Microstructure and Vacuum Leak Characteristics of SiC coating Layer by Three Different Deposition Methods Y. Kim Professor, Department of Materials Science and Engineering, College of Engineering, Kyonggi University, Suwon, South Korea. E-mail: ytkim@kgu.ac.kr J. Choi Graduate Student, Department of Materials Science and Engineering College of Engineering, Kyonggi University, Suwon, South Korea. E-mail: jwchoi0619@naver.com Abstract The Fukushima nuclear disaster has recently highlighted the danger of hydrogen leakage in nuclear reactors and the corresponding need to prevent or minimize hydrogen leaks through the coating of materials on a graphite substrate that are used to contain nuclear fuel. Silicon carbide (SiC) is a candidate coating material for heat exchangers for very high temperature gas cooled reactors (VHTR) owing to their high hardness, strength, and oxidation resistance. In this work, SiC is coated on a graphite substrate using three different methods: chemical vapor reaction (CVR), physical vapor transport (PVT), and chemical vapor deposition (CVD). The surface microstructure of the coating layers prepared by these methods were compared and analyzed by scanning electron microscopy (SEM). A custom device for gas (vacuum) leak tests was fabricated to compare the leak characteristics of the three types of coating layers. SiC coated specimens synthesized by the CVR method showed the best vacuum leak characteristics, in good agreement with the expectation from microscopic observations. It was concluded that the vacuum leak characteristics depend on the defects in the SiC coating layer rather than other parameters, such as the crystallinity of the coating layer. It is speculated that a SiC coating on a graphite substrate may be one measure for improving hydrogen leak resistance in the event of a nuclear reactor-related accident. Keywords: SiC, Vacuum leak, Chemical vapor reaction, Chemical vapor deposition, Physical vapor transport Introduction Hydrogen is well known to be a flammable and dangerous gas under certain conditions. Nuclear reactors are dangerous due to the potential for hydrogen explosions. Hydrogen leakage must be prevented or minimized by coating a leak resistant material on a graphite substrate for containing nuclear fuels in the event of a nuclear reactor-related accident. Silicon carbide (SiC) is a candidate coating material for heat exchangers for very high temperature gas cooled reactors (VHTR) owing to its high hardness, strength, and oxidation resistance, among other properties. [1, 2]. Chemical vapor deposition (CVD) is commonly used to manufacture high density SiC layers with high purity. SiC layers deposited by CVD also have good thermal and mechanical properties [3-6]. For the uniform and homogeneous deposition of silicon carbide on very large components, the gas flow and other process variables in a CVD reactor must be delicately controlled [7]. Although, CVD methods have been widely utilized to form coatings, the chemical vapor reaction (CVR) method is better suited for forming SiC coating layers on components with complex shapes such as spherical pebbles. In addition, the SiC conversion layer formed via the CVR method adheres well to graphite matrices [8]. The physical vapor transport (PVT) method has been a most successful and common method for the growth of bulk SiC crystals [9]. Therefore, the PVT method was also studied for comparison in this work. In this study, the relationship between microstructure and vacuum leak characteristics of SiC coating layers was studied for three coating methods. SiC was coated on graphite substrates using: CVR, PVT, and CVD. Microstructures of the surfaces of the SiC coating layers were observed by scanning electron microscopy (SEM). A custom device for vacuum and gas leak tests was fabricated to compare the vacuum leak characteristics of the different coating layers. Methodology A. Coating Methods Fig. 1 shows schematic diagrams for the different types of SiC coating equipment. For CVD coating, a tetrachlorosilane (SiCl 4 ) solution was used as the source material for silicon. Methane gas was used as a carbon source. Hydrogen was used for both controlling the atmospheric gas in the reactor and as a carrier gas for the vaporization of SiCl 4 by bubbling. Argon gas was also used to control the atmospheric gas in the reactor. The SiCl 4 solution was maintained at 0 C when bubbling. Hydrogen, argon, and methane gas were precisely controlled using a mass flow controller (MFC). The reaction temperature for the SiC deposition was fixed at 1300 C. The deposition time was fixed at 100 min. The length of the stainless steel input gas line was fixed at 30 cm. The reaction pressure was maintained to less than 1 Torr. The flow ratio for methane gas was fixed at 0.33 and that for the hydrogen gas was fixed at 20. A dense SiC coating was formed over the surface of the graphite substrate by a CVR process for the solid-solid and vapor-solid reaction in the SiO 2 (s)-c(s)-sio(v)-co(v) system. From the reduction of SiO 2 powder with a graphite 1192
