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.