Forms of Corrosion of Sintered Silicon Nitride Ceramics in Aqueous Environment at High-Temperatures and Pressures

Abstract

Pressureless-sintered Si₃N₄ with additives (Y₂O₃-Al₂O₃ and MgO) prepared with imide decomposed Si₃N₄ powder was severely corroded in water at high-temperature and pressure (300°C and 8.6MPa), resulting in weight loss and formation of corrosion product layer. The purpose of this work is to correlate the formation of corrosion product to the formation of pits and discuss on an aqueous corrosion pit as the fracture origin. After removal of corrosion product by ultrasonic cleaning in water bath, pits up to about 100μm in diameter were found distributed over the Si₃N₄ surface. Pit features in these sintered Si₃N₄ are different depending on the species of oxide additives; Si₃N₄ with Y₂O₃-Al₂O₃ forms deep pits with a thin protective corrosion product layer and that with MgO forms shallow pits with a thick non-protective corrosion product layer. This result in higher corrosion resistance for the former than the latter. Based on these results, schematic model for pitting corrosion of Si₃N₄ was proposed. The results also suggest that aqueous corrosion attack is practically important because it can lead to a strength degradation of engineering ceramics.