EUROFER97 interlayers for joints in helium-cooled divertor components

Journal of Nuclear Materials 436 (2013) 29–39

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Functionally graded vacuum plasma sprayed and magnetron sputtered tungsten/EUROFER97 interlayers for joints in helium-cooled divertor components T. Weber a,⇑, M. Stüber b, S. Ulrich b, R. Vaßen c, W.W. Basuki a, J. Lohmiller a, W. Sittel a, J. Aktaa a a

Karlsruhe Institute of Technology, Institute for Applied Materials-Materials and Biomechanics (IAM-WBM), Karlsruhe, Germany Karlsruhe Institute of Technology, Institute for Applied Materials-Applied Materials Physics (IAM-AWP), Karlsruhe, Germany c Forschungszentrum Jülich, Institute of Energy and Climate Research (IEK-1), Jülich, Germany b

a r t i c l e

i n f o

Article history: Received 27 July 2012 Accepted 4 January 2013 Available online 21 January 2013

a b s t r a c t Two coating technologies, magnetron sputtering and vacuum plasma spraying, have been investigated for their capability in producing functionally graded tungsten/EUROFER97 layers. In a first step, nongraded layers with different mixing ratios were deposited on tungsten substrates and characterized by nanoindentation, macroindentation, X-ray diffraction, transmission, Auger and scanning electron microscopy. The thermal stability of the sprayed layers against heat treatments at 800–1100 °C for 60 min was further analyzed. In a second step, the produced functionally graded layers deposited on tungsten substrates were joined to EUROFER97 bulk-material by diffusion bonding. The bonding and the graded joints were microscopically characterized and exposed to thermal cycles between 20 °C and 650 °C. Results from this study show that both coating technologies are ideal for the synthesis of functionally graded tungsten/EUROFER97 coatings. This is important in providing insights for future development of joints with functionally graded interlayers. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Very high surface erosion and heat loads caused by concentrated flux of plasma particles are major concerns for the design of divertor components of future fusion power plants. The current helium cooled divertor concept [1] is designed to handle surface thermal flow up to 10 MW/m2. Tungsten and its alloys are considered as refractory as well as structural materials. Due to the inherent brittleness of tungsten at low temperatures, its use as a structural material is limited to the high temperature region (>650 °C) of the component and hence, a joint to another structural material, the ferritic martensitic high chromium steel EUROFER97 [2,3], is necessary. However, the distinct difference in thermal expansion between tungsten (a = 4.4  106 K1 [4]) and EUROFER97 (a = 12.7  106 K1 [5]) causes a mismatch during thermal cycling. This results in stresses and subsequently leads to failure of the joint. An approach to reduce these stresses is to introduce a functionally graded layer between the materials to be joined. Previous elasto-visco-plastic finite element simulations had estimated the potential in the use of functionally graded materials [6]. Considering application relevant loadings, the effects of variation of the thickness and the transition function within the graded layer was analyzed. The evaluation of the resulting stresses and deformations have confirmed the advantages of a linearly graded ⇑ Corresponding author. Tel.: +49 176 62174699. E-mail address: [email protected] (T. Weber). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.01.286

interlayer between tungsten and EUROFER97. The production of the graded interlayer can be realized by the herein considered fabrication methods. Magnetron sputtering (PVD) and vacuum plasma spraying (VPS) have been identified [6] as promising methods to fabricate functionally graded layers. Two major advantages of the selected methods are: (i) the ability to achieve a full range of graded chemical composition for both materials and (ii) the avoidance of oxide formation, since both processes are conducted in vacuum. Both methods have already been successfully applied to the fabrication of functionally graded W and Cu coatings [7–10]. For many decades, tungsten carbide alloys are joined with steel in the drill head or in the cutting industry. The basic problem (thermal-induced stresses) is the same as in the tungsten/EUROFER97 system considered here. One way to avoid these stresses is to use mechanical joints, such as clamping, screwing or shrink joints [11]. However, these techniques are not available in the design of a divertor component, since they do not allow a helium-tight joint. The other alternative is brazing. Depending on part geometry and specific tungsten alloy a wide range of commercial brazing alloys consisting of copper, silver, zinc, palladium, gold, cadmium, nickel, manganese and/or cobalt, are available. The melting temperature of these alloys ranges from 600 °C to 1120 °C [12]. For this purpose a sufficient plasticity and thickness of the brazing alloy has to be taken into account for a successfully brazed joint. However, preceding brazing tests of tungsten with EUROFER97 at 1180 °C with a nickel-based brazing alloy failed for the specific divertor

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T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

