Study on Microstructure Characterisation of Three Different Surface Coating Techniques on 6082-T6 Aluminum Alloy

Abstract

The aim of this study was to investigate the detailed macro/microstructures of three different surface coating technologies on the 6082-T6 aluminum alloy surface, namely Plasma Electrolytic Oxidation (PEO), Hard Anodizing (HA), and Plasma Spray Ceramic (PSC). Some of the PEO coatings were sealed with silicates. The microstructure investigations were performed using optical microscope, scanning electron microscope equipped with EDX analyzer and X-ray diffractometer. The microhardness of the base material and the coated layer were evaluated using Knoop indenter. All the coated layers appeared as two different layers. The PSC coating was the thicker followed by HA then PEO coating. The HA coating had irregular surface appearance, contained some cracks and appeared as an amorphous structure. The PSC coating was loose and had a poor bond with the substrate. The structure of PSC coating exhibited a lamellar geometry configuring 14% α-Al2O3 and 86% γ-Al2O3. The presence of porosity and voids was also observed. The PEO coatings were uniform and comprehensive. It adhered quite well to the substrate consisting of alumina (α and γ) phases. The ratio of γ alumina to α alumina was 79:31%. The sealed PEO coating with silicate had much lower porosity especially in the outer layer. All the coatings exhibited higher hardness than the substrate. The PEO coating showed the highest hardness compared with other coatings. The hardness of the PEO coating improved as the readings were taken away from the base. It displayed a maximum value in the middle and then started to reduce. The PSC coating has the highest roughness value followed by HA and then PEO coatings.