Multilayer composite ceramic-metal-DLC coatings for sliding wear applications

The design of anti-friction coatings able to perform well in different wear conditions without lubricants requires a combination of adequate hardness and toughness, good adhesion, a low friction coefficient and a low wear rate. Recently introduced metal-diamond like carbon (DLC) coatings produced by magnetron sputtering of metals from targets, which are to a controlled extent covered with carbon from the chamber atmosphere, can be a step towards the achievement of such a combination. These coatings consist of an amorphous a:CH matrix with the possible incorporation of metal (Ta, W, Nb, Ti), metal carbide and/or graphite grains. Previous studies of Ti_x%-DLC coatings showed their good protective properties against abrasive, impact and single scratch wear, as well as a requirement for supporting interlayers to successfully apply such coatings to low-cost steels. In the present work an example of the selection of metal-ceramic Ti-TiN-TiCN supporting interlayers is given based on studies of their morphology, structure and mechanical properties. This resulted in the development of Ti-TiN-TiCN-[TiC-(Ti_x%-DLC)] multilayer composite coatings. Several coatings were prepared with the same supporting interlayer and a variation in the preparation of the Ti_x%-DLC layer. Ball-on-disc experiments were carried out to investigate these coatings in conditions of sliding wear against steel and cemented tungsten carbide balls. CrN, TiN and TiCN coatings were also deposited and tested in the same conditions to provide a reference. Low friction coefficients (below 0.2 at an air humidity of 50% RH) in combination with low normalized wear rates were found for multilayer coatings with upper Ti_20%-DLC and Ti_35%-DLC layers.