126 Journal of the American Ceramic Society-Zhu et al Vol. 82. Ne 0210310436x104 08 06 06 0.4 0.4 02 0 10910110210310410510107 0 0.002 0.004 0006 0008 cle Strain 90M 05 MPa Fig. 23. Evolution of the hysteresis loops during fatigue of the iC composite at 1300C under a ma 12b lution position of the SiC, O, phase, which resulted in the e. the tures have shown severe degradation that is caused by the 12 严1.2 decom icon monoxide(Si ases. 29,30,55 whereas fibers treated in an oxiding environment oxygen gas, air) showed slightly less degradation. 55 Only 25% of the initial strength of the fibers was retained after the treat- ment at 1300@C in argon. s The instability of Nicalon TM fiber at 06 high temperature in argon may be the reason for the similar y reep resistances at low stresses in air and argon(Figs. 13 and 0.4 When the matrix is enl 90 MPa 76 MPa ticulate(borosilicate) that is capable of sealing the outer sur- face and the cracked surface of the specimen at elevated tem- peratures, the oxidation of the interphase becomes minor. Thus 0 instability of Hi-Nicalon M or(Nicalon TM) fiber at high tem- perature in argon is the key factor that causes lower creep and atigue resistance in argon than in air. moreover, the sealing of Time. s cracks by glass may be more effective in air than in argon because there are more oxygen molecules available in air to react with the glass-forming particulate than in argon of the Hi-Nicalon TM SiC composite is almost the same as that Creep of Hi-NicalonTM/SiC, 1300 C, Ar of the enhanced SiC/SiC composite in which the matrix is the same as in the Hi-Nicalon TM/SiC composite but the fiber is icalonTM fiber in air(Figs. 10-12), althoug NicalonTM fiber had a greater creep resistance (one order of magnitude) in comparison to normal NicalonTM fiber 3 This observation indicates that Hi-Nicalon TM fiber does not improv 08F 08 the creep and fatigue resistance of the composite in air, if the fference in fiber architectures(satin-woven structure in the Hi-NicalonTNSiC composite and plain-woven structure in the enhanced SiC/SiC composite) is neglected. In fact, the effects of fiber architecture on the creep and fatigue of the omposites may not be minor. The bending extent of fibers in 120 MPa 60 MPa the satin-woven structure is smaller than that in the plain- 90 MPa woven structure. This observation may lead to less damage in the fiber bundles and. therefore. is beneficial to 0 0 and fatigue properties. The Hi-Nicalon TM/SiC composite and the enhanced SiC/SiC composite have the same creep and Time. 6 atigue resistance because the creep and fatigue resistance of oth composites is controlled by the matrix rather than the ibers or the fiber architecture. This result also explains why ne creep resistance of the Hi-NicalonTM/SiC composite is Fig. 24. Elastic modulus normalized by the value of the modulus higher than that of the enhanced SiC/SiC composite in argon nder the first loading(EE versus (a) the number of cycles for (Figs. 8 and 9) fatigue of the Hi-Nicalon TM/SiC composite in air at 1300C under ( Creep and Fatigue Interaction The interaction of creep-fatigue of CMCs has not been given mum stresses, and (c)the time required for creep of the Hi-NicalonTM/ much attention over the years. Holmes 8 reported that the fa- SiC composite in argon at 1300 C under different maximum stressestures have shown severe degradation that is caused by the decomposition of the SiCxOy phase, which resulted in the evo￾lution of carbon monoxide (CO) and silicon monoxide (SiO) gases,29,30,55 whereas fibers treated in an oxiding environment (oxygen gas, air) showed slightly less degradation.55 Only 25% of the initial strength of the fibers was retained after the treat￾ment at 1300°C in argon.55 The instability of Nicalon™ fiber at high temperature in argon may be the reason for the similar creep resistances at low stresses in air and argon (Figs. 13 and 14). When the matrix is enhanced by adding glass-forming par￾ticulate (borosilicate) that is capable of sealing the outer sur￾face and the cracked surface of the specimen at elevated tem￾peratures, the oxidation of the interphase becomes minor. Thus, instability of Hi-Nicalon™ or (Nicalon™) fiber at high tem￾perature in argon is the key factor that causes lower creep and fatigue resistance in argon than in air. Moreover, the sealing of cracks by glass may be more effective in air than in argon, because there are more oxygen molecules available in air to react with the glass-forming particulate than in argon. Another special result is that the creep and fatigue resistance of the Hi-Nicalon™/SiC composite is almost the same as that of the enhanced SiC/SiC composite in which the matrix is the same as in the Hi-Nicalon™/SiC composite but the fiber is normal Nicalon™ fiber in air (Figs. 10–12), although Hi￾Nicalon™ fiber had a greater creep resistance (one order of magnitude) in comparison to normal Nicalon™ fiber.31 This observation indicates that Hi-Nicalon™ fiber does not improve the creep and fatigue resistance of the composite in air, if the difference in fiber architectures (satin-woven structure in the Hi-Nicalon™/SiC composite and plain-woven structure in the enhanced SiC/SiC composite) is neglected. In fact, the effects of fiber architecture on the creep and fatigue of the composites may not be minor. The bending extent of fibers in the satin-woven structure is smaller than that in the plain￾woven structure. This observation may lead to less damage in the fiber bundles and, therefore, is beneficial to the creep and fatigue properties. The Hi-Nicalon™/SiC composite and the enhanced SiC/SiC composite have the same creep and fatigue resistance because the creep and fatigue resistance of both composites is controlled by the matrix rather than the fibers or the fiber architecture. This result also explains why the creep resistance of the Hi-Nicalon™/SiC composite is higher than that of the enhanced SiC/SiC composite in argon (Figs. 8 and 9). (3) Creep and Fatigue Interaction The interaction of creep–fatigue of CMCs has not been given much attention over the years. Holmes18 reported that the fa￾Fig. 23. Evolution of the hysteresis loops during fatigue of the Hi￾Nicalon™/SiC composite at 1300°C under a maximum stress of 120 MPa in air. Fig. 24. Elastic modulus normalized by the value of the modulus under the first loading (E/E° ) versus (a) the number of cycles for fatigue of the Hi-Nicalon™/SiC composite in air at 1300°C under different maximum stresses, (b) the time required for creep of the Hi-Nicalon™/SiC composite in air at 1300°C under different maxi￾mum stresses, and (c) the time required for creep of the Hi-Nicalon™/ SiC composite in argon at 1300°C under different maximum stresses. 126 Journal of the American Ceramic Society—Zhu et al. Vol. 82, No. 1