Failure modes of plasma sprayed WC–15%Co coated rolling elements

Abstract

This experimental study addresses the failure modes of plasma sprayed coatings in rolling contact. A high velocity plasma spraying system was used to deposit WC–15%Co coatings on the surface of 15 mm diameter 440-C bearing steel cones. These coatings were deposited in two different thickness. Rolling contact fatigue (RCF) tests were conducted using a modified four ball machine in conventional steel ball bearing and hybrid ceramic bearing configurations. These tests were conducted under various tribological conditions of contact stress and lubrication regimes at room temperature. Failure modes were investigated on the basis of surface and subsurface observations of failed coated rolling elements. Surface observations were made using conventional scanning electron microscopy (SEM) and light microscopy. Subsurface observations were made using fluorescent dye penetrant technique. Observations of debris generated during the RCF tests, changes in topography of lower planetary balls, electron probe microscope analysis (EPMA), microhardness/fracture toughness investigations and, coating microstructural studies are also included to aid the discussion. Two modes of failures, i.e., surface wear and coating delamination, were observed during this investigation. Coated rolling elements failed in either one or a combination of these two modes depending upon the tribological conditions during the RCF test. Surface wear was associated with asperity contact in the presence of microslip/sliding within the contact region. The process was accelerated in the later stages of RCF tests in the presence of wear debris due to additional mechanism of three body abrasion. Coating delamination was associated with the initiation/propagation of subsurface cracks, which resulted due to defects in the coating microstructure. These cracks propagated at the depths of orthogonal shear stress and maximum shear stress under the surface of wear track.