The influence of humidity on the sliding friction of brake friction material

Effects of humidity on the friction coefficients of brake friction materials at slow sliding speeds were studied. By measuring the static and dynamic coefficients of friction (COF) at high and low humidity levels, the mechanism of humidity-induced friction instability was investigated as functions of exposure time, applied pressure, and sliding speed. Tribological properties were examined using a pad-on-disk type tribometer and a 1/5 scale brake dynamometer. Results showed that the kinetic and static COFs were significantly affected by the exposure time in the humid air, showing saturation of the static COF within 2 s. The static COF increased in humid air due to the increased adhesion by water menisci produced at the contact junctions. On the other hand, the static and kinetic COFs increased as a function of pressure in dry air due to the increased real contact area at high pressure, while they were not affected by applied pressure at high humidity levels. The test results also showed that the humidity increased the amplitude of stick-slip and its critical speed, which explains high propensity of friction instability at slow speeds such as creep groan at humid weather conditions.