Protective coatings for plasma facing components in thermonuclear reactors

Refractory Metals & Hard Materials 11 (1992) 357-365

Protective Coatings for Plasma Facing Components in Thermonuclear Reactors* J. L i n k e , a M. A k i b a , b T. m n d o , b J. P. C o a d , c S. D e s c h k a a & E. Wallura a aResearch Centre Jiilich, KFA-Euratom Association, D-52425 Jiilich, Germany bJapan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki 311-01, Japan cJET Joint Undertaking, Abingdon, Oxon, UK, OX 14 3EA (Received 1 April 1993; accepted 22 April 1993)

Abstract: The plasma facing components in future thermonuclear confinement

experiments have to withstand high stationary heat loads during normal operation and severe thermal shocks during off-normal conditions (so-called disruptions). In these transient events on the first wall, energy depositions up to 2 MJ m - 2 can occur with pulse durations of the order of 1 ms. To improve the performance of the plasma and to protect it against high-Z impurities from metallic structures, boron carbide coatings have been used successfully in different fusion experiments. Thick coatings of this material have been prepared by plasma spraying, a technique which also offers potential for in-situ repair of damaged coatings inside the torus. Coatings with thicknesses of several hundred micrometers on different substrates (graphites, carbon fiber composites, stainless steel and refractory metals) have been tested in high heat flux test facilities at heat loads simulating the normal operation and disruption conditions. In addition, a limited number of coated tiles have been installed in fusion relevant tokamak experiments such as TEXTOR, JET or JT-60U.

INTRODUCTION The first wall protection in thermonuclear fusion devices consists of low-Z materials. In particular, carbon in the form of fine grain graphites or carbon fiber composites (CCs) is the most common material in existing tokamaks. To improve the resistance of plasma facing materials (PFM) against chemical erosion and, furthermore, to reduce the oxygen content in the plasma by gettering, even lighter elements such as boron or beryllium have been suggested and tested. 1-3 Boronization of the first wall in TEXTOR, and later in many other tokamak devices, which results in the formation of an amorphous hydrogenated boron containing carbon film some 10 nm in thickness, led to significantly lower metal, oxygen and carbon contamination in the plasma. Similar results have been obtained in JET with beryllium evaporated over the first wall. 357

Beside these thin coatings, bulk materials (metallic beryllium or graphites doped with boron) have been developed and/or evaluated for the application as PFM. 4 In addition, thick boron carbide coatings 5 based on different processing methods (conversion, chemical vapor deposition (CVD) or plasma spray) have been included in the test programs; here especially, the plasma spray technique seems to be a promising solution for future fusion reactors such as ITER (international thermonuclear experimental reactor) because the technique offers possibility for in-situ repair of eroded or damaged coatings in the neutron activated vacuum vessel of a tokamak by remote handling. The evaluation process of plasma sprayed coatings for next step fusion devices such as ITER was performed in three steps, which are shown schematically in Fig. 1. In a first step the processing parameters and the different operating regimes

358

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were evaluated in different laboratories. The substrate materials were selected according to the requirements in fusion devices; prime candidates are carbon, stainless steel and refractory metals. The resulting coatings were then characterized with respect to microstructure, porosity and adhesion strength. In a second step the performance of BaC coatings under fusion relevant heat loads was determined in high heat flux experiments; here, typical heat loads for normal and off-normal operation scenarios were simulated. Finally, those coatings which showed best performance in the simulation tests have been exposed to tokamak plasmas in different confinement experiments.

MANUFACTURING AND PROPERTIES OF PLASMA SPRAYED B4C COATINGS Thick boron carbide coatings are produced by plasma spraying in an inert gas atmosphere (e.g. Ar). Different operation regimes of the spray facilities have been applied which extend from the low pressure range (coatings in this report have been manufactured primarily by this method) via the atmospheric spray to the high pressure plasma spray (up to 2 bar). Also, graded B4C coatings especially adjusted for metallic substrates with high coefficients of thermal expansion (CTE) showed successful properties. 6 In all these processes, B4C powders of high purity have been used in order to avoid medium-Z or high-Z contaminations in the layer. During the process individual layers (each single layer having a thickness of c. 50 to 100/~m) have been deposited on substrates made from graphite, CCs, stainless steel or refractory metals. With this technique, coating thicknesses up to 1 mm and thicker are possible.

