Preparation Of Micro-arc Oxidation Coating On 7N01 Aluminum Alloy And Research On Its Wear Resistance And Corrosion Resistance

The 7N01 aluminum alloy used for railway vehicles has good strength,mechanical properties and forming ability,and is commonly used to manufacture the load-bearing components of the car body.However,wear and corrosion inevitably occur in the service process,which reduces the service life,so the surface strengthening of 7N01 aluminum alloy is necessary.In this paper,ceramic coatings were prepared on 7N01 aluminum alloy by micro-arc oxidation（MAO）technology.The effects of electrolyte composition and electrical parameters on the thickness,roughness and bonding strength of ceramic coatings were systematically studied.Scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）and X-ray diffraction（XRD）were used to analyze the micro morphology,element distribution and phase composition of the coating.The wear resistance and corrosion resistance of the ceramic coatings were also studied,and zinc oxide nanoparticles（Zn O NPs）were incorporated into the MAO coating to further improve its wear resistance and corrosion resistance.The results show that the ceramic coating obtained in the aluminate system has a medium thickness,the smallest roughness,the highest bonding strength,and the best surface quality.The electrolyte components were optimized orthogonally,and the optimized components were12 g/L Na Al O₂,1g/L Na OH,7 ml/L TEOA,and 2 ml/L H₂O₂.The electrical parameters were studied in turn by single factor variables,and the preferred electrical parameters included current density of 9 A/dm²,positive（negative）duty cycle of 30%（20%）,pulse frequency of300 Hz,and oxidation time of 30 min.The MAO coating has a porous structure with pores in a typical crater morphology,and there were a large number of pancake-like accumulation and small microcracks on the surface.The elements on the surface of ceramic coating were uniformly distributed,and the cross-section showed an obvious transition.The MAO coating was mainly composed ofγ-Al₂O₃ andα-Al₂O₃ phases.The friction coefficient of the MAO coating was lower than that of the 7N01 aluminum alloy substrate,and it decreased with the increase of load and speed.The wear amount of the MAO coating was only 20%of the substrate,and the width of the wear scar was only 50%of the substrate,indicating that the MAO coating has a good anti-friction and wear-resistant effect.There were a lot of pits and abrasive particles on the surface of the worn ceramic coating,and the wear mechanism was abrasive wear and adhesive wear.The load-bearing capacity of the ceramic coating is related to its compactness.The abrasive particles produced by wear can fill the recessed position of the coating,improve the density and roughness,and make it have good wear resistance.The ceramic coating with an oxidation time of 30 min had the longest dropping corrosion time（52 min）.After 10 days of immersion test,no obvious corrosion marks appeared on the surface of the MAO coating,and the specific mass loss was only 50%of the aluminum alloy matrix,which showed good corrosion resistance.Compared with the substrate,the corrosion potential of the MAO coating increased by 633.89 m V（-352.07 m V）,the corrosion current density decreased by 10 times(1.43×10^-7 A·cm^-2),the polarization resistance increased by 40 times(5.05×10⁵Ω·cm^-2),the corrosion resistance was significantly improved,and hole sealing process can reduce the corrosion current density 170 times(9.37×10^-9 A·cm^-2),further improve the corrosion resistance.The composite ceramic coating was prepared by adding different concentrations of Zn O to the electrolyte.It was found that with the increase of Zn O concentration,the growth of the coating was inhibited,the thickness gradually decreased,the roughness first decreased and then increased,and the bonding strength first increased and then decreased.There were many fine particles distributed on the surface of the composite ceramic coating,and the distribution became wider as the increase of Zn O concentration.Zn O particles were inertly added to the ceramic coating under the action of electrophoresis and melt coating,and first attached to the defect position of the coating.Therefore,the composite ceramic coating mainly contained three phases ofγ-Al₂O₃,α-Al₂O₃ and Zn O.Appropriate Zn O NPs concentration can improve the wear and corrosion resistance of the ceramic coating.The coating added with 2 g/L Zn O has the best anti-friction and wear resistance effect,the friction coefficient was the smallest（0.28）,the mass loss rate was less than 50%of the conventional ceramic coatin,and only 12%of the matrix.For the caoting added with 2 g/L Zn O,the specific mass loss of the immersion test was only 40%of that of the conventional coating,the corrosion current density was reduced by 500 times(2.91×10^-10 A·cm^-2),which was lower than that of the aluminum alloy matrix 4 orders of magnitude,and has the best corrosion resistance.