Diffusion bonding of zirconia to austenitic stainless steel
-Master thesis-
A.T J. van Helvoort
Supervisor: ir. R.H. Vegter Professor: Prof Dr. G. den Ouden Laboratory for Materials Science and Engineering Section Welding Technology and Non-Destructive Testing April 1999
I
Summary
This research deals with diffiision bonds of zirconia to AISI 316 and zirconia to zirconia using an AISI 316 foil, which were produced under various process conditions.
Zirconia is an interesting technical ceramic with some excellent properties such as good mechanical strength at elevated temperatures, good corrosion resistance and a low thennal conductivity. To use these properties in structural applications and by pass the poor properties such as a low toughness, it is desirable to bond the ceramic to a metal. AISI 316 austenitic stainless steel is a common type of stainless steel with some Mo to improve its properties at elevated temperatxires, for instance the creep resistance. Diffusion bonding is a possible sohd state bonding technique to join zhconia to AISI 316.
Experiments were carried out in which the bonding temperature (1000°C-1250°C), the bonding time (30,90 and 360 minutes) and the bonding pressure (2 and 15 MPa) were varied to determine the influences of these bonding parameters on the bond quality. The produced bonds were analysed by optical microscopy, scarming electron microscopy (SEM), X-ray diffraction technique (XRD) and electron probe microanalysis (EPMA). The shape of the AISI 316 part was varied to determine whether this could improve the bond strength. The different bonding geometries were modelled by finite element method (FEM) to determine the residual stress levels after cooling ofthe samples.
The experimental results show that all zirconia / AISI 316 combinations produced have a poor bond strength and that changes in the sample geometry does not result in a better bond quality. Experiments were also carried out on zirconia / AISI 316 foil / zirconia combinations. These bonds appear to have a higher bond strength. The experimental results can be explamed usmg the FEM calculations of the residual stresses.
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It was also found that during the cooling phase of the bonding process the tetragonal zirconia adjacent to the interface partly transforms to monoclinic zirconia, resulting in a constraint surface layer. The transformation appears to have a negative effect on the bond strength. During the bonding process Fe diffused into the zirconia. The tetragonal-monoclinic phase transformation and the diffusion of Fe result in a black layer on the zirconia interface. The layer thickness increases with increasing bonding temperature, bonding time and bonding pressure.
in
Samenvatting
Dit onderzoek betreft het diffiisielassen van zirconia aan AISI 316 en zirconia aan zirconia met behulp van een AISI 316 tussenlaag.
Zirconia is een interessant en veelbelovend technisch keramisch materiaal. Het heeft enkele uitstekende eigenschappen zoals een goede hoge temperatuur sterkte, een goede corrosieweerstand en een lage thermische geleiding. Om in praktische toepassingen optimaal van deze eigenschappen gebmik te maken en minder goede eigenschappen van het zirconia, zoals bijvoorbeeld de brosheid te omzeilen, is het veelal wenselijk het zirconia te verbinden met metalen. AISI 316 is een veel gebruikte austenitische roestvaste staalsoort dat een kleine hoeveelheid Mo bevat om de hoge temperatuur eigenschappen, bijvoorbeeld de kruipbestendigheid, te verbeteren. Diffiisielassen is een mogelijke manier om zirconia duurzaam aan AISI 316 duurzaam te verbinden.
Experimenten werden uitgevoerd waarbij het effect van de procesparameters op de laskwaliteit onderzocht zijn door de lastemperatuur (1000°C-1250°C), de lastijd (30, 90 en 360 minuten) en de aandrukkracht (2 en 15 MPa) te variëren. De lassen zijn onderzocht met behulp van optische microscopie, elektronenmicroscopie (SEM), Röntgendiffiractie (XRD) en röntgenmicroanalyse (EPMA). De vorm van het staalgedeelte is systematisch gevarieerd om na te gaan of deze aanpassing tot een betere verbinding leidt. De verschillende geometrieën zijn gemodelleerd met de eindige elementen methode (FEM) om de restspannüigen te bepalen.
Uit de experimenten blijkt dat de zirconia / AISI 316 combinaties een lage verbindingssterkte hebben. Het varieren van de geometrie bleek niet tot betere verbindingen te leiden. De zirconia / AISI 316 fohe / zirconia combinaties hebben een redelijke sterkte. De experimentele resultaten zijn te verklaren met behulp van de uitgevoerde eindige elementenberekeningen.
IV
Tijdens het afkoelen vanaf de procestemperatuur naar kamertemperatuur transformeert een deel van het tetragonaal zirconia aan het grensvlak tot monoklien zirconia, waardoor er een oppervlaktedruklaag gevormd wordt. Deze fasetransformatie is slecht voor de verbindingssterkte. Tijdens het proces diffundeert Fe in het zirconia. De fasetransformatie en de diffusie van ijzer in het zirconia leiden tot de vorming van een zwarte laag aan het zirconiaoppervlak. Deze laag neemt in dikte toe bij toenemende lastemperatuur, lastijd en aandrukkracht.
V
Contents:
Summary I
Samenvatting (Dutch summary) III
Contents V
1. Introduction 1
2. Theoretical Background 3 2.1. An mtroduction to diffusion bonding 3 2.2. Stages during bonding 4 2.3. Diffusion 7 2.4. Thermal stresses 9 2.5. Zirconia 10 2.6. AISI 316 stainless steel 12
3. Equipment and materials 13 3.1. Equipment 13 3.2. Materials and preparation 13 3.3. Testing device and analytic techniques 14 3.4. Finite Elements Method (FEM) 16
4. Results 18 4.1. Optical analysis 19 4.1.1. Visual analysis 19 4.1.2. The bonding surfaces 19 4.1.3. Cross sections 22 4.2. X-ray diffraction (XRD) and electron probe microanalyses (EPMA) 24 4.2.1. XRD results 24 4.2.2. EPMA results 25 4.3. Results FEM 26 4.3.1. "End-to-end" bonds 26 4.3.2. Notches 27 4.3.3. Foils 28 4.3.4. Summary 28
5. Discussion, conclusions and recommendations for further research 30
References 37
Appendix A: EPMA
Appendix B: FEM
Diffusion bonding of zirconia to austenitic stainless steel 1
1. Introduction
Each material has its own weak and strong specific properties. Ceramics can be used imder
severe circumstances as high temperature or in an aggressive environment and have some
specific optical and electrical properties. A disadvantage is the brittle character of this type of
materials. Metals are tough and strong and have proven their use in many applications, but in
general their corrosion and high temperature properties are rather poor. Combining of
ceramics and metals is expected to lead to better (combined) properties and is therefore an
important topic in materials research.
A method to join ceramics with metals is diffusion bonding. In this master thesis, the possible application of this technique is investigated for the combination zirconia / AISI 316 stainless steel. Zirconia is a very interesting technical ceramic. It has a low heat conduction coefficient, good mechanical and chemical properties at high temperatures, which makes it interesting for protecting metal against high temperatures, for instance in turbines. The thermal expansion coefficient is relatively high compared to that of other ceramics, which makes the material interesting for this bonding technique. The main feature of zirconia is that controlling the tetragonal-monoclinic phase transformation, the zirconia can be toughened.
AISI 316 is a very important austenitic stainless steel with a small amount of Mo to make it less susceptible for creep.
A literature study [1] on diffusion bonding of zirconia to metals showed that the interesting diffusion bonding combination zirconia/AISI 316 was hardly studied in detail so far.
However, a study by Derby [2] mdicates that good bondmg results should be expected for the zirconia/AISI316/zirconia diffusion couple.
The aim of this study is the determination ofthe possibihties of bonding zirconia to AISI 316 stainless steel by diffusion bonding. The influence of the bonding temperature, bonding time
Diffusion bonding of zirconia to austenitic stainless steel 2
and bonding pressure on the bond quality was investigated to estabhsh the opthnal process
conditions. Special interest was therefore paid to the physical phenomena that occur during
the bonding process at the interface and during cooling from the bonding temperature to room
temperature. In this study, emphasis is put on the microstructure ofthe zirconia.
With the finite elements method (FEM) the cooling is simulated to determine the residual
thennal stress levels. Different geometries of the steel part in the zirconia / AISI 316
combination have been modelled by FEM and experimentally tested to determine the best
geometry for the diffiision bond. The bonds made were studied by different techniques such as
electron probe microanalyses (EPMA), scanning electron microscopy (SEM), optical
microscopy and X-ray diffraction (XRD).
The theory of diffusion bonding is given in chapter 2. In this chapter the specific properties of zirconia that play a role in the process are also given. The experimental set-up and the equipment for analysing the bonds are discussed in chapter 3. The results of the bonding experiments and the FEM calculations are presented in chapter 4. In chapter 5 the results are discussed, summarised and conclusions are drawn. Recommendations for further research on this topic are also given in this chapter.
Diffusion bonding of zirconia to austenitic stainless steel 3
2. Theoretical Background
2.1. An introduction to diffusion bonding
During the diffusion bonding process two clean and smooth surfaces are pressed against each other at an elevated temperature and are held together until a bond is achieved. The mam feature of diffiision bonding is that there is no melting during the bonding process, unlike in the case of brazing or fusion welding. The technique is defined by Derby [3] as: "Diffusion bonding is a solid-state joining process by which two nominally flat surfaces are held together at an elevated temperature using an applied interfacial pressure". With this technique it is possible to join materials of the same type, like metals, or to join dissimilar materials like metals and ceramics.
The three main process parameters are temperature, pressure and time. The temperature is usually in the range 0.5 - 0.8 ofthe lowest melting temperature of the two materials. Here the temperature is given m Kelvin. The pressure is typically a fraction of the room temperature yield sfress. The last of the three parameters is the time. The holding time can vary from a few minutes to several hours. If the materials are dissimilar, their elastic properties and thermal expansion coefficients are also often different, which can cause problems. Therefore, the heating and cooHng to and from the bonding temperature must be done carefully to minimise thermal sfress. The process takes place at high temperatures and pressures, so creep of the metal can also play an important role. The bonds are expected to be capable to withstand high temperatures and the already mentioned advantage of joining dissimilar materials makes this technique very promising for special applications. However, like every other technique it has also disadvantages. Because of the high temperatures and vacuum conditions within the bonding chamber (usually <10"^ Pa), which are needed in the process, it is an expensive (a) (c) Ceramic
Metal
(b) Ceramic Ceramic
Metal Metal
Figure 2.1: Interface formation in a ceramic-metal diffusion bonding [4]. a) Hard ceramic and comparatively ductile metal surfaces come into contact at the asperity tips. b) Ceramic asperities indent the metal surface, which yields plastically under large local stress. c) Deformation continues primarily in the metal, coupled with diffusional mass transfer leading to shrinkage of the voids. d) Finally the bond is formed, ideally a perfect interphase boundary. Diffusion bonding of zirconia to austenitic stamless steel 4
technique. By the required carefully heating and cooling to minimise thermal stresses, the
process time can become rather long. Furthermore, the microstracture and properties of the
metal can change dramatically during the long holding time at high temperatiures.
The main ahemative techniques for jommg metals to ceramics are brazing and ultrasonic
bonding. In other fields of materials engineering, metal-ceramic combinations are also playing
a role like in coatmg metals with a thin fihn (CVD, PVD, plasma spraying, etc.), sintering and metal-matrix composites.
2.2. Stages during bonding
As mentioned before different types of materials can be bonded by diffiision bonding. In principle diffusion bonding of a ceramic to a metal is similar to that of a metal to a metal.
However, because ofthe relatively high elastic modulus ofa ceramic, compared with a metal, the stages in the process are somewhat different. In figure 2.1 a schematic picture of diffusion bonding of metal-ceramic is given. First, metal-metal diffusion bonding will be discussed after which attention will be given to the metal-ceramic bonding process.
In the initial stage of the diffiision bonding process the two surfaces have to make intimate contact. Even the flat and pohshed surfaces are at small scale still rough. Contact is only made between some asperities (figure 2.1a). Between these contact areas "voids" are present which can act as weak spots in the final bond and thus they must be eliminated during the bonding process to obtain good joints.
Derby et al. [3,5] proposed a three-stage theory. This theory is based on the shrinkage of the voids due to seven mechanisms observed during pressure sintering. bl
Figure 2.2: The mechanisms proposed for void closure, a) surface source mechanisms, b) bond-lined source mechanisms and c) bulk deformation mechanisms [61 Diffusion bonding of zirconia to austenitic stainless steel 5
In the first stage the area of contact becomes larger by means of instantaneous plastic
deformation (flow) due to the apphed pressure. The first stage is assumed to be time
independent. The stresses decrease and the second stage starts.
In the second stage diffusion, which is time dependent, plays the dominant role. In this stage
the voids have a small aspect ratio. In the third stage the voids are cylindrical and will be
partly eliminated.
The different mechanisms in the second and third step suggested are:
1) surface diffusion from a surface source to a neck;
2) volxmie diffusion from a surface source to a neck;
3) grain boundary diffusion froma n interfacial soiu-ce to a neck;
4) volume diffusion from an interfacial source to a neck;
5) power-law creep;
6) plastic yielding of original contacting asperities;
7) evaporation from a surface soxace to condensation at a neck.
Figure 2.2 shows the first six mechanisms. It is not entirely clear what is the contribution of
each mechanism to the total process, what is the order of these mechanisms and how they affect each other.
These mechanisms can be subdivided according to their driving force in the following three groups (see also figure 2.2). The driving forces are:
a) the difference in curvation ofthe surface, surface energy (1 and 2);
b) lowering the chemical potential (3 and 4);
c) the applied pressure (5 and 6). Figure 2.4: Void closure and deformation in a,b) metal-metal hond, c,d) ceramic- metal bond, a,c) before and b,d) after bonding process. Diffusion bonding of zirconia to austenitic stainless steel 6
The model is refined by Hill and Wallach [6]. They took the initial void shape as an ellipse,
whereas Derby modelled the surface as long parallel ridges with straight sides, hi figure 2.3
the geometric designs of the surface used by Derby (a) and Hill (b) are given. The
characteristic height and groove width they choose is 0.2- 2 \xm and 30-70 |j,m. By taking an
elliptical initial void shape the last two stages of Derby's model are combined. This
representation is more reahstic than the earlier model by Derby.
When the voids are spherical, surface sources do not contribute any more to the void
shrinkage. The seventh mechanism is thought to be not important in diffusion bonding and
Hill and Wallach do not take it into account.
