Correlations Between The Growth Behavior And Properties Of The Micro-arc Oxidation Coatings On Metal Surface In Basic Electolyte

With the development of science and technology,the service environment of the metal materials such as 6061 Al,Ti6Al4V,H62 Cu and Nd Fe B alloy are becoming more and more complex,and the functional characteristics for the materials become more demanding.Micro-arc oxidation（MAO）is a strengthening treatment technology on metal surface,which can prepare an MAO coatings with functional.However,the growth mechanism of the MAO coating in the micro-arc oxidation process is’t clear,which is’t conducive to the preparation and regulation of special functional MAO coatings.Numerous research studies have shown that the basic electrolyte solution played a decisive role in the growth process of the MAO coatings.Therefore,investigating the relationship between the basic electrolyte solution and the growth behavior and performance of the MAO caotings,can provide a possibility to freely design electrolyte and prepare the MAO coatings with specific functional properties.Based on this,this paper mainly carries out the following five aspects of research:1.In this work,the growth behavior,structure composition,friction and wear and corrosion protection properties of the MAO coatings on the surface of Al alloy were investigated in the basic electrolyte solution,which are silicate,phosphate,aluminate and fluorozirconate electrolytes.The results showed that the deposition of compound and oxidation of alloy substrate were occured in the MAO process.In the silicate,aluminate and fluorozirconate electrolyte,the growth of MAO coatings were dominated by deposition of the compound.And the outward growth behavior of MAO coatings was reflected.However,in phosphate electrolyte,the growth of MAO coating was mainly based on the oxidation of the substrate,the coating was characterized by inward growth.In the silicate electrolyte,the MAO coating is composed of amorphous Si O₂andγ-Al₂O₃ phase.In phosphate electrolyte,the MAO coating is mainly composed ofγ-Al₂O₃ phase,and the inner layer is a dense layer enriched with P-containing oxides.In aluminate electrolyte,the MAO coating is mainly composed ofα-Al₂O₃ phase.In fluorozirconate electrolyte,the MAO coating is mainly composed of t-Zr O₂ phase.The structure and composition of the MAO coatings are closely related to the friction and wear and corrosion protection performance.The composition of Al₂O₃and its composite oxides in the MAO coatings are exhibited excellent wear resistance.The composition of P amorphous in the MAO coating have performed excellent anti-corrosion and strong chemical stability.2.Acoording to the relationship between the growth behavior,structure and performance of the MAO coatings in basic electrolyte solution,the deposition effect of compounds in the MAO process could be enhanced by increasing the concentration of electrolyte,and meanwhile the Al substrate was inhibited from participating in the oxidation coating formation.The thermal control coating of the white MAO were designed and prepared on the surface of Al alloy,which have low absorption and high emission and space environment stability.At the same time,the black MAO coatings with high content of Vx Oy were prepared on Al alloy in silicate and phosphate electrolyte systems by adding soluble metal coloring salt of NH₄VO₃.All of these are guided by the demands of application.The results showed that the white MAO thermal control coatings with 81%Si O₂ and 64.6%Zr O₂ can be prepared in 70 g/L silicate and40 g/L fluorozirconate electrolytes,respectively.The solar absorptance and emissivity of Si O₂ are 0.233 and 0.91,and the Zr O₂ are 0.234 and 0.85,respectively.The P-V MAO coating which prepared in phosphate electrolyte system shows high absorptance and high emissivity of the thermal control performance,good space environment stability and excellent acid corrosion resistance.The solar absorptance is 0.9987,and the emissivity is 0.86.3.The growth behavior,structure,composition and friction and wear properties of the MAO coatings on Ti alloy were investigated in silicate,phosphate and aluminate electrolytes.The results showed that the growth behavior of MAO coating on Ti alloy is similar to that of Al alloy,indicating the deposition of compound and oxidation of alloy substrate occurs in the MAO process,and the electrolyte is the decisive factor affecting the growth of MAO coating.In the silicate electrolyte,the MAO coating is mainly composed of amorphous Si O₂.In phosphate electrolyte,the MAO coating is mainly composed of matrix oxide Ti O₂.In aluminate electrolyte,the MAO coating is mainly composed of Al₂O₃ and Al₂Ti O₅,which has shown high bonding strength and good friction and wear properties.The friction coefficient is 0.76 and the wear rate is1.19×10^-6 mm/N·m under the dry friction condition of 3N loading load,and the ultimate bearing load is 10 N.4.According to the relationship between the growth behavior,structure and performance of the MAO coatings in basic electrolyte solution,the degree of participation of electrolyte components and matrix components in the MAO process were controlled by increasing the concentration of aluminate and phosphate electrolyte.Guided by the application requirements of the MAO coating,the functional coatings were designed and prepared on the surface of Ti alloy and Ti fastener,and the working performance were studied.The results showed that the MAO coatings prepared in 40g/L aluminate electrolyte is mainly composed of Al₂O₃ and Al₂Ti O₅,and the surface hardness of the coating is 1140 HV,which is 3 times of that of the Ti alloy substrate,Under the dry friction condition of 15 N load,the wear rate is only 1.46×10^-6 mm/N·m.The MAO coating is almost completely grown into the substrate in phosphate electrolyte,which is mainly composed of Ti O₂ and the thickness is only 10μm.The results of 1500 times of torsion test show that the MAO coating has excellent performance of high bearing capacity and long service life.5.Non-valve metals such as Cu and Nd Fe B alloy can’t spontaneously form a complete dielectric barrier layer under the power-on state,so it is difficult to prepare MAO coatings on the surface of non-valve metals.According to the growth behavior of the MAO coating on the surface of valve metals such as Al and Ti alloys in the basic electrolyte solution,a dielectric barrier layer of Al PO₄ were constructed on the surface of Cu alloy and Nd Fe B alloy in high concentration aluminate electrolyte,and the preparation of the MAO coating on these non-valve metals were realized.The result showed that the breakdown voltage of MAO coating on Cu alloy surface is increased to750V in DC mode and 600V in AC mode,respectively.The electrochemical corrosion current density is reduced by three orders of magnitude.The MAO coating has no obvious corrosion damage after 732h neutral salt spray test.The electrochemical corrosion current density of the MAO coating on Nd Fe B alloy surface is reduced by one order of magnitude,and the neutral salt spray test for 732h shows that the MAO coating has the ability to inhibit the corrosion diffusion.This research work can be provided reference for design and preparation of the MAO coatings on other non-valve metals surface,and the application scope of the MAO technology is also expanded.