January 1999 Creep and Fatigue Behavior in Hi-Nicalon MSiC Composites at High Temperatures 20 um Fracture surfaces of Hi-Nicalon TM/SiC specimens creep- d in air at 1300oC and 120 MPa, showing that a glassy phaso the surface MPa, matrix cracking can occur via stress redistribution due te creep of the fibers (2) Effects of Fiber, Fiber Architecture, and Matrix behavior of composites is dependent on the cree of the matrix. the fibers the fiber architecture. and the inter- hases and interfaces between the fibers and the matrix. In the Hi-NicalonTM/SiC composite, the creep behavior of the en- hanced SiC matrix is not known. The amount of additives in the matrix also is not known. Therefore. it is ssible to compare the creep resistance of the matrix to that of the fiber to determine the load-transfer direction during creep. However all the testing stresses for creep of the Hi-Nicalon TM/SiC com- osite in air are higher than the proportional limit, which can be ssumed to be an approximate matrix cracking stress. The ma trix microcracking occurs during the initial application of a creep load; therefore, fiber bridging of the matrix cracks al ways operates, whether the creep rate of the fibers is higher or lower than the matrix. In the crack-bridging mechanism, the matrix-crack growth rate is governed by a process in which the (b) increase in the crack length is accompanied by an increase in the number of fibers that bridge the crack. This phenomenon Fig 22. Cracks and glassy phases(indicated by continues up to a steady-state condition that is produced by the Nicalon M/SiC specimen crept in argon at 1300oC and 45 MPa for competition between creating more bridged fibers as the crack h;(b)high-magnification image of the composite shown in Fig. 22(a) length increases and the fracture or creep of these fibers as more stress is transferred to them by the increased crack opening displacement. Therefore, the effects of free surfaces of the fiber and/or the matrix, which progressively tance of the bridge fibers on creep behavior of the composite closes the porosity and the access for oxygen toward the inter- are expected to be important phase and, consequently, stops the oxidation pro A special result is that the creep resistance of the H The annular porosity around the fibers decreases the Nicalon TM/SiC composite in air is higher than that in argon strength, which decreases as the gauge length increases. 13, 14 enhanced SiC/SiC composite II However, the opposite result is matrix produces a strong interface that is harmful in regard to observed for the creep of the standard SiC/SiC composite both strength and ductility. Therefore, annular porosity around (Figs. 13 and 14) he fibers and the silica formation on the free surfaces of the For the standard SiC/SiC composite, the strength and stress- fiber and/or the matrix both decrease the strength of the com- rupture life in air are always lower than those in argon or under posite. 20, 48-50 This observation is the primary reason for the vacuum, because of oxidation. 20,48-50 The oxidation of the lower creep and fatigue resistance of the standard SiC/SiC standard SiC/SiC composite includes two concurrent phenom- composite in air than in argon. However, the ena: oxidation of the pyrocarbon interphase, which creates an stability of Nicalon TM fiber at high temperature also should annular porosity around the fibers, and silica formation on the be considered. Fibers heat-treated in argon at high tempera-MPa, matrix cracking can occur via stress redistribution due to creep of the fibers. (2) Effects of Fiber, Fiber Architecture, and Matrix The creep behavior of composites is dependent on the creep of the matrix, the fibers, the fiber architecture, and the inter￾phases and interfaces between the fibers and the matrix. In the Hi-Nicalon™/SiC composite, the creep behavior of the en￾hanced SiC matrix is not known. The amount of additives in the matrix also is not known. Therefore, it is impossible to compare the creep resistance of the matrix to that of the fibers to determine the load-transfer direction during creep. However, all the testing stresses for creep of the Hi-Nicalon™/SiC com￾posite in air are higher than the proportional limit, which can be assumed to be an approximate matrix cracking stress. The ma￾trix microcracking occurs during the initial application of a creep load; therefore, fiber bridging of the matrix cracks al￾ways operates, whether the creep rate of the fibers is higher or lower than the matrix. In the crack-bridging mechanism, the matrix-crack growth rate is governed by a process in which the increase in the crack length is accompanied by an increase in the number of fibers that bridge the crack. This phenomenon continues up to a steady-state condition that is produced by the competition between creating more bridged fibers as the crack length increases and the fracture or creep of these fibers as more stress is transferred to them by the increased crack￾opening displacement. Therefore, the effects of creep resis￾tance of the bridge fibers on creep behavior of the composite are expected to be important. A special result is that the creep resistance of the Hi￾Nicalon™/SiC composite in air is higher than that in argon (Figs. 6 and 7). This result is the same as the creep of the enhanced SiC/SiC composite.11 However, the opposite result is observed for the creep of the standard SiC/SiC composite (Figs. 13 and 14). For the standard SiC/SiC composite, the strength and stress￾rupture life in air are always lower than those in argon or under vacuum, because of oxidation.20,48–50 The oxidation of the standard SiC/SiC composite includes two concurrent phenom￾ena: oxidation of the pyrocarbon interphase, which creates an annular porosity around the fibers, and silica formation on the free surfaces of the fiber and/or the matrix, which progressively closes the porosity and the access for oxygen toward the inter￾phase and, consequently, stops the oxidation processes.51–54 The annular porosity around the fibers decreases the fiber strength, which decreases as the gauge length increases.13,14 The silica formation on the free surfaces of the fiber and/or the matrix produces a strong interface that is harmful in regard to both strength and ductility. Therefore, annular porosity around the fibers and the silica formation on the free surfaces of the fiber and/or the matrix both decrease the strength of the com￾posite.20,48–50 This observation is the primary reason for the lower creep and fatigue resistance of the standard SiC/SiC composite in air than in argon. However, the thermodynamic stability of Nicalon™ fiber at high temperature also should be considered. Fibers heat-treated in argon at high tempera￾Fig. 22. Cracks and glassy phases (indicated by arrows) in the Hi￾Nicalon™/SiC specimen crept in argon at 1300°C and 45 MPa for 65 h; (b) high-magnification image of the composite shown in Fig. 22(a). Fig. 21. Fracture surfaces of Hi-Nicalon™/SiC specimens creep￾fractured in air at 1300°C and 120 MPa, showing that a glassy phase covered the surface. January 1999 Creep and Fatigue Behavior in Hi-Nicalon™/SiC Composites at High Temperatures 125