Investigation On Growth Mechanism Of Mao Coatings And Its Influence Factors

In this research, pure aluminum specimens were treated by a WH-1A micro-arcoxidation device in self-prepared electrolyte. The growth mechanism of micro-arcoxidation coatings, the chemical composition of amorphous material and the effect ofultrasound agitation on formation of mullite in the coatings were investigated.The results show that the coating/substrate interface is formed by stacking of a largenumber of tiny globular particles. Many irregular protrusions can be found on the interfacedue to the uneven growth of micro-arc oxidation coatings into the substrate. The coatingspresent clearly a two-layer structure, a compact inner layer with thickness of0.5~1μmand a porous outer layer with a large number of discharge channels and pores. The growthof the coatings is through repeated dielectric breakdown and reformation of the inner layer.XRD analysis of detached coatings suggests that micro-arc oxidation coatings on Al arecomposed of amorphous material, mullite, α-Al2O3and γ-Al2O3with mass proportions of56.9%,24.9%,3.8%and14.4%, respectively. Amorphous material, mullite and γ-Al2O3mainly distribute in the outer part of the coatings, while α-Al2O3mainly distributes in theinner part. DSC analysis result under heating rate of40K/min shows amorphous materialin the coatings will reorder its configuration at800-1250K, and will start to transform intocrystals at about1250K. The introducing of ultrasound agitation can decrease the criticalconcentration of silicate in electrolyte for the formation of mullite in micro-arc oxidationcoatings meanwhile increase the content of Si element and mullite in the coatings. Thepromotion effect of ultrasound agitation on formation of mullite is mainly due to thecavitations induced by ultrasound vibration, which can decrease the thickness of diffusionlayer between the coating and electrolyte, timely supply silicates to the coating/electrolyteinterface after they are consumed. Besides, the high temperature and pressure caused bycollapse of cavitations can promote the discharge of silicates and accelerate the formationreacts of mullite.