The characterization of the coating formed by Microarc oxidation on binary Al–Mn alloys

Abstract

In this study, effects of Mn content on behavior and properties of Microarc Oxidation (MAO) coatings produced on binary Al–Mn alloys and pure Al were investigated. Al–Mn alloys containing 1, 2, 4, 8 at.% Mn and pure aluminum produced using induction furnace under controlled atmosphere were coated in an alkaline electrolytic solution for 120 min. Surface roughness, thickness, microhardness, phase composition, microstructure and chemical composition of coatings were characterized by profilometry, Eddy Current test, microhardness measurement, XRD, SEM and SEM–EDS. All coatings composed of three regions: thin transition layer, dense inner region and porous loose outer region. Granular precipitates of α-Al2O3 were observed through dense inner region of coatings. With increasing Mn content, coating thickness increased from 111 μm to 169 μm. All coatings, regardless of substrate chemical composition, contained Al6Si2O13, γ-Al2O3, α-Al2O3 and considerable amount of amorphous phases. Increasing amount of Mn suppressed α-Al2O3 formation. It was seen that surface roughness and porosity formation tends to increase whereas the coating hardness tend to decrease with increasing Mn content. The coatings produced on Al–Mn alloys contained Mn along with Al, Si, O. The amount of Mn in the coatings increases with increase of Mn content in the substrates.