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G N Morscher, J.D. Cawley / Journal of the European Ceramic Society 22(2002)2777-2787 ses(Fig. 9)and a ne ew ulme-de pendence for fiber ment of Ceramic Components in a MS9001FA Gas Turbine, embrittlement would have to be determined in order to ASME.98-GT-186,1998 model these composites. It obviously takes a longer time 3. Kameda, T, Itoh, Y, Hishata. T. and Okamura, T. Develop- to fuse fibers together that have longer separation dis ment of Continuous Fiber Reinforced Reaction Sintered Silicon Carbide Matrix Composite for Gas Turbine Hot Parts Applica- tances. "Outside debonding"composites appear to be tion.(ASME,2000-GT67)2000. contro olled by the time it takes to oxidize through xidize through the 4. Heredia, F. E. McNulty. J. C, Zok, F. W. and E BN interphase layer, approximately 100 h at 815Cin Oxidation embrittlement probe for ceramic-matrix composites. J.Am.Cera.Soc.,1995,78.209 5. Filipuzzi. L, Camus, G. Naslain, R and Thebault, J, Oxidation mechanisms and kinetics of lD-SiC/C/SiC composite materials: I 5. Conclusions 6. Eckel, A J, Cawley, J. D and Parthasarathy, T, Oxidation of a ntermediate temperature strength degradation of ntinuous carbon phase in a nonreactive matrix. J. Am. Ceram. Sic/Sic composites is due to a"pest condition pri- Soc.1995,78,972-980. 7. Lara- Curzio. E. Ferber. M. K. and Tortorelli. P. F. Interface marily caused by the oxidation of the interphase separ- xidation and stress-rupture of Nicalon/SiC CFCC's at inter- ating the fibers and the matrix. Although, BN ediate temperatures. In Key Engineering Materials, Vols. 127 interphases are superior to carbon interphase compo- 131. Trans. Tech Publications, Switzerland, 1997, pp. 1069-1082. sites, they still exhibit significant degradation in stress- 8. Lara-Curzio, E, Stress-rupture of Nicalon/SiC continuous fiber rupture properties at intermediate temperatures. The cramic matrix composites in air at 950C.. Am 1997.80.32683272 main factor causing this strength degradation is the 9. Martinez-Fernandez, J and Morscher. G. N. Room and elevated fusion of fibers to one another in a matrix crack that is temperature tensile properties of single tow Hi-Nicalon, carbon exposed to the oxidizing environment. The amount of terphase CVI SiC matrix strength degradation is dependent on the kinetics for 20.2627-2636 Fusion of fibers to one another the number of matrix un.E. Y, Nutt. S. R. and Brennan, JJ. Oxi dation of BN-coated SiC fibers in ceramic-matrix composites. cracks, and the applied stress state. It was shown that Am Cera. Soc. 1996 the stress-rupture properties of SiC/BN/SiC composites 11. Jacobson, N.S., Morscher, GN,Bryant, D. R and Tressler, could be effectively modeled using an approach that R. E, High-temperature oxidation of boron nitride: Il, boron considers the probability of fiber failure in relation te ayers in composites. JAnm. Ceram. Soc.,1999.82.1473- the likelihood that the fiber had already been fused to its 12. Lin. H. T. and Becher. P. F. Effect of coating on lifetime of neighbor or the matrix. One important aspect of the calon fiber-silicon carbide composites in air. Materials Science model that was verified was the increased susceptibility d engineerin,1997,A231,143-150. to stress-rupture for composites with a greater number 13. Morscher, G. N, Tensile stress-rupture of SiC/SiCm mini- of matrix cracks composites with carbon and boron nitride interphases at elevated ecently, improvements have been made for BN- mperatures in air. J. Am. Ceram. Soc., 1997, 80, 2029-2042 14. Morscher. G. N. Hurst. J and Brewer. D. Intermediate-tem- nterphase composites. These include, Si-doped BN, perature stress rupture of a woven Hi-Nicalon, BN-interphase composites with more effective fiber spreading, and BN Sic-matrix composite in air. J. Am. Ceram Soc., 2000, 83, 1441- nterphases wh bonding Iding occur between the bn layer and the matrix rather than the 15. Morscher. G. N. and Hurst, J, Stress-rupture