Journal of the bridge the fiber-matrix gap for material with a 0. 15 um thick F. E. Heredia, J. C. MeNulty, F. w. Zok, and A. G. Evans, Oxidatio carbon layer as -170 h at 1073K and-7h at 373K. This upper 2097-100(1995) Embrittlement Probe for Ceramic-Matrix Composites, J. Am. Ceram. Soc., 78 temperature limit for the IRM, shown in Fig. 7, was determined by H-T. Lin and P. F. Becher, ""Effect of Fiber Coating on Lifetime of Nicalon Henager and Jones'and Lewinsohn et al. using tests that lasted Fiber-Silicon Carbide Composites in Air,"Mater. Sci Eng. A,231 a few hours. so that the transition from irm to oem would not S. Raghuraman, M. K. Ferber, J. F. Stubbins, and A.A. Oxidation Tests in SiC/SiC Composites",pp. 1015-26 in Co /ol The boundaries of the IRM and OEM regions, shown in Fig. 7, Ban sa ames ca ceramic societ westerwelle ohd egg have been defined by the available experimental data. However, Microstructural Chang the IRM region may extend to both lower and high temperatures and to lower Po values. The experimentally determined upper Proceedings of the 20th Anmal Conference on Composites, Advanced Ceramics, temperature limit results from the lack of fiber stability at higher Westerville, OH, 1996 Structures-B. Edited by V Greenhut. American Ceramic Society, temperatures, such that the FRM becomes dominant at higher Po values In other words, the fiber creep rate dominates sample failure long before carbon oxidation can play a role. A composite the 17th Annual Conference on Composites with a more thermally stable, creep-resistant fiber would support a by D. Cranmer, Ameri O. Unal, A. J. Eckel, and F. C. Laabs, "Mechanical Properties and Microstructure higher IRM regime. The experimentally determined lower temper- n to expect the IRM to continue Sorbide.mR下Cm由四cwSm ature limit is primarily a result of the limits of reasonable at lower temperatures, except that the oem boundary is likely to So277192381-94(1994 Jones, C. H. Henager Jr, and C. F. Windisch Jr, "High-Temperature love to lower Po, values with increased time, as discussed above Corrosion and Crack Growth of SiC-SiC at Variable Oxygen Partial Pressures, Likewise, the IRM could extend to lower Po, values, but the FRM becomes the dominant crack growth mechanism at low DC. F. Windisch Jr, C. H. Henager Jr, G. D. Springer, and R H. Jones, "Oxidation of the Carbon Interface in Nicalon-Fiber-Reinforced Silicon Carbide Composite, In summary, regardless of the temperature and oxygen pressure, J. dm. Ceram Soc, 80 [3]569-74(199 transition temperature(T> T )or at an oxide thickness lower than during Slow Crack Growth and the Result Jones, "Crack Bridging by SiC Fibers C. R. Jones. C. H. He tant Fracture Toughness of SiC/SiCr a critical value(d< d), and the OEM appears to operate at Composites,Scr. Metall. Mater, 33, 2067-12(1995) temperatures below the glass-transition temperature(I< g)and Kinetics of One- Dimeara s R. Naslain, and ).Thebault, "Oxidation Mechanisms and "L. Filipuzzi, G. Camu at an oxide thickness above a critical value(d>d) L. Filipuzzi, G. Camus, lain, and J. Thebault, "Oxidation Mechanisms and Composite Materials: I Ceram.Soc,77[2]467-80(1994) VI. Conclusions A, J. Eckel, J, D. Cawley, and T. A. Parthasarathy, "Oxidation Kinetics of a arbon Phase in a Nonreactive Matrix,J.Am. Ceran. Soc., 78 (41 Experimental weight loss and crack growth data support the 72-80(1995) conclusion that the oxygen-enhanced crack growth of Sic/Sic ISC. A. Lewinsohn, J. 1. Eldridge, and R. H. Jones, "Techniques for Measuring occurs by more than one mechanism, depending on the exper Interfacial Recession in CFCCs and th ications on Subcritical Crack Growth nental conditions. An OEM operates at temperatures s1373 Ceram, Eng. Sci. Proc., 19 [3] 19-26(1998). okan dation behavior of Sic-Fiber nd at high oxygen pressures; an IRM operates at temperatures