2023 Volume 18 Issue 5 Pages 239-248
Among Diamond-Like Carbon (DLC) materials, hydrogenated amorphous carbon (a-C:H) coatings may exhibit super-low friction coefficients (< 0.01) under vacuum. A running-in phase is observed in the first few cycles of sliding before reaching a steady state of super-low friction. In this study, a unique ultra-high vacuum tribometer, with 4 different axes of motions and a 6 axes force sensor, is used to characterize the tribological behavior of a steel pin / a-C:H coated flat contact. An original crossing-tracks method is proposed to dissociate the respective roles of tribofilm build-up and surface modifications on the a-C:H in the achievement of super-low friction. A previously build-up tribofilm on the steel counterface shortens the running-in phase by 18 cycles and reduces the initial friction coefficient by 32%. Sliding on a previously rubbed area of the coating allows an additional initial friction reduction of 36%. Morphological and chemical investigations on the a-C:H coated flat using Atomic Force Microscopy and X-ray Photoelectron Spectroscopy reveal that the initial nanoasperities were smoothed, eventually leading to shallow matter transfers and tribofilm thickening on the counterface. Carbon-to-carbon bonds rupture and a reorganization in the carbon structure leads to an increase of the sp2/sp3 hybridization ratio.
