Characterization of coatings obtained on aluminum alloy 6061 during soft sparking PEO in solid particles suspension

Plasma electrolytic oxidation (PEO) is a relatively novel method of obtaining thick and durable ceramic coatings on top of valve metals, such as aluminum and its alloys. A combination of electrochemical and thermochemical processes during PEO enables the incorporation of species present in the electrolyte inside the coating. In this systematic study, 6061 aluminum alloy was subjected to plasma electrolytic oxidation in solutions containing yttria-stabilized zirconia (ZrO 2 ), anatase (TiO 2 ), corundum (α-Al 2 O 3 ), and amorphous silica (SiO 2 ) under soft sparking regime (SSR). The electrolytes were modified by adding solid particle suspensions at concentrations of 3, 7, and 10 g L −1 . The aim was to enhance both corrosion resistance and mechanical strength. The resulting coatings were examined for surface morphology, phase composition, microhardness, tribological performance, and corrosion resistance. The results confirmed that particles can be incorporated into the coating during SSR, giving rise to distinct surface morphologies, higher hardness (from 862 ± 220 HV 0.05 for the base electrolyte to 1305 ± 298 HV 0.05 for the 3 g L −1 ZrO 2 variant), lower wear rate (from 70.97 ± 1.33 µm for the base electrolyte to 36.07 ± 0.82 µm for the 3 g L −1 TiO 2 variant at a load of 10 N), and improved corrosion resistance (charge-transfer resistances measured for the 10 g L −1 Al 2 O 3 bath increased by an order of magnitude compared to the Blank sample). The best outcomes were observed at the lowest particles loading (3 g L −1 ) for the SiO 2 - and ZrO 2 -enriched coatings. Higher concentrations were found to be optimal for the electrolytes containing Al 2 O 3 (10 g L −1 ) and TiO 2 (7 g/L −1 ).