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A method has been developed to simulate worn-in-service brake friction materials and rotors. The simulated thin linings and worn rotors were prepared by grinding the materials to a predetermined level and then subjecting the prethinned friction couple to a moderate inertial-dynamometer conditioning sequence. The friction materials were characterized by optical microscopy and thermogravimetric analysis. The rotors were characterized by surface roughness measurements and scanning electron microscopy. The simulated components were shown to have the same microstructure and chemical compositions as worn-in-service components. The extent of thermal and mechanical damage was found to be proportional to the duty cycles experienced. By the method described, the friction-affected layers of the simulated friction couple were found to be intermediate between those of a similar couple with 40,000 miles of light use and those of a couple with 3300 miles of medium-duty durability use.

The friction and wear characteristics of automotive friction materials are strongly dependent on the composition and microstructure of the rotor surface. In this study we investigated the compositional and microstructural changes occurring in the surface layers of cast iron brake rotors during dynamometer tests with a typical organic friction material. Rotors were studied in the as-manufactured, lightly ground and sanded, and as-burnished conditions, as well as after 30 stops from 60 mph at a deceleration rate of 15 ft/s2. Optical and scanning electron microscopes were used to examine the surfaces. Minimum disturbance of the microstructure was found in the sanded surface, but the as-manufactured and burnished surfaces exhibited considerable disturbance. After the 30 stops the pearlite was transformed locally into martensite. Composition analysis of the burnished rotor surface showed high magnesium content.