Preparation And Performance Of Al₂O₃/SiO₂ Micro-arc Oxidation Coatings On 5a06 Aluminum Alloy

In this paper,in order to change the porous structure and enhance the hardness of micro-arc oxidation(MAO)coatings,meanwhile meeting the requirements of wear resistance and protection performance of aluminum alloy surface,it was proposed to prepare Al2O3/SiO2 MAO coatings by adding nano-SiO2 particles to the electrolyte.Based on this idea,a suitable dispersant was added to the electrolyte to disperse nano-SiO2 particles,and finally,the Al2O3/SiO2 composite MAO ceramic coatings were prepared with appropriate electrical parameters on 5A06 aluminum alloy.Also,the wear resistance and protection performance were studied.721 spectrophotometer,transmission electron microscope(TEM)and scanning electron microscope(SEM)were used to study the dispersion effect of nano-SiO2 particles and the microstructure of MAO coatings with different dispersants added.The results show that,both SHMP and EDTA2Na had the better dispersibility and suspension stability of nano-SiO2 particles in the electrolyte.Compared with SHMP dispersant,the MAO coatings prepared with EDTA2Na added had the higher density,and its peak of hardness can reach 1352 HV.Besides,the coating had the better wear resistance and its volume wear rate reached 1.923×10-4 mm3/N·m,so EDTA2Na was selected as the dispersant of nano-SiO2 particles.Scanning electron microscope(SEM),energy spectrum analyzer(EDS)and X-ray diffractometer(XRD)were used to characterize the structure of the nanocomposite MAO coatings.The results show that,the composite of nano-SiO2 particles can reduce the roughness and increase the density of MAO coatings significantly.At the same time,nano-SiO2 particles can react with Al2O3 to form a mullite phade and distribute on the porous layer evenly.Within a certain range,the content of SiO2 in the nanocomposite coatings increased with the increase of the concentration of nano-SiO2 particles in the electrolyte.The silicon content in the nanocomposite coating can reach 42.6%when SiO2 concentration was 10 g/L.The effect of nano-SiO2 particles on the performance of MAO coatings were also studied by hardometer,friction-wear tester,salt spray and electrochemical experiments.The results show that,the hardness of the nanocomposite MAO coatings increased significantly,and its maximum hardness can reach 1777 HV,which was about 31.3%higher than that of the conventional coating.The nanocomposite MAO coatings had the better wear performance,which friction coefficient reduced significantly and volume wear rate reduced an order of magnitude compared with the conventional MAO coatings.The trace which was due to corrosion had not been obviously observed on the nanocomposite MAO coatings after the neutral salt spray test for more than 5000 hours.Compared with the conventional MAO coatings,the corrosion current density of the composite MAO coatings had reduced by an order of magnitude which showed the enhanced corrosion resistance.The influence mechanism of nano-SiO2 particles on MAO coatings was discussed.The nano-SiO2 particles can enter into the discharge channel during the discharge breakdown stage and participated in the micro-arc oxidation reaction,recombine into the layer during solidification,and form an Al2O3/SiO2 MAO composite coatings finally.With the recombination of SiO2,the mullite phase and ?-Al2O3 phase enhanced,which promote the hardness of MAO coatings.The wear resistance of MAO coatings was improved greatly because of the reduce of surface roughness and frictional shear stress between the film layer and the grinding ball.The density of MAO coatings was greatly increased so that its resistance to the corrosive medium had enhanced,then the corrosion resistance of MAO coatings was significantly improved.