| Microarc oxidation (MAO) is an advanced technique of processing materialby means of high energy density. MAO coatings have intensively attracted theattention of many research groups due to its desirable characteristics, especially thecoatings on the substrates of aluminium and titanium as well as their alloys. In recentyears, many research groups at home and abroad have been attracted to study thepreparation and characteristics of Ti alloy MAO coatings. Because MAO is acomplicated process, which is controlled by multifactor, different technical parameters,asdepositedMAO coatings show various properties. Moreover, the experimental andtheoretical results are not systematic or complete. Therefore, further research work onpreparation and characterization of MAO coatings on the Ti alloy is needed. In thisthesis,evaluated as a new material,MAO coatings on the substrates of Ti alloys arestudied systematically in which the effect of deposited parameters on coatingproperties is investigated. It is our goal to fabricate the MAO coatings with theoutstanding characteristics. We developed the highpowermultifunctional MAOpower supply system first, which can be outputted multiwavesand worked on twomodels. Using the PCA function of C8051F340 MCU, we build a highspeedandmultifunctioncontrol PWM circuit, with a real data collecting system of electricalcurrent and voltage. On the base of these, the effect of deposited parameters on MAOcoatings properties is studied.This thesis consists of three parts. In the first part, the effect of differentelectrolyte system (Na2SiO3, Na3PO4 and NaAlO2) and concentration on the criticalvoltage, growth characteristics, microstructure, phase component, and corrosionresistant properties of MAO coatings has been studied. The experiment results showthat by using the DC power supply and keeping the voltage constant, the criticalvoltage reduced gradually in various electrolyte systems with the increase of solution concentration. Further more, the critical voltage reduced rapidly in the NaAlO2electrolyte system. Both the MAO film thickness and surface roughness are increasedalong with the increase of solution concentration in the three electrolyte systems. Thecoating growth rate was highest in Na2SiO3 electrolyte system, while that was lowestin NaAlO2 electrolyte system. When the solution concentration increased, the powerefficiency (Pe, the energy consumed against the thickness of coatings) is increased inthe Na2SiO3 and Na3PO4 electrolyte system, while itdecreased in the NaAlO2 system.The coatings consist mainly of rutile and anatase in Na2SiO3 and Na3PO4 system. Therelative content of rutile gradually increased with the solution concentrationincreasing, especially in Na2SiO3 system. The coatings prepared in NaAlO2 systemare mainly composed of Al2TiO5,γAl2O3and a few anatase. Furthermore, the relativecontent ofγAl2O3is increased linearly with the increasing of solution concentration,while the Al2TiO5 changed on the contrary. The corrosion potential of the samplesincreased with the solution concentration in the three electrolyte systems. Thecorrosion resistance of samples are best in Na3PO4 system, while be worst in Na2SiO3system.In the second part, the effect of amplitude, frequency and duty cycle of theanodic voltage pulse and treating time on the growth characteristics, phase componentand morphology of MAO coatings has been studied by using the constant voltagemode. The experiment results show that the thickness, the micropores size, the surfaceroughness and the relative content of rutile increased with the increase of the anodicpeak voltage, while the number of the micropores and the relative content of anatasedecreased. With increase the frequency of anodic pulse, the thickness, the microporessize, the surface roughness and the relative content of rutile were gradually decreased,and the thickness was exponentially decreased, while the number of the microporesand the relative content of anatase were increased. Under high anodic pulse frequency,the coatings prepared by the change of anodic pulse duty cycle (da) show theproperties of coating are between anodic oxidation and MAO process. The growthsequence of the coating was decided by the electric field strength. With the increase ofda, the change of the thickness was divided into multiple stages. The number of themicropores was decreased, while the micropores size and the surface roughness weregradually increased. The coatings consist of rutile, anatase and other phase. And therutile only can be observed under high da. With increase the treating time, the numberof micropores, the surface roughness and the relative content of the rutile weregradually increased, while the micropores size and the relative content of the anatase was decreased. Especially, the change of the coating thickness can divides into threedifferent stages.In the third part, the effect of cathodic peak voltage, pulse duty cycle andtreatment time on MAO coatings characteristics are studied by using the constantvoltage mode, the phenomenon and mechanism of cathodic microarc discharge arealso analyzed. The experiment results show that the change of the cathodic peakvoltage or the pulse duty cycle can lead the coating thickness, the relative content ofrutile and the microhardnessto be a suddenly change when the other parameters werefixed. Further experiments showed the higher the amplitude and duty cycle ofcathodic voltage pulse can lead to cathodic microarc discharge on the sample surfacewhich has already grown certain thickness coating. Cathodic microarc discharge willsignificantly affect the characteristics of MAO coatings, under different cathodicpulse conditions, the coating growth rate, the phase componment, the microstructure,the microhardnessand the corrosion resistance of coatings was obviously different.Cathodic microarc discharge is impacted by many factors, such as the amplitude andwidth of cathodic voltage pulse, the amplitude and frequence as well as duty cycle ofanodic voltage pulse, and so on. The phenomenon of cathodic microarc discharge onlycan be generated on the sample which has already grown certain thickness coating,and it can not form coatings directly. By using cathodic microarc discharge, it cangreatly improve the coating growth rate and the power efficiency. Moreover, it canform MAO coatings with higher quality.In summary, the coatings with excellent properties have been prepared on TC4 Tialloys by using selfmademultifunctionalMAO preparation system. By usingcathodic microarc discharge, the coating growth rate is increased from 3.6μm/min to13.7μm/min, the corrosion potential of the sample are also increased from 0.503V(substrate of TC4 Ti alloy) to 0.094Vand the relative content of the rutile is morethan 80%. Furthermore, the coatings are closely combined with the substrate and havebetter mechanical properties. Above all, experimental data and results offer animportant reference for the preparation of MAO coatings with the more function,which also lay a solid foundation for us in further research.
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