## 3 Results and Discussion
### Lubrication Mechanism of WS\({}_{2}\)  
From the experimental results, compared with the base oil PAO6, the addition of 2H-WS\({}_{2}\) particles can reduce the friction coefficient, the maximum can be reduced by 35.4%, and the wear scar is relatively light and the furrows are shallow. The lubrication mechanism discussed below when multiscale 2H-WS\({}_{2}\) as additives are added alone in base oil. The schematic illustration of the lubrication mechanism under lubrication conditions of containing 2H-WS\({}_{2}\) is shown in Figure 14a. When the two friction surfaces were in contact, 2H-WS\({}_{2}\) deposited on the concave surface of the metal surface to fill the surface, thereby reducing the relative surface roughness and friction between sliding surfaces. The deposited 2H-WS\({}_{2}\) particles on the worn surface can also decrease the shearing stress, and hence reduce friction and wear. With the increase of friction time, the metal surface undergoes a chemical action to produce an iron oxide film that can effectively prevent direct contact between the metal surfaces, thereby significantly reducing wear between the metal surfaces. However, no obvious tungsten oxide or iron sulfide appeared during the whole friction process the Figure 11, and this is different from the results of the reference [20]. In this reference, [PERSON] et al. found that small quantities of WO\({}_{3}\) and iron sulfides were formed in the tribofilm. The reason for the different results may be due to the fact that the reaction temperature or load in this test cannot meet the reaction requirements. Therefore, at room temperature, the 2H-WS\({}_{2}\) used as a lubricant additive has friction-reduction and anti-wear effects mainly in the form of deposited films, and there is no oxidation reaction of the 2H-WS\({}_{2}\), and the chemical film is mainly composed of iron oxide in the process of friction. Moreover, the 2H-WS\({}_{2}\) with different sizes have different results under the same experimental conditions. In view  
Figure 13: Chemical mapping by EPMA of the wear track obtained when using 2H-WS\({}_{2}\) and ZDDP in oil, to lubricate the rubbed surfaces (40 mm/s, PAO+2H-WS\({}_{2}\)+ZDDP), (**a**) abrasion trace; (**b**) Fe; (**c**) C; (**d**) O; (**e**) W; (**f**) S; (**g**) P; (**h**) Zn. of the friction coefficient, small-sized 2H-WS\({}_{2}\) exhibit better friction-reducing properties, probably because small-sized 2H-WS\({}_{2}\) are more easily deposited on the concave surface, filling the surface, thereby reducing the relative surface roughness and the friction between sliding surfaces. In addition, small-sized 2H-WS\({}_{2}\) are more easily embedded, and they repaired the metal surface after scratched. The presence of 2H-WS\({}_{2}\) particles and ZDDP in PAO produce a greater reduction of friction coefficient and exhibit a better lubricating performance, according to the experimental results, and the maximum reduction is 47.2% compared with that of the base oil. In addition, the wear is the lightest, and there are no obvious furrows. When 2H-WS\({}_{2}\) and ZDDP are both added as additives into the base oil, the anti-wear and anti-friction properties of the lubricating oil are improved again, and the lubrication mechanism is discussed as follows. The schematic illustration of the lubrication mechanism under lubrication conditions of containing both 2H-WS\({}_{2}\) and ZDDP is shown in Figure 14b. Due to the presence of ZDDP, the rubbed surface will quickly form a tribofilm composed of zinc phosphate compounds [21; 22]. The chemical tribofilm formed by ZDDP could also have acted as a spacer to keep metal surfaces separated, which can also prevent oxidation of the metal surfaces and 2H-WS\({}_{2}\) because of its excellent oxidation resistance. Thereby, the metal surface is well protected, and the wear is reduced. The content of oxygen element in Figure 13d is obviously smaller than that in Figure 11d, indicating that the existence of ZDDP greatly reduces the formation of iron oxide film.