## 3 Results and Discussion
### Wear Resistance of G/6061 Al Composite  
Table 2 lists out the friction coefficient and the wear rate of G/6061 Al bearing 10 vol.% graphite. Figure 9 shows the SEM micrograph of the worn surface and EDS elemental maps of the regions marked by a white box in the worn surface. Table 2 manifests that wear rate of the HP sintered is the highest and that of the UHP-sintered is about one to three orders of magnitude lower. When the sintering temperature is raised from 450 \({}^{\circ}\)C to 550 \({}^{\circ}\)C at 5.2 GPa, the wear rate is increased from \(12.45\times 10^{-5}\) g/mm to \(17.92\times 10^{-5}\) g/mm. Raising the sintering temperature to 650 \({}^{\circ}\)C or higher, the wear rate of the UHP-sintered is substantially decreased, listed in Table 2. For example, the wear rate of the UHP sintered at 750 \({}^{\circ}\)C is \(0.15\times 10^{-5}\) g/mm, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Processing Parameters**} & \multicolumn{4}{c}{**UHP Sintered4 5.2 GPa**} & \multirow{2}{*}{**HP Sintered 35 MPa**} \\ \cline{2-2} \cline{5-6}  & **450 \({}^{\circ}\)C** & **550 \({}^{\circ}\)C** & **650 \({}^{\circ}\)C** & **750 \({}^{\circ}\)C** & **and 520 \({}^{\circ}\)C** \\ \hline Friction coefficient & 0.484 & 0.483 & 0.595 & 0.544 & 0.455 \\ Wear rate (\(10^{-5}\) g/mm) & 12.45 & 17.92 & 0.97 & 0.15 & 133.89 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Friction coefficients and wear rates of the G/6061 Al with 10 vol.% graphite. Figure 8: Fracture surfaces of **(a,b)** the 0 vol.% UHP and HP-sintered, and **(c,d)** that of the 5 vol.% G/6061 Al corresponding. which is only about 1% of that at 550 \({}^{\circ}\)C. In addition, it is also shown that the wear resistance of the UHP-sintered is proportional to its hardness. SEM observation shows that plastic deformation of 6061 Al exists on the worn surface during the wear test, seen in Figure 9. As wear progresses, partly adhesive delamination of the deformed 6061 Al layer occurs on the surface, which is the mark of adhesion wear. There are also grooves, spalling pits and wear debris on the worn surfaces, marking the abrasion wear. Figure 9a also shows that the delamination is serious on the worn surface of the UHP sintered at 550 \({}^{\circ}\)C and 5.2 GPa. With the elevation of the sintering temperature, the composite presents better resistance to delamination and spalling, shown in Figure 9b. The reason is that the G/6061 Al is more compact and harder (partly caused by Al\({}_{4}\)C\({}_{3}\)) when it is UHP-sintered at a temperature of 650 \({}^{\circ}\)C or 750 \({}^{\circ}\)C. It is also evident that the delamination is the main factor of weight loss of the UHP-sintered. Table 2 also exhibits that the friction coefficients are lower, but the wear rates are much higher when the 10 vol.% G/6061 Al composite is respectively HP-sintered and UHP-sintered at a temperature not higher than 550 \({}^{\circ}\)C. Elemental map provides that there is graphite distributed on the worn surface of the UHP sintered at a low temperature and the HP-sintered, seen in Figure 9b,c. Furthermore, a large amount of uniform graphite is found on the worn surface of the HP-sintered, of which the relative density is very low, shown in Section 3.2. Then, it can be induced that the lower the density of the composite, the easier the graphite will fall off and the better the lubrication will be. However, it is not that the lower the friction coefficient, the lower the wear rate. The relative density and hardness also have a great influence on the wear rate. The literature [3] also reported a similar law, in which TiC particles were considered to act as reinforcement in the Al/TiC/graphite composite and increase the wear resistance of the Al/TiC/graphite composite. Figure 9: SEM micrographs and EDS surface scanning results of worn surfaces of the composites bearing 10 vol.% graphite sintered at (**a**) 5.2 GPa and 550 \({}^{\circ}\)C, (**b**) 5.2 GPa and 750 \({}^{\circ}\)C and (**c**) 35 MPa and 520 \({}^{\circ}\)C, respectively.