Residual Stress Measurements Of Silicon NitrideAnd Coated Tungsten Carbide Rolling ContactElements

Abstract

High quality ceramic materials used in rolling element bearing applications show some practical advantages over traditional bearing steels. Silicon nitride has been found to have the optimum combination of properties suitable for certain high speed, low mass and high stiffness applications. Hybrid silicon nitride rolling element bearings are now supplied as standard components by manufacturers. Coatings may also be considered to enhance the capability of hybrid bearings for applications such as high speed and high temperature. A modified four-ball machine was employed to produce accelerated rolling contact fatigue failures of these materials. Silicon nitride verses steel, silicon nitride verses silicon nitride and tungsten carbide coating verses steel configurations were considered. The lubricated tests were performed at relatively high loads and speeds ie. maximum compressive stress from 4.5 to 8.1 GPa at 4000 to 10,000 r/min. Measurement of residual stresses is an important aspect of surface engineering. An X-ray method was employed since it is the only practical non-destructible means of measuring residual stress of these materials at the pre-test and post-test stages. Experimental testing, surface examinations and residual stress measurements are described for case study failures. Results are presented for a selection of surface failures of hot isostatically pressed silicon nitride and thermally sprayed tungsten carbide rolling elements. The role of the residual stress within these materials before and after rolling contact fatigue testing is discussed.