An insight on the MoS2 tribo-film formation to determine the friction performance of Mo-S-N sputtered coatings

Abstract Amorphous Mo-S-N coatings are known to provide excellent tribological properties in diverse environments due to easy sliding under the influence of MoS2 tribo-films. However, the role of nitrogen incorporation, the formation mechanism of MoS2 tribo-film at the sliding interface and the changes in the friction behaviour under different environments are not fully understood. In this study, an amorphous coating with 30 at. % N was deposited in a semi-industrial reactive direct current magnetron sputtering (DCMS) system, using a single MoS2 target in combination with a secondary plasma source. The coating was predicted to have either a Mo-S-N phase with N filling some of the S sites or a MoS2(N2) structure where the gas molecules prevent the formation of a crystalline lamellar structure. Tribological studies performed in vacuum and ambient air resulted in steady-state COF values of 0.03 and 0.15, respectively. High-resolution transmission electron microscopy (HRTEM) analysis performed on the wear-tracks revealed that the low coefficient of friction (COF) in vacuum was attributed to the formation of a thick and continuous lamellar tribo-film with a low amount of nitrogen. Contrarily, in ambient air, the surface oxidation disturbed the formation of a continuous MoS2 tribo-film from the amorphous coatings, leading to an increase in the COF and wear rate. This study shows through indirect measurements of the chemical composition of the as-deposited coating and wear debris that nitrogen is stored in gaseous form (N2) within the amorphous matrix and is released from the contact during sliding.