Tribological behaviour of Mo-S-N solid lubricant coatings in vacuum, nitrogen gas and elevated temperatures

Molybdenum disulphide (MoS2) is well known for the exceptional tribological properties in inert and high vacuum environments. On the other hand, these properties rapidly degrade in humid and higher temperature (>300 °C) environments which cause increase of the friction coefficient and substantial increase of the wear. Apart from this, MoS2 films also suffers of porous structure, very low hardness and low load-bearing capacity which limits its applications in terrestrial atmosphere and more demanding conditions. Mo-S-N coatings published in previous studies revealed greatly improved mechanical and tribological properties in humid environment – low coefficient of friction, very low wear rates and one order of magnitude higher hardness. However, to this date, knowledge about sliding response of these Mo-S-N coating is mainly limited to humid air. In this work, we tested Mo-S-N coatings with different nitrogen contents from 0 to 40 at.% of N in vacuum (10−2 Pa), nitrogen atmosphere and elevated temperatures. The coatings were deposited by High Target Utilisation Sputtering method. All tribological tests were performed using ball-on-disc testing rig. A maximum hardness of 8 GPa was measured for the 19 and 31 atomic % N-doped coatings. In all testing conditions, the pure MoS2 films had COFs in the range of 0.02–0.15 and wear rates of 1.2–22×10−6 mm3N−1 m−1. The COF and specific wear rates decreased with N additions. Mo-S-N films had COFs between superlubric 0.007 and 0.13 and wear rates of 0.08–3×10−6 mm3N−1 m−1. The best overall tribological performance was shown to be for the Mo-S-N films containing 31 at.% of N. This study concluded that Mo-S-N coatings performed better than MoS2 in all testing conditions.