The Impact of Composition in Non-steel and Low-Steel Type Friction Materials on Airborne Brake Wear Particulate Emission

In this work, airborne brake wear particulate matter (PM) emissions from a brake system were investigated by time-resolved and temperature-dependent measurement using a dynamometer. The measurement was performed for representative friction materials, 3 low-steel (LS) and 4 non-steel (NS), which are currently in worldwide use. The PM emission factor was found to be varied as large as by one order of magnitude depending on the composition of friction materials(pads). The airborne particle mass emissions from the LS materials ranged from 1.88 to 3.14 mg/km/vehicle, while the emissions from the NS ranged from 0.3 to 2.34 mg/km/vehicle, which is, in general, smaller than the LS. The time-resolved data imply that particle emissions in the extra-high-speed region of the WLTC cycle, where friction occurs at high temperature (T_disk > 150 °C), is much higher than in the low-speed region, and determines the total PM mass emission factor. It was found that the friction materials containing metals such as Cu and Sn (LS-2/-3 and NS-4/-5) exhibited a lower PM emission factor. This result suggests that copper and tin, which forms an effective lubricating tribolayer in the interface between the pad and disk at high temperature, remarkably reduces PM emissions. It has been also found that the surface roughness of worn brake pads is positively proportional to PM emissions according to surface topography analysis, which is consistent with composition effect. These findings suggest that tribological engineering to provide sliding frictional behavior at elevated temperature is crucial to reducing PM emissions.