WEAR REGIMES AND TRANSITIONS IN AL2O3 PARTICULATE-REINFORCED ALUMINUM-ALLOYS

The effect of the applied load on the unlubricated sliding wear behaviour of a 6061 Al alloy reinforced with 20 vol.% Al2O3 particles was studied. Experiments were performed using a block-on-ring type wear rig. Wear of the control alloy, i.e. the unreinforced 6061 Al, and the wear of the counterface (AISI 52100 steel) were also studied. Three wear rate regimes were observed in the composite: in region I, i.e. at low loads (less than 10 N) the wear rates were less than 2 x 10(-5) mm3 m-1. This was followed by a transition region where the rates increased by a factor of 10(2). In region II that covered mid-range loads the wear rates raised steadily, 10(-3) to 10(-2) mm3 m-1, up to 230 N where a second transition took place to a severe wear regime (region IH). In the unreinforced 6061 Al, only regions II and III were observed. At low loads the wear resistance of the composite (region I) was two orders of magnitude higher than that of the unreinforced alloy. In region H there was no significant difference between the wear rates of the unreinforced and the Al2O3-reinforced alloys. However, the transition from region II to III occurred at a lower load in the unreinforced 6061 Al (60 N). Metallographic studies performed to delineate the rate controlling wear mechanisms revealed that low wear rates in region I resulted from the load-bearing capacity of Al2O3 particles and the formation of transfer layers on the contact surfaces of the composites. When the applied load exceeded the fracture strength of the particles, particles at the surface were fractured and wear occurred by a process of subsurface crack growth. Al2O3 particles promoted crack nucleation and growth and acted as third-body abrasives resulting in wear rates similar to those in the unreinforced 6061 Al. The transition to severe wear rate regime is shown to be controlled by frictional heating to a critical temperature. This temperature was higher for the composite material for which the transition was postponed to higher loads.