## 3 Results and Discussion
### Tribological Properties  
#### 3.3.1 Friction Coefficient and Wear Rate  
To quantitatively assess the high-temperature tribological performance of the Al-Si-(Cu)-Mg alloys, the average friction coefficient (COF) and wear rate were measured at 25 \({}^{\circ}\)C, 150 \({}^{\circ}\)C, and 250 \({}^{\circ}\)C (Figure 6a,b). The tests aimed to elucidate the influence of the microstructural stability, interfacial behavior, and thermal effects on tribological responses. Both COF and wear rate values were determined based on the steady-state region, excluding the initial run-in period.  
The wear rate was calculated using Equation (1):  
\[\textit{Wear Rate }(WR)=\frac{V}{F\cdot d} \tag{1}\]  
where \(V\) is the wear volume (mm\({}^{3}\)), \(F\) is the applied load (N), and \(d\) is the sliding distance (m).  
Figure 5: SEM images of fracture surfaces after tensile testing for Alloy A and Alloy B at different temperatures: (**a**) Alloy A at RT, (**b**) Alloy B at RT, (**c**) Alloy A at 150 \({}^{\circ}\)C, (**d**) Alloy B at 150 \({}^{\circ}\)C, (**e**) Alloy A at 250 \({}^{\circ}\)C, and (**f**) Alloy B at 250 \({}^{\circ}\)C.  
At room temperature, Alloy A exhibited a COF of 0.38 and a wear rate of 485.22 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m, whereas Alloy B showed a lower COF of 0.30 but a significantly higher wear rate of 659.44 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m. The relatively higher COF of Alloy A is attributed to the enhanced shear resistance caused by the lattice distortion due to the uniformly dissolved Cu, along with the effective stress dispersion provided by finely distributed \(\alpha\)-Al(FeMn)Si phases. In contrast, the presence of \(\beta\)-AlFeSi and agglomerated Mg\({}_{2}\)Si in Alloy B likely resulted in non-uniform interfacial bonding, leading to a lower COF and increased wear due to particle detachment.  
At 150 \({}^{\circ}\)C, the COF for both alloys increased to 0.41, reflecting greater surface plasticity and interfacial adhesion due to thermal softening. Despite this, Alloy A maintained a relatively low wear rate of 561.15 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m, supported by stable tribofilm formation and a preserved microstructure. Alloy B, although momentarily benefiting from the pseudo-lubricating effects of Mg\({}_{2}\)Si, exhibited a slightly higher wear rate of 590.59 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m, likely due to the progressive impact of the microstructural heterogeneity.  
Under high-temperature conditions (250 \({}^{\circ}\)C), the distinction in the tribological performance became more pronounced. The thermal softening of the Al matrix and the coarsening of intermetallics significantly influenced the wear behavior. Alloy A exhibited a moderate increase in the COF (0.43) and wear rate (615.03 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m), demonstrating relatively controlled degradation. In contrast, Alloy B experienced a severe decline in wear resistance, with the COF reaching 0.42 and the wear rate increasing sharply to 823.67 \(\times\) 10\({}^{-6}\) mm\({}^{3}\)/N\(\cdot\)m. This deterioration is attributed to the fragmentation of acicular \(\beta\)-AlFeSi phases under the thermal stress and surface cracking induced by coarsened Mg\({}_{2}\)Si particles, which facilitated crack propagation and material removal.