ISSN 1027-4510, Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques, 2018, Vol. 12, No. 3, pp. 531–534. © Pleiades Publishing, Ltd., 2018. Original Russian Text © K.A. Anikin, A.M. Borisov, A.V. Zheltukhin, A.A. Zhukov, S.V. Savushkina, I.D. Fedichkin, V.N. Chernik, A.V. Apelfeld, 2018, published in Poverkhnost’, 2018, No. 6.

Characteristics of Thermal Control Plasma Electrolytic Coatings on Aluminum Alloy K. A. Anikina, A. M. Borisova, *, A. V. Zheltukhina, A. A. Zhukova, S. V. Savushkinab, I. D. Fedichkina, V. N. Chernikc, and A. V. Apelfelda a

Moscow Aviation Institute (National Research University), Moscow, 125993 Russia b Keldysh Research Center, Moscow, 125438 Russia c Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia *e-mail: [email protected] Received September 26, 2017

Abstract—The structure and properties of thermal control white and black coatings for space applications on AMg3 aluminum alloy, obtained by plasma electrolytic oxidation in silicate–alkaline electrolytes, are studied experimentally. The composition and structure of the coatings are studied using scanning electron microscopy, energy-dispersive microanalysis, and X-ray diffraction analysis. It is found that aluminosilicates dominate in the porous structure of the white slightly conductive coatings, while vanadium is present in the black coatings. The solar absorbance As and thermal emissivity ε are measured: As = 0.28, ε = 0.93 for the white coatings and Аs = 0.95, ε = 0.88 for the black coatings. These values correspond to the characteristics of EKOM thermal control paint coatings produced by Kompozit (Russia). The effect of atomic oxygen flux on the coatings under investigation leads to insignificant erosion of their surface, as compared to other materials. Keywords: AMg3 aluminum alloy, plasma electrolytic oxidation, solar absorbance , thermal emissivity, resistance to atomic oxygen flow, resistivity DOI: 10.1134/S1027451018030229

INTRODUCTION Thermal control coatings (TCC) are used for onboard-equipment units and spacecraft units, the surface of which is exposed to the electromagnetic radiation of the Sun, including far- and near-ultraviolet radiation, and charged particle fluxes [1, 2]. Examples of such products are hoods, tubes, and covers of telescopes, components of optoelectronic devices, equipment for remote sensing of the Earth, solar collectors, passive thermal protection systems for space vehicles, and other equipment. Paint coatings are often used as TCC, for example, coatings of the EKOM series produced by Kompozit [3]. There are also TCC based on composite materials [4] and blackened nickel [5]. Considerable attention has been paid recently to the possibility of creating TCC over constructional magnesium, aluminum, and titanium alloys by their plasma electrolytic treatment [6–15]. The relatively simple technology of such processing yields multifunctional ceramic-like coatings firmly adhered to the metal substrate on alloys of Mg, Al, Ti, and other valve metals [16]. One of the destructive factors of outer space in low near-Earth orbits (300–500 km) is the erosion of surface materials under the action of chemically active atomic oxygen [1, 2] formed during the dissociation of

oxygen molecules under ultraviolet radiation from the Sun. Polymer materials and, to some extent, metals undergo destruction. To protect the materials, thin (~1 μm) inorganic (SiO2, Al2O3), composite ceramic and other coatings are used. In the present work, the structure, composition, optical characteristics and resistance to the effect of atomic oxygen flux of white and black thermal control coatings obtained by plasma electrolytic oxidation (PEO) on AMg3 alloy are experimentally studied. EXPERIMENTAL The objects of investigation were rectangular plates 40 × 40 mm in size, 2 mm in thickness, made of AMg3 aluminum alloy with white and black coatings obtained by plasma electrolytic oxidation. Oxidation was carried out in the anode–cathode mode at a total anode and cathode current density of approximately 10 A/dm2 using experimental equipment of the Moscow Aviation Institute [16]. White coatings were obtained in a silicate–alkaline electrolyte; ammonium vanadate was added to this electrolyte to obtain black coatings. The thickness of the PEO coatings was measured with a VT-201 eddy current thickness gauge. The structure and composition were examined using a

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Fig. 1. SEM images of the black PEO coating, obtained in the backscattered electron mode, at various magnifications.

