EFFECT OF STARTING POWDER ON DAMAGE RESISTANCE OF SILICON NITRIDES

The role of starting powder in the resistance of silicon nitride (Si3N 4) ceramics to strength-impairing contact damage is studied. Si2N4 mat erials are prepared from three starting powders, at selectively increa sing hot-pressing temperatures to coarsen the microstructures: (i) fro m relatively coarse alpha-phase powder, essentially equiaxed alpha-Si3 N4 grains, with limited, slow transformation to beta-Si3N4 grains towa rd the latter half of the temperature range; (ii) from relatively fine alpha-phase powder, a more rapid transformation to beta-Si3N4, with a ttendant grain elongation; (iii) from fine beta-phase powder, an essen tially equiaxed beta-Si3N4 structure over the entire temperature range . The resulting microstructures thereby provide a spectrum of beta/alp ha phase ratios and grain sizes and shapes for investigation. Indentat ions with hard spheres (Hertzian test) are used to induce damage into the Si3N4 specimens. Examination of the indentation sites indicates a competition between brittle and quasi-plastic damage modes: in structu res with relatively equiaxed grains, the damage takes the form of clas sical cone cracking; in structures with large elongate grains, the dam age is distributed beneath the contact as grain-localized microfailure s within a subsurface ''yield'' zone. Bend tests on specimens containi ng single-cycle contact damage reveal those structures with well-devel oped cone cracks to be highly susceptible to strength degradation. The microstructures with the greatest resistance to strength loss are tho se formed from fine a-phase starting powder at intermediate firing tem peratures (1700 degrees C), with characteristic large, elongate beta g rains. Implications of the results in the context of other mechanical properties, e.g., toughness, wear, and fatigue resistance, are discuss ed.