Study On MAO Composite Process, Microstructure And Property Of The Coating Fabricated On AZ91D Magnesium Alloy

This topic mainly researches the micro arc oxidation(MAO) and sol-gel compositeprocess on the surface of AZ91D magnesium alloy.First,we research the effect of nano-SiCon the micro arc oxidation,then research the overlay technique of SiO2sol on the MAOcoatings,finally summarize the better composite process with MAO and sol-gel.Theexperimental results show the followings:After adding nano-SiC into the silicate-aluminate based composite electrolyte,nano-SiC has effect on the voltage during MAO.The thickness,morphology, density,roughness and corrosion resistance of the coating are also affected by it.After nano-SiCincorporated into the coating,the colour of the coating changes;The number of the porereduces,so as the and crack;The porosity of the coating decreases.With the increase ofnano-SiC, both the striking voltage and final voltage reduce,the thickness and roughness ofthe coating increase,the corrosion resistance of the coating decreases first and increaseslater,at last decreases.Taking all the experimental results into consideration,we think thatwhen the content of the nano-SiC in the electrolyte is4g/L, the coating obtained under thiscondition has a better performance.The coating obtained in the electrolyte which contains4g/L nano-SiC is overlaid byordinary SiO2sol and modified SiO2sol respectively.After overlaied the modified SiO2sol,the pores on the surface of the MAO coating disappear while some micro cracksappear.The cross-sectional of the coating presents two obvious structures, the modified SiO2sol adhere to the MAO coating uniformly,there is no crack between them.The overlaiedeffect of the modified SiO2sol is better than the overlaied effect of the ordinary SiO2sol.Use modified SiO2sol to overlay the MAO coatings, then solidify the samples underroom temperature, high temperature and gradient temperature respectively. The number ofthe crack on the surface of the gradient solidification composite coating is less than the othertwo, and the massive region on the surface is not obvious, it shows a good surfacemorphology.The hydrophobicity of the gradient solidification composite coating is alsobetter than the other two.Both immersion test and electrochemical experiment show that thecorrosion resistance of the gradient solidification composite coating is better than thecorrosion resistance of the other two.The results of the microstructure and the corrosionresistance of the composite coating indicate that the gradient solidification composite coating has a better performance.According to all the experiment results, a better composite process with MAO andsol-gel is summarized: Add4g/L nano-SiC in the electrolyte to prepare MAO coating, thenuse modified SiO2sol to overlay the MAO coating and solidify the SiO2sol under gradienttemperature,at last a composite coating with a better comprehensive performance can beobtained.