Microarc oxidation of pure aluminium in alumina containing electrolytes

ABSTRACT

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.

KEYWORDS:

• Aluminium
• microarc oxidation
• Al₂O₃
• slurry electrolyte
• microhardness

Disclosure statement

No potential conflict of interest was reported by the authors.