Pin-on-disc study of brake friction materials with ball-milled nanostructured components

Copper is an ingredient of the automotive disc brake pads with important functional role. On the other hand, copper is regarded as one of the most hazardous component of the particulate matter released by the brake linings. Legislation in several countries is progressively reducing the tolerated amount of copper in friction materials. In this work, a possible approach to the reduction of copper in brake friction materials is presented. Starting from a commercial, state-of-the art, non-asbestos organic friction material, different formulations have been prepared, changing the microstructure of some of the ingredients, namely copper and zirconia using high energy ball-milling. The wear behavior of the newly developed materials has been tested and validated using pin-on-disc wear tests. One interesting aspect observed is that the wearing out of copper fibres produces fine copper particles entering the friction layer, thus contributing to its compaction. This observation implies that copper powder is not strictly required as a component in the starting friction material to achieve anyway a satisfactory tribological behavior associated with a stable friction layer. Furthermore, the addition of milled components provided interesting indications to be explored further in view of the replacement of copper in brake friction materials.