Microstructure and detachment mechanism of friction layers on the surface of brake shoes

Automotive and truck brake shoe materials are complex composites of metals, ceramics, and polymers. These are designed to provide stable, reliable frictional performance over a range of vehicle operating conditions, exhibit acceptable wear life, and offer freedom from noise and sensible vibrations. During their use, friction brakes tend to form thin, third-body layers on their contact surfaces. Such tribo-layers play a part in frictional characteristics, and therefore it is of interest to study their structure and properties. This article describes the structure of the friction-induced layers that formed on commercially manufactured brake shoes on a trailer that was subjected to 4 years of highway driving. The layers consist of a thin, brittle crust of nanoscale particulate material that easily delaminates from the substrate. Polished cross sections were studied with both optical and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) of extracted flakes revealed the complex, fine structure of the friction-induced layer. Realistic brake friction models should not use bulk properties of the starting materials, but rather should incorporate the properties of the fine-grained friction layers.