Oxidation of Si3N4 and Sic by High Temperature—High Pressure Water Vapor

Abstract

Silicon nitride and silicon carbide have attracted great attention in recent years because of their potential application as high-temperature structural ceramics. Oxidation resistance is one of the most important properties in these applications. In air or oxygen, the oxidation believed to proceed as follows; $$ S{i_3}{N_4} + 3{O_2} \to 3Si{O_2} + 2{N_2} $$ ([1]) , and $$ SiC + 2{O_2} \to Si{O_2} + C{O_2} $$ ([2]) . A certain numbers of reports(1–3) argued that the oxidation rate was accelerated when water vapor coexisted with oxygen. However, the reaction of Si3N4 or SiC by H2O only has not been studied yet. We have studied the oxidation of Si3N4 (4) and SiC (5) powders by H2O under hydrothermal conditions where almost only high-temperature high-pressure H2O exist. Observed reactions are; $$ S{i_3}{N_4} + 6{H_2}O \to 3Si{O_2} + 4N{H_3} $$ ([3]) , and $$ SiC + 2{H_2}O \to Si{O_2} + C{H_4} $$ ([4]) . These reactions are quite different from the reactions in oxidative atmosphere without H2O. We discuss in the present paper the effects of H2O on high-temperature degradation of Si3N4 and SiC powders and ceramics based on the experimental results.