Friction instability induced by iron and iron oxides on friction material surface

This study analyzed the effect of ferrous wear particles, which can be transferred from gray iron brake discs to the friction material surface during brake applications, on friction instability. To simulate the chemical change of the sliding surfaces, ferrous particles, such as pure iron, Fe₂O₃, and Fe₃O₄ particles, were embedded on the sliding surface of a friction material, and their friction characteristics were investigated using a scale dynamometer. The results showed that the ferrous particles aggravated friction instability, showing larger stick-slip amplitudes and wider velocity ranges for friction instability than the bare specimen. The embedded ferrous particles increased the static coefficient of friction and produced larger stick-slip amplitudes in the order of Fe₂O₃, iron, and Fe₃O₄, while the friction material without ferrous particles showed the least instability. The surface energy and amount of high-pressure contact plateaus on the sliding surface were attributed to the high friction instability caused by the interfacial adhesion increased by ferrous particles at the sliding interface. This explained the frequent friction-induced noise and vibrations found with a corroded or relatively soft gray iron disc.