Humidity dependence on the friction and wear behavior of diamond-like carbon film in air and nitrogen environments

Diamond-like carbon (DLC) films were deposited on Si (100) wafers by a plasma enhanced chemical vapor deposition (PECVD) technique using CH4 plus Ar as the feedstock. The friction and wear behaviors of the resulting film sliding against Si3N4 balls were investigated on a ball-on-disk test rig in air and nitrogen environments at a relative humidity from 5% to 100%. The worn surface morphologies of the DLC film and the Si3N4 counterpart were observed on a scanning electron microscope (SEM), while the chemical states of some typical elements thereon were investigated by means of X-ray photoelectron spectroscopy (XPS). It was found that the DLC film recorded continuously increased friction coefficient and wear rate with increasing relative humidity in air. It showed linearly increased friction coefficient with increasing relative humidity in nitrogen, in this case the wear rate sharply decreased and reached the minimum at a relative humidity of 40%, which was followed by an increase with further increase of the relative humidity. The interruption of the transferred carbon-rich layers on the Si3N4 balls, and the friction-induced oxidation of the films in higher relative humidity were proposed to be the main reasons for the increases of the friction coefficient and wear rate. Moreover, the oxidation and hydrolysis of the Si3N4 ball in higher relative humidity, leading to the formation of a tribochemical film that mainly consists of silica gel on the wearing surface, were also thought to have effects on the friction and wear behaviors of the DLC films.