Effect of Starting Powder on Damage Resistance of Silicon Nitrides

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 α‐phase powder, essentially equiaxed α‐Si 3 N 4 grains, with limited, slow transformation to β‐Si 3 N 4 grains toward the latter half of the temperature range; (ii) from relatively fine α‐phase powder, a more rapid transfor‐mation to β‐Si 3 N 4 , with attendant grain elongation; (iii) from fine β‐phase powder, an essentially equiaxed β‐Si 3 N 4 structure over the entire temperature range. The resulting microstructures thereby provide a spectrum of β/α 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 α‐phase starting powder at intermediate firing temperatures (1700°C), with characteristic large, elongate β grains. Implications of the results in the context of other mechanical properties, e.g., toughness, wear, and fatigue resistance, are discussed.