## 3 Results and Discussion
### Tribological Results
#### 3.2.2 Anti-Wear Properties  
After the experiment, the wear tracks on the disc were straight, and each wear track length was 15 mm. Based on the anti-friction results mentioned above, in Figure 7, a few wear scars were selected for comparison. Figure 8 shows the SEM micrographs of disc surfaces after rubbing at different velocities. Figure 6: Friction curves of microscale/nanoscale 2H-WS\({}_{2}\) in the presence of ZDDP additive: (**a**) 10 mm/s; (**b**) 20 mm/s; (**c**) 30 mm/s; and (**d**) 40 mm/s. Figure 7: Average friction coefficient measured for all sample oils and their variances. In Figure 8, the wear scars at 30 mm/s are not as clear as the wear scars at 40 mm/s in the case of the same additives, and the furrow depths are not as pronounced as the latter. It is clear that the wear becomes more serious with the increase in sliding velocities. This is because the increase of velocity is bound to increase the total sliding length of the steel ball at the same experimental time. Therefore, under the same test conditions, the greater the velocity, the more obvious the wear. As a reference, the wear scars with additive-free PAO are shown in Figure 8a,b. There are lots of obvious furrows and a slight spalling on the friction surface. It can be speculated that the friction pairs mainly undergo microploughing and adhesive wear during the friction process due to the fact that it is difficult to avoid the direct contact between many asperities of the friction pair. The friction surface is prone to plastic deformation under a certain positive pressure, and the friction and wear is further aggravated when the abrasive particles enter the lubricating medium during the friction process. Compared with this, the abrasive wear is relatively light, and the furrows are shallow in the wear scars with adding 2H-WS\({}_{2}\), and the surface is more smooth (Figure 8c,d). This shows that the addition of 2H-WS\({}_{2}\) has a positive effect on reducing wear. The 2H-WS\({}_{2}\) are most likely to form a protective film on the friction surface, thereby reducing the direct contact between the friction pairs. Figure 8: SEM micrographs of disc surfaces after rubbing in (a) 30 mm/s PAO; (b) 40 mm/s PAO; (c) 30 mm/s PAO+2H-WS\({}_{2}\); (d) 40 mm/s PAO+2H-WS\({}_{2}\); (e) 30 mm/s PAO+2H-WS\({}_{2}\)+ZDDP; and (f) 40 mm/s PAO+2H-WS\({}_{2}\)+ZDDP. However, when the 2H-WS\({}_{2}\) and ZDDP are added at the same time, the wear is the lightest, and there are no obvious furrows. In addition, as can be seen from Figures 9 and 10, the wear width is much smaller than the addition of 2H-WS\({}_{2}\) alone at the same magnification. It can be inferred that the fatigue wear and galling are the main wear forms between friction pairs. The rubbed surfaces are separated by the friction-reaction film and deposition film during the friction process. Therefore, it can be concluded that the presence of 2H-WS\({}_{2}\) in lubricating oil can reduce wear and play a certain role in anti-wear. Moreover, the presence of ZDDP greatly improves the anti-wear properties of lubricating. These results confirm that the anti-wear performance is greatly enhanced under the combined action of 2H-WS\({}_{2}\) and ZDDP. The wear scars were characterized with a three-dimensional topography instrument, and the results are shown in Figure 9, from which the wear volume can be obtained. The wear volume of the wear scars of the sample oil at 10 mm/s and 30 mm/s are shown in Figure 10. In terms of sliding speed, the wear volume at 30 mm/s is much larger than that at 10 mm/s, almost 3-5 times more than that at 10 mm/s. At the same sliding speed, the wear volume decreases with the increase of 2H-WS\({}_{2}\) and ZDDP, indicating that the additives played a role in reducing wear. At a sliding speed of 10 mm/s, the wear volume of PAO was 0.117 mm\({}^{3}\). Compared with this, the maximum wear volume of the sample oil containing WS\({}_{2}\) was reduced by 61.16%, while the maximum wear volume of the sample oil with 2H-WS\({}_{2}\) and ZDDP was reduced by 74.38%.