Analytical Electron Microscopy of W-Core b-SiC Fibers for Use in an SiC-Based Composite Material for Fusion Applications Tea Toplis ˇek, Medeja Gec, Aljaz ˇ Ivekovic ´, Sas ˇa Novak, Spomenka Kobe, and Goran Draz ˇic ´* Jozef Stefan Institute, Department for Nanostructured Materials, Jamova cesta 39, SI-1000 Ljubljana, Slovenia Abstract: In this work, the interactions between tungsten ~W! and silicon carbide ~SiC! in Sigma TM SiC fibers at high temperatures were characterized using scanning and transmission electron microscopy. These fibers could have the potential for use in fusion-related applications owing to their high thermal conductivity compared with pure SiC-based fibers. The as-received fibers were composed of a 100-mm-thick shell of radially textured b-SiC grains and a 15-mm-thick tungsten core, composed of a few hundreds of nm-sized elongated tungsten grains. The interfaces between the tungsten and the SiC and the SiC and the outer coatings were sharp and smooth. After heat treatment at 1,6008C for 3 h in Ar, the tungsten core reacted with SiC to form a rough interface surface. Inside the core, W 5 Si 3 ,W 3 Si, and W 2 C phases were detected using energy-dispersive X-ray spectroscopy and electron-diffraction techniques. The mechanical properties of the fibers deteriorate after the heat treatment. Key words: SEM, TEM, silicon carbide, fibers, fusion I NTRODUCTION Silicon carbide ~SiC!-based fiber/ matrix composite materi- als are, owing to their low activation in a neutron flux, an operating temperature .1,0008C, and radiation defects re- sistance, practically the only nonmagnetic materials that could be used in structural applications in the next genera- tion of fusion reactors. Besides the mentioned properties, the composite should have a high wear resistance under the conditions of service, a resistance to structural or lattice damage owing to the impinging high-energy neutrons, a high thermal conductivity, and a gas impermeability ~Hase- gawa et al., 2000; Naslain, 2004; Andreani et al., 2006; Lässer et al., 2007; Novak et al., 2010!. The SiC should be in the cubic ~b! form, which is less prone to irradiation damage and with a very small amount of porosity. According to the present state of the art, a composite material prepared by chemical vapor infiltration ~CVI! and polymer infiltration and pyrolysis does not completely meet the required properties. Although the CVI method enables the production of pure SiC with very low neutron activa- tion, both methods are very slow and costly and/or result in an incomplete filling of the gaps between the fibers in the tows. The porosity at the micro and macro levels, the gas permeability, and the thermal conductivity are still not in the required range ~Chawla, 1987; Hasegawa et al., 2000!. The preparation temperature of the matrix material is ex- pected to be around 1,6008C, with an operating tempera- ture around 1,0008C. Insufficient thermal conductivity is one of the main drawbacks to SiC/SiC composites proposed for use in the structural parts of a fusion reactor beyond ITER. One possible solution to increasing the thermal conductivity is the incorporation of tungsten filaments in the composite material, with its intrinsic room temperature thermal con- ductivity being 170 W/ mK. With the proper amount and geometry of W filaments through the thickness in a SiC- based matrix, the requested thermal conductivity of 30 W/ mK should be achieved. As an alternative to pure W filaments, SiC-coated W or W-core SiC fibers could be used. The main objective of the work is the potential inter- actions between W and SiC in W-core SiC fibers at high temperatures. According to the literature data ~Chawla, 1987; Harris, 2002; Wawner, 2000!, the reaction products may have a detrimental or, if controlled, beneficial effect on the mechanical properties of the material. The W/SiC inter- faces were investigated with scanning and transmission elec- tron microscopy ~SEM and TEM! and microanalysis. The preparation of the electron-transparent sections of the ce- ramic fibers is a challenging task that often limits the use of TEM studies for such fibers. Various TEM sample prepara- tion methods were tested; the most efficient method com- bines a technique for preparing densely packed fiber/epoxy specimens and mechanical polishing to a thickness of ,5 mm, thus minimizing the time of the ion milling. Alternatively, the wedge-polishing method without any ion milling was also used. MATERIALS AND METHODS As model materials, different grades of Sigma fibers from TISICS Ltd, UK were used: SM 1040, SM 1240, SM 3156, and Hot Fiber. All the fibers consist of a 15-mm W-core with deposited SiC with various grades of purity and stoi- chiometry on the top. In some cases, different outer coat- © MICROSCOPY SOCIETY OF AMERICA 2013 *Corresponding author. E-mail: goran.drazic@ijs.si Microsc. Microanal. 19, S5, 136–139, 2013 doi:10.1017/S1431927613012506 https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1431927613012506 Downloaded from https:/www.cambridge.org/core. IP address: 54.191.40.80, on 10 Apr 2017 at 18:06:39, subject to the Cambridge Core terms of use, available at