Evolution of friction and wear in rack-railway gear–rack pairs under heavy-load dry sliding conditions

Rack railways commonly operate under extreme conditions—steep gradients, heavy loads, and dry sliding—where wear of the gear–rack pair becomes a critical limitation to both safety and cost-effectiveness. To address the lack of systematic optimization in material pairings, this study examines four gear–rack configurations: 18CrNiMo7-6/42CrMo, 18CrNiMo7-6/45 steel, 20CrMnMoA/42CrMo, and 20CrMnMoA/20CrMnTi. Short-term (104 cycles) and long-term (105 cycles) reciprocating dry-sliding tests were conducted on a ball-on-flat tribometer. The coefficient of friction and interfacial temperature rise were continuously monitored, 3D surface topography and material loss were quantified, and multiscale scanning electron microscope/energy-dispersive spectroscopy analyses were performed to elucidate the evolution and dominant mechanisms of friction and wear for each pairing. The findings indicate that the higher-hardness 45-steel rack delays wear initiation and yields the shallowest wear scar. However, its thin, brittle, mechanically mixed layer fractures readily under cyclic loading, generating spall pits that raise friction and increase the risk of contact fatigue. The softer 20CrMnTi rack generates debris that forms a thick, continuous, oxygen-rich transfer film, markedly lowering the friction coefficient but producing the deepest scar and the greatest material removal. Substituting 18CrNiMo7-6 with 20CrMnMoA as the gear material enhances wear stability and surface smoothness, fostering a dense oxide film conducive to long-term low wear. Considering frictional behavior, wear volume, and surface damage comprehensively, the 20CrMnMoA/42CrMo pairing performs best under both short- and long-term conditions, significantly prolonging gear life while causing only moderate rack wear. This study provides a scientific basis for material selection and hardness matching in rack-railway drive systems.