and stress-relaxa- and the bn layer. Consistent with the mechanistics tion of Sic/SiC composites at intermediate temperature Ceram Eng.Sci.Proc2001,22,539546. assumed in the model, for composites made with these 16. Yun, H. M. and DiCarlo, J. A, Time/temperature dependent modifications, the 500-h rupture stress increased from tensile strength of Sic and Al203-based fibers. In in Ceramic about 155 MPa for conventional composites to over 200 ansactions, ol. 74. Advances in Ce Matrix Composit MPa. For"outside debonding 500-h rupture stresses l. ed. N. P. Bansal and J. P. Singh. American Ceramic Society close to 250 MPa have been attained. this does not Westerville OH, 1996, pp. 17-26 necessarily eliminate the"pest regime"for these com- 17. Ogbuji, L. U.J. T, Identification of a Carbon Sublayer in a Hi- calon/ BN/SiC Composite. J. Mater. Sci. Letters, 199 posites; however, these approaches would significantly increase the stress range these composites could with 18. Curtin, w.A., Multiple matrix crack spacing in brittle matrix stand at intermediate temperatures in oxidizing envir- omposites. J. Am. Ceram Soc., 1991, 74, 2837. onments 19. Curtin, w. A. Ahn. B K. and Takeda. N, Modeling brittle and tough stress-strain behavior in unidirectional ceramic matrix opposites. Acta Mater., 1998, 46. 3409-3420 20. lyengar, N. and Curtin, w.A., Time-dependent failure in fiber References reinforced composites by fiber degradation. Acta Mater., 1997 21. Marshall. D. B. Cox.B. N. and Evans.A.G. The mechanics of 1. Brewer, D, HSR/EPM Combustor Materials Development Pro- matrix cracking in brittle-matrix fiber composites. Acta Metal. gram. Mater. Sci. Eng. 4,, 1999, A261, 284-291 1985.33.2013-2021 2. Grondahl, C. M. and Tsuchiya, T. Performance Benefit Assess- 22. Cao, H. and Thouless, M. D, Tensile tests of ceramic-matrixcases (Fig. 9) and a new time-dependence for fiber embrittlement would have to be determined in order to model these composites. It obviously takes a longer time to fuse fibers together that have longer separation distances. ‘‘Outside debonding’’ composites appear to be controlled by the time it takes to oxidize through the BN interphase layer, approximately 100 h at 815 C in air. 5. Conclusions Intermediate temperature strength degradation of SiC/SiC composites is due to a ‘‘pest’’ condition primarily caused by the oxidation of the interphase separating the fibers and the matrix. Although, BN interphases are superior to carbon interphase composites, they still exhibit significant degradation in stressrupture properties at intermediate temperatures. The main factor causing this strength degradation is the fusion of fibers to one another in a matrix crack that is exposed to the oxidizing environment. The amount of strength degradation is dependent on the kinetics for fusion of fibers to one another, the number of matrix cracks, and the applied stress state. It was shown that the stress-rupture properties of SiC/BN/SiC composites could be effectively modeled using an approach that considers the probability of fiber failure in relation to the likelihood that the fiber had already been fused to its neighbor or the matrix. One important aspect of the model that was verified was the increased susceptibility to stress-rupture for composites with a greater number of matrix cracks. Recently, improvements have been made for BNinterphase composites. These include, Si-doped BN, composites with more effective fiber spreading, and BN interphases where the debonding and sliding occur between the BN layer and the matrix rather than the fiber and the BN layer. Consistent with the mechanistics assumed in the model, for composites made with these modifications, the 500-h rupture stress increased from about 155 MPa for conventional composites to over 200 MPa. For ‘‘outside debonding’’, 500-h rupture stresses close to 250 MPa have been attained. This does not necessarily eliminate the ‘‘pest regime’’ for these composites; however, these approaches would significantly increase the stress range these composites could withstand at intermediate temperatures in oxidizing environments. References 1. Brewer, D., HSR/EPM Combustor Materials Development Program. Mater. Sci. Eng. A,, 1999, A261, 284–291. 