ournal I Amm Ceram Soc. 81 [10] 2738-40(1998) Platinum as a Weak Interphase for Fiber-Reinforced Oxide-Matrix Composites J. Wendorff. R janssen and N. Claussen Advanced Ceramics Group, Technische Universitat Hamburg-Harburg, D-21071 Hamburg, Germany Alumina/zirconia matrix composites with platinum-coated ready proved effective, and some other investigations show the sapphire fibers were fabricated using the technology of re potential for weak interfaces on oxide ceramic substrates. 9 action bonding of aluminum oxide(RBAO)as the synthesis Some authors have used mixtures of precious metals and a oute. The interfacial behavior of both sintered and an- fugitive phase to form porous coatings during processing, but nealed specimens was investigated by crack path observa- there are still limitations in view of coating homogeneity and ions and fiber pushout tests. The results show that pre degrading mechanical properties of the composites. 10), II The cious metals like platinum or related alloys can be used as compatibility of dense platinum and iridium coatings with alu- igh-temperature interphases for oxide/oxide composites. mina or nonoxide fibers and matrices has already been claimed, Weak interfaces were obtained both between fiber and but the possibility of controlling the interfacial properties with should be suitable to achieve a damage-tolerant behavior of the trin was presnforsed with adhi the a a dense conata mm. coating of the fibers was used to create a weak fiber/matrix L. Introduction interphase. The main emphasis was on investigating the inter- facial behavior of these composites using crack path observa- URE oxide fiber composites can be used at high tions and single fiber pushout tests ven in oxidizing environments even with ctive coatings to prevent degradation. The mage-tolerant fracture behavior of the composi Il. Experimental Procedure material is the control of the fiber/matrix interface. The inter- face usually has to be weak enough to allow fiber/matrix Reaction-bonded aluminum oxide(BAO) was used as ma- debonding and fiber pullout during fracture. For lower trix material, s, I6 The precursor powder was fabricated by at- trition milling of metal ceramic powder mixtures for 7 h in temperature regimes, control of matrix porosity or use of po- acetone. The starting composition was 40 vol% Al(ECKA AS ous oxide coatings is an appropriate tool to adjust the inter- 081, Eckart-Werke GmbH, Furth, FRG), 40 vol% Al203(Cer- facial properties. 2,3 However, at higher temperatures densification of the matrix takes place, leading to strong bond alox MPA 4, RWE-DEA AG, Brunsbuttel, FRG), and 20 vol% ing between fiber and matrix, preventing the required fiber/ partially stabilized ZrO2(TZ-2Y, Tosoh Co., Tokyo, Japan) matrix debonding. Therefore, several attempts have been made After drying and sieving(200 um mesh), the powder was pressed uniaxially together with sapphire fibers followed by to achieve weak interfaces, i.e., by coating the fibers with re- cold isostatic compaction at 300 MPa. The reinforcing fibers fractory metals(W, Mo, Cr), coatings like carbon which form fugitive reaction products, or dense oxides like ZrO2. 4. Lim- with a diameter of =120 um(Saphikon Inc, Milford, NH)we dip coated with a platinum slurry prior to composite fabrica- ited oxidation resistance of metals, poor reproducibility due to tion. First tests were also performed with fibers coated with nonuniform reaction, or severe fiber strength degradation due to reactions between coatings and fibers limits the applicability platinum by PVD. Both coating techniques were applied at of these coating concepts. Recently, improvements have been room temperature. Heat treatment was carried out in a conven- made using phosphate coatings like monazite. Fiber/matrix tional box furnace in air at ambient pressure. A two-step heat debonding and the absence of detrimental reactions between ing cycle was chosen to convert the metallic aluminum into lead to he matrix material at temperatures up to dense composite materials still require pressure-assisted sinter 1 100 and 1550C, respectively. Part of the samples were an- ng procedures like hot pressing, and furthermore, more com- nealed at 1550oC up to 50 h to investigate long-time stability of rehensive studies are necessary to investigate high-temperature havior 6, 7 After sintering and annealing, the samples were cut into slices of 0.4 mm thickness with a high-precision diamond saw 4 Another possibility is the use of precious metals or alloys th high melting temperatures like platinum or iridium. Ap (EXACT GmbH, Norderstedt, FRG). Further polishing was not plications as protective coatings or diffusion barriers have al necessary and therefore avoided in order to reduce damage of the interfaces between fibers coating, and matrix. Previous investigations confirmed that this procedure is suitable to ob- tain damage-free slices of the composite. 