The theory given above refers to metal-metal bonds. In the case of ceramic-metal bonds there are some deviations, since only the metal is assumed to be able to deform. Figure 2.4 gives a picture of this difference. In the case of metal-metal the bonding can take place at or across the initial interface. The interface can even totally disappear. In the case of ceramic-metal bonding the ceramic surface remains intact and will always remain visible after bonding.
The kind of interface is also important. Okamoto [7] classified interfaces in four groups.
Figure 2.5 shows the different groups and their concentration profile. Diffusive interfaces are typical for metal-metal bonding.
During bonding, at the high temperatures, reactions can occur between the metal and the ceramic or with the environment, for instance a metal oxide can be formed at the surface. By diffusion, elements can be solved in the other material or a new phase can be formed, for instance a low melting eutectic. These processes have an effect on the bond. Some will enhance bonding, others are disastrous for the bond strength. Figure 2.5: Types of interfacial structures, a) Non-reactive and non-penetrative Penetrative, c) Reactive, d) diffusive.[7] Diffusion bonding of zirconia to austenitic stainless steel 7
2.3. Diffusion
Diffusion mechanisms are an important part of the models describing diffusion bonding. During the bondmg process, at the elevated temperatures, atoms can change their position. When the temperature increases atoms become more mobile. The direction and the ease of their movement depend on the lattice structure, material, vibration frequencyetc . The driving forces for diffusion can be chemical reactions or concenfration gradients. Fick related diffusion to concenfration differences [8]. The flux J of diffusing particles and the concenfration profile are connected by the diffusion coefficient D, which is a measure for the ease of diffusion ofthe atoms through the material. This is described by Fick's first law (for one dimension):
with: J the flux (kg s'm-^) D the diffusion coefficient (m^ s'') X the position (m) C the concenfration (kg m'^)
Fick's second law describes the change of the concenfration profile in time during diffusion and is defined as (for one dimension):
dt ~ dx (2)
The change in the concenfration gradient is parabolic in time. The position x is proportional to the square root of D*t. The diffusion coefficient used in the laws of Fick can in general be fitted by:
(3) Figure 2.6: Schematic concentration profiles at successive times (0 with: Do the pre-exponential factor (m^ s"'); Q the activation energy (J mol '); T the temperature (K); R the gas constant (8.31 J mor'K *). With these equations the main features of diffusion are described. There is a threshold value Q to start the diffusion process. Atoms have to cross a certain energy hill before they come in a favoiirable lower energy valley. This activation energy depends on the lattice and the type of bonding between the atoms. In general diffusion is easier in metals with a metal bonding than in ceramics which are characterised by a stronger covalent or an ionic bondmg. This difference in type of bonding is expressed in the difference in activation energy for diffusion. The stronger type of bonding becomes also apparent in the higher melting point of ceramics. An increased temperature enhances the diffusion, because the mobility of atoms becomes higher. Beside the high temperature for plastic deformation in the first stage of the bonding process, high bonding temperatures therefore are also required for the diffusion mechanisms in the second stage of bonding. The temperature dependence of the diffusion coefficient has an exponential shape as expressed by equation (3). The pre-exponential diffusion coefficient, T-independent, is a material constant. It depends on lattice structure and size of the diffusing atoms. Diffusion is relatively easy across the surface. On the surface the atomic mobility is high. The mobility along grain boundaries is lower. The diffiision coefficient in the bulk is the lowest compared with that across the surface and along the grain boundaries. Atoms in the bulk are bonded and surrounded on all sites and have thereby a relatively low mobility. There is also an effect of the grain size on the diffusion rate. The grain size and hence the amount of grain boundaries is not constant during the bonding process. Due to grain growth at elevated temperatures the amount of grain boimdaries, which are fast diffusion paths, decreases and consequently, the diffusion rate decreases. When the process time is increased there is more time for the atoms to reach the equilibriimi position of lowest energy. The covered distance is proportional to the square root of D*time. In figure 2.6 the effect of time on the concentration profile is given. Describing the diffiision of atoms becomes more complex when there are more different atoms involved or when there are reactions taking place. For instance, reaction layers can grow on the interface into (one or both of) the parent materials. It BONDINC ROOM TEMPERATURE TEMPERATURE COOLING (xlO MPa) Figure 2.8: Schematic picture of Figure 2.7: Finite element models deformation of joint during a) (FEM) for the thermal stresses in a mechanical and b) thermal loading. steel-Si^N^ diffusion bond. N.B: the stresses are not homogeneous distributed [10]. °'c<°m °'c>°m Ceramic Ceramic ^^Tension^ Tension Tension Compression Compression Metal Metal Figure 2.9: Schematic illustration of the stresses andfailure modes due to differences in the thermal expansion coefficient a. The left picture is the situation as expected for the zirconia /AISI316 combination [11]. Diffusion bonding of zirconia to austenitic stainless steel 9 2.4. Residual thermal stresses As mentioned in section 2.1 the difference in thermal expansion between the two joining materials introduces thermal stresses. These stresses can deform or break the fresh bond during cooling down from bonding temperature to room temperature. At the high bonding temperature the bond is formed without sfresses. Generally speaking, after cooling the residual sfress pattem is very complex. The differences in thermal expansion coefficient between the two joining materials have to be kept as low as possible to avoid high residual thermal sfresses. The residual thermal sfress can be estimated with [9]: El*E2 „- (al-a2)Ar fA\ where Ej is the elastic modulus of material one (Nm"^); Ej is the elastic modulus of material two (Nm"^); ttl is the thermal expansion coefficient of material one (K"'); ttj is the thermal expansion coefficient of material two (K''); AT is the temperature change during cooling (K). The exact result is less important, because it is mostly far above the yield sfress of metals and thereby the metal will deform plastically to lower the sfress. In figure 2.7 an example of deformations and residual thermal sfresses is given in a diffusion bond of Si3N4 to steel [10]. In figures 2.8a and 2.9 the general effect of the thermal expansion mismatch is given schematically [11]. Thermal sfresses grow if the contact surface of the joint is increased. The thermal expansion coefficient of zirconia is 10.10"* K"' and that of AISI 316 is 17.10"* K"' (for other material properties see table 3.4). A possibility to lower thermal sfresses is the use of interlayers. A ductile interlayer can absorb part of the residual thermal sfress by plastic deformation. Interlayer materials can also be chosen on the basis of their medial thermal expansion coefficient. An interlayer can enhance the effective contact area, for instance by forming a low melting phase which improves the wettability and the adhesion to the ceramic or enhancing a reaction to occur that favours the bonding process. For the joining of ceramics to ceramics interlayers are always needed. 8w%Yj03 3000 - ! 2500h Figure 2.10: Phase diagram zirconia -yttria, M: monoclinic, T: tetragonal and C: cubic [13] Diffusion bonding of zirconia to austenitic stainless steel 10 The metals that are usefiil because of a matching (low) thermal expansion coefficient have in general a high melting point and a high yield pomt. Some examples of these hard metals are Re, W, Mo and V. Other metals are used for their low yield point. These metals can lower the thermal stresses by plastic deformation. Some examples of these soft metals are Ag, Al, Cu, Ni and Fe. Aimealing, in vacuum or air, can reduce stresses, but in most cases aimealing is accompanied by reactions. These reactions can enhance bonding, but it is also possible that brittle or gaseous phases are formed, which might lead to debonding. The finite element method (FEM) can be used to obtain an idea of the development and the distribution of the thermal stresses (see section 3.5.). Measuring the thermal stresses locally by a strain gauge. X-ray techniques or calculate fi-om dilatometer measurements is difficult. 2.5. Zirconia Zirconia is a very interesting technical ceramic. By its intemal structure it has some special properties. The properties which makes it very interesting for diffusion bonding are relatively high toughness, which is a weak point of ceramics in general, and a relatively high thermal expansion coefficient compared with other ceramics Some properties make zirconia interesting for specific applications. For instance, zirconia has a very low thermal conductivity, which makes it extremely usefiil in thermal barrier coatings. Zirconia also has a good chemical resistance. Therefore, it is used for instance in pumps in the chemical processing mdustiy [12]. The high rehability makes it useful for medical miplants. Its high diffusivlty for oxygen in the cubic phase, due to a high ionic defect concentration, is apphed in oxygen sensors. The relativity high strength at elevated temperatures and hardness are also reasons for increased use in the near future. Zirconia, ZrOz, is the oxide of zhconium. It exhibits three well defined polymorphs: the monoclinic phase (to 1170 °C), the teti-agonal phase (1170 °C-2370 °C) and the cubic phase (to the melting point of 2680 °C). This can be seen in the phase diagram of Zr02-Y203 given in figure 2.10. The special properties of zirconia ceramics are ascribed mainly to the occurrence ofthe tiansformation fiom the tetiagonal to the monoclinic phase (T-M tiansformation). The transformation is accompanied by a large volume change (3%-5%). This makes it impossible to manufacture large parts of pure zirconia. By addmg stabihsers as Mg, Y, Ce and Ca this phase transformation can be used such as is done in structural zirconia ceramics to improve the mechanical properties. These sinter additives form cubic oxides and a solid solution is Diffusion bonding of zirconia to austenitic stainless steel 11 generated. This makes it possible to design the desired properties of the ceramic. Zirconia added to other ceramics can improve the strength of these ceramics by the T-M transformation, for instance in zirconia toughened alumina. The tetragonal-monoclinic phase transformation will be deah with in detail later in this section. If insufficient stabiUsing oxide is added, partially stabihsed zirconia (PSZ) is produced, but the cubic phase can also be fully stabihsed so that no transformation occurs with a possible dramatic volume change. Also the tetragonal structure can be stabihsed by alloying the ceramic. The tetragonal polycrystalline zirconia (TZP) has excellent mechanical properties, when it is based on the zirconia-yttria system (see figure 2.10). The yttria content in the zirconia is the variable, which controls the properties. Minimal 1.8% Y2O3 is necessary to stabihse the tetragonal phase and the maximimi strength of TZP is obtained at about 3% Y2O3. Between the fully stabihsed cubic and tetragonal form, a wide range of mixtures of these structures is possible, which gives opportunities to control the properties of the ceramic. It should be remembered that also the processing conditions like sintering temperature determine the phases present. Beside the amount ofthe different phases, the size ofthe particles and their distribution have an important influence on the structure and thereby on the properties. This offers great opportunities to design the properties of the zirconia used if the relations between structure, grain size and properties are understood. 2.5.1. The T-M transformation This transformation is thoroughly studied, but there is no consensus on all aspects. The summary given here is extracted from [9, 14, 15]. The T-M fransformationoccur s during coohng. A re-fransformation is possible, which is exothermic. The tefragonal-monoclinic fransformationi s a martensitic transformation, which means that it occurs without atomic diffusion. The fransformation is induced by shear or dilatation (sfress induced reaction). Under consfrain a fransformation is not possible. Grinding or pohshing the surface can induce sfresses and so a fransformation,whereb y a compressed surface layer is formed. Only part ofthe tefragonal grains is fransformed and the amount of monoclinic phase drops with depth below the surface. The monoclinic phase has a larger volume than the tefragonal (a„= 5.156 A, c„= 5.304 A to at= 5.094 A, Ct= 5.177 A). Thereby the fransformation induces new sfresses, which can induce Diffusion bonding of zirconia to austenitic stainless steel 12 new transformations. Due to this phenomenon the process is autocataUtic and clusters of transformed grains are formed. The amoimt of volmne dilatation is about 4%. It is not clear whether the nucleation is homogeneous or heterogeneous. There is a critical grain size before the transformation starts (non-classical transformation). Subsequently, the transformation depends on the particle size. Coarse grains transform more easily during cooling. 