The correlation between contact stiffness and stick–slip of brake friction materials

Friction-induced stick–slip of brake friction materials was investigated by examining the correlation between contact stiffness (kn) and surface topography changed by material wear at the sliding interface. By changing the surface roughness and thickness of the friction material, friction instability during sliding at low speeds was analyzed using a scale brake dynamometer. The results showed that the intensity and frequency of stick–slip were determined by the relative magnitudes of tangential contact stiffness (kt), bulk stiffness (kb), sliding velocity, and the difference (Δμ) between the static (μs) and kinetic coefficients (μk) of friction and these results were supported by the contact theories proposed by Greenwood and Williamson. While Friction materials with rough surfaces showed small stick–slip amplitudes due to small tangential contact stiffness (kt), friction materials with high compressibility exhibited pronounced stick–slip because the amplitude inversely varied with the bulk stiffness (kb). The high stick–slip propensity observed from heat affected friction materials also supported the correlation of the surface condition and contact stiffness (kn).