Thermal response of plasma sprayed tungsten coating to high heat flux

Transcription

1 Fusion Engineering and Design 70 (2004) Short communication Thermal response of plasma sprayed tungsten coating to high heat flux X. Liu a,,l.yang a, S. Tamura b, K. Tokunaga b, N. Yoshida b, N. Noda c,z.xu a a Southwestern Institute of Physics, P.O. Box 432, Chengdu , Sichuan, China b Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasugakoen, Kasuga, Fukuoka , Japan c National Institute for Fusion Science, Toki, Gifu , Japan Received 12 May 2003; received in revised form 7 June 2004; accepted 16 June 2004 Available online 7 August 2004 Abstract In order to investigate the thermal response of tungsten coating on carbon and copper substrates by vacuum plasma spray (VPS) or inert gas plasma spray (IPS), annealing and cyclic heat load experiments of these coatings were conducted. It is indicated that the multi-layered tungsten and rhenium interface of VPS-W/CFC failed to act as a diffusion barrier at elevated temperature and tungsten carbides were developed after 1 h incubation time when annealing temperature was higher than 1600 C. IPS-W/Cu and W/C without an intermediate bonding layer were failed by the detachment of the tungsten coating at 900 and 1200 C annealing for several hours, respectively. Cyclic heat load of electron beam with 35 MW/m 2 and 3-s pulse duration indicated that IPS-W/Cu samples failed with local detachment of the tungsten coating within 200 cycles and IPS-W/C showed local cracks by 300 cycles, but VPS-W/CFC withstood 1000 cycles without visible damages. However, crack creation and propagation in VPS-W/CFC were also observed under higher heat load Elsevier B.V. All rights reserved. Keywords: Plasma sprayed W coating; Diffusion barrier; Annealing; Cyclic heat load 1. Introduction Tungsten has partially been selected as divertor armor tiles of ITER and its interactions with plasma Corresponding author. Tel.: ; fax: address: xliu@swip.ac.cn (X. Liu). have being intensively studied [1 4]. Because of heavy weight and brittleness of pure tungsten, at the present stage, tungsten coating on carbon and copper substrates attracts much attention, as the former has the advantage of full compatibility with the current support structure and the latter can provide simultaneously the joining of the W armour with the heat sink to make up plasma facing components. Plasma sprayed tungsten coating /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.fusengdes

2 342 X. Liu et al. / Fusion Engineering and Design 70 (2004) on carbon substrate has been used in current fusion devices, such as TEXTOR [5,6] and ASDEX-Upgrade [7,8]. Plasma spray can be divided into vacuum plasma spray (VPS) and inert gas plasma spray (IPS) according to different operation environments. The properties of plasma sprayed coating strongly depend on the parameters of the process. It is generally accepted that VPS-W coating on carbon substrate with rhenium (Re) layer interface acting as the diffusion barrier of carbon has good high heat flux properties [9,10]. For example, it has been reported [9,11] that the 550 m VPS tungsten coating with a single rhenium layer can sustain 10 MW/m 2 at 2 s pulse length for 1000 cycles without visible damage but W 2 C and WC phases were observed at the interface region of the VPS tungsten coating after cyclic heat loads. It is an indication that a single Re layer does not seem enough to prevent carbon diffusion towards tungsten coating, particularly under higher heat loads. In this work, thermal properties of VPS-W coated on CX2002U C/C fiber composite (CFC) with multi-layered tungsten and rhenium interface were investigated by means of high temperature annealing and cyclic heat loading. As comparisons, IPS-W coated graphite and copper were also tested. 2. Specimen for high heat flux tests The cross-sectional images of the tungsten coatings tested in this work are shown in Fig. 1. Detailed descriptions of the coating specimens are listed below: 1. The 0.5 mm thickness VPS-W coated CFC (CX- 2002U) has multi-layered tungsten and rhenium interface pre-deposited by physical vapor deposition (PVD), as shown in Fig. 1a, and heat treatments at 1000 C for several hours were performed in order to stabilize the microstructure of the sample [12]. The density of VPS-W was 92.5% of the theoretical value. The pre-deposited W and Re multi-layer on CX-2002U by PVD is expected to act as the diffusion barrier of carbon. Previous investigations indicated that the pre-deposited W Re multi-layer maintained its original structure below 1300 C and it was completely destroyed Fig. 1. Section images of the test samples: VPS-W/CFC with multi-layer W and Re interface (a), IPS-W coated on SMF700 graphite (b), IPS-W/Cu with graded transition interface (c) and IPS-W/Cu (d).

