interaction between plasma and tungsten carbide...

INTRODUCTIONKey issues to address in modern nuclear research include the choice of first wall materials and understanding the effects of plasma materials on a fusion device and how they interact. Particularly, plasma energy confinement time is strongly dependent on the first wall material selection and on how the wall interacts with edge plasmas (Hino & Yamashina 1996; Kirnev et al. 2001; Buzinskij et al. 1999). Recently, the use of coatings on material

surfaces to improve their quality and reduce their damage according to their application has been considered (Xu et al. 2006; Niknahad & Mannari 2016; Taha et al. 2010; Habibi et al. 2015).

One of the key elements that are effective on the core plasma conditions is the interaction between the surrounding surfaces and the plasma (Xu et al. 2006). The plasma should be chosen in a way that minimizes the impurity of production, the retention of hydrogen isotope, and neutron activation; beside these conditions it should also satisfy some mechanical and thermodynamic

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Philippine Journal of Science147 (3): 537-543, September 2018ISSN 0031 - 7683Date Received: 05 Mar 2018

Key words: first wall, scanning electron microscopy, thin film, tokamak, tungsten carbide, X-ray diffraction

Interaction Between Plasma and Tungsten Carbide Thin Films Coated on Stainless Steel

as Tokamak Reactor First Wall

Azadeh Jafari1*, Vahid Fayaz1, Sakineh Meshkani2, and S. Ali Asghar Terohid1

1Department of Physics, Hamedan Branch, Islamic Azad University, Hamedan, Iran2Plasma Physics Research Center, Science and Research Branch,

Islamic Azad University, Tehran, Iran

The physical properties of tungsten carbide (WC) thin film as a first wall material when it is exposed to the plasma of tokamak was studied in this research. In this regard, WC thin film was formed on grade 316L stainless steel – via the hot filament chemical vapor deposition method – to the sample installed on Iran tokamak 1 chamber and exposed to 300 shots of hydrogen plasma for a total duration of 11 s. For investigation of hydrogen plasma effects on morphology, crystalline structure properties, and roughness of the sample, X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and Raman spectroscopy analysis was performed. The experimental setup described and micrographs of the surfaces are shown. XRD analysis of WC thin film coated on stainless steel before and after plasma shots shows the changes in crystal structure. Based on the scanning electron microscopy images, it can be concluded that plasma exposure has created some cracks, holes, and lines. Also, the roughness of the sample after plasma shots decreased and it was observed that the thickness of WC thin film coated on stainless steel is reduced after plasma shots were introduced. Moreover, the weight loss of the uncoated sample was higher in comparison to the coated one. Finally, WC coating on the first wall of fusion device looks promising, but several open questions still remain to be solved.

Corresponding author: [email protected]

properties. The most mature technology that is currently used for power generation from fusion is the tokamak, which utilizes powerful magnetic fields to trap hot plasma in a circular trajectory inside a toroid reactor, wherein the fusion process takes place. Key findings in this paper are reported so that the effect of plasma on tungsten carbide (WC) thin films coated on tokamak first wall, as well as their interactions, will be described for the first time.

WC compounds have common industrial uses – they are hard materials and with high temperature resistance (Nerz et al. 1992). The properties of WC – such as wear resistance, high hardness, and chemically stability – are excellent (Zhong & Shaw 2011). In comparison, the typical hardness of WC is 1600 HV, while the hardness of mild steel is about 160 HV, which is lower by a factor of 10. Several techniques have been used to deposit WC, including pulsed laser deposition, chemical vapor deposition, sputtering, thermal evaporation, and molecular-beam epitaxy (Sickafoose et al. 2002). In this paper, the formation of WC on stainless steel 316L (S.S.316L) substrate was induced via hot filament chemical vapor deposition (HFCVD) method. S.S.316L was chosen since the first walls of Iran Tokamak 1 (IR-T1) are made of the same material. In this paper, the possibility of using WC as a suitable choice of material for IR-T1 first wall is investigated.

MATERIALS AND METHODSAn HFCVD system was used to prepare the WC thin film. The CVD apparatus setup with a stainless steel chamber equipped with diffusion and rotary pumps is used in these experiments. The HFCVD set-up is depicted in Figure 1. Pure and clean tungsten filament with a diameter of 0.5 mm was used as vapor and

heating source. The hot tungsten filament temperature was 1800 °C. The substrates consist of 10 × 10 mm2 S.S.316L, which was pre-etched by common protocol of alcohol, acetone, and deionized water. There was a distance of 1 cm between the filament and the substrate. First, vacuum pumps were used to pump the chamber to 1 × 10–5 Torr; afterwards, high purity argon gas was released. Moreover, the S.S.316L substrate was heated to reach the desired temperature (600 °C) as growth in a mixture of gases (85:15 for Ar:CH4) occurred in the chamber. The chamber was cooled to room temperature after the reaction time (t = 120 s) under Ar flow. Finally, the result was WC thin film formation on the S.S.316L substrate.

