## 3 Tests Results
### Low Speed and Heavy Load  
Figure 11 shows the friction torque curves of the SCF-modified three-layer composite bearing bush under three groups of different pressure levels, respectively. In the initial period of the test, the friction coefficient of the bearing bush first increased and then slowly decreased, and regular changes were obtained after a period of running-in process. According to the friction torque of the bearing bush under the condition of low speed and heavy load, the friction coefficient of the SCF-modified bearing bush is relatively stable under 28 MPa and 30.5 MPa. When the maximum pressure is 33 MPa, the friction coefficient of the SCF-modified bearing bush increases slowly. As can be seen from Figure 12, with the increase of experimental pressure, the wear depth of the bearing bush gradually increases. At 33 MPa, there is obvious material accumulation in the worn and non-worn parts of the bearing bush. With the increasing pressure, the difference in abrasion depth of the modified bearing bush becomes smaller. Therefore, combined with the dynamic change process of the friction coefficient of the bearing bush, it can be determined that the tribological performance of the SCF-modified bearing bush is improved. The excellent frictional properties of the SCF-modified bearing bush are more and more prominent under extremely high pressure. As can be seen from Figure 13a, there were many small holes on the surface of the composite material with carbon fiber added. The reason for this phenomenon may be related to the many small holes in the treatment layer of the fiber surface after the modification of carbon fiber. The appearance of these small pores increased the space for molecular flow between the modified materials, increased the activity of the lubricant phase polytetrafluoro-roethylene, and increased the free volume of the modified materials, resulting in a decrease in the glass transition temperature of the modified materials. In combination with the SEM  
Figure 11: Friction torque curve of SCF-modified three-layer composite bearing bush under different test pressures. image, it can be seen that the modified carbon fiber forms a relatively dense treated layer, and the hardness of the interfacial layer is improved, which can effectively reduce the movement and slip of molecular chains between self-lubricating materials, so the high-temperature resistance of the material can be improved to a certain extent. Combined with scanning electron microscope images, it can be seen that the surface of carbon fiber after modification is etched, and small gullies appear in the middle, making its surface more uneven and roughness increased, and changes in oxygen-containing groups reduce the surface energy required for coating of the treatment layer so that the intermolecular force is enhanced. Figure 12: Abrasion depth curve of SCF-modified three-layer composite bearing bush under different test pressures. Figure 13: SEM image of wear of SCF-modified bearing bush: (**a**) SEM image before wear; (**b**) SEM image under 28 MPa pressure class; (**c**) SEM image under 30.5 MPa pressure class; (**d**) SEM image under 33 MPa pressure class. As can be seen from Figure 13b, many particles are attached to the surface and grooves of carbon fiber. This is because the solid lubricant PTFE sticks to the surface of the carbon fiber. When the pressure is 28 MPa, the wear surface has a slight interface cracking and desticking phenomenon. It can be seen that only a small number of carbon fiber edges are stripped from the matrix, and the phenomenon of fiber pulling out is almost non-existent. There is a very small number of voids in the matrix, and this phenomenon occurs in the fiber-matrix with low pressure, resulting in the instability of the friction coefficient. As can be seen from Figure 13c, it is evident that when the pressure reaches 30.5 MPa, the fracture surface of the worn composite assumes a relatively flat profile.