Characterization and mechanical properties of coatings on magnesium by micro arc oxidation

Abstract

The commercial pure magnesium was coated by micro arc oxidation method in different aqueous solution, containing sodium silicate and sodium phosphate. Micro arc oxidation process was carried out at 0.060 A/cm(2), 0.085 A/cm(2) and 0.140 A/cm(2) current densities for 30 min. The thickness, phase composition, morphology, hardness, adhesion strength and wear resistance of coatings were analyzed by eddy current, X-ray diffraction (XRD), scanning electron microscope (SEM), micro hardness tester, scratch tester and ball-on disk tribometer, respectively. The average thicknesses of the micro arc oxidized coatings ranged from 27 to 48 mu m for sodium silicate solution and from 45 to 75 mu m for sodium phosphate solution. The dominant phases formed on the pure magnesium were found to be a mixture of spinel Mg2SiO4 (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg-3(PO4)(2) (Farringtonite) and MgO (Periclase) for sodium phosphate solution. The average hardnesses of the micro arc oxidized coatings were between 260 HV and 470 HV for sodium silicate solution and between 175 HV and 260 HV for sodium phosphate solution. Adhesion strengths and wear resistances of coatings produced in sodium silicate solution were higher than those of the ones in sodium phosphate solution due to high hardness of coatings produced in sodium silicate solution. Crown Copyright (C) 2012 Published by Elsevier B. V. All rights reserved.