Abstract
The formation of a tribofilm, as a layer on top of a coated surface and/or as a transfer-film on an uncoated counter-surface, implies a running-in process. When a coated part slides against an uncoated surface, whether or not the coating slides repeatedly over the same surface or encounters a fresh surface on each sliding cycle is likely to be a crucial factor in determining the practical lifetime performance. It will determine whether an effective tribofilm is able to form on the uncoated surface, and whether it takes part in influencing the subsequent tribological behaviour. Surprisingly, this is an often-overlooked aspect when assessing the tribological behaviour of coatings in the laboratory, even though it has important implications for the effectiveness of the coating in the intended application.
In the current work, the effect of repeated versus non-repeated reciprocating sliding was investigated for a hydrogenated DLC coating and a titanium-stabilized molybdenum disulphide coating sliding against an uncoated silicon wafer. The coatings were deposited onto sapphire spheres and experiments were carried out in ambient air and in vacuum conditions. In the case of repeated sliding the coated sphere slid in a reciprocating manner over the same surface on the silicon wafer. With the non-repeated sliding case the wafer was stepped sideways between each sliding cycle, so that the coating continuously encountered fresh silicon surface. The friction behaviour was recorded continuously as a function of the number of sliding cycles. After testing, the wear of both surfaces was investigated and the characteristics of the tribofilms formed on the coated and uncoated surfaces were analysed to determine the structural, chemical and topographical changes that had occurred. This provided new insights into the importance of tribofilm formation on coated and uncoated surfaces.