Journal of the American Ceramic Society-Zhu et al. Vol 81. No 9 15m Fig. 10. Micrograph depicting creep crack propagation paths in a specimen of the enhanced SiC/SiC composite crept in argon at 1300oC and a load of 90 MPa for 2 h SiC composite in air and argon at 1300 C are shown in Fig 8. temperatures. The borosilicate glass does not wet the surface At the same maximum stress the fatigue life of the enhanced and acts to seal the interior from oxidation 6 Although Fig. 12 SiC/SiC composite is much higher than that of the standard shows carbon interphase oxidation on the fracture surfaces, in most areas in the specimens, no oxidative attack exists and The results(Figs. 7 and 8)show that the addition of the carbon remains at the interfaces. Carbon interphase oxidation lass-forming, boron-based particulates in the SiC matrix will be inhibited as long as the glass is present creases creep and the fatigue life in SiC/SiC composites at high temperature in air. Observation of the crack-propagation (4 Creep and Fatigue Damage Evolution paths will give information and evidence to understand this Gradual modulus degradation during cyclic fatigue has been reported for the unidirectional and laminated ceramic compo ites at room temperature 35-39 and at elevated temper 3) Microscopic Damage and fracture tures.5, 16, 17 40 It has been shown that the gradual damage Creep and fatigue cracks are always found at the large pores growth accompanies modulus decrease in the CMCs under mong the fiber bundles(indicated by arrows in Fig. 9). When ue racks meet 0 fibers, debonding of interfaces between the To understand the damage evolution during fatigue fibers and the matrix occurs. The 0 fibers bridge crack faces enhanced SiC/SiC composite at 1300C, the Youngs and therefore decrease the driving force at the crack tip as a were measured. Figure 13 shows the evolution of the bridging component. 1-3,7-12 At 1300 C, the glassy phases be- strain hysteresis loops. The slope decreases and the width of come liquid and flow into cracks. At room temperature, they he loops increases as the number of cycles increases. The resolidify and become situated in the cracks(Fig 9(c).Around the large pores between the fiber bundles, the matrix is the last coating layer by the pure SiC(the light-colored layer), which the same as the matrix in the standard SiC/SiC composite Crack propagation is very straight in this layer but is deflected or discontinuous in the inner matrix( the darker-colored areas in Figs. 9 and 10). This observation means that creep propa- gation occurs in the matrix of the enhanced SiC/SiC composite Severe oxidation of the fibers and the matrix can be found occasionally in the specimens after long time tests in air( Fig 11). This type of oxidation surely decreases the failure time 星 The filling of the glassy phases in the cracks hinders the dif- fusion of oxygen along crack paths. As a result, the oxidation esistance in the enhanced SiC/SiC composite is improved Fracture surfaces under creep or fatigue in air show that the interfaces between the fibers and the matrix become hollow (Fig. 12(a), because the pyrolytic carbon layer at the interface reacts with oxygen,6 and forms gas that evaporates into the environment. There are many pores or holes in the matrix(Fig 12(b). Their size is the same as that of the glass-forming particulates(Fig. I(c). Therefore, it is thought that the par- Fig. I1. Micrograph showing the oxidation fibers and the matrix ticulates become glass and flow out to seal cracks at high after fatigue at a load of 90 MPa for 2. x 106 cyclesSiC composite in air and argon at 1300°C are shown in Fig. 8. At the same maximum stress, the fatigue life of the enhanced SiC/SiC composite is much higher than that of the standard SiC/SiC composite in air, and also is higher than that in argon. The results (Figs. 7 and 8) show that the addition of the glass-forming, boron-based particulates in the SiC matrix in￾creases creep and the fatigue life in SiC/SiC composites at high temperature in air. Observation of the crack-propagation paths will give information and evidence to understand this phenomenon. (3) Microscopic Damage and Fracture Creep and fatigue cracks are always found at the large pores among the fiber bundles (indicated by arrows in Fig. 9). When cracks meet 0° fibers, debonding of interfaces between the fibers and the matrix occurs. The 0° fibers bridge crack faces and therefore decrease the driving force at the crack tip as a bridging component.1–3,7–12 At 1300°C, the glassy phases be￾come liquid and flow into cracks. At room temperature, they resolidify and become situated in the cracks (Fig. 9(c)). Around the large pores between the fiber bundles, the matrix is the last coating layer by the pure SiC (the light-colored layer), which is the same as the matrix in the standard SiC/SiC composite. Crack propagation is very straight in this layer but is deflected or discontinuous in the inner matrix (the darker-colored areas in Figs. 9 and 10). This observation means that creep propa￾gation occurs in the matrix of the enhanced SiC/SiC composite. Severe oxidation of the fibers and the matrix can be found occasionally in the specimens after long time tests in air (Fig. 11). This type of oxidation surely decreases the failure time. The filling of the glassy phases in the cracks hinders the dif￾fusion of oxygen along crack paths. As a result, the oxidation resistance in the enhanced SiC/SiC composite is improved. Fracture surfaces under creep or fatigue in air show that the interfaces between the fibers and the matrix become hollow (Fig. 12(a)), because the pyrolytic carbon layer at the interfaces reacts with oxygen5,6 and forms gas that evaporates into the environment. There are many pores or holes in the matrix (Fig. 12(b)). Their size is the same as that of the glass-forming particulates (Fig. 1(c)). Therefore, it is thought that the par￾ticulates become glass and flow out to seal cracks at high temperatures. The borosilicate glass does not wet the surface and acts to seal the interior from oxidation.6 Although Fig. 12 shows carbon interphase oxidation on the fracture surfaces, in most areas in the specimens, no oxidative attack exists and carbon remains at the interfaces. Carbon interphase oxidation will be inhibited as long as the glass is present.6 (4) Creep and Fatigue Damage Evolution Gradual modulus degradation during cyclic fatigue has been reported for the unidirectional and laminated ceramic compos￾ites at room temperature35–39 and at elevated tempera￾tures.5,16,17,40 It has been shown that the gradual damage growth accompanies modulus decrease in the CMCs under fatigue loading.37,39 To understand the damage evolution during fatigue of the enhanced SiC/SiC composite at 1300°C, the Young’s moduli were measured. Figure 13 shows the evolution of the stress– strain hysteresis loops. The slope decreases and the width of the loops increases as the number of cycles increases. The Fig. 10. Micrograph depicting creep crack propagation paths in a specimen of the enhanced SiC/SiC composite crept in argon at 1300°C and a load of 90 MPa for 2 h. Fig. 11. Micrograph showing the oxidation fibers and the matrix after fatigue at a load of 90 MPa for 2.8 × 106 cycles. 2274 Journal of the American Ceramic Society—Zhu et al. Vol. 81, No. 9