Reinforced SiC, " J Nuc. Mater, 227, 130-37(1995). >700 K and low oxygen pressures. The IRM may operate at short litte, M. Gomina, and J. vicens, "TEM Observations of SiC-Sic times, with a transition to the oem at longer times, if the stress is Composites with a Carbon Interphase Layer Annealed in Air at High Temperatures, low enough that sample failure does not occur first. reep Behavior and Structural Characterization at The OEm results from the reaction of oxygen w m High Temperatures of Nicalon SiC Fibers,J Mater. Sci., 19, 3658-70 a glass layer on the fiber. The fracture stress of the fiber is I9C. H. Henager Jr. and R. H. Jones, "High-Temperature Plasticity decreased if this layer is thicker than a critical value(d> d) and and Subcritical Crack Growth in Ceramic Composites, the temperature below a critical value(T< Ty), such that a sharp H-T. Lin, P F. Becher, and P. F. Tortorelli, "Elevated-Temperature Static Fatigue rack can be sustained in the layer. Other possible, but not of a Nicalon-Fiber-Reinforced SiC Composite, Mater. Res. Soc. Symp. Proc., 365, demonstrated, OEMs include(1)glass-phase formation in the 435-40(1995 fiber-matrix interface and (2) fiber-strength reduction by reaction 2D. A. Woodford, D R Van Steele, J.A.Brehm, L.A. Timms, and JE.Palko Testing the Tensile Pre vith oxygen. E. Lara-Curzio, " Stress-Rupture of Nicalon/SiC Continuo Ceramic The IRM results from the oxidation of the interfacial layer and Composites in Air at 950oC,J. Am. Ceran. Soc., 80 [12]3268-72(1997). the resulting relaxation of the bridging fibers. Interface removal 2Y. T. Zhu, S. T. Taylor, M. G. Stout, D. P. Butt, and T. C. Lowe, "Kinetics of ontributes to the stress relaxation of the fiber that occurs by creep Thermal, Passive Oxidation of Nicalon Fibers, "J. Am. Ceram Soc, 81[3]655-60 The IRM occurs over a wide range of temperatures for d < de and (1998)- M. Takeda, A Urano, J. Sakamoto, and Y Imai, "Microstructure and Oxidative nay occur at T>T, and d> de. the IrM process has only been Degradation Behavior of Silicon Carbide Fiber Hi-Nicalon Type-S, "JNacl. Mater. observed in SiC/SiC with either carbon or bn as the fiber-matrix 258-63,1594-99(1998 interface material. Further research is needed to identify the W. H. Glime and J.D. Cawle ss Concentration Due to Fiber-Matrix Fusion eram. Soc,8loj2597-604(1998) specific temperature and Po, region in which the IRM and the OEM operate in SiCSic and whether there are conditions Exposure of Salt(NaCI) Water and Oxidation on the Strength of Uncoated and where both mechanisms also operate in glass-and oxide-matrix BN-Coated Nicalon Fibers, J Am Ceram. Soc-81171812-18(1998) Physics and Mechanics of Acknowledgments Temperatures, "J. Am. Cera. Soc., 79[3]591-96(1996) 2T. Helmer, H. Peterlik, and K. Kromp, "Coating of Carbon Fibers-The Strength the Office of Basic Department of Energy, under Contract No. DE-ACO6-75RLO 1830, with Battelle ies Degradation of sic Memorial Institute, which operates Pacific Northwest al Laboratory for the Fibers Below 850%C Mate ,13,680-83(1994) Department of Energy N. Okabe, I Murakami, I Y. Yoshioka, and H. Ichikawa, "Environmen- Deterioration and Damage of Ceramic Matrix Composites", in Proceedings of the by G. Pfendt American Ceramic Society, Westerville, OH, 1995. References 32R. H Jones and R. E. Ricker. "Mechanisms of Stress-Corrosion Cracking". in IA. G. Evans, F. w. Z M. McMecking, and Z. Z. Du "Models of Jones. .merican Socicty for Meals, Materils park. od, 1992. on. Edited by R. 3C. A Lewinsohn, C H. Henager Jr, and R H. Jones, "Environmentally Induced Composites,J. Am. Ceram 92345-52(199 Failure-Mechanism Mapping for Continuous-Fiber, Ceramic Composites", pp.bridge the fiber–matrix gap for material with a 0.15 mm thick carbon layer as ;170 h at 1073 K and ;7 h at 1373 K. This upper temperature limit for