## 3 Results and Discussion
### Wear Rate  
The wear rates variation versus the contents of La after a dry sliding distance of 114 m (equivalent to 600 s \(\times\) 0.19 m/s) is shown in Figure 6. The wear rate was defined as the volume loss of a material by wear per unit sliding distance. Because the wear rate depends just like the friction coefficient on a lot of parameters, this factor is to be found experimentally, and studying the wear mechanism by this quantity is useful in comparing wear rates for different categories of materials. The wear rate of the alloys with La addition were smaller than the matrix alloy, and it could indicate that the alloys with La addition exhibited improved wear resistance compared with the matrix alloy. Then, the AlSi5 Cu1 Mg + 0.6 wt % La alloys exhibited the least amount of wear rate. In addition, the wear rate of AlSi5 Cu1 Mg + 0.6 wt % La alloy decreased 48% further than the one of a matrix alloy under the normal load of 1.3 MPa. The effect of the contents of La on wear rate can be explained by using Equation (2) [33] as follows:  
\[wear\quad rate=\frac{RW}{3H}, \tag{2}\]  
where \(k\) is the wear coefficient and represents the wear intensity, \(W\) is the normal load, and \(H\) is the average hardness. The addition of rare earth element La made the grain refinement and improved the microhardness of the alloys. Then, the wear rate of the alloys decreased and the wear resistance of the alloys increased.  
Figure 5: EDS analysis of AlSi5 Cu1 Mg + 0.9 wt % La alloy.  
The wear rate of the samples under different normal loads at the sliding velocity 0.19 m/s is shown in Figure 7a. As shown, the wear rate was positively correlated with the normal load. It demonstrates that the wear resistance of the alloy deteriorated with increasing normal load. This is consistent with [PERSON]'s experiment [20]. [PERSON] stated that the change in slope of wear rate vs. normal load curve for as-cast alloy indicates the transition in the mode of wear from mild oxidative regime to severe metallic wear regime. The wear rates of AlSi5 Cu1 Mg + 0.6 wt % La alloy at different sliding velocity are shown in Figure 7b. With the increase of sliding velocity, the wear rate of alloy increases. It indicates that the wear state of AlSi5 Cu1 Mg + 0.6 wt % La alloy deteriorates as the increase of friction sliding velocity. This was because the frictional heat generated at a sliding interface. Steady state bulk temperature at the interface is given by Equation (3) [7]:  
\[T_{b}=T_{a}+\frac{\mu F\
u al_{b}}{A_{n}k_{m}}, \tag{3}\]  
where \(\mu\) is coefficient of friction; \(F\) is the normal force normal on the pin sample; \(v\) is sliding velocity; \(A_{n}\) is nominal area of contact; \(\alpha\) is fraction of heat diffusing to pin; \(K_{m}\) is thermal conductivity; \(T_{o}\) is sink temperature; and \(l_{b}\) is mean diffusion distance. When two surfaces slide against each other and work is done against friction, heat is generated at or very close to the sliding interface. This heat energy represents nearly all of the frictional work, since only a small proportion of it is stored in dislocation or other structural defects, or is used in the creation of new surfaces in debris particles or cracks. It is therefore easy to see that there will be a temperature rise near the contact. With the increase of sliding velocity, the temperature of the frictional surface rises. The hardness of metal material usually decreases with the increase of temperature [34].  
Figure 6: Wear rates variation under the normal loads of 1.3, 3.8, 6.3 MPa, and at a sliding velocity of 0.19 m/s versus the contents of La.