September 1998 Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature 2273 000 posite, whereas the time to rupture in air is much shorter than 1300c that in argon at a given stress in the standard SiC/SiC compos- ite. The time to rupture of the enhanced SiC/SiC composite is much longer than that of the standard SiC/SiC composite in air Although the creep rate of the enhanced SiC/SiC composite in argon higher than that of the standard SiC/SiC composite in the time to rupture of the enhanced SiC/SiC composite is still longer than that of the standard SiC/SiC composite. This phenomenon demonstrates that the addition of a glassy phase in he matrix of the enhanced SiC/SiC composite increases creep C=- Standard, Air rates, but much improves the total creep time to rupture in 10 argon. The reason for this result may be understood by the 10102103104105106107 match of the creep resistance between the fibers and the matrix ated Cycles to Failure SiC(NicalonTM) fibers is lower than that of the SiC ma trix.7,30-34 Creep of fibers transfers the stress onto the matrix Fig.8. Maximum stress versus number of cycles to failure of the and causes matrix cracks. Matrix cracking reloads the fibers enhanced SiC/SiC composite in air and the standard SiC/SiC compos- As the matrix creep resistance decreases, creep relaxation of ite in air and in argon under cyclic loading at 1300C he matrix may decrease the matrix cracking and stress con- centration near large pores, at which creep cracks are often Plots of the cyclic-fatigue life versus the maximum stress of he enhanced SiC/SiC composite in air and the standard SiC/ M2 030um (a ▲ Pores 25um for 2 hposite, whereas the time to rupture in air is much shorter than that in argon at a given stress in the standard SiC/SiC compos￾ite. The time to rupture of the enhanced SiC/SiC composite is much longer than that of the standard SiC/SiC composite in air. Although the creep rate of the enhanced SiC/SiC composite in argon is higher than that of the standard SiC/SiC composite in argon, the time to rupture of the enhanced SiC/SiC composite is still longer than that of the standard SiC/SiC composite. This phenomenon demonstrates that the addition of a glassy phase in the matrix of the enhanced SiC/SiC composite increases creep rates, but much improves the total creep time to rupture in argon. The reason for this result may be understood by the match of the creep resistance between the fibers and the matrix. As stated in the Introduction section, the creep resistance of SiC (Nicalon™) fibers is lower than that of the SiC ma￾trix.7,30–34 Creep of fibers transfers the stress onto the matrix and causes matrix cracks. Matrix cracking reloads the fibers. As the matrix creep resistance decreases, creep relaxation of the matrix may decrease the matrix cracking and stress con￾centration near large pores, at which creep cracks are often initiated.7 Plots of the cyclic-fatigue life versus the maximum stress of the enhanced SiC/SiC composite in air and the standard SiC/ Fig. 8. Maximum stress versus number of cycles to failure of the enhanced SiC/SiC composite in air and the standard SiC/SiC compos￾ite in air and in argon under cyclic loading at 1300°C. Fig. 9. Micrographs depicting crack propagation in specimens of the enhanced SiC/SiC composite (a) fatigued in air at 1300°C and a load of 90 MPa for 2.8 × 106 cycles, (b) crept in argon at 1300°C and a load of 90 MPa for 2 h, and (c) crept in air at 1300°C and a load of 90 MPa for 2 h. September 1998 Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature 2273