## 3 Results and Discussion
### Wear Mechanisms  
In this study, SEM morphologies of 90 N load worn surfaces were captured, as shown in Figure 10. Figure 10b,d,f show the enlarged SEM micrograph of the circle area of the dotted line referred in Figure 10a,c,e, respectively. When combined with the energy spectrum analysis results (shown in Figure 11), the morphologies exhibited that there were five different wear operating mechanisms: abrasion, oxidation, delamination, plastic deformation, and adhesion. Due to the easily oxidized nature of aluminum, frictional heating during sliding under high load that was caused the oxide film on the surface of the pin sample generated. Studies by [PERSON] et al. [27] and [PERSON] et al. [28] showed that the existence of the oxide film improved the wear behavior and reduced the friction coefficient because of the lubrication effect. At the same time, the oxide film  
Figure 9: Friction coefficient for samples at various Sliding velocity under a normal load of 60 N; (**a**) the average friction coefficient, (**b**) the friction coefficient in time, 0.188 m/s, (**c**) the friction coefficient in time, 0.377 m/s, (**d**) the friction coefficient in time, 0.565 m/s. prevented metal-to-metal contact, resulting in a low wear volume. EDS analysis of areas A and B, marked in Figure 10c,d, were revealed in Figure 11a,b. By contrast, more Fe was clearly present in the A area. The white powder (Figure 10c,d) represented the Fe-rich wear debris, and also consisted of some other elements, such as: O, Al, Si, and Cu. The wear debris was closely related to abrasive wear. Figure 10: SEM images of samples under a normal load of 90 N at sliding speed of 0.188 m/s. (a,b) Al-5 Si-1 Cu-0.5 Mg alloy, (c,d) SiCP/Al-5 Si-1 Cu-0.5 Mg composites before T6 treatment and (e,f) after T6 treatment. Because of its fragile nature, the oxide film tended to be broken into fragments at high applied loads during the sliding process. The oxide fragments did not prevent metal-to-metal contact, and thus a high wear volume occurred. Wear behavior of the material was largely dependent on the microstructure of subsurface [29]. As Figure 10a,b shows, delaminating grooves can be distinguished, as well as plastic deformations around the grooves. Such a feature may often be linked with delamination wear, which is the result of cracks beneath the surface that grow and eventually join each other and eventually extend to the pin surface, leading to partial detachment of the wear layers [30; 31]. As for the as-cast Al-5 Si-1 Cu-0.5 Mg alloy, at the applied load of 90 N, a combination of delamination and plastic deformation was the dominant wear mechanism, and oxidation wear and abrasion wear also contributed to the wear. As Figure 10c,d shows, many white powders (Fe-rich mechanical mixture), smooth flat surface (plastic deformation), and slight delamination wear were observed on the wear surface. These features indicated that oxidation wear and abrasion were the dominant wear mechanisms, and that plastic deformation and delamination also contributed to the wear. It can be concluded that SiCp improves the wear resistance of the matrix. Figure 10e,f show the SEM image of the SiCp/Al-5 Si-1 Cu-0.5 Mg alloy with T6 treatment under a load of 90 N. Besides some tiny scratches, a large area of smooth grooves appear on the wear surface due to the plastic deformation. There are no cracks on the grooves. It indicates that the worn surface could guarantee smooth sliding after plastic deformation. When compared with the SiCp/Al-5 Si-1 Cu-0.5 Mg alloy without T6 treatment, the wear resistance was significantly improved. The wear mechanisms were plastic deformation and abrasion wear. As shown in Figure 7a,c,e, oxides, plastic deformation and grooves were on the worn surface of alloys at a sliding speed of 0.377 m/s and the load of 60 N. These features indicated that oxidation wear and abrasion were the dominant wear mchanisms, and that plastic deformation and delamination also  
Figure 11: EDS maps of (**a**) area A and (**b**) area B.  
contributed to the wear.