Tribological Properties of WS2 Hexagonal Nanoplates Nanoflowers

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Tribological Properties of WS2 Hexagonal Nanoplates Nanoflowers ( tribological-properties-ws2-hexagonal-nanoplates-nanoflowers )

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Nanomaterials 2019, 9, 840 2 of 10 comprises other nanomaterials such as oxides, fluorides, and borides [16–20]. Among these different types of materials, transition metal chalcogenides have received great attention due to their special layered structure. WS2, as an important member of the transition metal chalcogenide material family, has attracted great attention for its intriguing electronic, electrochemical, and electrocatalytic properties, and for its extensive applications in field-effect transistors, energy storage, catalysis, and hydrogen storage media [21–28]. In addition, WS2 is an excellent solid lubricant due to its special layered structure, which is composed of strong S–W–S covalent bonds inside the layers, and the weak van der Waals force between the layers. The easy sliding between WS2 layers under small shear forces is often regarded as an important feature of its excellent lubricity [29]. Recently, WS2 nanomaterials with different morphologies have been synthesized, and their tribological properties and antifriction mechanisms have been studied. For example, Tenne et al. [11] investigated the tribological properties of fullerene-like WS2 nanoparticles as additives in a lubricating oil under harsh conditions, and the results showed that WS2 nanoparticles play a major role in alleviating friction and wear. Wu et al. [30] synthesized hollow WS2 spheres by a solvothermal process and compared their tribological properties with commercial colloidal MoS2 as an additive in liquid paraffin. Zhang et al. [12] prepared WS2 nanorods by using a self-transformation process and investigated the tribological performance of WS2 nanorods as an additive in lubricating oil. It was found that the antiwear ability of the base oil was improved by the addition of WS2 nanorods. Hu et al. [31] studied the tribological properties of WS2 and WS2/TiO2 nanoparticles dispersed in diisooctyl sebacate and found that the two nano-additives slightly affected the friction reduction effect, but WS2/TiO2 nanoparticles were found to remarkably improve the wear resistance of diisooctyl sebacate. All of the above studies have shown that WS2 nanomaterials with different morphologies help to improve the tribological properties of lubricating oils. However, these studies have only investigated the tribological properties of WS2 nanomaterials with a single morphology, and did not explore the antifriction properties and mechanisms of WS2 nanomaterials with a different morphology under the same working conditions. Previous studies investigating MoS2 nanomaterials have demonstrated the complex relationship between the morphology size and tribological properties of MoS2. For example, Xu et al. reported that the lubricity of the sheet-like nano-MoS2 is inferior to that of the micro-scale MoS2 in rapeseed oil [32]. However, Raboso et al. reported that the size and morphology of MoS2 did not have a significant effect on the friction and wear of the polyalphaolefin oil [33]. Therefore, it is valuable to study the tribological properties and friction reduction mechanism of WS2 nanomaterials with different morphologies. In this study, two different morphologies of WS2 nanomaterials—WS2 hexagonal nanoplates and nanoflowers—were synthesized by a different high-temperature solid-phase reaction process. The tribological properties of the two kinds of WS2 nanomaterials as additives in the paraffin oil were also investigated. 2. Materials and Methods 2.1. Reagents and Materials Tungsten and sulfur powders were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Tungsten trioxide and thiourea were obtained from the Aladdin Chemical Reagent Company (Shanghai, China). All chemical reagents were used directly without further purification. 2.2. Synthesis of WS2 Hexagonal Nanoplates In a typical method, high-purity tungsten and sulfur powder (W:S molar ratio of 1:3, S powder excess 50%) were poured into a steel kettle, and then the powders were mechanically ground with a speed of 300 rpm (rotations per minute) in a planetary ball mill for 12 h. Then, the ball-milled mixture was transferred into a stainless-steel reactor. The reactor was tightly closed and pushed into the middle

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