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

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.