Degradation of the surfaces exposed to the space environment

The presence of several atomic species in the LEO (Low Earth Orbits) could be considered one of the reasons for the degradation of the surfaces exposed to the Space Environment. At an average height of 400 Km (the altitude of International Space Station), the concentration of the main atomic species during the high sun activity are: 1.5 x 10(9) cm(-3) for atomic oxygen (AO), 1.6 x 10(8) cm(-3) for molecular nitrogen (N-2) and 1.4 x 10(8) m(-3) for atomic nitrogen (N). The energy with which the atoms collide with the surface of orbiting vehicle depends on the relative speed of the vehicle itself. For instance, the atoms colliding the International Space Station (ISS) (orbit average height: 400 Km; relative speed: 7.5 Km/s) have an energy of 8 eV for N-2, 5 eV for OA and 4 eV for N. The atomic oxygen is the most abundant species presents in LEO and it is considered the main responsible of the thermal, optical and mechanical alteration of the surfaces exposed to the Space Environment. Different hypothesis are reported in literature in order to explain the physical/chemical mechanisms that govern the material degradation in the Space, but no conclusion has been reached. In the energy range of few of eV, the main mechanism with which colliding atoms transfer its energy to the atoms of the surface is by phonons. In this paper the effect of an oxygen ion beam produced in the space environment simulator on materials for Space applications is studied in the frame of the thermal spike theory. Comparison between the measured erosion and the calculated one will be reported. The erosion mechanism will be modelled in order to understand the main thermodynamic parameters that govern the interaction between the atomic oxygen and the surface of the tested materials. (c) 2006 Elsevier Ltd. All rights reserved.