Invesigation On The Prearation And Selection Of Coloring Elements Of Microarc Oxidation Black Ceramic Coating Of Magnesium Alloy Based On First Principle

Based on the chromogenic principle of black ceramic coating,the band gap width and density of states distribution of MgO doped by Fe,Co,Ni and Cu were studied by first principles.The Fe doped black micro-arc oxidation ceramic coating was prepared by micro-arc oxidation technology,and the selection basis of doping elements in the black micro-arc oxidation ceramic coating was verified and discussed.The results are as follows:The first principle calculation showed that both Fe,Co,Ni and Cu element doping can reduce the band gap of MgO,and the band width decreases with the increase of doping concentration,which meets the requirement of ceramic coating doping theoretically.When Fe,Co,Ni and Cu are substituted for MgO,the MgO conduction band moves from the bottom to the low energy.Except for Fe,the valence band top does not change,the Co,Ni and Cu all make the MgO valence band top move to the high energy,thus reducing the band gap,Among them,the decreasing effect is in descending order:Fe>Co>Ni>Cu.According to the density of states distribution,the doping of these four elements introduces 3d and 4s energy levels,which affect the composition and distribution of MgO valence band and conduction band energy levels respectively.With the increase of Fe concentration,the color of the ceramic coating becomes darker,but when the concentration is too high,the ceramic layer is easy to ablate.When Fe/Mg is 3.5 at.%,the total color value of the black ceramic coating is 24.71.And the absorption rate in the visible light region is up to 96%.The effect of Fe doping is better than that of Cu doping,which is consistent with the calculated results.Combined with XRD and XPS,Mg1-xFexO was formed by Fe doped MgO ceramic coating.The corrosion potential of Fe doped and Cu doped black ceramic coating with thickness of 10?m is similar to that of the white ceramic coating.The corrosion current density is 10-9 A·cm-2,and the electrochemical impedance value is 105 ?·cm2,indicating that the doping elements do not change the phase of MgO ceramic coating.The band gap of the Fe-doped black ceramic coating decreases from 4.97 eV to 1.70 eV,the top of the valence band moves from 0.61 eV to 0.94 eV,and the bottom of the conduction band moves from 5.58 eV to 2.64 eV.The critical wavelength of light absorption of the Fe-doped black ceramic coating expands from 249.5 nm to 729.4 nm.It is proved that doping elements reduce the band gap of the ceramic coating to the wavelength range of visible light,which is the theoretical basis for the selection of coloring elements of MgO black ceramic layer,and The actual doping effect of doped elements depends on the conditions and parameters in the micro-arc oxidation process.