Abstract

The effect of alpha/beta phase on the mechanical properties and contact damage of silicon nitrides Si_3N_4) was investigated. Silicon nitride materials were prepared from two starting powders, at selective increasing hot-pressing temperatures to coarsen the microstructures: (i) from relatively coarse alpha-phase powder, essentially equiaxed alpha-Si_3N_4 grains, with limited, slow transformation to beta-Si_3N_4 grain; (ii) from relatively fine alpha-phase powder, a more rapid transformation to beta-Si_3N_4, with attendant grain elongation. The resulting micro-structure thereby provided a spectrum of alpha/beta phase ratios, grain sizes, and grain shapes. Fracture strength, hardness, and toughness were measured, and contact damage and strength degradation after indentation were investigated by Hertzian indentation using spherical indenter. A brittle to ductile transition in Si_3N_4 depended on alpha/beta phase ratio as well as grain size. Silicon nitride with elongated beta grains showed a superior, contact damage resistance.