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

Hudec, Tomáš, Roch, Tomáš, Gregor, Maroš, Orovčík, Ľubomír, Mikula, Marián and Polcar, Tomáš (2021) Tribological behaviour of Mo-S-N solid lubricant coatings in vacuum, nitrogen gas and elevated temperatures. Surface and Coatings Technology, 405, [126722]. (doi:10.1016/j.surfcoat.2020.126722).

Abstract

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.

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Contributors

Author: Tomáš Polcar

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