component geometry [13]. In addition, to enable neutron irradiated material to be recycling friendly, high activation elements such as Nb, Mo, Ni, Cu, Al, Si and Co should be avoided [14,15]. The use of silver-and copper-based brazing alloys would be at best a compromise due to its relative low mass compared to the bulk component. Another alternative would be a direct joint between tungsten and EUROFER97. However, diffusion bonding of both materials at 1050 °C does not result in an adequate final outcome [16]. The application of functionally graded tungsten/steel joints is not only limited to divertor component, but also widely used in plasma-facing materials, which are also actively cooled. Due to its low erosion rate, tungsten is an attractive candidate in the material selection, since the plasma is thus protected from contamination. 2. Experimental 2.1. Vacuum plasma spraying Four batches of non-graded samples composed of EUROFER97 and tungsten were produced by VPS at different mixing ratios. Subsequently, functionally graded coatings were produced with a thickness in the range of 200 lm–1 mm. The deposition process involved using a F4 plasma gun from Sulzer–Metco (Wohlen, Switzerland) with an output power of 50 kW and operating at a spray distance of 30 cm. Before coating, transferred arc cleaning [17] was employed. For the first and second batch, W powder (AMPERITÒ 140.067, H.C. Starck, Germany) with an average particle size of 9–13 lm (measured by laser light diffraction) was used. The mass fractions of Fe, O, N and C are lower than 0.1%. Because of difficult handling of the powder and its agglomeration as a consequence of the small particle size, another tungsten powder (AW3105A1, Eurotungstene, France) was used for the 3rd and following batches. It has an average particle size of 15–20 lm (also measured by laser light diffraction) and a purity better than 99.92 wt.%. The second raw material was EUROFER97 powder (spheroidized in nitrogen, see Table 1) with a particle size of d50 = 52.5 lm. In the first batch, the powder was used as-received. It was sieved for the second and the following batches so that the maximum particle size was limited to 57 lm. Both powders were injected separately into the plasma plume during the deposition process with an optimized feeding gas flow. The deposition of the sprayed coatings took place on sandblasted WL10-substrates, with the dimensions set to be 10 mm in height and 12.7 mm in diameter. WL10 is a creep-resistant tungsten alloy from the manufacturer Plansee (Austria). It contains 1 wt.% La2O3. The reason for the use of WL10 instead of pure W is the high creep resistance requirement at the head of the thimble, where temperatures above 1100 °C are expected in combination with stresses around 300 MPa [18]. 2.2. Magnetron sputtering The deposition experiments were done on a Leybold Z550 sputtering machine under a non-reactive RF-mode. The diameter of the target is 75 mm and its thickness is 4 mm. The target to substrate distance was set to be 45 mm. Prior to deposition, the surfaces of targets and substrates were cleaned by a 5 min pre-sputtering of

Fig. 1. Sketch of the experimental setup with segmented target und six substrates.

the target (by using a shutter in front of the target) and by a 15 min plasma-etching of the substrates (at an Argon gas pressure of 0.6 Pa). All deposition experiments were performed without substrate biasing, i.e. the substrates were grounded and unheated (effective substrate temperature was approximately 100–150 °C). The concept of manufacturing the thimble for the divertor component is to deposit a graded layer starting with 100% tungsten on a WL10 substrate. The graded layer should end up with a one hundred percent EUROFER97 concentration, so that the supporting steel structure can be connected by diffusion bonding. Therefore, various deposition experiments were carried out in order to evaluate the sputtering characteristics of the individual materials. Sputtering from both monolithic tungsten and EUROFER97 targets was done in the first experiments. In these experiments, a pure tungsten target and a EUROFER97 target were used. In order to synthesize the functionally graded W–FeCr layers, another experimental setup was applied: a ‘‘segmented’’ target which consists of two circular half plates, one made of tungsten, and the other made of EUROFER97 (see Fig. 1) was used. This setup allows deposition of monolithic W–FeCr mixed coatings of various compositions by placing a number of substrates opposite to the target along the projection line from the W-half plate to the EUROFER97 half plate. As a result, each substrate position has a specific W–FeCr composition. The synthesis of the functionally graded layers was then realized via a multilayer approach. In other words, a substrate sample is coated by several deposition processes. This is done by placing the same sample first at a position under the tungsten target, and then in the next process, at another position and finally, ends with the final deposition step where the sample is placed at a position under the EUROFER97 target. Consequently, a graded layer could be realized by a four steps multilayer design. For the magnetron sputtering experiments also the tungsten alloy WL10 from the manufacturer Plansee has been chosen as substrate. The diameter of each substrate is 12 mm and the height is 4 mm. Table 2 summarizes the process parameters and Table 3 lists the chemical composition of the EUROFER97 target. The tungsten target with a purity of 99.97 wt.% was produced by Plansee. 2.3. Microstructural analysis The porosity and composition ratios of the sprayed samples were analyzed by optical microscopy (OM) and scanning electron

Table 1 Chemical composition in wt.% of EUROFER97 powder used for the VPS-experiments [19]. Cr

C

Mn

V

W

Ta

Ni

Cu

Co

O

N

Si

S

9.70 ±0.01

0.097 ±0.001

0.38 ±0.0048

0.186 ±0.002

1.096 ±0.004

0.037 ±0.001

0.0143 ±104

0.0067 ±104

0.0035 ±104

0.023 ±0.003

0.048 ±0.002

0.023 ±0.0005

0.0073 ±104

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T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39 Table 2 Used process parameters in the PVD-experiments. Target material Type

W RF-Sputtering

EUROFER97 RF-Sputtering

W/EUROFER97 RF-Sputtering

W/EUROFER97 RF-Sputtering

Pressure Gas Bias–voltage Power Substrate temperature Layer thickness Deposition rate Distance target/substrate

0.6 Pa Argon 0V 500 W 100–150 °C 10.6 lm 3.6 nm/s 45 mm

0.4 Pa Argon 0V 400 W 100–150 °C 11 lm 0.7 nm/s 45 mm

0.6 Pa Argon 0V 200 W 100–150 °C <2 lm <0.27 nm/s 45 mm

0.8 Pa Argon 0V 250 W 100–150 °C 7 lm 0.24 nm/s 45 mm

Table 3 Chemical composition in wt.% of EUROFER97 (heat 993394) used as target material in the PVD-experiments [20]. Cr

