In-situ friction and wear responses of WS ₂ films to space environment: Vacuum and atomic oxygen

The Friction and wear behavior of both sputtered WS ₂ films with loose or dense microstructures were investigated in vacuum, atomic oxygen (AO) and vacuum/AO irradiation alternate environments. The loose microstructure of pure WS ₂ film composed of columnar platelets and vertical pores and the dense microstructure showed high compactness. The results revealed that the WS ₂ films exhibited a periodic fluctuant friction coefficient and high wear under the synchronization action of AO irradiation and friction (in-situ condition). The in-situ results also indicated significantly difference from that of conventional ex-situ methods. As the friction environments were switched from vacuum to in-situ AO, the friction coefficients of the two types of films increased from 0.03 to 0.07 and the wear life of them declined identically to a small value of ∼1.1 × 10 ³ cycles, although the wear life of loose WS ₂ film was up to 1.7 × 10 ⁴ cycles and that of the dense film was longer than 2.3 × 10 ⁴ cycles in vacuum. The friction and wear behavior did not depend on the film compactness but the anti-oxidation ability of WS ₂ itself. For in-situ AO irradiation condition, the high friction resulted from the formation of oxidation product (WO ₃ ) by AO and its delamination by frictional interaction was responsible for high wear.