Studies of the formation of α′ and α′-β′ sialon

Abstract

Recently α′ and α′-β′ composite sialon ceramics have received much attention because of the hardness and the better thermal shock properties of α′-sialon materials. Phase relationships determined in the α′ plane show that α′-sialon expands from the pure nitride side towards the alumina side and there exists a fairly big region of α′-sialon in equilibrium with β′-sialon, thus providing the possibility to fabricate α′-β′ composite sialon ceramics. Because of the small difference in composition between α′ and β′ sialons, transformations between α′ and β′ occur very rapidly at firing temperatures in the presence of an appropriate grain-boundary liquid phase. Therefore, to control the transformation is of importance for fabricating both α′ and α′-β′ composite sialons and for this an understanding of reaction mechanisms is necessary. This paper describes the reaction sequences under different firing circumstances of α′ and α′-β′ sialon with Y as “modifying” cation, doped with or without La 2O 3 as densifying additive. The effect of firing conditions on transformation between α′ and β′ and the role of La 2O 3 on densification as well as the formation of α′-sialon are discussed.