Rapid pulsed low energy electron bean irradiation of optical surfaces has proved to be a cost effective technique to test for radiation hardness and/or optical film adhesion. A rapid pulse (60 ns 130 ns) of electrons generates strong thermomechanical shock waves in the target surface to a depth of several micrometers. Spire''s pulsed electron beam sources (SPIPULSETh 300 and SPIPULSE 5000) are contaminantfree up to a fluence of 1. 0 cal/cm2 so that precise artifactfree optical scatter (e. g. bidirectional reflectance distribution function (BRDF)) measurements can be made before and after irradiation. Fluence reproducibility from shot to shot is 5 thus allowing direct comparisons of radiation responses of different samples or different irradiation sites to be made. Pulsed ebeams have been successfully applied to conductive and nonconductive optical surfaces such as metal and SiC mirrors optical baffles and ceramic optical coatings. A simple model is presented to demonstrate that radiationinduced conductivity (RIC) occurs during intense electron bombardment of any material. For alumina a time constant for the discharge of any residual charge build up that might occur during electron bombardment is estimated to be 30 picoseconds. For the ebeam radiation pulse lengths and fluence levels considered here alumina behaves as a conductive material. Several experimental examples are given including measurements of ebeam induced material blowoff for alumina. 1 .
In this paper properties of chemical vapor deposited (CVD) SiC and Si optical substrates for use in severe environments are presented. Important data on CVD SiC concerning the elastic modulus polishability scattering measurement thermal and cryogenic stability degradation due to atomic oxygen and electron beam are included. Further scattering measurement data and atomic oxygen degradation effects on CVD Si are also presented. These measurements show that CVD SiC substrates exhibit excellent polishability 1 A RMS) with low scatter good retention of mechanical properties up to 1500 C superior thermal and cryogenic stability (-190 C to 1350 C) and high resistance to atomic oxygen and electron beam degradation. VD Si substrates exhibit excellent polishability 2 A RNS) with low scatter and good resistance to atomic oxygen degradation. These preliminary results suggest that CVD SiC and Si are good optical substrates for severe environment such as outer space lasers combustion and synchrotron x-rays. 1 . 0
Experiments for studying the suitability of carbon films for IR optics exposed to soft X-rays are described. IR reflectors coated with amorphous hard carbon films are optically characterized before and after thermal cycling (from 100 K to 300 K) and before and after thermal exposure to flash X-rays.
A status report is presented on the obstacles and current research related to using CVD diamond as an optical material. Problems discussed include properties of CVD carbon deposits, including structure, thermal conductivity and oxidation resistance, which are relevant to the optical uses of diamond; absorption coefficient measurements on CVD diamond in the visible and IR; and a review of various aspects of the synthesis of CVD diamond, including the growth of transparent and translucent diamond, efforts to grow diamond at low substrate temperatures, and approches to reducing the optical scatter of as grown polycrystalline diamond films and windows. Particular attention is given to techniques for reducing optical scatter which involve modifying materials morphologies during the growth process by controlling nucleation density, renucleaton frequency, and/or the orientation of crystal faces at film surfaces; techniques for postdeposition polishing of the surface of CVD diamond films and windows; and optical applications for CVD diamond.
Anticipated space power systems which employ nuclear or solar dynamic power technologies may be required to reject waste heat at temperatures up to 950K. High temperature radiators will be required to have high thermal emittance surfaces which are durable to elevated temperatures as well as the operational space environment. Additional performance and durability threats such as extreme temperature variations and low earth orbital atomic oxygen impose challenging constraints upon the selection of suitable radiator surfaces. Traditional surface coatings which are acceptable for high emittance low temperature radiators may spall or degrade in an environment where large temperature changes high temperatures and atomic oxygen is present. Surface roughening and/or chemical modifications which produce high emittance surfaces that are an integral part of the radiator substrate may have much greater durability than coatings applied to radiator substrates. A variety of surface modification techniques have been evaluated for emittance enhancement of radiator surfaces. These techniques include: acid etching heat treating abrasion sputter texturing electro-chemical texturing arc texturing and atomic oxygen beam texturing. Candidate radiator surface materials investigated include: Nb-1 Zr Cu Ti Ti-6 Al-4 v 304 stainless steel Al6061-T6 Mo W and Ta. Results of durability evaluation of selected radiator surfaces in an atomic oxygen environment is also presented. 1.
Coatings of ceramic materials that exhibited high thermal absorptivities and emissivities were chemical vapor deposited on graphite and refractory metals. The coatings prepared were SiC and B4C and the substrates used were graphite molybdenum titanium and NblZr. The coatings were characterized with regard to adherence optical properties and response to potential harsh environments. 2.
The optical characteristics of spacecraft surfaces are fundamental parameters in controlling its temperature. Passive thermal control coatings with designed solar absorptance and infrared emittance properties have been developed for various space conditions and environments. In this total environment the coatings must be stable and maintain their desired optical and mechanical properties for the course of a particular mission ranging up to a lifetime of thirty years. This paper reviews stable polymeric and inorganic materials which we have developed for different orbital missions both near earth and geosynchronous. Physical characteristics of these coatings such as hardness flexibility and out gassing behavior are presented. The response of optical properties as a function of conditions such as ultravioletvacuum and atomic oxygen and also the function of time are discussed. 1.
