Wear of high-velocity oxy-fuel (HVOF)-coated cones in rolling contact

Abstract

An experimental approach using a modified four-ball machine was used to investigate the rolling-contact fatigue (RCF) performance of tungsten carbide cobalt (WC-12%Co) coatings deposited by the high-velocity oxy-fuel (HVOF) process. The coated rolling-elements were in the geometrical shape of a cone which replaced the upper ball of the modified four-ball machine. The RCF tests simulate the configuration of a deep groove rolling-element ball bearing, and the tests were conducted in a conventional steel ball bearing and hybrid ceramic bearing configurations. The coating thickness, Hertz contact stress and the test lubricants were varied to provide various tribological conditions during the tests. Three different coating thicknesses were investigated and the coatings were ground and polished to attain a good surface finish on the rolling elements. The effect of substrate hardness, coating thickness and fluid film thickness on the RCF performance have been investigated along with the coating failure modes. Coating wear has been analysed using a scanning electron microscope (SEM) and the results discussed with the aid of the coating microstructure, microhardness and talysurf analysis of the wear track. Test results reveal that the performance of the coating is dependent upon the combination of the substrate and the coating properties. Moreover, the coating failure is a function of surface and subsurface mechanisms depending upon the tribological conditions during the test. Thinner coatings did not fail at the interface, indicating a good adhesive strength between the coating and the substrate. The depth of the failure and its comparison with the orthogonal shear-stress depth has been discussed. The coating failure was observed to be a combination of surface wear and subsurface delamination.