The effect of Zn on the microarc oxidation coating behavior of synthetic Al–Zn binary alloys

Highlights: ► The coating thickness increased with Zn addition into substrate. ► The formation α-Al2O3 was suppressed and consequently hardness value of the coatings decreased with Zn content in the substrates. ► The increase in amount of Zn in Al, resulted in a rougher surface. ► Mullite and γ-Al2O3 phases existed on all coatings. α-Al2O3 phase was also detected on coated pure Al and Al–Zn alloys with 1–10 wt.% Zn. - Abstract: The synthetic Al–Zn binary alloys were prepared in a vacuum/atmosphere-controlled furnace with the addition of 1, 4, 8, 10, 15 and 33 wt.% Zn. The alloys and pure Al were coated by microarc oxidation in the electrolyte containing sodium silicate and potassium hydroxide for 120 min. XRD, scanning electron microscope (SEM)–energy-dispersive X-ray spectroscopy (EDS), profilometry and Vickers microhardness measurements were employed for the characterization of the coating. The higher amount of Zn in the substrate promoted the formation of porous coating. The outer regions of all coatings were loose and weak, while the inner regions were relatively denser and harder. The features on the coating surface became coarser with the increase of Zn content in the alloys, which resulted in the increase of surface roughness from 9.3 μm to 12.5 μm. The coating thickness increased from 80 μm to 120 μm with the increase of Zn content in the substrates. Mullite and γ-Al2O3 phases were formed on all coatings and α-Al2O3 phase was also formed on coated pure Al and Al–Zn alloys with 1–10 wt.% Zn. Increasing amount of Zn in the Al–Zn alloys suppressed α-Al2O3 formation and reduced the microhardness of the coatings. The Zn concentration was constant throughout the coating for the alloys with lower amount of Zn, though it decreased gradually from interface to surface for the alloys containing higher amount of Zn.