窒化ケイ素の曲げ強度に及ぼす研削加工の影響に関する実験的検討と数値シミュレーション

抄録

Three-point bending tests were conducted using specimens of sintered silicon nitride ground in six types of condition, which were prescribed by the combination of the mesh-size of grinding wheel and the cutting depth. It was revealed by the Weibull statistical analysis that the stength was higher and its scatter was less in specimens ground by using a grinding wheel with larger abrasive grains. The relative estimation of residual stress induced by grinding was tried by using the indentation-flaw method to evaluate the fracture toughness simply. In this case, the crack formation resistance ratio, which was defined as the apparent fracture toughness measured on the ground surface divided by that on the surface without residual stress, was proposed to correlate the stength depending on the grinding condition. The crack formation resistance ratio became larger with an increase of strength. The tendency was ascribed to the variation of the intensity of compressive residual stress depending on the grinding condition. A simulation was carried out to discuss the dominant factor to affect the relation between the strength distribution and the grinding condition. In the analysis, the fracture mechanics model for cracks located in the region subjected to tensile stress was combined with the Monte Carlo simulation. The simulated result presented similar characteristics of the strength distribution as observed experimentally. The characteristics of the strength distribution was successfully explained by introducing the concept of superposing distinct strength distribution depending on the crack geometry.