Effect of α/β phase ratio on microstructure and mechanical properties of silicon nitride ceramics

Highly densed silicon nitride ceramics with various alpha/beta phase ratios were produced by pulse electric current sintering process. The beta -phase content of Si3N4 in sintered materials varied from 20 to 100 wt% depending on the sintering condition. The microstructure was observed by scanning electron microscopy and investigated by image analysis. Young's modulus, hardness, fracture toughness, and strength were strongly dependent on the alpha/beta phase ratio. The fracture toughness increased from 4.6 MPa m(1/2) for 20-wt% beta -phase content to 8.2 MPa m(1/2) for 95-wt% beta -phase content, and the fracture strength showed a maximum value of about 1.6 GPa at 60-to-80-wt% beta -phase content.