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 MPa.m(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.