Reproducibility of physico-chemical properties in brakes emissions generated at the dynamometric bench

Reproducible generation, collection and physico-chemical characterization of brakes emissions are necessary when comparing particulates generated by different friction couples, brake designs or driving conditions. Furthermore, they are pivotal to provide universally reliable chemical information for source apportionment studies as well as for toxicological investigations and environmental assessments. Non-exhaust particulates generated by brakes are nowadays commonly collected at suitably designed variable inertia dynamometer benches, during selected braking test cycles in controlled conditions to avoid other contributions from the environment. In spite of being simplified models of a single real car semi-axis corner, dynamometric benches for brakes emissions collection still remain elaborate mechanical devices used for testing complex materials. Therefore, if the tribological interaction is not kept constant over different tests, significantly different local forces and temperatures might be expected at the tribological interface. This can eventually result in the generation of particulates with different granulometry and chemical composition, even if originated by the same braking device components. At this regard, the work here presented explores for the first time the capability of a brake dynamometer bench dedicated to emission tests to generate reproducible particulates. In particular, the assessment of the intra bench reproducibility over an extended period of time and number of tests is discussed. Specifically, the variability of physico-chemical properties such as particle size distribution, elemental composition and crystalline phase distribution is evaluated. In detail, it is observed that the elemental concentration of main constituents such as Iron and Oxygen are statistically equivalent during a set of ten tests, even at strict confidence level (α = 0.05). In addition, when statistical differences are observed in elemental concentration of secondary and minor constituents, their variability typically remains limited below the 10 % of the corresponding figure of merit. Similar findings are also observed when assessing the phase composition of main constituents compounds, such as Iron oxides, as well as the particle size distribution profiles. Conversely, higher relative variabilities up to 25 % are registered when assessing the phase composition of secondary and minor phases. The reported results are meant to provide a first critical assessment on the variability of both the size distribution and the chemical composition of the emissions generated by brakes in the laboratory environment typically used for their generation, measurements and collection.