Sustainable design of low-emission brake pads for railway vehicles: An experimental characterization

Non-exhaust particulate matter (PM) emissions deriving from transports are steadily increasing over last years. Nevertheless, there is a proven lack of up-to-date emission inventory guidelines, as well as a lack of related research studies. Rail transportation such as trains and subways contribute to non-exhaust PM emissions from different phenomena and specifically from braking events, during which PM of different dimensions can be emitted from both mechanical processes and high temperature processes. This study reports the development and application of an experimental procedure for the investigation of particulate matter PM₁₀, PM_2.5 and PM₁ emissions in the atmosphere produced by brake pads for railway vehicles. Two different test programs composed of several brake events each were conducted on a dynamometer test bench to simulate the brake performance of a railway vehicle during typical routes. PM emission levels were measured with an Electrical Low-Pressure Impactor at the exhaust air channel of the dynamometer. The tests were performed on an innovative sintered material and two commercial organic materials. For both test simulation routes, the three brake pads exhibited very similar friction performances and very similar average maximum temperature profiles. On the other hand, the innovative sintered material revealed a considerably better behavior in terms of wear resistance compared to the organic materials. Relationships between wear rate, train speed, applied forces and particle concentrations were detected in the wear processes occurring in the braking materials. PM concentrations produced by the sintered brake pad during both simulation tests were substantially lower than the organic pads and the differences were more pronounced for fine particles (PM_2.5) and ultrafine particles (PM₁). The shape of particle size distributions was similar for the three materials, with the maximum mass concentration of particles measured in the coarse particle region and two other modes in the fine and ultrafine particle regions. An increase in the production of ultrafine particles was observed for an increase in the temperature of the disk, while the production of coarse and fine particles remains substantially unaffected. The experimental results provided in this work can furnish the basis for developing the optimal formulations of brake pad materials with low environmental impact and for setting specific measures on train brake pads composition and production for the mitigation of non-exhaust emissions.