High-temperature cyclic fatigue in air of SiCf/SiC ceramic matrix composite with a pyrolytic carbon interface

A cyclic fatigue test of SiCf/SiC ceramic matrix composites was conducted at 1400°C and compared to the monotonic tensile test. The specimens were prepared with an interface layer of pyrolytic carbon and densified through chemical vapor infiltration. In the monotonic tensile test, at 1400°C, the specimen fractured at a strain of 0.35% with a proportional limit stress of 175 MPa, showing a typical fiber pull-out. However, after a prolonged cyclic test with increasing stresses from 65 to 95 MPa, the specimen fractured brittlely with almost no fiber pull-out. The microstructure analysis of the fracture surface showed different oxidation levels with respect to fracture locations, indicating that the crack propagated during the cyclic fatigue test. Transmission electron microscopy analysis revealed that the interface layer of pyrolytic carbon was removed by oxidation and oxide layers were formed on both sides of the fiber and matrix at the later stage of the cyclic test, resulting in a strong interface between the fibers and matrix and brittle fracture during the cyclic test at 1400°C.