Iron Oxide and Hydroxide Speciation in Emissions of Brake-Wear Particles from Different Friction Materials using X-ray Absorption Fine Structure

Abstract

Iron (Fe), the main component of non-exhaust particulates, is known to have variable health effects that depend on the chemical species of iron. This study characterized the possible contribution of iron oxides and hydroxides to airborne brake-wear particles under realistic vehicle driving and braking conditions with different brake-pad friction materials. We found significant differences in wear coefficients and PM10 emissions between non-asbestos organics (NAO) and European performance (ECE) brake pads. Iron was the dominant contributor to PM10 and PM2.5 brake-wear particles for both NAO and ECE. The iron concentration ratio in the particle mass were comparable to the disc to pads ratio measured by wear mass. The fact that magnetite, which is of interest with respect to health effects, was less abundant in NAO than in ECE suggested that tribo-oxidations occurred in NAO. Metallic iron is generated not only from abrasive wear but also from tribo-chemical reduction with magnetite as the starting material. We found differences not only in PM emissions between brake friction materials but also that the phase transformations of iron differed between friction materials. These differences were apparent in the distribution of iron oxides and hydroxides. Heat, tribo-oxidation, and tribo-reduction are intricately involved in these reactions.