LEO space environment exposure resistant behavior of Ag-Cu films

Ag-Cu films, deposited by arc ion plating and composed of face-centered cubic (fcc) Ag and AgCu alloy phases, had been exposed for 43.5 h in low earth orbit (LEO) space environment by a space environment exposure device (SEED) aboard the China Shenzhou-7 manned spaceship. The structure, morphology, composition and tribological property of the Ag-Cu films after the space environment exposure (SEE) were investigated using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS) and ball-on-disk tribometer, respectively. After the SEE, the surface layer of Ag-Cu film was partially oxidized to Ag2O, in which the Ag2O content was decreased from the surface to inner and the Cu element was absent. Furthermore, the exposed Ag-Cu film surface became color and exhibited lots of fine particle-like protuberances, but cracking and flaking phenomena that were common for the exposed Ag film, were not observed. Overall, the influence of space exposure on Ag-Cu film was slight compared to the pure Ag film. Sliding friction tests revealed that for the exposed Ag-Cu film, the friction coefficient was stable and the mean value was almost equivalent to one of the non-exposed Ag-Cu film.