Traction of Lubricated Rolling Contacts between Thin-Film Coatings and Steel

Tribological thin-film coatings can enhance the performance of mechanical components such as bearings and gears. Although a lubricant is present in most applications, the interactions of the lubricant with the coated surfaces are not always well understood. In the present study, Stribeck curves (i.e., traction coefficient vs. dimensionless film thickness lambda) were generated for lubricated rolling contact between coated and uncoated surfaces. Chromium nitride, tungsten carbide-reinforced amorphous hydrocarbon, and silicon-incorporated diamond-like carbon coatings were evaluated. Compositions, hydrogen concentrations, Raman spectra, and surface energies are reported for the films. A ball-on-flat test configuration was used in 5%, 50%, and 100% slide-to-roll conditions. The test lubricant was a polyalphaolefin containing rust and oxidation inhibitor additives only. Differences in traction performance were observed for different coating types. Traction coefficients decreased at high lambda with increased hydrocarbon content in the coating. Coating micro-texture and composition were believed to influence traction as lambda became small.