Moisture effect on velocity dependence of sliding friction in brake friction materials

The effect of moisture on the coefficient of friction (COF) of brake friction materials is studied using a pad-on-disc tribometer. A special attention is given to the velocity dependence of the COF at different humidity levels using friction materials with different hydrophobicities. The results show a strong velocity dependence of the COF, which is changed by humidity levels. The COF of the friction materials at low velocities is significantly influenced by the amount of in-situ menisci present at the sliding interface. Moreover, the velocity dependence of the COF is also affected by the hydrophobicity of the friction materials, showing pronounced velocity dependence of hydrophilic friction materials. This finding suggests a possible remedy to reduce friction instability under humid weather conditions by controlling the hydrophobicity of the brake friction material.