Preferred Orientation and Mechanical Properties of Pressureless Sintered Silicon Nitride

Mixtures of granular α-and needle-like β-silicon nitride particles could be obtained by adequate heat treatment of α-silicon nitride. By cold-pressing and pressureless sintering, the c-axis of the β-Si3N4 has a preferred orientation perpendicular to the cold-press direction. This orientation varied sharply in the vicinity of the specimen surface because of the β-Si3N4 grain growth along the surface. The degree of preferred orientation was related to the content of β-Si3N4 in the starting powder and to the molding pressure. Vickers microhardness and 3-point bending strength showed anisotropy due to the crystal orientation.