- Nano Express
- Open Access
Synthesis and Tribological Properties of WSe2Nanorods
© to the authors 2008
- Received: 10 July 2008
- Accepted: 2 October 2008
- Published: 25 October 2008
The WSe2nanorods were synthesized via solid-state reaction method and characterized by X-ray diffractometer, TEM, and HRTEM. The results indicated the WSe2compounds had rod-like structures with diameters of 10–50 nm and lengths of 100–400 nm, and the growth process of WSe2nanorods was discussed on the basis of the experimental facts. The tribological properties of WSe2nanorods as additives in HVI500 base oil were investigated by UMT-2 multispecimen tribotester. Under the determinate conditions, the friction coefficient of the base oil containing WSe2nanorods was lower than that of the base oil, and decreased with increasing mass fraction of WSe2nanorods when it was <7 wt.%. Moreover, the base oil with the additives was rather suited to high load and high rotating speed. A combination of rolling friction, sliding friction, and stable tribofilm on the rubbing surface could explain the good friction and wear properties of WSe2nanorods as additives.
- Growth mechanism
- Lubrication additive
- Tribological properties
- Rotating speed
The transition-metal dichalcogenides (including disulfide and diselenium) showed a wide variety of interesting physical properties, such as semiconducting, metallic, superconducting, and magnetic behavior [1–7]. WSe2 was an interesting member of the transition-metal dichalcogenides family. It was a semiconductor with a band gap in the range of 1.2–2 eV, which was useful for photovoltaic and optoelectronic applications [8–10]. WSe2 possessed a layered structure with the metal atoms (W) bonded covalently between the layers of chalcogen atoms (Se), and the remarkable feature of the WSe2 was highly antiphotocorrosive due to the observation of layered structure, which made it as a strong candidate in the development of high efficiency photoelectrochemical solar cells .
In the past few decades, the disulfides, such as MoS2 and WS2, had been extensively studied as lubrication additive on reducing friction and wear of rubbing pairs [12, 13]. The friction-and-wear mechanism had been discussed in great detail [14–19]. However, little work focused on the preparation of WSe2 nanorods, which was similar to the WS2 nanorods, especially the tribological properties of WSe2 as lubrication additive.
In this study, we reported a simple and benign method to prepare WSe2nanorods using W and Se (mole ratio 1:3) at 800 °C in an argon atmosphere. Moreover, the tribological properties of WSe2nanorods as additives in the HVI500 base oil were also investigated.
Preparation of WSe2Nanorods
All chemicals used in the experiment were from state reagent without any further purification. In a typical procedure, high-purity tungsten, selenium powders (mole ratio W:Se = 1:3), and agate balls with diameter of 8 mm were mixed in an agate jar and mechanically milled with QM-ISP2 apparatus for 50 h at 450 rpm. After ball milling, the mixture was pressed into cylindrical pellets with omnipotence tester (CSS44100, ChangChun, China). The pellets were put into a conventional tube furnace, and heated up 800 °C for 1 h followed the argon flow at the rate of about 20 sccm before cooling to the room temperature.
Structural characterization was performed by X-ray diffractometer (XRD) on D/max-2500 copper rotating-anode XRD with Cu Kα radiation (λ = 1.5406 Å) at 40 kV, 200 mA. The morphology and structure of samples was determined using TEM (JEM-2100HR, Japan) at 200 keV. The composition was characterized by energy dispersive X-ray spectroscopy (EDX, S-570, and Japan).
Tribological Properties of WSe2Nanorods as Lubrication Additive
Different mass fractions of WSe2nanorods were dispersed in the HVI500 base oil with ultrasonic vibration (1800 W power, 2000 Hz frequency) for 5 h without any active reagent, and then a series of suspended oil samples were obtained. The tribological properties of the base oil containing WSe2nanorods and the base oil were investigated using a ball-on-disk mode of UMT multispecimen tribotester at ambient condition. The morphology of the wear scar was examined using a Metallurgical microscope (MBA21000, Japan).
Characterization of WSe2Nanorods
The reported growth mechanism in literatures [22, 23], especially the solid-state reaction of MoS2 nanostructure, gave us much inspiration toward understanding the formation of WSe2 nanorods. Under the high temperature conditions, Se quickly evaporated and simultaneously reacted with W. This rapid reaction might lead to supersaturation and fast nucleation. Thus, numerous nuclei of WSe2 were initially formed in the vapor phase. When the initial clusters grew to the critical size, they began to form crystal flakes. Because of the instability of unsaturated dangling bonds, crystalline sheets began folding, and the dangling bonds of self-terminated planes stabilized into spherical or cylindrical crystal shapes. Thin-folded flakes could directly roll up and adopt the rod-like structure (Fig. 2c).
Effect of WSe2Nanorods on Tribological Properties
From the above results, WSe2nanorods as lubrication additive could improve tribological properties of the base oil. A rolling friction mechanism could explain the excellent tribological properties of nanoparticles as lubrication additive. In this study, the effect WSe2nanorods as lubrication additive could be attribute to the molecule bearing mechanism of rolling friction and sliding friction of the WSe2nanorods between the rubbing surfaces. In addition, rotating speed was very important in the experimental process. Under low rotating speed, the stability of liquid lubricant reduced, the load almost acted on protruding part of rubbing surfaces. When the load was increased, the lubricant film became unstable and easily splintered, the friction coefficient became high, as shown in Fig. 5b. But with the increase of rotating speed, the oil film became more and more stable, which could not only bear the load of the steel balls but also prevented any direct contact between the two rubbing surfaces. Moreover, when the shape of nonmaterial was destroyed at high load and high rotating speed, exfoliation of nonmaterial layer filled the rough contact surface and formed a stable thin film with base oil, which could decrease friction and wear happening on the rubbing surfaces.
WSe2nanorods of 10–50 nm in diameters and 100–400 nm in lengths were prepared successfully by solid-state reaction of the tungsten and selenium powders. The HVI500 base oil with addition of WSe2nanorods showed the best friction-and-wear properties. Tribological experiments indicated that the effect of WSe2nanorods as lubrication additive could be attribute to the molecule-bearing mechanism of rolling friction and the sliding friction of the WSe2nanorods between the rubbing surfaces. Moreover, a stable film on the rubbing surface could not only bear the load of the steel ball but also prevent any direct contact between the two rubbing surfaces.
This study is supported by the National Natural Science Foundation of China (Grant Nos. 60778040, 10804036, 60878039), the Science and Technology Bureau of Key Program for Ministry of Education (Item No. 207025), and the Development of the Science and Technology Planning Project of Jilin province (Item No. 20070514).
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