The effect of doping elements and oil additives on the tribological performance of boundary-lubricated DLC/DLC contacts

In the past decade the range of possible applications for diamond-like carbon (DLC) coatings has changed significantly: from their conventional use in low-stress applications without lubricants, to their use in high-stress applications under lubricated conditions. 

Despite this progress, there is still a lack of understanding of the lubrication mechanisms involving DLC coatings. Our idea for improving the boundary lubrication of DLC coatings is to dope them with various elements, thus making the coatings more surface-active, in order that they behave in a similar way to conventional metal-lubrication mechanisms. Accordingly, in this paper we present and compare the tribological performance of self-mated (DLC/DLC) non-doped and doped Ti–, W–, and Si–DLC coatings, lubricated with paraffinic mineral oil and using conventional anti-wear (AW) and extreme-pressure (EP) additives. For a comparison, conventional steel/steel contacts were also investigated. The results obtained under relatively severe contact conditions show the significant and beneficial influence of oil additives and DLC doping elements on the wear behaviour. Up to one order of magnitude less wear, as a result of using additives, suggests that efficient boundary lubrication with doped and non-doped DLC coatings can also be achieved and/or improved in self-mated DLC/DLC contacts by using conventional AW and EP additives. The coefficient of friction in DLC contacts increased with the use of additives, but it was always lower than that of steel contacts, thus maintaining the reputation of DLC contacts as low-friction coatings.