Providing predictable and optimised traction and breaking through tribo-chemical understanding of the wheel / rail interface

The chemical, physical, and tribological features of the wheel / rail contact are studied through a combination of environmental monitoring, chemical analysis, rheology, and tribo-testing. The work presents novel platforms for the combined analysis and physical testing of rail steel surfaces and oxide pastes, including friction mapping of surfaces, which have clear use in wheel / rail contact research as well as wider applications. X-Ray Diffraction identified eight iron compounds including iron oxides and iron oxide-hydroxides on the surface of operational rail-tracks. Trackside environmental monitoring revealed variation in conditions due to shaded areas and presence of vegetation. The railhead temperature is shown to lag behind that of the environment and occasionally below the dew point temperature; this is shown to happen most frequently in the early morning and can be related to the frequency of low adhesion events. A low adhesion condition was linked to the presence of wüstite, iron oxide-hydroxides, and small amounts of dew on the railhead. A newly proposed mechanism for the loss of traction due to oxides and debris on the railhead, in combination with small amounts of water, is supported by rheological and twin-disc tribo-testing which considers the extent of material entrainment as well as effect within the contact. Aqueous oxide pastes are shown to exhibit behaviours of Bingham-plastic fluids displaying both solid and liquid properties. High yield shear stress pastes are more entrained in the wheel / rail contact. High viscosity pastes more effectively transmit tractional forces through their layer and decrease the coefficient of traction less as a result. Results from the work provide new insights into how the surface of the railhead changes with environmental conditions as well as how oxide/water mixtures interact within the wheel / rail contact, including potential uses of viscous high shear stress pastes to control traction levels.