Oxidation of Non-oxide Ceramics

Abstract

Oxidation behaviour of silicon nitride, sialon and carbon fiber/carbon composite was discussed.
Oxidation of pure silicon nitride formed silica glass or cristablite on the surface. The oxidation rate followed the parabolic law with an activation energy of 60-70kcal/mol which was considered to be controlled by inward diffusion of oxygen through the oxidized layer. However, in the cases of most of silicon nitride materials, which were fabricated with sintering aid such as MgO or Y₂O₃, concentration of these impurities and formation of some silicates were observed in the oxidized layer. The activation energy was much higher such as 89kcal/mol for those materials. The oxidation rate was known to be limited by the rate of outward diffusion of the metallic impurity ions into SiO₂ layer. Microstructure of the oxidized layer also affected the oxidation behaviour. Oxidation mechanism of sialon was similar to that of silicon nitride, but sialon showed much better oxidation resistance.
Carbon matrix in carbon fiber/glassy carbon composite changed the microstructure from optically isotropic structure to graphite layer structure through optically anisotropic structure, when the composite was heat-treated up to 3000°C. The surface oxidation reaction of the composite, which was previously heat-treated at a temperature between 1500-3000°C, was analyzed at a temperature range of 650-850°C. The oxidation rate was faster than that of pyrolytic graphite, polycrystalline graphite and glassy carbon. The oxidation reaction, which initiated at the interface of fiber and matrix, progressed into graphite region, optically anisotropic region and isotropic glassy region. Activation energy for the oxidation reaction suggested that the oxidation was mostly governed by the structural defect in the composite.