Compositional effects of the brake friction material on creep groan phenomena

Creep groan (low frequency frictional vibration in the range of 200–500 Hz during braking at a very low vehicle speed) was studied by changing the relative amounts of ingredients in a brake friction material containing 12 ingredients. Twenty-nine friction material specimens with different relative amounts of the ingredients were manufactured according to a constrained mixture design. The difference (Δμ) between kinetic (μk) and static (μs) coefficients of friction for each formulation was measured to investigate creep groan propensity of each friction material since the creep groan is caused by stick–slip phenomena. Results showed that zircon (zirconium silicate), steel wool, and phenolic resin showed a tendency to increase Δμ. On the other hand, antimony trisulfide, cashew particles, and rockwool tended to reduce Δμ. Based on the results of this work, the initial formulation was modified to reduce Δμ. The modified friction material showed a smaller Δμ value than the initial friction material implying low propensity of creep groan phenomena during braking. The velocity dependence of the friction coefficient was also examined using the initial and the modified friction materials. The modified formulation exhibited a less negative μ–V relation than the initial formulation.