Evolution of Tribological Behavior and Wear Life of Brake Pads for High-Speed Trains

With the continuous improvement of high-speed train running speed and heavy load technology, the energy dissipation density and thermal shock between the braking pads- braking discs friction pair have also increased significantly, which undoubtedly constitutes a serious challenge for the friction and wear performance of the disc braking system, especially the braking pads material as a wearable and necessary part. In this paper, from the perspective of energy dissipation, the friction and wear behavior characteristics and wear life of dovetailed I-C type braking pads at a speed level of 350 km/h with different cumulative cycle times (service cycles) were thoroughly investigated using a high-speed pin-ring friction and wear tester instead of a 1:1 brake bench test equipment based on the principle of specific experimental parameter selection. In addition, semi-in-situ wear experiments were carried out in order to characterize in detail the damage mechanism of each component in the braking pads. The conclusions of this paper were as follows. The average friction coefficient of the pin-ring friction pair under different number of cyclic experiments was mainly distributed between 0.4 and 0.44, with certain fluctuation. 40 and 80 cyclic experiments resulted in similar wear surface morphology of the pin, with a large number of plow grooves, craters, fragmentation of reinforcing components, oxide adhesion, and edge spalling phenomena. The thickness and continuity of the friction layer on the wear surface of the pins (braking pads) increased with the growing number of cycles, and after 80 cycles, there were uneven thicknesses of friction layers enriched with Cu and Cr, bar attachments along the frictional sliding direction, and a large number of furrows on the ring surface. The 350 km/h condition of the braking pads was mainly characterized by abrasive and adhesive wear, and the reinforcing elements were fragmented and detached under cyclic shear and compressive stresses, especially the reinforcing elements immediately adjacent to the graphite elements. A large number of abrasive particles existed between the friction interfaces, resulting in severe abrasive wear. The abrasive particles may detach from the friction interface, be stored in the low-lying areas on the surface of the braking pads, or be compacted into a friction layer. The friction layer on the surface experienced a process of removal and generation under shear, plowing of the abrasive chips, and adhesion of the friction pair. The use of small-scale (laboratory-level) equipment and the adoption of certain experimental parameter selection principles could make the experimental results comparable with those of the 1:1 braking test bench. The average frictional dissipation energy of the braking pads at the speed level of 350 km/h was 0.113 cm³/MJ, which was close to that of the 1:1 braking test bench provided by the iron academy of science with the test result of 0.14 cm³/MJ. The wear life of the braking pads was 192 times, which corresponded to 341 times of emergency braking under the actual working condition of 350 km/h speed. The results of this paper could provide a reference for the improvement of the safety of the braking system and the development of new material systems for high-speed railroads from the point of view of tribological research.