13th International Conference on Fracture June 16–21, 2013, Beijing, China 6 All the specimens failed in a typical shear mode along their respective interlaminar shear planes. As shown in Fig. 5(a) and (b) for 2.5D-C/SiC, it can also prove that only the fewer weft interlocked yarns played a resisting role in the process of shear failure along the warp direction，while the more interlocked yarns resist to fracture and fracture surface is rough in the weft direction. As presented in Fig. 5(c) and (d), for 2D-C/SiC and 2D-SiC/SiC, the fracture surfaces are both somewhat smooth and clean, and the fiber bundle are clearly observed. Owing to a mismatch of the thermal expansion coefficients of the fibers and matrix, a certain mount of microcracks already exist in the SiC coating as well as in the SiC matrix upon cooling from the processing temperature for the three kinds of composites. And microcracks are formed in the coating that favor the in-depth diffusion of oxygen. When shear test is carried out at 1173K, the oxygen can diffuse through these microcracks and reach to the carbon interface and fibers. The oxidation kinetics is controlled by the gas-phase diffusion through the microcracks in the SiC coating, resulting in a nonuniform degradation of the carbon phases [15]. The oxidation of carbon interface and fiber causes degradation of bond between the fiber and matrix, so the fiber is easier pulling out owing to the weak interface, as illustrated in Fig. 6(a). Due to the inhomogeneous SiC coating and stress concentration in the vicinity of notches, failure of the specimens usually generates from the notches, resulting in severe oxidation at this site (Fig. 6(b)). Carbon fibers are rapidly oxidized at temperatures above 400°C, while the silicon carbide fiber possesses relatively high anti-oxidation temperature, as shown in Fig. 6(c) and (d), consequently, ILSS of 2D-SiC/SiC is higher than that of 2D-C/SiC. Figure 5. Optical micrographs of the failed 2.5D-C/SiC along the warp (a) and weft (b) direction, 2D-C/SiC (c)and 2D-SiC/SiC (d) specimens. a d c b
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