Vol.:(0123456789) 1 3 Tribology Letters (2018) 66:137 https://doi.org/10.1007/s11249-018-1085-z ORIGINAL PAPER Structural, Mechanical, and Tribological Properties of WS 2 -Al Nanocomposite Film for Space Application Xiaoming Gao 1  · Yanlong Fu 1  · Dong Jiang 1  · Desheng Wang 1  · Jun Yang 1  · Lijun Weng 1  · Ming Hu 1  · Jiayi Sun 1 Received: 21 May 2018 / Accepted: 6 September 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract In this paper, WS 2 -Al nanocomposite flms were deposited by closed-feld unbalanced magnetron sputtering to improve the microstructure and wear resistance of pure WS 2 flm. Results revealed that pure WS 2 flm presented a columnar microstruc- ture, but the growth of WS 2 platelets could be signifcantly suppressed by doping Al. Correspondingly, the WS 2 -Al com- posite flm showed a dense fber-like microstructure at low Al content (~ 3 at.%) and a featureless one at higher Al content (~ 6–12 at.%). The flm densifcation resulted in a signifcantly improved hardness from ~ 0.3 GPa for pure WS 2 flm to 2.9 GPa for WS 2 -3 at.% Al flm and to 4.7–5.7 GPa for WS 2 composite flms with higher Al contents of ~ 6–12 at.%, but the composite flms with higher Al contents were brittle. As a result, only the composite flm with Al content of ~ 3 at.% exhib- ited a much better wear resistance than pure WS 2 flm. In vacuum environment, the wear life of WS 2 -3 at.% Al composite flm about 1.5 × 10 6 cycles was much higher than that of pure WS 2 flm ~ 6.5 × 10 5 cycles, exhibiting a potential application in space technology. Keywords WS 2 -Al nanocomposite flms · Microstructure · Tribology · Vacuum 1 Introduction Space environment involves extreme temperatures (high or ultra-low temperature) and ultrahigh vacuum conditions, where oils and greases can hardly play these lubricating roles unless temperature-controlling and sealing devices are employed [1]. Under these conditions, solid lubrica- tion is an optimal candidate since some solid lubricants are insensitive to space-environmental temperatures and exhibit excellent tribological properties in vacuum environment [2]. However, the service life of spacecrafts has become longer and longer with the rapid development of space technology. Correspondingly, it is expected that the used solid lubricants may have a lower wear and better environmental adaptation. As for the environmental adaptation, some terrestrial tests are inevitable prior to the launch of spacecraft and hence the solid lubricant must be exposed in air environment. Even in space environment, solid lubricant sometimes may undergo a harsh oxidation condition, such as atomic oxygen in low earth orbit [3, 4], which can cause the oxidation and property deterioration of many materials. As summarized by Kannel and Dufrane [5], the demands on tribology for space mis- sions always grow faster than the solutions despite signif- cant advances in tribology. Therefore, numerous eforts have been paid to improve the wear endurance of solid lubricants for ensuring the high reliability of spacecrafts. Transition metal disulfde (TMD) flms have important applications as solid lubricants in space technology to decrease the friction and wear of moving parts [6]. It is well known that sputtered TMD flms normally exhibit a porous columnar microstructure, which may cause plenty of wear debris in the initial stage of friction due to the fracture of TMD columnar microplatelets and relatively limited wear endurance [7]. This can produce an adverse efect on the run- ning stability and service life of some fne-pitch gears (such as internal circular spline and external fexspline in har- monic driver that has been used in solar array drive mecha- nism, antenna pointing mechanism, etc), because there is no enough space to store too much wear debris for such friction pair. Previous studies reveal that the microstructure and wear * Ming Hu hum413@licp.cas.cn * Jiayi Sun sunjy@licp.cas.cn 1 State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China