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Nanomaterials 2019, 9, 840 3 of 10 of a tube furnace. The temperature of the tube furnace was raised to 650 ◦C at a rate of 5 ◦C min−1 in an atmosphere of N2 and the temperature was maintained at 650 ◦C for 2 h. Subsequently, the tube was gradually cooled to room temperature and the prepared powders were then obtained. 2.3. Synthesis of WS2 Nanoflowers The WS2 nanoflowers were synthesized according to the previous method [34] reported by us with minor modifications. This included 10 mmol of WO3, 60 mmol of sulfur powder and 140 mmol of thiourea were ground in a mortar for 30 min. Then 3 g of the ground mixture was loaded in an alumina boat. This boat was pushed into the hot zone of the tube furnace. The furnace temperature was maintained at 850 ◦C for 1 h in N2 atmosphere and then gradually cooled to room temperature. 2.4. Materials Characterization The X-ray diffraction (XRD) pattern was recorded by a Shimadzu LabX XRD-6000 X-ray diffractometer using a Cu Ka X-ray source operating at 40 kV and 30 mA with a scanning range of 10◦ to 80◦. A JSM-7001F field-emission scanning electron microscope (FESEM) and a JEM-2100 transmission electron microscope (TEM) were used to record the sample morphology. 2.5. Tribological Properties Test A UMT-2 tribotester (CETR, San Jose, CA, USA) was used to measure the tribological properties of the two WS2 samples. The prepared WS2 powders were dispersed into the paraffin oil by ultrasonic dispersion for 60 min which then resulted in the required lubricating oil with different WS2 contents. The tribological properties tests were performed in ball-and-disk mode with a load of 10–60 N and a rotational speed of 100–400 rpm for 30 min. The friction pair consisted of a ball with a diameter of 10 mm and a disc with size of Φ40 mm × 3 mm. The fixed upper sample (ball) is made of GCr15 bearing steel (AISI 52100) with a hardness of 62 HRC (Rockwellhardness) and the rotating lower sample (disk) is made of 45# steel. The surface of the steel disc was polished and cleaned with acetone before the test. The friction coefficient was automatically recorded during the contact friction, and the widths of the wear scars were measured by an optical microscope. The morphologies and elements of the wear scars on the surface of the lower disc were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). 3. Results and Discussion 3.1. Structure and Morphology Characterization The crystal structure and phase purity of the synthesized samples were verified by the XRD patterns, as presented in Figure 1. From the image, it can be seen that the diffraction patterns of the two samples were significantly different. The diffraction peaks of the nanoplates located at 14.30◦, 28.84◦, 32.74◦, 33.50◦, 39.50◦, 43.90◦, 49.70◦, 58.40◦, 59.80◦, 60.48◦, and 66.50◦ were assigned to the (002), (004), (100), (101), (103), (006), (105), (110), (008), (112), and (114) planes of WS2, respectively. Furthermore, a high and sharp (002) peak was observed from the XRD pattern, indicating that the WS2 nanoplates were stacked together with a highly ordered packing [35]. In contrast, only (002), (100), (101), and (110) peaks could be detected in the diffraction pattern of the nanoflower sample. Besides, the intensity of the (002) peak located at 13.76◦ was significantly weakened, and its position was shifted to the left by 0.56◦ from the standard card. This indicates that the number of stacks in the (002) layer of the WS2 nanoflowers was reduced, and the layer interval became larger [36]. All the diffraction peaks of the two patterns could be indexed to the hexagonal phase (p63/mmc space group) of WS2 (JSPDS No. 08-0237). No evidence of any other phases was detected, indicating that the samples were of high purity.PDF Image | Tribological Properties of WS2 Hexagonal Nanoplates Nanoflowers
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