| A coating with Ca and P elements was prefabricated on AZ91D magnesium alloy by micro arc-oxidation surface modification technology.Then a hydroxyapatite coating was deposited on micro arc-oxidation coating surface through electrochemical deposition technology.The phase compositions,morphologies,element compositions and thickness of the coating were studied by XRD,SEM,EDS and thickness tester,respectively.Electrochemical workstation and electronic analytical balance were used to study the corrosion resistant and degradation properties of the coatings in simulated body fluids.Finally using INSTRON tensile fatigue testing machine to research the coatings samples machine performance after soaking in simulated body fluid.The main results are as follows:1.Micro arc oxidation coating on magnesium alloy is mainly composed of MgO phase and Mg2SiO4 phase.The coating surface is rough,distributed with a large number of micro holes.It can be divided into two layers.The outer layer is looser with more defects while the inner layer is denser with less defects.The main components of the electrochemical deposition coating is hydroxyapatite phase and the coating is mainly composed of lamellar hydroxyapatite.2.Calcium gluconate,frequency and duty ratio have little impact on the micro-arc oxidation coating’s thickness,but they can affect on the coating’s surface morphologies and corrosion resistance in simulated body fluids.The micro-arc oxidation coating has a good comprehensive performance when the content of calcium gluconate,frequency and duty ratio are 4 g/l,600 HZ and 15%,respectively.The structure and morphology of hydroxyapatite in electrochemical deposition coating is affected by the deposition voltage and deposition time.As deposition voltage rising,hydroxyapatite structure gradually become more dense and its morphologies change from flake morphology into needle morphology.Deposition time can increase the coating’s thickness and quality,reducing the current density of this system,and increasing the quantity of flake hydroxyapatite.When the voltage is 5 V and depositing time is 120 min.,the electrochemical deposition coating has the best quality.3.The corrosion current density of micro-arc oxidation coating and micro-arc oxidation coating/electrochemical deposition composite coating in simulated body fluids is 0.65μA/cm2 and 0.03μA/cm2 respectively,it reduces significantly compered to1230μA/cm2 of magnesium alloy.Therefore coatings can improve the corrosion resistance,meanwhile they can also reduce the degradation mass of magnesium alloy in simulated body fluids.The micro-arc oxidation coating containing Ca and P element and the micro-arc oxidation/electrochemical deposition composite coating both have biological activity.4.Magnesium alloy suffers from a severe corrosion after soaking in simulated body fluids.Its tensile strength reduced from 299 MPa to 281 MPa and the fatigue life reduction from 69026 to 31400 times.Micro-arc oxidation/electrochemical deposition composite coating can effectively prevent the invasion of the corrosive medium than the micro-arc oxidation coating.It can slso make the magnesium alloy still keep its original mechanical properties in simulated body fluid. |
