It is generally accepted and well described that the mechanism by which extreme-pressure (EP) and antiwear (AW) additives reduce the friction and wear of metallic. surfaces under boundary lubrication is the formation of tribochemical films. Although investigations of diamond-like carbon (DLC)-coated surfaces showed improved tribological properties when lubricated by additivated oil, the mechanism responsible is not fully understood. Therefore, the aim of the investigation was to determine the mechanism responsible for the low-friction behaviour of W-containing DLC coatings when lubricated with polyalphaolefin (PAO) oil containing EP or AW additives and to obtain some further understanding to this important area. The results of the present investigation clearly show that low-friction behaviour of boundary-lubricated W-DLC coatings is governed by formation Of WS2-containing tribofilms on the steel countersurface or exposed steel substrate, which reduce friction by up to 50%. (c) 2004 Elsevier B.V. All rights reserved.