Fretting wear behavior of duplex PEO/chameleon coating on Al alloy

Plasma electrolytic oxidation (PEO) is an attractive technology for improving resistance to wear, heat and corrosion of aluminum alloys. PEO results in a hard, well-adhered alumina ceramic coating with a morphology which is graded from a dense region near the substrate interface to a porous outer region. Such properties mean that PEO can be an ideal underlying layer for the application of solid lubricants which can be entrapped in outer pores and provide reservoirs for the tribological contact lubrication. This study investigates the fretting wear behavior and adaptive mechanisms for a PEO-produced alumina surface of about 11–12 GPa hardness with a top layer of an MoS2/Sb2O3/C chameleon solid lubricating coating, the composition of which was designed to self-adapt in variable humidity environments for friction and wear reduction. Coupons of AA 6082 alloy were coated by the PEO process and then were over-coated by a burnishing process with a MoS2/Sb2O3/C chameleon coating to prepare a duplex coating combination. The coated surfaces were investigated using nanoindenation, Raman spectroscopy and scanning electroscopy and were then subjected to fretting wear tests against steel and alumina balls with variable amplitude (0–100 μm) and loads (10–100 N) in both humid air and in dry nitrogen environment conditions. The tests demonstrated low friction coefficients, considerable reduction in critical amplitude for the stick-slip transition, and self-adaptive tribological behavior in cycled environment tests. Friction coefficients of the order of 0.10–0.15 in humid air and 0.06–0.09 in dry nitrogen were recorded and linked with the surface self-adjustment from graphite to MoS2 lubrication, respectively, which was confirmed by Raman spectroscopy. The studies demonstrate the effectiveness of the PEO/chameleon duplex coating system for the friction reduction of and fretting wear in the gross-slip regime, as well as significantly reducing the critical amplitude of stick-slip transition for fatigue wear mitigation