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

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

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

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

New bonded assembly configuration for dynamic mechanical analysis of adhesives

A new sample configuration has been developed in order to study molecular mobility of an adhesive in a bonded assembly configuration by dynamic mechanical analysis. The torsional rectangular mode is used to provide a shear solicitation all along the adherend/adhesive interface. The initial mechanical properties of each assembly's constituent are first investigated as reference. The modulus of aluminum foils used as substrates exhibits a classic elastic component and a slight viscous part due to microstructural changes or stress relaxation. Four relaxation modes are highlighted and identified for epoxy adhesive tested as a bulk material. Its viscoelastic behavior is compared to the one of adhesive tested in assembly configuration. The relaxation modes of the adhesive remain visible in spite of the sample stiffening by aluminum foils. Relaxation modes comparison shows that the temperature of loss modulus associated with the mechanical manifestation of glass transition slightly increases for the assembly configuration. Energy losses during this relaxation are much higher in the assembly configuration. Influence of rigid aluminum substrates is discussed in terms of the adhesively bonded joint solicitation mode

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

Enthalpy relaxation phenomena of epoxy adhesive in operational configuration: Thermal, mechanical and dielectric analyses

Thermal cycling in space environment can cause physical aging of polymers used in structural adhesive bonded joint. Later, they can initiate failure. A methodology to follow physical aging effects on their thermal, mechanical and dielectric properties is applied to a commercial epoxy adhesive. The analytic description, using Tool, Narayanaswamy and Moynihan model gives a good description of the enthalpy relaxation. It is completed by a phenomenological analysis of the evolution of the adhesive thermal transitions, mechanical properties and molecular mobility. Testedsamples with bondedassembly are representative ofin service configurations. The influence of physical aging on the adhesive and the associated bonded assemblies is analyzed

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

Characterization of dissolved organic matter in a coral reef ecosystem subjected to anthropogenic pressures (La Reunion Island, Indian Ocean) using multi-dimensional fluorescence spectroscopy

La Saline fringing reef is the most important coral reef complex of La Reunion Island (southwestern Indian Ocean; 21 degrees 07'S, 55 degrees 32'E). This ecosystem is subjected to anthropogenic pressures through river inputs and submarine groundwater discharge (SGD). The goal of this study was to characterize the pool of fluorescent dissolved organic matter (FDOM) in different water bodies of La Saline fringing reef ecosystem using excitation-emission matrix (EEM) spectrofluorometry. From EEMs, we identified the different fluorophores by the peak picking technique and determined two fluorescence indices issued from the literature: the humification index (HIX) and the biological index (BIX). The main known fluorophores were present within the sample set: humic-like A, humic-like C. marine humic-like M, tryptophan-like T1 and T2, and tyrosine-like B1 and B2. In some samples, unknown fluorophores ("U") were also detected. The surface oceanic waters located beyond the reef front displayed a typical oligotrophic marine signature, with a dominance of autochthonous/biological material (presence of peaks: Tl>B1>A>T2>M>C; HIX: 0.9 +/- 0.4; BIX: 23 +/- 1.1). In the reef waters, the autochthonous/biological fingerprint also dominated even though the content in humic substances was higher (same relative distribution of peaks; HIX: 1.6 +/- 0.6; BIX: 1.0 +/- 0.1). Sedimentary and volcanic SGD showed very different patterns with a strong terrestrial source for the former (A>T1>C>B1 and A>C>B1; HIX: 9.8 +/- 2.0; BIX: 0.8 +/- 0.0) and a weak terrestrial source for the latter (A>81>U3>82>C and A>U4>C; HIX: 2.4 +/- 0.3; BIX: 0.9 +/- 0.0). In the Hermitage River, both humic substances and protein-like material were abundant (Ti>A>U5>B1>C>B2; HIX: 2.3; BIX: 1.4). We provide evidences for the presence of anthropogenic DOM in some of these water bodies. Some oceanic samples ( presence of peaks U1 and U2) were likely contaminated by oil-derived PAHs from ships navigating around the reef front, whereas the Hermitage River was highly impacted by sewage effluents, numerous in this coastal area of La Reunion Island. We conclude that multi-dimensional fluorescence spectroscopy (EEM) coupled to the determination of HIX and BIX is a good tool for assessing the origin and distribution of DOM in the coral reef ecosystems submitted to anthropogenic impacts. (C) 2011 Elsevier B.V. All rights reserved

Ultraviolet fixation of molecular contamination: physical model numerical implementation and validation

International audiencePhysical contamination results from a competition between the deposition of incoming contaminants and their reemission, depending on the surface temperature. However, interaction with ultraviolet light greatly inhibits reemission. Ultraviolet light can lead to chemical reactions, bonding contaminants to the substrate or nearby molecules. This effect can have important consequences in flight. This is especially true for hot surfaces that show no contamination in the absence of ultraviolet light due to their high temperature and can nevertheless be significantly contaminated through photofixation. Previous studies conducted by ONERA–The French Aerospace Lab with Centre National d’Etudes Spatiales allowed identifying the physical mechanisms involved in photofixation and led to a sufficiently simple model to be implemented in engineering software. In this model, the substrate excitation is followed by the reaction of adsorbed contaminants with excited sites. Contaminant chemical photofixation thus results from the competition between their reemission and their reaction with excited sites. This paper presents the implementation of this photofixation model in the numerical tool COMOVA and its validation. It was carried out by modeling test cases, including ground experiments. It demonstrated the qualitative coherence of the photofixation model. The simplest experimental cases with pure contaminants,which have analytical solutions, also quantitatively validated the physical model and its numerical implementation