17 - Superlow friction of a-C:H films: Tribochemical and rheological effects

Abstract

Friction and wear of diamond-like (DLC) coatings are strongly affected by the nature of the films—as controlled by the deposition process—and by the tribotesting conditions, including material parameters, mechanical parameters, kinematic parameters, physical parameters, and chemical parameters. The lowest friction-coefficient values of carbon-based films are observed for hydrogenated DLC films in inert environments. With values lower than 0.01, the vanishing of sliding resistance is often named “superlubricity,” or “superlow friction.” These very low friction coefficient values are not achievable with any DLC film, and only hydrogenated amorphous carbon (a-C:H) allows such friction reduction under a vacuum or inert environment. Superlow friction can be observed if contacting surfaces interact weakly during the friction process. However, this condition is also suggested with other materials in the case of ultralow friction. In the case of DLC films, both weak van der Waals interactions between smooth contacting tribofilms, combined with viscoplastic properties of hydrogen-rich DLC films, are the two key requirements to allow superlow interactions between flexible hydrocarbon chains. To achieve superlow friction from the running-in process, transfer film build-up is the first step to induce a transformation of the initial film and transfer film into contacting adaptive tribofilms.