Black anodic coatings for space applications: study of the process parameters, characteristics and mechanical properties

Black inorganic anodized aluminium alloys are used for managing passive thermal control on spacecraft and for avoiding stray light in optical equipment. Spalling of these coatings has sometimes been observed after thermal cycling on 2XXX and 7XXX aluminium alloys. This phenomenon could generate particulate contamination in satellites and may affect mission lifetime. In this work, the influences of the four main steps of the process (pretreatments, sulphuric anodizing, colouring and sealing) on the coating characteristics have been studied for a 7175 T7351 aluminium alloy. The chemical heterogeneity of the coating has been underlined, and its mechanical behaviour observed through crazing. Scratch-testing, used to evaluate coating adhesion to its substrate, revealed the negative impact of thermal cycling

Investigations into the coefficient of thermal expansion of porous films prepared on AA7175 T7351 by anodizing in sulphuric acid electrolyte

The aim of this study was to investigate the Coefficient of Thermal Expansion (CTE) of anodic films on 7175 T7351 aluminium alloy and to evaluate the influence of the film characteristics on this value. In particular, effects of porosity and post-treatments, such as coloring and sealing, were studied. Beam bending analysis was used as the experimental method and a numerical finite element model was developed to verify theoretical relationships hypotheses. The values determined and the choice of experimental method were then validated by comparing the experimental cracking temperature of anodic films with a theoretical value directly depending on the previously determined CTE

Mechanical behavior of black anodic films on 7175 aluminium alloy for space applications

Because of their low outgassing and their thermo-optical properties, black anodized aluminium parts are often used near optical instruments to manage thermal control in space applications. However, critical cases of flaking of the film were observed after simulated thermal ageing. To understand the mechanisms leading to flaking, the influence of the initial porosity of the film on its mechanical behavior during and after the black anodizing process has been investigated. The decrease of limit tensile stress with the porosity, the coloring and the sealing combined to thermal stresses due to the difference of thermal expansion coefficients between film and substrate have been shown to cause crazing in articular conditions. For high initial porosity films, thermal cycling ageing has a detrimental influence on the adhesion measured by scratchtesting. Numerical simulation has been used to simulate the combined effects of thermal stresses and film cracking on the stress field at the interface

Problématique d'adhérence des revêtements anodisés inorganiques noirs pour applications spatiales

Les revêtements anodiques noirs sur alliages d’aluminium sont utilisés sur les satellites en raison de leurs propriétés thermo-optiques. Toutefois, l’observation récente d’anomalies, consistant en une fissuration de certains de ces revêtements notamment après essais d’environnement, fait craindre une contamination particulaire potentielle des équipements environnants (instruments optiques, équipements électroniques…). Jusqu’à présent quelques essais mécaniques (pelage au scotch, scratch test) ont été réalisés afin d’évaluer ces anomalies critiques. Cependant, il nous est apparu nécessaire d’approfondir la compréhension des mécanismes de dégradation, et de mettre en oeuvre des essais quantitatifs, tels que le scratch test ou la flexion quatre points, dans le but de déterminer l’énergie d’adhérence de ce type de revêtements

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

Study of degradation mechanisms of black anodic films in simulated space environment

Black anodic films are used on aluminium alloys in space applications to provide specific thermo-optical properties to the surface. During the process, thermal and/or residual stresses can lead to crazing of the film in particular conditions. After thermal cycling performed to simulate the space environment, cases of film flaking have been observed. This constitutes potential risks for the satellite lifetime. Peel-test and scratch-test have been used to evaluate the evolution of the adhesion during ageing. For example, we showed that this phenomenon is due to crack propagation occurring at the beginning of ageing. A finite element model has then been developed to study more accurately the propagation mode of these cracks. Thermal stresses are likely to propagate cracks through the film in pure opening mode. When cracks are long enough, thermal stresses can generate propagation parallel to the interface. This would result in the decrease of the measured adhesion with a possibility of flaking

Étude et simulation des mécanismes de dégradation de revêtements anodiques noirs sur alliage d'aluminium pour applications en environnement spatial

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Flaking of black anodic films in space environment: Ageing and numerical simulation

International audienceBlack coatings can be used for managing passive thermal control on spacecraft and for avoiding stray light in optical equipment. Inorganic black anodizing of aluminium alloys is a solution to obtain films with a low outgassing and a sufficient thermal stability for this kind of application. Flaking of these coatings has sometimes been observed after thermal cycling on 2XXX and 7XXX aluminium alloys. This phenomenon could generate particulate contamination on satellites optics and may affect mission lifetime. In this work, the influence of thermal cycling on the adhesion of the films was investigated thanks to both a qualitative method (peel-test) and a quantitative method (scratch-test). It has been shown that differential dilatations and dehydration of the film result in stresses inside the film. A finite element model was developed to identify the mechanisms of crack propagation and bifurcation inside the film and thus explain the reasons of flaking. Especially, it has been show that cracks can bifurcate and propagate in the film along the interface under thermal solicitation resulting in a mixed opening / shearing mode loading at the crack tip