Suppression of moisture sensitivity of friction in carbon-based coatings

Tailoring of hydrogenated carbon film properties to minimize environmental effects on friction is accomplished by proper deposition techniques and choice of film composition. Magnetron sputter-deposition results in smooth surfaces and midrange hardness. Sulfur doping of hydrogenated carbon films has been successful in mitigating the humidity effects on friction. The stabilization of low friction in humid air can be attributed to the reduction of water adsorption on the surface. This reduction is verified by results of quartz crystal microbalance and temperature-programed desorption experiments. Even at 90% relative humidity, sulfur-doped films have less than one monolayer of water adsorbed on the surface. This reduction in water coverage is due to the decrease in the residence time of water on the surface. Residence time is related to the strength of the bonding between water molecules and the sulfur-doped surface. These results indicate that sulfur doping results in weaker bonding between water and the film surface.