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In comparison to CVD processed coatings the plasma sprayed B4C layers show higher porosity, which can be varied over a wide range depending on the spraying parameters; typical porosites are of the order of c. 10%. Compared with other material candidates for plasma facing components (PFC) the thermal conductivity of BaC is rather poor (for bulk material of hot pressed B4C values from 27 to 36 Wm-1 K-1 have been measured 7) due to the porosity of the sprayed coatings it can be assumed that the thermal conductivity of the sprayed layer is of the order of 20 Wm- 1 K- 1. The thermal loading of B4C coated materials will result in the formation of shear stresses due to a mismatch in the coefficients of thermal expansion of the coating (a = 5.7 x 10 - 6 K- ~) and the substrate (e.g. graphite EK 98: a = 3 . 8 x 10 -6 K-I; TZM: a = 5 . 3 x 1 0 -6 K-l; stainless steel: a = 1 6 . 2 x 10 -6 K-l). To measure the adhesion strength of the B4C layers, specimens of 40 mm diameter were bonded to metallic mounting bolts by means of an adhesive and then subjected to a tensile t e s t / T h e results from these tests are given in Fig. 2; in all cases the cracking occurred within the substrate material or within the adhesive. Thus, it could be concluded that the adhesion strength of the plasma sprayed B4C on the substrate exceeds 17 MPa on graphite and 22 MPa on the molybdenum-base alloy TZM. PERFORMANCE OF B4C COATINGS UNDER H I G H H E A T LOADS The stationary heat loads to PFC in future thermonuclear fusion experiments such as NET

Protective coatings in thermonuclear reactors

or ITER will be below 1 MW m -2 for the first wall during normal operation; plasma instabilities, so-called disruptions, will result in the deposition of energy densities of the order of 2 MJ m-2 over a time interval of 1 ms or less. For the divertor region these numbers are significantly higher (up to 3 0 M W m -2 and 12 M J m -2, respectively). To evaluate the performance of different candidate materials under these conditions, special high heat flux test facilities have been installed. Plasma sprayed B4C coatings have been evaluated both for normal and off-normal conditions in ion beam and electron beam test facilities.5,6,8 Experimental results described in this paper have been obtained from the MARION (Material Research Ion Beam Test Facility) and JEBIS facilities; the construction of these devices is shown schematically in Fig. 3. In MARION, coated specimens, 50 m i n x 5 0 m m and 5 0 m m × 2 5 m m in size, were exposed to energetic hydrogen ion beams (beam diameter 40 cm); the acceleration voltage was increased up to 30 kV. Prior to the experiments, careful calibration measurements were performed by evaluating the horizontal and vertical power density distribution. Under the above mentioned conditions, maximum power densities of 10.5 M-W m -2 have been determined in the beam center; maximum pulse durations were 10 s. In the high heat flux tests, power density or pulse length was increased stepwise; Fig. 4 shows some typical results from plasma sprayed B4C coatings on various graphites, CC material and TZM; the coating thickness was 125, 180 and 250 /~m. For 10 s pulses on the graphite substrates,

359

melting of the B4C occurred at 5.5 MW m -2 (CL 1116PT) and 6"0 MW m -2 (EK 98), respectively. Due to the poor thermal conductivity of the plasma sprayed B4C a high thermal gradient within the layer arises upon thermal loading. Caused by this thermal gradient and the mismatch in CTE between layer and substrate, cracks originate. In the case of delamination and loss of thermal contact, melting of the coating also occurred. This behavior was very pronounced at the remaining rims of the chipped-off regions. Compared with the fine grain graphites the refractory alloy TZM behaves marginally better for short pulses, an effect which can be attributed to the higher thermal conductivity of TZM. Similar tests on B4C coated CCs (CC1001G), which have been performed at pulse lengths of only 2 s, show a significant increase in the surface temperature

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with power density. This effect is mainly due to the relatively poor thermal conductivity ;t ~ of this anisotropic CC grade. The morphological changes in the B4C coatings caused by the applied heat loads have been investigated in metallo-/ceramo-graphic sections (cf. Fig. 5). At pulse durations of 10 s on B4C coated graphites beside some cracks no additional changes in the microstructure could be observed up to heat loads of 4 MW m- 2; the porosity corresponds to the as-deposited layer ( p - 2 0 % , deter-

