Superplastic Sinter‐Forging of Silicon Nitride with Anisotropic Microstructure Formation

The microstructure and mechanical properties of silicon nitride, produced by a superplastic sinter‐forging technique, were investigated. The obtained silicon nitride exhibited a highly anisotropic microstructure, where rod‐shaped grains tended to be aligned perpendicular to the forging direction. A very high bending strength of 2108 MPa as well as a high fracture toughness of 8.3 MPam 1/2 were achieved when a stress was applied perpendicularly to the pressing direction. This very high strength was considered to be due to the reduced flaw size by the superplastic sinter‐forging process and the steep R ‐curve behavior caused by the grain alignment.