## 3 Results
### Coefficient of Friction  
As shown in Figure 6, the coefficients of friction of two sets of friction pair LC2-1/QT350-22L and QT500-7/QT350-22L have an obvious variation trend under various conditions. In addition, the viscosity has also been taken into consideration and varies obviously. As the test temperature drops, the oil viscosity increases and the viscosity gradient increases. As the test temperature drops from 25 \({}^{\circ}\)C to \(-\)40 \({}^{\circ}\)C, the viscosity of hydraulic oil increases from 15.96 to 263.27 mPa\(\cdot\)s. The viscosity gradient increases with the temperature dropping. The minimum gradient of the oil viscosity with temperature is 0.71 mPa\(\cdot\)s/\({}^{\circ}\)C when the test temperature ranges from 25 \({}^{\circ}\)C to 0 \({}^{\circ}\)C. The viscosity of the hydraulic oil is 147.73 to 263.27 mPa\(\cdot\)s when the temperature drops from \(-\)30 \({}^{\circ}\)C to \(-\)40 \({}^{\circ}\)C, and there is the maximum viscosity gradient and it is 11.95 mPa\(\cdot\)s/\({}^{\circ}\)C. The viscosity gradient is increased by about 16.8 times. Figure 5: Variations of impact energy of the material with temperature. As shown in Figure 6a, the normal force is 20 N. At room temperature (25 \({}^{\circ}\)C), the coefficients of friction of the friction pairs LC2-1/QT350-22L and QT500-7/QT350-22L are relatively close, about 0.1201, indicating that the wear performance of the two sets of friction pairs is similar at room temperature. When the test temperature is \(-\)10 \({}^{\circ}\)C, the coefficients of friction of the two sets of friction pairs both decrease. The coefficient of friction of the former is about 0.0771, and the coefficient of friction of the latter is about 0.1021. The coefficient of friction of the former has a bigger decreasing rate than does that of the latter. When the test temperature is \(-\)20 \({}^{\circ}\)C or less, the coefficient of friction of the friction pair LC2-1/QT350-22L is basically stable at about 0.0753. However, the coefficient of friction of the friction pair QT500-7/QT350-22L increases significantly at \(-\)20 \({}^{\circ}\)C and is basically stable at \(-\)30 \({}^{\circ}\)C and \(-\)40 \({}^{\circ}\)C. The coefficient of friction of LC2-1/QT350-22L is much lower than that of QT500-7/QT350-22L. Figure 6b shows the average coefficient of friction of the two sets of friction pairs at different temperatures when the normal force is 30 N. At room temperature, the coefficients of friction of the LC2-1/QT350-22L and QT500-7/QT350-22L are 0.1535 and 0.1885, respectively. With the decrease in temperature, the coefficient of friction of the former also decreases. It stabilizes at about 0.085 at \(-\)40 \({}^{\circ}\)C. The coefficient of friction of the latter decreases first and then increases at \(-\)20 \({}^{\circ}\)C. As the temperature decreases from \(-\)10 \({}^{\circ}\)C to \(-\)20 \({}^{\circ}\)C, the coefficient of friction increases from 0.1633 to 0.1924. At \(-\)40 \({}^{\circ}\)C, the coefficient of friction is 0.2017. In short, the coefficient of friction of the friction pair LC2-1/QT350-22L is lower than that of QT500-7/QT350-22L. Meanwhile, the fictional coefficient of the former fluctuates less than does that of the latter under low-temperature conditions. This proves that the friction and wear process is relatively stable. As shown in Figure 6, comprehensive analyze the change law of oil viscosity and the coefficient of friction, for the friction pair LC2-1/QT350-22L, the variation of the coefficient of friction with temperature complies with the trend of the Stribeck curve. When the temperature drops from 25 \({}^{\circ}\)C to \(-\)20 \({}^{\circ}\)C, the lubrication state between the friction pairs is a mixed or boundary lubrication state. The coefficient of friction is stable when the temperature changes from \(-\)30 \({}^{\circ}\)C to \(-\)40 \({}^{\circ}\)C. It is inferred that the lubrication state changes into the mixed or hydrodynamic lubrication state.