CrC/a-C:H coatings for highly loaded, low friction applications under formulated oil lubrication

Abstract

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 due to their outstanding properties in terms of low friction coefficients under dry conditions. Under lubricated conditions the friction can be reduced once more by applying lubricants with special additives. For example 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. In order to overcome these problems 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. Hardness and abrasive wear were determined as a function of the Cr content and coating structure. With an increasing Cr content the morphology of the coatings became denser and the surfaces smoother. The hydrogen content in the coatings decreased continuously with increasing Cr contents and reached values close to 3 at% for chromium to carbon (Cr:C) ratios close to 1. Taking into account these hydrogen contents and sub-stoichiometric chromium carbide phases, a nanocomposite-like structure with chromium carbide phases embedded in an a-C:H binder matrix can be assumed. CrC/a-C:H coatings with Cr:C ratios in the range of 1, but having a notable amount of an amorphous a-C:H phase, showed to be an optimum solution under lubricated conditions with MoDTC because they exhibit both low friction and extremely low wear rates. The wear mechanism, especially the interaction with the friction modifier MoDTC will be discussed