Mechanical properties of chemically vapor deposited nonoxide ceramics

Plates of chemically-vapor-deposited nonoxide ceramics (Si/sub 3/N/sub 4/, SiC, and B/sub 4/C) up to 3 mm thick with high purity and density were prepared at high deposition rates (>1 mm/h). High-resolution electron microscopy of these materials reveals that there exist no glassy layers or secondary phases at any grain boundaries and multiple-grain junctions. Thus, degradation of strength and toughness is not observed up to 1500/sup 0/C; the strength and toughness of the Si/sub 3/N/sub 4/ and SiC increase with increasing temperature above approx. =800/sup 0/ and 1100/sup 0/C, respectively. The effects of preferred orientation, grain size (residual internal stress), and stoichiometry on the mechanical properties were also examined.