly crystalline F 2.5–7 4.9 4.9 and very natural!) P2O5 8–18 11.4 8.2 Substitutions in the HA structure are possible. Substi- TiO 2 Additions — 1.9 tutions for Ca, PO 4, and OH groups result in changes in the lattice parameter as well as changes in some of the properties of the crystal, such as solubility. If the OH − − [Ca 10 (PO 4)6(O,F) 2] as the A phase and β-wollastonite groups in HA are replaced by F the anions are closer to 2+ (CaO·SiO 2) as the W phase. These phases precipitate out the neighboring Ca ions. This substitution helps to at 870°C and 900°C, respectively. The composition of the further stabilize the structure and is proposed as one of residual glassy phase is (in wt%) 16.6 MgO, 24.2 CaO, the reasons that fluoridation helps reduce tooth decay as and 59.2 SiO 2. The properties of A-W glass-ceramic are shown by the study of the incorporation of F into HA and shown in Table 35.2. The applications include vertebral its effect on solubility. Biological apatites, which are the prostheses, vertebral spacers, and iliac crest prostheses. mineral phases of bone, enamel, and dentin, are usually Ceravital ® refers to a number of different compositions referred to as HA. Actually, they differ from pure HA in of glasses and glass-ceramics that were developed in the stoichiometry, composition, and crystallinity, as well as 1970s in Germany for biomedical applications. The only in other physical and mechanical properties, as shown in field in which Ceravital ® glass-ceramics are clinically Table 35.7. Biological apatites are usually Ca deficient and 2− 3− used is in the replacement of the ossicular chain in the are always carbonate substituted: (CO 3) for (PO 4) . For middle ear. In this application the mechanical properties of the material are sufficient to support the minimal applied loads. Bioverit I ® consists of two crystalline phases in a glass TABLE 35.7 Comparative Composition and Crystallo- matrix. The key crystalline component that makes this graphic and Mechanical Properties of Human Enamel, glass-ceramic machinable is mica. We have already Bone, and HA Ceramic described the idea behind machinable glass-ceramics in Cortical Section 18.10. For bioactivity, the other crystalline phase Enamel bone HA is apatite. The type and amounts of each phase depend on Constituents (wt%) the initial glass composition. Table 35.6 shows the typical Calcium, Ca 2+ 36.0 24.5 39.6 composition range of Bioverit I ® glass-ceramics. Compo- Phosphorus, P 17.7 11.5 18.5 (Ca/P) molar 1.62 1.65 1.67 sition 1 produces fluorophlogopite mica and apatite. Com- + position 2 produces tetrasilicic mica and apatite. There Sodium, Na 0.5 0.7 Trace Potassium, K + 0.08 0.03 Trace are several clinical applications of bioactive machinable Magnesium, Mg 2+ 0.44 0.55 Trace 2+ glass-ceramics, such as spacers in orthopedic surgery and Carbonate, CO 3 3.2 5.8 — middle ear implants. Fluoride, F − 0.01 0.02 — Chloride, Cl − 0.30 0.10 — Total inorganic 97.0 65.0 100 Total organic 1.0 25.0 —

35.7 HYDROXYAPATITE Absorbed H 2O 1.5 9.7 — Crystallographic properties The apatite family of minerals has the general formula Lattice parameters A10 (BO 4)6X2. In HA, or more specifically calcium hydroxy- ( ±0.03 nm) apatite, A = Ca, B = P, and X = OH. The mineral part of a 0.9441 0.9419 0.422 teeth and bones is made of an apatite of calcium and c 0.6882 0.6880 0.6880 phosphorus similar to HA crystals. Natural bone is ∼70% Crystallinity index 70–75 33–37 100 Crystallite size, nm 130 × 30 25 × 2.5–5.0 HA by weight and 50% HA by volume. Products after sintering Hydroxyapatite is hexagonal (space group is P63/m) with a = 0.9432 nm and c = 0.6881 nm. The hydroxyl ions >800°C HA + TCP HA + CaO HA lie on the corners of the projected basal plane and occur Mechanical properties at equidistant intervals [one-half of the cell (0.344 nm)] E (GPa) 14 20 10 Tensile strength (MPa) 70 150 100 along columns perpendicular to the basal plane and paral-

