The effect of / phase on the mechanical properties and contact damage of silicon nitrides ) 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 -phase powder, essentially equiaxed - grains, with limited, slow transformation to - grain; (ii) from relatively fine -phase powder, a more rapid transformation to -, with attendant grain elongation. The resulting micro-structure thereby provided a spectrum of / 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 depended on / phase ratio as well as grain size. Silicon nitride with elongated grains showed a superior, contact damage resistance.

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