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 (MoS 2 ) target in a reactive atmosphere. Nitrogen was introduced during the deposition with increasing partial pressures, resulting in a high N 2 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 to the range of 1.2-1.8, having gradient from metallic layer upto-the lubricious sulphide. Combining the ion bombardment with nitrogen incorporation, cohesive critical load (Lc1) reached 38N, 10 times more than MoS 2 coating. Observation using HRTEM revealed an amorphous structure and strong bonding with the substrate.
MoS 2 is the most widely used dry lubricant for low friction applications in vacuum environments. However, due to its lamellar nature it exfoliates during sliding, leading to high wear, high coefficient of friction (COF), and low stability. Here, we report the mechanical properties and the vacuum (10 −4 Pa) tribological performance of nitrogen-alloyed transition-metal-dichalcogenide (TMD-N) coatings. The coatings were deposited using a hybrid deposition method, that is, reactive direct current (DC) sputtering of MoS 2 target assisted by an additional plasma source. The tribological tests were performed at relatively low contact stresses to replicate real industrial needs. The interaction between different mating surfaces (coating versus steel, coating versus coating) has been reported. Additionally, the effects of loads on the sliding properties were also studied for coating versus coating interactions. A maximum hardness of 8.9 GPa was measured for the 37 atom % N-alloyed coating. In all mating conditions, the pure MoS 2 coating had COF in the range of 0.1−0.25 and the least specific wear rates were found to be 3.0 × 10 −6 mm 3 /N•m for flat and 2.5 × 10 −6 mm 3 /N•m for cylinder. As compared to MoS 2 coating, the COF and specific wear rates decreased with N additions. The COF was in the range of 0.05−0.1 for Mo−S−N coatings, while coating versus coating displayed the lowest specific wear rates (8.6 × 10 −8 mm 3 /N•m for flat and 4.4 × 10 −8 mm 3 /N•m for cylinder). Finally, the increase in load resulted in a decrease of COF, but an increase in the wear rate was observed. The detailed mechanism behind the behavior of the COF for the different mating conditions was presented and discussed. This work brings some important issues when testing transition metal dichalcogenide-based coatings under low contact stress conditions more appropriate for simulating real service applications.
Low stoichiometry, low crystallinity, low hardness and incongruencies involving the reported microstructure have limited the applicability of TMD-C (Transition metal dichalcogenides with carbon) solid-lubricant coatings. In this work, optimized Mo–Se–C coatings were deposited using confocal plasma magnetron sputtering to overcome the above-mentioned issues. Two different approaches were used; MoSe2 target powered by DC (direct current) or RF (radio frequency) magnetron sputtering. Carbon was always added by DC magnetron sputtering. Wavelength dispersive spectroscopy displayed Se/Mo stoichiometry of ~2, values higher than the literature. The Se/Mo ratio for RF-deposited coatings was lower than for their DC counterparts. Scanning electron microscopy showed that irrespective of the low carbon additions, the Mo–Se–C coatings were highly compact with no vestiges of columnar growth due to optimal bombardment of sputtered species. Application of substrate bias further improved compactness at the expense of lower Se/Mo ratio. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy confirmed the presence of MoSe2 crystals, and (002) basal planes. Even very low carbon additions led to an improvement of the hardness of the coatings. The work reports a comparison between RF and DC sputtering of MoSe2 coatings with carbon and provides a guideline to optimize the composition, morphology, structure, and mechanical properties.
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