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 x 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 +/- 10.06) and high wear rates (4.05 x 10(-4) mm(3)/m) in vacuum compared to ambient air (COF = 0.16 +/- 0.04, wear rate = 4.05 x 10(-4) mm(3)/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. (c) 2005 Elsevier B.V. All rights reserved.