Changes in structure and tribological properties of <italic>in situ</italic> oxygen passivated WS₂ film after a space exposure experiment outside the Chinese Space Station

The space environment, particularly highly reactive atomic oxygen (AO), often causes performance degradation and accelerated wear of solid-lubricating materials used in aerospace applications. In this study, an in situ oxygen-passivated WS2 lubricating film (W–S–Ti–O composite film) was deposited to withstand AO irradiation. The structural and tribological evolution of the film was examined after a six-month space exposure experiment conducted outside the Chinese Space Station. The results show that in situ oxygen passivation of sulfur vacancies in the WS2 film promoted the formation of a dominant WSxOy phase within the W–S–Ti–O composite film. This phase effectively suppressed excessive WO3 formation during prolonged AO exposure while maintaining a low friction coefficient. After space exposure, the film exhibited a low friction coefficient and a wear life exceeding 4.5 × 105 cycles. This performance is attributed to two main factors: (1) the presence of friction-induced spherical WO3 nanoparticles (approximately 11 nm) embedded in the transfer film, which promoted a transition from pure sliding to a mixed rolling–sliding regime; and (2) the retention of oriented WS2 (002) crystalline layers in the tribofilm, which mitigated the plowing effect of nanoparticles and prevented a significant increase in the wear rate.