Graphite is commonly used as a lubricating component in copper-based friction materials (Cu-MMCs). However, due to its susceptibility to oxidation and ablation at high temperatures, graphite significantly impacts tribological properties of copper-based friction materials under extreme high-energy braking conditions. In this study, copper-based friction materials containing different forms of graphite (oblate spheroidal, flaky, and spherical graphite) were fabricated by a powder metallurgy route. The evolution of microstructure, chemical composition, and tribological properties of these materials after oxidation at 600 and 800 ℃ were investigated. The results indicated that increasing the oxidation temperature promoted the formation of an oxide layer on the material surface. However, substantial oxidation and ablation occurred on the three materials after exposure to high-temperature oxidation. Among the three materials, the material containing spherical graphite after oxidation at 600 ℃ exhibited the lowest wear rate and highest friction coefficient, with abrasive wear identified as the primary wear mechanism. For the material containing flake graphite after oxidation at 800 ℃, the friction coefficient remained relatively stable, with minimal wear, where delamination and abrasive wear was the dominant wear mechanism.

As trains evolve towarded higher speeds and heavier loads, the surface temperature of brake pads gradually increased during braking. This rise in temperature accelerated the oxidation of copper-based friction materials, and the resulting oxide layer significantly influences their friction and wear performance. Among the three key components of copper-based friction materials, the lubricating component played a crucial role in maintaining the stability of the friction coefficient. It helped to form a lubricating film on the friction surface, reduced adhesion between friction pairs, and effectively lowered wear. Graphite, a vital constituent of the lubricating component, was commonly added to copper-based friction materials. However, its oxidation at high temperatures also significantly impacted the formation of oxide layers. Most current research focused on flake graphite, with relatively few comparative studies exploring the effects and mechanisms of different graphite types, such as oblate spheroidal, flaky, and spherical graphite. This study provided an in-depth comparison and analysis of the surface properties and oxide layers of copper-based friction materials containing different forms of graphite (oblate spheroidal, flaky, and spherical graphite) after high-temperature oxidation. It also investigated the friction and wear properties of these materials following high-temperature oxidation, clarifying the tribological behavior and mechanisms of these materials. The findings offered valuable insights for improving the high-temperature tribological performance of friction materials.

In this study, the evolution of surface composition and morphology of copper-based friction materials with different forms of graphite after high-temperature oxidation was explored from a tribological perspective. The surface characteristics and the impact of the oxide layer, both before and after oxidation, were comprehensively summarized. Furthermore, a series of tribological behavior and mechanism changes, such as alterations in the friction curve and trends in wear rate before and after high-temperature oxidation, were investigated using a reciprocating friction and wear tester.

As the oxidation temperature increased, the morphology of the oxide layer on the surface of copper-based friction materials underwent significant changes. For copper-based friction materials containing spheroidal graphite, CuO nanowires growed rapidly at 600 ℃, which notably affected both the heat dissipation and tribological properties of the material. Copper-based friction materials containing flake graphite, on the other hand, exhibited better lubrication effects under normal temperature and pressure. At 800 ℃, these materials had the lowest wear rate, and the friction coefficient remained relatively stable. This was because flake graphite tended to slide easily, causing peeling wear and exposing unoxidized graphite, which further enhanced lubrication. In contrast, the material containing spherical graphite exhibited the lowest wear rate and highest friction coefficient at 600 ℃, with stable friction performance. This was attributed to the superior bonding and heat dissipation properties of spherical graphite with the copper-based matrix, which enhanced its overall performance, making it more effective than the other two types of graphite in terms of tribological behavior.

Copper-based friction materials containing different forms of graphite were subjected to high-temperature oxidation tests. XRD and XPS analysis techniques were employed to examine the surface oxide layer, and the impact of this oxide layer on the friction and wear properties of the material was subsequently investigated. This study was crucial for enhancing the high-temperature friction and wear performance of brake pad materials.