C

Mn

V

W

Ta

Ni

Cu

Co

O

N

P

S

8.95 ±0.05

0.106 ±0.001

0.544 ±0.005

0.35 ±0.091

1.109 ±0.012

0.197 ±0.053

0.0157 ±0.0037

0.003 ±0.0021

0.0075 ±0.0026

0.0012 ±0.0001

0.036 ±0.001

<0.0002 –

<0.001 –

microscopy (SEM). For the OM investigations a DMLM from Leica was used and for the SEM investigations a Philips XL Series SEM was used applying a primary electron acceleration voltage of 30 kV. Cavities within the coatings appear to be very dark in the micrographs from OM, since the light is well absorbed there. This allows to determine an accurate porosity value. The composition ratios were calculated by evaluating the number of pixels corresponding to tungsten, EUROFER97 or a cavity. Since the contrasts of both materials are quite different by OM as well as by SEM, the composition ratios can be well determined. The chemical composition of the functionally graded magnetron sputtered coatings was analyzed using Auger electron spectroscopy (AES) before and after diffusion bonding with the EUROFER97 bulk material. The composition gradient along the coating was determined by analyzing an angled polish of the samples. The analysis was carried out using a Perkin–Elmer PHI 680 Auger Nanoprobe operating at a primary beam energy of 10 kV and an electron current of 20 nA. Sample cleaning was performed using 2 keV argon ions. Atomic concentrations were calculated using the elemental sensitivity factors SFe = 0.276, SCr = 0.739 and SW = 0.513 and the commercial software PHI-Multipak. The transmission electron microscope (TEM) investigations were performed at an accelerating voltage of 200 kV using a FEI Tecnai 20 FEG microscope equipped with a Gatan Image Filter (GIF) for EFTEM measurements. The TEM lamellas were prepared out of the sputtered coatings using a focused ion beam (FIB) for cutting and thinning. The X-ray diffraction (XRD) measurements were performed on a D-500 system (Siemens, Germany), using copper Ka radiation with a wavelength of k = 0.154056 nm and a scintillation detector. The step size was 0.04° and the angular range 37–90° on the 2h scale for both configurations (Bragg–Brentano (BB) and grazing incidence (GI) at an angle of incidence of 4°). Since the thicknesses of the magnetron sputtered coatings with mixed compositions are smaller than the penetration depth of the diffracted X-rays, a sharp W-peak from the substrate is superimposed on the diffraction signal from the coating. As a result, overlapping peaks in BB geometry complicate the grain size analysis and additional XRD measurements were performed in GI geometry, avoiding the substrate signal. Grain size and microstrain were computed with the Single Line Method (SLM) [21]. The SLM divides the integral breadth of a single reflex into Lorentzian and Gaussian shares via the ratio of full width at half maximum to integral breadth [22]. These shares were corrected from instrumental broadening using

a coarse grained tungsten reference sample. Grain size, D, and microstrain, e, are calculated from the corrected shares, bfL and bfG , according to:

D¼

e¼

k bfL cosðhB Þ bfG 4 tanðhB Þ

ð2:1Þ

ð2:2Þ

where ffiffiffi the Bragg angle. The root-mean-square microstrain ffiffiffiffiffiffiffiffiffi hBqis p he2o i ¼ p2  e is calculated and used for the further discussion. The local Young’s Modulus and the Berkovich Hardness were measured by a Nano Indenter XP device using a Berkovich tip at an indentation depth of 200 nm. Constant stiffness data measurements were obtained by oscillating the tip during indentation with a frequency of 45 Hz [23]. This measurement provided hardness, elastic modulus and stiffness data continuously along indentation depth. Hardness values were taken at a depth of 180–190 nm and compared to the calculated hardness resulting from the unloading curve [24], but no significant difference could be identified. In order to account for the abrasion of the tip, it was calibrated using a quartz crystal as well as pure tungsten raw material. By means of an atomic force microscope from the manufacturer Digital Instruments of the type Dimension 3100 a slight sink-in along all sputtered coatings could be detected as it is foreseen in the hardness calculation by Oliver and Pharr [24]. The Brinell–Hardness was measured on a Zwick testing machine with a spherical diamond tip, which has a diameter of 2 mm. The maximum applied force hereby was 1 kN, the holding time was 10 s and the loading rate was 10 N/s. The diameter of the indent in the specimen was determined by OM. Measured values obtained by the mentioned analyzing methods were compared with reference values belonging to pure bulk EUROFER97 plate material (heat 993402 in [20]) and pure sintered tungsten rod material from Plansee. Grinding with silicon carbide abrasive paper with successively decreasing grain size was the first step of the metallographic preparation of the samples, followed by a polishing procedure with diamond suspension. The diamond particle size in the suspensions was 9, 6 and 3 lm. The sample preparation procedure was finished by polishing with an oxide polishing suspension of 0.06 lm grain size.

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Fig. 2. SEM cross-section view of the sprayed layers showing (a) high porosity and unmelted particles in a first batch-sample; (b) multilayer-like structure of a second batchsample; (c) uniform micro structure of a third batch sample and (d) increase of thickness in the fourth batch. (Bright contrast = tungsten, dark contrast = EUROFER97).

