Formation And Characterization Of MAO Coatings Formed In Electrolyte Containing Rare-earth Salts On ZAlSi12 Alloy Surface

Cast aluminum alloys are wide applied in industry field due to its low density ,high specific strength and excellent processing properties.However, the poor wear resistance which has restricted the application of unprotected aluminum alloys, Micro-arc Oxidation is an effective and promising surface treat technique ,by using MAO method ,the film which situ-growed on the substrate can obtained . Consequently ,the hardness, wear resistance and corrosion resistance of the alloy was improved. The mechanism that the electrolyte have on the MAO process and characterization of ceramic coating were very complexes until now ,it hasn’t made an unique and definite cognition ,so there have many work to do to study the composition of electrolyte .The ceramic coating were prepared on the surface of ZAlSi12 alloys and a high power supply unit （WHD-30） treatment device was adopted for MAO process ,the potentiostatic method was adopted in this work ,the applied positive/negative potential of the acyclic was 420/120V respectively, predefined frequency at 100HZ,current density variated with the automatic adjustment ,an aqueous alkaline electrolyte of Na₂SiO₃ 8.0 g/L + NaOH 2.0 g/L + EDTA 2.0 g/L was formulated, an appropriate amount of additive Ce（NO₃）₃、La（NO₃）₃、Ce（SO₄）₂·4H₂O及Na5P3O10 were added into Na₂SiO₃ -NaOH （alkali-silicate）electrolytic system. The regularity that the effect of additive concentration and have on the characterizations of coating were investigated ,the coatings were characterized by eddy current thickness meter,SEM,XRDand EDS analysis methods .the result show that :the thickness of ceramic coating increased after added Ce（NO₃）₃ into electrolyte and the max value reach to 170μm,the coatings formed in the electrolyte contained Ce（NO₃）₃ are still composed byα-Al₂O₃ phase andγ-Al₂O₃ phase ,but the proportion ofα-Al2O₃ phase increased after added Ce（NO₃）₃ into electrolyte ;the thickness of the coating increased with the content of La（NO₃）₃ increased in the electrolyte ,and the max value reach to 146μm,it is also found by EDS that there was La element in ceramic coating. This indicates that La（NO₃）₃ contained in electrolyte has an effect on the process of oxidation and characteristics of micro-arc oxidation coatings. It can also established that ,the improved induced mass of films were obtained after add Ce（SO₄）₂ into Na₂SiO₃–NaOH system electrolyte ,a uniform, compacted and barrier coating can be formed on the surface of ZAlSi12 alloy, the Na₅P₃O₁₀ additive have an effect of chelating which can improve the utilization ratio of electrolyte and avoid generation of deposition, accordingly the discharge time was extended and the properties of coatings were anticipated to improved.