Abstract
The effects of fracture origin and microstructure on bending strength have been investigated with four kinds of Si3N4 ceramics. The mean grain sizes of three kinds of Si3N4 ceramics were 0.2μm and the other one was 0.4μm. The average bending strength of the weakest one was 1000MPa and the strongest one was 1500MPa. Three kinds of defects, pores, anomalously grown grains and inclusions (Fe) were identified as fracture origins. The stresses applied at these fracture origins were calculated from the bending strength and the locations of fracture origins. As the parameter to describe the size of fracture origin, the equivalent crack length was estimated. The strength of the high-strength Si3N4 ceramics with more than 1500MPa depended on both the grain size and the defect size same as conventional-strength Si3N4 and agreed with the Kishimoto's equation, from which the fracture strength could be estimated by using defect size, grain size and fracture toughness.
