High‐temperature fatigue crack propagation mechanism of orthogonal 3‐D woven amorphous SiC Fiber/SiC/YSi 2 ‐Si matrix composites

To elucidate degradation mechanisms attributable to high‐temperature fatigue crack propagation, a study was conducted of 3‐D woven SiC f /SiC CMC in which amorphous SiC fiber was used as a reinforcement material and in which a matrix was formed through low‐temperature melt infiltration. From a high‐temperature fatigue test conducted at 1373 K in the atmosphere with stress of 142 MPa or more, the fracture lifetime of newly developed SiC f /SiC CMC was found to be longer than that of SiC f /SiC CMC, which uses crystalline SiC fiber. Furthermore, repeatedly applying high temperatures during high‐temperature fatigue tests and using X‐ray computed tomography, fatigue cracks were found to propagate in a direction across 0‐degree fiber bundles that undergo stress. Electron mapping of regions with crack propagation revealed that oxidation eliminates boron nitride (BN), which has a crack deflection effect. The SiC fibers and matrix are fixed through the formation of oxides. Cracks propagate because of the consequent decrease in toughness of the SiC f /SiC CMC. In regions without crack propagation, fracture surfaces were not covered with oxides. These regions underwent forcible fracture in the final stage of the high‐temperature fatigue tests. From the test results presented above, SiC f /SiC CMC is considered to undergo fracture when the effective cross‐sectional area is reduced because of crack propagation accompanying oxidation and when the test load exceeds the tensile strength of the residual cross‐sectional area. However, some cracks in the matrix produced by a low‐temperature melt infiltration process were closed by oxides derived from YSi 2 . Because of crack closing, crack propagation is presumed to be avoided. Also, LMI‐CMC showed excellent high‐temperature fatigue properties at pressures higher than 150 MPa, which exceeds the proportional limit.