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 1725-1750degreesC for 2 h in N-2 gas flow and post-annealing at 1450degrees C for 4 It. 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 25-1600degreesC in N-2 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. (C) 2002 Elsevier Science Ltd. All rights reserved.