Synergistic Effects of Different Graphite on the Braking Performance of Cu-Matrix Friction Materials for High-Speed Trains Based on Pin–Disc Tests

Cu-matrix composites, with a constant graphite content but different proportions, for high-speed trains were prepared by a powder metallurgy technique. A reduced proportion pin–disc testing machine was used to study the braking performances of friction materials at different speeds. The results show that the microhardness and resistance to plastic deformation of the friction surface are significantly affected by the proper introduction of colloidal graphite. Compared with copper-plated graphite, colloidal graphite has more active surface free energy and can react readily with Fe to produce cementite. Cementite is a kind of metal compound with high strength that has a great supporting effect on the material surface. The cementite participates in the formation of a friction layer. As a result, the strength of the friction film is improved and the oxidation of Fe is reduced, leading to the generation of a friction film with strong antideformation ability. The transfer of material from the friction surface during high-energy braking is low, which stabilizes the coefficient of friction and decreases the wear volume. However, too many colloidal graphite particles results in many interfaces in the matrix and severely damage the matrix's continuity. With an increase in colloidal graphite mass fraction, the arrangement direction of the copper-plated graphite changes. When the colloidal graphite content is 1% and the copper-plated graphite content is 19%, the friction material shows a stable friction coefficient and has good tribological properties under high-speed braking at 350 km/h.