Progress of copper-based friction materials for high-speed train: Tribological property involving components, interfaces, and tribo-layers

With the continuous increase in operating speed of high-speed trains, enhanced safety and stability in braking systems are necessitated. Copper-based friction materials (CBFMs) are predominantly utilized in brake pads for high-speed trains exceeding 300 ​km/h. Substantial braking energy is dissipated by CBFMs through direct interaction with counterpart materials, and their pivotal role in maintaining the safety and reliability of high-speed braking systems is ensured. The components and intrinsic properties, the multiple variations at the interface, and the distinctive characteristics of brake conditions are regarded as the primary factors influencing the braking properties of CBFMs. Recent advancements in CBFMs and tribological properties are systematically explored in this review from three critical perspectives: components, interfaces, and tribo-layers. Firstly, the emerging trends in matrix, lubricant components, and abrasive components in CBFMs are detailed. Secondly, the correlation between interfacial and tribological properties at both micro and macro scales is investigated. Thirdly, the characteristics of tribo-layers at different scales and the associated wear mechanisms of CBFMs are examined. Lastly, the challenges CBFMs face and the constraints of multi-component synergistic design, evaluation methodologies, and novel wear mechanisms are highlighted.