2.6. AISI 316 Stamless Steel In the classification ofthe American Iron and Steel Institute (AISI) the 300 series are the austenitic stainless steels. The 304 and the 316 steels are the most common types with 16-18 % Cr, 10-12 % Ni and < 0.08 %C. AISI 316 contains some 2 % Mo to miprove the mechanical (creep) and anti-corrosive properties at elevated temperatures. The AISI 316 type is not very susceptible to forming secondary phases. Bonding force Zkconia • AISI 316 to Alumina or tuïbomolecular pump to HF generatof Tantalum 3 Water coding 1=1 Teflon (PTFE) Copper seal Viton seal Figure 3.1: Diffusion bonding furnace [4]. Figure 3.2: Centre of the furnace with the specimen, a) Pressure bar, b) W-Re thermocouple, c) AI2O3 table, d) Ta-susceptor and e) HF-coil (Cu, water cooled). Diffusion bonding of zirconia to austenitic stainless steel 13 3. Equipment and materials 3.1. Equipment In figure 3.1 the diffusion bonding furnace used for the experiments is shown. In figure 3.2 the heart ofthe furnace is shown in detail. The rounded pressure bar must ensure good lining ofthe specimen and guarantee a homogeneous pressure on the bonding interface. The steel part is placed on the side of the thermocouple to ensure a good measurement ofthe temperature on the bonding interface, because the steel has a good thermal conductivity. The temperature is measured by a Tungsten-Rhenium thermocouple (WRe5 to WRe26), which is positioned below the specimen. The heat is supplied by inductive heating using a HF-generator (1 MHz, 8kW, Himmel). A susceptor is placed in a coil of five windings. The inductive current in the cylinder will generate heat. The specimen placed in the middle of the cylinder is heated by radiation. The heating is controlled by a PID-unit (Eurotherm). During the process, a 6-channel recorder prints the temperature and the gas pressure. The standard program of a bonding experiment is schematically shown in figure 3.3. The program starts when the vacuum is about 1.10'^ Pa. The specimen is heated till 50 °C below the bonding temperature at a rate of 25 °C/min. After this, the specimen is heated at a rate of 5 "C/min to the process temperature. This procedure will prevent temperature overshoot. The bonding time is standard 90 min. After the bonding time, the specimen is cooled at a rate of 10 °C/min to 200°C. Then the cooling can not be controlled further and the specimen cools down exponentially to room temperature. The vacuum of about 4.10"'' Pa is obtained by a combination of a rotary vane vacuum pump and a turbomolecular pump. The gas pressure is measured by a Piranni cold cathode pressure gauge. The bonding force is supphed by an ah pressure cylinder and can vary between 0-2.5 MPa (small cylinder) and 14-37 MPa (larger cylinder). Most experiments were carried out under an applied mechanical presswe of 2 MPa to limit the plastic deformation ofthe steel specimen. 3.2. Materials and preparation The chemical composition of the zirconia (TZP) used was detennined by X-ray fluorescence technique and is given in table 3.1. The zirconia is hot-isostatic pressed fi-om 70 nm powder TCC) time (a.u.) Figure 3.3: Schematic temperature profile A) heating, B) bonding time, C) cooling, l)25°C/min, 2) 5°C/min, 3)process temperature, 4) -l(fC/min and 5) free cooling. Table 3.1: Chemical composition of the zirconia determined by XRF. element ZrO, CaO A1,0, SiO, HfO, Weight% (Wt%) 89.12 4.97 0.273 1.83 0.494 1.80 Atom% (At%) 92.15 2.80 0.620 2.29 1.047 1.09 Table 3.2: Chemical composition of AISI 316 determined by XRF and gas analyses (*). element _Cr M Mo _Si Mn Cu Je Wt% 16.65 10.46 2.03 0.808 0.029 1.45 0.281 0.023 0.036 67.27 At% 17.87 9.94 1.18 1.60 0.13 1.47 0.247 0.040 0.143 67.27 Diffusion bonding of zirconia to austenitic stainless steel 14 and it was supplied by Gimex b.v., Nieuwegein, The Netherlands in the form of rods. The rods were cut by a water cooled diamond saw to discs of 5 mm thick with a diameter of 11 mm. Polishing was carried out by the Department for Applied Physics, Delft University of Technology. The surfaces were ground and polished, using a diamond slurry xmtil a swface roughness R, = 0.03 |^m was obtained. The two surfaces were still parallel after polishing and no curvature was detected. The original material AISI 316 was present in rolled bars of 10 mm diameter and a grain size of about 14 |xm. Metal discs of 5 mm thickness were cut firomthi s bar by a water cooled SiC saw. The pohshing procedure was the same as for the zirconia discs. The composition of the AISI 316 stainless steels was determined by the X-ray fluorescence technique and the presence of hght elements C, N and S was determined with the gas analysis technique. The composition ofthe AISI 316 is given in table 3.2. Foils of AISI 316 steel were cut fiom the same bars by the water cooled SiC saw or made with a lathe and groimd and polished with 1 \im diamond slurry by hand. The adjusted geometries, notched and "comer notch", were prepared from the discs in a lathe. The notch was 1 mm wide and had a depth of 1 rmn. The ceramic and metal discs and foils were ulfrasonically cleaned in ethanol and stored in 2- propanol. Before use they were dried by blowing hot air. 3.3. Testing device and analysis techniques The sfrength of the bond was characterised by a shear sfrength test at room temperature. The shear sfrength test is a feasible testing method, which gives rehable information because the test parameters are well known and the outlining of the specimen in the testing machine is easy. Unfortunately, the sfrength data can not be compared properly with the three or four point bending test, which is a testing technique also carried out regularly to characterise diffusion bonds. In the case ofthe bending test the preparation of the testing specimen and its outlining is more froublesome and the tested volume is rather small. Tensile testing is difficult to obtain a purely tensile state and the tensile properties of ceramics are not their best properties. Unfortunately, there is no accepted standard for testing ceramic-metal diffusion bonds. Force Force Moving sample holder Figure 3.4: Shear testing device, a) side view, b) front view [12]. Table 3.3: Etching agents and techniques for optical microscopy [14, 16, 17, 18]. Etching agent composition etching reveals time/technique Kalling 5g CUCI2,100 ml 2-10 min Grain boimdaries ethanol, 100 ml HCl AISI 316, carbides Glyceregia HCl, HNO3, glycerol 6-60 s Grain boundaries AISI 316, cr-phase Vilella's reagent lg picric acid, 5 ml CT-phase, martensite HCl, 100 ml ethanol Marble reagent 4gCuS04,20 ml HCl, a-phase 20mlH,O Murakami's reagent 10gK3Fe(CN)„10g 1-3 min eventual secondary phases KOH, 100mlH,O boiling HF 40 % HF lh grain boundaries ZrO, Thermal etching 6hatll00°C grain boundaries ZrO, Diffusion bondmg of zirconia to austenitic stainless steel 15 In figure 3.4 the shear testing device is shown. It is mounted m a Tha test 2300 tensile and compressive testing machine. The ceramic part of the bond is fixed in the static part and the metal part is fixed in the moving holder. The shear stress is calculated by dividing the measured strength by the surface area. Optical microscopy was performed with a Leitz Neophot 2 optical microscope, using bright field, polarised hght and the interference contrast technique. Magnifications up to 1000 tunes can be obtained. For small magnifications a stereo microscope was available. For higher magnification scanning electron microscopy (SEM) was used (Jeol JSM 6400 F), equipped with EDX-analysis unit. After debonding, the bonding smrfaces were directly studied by microscopy. Cross-sections were made by sawing using a water-cooled diamond or SiC saw (in the case of steel). These cross-sections were embedded, groxmd with SiC paper (to Grid 600) and pohshed (with diamond slurry of grain size of 1 \xm). For SEM images the zirconia specimen was coated (Au for optical SEM pictures, C for EDX analyses) to make it sufficiently conductmg. For SEM and EPMA the specimens were not embedded during preparations. During grinding and polishing care was taken to avoid the disproportional wear of the metal part, as it is much softer than the ceramic part. Also care had to be taken that no phase transformation in the zirconia or larger deformations in the AISI 316 were induced. Different etching agents were used for AISI 316 (see table 3.3) to reveal possible secondary phases as the % phase and the 0 phase in the stainless steel. With Kalling the best resuhs were obtained, especially revealing grain boundaries and twinning. The ceramic can be etched by thermal etching or using HF or boiling H2PO3. HF was used to reveal the original structure of the zirconia. Complementary to the optical analyses, EPMA and XRD techniques were used. Line scans across the interface were made with a Jeol JXA 733 Electron Probe X-ray microanalyser (EPMA) to determine the element distribution profiles of the main elements. Four wavelength dispersive spectrometers and an energy dispersive spectrometer were used for this measurement, which is fully automated with the Tracer Nothem system TN 5500 and 5600. The elements that were measured are Zr, O, Y, Hf, Fe, Cr, Ni, and Mo. Al, Si and Mn were measured to ensure that these elements present did not influence the measurements of the main elements. The lattice structure of the zhconia was determined by X-ray diffraction (D5005 AXS diffractometer, diffracted beam monochromates). The 2-9-range varied from 20" to 80° in steps of 0.050° and a step time of 4 s. The acceleration voltage was 45 kV and the Figure 3.5: Studied geometries by Raewska: a) "end-to-end", b) "tappered" and c) "button bonds. Me: metal andKe: ceramic [20]. ZrO, 316 316 I 316 ZrO, ZrO, ZrOji ZrO, r Figure 3.6: Studied geometries is this survey: a) "end-to-end" or direct, b) notch, c) corner notch and d) foil. W^ï lis ill • ' ^ 1 Ih -mniiuiiiiJii Ullllllllllllll ÜHIIIIIIIIIIU + Figure 3.7: Standard mesh used for FEM calculations. Zirconia on left side, AISI 316 right side Diffusion bonding of zirconia to austenitic stainless steel 16 measuring current was 30 nA. The X-ray diffraction pattem was measured with Cu Ka radiation. 3.4. Finite Elements Method (FEM) hi this investigation FEM is used to calculate the development and distribution of the residual sfresses in the bonded specimen. At the starting point of the calculation, the cooling starts and the bond is considered to be perfect. The sfresses modelled are only caused by thermal mismatch. hi hterature on diffusion bondmg FEM calculations are often reported [4, 12, 18, 19, 20] to explain experimental resuhs or to refine the common models on diffusion bonding. Different shapes and geometries for diffiision bonds are suggested and attention is paid to the level of residual sfresses, calculated by FEM. Suganuma [21] compared circular and rectangular bond face joints. Raewska [20] compared the residual sfresses of dhect bonds (end-bonds) to conical and tapering bonds (see figure 3.5). Lado et al. [22] compared metal encircled by ceramic and ceramic encfrcled by metal bonds with end-bonds. Some of these bonds showed lower levels of thennal sfresses, especially at the edge. A further advantage of these bonds is the help of the shape in outlining of the specimen. In this research project FEM is used to compare the influence of different geometries on the sfress levels and eventually to explain certain features like cracks in the microstructure. The various shapes investigated here are shown in figure 3.6. The choice to vary the shape of the metal part originates from the fact that metal can be more easily shaped than the brittle ceramic part. The different geometries will be compared with the "end to end" form and with each other. FEM calculations are also used to determine the ideal shape, dimensions and place ofthe notch and the effect of the thickness in the case ofthe "foil" specimens. The FEM calculations were carried out with the program MARC 7.1 on a Digital DEC 3000 work station. The preparation of the model and the evaluation of the results are carried out using the program Mentat 3.1.0. The standard model used for the calculation is depicted m figure 3.7. The size of the discs modelled is 5 mm thick and 10 mm in diameter. Because of the cylindrical symmetiy a "body of revolution" shnphfies the model and an axisymmetric FEM calculation can be done. This approach shortens the computing time and is more Table 3.4: Material properties used in FEM calculations. Property ZrO, AISI316 E (GPa) 205r231 172r251 V 0.3[23] 0.34[25] a(10-*K-') 10[23] 16.5[26] pdO^kgm-^) 6.05[24] 7.9[26] conductivity (W/mK) 2 [24] 16r26] 800[231 500r261 plasticity (kN/mm^) none 196 (0°C) [261 178 (SO^C) [261 160 (100°C)[261. 130 (250°C)[261 108 (520°C )r261 D Bonding Fixed side pressure in X direction 2 MPa Fixed side in direction y syinm.