Rolling contact fatigue of post-treated WC–NiCrBSi thermal spray coatings

The aim of this experimental study was to comprehend the relative rolling contact fatigue (RCF) performance and failure modes of functional graded WC–NiCrBSi thermal spray coatings in the as-sprayed and post processed state, by means of Hot Isostatic Pressing (HIPing) and vacuum heating. Functional graded WC–NiCrBSi coatings were deposited by a JP5000 system. HIPing was carried out at two different furnace temperatures of 850 and 1200 °C, while vacuum heating was performed at the elevated temperature of 1200 °C. The rate of heating and cooling was kept constant at 4 °C/min. Rolling contact fatigue tests were conducted using a modified four ball machine under various tribological conditions of contact stress and configuration, in full film elasto hydrodynamic lubrication. Results are discussed in terms of the relative RCF performance of the as-sprayed and post-treated coatings, and also surface and sub-surface examination of rolling elements using scanning electron microscope (SEM), light microscope and surface interferometry. esults reveal that performance of the coating was dependant on the microstructural changes due to post-treatment. Coatings heat-treated at 1200 °C displayed superior performance in RCF testing over the as-sprayed coatings at all stress levels (2, 2.3, 2.7 GPa) with emphasis on RCF performance at lower stress load of 2 GPa, where no failure occurred. Improvement in RCF performance was attributed to the diffusion between the carbides and matrix resulting in improved strength. At higher levels of contact stress, failure was surface initiated, and was attributed to initiation and propagation of micro-cracks at the edge of rolling contact region which led to coating delamination.