Transition of the friction behaviour and contact stiffness due to repeated high-pressure contact and slip

A rapid increase in the friction coefficient can occur during the running-in between the wheel and rail. Although it has been found that the running-in process depends on the initial topography, the difficulty in obtaining accurate non-destructive interfacial measurements has hindered systematic investigations. In this work, four interfaces, which have different initial topographies, were continuously monitored using ultrasound reflectometry until they became conformal. A contact pressure representative of that in a wheel-rail interface was achieved by using a high-pressure torsion test approach. The transition of contact stiffness and friction coefficient with repeated slip and their relationship were investigated. Based on the experimental results, a mechanistic model for the running-in process of the contact interface was proposed. These findings will help in understanding the running-in process of the wheel-rail interface and assist in managing the wheel and rail appropriately to improve safety. A common insight into the running-in process for metal-to-metal contacts under high contact pressures has also been developed.