## 3 Results and Discussion
### Worn Surfaces  
As it can be seen in Figure 10, the scanning electron microscope (SEM) micrographs of worm out surfaces for AlSi5 Cu1 Mg + 0 wt %, 0.3 wt %, 0.6 wt %, 0.9 wt % La alloys subjected to 0.19 m/s sliding velocity and 3.8 MPa normal load. A lot of valuable information on the mechanism of wear can be provided by studying the worn surfaces [36]. The worn surface of matrix alloy tested at the sliding velocity of 0.19 m/s and the normal load of 3.8 MPa exhibiting parallel grooves and scratches together with rows of furrows and smearing in plastically deformed regions are shown in Figure 10a. These parallel grooves and scratches on the worn surface indicate the occurrence of abrasive wear, which resulted from abrasive contact between the hardened steel disc and aluminium pin. Plastic deformation of surface with rows of furrows is an indicator of adhesive wear. La content increased to 0.3 wt %. As shown in Figure 10b, the shallow craters disappeared, which means that the adhesion wear was reduced. When the content of La increased to 0.6 wt %, the craters on the worn surface almost disappeared, as shown in Figure 10c. It indicates that the wear mechanism of AlSi5 Cu1 Mg + 0.6 wt % La alloy tested at the sliding velocity of 0.19 m/s and the normal load of 3.8 MPa is mainly abrasive wear because of the maximum hardness, as shown in Figure 4. Nevertheless, by increasing the content of La to 0.9 wt %, as shown in Figure 10d, the adhesive wear appeared again, and the size of the craters is greater. This indicates that the wear mechanism of AlSi5 Cu1 Mg + 0.9 wt % La alloy tend to be adhesive, due to the deterioration of modification effect. When surfaces come into contact with each other, the films that naturally occur on surfaces often separate the atoms or molecules so there is no tendency for adhesion. However, if the force pushing contacting surfaces becomes sufficient locally at spots in the real area of contact, films can break down and the atoms or molecules of the mating surfaces can make atomic contact and atomic bonding can occur. Then, the surface of the material with less hardness does fall off and stick to the surface of the material with higher hardness. It is called a \"metal transfer\" [37] phenomenon. Furthermore, the hardness of alloy increases because of the modification by the rare earth element La, the hardness ratio of the steel-disk and the aluminium-pin decreases, so the wear mechanism tends to be abrasive wear, and the adhesive wear tends to be in a secondary position. The SEM micrographs of worn surfaces for AlSi5 Cu1 Mg + 0.6 wt % La alloy tested at different conditions are shown in Figure 11. As it can be seen in Figure 11a,b, with the increase of friction load, the characteristics of adhesive wear are more obvious. There is a large proportion of pits in the Figure 11b, and a clear gully is shown in Figure 11a. It was mainly due to rupture of the oxide film (as shown in Figure 11b region A; the EDS analysis of region A is shown in Figure 12) in the friction surface, the new exposed matrix aluminium came into direct contact with the disc under the higher normal load of friction condition. Figure 13 schematically shows the role of normal load for wear. In addition, the hardness of the oxide film was higher than that of the new exposed matrix aluminium. Asperities on the friction plate pierced into the alloys, promoting the formation of pits. Figure 9: Coefficient of friction (COF) of AlSi5 Cu1 Mg + 0.6 wt % La alloy: (**a**) 1.3 MPa normal load; (**b**) 3.8 MPa normal load; (**c**) 6.3 MPa normal load. Figure 10: SEM micrographs of worn out surfaces for (**a**) AlSi5 Cu1 Mg; (**b**) AlSi5 Cu1 Mg + 0.3 wt % La; (**c**) AlSi5 Cu1 Mg + 0.6 wt % La; (**d**) AlSi5 Cu1 Mg + 0.9 wt % La alloys subjected to 0.19m/s sliding velocity and 3.8 MPa normal load, and the high magnification micrographs are on their right side.