3 X. Liu et al. / Fusion Engineering and Design 70 (2004) at extremely high temperature, such as 2800 C [12,13]. However, the microstructural changes of the multi-layered W and Re interface at medium temperature ranges of C and the critical failure temperature of the diffusion barrier, at which tungsten carbides were developed, were unclear so far. In present experiments, firstly, an annealing experiment was carried out to investigate the microstructural changes of the interface layers and the formation of tungsten carbides at the interface region. Then cyclic heat load tests under different heat fluxes were also performed. 2. IPS-W coating on SMF700 graphite (made in China, its thermo-mechanical properties can be found in Ref. [14]) and copper were also prepared, having lamellar structure and high porosity of about 16%. The section morphologies of IPS-W coating are shown in Fig. 1b d, of which Fig. 1c shows tungsten-coated copper with W and Cu transition interface (W/Cu (T) for short). In the graded transition layer, W/Cu ratio was increased from 0 to 100% by 20% step within the thickness of 300 m. Then 100% W layer of 200 m thickness was sprayed on the graded transition layer. Both the thicknesses of the tungsten coating on graphite and copper were 500 m. Tensile bond strength of the W/Cu and W/Cu (T) coating was measured to be about 36 and 58 MPa, respectively. The size of all coated specimens is 10 mm 10 mm 5 mm with an exception of 10 mm 6mm 5mm for electron beam anneal experiments. VPS-W specimens are cut from the edge region of a large block of about 160 mm 100 mm 15 mm and IPS-W specimens are cut from a block of 50 mm 50 mm 10 mm. The morphology and composition of the coating are examined by a SEM equipped with an energy dispersion X-ray spectroscopy (EDS) and X-ray diffraction spectroscopy (XRD). Results of the heat treatment of IPS-W coating in a vacuum furnace indicated that most of IPS-W/Cu samples without transition layers failed by the detachment of the coating from the copper substrate by 900 C annealing for 4 8 h, but W/Cu with graded transition interface did not fail at this temperature. The release of residual stresses (particularly at the interface) arising from coating deposition process, and different thermal contractions of tungsten coating and copper substrate due to the CTE (coefficient of thermal expansion) mismatch, induce the debonding of W/Cu coating during the annealing, as CTE of copper is four times larger than that of tungsten. Better bonding capability of W/Cu (T) is reasonable since the thermal mismatch of W and Cu has been effectively relaxed by graded transition interface. Some of IPS-W/C sample also failed showing the detachment by 1200 C annealing for 4 8 h. Fig. 2 shows the X-ray diffraction patterns of the joint surface of the tungsten coating and graphite after 1200 C annealing for 2 and 8 h. It can be seen that most of tungsten in the interface turned to W 2 C phase by solid reaction with carbon by 1200 C annealing for 2 h, and almost complete WC structure was found for the case of 1200 C annealing for 8 h. However, no failure was found at 1100 C anneal for 8 h or 1200 C annealing for 2 h, where part of carbon atoms diffused from graphite substrate was still in elemental phase at the interface region (see Fig. 2a), which may be a main reason for keeping the integrity of IPS-W/C in these cases. These results indicate that intermediate layer or transition interface is beneficial to both tungsten-coated carbon and copper, in particular to tungsten-coated copper because of the lack of mutual solubility of W and Cu. 3. Experiments and results 3.1. Annealing Fig. 2. X-ray diffraction patterns of the joint interface of IPS-W/C after 1200 C annealing for 2 h (a) and 8 h (b).

