The Preparation And Performance Of Micro Arc Oxidation Coatings On Mg-8Li-2Ca Alloy

Micro-arc oxidation?MAO?coatings were obtained on Mg-8Li-2Ca alloy in an alkaline Na₃PO₄--Ca[C₃H₇O₆P]base solution.The effects of voltage,frequency,the content of Ca[C₃H₇O₆P],time and duty cycle on the microstructure,composition and properties of the coating were researched.The formation and corrosion mechanisms of the coating were researched.The surface and cross-sectional morphologies,elemental compositions and phase of the obtained coatings were investigated by scanning electron microscopy?SEM?,energy dispersive X-ray spectroscopy?EDS?and X-ray diffraction?XRD?,respectively.The corrosion behavior of the coatings was evaluated by potential dynamic polarization?PDP?curves,electrochemical impedance spectroscopy?EIS?and hydrogen evolution experiment in SBF solution.The hardness,friction and wear property were evaluated by micro hardness test and friction and wear test.Results show that the optimum parameters are 450V of voltage,700Hz of frequency,0.02mol/L of Ca[C₃H₇O₆P]concentration,30min of treatment time and 50%of duty cycle.The MAO coating consists of MgO,Ca₂H₂P₃O₄,Ca₅?OH??PO₄?₃.The corrosion resistance after micro arc oxidation is significantly increased.Compared with substrate,the hydrogen evolution rate is decreased evidently,the corrosion potential moves rightwards 1.2V at least,and the corrosion current density decreases by two orders of magnitude at least.The wear mechanism of the abrasion between the coating and GCr15 steel ball is mixed mechanism of fatigue wear and abrasive wear.The formation of micro arc oxidation coating is divided into two steps:the first step is anodic oxidation,the substrate is corroded,then the corrosion product MgO layer is formed on the surface of alloy;the second step is the corrosion product layer break down,PO₄^3-,OH-enter the discharge channels,micro arc oxidation reactions occur on the surface,micro arc oxidation coatings which contains the elements P and Ca are formed.When the coating is immersed in SBF solution,the corrosion begins at the interface of coating and substrate.Corrosion spreads to substrate and coating at the same time.The cracks form in the coating,and then connect with each other,eventually the coating falls off from the substrate.The bioactive Ca₃?PO₄?₂?OH?phase is deposited on the coating during the corrosion process.