Microstructure and Statistical Properties of Bending Strength and Fracture Toughness of Silicon Nitride Ceramics

On four kinds of silicon nitride ceramics, the statistical properties of bending strength and fracture toughness were investigated. On these properties, universal estimation based on Weibull statistics and local fracture model was attempted. And the influences of microstructure on statistical properties of bending strength and fracture toughness were studied. In the analysis based on Weibull statistics, the equations which universally evaluate bending strength of smooth specimen and fracture toughness of pre-cracked specimen by means of Weibull stress were derived. According to this analysis, if it is assumed that the fracture of pre-cracked specimen occurs from crack front or defect in front of crack tip, it is predicted that Weibull modulus of fracture toughness equals to 4 independently of materials. However, fracture toughness and its Weibull modulus predicted by Weibull stress differed from the measured values, and their correlation was different among materials. The reason may be that aforementioned assumption is not consistent as suggested by the observation of fracture surfaces. On the other hand, in the analysis based on local fracture model, the intrinsic strength of grain boundary was estimated with the use of bending strength, defect size and grain diameter by means of point stress model in which the fracture depends on intrinsic strength of grain or grain boundary. Then the fracture toughness of pre-cracked specimen was predicted by the intrinsic strength of grain boundary. As a result, the predicted fracture toughness showed a tendency to be proportional to grain diameter in good agreement with experimental results.