The Friction and wear behavior of both sputtered WS2 films with loose or dense microstructures were investigated in vacuum, atomic oxygen (AO) and vacuum/AO irradiation alternate environments. The loose microstructure of pure WS2 film composed of columnar platelets and vertical pores and the dense microstructure showed high compactness. The results revealed that the WS2 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 similar to 1.1 x 10(3) cycles, although the wear life of loose WS2 film was up to 1.7 x 10(4) cycles and that of the dense film was longer than 2.3 x 10(4) cycles in vacuum. The friction and wear behavior did not depend on the film compactness but the anti-oxidation ability of WS2 itself. For in-situ AO irradiation condition, the high friction resulted from the formation of oxidation product (WO3) by AO and its delamination by frictional interaction was responsible for high wear. (C) 2018 Published by Elsevier B.V.