Tribology of Co-sputtered Nanocomposite Au/MoS2 Solid Lubricant Films over a Wide Contact Stress Range

Co-sputtered nanocomposite metal/MoS2 solid lubricant films are traditionally used in high contact stress applications (typically around 1 GPa) because they are hard and conform to the Hertzian contact model, i.e., the coefficient of friction (μ) decreases with increasing contact stress. We are investigating whether appropriate modifications can be made in these films that could make them also work in low contact stress applications, especially sliding electrical contacts (e.g., slip rings) that would benefit from the higher conductivity and environmental robustness of these films. To this end, and also more generally to increase our understanding of how film composition affects performance, we studied the friction and endurance of co-sputtered Au/MoS2 films in sliding contact in N2 gas at two vastly different contact stresses, 730 and ∼0.1 MPa. Seven different film compositions were studied, with Au contents in the range 42-100 at.%, as well as pure MoS2. The results showed that co-sputtered Au/MoS2 films outperformed both pure sputtered MoS2 films and pure sputtered Au films. Optimum films at high contact stress (i.e., those that exhibited the lowest μ and highest endurance) were films with lower Au contents (i.e., 42 and 59 at.% Au). In contrast, at low contact stress, films with moderately high Au contents (i.e., 75 and 89 at.% Au) performed the best. For films that did not fail by the end of the 2000 m test, Auger Nanoprobe analysis revealed that lubrication was provided by a thin film (∼1 nm thick) of relatively pure MoS2, regardless of the contact stress. Based on these results, we hypothesize that at high contact stresses, the low Au content provides optimum amounts of MoS2 in the contact region, while at low contact stresses, the higher Au contents limit the amount/size of MoS2 particles that are transferred to the opposing surface, providing a thinner, more uniform transfer film. The results indicate that with appropriate optimization of the metal:MoS2 ratio, co-sputtered nanocomposite metal/MoS2 films can be applied to a much wider range of contact stresses than previously studied.