Strength-toughness design and braking behavior study of coatings for 400 km/h high-speed train brake discs

The pursuit of 400 km/h operating speeds for high-speed trains imposes more demanding requirements on brake disc performance. During emergency braking, immense kinetic energy must be rapidly converted into thermal energy, subjecting the disc surface to extreme thermomechanical loads. This study developed a Mn-alloyed Stellite 21 cobalt-based coating via laser cladding on full-scale 24CrNiMo steel brake discs. The resulting coating, designated LC-disc, exhibited excellent forming quality without cracks or pores. The addition of Mn modified the coating's microstructure and activated multiple strengthening mechanisms, including a transformation-induced plasticity (TRIP) effect and increased geometrically necessary dislocation density. These led to significant improvements in mechanical properties: ultimate tensile strength reached 1137 ± 20 MPa (25 % increase) and elongation reached 23.3 % ± 1.2 (155 % increase). Thermal fatigue crack initiation and propagation were also effectively suppressed. The LC-disc passed full-scale bench tests under 400 km/h braking conditions. While the conventional Stellite 21 coating developed cracks due to thermal gradients and residual stresses, the Mn-modified variant remained crack-free. The friction coefficient stayed between 0.28 and 0.45 during durability tests, meeting application requirements. The maximum braking temperature reached 813 °C, with simulations showing approximately 10 % deviation from experimental values. This work demonstrates the potential of Mn-modified Stellite 21 coatings for crack-resistant braking systems in ultra-high-speed trains.