Effects of Coating Thickness on the Microstructure and Properties of Black Micro-Arc Oxidation Ceramic Coating on 6061 Aluminum Alloy

In order to reveal the influence law of coating thickness on the blackness， corrosion resistance and hardness of black micro-arc oxidation coatings on aluminum alloy for promoting their practical application，6061 aluminum alloy was treated by micro-arc oxidation in silicatevanadate composite solution systems，and black ceramics coatings with different coating thickness on aluminum alloy were obtained through controlling micro-arc oxidation time.The surface structure， composition and roughness of the black ceramic coatings were characterized by thickness gauge， roughometer， SEM and EDS.Moreover， the effects of coating thickness on the blackness， microhardness and corrosion resistance of the black ceramic coatings were studied by using colorimeter， Vickers hardness tester and electro-chemistry workstation.Results showed that the coating thickness had great influence on the surface morphology， roughness， color features， microhardness and corrosion resistance.As the micro-arc oxidation time increased， the thickness of coatings increased， the surface of the coating became rougher， the number of discharge micropores on the surface declined， and the diameter of the discharge micropores increased.When the thickness increased to 30.786 μm， the size of discharge micropores increased significantly， reaching 10 μm.As the coating thickness increased， the blackness and microhardness of ceramic coatings improved， while the corrosion resistance increased firstly and then decreased.When the thickness reached 20.781 μm， the ceramic coating had higher microhardness and blackness， and showed the best corrosion resistance， with the self-corrosion current density of 1.093×10-6 A/cm2 in 3.5%(mass fraction) NaCl solution.