Quanta 600 scanning electron microscope with a Trident XM-4 energy dispersive attachment for elemental analysis and an Empyrean X-ray diffractometer (PANalytical) with CuKα radiation. The solar absorbance As was measured using an integrating spectrophotometer in the wavelength range 400–2500 nm and an FM-59M photometer. The thermal emissivity ε was determined using an IKS-40 infrared spectrophotometer and a TRM-3 radiometer. To assess the electrical resistivity of the PEO coatings, the resistance between the metal substrate and conductive carbon adhesive tape with an area of 1 cm2, glued to the surface of the coating, was measured using an Advantage gigaohmmeter. To compare the properties, white (EKOM1) and black (EKOM2) thermal control coatings were used. The coating samples were irradiated with an atomic oxygen flux using a simulator stand with a magneto5.4 Elements O Na Mg Al Si V

4.8 Si

I, arb. unit

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wt % 37.77 6.29 0.40 12.94 38.66 3.94

at % 51.50 5.97 0.36 10.46 30.03 1.69

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Fig. 2. Results of energy-dispersive microanalysis of the black PEO coating.

plasmadynamic accelerator of oxygen plasma (Institute of Nuclear Physics of the Moscow State University) [2]. The equivalent fluence of oxygen atoms with an average energy of 30 eV was 12 × 1021 cm–2, which corresponds to one year of operation of the International Space Station in near-Earth orbit. The weight loss of the samples after irradiation with atomic oxygen was determined with the help of an ADV-200 analytical balance. RESULTS AND DISCUSSION The thickness of the white and black PEO coatings was 150 and 50 μm, respectively. Scanning electron microscopy shows that the PEO coatings on aluminum and its alloys have a porous structure; the porosity is approximately 7%. This is demonstrated by SEM images of the white [16] and black PEO coatings (Fig. 1). The porous structure causes the diffuse scattering of light [7]. Electron probe X-ray microanalysis shows that aluminosilicates predominate in the composition of the white coatings [16], while in the black coatings, vanadium is additionally present, apparently, in the form of oxide compounds (Fig. 2) which provides this color. According to X-ray diffraction analysis, the black PEO coating is amorphous (Fig. 3). The amorphous halo with a maximum at 2θ = 22° is possibly associated with a low-symmetry monoclinic phase of SiO2. The diffraction peaks of aluminum are due to the partial X-ray transparency of the coatings. The white PEO coatings are characterized by the presence of diffraction peaks of the γ-Al2O3 and α-Al2O3 phases [16]. Figures 4 and 5 present the reflectivity R spectra of the coatings in the visible and infrared ranges, respectively. The reflectivity of the white PEO coatings is 0.7–0.75 in a wide range of wavelengths from 400 to 2400 nm, while the same coefficient for the black coatings does not exceed 0.1 in the same wavelength

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Al 220 0.8 Al 311

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0.4 30 Al 222

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1200 1600 λ, nm

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Fig. 3. X-Ray diffraction pattern of the black PEO coating.

range. In the infrared region of 3–25 µm, the reflection coefficients of the white and black PEO coatings differ insignificantly and are in the range of 0.05–0.25. The solar absorbance As and thermal emissivity ε, calculated according to [1, 2] are As = 0.28 and ε = 0.93 for the white PEO coatings and As = 0.95 and ε = 0.88 for the black ones. Practically the same values of As and ε were obtained in the measurements with an FM59M photometer and a TRM-3 radiometer. The measured values correspond to the requirements imposed on the TCC [1, 2]. They are close, in particular, to the optical properties of the EKOM1 and EKOM2 thermal control coatings produced by Kompozit. According to [8], the effect of atomic oxygen flux on the white PEO coatings leads to slight erosion of its surface and does not affect their reflectivity in the visible region of the spectrum. Based on weight measurements in [8], the sputtering yield of the PEO coatings upon irradiation with atomic oxygen flux was determined, the value of which turned out to be ~0.05 at/at O. The results of simulation of the sputtering of α-Al2O3 by oxygen atoms with an energy of 20–50 eV made it possible to conclude that the observed erosion is caused by the collision mechanism of sputtering. It is found that the erosion of the PEO coatings under the effect of an atomic oxygen flux is much less than the erosion of many carbon and polymer composite materials used on the outer surface of space vehicles [2]. Similar results were obtained in the present work for the black PEO coatings. The experimentally observed erosion of PEO coatings under the action of atomic oxygen flux was no more than 0.5% of their weight.

Fig. 4. Reflectivity R of the (1) white (As = 0.28) and (2) black (As = 0.95) PEO coatings in the wavelength range of λ = 200–2500 nm.

Measurement of the electrical resistivity of the PEO coatings showed that they are slightly conductive. The resistivity of the white PEO coating and the white EKOM1 coating are comparable: it is ~105 Ω m. The resistivity of the black PEO coating was found to be two orders of magnitude smaller than that of the white PEO coatings. Such electrical characteristics provide antistatic properties which are necessary for TCC [1, 2].

