| In this paper,in order to study the difficulties that are closely related to the performance of the micro-arc oxidation film layer,such as the unclear partial discharge mechanism in the micro-arc oxidation process,the controversial temperature distribution,and the complex law of the second-phase particles,we designed and built an in-situ continuous observation system.The microstructure corrosion behaviors and errosion resistance of the oxidation layer were systematically studied.The micro arc oxidation behavior and mechanism of aluminum alloy in Na2SiO3-Na5P3O10-CH3COONa electrolyte system.The micro arc oxidation/strengthening particle composite film was prepared by adding the second phase particles.The film formation mechanism of the micro arc oxidation/strengthening particle composite film was studied by means of microstructure observation and film analysis.The wear resistance and corrosion resistance behavior of the micro-arc oxidation/(ZrO2 and h-BN)composite film were compared and analyzed.Based on the theoretical analysis,the discharge characteristics,the growth rule and mechanism of the composite coating in the process of micro arc oxidation are discussed,and the key technical parameters for the preparation of high-performance composite coating are defined.The main research results are as follows:(1)The behavior of partial micro arc discharge in the process of micro arc oxidation is continuously observed in situ.The high temperature produced by micro arc discharge makes the base metal and its oxides melt or even gasify.The center of the molten pool is the discharge channel.With the process of micro arc oxidation,the water vapor in the electrolyte escapes along the channel of the molten pool,and the volume of the molten substance increases.When the melting range reaches the film thickness,the oxide film breaks down and the molten substance solidifies rapidly under the cooling effect of electrolyte.As the charge conduction spark discharge stops,the molten pool stops increasing,and its size is consistent with the film thickness.Due to the influence of growth mechanism,the micro arc oxidation film has no layered structure,no interface between loose layer and dense layer,and the new molten pool is continuously generated to cover the old molten pool,which makes the film continuously thickened.(2)After the second phase particles are added to the electrolyte,some of the second phase particles are "captured" by the pool formed by the micro arc during the reaction process,and then enter the pool on the surface of the micro arc oxidation film,so that the number of the pool on the surface of the micro arc oxidation film is significantly reduced.Part of the second phase particles agglomerate on the surface of the micro arc oxidation film in the form of adsorption and engagement,forming a micro convex body.In addition,some of the second phase particles fused into the micro arc oxidation film at high temperature.(3)The second phase particles on the surface of the composite film formed and enriched by electrophoresis,adsorption and diffusion,However,the second phase particles at the bottom of the micro arc oxide film are caused by the breakdown discharge,After the melt at the bottom of the molten pool is ejected out along the discharge channel,a negative pressure is formed inside the channel,As a result,the second phase particles in the solution enter into the molten channel with the solution and deposit at the bottom,forming a composite film of micro arc oxidation.(4)The comparative analysis shows that the friction coefficient of the micro-arc oxidation/h-BN composite coating can be reduced to about 0.1 after adding different second-phase particles,and its wear resistance is better than the micro-arc oxidation/ZrO2 composite coating.Because of the graphite-like layered structure of h-BN particles,the molecular force between the layered h-BN can replace the partial force between the grinding pairs during the friction process,The electrochemical test results show that the self-corrosion current density of the micro-arc oxidation/ZrO2 composite coating can be reduced to 6.633×10-7A·cm-2,which is 92.4%lower than that of the uncomposite micro-arc oxidation film.The data show that the corrosion resistance of the micro-arc oxidation composite film prepared by adding ZrO2 is better than that of the micro-arc oxidation/h-BN composite coating.(5)Ti-6Al-4V titanium alloy matrix as process comparison,The phase transformation of the second phase particles in the composite film of micro arc oxidation under the action of the temperature field in the molten pool.The high temperature phase remains in a multiphase state in the freezing effect of rapid cooling of molten pool.The low temperature phase is produced by the second phase transformation of the high temperature phase in the region where the temperature field of the molten pool acts for a long time.The lowest temperature is higher than 2123k and the highest temperature is not lower than 3683k.The continuous active time of molten pool is shorter in the early stage of micro arc oxidation(ms),and increases with the increase of film thickness(s).The temperature gradient of molten pool is very large,about 70K/?m.(6)The second-phase particle compounding process is divided into two steps.One is that the second-phase particles reach the anode surface,including three mechanisms of turbulent diffusion,pressure differential drive and charged migration;And the other is that the particle compound into the coating,including four models of stamping closure,meshing fusion,high impact and regional recombination.The strengthening mechanism of the second phase particles on the composite film is element doping.The way it enter the composite coating is to adhere to the surface during the cooling process of the molten pool,and wrapped into the coating by the molten pool in the next active period of the molten pool. |
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