-as : Heat flux Q Figure 3.8: Applied boundary conditions for FEM calculations Diffusion bonding of zirconia to austenitic stainless steel 17 accurate than a 2-dimensional model. Each disc is divided into 400 square elements and the mesh is refmed towards the interface and the edge. This is expressed in a bias factor of 0.25. The used material properties are given in table 3.4. Only the yield stress of the steel has a temperature dependence in this approach. No phase transformation or chemical reactions with thermal or stress effects are modelled. The calculation is only carried out to simulate cooling of a perfectly bonded material combination. Linear elastic and ideal plastic behavioxu- is presumed according to the Von Mises criteria. The starting pomt of the calculation is 1200 "C, the process temperature. At this pomt it is assumed that the two materials are well and stress free bonded. The cooling rate is taken the same as in the experiments, 10 °C/min and the programs ends after 599 steps at a temperature of 25 °C. The heat flux through the outer surfaces (see figure 3.8) necessary to reach this cooling rate is about 1.2 • 10"^ W/mm^. No heat flux is assumed on the symmetry surface (y = 0). The bonding pressure is present during the entire experiment and not just during the bondmg time. Raewska [19] showed with FEM calculations of metal-ceramic diffusion bonds that for a correct modellmg the pressure should be taken mto account during cooling. In figure 3.8 all four boimdary conditions used in the model are presented. The standard model of figure 3.7 is adjusted for the "notch", "comer notch" and foil specimen. From the model the ideal shape should be determined. The notch tip is taken smoothly rounded. For this form some triangle shaped elements are taken. In the case of the zfrconia/steel foils/zfrconia models, different thickness ofthe foils are taken. In this case, the zhconia discs have the same mesh as in the standard model. The number of elements in the foils is varied with thickness and in the foil the bias is taken 0 in the x direction and 0.25 in the y direction. From the calculated stresses and deformations, the normal 11-sfress, the normal 22-sfress and the 12-shear sfress in the specimen were selected as being the most interesting. The global sfress pattem and the sfresses on the bonding surface and just below this surface in the ceramic and in the steel part were analysed. For specific locations, such as the interface, only qualitative predictions can be made. However, FEM can give a first indication if certain geometry results in lower sfress. A lower residual sfress level is expected to give better bonds. As the measuring of the sfress levels, for instance from dilation measurements or by X-ray techniques, is difficuh and making and testmg bonds of different shapes is time consuming, FEM models can save thne and material. Table 4.1: List of diffusion bonding experiments of zirconia/AISI 316, (*) unstable temperature profile. Type Temperature Bonding pressure Bonding time Number of °C MPa min experiments EfJ ect process parameter: "end to end" 1000 15 90 4 "end to end" 1050 15 90 3 "end to end" 1100 15 90 1 "end to end" 1100 2 90 3 "end to end" 1150 2 90 3 "end to end" 1200 2 90 3 "end to end" 1250 2 90 3 "end to end" 1200 2 30 1 "end to end" 1200 2 360 1 Other geometries: Notch 1200 2 90 3 "C-notch" 1200 2 90 3 0.4 mm foil 1200 2 90 1 0.5 mm foil 1200 2 90 3 Other experiments (not discussed): "end to end" 1200-1400 (*) 2 360 2 hand made notch 1100-1200 (*) 2 90 2 hand made notch 1200 2 90 1 0.2mm 18/8 foil 1200 2 60 1 Diffusion bonding of zirconia to austenitic stainless steel 18 4. Results In this chapter the resuhs of the experiments on diffusion bonding of zirconia to AISI 316 are described. All experiments are hsted in table 4.1. In the first experiments "end to end" bonds were made at a bondmg temperature of 1000 °C (0.74 TJK)) to 1100°C (0.80 TJK)). The bondmg time was 90 minutes and a mechanical load of 15 MPa was applied. The bonded specimens did not show any mechanical strength. Furthermore, a significant deformation of the steel part was observed. Therefore the bonding pressure was lowered to 2 MPa, whereas the bonding temperature was increased from 1100°C to 1250 °C (0.89 T„ (K)) and also longer and shorter bondmg tunes (30 to 360 mm) were used. However, this did not resuh m sfronger bonds. "Notch" and "comer notch" specimens bonded at 1200 °C for 90 minutes and with a load of 2 MPa were also foimd to have no sfrength. Better results were obtained m the case of the combmation zhconia/ AISI316 foil/ zhconia produced under the same process conditions. First the optical analyses will be discussed in section 4.1. The visual analysis without the help of optical instruments is presented in section 4.1.1. In section 4.1.2. the surfaces of contact are subjected to a closer investigation by the use of optical microscopy and scanning elecfron microscopy (SEM). In section 4.1.3. cross sections of the specimens are freated. The "end-to- end" bonds ofthe first series experiments are discussed in detail, after which the deviations m structure of the other type of bonds, such as bonds with notches and foils will be given if differences are present. In section 4.2. the results of the XRD and EPMA measurements will be given. In section 4.3. the resuhs of the FEM calculations ofthe four geomefries are presented. First the sfresses in the "end-to-end" bonds will be discussed and after that the sfresses in the notch and foil models. Attention was paid if FEM could help explaming the optical analyses, for instance cracks and failure paths. Figure 4.1: Schematic picture of a zirconia /AISI216 foil/ zirconia combination after bonding. Figure 4.2: Zirconia surface after failure of a zirconia/AISI316 combination (90 minutes bonding time at a temperature of1200 °C with 2 MPa pressure, 200 x). Figure 4.3: SEM image of a zirconia surface after failure of a zirconia/AISI316 combination (90 minutes bonding time at a temperature of1200 °C with 2 MPa pressure) (10000 x). Diffusion bonding of zirconia to austenitic stainless steel 19 4.1. Optical analysis 4.1.1. Visual analysis To obtain insight in the structure of the diffusion bonds obtained, a visual exammation was carried out. Of all specimens only the foil specimens showed any bond strength. The others fractured during or directly after the bonding process On the ceramic side the impression of the mechanical pressure bar is visible. This dimple becomes larger with increasing temperature, which indicates that the ceramic becomes more ductile at higher temperatures. At higher bondmg temperatures the dimple becomes dark and the surrounding becomes brighter. On the other parts of the ceramic no colour changes are visible. The dimple in the case of foil spechnens is larger than the dimple of "end to end" specimens made imder the same process conditions. This indicates that the temperature was higher than denoted by the thermocouple. The thermocouple was placed against a zirconia part in the case of foil experiments, which has a poorer heat conduction than the steel part placed against the thermocouple in the case of "end to end" bonds. Furthermore, both ceramic parts deformed during the process and sometimes cracks are visible (see figure 4.1). Because of this deformation the specimens did not fit properly anymore in the shear testing device and no sfrength data could be obtained. The metal part also deformed sfrongly in all the experhnents. The change in diameter and height increased with increasing temperature and bonding pressure. On the bottom of the metal, a print ofthe table on which the specimen was placed, was observed. The outer surface became rough. In some experiments the outlining of the specimens was not perfect in line, which caused an unbalanced pressure distribution. 4.1.2. The bonding surfaces After failure of the bond the two surfaces of contact were examined. Both surfaces will be discussed separately below. The ceramic side In figure 4.2 the ceramic surface of a specimen (90 minutes bonding thne at a temperature of 1200 °C with 2 MPa pressure) is depicted. The used bonding conditions are taken as reference, because from literature they are expected to be the conditions for which bonding of 1000-1050 1200 1250 a) b) c) Figure 4.5: Schematical picture of misaligned specimen, a) zirconia surface, b) cross section bonded specimen and c) AISI 316 surface. Figure 4.6: Zirconia surface after failure of a zirconia/AISI316 foil/zirconia combination (90 minutes bonding time at a temperature of1200 °C with 2 MPa pressure, 1000 x). Diffusion bonding of zirconia to austenitic stainless steel 20 zirconia to this type of stamless steel is possible. Below the apphed temperature no strength was measured [2]. The white spots visible m figure 4.2 are large zhconia grains (~ 5 ^im) hnbedded in a matrix of small grains (~ 0.5 |j,m) (see figure 4.3). This structure is similar to that of the original material. Due to the high temperature the ceramic was thermally etched and the grams became visible. The etchmg effect is less pronounced when there is no steel pressing on the ceramic. Probably the etching is not pxirely thermal, but diffusion of some alloymg elements from or to the ceramic also plays a role. Systematic exammation shows that the higher the bonding temperature the clearer the spots. The larger grams are randomly distributed and have approxhnately the same size. The darker strings on the zhconia (see figure 4.2) are no second phase, but only smuts of a long shaped cave present in the opposhe metal part. The appearance of the joints made below 1050 °C is different, hi these joints, many little black spots are visible, covering the zirconia sample, which are spotty smuts of holes on the metal site. The black strings are relatively small and are buih up of little black spots. The string pattem has a size shnilar to the grain size in the metal. This suggests that the holes in the metal combine at higher temperatures. An explanation could be the formation of microflaws on the metal surface due to shrinkage effects. Spechnens made at or above 1250 °C have also a different appearance. The cracks become smaller and the white spots become brighter. The frendsdescribe d above are depicted schematically in figure 4.4. In a few experiments, the outlinmg of the ceramic and stainless steel discs was not perfect. If there was no metal on the opposite side the ceramic had still the bright colour ofthe original pohshed ceramic (moon sickle shaped area), whereas the area of contact was black (see figure 4.5a). The ceramic surface ofthe "notch" and "comer notch" bonds had the same appearance as that of the "end to end" bonds. The area of contact is also black. At the edges of the zhconia, a cfrcle as broad as the notch depth is visible, where the colour ofthe ceramic is brighter. In the broken foil bonds made at 1200 °C the same features can be observed, only the smuts form a tangled network, especially near the edge of the spechnen. Square spots are visible on the ceramic (see figure 4.6), which appear to be metal. Some of these square spots are black, others are bright. With Normansky differential interference contrast microscopy and dark field microscopy it was found that the whole zhconia interface was covered with stainless steel remnants. These foil bonds also failed macroscopically at the ceramic metal interface, but the Figure 4.7: Stainless steel surface after failure of a zirconia /AISI316 combination (90 minutes bonding time at a temperature of1200 °C with 2 MPa pressure, 200 x). Figure 4.8: White string with Si02 Figure 4.9: Backscatter image of stainless containment on SEM image of stainless steel surface after failure of the zirconia/ steel surface after failure of the zirconia/ AISI 316 combination (90 minutes bonding AISI 316 combination (90 minutes bonding time at a temperature of1200°C with 2 time at a temperature of1200°C with 2 MPa pressure, 500 x). MPa pressure, 10000 x). Diffusion bonding of zirconia to austenitic stamless steel 21 metal part seems to be more affected as appears from the broken pieces of metal on the zirconia side and the occurrence of a dense crack pattem. The metal side hi figure 4.7 the metal contact surface of a bond made under the standard bonding conditions is shown. As in the case of the zirconia interface, white spots are also visible on the metal interface. These spots are prints of the large zhconia grains. With SEM under higher magnification (lOOOOX) the prints of the smaller zirconia grains also become visible (see figure 4.8). Two kinds of strings are present on the metal surface, black and white strings. The black strings are long shaped caves. They appear to have the shape of grain boundaries, but on a back scatter image (figure 4.9) in which the grains are also visible, the strings seem to follow the grain boundaries only partly. The white strings have a "washed out" appearance. From SEM images it becomes clear that they he over the print pattem of the zhconia grains, which indicates that they are formed later in the process than the print pattem (see figure4.8) . With EDX it was found that these strings are Cr richer and also contain some contamination of hghter elements and SiOz- At higher temperatures or longer bonding times there are more white strings. Beside these strings there are bigger white areas, especially near the edges of the spechnen bonded and in bonds made at the higher temperatures. On these "white lakes" the white spots contours have a comet like shape, radial directed, pointing towards the outside (see figure 4.10). On some specimens some brown areas are visible, which are assumed to be due to local surface oxidation of the metal. The appearance of spechnens made below 1050 °C and above 1250 °C is deviating from normal behaviour in a similar way as the microstracture of the ceramics at these temperatures, hi the original stainless steel a distinct difference between the inner and outer side of the etched surface was found. The inner side showed a fine pattem of deformation bands caused by the rolling of the metal, whereas the outer side is plane. These differences between the inner and outer side are not visible anymore after the bonding process. On the metal surface which have not been in contact with zhconia due to outhning problems, the grains are clearly visible due to thermal etching during the bonding process (see figure 4.5c). Figure 4.10: Comet shaped spots on stainless steel surface after failure of the zirconia/ AISI316 combination. The upper side is directed to the outside ofthe specimen. (90 minutes bonding time at a temperature of1250°C with 2 MPa pressure, 1000 x). Figure 4.11: Cross-section of zirconia/ AISI316 foil/zirconia combination made at 1200°C, 90 min and 2 MPa (lOOx). a) with polarised light and b) with bright field optical microscopy. Foil is etched with Kalling. Diffusion bonding of zirconia to austenitic stainless steel 22 The "notch" specimens do not show many differences with the above described "end-to-end" specimens, except that there is a circle as broad as the notch depth, where the metal is thermally etched. In the case of zirconia/ AISI316 foil/ zirconia spechnens, the metal surface has a different stmcture. It has a denser crack pattem and the cracks are partly connected forming a network, especially on the outer side of the specimen. The square points visible on the zhconia are also visible on the metal foil surface as holes. 