*4.2. Effect of Temperature on Wear Mechanisms*

Friction Stir Processing improves the defects in the as-cast composites [25] and promotes the performance improvement of the composites [47]. Figure 11a is the photo of the cross-section of the FSLW joint; Figure 11b is the SEM photograph of the as-cast composite material area at point A not affected by FSP in Figure 11a; Figure 11c is the SEM photo of the nugget area at point B in Figure 11a. As shown in Figure 11b, the SiC particles are agglomerated and unevenly distributed, and there are casting shrinkage defects in the as-cast SiCp/ZL101. The microstructure of the as-cast SiCp/ZL101 is significantly improved after FSP (Figure 11c). The SiC particles are refined and evenly distributed, and casting shrinkage cavities are eliminated. The composite material after stirring is denser, and the wear surface maintains better compactness and continuity [46,48]. Therefore, the friction process at each temperature is relatively stable, and the fluctuation range of the friction coefficient is small. A stable friction coefficient can lead to better braking effects, such as better thermal stability, the reduction of the vibration of the braking system, low noise, controllable braking safety and so on [37,38].

**Figure 11.** (**a**) Schematic diagram of the cross-section of FSLW joint; (**b**) SEM photograph of the as-cast composite material area at point A not affected by FSP in Figure 11a; (**c**) SEM photo of the nugget area at point B in Figure 11a.

When the friction and wear test was carried out at 30 ◦C, the micro-protrusions on the surface of the composite material were plastically deformed under the cutting action of the grinding ball and the rotating torque; a large amount of heat was generated locally, resulting in the partial oxidation of the material and it falling off the surface to form wear debris, leading to pitting on the wear surface. This phenomenon exhibited characteristics of oxidation wear. The fine scratches on the wear surface along the sliding direction were mainly caused by the reinforcement particles shed from the matrix [49], indicating that abrasive wear also occurred during the sliding friction process [50]. It is worth noting that the stirred composite materials show very shallow grooves after sliding wear, which may be attributed to the following two factors. First, the SiC particle becomes smaller and has a good interfacial bond with the Al matrix after the stirring treatment. Good interfacial bonding persists at 30 ◦C and allows load transfer across the particle/matrix interface. Therefore, only a small number of SiC particles fracture and detach from the Al matrix. Secondly, since there is no obvious accumulation of reinforced particles after the stirring treatment [51], the evenly distributed small-sized and high-hardness SiC particles improve the hardness, strength and wear resistance of the composite material [52], which can act as a hard barrier against scratching and plowing by abrasive particles [53]. These two factors lead to the formation of shallow grooves in the wear surface.

The plasticity of the wear surface is improved with the increase in temperature. Figure 12 is a schematic diagram of fatigue crack growth on the wear surface. As shown in the figure, due to the repeated extrusion of the grinding ball, microcracks nucleate at the stress concentration of the friction surface, gradually break through the wear surface, and then grow and connect with each other during the sliding process. Eventually, the surface metal will fall off and become wear debris. This statement is confirmed by the cracks on the wear debris in Figure 7c,d and the pits with irregular edges formed by the shedding of wear debris in Figure 4c1,d1. The wear surface exhibits fatigue wear behavior at this stage. The initiation and growth of the fatigue cracks are further accelerated with the increase in temperature; the size of the wear debris is larger; the wear extent and wear scar depth reach the maximum at 150 ◦C. The fatigue wear leads to the shedding of large pieces of wear debris at 150 ◦C (Figure 7c), and the wear surface is rough and uneven. Therefore, the fluctuation range of the friction coefficient becomes significantly larger after 1500s (Figure 6c), which shows significant differences with the friction coefficient curves at other temperatures. As shown in Figure 3c, there were wavy folds caused by plastic deformation on the wear surface at 150 ◦C. When the surface profiling method was used to detect the wear extent and wear scar depth, if the sampling part was just near the wavy folds, it would cause a large deviation between different measurements. Therefore, the large dispersion of wear extent and wear scar depth at 150 ◦C can be attributed to the combination of the high roughness of the wear surface and the morphology of the measurement sampling part.

In Figure 4d1, the irregular tear ridges are shown after the wear debris breaks away at 200 ◦C, and the wear scar depth is significantly smaller, showing the characteristics of adhesive wear [53], indicating that the further improvement of the plasticity of the composite material at high temperature leads to the transition of the wear behavior to adhesive wear. As the temperature further increases, the softening of the Al matrix leads to the cracking or loosening of the SiC particles. As shown in Figure 9, the EDS component analysis of the abrasive debris at 300 ◦C shows the presence of small-sized SiC particles and oxide particles. Figure 13 is a schematic diagram of the sliding friction process at high temperature. As shown in Figure 13b, these small-sized SiC particles and oxides aggregate between the wear surface and the grinding ball, and gradually form a mechanically mixed layer on the wear surface under the action of cyclic load [54]. Elastic deformation occurs on the wear surface at high temperature [55], and this mechanically mixed layer is more likely to cause shear instability on the sliding friction surface [56,57]. Meanwhile, the wear surface maintains better compactness and continuity due to the denser composite material after stirring [46,48], and is not easily damaged by the shear force of the mechanical mixing layer, which further reduces the severity of the microplowing action caused by interactions between abrasive particles. The combined effect of appealing factors leads to a reduced wear rate at elevated temperatures. On the other hand, the plastic deformation resistance of the composite decreases due to the increase in temperature. The wear debris adhered to the grinding ball forms a clear furrow on the wear surface; the wear does not progress further, due to the lubrication of the high-hardness mechanical mixed layer [58]. Therefore, there were no pits on the wear surface caused by the shedding of large-sized wear debris at 250 ◦C and 300 ◦C, and the depth of the wear scars did not continue to increase.

**Figure 13.** (**a**) The SiC particles started to loosen and detach from the matrix during sliding friction at high temperature; (**b**) SiC particles and oxides aggregated on the wear surface to form a mechanically mixed layer.
