Dry sliding wear of particulate-reinforced aluminium alloys against automobile friction materials

Abstract

Two magnesium/silicon aluminium alloys each reinforced with 20 vol.% SiC particulates have been worn against three makes of automobile friction linings (brake pads). Two of the friction linings are commonly used against cast iron rotors in automobile braking systems while the third has been specifically formulated for use against aluminium metal matrix composite (MMC) brake rotors. Wear processes at the interfaces of the specific rotor-pad combinations have been characterised through the analysis of friction traces and the use of electron microscopy. For aluminium MMCs sliding against organic pads formulated for use against cast iron, wear rates are low and friction traces constant due to the formation of a solid lubricant graphitic layer at the wear interface. When these MMCs are worn against semi-metallic pads formulated for use against cast iron, wear rates are extremely high due to three body abrasion which leads to subsurface delamination and early melt wear in the MMC. For an aluminium MMC developed for its use in automobile brake rotors sliding against a semi-metallic pad specifically formulated for its use against MMC brake rotors, wear rates at low loads are low although friction traces are highly irregular and fracture of the SiC particulates occurs at the lowest load and sliding velocity. This fracture of SiC is induced by the abrasive action of hard alumina particles within the pad. At high loads and sliding velocities cohesiveness of materials within the pad is poor and the wear rate of the MMC is extremely high. If the structure and composition of friction linings are arranged correctly, the wear resistance and frictional properties of aluminium MMC brake rotors are superior to those of cast iron brake rotors.