7 For initial test of the interfacial behavior. Vickers indentations were used. Crack R.J. Kerans--contributine editor ath propagation at unstable crack growth was investigate with a specially designed cleavage apparatus using notched slices of composites, based on an idea of Morgan and Mar- Manuscript No, 190455. Received December 22, 1997; approved June 30, 1998. shall. Crack propagation during loading was investigated un- der an optical microscope. Furthermore, single fiber pushout tests were performed in Supported by the German Research Foundation(DFG)under Contract No Ja strength from the peak value splacement curves Member, American Ceramic Society The apparatus used is described in elsewhere I8 2738
Platinum as a Weak Interphase for Fiber-Reinforced Oxide-Matrix Composites J. Wendorff, R. Janssen,* and N. Claussen* Advanced Ceramics Group, Technische Universität Hamburg-Harburg, D-21071 Hamburg, Germany Alumina/zirconia matrix composites with platinum-coated sapphire fibers were fabricated using the technology of reaction bonding of aluminum oxide (RBAO) as the synthesis route. The interfacial behavior of both sintered and annealed specimens was investigated by crack path observations and fiber pushout tests. The results show that precious metals like platinum or related alloys can be used as high-temperature interphases for oxide/oxide composites. Weak interfaces were obtained both between fiber and coating as well as between matrix and coating, which should be suitable to achieve a damage-tolerant behavior of the composite. I. Introduction PURE oxide fiber composites can be used at high temperatures even in oxidizing environments even without additional protective coatings to prevent degradation. The key factor for damage-tolerant fracture behavior of the composite material is the control of the fiber/matrix interface. The interface usually has to be weak enough to allow fiber/matrix debonding and fiber pullout during fracture.1 For lowertemperature regimes, control of matrix porosity or use of porous oxide coatings is an appropriate tool to adjust the interfacial properties.2,3 However, at higher temperatures, densification of the matrix takes place, leading to strong bonding between fiber and matrix, preventing the required fiber/ matrix debonding. Therefore, several attempts have been made to achieve weak interfaces, i.e., by coating the fibers with refractory metals (W, Mo, Cr), coatings like carbon which form fugitive reaction products, or dense oxides like ZrO2. 4,5 Limited oxidation resistance of metals, poor reproducibility due to nonuniform reaction, or severe fiber strength degradation due to reactions between coatings and fibers limits the applicability of these coating concepts. Recently, improvements have been made using phosphate coatings like monazite. Fiber/matrix debonding and the absence of detrimental reactions between the components lead to encouraging results. However, fully dense composite materials still require pressure-assisted sintering procedures like hot pressing, and furthermore, more comprehensive studies are necessary to investigate high-temperature behavior.6,7 Another possibility is the use of precious metals or alloys with high melting temperatures like platinum or iridium. Applications as protective coatings or diffusion barriers have already proved effective, and some other investigations show the potential for weak interfaces on oxide ceramic substrates.8,9 Some authors have used mixtures of precious metals and a fugitive phase to form porous coatings during processing, but there are still limitations in view of coating homogeneity and degrading mechanical properties of the composites.10,11 The compatibility of dense platinum and iridium coatings with alumina or nonoxide fibers and