Brake performance and emission behaviors of brake materials on a sub-scale dynamometer

Brake materials represent an important source of air pollution, especially in urban areas, where they can contribute to approx. 21 % of the traffic-related particulate matter emission. For this reason, the design of new brake materials with low emissions is a topical issue. In addition to low emissions, the design of new friction materials has to ensure excellent performance with stable coefficients of friction and low wear rate. Due to the several requirements that these materials need to fulfill, their development and testing are complex and intercorrelated. Good performance and low emission strongly depend on the mechanisms acting at the disc-pad interfaces. In this thesis, a brake dynamometer testing protocol is developed to better understand the relationships of the braking parameters with the brake performance and emission behavior, correlating them with the surface characteristics. The surface characteristics were investigated with a-posteriori analysis, in terms of extension of the contact area, degree of compaction of the wear particles and relevant composition. The work is focused on the bedding process and the influence of the braking parameters on the frictional, wear and emission behaviors. Regarding the bedding process, run-in, transition stage and steady states were identified as concerns the frictional, wear and emission behaviors. The frictional behavior gets stabilized by the extension of the secondary plateaus, whereas the wear and emission behaviors are stabilized as their degree of compaction increases. The influence of pressure and velocity under mild sliding conditions were studied for a low-met and NAO material, the two most common types of friction materials. The low-met material featured a more stable and higher friction coefficient and lower wear and emissions than the NAO material. The wear behavior is strongly affected by pressure for the NAO material, and for the low-met material, velocity is very influential. Emissions follow a cube relationship with velocity for both materials. The significant differences in the observed behaviors are explained in terms of the different features of the surfaces. The NAO material featured a smooth and uniform surface, with higher coverage than the low-met material, on which steel fibers play important adhesive and abrasive actions. From tests under mild sliding conditions of several friction materials sliding against cast-iron discs, a linear relationship is found between the specific wear rate and the emission factor. This relationship identifies a wear rate below 2.5 10-14 m2/N complying with the Euro 7 limitation of 3 mg/km/vehicle after 2034. Among the friction materials sliding against cast iron discs, the NAO material and only one friction material displayed an emission factor below the limit of 3 mg/km/vehicle. In addition, the emission factor of low-met material sliding against a cermet-coated disc was lower than this limit. These observations confirm that the NAO materials and coated discs are effective systems to mitigate emissions, whereas further efforts are required to improve the emission behavior of low-met materials. Interestingly, the low-met materials with a reduced presence of secondary plateaus featured higher wear and emissions. Regarding the brake performance, under severe sliding conditions, the NAO material displayed worse frictional and wear behaviors than the reference low-met material. For high-pressure ranges, the effect of pressure is to cause a monotonic decrease in the friction coefficient. The effect of temperature on the friction coefficient causes an increase in the friction coefficient when the tribo-oxidative processes are contained up to 300 °C. For combinations of high velocity and temperature, the tribo-oxidative processes are high enough to form a thick glaze layer on the surfaces. The glaze layers were correlated to a lubricating effect, or fade effect, at disc temperatures above 400 °C, especially when their extension covered the steel fibers. The cermet-coated disc displayed the same fade behavior at high velocity-temperature values, although at low velocities and high temperatures, friction instability was observed and related to larger but fewer patches originating to a significant extent from material transfer from the disc. The friction instability in the coated disc was ascribed to the different tribo-oxidative behavior in the formation of ‘glazes’ due to the low source of iron in the disc material.