MoS₂/WS₂ Nanosheet-Based Composite Films Irradiated by Atomic Oxygen: Implications for Lubrication in Space

The atomic oxygen (AO) in the low earth orbit (LEO) has an irreversible degradation impact on the tribological properties of MoS2 lubricating films widely used in space. Here, we demonstrate that high irradiation tolerance combined with low friction and wear can be realized at a composite comprising the MoS2 matrix with a small amount of WS2. By comprehensively using microstructure characterization and molecular dynamics (MD) calculations, we reveal that the amorphous structure of a pure MoS2 film is the most susceptible to be oxidized due to its loose and disordered structure. In comparison, films with a highly ordered (002) crystal plane parallel to the substrate can significantly inhibit the diffusion of AO, e.g., the oxidation depth of a MoS2/WS2 composite film with an addition of 3.9 atom % WS2 is below 20 nm after a dose of 5.4 × 1019 atoms·cm–2 irradiation. Specifically, the structure of composite films during long-term AO irradiation experiences a transformation process from the MoS2 crystal to the MoOx nanocrystal to amorphous. Compared with the rapid deterioration of tribological properties for the pure MoS2 film, the MoS2/WS2 composite film maintains a low friction coefficient of 0.015 and wear rate of 2.03 × 10–7 mm3·N–1·m–1 after a dose of 5.4 × 1019 atoms·cm–2 irradiation, almost identical to the unirradiated samples.