## 3 Results and Discussion
### Wear Rates  
As shown in Figure 5, the wear rates for the three alloys are plotted against applied loads. The wear rates increased with load. The wear rate of the as-cast Al-5 Si-1 Cu-0.5 Mg alloy was the largest among the ones of the tested alloys. At low load (15 N), the wear rate of the as-cast SiCp/Al-5 Si-1 Cu-0.5 Mg \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Samples** & **\(\rho\)/(g:cm\({}^{-3}\))** & **Porosity/\%** & **Microhardness/HV** & **UTS/MPa** \\ \hline Al-5 Si-1 Cu-0.5 Mg & 2. 695 & 0.55 & \(80.17\pm 3.50\) & \(191.56\pm 4.77\) \\ Al-5 Si-1 Cu-0.5 Mg + SiCp & 2. 699 & 0.61 & \(87.06\pm 4.39\) & \(238.62\pm 5.19\) \\ Al-5 Si-1 Cu-0.5 Mg + SiCp + T6 & 2. 698 & 0.65 & \(101.86\pm 1.59\) & \(273.64\pm 6.47\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Some properties of the samples used in the wear tests. Figure 4: (**a**) TEM image and (**b**) EDS (energy dispersive spectroscopy) pattern of precipitates of SiCp/Al-5 Si-1 Cu-0.5 Mg composite after aging treatment. Figure 3: Microstructures of (**a**) Al-5 Si-1 Cu-0.5 Mg alloy as cast, (**b**) SiCp/Al-5 Si-1 Cu-0.5 Mg composites, and (**c**) T6 heat-treated SiCp/Al-5 Si-1 Cu-0.5 Mg composites. showed almost the same as the one of the base alloy. However, when the load upped to 90 N, the wear rate of the as-cast SiCp/Al-5 Si-1 Cu-0.5 Mg was only about half of the one of the matrix alloy. The results indicated that the addition of SiCp significantly improved the wear resistance of the Al-5 Si-1 Cu-0.5 Mg alloy. Especially, treated by T6, the SiCp/Al-5 Si-1 Cu-0.5 Mg alloy achieved further improvement of the wear resistance. The wear rates of the three alloys were inversely proportional to the micro-hardness of the alloys, which was consistent with the Achard's law [22]. It has been reported that the wear behavior of the alloys was affected by their microstructure and mechanical properties [23]. As mentioned before, the as-cast Al-5 Si-1 Cu-0.5 Mg alloy microstructre consisted of an \(\alpha\)-Al matrix that was surrounded by the eutectic Si phase of the needle or lamellar shape, and the \(\gamma\) phase of the intensive bone shape. This configuration produced stress concentration on the boundary of the \(\alpha\)-Al matrix, and the fracture occurred on the boundary interface, which caused the wear debris to fall out. With appropriate addition of SiCp, the shape of eutectic silicon was changed to a fine acicular or short stick or even a granular-like shape, and the \(\gamma\) phase was also distributed uniformly. Therefore, the interface bonding strength of the secondary phase with the matrix was reinforced, the mechanical and wear resistance property was improved, and the wear rate was decreased. Treated by T6, the eutectic silicon was spheroidal in shape, and some light grey \(\gamma\) phases were precipitated. As a result, the wear resistance property was improved. Figure 6 illustrates that the wear rate decreases with the sliding speed increasing. As the sliding velocity increased, the worn surface became smooth and the amount of delaminating groove was decreased (Figure 7). At a higher sliding velocity, more frictional heat was generated. The heat generation could increase the plastic deformation on the worn surface [23]. The deformation improved the hardening of the alloys. Therefore, the wear resistance property was improved. Figure 5: Wear rate values for samples under various applied loads. Figure 6: Wear rate values for samples at various Sliding velocity under a normal load of 60 N.  
Figure 7: Worn surfaces of different alloys under a normal load of 60 N at different sliding speeds; (**a**) Al-SSi-1 Cu-0.5 Mg, 0.377 m/s, (**b**) Al-5 Si-1 Cu-0.5 Mg, 0.565 m/s, (**c**) SiCp/Al-5 Si-1 Cu-0.5 Mg, 0.377 m/s, (**d**) SiCp/Al-5 Si-1 Cu-0.5 Mg, 0.565 m/s, (**e**) SiCp/Al-5 Si-1 Cu-0.5 Mg with T6, 0.377 m/s, (**f**) SiCp/Al-5 Si-1 Cu-0.5 Mg with T6, 0.565 m/s. It has been recognized that under the condition of sliding with loading, strain hardening will be developed.