substrate, SiO vapor was created and infiltrated into the graphite substrate. Finally, a SiC conversion layer formed from the vapor-solid reaction of SiO and graphite [10]. The SiC coating was deposited by CVR at a temperature of 1600 C for 160 min at a pressure of 10-2 Torr. leak results were adequate for comparing the gas leak characteristics of the three different types of specimens. Figure 1: Schematic of SiC deposition equipment. (a) CVD, (b) CVR, and (c) PVT methods. In the PVT process, source material is evaporated or dissociated at the bottom of the crucible and crystal growth occurs in colder regions of the crucible. An overview of a typical furnace used in the three regions of interest can be defined within the crucible during growth: the source material region, the growth region, and the gaseous region between the source and growth regions [11]. During growth, the growth zone is held at a slightly lower temperature than the source material region, producing a temperature gradient between 1 10 C/mm. The source material is evaporated or dissociated under proper conditions and re-condenses in the colder regions of the crucible to promote crystal growth. Graphite crucibles are used because they are inexpensive and relatively stable at these temperatures. The high carbon vapor pressure associated with graphite at elevated temperatures has not been detrimental to growth when SiC powder is used as a source material. PVT was conducted at a temperature of 1938 2020 C for 10 h at a pressure of 200 Torr [12]. B. Leak test device A custom laboratory device fabricated for gas leak tests is shown in Fig. 2. This device was designed to compare the gas leak characteristics of specimens before and after coating. The diameter and height of the graphite and stainless steel (SS) substrates (reference specimens) for gas leak tests were 1 inch and 1.5 mm, respectively. The vacuum level profile of the test chamber with the SS standard specimen during pumping is shown in Fig. 2(b). The vacuum of the test chamber decreased drastically in 6 h and then stabilized near 2.0 10-3 Torr for up to 24 h. Vacuum leak characteristics were measured with SiC specimens deposited by CVR, PVT, and CVD methods. Tests were conducted after 6 h of pumping by a rotary pump. The pump was then turned off and left in a completely closed state for 24 h. The device was originally designed to introduce hydrogen gas into the vacuum space for leak tests. However, no gas was provided in this study as the vacuum (a) (b) Figure 2: (a) A custom device for gas leak tests and (b) the vacuum leak profile of the test device. The vacuum level of the specimen chamber was measured after maintaining atmospheric pressure for 1 h. The abscissa in (b) indicates the hours spent pumping the device. Results and Discussion A. XRD analysis Fig. 3 shows the XRD patterns of SiC coated using CVR, PVT, and CVD. Silicon carbide peaks were observed in all specimens. However, the highest intensity was observed in the specimen deposited by the CVD method and the lowest intensity in the specimens deposited by the PVT method. Note that the scales of the vertical axis in Fig. 3 are different. Although carbon peaks were observed in all specimens and unavoidable, the SiC peaks are dominant in the specimen deposited by CVD, as shown in Fig. 3(c) unlike the other two specimens. This XRD result implies that the quality of CVD SiC coating layers is better than those deposited by the other methods. 1193
