T. Oikawa

Introduction The fracture toughness as well as oxidation resistance of sintered silicon nitride is known to be dependent on morphology of its grain-boundary glass phase In this work, high temperature fracture toughness tests were carried out using a HIP-sintered silicon nitride with certain amount of oxide additives and a HIP-sintered sialon with extremely thin glass phase [4], both of which had /3-crystalline structure. In /3-sialon structure, a small amount of Si and N in (3-Si 3 N 4 structure is replaced by Al and O, respectively, and it is considered that mechanical properties of the present sialon grain are similar to those of silicon nitride grain. So, the main difference of these two materials is the amount of grain-boundary glass phase. Temperature dependence of fracture toughness and fracture mechanisms of the two materials were discussed in detail, taking the characteristics of grain-boundary glass phase into consideration