Space environmental effects on MoS2 and diamond-like carbon lubricating films: Atomic oxygen-induced erosion and its effect on tribological properties

Effect of 5 eV atomic oxygen beam exposure on the surface properties of sputter-deposited molybdenum disulfide (MoS2) and diamond-like carbon lubrication films is experimentally evaluated with relevance to space environmental effect in low Earth orbit. X-ray photoelectron spectra indicate that the loss of sulfur and oxidation of molybdenum at the atomic oxygen-exposed MoS2 surface are significant. Depth profiles of sulfur and molybdenum indicate that the oxidation is restricted within 3 nm from the surface. This is due to the fact that Mo oxide plays as a protective layer against further atomic oxygen attack. The surface oxidation affects the friction coefficient, however it is recovered with sliding. Due to the delamination of oxide layer, wear-life of the film is reduced dramatically by certain exposure conditions. On the other hand, no severe oxidation states of carbon atoms are detected at DLC surface from the synchrotron radiation photoemission spectra. In contrast, loss of the DLC film itself is measured by Rutherford backscattering spectroscopy. These analytical results can be explained by the oxidative gasification of carbon atoms at DLC surface.