matrices has already been claimed, but the possibility of controlling the interfacial properties with these coatings has not been demonstrated yet.12–14 In the present study, a reaction-bonded alumina/zirconia matrix was reinforced with sapphire fibers. A dense platinum coating of the fibers was used to create a weak fiber/matrix interphase. The main emphasis was on investigating the interfacial behavior of these composites using crack path observations and single fiber pushout tests. II. Experimental Procedure Reaction-bonded aluminum oxide (RBAO) was used as matrix material.15,16 The precursor powder was fabricated by attrition milling of metal ceramic powder mixtures for 7 h in acetone. The starting composition was 40 vol% Al (ECKA AS 081, Eckart-Werke GmbH, Fürth, FRG), 40 vol% Al2O3 (Ceralox MPA 4, RWE-DEA AG, Brunsbüttel, FRG), and 20 vol% partially stabilized ZrO2 (TZ-2Y, Tosoh Co., Tokyo, Japan). After drying and sieving (200 mm mesh), the powder was pressed uniaxially together with sapphire fibers followed by cold isostatic compaction at 300 MPa. The reinforcing fibers with a diameter of ≈120 mm (Saphikon Inc., Milford, NH) were dip coated with a platinum slurry prior to composite fabrication. First tests were also performed with fibers coated with platinum by PVD. Both coating techniques were applied at room temperature. Heat treatment was carried out in a conventional box furnace in air at ambient pressure. A two-step heating cycle was chosen to convert the metallic aluminum into Al2O3 and sinter the matrix material at temperatures up to 1100° and 1550°C, respectively. Part of the samples were annealed at 1550°C up to 50 h to investigate long-time stability of the coatings. After sintering and annealing, the samples were cut into slices of 0.4 mm thickness with a high-precision diamond saw (EXACT GmbH, Norderstedt, FRG). Further polishing was not necessary and therefore avoided in order to reduce damage of the interfaces between fibers, coating, and matrix. Previous investigations confirmed that this procedure is suitable to obtain damage-free slices of the composite.17 For initial test of the interfacial behavior, Vickers indentations were used. Crack path propagation at unstable crack growth was investigated with a specially designed cleavage apparatus using notched slices of composites, based on an idea of Morgan and Marshall.6 Crack propagation during loading was investigated under an optical microscope. Furthermore, single fiber pushout tests were performed in order to determine the interfacial strength from the peak value of the force–displacement curves. The apparatus used is described in detail elsewhere.18 R. J. Kerans—contributing editor Manuscript No. 190455. Received December 22, 1997; approved June 30, 1998. Presented at the 1st Conference on Composites at Lake Louise ’97, Lake Louise, Canada, October 12–17, 1997. Supported by the German Research Foundation (DFG) under Contract No. Ja 655/2.*Member, American Ceramic Society. J. Am. Ceram. Soc., 81 [10] 2738–40 (1998) Journal 2738
October 1998 Communications of the American Ceramic Sociery 2739 Fig. 1. Cross section of dense RBAO body (98.6% TD) with 3. Platinum-coated sapphire fiber pushed out of dense RBAO 1550C. It was decided not to polish the sample in order to prevent ix showing debon nd ductile deformation of the coating remain visible on the surface of the sappie 5 damage to the ductile interphase. Therefore, smal defects fro the ductility of the metal phase. 2 Both matrix and coating show no differences between sintered specimens and samples lI. Results and discussion annealed for up to 50 h in air Vickers indentations gave first indications about a weak vith continuous fiber reinforcement can be fabricated. because metal interphase. The cracks introduced by, the indentor run of the good flowing characteristics of the powder and the plas- tic deformability of the metallic aluminum during pressing. almost defect-free green compacts can be obtained by simple matrix are weak enough to allow the desired crack deflection tion is limited because of the size and rigidity of the sapphire and debonding. In Fig. 2, a sample cracked in the cleavage fibers as well as the simple hand laying technique used for apparatus is shown. In this case the debonding has mainly green body fabrication, fiber volume contents up to =20% were taken place between fiber and coating and coating deforms achieved. High green densities of >60%TD and the volume plastically. The smooth fiber surfaces prove the absence of expansion related to the oxidation of aluminum to alumina detrimental reactions between fiber. matrix and coating These results already show the suitability of platinum coat- and. therefore no defects are formed. 