Lubricated PVD CrAlN and WC/C coatings for automotive applications

Demand on a better efficiency and higher performance under environmentally friendly aspects is an actual trend for automotive applications and machine components. This can be achieved by vacuum coatings such as DLC (diamond-like carbon), which offer excellent results in e.g. fuel injection systems, piston pins, gears and bearings. Nowadays, DLC has become very interesting for lubricated tribological contacts, because of its outstanding tribological performance under boundary lubrication conditions, which provides low friction losses and high wear resistance. Similar to DLC CrAlN promises performance increase especially in lubricated tribological contacts, where high ductility and wear resistance are needed. Surface properties of DLC and CrAlN coatings are not analogous to uncoated steel surfaces; thereby their wettability with lubricants is different from that of conventional steel surfaces. Hence, finding an optimum PVD (physical vapor deposition) coating/lubricant pair is still a challenge with regard to improvement of system performance and efficiency. In this study the wettability of selected PVD coatings with different lubricants and the effect of their wettability on their tribological behavior were investigated. The PVD coatings are WC/C and CrAlN; the lubricants are polyalphaolefin, polyglycol, synthetic ester and mineral oil, which are mixed with the additives ZnDTP, MoDTP and S-P. The wettability of the PVD coatings with lubricants is characterized by means of adhesion energy. The tribological tests were carried out using a pin-on-disk and twin disk tribometer. The friction behavior of PVD coatings and their wettability with the lubricants were compared. It is found that the wettability of the PVD coated surface can be optimized by means of adapting the adhesion energy, which in return leads to an optimized friction behavior.