Pressureless-sintered Si
3N
4 with additives (Y
2O
3-Al
2O
3 and MgO) prepared with imide decomposed Si
3N
4 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 Si
3N
4 surface. Pit features in these sintered Si
3N
4 are different depending on the species of oxide additives; Si
3N
4 with Y
2O
3-Al
2O
3 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 Si
3N
4 was proposed. The results also suggest that aqueous corrosion attack is practically important because it can lead to a strength degradation of engineering ceramics.
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