Effect Of Dielectric Barrier Layer On PEO Ceramic Coating's Performance And Process Energy Consumption

Plasma electrolyte oxidation (PEO) technology provides a new platform for surfacemodification of light-metallic materials. The films formed by PEO combine the performanceof both anodic oxide films and ceramic films, which broadens the application field oflight-metallic materials. Many researches related to the PEO films' properties have been downat home and abroad. Because of its high energy inefficient PEO technology cannot be usedwidely. Based on this, the AZ31Mg alloy was used as the working electrode. The influence ofthe dielectric barrier layer's (DBL) characteristics and solvent composition on the breakdowndischarge, process energy consumption, the gas envelops and performance of the PEOcoatings were investigated.(1) Analyzed the influence of chemical conversion process conditions on conversionfilms' thickness, porosity, corrosion resistance and the phase composition. The result showthat: The thickness of conversion coating approximates linear growth with conversion time in30minutes, its linear function is d=0.43t-0.44(R2=0.999); compactness of conversioncoating improves with the extension of conversion time; conversion film can help improvecorrosion resistant performance of the magnesium alloy to some extent; the crystal phase ofconversion film mainly includes MgO and MgAl₂O₄, conversion time has great influence oncomposition of crystal phase.(2) In order to reduce the adverse effects of anodic oxidation during PEO process, thedielectric barrier layer was prepared through chemical conversion pretreatment. The effect ofdielectric barrier layer on the breakdown discharge, process energy consumption, the gasenvelops and performances of the PEO films were investigated. The experimental resultsshow that the pretreated DBL has significant effect: The thickness of DBL has remarkableinfluence on current density, balanced current and stationary discharge of PEO process; thegrowth rate of ceramic membrane doubled; adhesive strength, abrasion resistance andcorrosion resistance of ceramic membrane improve obviously because of MgAl2O4increased;energy consumption of PEO process decreases greatly, it bottoms out lowest when thethickness of conversion film is8.3μm. (3) The effect of methanol on the gas enveloping process was analyzed. The influence ofmethanol concentration on discharge characteristic, process energy consumption, and the PEOfilms' performance were discussed. The experimental data shows that: the concentration ofmethanol has great impact on current density and balanced current of PEO process; the energyconsumption of PEO process has the very big relations with methanol concentration, when theconcentration of methanol is16%(volume fraction), energy consumption is lowest; thickness,adhesive strength, abrasion resistance and corrosion resistance of ceramic membrane almosthas nothing to do with methanol concentration.(4) According to the study shows that (1) and (2), the mechanisms of PEO process'smass and heat transfer as well as DBL's mechanism of action were analyzed.(5) The results indicated that:1) DBL mainly impacts the electric power distribution ofthe initial stage of PEO process by its good dielectric properties, so that the gas phase mainlycomes from solvent evaporation process other than electrolysis of solvent. By this, the energyconsumption of the reaction was reduced.2) The DBL changed the crystal phase compositionand morphology distribution of PEO films to influence their performance.3) Because of itssmaller evaporation enthalpy and electrolytic energy consumption than water, methanol canmake energy consumption of the gas envelope's formation process reduced.4) The methanoladded to the solution affects discharge points through changing thermodynamics environmentof PEO reaction.