1.0 0.8 0.6 R

20

0.4 0.2

2 1

0 5

10

15 λ, μm

20

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Fig. 5. Reflectivity R of the (1) white (ε = 0.93) and (2) black (ε = 0.88) PEO coatings in the wavelength range of λ = 3–25 µm.

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CONCLUSIONS The structure, composition, and characteristics of white and black thermal control coatings obtained on AMg3 alloy by plasma electrolytic oxidation are studied experimentally. The TCC obtained by plasma electrolytic oxidation have a porous structure, and the porosity is approximately 7%. Aluminosilicates are predominant in the white coatings, and the black coatings additionally contain vanadium, apparently, in the form of oxide compounds, which provides this color. In the white PEO coatings, the solar absorbance is As = 0.28 and the thermal emissivity is ε = 0.93; for the black coatings, they are Аs = 0.95 and ε = 0.88. The PEO coatings exhibit high resistance to the effects of atomic oxygen flux. The black and white PEO coatings are slightly conductive, with a resistivity of ~105 and ~103 Ω m, respectively. In general, PEO coatings can serve as thermal control coatings with suitable optical and electrical characteristics. ACKNOWLEDGMENTS We are grateful to A. A. Ashmarin for conducting X-ray structural analysis. The work was supported by the Grant Council of the President of the Russian Federation, project no. MK-524.2017.8. REFERENCES 1. D. G. Gilmore, Spacecraft Thermal Control Handbook, Vol. 1: Fundamental Technologies (American Institute of Aeronautics and Astronautics, Reston, VA, 2002). 2. Model of the Cosmos, Vol. 2: Space Medium Influence onto Materials and Equipment of Spacecrafts, Ed. by M. I. Panasyuk and L. S. Novikov (Knizhnyi Dom Universitet, Moscow, 2007) [in Russian].

3. http://www.kompozit-mv.ru/. 4. A. A. Zhukov, A. S. Korpukhin, V. P. Lavrishchev, O. A. Dyukareva, and O. A. Kazantsev, RF Patent No. 2503103, Byull. Izobret., No. 36 (2013). 5. S. A. Zhukova, A. V. Tyutyugin, B. I. Zadneprovskii, A. G. Zaitsev, E. A. Grin’kin, and V. E. Turkov, RF Patent No. 2467094, Byull. Izobret., No. 32 (2012). 6. C. S. Kumar, A. Sharma, K. Mahendra, and S. Mayanna, Sol. Energy Mater. Sol. Cells 60, 51 (2000). 7. A. M. Borisov, K. E. Kirikova, and I. V. Suminov, Fiz. Khim. Obrab. Mater., No. 2, 18 (2011). 8. A. M. Borisov, L. A. Zhilyakov, K. E. Kirikova, et al., Fiz. Khim. Obrab. Mater., No. 5, 27 (2012). 9. R. V. Alyakretskii, D. V. Ravodina, T. V. Trushkina, et al., Tr. Inst.–Mosk. Aviats. Inst. im. Sergo Ordzhonikidze, No. 74, 1 (2012). 10. A. E. Mikheev, A. V. Girn, S. S. Ivasev, and I. V. Evkin, Vestn. Sib. Gos. Aerokosm. Univ. im. Akad. M. F. Reshetneva, No. 3 (49), 217 (2013). 11. H. Li, S. Lu, X. Wu, and W. Qin, Surf. Coat. Technol. 269, 220 (2015). 12. S. Lu, W. Qin, X. Wu, X. Wang, and G. Zhao, Mater. Chem. Phys. 135, 58 (2012). 13. L. Wang, J. Zhou, J. Liang, and J. Chen, Appl. Surf. Sci. 280, 151 (2013). 14. Z. Yao, Q. Shen, A. Niu, B. Hu, Z. Jiang, Surf. Coat. Technol. 242, 146 (2014). 15. Q. Xia, J. Wang, G. Liu, et al., Surf. Coat. Technol. 307, 1284 (2016). 16. A. V. Apelfeld, P. N. Belkin, A. M. Borisov, V. A. Vasin, B. L. Krit, V. B. Lyudin, O. V. Somov, V. A. Sorokin, I. V. Suminov, and V. P. Frantskevich, Modern Technologies for Modifying Materials’ Surface and Applying Protective Coatings, Vol. 1: Microarc Oxidation (Renome, Moscow–St.-Petersburg, 2017) [in Russian].

Translated by O. Zhukova

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