the IRM, shown in Fig. 7, was determined by Henager and Jones19 and Lewinsohn et al.,33 using tests that lasted a few hours, so that the transition from IRM to OEM would not require a much longer duration test. The boundaries of the IRM and OEM regions, shown in Fig. 7, have been defined by the available experimental data. However, the IRM region may extend to both lower and high temperatures and to lower pO2 values. The experimentally determined upper temperature limit results from the lack of fiber stability at higher temperatures, such that the FRM becomes dominant at higher pO2 values. In other words, the fiber creep rate dominates sample failure long before carbon oxidation can play a role. A composite with a more thermally stable, creep-resistant fiber would support a higher IRM regime. The experimentally determined lower temper￾ature limit is primarily a result of the limits of reasonable experimental times. There is reason to expect the IRM to continue at lower temperatures, except that the OEM boundary is likely to move to lower pO2 values with increased time, as discussed above. Likewise, the IRM could extend to lower pO2 values, but the FRM becomes the dominant crack growth mechanism at low pO2 values. In summary, regardless of the temperature and oxygen pressure, the IRM appears to operate at temperatures exceeding the glass￾transition temperature (T . Tg) or at an oxide thickness lower than a critical value (d , dc), and the OEM appears to operate at temperatures below the glass-transition temperature (T , Tg) and at an oxide thickness above a critical value (d . dc). VI. Conclusions Experimental weight loss and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiCf /SiC occurs by more than one mechanism, depending on the experi￾mental conditions. An OEM operates at temperatures &1373 K and at high oxygen pressures; an IRM operates at temperatures .700 K and low oxygen pressures. The IRM may operate at short times, with a transition to the OEM at longer times, if the stress is low enough that sample failure does not occur first. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber. The fracture stress of the fiber is decreased if this layer is thicker than a critical value (d . dc) and the temperature below a critical value (T , Tg), such that a sharp crack can be sustained in the layer. Other possible, but not demonstrated, OEMs include (1) glass-phase formation in the fiber–matrix interface and (2) fiber-strength reduction by reaction with oxygen. The IRM results from the oxidation of the interfacial layer and the resulting relaxation of the bridging fibers. Interface removal contributes to the stress relaxation of the fiber that occurs by creep. The IRM occurs over a wide range of temperatures for d , dc and may occur at T . Tg and d . dc. The IRM process has only been observed in SiCf /SiC with either carbon or BN as the fiber–matrix interface material. Further research is needed to identify the specific temperature and pO2 region in which the IRM and the OEM operate in SiCf /SiC and whether there are conditions where both mechanisms also operate in glass- and oxide-matrix composites. Acknowledgments This research was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy, under Contract No. DE-AC06-75RLO 1830, with Battelle Memorial Institute, which operates Pacific Northwest National Laboratory for the Department of Energy. References 1 A. G. Evans, F. W. Zok, R. M. McMeeking, and Z. Z. Du, “Models of High-Temperature, Environmentally Assisted Embrittlement in Ceramic-Matrix Composites,” J. Am. Ceram. Soc., 79 [9] 2345–52 (1996). 2 F. E. Heredia, J. C. McNulty, F. W. Zok, and A. G. Evans, “Oxidation Embrittlement Probe for Ceramic-Matrix Composites,” J. Am. Ceram. Soc., 78 [8] 2097–100 (1995). 3 H-T. Lin and P. F. 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