Due to the space vacuum, thermal regulation of spacecrafts is passively managed by radiative exchanges between its external surfaces and the environment. Black inorganic anodized aluminium alloys are often used for their thermo-optical properties (αs≥0.93; εn≥0.9). However, many cases of flaking have been observed after thermal cycles carried out to simulate the space environment. In orbit, such particles could contaminate instruments of the satellite. The aim of this study is to evaluate the influence of the ageing on such films adhesion.

The process used is a sulphuric anodizing, followed by an inorganic colouring and a sealing. It has been shown that the porosity and the thickness of the anodic film have a major influence on the mechanical behaviour during adhesion measurement performed by scratch tests and 4-point bending tests.

Black anodized samples with different porosities have been submitted to thermal cycles. The influence of the pressure and temperature on the adhesion has been observed. Two main mechanisms have been highlighted: the difference of thermal expansion coefficient between film and substrate and the dehydration of the film. Finite element modelling illustrated the mechanical state at the interface between the substrate and its cracked or non cracked anodic film during thermal loads (process and cycling). Particularly, the presence of cracks can either be detrimental or beneficial on the interface loading depending on the mechanical properties of the film which change with the porosity.

SiC-derived carbon films have been produced by etching silicon carbide in mixed gas environment containing chlorine and hydrogen^1-2. Hydrogen plays a crucial role in the SiC derived carbon films. But the detailed role of hydrogen on the carbon formation and its structure and properties is still not well understood. In this study, the effect of hydrogen content on the structure and mechanical properties of carbon films modified from sintered SiC by mixtures of chlorine–hydrogen gas at 1000 °C for 20 hours was investigated. Based on the structural analysis of carbon films produced, conversion from silicon carbide proceeded faster with increasing chlorine content at a high temperature. The rate of carbon formation determined by SEM observation was decreased with increasing hydrogen content. The hardness and elastic modulus of the carbon films tended to decrease with increasing hydrogen content. By varying the concentration of hydrogen, the ratio between the crystalline graphite and amorphous carbon was changed. The different bonding types and crystallinity of the carbon films directly correlated with their mechanical properties. The possible mechanism for the lowering mechanical properties has been discussed based on the analysis of HREM and Raman spectroscopy observation.

¹A. Nikitin and Y. Gogotsi, “Nanostructureed Carbide-Derived Carbon,” Encyclopidia of nanoscience and nanotechnology, Vol. 7 p 553-573 (2004),

² Hyun-Ju Choi, Jeon-Kook Lee, and Dae-Soon Lim, “Tribology of Carbon Layers Fabricated from SiC under the Different H2/Cl2 Gas Mixtures,” Accepted for Publication in Journal of Ceramic Processing Research (2008).

The use of porous metal coatings is ever increasing in renewable-energy system applications as solar cells and hydrogen fuel cells. In particular, the scale of porosity in metal coatings is particularly important to their catalytic performance. Potentially just as important is the mechanical stability of the porous coating in these devices. A series of rate-dependent deformation tests are now conducted to better understand operative deformation mechanisms in the evaluation of strength as the porous support structure changes across multiple length scales, i.e. from the micro-to-nano. Tensile testing is used to evaluate commercially available, free-standing silver membranes with constituent micron-to-submicron porosity. Scratch testing of porous silver-coated substrates permits evaluation of nanoscale porous structures. Preliminary findings indicate that the strain-rate sensitivity of tensile tested specimens is found to increase as length scale decreases. The trends are similar to those experimental results reported for bulk nanocrystalline metals. Underlying structural features that can contribute to this mechanical behavior include pore size, filament or strut size, and the grain size within. These features of length scale are evaluated, and then assessed for the scratch hardness behavior of nano-porous silver coatings.

This work is supported through the J.W. Wright Endowment in Mechanical Engineering at Texas Tech University.