(a) formed, small crystallites and crystal facets were observed at high magnification ( 10,000). Thus, it is expected that the vacuum leak characteristics of CVR specimens will be superior. Very large crystals were observed on the coating layers made by the PVT method. The size of some crystallites exceeded 50 micrometers in diameter. There were many pores and voids on the surface and the PVT film appears less dense than the CVR and CVD specimens. Therefore, the highest vacuum leaks were expected for the PVT specimens. Judging from the figures at the bottom of Fig. 4, the CVD formed surfaces were smoother and denser than any of the other specimens. However, a few voids and cracks were found at lower magnification. Well-formed crystals with clear facets and grain boundaries were also observed at higher magnification ( 10,000). Although the CVD film has a dense structure and appears to have a high degree of crystallinity, the voids and cracks observed at low magnification suggest that the vacuum leak characteristics will be inferior to the PVT sample. (b) (a) CVR: x1,000 (b) CVR: x3,000 (c) Figure 3: XRD patterns of specimens coated with SiC using three different deposition methods. (a) CVR, (b) PVT, and (c) CVD. B. SEM analysis Fig. 4 shows SEM surface micrographs of SiC coating layers fabricated by CVR, PVT, and CVD methods. As shown in Fig. 4, the CVR coating layers appear smooth and densely coated without voids or pores on the surface. Moreover, well (c) CVR: x10,000 1194
(d) PVT: x1,000 (h) CVD: x3,000 (e) PVT: x3000 (f) PVT: x10,000 (i) CVD: x10,000 Figure 4: SEM surface micrographs of SiC coating layers deposited by CVR, PVT, and CVD methods. Method of deposition and magnifications were indicated under the each figure. B. Vacuum leak test Fig. 5 shows the vacuum leak test results for SiC coated specimens deposited by CVR, PVT, and CVD methods. Reference data were obtained before the main experiment by conducting the leak test with two standard specimens: a stainless steel plate and a graphite substrate before coating. The pure graphite substrate without coating reached 760 Torr in 90 s. The stainless steel plate reached 4.5 10-2 Torr in 24 h after turning off the vacuum pump. (g) CVD: x1,000 Figure 5: Vacuum leak test results for SiC coated by CVR, PVT, and CVD methods. 1195
Specimens coated with SiC using PVT and CVD methods showed fast vacuum leaks. Only 13 min 14 s and 30 min 02 s elapsed to reach 760 Torr for the PVT and CVD films, respectively. The specimen coated by CVR showed the slowest vacuum leak compared to the other specimens. The vacuum level reached 3.5 Torr after 24 h. The results of the vacuum leak test show that the CVR coating layers have the best vacuum leak characteristics, in good agreement with the expectation from microscopic observations by SEM. As mentioned earlier, the crystallinity of the CVD coating layers was the highest among all specimens, as shown by XRD. Therefore, the degree of crystallinity of the coating layer is not the main factor for optimizing the leak characteristics of coating layers. Instead, defects such as pores, voids, and micro-cracks are more important factors for determining the vacuum leak resistance. It is expected that CVD specimens would show better vacuum leak resistance over that for CVR if the there are no defects in the coating layers, achieved through longer coating times or by double coating, as the quality of the CVD coating layers would then be superior to the other specimens studied here. As seen in Fig. 5, a SiC coating layer proved to be effective in increasing leak resistance regardless of the particular coating method, although the degree of resistance is different. Therefore, it is speculated that the SiC coating on graphite substrates may offer one way to enhance hydrogen leak resistance for mitigating nuclear reactor-related accidents. Conclusions The vacuum leak characteristics of SiC layers coated by three different methods (CVR, PVT, and CVD) were compared and explained using their microstructures. The specimens coated using CVR showed the smallest vacuum leaks, where the vacuum level was maintained at 3.5 Torr for 24 h after turning off the vacuum pump. Considering that a graphite specimen without a coating reached 760 Torr in 90 s, it is believed that the SiC coating deposited by CVR on graphite may be one way of minimizing hydrogen gas leaks. Although the CVD coating layer had the highest crystallinity, these films did not have the lowest leak rates. The vacuum leak characteristics of coating layers depended mainly on defects such as voids, pores, and micro-cracks rather than crystallinity, crystal size and shape, etc. The vacuum leak characteristics of specimens having a similar defect density by the three different methods are worth comparing in the future. Hydrogen gas leak tests with the specimens studied here will be conducted in the near future. Acknowledgements This work was supported by Kyonggi University Research Grant 2014. Operated in a Severe Environment, J. Kor. Ceram. Soc., vol. 48, no. 1, pp. 52-56, 2011. [3] T. Hirai and M. 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