Effects of nitrogen pressure on properties of sintered reaction-bonded silicon nitride

We prepared sintered reaction-bonded silicon nitride ceramics by using yttria and magnesia as sintering additives and evaluated the effects of nitrogen pressure (0.1-1.0 MPa) on their microstructure, bending strength, fracture toughness, and thermal conductivity. The ratio of & beta; phase in the nitrided compacts varied with the pressure and increased with increasing it. Since many & beta; grains in the nitrided compacts were formed and interlocked each other with a stable three-dimensional structure which restricted the shrinkage during the sintering procedure, many pores remained in the sintered body. Under the middle pressure (0.3-0.5 MPa), the grains grew large because the number of formed nuclei was small. On the other hand, under the high pressures (0.8-1.0 MPa), the grains were relatively fine and uniform because of a large number of nuclei. Since the porosity and grain length depended on the nitridation mechanism, which was affected by the nitrogen pressure, the properties largely varied accordingly. The nitridation at 0.1 MPa gave the best properties in this study.