Different Cu-based brake pads applied in high-speed railway trains were fabricated by the co-addition of Cr and CrFe particles, and the influence of the Cr/CrFe ratio on the tribological behavior of the powder metallurgy brake pads were studied by a reduced scale testing apparatus with the pad-on-disc configuration under various braking speeds. The results indicated that the Cu-based brake pad containing 6 wt% Cr and 4 wt% CrFe exhibited the highest and the most stable friction coefficient as well as the lowest wear loss when the braking speed was higher than 300 km/h. Moreover, a new reasonable explanation is provided for the effect of Cr and CrFe particles during the braking process. The excellent braking properties are attributed to the synergistic effect of Cr and CrFe on promoting the formation and stabilization of a tribofilm. Cr particles, which have high reactivity with Fe and O, act as a steady source of fine oxides in the tribofilm, and CrFe particles bear the load and strengthen the subsurface in the position near the friction surface. It is clear that the application of an appropriate ratio between Cr and CrFe can develop Cu-based brake pads suitable for more serious braking conditions.

Argon gas atomized Cu-Fe prealloyed powder was used to prepare Cu-based composites, and the effect of matrix alloying of Fe on microstructure and friction and wear properties of the brake pad material was systematically investigated on MS3000 friction and wear tester. The results indicate that matrix alloying of Fe induces the precipitation of uniformly distributed iron rich phase interior of grains, the segregation of iron rich phase along SiO₂-matrix interface and the formation of pearlite in the vicinity of graphite-matrix interface, which favors the strengthening of copper matrix, improves interfacial bonding and protects the third body. The prealloyed sample exhibits relative high friction coefficient and the enhanced friction stability as well as the reduced wear loss when the braking speed is lower than 200 km/h. At higher braking speed (>200 km/h), the breakage of iron rich phase leads to the unstable friction coefficient and the high wear loss.

Cu-based brake pads applied in high-speed railway trains containing Ni-coated graphite flake and uncoated graphite flake were fabricated by powder metallurgy. The braking properties of the brake pads were investigated by a scaled down testing apparatus with the pad-on-disk configuration under various braking speeds and braking pressures. Compared with the brake pads containing uncoated graphite flake (designated GF), the brake pads containing Ni-coated graphite flake (designated NGF) exhibits a similar braking performance at lower braking speed and pressure. However, NGF shows more stable friction coefficient, lower linear wear loss, and lower maximum temperature during the braking process at worse braking conditions, e.g., 350 km/h, 1.5 MPa. The Ni-coating on the surface of Ni-coated graphite can transfer the mechanical bonding between copper and graphite to diffusion bonding so that there is a stronger interface bonding between copper and Ni-coated graphite. Further, the multiple linear regression analyses reveal that the mean friction coefficient of NGF is more sensitive to braking pressure than braking speed because of the better thermal resistance of NGF, while the mean friction coefficient of GF and the linear wear loss are mainly affected by braking speed.