Chemical characteristics, mechanical and thermo-optical properties of black anodic films prepared on 7175 aluminium alloy for space applications

Black anodic coatings are usually used on space vehicles for their thermo-optical properties. This paper presents a study of the chemical characteristics of these black anodic films, as well as their mechanical and thermo-optical properties as a function of the operational conditions of the process. The influence of the anodizing parameters was seen to affect the morphology of the coating (thickness and porosity). In particular, the electrolyte temperature was found to have a major impact on the porosity as it modifies the chemical kinetics during anodization. The impact of the preparation steps on Young's modulus was also investigated. The colouring and sealing steps thus modified the mechanical properties at the coating surface, resulting in a Young's modulus gradient in the film. Finally, a decrease of film porosity seemed to limit the risks of crazing then flaking, while thermo-optical properties were not affected.