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Creep groan is a low-frequency (20-300Hz) self-excited brake vibration caused by stick-slip phenomena at the friction interface observed at very low vehicle speed. The creep groan propensity of friction materials is closely related with the difference (Δμ) between the static (μs) and the kinetic (μk) coefficients of friction. In this study, a NAO brake pad material was used as a base formulation and the abrasives tested were commercial grade of black iron oxide, chromite, zirconium oxide, magnesium oxide and aluminum oxide. Experimental results were obtained by testing seven different friction material formulations, in which the type of abrasives or its hardness or its particle size was changed in order to explore the impact of these variables on the stick-slip occurrence. A laboratory-scale tribometer was used to investigate the influence of different types of abrasives and their physical properties in the stick-slip.

This work presents the modeling and investigation of the mechanical behavior of brake pad materials with high viscoelastic content, reinforced with particles and fibers. A simplified model is presented, with the purpose of prediction of the elastic modulus of this class of material. The model consists in the modification of the Rule of Mixtures, with the introduction of a term referring to the particle phase present in the composite material. It is introduced an alignment factor, in order to adjust the magnitude of orientation of the fibers in the stage of material compaction process. Also, the influence of the viscoelastic phase is modeled according to the percolation theory. Three standard brake pad materials with different viscoelastic contents were produced and tested. The composite materials produced were tested by three-point bending, in order to determine their mechanical properties.