Relation between Microstructure and Fracture Toughness in Hot-Pressed Silicon Nitride Ceramics.

A change in dominant toughening mechanism with microstructure in HP-Si3N4 ceramics has been investigated through the model calculation of toughening by crack deflection and through the atomic force microscope (AFM) observation of crack-tips. The microstructure of HP-Si3N4 was controlled by sintering temperature, ranging from 1773K to 2073K. The grain size was almost constant, 0.14-0.16μm, from 1773K to 1923K, and the aspect ratio increased from 1.6 at 1773K to 9.4 at 1923K. After the aspect ratio reached a maximum at 1923K, it decreased and the grain size increased. The aspect ratio was 5.6 and the grain size was 0.36μm at 2073K. The contributions of aspect ratio and grain size to toughening were separately evaluated using these specimens.Toughening by crack deflection was estimated by model calculation and by the crack shape in AFM image of crack-tips. Toughening by crack bridging was estimated by roughness at the upper and lower crack planes in the AFM images. When crack bridging contributed to toughening, stress was produced between the upper and lower crack planes, strain resulted from this stress and roughness resulted from this strain. When crack bridging didn't contribute to toughening, roughness wasn't observed. The model calculation and AFM results show that when the grain size was constant and the aspect ratio increased, the dominant toughening mechanism was crack deflection. When the grain size began to increase crack bridging came to contribute to toughening and the dominant toughening mechanism was the combination of crack deflection and crack bridging.