| Refractory high-entropy alloys（RHEAs）have the potential to be used in high temperature structural components due to their excellent room temperature and high temperature mechanical properties.However,they contain elements such as V,Mo or W that are easy to form volatile oxides,resulting in poor high temperature oxidation resistance,which has become the key limiting factor for their high temperature applications.In this paper,micro-arc oxidation（MAO）coatings were prepared on Nb Mo Ta W RHEA,and the formation process of the coatings was investigated and the growth mechanism of the coatings was analyzed.Optimize the MAO process by regulating the electrolyte concentration and electrical parameters,and its influence on the chemical composition,microstructure and high temperature oxidation resistance of the coatings were explored.The high temperature resistant sealing coating was applied to the optimized MAO coating to obtain the MAO/high temperature resistant sealing composite coating.The chemical composition,microstructure and high temperature oxidation resistance of the composite coating were investigated,and compared with the single coating.In addition,the failure mechanism of MAO coatings after high temperature oxidation and the high temperature oxidation resistance mechanism of composite coatings were analyzed.The main conclusions are as follows:During the formation of the MAO coating,the substrate elements and solute ions all participate in the coating-forming reaction,and the coating components are mainly Si O2,Mo O3,Nb2O5,Ta2O5,and WO3.The initial stage of MAO is dominated by the formation of granular oxide,the particles are deposited on the substrate to form an insulating coating.After reaching the breakdown voltage,a large number of particles and discharge micropores are observed on the substrate.Na2Si O3concentration had no effect on the chemical composition of the coating,but had effect on the microstructure,with the concentration increasing,the particle aggregation phenomenon decreased and the coating thickness increased.The microstructure of the coating is closely related to the high temperature oxidation resistance.The coating prepared by 20 g/L Na2Si O3 had the smallest porosity and the thickest coating,and it is difficult for Oxygen to pass through the composite coating and contact with the substrate alloy.After oxidation at 1200℃for 1 h,the oxidation layer thickness and oxidation weight gain are the smallest,and had the best high temperature oxidation resistance.The MAO coatings prepared with different electrical parameters were dominated by Si O2,Mo O3,Nb2O5,Ta2O5,and WO3.The microstructure of the coatings changed with the electrical parameters,and when the frequency-duty cycle is constant,the higher the voltage,the rougher the coating surface,the larger the micropore size,and the thicker the coating.When the voltage is constant,as the frequency increases and the duty cycle decreases,the surface roughness of the coating decreases,the micropore size decreases,and the number of micropores increases,but the coating thickness does not change.After oxidation at 1200℃for 1 h,Nb14W3O44 and Ta16W18O94were formed on the samples prepared with different electrical parameters,and many columnar and flaky oxidation products gathered at the pores.The high temperature oxidation resistance of the coating changes with the microstructure.The MAO sample prepared with electrical parameters of525 V,600 Hz-8%had the thinnest high temperature oxide layer and the smallest oxidation weight gain,showed the best high temperature oxidation resistance.The chemical composition of the composite coating is dominated by Al,Al2O3 and Si O2.Many particles and flakes can be observed on its surface,and the coating thickness is about 148μm.After oxidation at 1200℃for 1 h,only strong diffraction peaks of Al2O3 were detected on the composite coating samples,and the substrate alloy was not oxidized.The composite coating was cured by secondary molding at high temperature,and the surface became smooth and compact,with excellent high temperature oxidation resistance.The existence of holes and cracks in the MAO coating,the volatilization of Mo O3 and WO3,and the volume expansion due to the large volume ratio of the substrate element oxides to the alloy aggravate the oxidation of the substrate alloy,resulting in the failure of the MAO coating.The composite coating prepared by sealing the MAO coating with a high-temperature resistant closure coating will be formed by secondary curing at high temperature.The formed composite coating is thick and compact,and it is difficult for O to pass through the composite coating and contact with the substrate alloy,which showed excellent high temperature oxidation resistance. |
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