Relationships between third body flows, load-bearing mechanisms and particle emissions in automotive braking

Abstract

During automotive braking, the third body generated by friction forms a tribological circuit that feeds the contact between the disc and the pad and provides the energy dissipation capacities of the disc brakes. This results in emissions of airborne matter particles, composed of fine and ultrafine particles, are mostly carcinogenic metal oxides. In order to reduce these particle emissions, it is important to investigate the factors involved in their formation, and in particular the link between the particle emissions dynamics, the tribological circuit and the thermomechanical load in the contact.This study focuses on a martensite stainless steel as an alternative to a lamellar graphite cast iron usually used for automotive brake discs. Thus, 2 disclining friction pairs are studied in the objective to evaluate the effect of disc material and braking conditions on the production and the emission of particulate matter during braking by friction. This paper presents the results obtained with a sintered metal matrix composite lining, the steel discsintered metal lining couple being used for motorcycle applications. Tests were carried out on a laboratory tribometer dedicated to braking simulation. Two situations of automotive braking, typical of urban and suburban traffic, were modeled and studied. Using a visible high speed camera, the evolution of the disc friction track in terms of morphology was observed. In addition, an infrared thermal camera was used to visualize the thermal dissipation localization. Particle emissions were studied in operando thanks to a sampling chamber under controlled air flow and two particle matter analyzers providing a particle rate by size in the range [6 nm; 10 μm]. Furthermore, an impactor was used to collect emitted particles for post-mortem chemical composition analyses. Pad and disc friction surfaces were analyzed using scanning electron microscopy and energy dispersive spectroscopy.During these tests, an evolution of the size and density profile of the particles collected by the analyzers was observed, and was correlated to a thermomechanical localization in the form of hot bands or hot spots according to the cases studied. The analyses of rubbed surfaces, crossed with these localizations, allowed us to develop scenarios favoring the feeding of the rate of coarse, fine or ultrafine particles.