The tribological behaviour of the boundary layer between the brake pad and the disk is determined by chemical and physical processes on different time scales. To get a better understanding of these dynamic processes it is useful to separate the external influences caused by the brake system when macroscopically measuring the coefficient of friction. Furthermore a close control of the influencing parameters such as speed, pressure and temperature is necessary. Both can be achieved by using a scaled laboratory tribometer. Tests conducted on the High-Load-Tribometer at the Institute of Dynamics and Vibrations in Braunschweig show that temporary changes of input parameters can have a long lasting influence on the friction process. It is observed that a variation of the initial disk temperature can cause a temporary increase of the coefficient of friction. This is taking place at disk temperatures above approximately 100 °C and can therefore be separated from other thermally induced effects such as the well-known fading effect. This increase of the coefficient of friction is repeatable but it needs several brake applications at lower temperatures for the friction material to recover. This means the friction material can even memorize effects that occur under relatively mild conditions. Understanding these phenomena is very important for the interpretation of test results from standard test procedures or for the definition of run-in sequences.
Lars Wilkening, Hans-Guenther Paul, Georg-Peter Ostermeyer - Technische Universität Braunschweig, Federal-Mogul Friction Products GmbH