Microstructures and flexural properties of high temperature-pyrolyzed PIP-SiC/SiC composites

Near-stoichiometric SiC fiber composites with polymer-derived high-crystallinity SiC matrices were produced and were characterized their microstructures and mechanical properties. The material was produced through repeated polymer-impregnation and pyrolysis (PIP) cycles of Tyranno™-SA sintered SiC fibers, following pyrolytic carbon and SiC bi-layer interphase deposition. Co-polymer of polycarbosilane (PCS) and polymethylsilane (PMS) was used as the precursor for near-stoichiometric SiC matrix. The final heat treatment was performed at 1100-1800°C in flowing commercial-purity argon. The produced composites were dense in general with small macro-porosity. Transmission electron microscopy revealed that the matrix microstructures after heat treatment at >1500°C consisted of fine layers of SiC crystallites in amorphous SiC and carbon matrix. High temperature heat treatment promoted crystallization at the expense of porosity. No degradation of Tyranno-SA fiber was identified after heat treatment at 1800°C. PIP-composites with well-crystallized SiC matrices exhibited flexural fracture behavior very similar to that of chemically vapor infiltrated composites. The composites maintained ambient temperature strength up to 1000°C in air and to 1300°C in argon.