Contact surface topography and system dynamics of brake squeal

Brake noise is an example of noise caused by vibrations induced by friction forces. During brake operation, the friction between the pad and the disc can induce a dynamic instability in the system. Brake squeal can occur in the frequency range between 1 and 20 kHz, and more. Approaches to this problem are usually confined to the analysis of the system dynamics or to the tribological studies (analysis of the contact surfaces). In a previous dynamic analysis, the squeal events treated in this paper were identified to be dynamic instabilities of the brake system. This paper presents an experimental tribological analysis performed in parallel to the dynamic one and aimed to highlight the role of the contact problems in the squeal phenomena. The topography of the contact surface after braking phase with and without squeal is presented. The features of the third body and the surface topography were found to be completely different in the two cases. After squeal events the topography of the surface and the internal structure of the pad material close to the contact show several cracks and material exfoliations; on the contrary, after braking without squeal the contact surfaces were found to be smooth and compact. The analysis of the system vibrations during squeal and the numerical analysis of the contact stresses allow to relate these features to the fatigue excitation due to the local oscillation of the contact stresses at the contact surface at the squeal frequencies. The surface topography after squeal, by revealing several fatigue evidences, points out a further problem, i.e., the possible increasing of wear rates that are consequent to the squeal vibrations.