4.1.3. Cross-sections The cross-sections of the two parts of the specimen remaining after failure show some interestmg features. On the ceramic side below the contact surface, a black layer is visible. This layer is visible by eye, under a stereomicroscope and with polarised light microscopy. Using normal light microscopy there is hardly any difference visible between this layer and the underlymg material. In figure 4.11a a cross-section of a zirconia/AISI316/zirconia bond (foil thickness 0.4 mm) is depicted obtained imder polarised hght conditions and in figure 4.11b a cross section is shown obtained under bright field conditions. It was found that all geometries have this black layer just below the zirconia interface. In figure 4.11b the layer appears smooth, whereas the underlying material has little holes. This is the only difference between the black layer and the bulk zirconia, which can be seen with normal light microscopy. The transition fiom the black layer to the bulk zhconia is not completely sharp as can be seen in figure 4.1 la taken with polarised hght. The Normarski Differential Interference Contrast technique, which reveals height differences, was also used to analyse the layer. It tums out that the thickness of the layer is much larger than the curvature of the surface at the edge of the specimen caused by the grinding and pohshing of the specimen and that the layer is no optical effect introduced by the preparation of the cross-section. It should be noted that the black layer is only present where the steel pressed against the ceramic. This is schematically shown in figure 4.5b. Edges ofthe ceramic where no metal was present, due to outlining problems, did not show a black layer. Also on the other sides ofthe ceramic part, there no black phase was visible, although it was assumed to have had the same temperature as the interface. On a comer of the ceramic of a not well-lined specimen, the black layer curves around the comer (see figure 4.5b). During grinding and pohshing it became clear that the black layer is harder than the underlying zirconia. The hardness of the original zirconia is about 1200 HV [23]. Layer thickness vs. temperature • 15 Mpa, 90 min •2 Mpa, 90 min A2 Mpa, 30 min ® 2 Mpa, 360 min 950 1050 1150 1250 Temperature C Figure 4.12: Thickness of black layer with bonding temperature. Figure 4.13: Cross-section of metal part bonded at 1200 "Cfor 90 min and 2 MPa bonding pressure. Etched with Kalling (6.3 x). Dashed line denotes the original shape of the metal part (5x10 mm). Diffusion bonding of zirconia to austenitic stainless steel 23 The thickness of the black layer hes between 25 )im at 1000 °C (15 MPa) and 200 ^im at 1250 °C (2 MPa) and it seems to grow with bondmg temperature, bonding thne and apphed pressure (see figure 4.12). hi figure 4.13 a cross-section of the metal part of a joint bonded at 1200 °C for 90 minutes is depicted. The dashed line gives the origmal shape, showing the deformation. In the middle of the steel part the rolling texture is still present, but not as clearly visible as in the original AISI 316. At the highest bonding temperature the rolhng texture ahnost vanishes. The gram size varies strongly and also many twins are present. There is no indication that a second phase, for instance the x-phase or the 0-phase, has formed during the bonding process. Kalling and glyceregia reagents gave the best etchmg results. The other etching reagents did not lead to any effect. At the outer side an oxide layer is visible. It was also found that during the bondmg process gram growth took place. The original gram size was 14 |j,m, whereas spechnens bonded at 1200 °C for 90 minutes have a mean grain size of 180 |im. With optical microscopy no reaction layer in the metal was visible. Furthermore, it was found that m notch specimens the black layer in the zhconia is thm at the outside. Away from the outer side, on the depth where the notch in the metal ends, the layer in the zirconia became as thick as in "end to end' specimens. The cross-sections of the foil specimen show the same features. The black layer in the zirconia adjacent to the foils is approximately as thick as in the specimens made under the same bonding conditions in the "end-to-end" bonds. The black layers on both sides of the foil have not exactly the same thickness. Also both interfaces are not similar. One side is sfraighter than the other is. Above the black layers a brighter band of 1 mm width is visible in the zhconia. The mean grain size of a steel foil after diffiision bonding at 1200 °C is about 200 fj,m. The main orientation of the grain boundaries is roughly in the direction of the cenfral axis of the specimen, from interface to interface. Near the edge the foil contains big pores and cracks. 10000 1000 A VS 100 H 10 4 20 30 40 50 60 70 80 26 Figure 4.14: XRD 2-Q-plot ofthe ZrO2 bonding surface after failure (1200° C, 90 min, 2 MPa), scale: square of cps scale. The theoretic monoclinic and tetragonal peaks are indicated. d-écale Figure 4.15: Detail of XRD plot, d-scale (corresponding to 72°<2Q<7f of figure 4.14) of the Zr02 bonding surface, the tetragonal 004-400 "doublepeak". Upper line is the measured line, lower line is the adjusted line. Diffusion bondmg of zirconia to austenitic stamless steel 24 4.2. Results of the XRD and EPMA measurements 4.2.1. XRD X-ray diffraction was carried out on the surface ofthe zirconia part of a bond made at 1200°C for 90 min and on the original material, to determine the crystallographic stmcture of the zirconia and its eventual changes due to the bonding process. The diffraction pattem is shown in figure 4.14. By taldng the square ofthe counts per second a clearer picture is obtained in the low coimt region. The large peaks are from the tefragonal stmcture. The total pattem is somewhat shifted, compared with reference data, probably due to sfresses in the layer, but its characteristic shape is clearly present. The double peaks originate from the a and c axis of the tefragonal stmcture. Figure 4.15 is a magnification of the 004-400 "double peak" of figure 4.14. Here the square of the counts per second is plotted as function of the plane spacing. The calculated d values are close to those given in literature. The measured values are a= 5.097 A and c=5.182 A. In hterature [13] the given values for the axis are a = 5.094 A and c = 5.177 A. Figure 4.15 shows some other aspects of the measurement. In the upper lme every peak consists of two peaks ofthe two Cu Ka radiations used. The lower line has been the corrected for this double effect. In the middle of the two peaks an elevation is visible. This broad middle peak originates from the cubic stracture of zhconia and its plane spacing values hes in the middle of the a and c axis of the tefragonal stmcture. The width of the double peaks is probably due to the alloymg elements in the zhconia. All other "tefragonal double peaks" in figure 4.14 have this cubic "middle peak", only there they are less visible by the height ofthe peaks. From these results it becomes clear that the original zirconia was not purely tefragonal at the start, but that the "cubic- contamination" was not so pronounced. The cubic phase originates from the production process ofthe zhconia and was also present on the XRD scan of the original material. A second type of peaks visible in figure 4.14 originates from the monoclinic phase. They were not so clearly visible in the diffraction graphs of the original material. In the XRD plot of the bonded sample the monochnic peaks are still small (only the 28° and 31.5° 2-0 peaks are clearly above the backgroimd), which implies that the monoclinic content is small. It is obvious that the depth ofthe X-ray beam should be taken into account. The penefration depth in the zirconia of the X ray beam is about 30 )j,m. a) 20 \im EPMA scan over zirconia/fpil/zirconia 80 Zr02 ZxOi 70 j ^ 60 + 50 • Zr HFe • O 30 20 AISI 316 10 0 200 400 600 800 1000 distance (^m) b) 20 EPMA scan over zirconia/foil/zirconia 20 J 18 •- Zr02 Zr02 16 -- 14 -- 12 -- lAY • Hf o 10 -- |*Cr B 8 -- INI 6 -- 4 -- AISI 316 0 i 200 400 600 distance (|im) c) 1 |im EPMA scan over interface 80 Zr02 AISI 316 70 • • • • • • • • 60 50 lAZr IFe I 40 • O 30 i A A • • A • • • 20 -¬ 10 -¬ J 0 É-B-a- 10 15 20 distance (^m) Diffusion bonding of zirconia to austenitic stainless steel 25 4.2.2. EPMA To determine the element distribution m the vicinity of the zirconia/ AISI316 interface, an EPMA-scan was made of a ZrOz/Blófoil/ZrOj combmation with a foil thickness of 400 [im, bonded for 90 minutes on 1200°C. A foil combination was taken instead of other geometries, because this was the only combination that showed good bond strength. Two lme scans were made. In Appendix A the results of the EPMA scans with step size 20 (xm and 1 |j,m are shown m tables and graphs. In figure 4.16 the most interestmg graphs are given. The statistical values of the measurement are given per element and the chosen line (K, L, M) per element which is measured are given in the Appendix A. Here only the results relevant for the present research are discussed. The acceleration vohage was 12 KV and the sample was sputtered with C to make the ceramic part conductive. Some measurement points did not show correct counting statistics and were cancelled. The values of oxygen show a high standard deviation and low oxygen values should be interpreted with care. The measurement of Al, Mn and Si was carried out because these elements were present in an amount that could give distortion of the measurement of other elements. The counting thne for these elements was short. The scan with a step size of 20 |j.m was made over a distance of 1000 |j,m, startmg in the zhconia, through the black layer, the hiterface, metal foil, interface, black layer and ending again in the zhconia. It appears that at the interface an enrichment of Fe and Ni occurs. The Cr content is slightly decreased. Si and Fe appear to diffuse into the zhconia. In terms of the atomic contents measurement no transition was visible in the region where the black layer/zirconia interface should be. Also there was no indication from the EPMA results for the presence of the bright band in the zirconia, which is visible in figure 4.11. The scan with a step size of 1 |j,m made across the ceramic/metal hiterface over a distance of 20 (im, starting in the black layer and ending in the metal, to get an idea of the processes taking place on the hiterface during the bonding process. In figure 4.16e the diffusion of Fe in the black layer is clearly visible. The dashed hne represents the amount of Fe present in the original zhconia. The other elements do not show such an enrichment m this area. In other cases the standard deviation of the measurement is too high to draw conclusions. d) 1 nm EPMA scan over interface 18 Zr02 16 • • • 14 12 lAY 10 + mmm • Hf INI 8 • Cr 6 AISI 316 4 + 2 • • • • A • *-4 10 1 15 20 distance (^m) e) 1 um EPMA scan over interface for Fe 0,8 J 0,7- 0,6-- 0,5-- E 0,4- • Fe • • Q(0 0,3-- 0,2 - • • 0,1 0-- 2 3 4 5 distance dm) Figure 4.16: Results of EPMA measurement at the zirconia/AISI 216 interface. a,b) EPMA scan with step size of 20 [im, ceramic/metal interfaces at 300 \xm and 700 \im. c,d) EPMA scan with step size of 1 \x,m, (right interface of figure 4.16a), ceramic/metal interface at about 12 \m. e) EPMA scan with step size ofl\m of zirconia/AISI316 foil/zirconia bond for Fe in zirconia, ceramic/metal interface at about 12 \m. Dashed line denotes the iron content in the original zirconia. Table 4.2: Geometries modelled by FEM, sizes in mm, models a,f,g and i(d=0.5) were experimentally investigated. a) Standard f) Notch D (detail^ > 1 p 5 1.4 ZrOj AISI316 > Zi-O, AISI316 10 b) Standard, cooled from lOOOOC g) Comer notch A > < A 5 AISI316 > ZrOj 10 c) Notch A ^ h) Comer notch B > U J 1 >'< 1 < 5 6 f ZrO, AISI316 ZtO, AISI316 10 < > 10 d) Notch B (detail) i)Foil(0.1 ZiO, 316 ZiO, ZrOj AISI316 10+ d e) Notch C (detail) > JLS. 1.6 ZrO, AISI316 Diffusion bonding of zirconia to austenitic stainless steel 26 4.3. Results FEM With the fmite element method (FEM), the cooling of a diffusion bonded zirconia/AISI 316 combination was modelled to obtain information ofthe development of residual stresses. The results of these calculations will be presented first, after which the modifications of the standard geometry, notches and foils, wiU be given. Subsequently, the results ofthe different calculations will be compared. The characteristics of all models are given in table 4.2. At the end of this section, the FEM calculations will be summarised. In Appendix 2 all graphs and stress patterns will be given, as obtained with the aid of the computer program Mentat. The most interesting stresses are the an and normal stresses and the shear stress around the interface. The stress levels around the interface are the most hnportant for the processes occurring during cooling of the bond and the quality of the final bond. For the case of the "end-to-end" bonds the stress values are also given for a line just below the outer surface of the specimen. 