642 ...... Ceramics in Biology and Medicine this reason you will sometimes see biological apatites stromal component) of the bone. The ideal microstructure referred to as carbonate apatite and not as hydroxyapatite for regeneration of cortical bone is an interconnected or HA. porosity of 65% with pore sizes ranging from 190 to For use in biomedical applications, HA is prepared in 230 µm. The ideal graft substitute for cancellous bone one of two forms: either dense or porous. would consist of a thin framework of large (500–600 µm) We will now discuss interconnected pores. how these two forms are DENSE HA Several methods have produced and some of the Porosity <5 vol% been used to produce applications for each. Pore size <1µm diameter porous HA ceramics. The methods used to Grain size >0.2 µm Remember that historically produce dense HA are much of ceramic process- ones that we have already ing was concerned with encountered in Chapter 23. The simplest is to dry press trying to produce components that were fully dense. So to HA powder. Mold pressures are typically 60–80 MPa. The produce porous components, particularly where we need powder may also be mixed with a small amount of a to control pore size, often requires ingenuity and a rethink. binder. Suitable binders are 1 wt% cornstarch and water, We discussed porous ceramics in Section 23.15, so will steric acid in alcohol, or low-molecular-weight hydrocar- just point out special features for producing porous HA bons. After pressing, the green ceramic is sintered in air. ceramics. Sintering temperatures are up to 1300°C with dwell times at peak temperature of several hours. ᭿ Sintering HA powders By using hot isostatic pressing (HIP) techniques, we ᭿ Make a cement can to achieve densification at much lower temperatures ᭿ Use a natural template (900°C vs. 1300°C). The use of lower sintering tempera- tures not only saves on energy costs but prevents the for- Sintering HA powders, or a mixture of suitable reac- mation of other calcium phosphate phases, such as TCP, tant powders, uses naphthalene particles that volatilize which usually form when HA is sintered at T > 900°C. during heating to create an interconnected porous network. HIPing has also been used to form HA ceramics. Similar approaches have been used to produce foam glass HIPing results in products with more uniform densities (Section 26.9) and in the fugitive electrode method of than those obtained by uniaxial pressing and higher com- producing multilayer chip capacitors (MLCCs) (Section pressive strength. 31.7). The disadvantage of both hot pressing and HIPing is By mixing water-setting HA cements with sucrose the equipment costs. granules, porosity can be There are many appli- created when the sucrose cations for dense HA in RESORPTION is dissolved prior to the both block form and as Resorption is the process of reabsorbing biological cement setting. One particles as listed in Table material. example of an HA cement 35.8. One important appli- is that obtained by the fol- cation is replacements for tooth roots following extraction. lowing reaction in an aqueous environment: The implants help minimize alveolar ridge resorption and maintain ridge shape following tooth loss, which affects Ca 4(PO 4)2 + CaHPO 4 → Ca 5(PO 4)3OH (35.1) millions of people. The particular advantage of porous HA is that it The natural-template method was developed in 1974. permits ingrowth of tissue into the pores, providing bio- It can produce porous HA powders. A suitable template logical fixation of the implant. The minimum pore size was found to be the calcium carbonate (CaCO 3) skeleton necessary is ∼100 µm. When used as a bone graft substi- of reef-building corals, such as those found in the South tute, the porous HA should mimic the framework (or Pacific. The reaction to produce HA involves a hydrother- mal exchange reaction of carbonate groups with phosph- ate groups, which can occur via the following chemical reaction: TABLE 35.8 Applications for Dense HA Ceramics Application Form 10CaCO 3 + 6(NH 4)2HPO 4 + 2H 2O → Augmentation of alveolar ridge for better denture fi t Blocks Ca 10 (PO 4)6(OH) 2 + 6(NH 2)CO 3 + 4H 2CO 3 (35.2) Orthopedic surgery Blocks Target materials for ion-sputtered coatings Blocks The HA structure produced by this exchange reaction Filler in bony defects in dental and orthopedic surgery Particles replicates the porous marine skeleton, including its inter- Plasma sprayed coatings on metal implants Particles connected porosity. Hydroxyapatite grown on Porites and Filler in composites and cements Particles Goniopora coral skeleton templates can be used to mimic

35.7 Hydroxyapatite ...... 643 the stroma of cortical bone and cancellous bone, 25 respectively. E Ductile Brittle εf (%) (GPa) E (Cortical bone) 10 100 x 35.8 BIOCERAMICS IN COMPOSITES εf The main reason for forming composites is to improve the 8 80 x mechanical properties, most often toughness, above that of the stand-alone ceramic. For bioceramic composites we E often are trying to increase K and decrease E. IC 6 Bioactive 60 The first bioceramic composite was a stainless-steel ® fiber/bioactive glass composite made of Bioglass 45S5 x and AISI 316L stainless steel. The composite was made x by first forming a preform of the discontinuous metal 4 40 x fibers, then impregnating it with molten glass, and finally x heat treating the composite to develop the desired mechan- x ical properties. 2 εf 20 For effective stress transfer between the glass matrix E and the reinforcing metal fibers when the composite is x under load, there must be a strong glass–metal bond. This 0 0 requires that the glass wet the metal surface during 0 10 20 30 40 50 processing. Wetting is achieved by oxidizing the metal vol. % HA fibers before they are immersed in the glass. Chemical FIGURE 35.7 Effect of volume fraction of HA on E and strain to analysis across the glass–metal interface showed that failure of HA-reinforced PE composites, in comparison to cortical there is Fe diffusion from the oxide into the glass and Si bone. diffusion from the glass into the oxide. The composition gradient across the interface indicates chemical interac- tion between the two phases, which leads to improved Other current bioceramic composites of interest are adhesion. Assuming that the fibers are aligned in the direction ᭿ Ti-fiber-reinforced bioactive glass ᭿ of the applied load, and that there is good adhesion with ZrO 2-reinforced A-W glass the matrix such that the elastic strains are equal in both ᭿ TCP-reinforced PE components, we can write ᭿ HA-reinforced PE

