Tribological nature of squealing disc brakes

Sammanfattning: In the stiff competition between car manufacturers, silent and vibration free brakes hasbecome increasingly important. The aim of the present work is to increase the knowledge of the fundamental friction mechanisms operating in brake systems and to study the parameters influencing disc brake squeal generation.Characterisation of the rubbing surfaces at the microscopical level revealed that only parts of the pad surfaces were in contact with the brake discs during braking. This effect is due to the complex microstructure of the pads. Based on these observations, a qualitative model for the wear and friction properties is discussed. Due to the complex contact situation, the dynamical friction behaviour does not follow classical models.A test-rig has been designed and installed in order to evaluate squeal and friction properties of car disc brakes. A software based measurement system has been especially developed to allow for flexibility in data collection and analysis. Several parameters have been studied with respect to brake squeal generation: the influence of brake pad geometry, the effect of solid lubricant additions to the friction material and the influence of brake disc topography.It was found that the friction force was independent of pad size while the squeal properties were highly dependent on the pad surface extension in the sliding direction. The addition of Cu2S to an especially developed pad friction material reduced both the friction and the squeal generation. By introducing small pits in the brake disc surface the coefficient of friction was lowered and brake squeal totally eliminated. This work has pointed out several methods to reduce disc brake noise and that an accurate descriptions of the friction mechanisms operating in brakes is of major importance for the understanding and prevention of brake squeals.