The role of surface protective additives becomes vital when operating conditions\nbecome severe and moving components operate in a boundary lubrication regime. After\nprotecting film is slowly removed by rubbing, it can regenerate through the tribochemical\nreaction of the additives at the contact. However, there are limitations about the\nregeneration of the protecting film when additives are totally consumed. On the other hand,\nthere are a lot of hard coatings to protect the steel surface from wear. These can enable the\nfunctioning of tribological systems, even in adverse lubrication conditions. However, hard\ncoatings usually make the friction coefficient higher, because of their high interfacial shear\nstrength. Amongst hard coatings, diamond-like carbon (DLC) is widely used, because of its\nrelatively low friction and superior wear resistance. In practice, conventional lubricants\nthat are essentially formulated for a steel/steel surface are still used for lubricating machine\ncomponent surfaces provided with protective coatings, such as DLCs, despite the fact that\nthe surface properties of coatings are quite different from those of steel. It is therefore\nimportant that the design of additive molecules and their interaction with coatings\nshould be re-considered. The main aim of this paper is to discuss the DLC and the\nadditive combination that enable tribofilm formation and effective lubrication of\ntribological systems.
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