Multiscale boundary frictional performance of diamond like carbon coatings

This paper investigates the frictional performance of a PECVD hydrogenated undoped DLC coating and the alternative super-finished gear steel surface pairs. A tribometer replicates the conditions found in the conjunction of a performance racing transmission spur gear pair. In mixed and boundary regimes of lubrication the DLC contact is shown to have lower friction despite having comparable surface topography when measured using optical interferometry. To determine the mechanisms responsible for improved friction of DLC coated tribo-pairs AFM is used. It is shown that the nanoscale surface topography of the investigated DLC coated pairs reduces the real asperity contact area formed at the same load carrying capacity. This highlights a contributing mechanism for improved friction with the DLC coating. •Improvements in frictional performance of DLC coated pairs in comparison with steel contacting pairs under mixed regime of lubrication.•Multiscale measurements of tested samples are reported using optical as well as AFM and SEM microscopic techniques.•The analysis shows reduced real asperity load bearing contact area for the DLC-on-DLC tribo-pairs in comparison with the counterpart Steel pairs.•This is shown to be the underlying mechanism for reduced friction under boundary regime of lubrication.