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The present work presents evaluation of the sliding surface morphology of brake pads during stick-slip. A low-metallic (LM) and a Non Asbestos Organic (NAO) brake friction materials were subjected to slide against a brake disc under conditions favorable to produce stick-slip phenomenon. The experiments were conducted in a laboratory-scale tribometer, which was especially designed to test brake pads used in vehicle. Delta torque divided by slip time (dT/dtslip) was the parameter used to quantify stick-slip propensity. In addition, optical microscope images of the material's surface were obtained at different stages of the braking test. These images were post-processed in appropriate computational software and by means of the segmentation technique, the real contact area, size and amount of contact plateaus related to the brake pad surface were estimated. This technique was effective to quantify the differences in the sliding surface morphology during low speed braking test.

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.