Research On The Three-dimensional Structure And Growth Characteristics Of Micro-arc Oxidation Coatings On 1060 Aluminum

Micro-arc oxidation(MAO)coatings are prepared on aluminum under constant current mode.The phenomenon and mechanism of anomalous gas evolution during MAO are investigated.The free-standing coatings are obtained by dissolving the aluminum substrate via an electrochemical method.The chemical composition and 3D structure,including surface,inner structure,aluminum/coating interface and fracture cross-section of MAO coatings in different growth stages are analyzed via EDS,XRD and SEM.The formation of the 3D structure of the MAO coating is explained by a molten zone model,the growth mechanism of MAO coating is discussed.In addition,the MAO coatings are prepared in silicate,phosphate and their mixed electrolyte,respectively.The effects of electrolyte to the 3D structure and formation mechanism of MAO coatings are discussed according to the structure and elemental distribution.The results show that the hydrogen and oxygen mixture is the main gas escaping from aluminum anode during MAO in the single pulse mode.The release volume of anode gas is much higher than that of conventional electrochemical reaction product.In Na2SiO3-Na5P3O10 electrolyte system,the surface of coatings is dominated by a crateror pancake-like structure of alumina surrounded by the Si-rich nodules.A barrier layer which consists of small spherical particles appears at the aluminum/coating interface.MAO coatings gradually evolve to a distinct three-layer structure as the coating thickens,including a barrier layer,an inner porous structure and a rough outer surface.There are obvious cavities exist between the inner and outer layer when the coatings are thicker.The molten zone caused by plasma discharge is the basic unit of MAO coating growth.The MAO coatings are formed by both the oxidation of the aluminum substrate and the deposition of electrolyte compounds.The oxidation of substrate occurs in the molten zone,leaving a pancake-like structure with a central hole after solidification.Around the molten zone,the electrolyte compounds are deposited under the thermal effects to form porous nodules.The unevenness of temperature distribution and the cooling rate in the molten zones result in the structure distinction along the section of coating.The difference in the volume and lifetime of the molten zone at different discharge periods result in various 3D microstructure of MAO coating.The coating is mainly formed by the deposition of electrolyte compounds in silicate electrolyte,the coating has a faster growth rate,but the surface is rough.There is a large amount of mulite in the coating.The Si elements are mainly distributed on the outer coating.The oxidation of aluminum is weak,so the aluminum/coating interface is flat.In phosphate electrolyte,the formation of coating depends greatly on the oxidation of the aluminum.This is conducive to form a dense structure.The main component of coating formed in phosphate electrolyte is Al2O3,while trace amounts of P in the coating may be the residue of the electrolyte,which are mainly distributed in the inner-layer of coating.Reasonable use of silicate and phosphate mixed electrolyte can play an active role in regulation of the structure and composition of the MAO coating.