Study On The Strong Toughness Of Black Micro-arc Oxidation Coating On Aluminum Alloy

In high precision optical instruments or laser weapon systems,it is often necessary to absorb or shield stray light generated from outside or inside.The black film has the ability to absorb all wavelengths including visible light,and is the preferred solution for light absorption.The actual service process of the instrumentation often faces the vibration and temperature changes generated by the external environment or the operation of the instrumentation,which may lead to the peeling of the film if the bonding force between the coating and the substrate is insufficient.As an emerging surface treatment technology,micro-arc oxidation technology can produce a black coating with high hardness,strong bonding and wear resistance.The black micro-arc oxidation coating with high light absorption and strong toughness is prepared by colorant selection and process optimization using 6061,2A12 and 7075 aluminum alloys as substrates,and the main contents and conclusions are as follows.（1）The effect of colorants NH₄VO₃and K₂Cr₂O₇on the performance of black coatings formed in composite electrolytes of sodium silicate and sodium hexametaphosphate is studied with 6061 aluminum alloy as the substrate,respectively.The absorption of the two black coatings is about 98%in the range of UV to visible light.The cross-section of the V-containing black coating is loose in the inner layer and dense in the outer layer,and the presence of delamination weakens the bonding force between the coating and the substrate.The Cr-containing black coating has higher crystallinity,and K₂Cr₂O₇can promote the formation ofα-Al₂O₃,which greatly improves the hardness and wear resistance of the coating.The fracture toughness of the coating is evaluated by drop hammer impact test,and the Cr-containing coating do not peer off after impact at a height of 30 cm.（2）2A12 aluminum alloy can not be film-formed in the electrolyte with K₂Cr₂O₇as the colorant directly,and a two-step process is used to prepare black coating.In the first step,the composite electrolyte of sodium silicate and sodium hexametaphosphate and the composite electrolyte of sodium silicate and sodium citrate are respectively used to prepare the prefabricated coating.The former coating has large gaps and holes between the coating and the substrate,while the latter is relatively dense with fewer holes.The black coating is prepared in the second step by using the electrolyte with K₂Cr₂O₇as the colorant.All the black coatings had similar physical phase composition,dominated byα-Al₂O₃.According to the results of friction,drop hammer impact tests on the coatings,the coatings prepared by using the composite electrolyte of sodium silicate and sodium citrate in the first step has better performance.（3）A black micro-arc oxide coating with high strength and toughness is prepared on 7075 aluminum alloy by the two-step process using the composite electrolyte of sodium silicate and then the electrolyte containing K₂Cr₂O₇.At the end of the friction with a duration of 20 min,the friction coefficient of the black coating is about 0.22.Only a small area of the coating is peeled off at a drop hammer impact height of 40 cm.