4 344 X. Liu et al. / Fusion Engineering and Design 70 (2004) Fig. 3. A diagrammatic sketch for electron beam annealing experiments. In order to investigate the microstructural changes of W Re multi-layer of VPS-W/CFC under high temperature annealing ( C), an anneal test of the VPS-W/CFC specimens with a slope side was performed in an electron beam heat load facility [15], where a focused electron beam with a 4 mm diameter heated not only the cross-section of the tungsten coating but also part of the declined tungsten surface as shown in Fig. 3. The samples were put on a copper heat sink actively cooled by water. The surface temperature of the irradiated area was measured by a two-color optical pyrometer, and kept constant by carefully adjusting the filament current of the electron beam source. It was found that the microstructure of multi-layer tungsten and rhenium kept its original structure when the annealing temperature was below 1300 C, but the boundaries of tungsten and rhenium layers became blurred with increasing the annealing temperature and finally disappeared by the annealing at 2000 Ceven for only one minute due to the mixture of W, Re and C at the interface. A new phase was developed for longer time annealing (1 h) as shown in Fig. 4. The thickness of the new phase reached to 200 m for the 1800 C annealing and in some area, the surface tungsten layer has completely changed to the new phase for the 2000 C annealing. The inhomogenous structure of the new phase as shown in Fig. 4 could be related to the inhomogeneity of temperature of the tungsten coating (only average surface temperature was kept constant in this experiment). Micro-hardness of the coated layer with the newly formed tungsten carbide phase (see Fig. 4) was measured by using a nano-indenter BNT The size of the indentation was less than 2 m, but it was large enough to avoid the effects of the dense cavities and small grain size. Micro-hardness for the samples annealed at 1800 and 2000 C are plotted in Fig. 5a and b, respectively, in which 10 measurements averaged out to one data point. The newly formed layers show very high hardness of about Hv, which is about four times higher than that of the original VPS-W layer (about 400 Hv). It is considered that such high hardness can be obtained only by the formation of tungsten carbide. In the case of 1800 C annealing, the hardness at the interface region of about 20 m thickness, where W Re multi-layers were originally formed, is about Fig. 4. Cross-section of VPS-W/CFC after 1 h annealing at 1600 C (a), 1800 C (b) and 2000 C (c).

5 X. Liu et al. / Fusion Engineering and Design 70 (2004) Table 1 Experimental conditions and results of electron beam cyclic tests Specimen type Thickness ( m) P av (MW/m 2 ) Pulse/interval (s) Surface temperature ( C) Cycles Results IPS-W/Cu / Failure IPS-W/Cu (T) 200 (W) (interlayer) 35 3/ Failure IPS-W/C / Local cracks VPS-W/CFC / No failure 750 Hv. This rather low hardness indicated that only part of tungsten carbide was formed in this region and the dominant phase was the mixture of Re and C. In contrast, the entire coated layer at the central region of irradiated zone has rather homogenous hardness for the 2000 C case. However, it must be stressed that the thickness of the coating in this region has been reduced by several percents, which indicates that the original W Re layers have been dissolved in CX-2002U C/C composite due to the mutual diffusion of Re and C, es- Fig. 5. Hardness as a function of distance from the interface between CX2002U and VPS-W after annealing at 1800 C for 1 h (a) and 2000 C for 1 h (b). pecially rapid diffusion of active carbon atoms towards the Re and W layers at high temperature. Similar phenomena have been observed in CVD-W/Mo coating, where molybdenum substrate dissolves tungsten coating and induces the reduction of the thickness of CVD- W coating at 2500 C [16]. The results indicate that the W Re multi-layer acts no longer as effective diffusion barrier for carbon in this case Cyclic heat load tests Cyclic heat load testing of plasma sprayed tungsten coatings was performed by a 60 kev electron beam. The samples were mechanically fixed on a copper heat sink actively cooled by water. The heat flux absorbed by the specimen was determined from the measured current through the specimen and incident energy density can be changed by adjusting electron beam diameter and current. In the present experiment, an electron beam of 4.5 mm diameter with Gaussianlike spatial distribution was used. Average power density was 35 MW/m 2 and the maximum heat flux was 50 MW/m 2 in the central zone of 1.5 mm diameter. The pulse/interval lengths were 3/40 s. The interval time is chosen to ensure every thermal cycle to cross the ductile-to-brittle transition temperature (DBTT) of the tungsten coating. The aim is to simulate the worst conditions for normal operation of plasma in fusion devices. The surface temperature of the samples was measured by a two-color optical pyrometer ( C), which was calibrated with a thermocouple before measurements. The temperature of Cu heat sink, detected by a thermocouple, showed a rise of only several degrees during the heat load. Table 1 lists the experimental conditions and main results of the electron beam cyclic tests, in which P av denotes average power density determined by the measured current through the specimen. An important factor influencing surface temperature is the thermal conduction from the coating to the substrate or heat sink.

