Understanding the Formation Mechanisms of Leaf Layers on the Rail Surface using a High-Pressure Torsion Test

For decades, rail systems with the ability to carry large numbers of passengers over long distances have been the most carbon-efficient form of public transport and they have become an essential part of our daily mobility. In the UK, however, many passengers express dissatisfaction with rail delays. These are often caused by factors such as congestion, signalling problems and low friction resulting from leaf layers. The formation mechanism of the leaf layer is still largely not understood..

This study used an HPT test machine to simulate the actual wheel-rail contact interface to study how leaf layers are formed on the rails and depleted by cycles aimed to simulate a wheel passage over the layer. This experiment can help us better understand and study the basic mechanisms of leaf layer formation and lay the foundation for studies on how to remove the black layer more efficiently in the future.

The tests used different ratios of leaves to water, but the final results seemed similar. After the 4th cycle, a black layer formed, and the coefficient of friction (CoF) maintained a stable value from the 5th to the 8th cycle. After the 8th cycle, the value of CoF increased. At the same time, the thickness of the leaf layer on the wheel and rail specimens continued to decrease, which also indicated that the leaf layer was wearing away and eventually metal-to-metal contact occurred.

Such results are consistent with the results of experiments carried out on track, which also proves that this is a relatively reliable experiment. It provides a viable test platform for assessing low friction mitigation products.