Transfer layers in tribological contacts with diamond-like coatings

The tribological properties of amorphous hydrogenated carbon (a-C:H) films deposited on steel AISI440 B in a radio frequency (RF) assisted plasma were studied. The films were studied using a pin-on-disc machine with steel AISI 52100 and alumina as the counterface materials. The sliding velocities varied from 0.1 to 3.0 m/s and the normal forces from 5 to 40 N. The tests were carried out unlubricated in room air at 22±2 °C temperature and with 50±5 % relative humidity. Tribofilms formed on the pin and coating wear surfaces were studied by secondary ion mass spectrometry (SIMS). The coefficient of friction strongly depended on both the sliding velocity and the load. For the steel pin sliding against coating the coefficient of friction varied from μ=0.42 to μ=0.1 and for the alumina pin sliding against coating the coefficient of friction varied from μ=0.13 to μ=0.02. The formation of a tribofilm on the pin had a significant effect on both the friction and the wear properties of the coating.