Fatigue and delamination resistance of overlay coatings is critical to their performance in tribological applications involving Hertzian loading. This study addresses the influence of coating thickness and contact stress fields on the fatigue/delamination resistance of thermal spray (WC-12%Co) coatings, deposited by a JP-5000 system. These coatings were deposited in three different thicknesses on the surface of 440-C steel substrate cones. Fatigue tests were conducted using a modified four-ball machine under various tribological conditions of contact stress and configuration. Results are discussed in terms of Hertzian contact stress fields, coating thickness and Scanning Electron Microscope (SEM) observations to comprehend the performance and ascertain the fatigue failure modes of coated rolling elements. These results indicate that by appropriate control of coating thickness and tribological conditions, it is possible to achieve a fatigue life in excess of 70 million stress cycles without failure. Further studies in this field can thus trigger an area of new novel applications of thermal spray coatings.

A non-destructive experimental approach was adapted to investigate the variations in residual stress fields within thermal spray coatings. WC-Co coatings produced by a HVOF technique were considered for concentrated rolling sliding contacts in this study. These coatings were produced in various thicknesses on various substrates. Residual stress measurements were made using an x-ray diffraction technique, along and across the rolling direction. A modified four-ball machine was used to conduct rolling contact fatigue tests under various tribological conditions of contact stress, lubrication and contact configuration. Residual stress measurements were made before and after the tribological tests. Failed rolling elements were analyzed using scanning electron microscopy, electron probe microscopy and surface interferometry. Results indicate that the magnitude of compressive residual stress attenuates during fatigue failure. The magnitude of attenuated residual stress was dependent upon the type of tribological failure. This attenuation of residual stress was attributed to the microcracking of coating under the influence of contact stress.

The full potential of rolling element bearings operating in specialised conditions such as high speed and corrosive environments are realised using surface coatings. Tungsten Carbide coating by thermal spray HVOF and D-Gim processes are considered for these applications. An experimental approach using a modified four-ball machine simulates the tribological conditions within a rolling element bearing. The fatigue failure modes of the tungsten carbide coating in rolling contact with steel and silicon nitride are examined using conventional surface analysis techniques. The stress fields within the coating are examined using traditional contact theory and residual stress measurement by X-ray diffraction. The residual stress measurements of the pre-test coating, the contacting surface and the fatigue failures are described. Results of residual stress relating to orientation, failure depth, coating thickness are discussed along with the fatigue failure mode.