## 3 Results and Discussion
### Friction Coefficient  
The relationships between the frictional coefficients and applied loads are shown in Figure 8. The friction coefficient of the alloy treated with T6 was minimal under the same test conditions. This value decreased to its minimum (0.3255) at the lowest load (15 N). The collected friction coefficient values of the tested alloys under different applied loads are listed in Table 3.  
[PERSON]'s model for friction [24] regards friction as the resistance of asperities on one surface riding over the asperities of the matching surface. Under the same applied load, the friction coefficient and wear volume of the as-cast Al-5 Si-1 Cu-0.5 Mg were all at maximum levels. The result showed that the wear debris fell out significantly between the asperities of the matching surface. This debris prevented the contact surfaces from sliding smoothly, and it significantly increased the friction force. The wear volume of the alloy with T6 treatment was minimal, few wear debris fell out, and the friction coefficient was low. In general, there is a proportional relationship among the friction coefficient and the microstructure or mechanical properties of the material [23; 25].  
The friction coefficient values increased with the applied load. When the two surfaces interacted, contact does not take place over the entire surface area. The real contact area consists of very small contact points at asperity tips, which are called micro-contacts [26]. When load was applied on the sample, the micro-contacts needed to have enough plastic deformation to bear the applied normal load. The higher the load, the greater the extent of the plastic deformation, which led to larger tribo-surface  
Figure 8: Coefficient of friction (COF) values for samples under various applied loads.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Samples** & **15 N** & **30 N** & **60 N** & **90 N** \\ \hline Al-5 Si-1 Cu-0.5 Mg & 0.343 \(\pm\) 0.003 & 0.364 \(\pm\) 0.0025 & 0.390 \(\pm\) 0.0025 & 0.401 \(\pm\) 0.0025 \\ Al-5 Si-1 Cu-0.5 Mg + SiCp & 0.335 \(\pm\) 0.0025 & 0.358 \(\pm\) 0.0025 & 0.375 \(\pm\) 0.0025 & 0.388 \(\pm\) 0.0025 \\ Al-5 Si-1 Cu-0.5 Mg + SiCp + T6 & 0.326 \(\pm\) 0.0025 & 0.350 \(\pm\) 0.0025 & 0.368 \(\pm\) 0.0025 & 0. 373 \(\pm\) 0.003 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Detailed Coefficient of friction (COF) values for samples at various applied loads.  
removal. Consequently, the contact area between the friction pairs increased as the load increasing, as well as friction. From the view of [PERSON]'s model for friction, at high loads, wear debris increased and became lodged between the asperities that resisted the motion at the interface. As a result, the friction force and friction coefficient increased.  
Figure 9a illustrates that the friction coefficient values decreasing with the sliding speed increasing. As Figure 7b,d,f shows, smooth worn surface were observed on the samples without oxide layer. The smooth worn surface could be ascribed to the softening of the alloy at higher surface temperature during sliding at a high speed. The smooth worn surface led to a low friction coefficient value at a high sliding speed. Meanwhile, the fluctuation amplitude of the friction coefficient was reduced, as shown in Figure 9b-d.