*3.2. Microstructure Morphology of Wear Surface*

Figure 4 is a schematic diagram of the microstructure morphology of the wear surface at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C. As shown in Figure 4a, there are a large number of pits on the wear surface at 30 ◦C due to the shedding of wear debris, and these pits are connected to each other, resulting in uneven wear surfaces. The high-magnification image of the SEM (Figure 4a1) shows that there are fine furrows on the surface of the wear scar. The wear surface at 100 ◦C (Figure 4b,b1) shows obvious traces of plastic deformation due to extrusion, and the amount of wear debris falling off is reduced compared with that at 30 ◦C, and it begins to show obvious peeling marks. At 150 ◦C, the amount and area of the wear debris shedding on the wear surface (Figure 4c) increase significantly, and the SEM high-magnification image (Figure 4c1) shows obvious peeling marks. The wear surface above 200 ◦C shows significant plastic deformation traces. Due to the extrusion of the grinding ball, the metal softened at high temperature overflows at the edge of the wear scar to form an obvious flash-like structure (Figure 4d–f). The high-magnification image of the SEM (Figure 4f1) shows that fatigue cracks are generated on the wear scar surface under the action of cyclic stress.

**Figure 4.** (**a**–**f**) Low magnification SEM micrographs of the microstructure of the wear surface at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C; (**a1**–**f1**) High magnification SEM micrographs of the structure of area A in a–f.

Figure 5 is a graph of the wear extent curve, wear scar depth curve and average friction coefficient curve at each experimental temperature. As shown in Figure 5a,b, as the temperature increases, the wear extent and wear scar depth first decrease and then increase, and then show a downward trend after reaching the highest level at 150 ◦C. The wear extent above 200 ◦C is equivalent to about 35% of the wear extent at room temperature; the wear extent and wear scar depth show the smallest dispersion at 30 ◦C, and the largest dispersion at 150 ◦C. As shown in Figure 5c, the average friction coefficient does not show significant differences with the change of temperature, and is stable at around 0.4; the average friction coefficient has the largest dispersion at 300 ◦C.

**Figure 5.** (**a**) Wear curves at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C; (**b**) Wear scar depth curves at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C; (**c**) Curves of the average friction coefficient at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C.

Figure 6 is a graph of the friction coefficient changing with time at various experimental temperatures. As shown in the figure, at each experimental temperature, the friction coefficient does not show large fluctuations with time, reflecting a stable friction performance.

**Figure 6.** (**a**–**f**) Curves of friction coefficient versus time at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C.

#### *3.3. Morphology of Wear Debris*

Figure 7 shows SEM micrographs of wear debris at 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C. As shown in Figure 7a, the abrasive debris consists of a small amount of large-sized massive abrasive debris and a large amount of powdery abrasive debris at 30 ◦C. As the temperature increases, the powdery abrasive debris decreases greatly (as shown in Figure 7b). The size of the massive abrasive debris reaches the maximum at 150 ◦C. As the temperature rises further, the powdery abrasive debris begins to increase, and the size of the debris tends to be consistent (as shown in Figure 7e,f); obvious cracks appear on the surface of the massive wear debris at 150 ◦C and 200 ◦C.

**Figure 7.** (**a**–**f**) SEM micrographs of wear debris of 30 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C.

An energy-dispersive X-ray spectrometer was used to analyze the composition of the wear debris of 30 ◦C and 150 ◦C. As shown in Figure 8, besides the Al and Si elements contained in the matrix material, there are more O elements in the wear debris. Since the grinding balls are Si4N3 ceramic balls with high hardness and stable chemical properties, it shows that the O element in the grinding debris comes from the oxidation of the composite material during the wear process.

An energy-dispersive X-ray spectrometer was used to analyze the composition of the wear debris at 300 ◦C. As shown in Figure 9, the EDS component analysis shows the presence of Al, Si, O and C. Since the friction and wear experiments were carried out in air atmosphere, the surface of the exfoliated SiC particles was easily stained with the oxide produced by the Al matrix. Therefore, according to the EDS analysis results in Figure 9b, it is judged that the particles in Figure 9a are SiC particles.

**Figure 8.** (**a**,**a1**) The EDS composition analysis of wear debris of 30 ◦C, the red plus sign in (**a**) is the EDS sampling point; (**b**,**b1**) EDS composition analysis of wear debris of 150 ◦C, the red plus sign in (**b**) is the EDS sampling point.

**Figure 9.** (**a**,**b**) The EDS analysis result of wear debris at 300 ◦C, the red plus sign in (**a**) is the EDS sampling point.

#### **4. Discussion**

#### *4.1. Effect of Temperature on the Morphology of Wear Scars*

The roughness of sliding friction surfaces plays a crucial role in the wear process [45]. When the grinding balls first come into contact with the composite surface, they only make contact at a few rough points where these micro-protrusions cover only a small portion of the surface area. As a result, very high stresses are generated in these small surface areas and wear occurs at these points [46]. Figure 10 is a schematic diagram of the wear surface morphology change from low temperature to high temperature. As shown in Figure 10a, when the temperature is low, the plastic deformation of the contact surface is small, and the uneven contact point cracks and breaks under the cyclic shearing action of the grinding ball; the wear process at 30 ◦C fits this type (Figure 3a). In addition, due to the higher fracture energy [46], the plastic deformation of the metal caused by wear at 30 ◦C is small; a silverwhite rough wear surface is finally formed. As the temperature increases, the plasticity of the friction surface of the composite material improves; the uneven contact points on the

friction surface are fractured due to the large plastic deformation under the sliding shear of the grinding ball and accumulate to the advancing side of the grinding ball. With the increase in accumulation, due to insufficient shear force, the grinding ball will move on over the accumulated metal. Wavy folds are formed on the wear surface under this cyclic friction (as shown at point A in Figure 10b). Since the plasticity of the composite material is further enhanced with the increase in temperature, more metal needs to be accumulated on the advancing side of the grinding ball to generate sufficient shear resistance (Figure 10c). Therefore, the spacing of the wavy folds on the wear surface gradually increases (as shown in Figure 3b–d).

**Figure 10.** (**a**) Schematic diagram of the change of the wear surface morphology during the friction and wear process at room temperature; (**b**) Schematic diagram of the formation of wavy folds on the wear surface when the temperature of the friction and wear test increased; (**c**) The temperature of the friction and wear test was further increased; the spacing of the wavy folds on the wear surface increases.

When the temperature reaches 200 ◦C, the increased plasticity of the composite leads to the improvement of the fluidity, leading to flashing at the edge of the wear scar due to composite spillage (as shown in Figure 4d–f). When the temperature exceeds 250 ◦C, the wear surface exhibits extensive plastic deformation. Therefore, the wear groove track formed parallel to the sliding direction is relatively clear (as shown in Figure 4e,f).
