Tribological Behavior of Amorphous Hydrogenated Carbon Films on Silicon

Unlike polycrystalline diamond films, amorphous hydrogenated carbon (AHC) films can be deposited at room temperature, are amorphous in atomic structure, and form very smooth surfaces. Amorphous hydrogenated carbon film consists of very small 10–20 Å sp² bonded (graphitic) clusters captured in a largely sp³ coordinated, partially hydrogenated random network of covalently bonded carbon. Because of the extreme stiffness of the carbon-carbon bond, this hydrocarbon composite, less dense even than graphite, exhibits hardness rivaling that of the hardest ceramics. We report a systematic study of the tribological characteristics of AHC films deposited on silicon substrates by radio frequency plasma assisted chemical vapor deposition. The friction and wear behavior of these films in sliding contact with a steel ball without any lubrication was evaluated as a function of film deposition conditions, contact stress, sliding speed, sliding distance, and relative humidity. The friction coefficient and the wear of both the contacting surfaces were found to increase with relative humidity. At low relative humidity, (a) the films exhibited friction coefficients in the range of 0.05–0.16 under a contact stress ranging from 0.83 to 1.66 GPa and a sliding speed ranging from 0.03 to 1 m/s and (b) the wear rates of the film and the steel ball were significantly lower than that of other hard coating, such as TiN or TiC, evaluated under similar conditions. These results are very encouraging for some tribological applications of AHC films.