Tribological Behavior of MoS₂-TiCr/C Coating in Atmospheric and Seawater Environment

MoS₂ has excellent lubrication performance resulting from its interlayer shear properties, so it often serves as a solid lubricant in various industrial fields. It is particularly useful for mechanical equipment components such as bearings, gears, and bolts. Unfortunately, the outstanding lubrication performance of MoS₂ is easily affected by H₂O and O₂, which makes MoS₂ solid lubrication coating typically present a short service life in high humidity and aerobic environments. Doping MoS₂ with elements is a reliable and promising method to improve the compactness of the microstructure, mechanical properties, friction, and wear performance of MoS₂-based composite coating. At present, a large number of studies have focused on exploring the friction and wear behavior of MoS₂-based composite coatings in a vacuum, inert gas, or atmospheric environments, with few studies reporting the friction and wear behavior of MoS₂-based composite coatings in the marine environment. To explore the possibility of the application of MoS₂-based coatings in marine environments, pure MoS₂, MoS₂-TiCr, and MoS₂-TiCrC coatings were deposited on the surface of a titanium alloy (Ti-6Al-4V) using a non-equilibrium magnetron sputtering system. Electron probe microscopy (EPMA), XRD, Raman spectroscopy, SEM, and nanoindentation were carried out to systematically characterize the chemical composition, phase structure, microstructure, and mechanical properties of the coatings. The friction and wear behavior of the coatings in atmospheric and artificially simulated seawater environments were evaluated in detail using a ball-on-plate tribometer. The research results revealed that doping with Ti, Cr, and C elements in MoS₂ changed the crystal structure of MoS₂ from long-range order to short-range order or disorder (grain refinement), reduced the crystallinity of the composite coatings, and increased the microstructure density, hardness, and elastic modulus of the composite coatings. In the atmospheric environment, both the friction coefficient (0.083 4) and wear rate 0.75×10⁻⁶ mm³/(N·m) of MoS₂-TiCr coating were the lowest, which was accounted for its dense microstructure and exceptional mechanical properties. In artificially simulated seawater environments, due to the dominant role of seawater lubrication, the friction coefficients (μ) of pure MoS₂, MoS₂-TiCr, and MoS₂-TiCrC coatings were almost identical, with values of 0.063 7, 0.062 3, and 0.062 9, respectively. It was worth noting that the MoS₂-TiCrC coating had the lowest wear rate in a seawater environment 0.27×10⁻⁶ mm³/(N·m), which was ascribed to the dissociation of a large number of H₂O molecules into -H and -OH under the action of frictional heat, passivating the suspended C bonds on the worn surface, and reducing the wear of the composite coating. In addition, the pure MoS₂ coating had the highest wear rates under both atmospheric and seawater environmental conditions, with values of 25.54×10⁻⁶ mm³/(N·m) and 28.16×10⁻⁶ mm³/(N·m), respectively. Under the combined action of seawater corrosion and mechanical wear, MoS₂-TiCr coating with higher Ti and Cr contents exhibited significant peeling corrosion behavior, resulting in a higher wear rate of MoS₂-TiCr coatings in the seawater environment 2.94×10⁻⁶ mm³/(N·m). The research results provided an understanding of the failure and lubrication behavior of MoS₂-based composite coatings in marine environments, providing certain scientific guidance for the application of MoS₂-based coatings in marine environments.