Hydroxyapatite-reinforced PE is a good illustration σ = σ V + σ V (35.3) c f f m m of a composite that can have properties that are not available in a single material. These composites were The subscripts c, f, and m refer to the composite, fiber, developed as a bone replacement that would have a matched and matrix, respectively, and V is the volume fraction of modulus, be ductile, and bioactive. Figure 35.7 shows

each phase. If we assume 45 vol% of steel fibers and σf = how increasing the volume fraction of HA to 0.5 in a 530 MPa and σm = 42 MPa, then σc = 262 MPa. This value, composite can be achieved with E in the range of that of which is close to experimentally measured values, repre- cortical bone. When the volume fraction of HA in the sents a signifi cant strengthening above that of the glass composite is increased above about 0.45 the fracture alone. mode changes from ductile to brittle. For clinical applica- One of the potential problems associated with forming tions a volume fraction of 0.4 has been found to composites is that of mis- be optimum. The HA- match in α between the two reinforced PE composite components, which is sig- AISI 316L is designated commer- TM nificant for glass and steel. −6 −1 cially as HAPEX and For reinforcing fibers for α = 20.0 × 10 °C (to 200°C) several thousand patients

which the difference in α −6 −1 have received middle ear with the glass phase is even α = 21.8 × 10 °C (to 400°C) implants made from this

greater than that with steel, ® material. The technology e.g., Ti, it is necessary to BIOGLASS 45S5 was granted regulatory

change the composition of −6 −1 approval by the FDA in the the glass to lower its α. α = 18.0 × 10 °C (to 450°C) United States in 1995.

644 ...... Ceramics in Biology and Medicine 35.9 BIOCERAMIC COATINGS the plasma temperature may exceed 10,000°C!) so the mechanical properties of the metal are not compromised. Applying a glass or ceramic coating onto the surface of a The coating thickness typically averages 40–60 µm with substrate allows us to have the best of both worlds. We a residual porosity <2%. have the bulk properties of the substrate and the surface Hydroxyapatite coatings prepared by plasma spraying properties of the coating. There are three main reasons for typically contain considerable amounts of amorphous applying a coating: calcium phosphate and small amounts of other crystalline phases. Heat treating the coating can increase crystallinity 1. Protect the substrate against corrosion and also improve the adhesion to the substrate. However, 2. Make the implant biocompatible this process is not usually done because of economic 3. Turn a nonbioactive surface into a bioactive one factors and concerns about the adverse effects it might have on the mechanical properties of the substrate. There are four substrate-coating combinations: In addition to plasma spraying other methods have been used to apply HA coatings: 1. Polycrystalline ceramic on ceramic 2. Glass on ceramic ᭿ Electrophoretic deposition when line-of-sight deposi- 3. Polycrystalline ceramic on metal tion is not possible 4. Glass on metal ᭿ Sputtering when very thin coatings are needed ᭿ HIPing when we need a very dense material Bioceramic coatings are often used on metallic sub- strates in which the fracture toughness of the metal is Electrophoretic deposition (see Section 27.6 for a combined with the ability of the coating to present a bioac- description of the technique) can be used to coat porous tive surface to the surrounding tissue. The use of a bio- surfaces that cannot be completely coated by line-of-sight ceramic coating on a metal implant can lead to earlier techniques such as plasma spraying. But the adhesion of stabilization of the implant in the surrounding bone and the HA particles to the substrate and each other is weak extend the functional life of the prosthesis. Under the and high-temperature sintering after deposition is usually proper conditions a cementless prosthesis should remain necessary. functional longer than a cemented device in which stabi- Sputtering has been used to produce thin (1 µm) HA lity is threatened by fracture of the bone cement. coatings. The deposited films are amorphous because The important ceramic coatings are HA and TCP. We the sputtered components do not possess enough kinetic described the structure and properties of HA, a bioactive energy to recombine in a crystalline form. Heat treatment ceramic, in some detail in Section 35.6. Tricalcium phos- at 500°C is enough to crystallize the amorphous film. phate is a resorbable bioceramic. It occurs in two poly- Durability of thin sputtered films in the body has not yet morphs, α-whitlockite and β-whitlockite. The β form is been demonstrated. the more stable. When TCP is implanted into the body it HIPing is a technique we encountered earlier, but not will eventually dissolve and be replaced by tissue. The role in the context of forming films. If a metal implant is of resorbable bioceramics is to serve as scaffolding, per- coated with HA particles then HIPing can be used to form mitting tissue infiltration and eventual replacement. Essen- a dense adherent coating. To achieve a uniform application tially, this is the same function as bone grafts. Tricalcium of pressure on the HA particles an encapsulation material phosphate has been clinically applied in many areas of (e.g., a noble metal foil) is necessary. As mentioned earlier, dentistry and orthopedics. Bulk material, available in HIPing allows the use of lower sintering temperatures dense and porous forms, is used for alveolar ridge aug- than pressureless techniques; as a result there is less mentation, immediate tooth root replacement, and maxil- chance of altering the microstructure or mechanical pro- lofacial reconstruction. However, because bulk TCP is perties of the metal substrate. mechanically weak, it cannot be used in load-bearing There are several requirements for HA coatings used applications. Therefore, TCP is often used as a coating on for prosthetic devices: metal substrates. The most widely used method for applying coatings of ᭿ Correct crystalline phase HA and TCP is plasma spraying. We already described ᭿ Stable composition this technique in Section 27.5; it is one of the methods ᭿ Dense used to produce thermal barrier layers. Plasma spraying ᭿ Good adhesion to the substrate uses a plasma, an ionized gas, that partially melts the HA ᭿ High purity particles and carries them to the surface of the substrate. ᭿ No change to the substrate For HA coatings the starting material is pure 100% crys- talline HA particles in the 20–40 µm range. One of the Plasma sprayed coatings often contain a mixture of advantages of plasma spraying is that the substrate remains crystalline and amorphous phases, which may be undesir- at a relatively low temperature (generally less than 300°C; able. The adhesion of plasma-sprayed HA coatings to

