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.
In an attempt to explain the electrochemical effects observed on an original potentiometric gas sensor, the influence of oxygen pressure and temperature on the electrochemical response of the device were largely investigated. The sensor consists of a solid electrolyte associated with two different electrodes, one in platinum the other in gold, located in the same gas mixture. A thermodynamic model is proposed, based first on the competitive adsorption of two oxygen species on the metals and second on the capacitance effects which may occur at the metal solid electrolyte interface. The simulation of the model provides a satisfactory solution if one of the oxygen species is considered as a metastable particle responsible for the electrostatic potential that appears at the metal‐solid electrolyte interface. Additional electrostatic experiments confirm the role of oxygen in the potentials of platinum and gold which appear as ideally polarizable electrodes. © 1999 The Electrochemical Society. All rights reserved.
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