Study on the tribological properties of PVD polymer-like carbon films in air/vacuum/N2 and cycling environments

Abstract

Polymer-like carbon (PLC) based thin films were firstly prepared using reactive magnetron sputtering. Then high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) with energy dispersive X-ray analysis (mapping) were used to evaluate the composition and nanostructure. Ball-on-disk sliding wear tests were carried out for detailed investigation of the tribological behaviour of as-deposited PLC films under 3 constant environments, namely ambient air (AA), dry nitrogen (DN) and high vacuum (HV), respectively, and 4 cycling environments, namely AA-HV, AA-DN, HV-DN and AA-HV-DN, respectively. After that, X-ray photoelectron spectroscopy (XPS) was carried out to analyse the chemical states of the elements involved in the sliding tests. Results showed that 1) wear tracks under 3 different constant environments exhibited similar oxidation degree, which was controlled mainly by the exposure time to ambient air; 2) The sp2/sp3 ratio (related to graphitisation extent) increases with increasing CoF; 3) transfer layer quickly built up at the contact surface of the counterpart ball; 4) for dry nitrogen environment, its purity in the test chamber determined the final CoF level, and interruption on the nitrogen filling process merely caused pause on the CoF descending trend, without introducing any negative effects; 5) the PLC film behaves reversible under most types of cycling environments, with roughly the combined, superimposed effects of each environmental components; 6) different environment-related lubrication mechanisms of hydrogenated carbon films were discussed; 7) the combination of ambient air and high vacuum environments was found to be detrimental, which should be avoided in tests or services where possible.