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mined by quantitative image analysis). If the heat load is increased to c. 5 MW m-2 sintering processes become dominant and the porosity is reduced to p--15%. At 5"5 MW m -2 melting occurs; the recrystailized structure is rather dense with a remaining porosity of -<2% (with possibly increased thermal conductivity values). Heat loads which occur during plasma disruptions have been simulated in electron beam facilities; by focussing the beam on relatively small surface areas sufficient energy densities can be applied to B4C coated test specimens. In addition to JEBIS (cf. Fig. 3(b)) other experimental facilities (e.g. the Juelich Divertor Test Facility in Hot Cells, JUDITH 6) have also been used. The typical heat load conditions applied in electron beam tests were energy densities of 2 MJ m-2 at pulse durations of the order of 1 ms (corresponding to the requirements for devices like NET or ITER). In these experiments, besides morphological changes and material damage in the coating, erosion processes due to sublimation of B4C and chipping also occurred and were investigated by quantitative methods. Typical diagnostics applied here were weight loss measurements and profilometry on the erosion craters, using mechanical and optical methods. Compared to the MARION test stand JEBIS operates at much higher acceleration voltages. In the experiments described here the electron energy was 80 keV, which, as a consequence, results in a deep penetration of the particles into the coating; for low-Z materials such as B4C a volumetric heating occurs down to a depth of several tens of micrometers. Due to the energy densities which have to be applied in these tests the loaded surface area is small. In general the beam profile is Gaussian shaped; typical beam diameters are of the order of 5 mm. Since part of the incident energy is reflected (due to back-scattered electrons), a careful calorimetry has been applied to determine the local heat fluxes. In Fig. 6 the measured erosion weight loss is plotted versus coating thickness for different substrate materials; in these tests, five pulses of 1.2 ms duration with 2.15 MJ m -2 (absorbed energy density) have been applied. Coatings on stainless steel substrates (316L-SA) show significantly higher erosion rates if compared to those on CCs (PCC-2S, CC 1501GR). This effect may be attributed to the relatively poor thermal conductivity of the metal; on the other hand the residual stresses are less in the CC coated samples

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due to the better CTE-matching. The crater depths determined by profilometry are 0.12 to 0.18 m m maximum, and 0"05 m m on the average. The crater profile reveals a characteristic stepped structure (see Fig. 7(a)), with a step height of 60 /~m. This is in good agreement with the microstructure of the coating which has been deposited by spraying subsequent layers of c. 50 to 60/~m per pass. The volumetric heating by the 80 keV electrons occurs on a similar scale. Single shot electron beam tests at otherwise identical loading conditions on preheated specimens (T~700°C) showed significantly lower weight losses as compared to specimens loaded at room temperature. The crater depth is similar in both cases, but wider in the room temperature case. Possible explanations for this effect are outgassing or reduced residual stresses in the case of preheated coatings. Typical morphological changes in the plasma sprayed B4C layers due to simulated disruption effects are shown in Fig. 7(b) and (c). The outer zone of the erosion crater (where individual B4C layers have delaminated) shows the typical appearance of as-deposited B4C coatings; this structure is also typical for the coating shown in Fig. 5(a). In the beam center, however, local melting occurs; the resolidified surface is characterized by a dense packing of crystallites (10-20 /~m) and low porosity (cf. also Fig. 5(c)). PERFORMANCE OF B4CCOATINGS IN TOKAMAKS

Experiences with plasma sprayed B4C coatings on graphite composites and CCs have been obtained

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(c) Fig. 7. Disruptionsimulationon plasma sprayed B4Ccoatings (2.1 MJ m -2, 1.2 ms, five shots) in the electron beam facility JEBIS: (a) macroscopiealappearance of the erosion crater; (b) SEM micrograph from the outer zone of the erosion crater; (c) SEM micrograph from the bottom of the crater (beamcenter). in three tokamak devices: TEXTOR (J/ilich, Germany), JT-60U (Naka, Japan) and JET (Culham, UK). The coated tiles and the position during plasma exposure are shown in Figs 8-10.

