Influence of EP additive concentration on the tribological behaviour of DLC-coated steel surfaces

The influence of the concentration of a conventional extreme-pressure (EP) additive on the wear and friction behaviour of DLC-coated steel surfaces was investigated. Special emphasis was put on exploring a DLC/steel combination operating under boundary lubricated conditions and on determination of how the DLC against steel situation differs from conventional steel against steel contacts in boundary lubricated sliding. Tests were performed in a load-scanning reciprocating test rig, which allowed a wide spectrum of loads to be tested in a single test. The sliding speed was set to 0.1 m/s and the contact pressure was in the range from 2.4 to 5.6 GPa. nvestigation indicated that the life of boundary lubricated DLC-coated surfaces involves several stages. After running-in, the DLC coating, although unable to form a tribofilm on top of the coating, takes an active part in forming a low friction tribofilm on the uncoated steel counterpart or on the exposed steel substrate, which eventually leads to the final wear out of the coating. Further, the concentration of additive has a significant influence on the tribological behaviour of DLC-coated steel surfaces. Contrary to the common rule for steel/steel contacts, reduction of the EP additive concentration may lead to improved tribological behaviour of DLC-coated surfaces.