IR-T1 is a research tokamak that is located at the Plasma Physics Research Center (PPRC). IR-T1 is an air-core tokamak without a copper shell with minor and major radii of 12.5 and 45 cm, respectively. The experimental conditions for the ohmically heated hydrogen plasma used in the setup are characterized as follows: plasma current of Ip = 20–30 kA, electron temperature of Te = 200 eV, and toroidal magnetic field of Bt = 0.7–0.8 T. This apparatus consists of a circular S.S.316L vacuum vessel with a minor radius of 15 cm and two toroidal breaks (Ghoranneviss et al. 2003; Jafari et al. 2016a). The IRT1 chamber is illustrated in Figure 2. IR-T1 Tokamak chamber material also utilized S.S.316L. The positions of samples on the IR-T1 tokamak are depicted in Fig. 2(c), whereinthe samples were installed on the tokamak chamber and exposed to 300 shots of hydrogen plasma within a total duration of 11 s. Several analytical techniques were applied to characterize the samples. Crystalline structures were analyzed via X-ray diffraction (XRD). The diffraction patterns were recorded on a STOE–XRD diffractometer with CuKα radiation (λ = 1.54060 Å) scanning rate of 0.2 s at 2θ = 10–90° and step size of 0.02°. Surface morphology analyses were observed by top view images from a scanning electron microscopy (SEM, Leo 440i). Also, using an atomic force microscope (AFM), the surface roughness was studied. The facility consists of an AFM (SPM Auto Probe CP, Park Scientific Instruments, USA) in contact mode with a low-stress tantalum nitride tip of less than 200 Å radii. AFM analysis in contact mode was used to study surface topography and roughness at room temperature. The grown boron carbide nanostructures were examined using a Raman spectrometer (Almega Thermo Nicolet) with a Nd:YLF laser (532 nm wavelength). The chemical composition characteristics of the prepared sample were evaluated using an X-ray photoelectron spectrometer (XPS; SPECS XP Flex mode, Germany; equipped with a hemispherical PHOBIOS energy analyzer). The thicknesses of the films were measured using a Dektak3 profilometer.Figure 1. Schematic of HFCVD setup.

Jafari et al.: Plasma-WC Film Interaction on Tokamak Stainless Steel Wall

Philippine Journal of ScienceVol. 147 No. 3, September 2018

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RESULTS AND DISCUSSIONThe weights of the uncoated S.S.316L samples before and after exposure were 8.3224 g and 7.3201 g, respectively, while the weights of the coated samples before and after exposure were 8.9309 g and 8.9302 g, respectively. As is clear, the weight loss of the uncoated sample was 0.0023 g, while the weight loss of the coated one is 0.0007 g. This implies that the sputtering rate from the coated sample was much less than the uncoated S.S.316L sample, which could lead to less plasma contamination. The higher weight loss of the steel in comparison with the WC thin film might have been due to the disruption and high temperature loading on the samples and the low melting point temperature of the steel (melting point of S.S.316L is about 1370–1450 °C and the melting point of WC is 2780–2830 °C).

XRD is an analytical technique used for phase identification of a crystalline material and sample purity. The XRD patterns of the sample before and after tokamak plasma shots are shown in Figure 3. The diffraction peaks related to different crystalline planes of WC are observed, and there were no diffraction peaks related to the S.S.316L, which confirms the successful growth of WC on the stainless steel substrates. Moreover, in the XRD pattern of the WC thin film – at 33.11°, 47.21°, 74.15°, and 87.10°– two peaks were observed that could be related to WC(001), WC(101), WC(111), and WC(201) crystallographic orientations, respectively. Also, W2C phases are formed in 37.10°, 40.41°, and 52.80°. As it can be seen, the phases of W formed as a result of incomplete carburization reaction circumstances. Also, in the XRD pattern after plasma

shots – at 2θ=25° – a peak was observed that could be attributed to WC(002). Another interesting finding was that the intensity of W2C diffraction peaks would decrease after plasma shots. To confirm this issue, the intensities of the W2C peaks before and after plasma exposure was compared for W2C peak at 2θ=40.5°. The researchers found that the peak intensity before plasma shot is about 1.8 times the peak intensity after the shot. It is known that there are several factors affecting the intensity of XRD peaks such as crystallite size, microstrain, crystal defects, and preferred orientation. Therefore, it might be concluded

Figure 2. The image of (a) chamber and (b) first wall of IR-T1 Tokamak, the schematic of samples setup position on IR-T1 Tokamak (c).

Figure 3. XRD pattern of WC coated on stainless steel 316 L, (a) before and (b) after plasma shots.



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that plasma interactions can partially change crystallinity (Jafari et al. 2016b).

The Raman spectra, reported in Fig. 4, show the appearance of the well-resolved D and G peaks (at 1355 and 1575 cm–1) – typical of the formation of nano-structures of amorphous carbon. This could mean that some amorphous carbon existed in the WC thin film in the experiment. From the Raman spectra, it can be seen that the content of amorphous carbon was increased after plasma shots because plasma would interact with carbon atoms in WC thin film coated on the first wall. The presence of an XRD peak at around 2θ=25º in the XRD pattern could be an indication of this reaction (Manoj & Kunjomana 2012).

