Friction Control Performance of Top-of-Rail Friction Modifiers under Different Application Conditions

Railway transportation is undergoing a transformation, where operation and maintenance are now equally prioritized alongside construction and development. The lubrication technology of the wheel-rail interface, comprised of top-of-rail friction control technology and rail gauge lubrication technology is gradually becoming a critical aspect of wheel-rail material maintenance. However, there is currently limited research reporting on the application conditions and friction control performance of top-of-rail friction modifiers with different substrates. This paper investigated the friction control performance and mechanisms of five types of top-of-rail friction modifiers under varying wheel-rail surface roughness and application amounts. Firstly, the viscosity, density, elemental composition and major element content of each top-of-rail friction modifier were analyzed using a rotational viscometer, electronic balance and electron microscope. Then surface roughness parameters of initial wheel-rail rolling contact specimens were obtained using a twin-disc testing rig, and the influence of roughness on lubrication state parameters was derived based on an empirical formula. Subsequently, the influence of five types of top-of-rail friction modifiers on wheel-rail adhesion and wear was tested on the twin-disc testing rig, including their effects on the adhesion coefficient, retentivity, friction characteristics, wear rate and surface roughness. Refined based on the test results, three comprehensive testing metrics were proposed: adhesion coefficient controlling performance, retention performance and wear reduction performance. The results indicated: (1) reasonable application of top-of-rail friction modifiers could transition the wheel-rail interface from dry friction to mixed lubrication, resulting in moderate levels of adhesion coefficient and positive friction characteristic; (2) the distribution amount and shearing strength of top-of-rail friction modifiers were key parameters affecting the adhesion behavior of wheel-rail contact interface. Increasing the surface roughness of the wheel and rail surface roughness or reducing the application amount of top-of-rail friction modifiers could increase the load-bearing capacity of the metal surface asperities, thereby avoiding low adhesion phenomenon. Base materials with low shear strength, such as lubricating oil and grease could enhance the lubrication performance of the material; (3) the safe application amount of different base top-of-rail friction modifiers from high to low was the water-based, the oil-based, and the grease-based top-of-rail friction modifier. The higher the adhesion coefficient produced by the application of top-of-rail friction modifier, the poorer their retention performance at the wheel-rail interface; (4) the application of top-of-rail friction modifiers significantly reduced wheel-rail wear. The wear reduction performance from high to low followed the order of the winter water-based friction modifier, the oil-based top-of-rial material, the mixed water-based friction modifier, the grease-based top-of-rial material and the summer water-based friction modifier. The research results could offer the oretical backing for the development, testing and selection of top-of-rail friction modifiers.