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.