Effect of starting powder on damage resistance of silicon nitrides

Abstract

The role of starting powder in the resistance of silicon nitride (Si(3)N(4)) ceramics to strength-impairing contact damage is studied. Si(2)N(4) materials are prepared from three starting powders, at selectively increasing hot-pressing temperatures to coarsen the microstructures: (i) from relatively coarse alpha-phase powder, essentially equiaxed alpha-Si(3)N(4) grains, with limited, slow transformation to beta-Si(3)N(4) grains toward the latter half of the temperature range; (ii) from relatively fine alpha-phase powder, a more rapid transformation to beta-Si(3)N(4), with attendant grain elongation; (iii) from fine beta-phase powder, an essentially equiaxed beta-Si(3)N(4) structure over the entire temperature range. The resulting microstructures thereby provide a spectrum of beta/alpha phase ratios and grain sizes and shapes for investigation. Indentations with hard spheres (Hertzian test) are used to induce damage into the Si(3)N(4) specimens. Examination of the indentation sites indicates a competition between brittle and quasi-plastic damage modes: in structures with relatively equiaxed grains, the damage takes the form of classical cone cracking; in structures with large elongate grains, the damage is distributed beneath the contact as grain-localized microfailures within a subsurface "yield" zone. Bend tests on specimens containing single-cycle contact damage reveal those structures with well-developed cone cracks to be highly susceptible to strength degradation. The microstructures with the greatest resistance to strength loss are those formed from fine a-phase starting powder at intermediate firing temperatures (1700 degrees C), with characteristic large, elongate beta grains. Implications of the results in the context of other mechanical properties, e.g., toughness, wear, and fatigue resistance, are discussed.