Bending Strength of the Seeded and Tape Cast Si₃N₄ before and after Oxidation Exposure in Air at 1500°C

Silicon nitride ceramics was prepared by tape casting nonaqueous ceramic slurries, laminating the green ceramic tapes, and gas pressure sintering in nitrogen atmosphere. Lu2O3 and SiO2 were used as the sintering additives, and 3 wt.% β-Si3N4 seed was added to enhance β-Si3N4 grain growth unidirectionally. The seeded and tape-cast Si3N4 showed very good high temperature bending strength at 1500oC, when the stress applied along with the grain alignment direction. This was attributable to the formation of a high melting point grain boundary phase and the fibrous Si3N4 grains alignment. After exposure in air at 1500oC for up to 100 h, the oxidation products formed on the Si3N4 surface consist of Lu2Si2O7 and SiO2. The bending strength of the oxidized and tape-cast Si3N4 was degradation, the strength decrease was associated with the formation of new defects on the surface and the interface between the oxide layer and the Si3N4 bulk.