FIBROUS MONOLITHIC CERAMICS .3. MECHANICAL-PROPERTIES AND OXIDATION BEHAVIOR OF THE SILICON-CARBIDE BORON-NITRIDE SYSTEM

Fibrous monolithic ceramics were fabricated in the SiC/BN system. The microstructure consists of high-aspect-ratio polycrystalline cells of SiC separated by cell boundaries of BN. The fibrous monolith with aligned cells fails non-catastrophically in flexure, with delamination cracking along the BN cell boundaries. Indentations cause controlled damage on the surface, but no strength-degrading flaws. The flexural strength is in the range 300-375 MPa, and the estimated shear strength is almost-equal-to 14 MPa. The SiC/BN fibrous monoliths also show excellent resistance to high-temperature oxidation in air. Noncatastrophic fracture behavior is observed at room temperature after heat treatments between 1200-degrees and 1500-degrees-C. The SiC cells on the surface are oxidized to form a protective silicate scale, which prevents deterioration of the BN cell boundaries.