4.3.1. "End to end" bonds In figure 4.17a,b,c the stiress pattem in the zirconia/AISI 316 combination is given. The stress distribution along a line in the x-direction, just below the surface, is also given numerically in a graph (figure 4.18). In the zirconia, at the left side of figure 4.18, the stress component is tensile, in the metal compressive. On the metal side does not drop to zero because of the fourth boundary condition in which this side is fixed m the x direction. It is assumed that the effect of the fourth boundary condition does not affect the results of the calculation around the interface. The ceramic part experiences a tensile stress in the vicinity of the interface, cjn is high compared to a22 and T12 (shear stress). From figure 4.18 it can be seen that the stress development aroimd the interface is the most interesting to characterise the bond. This is the place where the bond is formed and is expected to fail. The other figures given here are taken from the area from the edge to the middle of the specimen, around the interface. This is the area in which the different geometries will be compared with each other. a) b) c) Figure 4.17: Global pictures of residual stresses caused by cooling from 1200°Cto 25° C, modelled by FEM. Values in MPa. The lower part is the zirconia, the left side is the outer side, the right side is the centre line, a) djj normal stress, b) 1,2 shear stress and c) a22 normal stress. Left side is outer side, right side is centre line of a specimen. stresses along outer surface 3,00E+08 2,00E+O8 .... » A 1,00E+08 • A Jl n g Ê gnnnn III 1 lllllfe A ^ in j^il^ ISÜBlillB&ffl 11 •sigma11 -> -4 -3 -2 -1 1 ^ • 1 2 3 4 !i B Sigma 12 i A Sigma 22 I -1,00E+08 iP^ • • • • • -2,00E+08 ••••• A -3,00E+08 • Zr02 AISI316 -4,00E+08 distance to interface (mm) Figure 4.18: Stress values of "end-to-end" bond, calculatedfor the first nodes below the outer surface, calculated by FEM. Diffusion bonding of zirconia to austenitic stainless steel 27 4.3.2. Notches Different shapes and sizes of notches have been modelled (see table 4.2). The results of the FEM calculations show that the stress pattem m the interface region does not change significantly if the notch tip is taken straight instead of round. The discussed geometries below are all modelled with a round notch tip. A notch can change the stress level around the edge of the specimen. The stress levels are lower at the hiterface, regardless the position and size of the notch compared to the "end-to- end" bonds. For the ceramic side the position of the notch m the metal part is important. Notches far from the interface (1 mm) result in higher sfresses near the edge than in the case of "end-to-end" bonds. A deep (1.6 mm) and narrow (0.5 mm) notch just below the interface (0.5 mm) gives the lowest sfress level in the ceramic, especially further from the edge where the normal sfresses are almost half of the normal sfresses in the "end-to-end" bonds. For the metal side the picture is not so clear. A long and small notch results in a lower shear sfress. The CTh fluctuate and the is opposite (tensile) to that of the "end-to-end" bonds. The sfress levels at the hiterface and in the ceramic are considered to be more important for the quality of the bond than the sfresses in the metal, which is assumed to deform and thereby to lower the other sfress levels. A simple lathe can not make the long and narrow notches, because the chisel is too broad. The deeper the notch the broader it would become. The most ideal notch that can be manufactured hes 0.2 mm below the interface, is 1.0 mm deep and 1.5 mm broad. The resuhs of the calculations of the different notches are presented in the graphs of Appendix Bl and show that the sfresses become higher than those of the most ideal, long and small notch, but they show the same sfress profile. However, the sfress levels around the interface are the lowest for the specimen with the broader and reahstic notch and not for the long small notch. Also a "notch" on the surface is modelled, with dimensions 1x1 mm. This "comer" notch shows lower residual sfresses, especially on the shifted edge point on the interface and in the ceramic. This could be ascribed to the smaller interfacial area. Therefore also a new model was made. The ceramic and metal parts were both 5 mm high and had a radius of 6 mm instead of 5 mm. On the interface a 1x1 mm cube of metal was virtually removed to get the "comer" notch, which resulted in an interface length of 5 mm. The sfress level increased shghtiy, but was still lower than in the case of the "end-to-end" bonds. The size of the interface area is therefore an important parameter of the residual sfresses in the bond, but it is sigma 11 stress along outer surface 3.00E+Q8 2,00E+08 I 1,O0E+O8 •end to end « O.OOE+OOÉ» • foil 0,5mm • notch D I -1,00E+08 • comer notch A -2,00E+08 + -3,00E+08 + -4,00E+08 distance to interface (mm) Figure 4.19: Graph of g^ patterns of the four geometries, calculatedfor first nodes below the outer surface. Diffusion bonding of zirconia to austenitic stainless steel 28 difficult to compare these two comer notch specimens, because the amount of material that cools and shrinks is different, hi Appendix Bl the first described "comer" notch of 4 mm interface is used for the calculations and it is compared with the other geometries. Specimens wh this notch are made and tested, because the ahn of this study was to join samples of 5 mm radius and not of 6 mm radius. 4.3.3. Foils FEM calculations were also carried out to determine the stress level in the case of zirconia/AISI316-foil/zirconia bonds. The thickness of the foil was varied (0.1-0.2-0.5-1.0-2.0 mm). The thinner the foil, the lower the residual stresses. In the metal the stress level as function of the thickness fluctuates in the range of the interlayer thickness between 1 and 2 mm for unclear reasons (see Appendix B2). This fluctuationlook s similar to the fluctuations as observed around the interface in figure4.18 . In the ceramic and on the interface, the course is straight and undisturbed. The foil of 0.5 mm is compared to the notch and dhect bonds. The normal stress and the shear stress are clearly lower m the foil than in the "end to end" bonds, both m the ceramic, as across the interface and in the metal. In the case of zirconia/AISI316 foil/zhconia bonds and "end to end" bonds, CT22 is almost the same in the ceramic part. At the interface the value of the a22 hes between the values of other geometries. The value ofthe normal stress ^22 in the foil has the highest stress level m the foil compared with the other geometries. The thicker the foil, the more the stress levels resemble the stress levels in the direct bond. The foil of 2.0 mm shows the same pattem in the stress levels as the direct bond, especially for the shear stress. The stress levels are ofthe same order of magnitude as in the case of "end-to-end" bonds. 4.3.4. Summary Because of the different geometries, stress levels and positions in the bonds, which were discussed above, the total picture can be somewhat diffiise. Here a short summary is given. Figure 4.19 gives the normal stress CTj,i n the ceramic for the four geometries, just below the outer surface (along a line through the spechnen as in figure 4.18). It should be noted that the highest stresses in the "comer" specimen are present somewhat further from the outer surface. 11 normalatfks* on ceramic sld* 22 normal «tr*as on lh« InUfaco 22 Normal altvaa on 9.D0S>I 3.oos*oa ••nd ta and pioltO.Smm Anoleh 3-.... ' •a.aae*ai * *; * -4.006*01 <.0OEH laUna« la adg* (mm) dlolanc* lo adga {mm} 11 normal straas en tha Inlarfaea 12 ahaar atraaa on tha Ititarfaca 22 normal atraaa on tha intafaca a.aQE*OB 9,0OG'O> , o,oe- 4.00B*0B •,ooe*o* 7,oaE*'0> - •2,0fi*07 „ a,ooG*os' ^•nd lo and ««nd ta and M(oll0.amm Anoian 1 4,0DE*0> • Sé " 3,aoe*oa O.OOH'OO 3.ooE*oa I.OOS'OI ' • • * • • ' » « • • .i.ooE*oa o,ooe*oo 1 -i,ooE*oa t 2 3 4 dlalanoa to adga (mm) dlalanc* to odga (mm) dlaUnOB lo adga (mm) 11 normal atraaa on matal alda 12 Shaar atraaa on matal alda 22 normal alraaa on matal alda ,06*07 ,0E*OO 2,oaE*ai .06*01 S""* * * . . 1.006*01 .OE*o; Mfoll D.9 mm 2 0.001*00 ,os*o; ,0E*o; .j.ooB*oa •3^0E*Oi dlalanoa lo adga (mm) dlalanoa lo adga (mm) dlalanoa to adga (mm) Figure 4.20: Stress patterns around the interface for the four geometries. First coIumn:aji, second column: Xu, and third column: 022- First row: ceramic side near interface, second row: interface and third row: metal side. Diffusion bonding of zirconia to austenitic stainless steel 29 As shown before the stress level in a zhconia/AISBló combination with a notch is only slightly better than that in a combination with an "end-to-end" bond. The stress level in a zhconia/AISI 316 foil/zhconia combination is the lowest, whereby an increasing foil thickness, increases also the residual stresses. The stress levels in the direct bond are given in figures 4.17 and 4.18. Specimens with a notch are in the best case shghtiy better than the direct bonds if the residual stress levels are compared in the region around the interface, where the highest stresses are present. Zhconia/ AISI316 foil/ zirconia combinations have the lowest stress levels compared with the other geometries, whereby the thinnest foils resuh in the lowest residual stress levels. The stress values of the four geometries are given in figure 4.20 in which the three stress types are shown, in the zhconia, at the interface and in the metal. Temperature oC Figure 5.1: Ellingham diagram of oxides [27]. Diffusion bonding of zirconia to austenitic stainless steel 30 5. Discussion, conclusions and recommendations for further research In this chapter the resuhs ofthe experiments on diffusion bonding of zirconia to AISI 316will be discussed. Fhst the black layer is analysed. The occurrence of the black layer will be related to the poor bond strength and the residual stresses, as calculated by the FEM. Conclusions and recommendations for further research are given and summarised at the end of this chapter. One ofthe most important findings of this study on the bonding process is the occurrence of a black layer in the zirconia along the zhconia/AISI 316 interface after the bonding process. Although clearly visible by eye, it can not be seen using optical or scanning electron microscopy. It can, therefore, easily be missed, especially when the layer is not very thick. The layer does not consist of decomposed zhconia, as can be seen from the Ellingham diagram of figure 5.1 [27], because zhconia is a very stable oxide. Under the process conditions (P^z = 1.10 'bar) the decomposition temperature is above 2500 °C. Fhst the explanation of the black layer is sought in the T-M fransformation.Fro m literature the tefragonal-monoclinic (T-M) fransformationi s well known for tefragonal zirconia surface layers [13, 14]. The fransformed surface layer is a compressive layer, which is the result of a spontaneous fransformation of the tefragonal phase to the monoclinic phase, which has a larger volume. The explanation for this compressive sfress surface layer, given in literature, is that the transformation can possibly occur because of the absence of a hydrostatic consfraint near the free surface, whereas in the bulk the hydrostatic consfraint makes the fransformation less likely. The compressive surface layer improves the fracture sfrength considerably, because flawsca n hardly grow out under sfress. The fransformationma y be induced by shear sfress, for instance by grinding the surface [13] or by dilatation [28]. The X-ray diffraction reveals that during the bonding process (in the cooling stage) monoclinic zirconia has formed (see figure 4.15). This measurement only reveals the stracture of a layer of 30 [xm at the zirconia surface. The monoclinic peaks are small and therefore the monoclinic content must be low. Based on the theories given in literature, only the large grains on the surface are expected to fransform.Ther e the thermal sfresses are so high and the sfresses are not hydrostatic that the fransformationmigh t be possible. In the work concemed with the depth of the fransformation[13 , 14], the profile is determined by XRD after which a thin layer is removed and again the specimen is scanned to determine the monoclinic content. monoci. -o- tetr. Pl acticallf/ no st ress grain boundary 2 Figure 5.2: Schematic model ofa transformed surface layer and the tetragonal bulk material. The plastic strain corresponding to the transformation results in elastic compressive stresses in the transformed part ofthe material. The compensating tensile stresses are spread out over the bulk material. The compressive stresses can be relaxed near grain boundaries as shown in 5.2.2. This will cause relatively high localised tensile stresses in the tetragonal structure which can promote fracture at these locations [14]. Diffusion bonding of zirconia to austenitic stainless steel 31 This sheer method is a recommendation for further research on the black layer developed during diffusion bonding, to fmd the transformation profile as a function ofthe depth and the effect of the bonding parameters (bonding temperature, thne and pressure) on the transformation. However, grinding and pohshing, but also etching might induce new stresses on the zircoiüa surface and thereby cause fiirthertransformation . If the black layer consists of a region in which the monoclinic phase is present, the layers observed are much larger than reported m hterature. The transformed layer thickness hes normally around 10-100 |j,m. The layers observed here are 25-200 fxm thick. This can be due to the preparation of the layer, the steel present during the diffusion bonding process, the cooling rate, starting temperature of the transformation and the bonding pressure. In the referred literature on the transformed layer, the transformed material is obtained by grinding and not by cooling from high temperatures. Beside the XRD results there are more indications that the T-M transformation occurs. The black layer is a constraint layer. The hardness is compared to that of the bulk zirconia significant higher. In further research the hardness development can be an interesting feature to study. With bright field and with Normansky differential interference contrast microscopy, pits can be seen in the bulk zirconia, whereas the black layer is smooth. Using SEM to study a broken bond, which partly failed in the zhconia, it becomes clear that in the zirconia the large grains fracture along the grain boundaries, whereas in the consfraint layer the large grains break. The fracture ofthe big grains in the bulk zhconia, away from the consfraint surface layer may be the cause of the pits in the zirconia. Berg [14] gives in his work the explanation for this effect (see figure 5.2). Below the consfrained surface layer, a tensile sfress is present, which causes intergranular cracking ofthe zhconia. The black colour can originate from the distorted lattice in this consfraint layer. It is also reported in other martensitic fransformations [29]. With polarised light the polarisation direction is changed and therefore a part of the reflected light can not pass the analysator, which is seen as a less bright region through the microscope. The preparation prior to bonding (sawing, grinding and pohshing) of the zirconia surface certainly will have its effect on the fransformation and therefore on an explanation of the black colomr. XRD scans should be made from other sides of the specimens and not only from the interface, to determine whether the monoclinic phase is formed there. This gives an idea of the effect of the preparation ofthe interface on the phase fransformation. The other sides did Diffusion bonding of zirconia to austenitic stainless steel 32 not show a change of colour. Also the parts of misalmed specimens which where not pressed against steel on the bonding surface and did not tum black should be scanned to detemiine whether the phase transformation took place. The transformation is promoted if the content of ythia or hafiiia m zirconia is lower, but no diffusion of these elements away from the interface was detected by EPMA. Like zhconia, yttria