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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]

Oxygen-doped transition metal disulfides solid lubricating films have been less investigated due to the widespread viewpoint of the detrimental effect of oxidation on tribological properties. In this study, the introduction of oxygen into the deposited W-S-O film was achieved through sputtering WS 2 /WO 3 composite targets. The hardness of W-S-O film has been found to increase threefold, accompanied by a 50% reduction in the friction coefficient and an order of magnitude increase in wear life. The compact structures and the embedded WO 3 nanocrystals in the film contribute to the improved tribological property of the W-S-O film by improving the load-bearing capacity. Meanwhile, the presence of WO 3 phase within the W-S-O film does not hinder the formation of the transfer film during friction. [Display omitted] • The presence of WO 3 nanocrystals in the W-S-O film resulted in a substantially thick equiaxed crystal layer. • The W-S-O film exhibited lower friction coefficient and longer wear life compared to that for pure WS 2 film. • The WO 3 phase was expelled from the tribo-surface without hindering transfer film formation during friction. [ABSTRACT FROM AUTHOR]

Sputtered WS 2 films can be modified by adding some amount of hetero-elements to overcome the porous columnar microstructure and thereby to enhance the wear resistance. However, the recession of tribological properties of WS 2 -hetero-element composite films in low earth orbit environment would be a potential danger because most hetero-elements succumbed to oxidation by atomic oxygen. Herein, by altering deposition argon pressure to adjust the plasmas bombard function on growing WS 2 film, the characteristic porous microstructure disappeared. At the optimization condition, the sputtered WS 2 film exhibited a dense microstructure, high ratio of S to W, and thereby much better wear-resistance. However, the excessive bombardment would result in the prominent loss of S atoms in WS 2 film and the deterioration of tribological properties. [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: Pure WS2 and WS2/Cu multilayer films were deposited by radio frequency sputtering method and the erosion behavior caused by moisture was studied. For the pure WS2 film, the morphology, microstructure, composition and also the tribological properties experienced substantial change upon storage in humid air environments. Depth XPS and Raman analysis showed that the pure WS2 film suffered deep oxidation due to the open structure, but the compact WS2/Cu multilayer film resisted oxidation very well. For the multilayer film, the oxidation was restricted on the surface layer owing to the protective effect of the reactive product cladding layer of W and Cu oxidation and the compact microstructure. The open lamella structure of pure WS2 film allowed moisture to diffuse into the film interior, and so the deep oxidation, evolution of structure and Fe substrate corrosion contributed to the deterioration of tribological properties. The WS2/Cu multilayer film exhibited better deep oxidation resistance even after storage for long durations in humid air environments, and accordingly still showed low and stable friction coefficient and long wear life. [Copyright &y& Elsevier]

Abstract Characteristic porous microstructure causes poor wear resistances of sputtered transition metal dichalcogenides (TMDs) films that have important applications in space technology. TMDs/metal multilayer films exhibit improved microstructural and tribological properties, but which are unreliable due to the mismatch in thermal/mechanical properties between them as well as the metal sensibility to atomic oxygen (AO). In this study, MoS 2 /WS 2 nano-scale multilayer films were fabricated to improve the microstructural, mechanical, tribological and environment-adaptive properties of TMDs films. Results revealed that as compared with MoS 2 or WS 2 single-layer film, the multilayer film exhibited a dense microstructure, strong (002) texture and high hardness. Correspondingly, it showed a significantly improved wear resistance in vacuum, whose wear life was ~one order longer than those of single-layer films. Simulation tests revealed that even drastic thermal shock could not cause the delamination of multilayer film, which was observed from TMDs/metal multilayer system; and the oxidation from AO irradiation was restricted into film surface layer (≤10 nm). These results indicated that the microstructural, mechanical and tribological properties of TMDs films were significantly improved by the nano-scale multilayer design, and the fabricated multilayer film had potential applications in space technology. Graphical abstract Unlabelled Image Highlights • WS 2 /MoS 2 nano-scale multilayer film was designed and fabricated. • Porous microstructure was significantly restrained by multilayer fabrication. • Multilayered film exhibited better mechanical, wear-resistant and space-environment-resistant properties. [ABSTRACT FROM AUTHOR]

