Abstract
Wear dust in the boundary layer between a brake pad and rotor has a crucial influence on the friction and wear behavior of brakes. Studies of the wear dust show that during the friction process, characteristic geometric structures and grain sizes are formed on the friction surfaces. These very stable self-organization forms result from the dynamics of the wear dust in the boundary layer, and have been successfully used for mesoscopic and macroscopic friction force descriptions based on balance equations [1].
In order to better describe the formation and development of these characteristic structures, the Wear Debris Investigator (WDI) has been developed at IDS. This device is used to quantitatively describe the self-organization forms of the wear debris in the boundary layer, and in particular, to describe the grain size as a function of the external loading conditions.
This apparatus consists of a rotating horizontal glass plate and a counter-body (punch), which are pressed against one another with a predefined normal force. Before the start of each experiment, "artificial" wear particles are scattered between the glass disc and the punch. The motion of the particles is then tracked using a camera-mirror setup, while the total normal force and friction torque are measured. In this way, the optical observations can be considered alongside corresponding quantitative results.
In first experiments, it has been shown that the formation of tribologically relevant selforganization structures beyond the classical patches can be observed within the friction boundary layer. Rollers can be formed, for example, which move with half of the relative velocity between the rotor and the pad. Patches are more likely to develop at higher normal forces and lower rotational speeds, whereas the rollers only form at lower normal forces, but also at higher velocities. In addition to normal force and speed, the material properties and particle sizes are equally influential to the formation and development of such structures.
Precisely how the formation of these self-organization structures depends on the material properties, the normal force, and the speed, is a current subject of research at IDS. This work provides first insights into these topics.