| ZK60alloy is a typical wrought alloy with superior properties，especially for its highstrength, however some protective measures should be taken to enhance its corrosionresistance so that its application could be further expanded. Micro-arc oxidization (MAO) isa new promising surface treatment method of forming ceramic coatings on Al, Ti, Mg andtheir alloys. The surface properties, such as wear and corrosion resistance and electricalinsulation can be considerably improved by MAO.In this paper, MAO method was used tofabricate protective ceramic coatings on ZK60alloy. A dual electrolytes system containingaluminate and phosphate and constant voltage power mode were employed to investigateeffect of different voltage acting ways and electric parameters on MAO coatings,and alsohave a initial disscuss about growing mechanism and properties of the coatings.Employing constant voltage power mode,first keep voltage constant during MAOprocess, effect of positive voltage on microstructure and corrosion resistance of MAOcoatings formed on ZK60Mg alloy were investigated.The results show that, the coatingsgave a faster growth rate as voltage increasing,but increase to some extent, the coatingformed was easily to be loose with poor corrosion resistance.MAO coatings were thinnerbut with better corrosion resistance under280v.Generally speaking, if always keep voltageconstant, weak spark discharges with insufficient breakdown strength would appear at laterstage of MAO process.It’s no good for growing of the coatings.On this basis,a concept of voltage increasing mode was suggested to improve thecoating quality.280v as a base voltage,two steps and three steps voltage increasing modewere respectively investigated, voltage increase extent was also investigated to show thedifferences of formed MAO coatings.The results show that, after employing these two kindsof voltage increasing mode, the coatings experienced re-arcing, re-breakdown and a rapidregrowth process which is almost the same as our expected goals. Rougher and thickercoatings were formed with increase value of voltage.The MAO coatings fabricated undervoltage increasing mode by two steps from280V to360V (280V-360V) exhibits bestcorrosion resistance.Employing above voltage increasing mode, by means of single variable experiments,effect of negative voltage,duty cycle,frequency and oxidation time were investigated and theparameters were also optimized.The results show that, higher negative voltage helped to have an intensified MAO reaction,thus thicker coating with better corrosion resistance wasformed.Excessive duty cycle would lead to big size arc and gain damaged coatings withpoor corrosion resistance.MAO process was a little special as requiring low frequency,theformed coatings were thick but uneven.As oxidation time increase,there’s a balance betweengrowing and dissolving of MAO coating, thus the coating thickness was first increase thendecrease,same change with its corrosion resistance.The optimzied electric parameters wereas follows,positive voltage:280-360v,negative voltage:20v,duty cycle:30%, frequency:600Hz, oxidation time:15min.A series of MAO coatings were fabricated with optimized parameters. Growing modeland mass transfer model of MAO process under voltage increasing mode were set upthrough observation of morphologies of MAO coating,changing of coating phases,variationof element contents during MAO process.It seems that typical crater-like porous could befound on the coating surfaces and the coatings are characterized with typical loose outerlayer and dense internal layer with a wavy-jagged interface between substrate andcoating.MAO process under two steps voltage-increasing mode can be divided into fourstages: anodizing, spark discharge, micro arc discharge and tiny spark discharge. The mainphase of coating are MgAl2O4and MgO.Mass transfer during MAO process was mainlyconcerning Mg from ZK60substrate and Al,O from electrolyte.The properties of optimized MAO coating were tested by Vickers hardness, Scratch,Immersion, Electrochemistry and Friction and wear experiments.The results show that,MAO coatings formed under optimized parameters have a good formability and highbonding strength with a critical load of24.59N. Compared with the ZK60substrate, thehardness is improved obviously which was about8times, corrosion current of MAO coatingdecreases by4order of magnitude which exhibit excellent corrosion resistance, frictioncoefficient of the coating decreases under the same load which has an effect to lessen wear. |
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