Brake discs are safety elements that are constantly solicited in the field of land transport. They must be able to ensure braking performance and dissipate large quantities of energy while limiting nuisances (wear, noise, particle and VOC emissions). Moreover, during braking, the contact evolves in response to thermomechanical deformations, thermoelastic instabilities and wear, which are the source of thermal localizations, such as hot bands or hot spots [1]. The resulting contact localizations lead to extreme stresses (pressure, shear, temperature), capable of transforming the materials close to the friction surfaces (phase change, adsorption, corrosion, plasticity, damage...) [2,3]. They gradually lead to the formation of a surface layer called TTS (Superficial Tribological Transformation) which is a privileged site of particle detachment [4]. These microstructural modifications are thus determinant of the tribological behavior and wear processes, as a source of third body and therefore of particle emission [5,6]. In the context of automotive braking , stainless steels with differentiated microstructure are studied. They are compared in a perspective of reduction of particle emissions to a reference lamellar graphite cast iron brake disc. The braking behavior is studied in the case of a composite brake pad with organic matrix . A specific experiment is developed on a test bench of the laboratory dedicated to the transport braking. The design of the assembly and the samples (disc and pad) as well as the choice of the experimental protocol are based on a transient thermal analysis by finite elements in order to reproduce braking solicitations representative of urban traffic. Thermomechanical stress is evaluated by IR thermography of the friction track and by thermocouples placed on the near surface at different depths and radial positions of the disc and pad. Particle emissions are collected thanks to the confinement of the assembly in a chamber crossed by a constant air flow with controlled hygrometry. They are characterized at a frequency of 10 Hz over the range [6 nm - 10 µm]. Total wear is estimated by profile measurements of the rubbed surfaces (discs and pads) before and after the tests, while capacitive sensors provide indicators of wear evolution during the test campaign. Disc and pad samples are taken before and after the test in accordance with the thermal location history. They are analyzed by scanning electron microscopy in order to characterize the third body layer and the microstructural evolutions in the near surface (FIB section). This oral communication presents the experiment developed and the results obtained . The surface tribological transformations are discussed according to the solicitation history, the characteristic tribological mechanisms of the load-bearing areas and the velocity accommodation, in order to reconstruct the wear processes and their correlation with particles emissions. References : [1] Panier, S., Dufrenoy, P., Weichert, D. An experimental investigation of hot spots in railway disc brakes. Wear 256(7–8) 764-773 (2004). [2] Kasem, H., Brunel J.F., Dufrenoy P., Siroux M., Desmet B., Thermal levels and subsurface damage induced by the occurrence of hot spots during high-energy braking. Wear 270(5–6) 355-364 (2011). [3] Ganeev, A., Nikitina, M., Sitdikov, V. et al. Effects of the Tempering and High-Pressure Torsion Temperatures on Microstructure of Ferritic/Martensitic Steel Grade 91. Materials 11(4) 627 (2018). [4] Boher, C., Vidal, V., Cabrol, E., Berthier, Y., Rezai-Aria, F., Shear mode M3 in the first sites of ductile metallic alloys: some considerations on the physical mechanisms leading to internal particle flows, Wear 426–427 1152-1162 (2019). [5] Collignon M. et al. Braking performance and influence of microstructure of advanced cast irons for heavy goods vehicle brake discs. Proc IMechE Part J: Journal of Engineering Tribology 227(8) 930–940 (2013). [6] Blau P. J., Elevated-temperature tribology of metallic materials, Trib. Int. 43(7) 1203-1208 (2010).

Mr. Mathis Briatte, Doctorant au LaMcube, Laboratoire de Mécanique, Multiphysique, Multiéchelle (LaMcube) - UMR 9013; Mr. Alexandre Mege-Revil, Maître de conférences, Laboratoire de Mécanique, Multiphysique, Multiéchelle (LaMcube) - UMR 9013; Mr. Yannick Desplanques, Professeur des universités, Laboratoire de Mécanique, Multiphysique, Multiéchelle (LaMcube) - UMR 9013; Mr. Pierre-Olivier Santacreu, Head of Department Tech.Watch & Breakthrough, APERAM Stainless Steel, Research Center, BP15, 62330 Isbergues