Toughening Mechanism of Particle Dispersed SiC Composites

Toughening mechanism of particle dispersed SiC composite was investigated from the stand point of the toughening places, crack tip and crack wake. Comparing the measurement results of toughness with SEPB method and SEVNB method, two toughening effects working at crack tip and in crack wake were separated. Measurement results of residual strain in SiC with XRD indicated compressive residual strain in particle SiC composite according to the kind of particle and its content. A linear relationship was found between the average compressive residual strains and the toughening effects at crack front independent to the kind of particles and their content, suggesting the toughening effect is caused by the generation of compressive stress field, At high temperature, residual strain was measured to decrease with XRD. Corresponding to the decrease, the toughening effect at crack tip decreased at high temperature. These results indicate that the thermal residual stress, which caused in the cooling process from the sintering temperature to the room temperature because of the higher coefficient of thermal expansion of dispersed particles than matrix SiC, reduced the stress intensity factor at the crack tip and the toughness of SiC increased. As for the toughening effect in crack wake, it was related to the fracture mode and the dispersed particle size. The toughening effect in crack wake was significant in the case of the fracture mode of particle boundaries and larger particle size. The temperature dependence of the toughening effect in crack wake was not large. These results indicate that the toughening effect in crack wake is caused by the particle bridging between fracture surfaces.