The role of oxidation in the friction and wear behavior of solid solution Cu-Al alloys in reciprocating sliding contact with sapphire and D2 tool steel

Abstract

The tribological behavior of dilute solid solution Cu-Al alloys in sliding contact with sapphire and D2 steel was investigated. The worn and unworn contacting surfaces and wear debris were characterized using an appropriate combination of X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, optical microscopy, optical stereomicroscopy, hardness and X-ray photoelectron spectroscopy (XPS). Wear testing was performed with a slow speed reciprocating wear tester with cylinder-on-flat geometry. The coefficient of friction (COF) was recorded for each wear cycle during run-in and periodically recorded during the steady state wear regime. Cumulative wear rates were measured for both the Cu-Al alloy flats and slider cylinders. The wear rate of the solid solution Cu-Al alloys was found to increase with increasing aluminum content. Worn alloy surfaces exhibited increasing mechanical damage and decreasing copper oxide coverage as aluminum content increased. High aluminum contents were found to promote planar slip, adhesive wear and the formation of metallic wear debris. Increased adhesion and metallic transfer resulted in a parallel increase in the COF. Surface segregation of aluminum was observed in the wear track areas for all alloy compositions. Angle-resolve XPS (ARXPS) analysis showed significant aluminum enrichment of the surface for as-polished and partially sputter-cleaned Cu-6wt.%Al alloys. For the tribosystem described, the friction and wear behavior of solid solution Cu-Al alloys is dependent on the adherence of the wear-induced copper oxide to the underlying substrate. Neither increased alloy hardness nor the ability to strain harden increased wear resistance. Instead, as aluminum content increased, a substantial increase in the adhesive wear of the Cu-Al alloy flats was observed during the run-in period. Surface segregation of aluminum resulted in the formation of an aluminum oxide layer. The incompatibility of A1 2O 3 and the metallic substrate creates an interface susceptible to disruption by surface shear forces during the wear process. The increased interfacial adhesion results in alloy transfer to the opposing slider and an increase in wear.