Comparison of Black Oxide and Tungsten Carbide-Reinforced Diamond-Like Carbon (WC/a-C:H) Surface Treatments for Rolling Element Bearings

Surface-initiated damage increasingly limits the service life of rolling element bearings in challenging environments. In particular, operation in the boundary lubrication regime and/ or conditions that produce excessive rolling element-raceway slip require the use of surface treatments on roller bodies and raceways to extend surface durability and component life. The tribological performance of two approaches is compared and contrasted: black oxide chemical conversion treatment and tungsten carbide-reinforced diamond-like carbon (WC/a-C:H) thin film hard coating. Black oxide treatment is performed on both rollers and raceways, whereas WC/a-C:H coatings are deposited on rollers only. The cross-sectional layer structure is compared using transmission electron microscopy, and mechanical properties are measured using nanoindentation. Ring-on-ring friction and wear testing is conducted to obtain Stribeck curves and evaluate adhesive wear resistance and cyclic acceleration performance. Tapered roller bearing fatigue test results are reported for treated bearings. Results from both bench-scale tribological tests and bearing fatigue tests indicate the superiority of WC/a-C:H coatings in protecting against severe adhesive wear damage and extending bearing fatigue life in boundary-lubricated conditions.