Investigation of Tribological Behavior and Airborne Emissions During the Bedding Stage

Abstract

Tribological transitions are frequent when two new surfaces are placed in contact under load and sliding conditions. These transitions usually occur not only by the changes on the surface roughness, but also by the characteristics of the third body formed between the mating surfaces. The present work investigates the tribological behavior and the related airborne particles emissions of a Cu-free brake friction material during the initial stage of braking (bedding), starting from a virgin material. The tribo-tests have been conducted using a reduced-scale dynamometer, and the number distribution of the airborne particles was simultaneously measured by an Optical Particle Sizer (OPS). The tests were stopped with 25, 50, 100 and 130 brake events in order to analyze the surface of the friction material. Bedding showed two distinct stages regarding the friction and emissions behaviors. The first is characterized by a pronounced increase in both friction coefficient and particles emissions, achieving the highest value around 35 brake events. Successively, a smooth decrease is observed until reach the steady-stage of the friction coefficient. The friction layer formed on the surface of friction material seems to be correlated with the tribological and emissions results; in which the achievement of the state-state on the friction coefficient is coincident with the progressive compaction and area-increase of the secondary plateaus.