A Low Earth Orbit (LEO) Atomic Oxygen Resistant (AOR) thin film coating has been developed by Sheldahl Inc. for use on the Space Station Freedont Photo Voltaic Solar Array Panels. The AOR silicon oxide (SiOx) coating was developed in a irianufacturing system and lends itself to reasonably large production quantities. Test results of the SiOx coating on polyiinide versus uncoated polyiiaide are presented along with a discussion of the nanufacturing process used. The substrate used for this program was Dupont type H Kapton! The major areas of testing were environnental survivability (atomic oxygen resistance optical properties and thermal shock) and physical/mechanical characterization (adhesion blocking flexibility and abrasion resistance). 2.
Spacecraft and space structures have an obvious need for thern*ial control coatings to inininiize tentperature excursions due to sun/shade cycles and to dissipate internally developed heat. Sheldahl Inc. ha produced a new thermal control coating utilizing TefloriA. F. 2400 (amorphous fluoropolymer) a product recently developed by the Dupont Co. With this new Teflon coating ce/c ratios of 0. 14 on aluminura are easily obtainable. Sheldahl''s coatings have been prepared on a range of substrates and tested for space conipatibility. Testing done to date includes temperature cycling salt fog exposure vacuum bakes vacuum outgassing abrasion testing and sortie synergistic effects. 2.
The performance of selected optical parts of the Mars Observer Laser Altimeter (MOLA) is considered. Test results indicate that Schott RG-830, RG-850 filter glass, and the Rolyn Optics Neutral Density filters are essentially immune to levels of radiation an order of magnitude larger than that expected for the MOLA spacecraft. The Corion LG-840 filters are shown to be relatively safe for this application, but exposures to levels higher than 15 kilo-rads(Si) have a severe effect on this material. The BK-7 prism appears to be acceptable for the relatively benign environment required for MOLA. It is concluded that the optical performance of the components tested is not degraded by exposure to the whole life dose expected for the Mars Observer.
Chemical, optical and mechanical characteristics of two new fluoro-plastic materials, Teflon AF 1600 and Teflon AF 2400, are discussed, and several potential application possibilities are suggested. These materials have the unique properties of being optically transparent in the visible and near IR. They have very low indices of refraction and exhibit a relatively low dispersion. It is concluded that the Teflon AF family of fluoroplastics will offer significant uses in both bulk and thin film applications.
A number of optically important materials such as ZnS, SiO2, SiO2-TiO2, GaAs, and heavy metal fluoride (e.g., ZBLAN) glasses are subject to moisture- and/or liquid water-induced crack growth. A notable exception to this behavior appears to be Si. Such environmentally enhanced crack growth can lead to ultimate failure in service at stresses well below those expected from normal strength tests. The sensitivity of a material to water can be obtained by determining a crack growth parameter, N. This parameter can be combined with other easily obtainable fracture information which include measures of the strength and strength distribution to create a lifetime design diagram using fracture mechanics concepts. Methods for determining these fracture parameters including direct crack growth measurements and dynamic fatigue are reviewed, and the influence of environmental water on the materials is discussed. Crack growth mechanisms including physical (dielectric) and chemical reaction mechanisms are discussed, and lifetime design diagrams which can be used to determine stress levels in service are presented.
Infrared optically black baffle surfaces are an essential component of many advanced optical systems. All internal surfaces in advanced infrared optical sensors that require stray light management to achieve resolution are of primary concern in baffle design. Current industrial materials need improvements to meet advanced optical sensor systems requirements for optical, survivability, and endurability. Baffles are required to survive and operate in potentially severe environments. Robust diffuse-absorptive black surfaces, which are (1) thermally and mechanically stable to threats of X-ray, launch, and in-flight maneuver conditions, with specific densities to allow an acceptable weight load, (2) handleable during assembly, (3) cleanable, and (4) adaptive to affordable manufacturing, are required as optical baffle materials. An overview of recently developed advanced infrared optical baffle materials, requirements, manufacturing strategies, and the Optics MODIL (Manufacturing Operations Development and Integration Laboratory) Advanced Baffle Program is discussed.
Quantitative measurements of outgassing for Martin black and a variety of metallic, diffuse absorptive baffle materials under development for stray light management are reported here. Outgassing measurements were made during pumpdown from atmosphere at room temperature. Mass scans indicate water was the major outgassing species for all materials tested. Calibrated measurements of water vapor outgassing as a function of time were also made for each baffle material. Most baffle materials exhibited total water vapor outgassed during pumpdown of between 1 x 10 exp -5 and 4 x 10 exp -5 moles/sq cm. Plasma sprayed beryllium, currently under development exhibited approximately an order of magnitude lower total water vapor outgassed during pumpdown.
Spacecraft in low earth orbit experience a variety of environments which are potentially damaging to materials and to optical systems including electronic controls and components. The low earth orbit (typically 400 km) has a significantly different set of environments than higher orbits. The environments vary not only with altitude but also with inclination. This paper deals with the environment that the Space Station Freedom will experience and with some of the effects on the materials and electronic components that will comprise the optical systems on the station. Specific optical systems are not addressed but the information presented is general and does apply to optical systems.