The wear of brake materials from passenger vehicles is a major source of particulate matter in urban areas. The upcoming Euro 7 regulation will impose limits on the PM that originate from brake materials attesting to the several efforts to reduce pollution by the European Commission. A strategy to reduce the hazard of brake emissions is to design brake couples with lower emissions and without hazardous compounds in their formulations. In particular, a direct strategy is the mitigation of the wear of the brake disc by increasing its hardness through the deposition of a hard coating. Along with these environmental requirements, brake materials should fulfill their basic function. They should have excellent performance: stable friction coefficient and low wear over a wide range of braking conditions. In this work, a critical comparison is presented regarding the frictional, wear and emission behaviors at mild sliding conditions and the brake performance at severe braking conditions of two brake couples: a Cu-free friction material slid-ing against a conventional cast iron disc and a Cu-less friction material sliding against a cermet-coated disc. The cermet-coating was deposited by HVOF technique. The two brake couples were tested on a reduced-scale dynamometer with two testing procedures. Brake emissions were measured by an Optical Particle Sizer and are assessed with a specific brake cycle to map the frictional, wear and emission behaviors under mild sliding conditions as a function of pressure and velocity. The brake couple having the cermet-coated disc showed lower wear and emissions at every braking condition. Regarding the brake performance, the frictional response of the brake couples was investigated, including the influence of the disc temperature, with a specific brake cycle developed to obtain pvT-µ Maps under severe sliding conditions. The two brake couples showed comparable frictional behaviors below 300°C. Moreover, a posteriori observations by Scanning Electron Microscopy (SEM) were used to explain the wear processes acting on the surfaces at the different braking conditions. For both the brake couples, the fade effect was found for v-T combinations, above 300 °C and 140 km/h, and was correlated to the primary plateaus being progressively covered at increasing T by a thick layer of iron oxide. Finally, a peculiar behavior, friction instability was observed for the cermet-coated disc above 300 °C and was correlated to an increase of the wear rate and a lower degree of compaction of the iron-oxide particles into the friction layer of the pad surfaces.
Dr. Stefano Candeo, PdD Student, University of Trento; Dr. Francesco Varriale, Testing Engineer, Brembo; Dr. Ana Paula Nogueira, Material Specialist Engineer, Brembo; Prof. Stefano Gialanella, Professor, University of Trento; Prof. Dr.-Ing. Giovanni Straffelini, Professor, University of Trento