Friction, Wear and Emission in Brakes

Brakes are tribological high-load contacts. Such contacts are characterized by the fact that the wear provides material in the friction contact, which can generate additional tribologically active contact surfaces there. In addition to the natural contact areas, wear dust in the boundary layer forms temporary contact areas. These contribute significantly to the resulting friction power.

Over the last 20 years, scientific research has focused on the dynamics generated by wear in the boundary layer. The growth and destruction of the local contact areas formed by wear dust have been successfully used to explain time-variant effects. Thus, dynamic friction laws have been developed along with spatially resolved simulations that reflect the evolutions of surface geometry very precisely.

Following investigations into friction dynamics, the demand for time-resolved predictions of wear, and in particular the wear dust load during braking is becoming increasingly important. For the description of brake dust loads, we have now set up highly automated high-precision optical measurements combined with a certificated particulate matter measurement system (TSI EEPS 3090) and several low-cost particulate matter sensors, which investigate wear and wear dust emission during braking over long periods of time. These measurements have shown that the wear and particle emission intensity also has its own time dynamics. The wear dust moves in the boundary layer between pad and disc. It is not only originally responsible for the construction of contact patches, but also fills reservoirs near the surface in various ways. The cavities are filled once and then emptied again. These reservoir dynamics seem to be characteristic for the output of wear dust in high load contacts in transient operation and modulates not only the friction coefficient but also the wear intensity in the friction contact.