and hafiiia are very stable oxides, which will not decompose under the process conditions. It is clear that the T-M fransformation occurs in the present case and thereby the formation of a consfraint layer. Some doubts arise by the observed thickness of the layer and the fact that the thickness increases with bonding temperature. Based on the phase diagram of figure2,1 0 and literature [15, 30] the startmg temperahire of the fransformation is estimated around 950 °C during cooling (the reverse fransformation during heating starts at 1150 °C). The period before the starting temperature is less hnportant if the fransformation can be seen as a purely martensitic fransformation. There are no indications that support the idea that a higher temperature before the martensitic fransformationhelp s the extend of the fransformation.Wit h acoustic emission Clarke [30] determined the starting point of the martensitic fransformation in zirconia. This technique can be used to examine whether the starting point of the fransformation is indeed mdependent ofthe diffusion bonding temperature. It also can give an indication at what temperature eventually debonding or cracking of the diffusion bonds occurs. There is another effect of the higher bondmg temperature that can form the explanation of the broader layers. A higher bonding temperature will cause higher thermal sfresses at the starting temperature of the transformation, which is expected to be constant. Because the fransformation is induced by sfresses, these thermal sfresses could be the explanation of the broadening of the layer at higher bonding temperatures. With FEM the situation is calculated and it tumed out that csn, cs^i and are all three higher at 950 °C for bonds made at 1200 °C than for bonds made at 1000 °C. Also a higher bondmg pressure is expected to resuh in higher sfress at the starting temperature ofthe T-M fransformation and so promoting the layer thickness. FEM calculations should be made to verify this assumption. There is at the moment no explanation for the effect of the bonding time on the layer thickness and further research is needed. The layer thickness in the "end to end" bonds is similar as in the foil bonds. This hidicates that the layer is not formed at low temperatures under the influence of sfresses, because the Diffusion bondmg of zirconia to austenitic stainless steel 33 Stress levels differ significantly at low temperatures (T<200''C) between these two geometries. Only at high temperature (T>900°C) the residual shess levels of these geomehies are comparable. As stress levels are assumed to play an important role in the induction of the transformation, it might be concluded that the formation ofthe T-M transformated layer takes only place at high temperatures under the influence of thermal stresses. Within this chapter three types of stresses are discussed. They might be mixed up during the discussions and maybe interact with each other. The stresses are: 1) stress layer caused by T¬ M transformation, forming a constraint surface layer, 2) thermal residual shess at the starting of the hansformation (T = PSO^C) which is a possible explanation of 4.5b, because it determines where on the surface the transformation starts and 3) the residual stresses (calculated by FEM) after cooling to room temperature (figiire 4.19). Note that the transformation can be auto-catalytic, because it is induced by stresses and again develops stresses during the transformation. In hterature [1] on dif&sion bonding of zhconia to metals the black layer is seldom described. The only reported black layer occurs in the case of diffusion bonding of zirconia to carbon steel [31, 32]. Other metals bonded to zirconia (Cu, Pd, Ni, NiCr), some even at much higher temperatures (Pt) and with comparable thermal expansion mismatch, do not show the black layer. Only few authors report the monoclinic phase after the bonding process. However, the composition and the exact process conditions are very important for the monoclinic transformation and comparing the outcome is therefore difficult. The strong optical effect of the black layer is not reported in literature on the T-M transformation, although it is a far more simple technique than the XRD to trace the transformation, if the transformation is the complete explanation for the colour change. The doubts mentioned above and the data from literature indicate that there must be an effect of the steel. The steel bonding partner was not taken into account in the discussion of the black layer so far. The T-M fransformation is so far taken as only possible explanation for the optical effect. From the EPMA scans it becomes clear that some hon diffuses into the zhcorha, although there is no diffusion on a large scale. It is well known that small amounts of metal ions can colour glasses. Moya et al. [33] studied the appearance of a black colouring of PSZ during sintering and cooling. They detected with elecfron spin resonance (ESR) some Fe^* and Fe^* under reducing conditions at the grain boundaries. The amount of hon ions was very low (content: 0.007 wt%). Under oxidising chcumstances the zirconia did not become black, because the hon ions were reduced and therefore not detectable. According to Moya et al. the colour effect of sintering PSZ is only Figure 5.2: Schematically picture of black layer and iron content, to support the idea that the abrupt ending of the black layer can not be explained by a drop in the iron content Diffusion bonding of zirconia to austenitic stainless steel 34 due to the iron ions. Iwamoto [31, 32] used the same technique and also reports Fe^* and Fe^^in the black layer of a zhconia/carbon steel bond. The observed layer is only about 10 iim thick. The dark layer of the zhconia/carbon steel bond was clearly visible with SEM, which was not the case in the present study of the zhconia/AISI 316 combination. The EPMA result shows that the Fe content in zirconia is decreasing very slowly, after a few microns from the interface. Although only for the fhst 10 jxm of the black layer the Fe contents is determmed precisely, and further only with steps of 20 ^im, it is expected that profile is rather flat in the black layer till the Fe bulk concenfration is reached. The visible black layer ends rather abmptly and does not appear to become gradually less dark. The effect of iron can therefore also not completely explain the appearance of the black layer (see figure 5.3). Summarising it can be concluded that T-M fransformation as weU as the iron in the zirconia are present and will therefore have theh effects on the colouring of the surface layer. The colouring is most likely caused by the sfress field, which is a result from the T-M fransformation. The sfress field becomes visible by the iron present in the sfress field, where it is probably present in an ionic form. The T-M fransformation on the zirconia bonding surface will not stimulate the forming of a good bond, because of the change in volume of the zirconia during the cooling. The deformation due to the transformation is about 4.7% [15]. The thermal deformation will be about 1%, calculated by the difference in thermal expansion coefficient. An other type of zhconia, where the tefragonal phase is more stabihsed for instance by a higher ytfria content and so will not fransformt o the monoclinic phase, might show higher bond sfrength. Another reason for the poor bondmg quality is the presence of residual thermal sfresses. In the case of "end to end" and "notch" spechnen the by FEM calculated sfress levels are exfremely high and the bond will break down during coolmg. In the case of zhconia/AISI316 foil/zirconia bonds the sfress levels are much lower. Foil specimens show some sfrength and in the zhconia some cracks occur. Unfortunately the specimen could not be tested properly because of the cracks and the deformation of the zirconia but one can conclude that the bond sfrength is higher than in the case of "end to end" bonds. In the foils the level increase in the metal part, compared to the geometries calculated. This mcrease m the CJ22 direction explains the denser crack pattem in the foil, as observed during the microscopic analyses. Diffusion bonding of zirconia to austenitic stainless steel 35 Cracking seems to be intergranular, starting from the edge ofthe foil. The sfrength should be determined in fiuther research and be compared with the findings of Derby [2]. In Derby's study the pressiu-e was 10 MPa instead of the 2 MPa used here. In the present case the bonding pressure of 2 MPa is thought to be sufficient. Derby does not report the formation of a monoclinic phase. The zirconia used there could have a higher yttria content or smaller zirconia grain, by which the T-M fransformation and its negative effects on the bond sfrength are repressed. Another diffusion couple, that shows a better bond sfrength in the "end to end" configuration should be used to test the FEM resuhs for different geometries and the effect of the geometry on the bond sfrength. For instance the combhiation zhconia/nickel [12] could be used, because the sfresses in these bonds are purely due to the differences in thermal expansion coefficient. This combination has a lower thermal mismatch than in the case of zirconia and AISI 316. If the T-M phase fransformationi s understood properly it could be incorporated in the FEM calculations Conclusions: Zirconia/AISI316 diffiision bonds have a poor bond sfrength, mainly due to the high thermal sfresses formed during cooling and the T-M phase fransformation on the interface. Zhconia/AISI316 foil/zirconia diffiision bonds show good bonding, probably due to the lower thermal sfresses. FEM calculations show that modifying the geometry of the metal part by notches only slightly reduces the residual sfress levels. The sfress levels for the zirconia/AISI 316 foil/zhconia combination are significantly lower and the sfress levels increase with foil thickness. During the cooling phase, some (large) zirconia grains at the hiterface can fransform from the tefragonal to the monoclinic phase, hereby forming a consfraint surface layer. Fe diffiises into the zhconia during the bonding process During the bonding process a black layer is formed at the zirconia interface, which is probably due to a combined effect ofthe T-M fransformation and the diffiision of iron into the zirconia. Diffusion bonding of zirconia to austenitic stainless steel 36 The thickness of the black layer mcreases with incrcasmg bondmg temperature, bonding time and bonding pressure. In all bonding experhnents the metal part deformed strongly and the mean gram size increased strongly. In the zirconia/AISI316 foil/zhconia combinations, the zhconia deformed and in some cases cracked. Recommendations for further research: Zhconia with an other composition, for instance one that is less susceptible to the T-M transformation by yttria amount or grain size, should be tested as diffusion bondmg partaerto AISI316. The T-M transformation temperature can be determined by studying the bonding process with acoustic emission. This could reveal the influence of the bonding time, temperature and pressure on the transformation temperature. Acoustic emission could also reveal when debonding of the specimen or crack formation in the metal part occurs. The results of the FEM calculations should experimentally be verified using a diffusion bonding combination with higher bond strength, for instance zhconia/nickel. The effect of the cooling rate on the T-M transformation, as well as on the bond shength should be investigated. The effect of the microstracture and the foil thickness on the bond strength of foil bonds should be experimentally determined. The composition of the square metal remnants on the zirconia of foil bonds can be determined by EDX and forms an interesting point for further research to understand the bonding principle. Diffusion bonding of zirconia to austenitic stainless steel 37 References 1) A.TJ. van Helvoort, Diffiision bonding of zirconia to metals, third year literature survey, Delfl University of Teclmology, 1997. 2) CD. Qin and B. Derby, Diffusion bonding between a stainless steel and zhconia. Journal American Ceramic Society, 76 (1993), 232-234. 3) B. Derby and E.R. Wallach, Theoretical model for Diffusion Bonding. Metal Science 16 (1982), 49-56. 4) B.J.T. Stoop, Diffusion Bonding of Silicon Nitride to Austenitic Stainless Steel, PhD. thesis. Delft University of Technology, 1991. 5) B. Derby and E.R. Wallach, Diffusion Bonding: Development of Theoretical Model. Metal Science, 18 (1984), 427-431. 6) A. Hill and E.R. Wallach, Modelling Solid-State Diffusion Bonding. Actametall., 37 (1989), 2425-2437. 7) T. Okamoto, Interfacial Stmcture of Metal-Ceramic Joints. ISIJ hitemational, 36 (1990), 1033-1040. 8) D.A. Porter and K.E. Easterlmg, Phase transformations in metals and alloys. Van Nostrand Remhold (Int) Co. ltd, Wokingham, 1988. Diffiision bonding of zirconia to austenitic stainless steel 38 9) T.C. Bor, Het diffiisielassen van Si3N4 met een Ni-tussenlaag, Graduation thesis. Delft University of Technology, 1994. 10) K. Suganuma and T. Okamoto, Interlayer bonding methodes for ceramic / metal systems with thermal expansion mismatch. In: Fundamentals of diffiision bondmg, proceedmgs of the first Seiken symposium on interfacial stmcture and diffiision bonding. Tokyo, Japan, 2-4 december 1985. 11) LD.Cawley, Introduction to Ceramic-Metal joining. In: Proceeding of symposium Metal-Ceramic Joining, Detroh, Michigan, 8/9 October, 1990. 12) R.H. Vegter, Diffiision Bondmg of Zhconia to Sihcon Nitride, Design Engmeering in Materials Science and Materials Technology thesis. Delft University of Technology, 1996. 13) R. Stevens, Zirconia and zirconia ceramics. Magnisium Elekhon Ltd, Twickenham (UK), 1986. 14) P. Berg, Zhconia Ceramics and Mechanical Surface Interactions, PhD thesis, Eindhoven University of Technology, 1992. 15) M.Ruehle, Phase transformation in Zr02-containing ceramics: n. The Martensitic reaction in t-ZrOj, In: Proceeding of the Second Intemational Conference on the Science and Technology of Zirconia, Stuttgart, Germany, June 21-23,1983. Diffusion bonding of zirconia to austenitic stainless steel 39 16) J.R. Davis, ASM Specialty Handbook Stainless Steels, Materials Park, ASM Intemational, 1994. 