Figure 4 provides evidence for carbon deposition, thus providing the simplest explanation for the reduction in the XRD intensities with plasma exposure, as seen in Fig. 3. Also, it could be concluded that after the plasma shots, ID/IG would increase and might subsequently cause some disorder in the WC coated on steel. This was in agreement with the morphology images of the WC by HFCVD.

Figure 4. Raman spectra of WC coated on stainless steel 316 L, (a) before and (b) after plasma shots.

The SEM images of tungsten carbide on substrate – before and after shots with different magnification – are shown in Figure 5. From Fig. 5(a), the morphology of WC grains was found to be continuous and dense with an average grain size of 70 nm.

Also – as shown in Fig. 5(c) – linear cracks could have been appeared on the surface in the micrometer scale, which could have been due to the collisions of plasma particle with WC thin film atoms. Cracks in thin films are often caused by differential thermal expansion in thermal

Figure 5. FESEM images of WC coated on stainless steel 316 L, (a) before plasma shots, (b) and (c) after plasma shots.

cycling (Chen et al. 2008; Sickafoose et al. 2002). On the other hand, the incidental charged particle may simply be reflected in the form of a neutral particle and go back to the plasma, where it will be ionized once again.

Also, it seems that some holes that have been created due to mechanical collisions and ablation. IR-T1 is a small research tokamak and there is no fusion reaction in the reactor. According to the reduction of plasma current and plasma confinement time, the researchers hypothesize that impurities entering from the wall will also radiate and subsequently quench the plasma if they reach the core. The same result can also be seen from a study by Merola et al. (2010).



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As thickness measurement confirmed this description, the thicknesses of deposited samples were tested with a surface profiler with the accuracy of 10 nm. On the surface of the substrate before performing the deposition, a scotch tape was placed and eventually removed after the measurement was done. In this manner, the difference between the surface of thin film and the substrate is measured. In this method of thickness measurement, moving a probe along the surface to maintain a constant force allows for the acquisition of surface height information along the scan line. It was observed that the thickness of WC thin film

Figure 6. XPS W4f and C1s spectra of WC coated on stainless steel 316 L before and after plasma shots.

Figure 7. AFM images of WC coated on stainless steel 316 L, (a) before and (b) after plasma shots.

coated on stainless steel would reduce after plasma shots from 0.73 µm to 0.66 µm.

XPS measurements were performed before and after plasma shots in order to analyze the chemical state. The concentration was calculated by dividing the W4f and C1s peak intensities by the relative sensitivity factor for each element (Wagner et al. 1979). The sets of replicate spectra are shown from different spots in Figure 6(a) and (b). The spectrum (a) was successfully synthesized with W4f7/2, W4f5/2, and W5p3/2 peaks of WC (McGuire et

al. 1973). As shown in Fig. 6(a) for the W4f spectrum, two peaks corresponding to W4f7/2 and W4f5/2 – seen located at 31.5 and 33.5 eV, respectively – could be assigned to WCx. Fig. 1c shows the C1s peak. A carbidic peak at 283 eV, corresponding to the chemical bonding between W and C, was also observed.

The AFM image and the roughness (RMS) values of the sample before and after plasma shots are displayed in Figure 7(a) and (b).The comparison of these images indicated that sharp nano tips changed into flat nano tips on the surface. As expected, it can be seen from these figures that the plasma interaction with surface atoms smoothened the surface (Jafari et al. 2016c). The smoothening process can be the result of both (i.e., surface atoms and sputtering incident charged particles). Because of the plasma shots and collision of particles, the roughness of the sample after plasma shots decreased. During the process of interaction between plasma and first wall material, after a series of scattering events, the plasma energetic particles could be implanted into the surface and could remain in the material (Sickafoose et al. 2002; Buzinskij et al. 1999).



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CONCLUSIONIn this work, WC thin film on S.S.316L was formed via HFCVD method for investigation of the IR-T1 tokamak hydrogen plasma effect on WC as a first wall material in tokamak. Afterwards, WC coated on S.S.316L samples was installed on the IR-T1 chamber and exposed to 300 shots of hydrogen plasma within a total duration of 11 s. Using XRD, SEM, XPS, and AFM, plasma shot effects on crystalline structure, surface morphology, and surface roughness before and after exposure were studied. XRD spectra showed that the intensity of W2C diffraction peaks decreasing after plasma shots might cause partial amorphization of the crystallite film. Based on the analysis of Raman and XRD spectra, the researchers conclude that the content of amorphous carbon increases after plasma shots since plasma interacts with carbon atoms in WC thin film. Also, the AFM results show that because of the plasma shots and collision of particles, the roughness of the sample after plasma shots decreases. Based on the SEM images, it can be concluded that plasma exposure can create some cracks, holes, and lines in some zones of the sample.

ACKNOWLEDGMENTSThe Department of Physics, Hamedan Branch, Islamic Azad University, Hamedan, Iran is gratefully acknowledged for the funding support and making this research study feasible.

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