Abstract
The surface of phenolic composite material was continuously observed through borosilicate glass (BK7) during friction to verify the tribo-reduction of copper oxide (CuO) taking place at the interface. Two types of pad samples were slid against a rotating BK7 glass disk. These pad samples were made up of selected components, one of which contained CuO as an additive, from a commercial brake pad. During rubbing, dark wear tracks appeared on both pads, where debris generation and its flow were so frequent that the temporal change of these tracks was extremely fast. Using laser microscope after the friction test, small metallic spots were clearly visible on the dark wear tracks of the pad that had contained CuO. The EPMA analysis of these spots showed that the main component was Cu itself. This means that CuO was reduced to Cu, i.e. tribo-reduction took place on the dark wear tracks. Pad friction surface was also observed during friction in relatively dimmed light conditions. The orange luminescence from some spots on the dark wear tracks appeared. The orange color suggested that the temperature of these spots reached significantly high. Heating experiments evidenced that CuO powder, when mixed with phenolic resin powder, was reduced to Cu at 300 °C or higher associated with the weight decrease of mixed powder. Therefore, the luminescence at the surface indicated that the temperature was high enough to cause the degradation of phenolic resin, which promoted the reduction of CuO. It was thus confirmed from the chemical composition and luminescence color that CuO in phenolic composite material was reduced to copper during friction.
