Effects of Nitrogen Modification of Porous PVD–MoS₂ Coatings on the Tribological Behavior under Rolling–Sliding Conditions in Vacuum

In order to improve the tribological performance of PVD–MoS₂ coatings, which are frequently used as a solid lubricant for operating in challenging environments, e.g., in a vacuum, they can be modified with nitrogen. This work evaluates the tribological behavior and a possible compaction occurring during the initial tribological load in the rolling contact for pure and nitrogen-modified PVD–MoS₂ coatings in a vacuum. Short-running tests (1000 cycles) of coated steel discs paired with uncoated steel discs made from 100Cr6 (1.3505, AISI 52100) were conducted on a two-disc tribometer. The slide-to-roll ratio of 10.5% was kept constant, while the load was varied in two steps from 1.1 GPa to 1.6 GPa. Subsequently, a comparison was made between the worn and the pristine coatings by means of nanoindentation and an optical analysis of the wear track. The formation of a load-bearing solid lubrication was achieved for both MoS₂-variants. The main differences affected the material transfer and wear mechanisms. The worn coatings reached a similar wear coefficient of 4 × 10⁻⁶ mm³N⁻¹m⁻¹ and a possible compaction of the coatings was found, indicated through an increased indentation hardness (for MoS₂ 1158% and MoS₂:N 96% at a 1.1 GPa load). The assumed tribological mechanism changed with nitrogen modification, but scales with increasing load. The nitrogen-modified MoS₂ coating showed less compaction than pure MoS₂, while the frictional behavior was improved by a 17% reduction of the coefficient of friction.