COMPLEX INVESTIGATIONS OF NEW THERMAL CONTROL COATINGS Grigorevskiy A.V., Kiseleva L.V. JSC «Composite», 4, Pionerskaya str., Korolev, Moscow region, 141070, Russia tel./fax +7 (495) 513-2020 E-мail: info@kompozit-mv.ru ABSTRACT Complex investigations of new thermal control coatings (TCCs) intended for spacecraft operating at highaltitude orbits (GEO, HEO) have been carried out. Changes of optical properties under combined exposure to space environment have been measured. INTRODUCTION for the effect of space environment, service conditions of spacecraft can vary to a great extent. This is determined by spacecraft orbits where they operate: low-earth orbit (LEO), high-elliptical orbit (HEO) passing through the earth radiation belts, geostationary earth orbit (GEO), interplanetary transfer orbit, etc. But whatever the orbit is, there exists ionizing radiation that produces in outer skin of a spacecraft absorbed doses from 10 2 to 10 9 rad annually, solar electromagnetic radiation, vacuum, and temperature changing. To maintain the thermal balance of a spacecraft in service, its skin is covered with thermal control coatings with certain optical characteristics (A s and ε) that change in operation conditions. In addition to standard process parameters like good spreading capacity, adhesion, etc. there are a number of other requirements imposing on external coatings of spacecraft with long-term lifetime: - stability of and ε in operation conditions under exposure to damaging environmental factors and thermal cycling; - low outgassing level with m/m 1.0% and content of volatile condensable material 0.1%; - low specific volume resistance (ρ v 10 6 Ω m) for coatings of spacecraft that enter the earth radiation belts. At present, the K-208 type (OSR) TCCs in the form of square quartz plates 20-40 mm in width and 100-200 mcm in thickness with Al- or Ag-sputtered layer that are glued to radiator substrate, are applied in spacecraft operating in GEO and HEO. These coatings are expensive, fragile and practicable non-feasible in use. Besides, if the necessity of replacement of radiator elements occurs, they are almost unrepairable. The paint TCCs have no such shortcomings though they have very low radiation stability at long-term lifetime of spacecraft [1]. Tests showed that developmental paint radiation-resistant TCCs based on new pigments and binders are much more stable to combined action of space environment as compared with available TCCs. EXPERIMENTAL TECHNIQUE To produce TCCs, radiation-resistant pigments and binders had to be chosen. For this purpose, there were prepared samples of binders in the form of films applied to substrates, 30 mm in diameter, made of AMg6 aluminum alloy and then hardened at room temperature. Tablets were formed by compacting powders of white inorganic compounds. To do this, powder of pigment was filled in the metal punch arranged on plane polished surface of a die made of a stainless steel. Using the hydraulic press the powder was compressed by movable die until a solid tablet 3-4 mm in thickness was formed. Rated pressure required to form tablets from powder depends on hardness of pigment particles under study varying within 12-15 MPa. Using the «Cary 500» spectrophotometer, reflectance spectra within the wavelength range of 0.2-2.5 mcm have been taken for pigments, binders and TCCs before and after irradiation. Absorptance A s was calculated using the reflectance spectra and data on extraterrestrial solar spectral density - S(λ) [2]. The most radiation-resistant pigments and binders, in optical properties, were chosen according to their resistance to proton radiation that provides the greatest deterioration of TCCs in GEO and HEO as it is confirmed by numerous experimental data. Proton irradiation of pigments and binders was performed using the UV-1/2 test facility [3]. The UV- 1/2 (see Fig. 1) was designed for studying physicalchemical properties of materials and coatings under separate and combined action of space damaging factors (vacuum as low as 10-5 Pa, electrons and protons with energies up to 50 kev, solar electromagnetic radiation up to 10 SEE (solar exposure equivalent), temperature T = ±150 о С) and forecasting changes of their properties for long-term operation.
