Using Tribo-Chemistry Analysis to Understand Low Adhesion in the Wheel-Rail Contact

Low adhesion between wheel and rail is a recurrent problem for the rail industry. Low adhesion can lead to wheel slides and slips during acceleration and deceleration, which can cause large amounts of damage to the wheel and rail as well as causing safety issues and delays if a train cannot accelerate or decelerate when necessary. Adhesion in the wheel-rail contact is affected by the third body layer which is present in the contact patch between wheel and rail. It is composed naturally from steel wear debris and iron oxides, but often contains other contaminants such as organic matter, ballast dust, soil and grease. Different environmental conditions such as temperature, precipitation and humidity change the properties of this third body layer and therefore change adhesion conditions on the railway. Low adhesion has been well documented throughout the autumn season due to organic contamination, but also takes place throughout the year when no visible contamination is seen on the railhead, known as the “wet-rail” phenomenon. It is thought to occur when there are low levels of water on the railhead, formed by dew, mist or light rain, rather than heavy rain. The conditions and mechanisms that cause the phenomenon are not fully understood. Low adhesion does not occur very often and under what is likely to be a narrow window of conditions, which means that it can be difficult to simulate and study. The aim of this work was to use a combination of tribology and chemistry to better understand the cause of low adhesion throughout the year, known as the wet-rail phenomenon. It investigated low adhesion conditions that occur all year round, initially focusing on the role of iron oxide in low adhesion as it has previously been hypothesised that oxides could play a major role in the wet-rail phenomenon. Testing was carried out over a range of conditions on three different tribological test rigs to attempt to simulate low adhesion due to the wet-rail phenomenon, which produced valuable information about the causes of low adhesion. It was found that, under certain conditions, a combination of iron oxides and water could cause low adhesion in a simulated wheel-rail contact. Test methods were designed to simulate the wet-rail phenomenon, which can be used as a platform to better understand the causes of low adhesion and to test future mitigation methods.