Masotti, D., Ferreira, N., Neis, P., Menetrier, A. et al., "Evaluation of Creep Groan Phenomena of Brake Pad Materials Using Different Abrasive Particles," SAE Technical Paper 2014-01-2518, 2014, doi:10.4271/2014-01-2518.
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. The results showed that abrasive particle size and hardness significantly affect the propensity of stick slip. Abrasives with high hardness showed relatively large stick-slip amplitudes in comparison to that with lower hardness. High propensity of stick-slip also was observed in tests of samples with the same abrasive but with larger particle size.