Effect of Na2SiO3·5H2O concentration on microstructure and mechanical properties of plasma electrolytic oxide coatings on AZ31 Mg alloy produced by twin roll casting

In this study, AZ31 Mg alloys produced by twin roll casting (TRC) were coated by plasma electrolytic oxidation (PEO) in solution of potassium hydroxide (KOH) and different concentration of sodium metasilicate pentahydrate (Na2SiO3·5H2O) electrolytes at 0.140 A/cm2 current density for 60 min. The phase structure, morphology, hardness, adhesion strength, surface roughness and wear resistance of the PEO coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), micro-hardness tester, micro-scratch tester, profilometer and tribometer, respectively. The XRD results indicated that Forsterite (Mg2SiO4) and Periclase (MgO) phases were formed on the surface of the coated magnesium alloys. The surface of the coatings is very porous and rough due to the presence of micro-discharges through PEO process. It was seen that the wear rate, hardness and surface roughness of the coatings were increased when the concentration of Na2SiO3·5H2O electrolyte was increased.