Martin Morgeneyer

With the implementation by the European Union since the 1980s of strict measures to reduce emissions of air pollutants (PM10, PM2.5, NH3, CH4, O3, NOX, SOX, VOCs (volatile organic compounds), etc.), these emissions have fallen from a total of nearly 65 million tons in 1990 to around 20 million tons in 2021, according to data from the European Environment Agency. In the road transport sector, the implementation of the first European emissions standard in 1992 and the electrification of vehicles have made it possible to reduce exhaust emissions. As non-exhaust emissions increased, the European Commission introduced thresholds for braking systems for certain vehicle categories in the future Euro 7 standard. Car manufacturers are looking for solutions to reduce brake particle emissions, including the modification of the composition of the brake pads and discs. This literature review aims to present the state of the art of a set of parameters that can influence brake particle emissions. The parameters highlighted here include the raw materials and manufacturing process parameters of the brake pads, the composition of brake discs, some test parameters, and some characteristics of brake pads and discs. A brief analysis of the tribological mechanisms that could be involved in particle emissions is also described.

The present contribution showcases the potential brake emission reduction with Cr₂O₃ (chromium oxide) and WC-CoCr (tungsten carbide–chromium–cobalt) rotor coatings, as realized in our joint public–private research consortium. Particulate matter (PM) emissions from automotive braking systems have been characterized using a pin-on-disc tribometer equipped with particle measurement devices: a CPC (Condensation Particle Counter), an APS (Aerodynamic Particle Sizer), an SMPS (Scanning Mobility Particle Sizer), and a PM_2.5 sampling unit. Brake pad samples made from the same low-steel friction material were tested against a grey flake cast iron disc and two types of custom coated discs: a Cr₂O₃-coated disc and a WC-CoCr-coated disc. The friction pairs were investigated at a constant contact pressure of 1.2 MPa while the sliding velocity varied during the test, starting with 25 sequences at 3.6 m/s, followed by 19 sequences at 6.1 m/s, and finishing with 6 sequences at 11.2 m/s. The test results show encouraging 64% to 84% reductions in particle number (PN) emissions between 4 nm and 3 µm and 84% to 95% reductions in mass emissions (PM_2.5) thanks to the respective coated discs. SEM-EDXS (Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy) analyses show that the hardness and roughness of the discs, the chemical reactivity (oxidation), and the abrasiveness of the three friction pairs are parameters that might explain this reduction in emission.

This study focuses on the influence of different brake pad formulations on the emission of particulate matter coming from car braking systems. The brake particles were characterised using a pin-on-disc bench and some particle measuring devices such as CPC, APS, SMPS and a PM₁₀ sampling unit. Seven samples of brake pad materials of different compositions (1 NAO and 6 Low Steel) were tested against grey cast iron discs. The results presented in this work show differences in particle number concentration and PM₁₀ emission factor between the different friction materials tested. Three friction materials, LS04, LS06 and NAO01, reduce particle number emissions by up to 71% and PM₁₀ emissions by up to 57%. On the other hand, this reduction in particulate emissions goes along with a reduction of 20% to 27% in the coefficient of friction. The microscopic analyses carried out on the test parts (pins and discs) show differences between the most emissive and the least emissive friction pairs, which may explain the differences observed in particle emissions. Correlations between the emission of particles and the concentration of iron of the PM₁₀, as well as the steel fibre content in the formulas, were found.

The characterization of airborne submicrometric composite structures’ debris is a challenge in the field of environmental monitoring and control. The work presented here aims to develop a new quantitative method to measure elemental mass concentrations via particle sampling and Transmission Electron Microscopy—Energy-Dispersive X-ray Spectroscopy (TEM-EDS). The principle is to collect airborne particles on a porous TEM grid, then add a certain mass of reference particles, and compare the relative mass percentages of elements from reference and sample particles via EDS. Diverse pairs of airborne particles (RbCl, CsCl, NaCl, SrCl₂, Ga(NO₃)₃, braking particles) were deposited on one TEM grid, and the experimental elemental mass ratios were measured by EDS and compared with the theoretical values. Results show that the quantitative and homogeneous collection of reference particles, such as RbCl, on the TEM grid could be suitable. For all the tested conditions, the absolute deviations between the theoretical elemental mass ratios and the experimental ratios remain lower than 8%. Thus, the mass concentration of Fe from the braking aerosol is calculated as 107 µg/m³. Compared to the cumbersome real-time instrument, this new method for mass characterization appears to be convenient, and requires a short time of aerosol sampling at the workplace. This approach ensures safety and practicability when assessing, e.g., the exposure risk of hazardous materials.

The characteristics of powders on a bulk scale are heavily influenced by both the material properties and the size of their primary particles. Throughout the stages of storage and transportation in the powder processing industry, various forms of deformation and stress, such as pressure and shear, impact these materials. Recognizing the point at which a powder undergoes yielding becomes particularly significant in numerous applications. There are also times when the level of stress needed to maintain it must be understood. The measurement of powder yield and flow properties remains a challenge and is addressed in this study. As part of the European collaborative project, a number of shear experiments were performed using two shearing devices: the Schulze ring shearing device and the Anton Paar Powder Cell (APCC). These experiments have three purposes: (i) test reproducibility/consistency between two shear devices and test protocols; (ii) relate bulk behavior to microscopic particle properties, focusing on bulk density and thus the effect of cohesion between particles; and (iii) investigate the influence of the temperature of heated powders on the powder’s flow properties, which is important for industrial reactors. Interestingly, for samples with small particle sizes, bulk cohesion increases slightly, but bulk friction increases significantly because of particle interaction effects. The experimental data not only provide useful insight into the role of microscopically attractive van der Waals gravitational and/or compressive forces on the macroscopic flow behavior of bulk powders but also have industrial relevance. We also provide robust data of cohesive and attritional fine powder for silo design used for calibration and validation of silos, models, and computer simulations.

The characteristics of powders on a bulk scale are heavily influenced by both the material properties and the size of their primary particles. Throughout the stages of storage and transportation in the powder processing industry, various forms of deformation and stress, such as pressure and shear, impact these materials. Recognizing the point at which a powder undergoes yielding becomes particularly significant in numerous applications. There are also times when you need to understand the level of stress needed to maintain it. Measurement of powder yield and flow properties remains a challenge and is addressed in this study. As part of the CALIPER collaborative project, a number of shear experiments were performed using two shearing devices: Schulze ring shearing device and Anton Paar Powder Cell (APCC). These experiments have three purposes: i) Test reproducibility/consistency between two shear devices and test protocols. ii) To relate bulk behavior to microscopic particle properties, focusing on bulk density and thus the effect of cohesion between particles; iii) Investigate the influence of the temperature of heated powders on the powder flow properties, which is important for the industrial reactors. Interestingly, for samples with small particle size, bulk cohesion increases slightly, but bulk friction increases significantly, because of particle interaction effects. The experimental data not only provide useful insight into the role of microscopically attractive van der Waals gravitational and/or compressive forces on the macroscopic flow behavior of bulk powders, but also have industrial relevance. We also provide a robust datas of cohesive and attritional fine powder for silo design used for calibration and validation of silos, models and computer simulations.

This paper addresses a need for information on nanoparticle emissions and related issues such as worker exposure, filtration efficiency, and dustiness. A survey has been conducted on the working conditions and safety measures used in thermal spray companies and the results compared to scientific literature and previous surveys. Responses to questions on matters of health and safety reveal a lack of information and awareness of the risks posed by the emissions of ultrafine particles generated by thermal spraying processes.