Airborne particulate matter from brake systems: An assessment of the relevant tribological formation mechanisms

The growing awareness on the best strategies for improving environmental conditions, particularly in urban areas, has fostered the interest for those sources releasing particulate matter (PM) in the atmosphere, including non-exhaust emissions from road vehicles. These are becoming increasingly important and a main contributor is the PM from brake systems. The search for better brake materials, as concerns their PM emission behavior, requires the knowledge of the wear mechanisms at the pads-disc interface. The focus of this paper, based on recent literature results, is to identify and discuss the specific tribological phenomena leading to the formation of coarse (e.g., PM10 and PM2.5), fine (PM1) and ultrafine airborne PM. The in-depth understanding of the formation mechanisms of airborne PM is a prerequisite for designing new friction material formulations, possibly containing green ingredients. Considering the essential role in supporting the research efforts in this field, the main experimental approaches, based on dynamometer bench tests and pin-on-disc tests, are also recalled and discussed.