BRENNAN: INTERFACIAL CHARACTERIZATION interface was developed that exhibits improved ther. saep tes anca s pnost anin g i he god se er this roeraney SiC/SiC composite for advanced gas turbine combus- tor applications. The fiber selected, Sylramic crys REFERENCES talline stoichiometric SiC. was found to exhibit improved thermal and environmental stability in the I. Wang, H, Singh, R and Goela,JJ.Am. CeramSoc. melt-infiltrated(MD)composites compared with the 2. Lipowitz, I. Rabe, I. Zangvil, A. and Xu, Y. Ceram. Eng carbon-rich Hi-Nicalon type SiC fiber. The instability Sci.Proc,l997,18(3),147 of the Hi-Nicalon fiber in the MI composite was 3. Srinivasan, G, Venkateswaran, V and Lau, S.Ceram of a thin, carbon-rich cial layer between the fiber and the BN fiber coating 4. Xu Y, Cheng, L. and Zhang, L. Carbon, 1999,37(8) during composite fabrication which, on subsequent 5.Luthra, K Singh, R and Brun, M. Am. Ceram Soc. Bull. exposure in oxidizing environment under stress, oxid- 993,72(7),79 ized away leading to silica formation and a strong 6. Luthra, K, Singh, R an M interfacial bonding state. The high proportional lim dings of the 6th Europe or matrix microcracking stress, of the Sylramic SiC 0-27 September 1993, ed. R. Naslain, J. Lamon and D fiber MI composites results in a high tensile fatigue Doumeing, Woodhead Publishing Ltd, Cambridge, UK, limit with very little or no degradation in composite 993,p.429 properties. 7. Corman, G, Brun, M, Meschter, P and Luthra, K. in Pr Issues still of concern for use of these composites ceedings of the 39th International SAMPE Symposium, Society for the Advancement of Materials and Process in advanced gas turbine combustor applications include the occasional weak and brittle composite due 8. Corman, G, Heinen, J and Goetz, R, ASME Paper 95. to incompletely sintered Sylramic SiC fibers witl G7-387. Presented at the 40th ASME International Gas rougher than normal surfaces, and the degradation of ne Congress and Exposition, Hous- composite properties in hot moist environments due 9. Corman, G, Brun, M. and Luthra, K ASME Paper 99. to accelerated oxidation of the bn fiber/matrix inter GT-234. Presented at the 44th ASME Gas Turbine and face. While the former concern has been reduced by Aeroengine Technical Congress, Exposition and Users changes in the processing of the Sylramic SiC fibers Symposium, Indianapolis, IN,7-10 June 1999 which ensure that sintering goes to completion, the 10. Anmual Book of ASTM Standards Section 15, vol. 15.01 atter concern has only been partially alleviated by adelphia, PA, 2000, p. 333 doping the Bn with Si. Additional work is necessary 11. Wortham, D. NASA CR-185261, National Aeronautics and to solve the high-temperature moisture degradation of Space Administration(NASA), Washington, DC, 1990 the Bn interface in these composites 2. Weihs, T, Sbaizero, O, Luh, E. and Nix, W. J.Am Ceram. Soc.. 1991. 74. 535 13. Liu, H, Zhou, L. and Mai, YJ.Am. oC,1995, acknowled pon work supported by Nas d like to 14. Cooper, R and Chyung, K.J. Mater. So 2.3148 wn and R. Wong of 15. Morscher, G, Bryant, D and Tressler, R ng. Sci. UTRC fo of the composite Proc., 1997, 18(3,5254628 BRENNAN: INTERFACIAL CHARACTERIZATION interface was developed that exhibits improved ther￾mal and mechanical properties over conventional CVI SiC/SiC composite for advanced gas turbine combus￾tor applications. The fiber selected, Sylramic crys￾talline stoichiometric SiC, was found to exhibit improved thermal and environmental stability in the