Effect of different rare earth cations on the densification behaviour of oxygen rich alpha-SiAlON composition

Five different stabilizing cations (Y, Yb, Dy, Sm, Nd) were investigated on an alpha-SiAlON composition, deliberately shifted to the Al2O3 rich area corresponding to M(0.445)Si(9.998)Al(2.003)O(0.668)N1(5.333), with respect to the reaction sequences, densification behaviour and mechanical properties. The analysis of the reaction sequences show that the formation of alpha-SiAlON involves the intermediate formation of aluminate, garnet, melilite and a nitrogen rich liquid containing the respective cation. The sintering rate is influenced by volume fraction of the garnet and melilite phase which varies for different cations. Dy2O3 shows the best densification behaviour which is followed by Sm2O3 and Nd2O3. The fracture toughness for all are comparable while the hardness value seems to decrease with the increasing ionic size of the cations due to the lower proportions of alpha-SiAlON phase. The present composition favours the removal of melilite from the as cooled product as well as good densification. (C) 1999 Elsevier Science Limited and Techna S.r.l. All rights reserved