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

Abstract

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.