J. Linke, M. Akiba, T. Ando, J. P. Coad, S. Deschka, E. Wallura

362

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A graphite test limiter (10 c m x 6 cm) made from fine grain graphite EK 98 has been coated in a low-pressure plasma spray process with a B4C layer of c. 200 p m thickness. Limiters with this geometry (see Fig. 8) can be mounted on a moveable support and introduced into the TEXTOR vacuum vessel by a vacuum lock (without breaking the vacuum in the toms). Depending on the plasma heating (ohmic discharges plus 2 x 1.7 MW neutral beam injection) and on the radial position of the test limiter, heat loads up to 30 MW m-2 can be applied for pulse durations of c. 3 s. In addition to plasma heating the limiter temperature can be controlled by an internal heater. Surface temperatures and macroscopic changes on the limiter surface can be recorded during/ between plasma discharges with a CCD camera from a viewing port on the top of the vessel. The bulk temperature of the limiter is measured with thermocouples. In addition, spectroscopical measurements can be applied to determine the temporal evolution of the intensities of different emission lines. 9 Compared to other test limiter experiments with boron doped (bulk) graphites the plasma sprayed B4C coating already melted at relatively

Protective coatings in thermonuclear

363

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position with maximum heat loads close to the separatrix (see Fig. 9). A total of 360 discharges has been applied during this period, 172 discharges with neutral beam heating. The NB heating power was typically 20 M W for a duration of 2 s. The divertor heat flux was c. 5 M W m-2; a surface temperature of c. 700°C has been measured. At the end of the operation period the coated tiles were removed; detailed inspection revealed that the major fraction of the coating remained adherent. A slight melting occurred at the edge of the tiles (see Fig. 9). This effect could be avoided by a moderate tapering of the edges of the CC tiles before plasma spraying. Three B4C coated tiles made from a felt type CC material (A05) have been installed in the upper X-point region of the JET toms (see Fig. 1011). From a scientific viewpoint this was a dual purpose experiment: on the one hand the performance of plasma sprayed coatings under realistic wall loading in large tokamak devices was of interest, on the other hand the coated tiles provided a boron source element for the analysis of redeposition processes in the divertor channel. The three tiles (SC44, SC39 and SC47; length:

low power loads. The molten material agglomerated with droplet sizes of a few millimetres down to about 0.1 mm. Figure 1 l(a) shows the morphology of these features in the scanning electron microscope. By energy dispersive X-ray analysis (EDX) significant amounts of boron could be detected at the sharp-edged side of the droplet; at the central part, however, carbon is the dominant element (Fig. l l(b)). On the uncovered graphite surface a continuous boron-containing film is still detectable by EDX. Numerical calculations of the heat transport and temperature distribution of a two layer system have shown that the thermal conductivity of the layer is only about 2 = 2 - 4 W m-1 K-1.9 Since this value is almost one order of magnitude smaller than 2 values reported for bulk B4C7 it also has to be assumed that the adhesion of the plasma sprayed coating was poor, which again results in low heat transfer coefficients and high surface temperatures. Plasma sprayed B4C coatings with 150 # m thickness on CCs (CX 2002U, PCC-2S) were exposed to plasma discharges in JT-60U for a 2 month period. 1° The tiles (length = 107 m m ) w e r e mounted at the bottom of the vacuum vessel in a

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364

J. Linke, M. Akiba, T. Ando, J. P. Coad, S. Deschka, E. Wallura

10-12cm) formed a poloidal set across the regions of maximum plasma surface interaction; 370 X-point discharges of about 5 s duration were applied. After removal from the torus the tiles were inspected by SEM and analysed by different methods such as RBS, PIXE, NRA and EDX. 11 Slight misalignments of the individual tiles resulted in localized heat loads (hot spots), primarily at the edges of the tiles due to the small incident angle of the plasma particles. One of the B4C coated tiles (SC47) was severely eroded on its edge adjacent to the sector break. SEM analysis from this edge revealed an almost complete removal of the boron carbide layer. The major fraction of the coating was, however, intact and appeared to be overlaid by a smoother deposit. The typical morphology of this material, which is shown in Fig. 11(c), is characterized by a widespread network of cracks with a typical mesh size of 0.2 to 0-5 mm. Detailed EDX analyses from this structure on different positions on the tile show a very uniform result, namely, spectra which indicate the presence of carbon and oxygen as main constituents of the deposit. The recorded carbon to oxygen ratios vary considerably; some spectra also emphasize the presence of boron. Beryllium which has been evaporated several times during the test phase is undetectable by EDX methods. Localized spots (marked 'B' in Fig. 11(c)) show a partial removal of the coating; at the edge of these areas resolidified B4C is detectable (marked 'C'). EDX analyses from these features show a boron rich and oxygen-free material for area 'C' and predominantly carbon and oxygen for area 'D'. High resolution SEM micrographs from the latter area reveal the characteristic structure of redeposited material. Other features observed by SEM on tile SC47 are melt regions which, with respect to morphology and size, are almost identical to those detected on the TEXTOR limiter (Fig. 11(a)). In contrast to tile SC47 which was the hottest of the three tiles, samples SC39 and SC44 obviously did not reach the melting temperature of B4C;here, no localized melt regions could be detected. CONCLUSIONS Boron doped graphites or boron-containing coatings tumed out to be resistant against chemical erosion and to reduce the impurity content in the