17) G. Petzow, Metallographischen, Keramographischen, Plastographisches Aetzen, Gebraeder Bomtraeger Berlin, Stuttgart, 1994. 18) J.P.H.M. Kmgers, Joining silicon carbide to austenitic stainless steel through diffusion welding, PhD. thesis. Delft University of Technology, 1993. 19) S. Raewska, Verbessertes Modell zur Bestimmung des Spannungszustands waehrend der Abkuehlphase beim Diffusionsschweissen, Schweissen und Schneiden, 49(1997), 100-104. 20) S. Raewska, Moeglichkeiten zur Spannungsreguherung in Metall-Keramikverbindungen, Schweissen und Schneiden, 50(1998), 361-365. 21) K. Suganuma and T. Okamoto, Influence of shape and size on residual stress in ceramic/metal joining. Journal of Materials Science, 22(1987), 2702-2706. 22) L.N. Lado, V.R. Evdokhnov and S.N. Shubm, Diffusion bonding sihconised graphite to stainless steel for end sealing components, Welding hitemational, 12(1998), 745-746. 23) N.A. Waterman and M.F. Ashby (eds), Elsevier Materials Selector, 2"** edition, Elsevier Apphed Science, London, 1997. Diffiision bonding of zirconia to austenitic stainless steel 40 24) Data sheet ceramic materials, Ghnex b.v., Nieuwegein, The Netherlands, 1995. 25) Chronium Nickel Stainless Steel Data, The Intemational Nickel Company Inc., New York. 26) C.W. Wegst, Stahlschluessel, 17. Auflage, Verlag Stahlschluessel, Wegst GmbH, Marbach, 1995. 27) J. Herenquel, MétuUurgie speciale, tome EI, b. Zirconium et ses alhages. Commissariat a l'Énergie Atomique, Paris, 1962. 28) CA. Andersson and T. Gupta, Phase stability and transformation toughing in zhconia. Advances in Ceramics, Vol. 3, The American Ceramic Society, Columbus, OH, 1981! 29) J.H. Wayman, The martensitic reaction. Advances in Ceramics, Vol. 3, The American Ceramic Society, Columbus, OH, 1981. 30) D.R. Clark, Acoustic Emission Characterization of the Tetragonal-Monoclinic Phase Transformation in Zirconia, In: Proceeding of the Second Intemational Conference on the Science and Technology of Zirconia, Stuttgart, Germany, June 21-23,1983. Diffusion bonding of zirconia to austenitic stainless steel 41 31) N. Iwamoto, H. Yokoo, Y. Makino and R. Shikata, Reaction of zirconia with carbon steel in a vacuum condition. Transactions of JWRI, 15 (1986), 205-206. 32) N. Iwamoto, H. Yokoo, Y. Makino and R. Shikata, Reaction and interfacial characterization on joining of zirconia to carbon steel. Transactions of JWRI, 16 (1987), 91-96. 33) J.S. Moya, R. Moreno and J. Requena, Black Color hi Partially Stabihzed Zhconia, Journal of the American Ceramic Society, 71(1988), C479-C480. Appendix A: EPMA-results 1 Appendix A: EPMA-results Table 1: Measured lines per element: Element Line 0 K Si K Mn K Ni K Zr L Hf M Al K Cr K Fe K Y L Mo L Table 2: Results EPMA scan of zirconia /AISI 316/zirconia bond made at 1200 C, 90 min, and 2 MPa. Scan step size was 20 \m. Pos. 0-K Al-K Si-K Cr-K Mn-K Fe-K Ni-K Y-L Zr-L Mo-L Hf-M nm At% At% At% At% At% At% At% At% At% At% At% 0 66,29 0,11 0,03 0,02 0 0 0,01 1.4 31,73 0,01 0,41 20 66,06 0,09 0,08 0,02 0,02 0,02 0 1,43 31,88 0 0,4 40 66,16 0,14 0,09 0,02 0,05 0,01 0 1.36 31,77 0 0,41 60 66,79 0,15 0,13 0,01 0,02 0 0 1,55 30,94 0,01 0.4 80 66,01 0,22 0,21 0,05 0,1 0,02 0 1,4 31,59 0 0.41 100 66,12 0,16 0,2 0,02 0,11 0,02 0 1,49 31,49 0 0,39 120 66,3 0,88 0,1 0,53 0,57 0,01 0,01 1,33 29,86 0,01 0,39 140 65,87 0,11 0,12 0,01 0,04 0,04 0,04 1,67 31,71 0 0,4 160 66,14 0,13 0,09 0,07 0,1 0,03 0,01 1,72 31,27 0,01 0,42 180 65,94 0,14 0,14 0,03 0,05 0,03 0,03 1,53 31,7 0 0,39 200 65,82 0,17 0,21 0,07 0,12 0,07 0 1,5 31,62 0 0,41 220 65,91 0,25 0,33 0,1 0,15 0,06 0,02 1,44 31,35 0 0,39 240 66,51 0,14 0,09 0,03 0,08 0,02 0,01 1,48 31,22 0 0,41 260 66,31 0,28 0,46 0,08 0,26 0,08 0 1,51 30,63 0 0,39 280 65,86 0,22 0,35 0,09 0,18 0,14 0 1,45 31,31 0,01 0,4 300 0,99 0 0,66 17,32 0,75 68,42 10,63 0,01 0 1,22 0 320 0,85 0,02 1 17,88 1,1 67,4 10,5 0 0 1,25 0,01 340 0,67 0 1,14 17,94 1,28 67,5 10,54 0 0 1,21 0,02 360 0,8 0 1,22 18,14 1,34 66,93 10,32 0 0 1,25 0 380 0,77 0 1,33 18,07 1,48 66,75 10,4 0 0 1,19 0,1 400 0,85 0 1,31 18,08 1,56 66,55 10,37 0 0,01 1,27 0 420 0,81 0,02 1,32 17,98 1,55 66,76 10,38 0 0 1,23 0 440 0,77 0 1,34 17,77 1,64 67 10,27 0 0 1,21 0 460 0,8 0 1,39 17,62 1,53 66,97 10,46 0,01 0,01 1,21 0 480 0,98 0 1,36 17,79 1,55 66,93 10,19 0 0 1,2 0 500 0,74 0 1,28 17,7 1,52 67,22 10,33 0,01 0 1,21 0 520 0,81 0 1,32 17,82 1,54 66,93 10,34 0 0,01 1,23 0 540 0.7 0,04 1,39 17,56 1,55 67,17 10,41 0 0 1,18 0 Appendix A: EPMA-results 2 560 0.65 1,31 17.61 1,92 64,9 9,79 0,01 0 1,13 0,01 580 0,67 0,02 1,38 18 1,48 66,92 10.33 0 0 1,2 0.01 600 0,73 0 1,31 17.82 1,5 67 10,38 0,04 0 1,22 0 640 - 0,45 1,44 16.28 1,63 59.29 9,26 0 0 1,11 0.01 660 0,7 0,02 1.02 17.47 1,12 67,78 10,68 0 0 1,21 0.01 680 0,78 0,02 0,8 17,04 0,82 68,75 10,5 0 0,01 1,27 0 700 0,89 0 0,51 15,73 0,27 70,32 10,94 0 0.01 1,34 0 720 66 0,09 0,24 0,23 0,08 0,2 0,06 1,85 30,25 0 0,41 740 65,71 0,17 0,46 0,09 0,12 0,03 0,04 1,57 31,38 0 0,42 760 65,3 0,11 0.2 0,12 0,11 0 0,02 1,83 31,86 0,02 0,42 780 66,08 0,16 0.3 0,06 0,08 0 0 1,71 31,21 0 0,4 800 65,72 0,14 0.2 0,16 0,03 0,05 0,01 1,53 31.71 0,01 0,43 820 65,89 0.11 0,17 0,05 0,04 0,02 0 1,55 31.76 0,01 0,42 840 65,54 0.1 0,06 0,07 0,05 0.02 0,04 1,68 31,99 0,01 0,44 860 65,09 0.1 0,02 0 0,01 0,02 0,01 1,6 29,73 0,01 0,4 880 64,86 0,08 0,08 0,19 0,03 0 0 1,8 30,64 0 0,4 900 65,94 0,1 0,05 0 0,02 0 0 1,58 31,91 0 0,42 920 66,31 0,11 0,03 0.03 0 0 0 1,75 31,34 0 0,43 940 66,22 0,09 0,08 0 0 0 0.01 1,65 31.53 0 0,41 960 66.16 0,09 0,11 0.01 0,01 0,04 0 1,65 31,5 0,02 0,42 1000 65,82 0,27 0,35 0.02 0 0,01 0 1,71 31,4 0 0,43 Table 3: Results EPMA scan of left interface (between positions 280 (j,m and 320 |j,m of table2) of zirconia / AISI 316 /zirconia bond made at 1200^C, 90 min. and 2 MPa. Scan step size was 1 ^m. Pos. 0-K Al-K Si-K Cr-K Mn-K Fe-K Ni-K Y-L Zr-L Mo-L Hf-M ixm At% At% At% At% At% At% At% At% At% At% At% 0 65,9 0,38 0,74 0,11 0,37 0,21 0,03 1,38 30,49 0 0,38 1 65,72 0,09 0,23 0,07 0,07 0,22 0,04 1,41 31,75 0 0,39 2 66,03 0,16 0,25 0,08 0,08 0,24 0 1,36 31,41 0 0,39 3 66,06 0,14 0,25 0,1 0,08 0,24 0,01 1,41 31,33 0 0,38 4 66,31 0,17 0,23 0,13 0,1 0,34 0,02 1,4 30,9 0 0,39 5 66,26 0,17 0,28 0,15 0,16 0,35 0,04 1,4 30,81 0 0,39 6 66,43 0,15 0,2 0,19 0,07 0,47 0,04 1.39 30,65 0,01 0,39 7 66,44 0,13 0,27 0,33 0,17 0,57 0,08 1,49 30,13 0 0,38 8 66,5 0,15 0,27 0,25 0,11 0,73 0,11 1,53 29,97 0,02 0,37 9 39,4 0,08 0,42 8,39 0,28 25,79 3,53 0,89 20,44 0,49 0,31 10 1,06 0,02 0,81 16,96 0,62 68,96 10,18 0,02 0,09 1,29 0 11 1,13 0 0,73 17,34 0,63 96,01 9,91 0 0,03 1.23 0 12 1,06 0,02 0,75 17,43 0,6 68,84 10,01 0 0,01 1,28 0 13 1,09 0 0,75 17,46 0,56 69,08 9,8 0 0,01 1,24 0,01 14 0,98 0,04 0,7 17,62 0,69 68,98 9,72 0 0,03 1,23 0 15 0,98 0 0,81 17,65 0,67 68,82 9,84 0 0,01 1,22 0,01 16 1,02 0 0,77 17,64 0,75 68.63 9,96 0 0 1,24 0 17 1,09 0 0,79 17,53 0,76 68,73 9,88 0 0 1,21 0 18 1,13 0 0,85 17,48 0,78 68,68 9,85 0 0,01 1.22 0 19 1,12 0 0,83 17,73 0,89 68,38 9,84 0 0,01 1,19 0,01 20 1,09 0,04 0,89 17,69 0,83 68,38 9,84 0 0,01 1,23 0 Appendix A: EPMA-results 3 Table 4: Results EPMA scan of right interface (between positions 680 |4.m and 720 |j,m of table2) of zirconia / AISI 316 / zirconia bond made at 1200° C, 90 min. and 2 MPA. Scan step size was 1 \xm. Pos. 0-K Al-K SI-K Cr-K Mn-K Fe-K NI-K Y-L Zr-L Mo-L Hf-M M,m At% At% At% At% At% At% At% At% At% At% At% 0 65,88 0,16 0,4 0,08 0,06 0,12 0,01 1,49 31,36 0,02 0,42 1 65,21 0,2 0,62 0,15 0,1 0,17 0,04 1.45 31,64 0 0,43 2 65,12 0,13 0,52 0,13 0,04 0,22 0,06 1,45 31,88 0 0,43 3 64,86 0,25 0,8 0,32 0,15 0,21 0,03 1.98 30,97 0 0.43 4 65,3 0,23 0,77 0,17 0,16 0,28 0,09 1.62 30,96 0 0.42 5 65,25 0,2 0,73 0,15 0,06 0,25 0,06 1.54 31,35 0 0,42 6 64,91 0.2 0,64 0,19 0,15 0,33 0,05 1,6 31,53 0 0,42 7 65,09 0,21 0,58 0,15 0,1 0,37 0,06 1,5 31,48 0,02 0,43 8 65,07 0.23 0,7 0.2 0,11 0,41 0,1 1,54 31,22 0 0,43 9 64,96 0,17 0,54 0,19 0,1 0,48 0,08 1,56 31,49 0,01 0,43 10 64,77 0,25 0,81 0,21 0,04 0,61 0,14 1,54 31,21 0 0,42 11 62,29 0,26 1,16 0,46 0,06 2,24 0,36 1,56 31,16 0,04 0,42 12 12,41 0,22 1,26 13,16 0,14 55,64 7,85 0,55 7,67 1 0,11 13 1,03 0,04 0,48 15,72 0,28 70,23 10,8 0,02 0,06 1,33 0,01 14 1 0.02 0,48 16 0,23 70,2 10,71 0 0,04 1.3 0 15 0,96 0 0,53 15,83 0,25 70,37 10,72 0 0,02 1,32 0 16 1 0 0,5 16,29 0,29 69,8 10,79 0 0,02 1,31 0 17 0,93 0,02 0,51 16,13 0,32 70,16 10.62 0 0,01 1.3 0,01 18 0,96 0 0,54 16,36 0,32 69,85 10,64 0,01 0,01 1,32 0 19 0,99 0,02 0,54 16,24 0,33 69,74 10,81 0,01 0 1,3 0,01 20 0,92 0,02 0,61 16,39 0,38 69,74 10,64 0 0,02 1,29 0 AppenciixA.1 EPMA 20 micron Main elements 80 Zr02 AISI 316 Zr02 70 ••••••••••••••• 60 50 + •Zr §40 + BFel • O 30 T 20 10 immmmmm®mmmmmmmilittttttttttttAtt ktttmmmmmmmmmmsm-é 0 100 200 300 400 500 600 700 800 900 1000 distance (^m) Main alloying elements 20 Zr02 Zr02 18 + 16 14 -• 12- AY i 1 • Hf i 10 • Cr • Ni 8 6 + AISI 316 4 - ^AAAAAA^^AAAAAA I ^A^AAAAAA^AAA 4 iiiiisil 0 100 200 300 400 500 600 700 800 900 1000 distance {(xm) Page 1 AppendixA.IEPMA20 micron Other elements 2,5 Zr02 j AISI 316 i Zr02 I • • j 1,5 B j 1 ^•l-gB « H B B |4Mn I E •• .•r !Si I »Mo| IB B j 4 ij if 0.5 • B 1 • • 1 >il»««»>»»««««» \ i h- l::::#ïiBiie5: 100 200 300 400 500 600 700 800 900 1000 distance (^m) Page 2 I AppendixA.2 EPMA 1 micron left Main elements 120 Zr02 AISI 316 100 + 80 + • ••••••• •o • Fe I ^ 60 AZr 40 20 o*—a—®—»- mmm ttttttttttt 8 10 12 14 16 18 20 distance (^m) Main alloying elements 20 Zr02 AISI 316 18 ^••••••••4 16 - 14 - 12 -- INI lAY £10 + • MB • B S H é • Hf I 2 + AAA AAAAA li t 1 i i w ^ $ • • 14 16 18 20 8 I'O 12 distance (nm) Page 1 AppendixA.2 EPMA 1 micron left Fe in zirconia (detail) • Fe 4 5 distance (mu) Other elements 1,4 Zr02 AISI 316 1,2 • A B <> 0,8 • II • • • * . * É . 0,6 m A 0,4 • • • • • • • 0,2 -f X X X • X K • • • X OA—A—A—A—A—A—*—A—^ 4- H >€ )i€ )l£ i Page 2 AppendixA.3 EPMA 1 micron right Main elements 80 Zr02 AISI 316 70 m É • ••••••••• 60 + 50 + AZr BFel è 40 + • O 30 IAAAAAAAAAAA 20 I I 10 I •m—m- B • B • B B ttttttti 10 12 14 16 18 20 distance {\im) Main alloying elements 18 Zr02 AISI 316 16¬ • ••••• 14-¬ 12 AY B B 10 • Hf I • Nl 8 • Crl 6¬ 4 -¬ 2- A A AAAAAAAA l--4-4-4--l-J--i--t--t-t-4 * • 8 10 12 14 16 18 20 distance {\im) Page 1 AppendixA.3 EPMA 1 micron right Fe in zirconia (detail) 0,8 Zr02 0.7 + 0,6 0,5 + • Fe 0,3 0,2 0,1 —I 1_ H 1 4 5 7 8 9 distance (mu) Other elements 1,4 Zr02 i 1.2 1 + AISI 316 XAI 0,8 + • Si • Mn A Mo 0,6 • • • • • • • 0,4* 0,2 X X X X X ^ X • • • X • • • * X 0?—A—A—A—A—A—A—A—A—*—A- -X— 8 10 12 14 16 18 20 distance (^m) Page 2 0*am«tri«*: 11 normal «tr**» In cvrsmlc OaomalrUa: 11 n • amalrlaB: 33 nvrmal ttrmtt In earamlo a,noB.M i,miE*o> g O.DOE-.00 0.0(1 e*oD •«••Kilah • I .1.00E.OB Xci«lBhC [ .t.DflE*OB Anolsh D (J ' J.006+OB " -2,0DE*0» J,O0E*0B • * I <,O0E*OB ^.O0E*O> Gaomatrt**: 11 normal strsss on Inlarfac* O«om«trlas: 12 shaar atraaa on Intarfaea Gaomatrlaa: 22 normal atrass on Intarfaea S.aOE*OB ; ; i : 4,00E»0» -2.0E*07 ••nd lo and -J -4,OE*07 +naiol> B t 2,006 *0B 4">oleh B Xnsish C g •B.oE*or IxnolühC An«l«h D Aooleh D , X » J. X » * • «emar noleh A 5 1,00B»0B • • • • I m Z 9 . X XAAAAAA.)^— • *ï«x** + Ï * + • t . ^ •1.0E*0« ;::::ttït:Jï" > A 'A ^ ft' 1 3K W* ?t *A »• J 1 J ,* ' •I,3E-»0B .• DlaUne* lo adfla (mm) Oaemalrlas: 11 atraaa on matal aid* Gaomatrlaa: 12 ahaar «traas on matal Oaomatrlas: 22 normal straaa on matal ,0E*OT .0E*00 S,OOE*OB X l.aOB*OB •and lo and (atandanl) ^*A*A«2ï;x, •and lo and (awndard) •fnoHh B g 1^0fl*OI ' 4-nolali B I.. Xnstth C xï'^x sJÏï*4* Xnoleh C Anoloh D 1 O.OOI*OD I Anoleh D X w X * ,06*07 •nar nolnh A •eomar nolch A X X X X ^ .i,ooE*oa \>i$Z^4* I Z • • •* * " * .0E*0T + " i * • ! ^,00E+O« .OE'OB + ,aB*D> dlalanoa la adga (mm) tflalanea lo adga (mm) AppendixB.l: Residual stress data for different geometries (see table 4.2), calculated by FEM. 1 Foil: 11 normal aUMS caramlealdo Foil: 12 ahaar straaa earamlc alda Foil: 22 normal straaa on earamlc alda 2,O0E*OB 1.90E*oa 1,D0G»0B g B,00e*07 2,0E*0« • £ ^.OE«0T M Al.on, 5 0,006*00 m m X2.0fr 1 ..„.„ A 5 n .8,006*07 «•1!t ! 1 ' < < ' » 11»—» » * * A ft s .i.ooe*o» •1.BDS*Da xxxxxxxxxxx X X X dltUnoa to adga (mm) .a,oos*oB Foil: 11 normal atraaa on Intarfaoa FotI: 22 nonnal atr*aa en lnl«rfac« e,OE*i 2,OE*07 .1 B,OE*t 0,OE*DO ' ^ A * ' X -2.aE*07 • • 0,1 mm . 3,0E»C X X X 3E S 2 £ ^.OE+07 • • D,Jmm HD,S mm • •«••!Ii I » • * 2.0E-C j^XXXXXX X X A1,0 mm 1 .B.aE*OT X2.e mm I- 4I,OE*07 , A A * X ^.OE- V X -1.06- -(,0E*aB X X X X X X X -1.SE*0I .to odg. Imm) dlalanoa to adgo(mm) Foil: 12 ahaar strasa In matal Foil: 22 normal sirass In matal 3.oa*0B 1,0E*0B 2.5E*0B S,OE*0T 2.0B-0B t f. W 9- ^ t,BE»OB • • X 07 I 1.0et0B X X " ! -LOE'OB I «.0e*07 •t.SE •2,SE*aB -1.0E.OB :xxxxx X X X X X -3,0B»0i toRso la oilaa (mm) AppendixB.2: Residual stress data for foil specimens calculated by FEM. S,000e+01 ii-ittiEbbfCiJËicau&Qii 6.500e+02 iiiiHigBtiaeBEBHaBEsS 4,000e<«2 J.SOOetOl llllilHUïa^'SESeBBB (:.160s<02 l.BOOBtOl 4.6006*02 2,3006*02 •l.OOOe+OO ^^^^^^^^^^^^^^^^^^^ 3.K0e*02 g 1.450e+02 -1.800e<01 2,700d*02 as 6.000e*ul -3.500e«01 1.7508+02 , -2.500e*01 -6.a)0e b) c) 2,600e*02 | *01 lililSE 3 anUBUHSfi 2,0008*02 1.800e»02 I •01 l,500e*02 IjjlUI B BB,CBKIUn5 1,1006-02 I *00 1.0000*02 iiiiiiim BUBnHnBM I •00 5.000e*01 4.0006*01 I *01 O.OO'I'S+OO -3.000e* lii i; ïi:£b[!srl!'.ï{r!I.E.oCI 11 t- libü'fiBSaBliHliiCbl 6.ugOe*01 2,2006*02 6.000e*0i II ii litaERBBaSiKSt-]::! Ij li lEBtciauByiiiiiuu 4,S00e*0i 1,8406*02 2.200e*01 n U IkbïBBESBIiïBiiSlH S H SÊKSHnHSEMSBli 3.S0O6*«L 1,4806*02 1,6006*01 •U n MBkHHIkKHlIltHH! 2.40Oe*Ol 1,1206*02 -6,4006*01 WÏ5ÏSSSSSSE5SSSÏSS 1.2OOe*01 1 7,5006*01 3.200e*«l 0,0006*00 I 4.0006*01 |-i.300e+02 -1,200»»01 I 4.0006*00 .6806*02 -2.4006*01 I :-3,200o*01 -2.0C^)e*« -3.6006*01 i •2.44t«*0J euKi ssissssEsssE -e.eoo6'Oi niiui •••••••BMBnci nUHi uiEBaBaaiBHBB -4,«l06*01 I •2.8206*02 BnailBhBBKBKBBHIIiaB l,040e«0£ I SCm BBS- J.bL. |-G,OOOe*01 •3.200e*02 1 e LnsaiEBBb »• 1,4006*02 L rikKbuBB IBu A i AEfa L u _ 11 E lüEk. br» L L L l b B C_[ ^ l !it_,_L_st_v,™J h) i) j) Appendix B,3: Stress patterns calculated by FEM for a,b,c) Notch D, d,e,f) Corner notch A and h,ij) 0.5 mm AISI 316 foil. Values in MPa. First column: 11 normal stress, second column: 12 shear stress and third colimin: 22 normal stress. First row: ceramic side, second row: interface and third row metal side.