19 In accordance with the and both matrix and fiber. As shown in Fig 3, debond can results of Lange 20 these defect-free compacts can be sintered in occur between all components of the composite simulta- air without cracking. Final densities of 98.6% TD were ob- interphase itself. The corresponding pushout diagram(Fig. 4) verage grain size of the sintered specimens was 1.5 and 1.7 shows no sharp debonding peak stress but a soft transition um for a sintering time of 2 h and an annealing time of 50 h between debonding and sliding out of the fiber, correlated with respectively. It can be concluded that grain coarsening is al the ductile deformation behavior of the platinum coating. After most completely inhibited by the zirconia particles.2I debonding, pushout forces decrease because of the lower co- during processing. No indications of detrimental reactions be- the decreasing embedding length of the fiber. However, the ditionally, no gaps etween fibers, matrix, and coating were lated from the maximum of the pushout curves are between K,aBAO=7.5×1056-9×10°k4=7×106-85×10 50 F·482N atd=19.67 um Fig. 2. Cracked sample showing debonding between platinum slurry coating and fiber. The interphase deforms plastically
III. Results and Discussion Using RBAO as matrix material, homogeneous composites with continuous fiber reinforcement can be fabricated. Because of the good flowing characteristics of the powder and the plastic deformability of the metallic aluminum during pressing, almost defect-free green compacts can be obtained by simple powder metallurgical processing. Although fiber volume fraction is limited because of the size and rigidity of the sapphire fibers as well as the simple hand laying technique used for green body fabrication, fiber volume contents up to ≈20% were achieved. High green densities of >60% TD and the volume expansion related to the oxidation of aluminum to alumina (DV 4 28%) lead to reduced shrinkage on sintering compared with conventional alumina ceramics.15 Therefore, the stress regime in these composites is expected to be at a very low level and, therefore, no defects are formed.19 In accordance with the results of Lange20 these defect-free compacts can be sintered in air without cracking. Final densities of 98.6% TD were obtained by pressureless sintering in air for 2 h at 1550°C. The average grain size of the sintered specimens was 1.5 and 1.7 mm for a sintering time of 2 h and an annealing time of 50 h, respectively. It can be concluded that grain coarsening is almost completely inhibited by the zirconia particles.21 As shown in Fig. 1, the platinum coating remains fully intact during processing. No indications of detrimental reactions between the components of the composite have been found. Additionally, no gaps between fibers, matrix, and coating were formed due to low CTE mismatch (aAl203 ≈ 7 × 10−6 –8.5 × 10−6 K−1, aRBAO ≈ 7.5 × 10−6 –9 × 10−6 K−1, aPt ≈ 9 × 10−6 K−1) and the ductility of the metal phase.12 Both matrix and coating show no differences between sintered specimens and samples annealed for up to 50 h in air. Vickers indentations gave first indications about a weak metal interphase. The cracks introduced by the indentor run inside the coating around the fibers. Using the cleavage apparatus, it can be shown that, even at unstable crack growth, both the interface fiber/coating as well as the interface coating/ matrix are weak enough to allow the desired crack deflection and debonding. In Fig. 2, a sample cracked in the cleavage apparatus is shown. In this case the debonding has mainly taken place between fiber and coating and coating deforms plastically. The smooth fiber surfaces prove the absence of detrimental reactions between fiber, matrix, and coating. These results already show the suitability of platinum coatings as a weak interphase for oxide composites. First single fiber pushout tests confirm weak bonding between the coating and both matrix and fiber. As shown in