Influence of surface energy on the interactions between hard coatings and lubricants

Generally, hard coatings improve the tribological properties of contact surfaces under dry sliding, while under lubricated conditions their inertness hinders the possibility of boundary film formation. For steel surfaces, the mechanisms of boundary film formation are well known. However, this is not the case for coated surfaces, where the end result might be high friction and wear. It is well known that surface structure has a crucial influence on the adsorption properties of the surface, while there is almost no information on the influence of surface energy on the formation of an adsorbed boundary film. The aim of the present research was to investigate and understand interactions between hard coatings and lubricants, and to define correlations between the surface properties of the liquid and solid phase and their tribological behaviour. For this purpose surfaces of DLC coatings and lubricants were characterized using the tensiometric method, and tribologically tested under boundary lubricated conditions. The results of the investigation do not show any direct correlation between surface energy, wettability of the surface and tribological performance of the lubricated surface. However, they indicate that the surface properties of the solid and liquid phase influence solid surface wettability and its tribological performance.