Effect of Pressureless Sintering and Gas Pressure Sintering on Properties of Silicon Nitride Ceramics

Using silicon nitride powder as the main raw material to prepare silicon nitride ceramics by using pressureless sintering and gas pressure sintering. Effects of the two methods on the density, hardness, strength and toughness of the silicon nitride ceramics were studied. Oxidation resistance and surface morphology of the silicon nitride ceramics were examined at 1300 ℃ for 5 h. It is found that mechanical properties of the silicon nitride ceramics prepared by using the two methods were similar. However, the properties of the ceramics prepared by using the gas pressure sintering were relatively stable and had higher fracture toughness, with an average value of 12.6 MPa·m^1/2. Surface of the Si₃N₄ was oxidized to glass phase SiO₂ with obvious cracks at 1300 ℃ for 5 h. Compared with pressureless sintered one, the gas pressure sintered sample showed stronger oxidation resistance and high temperature strength. The average oxidation gain rate and flexural strength were 0.014% and 304.2 MPa, respectively.