Research On Growth Characteristics And Composite With Second-phase Particles Of MAO Coatings On 7075-T6 Alloy

Micro-arc oxidation?MAO?coating was prepared by using a bipolar DC pulse power source in a mixed electrolyte of Na₂SiO₃·9H₂O-?NaPO₃?₆-NaOH with electrical parameters of forward current density 15 A/dm²?the negative is 0?,frequency 500 Hz and duty ratio 1:1.The electrolytes were doped with HBN of different particle diameters and concentrations and of CeO₂ and Nd₂O₃ rare earth particles of different concentrations to prepare micro-arc oxidation coatings of the composite particles,respectively.The spark discharge phenomenon during the breakdown process was studied,including the surface state such as spark color,dimensional change and discharge duration;the effects of soft spark discharge on the surface roughness,thickness,friction properties and corrosion resistance of the coatings;energy loss of micro-arc oxidation process,characteristics of the internal structure,wear resistance and corrosion resistance of the composite coatings were analyzed.The effect of HBN and CeO₂,Nd₂O₃ rare earth particles on MAO coatings was carried out.The results show that the spark changes from white to yellow with increasing of size and the discharge duration during the micro-arc oxidation process.The oxidation energy consumption increases rapidly within middle stages,and the growth of energy consumption in the later period is extremely slow.The uniform discharge breakdown process has a leveling effect on the surface of the oxide coating,especially the soft spark breakdown stage in the early stage of MAO process.Compared with the anodic oxidation?AO?coating,the wear resistance and corrosion resistance of the coating after the soft spark discharge breakdown are greatly improved,and the specific wear rate and the self-corrosion current density are reduced by about 72.9%and 50.3%,respectively.Improvement of the performance of the coating to slow down with the oxidation time increasing,which was 91.7%and 84.1%lower than that of the AO coating.The overall energy consumption of micro-arc oxidation increases by about50¹00 kJ because of HBN and rare earth particles.The composite of the particles accelerates the breakdown of oxide coating,causing the voltage to reach the breakdown voltage by about 67s and 205s earlier,respectively,and the quality of coating was improved by prolong the soft spark discharge.The thickness and surface roughness of coatings increase with the oxidation time,which growth is stagnant in the later stage.The surface roughness of both of composite coatings increases obviously.HBN promotes the growth of MAO coating,while CeO₂ and Nd₂O₃ on the contrary.The density of coatings of composite HBN and rare earth particles is both higher than that of uncomposite.The density of coating prepared in electrolyte with concentration of 5 g/L Nd₂O₃+5 g/L CeO₂ and 40 min oxide time is up to 35.76%compared with the particle-free;the adhesion of coating layer is slightly increased by about5.27%¹1.15%,but it is tested that the uniformity of the internal structure of the composite coatings is greatly improved.Through the SEM and EDS microscopic observation,it is found that most of HBN particles are deposited on the surface of coating or near-surface layer,when only a small amount of BN phase is found inside by XRD phase analysis;a lot of CeO₂ and Nd₂O₃ particles can be found on surface in the forms of molten and granules,which can also enter the layer more at the same time and also fill the defects.A large amount of Ce and Nd elements and the second phase of paticle can also be captured by EDS and XRD,respectively.The improvement of friction behavior and corrosion resistance of HBN and rare earth particle composite membranes is also more prominent.The two kinds of granular composite membranes showed a consistent downward trend in friction coefficient and specific wear rate,which could be reduced by 13.8%,28.6%and40.2%,86.4%,respectively;the abrasive wear was less,and the wear was more slight than uncomposite,showing better wear resistance.The self-corrosion current density and potential of the samples were calculated by Tafel curve.The self-corrosion current density of the two composite films decreased by 80.55%and 51.4%at least,respectively,and even decreased by nearly one order of magnitude.The artificial seawater immersion test and salt spray test of the two kinds of granular composite membranes showed that the surface corrosion morphology of the HBN composite membrane sample after 1800 h artificial seawater immersion was greatly improved,compared with the corrosion of the matrix and uncomplexed.In addition to less corrosion through two corrosion tests,the weight gain of samples with rare earth composite decreased by 77.1%and 64.1%,respectively,and the growth curve of the weight gain was more flat,which showed better seawater erosion resistance and salt spray corrosion resistance.