Effect of temperature on the sliding wear performance of Al alloys and Al matrix composites

The effect of ceramic particulate and graphite additions on the high temperature dry sliding wear resistance of two Al alloys was studied. The experiments were performed using a ring-on-flat sliding contact configuration against hardened SAE 52100 bearing steel counterfaces on an apparatus built for testing at controlled temperatures. Conditions were selected such that the materials in contact were kept in an isothermal atmosphere and the generation of frictional heat was minimised by the use of a low load (11.55 N) and sliding speed (0.1 m s(-1)). For unreinforced 6061 Al and A356 Al alloys a transition from mild to severe wear occurred in the ranges 175-190 degrees C and 225-230 degrees C respectively. With the addition of 20 vol.% Al2O3 to 6061 Al, the mild to severe wear transition was raised to a range between 310-350 degrees C, Likewise, an addition of 20 vol.% SiC to the A356 Al increased this transition to 440-450 degrees C. A hybrid A356 Al composite containing 20 vol.% SiC and 10 vol.% graphite remained in a mild wear regime at the highest test temperature of 460 degrees C. All the reinforced alloys were able to withstand considerable thermal softening effects while remaining in a mild sliding wear regime. This is attributable to the formation of protective transfer layers of comminuted reinforcing particulates and transferred steel debris from slider counterfaces. Graphite in the hybrid composite introduced greater mild wear losses compared with the other composites due to increased friability and contact surface extrusion effects. The absence of severe wear phenomena in this composite contributes to the inhibition of comminution and fracture by graphite entrained in the surface tribolayer.