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

Abstract

Black 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 shown 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.

Preparation and Characterization of Electrodeposited Aluminum Oxide Thin Films

“…The biggest threat to spacecraft performance and life are the temperature anomalies during its orbital mission. Due to alternating passages of spacecraft in the earth shadow and in front of sun, a spacecraft experiences thermal cycling in the space vacuum 3,4 . This thermal cycling in spacecraft, orbiting the earth, produces thermal stresses inside the vehicle 5 and causes temperature variations in the range of -1400 o C to +1400 o C 6 .…”

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

“…The Young modulus of the film, Efi was measured by nanoindentation as a function of the porosity and will be the aim of a forthcoming paper [8]. To sum up, the modulus decreased from 80 GPa to 20 GPa for films with porosities increasing from 10% to 50%.…”

“…Before the launch, parts of satellites are submitted to thermal cycles under vacuum to simulate the space environment. Cases of flaking of black anodic films have been observed after those tests when pulling-off tapes used to fix thermocouples [1].…”

Alternative black coatings prepared on aluminium alloy 1050

“…First, it was placed in a solution of cobalt acetate (200 g L −1 ) regulated at 43 ± 2°C for 15 min to impregnate the pores then, second, the impregnated sample was dipped in a solution of ammonium sulphide (30 g L −1 ) at room temperature for 10 min. A precipitation of CoS black pigments occurred in the porous anodic films as described by Goueffon et al [19][20][21][22][23]. After each step, the samples were immediately rinsed in distilled water to remove the excess solution.…”