Effects of Ni-Coated Graphite Flake on Braking Behavior of Cu-Based Brake Pads Applied in High-Speed Railway Trains

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 padon-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.