Friend or Foe? Revising the Role of Oxygen in the Tribological Performance of Solid Lubricant MoS2

Molybdenum disulfide (MoS2) is a solid lubricant used in various forms, such as a dry lubricant by itself or as a component of a more complex coating. In both these forms, the effect of oxygen contamination on the sliding properties of the MoS2 coatings is traditionally considered detrimental, resulting in expensive technological processes to produce pure MoS2. Here, it is shown that the high oxygen content does not necessarily hinder the solid lubricant properties and may even result in a lower friction and wear when compared to pure MoS2. Mo-S-O coatings were fabricated by unbalanced magnetron sputtering and tribologically tested under vacuum conditions. Oxygen caused amorphization of the as-deposited coatings but did not prevent the triboactivated formation of an ultra-thin crystalline MoS2 tribolayer with the incorporated oxygen. Such an imperfect tribolayer was found to reduce the coefficient of friction to 0.02, a value lower than that of pure MoS2. Moreover, owing to the higher density and hardness of oxygen-containing films, the wear rate was also found to be lower. Molecular dynamics simulations performed using a newly developed Mo-S-O force field confirmed that such an imperfect tribolayer can mitigate friction in a manner comparable to that of MoS2.