The evolution of brake pad material caused by brake loads is a key concern in tribology. Indeed, this evolution has a direct impact, on the one hand, on the management of the tribological circuit (wear etc.) and, on the other hand, on the extent of the area in contact. In literature, the effect of brake loads on the microstructural evolution, however, has so far not been intensively investigated. This challenging issue resulted from the complexity of brake loads, which combined mechanical, thermal, physicochemical loadings etc.

To understand the problem without tackling it in all its complexity, this study proposes an experimental approach where physics is decoupled but inspired by the braking sequence.

Two experimental approaches of thermal gradient test and hot-compression tests are carried out to highlight the effects of thermal (temperature gradient) and mechanical (compression) solicitation. The brake pad material used in this work was a sintered metal matrix composite with graphite and ceramic inclusions which derived from an industrial formulation for high energy railway brake. The evolution of brake pad material is characterized by Scanning Electron Microscopy (SEM) coupling with Energy-dispersive X-ray Spectroscopy (EDS). Sequentially, the obtained results are discussed by associating with the strain fields collected by a compressive test equipped with a Digital Image Correlation (DIC).

By the thermal gradient test, the surface oxidation is identified as the principal evolution. Different oxidation behavior between various components in the metal matrix and the ceramic inclusions were figured out in considered temperature range. Combining thermal solicitation with mechanical solicitation in the hot-compression test, deformation of the metal matrix and the graphite inclusions, which was identified as primarily responsible for the evolution of elastic modulus as a function of the pressure, was tracked in the heterogeneous material. This work provides a baseline for predicting the evolution of friction material during the real braking sequence.

Dr. Hoang-Long Le Tran, Ingénieur de recherche, University of Lille; Dr. Anne-Lise Cristol-Bulthé, University of Lille; Dr. Vincent Magnier, University of Lille