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.