Boron carbide coating deposition on tungsten and testing of tungsten ...

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ISSN 10637788, Physics of Atomic Nuclei, 2015, Vol. 78, No. 14, pp. 1640–1642. © Pleiades Publishing, Ltd., 2015. Original Russian Text © A.A. Airapetov, L.B. Begrambekov, O.I. Buzhinskiy, A.V. Grunin, A.A. Gordeev, A.M. Zakharov, A.M. Kalachev, Ya.A. Sadovskiy, P.A. Shigin, 2014, published in Yadernaya Fizika i Inzhiniring, 2014, Vol. 5, Nos. 11–12, pp. 961–963.

TECHNOLOGY OF NUCLEAR MATERIALS

Boron Carbide Coating Deposition on Tungsten and Testing of Tungsten Layers and Coating under Intense Plasma Load A. A. Airapetova, L. B. Begrambekova, *, O. I. Buzhinskiyb, A. V. Grunina, A. A. Gordeeva, A. M. Zakharova, A. M. Kalacheva, Ya. A. Sadovskiya, and P. A. Shigina aNational Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe sh. 31, Moscow, 115409 Russia b

State Research Center Troitsk Institute for Innovation and Fusion Research (TRINITI), Moscow oblast, 142190 Russia *email: [email protected] Received February 23, 2015

Abstract—A device intended for boron carbide coating deposition and material testing under high heat loads is presented. A boron carbide coating 5 µm thick was deposited on the tungsten substrate. These samples were subjected to thermocycling loads in the temperature range of 400–1500°C. Tungsten layers deposited on tungsten substrates were tested in similar conditions. Results of the surface analysis are presented. Keywords: first wall materials, PFM, tungsten, boron carbide, thin films, protective coatings, plasma deposi tion, sputtering, film deposition, heat loads, heat testing. DOI: 10.1134/S106377881514001X

INTRODUCTION At present, tungsten is proposed as a plasmacon tacting material of the ITER diverter. It is known that erosion, defoliation of the surface, and formation of dust are observed under heat and corpuscular loads corresponding to ITER conditions [1–5]. It was shown in [6] that it is possible to protect the ITER tungsten tiles from the action of plasma using in situ renewable protective boron carbide coating (В4С). The coating deposition is possible in the regular ITER discharge from carborane (B12C10H2) vapor [7]. EXPERIMENTAL DEVICE One vacuum chamber is used in the device (Fig. 1), in which a plasma chamber, a system of the formation of the flux of ions and electrons drawn from plasma on the target, carbon and boron targets, a device for posi tioning targets and samples, and other construction elements are located. The deposited sample (sub strate) is installed on a movable input and is placed in a vacuum chamber via the gateway chamber. A mass spectrometer with differential pumping is also included in the composition of the setup, making it possible to control the composition of the working gas in the process of the experiment. Pumping is per formed by spiral and turbomolecular pumps; the lim iting vacuum is 2 × 10–6 Pa. The device is completely automated, and the com plete experimental cycle is performed according to the preliminarily set program.

MODES OF FUNCTIONING OF THE DEVICE To deposit the boron carbide coating, the working gas is fed to the vacuum chamber and the discharge is ignited between the filament tungsten cathode and anode. Voltage negative with respect to the anode is applied to the boron and graphite targets. Plasma ions sputter the target surface, and sputtered atoms are deposited on the substrate surface, forming the boron carbide coating. If necessary, ion bombardment of the substrate is performed prior to the beginning of the target sputtering. The potentials on the targets are also varied with the aim of higher sputtering of boron or carbon for the formation of the transition layer improving the coating adhesion. To study the behavior of materials and coatings under thermal and corpuscular impacts, the studied samples are fixed on a special holder, placed in a posi tion occupied by targets during the coating deposition, and irradiated with the ion or electron flux. It is possible to obtain a power as high as 4 kW in the stationary and pulse irradiation mode during testing of materials. The sizes of studied samples are limited to a circle with the area of 104 mm2. The control block makes it possible to perform tests with the pro grammed variation of the irradiation power and the beam shape. Cyclic irradiations are possible with the pulse duration from 1 ms and at any filling.

