# Microstructure and Properties of AA6061/SiCp Composites Sintered under Ultra High-Pressure
## 3 Results and Discussion
### Research on Wear Resistance  
#### 3.6.1 Effect of Volume Fraction of SiCp on Friction and Wear Properties  
Figure 15 shows that the wear rate of the AA6061/N-SiCp composite decreases gradually with the increase in the volume fraction of the N-SiC particles. When N-SiCp increases from 3 vol.% to 15 vol.%, the wear rate decreases from \(0.55\times 10^{-5}\) g/m to \(0.1\times 10^{-5}\) g/m, indicating the wear resistance is improved. This is consistent with the conclusions reported by [PERSON] et al. [36], who studied the dry sliding friction and wear properties of AA2009/20 vol% SiCp. Figure 14: SEM micrograph of the fracture surface of AA6061/5 vol.% N-SiCp composites produced sintered at 450 (a), 550 (b), 600 \({}^{\circ}\)C (c), respectively. Figure 13: Fracture morphology of AA6061/3 vol.% N-SiCp composite sintered at 550 \({}^{\circ}\)C.  
Figure 16 shows the friction and wear morphology of AA6061/N-SiCp composites with different content. For AA6061/3 vol.% N-SiCp in Figure 16a, the wear model of the composite is mainly abrasive wear and slight adhesive wear. There are many furrows with different depths on the surface of the sample. The reinforcement effect of SiC is not obvious at the low content of N-SiC. During the wear process, the surface temperature increases, and the Al matrix softens. Under the friction of the grinding disc, the surface layer plastically deforms. Due to the intermolecular force, the Al substrate is cold welded to the surface of the grinding disc. Under the effect of shear stress, the welded joint is sheared and adhered to the surface of the grinding disc, causing the formation of a peeling block. As the friction and wear process progresses, the adhered Al matrix is detached from the surface and forms abrasive grains. Under the effect of the normal load or the forward load of the grinding disc, the Al substrate is cut and the surface is shaped, forming a large number of furrows. For AA6061/5 vol.% N-SiCp, as shown in Figure 16b, the wear form is mainly abrasive wear and adhesive wear. The furrow depth becomes shallow, but the area of the peeling block formed by adhesive wear becomes large. Figure 16c shows the wear morphology of the AA6061/10 vol.% N-SiCp composite. Generally, a greater hardness of the composite surface means better resistance to deformation, and smaller wear rate (only \(0.25\times 10^{-5}\) g/m for AA6061/10 vol.% N-SiCp composite). It can be viewed that the number of surface furrows is small, and the wear mechanism is mainly stripping wear. For AA6061/10 vol.% N-SiCp, there are obvious agglomeration phenomena in the microstructure, and defects such as agglomerated pores. Under the effect of tangential load and normal load, the grinding disc is deformed on the sub-surface of the Al matrix. Cracks are nucleated at the pore locations, and the cracks grow or propagate under the external load, which finally shear to the surface and form a peeling layer. For AA6061/15 vol.% N-SiCp, the wear rate is only \(0.1\times 10^{-5}\) g/m. In the meantime, the wear model exhibits mainly fatigue wear, as shown in Figure 16d. Slight scratches and fatigue cracks can be inspected on the surface. For the high content of AA6061/N-SiCp, the surface hardness is high. The periodic load during the wear process causes the subsurface layer to plastically deform, bulge, and generate fatigue cracks. The excessive interface will also reduce the bonding strength, and the convex portion will be crushed to form wear debris. At this time, the abrasive chips are mainly composite particles of N-SiCp and Al particles, and the roundness is good. It is different from the polygonal micro-SiC cutting into the Al substrate. The bonding of the Al substrate and the grinding disc is appropriate, resulting in the enhancement of the wear resistance. The AA6061/N-SiCp composites have a very small size of N-SiCp, and the surface damage is less after the formation of abrasive chips; therefore, it is suggested that the wear resistance is better.