Abstract The WS 2 /CrN bilayered films with different interface morphologies between WS 2 and CrN layer were prepared by physical vapor deposition. By controlling the deposition parameters, the surface of CrN interlayer exhibited compact granular structure when deposited under low partial pressure of Ar while a cone array-like surface appeared under high Ar partial pressure. The surface texture of the CrN interlayer had a great effect on the tribological performance of the WS 2 /CrN film. The compact CrN textured WS 2 film exhibited a wear life about 2.6 × 105 cycles while the optimal cone array-like CrN textured WS 2 film showed a much longer wear life reaching to 6.8 × 105 cycles, which was more than 2.6 times of that for compact CrN textured WS 2 film and 3.5 times of that for WS 2 film without the CrN interlayer. The improved tribological property of the optimal cone array-like CrN textured WS 2 film was attributed to the high adhesion between the WS 2 layer and the CrN interlayer. Meanwhile, the reservoir and replenishment of the WS 2 lubricant by the CrN nanocones during the friction made a significant contribution for the long wear life. Highlights • Compact and cone array-like CrN interlayers were deposited under different Ar partial pressure. • Different tribological performances would be resulted by tailoring the interface of WS 2 /CrN bilayered film. • The reservoir and replenishment of the WS 2 lubricates by CrN nanocone array contribute to the tribological performances. [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]

Thin Cr-Nb-N films with Nb content ranging from 5.1 to 19.5 at.% were prepared onto the AISI 440C substrates by reactive magnetron sputtering of the Cr and Nb in Ar-N 2 gas mixture. The effects of the Nb content on the microstructure and the corrosion behavior in 3.5 wt% NaCl solution were investigated. All the deposited films showed a typical surface feature of granular structure. The Cr-Nb-N films with 5.1 at.% and 10.9 at.% Nb showed a duplex layers consisting of a dense and coherent layer near film-substrate interface, followed by a columnar layer differed from the uniform columnar structure for pure CrN. While for Cr-Nb-N films with higher Nb content, there was no obvious dense layer but a columnar structure. The low Nb content Cr-Nb-N coated samples showed a positive shift of both corrosion potential and the pitting potential compared to the pure CrN coated sample while the pitting potential decreased for high Nb content Cr-Nb-N coated samples. The Cr-Nb-N film with Nb content of 10.9 at.% possesses the best corrosion resistance due to the high polarization resistance resulted from the modification of the microstructure by the Nb incorporation. [ABSTRACT FROM AUTHOR]

To improve the tribological properties of WS film both in vacuum and in humid air conditions, its microstructure was optimized by doping different concentrations of Cu via radio frequency co-sputtering method. The film microstructure and composition were investigated by field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, grazing incidence X-ray diffraction and high-resolution transmission electron microscopy. It was verified that Cu was presented in amorphous phase in the WS matrix and could also induce amorphization and densification of the composite films gradually. The film microstructure changed from coarse columnar platelet structure at low Cu content (0-5.8 at.%) to transition structure with two separate layers at increased Cu content (11.5-16.2 at.%) and to a featureless structure at high Cu content (above 24.4 at.%). The mechanical and tribological properties of films were evaluated using the scratch tester and ball-on-disk tribometer, respectively. It was found that the incorporation of a suitable content of Cu dopant could significantly improve the film toughness, but excess amount of Cu dopant lead to high brittleness. All the composite films exhibited much lower wear rate and longer wear life than those of pure WS film both in vacuum and in humid air conditions. The wear mechanisms were proposed after correlating the mechanical performance with film microstructure. [ABSTRACT FROM AUTHOR]

