Metallic Surfaces under Interplanetary Medium - Degradation Mechanisms and Protection Possibilities

Abstract

Thin metallic films are commonly used in space industry. Important applications are e.g. multilayer insulation blankets (MLI) or solar sail membranes. In a sufficiently large distance from the Earth atmosphere, the solar wind and electromagnetic radiation are the dominating factors for material degradation. The solar protons while penetrating the metals recombine to neutral hydrogen atoms and then form molecular hydrogen bubbles. Their high concentration within the metals’ lattice has a direct influence on their physical properties. Up to now, no material that was exposed to the interplanetary space conditions has been returned to Earth. Therefore, studies both theoretical and experimental carried out in the terrestrial laboratories are necessary to predict the changes in the mechanical and thermo-optical properties of the metals. The aim is to present the basics behind formation processes of molecular hydrogen bubbles as well as their influence on physical properties of exposed metals to the solar wind. The other degradation mechanisms are discussed as well, e.g. the delamination processes caused by corpuscular radiation. Ideas of protection possibilities of the metals are drawn. The experimental studies were performed by use of the Complex Irradiation Facility (CIF) of the DLR’s Institute of Space Systems in Bremen, Germany. The CIF’s linear proton accelerator allows to irradiate specimens to well defined proton fluxes. A series of experiments have been made to simulate the growth of the bubbles at different physical conditions which represent those at the interplanetary space. The post-degradation studies of the exposed metallic films have been performed by use of an electron microscope and number of image-processing methods to analyze the morphology of the specimens. It has been proven that at the early stage of the bubble growth the average bubble radius R evolves according to R~t^1/3 law. One of the consequences of the molecular hydrogen bubbles formation is a significant increase of the metallic surface roughness or changes of the samples’ thermo-optical properties. However, the bubble formation depends strictly on physical conditions at which the samples are exposed to the solar wind protons. Dependency on bubble formation mechanism due to temperature as well as proton kinetic energy is given.