Comparison of fracture characteristic of silicon nitride ceramics with and without second crystalline phase

摘要

The comparison of microstructure and fracture characteristics of gas-pressure-sintered (GPS) Si_3N_4 ceramics with and without second crystalline phase at the junction regions was investigated by a combination of SEM, HRTEM and micro-indentation techniques. The bimodal microstructure composed of many large and fine rod-like Si_3N_4 grains was made by carbothermal reduction treatment (CRT). Most of triple regions were comprised of Y_3AlSi_2O_7N_2 as a second crystalline phase while all of grain boundaries and interfaces between Si_3N_4 and Y_3AlSi_2O_7N_2 phase were an amorphous layer of about 2 nm in thickness. Although a few large, rod-like Si_3N_4 grains were also observed in the sample without CRT, but most of junction regions existed with an amorphous phase. The value of fracture toughness in the sample with second crystalline phase at the junction regions by CRT was 6.6 MPa m~(1/2) This was slightly higher than the value without second crystalline phase (5.1 MPa m~(1/2)), The main reason for the slight increase of the fracture toughness, even though showing well-developed bimodal microstructure, is the formation of the Y_3AlSi_2O_7N_2 phase at junction regions, which led the transgranular fracture of Si_3N_4 grains at local regions.