Phase Relationships in Y-Si-Al-O-N Ceramics

Abstract

During the last fifteen years of silicon nitride development, yttrium oxide has emerged as the best sintering additive for producing fully dense materials which combine good room-temperature strength with reasonable high temperature strength at temperatures in excess of 1000°C. ß’-sialons, which offer advantages over silicon nitride of easier fabrication, also require an additive for densification and again, yttria has proved to be the most satisfactory. The high temperature properties of these materials are determined mainly by the grain boundary phase assemblage and this in turn is determined by phase relationships in the yttrium sialon system at the sintering temperature, at any post-preparative heat treatment temperature and also at the likely operating temperature of the material. The amount of work involved in determining phase relationships in a five-component system over a wide temperature range is extremely large; researchers have therefore focused their attentions on localised regions of the system which are of particular relevance to commercial materials. A good summary of previous work in the yttrium sialon and other sialon systems has been given by Jack¹. The present paper summarises the current state of understanding of phase relationships in the yttrium sialon system and includes more recent work carried out at Newcastle on previously unexplored regions of the system.