Friction performance of Cu-based brake pad for high-speed railway trains

The brake pad is the key component in the braking system of high-speed railway trains. The running speeds of commercial high-speed railway trains in China can reach higher than 350 km·h⁻¹. The friction coefficient of brake pads is a key factor determining the safety of any vehicle brake, and a high and stable friction coefficient is ideal for ensuring the safety of the braking system. In practical applications, the friction coefficient can vary because of the changes in the working conditions, such as sliding speed, braking pressure, and temperature between contact surfaces. Under severe conditions, such as high-speed braking and overload, the friction coefficient decreases markedly, which lengthens the braking distance and braking time. Based on the friction performance collaborative regulation theory of powder metallurgy friction materials, a Cu-based friction material was designed. The performance of the brake pad was tested on the full-scale dynamometer, and the characteristics of the friction film were analyzed in detail. Results show that the brake pad exhibits high stability of the friction coefficient, low wear loss, and the capability to protect the brake disc. Both the instantaneous friction coefficient and average friction coefficient of the developed brake pad meet the requirements of the TJ/CL307—2019 technical condition. The stability of the friction coefficient is 0.0015. The recession of the friction coefficient from 250 to 380 km·h⁻¹ is as low as 0.027. The average friction coefficient at 380 km·h⁻¹ remains at the relatively high value of 0.35, and the average wear loss is only 0.06 cm³·MJ⁻¹. The excellent friction and braking performance of brake pads can be attributed to the formation of friction films with high strength and toughness and low transfer rate. The friction components with large particle sizes are used as external motion obstacles to nail the friction film. The submicron wear debris in the friction film serves as the meshing point between the friction film and the dual disc to provide friction resistance, thus maintaining the friction coefficient during high-speed braking. Oxides are continuously supplied by adding easily oxidized components, and the nanosized oxides generated by the severe grinding process are used in the dispersion-strengthening phase. The multiscale particles synergistically enhance the dynamic stability of the friction film. The metal oxide layer on the friction surface reduces and stabilizes the friction coefficient and enhances the wear resistance because it prevents metal–metal contact between the brake pad and the brake disc. The fade phenomenon that occurs under high braking speed and overload conditions is effectively prevented.