In order to investigate the fracture process of a sintered silicon nitride at high temperatures, tensile, static creep, and fatigue tests were carried out at temperatures from room temperature up to 1573K, and the fracture surface of each specimen was carefully observed by means of optical and scanning electron microscopes. Fracture of the silicon nitride can be classified into two types from the viewpoint of cracking morphology. One is “single cracking type” where only a single crack brings about the fracture. As the crack is initiated at the largest pre-existing defect, distribution and size of the defects greatly affect the fracture strength. The other is “multiple cracking type” where several cracks were observed on the fracture surface as well as on the specimen surface. As the cracks are initiated by viscous sliding of glassy phase along grain boundaries, pre-existing defects affect little the fracture strength. The latter type is apt to appear in the tests with lower tensile stress or lower stress rate at higher temperature.