2.4. Heat treatment

3. Results and discussion

The mechanical properties and microstructure of EUROFER97 can be recovered by normalization at 980 °C for 30 min followed by tempering heat treatment at 760 °C for 90 min [20]. In this work, the second heat treatment was considered only in order to preserve the samples from high thermally induced stresses. Air quenching is necessary for normalization to allow an adequate cooling rate for the formation of martensitic phases. A high cooling rate is not required for the second heat treatment, where martensitic phases were transformed into ferritic phases with a high dislocation density. Thus, the heat treatment and cooling down processes were performed in a vacuum to avoid formation of oxides. In a previous work [25], it was demonstrated that sintered EUROFER97 regains its original Berkovich hardness value after tempering heat treatment at 760 °C. However, a measurement of the EUROFER97 powder used in this work revealed that the hardness is already increased due to the spheroidization process and not only caused by the thermal spraying. Additional heat treatments beyond 760 °C were performed at 800 °C/900 °C/1000 °C/1100 °C for 60 min in vacuum revealing the thermal stability of the produced composites.

3.1. Vacuum plasma spraying

2.5. Diffusion bonding The experimental setup for the diffusion bonding performed in this work was the same as previous studies [16,26]. The temperature was 800 °C, the pressure 60 MPa, the bonding time 2 h and the vacuum pressure better than 105 mbar. In preparation for the diffusion bonding, the surfaces to be joined were polished with diamond suspension as described in Section 2.3 and cleaned twice in an ultrasonic bath with acetone for 5 min.

2.6. Thermal cycling Structural materials and components in fusion reactors based on the Tokamak principle are subjected to thermal cycling during the operation of a reactor. In order to assess the suitability of the produced joints in this work, thermal cycling tests were performed in vacuum. The off-time at room temperature lasted 1 h and was followed by a dwell time period for 19 h at 650 °C. The ramping of the temperature curves lasted about 20 min when heating up with a maximum heating rate of 1.9 K/s and about 1 h when cooling down with an initial rate of 1.0 K/s.

3.1.1. Constitution and properties of sprayed monolithic coatings Although the fabrication of functionally graded vacuum plasma sprayed layers was already done with W and Cu materials, the spraying process had to be adapted and optimized step by step for tungsten and EUROFER97. This optimization procedure was initiated by the production of non-graded samples comprising four batches, whereby one aspect was improved in each batch. The process parameters used in the 4th batch were then applied in the subsequent deposition of graded coatings. Finally, the successfully produced functionally graded sprayed W/EUROFER97 coatings could be used as interlayer for diffusion bonding between tungsten and EUROFER97 bulk material. The first aspect was to check the feasibility of producing composites consisting of EUROFER97 and tungsten. This was realized in the first batch of samples. These samples exhibited a rather high porosity up to 20% (determined by optical measurement) and unmelted EUROFER97 particles (Fig. 2a). Thus, the process parameters were optimized and the EUROFER97 powder was sieved before the second batch of samples was fabricated. Fig. 2b illustrates the surface morphology of the polished cross section and reveals a low porosity (<4%) along all samples in the second batch. The spherical shape of the used EUROFER97 powder particles which has a darker contrast than tungsten in the SEM images can be recognized sometimes, but is deformed due to melting of the particle in the most cases. Since both materials were injected separately into the plasma plume, their trajectories do not hit necessarily the same spot on the substrate. The combination of this circumstance with the movement of the plasma gun means that both materials are deposited alternately and not simultaneously. As a consequence, the microstructure is multilayer-like (Fig. 2b). Subsequently, both trajectories were focused on each other so that a uniform microstructure in each spatial direction could be achieved in the third batch (Fig. 2c). The fourth step confirmed the possibility to produce thick coatings, with an approximate thickness of 1 mm. This is necessary to reduce thermal induced stresses in the considered issue [4]. Although the quality characteristics of the fourth batch coatings are satisfactory, the adhesion is not sufficient, as gaps in the substrate were observed, as illustrated in Fig. 2d. This issue will be discussed later in this chapter. Based on the images taken by optical and scanning electron microscopy, a microstructure of good quality with no cracks can be observed. However, cavities with a size up to 20 lm are strewn along the sample volume, identifiable by the black areas in Fig. 3.

T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

33

Fig. 3. SEM images showing VPS samples of the 4th batch with three different mixing ratios.

Nanoindentation usually allows measurement of local mechanical properties like Young’s Modulus and Berkovich–Hardness in the scale of few microns being quite interesting for the plasma sprayed samples with mixed ratios. Unfortunately, the intermixture of tungsten and EUROFER97 is too high, impeding a suitable measurement. The results from the macroindentation tests reflect mechanical properties on a scale of a few millimetres, so that the local influences from the EUROFER97 particles measuring up to 57 lm are averaged. A hardness increase proportional to the tungsten concentration can be observed within each batch in all samples. While the hardness values of the first batch samples are lower due to the

high porosity, the values corresponding to the other batches are lying in-between those of the reference materials. Each data point in Fig. 4 represents the quantile-quartiles of 10 indents. A few cracks around the indents as well as a slight sink-in effect can be observed with the optical microscope for the first and second batch samples. The indents in the third and fourth batches contain no cracks. In Fig. 4, 1three red arrows mark the samples, whose layer thicknesses are too low for a valid hardness test. As the indentation depth is in

1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article.

