Synthesis and structural properties of Mo-S-N sputtered coatings

Transition-metal-dichalcogenide coatings provide low friction because of characteristic low shear strength along the basal plane of the lamellar structure; however, the material can easily degrade through exfoliation and poor adhesion to the metallic substrates. In this work, an innovative approach was employed to improve the coating's adhesion. A secondary plasma source was used during deposition to generate an additional charged particle flux which was directed to the growing film independently of the magnetron cathode. Therefore, Mo-S-N solid lubricant films were deposited by DCMS from a single molybdenum disulphide (MoS₂) target in a reactive atmosphere. Nitrogen was introduced during the deposition with increasing partial pressures, resulting in a high N₂ content in the doped films (37 at. %). The variation in incident ion energy and flux of energetic species bombarding the growing film allows for the control of the S/Mo ratio through selective re-sputtering of sulphur from the film. The S/Mo ratio was progressively increased in the range of 1.2–1.8, resulting in a gradient from a metallic layer to the lubricious sulphide. Combining the ion bombardment with nitrogen incorporation, the cohesive critical load (Lc1) reached 38 N, 10 times more when compared to pure MoS₂ coating. Observation using HRTEM revealed an amorphous structure and strong bonding with the substrate.