Effect of the Coke/Flake Graphite Ratio on the Microstructure and Properties of Cu-Based Powder Metallurgy Friction Materials

The effect of the coke and flake graphite ratio (C/F ratio) on the microstructure formation and properties of Cu-based friction materials was investigated. The density, hardness, friction coefficient, braking properties of the Cu-based friction materials with different C/F ratios were tested. The microstructures, wear morphology, and frictional subsurface of the Cu-based friction materials were characterized by SEM. With the increase in coke content, the proportion of flake graphite with long strips shape decreased, and the coke with clump shape increased. The density of Cu-based friction material decreased continuously with the increase in the C/F ratio. The hardness of the Cu-based friction material decreased first and then increased gradually with the increase in the C/F ratio. With the increase in C/F ratio, the friction coefficient increased, and the wear loss decreased. Moreover, during the braking process, the surface temperature of the Cu-based friction materials first increased and then decreased, and the braking time decreased first and then increased with the increase in C/F ratio. The wear mechanism of the Cu-based friction materials was abrasive wear, fatigue wear, and oxidative wear. The failure of the Cr-Fe particles was oxidation, crushing, and wear. Relatively, the Cu-based friction material with C/F ratio of 7/2 had high friction coefficient, low wear loss, low surface temperature during braking, and low braking time, which indicated better combination braking properties.