The role of humidity and oxygen on MoS2 thin films deposited by RF PVD magnetron sputtering

MoS2 is usually applied as thick films (1 μm and above) on sliding counterparts to decrease friction and wear. Thick films of MoS2 generally grow columnar-like and the bending or fracture of these columns during sliding are supposed to be the origin of the good tribological performances. In the present work, we studied the tribological behavior of 200 nm MoS2 films prepared by RF magnetron sputtering, with emphasis on the friction mechanisms. We performed ball-on-disc tests at different values of residual humidity, in pure oxygen atmosphere and at different temperatures, in order to disentangle the role of water and molecular oxygen during sliding. We found that, despite the fact that the inner structure of these thin films is not lamellar, the tribological behavior is similar to thicker ones. The friction in absence of humidity is well below that in standard conditions and the lifetime of the film is strongly enhanced. We observed similar performances in humid air while keeping the sample at 75 °C. Our results clearly demonstrated that absorbed water causes the deterioration of the lubricating properties of the films, while film oxidation plays only a marginal role.