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T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

3.2. Magnetron sputtering 7

WL10 EUROFER97

5 4 3 2 1 0

1st batch

2nd batch

3rd batch

4th batch

0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

Brinell hardness in GPa

6

tungsten concentration in vol.% Fig. 4. Brinell–Hardness of the VPS samples.

the range of 50–80 lm and the layer thickness is about 200 lm, an increased hardness value caused by the substrate effect can be substantial in these cases. Heat treatments at different temperatures for 1 h revealed insights on the thermal stability of the sprayed tungsten/EUROFER97 composites. For all the tested temperatures, no cracks were visible by the SEM, neither in the EUROFER97 nor in the tungsten regions (Fig. 5). Gaps between both materials arose when the temperature was equal or higher than 1000 °C. However, the temperature should not exceed 800 °C as the intermetallic phase Fe2W will be formed already at 900 °C. It can be deduced that the second heat treatment of EUROFER97 at 760 °C/90 min, as discussed in Section 2.4, can be applied without damaging the microstructure of the sprayed composites. 3.1.2. Sprayed functionally graded coatings After the characterization of ungraded samples, functionally graded coatings were produced by VPS. The gradient was realized by adjusting the powder feeding rate during the deposition process. The process parameters were the same as already used for the production of the fourth batch. Unfortunately, the graded coatings peeled off from the WL10 substrates after deposition. The interfacial strength is insufficient for the high thermally induced stresses coming from the material gradient. The reason for the poor interfacial strength might be that the tungsten substrate roughness was not high enough as bonding is only formed by mechanical clamping. However, the functionally graded layers were of high quality with a porosity lower than 4% and free of cracks. To overcome the lack of interfacial strength, a thin vanadium foil with a thickness of 20 lm was used for the bonding to WL10. No additional production step is necessary as the V-interlayer can be joined to the tungsten using the same diffusion bonding parameters. Such an interlayer is necessary, since diffusion at 800 °C between two tungsten surfaces is not sufficient. The SEM analysis of the diffusion bonding experiment with EUROFER97 and WL10 bulk material revealed no cracks at all (Fig. 6). One diffusion bonded sample was cut into a tube with 12 mm in diameter and 1 mm in wall thickness by wire eroding. This sample was subsequently thermally cycled 10 times in vacuum between 650 °C and room temperature. The result of this test is shown in Fig. 7. No cracking occurred within the vanadium and VPS interlayers, proving that their microstructure is capable to withstand the thermal cycling. In some cases, short cracks with a length of 5– 20 lm can be recognized by SEM along the welding line, coming from the outer faces.

3.2.1. Constitution and properties of magnetron sputtered monolithic coatings The development of the functional graded coatings was achieved by step-wise experiments. The first step proved the feasibility to sputter pure EUROFER97 and pure tungsten coatings on tungsten and WL10 substrates, respectively. The cross section view in Fig. 8 of the layers confirms that a layer thickness of at least 10 lm was achieved. All sputtered coatings in this work exhibit a dense micro structure with no remarkable porosity after a slight polishing with oxide polishing suspension. The second step was the realization of coatings with mixed tungsten/EUROFER97 concentrations on a WL10 substrate (Fig. 9). In this experiment, six substrates were placed stationary beneath the segmented target. In order to reduce sputtering time, the layer thickness was kept around 2 lm. For the purpose of reducing thermal induced stresses in a graded joint between WL10 and EUROFER97, a gradation range from 0 to 100% is beneficial. The achieved range here was between 33 and 87 at.% for the tungsten concentration. Thus, the gain of shielding plates was investigated, but unfortunately not fruitful. The reason therefore was an inappropriate influence on the electromagnetic fields, preventing a stable plasma and deposition process. The use of two separate targets [7,27] inclined to each other and inclined to the substrate might be a possibility to achieve a higher gradation range. Diffraction patterns recorded in Bragg–Brentano configuration show the a-phase for all samples. (1 1 0) a-W, (2 0 0) a-W, (2 1 1) a-W and (2 2 0) a-W peaks are identified for pure W and alloyed samples (Fig. 10a). Pure EUROFER97 only exhibits the (1 1 0) apeak in the examined 2h-range. It can be noted that with decreasing tungsten concentration the (1 1 0) peak, having the strongest intensity, shifts from the 2h position of the a-W phase to the position of a-Fe. In grazing incidence geometry (Fig. 10b), additionally, b-W is observed for pure W and for alloys with a W-concentration cW P 60 at.%. It seems that due to texture this phase has no planes parallel to the surface, and hence is not seen in BB geometry. Alloys with cW 6 44 at.% exhibit extremely broadened peaks in both setups. The results obtained by applying the SLM are displayed in Fig. 11. Fitting of peaks in BB geometry was not possible for alloys with 60 at.% 6 cW 6 87 at.% due to overlapping with the W-peak from the substrate. Therefore, for these alloys results are only obtained from measurements in GI geometry. For pure EUROFER97 and pure W, largest grain sizes and smallest microstrain were obtained. Values are typical for sputter-deposited films and they are comparable to literature values [28]. The alloys can be divided into two main regions: Alloys in the range of 60 at.% 6 cW 6 87 at.% exhibit reduced grain size and enhanced microstrain as compared to the pure materials. Alloys in the range of 34 at.% 6 cW 6 44 at.% have smaller grain sizes, which are as small as 2 nm, and have extremely high microstrain values. Therefore, it is concluded that these alloys are of extremely fine-grained or even amorphous structure. Differences between the two geometries occur, since the probed lattice planes are oriented differently with respect to the sample surface. Images taken with the TEM confirm the results from the XRDanalysis. The grains of the pure sputtered EUROFER97 coating (Fig. 12a) exhibit a rather round shape, while the grains of the coating with 87 at.% W concentration (Fig. 12c) are of columnar shape (perpendicular to the substrate). In contrast, the sample with 44 at.% W (Fig. 12b) does not provide any pattern due to its amorphous microstructure. This is different from the original EUROFER97 bulk material, which has a microstructure composed of grains with a size of 11–13 lm [20].