Fig. 3, debonding can occur between all components of the composite simultaneously, combined with a plastic deformation of the platinum interphase itself. The corresponding pushout diagram (Fig. 4) shows no sharp debonding peak stress but a soft transition between debonding and sliding out of the fiber, correlated with the ductile deformation behavior of the platinum coating. After debonding, pushout forces decrease because of the lower coefficient of friction for sliding compared to static friction and the decreasing embedding length of the fiber. However, the detailed investigation of frictional sliding is beyond the scope of this paper. The values of the interfacial shear stress calculated from the maximum of the pushout curves are between Fig. 1. Cross section of dense RBAO body (98.6% TD) with sapphire fiber, PVD-coated with platinum, after annealing for 50 h at 1550°C. It was decided not to polish the sample in order to prevent damage to the ductile interphase. Therefore, small defects from cutting remain visible on the surface of the sapphire fiber. Fig. 2. Cracked sample showing debonding between platinum slurry coating and fiber. The interphase deforms plastically. Fig. 3. Platinum-coated sapphire fiber pushed out of dense RBAO matrix showing debonding and ductile deformation of the coating. Fig. 4. Typical pushout curve of platinum-coated sapphire fiber (recalculated taking elastic deformation of the testing apparatus into account). October 1998 Communications of the American Ceramic Society 2739
ations of the American Ceramic Sociery Vol 81. No. 10 30 and 50 MPa and therefore suitable to References sired fiber/matrix interaction with crack d and fiber A G. Evans and F. w. Zok, Review: The Physics and Mechanics of Fibre-Reinforced Brittle Matrix Composites, J. Mater. Sci, 29, 3857-96 So far. no substantial differences have been observed be tween sintered and annealed samples. Similar load/deflection Oxide Composites" Pais(Bordeaux, France, 199 the 6th Europ curves were obtained in both cases. A certain variety of pea naslainence stresses(interfacial strength values)may be due to inhomoge 3T. J. Mackin, J. Y. Yang, C. G. Levi, and A. G. Evans, ""Environmentally neous platinum coating of the fibers by the simple dip-coating Compatible Double Coating Concepts for Sapphire Fiber-Reinforced y-TiAl, technique, but nevertheless, the interfacial properties remain unchanged. Therefore, any degradation of the platinum inter- Composites." Am. Ceram. so;76图2y phase due to corrosion or evaporation can be neglected and SM. A. Stough, J. R. Hellmann, and M. S. Angelone, ""Interfacial Degradation stifles the expensive coating with precious metals. Looking at Ceramic Transactions, Vol 46, Chances in Ceramic- Matrir Composites .Edited the ductile deformation behavior of the platinum coating, a by J. P Singh and N P Bansal. American Ceramic Society, Westerville, OH, 1994 significant reduction of the coating thickness should be pos- " Ceramic Composites of Monazite and sible. Depending on the surface roughness of the fibers, coatin Alumina, J. Am. Ceram. Soc., 78[6] 1553-63(1995) 'D. B. Marshall, J. B. Davies, P. E. D. Morgan, and R. M. H that the precious metal is the dominant parameter. Neverthe Lake Louise, Canada, October 12-17, 1997 ss, with commercially available oxide fibers like polycrystal- B J. Dalgleish, E. Saiz, A P. Tomsia, R. M. Cannon, and R O. Ritchie line Al2O, fibers, thin platinum coatings, and the benefits of the Interface Formation and Strength in Ceramic-Metal Systems, ' Scr. Metall. RBAO process, composite prices below $700 "R K. Bordia and A Jagota, ""Crack Growth and Damage in Constrained ossible at fiber volume contents of =35% t Regarding the Sintering Films, "J.Am. Ceram Soc., 76[10]2475-85(1993) igh-temperature capabilities, such composites can have rea- oR S Hay, Fiber-Matrix Interfaces for Alumina Fiber-YAG Matrix Com- onable commercial relevance. Therefore, we think that plati IR S Hay, T May, and C. Cooke, ""Molybdenum-Palladium Fiber-Matrix num coatings represent a viable means to control the interfacial terlayers for Ceramic Composites, "Ceram. Eng. Sci. Proc., 1515] 760-68 properties of pure oxide composites for high-temperature use. (1-H w. Carpenter,JWBohlen, and w.s. Steffier,"Method of Forming a Ductile Fiber Coating for Toughening Non-oxide Ceramic Matrix