6 346 X. Liu et al. / Fusion Engineering and Design 70 (2004) Fig. 6. Peak surface temperature of IPS-W coatings vs. number of cycles. For the two kinds of W/Cu coating, Cu and W transition interface improves the thermal conduction and induces lower surface temperature of W/Cu (T). In the cases of tungsten-coated carbon, higher surface temperature of IPS-W/C compared with VPS-W/CFC caused by lower thermal conductivity of IPS coating and SMF700 graphite. Generally, it is believed that the high porosity and impurity level, such as oxygen, will significantly reduce the thermal conductivity of IPS coating, although no detailed data were reported. Additionally, SMF700 graphite [14] has lower thermal conductivity than CX-2002U C/C composite [17] by a factor of about 4. For IPS tungsten coating (W/C, W/Cu and W/Cu (T)), failures by cracking and debonding were found after different cycles. Since all these failures could degrade the thermal conduction of specimen to a certain extent and induce surface temperature rise, the rapid increase of surface temperature usually should be a signal of the failure occurrence. Fig. 6 shows a relationship of peak surface temperatures versus number of cycles. The data points in this figure come from the average of adjacent ten temperature measurements except the situation where the temperature jumped up between succeeding two cycles. It is demonstrated that the peak surface temperatures of W/Cu, W/Cu (T) and W/C jumped to a higher level (about C) after 111, 172, 300 cycles, respectively. Local debonding of W coating was found and a protruding surface was observed by 113 cycles and 174 cycles, respectively for W/Cu and W/Cu (T). Further thermal cycles will lead to the melting of coatings. Metallographic observations indicated the fatigue fracture for both of the coating specimens occurred at the interface of the coating and the substrate. In the case of IPS-W coated SMF700 graphite, local cracks appeared at the interface and in the surface layer of tungsten coating after 300 cycles, showing the characteristic feature paralleled to the surface. These could be due to the lamellar structure and local thermal stress resulting from high temperature gradient in the coating and interface as post-mortem EDS analysis indicated that WC phase had formed at the interface region. Meanwhile the cracks degraded the thermal conduction of the sample and induced a higher surface temperature as shown in Fig. 6. However, obvious crack propagation was not observed by further thermal cycles, this was in coincidence with the stable surface temperature from 300 to 400 cycles as shown in Fig. 6. It is believed that high porosity of IPS coating provides a crack-arresting mechanism, so that stresses arising in IPS coating during thermal load can be relieved by pores and limited crack propagation. Garcia-Rosales et al. [9] reported that IPS-W coated graphite showed localized cracks but did not induce large failure. Present experiments are in good agreement with their results. Post-mortem investigations indicated that no visible damages were found in the case of VPS-W/CFC by 1000 cycles. Temperature measurements showed that the peak surface temperature was around 750 Catthe beginning stage of the cyclic load and many bright spots appeared in the irradiated zone. These spots were gradually reduced with the increase of cycles and finally disappeared after about 200 cycles and the maximum surface temperature sustained on a slightly higher level ( C) up to 1000 cycles. Taking into account that these bright