Interaction of oil additives and the influence of surface roughness for carbon based coatings in industrial applications

Diamond-like carbon (DLC) coatings, especially amorphous hydrogenated a-C:H and metal containing a-C:H:Me coatings are well established in the automotive industry as standard solutions to eliminate wear problems of highly loaded components. A characteristic trait of DLC coatings are the low friction coefficients under dry conditions. Under lubricated conditions the friction can be reduced once more applying lubricants with special additives. So the friction modifier MoDTC (Molybdenum-Dithio-Carbamate) is used in many engine oils. Metal free a-C:H coatings were found to interact in a negative way with MoDTC in high concentrations, especially under high loads in the tribological contact under mixed lubrication. When the amount of the additive reaches a certain level, even DLC coatings can be decompose under such severe conditions. This will be discussed with MoDTC as an example. A new developed coating, build up from CrC/a-C:H, showed contrary an extremely high compatibility to additives in lubricants. It is not affected by MoDTC and results in a further reduction of the friction coefficient. The resulting reduced surface roughness contributes further to the friction reduction. Beside the optimisation of this coating system the behaviour of the tribosystem of the coating and the additive was investigated with surface sensitive methods in detail for a better understanding of tribochemical reactions. The CrC/a-C:H coatings with different Cr contents were prepared by reactive magnetron sputtering, and tested systematically under lubricated conditions with and without MoDTC. The results were compared with those obtained for pure a-C:H. The wear mechanism, especially the interaction with the friction modifier MoDTC will be discussed.