TRIBOLOGICAL PROPERTIES OF MATERIALS
The surface properties are among the crucial influences for tribological behavior in almost every application. For boundary and mixed lubrication this is quite obvious. However, for full film lubrication, less importance is placed to this point due to predominant relevance of fluid film parameters. However, fluid film formation also depends on solid-liquid interaction, thus also on surfaces and their physical and chemical interactions with lubricants. Wetting and surface energy are often used to describe these phenomena. However, in tribological studies these parameters are very simplified – and more typically – neglected. One of the reasons is that their influence on friction and wear is not yet well understood and that there exists no generally accepted model to encounter these phenomena in engineering design. Moreover, recent studies also show that contact angle does not represent properly the wetting behaviour, but instead, spreading parameter must be used, especially in spreading-wetting regime.

In this work, we present experimental and theoretical results of surface energy and wetting and their effects on friction of several engineering-relevant surfaces and liquids, such as steel, ceramics, DLC coatings, polymers and various base oils and water. We present significant differences in wetting behavior with water and different lubricating oils and the influence on tribological behavior depending on these properties for different contacts. The mechanism for the solid-liquid slip, which combines the wetting behaviour, surface energy and adsorption is also explained.

Moreover, wetting behaviour also affects the tribochemical interactions at the interfaces. We present an example of DLC-lubricated interfaces and we discuss the adsorption mechanisms of alcohols and fatty acids on various DLC coatings, in correlation to wetting properties. In spite of a notable amount of studies about the reactivity between the diamond-like carbon (DLC) coatings and various oils and additives, fundamental chemical and physical effects of base oils and simple hydrocarbons, as well some isolated reactive functional groups were investigated only scarcely and only recently. The results show that DLC coatings are very sensitive toward different polar molecules and that by defining their and lubricant properties, these interfaces can be substantially improved and tailored for different functionalities.