Effects Of Process Parameters On The Growing Proceeding Of Black Ceramic Coatings Prepared By Micro-arc Oxidation Of Aluminum Alloy

Micro-arc oxidation(MAO) of aluminum alloys is a new technique that apply micro-plasma discharge to make a ceramic-like coating at the surface of aluminum alloy .This coating have high hardness,corrosion resistance and dielectric properties also have well-adhered with the aluminum and its alloys. Black ceramic coating widely applied to protection of materials,optical properties and decoration due to special optical properties and ceramic properties.By a series of MAO experiments, I have studied the effect of process parameters to film forming law of black ceramic coating of Aluminum alloy. Using SEM, I have analyzed the micro-shape of ceramic oxidation film. Otherwise, I have studied the wear proof effect in the different composition by the test machinery of the pin-disk model.By the analysis of the result of experimentation, it show the thickness of film increased with the increasing of concentration of H₃BO₃ and KOH solution , but the hardness first increased and then decreased. The black saturation increased firstly and then decreased. Moreover, the black saturation is not high. While the concentration of H₃BO₃ is more than 18g/l, it presents brown in the film . It not only increased the black saturation greatly, but also increased the wear proof of film with the adding of Na₃PO₄ solution. With the increase of the concentration of Na₃PO₄ solution, the thickness of film increased. The hardness and black saturation first increased and then decreased and the change of black saturation is significant. While the concentration of Na₃PO₄ solution exceeds a certain value, it would ruin the oxidation film for the arc-light discharge. The thickness of film increased with the increasing of concentration of Na₂WO₄ solution, but the hardness and black saturation first increased and then decreased. The decreased degree is smaller than Na₃PO₄ solution and the black saturation is better especially. The film is dense and uniform. The effect of increasing wear proof is very significant.During the experiment, if the current density is not enough ,it hardly shape the whole black ceramic coating. While the current density was reached, the thickness increased with the increasing of current density .But the hardness and black saturation first increased and then decreased. While the current density exceeds 60A/dm² , it would ruin the oxidation film for the arc-light discharge. The thickness of film increased with the increasing of oxidation time. MAO has two stages. The first stage is mainly the growth of amorphous oxide-film. It only has the offwhite and grey-black oxidation film of with thin thickness and low hardness; the second stage is the growth of ceramic oxide-film. By the action of high temperature and high-pressure that produced by micro-plamsa discharge, amorphous oxide-film melts quickly, then freezes quickly by the action of cool electrolyte. Amorphous oxide melts into ceramic oxide-grain. The hardness and black saturation increased greatly.