## 3 Results and Discussion
### Tribological Results  
#### 3.2.1 Friction-Reduction Properties  
The friction coefficient curves of PAO and two sizes of 2H-WS\({}_{2}\) particles at four sliding velocities are plotted in Figure 5. The purpose of the test is to investigate the lubricating properties of the synthesized 2H-WS\({}_{2}\) in PAO. The tribological behavior of 2H-WS\({}_{2}\) in the presence of the ZDDP additive in PAO was also evaluated, and the results are shown in Figure 6. Moreover, the important fluctuations in friction experiments can be observed in Figures 5 and 6. In addition, in order to reflect the friction coefficients more intuitively at different sliding velocities, the average friction coefficients and the variances of all sample oils are reported in Figure 7. It shows that the friction coefficient increases gradually with the increase of the sliding velocity. Friction coefficients ranged from 0.138 to 0.175 when only PAO was added, whereas the friction coefficients of sample oils containing additives are all lower than that of base oil under the same test conditions. Therefore, the frictional benefits of the 2H-WS\({}_{2}\) and ZDDP are observed compared to PAO alone in Figure 7.  
In Figure 5, the lubrication performance becomes more stable as the sliding velocity increases. Figure 7 reveals that the maximum reduction of friction coefficient is 31.3% when the 500 nm of 2H-WS\({}_{2}\) was added to base oil, and the maximum reduction of friction coefficient is 27.4% in the case of adding 2 \(\upmu\)m of 2H-WS\({}_{2}\) compared with that of base oil. Thus, the following conclusions can be drawn according to the experimental results: Firstly, the addition of 2H-WS\({}_{2}\) in base oil resulted in a significant reduction in the friction coefficient under appropriate working conditions. Secondly, in general, the sample oil containing smaller size 2H-WS\({}_{2}\) particles showed a better lubricating effect. Figure 6 shows the friction curves of the sample oil containing both ZDDP and 2H-WS\({}_{2}\), and then compares them to the curve with better lubrication effect in Figure 5. The addition of ZDDP further reduces the friction coefficient compared to the addition of 2H-WS\({}_{2}\) alone, as shown in Figure 6. When we combine this with Figure 7, we can see that the maximum reduction in friction coefficient was 35.4% when the 500 nm of 2H-WS\({}_{2}\) and ZDDP were both present, and the maximum reduction of friction coefficient was 47.2% in the case of adding both 2 \(\upmu\)m of 2H-WS\({}_{2}\) and ZDDP. Therefore, some conclusions can be drawn: Firstly, the presence of 2H-WS\({}_{2}\) and ZDDP in PAO can cause greater reduction of friction coefficient and exhibit a better lubricating performance, and there is a positive interaction between the two additives. Secondly, in the presence of ZDDP, the 2H-WS\({}_{2}\) particles with larger size have the better friction-reduction performance compared with that of other sample oils. Figure 4: SEM images of micro-WS\({}_{2}\) at different magnifications, (**a**) 5000\(\times\); (**b**) 8000\(\times\). Figure 5: Friction curves obtained for the reference sample oil (PAO) and the microscale/nanoscale 2H-WS\({}_{2}\) in PAO, both with respect to four sliding velocities: (**a**) 10 mm/s; (**b**) 20 mm/s; (**c**) 30 mm/s; and (**d**) 40 mm/s. Figure 6: _Cont._  
The average friction coefficient and its variance of each sample oil at four sliding velocities are shown in Figure 7. The friction-reduction effect of the sample oil containing additives was different at four sliding velocities. Some of them had a poor friction-reduction effect, while others had a very good one. For example, at the sliding velocity of 20 mm/s, the anti-friction effect was not prominent, as the friction coefficients were reduced only by 4.9-15.3% compared with that of PAO alone. However, at 30 and 40 mm/s sliding velocities, the additives apparently played a positive role in reducing friction, as the friction coefficients were greatly reduced by 22.7-47.2%.