35.9 Bioceramic Coatings ...... 645 metal substrates is principally mechanical, and so surface roughness of the substrate plays an important role. Bioactive glass coatings are also important for implant devices. These are usually applied by one of the following techniques: ᭿ Enameling ᭿ Flame spraying ᭿ Dip coating Flame spraying is similar to plasma spraying except that the carrier gases are not ionized and the temperatures are considerably lower than in plasma spraying. In dip coating, the metal implant is preoxidized to provide a suit- able surface for wetting of the molten glass. The heated metal is then dipped into the molten glass. Enameling is a traditional method of applying glass FIGURE 35.8 Lithium calcium borate glass microspheres coatings and uses a particulate form of the glass called a produced by passing through a fl ame at 1400°C. frit, which is formed when molten glass is quenched in water. The resulting coarse particles of the frit are ground etration ∼10 mm). For the radioactive material to reach the to a fine powder that is applied to the metal substrate by site of the tumors between 1 and 15 million microspheres painting, spraying, or dipping. The coated article is then are injected into the hepatic artery, which is the primary heated to soften the glass and form a uniform coating. In blood supply for the target tumors. Treatment time takes traditional enameling the adhesion between the glass and 2–4 hours. The size of the microspheres is 15–35 µm in metal is improved by using what enamellers call a “ground diameter, which allows the blood to carry them into the coat.” This is a mixture of metal oxides that reacts chemi- liver, but they are too large to pass completely through the cally with both the metal and the glass enabling the forma- liver and enter the circulatory system. The microspheres tion of a chemical bond. However, this approach has not concentrate in the tumor because it has a greater than proved to be successful with bioactive glasses and alterna- normal blood supply. And there they irradiate it with β- tive approaches are being used. particles. Since the half-life of 90 Y is 64.1 hours, the radio- activity decays to a negligible level in about 3 weeks. 35.10 RADIOTHERAPY GLASSES Although the use of radiotherapy glass spheres in treating liver cancer is still at a relatively early stage the Radioactive yttrium aluminosilicate (YAS) glasses have results appear promising. The commercially available been used to provide in situ irradiation of malignant product called TheraSphere TM made by MDS Nordion is tumors in the liver. Although primary liver tumors are approved in the United States and Canada for treating relatively rare in the United States (about 3000–4000 patients with inoperable liver cancer. Other medical appli- deaths per year in the United States, 1.2 million world- cations for these glass spheres have been considered such wide) they are almost always lethal. And most of these as the treatment of cancers of the kidney and brain. tumors are inoperable due to various medical complica- Figure 35.8 shows lithium calcium borate (LCB) glass tions. Irradiating the tumors inside the body allows the microspheres that were spheroidized by passing them through use of large ( >10,000 rads) localized doses of radiation a high-temperature flame. A similar process is used to make directly to the tumor while minimizing damage to sur- the smaller YAS microspheres. The LCB microspheres are rounding healthy tissue. And this procedure represents an subsequently converted into hollow hydroxyapatite micro- important tool in treating this disease. spheres that have potential application in drug delivery. YAS glasses are particularly suitable because they are

