Influence of contact pressure and velocity on the brake behaviour and particulate matter emissions

Abstract

In this work, the braking performance and the emission behavior of airborne wear particles of a low-metallic and NAO friction material have been investigated using a reduced-scale dynamometer, adapted for emission measurements. A specific braking cycle has been developed to assess the influence of velocity and pressure on the tribological performance of friction materials sliding against a cast iron disc. The adopted brake cycle consists of a factorial design of blocks of repeated brake stops, where each block was intercalated with a so-called reference block, meant to recover the initial characteristics of the sliding surfaces. A progressive stabilization of the tribological parameters was observed during the blocks featuring a higher initial velocity. The relevant values of these parameters would depend on the extension of the secondary plateaus in the friction layer. At high pressure and velocity, the low-metallic friction material exhibited a higher wear rate and lower friction coefficient than the NAO material, although the NAO material displayed a weaker recovery behavior regarding the friction coefficient. The emissions of the NAO material were found to be lower and more stable than the low-metallic material. In general, the emissions were found to be correlated with the initial brake velocity mainly, through a cubic relationship, whereas the emissions did not show a clear dependence on contact pressure.