Diffusion Bonding of Zirconia to Austenitic Stainless Steel
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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. n 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