The tribological performance of DLC-coated gears lubricated with biodegradable oil in various pinion/gear material combinations

Biodegradable oils possess good tribological properties and are less harmful to the environment than conventional oils; however, they suffer from a poor oxidation stability, which limits their operating temperature. Diamond-like carbon coatings, with their low-friction properties, could offer an innovative solution to prevent excessive frictional heating in machine components and reduce the oxidation of biodegradable oils and hence prolong the lifetime of the oil. In this study, we looked at (1) the effect of pinion/wheel material combinations (steel/steel, W-DLC/W-DLC, steel/W-DLC and W-DLC/steel) on the wear resistance and (2) the frictionally induced oil temperature rise with these material combinations, by using FZG gear-scuffing test procedure. The results show that the contacts of the W-DLC/W-DLC-coated gears could importantly reduce the oil temperature across the whole range of loads and could provide satisfactory wear resistance up relatively high loads (about 1.4 GPa). The steel/steel gear pairs showed the lowest wear over the whole range of loads, but produced more frictional heating. Both “mixed” combinations (steel pinion/W-DLC-wheel and W-DLC-pinion/steel-wheel) performed significantly worse than the self-mated combinations, they exhibited extremely high wear on the non-coated steel gears and gave similar frictional heating as the steel/steel gear contacts.