The effect of contact area on velocity weakening of the friction coefficient and friction instability: A case study on brake friction materials

Newly found evidence on the negative slope of the friction-velocity curve is described in terms of the contact area on the surface of brake friction materials. The tribotests were performed using two commercial friction materials with and without steel fibers. The results showed that velocity weakening friction was pronounced in the friction materials containing steel fibers, which facilitated high pressure contact areas by the agglomeration of wear debris. The size of the high pressure contact area increased as the sliding velocity decreased, which supported the correlation among velocity weakening friction, friction instability, and high pressure contact areas. The pronounced velocity dependence of the friction coefficient due to transient contact area was highly relevant to noise propensity of the friction material. •The sliding surface of a brake friction material shows wide pressure distribution.•The size of the high-pressure contact area changes with the sliding velocity.•μ-v character of the friction material is affected by high-pressure contact areas.•Large stick-slip amplitudes are germane to velocity weakening friction phenomena.•Velocity dependence of the contact area affects the friction and noise propensity.