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Third bodies, also termed friction layers, tribofilms or secondary contact patches, are layers of more or less compacted wear debris between pads and rotor of a disc brake. Our approach of assessing the sliding behavior and friction properties induced by third bodies has been: i) structural characterization after AK-master test procedure, ii) sliding simulation of model structures similar to the observed ones but with simpler and well defined compositions, and iii) verification of simulation results by pin-on-disc tests with artificial third bodies showing the same microstructures and compositions as the model structures. The idea was to simulate structure formation during real braking conditions by high energy ball milling of appropriate powder blends.

The Focused Ion Beam (FIB) technique combined with Transmission Electron Microscopy (TEM) was used here for the first time to reveal the microstructure of thin surface layers on brake pads and rotors after a run-in period during which a stable coefficient of friction had developed. Generally the outermost layers of both, pad and rotor, comprise a complicated structure of nanocrystalline metal and metal oxide particles embedded in an amorphous matrix. Obviously oxidation plays a major role and the third body seems to behave like a granular medium. The contact areas were no protuberant plateaus in the regarded case. Therefore the flow of the granular third body layer seems to control friction force rather than ploughing of asperities.