Mechanical properties of chemically vapor deposited nonoxide ceramics

Plates of chemically-vapor-deposited nonoxide ceramics (Si3N4, SiC, and B4C) 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 15000C; the strength and toughness of the Si3N4 and SiC increase with increasing temperature above approx. =8000 and 11000C, respectively. The effects of preferred orientation, grain size (residual internal stress), and stoichiometry on the mechanical properties were also examined