᭿ 35.11 PYROLYTIC CARBON Not toxic HEART VALVES ᭿ Easily made radioactive ᭿ Chemically insoluble while the glass is radioactive Carbon is an important bioceramic. It combines out- The sol-gel process has been used to produce high standing biocompatibility and chemical inertness. Carbon purity YAS glass spheres. The radioactive isotope pro- exists in many forms, some of which have been discussed duced when the YAS glass spheres are irradiated is 90 Y, a in earlier chapters. The most important form of carbon β emitter with a half-life of for biomedical applications 64.1 hours. The average is a type of pyrolytic penetration of β-particles LTI graphite known as the low- (electrons) in human tissue Low T refers to the forming T < 1500°C. For ceramics temperature isotropic form is 2.5 mm (maximum pen- 1500°C is not a high T. (LTI carbon).

646 ...... Ceramics in Biology and Medicine Low-temperature isotropic carbon is an example of what are referred to as turbostratic carbons. These have a disor- dered structure based on graphite (and thus are also called turbostratic graphite). In turbostratic carbon the ABABA stacking sequence is disrupted through random rotations or displacement of the layers relative to each other. The indi- vidual LTI carbon crystallites are only ∼10 nm in size and are arranged randomly in the bulk material. This microstructure leads to the material having isotropic mechanical and physical properties, unlike graphite in which the properties are highly anisotropic. The density and mechanical properties of LTI are influenced by the number of carbon vacancies in each of the layers and distortions FIGURE 35.9 LTI pyrolytic carbon-coated heart valve. within each plane. The densities range from 1400 kg/m 3 up to a theoretical maximum of 2200 kg/m 3. High-density LTI carbons are the strongest bulk form materials. The first use of LTI carbon in humans for pros- of turbostratic carbon; we can increase their strengths thetic heart valve was in 1969. The majority of artificial further by adding Si. The material then consists of discrete heart valves currently use Si-alloyed LTI pyrolytic submicrometer β-SiC particles randomly dispersed in a carbon. matrix of roughly spherical micrometer-sized subgrains of pyrolytic carbon; the carbon itself has a “subcrystalline” turbostratic structure, with a crystallite size typically 35.12 NANOBIOCERAMICS <10 nm. This is analogous to the microstructure produced during precipitation hardening of metals. By 20.12, there will be books on the uses of nanoparticles A chemical vapor deposition (CVD) process (see and there are already hundreds of research papers. There Section 28.4) involving the codeposition of carbon and may also be books discussing the toxicity of these mater- SiC is typically used to produce the LTI–Si alloys. Two ials. The asbestos fibers linked to respiratory illness have possible reactions are widths <250 nm; amphibole (red or blue asbestos) fibers are ∼75– ∼240 nm wide, therefore definitely counting as 1. Decomposition of propane: nanoparticles. Examples of a microbarcode made by Corning are

C 3H8 → 3C + 4H 2 (35.4) shown in Figure 35.10. The information is coded into the small glass bars so that they fluoresce. The pattern can 2. Decomposition of methyltrichlorosilane then be read by illuminating the glass with UV; otherwise it is not only too small to see but the information could

CH 3Cl 3Si → SiC + 3HCl (35.5) not be detected. The magnetite nanocrystals we discussed in Chapter The articles to be coated are suspended within a flui- 33 are used by nature in ways we do not fully understand, dized bed of granular particles, usually ZrO 2. The reactions but they appear to allow certain species to detect the take place in the range of 1000–1500°C and the products earth’s magnetic field and use it to navigate. coat the components as well as the ZrO 2 particles. TiO 2 nanoparticles are used in sunscreen to protect the One of the major applications for LTI carbon is in skin from UV radiation. The particles used for this appli- making prosthetic heart valves as shown in Figure 35.9. cation are typically 10–100 nm in diameter and block both This is one of the most demanding applications for bio- UVA (320–400 nm) and UVB (290–320 nm) irradiation.

FIGURE 35.10 Microbar codes from Corning. The cyllindrical bars are typically 100 µm long and 20 µm in diameter.

35.12 Nanobioceramics ...... 647 Veneer Veneer

Bulk ceramic

Dentine Pulp

Dentine

FIGURE 35.11 Tooth restoration.

