Friction instabilities on corroded surfaces of automotive brake materials

"The recent paradigm shift related to regenerative/blended braking in electric vehicles led to the decrease in usage of friction brakes, resulting in accumulation of rust on the friction surfaces of brake components [1]. This is due to a ?less effective refreshment? of friction surfaces in wear process, but also as a consequence of exposing automotive brakes to harsh environmental conditions causing corrosion on the friction surface. These factors alter the chemistry of friction layer and affect friction mechanisms, which can significantly disturb the braking performance. The goal of this paper is to address the changes in surface morphology and friction performance of a brake couple after subjecting it to corrosive environment. Gray cast iron rotor was tested against a laboratory prepared non-asbestos organic (NAO) brake pad. Tested samples were subjected to corroding environmental conditions as per SAE J2334 (Laboratory cyclic corrosion test) recommended practice. The most severe sections of scaled down SAE J 2522 procedure, based on laws of physics, were adopted to design friction tests (UMT, Tribolab) [2,3]. The surface morphology and friction mechanism of the tested rotors and pads were analyzed using scanning electron microscopy (FEI Quanta FEG 450), equipped with energy dispersive x-ray microanalysis (Oxford detector, Inca Systems) and X-ray diffraction (Rigaku MiniFlex II). When compared to ?normal samples?, the corroded brake samples exhibited significantly higher friction instabilities. These phenomena were correlated to changing wear and friction mechanisms, induced by the presence of newly formed corrosion products. Abrasive wear mechanisms were aggressive to surfaces, which led to formation of a weak and instable friction layer comprised of abraded metal oxide particulates compressed together. When still present, this weak layer exhibited higher friction level but was suddenly easily removed which led to a sudden change in the friction level. The observed formation and removal of instable friction layer continued until wear removed the corrosion products embedded in pads and rotors. Obviously, the results from friction tests relate to the difference in chemistry and morphology of friction layers developed on corroded and normal surfaces of both rotor and pad. These data confirm the vulnerability of current materials when used in vehicles with regenerative braking. The new materials with better corrosion resistance would be beneficial for situations with blended braking scenarios in vehicles with regenerative braking. References: [1] Antanaitis, D., ""Effect of Regenerative Braking on Foundation Brake Performance,"" SAE Int. J. Passeng. Cars ? Mech. Syst. 3(2):14-30, 2010, https://doi.org/10.4271/2010-01-1681. [2] Hazhir Athari, Friction material downscaling and its effect on brake system performance, SAE Brake colloquium 2017, 17BC-0056. [3] Sai Krishna Kancharla, Effect of Humidity and testing strategy on Friction Performance of model brake pads containing Nano-additives. Southern Illinois University at Carbondale; 2018. "