Compos- IV. Conclusions ites,"U.S.Pat.No.5162271,1992 IK. L Luthra,""Ceramic Composite, "U.S. Pat. No 4 921 822, 1990 Reinforced e Using the bao technique, homogeneous fiber rein with Noble Metal Coated Ceramic Fibers. ' U.S. Pat. No 4 309 1989 orced oxide matrix co ites can be manufactured by simple ISN. Claussen, T. Le, and S. Wu, ""Low Shrinkage Reaction Bonded Alu- powder metallurgical processing and pressureless sintering Thin platinum coatings are suitable as compatible inter- ace layers between fibers and the matrix providing weak in 7J. Wendorff, R Janssen, and N. Claussen, "Model Experiments on Pure terfaces and ductile deformation behavior during fracture Oxide Composites, Mater. Sci. Eng. A, in pres These coatings are suitable for long-time use in oxygen- High Temperatures for Interface Characterization of Ceramic/Ceramic Com- ch atmospheres even at high temperatures First results indicate that very thin coatings should be posites,Pp:ng and Stan tion(Hamburg, Germany ) Edited by P. J. Hoe K Schulte, and H. Wittich. Woodhead Publishing Ltd, Cambridge, U.K., 1995. composite. Therefore, the overall price of such a composite Reinforced BAo, Ceram. Eng. san. noc. I5 5)36560(199-phire should be tolerable rained by Dens indrical Cores, Acta Metall. Mater, 37 [2]697-704(1989) ID Holz, M. Roger, R. Janssen, and N. Claussen, "Mechanical of Reaction-Bonded Al, OyZrO, Composites, " Ceram. Eng. Sci. Proc. 23J. 1. Eldridge,"Elevated Temperature Fiber Push-Out Testing, " Mater fThe calculation is based on a price for commercial fibers of about Res. Soc. Symp. Proc., 365, 283-90 er Push-Out Testing Apparatus for he market price assumed for platinum is -s 24H Janczak, L Rohr, P. Schulz, and H P, Degischer, ""Grenzflachenunt The stock market price has been in the close suchungen an endlosfaserverstarkten Aluminiummatrix Verbundwerkstoffen fur for the last 12 months die Raumfahrttechnik, ""Oberflachen, 6, 17-19(1995)
30 and 50 MPa and therefore suitable to achieve the desired fiber/matrix interaction with crack deflection and fiber bridging.17,22–24 So far, no substantial differences have been observed between sintered and annealed samples. Similar load/deflection curves were obtained in both cases. A certain variety of peak stresses (interfacial strength values) may be due to inhomogeneous platinum coating of the fibers by the simple dip-coating technique, but nevertheless, the interfacial properties remain unchanged. Therefore, any degradation of the platinum interphase due to corrosion or evaporation can be neglected and justifies the expensive coating with precious metals. Looking at the ductile deformation behavior of the platinum coating, a significant reduction of the coating thickness should be possible. Depending on the surface roughness of the fibers, coating thickness down to 0.2 mm or even below should be suitable. Calculating the overall price of such a composite, it is obvious that the precious metal is the dominant parameter. Nevertheless, with commercially available oxide fibers like polycrystalline Al2O3 fibers, thin platinum coatings, and the benefits of the RBAO process, composite prices below $700/kg should be possible at fiber volume contents of ≈35%.† Regarding the high-temperature capabilities, such composites can have reasonable commercial relevance. Therefore, we think that platinum coatings represent a viable means to control the interfacial properties of pure oxide composites for high-temperature use. IV. Conclusions ● Using the RBAO technique, homogeneous fiber reinforced oxide matrix composites can be manufactured by simple powder metallurgical processing and pressureless sintering. ● Thin platinum coatings are suitable as compatible interface layers between fibers and the matrix providing weak interfaces and ductile deformation behavior during fracture. ● These coatings are suitable for long-time use in oxygenrich atmospheres even at high temperatures. ● First results indicate that very thin coatings should be sufficient to achieve a damage-tolerant fracture behavior of the composite. Therefore, the overall price of such a composite should be tolerable. References 1 A. G. Evans and F. W. Zok, ‘‘Review: The Physics and Mechanics of Fibre-Reinforced Brittle Matrix Composites,’’ J. Mater. Sci., 29, 3857–96 (1994). 