There is some concern that these nanoparticles (and those of ZnO) are so active that they might catalyze the break- down of DNA, but they do not appear to penetrate the outer layers of the skin. The positive aspect of this is the potential for using these same TiO 2 nanoparticles for photo-killing of malignant cells—known as photody- namic therapy. TiO 2 and ZnO particles are actually being coated with silica so that the particle surface is more inert. (A use for core-shell nanoparticles.)

FIGURE 35.12 (a) Schematic of the abalone shell. (b) Abalone 35.13 DENTAL CERAMICS slabs. The felspathic porcelains (porcelain’s based on feld- Biomimetics can poten- DENTAL RESTORATIONS spar) are used as the veneer tially lead to an enormous Feldspathic veneers to “cap” the front of a tooth subset of ceramics. Not Porcelain jacket crowns (PJCs) for cosmetic reasons; these only are the materials Metal-ceramic crowns veneers are ∼500 µm thick. important but their topo- Inlays and onlays Today, this material is logy and microstructure Implants mainly replaced by glass, are also special. The aim is although the name may not to mimic natural materials have changed. Leucite is added to modify the thermal that have special behaviors, but to do it with “better” (i.e., expansion coefficient. Dicor is the glass-ceramic devel- ceramic) materials. oped by Corning for construction of replacement teeth. The tooth is cast as a glass using a lost-wax mold and then Shells (especially the abalone shell). Biological organisms cerammed. Alumina has also been used to form the tooth, can deposit inorganic layers. The abalone shell consists although porosity causes failure during “use.” One way of layers of aragonite with ∼5 wt% organic material to improve this is to infiltrate the alumina with a lantha- (protein, etc.) between the layers to toughen it. A sche- num-containing glass (known commercially as In-Ceram). matic cross section and a scanning electron micro- The different restorations are shown schematically in scopic (SEM) image of the aragonite layers are shown Figure 35.11. in Figure 35.12. Coral . Coral is used “as harvested” or after processing in the manufacture of bone grafts. It is a natural porous 35.14 BIOMIMETICS ceramic and can be converted to porous HA (see Section 35.7). Surgeons in the United States perform The topic of biomimetics was actually mentioned earlier ∼500,000 bone grafts each year. but not using the new name. The principle underlying the Petrifi ed wood . A natural material is infiltrated by a development of biomimetics is “learning from nature.” ceramic.

648 ...... Ceramics in Biology and Medicine FIGURE 35.13 Wood converted into ceramics by a reactive melt-infi ltration process.

Petrified wood provides a good illustration of the sur- the wood used. Beech, pine, and rattan produce very dif- prising potential of biomimetics in a way that is analogous ferent microstructures which are of course different in to the conversion of coral. Wood can be intentionally cross section and longitudinally. In this example, the infil- converted into biomorphic, microcellular SiC-based trating liquid was a molten Si-M alloy (Mo, Ta, Ti, and Fe ceramics using a reactive melt-infiltration process, or more were explored as the metal, M). For Mo, this produces an specifically liquid-Si infiltration (LSI). The images shown MoSi 2/Si composite. The LSI technique has been used in Figure 35.13 illustrate the possibility: the microstruc- previously to produce SiC and Si 3N4; the special feature ture of the resulting composite depends on the nature of here is the biomimetic structure produced by the wood.

CHAPTER SUMMARY Bioceramics is a relatively new fi eld and an increasingly important one. Bioceramics are implanted into the human body to replace existing parts that have become diseased, damaged, or worn out. More than 1 million hip prostheses using alumina components have been performed. Alumina is a very important bioceramic because it is biocompatible—it does not produce any adverse reactions in the body. One of the disadvantages of alumina is that it is also an example of what is termed a nearly inert bioceramic: it does not allow interfacial bonding with tissue. When bioactive ceramics and glasses are implanted into the body they undergo chemical reactions on their surface leading to strong bond formation. The most important bioactive ceramic is hydroxyapatite, which is very similar to the mineral part of teeth and bones. Hydroxyapatite is brittle and mechanically weak, but if it is combined with a polymer a composite can be produced that is ductile and has E close to that of bone. Hydroxyapatite and other bioactive ceramics and glasses are often used as coatings on metal supports. This allows the excellent mechanical properties of the metal to be combined with the biocompatibility of ceramics. Another ceramic material that is used in the form of a coating is pyrolytic carbon. The application here is artificial heart valves, which is a very demanding materials application: reliability is critical. We concluded the chapter with a brief discussion of biomimetics (which is related to bionics, etc.) and emphasize that this will become a large field in itself but the materials must be understood to take full advantage of this potential.

GENERAL REFERENCES Hench, L.L. and Wilson, J.K. (Eds.) (1993) An Introduction to Bioceramics , World Scientific, Singapore. A collection of chapters on different aspects of bioceramics written by experts in the field. This is an excellent resource.

