Wear of aluminium MMCs against automobile friction materials

Abstract

Two magnesium/silicon aluminium alloys each reinforced with 20 vol. % SiC particulates have been worn against three different automobile friction linings (brake pads). Two of the friction linings are commonly used against cast iron brake rotors while the third has been formulated for use against aluminium 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 optical and electron microscopy. Models on the interdependence of friction and wear, and models of wear mechanisms for aluminium MMCs and cast iron sliding against friction materials have been proposed and discussed. For an aluminium MMC sliding against an organic pad formulated for use against cast iron, wear rates are low and friction is constant due to the formation of a solid lubricant layer at the wear interface. When this MMC is worn against a semi-metallic pad formulated for use against cast iron, wear rates are extremely high due to two and three body abrasion which lead 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 irregular and fracture of the SiC particulates occurs at the lowest load and sliding velocity. This fracture of SiC is caused 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. At the highest load/sliding velocity combination, the wear resistance of the MMC is inferior to that of its unreinforced matrix. If the structure and composition of friction linings are arranged correctly, the wear resistance and frictional performance of aluminium MMC brake rotors are superior to those of cast iron brake rotors. In addition, the lower density of aluminium MMCs provides for an economic advantage over cast iron with respect to efficient use of fuel, and fabrication expenses.