## 4 Discussion  
Combining the coefficient of friction, wear volume, and surface morphology of the two types of pin samples, the shape and size of wear debris are comprehensively analyzed in various temperature tests. Each test result of the QT500-7 test ground increased when the material was exposed below \(-20\)\({}^{\circ}\)C. According to the experimental results, the impact energy of the material has an effect on its tribological properties. The impact energy of QT500-7 decreases sharply between \(-10\)\({}^{\circ}\)C and \(-30\)\({}^{\circ}\)C, and its change rate is bigger than that of LC2-1, (as shown in Figure 5). The fatigue wear of materials is aggravated at this test temperature, so the tribological properties are the worst, and the wear damage is more serious. When the temperature is \(-30\)\({}^{\circ}\)C or \(-40\)\({}^{\circ}\)C, the brittleness of the material increases so that the surface of the material is prone to fatigue and wear and is vulnerable to serious damage. From the shape and size of wear debris, it can be inferred that the wear debris has entered the friction pair again, causing the friction pair to undergo \"three-body\" wear, which in turn aggravates material wear. During this wear process, the hydraulic oil film is destroyed, and the lubrication performance of the hydraulic oil worsens. Based on the above analysis, the material LC2-1 with stable wear performance is more applicable for being processed and manufactured into parts for low-temperature environmental conditions, as there is no serious fluctuation in the temperature range from \(-40\)\({}^{\circ}\)C to room temperature. Therefore, it is imperative to explore the wear mechanism of LC2-1/QT350-22L at different temperatures. Figure 10: Morphology of abrasive grains of the friction pairs of the two materials at different temperatures. (**A**–**C**) are morphology of abrasive grains of the friction pair of LC2-1 with QT350-22L when the test temperatures are \(25\)\({}^{\circ}\)C, \(-20\)\({}^{\circ}\)C and \(-40\)\({}^{\circ}\)C, respectively; (**a**–**c**) are morphology of abrasive grains of the friction pair of QT500-7 with QT350-22L when the test temperatures are \(25\)\({}^{\circ}\)C, \(-20\)\({}^{\circ}\)C and \(-40\)\({}^{\circ}\)C, respectively. Figure 11 shows the morphology of the unworn and worn surfaces of the disc paired with LC2-1 at different test temperatures. The normal load is 20 N and the rotary speed is 300 r-min\({}^{-1}\) in these tests. The left part of each picture is the original unworn surface morphology of the disc samples, and the right part is the worn surface morphology of the disc samples. As shown in Figure 11, when the test temperature is room temperature (25 \({}^{\circ}\)C), there is obvious deformation at the edges of the wear mark. This is due to the ductility of QT350-22L and the fact that the wear debris is only partially separated from its body. In addition, there are some ploughs on the worn surface, a morphological feature of abrasive wear. At the test temperature of \(-20\)\({}^{\circ}\)C, the worn surface has uniform, fine and shallow ploughs. The edges of the wear marks are neatly accompanied by the accumulation of material, except for the only difference that the wear debris is inlaid in the ploughs, indicating the existence of \"three-body\" abrasive wear. As the test temperature drops to \(-40\)\({}^{\circ}\)C, the wear edge is still neat but there is no obvious accumulation. The most remarkable thing is that there are traces of material peeling off the edges. This is mainly due to the brittleness of the material increasing with the decrease in temperature. The wear debris leaves the body, and the material appears slight fatigue wear. To sum up, Figure 12 shows a schematic diagram of the wear process of the friction pair during sliding under the certain normal force (F) and rotational speed (\(\omega\)) conditions. The friction pair material is immersed in hydraulic oil.