Friction properties of ta-C and a-C:H coatings under high vacuum

The tribological behaviour of ta-C and a-C:H carbon films was analysed by pin-disc tests under high vacuum (< 10(-3) Pa). The tests were executed on a commercial High Vacuum Tribometer (CSM-Instruments) and compared with results obtained under controlled atmospheric pressure conditions. In this work, we present the comparison of the friction coefficients under high vacuum and under controlled atmosphere conditions of two sets of carbon films, one prepared by Filtered Cathodic Vacuum Arc (FCVA; giving rise to hydrogen-free carbon), the other by Plasma Enhanced Chemical Vapour Deposition (PECVD; leading to hydrogenated carbon). The structure of these films was analysed by Raman and Rutherford BackScattering (RBS) spectroscopy and Elastic Recoil Detection (ERD) as well as by X-rays reflectometry (XRR). We will demonstrate that under vacuum, FCVA carbon layers do not slide (mu=0.5-0.7) but PECVD carbon films can have a good sliding behaviour (mu = 0.1 - 0.3). An inverse behaviour was observed under controlled atmospheric conditions. These first results indicate a predominant role of the structure of the films, as analysed by Raman and RBS-ERD spectroscopy as well as by XRR, on the tribological characteristics of the carbon films. In particular, the influence of hydrogen concentration seems to be directly linked to the vacuum tribological behaviour of the coatings. (c) 2005 Elsevier B.V. All rights reserved.