Structure, mechanical and tribological properties of Mo-S-N solid lubricant coatings

Solid lubricant coatings deposited by plasma-assisted deposition techniques represent modern way to reduce the coefficient of friction in lubricant-free sliding contacts. Molybdenum disulphide (MoS₂) is perhaps the most known and applied solid lubricant coating; however, its use is strictly limited by low hardness and environmental sensitivity. To improve mechanical and tribological properties, we doped MoS₂ coating with nitrogen. Mo-S-N self-lubricant films were deposited by pulsed d. c. High Target Utilisation Sputtering (HiTUS) in reactive atmosphere. The effect of deposition conditions on chemical composition, structure and mechanical properties of MoS_x and Mo-S-N coatings was studied; films with the most promising properties have been selected for tribological testing. MoS_x film with the elemental composition sulphur to molybdenum (S/Mo ratio) 1.6 exhibited the coefficients of friction (COFs) in humid air 0.17 and 0.07 for loads 2 and 15 N, respectively. Mo-S-N films were prepared with nitrogen content in a range of 11 to 50 at., whereas S/Mo ratio varied from 1.35 to 0.4. Mo-S-N films were amorphous or nanostructured with nanograins of molybdenum disulphide. Hardness increased with N content up to 14 GPa for film with the highest content of nitrogen. Friction behaviour in humid air was evaluated using a ball-on-disk tribometer. Globally, the doping with N resulted in hardness in Mo–S–N films one order of magnitude higher and wear rate two orders of magnitude lower than in an undoped one, keeping the friction coefficient at the same level or even lower. These coatings showed remarkable friction coefficients in humid air from 0.28 to 0.05 with loads from 2 to 15 N, respectively. The excellent friction properties were attributed to the formation of a thin molybdenum disulphide tribofilm at the sliding interface. HiTUS represents a very promising way of producing thin films on the thermally sensitive substrates (e.g. bearing steel) with desired properties.