Chapter Summary ...... 649 LeGeros, R.Z. and LeGeros, J.P. (Eds.) (1998) Bioceramics, Volume 11, Proceedings of the 11 th International Symposium on Ceramics in Medicine , World Scientific, Singapore. The most recently published proceed- ings in this annual conference series. Covers the latest developments in all aspects of the field of bioceramics. Marieb, E.N. (1998) Human Anatomy and Physiology , 4th edition, Benjamin Cummings, Menlo Park, CA. For a more detailed description of bone. Park, J.B. (1984) Biomaterials Science and Engineering , Plenum, New York. A textbook covering all aspects of biomaterials. The section covering ceramic implant materials is quite brief, but there is information about polymers and metals and a general background about the field. Ravaglioli, A. and Krajewski, A. (1992) Bioceramics: Materials, Properties, and Application, Chapman & Hall, London. Covers the history of bioceramics, scientific background to the field, and state of the art as of 1991. Shackelford, J.F. (Ed.) (1999) Bioceramics: Applications of Ceramics and Glass Materials in Medicine Mater. Sci. Forum , 293 . A recent monograph on bioceramics. Williams, D.F. (1999) The Williams Dictionary of Biomaterials , Liverpool University Press, Liverpool. The latest collection of definitions used in the field of biomaterials. Yamamuro, T., Hench, L.L., and Wilson, J. (Eds.) (1990) Handbook of Bioactive Ceramics, Volume I: Bioactive Glasses and Glass Ceramics, Volume II: Calcium Phosphate and Hydroxylapatite Ceramics , CRC Press, Boca Raton, FL. A collection of articles on bioactive and resorbable bioceramics.

JOURNALS AND CONFERENCE PROCEEDINGS The following journals and conference proceedings contain reports of advances in bioceramics. Bioceramics. This is the title of the Proceedings of the International Symposium on Ceramics in Medicine. The symposium has been held annually since 1988. Biomaterials. 1980–present (volume 27) Biomedical Materials and Engineering. Covers all aspects of biomaterials and includes details of clinical studies. Usually found with medical journals rather than physical science or engineering publications. Journal of Biomedical Materials Research. (Now volume 80; 4 volumes in both ‘A’ and ‘B’.) Journal of Materials Science Materials in Medicine. Emphasising the materials science aspects. Journal of the American Ceramic Society. Covers the entire ceramics fi eld including bioceramics. Nature. A journal with a wide readership. Publishes important news articles having a significant impact on the field.

SPECIFIC REFERENCES Annual Book of ASTM Standards , 13.01, Medical Implants , ASTM, Philadelphia. The American Society for Testing and Materials (ASTM) has developed several standards related to bioceramics for surgical implants: the Annual Book of ASTM Standards . Bokros, J.C., LaGrange, L.D., Fadali, A.M., Vos, K.D., and Ramos, M.D. (1969 ) J. Biomed. Mater. Res. 3, 497. First use of carbon heart valves in humans. Bonfield, W., Grynpas, M.D., Tully, A.E., Bowman, J., and Abram, J. (1981) “Hydroxyapatite reinforced polyethylene—a mechanically compatible implant,” Biomaterials 2, 185. Brömer, H., Pfeil, E., and Käs, H.H. (1973) German Patent 2,326,100. Patent for Ceravital. Chakrabarti, O., Weisensel, L., and Sieber, H. (2005) “Reactive melt infiltration processing of biomorphic Si–Mo–C ceramics from wood,” J. Am. Ceram. Soc. 88 (7), 1792. DiB: Defi nitions in Biomaterials (1987) D.F. Williams (Ed.), Elsevier, Amsterdam, 6. Includes a discussion of biocompatibility. Ducheyne, P. and Hench, L.L. (1982) “The processing and static mechanical properties of metal fibre rein- forced bioglass ®,” J. Mater. Sci. 17 , 595. Diffusion across a bioglass-metal interface. Galletti, P.M. and Boretos, J.W. (1983) “Report on the Consensus Development Conference on Clinical Appli- cations of Biomaterials” J. Biomed. Mat. Res . 17 , 539. Defined “biomaterial.” Hench, L.L. (1991) “Bioceramics: From concept to clinic,” J. Am. Ceram. Soc . 74 , 1487. An excellent review on this topic. Hench, L.L. and Andersson, O. (1993) “Bioactive glasses,” in An Introduction to Bioceramics , edited by L.L. Hench and J.K. Wilson, World Scientific, Singapore, 41. Klawitter, J.J. and Hulbert, S.F. (1971) “Application of porous ceramics for the attachment of load bearing orthopedic applications,” J. Biomed. Mater. Res . 2, 161. Determined that a minimum pore size of 100 µm is necessary for bone ingrowth. Mehmel, M. (1930) Z. Kristallogr . 75 , 323. One of two original determinations of the structure of hydroxy-

apatite. The structure was originally taken to be the same as that of fluorapatite, Ca 10 (PO 4)6F2, which had already been resolved.

