Despite much progress in recent years, the phenomena that occur in brakes due to friction are currently not fully understood. Dynamic contact structures in the boundary layer have been shown to be responsible for variations of the coefficient of friction, which can be found in all brake measurements. Investigations of high-frequency vibrations by means of laser vibrometry are typically focused on operating vibration analyses of the rotating disc or the brake components during the brake application. In order to gain insights into the causes of such vibrations, it is necessary to understand the formation mechanism in the friction boundary layer. For this purpose, balance equations for the dynamic coefficient of friction were developed at the Institute of Dynamics and Vibrations (IDS) by theoretical consideration, and high-precision measurement procedures were carried out for a statement about the tribology parameters. In the context of these investigations, a vibration signature between 20 - 80 kHz has been observed which can be found in all brakes. In connection with the friction theories, it is an open question as to whether these high-frequency oscillations are not responsible as a kind of trigger for Noise Vibration Harshness problems. In addition, it is a problem that the parking sensors installed in the vehicle work on an ultrasonic basis in the same frequency range and can therefore lead to failure due to these frequencies. Therefore, this paper is focused on frictioninduced vibrations between brake pad and brake disc in the high-frequency range. For this purpose, brake measurements were performed on differently scaled pin-on-disc tribotesters and on complete brake systems. The resulting vibrations were recorded using a laser vibrometer and an ultrasonic microphone.

Johannes Otto, TU Braunschweig, Germany; Jan Malte Sandgaard, Georg-Peter Ostermeyer