tokamak plasmas. However, doping of graphites and CCs with boron in general degrades important physical parameters such as thermal conductivity; on the other hand, thin amorphous BnC coatings which have been deposited from the gas phase have only a limited durability. Therefore, thick BaC coatings produced in a plasma spray process are considered as an interesting alternative material candidate for the plasma facing side of existing tokamaks and for the first wall in next step fusion devices such as ITER. Today, plasma sprayed coatings can be produced with thicknesses up to several millimetres on different substrates; most interesting for fusion applications are carbon based materials (graphites, CCs), stainless steel or refractory metals (Mo, TZM, W). Different techniques (from low pressure to high pressure plasma spray), including the deposition of graded systems, especially on substrates with high coefficients of thermal expansion, have been optimized. Plasma sprayed coatings can be produced with rather good adhesion properties; however, due to the nature of B4C and due to the remaining porosity the thermal conductivity is limited. Heat flux experiments performed in ion beam and electron beam test facilities have shown that power densities up to c. 4 MW m-2 can be tolerated by BaC coated graphite tiles at pulse durations up to 10 s. Experimental data with stationary heat loads of < 1 MW m-2, which are expected for the first wall in ITER, are not available yet; a reliable technical solution seems to be feasible. Simulated disruptions with ITERrelevant parameters (2 MJ m-2; 1 ms) on plasma sprayed coatings result in a non-negligible erosion of the test specimens. The existing data give evidence that thick sacrificial layers produced by plasma spray techniques (thickness of the order of 1 mm) have the potential to survive at least several tens of disruptions. First results with BnCcoatings on different CCs are rather promising; heat loads up to several MW m-2 can be tolerated at pulse lengths of about 5 s. However, a precise shaping of passively cooled tiles or active cooling is necessary to avoid hot spots; otherwise severe erosion by sublimation and/or melting becomes significant. In future tokamak devices the plasma spray process offers the possibility to repair eroded or damaged coatings in situ by remote handling techniques.

Protective coatings in thermonuclear reactors

ACKNOWLEDGEMENTS The authors would like to thank G. Schnedecker, K. Born, D. J~iger and W. Mall6ner for the preparation of plasma sprayed coatings and H. Hoven for ceramographic investigations on the test samples. REFERENCES 1. Winter, J. etal., J. Nucl. Mater., 162-4 (1989) 713-23. 2. Hirooka, Y. et al., J. Vac. Sci. Technol., A8 (3) (1990) 1790-8.

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3. Thomas, E R., J. Nucl. Mater., 176/177 (1990) 3. 4. Linke, J. etaL, J. NucL Mater., 176/177 (1990) 856. 5. Nakamura, K. et al., J. Nucl. Mater, 196-8 (1992) 627-32. 6. Laan, J. G. van der et aL, Proc. 17th Syrup. on Fusion Technology, Rome, 1992. 7. Goodfellow GmbH, Table of Ceramics Data, Goodfellow Catalogue 1990/91, Eschborn, Germany, p. 33. 8. Deschka, S., Linke, J., Nickel, H. & Wallura, E., Fusion Eng. andDesign, 18 (1991) 157. 9. Phillips, V. et al., J. Nucl. Mater., 196-8 (1992) 1106-11. 10. Ando, T. et al., Proc. 17th Syrup. on Fusion Technology, Rome, 1992. 11. Coad, J. P., Farmery, B., Linke, J. & Wallura, E., Proc. 8th Int. Conf. on Solid Surface (ICSS-8), Den Haag, 1992.