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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]

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 A novel approach by texturing the WS 2 film with the nanocone-array Ni interlayer was adopted to improve the tribological property of WS 2 film particularly prolonging the wear life for application in space. The WS 2 /Ni bilayer films were prepared by sputtering WS 2 layer onto electrodeposited Ni interlayers with different surface morphologies. The micro-structures and the tribological properties of the prepared WS 2 /Ni bilayer films were investigated. The results showed that with the existence of the nanocone-array Ni interlayer, the textured WS 2 film showed a cabbage-like structure due to the growth of the WS 2 columnar plates surrounding the Ni nanocones. The cabbage-like WS 2 /Ni bilayer thin film was benefited for improving the lubrication properties with a long wear life of 4.9 × 105 cycles while the wear life was only 0.7 × 105 cycles for the pure WS 2 and 3.2 × 105 cycles for rough Ni textured WS 2 film. The improved tribological property was mainly attributed to the reserve of the WS 2 lubricate by the Ni nanocones as well as the relative low friction coefficient of the nanocone-array Ni interlayer due to the obvious reduced real contact area. Highlights • A novel cabbage-like WS 2 /Ni bilayer film was prepared without any template. • The cabbage-like WS 2 /Ni bilayer lubricating film showed an improved lifetime. • Reservoir of the WS 2 lubricates by Ni nanocones resulted in the improved property. [ABSTRACT FROM AUTHOR]

Tribological properties and wear mechanisms of TiN films deposited by arc ion plating (AIP) against Si 3 N 4 , Al 2 O 3 and steel balls were comparatively investigated under different loads and speeds. The tribological properties of the TiN films were evidently influenced by load, speed and counterball. The different friction coefficients were related to the real contact state of the friction interface of the tribopair. The TiN films against the ceramic balls mainly were typical polishing wear, which resulted from the interaction between the preferential formation of the Ti oxide layer on the wear track surface and the small particles from ceramic balls. For steel ball, the platelets of Fe 3 O 4 adhered to the wear track surface did not generate polishing effect. [ABSTRACT FROM AUTHOR]

In the present work, WS 2 –MoS 2 nanocomposite films were sputtered, based on which solid/liquid-lubrication systems were realized by combining the WS 2 –MoS 2 films with space oils including MACs (multialkylated cyclopentanes) and CPSO (chlorinated-phenyl with methyl-terminated silicone oil). The tribological behaviors of the solid/liquid systems were investigated in vacuum. It was revealed that in comparison to pure WS 2 film, the WS 2 –MoS 2 composite film exhibited an enhanced tribological performance. Moreover, our results identified that the WS 2 –MoS 2 /MACs system integrated better the advantages of the solid and liquid lubrications and thereby a synergistic lubricating effect was generated. A tribological model was established to explain the synergistic lubricating mechanism. [ABSTRACT FROM AUTHOR]

Discharge phenomena in electrolytes were discovered many decades ago. However, the research and development of plasma oxidation in electrolytes for surface engineering have only truly been actively performed in recent years, particularly stimulated by a review paper entitled Plasma Electrolysis for Surface Engineering published in 1999. Among plasma electrolysis processes, plasma electrolytic oxidation (PEO) caught the most attention from academic and industrial communities. However, PEO has been largely restricted to valve metals, such as Al, Mg and Ti and their alloys. This research intends to explore the current boundaries of the PEO process and extend it into non-valve metals. The paper therefore first describes the coating growth mechanisms in non-valve metals, which are different from the conventional PEO (conversion) process on valve metals. Particular emphasis is given to revealing details of the coating deposition behavior through direct observations and analysis of non-valve metallic substrates and their coatings formed at different timescales of the PEO process. Copper and ferrous materials are chosen as appropriate non-valve metal substrates and their coating properties in terms of mechanical, corrosion, thermal and electrical aspects are summarized. The results show that high hardness and adhesion strength, high corrosion resistance, good thermal and electrical insulation performances are achievable for PEO-derived coatings on non-valve metals. These properties also open doors for emerging applications, for example in electrical vehicles. Such applications are presented by reporting the coating application on an e-motor bearing and brake disc as two examples. Future research perspectives are discussed with a hope to stir a new wave of PEO research for non-valve metals as realized by the first review paper for the previous wave of plasma electrolysis research. [ABSTRACT FROM AUTHOR]

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 0–20.3at.%. 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 9.0at.% 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 1.0GPa). The wear mechanism was discussed in terms of the rearrangement of wear track and formation of transfer film. [ABSTRACT FROM AUTHOR]

MoS 2 -Cr 3 O 4 composite films were prepared by the combination method of PVD and liquid phase synthesis. The microstructure and tribological properties of films have been investigated. The results indicate that the size of Cr 3 O 4 varied from 5 nm to 500 nm and the shape of Cr 3 O 4 changed from nanospheres to nanorods by adjusting concentration of CrCl 3. The MoS 2 -Cr 3 O 4 composite films developed excellent wear resistance and much lower friction coefficient than sputtered MoS 2 both in vacuum and 45%RH air. During friction, the Cr 3 O 4 @MoS 2 core-shell structure with Cr 3 O 4 core of high hardness and soft MoS 2 shells were formed and played the role of nano-bearing, which reduced the friction and wear of the composite film. Besides, better oxidation resistance and tribological property of the MoS 2 -Cr 3 O 4 composite films in humid air were attributed to the Cr 3 O 4 nanoparticles, which occupied the most of dangling bond sites. Unlabelled Image • Cr 3 O 4 nano-particles are connected to dangling bonds of the sputtered MoS 2 film. • Occupied dangling bonds by Cr 3 O 4 can much improved the antioxidant of MoS 2 film. • The Cr 3 O 4 @MoS 2 core-shell structure formed and played a role of nano-bearing. • The Cr 3 O 4 greatly improves tribological properties of films both in air and vacuum. [ABSTRACT FROM AUTHOR]

To achieve the desired levels of performance and durability for bearings applied in space, a novel solid-liquid dual lubricating system was established via the WS2-MoS2 composite films sliding against oil-impregnated porous polyimide (PPI), in which polyalphaolefin (PAO) oils with different viscosities were used, respectively. The contact angle measurement indicated that the PAO oil had good wettability on the surfaces of the composite film and the PPI. The vacuum tribological behaviors of single WS2-MoS2 composite film and the dual lubricating system were mainly evaluated under different loads and speeds. In comparison with single WS2-MoS2 composite film, the dual lubricating system exhibited the low friction coefficient and synergistic lubricating effect for the low-viscosity PAO oil under severe sliding conditions. It was concluded that the cleavages of the WS2 and MoS2 crystals along the basal plane were more sufficient and formed small and thin WS2 and MoS2 platelets in the dual lubricating system. Meanwhile, it was found that oil-impregnated PPI readily released PAO lubricating oil with low viscosity, which further decreased friction on the contact interface. The lubricating mechanism of the dual lubricating system was also revealed after correlating the tribological behaviors of the different lubricating systems. [ABSTRACT FROM AUTHOR]