T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

35

Fig. 5. SEM images showing the top view of the polished heat treated VPS samples (2nd batch).

Fig. 6. (a) Cross-section view of the VPS layer diffusion bonded to EUROFER97 and WL10. (b) The bonding to WL10 has been realized by a vanadium interlayer with 20 lm thickness.

Fig. 7. Effects of the thermal cycling between RT and 650 °C of the diffusion bonded parts. (a/b) Inner face, (d/e) outer face of the cross section view and (c) cutaway of the tube.

A Berkovich tip was used at an indentation depth of 200 nm, being roughly 1/10th of the layer thickness. By means of a surface profile analysis, it was checked that the surface roughness is low enough, such that the results of the indentation analysis are not

influenced. The mean roughness index Ra for all tested samples was lower than 32 nm, being about 1/6th of the indentation depth. The measured Young’s moduli of the pure sputtered coatings are equal to those of the bulk materials within the measuring

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T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

Fig. 8. SEM images showing a cross section view of the (a) sputtered pure EUROFER97 and (b) pure tungsten coatings.

(a) relative counts (scaled indvidually)

α−W (110)

30

40

α−W (200)

50

(b)

0%W 100%FeCr 34%W 66%FeCr 44%W 56%FeCr 60%W 40%FeCr 74%W 26%FeCr 84%W 16%FeCr 87%W 13%FeCr 100%W 0%FeCr

α−Fe (110)

α−W (211)

60

70

α−W (220)

80

90

2θ

relative counts (scaled indvidually)

Fig. 9. SEM images showing the view from the top of the sputtered mixed EUROFER97/tungsten coatings.

0%W 100%FeCr 34%W 66%FeCr 44%W 56%FeCr 60%W 40%FeCr 74%W 26%FeCr 84%W 16%FeCr 87%W 13%FeCr 100%W 0%FeCr

α−Fe (110)

β−W (200)

30

β−W (222) α−W (110)

40

α−W (200)

50

α−W (211)

60

70

β−W (400) α−W (220)

80

90

2θ

Fig. 10. XRD spectra of the sputtered coatings measured with (a) Bragg–Brentano and (b) grazing incidence configuration.

inaccuracy. Considering the coatings with mixed composition, the measured Young’s moduli can be found between those of the pure materials, showing proportionality to the tungsten concentration. However, since the measuring inaccuracy is high (about 30 GPa) due to the low layer thickness, these results are not published here. Fig. 13 shows the results of the Berkovich hardness measurements of both pure sputtered layers and the six coatings with mixed composition. Each data point in Fig. 13 represents the quantile-quartiles of 20 indents. The high hardness values of the sputtered coatings may point to the presence of a

nano-crystalline microstructure [30], residual stresses [31,32] and a high dislocation density. However, it is noted that argon impurities at concentrations below 1 at.% can dramatically increase the hardness of sputtered tungsten layers [33]. Although such low argon concentrations cannot be completely ruled out by AES, it can be expected that the argon concentration is negligible in view of a bias voltage of 0 V. Similar high Berkovich hardness values were already measured in sputtered nanocrystalline tungsten films, chromium tungsten nitride coatings and sputtered tungsten nitride thin films [34–36].

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T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

(a) 80

(b) 0.06

70

Bragg−Brentano

Bragg−Brentano

grazing incidence

grazing incidence

0.05

rms micro strain

grain size in nm

60 50 40 30

0.04 0.03 0.02

20 0.01

10 0

0

20

40

60

80

100

0

0

20

W concentration in at.%

40

60

80

100

W concentration in at.%

Fig. 11. Results obtained from the Single Line Method: (a) grain size and (b) microstrain dependent on alloy composition. Alloys can be subdivided into two regions: amorphous (34 at.% 6 cW 6 44 at.%) and crystalline (60 at.% 6 cW 6 87 at.%).

(a)

(b)

vacuum

coating

(c)

substrate

Fig. 12. Bright field EFTEM-images of sputtered coatings (a) 91% Fe/9% Cr [29], (b) 44% W/51% Fe / 5% Cr and (c) 87% W/12% Fe/1% Cr (concentrations in at.%). The arrows are pointing to some selected grain boundaries.

30

Berkovich−Hardness in GPa

25

20

15

10 tungsten ref. val.

5 EUROFER97 ref. val.

0 0

10

20

30

40

50

60

70

80

90

100

W conc. in at. %

Fig. 14. Sputtered graded coating on a WL10 substrate joint to EUROFER97 by diffusion bonding at 800 °C.

Fig. 13. Berkovich hardness of the sputtered coatings with different composition ratios.

3.2.2. Magnetron sputtered functionally graded coatings After successful preparation and analysis of the sputtered tungsten/EUROFER97 layers with different compositions, the synthesis of graded coatings was performed as described in Section 2. Finally, a total of four layers with different mixing-ratios were deposited on top of each other. The bandwidth achieved ranges from

33 at.% up to 82 at.% tungsten (determined with EDX). Diffusion bonding of this coating with EUROFER97 bulk material was realized at 800 °C. Fig. 14 shows a SEM cross section image of the welded four-staged graded layer. Occasionally, cracks perpendicular to the substrate within the coating can be observed. Further characterization by nanoindentation determines elastic modulus and Berkovich hardness along the joint (Fig. 15). The results of

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T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

(a) 500

100

measurement A measurement B 80

fraction in at.%

Youngs’s modulus in GPa

400

300

200

W Fe Cr before bonding after bonding

60

40

20 100

WL10 0 −20

(b)

−10

sputtered coating 0

10

EUROFER97 20

30

0 40

distance to subtrate surface in μm 30

−5

0

5

10

15

distance to substrate surface in μm Fig. 16. Concentration gradient measured by AES of the sputtered graded coating (a) before and (b) after diffusion bonding.

measurement A measurement B

Berkovich hardness in GPa

25

20

15

10

Fig. 17. Diffusion bonded sputtered layer, which has been thermally cycled between RT and 650 °C.

