Degradation of SiC/SiC composite due to exposure at high temperatures in vacuum in comparison with that in air

Room temperature residual strength of the SiC/SiC composite exposed in vacuum at high temperatures (823-1673 K) was studied and compared with that exposed in air. The vacuum-exposed composite showed only the fiber-pullout type fracture, and the pullout length increased with increasing exposure temperature and time, while the fractured mode of the air-exposed one changed with progressing oxidation; from the fiber-pullout type to the nonfiber-pullout one characterized by the overall fracture perpendicular to the tensile axis without fiber-pullout. The reduction in residual strength in the case of vacuum exposure was attributed mainly to the extension of the decomposition-induced defects on the fiber surface into fiber, while that in the case of air exposure mainly to the extension of the crack made by premature fracture of the SiO ₂ layer into the fiber. A simple model based on the kinetics of the growth of the defects and fracture mechanics was presented to describe the variation of composite strength as a function of exposure temperature and time for the vacuum exposure, which could describe the experimental results.