Effect of cyclic oxidation temperature on compressive strength of Cu-based brake pad for high-speed train

The cyclic oxidation tests of Cu-based brake pads at temperatures of 25, 300, 400, 500, 550, 600, 650 and 700 °C were conducted to simulate the oxidation of the friction surface under different braking speeds. Furthermore, the effect of cyclic oxidation temperature on the compressive strength was investigated. The results show that the cyclic oxidation process of Cu-based brake pads can be divided into three stages: mild oxidation (≤400 °C), moderate oxidation (500–600 °C) and severe oxidation (≥650 °C). Mild oxidation results in the formation of spherical Cu ₂O particles. The surface is covered by Cu ₂O and Fe ₂O ₃ nanosheets for moderate oxidation. Severe oxidation is characterized by a continuous thick CuO film, Fe ₂O ₃ nanosheets, and eroded graphite particles. The compressive strength and toughness of Cu-based brake pads show a declining trend as the cyclic oxidation temperature rises. The compressive strength of the Cu-based brake pad after oxidized at 700 °C, recorded at 30.56 MPa, is considerably lower than that of the original samples, which was 132.2 MPa. This reduction in compressive strength is attributed to the embrittlement of the metallic matrix and the formation of pores from graphite oxidation.