5

WL10 0 −20

−10

sputtered coating 0

10

EUROFER97 20

30

40

4. Summary and conclusions

distance to subtrate surface in μm Fig. 15. (a) Young’s modulus and (b) Berkovich hardness of the diffusion bonded graded sputtered coating with EUROFER97.

the ungraded layers (shown in Fig. 13) were largely reproducible. However, a distinction between the individual layers of the fourstepped graded coating is not possible, due to a large scatter in the data. An advantage of the AES with respect to the EDX analysis is that light elements (O, Ar, C, etc.) can be detected more precisely. In this case, the accurate determination of the argon concentration was not possible, as the Auger electrons from tungsten and argon have similar congruent energies. However, an increased oxygen and carbon concentration due to a corresponding phase formation can be ruled out. This is in contrast to the diffusion bonding of EUROFER97 and tungsten at 1050 °C [26], where a concentration of 19.2 at.% carbon has been detected inside an interlayer, which is created during the joining procedure. In our case, a significant change in the chemical composition of the elements in the sputtered layers due to a consequence of diffusion processes could not be found based on the AES analysis (Fig. 16). Also, the diffusion bonded sputtered sample was cut into a tube with 12 mm diameter and 1 mm wall thickness by wire eroding and subsequently thermally cycled 10 times in vacuum between 650 °C and room temperature. Again, short cracks were found along the welding line, propagating from the outer faces. Anyhow, the inner part of the joint is still intact (Fig. 17), in spite of the preexisting vertical cracks prior to thermal cycling.

Two fabrication methods (vacuum plasma spraying and magnetron sputtering) have been successfully applied to the production of functionally graded EUROFER97/tungsten coatings. Both coatings are suitable as interlayer for joining EUROFER97 and tungsten bulk material by diffusion bonding at 800 °C. According to the analysis methods used here, no phase formation or change in chemical composition in the functionally graded layers could be detected after the bonding process. The produced joints were thermally cycled 10 times between 20 °C and 650 °C and proved their basic capability to withstand the operating conditions in a future fusion Tokamak reactor. This is an improvement in comparison to the direct diffusion bonding of EUROFER97 and tungsten at 1050 °C, where all samples fell apart in the post bonding heat treatment [16]. Certainly, the low welding temperature of 800 °C, which avoids the austenite phase formation of EUROFER97 at T > 800 °C [2] and reduced thermally induced stresses during the cooling down, is a very positive circumstance. In addition, the gradation helps to reduce thermally induced stresses as it has been assessed numerically in [6]. However, the real gain and applicability for use of the functionally graded interlayers has to be confirmed with additional experiments. The question on whether crack growth begins with further thermal cycling is not clear. Thus, thermal cycling tests with higher number of cycles are necessary to clarify which joining variant is the best. The interfacial strength relating to fracture and delamination is an important element for the performance of this joining technique. This issue has not yet been addressed. A finite element

T. Weber et al. / Journal of Nuclear Materials 436 (2013) 29–39

analysis considering crack propagation within the tungsten region revealed that the fracture strength of tungsten is too low to stop cracking once it has been initiated [37]. The reason for that is that the fracture strength of tungsten is inherently low and the stress intensity factor increases with increasing crack length. Nevertheless, the thermal cycling experiments (see Fig. 7) showed that small cracks are tolerated without abruptly initiating spontaneous crack propagation. As a result of plastic deformations at the crack tip, the materials apparently have a certain crack growth resistance (R-curve behavior). Hence, the produced joints are capable of withstanding mechanical and thermal loads during the manufacturing process. The more relevant question is whether the subcritical crack growth due to thermal fatigue will lead to a failure in the joint. Although great theoretical effort has been done [38–41] in understanding and predicting interfacial cracking between two dissimilar materials, it is still a very complex issue when adapting this knowledge to the geometry used here. The stress fields are complex due to the significant nonlinearity in the material properties originated from plastic deformation and temperaturedependent properties. In any case, it is clear that the substantially reduced load on the interfaces of functionally graded joints maintain an advantage compared to direct or brazed joints. So, although the mentioned theoretical assessment of interfacial strength still has to be done, the performed experiments in this work indicate a high potential of this approach. Acknowledgements This work was carried out within the Nuclear Fusion Program of the Karlsruhe Institute of Technology and supported by the European Communities under the contract of Association between EURATOM and Karlsruhe Institute of Technology within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The authors thank to Mr. T. Weingärtner for the AES analysis, Mr. R. Lindau for the supply of EUROFER97 powder, and Dr. H. Gesswein for the XRD measurements, who all are affiliated to the Institute for Applied Materials. References [1] P. Norajitra, A. Gervash, R. Giniyatulin, T. Ihli, W. Krauss, R. Kruessmann, V. Kuznetsov, A. Makhankov, I. Mazul, I. Ovchinnikov, Fusion Eng. Des. 81 (2006) 341–346. [2] R. Lindau, M. Schirra, Fusion Eng. Des. 58–59 (2001) 781–785. [3] R.L. Klueh, D.S. Gelles, S. Jitsukawa, A. Kimura, G.R. Odette, B. van der Schaaf, M. Victoria, J. Nucl. Mater. 307–311 (2002) 455–465.