Highlights: [•] Pure WS2 and WS2-Al composite films with different structures were prepared. [•] The compactness of WS2 film was significantly improved due to incorporation of Al. [•] Different mechanisms of atomic oxygen resistance of both the films were discussed. [•] Films before and after AO irradiation kept the unchanged tribological properties. [•] The composite films showed much better wear resistance than pure WS2 film. [Copyright &y& Elsevier]

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]

Highlights: [•] A novel kind of nanostructured WS2–Ni composite films was prepared. [•] The introduction of dopant Ni induced the microstructural change from loose columnar platelet structure with abundant inherent porosity to denser structure. [•] The WS2–Ni composite films exhibited better anti-oxidation capability compared with the pure WS2 film. [•] The suitable doping of Ni in the WS2 film induced film brittleness to a lesser extent, but was able to improve adhesion strength effectively. [•] The composite films with low Ni content (about 5.2at%) showed longer wear life and about sevenfold better than that of pure WS2 film in humid air. [Copyright &y& Elsevier]

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]

MoS 2 film deposited was deposited by closed-field unbalanced-magnetron sputtering (CFUBMS), and its structural and tribological properties were modified by the heat treatment at temperature range of 100–500 °C in vacuum environment. Results revealed that as-deposited MoS 2 film was composed of MoS 2 plus a little MoO 3 with a dense amorphous-like structure. After the vacuum heat treatment, MoS 2 film was still very compact, but its microstructure was changed from amorphous-like to nano-crystalline states with the increase of heat treatment temperature. As a result, a MoS 2 film with a microstructure of MoS 2 nanocrystals embedded into its own amorphous matrix was obtained at middle temperature of 200 °C. Furthermore, MoO 3 phase in the film was reduced to MoO 2 after the vacuum heat treatment at high temperatures of 400 and 500 °C. Friction tests revealed that the wear resistance of MoS 2 film in vacuum environment was significantly improved by the vacuum heat treatment at suitable temperature. The wear life of vacuum-heat-treated MoS 2 film at 200 °C for 3 h was ~6.2 × 105 r, about four times longer than that of as-deposited MoS 2 film. Unlabelled Image • Amorphous-like MoS 2 film changed structure to nanocrystal states, embedded into matrix to improve wear resistance by vacuum-heat-treatment. • Its structure and wear resistance may be significantly modified by vacuum-heat-treatment. • Film structure changed from amorphous-like to nanocrystal states by vacuum-heat-treatment. • A special structure of MoS 2 nanocrystals embedded into its own amorphous matrix was obtained at suitable temperature of heat treatment. • Wear life of vacuum-heat-treated MoS 2 film at 200 °C was about four times longer than that of as-deposited MoS 2 film. [ABSTRACT FROM AUTHOR]

In this paper, WS2-Al nanocomposite films were deposited by closed-field unbalanced magnetron sputtering to improve the microstructure and wear resistance of pure WS2 film. Results revealed that pure WS2 film presented a columnar microstructure, but the growth of WS2 platelets could be significantly suppressed by doping Al. Correspondingly, the WS2-Al composite film showed a dense fiber-like microstructure at low Al content (~ 3 at.%) and a featureless one at higher Al content (~ 6-12 at.%). The film densification resulted in a significantly improved hardness from ~ 0.3 GPa for pure WS2 film to 2.9 GPa for WS2-3 at.% Al film and to 4.7-5.7 GPa for WS2 composite films with higher Al contents of ~ 6-12 at.%, but the composite films with higher Al contents were brittle. As a result, only the composite film with Al content of ~ 3 at.% exhibited a much better wear resistance than pure WS2 film. In vacuum environment, the wear life of WS2-3 at.% Al composite film about 1.5 × 106 cycles was much higher than that of pure WS2 film ~ 6.5 × 105 cycles, exhibiting a potential application in space technology. [ABSTRACT FROM AUTHOR]