Surface Modification of Polymers, Paints and Composite Materials Used in the Low Earth Orbit Space Environment
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Surface Modification of Polymers, Paints and Composite Materials Used in the Low Earth Orbit Space Environment Jacob I. Kleiman Integrity Testing Laboratory Inc. 80 Esna Park Drive, U#7-9, Markham, Ontario, L3R 2R7, Canada ABSTRACT Many organic-based materials exposed to low Earth orbit (LEO) environment undergo dramatic changes and irreversible degradation of physical characteristics. While many protective schemes are used to reduce the effects of LEO environment, protection of such materials in LEO still remains a major challenge, especially for future long duration missions or space stations. In addition to the traditional protective coating approaches, surface modification processes were proposed and successfully used as an approach to protect polymers, thermal control paints and other components and structures from LEO environment. Among them two surface modification processes, the PhotosilTM and the ImplantoxTM that used new approaches in silicon functionalization, as in the PhotosilTM, and a modified ion implantation process, as in the ImplantoxTM allowed to incorporate up to 36 at.% of Si into the upper surface layer regions of the treated polymers, composite materials, thermal control paints and other high-performance organic materials. The tests conducted in plasma and fast atomic oxygen (FAO) beam facilities at comparable to LEO fluencies (~ (1-2)Al020 at.O/cm2 ) demonstrated erosion yields lower than -10-26 cm3at, unchanged thermal optical properties, where important, and excellent thermal match between the treated layers and the bulk of the treated materials. After FAO testing, the ImplantoxTM treated samples were clear and transparent, with a glassy-like shiny surface with no signs of any surface erosion. INTRODUCTION Atomic oxygen (AO) is the predominant species in the low Earth orbit (LEO) environment at altitudes between 200 and 700 km [1,2]. The number density of AO at about 250 km altitude is 109 atomsAcm-3 that corresponds to the density of residual gas in a vacuum of 10-7 Torr. However, due to the high orbital velocity of orbiting vehicles (~ 8 km/sec at Space Shuttle altitude), the flux is high, being of the order of 1014 atomsAcm-2Asec-1. Furthermore, this high orbital velocity corresponds to collisions with oxygen atoms having a kinetic energy of about 5 eV. Atomic oxygen having kinetic energy of about 1-5 eV is commonly referred to as fast (FAO) or hyperthermal (HAO) atomic oxygen. When exposed to FAO, polymeric materials, graphite and carbon-fiber reinforced plastic (CFRP) composites and many thermal control paints have been shown to undergo significant accelerated surface erosion and mass loss [2-6]. The materials eroded in space exhibit strongly altered surface morphology on a micrometer scale (Figs. 1 and 2), having a roughened “carpet-like” texture [3] and irreversible degradation of various material properties.
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a
b LEO AO flux direction
Figure 1. a) SEM image of a PMMA sample eroded by atomic oxygen beam in a six hour accelerated test (acceleration factor of appro
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