Microstructure of the Oxidized Surface of Rare-Earth Disilicate - Silicon Nitride Ceramics

Abstract

Silicon nitride ceramics are being considered for use as structural materials in high-temperature oxidizing environments. Silicon nitride itself is extremely oxidation resistant due to the protective SiO2 layer, which results from exposure to high oxygen partial pressures at elevated temperatures. However, hightemperature exposure of silicon nitride sintered with oxide additives leads to accelerated oxidation attributable to the enhanced oxidation of Si3N4 dissolved in a silicate phase present at the surface. Since diffusion of additive and impurity cations through an amorphous grain-boundary phase from the bulk to the surface oxide scale is rate limiting, obtaining crystalline phases in equilibrium with SiO2 should eliminate the driving force for cation diffusion and improve oxidation resistance. Characterization by electron microscopy of the oxidized surfaces resulting from high-temperature oxidation is necessary to understand and solve these problems.