## 5 Conclusions  
In the present study, the viscosity of hydraulic oil and the impact strength of the two materials were measured via viscometer and mechanical bench. The tribological properties of the two typical materials of hydraulic motor's rotor were analyzed at different ambient temperatures, especially at low temperature. A series of tribological experiments were performed to explore the wear process and mechanism of two sets of friction pairs. and to select the suitable material for the rotor. There are some conclusions drawn as follows. 1. Temperature has a significant effect on material properties and the viscosity of lubricating oil. The viscosity gradient increases with temperature dropping. As the test temperature drops from 25 to \(-\)40 \({}^{\circ}\)C, the viscosity of hydraulic oil increases from 15.96 to 263.27 mPa-s. The viscosity gradient increases with temperature dropping. The minimum gradient of oil viscosity with temperature is 0.71 mPa-s/\({}^{\circ}\)C when the test temperature ranges from 25 \({}^{\circ}\)C to 0 \({}^{\circ}\)C. The viscosity of hydraulic oil is 147.73 to 263.27 mPa-s when the temperature drops from \(-\)30 \({}^{\circ}\)C to \(-\)40 \({}^{\circ}\)C, and there is the maximum viscosity gradient and it is 119.5 mPa-s/\({}^{\circ}\)C. The viscosity gradient is increased by about 16.8 times. The impact energy of the two types of materials decreases as the temperature decreases. However, the variation trend of the impact energy of LC2-1 is not as obvious as that of QT500-7. For the material QT500-7, there are two consecutive subintervals of sharp decreases from 97.3 J to 42.2 J and then to 18.0 J as the temperature drops from \(-\)10 \({}^{\circ}\)C to \(-\)20 \({}^{\circ}\)C and then to \(-\)30 \({}^{\circ}\)C. The percentage decreases are \(-\)56.6% and \(-\)57.3%, respectively. It is worth mentioning that the brittle-ductile transition temperature of QT500-7 is between \(-\)10 \({}^{\circ}\)C and \(-\)30 \({}^{\circ}\)C.
2. Each test result of the QT500-7 ground test is worse when the tribological experiments are carried out at \(-\)20 \({}^{\circ}\)C. There is a maximum difference of around \(-\)20 \({}^{\circ}\)C. The wear volume of material QT500-7 sharply increases and reaches the maximum of 0.107 mm\({}^{3}\) when the normal force is 20 N, which is 3.65 times as much as the wear volume of the LC2-1. When the normal force is 30 N, the wear volume of material QT500-7 and LC2-1 are 0.125 and 0.036 mm\({}^{3}\), respectively. The wear volume of former is 3.4 times that of the latter. The results indicate that the impact energy of the material affect its tribological properties since the impact energy of the material QT500-7 decreases sharply at the temperature of \(-\)10 to \(-\)30 \({}^{\circ}\)C.
3. Under low-temperature conditions, both materials experience fatigue wear while sliding with QT350-22L, indicating that the low-temperature environment will accelerate the fatigue of the material, but the temperature at which fatigue wear occurs differs from material to material. The material QT500-7 undergoes severe fatigue wear all over its surface at \(-\)20 \({}^{\circ}\)C, whereas LC2-1 has insignificant fatigue wear at \(-\)40 \({}^{\circ}\)C. This is mainly because QT500-7 has a ductile-brittle transition between  
Figure 12: Schematic diagram of the lubrication and wear process. \(-10\)\({}^{\circ}\)C and \(-30\)\({}^{\circ}\)C such that the wear is most serious. However, LC2-1 has no obvious ductile-brittle transition within the test temperature range. * The wear resistance of LC2-1 in a low-temperature environment is superior to that of QT500-7. The friction pair composed of LC2-1 with QT350-22L has better tribological characteristics. 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.