Railway disc brakes are safety components subjected to restrictive standards in terms of braking performance. Friction must remain the same regardless of use and environmental conditions. Recently, new constraints have stacked upon these requirements, related to the evolution of disc brake use, and raising new concerns about friction stability. Firstly, the development of complementary braking technologies leads to extended service life of disc brake components on rolling stock, exposing them to ageing issues. Secondly, in an effort to lighten train structures and increase service speeds, fewer braking systems must dissipate higher amounts of energy. And finally, the increasing traffic density leads to uneven rolling conditions, with pronounced acceleration and deceleration phases. All this may expose disc brakes to severe braking conditions and affect friction stability in a way that is currently misunderstood. Industrial tests performed by Alstom Flertex showed that a severe braking history can significantly affect friction. A full scale dynamometer, equipped with a steel disc and sintered copper-iron brake pads, was exposed to repeated emergency braking. In order to study friction evolution with braking history, service braking performance was characterized beforehand and afterwards. The results show that friction after the emergency braking series was reduced by about 10% as compared to before. The aim of this work is to establish a relationship between severe braking history and friction evolution. On a reduced scale pin-on-disc tribometer, similar disc and pad materials were exposed to either service or severe braking. Friction evolution was characterized beforehand and afterwards under service conditions. Infrared thermography and thermocouples embedded close to the pad and disc surfaces were used to monitor the heat dissipation and contact localization. The evolution of the disc surface was recorded by a high speed camera, and displacement sensors provided information about the pad wear and disc thermomechanical distortion. Surfaces rubbed during full scale and reduced scale experiments were analyzed using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray spectroscopy (EDX), in order to characterize differences in sliding velocity accommodation and wear mechanisms between high and low friction circumstances. The oral presentation will focus on the analysis of the friction decrease process, from full scale observations to deeper reduced scale experiments. Similar friction behaviours and temperature levels are observed at both scales, under service and severe braking conditions. Reduced scale experiments produced a friction decrease similar to what was observed on the industrial dynamometer. Further analysis at reduced scale show that the friction deviation is correlated with smaller apparent contact area, attributed to uneven pad wear and disc thermomechanical distortion during severe braking. Service brake applications following the friction decrease show a slow increase in friction back to its former level. Possible causes of the reduced friction, and the recovery process, are discussed based on multi-scale surface analysis, regarding the evolution of load-bearing zones in terms of distribution, size and composition. Friction surfaces after full scale industrial tests are also analysed, to identify the mechanisms and processes observed during reduced scale analysis.

Mr. Sylvain Delattre, PhD Student, Alstom Flertex SAS; Dr. Anne-Lise Cristol, Lecturer, Ecole Centrale de Lille; Prof. Philippe Dufrénoy, Professor, Polytech Lille; Prof. Yannick Desplanques, Professor, Ecole Centrale de Lille; Dr. Michèle Henrion, R&D and Application Engineering Manager, Alstom Flertex SAS