Evolution in microstructure and compression behaviour of a metallic sintered friction material after braking

Due to the complexity of friction materials, the characterization of the tribological properties is prioritised over the bulk material properties even though the tribology is expected to be influenced by the material behaviour. The extent of this relationship is still unknown and further knowledge is required to account for the load history and evolution of the bulk properties. With this view, the compression behaviour and microstructure of a semi-metallic friction material with reduced formulation were investigated before and after a braking program. The thermal loading was monitored with inserted thermocouples. Uniaxial compression tests coupled with Digital Image Correlation (DIC) show significant changes in the worn material, which develops a compression behaviour similar to that of a tri-layered material. The microstructural analysis indicates microcracking of the metallic matrix and carbon diffusion in the Fe-phase. The thermal loading was found to be the key parameter controlling both the friction behaviour and evolution of the material properties. The expected effects of material evolution on the contact uniformity, durability and tribology are discussed. •The compression behaviour changes remarkably; a tri-layer material is formed.•In-service temperature controls the tribological response and mechanical/microstructural changes.•The hardness of the sub-layer adjacent to the contact surface is increased by C diffusion.•The development of a macrocrack in the intermediate layer induces high strain localisation.•The bulk mechanical changes affect the braking performance.