## 2 Materials and Methods
### Experimental Methods of Tribological Properties  
All of the tribological experiments were conducted using a testing machine (3 KW testbed, Wuhan University of Technology, China, as shown in Figure 4) to determine the tribological properties of LC2-1 and QT500-7 sliding contact on QT350-22L in a low-temperature hydraulic oil environment. The wear track of the pins after experiments was observed and measured using CLSM, while the wear surface was investigated using scanning electron microscope (SEM) (VEGA3, TESCAN, Brno, Czech Republic). The wear volume was calculated based on the measured results using the integral principle. A microscope (BX3M, Olympus, Tokyo, Japan) and a ferrography analyzer were used to observe and analyze the wear particles in the oil, respectively, after the test. As shown in Figure 4, a low-temperature cavity (5) was fixed on the testbed to provide a low-temperature environment. The disc (11) and pin (12) were installed at the front of the rotating axle (4) and the axle (6) by using fixtures, respectively. Hydraulic oil was injected into the house through the flow path on the axle (6). Low-temperature equipment (FP89-HL, Julabo, Seelbach, Germany offered the circulating cooling medium to copper tube (9) installed in the cavity's interior. The circulating cooling medium came into and out of the cavity through the oil ports (14 and 16). The refrigerant was impelled by the compressors to recirculate in the copper tube to cool the coolant (10), which provided a low-temperature environment in the cavity. A digital temperature sensor (13) was fixed into the cavity to monitor the temperature in real time. The temperature signal collected is directly transmitted to the temperature control system of the refrigerator, forming a closed-loop control of temperature. The temperature of the hydraulic oil in the low-temperature oil chamber was real-time controlled. During the experiment, the low temperature was kept stable with a fluctuation of less than 1 \({}^{\circ}\)C.  
Figure 3: (**a**) Impact testing machine (ZBC2302-B). (**b**) Sample. (**c**) Impact sample installation diagram. The testing machine was driven by a converter motor (1) whose rotary speed was controlled by a frequency conversion controller. The axle was pushed to slide by loading screw (8) compression spring so that the pin was pressed against the surface of the disc. The normal force was detected by a loading sensor (7). Meanwhile, a torque sensor (3) (measurement error: \(\pm\)2%) was employed to measure tangential torque. The data of the normal force, the tangential force and the temperature could be recorded automatically on the computer. The entire wear test was divided into 24 groups. The experiments were performed at room temperature (25 \({}^{\circ}\)C), 0 \({}^{\circ}\)C, \(-10\) \({}^{\circ}\)C, \(-20\) \({}^{\circ}\)C, \(-30\) \({}^{\circ}\)C and \(-40\) \({}^{\circ}\)C, respectively. During the test, the ambient temperature was always kept stable. In addition, the normal force was set to 20 N and 30 N, respectively, to study its influence on the two typical materials. The rotary speed of the converter motor was 300 \(\mathrm{r/min^{-1}}\). The experiments were carried out under each frictional condition for 120 min. The test plan is shown in Table 2.  
The center line of the pin was located \(11\) mm eccentrically from the center line of the disc. The coefficient of friction was measured and calculated using the following Equation (1):  
\[f=T/(0.011*N) \tag{1}\] \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Test Conditions**} & \multicolumn{8}{c}{**Temperature/\({}^{\circ}\)C**} \\ \cline{2-7}  & \multicolumn{8}{c}{**(Rotary Speed 300 \(\mathrm{r/min}\); 120 min)**} \\ \hline Normal & \multicolumn{2}{c}{20 N} \\ \cline{2-7} force & \multicolumn{2}{c}{30 N} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Related parameters of the test plan.