Improving rolling contact fatigue behavior in wheel–rail systems using a slag powder–oil mixture friction modifier

This study investigates the effect of different top-of-rail friction modifier (FM) formulations on the rolling contact fatigue (RCF) behavior at the wheel–rail interface. Three different FMs were examined: flange oil, slag powder, and a hybrid oil + slag powder mixture. A twin-disc test was conducted over 700,000 cycles to evaluate crack initiation and propagation. Surface and subsurface damage characteristics were analyzed using scanning electron microscope (SEM) micrographs. The results show that under flange oil conditions, RCF cracks propagate deeper beneath the surface (127 µm), and the presence of liquid oil promotes crack opening and expansion due to hydraulic pressure within the microcracks. In contrast, slag powder (FMA) alone caused severe wear and surface delamination, demonstrating the lack of protective film formation. However, when combined with slag powder + oil (FMB), the cohesive third body layer formed by viscous oil and slag powder particles effectively restricted subsurface crack growth. The FMB formulation reduced the average subsurface crack length by approximately 42% compared to the flange oil condition and by approximately 35% compared to the slag powder-only condition. Similarly, it successfully reduced the average crack depth by up to 77% in rail discs. This improvement demonstrates the hybrid FM's ability to balance local stresses and suppress hydraulic crack propagation. In general, the findings indicate that hybrid FM, composed of oil and solid waste-based particles, can significantly increase RCF resistance by balancing lubrication and surface protection mechanisms. This highlights that it offers a promising approach for extending the service life of wheel/rail systems.