Wear and Friction-Induced Vibration of Brake Friction Materials with Different Weight Average Molar Mass Phenolic Resins

The friction and wear of brake friction materials containing phenolic resins with different weight average molecular masses (Mw = 2.2–6.1 kg/mol) were investigated using a Krauss-type tribometer and a reduced-scale dynamometer. The results demonstrated that the friction level, wear rate, and friction instability were strongly affected by the Mw of the phenolic resin, attributed to the different shear strengths of the friction materials. The high-Mw phenolic resin exhibited an increased friction level while reducing the wear rate, with both effects more pronounced at temperatures above the phenolic resin decomposition temperature. The friction materials containing the higher-Mw phenolic resins also showed an improved friction instability, revealing a lower critical velocity for the incidence of friction oscillation. The stiffness measured from the friction material revealed that the Mw did not change the surface stiffness after the friction tests, indicating that the improved friction-induced vibration of the friction material with the high-Mw phenolic resin was attributed to an increased matrix stiffness rather than to the sliding surface.