Measurement of Indentation Fracture Toughness of Silicon Nitride Ceramics: I, Effect of Microstructure of Materials

Effect of microstructure of silicon nitride on the fracture toughness, KIc evaluated by the IF method was studied with various indentation loads ranging from 49 N to 490 N, since practical assessment of fracture toughness of small Si3N4 parts is needed in the ceramic ball bearing market. The plot of KIc against the as-indented crack length revealed the rising R-curve behavior for the coarse Si3N4 and slight R-curve for the fine Si3N4. By comparing KIc estimated from the SEPB and IF methods using 4 different equations, it was revealed that the IF equation which gave the nearest value to KIc from SEPB was different depending on the microstructures. These results were discussed in conjunction with their R-curve behavior and the effective crack length in the SEPB specimens.