spots were not observed for polished tungsten surface, we believed that this phenomenon resulted from local overheating of the surface tungsten particles, in particular the protruding parts of sprayed particles due to their one- or two-dimensional contact with the periphery, and the un-uniform spatial distribution of heat flux was not the main reason. In fact, the irradiation under higher heat flux as shown in the next section induced a relatively smooth irradiated surface only by several cycles and a lot of small fragments ( 0.2 m), which could come from the splashing of

7 X. Liu et al. / Fusion Engineering and Design 70 (2004) Fig. 7. Surface SEM images of VPS-W/CFC after five cycles (a) and 11 cycles (b), and the section SEM image (C) at 70 MW/m 2 and 12 s pulse. the melted tungsten particles or the evaporation of the protruding part of surface particles, were deposited on the periphery of irradiated zone Cyclic heat load tests of VPS-W/CFC under higher heat flux In order to investigate the high heat flux behaviors of VPS-W/CFC at heat load conditions where surface temperature is over recrystallization temperature, a thermal cycle test was performed by stepwise increase in heat flux until the surface temperature of VPS- W/CFC is up to C. No visible cracks were found after 60 cycles, whereas cracks were observed in the similar experiments performed in the heat load facility in Kyushu University of Japan where an electron beam with spot-like profile was used. A crack crossed irradiation area was found by five cycles at 70 MW/m 2 and 12 s pulse (surface temperature of VPS-W/CFC is C too). The original cracks became longer and wider, and new cracks were created by further thermal cycles as shown in Fig. 7a and b. SEM image of the cross-section revealed that the cracks propagated into the inner of tungsten coating and ended at rhenium layers due to its higher ductility (Fig. 7c). In previous experiments, heat load tests of VPS- W/CFC were performed by spot-like [10] and uniform electron beam [12], and cracks were found only in the former. Crack creation was attributed to the local thermal stress coming from the un-uniform electron irradiation in Refs. [10,12]. Considering that cracks do not appear in the present experiment (a Gaussian-like electron beam was used), it is expected that the local thermal stresses does not exceed the ultimate stress of tungsten coating in this case. Of course, detailed stress analysis is required to confirm this. Nevertheless, it can be seen from Fig. 7, once cracking starts, crack propagating and extending will take place by further cycles, which is different from the situation of IPS-W coating discussed in the section above. Usually, it is believed that the release of elastic strain energy as the stresses become relaxed can offer the driving force to cracking and coating debonding, and the quantity of energy released rises linearly with coating thickness. Therefore, the high elastic strain energy stored within such thick (0.5 mm) VPS coating arising from thermal deformation during the heat load drives crack propagation, though porosity in VPS coating can be more or less helpful to stress release and to stop crack propagation. Additionally, local plastic deformations were also observed on the surface of the tungsten coating in this case, which commonly appeared at the top and the periphery of cracks as shown in Fig. 8. It indicated that Fig. 8. SEM image of surface crack and local plastic deformation of VPS-W/CFC after 11 cycles, 70 MW/m 2, 12 s (surface temperature: C).

