Ultralow Macroscale Friction and Wear of MoS₂ in High-Vacuum Through DC Pulsing Optimization, Nitridization, and Sublayer Engineering

The reliability and durability of devices that operate in vacuum and are subjected to wear are critical issues. The concerns rise at the macroscale wherein the contact area is large, yet the contact stress is in the GPa range. This paper reports on the design and fabrication of a coating comprising nitridized molybdenum disulfide (MoSN) supported by a novel nanolayered coating composed of carbon with subnanometer-thick periodic albeit discrete Cr interlayers. The MoSN coatings are deposited onto mating surfaces using pulsed-DC magnetron sputtering in an atmosphere with varying N₂ to argon (Ar) ratios. Pulsed-DC parameters, such as pulse duty and frequency, together with nitrogen content, are systematically optimized to achieve superior mechanical and tribological properties. The results reveal the importance of optimizing pulsing parameters during deposition as they significantly alter the properties of MoS₂. The outcome is fabrication of a coating that despite having a low thickness, exhibits extremely low friction and record-breaking macroscale wear rate in high vacuum compared to the values reported in the literature by date. Lastly, the analysis confirms the prediction of theoretical studies regarding the release of entrapped nitrogen in gaseous form during the wear process without disrupting the formation of a stable solid lubricant tribofilm.