In order to satisfy the copper content regulations in North America, copper-free non-asbestos-organic (NAO) brake pads are being developed with various raw materials to replace copper. Copper fibers are used in brake pads to improve pad wear by reinforcing the friction materials and to stabilize the friction coefficient by spreading frictional interfaces. On the other hand, it is known that copper-free NAO pads have an unstable friction coefficient and excessive pad wear degradation at high temperatures. For these technical issues, the authors conducted research from two viewpoints. The first one is the number of organic components, such as phenolic resins and cashew particles. The second one is the addition of catalysts to evaluate the impact on the decomposition process of organic components.
First, a wear test controlled by temperature was performed on brake pads containing a different number of organic components. The authors found that the lower the number of organic components in the pad, the more stable the friction coefficient is, at temperatures above 300 °C, which is the decomposition temperature of organic components. Additionally, the formation of a transfer film was observed on the rotor surface when the number of the organic components was small, but not otherwise. Furthermore, it was also found that the extent of wear at 300 °C or higher depends on whether the transfer film was formed at a temperature exceeding 300 °C. To verify that this phenomenon is caused by the decomposition of organic components, the stability of the friction coefficient in a two-component system, produced from an inorganic catalyst and a phenolic resin, was evaluated. Thus, the authors found that the friction coefficient was stabilized with titanate. Based on this study, wear tests with and without titanate were conducted. It was found that the addition of titanate improved the stability of the friction coefficient and wear resistance.
In conclusion, the number of organic components and the addition of the catalyst are strongly related to the formation of the transfer film on the rotor surface, the stability of friction coefficient and the amount of wear. Decreasing the number of organic components or adding the catalyst can improve the wear and friction coefficient stability. This study is an important finding from the viewpoint of environmental conservation, to suppress the emission of brake dust even under high-temperature conditions in which the wear generally increases.