Examination of wetting behavior of lubricants on PVD-coated surfaces and its influence on tribological characteristics

Examination of wettability of the PVD coated surfaces with lubricante and influence of the wettability on the tribological characteristics. The application of the high quality PVD (Physical Vapour Deposition) coatings in machine elements is gaining weight. The PVD coatings increase not the tribological performance of these machine components, but also their lifetime. Especially in mixed friction, e.g. roller bearings or gears, the tribological elements are subjected to high wearing loads. These elements in tribological systems can be protected against wear in mix friction by using additives in lubricants, such as EP (Extreme Pressure) and/or AW (Anti-Wear) or the modification of the surfaces with PVD coatings. These additives react with the surface and build on the uncoated surfaces of machine elements by chemical reaction a protecting tribological layer. This protecting layer separates the contact pairs and so reduces their wear out. The interactions between the additives and the PVD coated machine parts differs from the uncoated. The most of the PVD coatings are chemically inert, such that a chemical reaction between contact surfaces and additives is not possible. Though the additives can influence the physical interactions between lubricants and PVD coated surfaces. In this study the interactions between lubricants, such as Mineral oil, Polyglycol (PG) and Polyalphaolefln (PAO) with and without ZDDP and Mo additives and a carbon based type PVD coating ZrCg (graded zirconium carbide) and chromium based PVD coating CrAIN (Chromium aluminium nitride) are examined. The wetting behaviour between lubricants and PVD coatings is determined by means of contact angle measurements. The spreading coefficient, which delivers information about the wettability, is calculated for each lubricant PVD coating pair. The tribological performance between the ZrCg and CrAIN coated surfaces ' and four chosen lubricants are examined in a ball on disc tribometer. The results show that the calculated spreading coefficient can be correlated directly with the friction coefficients.