Influence of additives on microstructure and property of microarc oxidized Mg–Si–O coatings

Ceramic coatings were fabricated on ZK60 magnesium alloy substrate by microarc oxidation (MAO) in Na2SiO3–KOH base electrolyte with four kinds of additives (i.e. KF, NH4HF2, C3H8O3 and H2O2). The effects of these additives on microstructure and property of coatings were investigated. The surface morphology, phase composition and corrosion resistance of the ceramic coatings were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and simulation body fluid (SBF) immersion test respectively. It is found that different additives can change the spark discharge phenomenon during microarc oxidation. It is proved that both potassium fluoride (KF) and ammonium bifluoride (NH4HF2) promote discharge and accelerate reaction while the introduce of glycerol (C3H8O3) leads to the refining of sparks and reduction of thermal effects. Results also demonstrate that the introduce of hydrogen peroxide (H2O2) contributes to the increase of coating surface roughness and enlargement of surface micropore size. XRD results indicate that the ceramic coatings are mainly composed of Mg2SiO4, MgSiO3 and SiO2. The introduce of H2O2 hinders the reaction between SiO2 and MgO and creates favorable conditions for the formation of the MgO phase. The ceramic coatings formed in base electrolyte containing 7 g/L NH4HF2 and 5 mL/L C3H8O3 exhibit the highest corrosion resistance.