2. Grondahl, C. M. and Tsuchiya, T. Performance Benefit Assessment of Ceramic Components in a MS9001FA Gas Turbine, ASME. 98-GT-186, 1998. 3. Kameda, T., Itoh, Y., Hishata, T., and Okamura, T. Development of Continuous Fiber Reinforced Reaction Sintered Silicon Carbide Matrix Composite for Gas Turbine Hot Parts Application. (ASME, 2000-GT-67) 2000. 4. Heredia, F. E., McNulty, J. C., Zok, F. W. and Evans, A. G., Oxidation embrittlement probe for ceramic-matrix composites. J. Am. Ceram. Soc., 1995, 78, 2097. 5. Filipuzzi, L., Camus, G., Naslain, R. and Thebault, J., Oxidation mechanisms and kinetics of 1D-SiC/C/SiC composite materials: I, an experimental approach. J. Am. Ceram. Soc., 1994, 77, 459– 466. 6. Eckel, A. J., Cawley, J. D. and Parthasarathy, T., Oxidation of a continuous carbon phase in a nonreactive matrix. J. Am. Ceram. Soc., 1995, 78, 972–980. 7. Lara-Curzio, E., Ferber, M. K., and Tortorelli, P. F., Interface oxidation and stress-rupture of Nicalon/SiC CFCC’s at intermediate temperatures. In Key Engineering Materials, Vols. 127– 131. Trans. Tech. Publications, Switzerland, 1997, pp. 1069–1082. 8. Lara-Curzio, E., Stress-rupture of Nicalon/SiC continuous fiber ceramic matrix composites in air at 950 C. J. Am. Ceram. Soc., 1997, 80, 3268–3272. 9. Martinez-Fernandez, J. and Morscher, G.N., Room and elevated temperature tensile properties of single tow Hi-Nicalon, carbon interphase CVI SiC matrix minicomposites. J. Eur. Ceram. Soc., 20, 2627–2636. 10. Sheldon, B. W., Sun, E. Y., Nutt, S. R. and Brennan, J. J., Oxidation of BN-coated SiC fibers in ceramic-matrix composites. J. Am. Ceram. Soc., 1996, 79, 539–543. 11. Jacobson, N. S., Morscher, G. N., Bryant, D. R. and Tressler, R. E., High-temperature oxidation of boron nitride: II, boron nitride layers in composites. J. Am. Ceram. Soc., 1999, 82, 1473– 1482. 12. Lin, H. T. and Becher, P. F., Effect of coating on lifetime of Nicalon fiber-silicon carbide composites in air. Materials Science andEngineering, 1997, A231, 143–150. 13. Morscher, G. N., Tensile stress-rupture of SiCf/SiCm minicomposites with carbon and boron nitride interphases at elevated temperatures in air. J. Am. Ceram. Soc., 1997, 80, 2029–2042. 14. Morscher, G. N., Hurst, J. and Brewer, D., Intermediate-temperature stress rupture of a woven Hi-Nicalon, BN-interphase, SiC-matrix composite in air. J. Am. Ceram. Soc., 2000, 83, 1441– 1449. 15. Morscher, G. N. and Hurst, J., Stress-rupture and stress-relaxation of SiC/SiC composites at intermediate temperature. Ceram. Eng. Sci. Proc., 2001, 22, 539–546. 16. Yun, H. M. and DiCarlo, J. A., Time/temperature dependent tensile strength of SiC and Al2O3-based fibers. In in Ceramic Transactions, Vol. 74. Advances in Ceramic-Matrix Composites III, ed. N. P. Bansal and J. P. Singh. American Ceramic Society, Westerville OH, 1996, pp. 17–26. 17. Ogbuji, L. U. J. T., Identification of a Carbon Sublayer in a HiNicalon/BN/SiC Composite. J. Mater. Sci. Letters., 1999, 18, 1825–1827. 18. Curtin, W. A., Multiple matrix crack spacing in brittle matrix composites. J. Am. Ceram. Soc., 1991, 74, 2837. 19. Curtin, W. A., Ahn, B. K. and Takeda, N., Modeling brittle and tough stress–strain behavior in unidirectional ceramic matrix composites. Acta Mater., 1998, 46, 3409–3420. 20. Iyengar, N. and Curtin, W. A., Time-dependent failure in fiberreinforced composites by fiber degradation. Acta Mater., 1997, 45, 1489. 21. Marshall, D. B., Cox, B. N. and Evans, A. G., The mechanics of matrix cracking in brittle-matrix fiber composites. Acta Metal., 1985, 33, 2013–2021. 22. Cao, H. and Thouless, M. D., Tensile tests of ceramic-matrix 2786 G.N. Morscher, J.D. Cawley / Journal of the European Ceramic Society 22 (2002) 2777–2787