Fracture Toughness- and Hardness-Dependent Polishing Wear of Silicon Nitride Ceramics

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Abstract
Production ball grinding and polishing apparatus were employed as tribometers. The volumetric wear rates of near-net-shape silicon nitride (Si3N4) ceramic ball blanks and the sphericity of the final-polished bearing balls were used to examine the premises that only homogeneous Si3N4 with isotropic structural integrity can be converted into precision balls, and that the abrasive or polishing wear rate of the materials is inversely proportional to Klc 3/4·Hv 1/2 (the Evans-Wilshaw wear relationship). One hot-pressed (HP) and five hot-isostatically-pressed (HIP) silicon nitride ceramics were processed into 1.27 cm (0.5 in.) diameter bearing balls under identical preparatory conditions, after having determined the hardness (Hv) and fracture toughness (Klc) of the ball stocks by Vickers indentation and crack-tip-extension-indicated Klc measurements. The volume removed from the ground and polished balls was monitored periodically until the desired ball diameter was reached. The actual volumetric wear rates, y in units of m3/s, linearized over the periods of constant grinding load and plotted as a function of each x = 1/(Klc 3/4 · Hv 1/2), yielded the equation y = 4.26 · 10−9x −7.45 · 10−12, with a correlation coefficient of 0.935. The corresponding wear rates represented by the slopes of the regression-analyzed volume loss data resulted in the near-identical equation of y = 4.26 · 10−9 x − 7.50 · 10−12, with a correlation coefficient of 0.876. These excellent curve-fits, combined with the results of diametral metrology of each processed bearing ball confirmed that a) only isotropic HIP-Si3N4 could be polished into 1.27 cm (0.5 in.) diameter precision-spherical rolling elements, and b) the Evans-Wilshaw wear relationship was an excellent predictor of the abrasive (polishing) wear of silicon nitride ball ceramics.