Characterization of transfer layers on steel surfaces sliding against diamond-like hydrocarbon films in dry nitrogen

Abstract

Carbon-rich transfer layers on sliding contact surfaces play important roles in the tribological performance of diamond-like hydrocarbon (DLHC) films. In this study, we investigated the nature of these layers formed on M50 balls during sliding against DLHC films (1.5 mu m thick) prepared by ion-beam deposition. Long-duration sliding tests were performed with steel balls sliding against the DLHC coatings in dry nitrogen at room temperature, approximately 22 +/- 1 degrees C. Results indicated that the friction coefficients of test pairs were initially about 0.12 but decreased steadily with sliding distance to 0.02-0.03 and remained constant throughout the tests, which lasted for more than 250000 sliding cycles (approximately 30 km). This low-friction regime appeared to coincide with the formation of a carbon-rich transfer layer on the sliding surfaces of M50 balls. Micro-laser Raman spectroscopy and electron microscopy were used to elucidate the structure and chemistry of these transfer layers and to reveal their possible role in the wear and friction behavior of DLHC-coated surfaces.