Tribological Properties of MoS₂-TiNb Composite Solid Lubricating Film under Air-Vacuum Cycling Condition

Reusable spacecrafts can travel freely between the earth's surface and outer space several times, which is one of the important methods to reduce the cost of space missions. Therefore, research on reusable solid lubricating films under the air-vacuum cycling condition is of great significance to ensure the stability, efficiency and low-cost operation for such spacecrafts. To improve the tribological properties of solid lubricating films in vacuum-atmosphere environment, MoS₂-TiNb composite solid lubricating films were fabricated by the magnetron co-sputtering method with both β-TiNb alloy and MoS₂ targets. The tribological performance of MoS₂-TiNb composite film under air, vacuum, and air-vacuum cycling conditions, was markedly enhanced when the doping amount of β-TiNb alloy reaches 10%. The friction coefficient of the MoS₂-10%TiNb composite film in the air condition was 0.065 and the friction life was 4.2×10⁵ r. The friction coefficient of MoS₂-10%TiNb film in the vacuum condition was only 0.010 and the friction life arrives to 7.2×10⁵ r. Remarkably, MoS₂-10%TiNb film could also maintain their initial friction coefficient over 10 times under air-vacuum cycling conditions. Raman spectroscopy, scanning electron microscope energy dispersive spectrometer (SEM-EDS), and transmission electron microscope (TEM) measurements were applied to analyze the friction tracks, transfer films, and wear debris after friction test under different conditions. The results revealed that a highly crystalline MoS₂ sliding interface layer forms on the surface of the amorphous composite film during the friction process. This layer led to a reduced friction coefficient under vacuum condition. Concurrently, the presence of the β-TiNb alloy mitigated the oxidation of the lubricating film and diminished the sliding interface layer's sensitivity to water and oxygen, which contributes to a lower friction coefficient under atmospheric conditions. This was attributed to the tendency of β-TiNb elements to enter the edge position of MoS₂ crystals, filling the defects caused by the absence of S during sputtering deposition. Meanwhile, the composite film will produce a certain MoO₃ during the friction process, so that the friction coefficient increases slightly. Upon re-entry into vacuum conditions, the highly crystalline sliding interface layer exhibited self-repairing properties due to friction-induced reconstruction of MoS₂ sliding interface layer on the surface of composite films, and then its tribological performance was self-recovered once again. Meanwhile, the pre-formed hard oxide particles (MoO₃) in the air environment would be wrapped by the rapidly formed MoS₂, which furtherly promoted the wear resistance of the MoS₂-10%TiNb composite film. These results indicated that MoS₂-10%TiNb film was a high-performance solid lubricating film that could be reused under the air-vacuum cycling conditions for several times. Our work paved a new path to break through the performance limitations of the solid lubrication film by the synergistic effect of multiple metals alloys.