Xiaoming Gao

Microstructure Evolution and Enhanced Tribological Properties of Cu-Doped WS2 Films

AbstractnTo improve the tribological properties of WS2 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 WS2 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 WS2 film both in vacuum and in humid air conditions. The wear mechanisms were proposed after correlating the mechanical performance with film microstructure.

Tribological behavior of WS2-based solid/liquid lubricating systems dominated by the surface properties of WS2 crystallographic planes

In this paper, WS2-based solid/liquid systems were established successfully by combining pure WS2 films with FCPSO (trifluorinated-propyl and chlorinated-phenyl with methyl terminated silicone oil) and SiCH (silahydrocarbons) space oils, and the tribological performances and mechanisms were investigated. The results showed that the tribological properties of the WS2 film were improved greatly when associated with the SiCH oil, and hence this composite system exhibited a low/stable friction coefficient ( 3 × 106 sliding cycles) both in vacuum and air environments. However, the reverse effect was obtained from the WS2 film/FCPSO system. The selective adsorption of the WS2 crystallographic planes with the specific oils seemed to dominate the tribological performance of the composite systems. The SiCH and FCPSO oils have an affinity for the base and edge planes, respectively, of the WS2 films in their composite systems. The combination of the WS2 base plane with SiCH was more advantageous for the formation of perfect lubricating and transfer films at the friction contact area, resulting in improved friction and wear performances. This result provides a significant way for us to design solid/liquid lubricating systems based on lamellar solid lubricants.

Structural, Mechanical, and Tribological Properties of WS2-Al Nanocomposite Film for Space Application

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 (~u20093xa0at.%) and a featureless one at higher Al content (~u20096–12xa0at.%). The film densification resulted in a significantly improved hardness from ~u20090.3 GPa for pure WS2 film to 2.9 GPa for WS2-3xa0at.% Al film and to 4.7–5.7 GPa for WS2 composite films with higher Al contents of ~u20096–12xa0at.%, but the composite films with higher Al contents were brittle. As a result, only the composite film with Al content of ~u20093xa0at.% exhibited a much better wear resistance than pure WS2 film. In vacuum environment, the wear life of WS2-3xa0at.% Al composite film about 1.5u2009×u2009106 cycles was much higher than that of pure WS2 filmu2009~u20096.5u2009×u2009105 cycles, exhibiting a potential application in space technology.

Influence of Deposition Temperature and Pressure on Microstructure and Tribological Properties of Arc Ion Plated Ag Films

The films deposited at low temperature (LT-films) have increasingly attracted theoretical and technical interests since such films exhibit obvious difference in structure and performances compared to those deposited at room temperature. Studies on the tribological properties of LT-films are rarely reported in available literatures. In this paper, the structure, morphology and tribological properties of Ag films, deposited at LT (166 K) under various Ar pressures on AISI 440C steel substrates by arc ion plating (AIP), are studied by X-ray diffraction (XRD), atomic force microscopy (AFM) and a vacuum ball-on-disk tribometer, and compared with the Ag films deposited at RT (300 K). XRD results show that (200) preferred orientation of the films is promoted at LT and low Ar pressure. The Crystallite sizes are 70 nm–80 nm for LT-Ag films deposited at 0.2 Pa and 0.8 Pa and larger than 100 nm for LT-Ag films deposited at 0.4 Pa and 0.6 Pa, while they are 55 nm–60 nm for RT-Ag films deposited at 0.2 Pa–0.6 Pa and 37 nm for RT-Ag films deposited at 0.8 Pa. The surfaces of LT-Ag films are fibre-like at 0.6 Pa and 0.8 Pa, terrace-like at 0.4 Pa, and sphere-like at 0.2 Pa, while the surfaces of RT-Ag films are composed of sphere-like grains separated by voids. Wear tests reveal that, due to the compact microstructure LT-Ag films have better wear resistances than RT-Ag film. These results indicate that the microstructure and wear resistance of Ag films deposited by AIP can be improved by low temperature deposition.