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[4] Plansee AG, Reutte, Austria, Tungsten – material properties and applications, material catalogue, 2010. [5] A. Paúl, A. Beirante, N. Franco, E. Alves, J.A. Odriozola, Mater. Sci. Forum 514– 516 (2006) 500–504. [6] T. Weber, J. Aktaa, Fusion Eng. Des. 86 (2011) 220–226. [7] C. Wang, P. Brault, C. Zaepffel, J. Thiault, A. Pineau, T. Sauvage, J. Phys. D Appl. Phys. 36 (2003) 2709–2713. [8] G. Pintsuk, I. Smid, J.-E. Döring, W. Hohenauer, J. Linke, J. Mater. Sci. 42 (2007) 30–39. [9] J.-E. Döring, R. Vaßen, G. Pintsuk, D. Stöver, Fusion Eng. Des. 66–68 (2003) 259– 263. [10] G. Pintsuk, S.E. Brünings, J.E. Döring, J. Linke, I. Smid, L. Xue, Fusion Eng. Des. 66–68 (2003) 237–240. [11] I.T. Northdrop, J. S. Afr. Inst. Min. Metall. 87 (1987) 125–135. [12] J. Willingham, Filler metals and fluxes for brazing tungsten carbide, johnson matthey metal joining, 2012. [13] T. Chehtov, J. Aktaa, O. Kraft, J. Nucl. Mater. 367–370 (2007) 1228–1232. [14] G. Butterworth, J. Nucl. Mater. 179–181 (1991) 135–142. [15] G. Butterworth, L. Giancarli, J. Nucl. Mater. 155–157 (1988) 575–580. [16] W.W. Basuki, J. Aktaa, J. Nucl. Mater. 86 (2010) 9–11. [17] K.J. Hollis, R.G. Castro, R.P. Doerner, C.J. Maggiore, Fusion Eng. Des. 55 (2001) 437–447. [18] V. Widak, P. Norajitra, Fusion Eng. Des. 84 (2009) 1973–1978. [19] C. Adelhelm, 2011, Karlsruhe Institute of Technology – Institute for Applied Materials, Karlsruhe, Germany, Private communication. [20] E. Materna-Morris, Ch. Adelhelm, S. Baumgärtner, B. Dafferner, P. Graf, S. Heger, U. Jäntsch, R. Lindau, C. Petersen, M. Rieth, R. Ziegler, H. Zimmermann, Structural material EUROFER97-2: characterization of rod and plate material: structural, tensile, Charpy and creep properties, Forschungszentrum Karlsruhe, Internal, Report 31.40.04, 2007. [21] T.H. de Keijser, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, J. Appl. Crystallogr. 15 (1982) 308–314. [22] J.I. Langford, J. Appl. Crystallogr. 11 (1978) 10–14. [23] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564–1583. [24] W. Oliver, G. Pharr, Mater. Res. Soc. 19 (2003) 3–20. [25] T. Weber, Z. Zhou, D. Qu, J. Aktaa, J. Nucl. Mater. 414 (2011) 19–22. [26] A. von der Weth, H. Kempe, J. Aktaa, B. Dafferner, J. Nucl. Mater. 367–370 (2007) 1203–1207. [27] P. Plantin, A.-L. Thomann, P. Brault, P.-O. Renault, S. Laaroussi, P. Goudeau, B. Boubeker, T. Sauvage, Surf. Coat. Technol. 201 (2007) 7115–7121. [28] I. Djerdj, A. Tonejc, A. Tonejc, N. Radic, Vacuum 80 (2005) 151–158. [29] W. Basuki, 2012, Karlsruhe Institute of Technology – Institute for Applied Materials, Karlsruhe, Germany, Private communication. [30] J.R. Trelewicz, C.A. Schuh, Acta Mater. 55 (2007) 5948–5958. [31] G. Abadias, S. Dub, R. Shmegera, Surf. Coat. Technol. 200 (2006) 6538– 6543. [32] Y.X. Yao, Y. Zhu, Key Eng. Mater. 259–260 (2004) 180–185. [33] Paturaud et al., Surf. Coat. Technol. 86–87 (1996) 388–393. [34] H. Sun, Z. Song, D. Guo, F. Ma, K. Xu, J. Mater. Sci. Technol. 26 (2010) 87–92. [35] M. Wen, Q. Meng, W. Yu, W. Zheng, S. Mao, M. Hua, Surf. Coat. Technol. 205 (2010) 1953–1961. [36] P. Hones, R. Consiglio, N. Randall, F. Lévy, Surf. Coat. Technol. 125 (2000) 179– 184. [37] T. Weber, M. Härtelt, J. Aktaa, Considering brittleness of tungsten in failure analysis of helium-cooled divertor components with functionally graded tungsten/EUROFER97 joints, Eng. Fract. Mech., in press, Corrected Proof. [38] J.H. You, H. Bolt, J. Nucl. Mater. 299 (2001) 1–8. [39] C.F. Shih, R.J. Asaro, J. Appl. Mech. 55 (1988) 299–316. [40] C.F. Shih, R.J. Asaro, J. Appl. Mech. 56 (1989) 763–779. [41] M. Olsson, A.E. Giannakopoulos, Int. J. Fract. 85 (1997) 81–97.