Railway transport is considered as a sustainable solution with a low contribution to pollutant emissions. Nevertheless, in underground railway enclosures, several studies show that the concentration of particles is of particular concern1. It has been shown that the source of particle emissions is closely related to brake wear emissions2. During a braking event, the high dissipated energy in the contact results in large frictional heat dissipation and wear processes of both pads and disc, resulting in the generation of particles and gases. The generated particles are used by the contact, namely forming the third-body layer, whereas a part of the matter flows is ejected out of the tribological circuit, namely the wear flow3. Thus, wear depends on the ability of the tribological system to control a matter flow balance while maintaining the friction performance. The involved mechanisms are complex, limiting the relevance of predictability of theoretical models4. Experiments, which are carried out at the system scale, remain necessary to quantify wear and then the service life of friction parts. However, these tests have a significant implementation time, high costs and with a limited scope. Therefore, having a scaled down brake system would be very suitable. This paper deals with an original development of a downscaling strategy that allows the reproduction of braking performances and wear of railway disc-brake systems. This approach is based on a complete understanding of the thermomechanical phenomena occurring during braking with respect to tribology. Indeed, a thermal approach based on heat transfers and contact surface temperature is necessary but not sufficient, as the thermomechanical phenomena largely affect sliding-wear processes5. The new thermomechanical approach involves the kinetics of thermal localization as well as the contact dynamics that drives the tribological circuit. Tests were carried out on a full-scale bench equipped with a railway brake with a cast iron disc and organic pads. Since wear is a matter of brake usage, tests are based on a given subway route and various endurance conditions. The results allow developing a new brake design convenient at the reduced scale, thanks to a numerical model to adapt and transpose the braking conditions. Highly instrumented tests were carried out at both scales to consolidate the experimental strategy. Thermocouples, distributed in disc and pads are used for temperature measurements near the sliding surface. Temperature maps provided in addition to IR thermography monitoring of the disc-track allow to characterize the evolution of thermal localizations and related load-bearing areas in the contact. The wear behavior is monitored at the full scale using an original setup combining displacement laser sensors with a laser line profiler and a high frequency airborne particle sizer device. Indicators of the tribological circuit, wear and the kinematic accommodation of the disc-lining contact are evaluated in order to characterize and quantify the wear processes in correlation with the dynamics and the history of the solicitation of the contact. This oral communication will present the thermomechanical downscaling strategy. Experimental results relative to both scales will be detailed and discussed. The project is funded by the French government as part of the PIA, Invest for the Future Plan, now integrated into France 2030, and operated by ADEME, the French ecological transition agency. References: 1. Kwon, SB., Namgung, HG., Jeong, W. et al. Transient variation of aerosol size distribution in an underground subway station. Environ Monit Assess 188, 362 (2016) 2. Namgung HG., Kim JB., Kim MS. et al. Size distribution analysis of airborne wear particles released by subway brake system. Wear 372–373,169-176 (2017) 3 Desplanques, Y. & Degallaix, G. Genesis of the Third Body at the Pad-Disc Interface: Case Study Of Sintered Metal Matrix Composite Lining Material. SAE Int. J. Mater. Manuf. 2, 25–32 (2010). 4. Berthier, Y. Experimental evidence for friction and wear modelling. Wear 139, 77–92 (1990). 5. Cristol, AL., Desplanques, Y. & Degallaix, G. Coupling between friction physical mechanisms and transient thermal phenomena involved in pad–disc contact during railway braking. Wear 263, 1230–1242 (2007).

Mr. Joseph Frangieh, PhD Student, Centrale Lille; Dr. Edouard Davin, Lecturer, Centrale Lille Institut - LaMcube; Dr.-Ing. Florent Brunel, Research Engineer, Centrale Lille Institut - LaMcube; Prof. Philippe Dufrenoy, Professor, University of Lille; Prof. Yannick Desplanques, Professor, Centrale Lille Institut; Mrs. Michèle Henrion, R&D and Application Engineering Management, Alstom Flertex SA; Mr. Guillaume Funck, Site Engineering Director, Alstom Flertex SA; Dr. Ruddy Mann, Friction Brake Expert, Alstom SA