The design of new brake materials capable of reducing emissions is a very important and topical issue. A deep understanding of tribological mechanisms enables the design of friction materials with low emission characteristics. In addition to low emissions, the design of new friction materials must ensure excellent performance with stable coefficients of friction and low wear. Furthermore, the development and characterization of new materials take time and effort as there are many aspects to be considered and even small changes can generate substantial behavioural alterations. In this study, to speed up procedures and provide a streamlined tool, a sub-scale dyno with a simplified cycle was used to investigate the correlations between the parameters of interest: friction, wear, and emissions. The sub-scale dyno was set up for the evaluation of emissions and it is embedded with an LVDT sensor for wear evaluations. Wear is expressed as the depth of wear (µm/brake) from LVDT measurement of different brake stops. Emissions were measured with an Optical Particle Sizer in the range of 0.3-10 µm and expressed in terms of mean concentration/brake. The brake cycle consists of a factorial design of experiment with brake blocks of repeated brake stops. Each brake block has fixed brake parameters of contact pressure and initial sliding velocity. The brake blocks were randomized in order to minimize the order effects. Different friction materials (low-met, NAO, inorganic) were tested sliding against cast-iron discs. Initial sliding velocity was found a markedly more influencing parameter on the wear and emissions behaviours than contact pressure. In particular, the low-met friction material displayed an approx. cubic and monotonous dependence of the wear and emissions with initial sliding velocity. Whereas the NAO material showed a lower dependence with the initial sliding velocity. On the other hand, the inorganic material displayed a, not monotonous, behaviour regarding the wear and emissions at high velocity. Additionally, the characteristic relationships between the wear and emissions are proposed to describe the different tribological mechanisms.
Brembo S.p.A: Dr.Eng, Mara Leonardi; University of Trento: Dr. Eng. Stefano Candeo, Giovanni Straffelini