Transfer Film Composition and Characteristics in Copper-Free NAO Brake Pads

Copper-free non-asbestos-organic (NAO) brake pads have been developed to satisfy the copper content regulations in North America. Copper-free NAO brake pads are required to have a stable friction coefficient owing to the electrification of the control systems, as well as to exhibit improved wear resistance to reduce brake dust emissions. Our previous study indicated that the transfer film formed on the rotor surface affects both the friction coefficient stability and amount of wear. In this study, we investigated how different types of inorganic fillers affect the transfer film formation and its composition in a wear test controlled by temperature. It was confirmed that the main component of the transfer film was iron oxide derived from the rotor. Furthermore, the contained components changed according to the appearance of the rotor surface after each wear test. When the brake pad contained potassium lithium titanate, a transfer film was formed uniformly and stably, the amount of wear was small, and the friction coefficient was stable. In addition, the iron oxide of the transfer film incorporated components such as titanium and barium that were dispersed uniformly. The hardness of the rotor surface after the test was higher than that of the pure iron oxide. This study characterized the composition and physical properties of the transfer film obtained on the rotor surface when using copper-free NAO brake pads. Increased control of the state of this interface may be used to improve the friction characteristics of these systems in the future. Therefore, controlling the transfer film on the rotor surface, which changes during friction, is an important factor to consider when improving brake pads.