3 5 1 2 4 8 7 10 9 6 11 12 LiF NaF ZnP BaF2 CeF3 Scandium fluoride Calcium silicate microwollastonite LimCarb 2XK KAlO2 Mg(AlO2)2 Ca(AlO2)2 KTiF6 MgO CaO К-208cр LiAlO2 K2ZrF6 Li2SiO3 SiO2 Al2(SiO3)3 KBF6 Ba(OH)2 Al(OH)3 MgF2 Sodium molybdate SrCO3 Lanthanum fluoride Calcium hydroxide Yttrium oxalate Yttrium oxide Magnesium carbonate Gallium oxide Zinc carbonate Fig.1 Schematic diagram of computer-aided UV-1/2 test facility. 1 - vacuum chamber; 2 table for samples; 3 thermostat; 4 - pumping and vacuum monitoring system; 5 - measurement unit; 6 - space simulators; 7 - electron accelerator; 8 - proton accelerator; 9 - simulator of solar radiation (UV-source); 10 forming optical device; 11 - solar simulator control box; 12 accelerators control box. Basic formulas of enamel compositions have been calculated and prepared based on chosen organic (acryl resin) and non-organic (liquid lithium glasses) binders and pigments. When producing the basic formulas, pigmentation of compositions was equal 6.0 for liquid lithium glass. Enamel compositions were prepared in the porcelain ball grinder. Dispersion time prior to making homogeneous composition was 1-1.5 hrs in dependence on pigmentation level, dispersivity and hardness of the pigment. In such a way, some compositions have been prepared. It was found that these pigments make possible to produce homogeneous dispersions. The enamels were applied on AMg6 aluminum alloy samples preliminary treated with a sandpaper and degreased with an acetone. RESULTS AND DISCUSSION The major components in TCC production are pigments and binders that are the ones that specify radiation resistance of spacecraft coatings in service conditions. To prepare TCC samples, the liquid lithium glass was chosen as the binder. The pigment was chosen using the results of proton irradiation of tablets made from different white powders. 0,10 0,00 0 1E+16 2E+16 3E+16 4E+16 5E+16 6E+16 Fig. 2. Dependence of А s on proton fluences for different pigments. Fig. 2 and 3 shows behavior of solar absorptance А s of different pigments in dependence on proton fluences. The dotted red line presents data for ZnO pigment that is used widely for various TCCs. Pigments and binders were irradiated by 20-keV protons with flux density of 10 12 cm -2 s -1. Samples of developmental TCC compositions have been prepared based on liquid lithium glass binders and mixtures of pigments consisting of complex compounds of oxides and fluorides of alkaline-earth elements (Ba, Ca, Mg) with stabilizing additives. To evaluate the changes of coating properties for longterm service in GEO the combined tests were carried out: outgassing tests; thermal cycling tetst; durability to combined effect of space environment (protons, electrons and UV solar radiation) with measurements of optical and electrical properties of coatings. The change of of developmental coatings for in- GEO service during 15 years was evaluated based on comparative results of combined irradiation of developmental TCCs: ECOM-1 having the 15-year forecast made earlier by Dr. Vassiliev [4] within the boundaries of the prediction model, and TП15-4.5-1.5 2
for which there are data of environmental tests in GEO [1]. EKOM-1 2 167Li 254 ТП EKOM-1 2 166Li 255ТП 239Li 243Li Cs2CO3 MgCO3nH2O CaO CaSO4 SnO2 CaWO4 Magnesium hydroxide Li2CO3 BaSO4 BaSiF6 BaCO3 MgO CaO BaWO4 CaWO4 Baryta Li2CO3 ZnO whiskers. Li2SiF6 ZnF2 0,650 0,550 0,450 0,350 ТП15 243Li 0,250 166Li 0,150 167Li 0,050 0,0E+00 1,0E+16 2,0E+16 3,0E+16 4,0E+16 5,0E+16 6,0E+16 Fig. 4 Behaviour of solar absorptances on proton flux Ф (F, р/cm 2 ). 0,10 0,80 0,70 EKOM-1 AK-512b AK-512b 0,00 0 1E+16 2E+16 3E+16 4E+16 5E+16 0,60 Fig. 3. Dependence of А s on proton fluences for different pigments. Table 1 shows details of combined irradiation. Table 1 Parameter Unit Value Vacuum, Р Pa 10-4 Temperature of samples, Т 0 С 45±5 Energy of protons, Еp kev 20 Flux density of protons, φ р cm -2 s -1 4*10 11 Energy of electrons, Ее kev 40 Flux density of electrons, φ е cm -2 s -1 2*10 12 UV-radiation factor, Еs UV SEE 2-3 Fig. 4 shows behaviour of solar absorptances on combined irradiation time. When irradiation was ended the final electron flux Фе and SEE Нs (does not shown in the Fig.) were from 1.3*10 17 to 1.8*10 17 cm -2 *s -1 and 200 UV SEE respectively. Here can be seen that radiation resistance of developmental TCCs (samples 239Li, 243Li, 166Li, 167Li) is essentially greater as compared with the coatings ECOM-1 and TП15. Fig. 5 presents data of environmental tests (AK-512b and TП15 [1]) and laboratory tests (ECOM-1 [5]). 