## 3 Results and Discussion
### Wear Behavior
The reason for the reduction of the COF in the composite added with 5% B\({}_{4}\)C can be related to the increased hardness of the composite (Figure 11) as finer Mg\({}_{2}\)Si particles can support more load, and decrease the contact area between the pin and the sliding disc. This indirectly prevents the pin surface from acquiring more scratches. In addition, [PERSON], et al. [47] reported that the segregation of boron which acted as a dry lubricant between the sliding surfaces, leads to a decreased friction coefficient. However, with a further increase in the B\({}_{4}\)C content to 10%, the COF increases to 0.45. Due to the agglomeration of B\({}_{4}\)C particles in 10 wt% B\({}_{4}\)C hybrid composite, the minority of the Mg\({}_{2}\)Si particles appeared in coarse size. The fracture of the large and brittle Mg\({}_{2}\)Si could initiate the crack propagation in the matrix, and small Mg\({}_{2}\)Si particles are pulled out from the matrix, which further acts as the source for abrasive wear, causing the COF to increase. The increasing COF could also is due to the fragmentation of boron oxide layer. B\({}_{2}\)O\({}_{3}\) layer occurred when the pulled out B\({}_{4}\)C particles reacted with the atmosphere. Its delamination during dry sliding formed scratches and grooves on the pin surface that increased the friction between the pin and the sliding surface [48]. An increase in the applied load to 40 N increases the COF of the composites compared to the 20 N applied load; however, the Al-20%Mg\({}_{2}\)Si composite added with 5 wt% B\({}_{4}\)C owns the lowes COF (0.52) compared to other fabricated composites and addition of 10 wt%  
Figure 15: Coefficient of friction of Al-20%Mg\({}_{2}\)Si composite reinforced with different amount of B\({}_{4}\)C particles. B4C incresed the COF to 0.54, which is similar with the trend of COF of the composites under 20N. Figure 16a-f demonstrates the worn surface of the fabricated composites added with 0, 5, and 10 wt% B4C particles under the applied loads of 20 and 40 N. The worn surface of Al-Mg2 Si composite (Figure 16a) displays several continuous grooves parallel to the sliding direction. Moreover, the counterfaces between the pin and the sample surface increased; hence, more materials were detached from the sample's surface. This detachment may be due to the adhesive friction that subsequently increased the wear rate (Figure 14). The principal wear mechanisms of the samples were abrasion and delamination wear. By increasing the load to 40 N, delamination and adhesion govern the wear mechanisms, in which extensive pits and large size of delamination flakes exist on the worn surface, indicating the alteration of wear mechanism to severe wear (Figure 16b). With the addition of 5 wt% B4C particles to the Al-20%Mg2 Si composite, the delamination pit areas as well as the groove width decreased, illustrating a mild abrasive wear mechanism, as shown in Figure 16c. This is consistent with decreasing wear rate and coefficient of friction results depicted in Figures 14 and 15, respectively. Figure 16: SEM micrographs of the worn surface of Al-Mg2 Si composite samples with various B4C contents (**a,b**) 0 wt%, (**c,d**) 5 wt%, and (**e,f**) 10 wt% under 20 N (left) and 40 N (right) applied loads. On the other hand, as observed in Figure 16e, by adding 10 wt% B4C to the Al-20%Mg2 Si composite, deep parallel grooves with large widths indicate a severe abrasion, which is regularly produced by rigid particles between the pin and disc that cut into the worn surface. As observed in Figure 3d, the introduction of a high percentage of B4C particles (10 wt%) to the Al-20%Mg2 Si composite caused the B4C particles to cluster in some regions in the matrix with a non-uniform distribution. Therefore, during the sliding wear test, the agglomerated B4C particles were pulled out from the matrix, fragmented, and became entrapped between the pin and disc. As a result, abrasion took place, leading the material to be omitted from the abrasion groove Figure 16e.