Influence Of A Kind Of Additive Or Ultrasound On Micro-arc Oxidation Of Aluminium

In this thisis, pure aluminum were treated by a homemade WH-1A micro-arc oxidation(MAO) device. The research inclouds two parts: First, influences of the additive of hexametaphosphate on voltage of micro arc oxidation and phase compositions, morphologies and structures of micro-arc oxidation coatings. Furthermore, growth mechanism of micro-arc oxidation coatings has also been investigated by the two-step method; Second, growth mechanism of the early-formed micro-arc oxidation coatings affected by ultrasound has been investigated.The results show that the additive of hexametaphosphate makes voltage of micro-arc oxidation increased, and facilitate silicate in the electrolyte to participate into the coatings and formed mullite phase; At the same time, the additive of sodium hexametaphosphate makes thickness and surface roughness of the coatings increased, too. Moreover, morphologies of the coating/substrate interface that formed in the silicate electrolyte is composed of broken small spherical morphologies, while morphologies of coating interface/substrate that formed in the compound electrolyte, which is the former silicate electrolyte added with the additive of sodium hexametaphosphate, is composed of integral small spherical morphologies. With micro-arc oxidation time increases, the uneven degree of inward and outward growth interface of the coatings are increasing.The inward growth of the coatings mainly depends on breakdown and discharge that happened in large cavities. While in the dense region of the coatings, the inward growth may not occur for up to 20 min, thickening in this region is more dependent on the outward growth of the coatings.In the early stage of micro-arc oxidation, ultrasound has no significant effects on the voltage of micro-arc oxidation and phase composition of micro-arc oxidation coatings. Ultrasound makes surface discharges and depositions on the coating surfaces more uniform. And cross-section morphologies of micro-arc oxidation coatings show that under the effect of ultrasound, the coating thickness increases uniformly and fewer defects occur in the film, but more cracks were left in the coatings. Electrochemical experiment results show that ultrasound makes the micro-arc oxidation coatings more uniform, especially for the substrate/coating interface which greatly enhance corrosion resistances of the coatings, but many cracks that be left in the coatings also reduce corrosion resistances of the outer coatings.