Effect of test atmosphere on the tribological behaviour of the non-hydrogenated diamond-like carbon coatings against 319 aluminum alloy and tungsten carbide

Diamond-like carbon (DLC) coatings are potential candidates for dry and near-dry machining of aluminum alloys as aluminum shows much less tendency to adhere to the DLC surfaces compared to other industrial coatings such as TiN and CrN. In this study, non-hydrogenated DLC coatings were magnetron sputtered on M2 steel discs. The effects of the test atmosphere on their tribological behaviour against 319 Al and tungsten carbide (WC) were investigated by performing pin-on-disc tests in vacuum (6.65 x 10(-3) Pa), nitrogen, and air with humidity levels varying from 0 to 85% RH. The DLC coatings exhibited high coefficient of friction (COF >= 0.44) and wear rates (>= 3.28 x 10(-5) mm(3)/m) in vacuum, nitrogen, and dry air (0% RH) against both 319 Al and WC. The 319 Al caused higher wear damage to the DLC coatings compared to WC in vacuum, nitrogen, and dry air (0% RH). Material transfer that occurred as a result of adhesion to the counterface appeared to be the dominant wear mechanism in vacuum and in nitrogen. Introduction of air into the test chamber promoted abrasive and oxidational wear of the DLC coatings. Carbonaceous transfer layers were formed on the surfaces of 319 Al and WC run against the DLC coatings in humid air (>= 20% RH). Tests done under vacuum after an initial running-in in ambient air have resulted in very low COF values in the range of 0.006-0.02. This very low COF regime was attributed to the formation of carbonaceous transfer layers on the counterface during sliding in ambient air, as well as the adsorption of water vapour on the DLC coating surface. (c) 2005 Elsevier B.V. All rights reserved.