Morphology evolution of Ag alloyed WS2 films and the significantly enhanced mechanical and tribological properties

• The WS2–Ag thin composite films with columnar structure were prepared.
• Ag induced the size of platelets and pores decreased but number of those increased.
• Suitable doping of Ag induced film brittleness, but improved its adhesion strength.
• The composite films with low Ag content (about 9.0 at.%) showed longest wear life.
• That provided a feasible way of improving wear resistance of space lubricant films.

The WS2–Ag composite films were prepared by radio frequency co-sputtering method. The effects of alloying Ag content on composition, microstructure, mechanical properties and friction behaviors have been analyzed by X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) and high resolution transmission electron microscope (HRTEM), scratch tester, nano-indentation tester and ball-on-disk tribo-tester. The Ag addition was in present of nanocrystalline phase in the boundary of the crystalline WS2 matrix and induced morphology change, but could not completely prevent the columnar platelets. The columnar platelet was composed of a great deal of nanocrystalline and a small amount of amorphous WS2 phase. There were no substantial variations in the hardness of the composite films when the Ag content was in the range of 0–20.3 at.%. The suitable amount of Ag content was benefited for improving the film adhesive strength and wear resistance both in vacuum and humid air environment. Particularly, the composite film with 9.0 at.% Ag exhibited the longest wear life (about 7.6 × 105 cycles in vacuum, 6.7 × 105 cycles in humid air) under high Hertzian contact pressure (as high as 1.0 GPa). The wear mechanism was discussed in terms of the rearrangement of wear track and formation of transfer film.