Iron Oxide and Hydroxide Speciation in Emissions of Brake Wear Particles from Different Friction Materials Using an 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 factors and PM10 and PM2.5 emissions between non-asbestos organic (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 (PM) was 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 that there were differences in PM emissions between brake friction materials, and 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.

Light-Duty Vehicle Brake Emission Factors

“…GCI discs are popular due to their advantages but, at the same time, are associated with high wear. In general, in GCI with ECE high-emitting brakes, the disc contributes around 60% to total mass loss [16,77,82,83]. With drum brakes, the contribution is lower; one study found 37% [83].…”

Brake Wear and Airborne Particle Mass Emissions from Passenger Car Brakes in Dynamometer Experiments based on Worldwide Harmonized Light-Duty Vehicles Test Procedure Brake Cycle

“…The conditions required to control the test brake, the tire dynamic load radius and inertia [34], and the nominal wheel load to disc mass ratio (WLn/DM) required to define the brake temperature are given in Table A1. Three enclosure/tunnel types were used (Table A1): Type A, the enclosure/tunnel (inner diameter φ80.7) equivalent to JASO [9] used in previous studies [35,36]; Type B, enclosure type A changed to a height of 700 mm and tunnel type A changed to an inner diameter of φ80.7); and Type C, enclosure type A changed to a depth of 200 mm, and tunnel type A changed to an inner diameter of φ208.3 as in [37]. The brake assembly was fitted to the dynamometer using universal style (L0-U) brake fixings as described in GTR24 [7].…”

“…The sampling of brake wear particles was based on the JASO C 470 test method [9] and previous studies [35,36]; the process consists of an enclosure with a brake assembly inserted downstream of air supplied through a HEPA filter and a constant-flow sampling tunnel (25°C standard for enclosure/tunnel Type A, 20°C standard for enclosure/tunnel types B and C) (Table A1) because the GTR24 test method was not defined at the time of this experiment [7]. The cooling air flows in a rightto-left direction when the brake disc is viewed from the front, whereas the disc rotates in a counterclockwise (CCW) direction [7] or clockwise (CW) in some of experiments.…”

“…Table B1 summarizes mass percentages of elements in the brake pads and discs used in this study. The elements of brake pads (Table B1) and discs (Table C1) were measured by wavelength-dispersive X-ray fluorescence (WD-XRF) analysis (ZSX Primus II, Rigaku, Corp.) [36].…”

“…Sampling of PM10 and PM2.5 was conducted with a multi-cascade impactor (MCI) (MCI-20, Tokyo Dylec Corp.) according to the methods reported by [35,36] and JASO test protocol [9] on a Teflon filter (Fluoropore FP-500-100, 47 φ for PM10 and PM2.5, Sumitomo Electric Fine Polymer Corp.) or a Teflon filter with a support ring (PT47, 47 φ, Measurement Technology Laboratory). Aspiration was from a isokinetic sampling nozzles.…”