Effect of Heat Treatment on Microstructure and Mechanical Properties of Stoichiometric SiC/SiC Composites*1

Polymer impregnation and pyrolysis (PIP) is one of the most attractive fabrication processes for silicon carbide (SiC) composites due to the shape flexibility, mass production and relatively low cost. In particular, advanced PIP SiC/SiC composite with high-crystallinity and near-stoichiometric composition is expected to have superior thermo-mechanical properties including good oxidation resistance, due to the reduction of impurities and well-organized crystal structure. Additionally, by applying a thin carbon interphase by chemical vapor infiltration (CVI), control of the crack propagation and oxidation resistance are also achieved. In this study, a CVI + PIP hybrid process based on the recently developed stoichiometric PIP process was performed. Specifically, matrix crystallization was enhanced by heat treatment in Ar, and its effect on microstructures and mechanical properties were evaluated. Stoichiometric SiC/SiC composites exhibit superior flexural strength up to 1573K in Ar and 1273K in air. This is because the stoichiometric composition in PIP-SiC matrix reduces inner oxidation by impurities. Also, the thin pyrolytic carbon interphase tailored by CVI process effectively controls crack propagation at fiber and matrix interphase. In a similar manner, the microstructure of the stoichiometric PIP-SiC matrix, constructed by the mixture of amorphous SiC and highly crystalline SiC, was stable against the high-temperature heat exposure up to 1773K in Ar. In particular, stoichiometric SiC/SiC composites heated at 1773K in Ar provides superior stability of mechanical properties up to 1573K even in air atmosphere, although extensive crystallization, in the case of heat treatment at 1973K in Ar, caused brittle composite fracture