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

Four kinds of Si3N4–RE2Si2O7 (RE: Nd, Sm, Y, Yb) ceramics have been fabricated by hot pressing at 1725–1750°C for 2 h in N2 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 Si3N4–RE2Si2O7 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<Nd<Y<Yb. Flexural strength was examined at temperature in a range of 25–1600°C in N2 gas flow. The high temperature strength of Si3N4–Yb2Si2O7 or Si3N4–Y2Si2O7 ceramics is better than that of Si3N4–Sm2Si2O7 or Si3N4–Nd2Si2O7 ceramics. There is a roughly linear relation between the strength retention and the liquid phase formation temperature. Present results indicate that Si3N4–RE2Si2O7 ceramics with high strength retention can be fabricated by adopting additives with small ionic radius and high ionic charge.