Vacuum tribological behavior of the non-hydrogenated diamond-like carbon coatings against aluminum: Effect of running-in in ambient air

Magnetron sputtered non-hydrogenated diamond-like carbon (DLC) coatings have been tested against 319 Al alloy (Al–7% Si) pins in ambient air (52% RH), and under vacuum (1.07 × 10−2 Pa). When tested at 4.9 N applied load and 0.12 m/s sliding speed, DLC coatings showed high coefficient of friction (COF = 0.52 ± 0.06) and high wear rates (4.05 × 10−4 mm3/m) in vacuum compared to ambient air (COF = 0.16 ± 0.04, wear rate = 4.05 × 10−4 mm3/m). A very low COF regime (0.006–0.02) was observed under vacuum when the tribopair was first run in ambient air for some sliding distance at the same load. Longer running-in periods and slower sliding speeds in ambient air extended the duration of the subsequent very low COF regime in vacuum. A model that considers the formation of a tribolayer on the counterface during running-in period was proposed. Formation of this tribolayer together with adsorption and dissociation of the atmospheric water on the sliding surfaces were suggested as the possible mechanisms that minimize the COF.