Further improvement in mechanical properties of highly anisotropic silicon nitride ceramics

Si3N4ceramics were fabricated by tape casting of a raw-powder slurry seeded with three types of rodlike β-Si3N4particles. The effects of seed size on the microstructure and mechanical properties of the sintered specimens were investigated. All the seeded and tape-cast silicon nitrides presented an anisotropic microstructure, where the elongated grains grown from seeds were preferentially oriented parallel to the casting direction. The orientation degree of these grains, f0, was affected by seed size, and small-seed addition led to the highest f0value. This material exhibited high bending strength (∼1.4 GPa) and high fracture toughness (∼12 MPa.m1/2) in the direction normal to the grain alignment, which were attributed to the highly anisotropic and fine microstructure.