Preparation And Performance Characterization Of Micro Arc Oxidation Coatings On AlTiCrVZr High Entropy Alloy

High entropy alloys(HEAs)have attracted much attention due to their excellent mechanical properties,wear resistance and corrosion resistance.Refractory high entropy alloys(RHEAs)have excellent mechanical properties at room and high temperature,which are expected to become another important candidate material in the high temperature field.However,most of the existing RHEAs contain V,Mo,W elements,which are easy to form volatile oxides at high temperature,resulting in poor high oxidation resistance.As a result,the application of RHEAs in the high temperature field has been greatly limited.In this paper,oxide ceramic coatings were prefabricated by micro arc oxidation(MAO)on AlTiCrVZr.The effects of electrolyte composition,concentration and electrical parameters on the microstructure,composition,high temperature oxidation resistance and corrosion resistance of the MAO coatings on AlTiCrVZr were investigated.The growth mechanism and high temperature oxidation resistance mechanism of the MAO coatings on AlTiCrVZr were revealed.The results are as follows:The surface and cross-sectional morphologies of the MAO coatings prepared in the electrolytes with different compositions and concentration are quite different.The compositions of the MAO coatings prepared in the Na₂SiO₃+(NaPO₃)₆+NaOH+Na₂B₄O₇+KF electrolyte are Al₂O₃,TiO₂,Cr₂O₃,CrO₃,V₂O₅,ZrO₂ and SiO₂.The MAO coating with less micropores and micro cracks formed in the 50 g/L Na₂SiO₃+25 g/L(NaPO₃)₆+5 g/L NaOH electrolyte has a smooth and compact surface.With the increase of electrolyte concentration,the composition of MAO coatings are mainly Al₂O₃,TiO₂,Cr₂O₃,ZrO₂ and SiO₂,which have excellent high temperature oxidation resistance.After high temperature oxidation at 800? for 20 h,the oxidation rate of the substrate is 4.90 mg/(cm²·h).The composition and concentration of electrolyte and the oxidation rate of the MAO coating are formed in the 30 g/L Na₂SiO₃+15 g/L(NaPO₃)₆+3 g/L NaOH electrolyte was up to 4.97 mg/(cm²·h),which is higher than that of the substrate.The spalling of MAO coating at high temperature results in the loss of its protective effect on the substrate.The MAO coatings prepared in the electrolytes with different compositions and concentrations have good corrosion resistance in the simulated seawater for 30 days.With the increase of voltage,the MAO coatings gradually become smooth and compact.The main components of the MAO coatings formed under different voltages are Al₂O₃,TiO₂,Cr₂O₃,V₂O₅,ZrO₂ and SiO₂.After oxidation at 800? for 20 h,the MAO coating prepared at 420 V has good high temperature oxidation resistance and corrosion resistance,the oxidation rate is 3.97 mg/(cm²·h)and the corrosion rate is 0.0199 mg/(cm²·d).As the voltage is constant,the MAO coatings prepared under high frequency and low duty cycle are more compact,with less micropores and uniform distribution.In particular,the MAO coating formed under the frequency of 800 Hz and duty cycle of 6%has better high temperature oxidation resistance and corrosion resistance than the substrate.The oxidation rate of this coating is 3.92 mg/(cm²·h)and the corrosion rate is 0.0070 mg/(cm²·d).During the first 50 seconds,the surface and cross-sectional morphologies of the MAO coatings on AlTiCrVZr have change greatly,and the elements from the substrate and solute ions from the electrolyte are more involved in the coating-forming reaction.In the range of 60 s-300 s,the coating tends to be smooth and compact.The molten materials produced by discharge breakdown fill the uneven area,and solute ions play a dominant role in the coating-forming reaction.In the process of coating formation,the existing forms of the oxide formed by Al,Ti,Cr,Zr and Si elements have changed,but the MAO coatings at different times all contain Al₂O3,ZrO₂,TiO₂,SiO₂ and Cr₂O₃,which are with good high temperature oxidation resistance.A small amount of V2O5 exists in the MAO coatings,and its content first increases and then decreases.There are a lot of oxides such as Al₂O₃,ZrO₂,TiO₂,SiO₂ and Cr₂O₃ in the prefabricated MAO coating on AlTiCrVZr,which can effectively block the inward diffusion of O and the outward diffusion of V,improving the high temperature oxidation resistance of the alloy.It will be the focus of the next research work to prepare MAO coatings with high density,thick and strong interface bonding on RHEAs and further inhibit the formation of volatile oxides in the oxide coating.