This study aimed to prepare self-lubricating micro-arc oxidation (MAO)/graphite composite coatings on 6061 aluminium alloy by adding graphite nanoparticles into an alkaline electrolyte during the MAO process. The microstructure, phase composition and element distribution of MAO coatings were studied using Scanning Electron Microscope (SEM), x-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS). The results showed that graphite nanoparticles can be coated uniformly in the MAO coatings, and the porosity and surface roughness of the MAO coatings were reduced by the addition of graphite nanoparticles. However, when the content of graphite nanoparticles was more than 6 g l−1, graphite aggregation occurred. The wear resistance of the coatings was investigated using the ball-on-disk friction tester. After incorporating the graphite nanoparticles, the MAO coatings showed excellent self-lubricating properties and a reduction in the friction coefficient during the wear process. In addition, the MAO coatings showed the best result with the addition of 4 g l−1 graphite nanoparticles compared with all the other composite coatings under investigation. In conclusion, the incorporation of graphite nanoparticles improved the wear properties of the MAO coatings.
Micro-arc oxidation (MAO) coating was initially prepared on 6061 Al alloy, and subsequently coated with Ag using magnetron sputtering. Laser beam scan (LBS) treatments were then applied to infiltrate the sputtered Ag into the MAO coating in order to simulate the corrosion behaviour of the MAO coating during the neutral salt spray test (NSST). Cross-sectional morphologies and Ag element distribution maps were studied by using the field emission scanning electron microscopy (FESEM) on the MAO-Ag coated samples after LBS infiltration. The results showed that there existed the microcracks with tree-root or lightning shape within the MAO coating. Interconnected aggregates, including micro-pores, large cavities far beneath the surface and tree-root like micro-cracks, served as complex corrosion channels during salt spray corrosion. Through these corrosion channels, the salt spray penetrated gradually into the interface between the MAO coating and the original Al substrate, thus causing corrosion of the Al substrate during the NSST. The LBS treatment is presented as a method to explore the subtle micro-crack and pore channels associated to the MAO coating and to provide information that paves the way towards understanding of corrosion phenomena in these porous systems.
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