Study Of The Microstructure And Properties Of Ceramic Coatings Prepared By Micro-arc Oxidation On Surface Of7075Super-hard Aluminum Alloy

Aluminum and its alloys are widely applied due to its advantages such as lightweight, highspecific strength, good toughness and formability, but some shortcomings such as low hardnessand poor corrosion resistance and abrasive resistance limit its application. Special and excellentperformances can be obtained after ceramic technology on surface of aluminum and its alloys.Technology of micro-arc oxidation (MAO) is an emerging surface treatment process onaluminum, magnesium, titanium and their alloys. In-situ ceramic coatings formed by micro-arcdischarge have high hardness, good corrosion resistance, good abrasive resistance and excellentadhesion to substrate. Therefore, the technology is widely used in the field of machinery,electron, spinning and aerospace.Electrolyte parameters, electrical parameters, alloying elements in matrix material are keyfactors which influence the performance of ceramic coatings on aluminum alloys. Orthogonalexperiment was used in this paper to optimize the best electrolyte parameters to preparemicro-arc ceramic coatings on aluminum alloy7075in electrolytic systems of sodiumaluminate, sodium silicate, sodium borate and sodium phosphate. Based on the best electrolyteparameters above, electrical parameters such as current density, duty cycle, frequency, andoxidation time were optimized by using single factor analysis under experimental index ofthickness, micro-hardness and microstructure of ceramic coatings. Corrosion resistance,abrasive resistance and mechanical properties were characterized of ceramic coatings preparedin different oxidation time, influences of alloying elements in substrate during the process ofmicro-arc oxidation were analyzed. Conclusions obtained during the experiment were listedbelow:By orthogonal experiment, the best electrolyte parameters to prepare micro-arc ceramiccoatings on aluminum alloy7075in electrolytic systems of sodium aluminate, sodium silicate,sodium borate and sodium phosphate were:Sodium aluminate: sodium aluminate9g/L, sodium hydroxide1g/L, trethanolamine6ml/LSodium silicate: sodium silicate8g/L, sodium hydroxide1g/L, trethanolamine6ml/LSodium borate: sodium borate15g/L, sodium hydroxide1g/L, trethanolamine6ml/LSodium phosphate: sodium phosphate12g/L, odium hydroxide1g/L, trethanolamine6ml/LThickness, micro-hardness, surface and cross-section morphology of ceramic coatingsprepared in the electrolyte above were tested and analyzed, sodium aluminate system waschosen for its best ceramic coatings quality. By using the best electrolyte parameters and singlefactor analysis, effects of anodic current density and cathode/anodic current density ratio,posive/negative duty cycle, frequency and oxidation time on coating properties were discussedin detail. The optimum electrical parameters were proposed as follows: anodic current densityof10A/dm2, cathode/anodic current density ratio of0.7, positive duty cycle of15%, negative duty cycle of10%, frequency of300Hz, oxidation time of45min.The microstructure and morphology of ceramic coatings were observed by using scanningelectron microscope (SEM) and X-ray diffraction (XRD). The results showed that,embossments like “volcanic vent” were presented on the surface of MAO coatings prepared inthe optimal technological parameters with diameters of micro-holes between1and3μm. Inaddition, metallurgical bonding mode like zigzag was observed between coating and substrate,coatings were continuous and compact with thickness of31.1μm and micro-hardness ofHV1080. Coatings were mainly composed of γ-Al2O3and little-Al2O3.Properties of ceramic coatings prepared in different oxidation time were characterized,results showed that, corrosion resistance and abrasive resistance were improved significantly.Corrosion hadn’t occurred on ceramic coatings prepared under the best technical parametersafter salt spray test of240hours, its corrosion potential(-0.589V) was higher thansubstrate(-0.772V), and its corrosion current density (1.281×10-9A/cm2) had decreased4ordersthan substrate(1.281×10-9A/cm2). At the same time, the abrasive resistance had improved by3orders while the mechanical properties just lost little.In order to research the influences of alloying elements such as Cu, Mn, Mg, Si and Zn onthickness, micro-hardness, microstructure and phase composition, a series of aluminum alloyswhich contained different content of alloying elements were smelted, and micro-arc oxidationwas carried out under the same technical parameters, results showed that, the phase of ceramiccoatings prepared on different substrates were composed of γ-Al2O3and little-Al2O3. elementCu and Si can promote the formation of-Al2O3, while suppression was reflected when theircontent was too high. Auxo-action was presented when mass percent of element Mg below0.8,while inhibitional effect was presented when mass percent of element Mg between0.8and2.5,besides, element Mg promote the process of micro-arc oxidation with unconspicuous effect.Element Zn had strong inhibition effect on the formation of high temperature phase of alumina,and the effect became stronger with the content of Zn increasing.