抄録
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.
