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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. [ABSTRACT FROM AUTHOR]

Abstract: Ag–Cu alloy film was deposited by arc ion plating (AIP). Atomic oxygen (AO) irradiation experiments were conducted using a ground AO simulation facility. The structure, morphology, composition and tribological property of the Ag–Cu alloy film before and after AO irradiation were investigated and compared with Ag film using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS) and ball-on-disk tribometer, respectively. In depth XPS analysis clearly shows that the affected thickness of Ag–Cu alloy film was greatly reduced and the oxidation product was mainly Ag2O, but AgO and Ag2O in case of pure Ag film. As a result, the Ag–Cu alloy film exhibited better AO resistant behaviors, and showed a stable friction and low wear after the AO irradiation. The AO resistant behaviors of the Ag–Cu alloy film were discussed in terms of the film microstructure. [Copyright &y& Elsevier]

The rf-sputtered MoS 2 -Au composite films were exposed in real low earth orbit (LEO) space environment by a space environment exposure device (SEED) aboard China Shenzhou-7 manned spaceship. The ability of the composite films resistant to atomic oxygen (AO) was investigated using X-ray photoelectron spectroscope (XPS), X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) equipped with an X-ray energy dispersive spectroscopy (EDS). The results showed that the microstructure of the composite film was densified because the growth of MoS 2 platelets was suppressed to some content due to the addition of Au, although it was still characterized by a typical dendrite-like morphology. No obvious change in the morphology, phase structure, element composition and friction property was observed from the space exposed and non-exposed composite films, indicating that the composite film exhibited a better anti-oxidation ability as exposed in LEO environment. The improved anti-oxidation ability was attributed to the more compact microstructure and a passivation effect that the dangling bonds at the edge planes of MoS 2 platelets were probably occupied by the partial doping atoms. [ABSTRACT FROM AUTHOR]

Radio frequency-sputtered MoS 2 films had been exposed for 43.5 h in real low earth orbit (LEO) space environment by a space environment exposure device (SEED) aboard China Shenzhou-7 manned spaceship. The composition, morphology, phase structure and friction property of the exposed films were investigated using X-ray photoelectron spectroscope (XPS), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), X-ray energy-dispersive spectroscopy (EDS) and ball-on-disk tribometer. XRD and EDS results revealed that the as-deposited MoS 2 films were characterized by a MoS x O y phase structure, in which x and y values were determined to be ∼0.65 and 1.24, respectively. XPS analysis revealed that due to space atomic oxygen attack, the film surface was oxidized to MoO 3 and MoS x O y with higher O concentration, while the partial S was lost. However, the affected depth was restricted within the surface layer because of protective function of the oxidation layer. As a result, the friction coefficient only exhibited a slight increase at initial stage of sliding friction. [ABSTRACT FROM AUTHOR]

Ag films, deposited by arc ion plating, had been exposed for 43.5 h in real 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 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. Depth XPS and XRD analysis revealed that after the SEE, the surface layer of Ag films was partially oxidized to Ag 2 O, the cracking and flaking phenomena could be observed from the film surface by FESEM, and so the Ag films lose its metallic shine and became gray. As a result, the tribological performance was deteriorated and the high/unstable friction was obtained. [ABSTRACT FROM AUTHOR]

Abstract To clarify the ability of reactive metals alloyed transition metal disulfides (TMDs) films resistant to low-earth-orbit (LEO) space exposure, the MoS 2 -16 at. % Ag film was sputtered and exposed in long-term atomic oxygen (AO) environment. Results revealed the MoS 2 -Ag composite film could be severely oxidized by the AO irradiation. The 2H-MoS x O y matrix was severely oxidized to MoO 3 and MoO 2 , and the alloyed Ag was partially oxidized to Ag 2 O. The volume expansion from the oxidation of Ag to Ag 2 O was responsible for the cracking and thereby severe oxidation of brittle 2H-MoS x O y platelets in the composite film. Furthermore, the re-growth of Ag induced by the AO irradiation was also observed in the initial stage of AO irradiation. Due to the oxidization, the film morphology was changed from the characteristic dentrite-like surface for the non-irradiated film to the terrace-like one for the irradiated film and eventually to the chrysanthemum-like one as increasing the AO fluence. Wear tests revealed that the wear resistance of MoS 2 film in vacuum environment could be significantly improved. However, the wear resistance of MoS 2 -Ag composite film was obviously deteriorated because of the severe oxidation from the AO irradiation. The wear life of MoS 2 -Ag composite film was sharply reduced from ∼7.6 × 105 cycles before the AO irradiation to ∼4.9 × 104 cycles after the AO irradiation of 5.6 × 1026 atoms∙m−2. These results indicated that for the selection of solid lubricants in LEO space environment, the reactive metals alloyed TMDs films should be excluded as possible as to eliminate potential risks. Graphical abstract Image 1 Highlights • MoS 2 -Ag composite film could be severely oxidized by long-term AO irradiation. • The oxidation production was mainly MoO 3 , MoO 2 and Ag 2 O. • The AO irradiation caused not only the oxidation but also the re-growth for the doped Ag. • The oxidation led to the cracking and morphological change of MoS 2 -Ag film. • Wear endurance of MoS 2 -Ag film was significantly reduced by AO irradiation. [ABSTRACT FROM AUTHOR]

The microstructure and wear resistance of sputtered MoS 2 films can be improved by adding some amount of metals. However, the oxidation-resistance of MoS 2 -metals films would be worried because some metals are sensitive to atomic oxygen (AO). In this paper, the inert Sb 2 O 3 was introduced into the MoS 2 film, and its effect on the structure and anti-oxidation ability of the MoS 2 film was clarified. XRD and SEM results revealed that due to the addition of Sb 2 O 3 , the growth of MoS 2 micro-platelets was almost suppressed, and so the composite film was amorphous and very compact. XPS analysis indicated that for the composite film, the influence by the AO irradiation was restricted into the film surface layer. Namely, the composite film exhibited a better anti-oxidation ability in the AO environment. [ABSTRACT FROM AUTHOR]