Surface Modification Technologies for Durable Space Polymers
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Introduction Polymer materials, paints, graphite, and polymer-based composites exposed to space environmental factors such as atomic oxygen (AO), ultraviolet radiation, and extreme thermal cycling conditions in low Earth orbits (LEOs) and charged particles in geosynchronous orbits (GEOs) have been shown to undergo significant accelerated deterioration of their major structural and functional properties that include surface erosion, mass loss, and thermal-optical properties changes. Higher energy erosion processes, such as physical sputtering, are well understood for polymers. Chemical reactions of polymer oxidation at thermal energies, including those by thermal AO, are also well understood, yet only a few attempts have been made to study the basic aspects of the accelerated mechanism of polymer erosion by fast atomic oxygen (FAO).1,2 This situation is due mostly to the absence
of theoretical approaches for this very special energy range. The way the kinetic energy of fast reactive particles is transferred to chemical reactions is still not clear on a microscopic level. Correlations were found between the FAO erosion yield of hydrocarbon polymers and their chemical content and structure.3 A very important feature found in the relationship was the observation that carbon films and graphite were more resistant to FAO than hydrocarbon polymers, which was later confirmed by long-duration flight data.4 This finding implied that one could not expect substantially increased erosion stability in hydrocarbon polymers and composite materials. Most polymers have FAO erosion yields of about (1–4) × 10−24 cm3/atom AO. Perfluorinated polymers are an exception. Because of fluorine atoms in their bonding structure, their erosion yields are much lower.1,4 Although it was
MRS BULLETIN • VOLUME 35 • JANUARY 2010 • www.mrs.org/bulletin
once thought that perfluorinated polymers might be the answer to the problems of polymers in LEO, there are other factors such as combined effects between AO and vacuum ultraviolet (VUV) and/or ionizing radiation or the influence of soft x-rays with energies from 0.12 to 12 keV (10 to 0.10 nm wavelength) that increase the erosion yields to unacceptable levels. Materials having erosion yields of the order of 10−24 cm3/atom are unsuitable for long-term use in the LEO environment and for space in general. In general, the cost of developing new polymer materials and paints for use in space is very high because of constraints on bulk optical, thermal, and mechanical properties. Presently, a number of different technological solutions are offered to solve the problems discussed previously5–7 (see also the references therein). Protection is provided by metals or by stable inorganic compounds (mostly by oxides or oxide-based surface structures). Oxide coatings are often deposited by one of a few advanced deposition techniques such as electron-beam deposition or magnetron sputtering.8 Also, specially selected or synthesized materials are used that are able to form oxide(s)based compounds in a top surface layer
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