Lubrication of Silicon Nitride and Silicon Carbide by Water: Running in, Wear and Operation of Sliding Bearings

Abstract

The purpose of this work was to establish the conditions for the operation and break-in of water-lubricated ceramic bearings. The experiments consisted of sliding 1/4″ silicon nitride or—carbide balls against pre-polished disks of the same material in water until tribochemical wear generates smooth conformal surfaces that allow hydrodynamic lubrication (μ<0.002) by very thin water films. This “running in” was performed at various sliding speeds (0.01-4m/s) and loads (0.5-20N). The minimum sliding speed for low friction were 0.04m/s for silicon nitride and 0.5m/s for silicon carbide, much lower than for conventional bearings. The load carrying pressures were 60-80MPa, which is higher than the usually pressures of thrust bearings. The hydrodynamic fluid film thickness was estimated with a standard integration of Reynolds' equations modified for circular geometry, it was to be 5-15nm for silicon nitride, 25nm for silicon carbide. Operation over long distances (80km) allowed us to measure the wear rate during hydrodynamic lubrication; this was found to be <2×10⁻¹¹mm³/nm, a rate acceptable for industrial application. A novel method completed during this work allows the determination of the wear rate during run-in. It varies with sliding velocity for silicon nitride, from 1 to 6×10⁻⁵mm³/nm; it is constant at 4×10⁻⁶mm³/nm for silicon carbide.