Microarc oxidation of pure aluminium in alumina containing electrolytes

In this study, oxide coatings produced on commercially pure Al via 40-min microarc oxidation process in electrolytes containing Na2SiO3, KOH and α-Al2O3 powder were characterized by SEM-EDS, profilometry, XRD and microhardness tests. The addition of α-Al2O3 particles into the electrolyte resulted in increased coating thickness and roughness. The coating thickness values of the samples varied between 35 and 135 µm. Increasing the amount of α-Al2O3 particles in the electrolyte resulted in mechanical deposition/encapsulation of α-Al2O3 particles and/or chemical reaction with constituents of the coatings. This chemical reaction increased the microhardness of the coatings and resulted in preferential growth of mullite (3Al2O3.2SiO2) crystal in the outer glassy region of the coatings. Outer regions of the coatings comprised 3Al2O3.2SiO2, γ-Al2O3 and κ-Al2O3 phases with chemically or mechanically bonded α-Al2O3 particles, and in the inner regions were mostly γ-Al2O3 and κ-Al2O3 phases along with α-Al2O3 phase formed through phase transformations.