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In the first experiments we obtained tungsten sam ples with the boron carbide coating 5 µm thick. The coating composition was estimated by the energydis persive spectrometry (EDS) method, which showed the stoichiometric ratio B : C = 4 : 1. This sample was subjected to heat testing. Thermocycling was performed in the temperature range of 400–1150°C. Each cycle consisted of heating for 8–10 s and cooling for 30–40 s. A total of 60 cycles were performed. After thermocycling, all measured parameters (weight, composition, microrelief, adhe sion) did not vary.

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THERMOCYCLING OF THE TUNGSTEN LAYER DEPOSITED ON TUNGSTEN A coating 200 nm thick deposited on a tungsten substrate was studied. The coating was formed by tungsten atoms sputtered by argon plasma ions from the tungsten target. Both the substrate and the sput tered tungsten target were plates of tungsten used for manufacturing the ITER diverter. The tungsten sub strate was annealed in vacuum for 1.5 h at the temper ature of 1500°C, and it was degassed at the tempera ture of 1150°C immediately before the coating depo sition. The surface substrate before the coating deposition was irradiated with argon ions in order to sputter the layer of surface oxides. Thermocycling was performed in the temperature range of 300–1200°C. A total of 300 cycles were performed. Each cycle included heating for 10 s and cooling for 40 s. As early as after 5–10 heating cycles, cracks 0.1–0.3 µm thick appeared between coating regions and blisters were seen on their surface, which developed, as shown by further observations, on the boundary between the substrate and the deposited layer. Blisters with the size of 2–3 µm (small blisters) were formed almost imme diately adjacent to each other. Large blisters had sizes of 5–7 µm. The distances between them were about 5–7 µm. Lids of some of blisters were broken. After 15–20 cycles, a large fraction of boundary cracks were not noticeable and the boundaries between separate regions were not discernible. Shapeless large blisters, in which a large fraction of neighboring small blisters were united, were seen on the coating surface. Their sizes could vary from 5 to 12 µm. At the same time, the sizes of large blisters formed earlier did not change noticeably. At the following stage of the modification (20–30 cycles), the coatings of the lid of both types of large blisters descended, and the destruction of their lids began, which continued to the termination of tests. It was concluded that stresses in the coating devel oped in the process of its crystallization during ther mocycling, which might also determine the further modification of blisters and the destruction of their lids. PHYSICS OF ATOMIC NUCLEI

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Vacuum chamber of the device for the deposition of coat ings and testing of materials: (1) vacuum volume; (2) plasma chamber; (3) cathodes; (4) anodes; (5) movable input of the anode; (6) graphite target; (7) boron target; (8, 9) movable inputs of targets; (10, 11) antidynatron electrodes; (12, 13) movable inputs of antidynatron elec trodes; (14) substrate; (15) movable input of the substrate; (16) movable input seal.

CONCLUSIONS A simple and maintenancefriendly automated device was created that makes it possible to study the regularities of the formation and elaborate the deposi tion modes of one and twocomponent coatings and also to study the behavior of materials and coatings during thermocycling and irradiation with ion and electron fluxes with high power density. A boron carbide coating with thickness of 5 µm was deposited on tungsten. The coating was subjected to thermocycling in the temperature range of 400– 1150°C. No modification of the coating or worsening of its adhesion was revealed. Thermocycling of a tungsten layer with thickness of 200 nm deposited on the tungsten substrate in the temperature range of 300–1200°C was performed. The formation of blisters between the deposited layer and the tungsten substrate located close to each other was found. Their subsequent development, in the long

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run, led to the separation of the greater part of the deposited layer from the substrate.

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Translated by L. Mosina

REFERENCES 1. K. Krieger, A. Geier, Y. Gong, et al., J. Nucl. Mater. 313–316, 327 (2003).

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