The tribological performance of 6061 aluminum alloy/graphite composite materials in oil lubrications with EP additives

A thrust-on-washer adapter was applied to simulate surface contacts under oil lubrications. The upper, ring-style specimens were prepared using A356.0 aluminum alloy material; the lower, disk-style specimens were prepared using a composite material of 6061 aluminum alloy and graphite. The load applications were increased so that friction contacts ranged from hydrodynamic or mixed lubrication to boundary lubrication. The tribological mechanism and antiseizure performance of oil with different extreme pressure (EP) additive concentrations was shown to relate closely to friction coefficients, Stribeck curves, electrical voltage variations, wear mechanisms, and chemical products in wear debris. The experimental results reveal that the occurrence of surface seizure is dependent upon the value of the oil temperature rise per second, rather than the oil temperature itself. When the initial oil temperature was kept at 25°C, raising the EP additive concentration shortened the time before surface seizure occurred. When the initial oil temperature was 70°C, an increase in the EP additive concentration fully prevented surface seizure. Under boundary lubrication conditions, the EP additive alone was unable to generate chemically reactive films regardless of oil temperatures; the antiwear performance occurred only in tribosurfaces when operating in hydrodynamic or mixed lubrication regimes.