The aim of this research is to characterize the single ingredients of a brake system friction material compound in terms of tribological parameters (coefficient of friction and specific wear rate) and airborne particle emissions (Particulate Matter, Particle Number). The first deals with the development of a material database available to be used as input for a simulation tool; the second is related to investigate how one single ingredient contributes to the total airborne emissions since, besides friction and wear, airborne particle emission has become a core parameter when evaluating disc brake performance due to its adverse health effects. A common friction pad material is composed of four different material categories: abrasives, reinforcing fibres, lubricants, and fillers, for a total of more than thirty-five ingredients in the mixture. A Design of Experiment study was conducted to investigate the ingredients having more impact on the friction material performance in terms of coefficient of friction and wear; twelve ingredients remained. One ingredient from each category was chosen and produced as pin to investigate the topic. Pins are made of the material of interest and a binder agent to keep fibres and/or powders together. Pins made of a commercial European brake friction material were also produced as a reference. Cast iron discs extracted from commercial brake rotors were used as counterpart. Tests were executed using a pin-on-disc tribometer designed for airborne emission studies, involving a Condensation Particle Counter and a Dust Trak to monitor particles number and particle sizes and masses respectively. Coefficient of friction evolution, airborne particle masses (PM1, PM2.5, PM4, PM10, PMTOT) and number concentrations (PN) were measured during the tests. Specific wear rate (Ka) was calculated using the Archard’s wear Law, since both the pins and the discs were weighted before and after the tests execution to evaluate the loss of mass. In terms of particles emission, the results show that the abrasive and metal fibre emit much more particles in terms of mass and number than the lubricant and filler materials. The abrasive shows the highest value of specific wear rate, one order of magnitude higher than the one of all the other ingredients. This shows that wear and particles emission are closely correlated. Metal and abrasive samples return the highest coefficient of friction values. Lubricant is the ingredient showing the lowest values for all the interested parameters. Limitations of this study can be resumed as follows: just four ingredients out of more than thirty-five were tested in a single working condition, corresponding to a mild wear braking event from a city cycle. Further tests will be executed exploring different physical conditions and analysing more ingredients. The addition of a binder agent in the pins production may have an influence on the tribological properties and airborne particle emissions. However, since it is necessary in the samples preparation because it guarantees the structural integrity of the specimens, its possible effect had to be considered negligible. This work focusses on a novel approach to study the airborne particle emissions of single ingredients of a friction material mixture, aiming to understand which raw material emits the most. The knowledge is important since it can be used to build a database for developing environmentally friendly friction material mixtures to satisfy the Euro 7 regulations, referring to PM10. Te present research also provides a tribological characterization of the ingredients that can be used as inputs for a simulation tool in the design of a novel friction material compound. This paper presents an experimental comparison of airborne particle emissions and tribological properties for samples made of an ingredient of a friction material mixture and a binding agent. Four samples were produced and tested; another specimen, made of the original friction material compound, was investigated as reference. Major conclusions are: • higher the specific wear rate is, higher PM10 and the number of particles emitted are. • Lubricant and Filler show very low values of PM10 and PN, while the Abrasive shows the highest values. • Materials which wear the most tend to emit fine particles rather than coarse ones.
Ing. Francesco Varriale, Innovative Simulation Methodologies Specialist, Brembo S.p.A.; Ing. Francesco Varriale, Innovative Simulation Methodologies Specialist, Brembo S.p.A.