melt-infiltrated (MI) composites compared with the carbon-rich Hi-Nicalon type SiC fiber. The instability of the Hi-Nicalon fiber in the MI composite was related to the formation of a thin, carbon-rich interfa￾cial layer between the fiber and the BN fiber coating during composite fabrication which, on subsequent exposure in oxidizing environment under stress, oxid￾ized away leading to silica formation and a strong interfacial bonding state. The high proportional limit, or matrix microcracking stress, of the Sylramic SiC fiber MI composites results in a high tensile fatigue limit with very little or no degradation in composite properties. Issues still of concern for use of these composites in advanced gas turbine combustor applications include the occasional weak and brittle composite due to incompletely sintered Sylramic SiC fibers with rougher than normal surfaces, and the degradation of composite properties in hot moist environments due to accelerated oxidation of the BN fiber/matrix inter￾face. While the former concern has been reduced by changes in the processing of the Sylramic SiC fibers which ensure that sintering goes to completion, the latter concern has only been partially alleviated by doping the BN with Si. Additional work is necessary to solve the high-temperature moisture degradation of the BN interface in these composites. Acknowledgements—This paper is based upon work supported by NASA contract NAS3-26385. The author would like to thank G. McCarthy, B. Laube, B. Brown and R. Wong of UTRC for the microstructural analyses of the composite samples, and G. Linsey and G. Ojard of Pratt & Whitney for their technical support during the course of this program. REFERENCES 1. Wang, H., Singh, R. and Goela, J. J. Am. Ceram. Soc., 1995, 78, 2437. 2. Lipowitz, J., Rabe, J., Zangvil, A. and Xu, Y. Ceram. Eng. Sci. Proc., 1997, 18(3), 147. 3. Srinivasan, G., Venkateswaran, V. and Lau, S. Ceram. Eng. Sci. Proc., 1995, 16(4), 63. 4. Xu, Y., Cheng, L. and Zhang, L. Carbon, 1999, 37(8), 1179. 5. Luthra, K., Singh, R. and Brun, M. Am. Ceram. Soc. Bull., 1993, 72(7), 79. 6. Luthra, K., Singh, R. and Brun, M. in High Temperature Ceramic Composites, Proceedings of the 6th European Conference on Composite Materials, Bordeaux, France, 20–27 September 1993, ed. R. Naslain, J. Lamon and D. Doumeing, Woodhead Publishing Ltd, Cambridge, UK, 1993, p. 429. 7. Corman, G., Brun, M., Meschter, P. and Luthra, K. in Pro￾ceedings of the 39th International SAMPE Symposium, Society for the Advancement of Materials and Process Engineering, Covina, CA, 1994, p. 2300. 8. Corman, G., Heinen, J. and Goetz, R., ASME Paper 95- GT-387. Presented at the 40th ASME International Gas Turbine and Aeroengine Congress and Exposition, Hous￾ton, TX, 5–8 June 1995. 9. Corman, G., Brun, M. and Luthra, K., ASME Paper 99- GT-234. Presented at the 44th ASME Gas Turbine and Aeroengine Technical Congress, Exposition and Users Symposium, Indianapolis, IN, 7–10 June 1999. 10. Annual Book of ASTM Standards Section 15, vol. 15.01. American Society for Testing and Matetrials (ASTM), Phi￾ladelphia, PA, 2000, p. 333. 11. Wortham, D. NASA CR-185261, National Aeronautics and Space Administration (NASA), Washington, DC, 1990. 12. Weihs, T., Sbaizero, O., Luh, E. and Nix, W. J. Am. Ceram. Soc., 1991, 74, 535. 13. Liu, H., Zhou, L. and Mai, Y. J. Am. Ceram. Soc., 1995, 78, 560. 14. Cooper, R. and Chyung, K. J. Mater. Sci., 1987, 22, 3148. 15. Morscher, G., Bryant, D. and Tressler, R. Ceram. Eng. Sci. Proc., 1997, 18(3), 525