8 348 X. Liu et al. / Fusion Engineering and Design 70 (2004) cracks could be ceased by local plastic deformations too. 4. Summary Based on the anneal and cyclic heat load tests of plasma sprayed tungsten coating on carbon and copper substrates, the thermal responses of the tungsten coating tested in this work can be summarized as follows. Firstly, IPS tungsten-coated copper shows the worst thermo-mechanical properties among all tested coatings, although some improvements have been achieved by using graded transition interface. Problems related to W coated copper are low adhesion and the high residual stresses following the deposition process due to the CTE difference of W and Cu. An appropriate transition interface is required to obtain high quality W coating on Cu. Secondly, for IPS-W/C, both the results of annealing and cyclic loading indicated the formation of brittle tungsten carbides at the interface, which strongly degraded the bonding capability of the coating. Although local cracks formed in the surface layer of the tungsten coating and at the interface of W and graphite and induced a higher surface temperature after 300 cycles under 35 MW/m 2 heat flux, further cycles did not induce disabling failure of the coating due to crack-arresting mechanism of porosity. The experimental results indicated that a suitable intermediate layer for prohibiting carbon diffusion is also critical. VPS-W/CFC coating with a diffusion barrier of W and Re multi-layers shows the best thermal properties. No microstructural changes were found at 1300 C annealing for 1 h. However, the mixture of W, Re and C at the interface was observed with the increase in annealing temperature. Tungsten carbides were developed at interface while temperature was beyond 1600 C for 1 h incubation time and the thickness of tungsten carbides rapidly increased with temperature rise. It is an indication that this kind of VPS tungsten coating can be safely used below 1300 C but should not be used at much higher temperatures. Thermal fatigue tests showed no visible damages with 1000 cycles under a surface heat flux of 35 MW/m 2 heat flux for 3 s duration, but crack formation and crack propagation were observed under higher heat load (70 MW/m 2, 12 s pulse duration, surface temperature: C) by using spot-like electron beam. The crack creation was attributed to local thermal stresses coming from un-uniform electron irradiation and the crack propagation was due to high elastic strain energy within the coating arising from heat loading. Meanwhile local plastic deformations were also found, and the crack propagation could be prohibited more or less by porosity and local plastic deformations. References [1] M. Merola, I. Bobin-Vastra, A. Cardella, et al., Manufacturing of a full scale baffle prototype for ITER with a CFC and W plasma spray armour, Fus. Eng. Des (2000) 289. [2] N. Noda, V. Philipps, R. Neu, A review of recent experiments on W and high Z materials as plasma-facing components in magnetic fusion devices, J. Nucl. Mater (1997) 227. [3] N. Yoshida, Review of recent works in development and evaluation of high-z plasma facing materials, J. Nucl. Mater (1999) 197. [4] X. Liu, N. Yoshida, N. Noda, F. Zhang, Z. Xu, Y. Liu, Erosion and erosion products of tungsten and carbon-based materials irradiated by a high energy electron beam, J. Nucl. Mater (2003) 399. [5] T. Tanabe, M. Wada, T. Ohgo, V. Philipps, M. Rubel, A. Huber, J. von Seggern, K. Ohya, A. Pospieszczyk, B. Schweer, TEXTOR team, application of tungsten for plasma limiters in TEXTOR, J. Nucl. Mater (2000) [6] M. Rubel, V. Philipps, A. Pospieszczyk, T. Tanabe, S. Kotterl, Overview of fuel retention in composite and tungsten limiters, J. Nucl. Mater (2002) 111. [7] K. Krieger, H. Maier, R. Neu, ASDEX upgrade team, conclusions about the use of tungsten in the divertor of ASDEX upgrade, J. Nucl. Mater (1999) 207. [8] R. Neu, R. Dux, A. Geier, et al., New results from the tungsten programme at ASDEX upgrade, J. Nucl. Mater (2003) 116. [9] C. Garcia-Rosales, S. Deschka, W. Hohenauer, et al., Highheat-flux loading of tungsten coatings on graphite deposited by plasma spray and physical vapor deposition, Fus. Technol. 32 (1997) 263. [10] K. Tokunaga, N. Yoshida, N. Noda, T. Sogabe, T. Kato, High heat load properties of tungsten coated materials, J. Nucl. Mater (1998) 998. [11] S. Deschka, C. Garcia-Rosales, W. Hohenauer, et al., Manufacturing and high heat flux loading of tungsten coatings on fine grain graphite for the ASDEX upgrade divertor, J. Nucl. Mater (1996) 645. [12] K. Tokunaga, N. Yoshida, N. Noda, et al., Behavior of plasmasprayed tungsten coatings on CFC and graphite under high heat load, J. Nucl. Mater (1999) [13] K. Tokunaga, T. Matsubara, Y. Miyamoto, et al., Changes of composition and microstructure of joint interface of tungsten coated carbon by high heat flux, J. Nucl. Mater (2000) 1121.

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