2 A. Kristofferson, A. Warren, J. Brandt, and R. Lundberg, ‘‘Reaction Bonded Oxide Composites’’; pp. 151–58 in Proceedings of the 6th European Conference on Composites Materials (Bordeaux, France, 1993). Edited by R. Naslain. 3 T. J. Mackin, J. Y. Yang, C. G. Levi, and A. G. Evans, ‘‘Environmentally Compatible Double Coating Concepts for Sapphire Fiber-Reinforced g-TiAl,’’ Mater. Sci. Eng., A, 161, 285–93 (1993). 4 J. B. Davis, J. P. A. Löfvander, and A. G. Evans, ‘‘Fiber Coating Concepts for Brittle-Matrix Composites,’’ J. Am. Ceram. Soc., 76 [5] 1249–57 (1993). 5 M. A. Stough, J. R. Hellmann, and M. S. Angelone, ‘‘Interfacial Degradation and Surface Modification in Zirconia-Coated Sapphire Fibers’’; pp. 839–49 in Ceramic Transactions, Vol. 46, Advances in Ceramic-Matrix Composites II. Edited by J. P. Singh and N. P. Bansal. American Ceramic Society, Westerville, OH, 1994. 6 P. E. D. Morgan, D. B. Marshall, ‘‘Ceramic Composites of Monazite and Alumina,’’ J. Am. Ceram. Soc., 78 [6] 1553–63 (1995). 7 D. B. Marshall, J. B. Davies, P. E. D. Morgan, and R. M. Housley, ‘‘Design of Ceramic Composites for high Temperature Oxidizing Environments’’; TNM5 in Proceedings of the 1st Conference on Composites at Lake Louise ’97, Lake Louise, Canada, October 12–17, 1997. 8 B. J. Dalgleish, E. Saiz, A. P. Tomsia, R. M. Cannon, and R. O. Ritchie, ‘‘Interface Formation and Strength in Ceramic–Metal Systems,’’ Scr. Metall. Mater., 31 [8] 1109–14 (1994). 9 R. K. Bordia and A. Jagota, ‘‘Crack Growth and Damage in Constrained Sintering Films,’’ J. Am. Ceram. Soc., 76 [10] 2475–85 (1993). 10R. S. Hay, ‘‘Fiber–Matrix Interfaces for Alumina Fiber–YAG Matrix Composites,’’ Ceram. Eng. Sci. Proc., 14 [9–10] 922–30 (1993). 11R. S. Hay, T. May, and C. Cooke, ‘‘Molybdenum–Palladium Fiber–Matrix Interlayers for Ceramic Composites,’’ Ceram. Eng. Sci. Proc., 15 [5] 760–68 (1994). 12H. W. Carpenter, J. W. Bohlen, and W. S. Steffier, ‘‘Method of Forming a Ductile Fiber Coating for Toughening Non-oxide Ceramic Matrix Composites,’’ U.S. Pat. No. 5 162 271, 1992. 13K. L. Luthra, ‘‘Ceramic Composite,’’ U.S. Pat. No. 4 921 822, 1990. 14K. L. Luthra, ‘‘Process for Producing a Ceramic Composite Reinforced with Noble Metal Coated Ceramic Fibers,’’ U.S. Pat. No. 4 933 309, 1989. 15N. Claussen, T. Le, and S. Wu, ‘‘Low Shrinkage Reaction Bonded Alumina,’’ J. Eur. Ceram. Soc., 5, 29–35 (1989). 16N. Claussen, R. Janssen, and D. Holz, ‘‘Reaction Bonding of Aluminum Oxide (RBAO) Science and Technology,’’ J. Ceram. Soc. Jpn., 103 [8] 749–58 (1995). 17J. Wendorff, R. Janssen, and N. Claussen, ‘‘Model Experiments on Pure Oxide Composites,’’ Mater. Sci. Eng. A, in press. 18J. Wendorff, R. Janssen, and N. Claussen, ‘‘The Fiber Push-Out Test at High Temperatures for Interface Characterization of Ceramic/Ceramic Composites’’; pp. 69–74 in Proceedings of the 2nd European Conference on Composites Testing and Standardization (Hamburg, Germany). Edited by P. J. Hogg, K. Schulte, and H. Wittich. Woodhead Publishing Ltd., Cambridge, U.K., 1995. 19J. Wendorff, D. E. Garcia, R. Janssen, and N. Claussen, ‘‘Sapphire-Fiber Reinforced RBAO,’’ Ceram. Eng. Sci. Proc., 15 [5] 364–70 (1994). 20F. F. Lange, ‘‘Densification of Powder Rings Constrained by Dense Cylindrical Cores,’’ Acta Metall. Mater., 37 [2] 697–704 (1989). 21D. Holz, M. Ro¨ger, R. Janssen, and N. Claussen, ‘‘Mechanical Properties of Reaction-Bonded Al2O3/ZrO2 Composites,’’ Ceram. Eng. Sci. Proc., 15 [5] 651–58 (1994). 22J. I. Eldridge, ‘‘Elevated Temperature Fiber Push-Out Testing,’’ Mater. Res. Soc. Symp. Proc., 365, 283–90 (1995). 23J. I. Eldridge and B. T. Ebihara, ‘‘Fiber Push-Out Testing Apparatus for Elevated Temperatures,’’ J. Mater. Res., 9 [4] 1035–42 (1994). 24H. Janczak, L. Rohr, P. Schulz, and H. P. Degischer, ‘‘Grenzfla¨chenuntersuchungen an endlosfaserversta¨rkten Aluminiummatrix Verbundwerkstoffen fu¨r die Raumfahrttechnik,’’ Oberfla¨chen, 6, 17–19 (1995). h † The calculation is based on a price for commercial fibers of about $500/kg, $10/kg for RBAO matrix, a fiber volume content of 35%, fiber diameter of 25 mm, coating thickness of 0.2 mm. This leads to an overall platinum content of 0.6 vol% of the composite. The market price assumed for platinum is ≈$14,850/kg plus 10% handling charge. The stock market price has been in the close vicinity of this value for the last 12 months. 2740 Communications of the American Ceramic Society Vol. 81, No. 10