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

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 K_lc ^3/4·H_v ^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 (H_v) and fracture toughness (K_lc) of the ball stocks by Vickers indentation and crack-tip-extension-indicated K_lc 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/(K_lc ^3/4 · H_v ^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.

Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference In San Diego, California, October 19–21, 1992

KEY WORDS:

• Ceramic Balls
• Polishing Wear
• Silicon Nitrides
• Isotropy
• Wear Factors
• Hardness
• Fracture Toughness
• Wear Equations
• Ball Grades

Notes

Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference In San Diego, California, October 19–21, 1992