Responses of TMDs-metals composite films to atomic oxygen exposure

Abstract

To clarify the ability of reactive metals alloyed transition metal disulfides (TMDs) films resistant to low-earth-orbit (LEO) space exposure, the MoS2-16 at. % Ag film was sputtered and exposed in long-term atomic oxygen (AO) environment. Results revealed the MoS2-Ag composite film could be severely oxidized by the AO irradiation. The 2H-MoSxOy matrix was severely oxidized to MoO3 and MoO2, and the alloyed Ag was partially oxidized to Ag2O. The volume expansion from the oxidation of Ag to Ag2O was responsible for the cracking and thereby severe oxidation of brittle 2H-MoSxOy platelets in the composite film. Furthermore, the re-growth of Ag induced by the AO irradiation was also observed in the initial stage of AO irradiation. Due to the oxidization, the film morphology was changed from the characteristic dentrite-like surface for the non-irradiated film to the terrace-like one for the irradiated film and eventually to the chrysanthemum-like one as increasing the AO fluence. Wear tests revealed that the wear resistance of MoS2 film in vacuum environment could be significantly improved. However, the wear resistance of MoS2-Ag composite film was obviously deteriorated because of the severe oxidation from the AO irradiation. The wear life of MoS2-Ag composite film was sharply reduced from ∼7.6 × 105 cycles before the AO irradiation to ∼4.9 × 104 cycles after the AO irradiation of 5.6 × 1026 atoms∙m−2. These results indicated that for the selection of solid lubricants in LEO space environment, the reactive metals alloyed TMDs films should be excluded as possible as to eliminate potential risks.