The effect of additives on sintering behavior and strength retention in silicon nitride with RE-disilicate

Abstract

Abstract Four kinds of Si 3 N 4 –RE 2 Si 2 O 7 (RE: Nd, Sm, Y, Yb) ceramics have been fabricated by hot pressing at 1725–1750°C for 2 h in N 2 gas flow and post-annealing at 1450°C for 4 h. The effect of rare-earth oxide additions on the densification process was investigated by measuring the shrinkage of the compact. Results revealed that the rare-earth oxides affect the densification process of Si 3 N 4 –RE 2 Si 2 O 7 ceramics in which the shrinkage temperature varies with the types of rare-earth additives. The Si–RE–O–N liquid phase formation temperature can be estimated from the onset temperature of final shrinkage stage, and the temperature is in the order of Sm 2 gas flow. The high temperature strength of Si 3 N 4 –Yb 2 Si 2 O 7 or Si 3 N 4 –Y 2 Si 2 O 7 ceramics is better than that of Si 3 N 4 –Sm 2 Si 2 O 7 or Si 3 N 4 –Nd 2 Si 2 O 7 ceramics. There is a roughly linear relation between the strength retention and the liquid phase formation temperature. Present results indicate that Si 3 N 4 –RE 2 Si 2 O 7 ceramics with high strength retention can be fabricated by adopting additives with small ionic radius and high ionic charge.