Spherical-Impact Damage and Strength Degradation in Silicon Nitrides for Automobile Turbocharger Rotors

Spherical‐impact damage to two silicon nitrides is investigated. Gas‐pressure‐sintered silicon nitride exhibits an elastic response to impact by spherical partially stabilized zirconia particles, resulting in Hertzian cone‐crack initiation in the sintered body. Pressureless‐sintered silicon nitride, on the other hand, demonstrated an elastic/plastic response, with median/radical‐crack initiation. These differences in behavior are due to their microstructural differences as well as to the different hardness values of the silicon nitrides in relation to those of the PSZ spheres. The postimpact bend strength of silicon nitrides is also degraded when crack length exceeds the inherent flaw size.