Oxidation behavior of Cu-based brake pad for high-speed train

Cu-based brake pad for high-speed train undergoes cyclic oxidation due to the generation and dissipation of friction heat during braking. The oxidation behavior of the Cu-based brake pad was investigated via isothermal oxidation at 300, 400, 500, 600 and 700 °C for up to 50 h. The results show that the oxidation of the Cu-based brake pad presents multiple stages. The combination of the oxidation of Cu and Fe and the oxygen diffusion controls the oxidation process in the earlier stage, while the oxidation of graphite plays a more important role in the later stages above 500 °C. The Cu₂O nanoclusters are firstly formed by the oxidation of copper, then CuO nanowires, and finally fine and coarse equiaxed grains are generated. The rise in temperature promotes the growth and densification of Fe₂O₃ nanosheets, which grow on the Fe₃O₄ layer. However, Fe oxides are gradually covered by Cu oxides because of the larger volume expansion of Cu oxides. The connected pores formed by the graphite burn-off provide oxygen diffusion channels for internal oxidation. The improved surface microhardness is attributed to the formation of oxides.