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Т, years Fig. 5 Changes of in dependence on lifetime in GEO according to data of environmental tests (AK-512b and TP15) and laboratory tests (ECOM-1) Estimation of coating was performed by comparing its change of with degradation of the TP15 coating in space conditions, with forcast for ECOM-1 obtained in on-ground tests, and with experimental curves of developmental coatings obtained in this work. Approximating curves were matched for all the curves (power, linear and logarithmic dependencies) with approximation reliability R 2 no worse than 0.98. Using these curves there was calculated the equivalent proton fluence for which degradation of under combined irradiation was exactly the same as degradation of the TCC TP15 in space environment as well as the forecast of the ECOM-1 in dependence on operation time in GEO (years). 3
EKOM-1 2 167Li 254 ТП EKOM-1 3 166Li 255ТП 239Li 243Li Степенной (167Li) Степенной (EKOM-1 2) Степенной (254 ТП) 0,75 0,65 0,55 0,45 0,35 0,25 0,15 167Li y = 3E-06x 0,3244 y = 3E-08x 0,4337 243Li 166Li y = 1E-07x 0,3846 0,05 0,0E+00 1,0E+16 2,0E+16 3,0E+16 4,0E+16 5,0E+16 6,0E+16 7,0E+16 Fig. 6 Behaviour of according to test results for TCCs under study with approximating curves (power dependence). Then the equivalent proton fluence was inserted into the appropriate formula for the developmental TCC thus getting for specific life-times of TCCs in GEO (from 1 to 15 years). 0,80 0,70 0,60 EKOM-1 y = 0,1611Ln(x) + 0,3433 Логарифмический (EKOM-1) Логарифмический ( ) y = 0,1002Ln(x) + 0,4307 EKOM-1 Measurement results of emissivity ε and specific volume resistance ρ v of the developmental TCC after combined irradiation are given in Table 2. Table 2 Sample ε ρ v, Ω*m ECOM-1 N 2 0.84 1.7*10 5 ECOM-1 N 3 0.85 2.2*10 5 254TP 0.94 1.4*10 5 255TP 0.945 1.6*10 5 166Li 0.955 1.6*10 4 167Li 0.95 2.0*10 4 243Li 0.96 6.2*10 4 239Li 0.96 7.3*10 4 Thermal cycling of developmental TCC samples (200 cycles within the temperature range ± 100 o С) was performed as well. Adhesion of the coating to a substrate was not changed. CONCLUSION. Owing to the carried out researches the thermal control coating with the below mentioned properties is available: Absorptance, 0.1-0.12 Emittance, ε 0.94 Specific volume resistance, ρ v, Ω*m 10 5 Mass, g/m 2 200-250 Adhesion: tearing failure stress (AMg6-45 type alloys) according to GOST 1476-69 According to GOST 15140, grade Outgassing according to GOST R 50109: TML, % CVCM, % 2 1.0 0.1 (15 years in GEO) 0.33 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Т, years Fig. 7 Changes of according to the results of environmental (TP15) and on-ground (ECOM-1) tests with approximating curves (logarithmic dependence). Figs. 6 and 7 show changes of according to the results of TCC tests with approximating curves. Then averaging of all the data was done with extreme values discarding. Therefore, an average value of for 15-year operation of developmental coating in geostationary orbit was obtained; av =0.32 if compared with TP15 and av =0.29 if compared with ECOM-1. REFERENCES. 1. Tenditnyi V.A., et al., Laboratory and in-flight tests of spacecraft thermal control coating degradation, Proceeding 6-th International Symposium Materials in a Space Environment, ESTEC, Noordwijk, The Netherlands, September 19-23, 1994, pp. 113-122. 2. Space model, under acad. Vernov S.N., v.2, Мoscow, MSU press, 1983. 3. Khassanchine R.H., Kostyuk V.I., Belyakov N.A., Bajdaev D.V. On studying effect of combined action of damaging factors of space environment on outgassing process of non-metallic materials. Problems of atomic science and technology, iss. 1-2, 2006, p.p. 44-48. 4. Vasiliev V., Grigorevskiy A., Gordeev J.: Mathematical simulation methods in forecasting the change of integral and spectral optical characteristics 4
of spacecraft external surface materials and coating, Protection of Materials and Structures from Space Environment, ICPMSE-6, Toronto, Canada, May 2002, pp. 543-549. 5. Grigorevski A., Gordeev J., Gurov A., Kiseleva L., Shuiski M.: Study of behaviour of the new thermal control coating EKOM-1 in flight and laboratory experiments under exposure to simulated separate and complex factors of space environment., Proc. 8-th Int. Symp., Mat. in a Space Envir. and 5-th Int. Conference Protection of Materials and Structures from Space Environment, Arcachon, France, June 5-9, 2000. 5