650 ...... Ceramics in Biology and Medicine Merwin, G.E., Wilson, J., and Hench, L.L. (1984) in Biomaterials in Otology , edited by J.J. Grote, Nijhoff, The Hague, pp. 220–229. Description of bioglass. Náray-Szabó, S. (1930) Z. Kristallogr . 75 , 387. The other paper on the structure of hydroxyapatite. Posner, A.S., Perloff, A., and Diorio, A.F. (1958) “Refinement of the hydroxyapatite structure,” Acta Cryst . 11 , 308. Determined atom positions, bond lengths, and lattice parameters for the HA crystal structure. Roy, D.M. and Linnehan, S.K. (1974) “Hydroxyapatite formed from coral skeletal carbonate by hydrothermal exchange,” Nature 247 , 220. Developed the natural-template method for making porous HA. Sarakiya, M. (1994) “An introduction to biomimetics: A structural viewpoint,” Microsc. Res. Techn. 27 , 360. A very useful introduction.

Sato, T. and Shimada, M. (1985) “Transformation of yttria-doped tetragonal ZrO 2 polycrystals by annealing

in water,” J. Am. Ceram. Soc . 68 , 356. Described the martensitic transformation in ZrO 2 in aqueous environments. Wang, Q., Huang, W., Wang D., Darvell, B.W., Day, D.E., and Rahaman, M.N. (2006) “Preparation of hollow hydroxyapatite microspheres,” J. Mater. Sci: Mater. Med. 17 , 641.

WWW www.mds.nordion.com/therasphere/ The TheraSphere ® web site.

EXERCISES 35.1 Briefly compare and contrast the suitability of metals, ceramics, and polymers for use in biomedical applica- tions. In your answer consider the following factors: biocompatibility, mechanical properties, and ease of processing.

35.2 Alumina (Al 2O3) ceramic implants are required to have a small grain size ( <4.5 µm). (a) Why do you think a small grain size is important? (b) How does the addition of MgO to the powder mixture help to keep the grain size small? (c) Are there any other ways that could be used to limit the extent of grain growth? 35.3 Explain why tetragonal zirconia polycrystal (TZP) and Mg-partially stabilized zirconia (PSZ) ceramics have higher toughness than alumina ceramics. 35.4 The Weibull modulus of an alumina bioceramic is given as 8.4. (a) What does a value of m = 8.4 imply? (b) For an implant made out of a metal the value of m ∼ 100. What implications would this have for lifetime predictions for the metal component compared to the alumina component? (c) How does the value of m affect the design of a component for a load-bearing application?

® 35.5 The composition of Bioglass 45S5 is 45% SiO 2, 24.5% Na 2O, 24.4% CaO, and 6% P 2O5. (a) Classify each of the oxide constituents of 45S5 as either network formers, modifiers, or intermediates. (b) What would the composition of Bioglass ® 45S5 be in mol%? (c) Explain briefly how the structure of 45S5 differs from the silicate glasses described in Chapter 7 and what implications this difference has on the properties of 45S5. 35.6 The substitution of ions in the HA structure can change the lattice parameters of the unit cell. Explain how you think the substitution of Ca 2+ for the following ions would change both the a and c lattice parameters: Sr 2+, Ba 2+, Pb 2+, Mg 2+, Mn 2+, and Cd 2+. 35.7 According to ASTM Standards [American Society for Testing and Materials (1990) Annual Book of ASTM Standards , Section 13, F 1185–1188] the acceptable composition for commercial HA is a minimum of 95% HA, as established by X-ray diffraction (XRD) analysis. Describe how XRD can be used to determine phase proportions in a mixture. 35.8 We mentioned that in steel fibers/bioactive glass composites it was important that there was chemical interac- tion between the two components to ensure stress transfer during loading. We have just formed a bioceramics company and want to hire you as a consultant. We want you to determine whether such interactions have occurred in a composite we have just made. What analytical technique or techniques would you use for your evaluation? Explain the reasoning behind your answer and some of the pros and cons of the technique or techniques you chose. 35.9 One of the advantages of plasma spraying for producing HA coatings on metallic implants is that the substrate temperature can be kept relatively low. What possible mechanisms can lead to a loss in the mechanical strength of a metal if it is exposed to high temperatures? (You can limit your discussion to the cases of Ti alloys and stainless steel.) 35.10 Explain why the mechanical properties of turbostratic carbons such as LTI are different from those of gra- phite. Both are forms of carbon and are chemically identical.

Chapter Summary ...... 651