Comparison study of silicon carbide coatings produced at different deposition conditions with use of high temperature nanoindentation

Journal of Materials Science, Oct 2016

The elastic modulus and hardness of different silicon carbide (SiC) coatings in tristructural-isotropic (TRISO) fuel particles were measured by in situ high temperature nanoindentation up to 500 °C. Three samples fabricated by different research institutions were compared. Due to varied fabrication parameters the samples exhibited different grain sizes and one contained some visible porosity. However, irrespective of the microstructural features in each case the hardness was found to be very similar in the three coatings around 35 GPa at room temperature. Compared with the significantly coarser grained bulk CVD SiC, the drop in hardness with temperature was less pronounced for TRISO particles, suggesting that the presence of grain boundaries impeded plastic deformation. The elastic modulus differed for the three TRISO coatings with room temperature values ranging from 340 to 400 GPa. With increasing measurement temperature the elastic modulus showed a continuous decrease.

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Comparison study of silicon carbide coatings produced at different deposition conditions with use of high temperature nanoindentation

Comparison study of silicon carbide coatings produced at different deposition conditions with use of high temperature nanoindentation Nadia Rohbeck 2 Dimitrios Tsivoulas 1 2 Ian P. Shapiro 2 Ping Xiao 2 Steven Knol 0 Jean-Michel Escleine 4 Marc Perez 4 Bing Liu 3 0 Nuclear Research and Consultancy Group (NRG) , PO Box 25, 1755 LE Petten , The Netherlands 1 Clean Energy/Nuclear Services, Amec Foster Wheeler , 601 Faraday Street, Birchwood Park, Warrington WA3 6GN , UK 2 School of Materials, The University of Manchester , Oxford Road, Manchester M13 9PL , UK 3 Institute of Nuclear and New Energy Technology (INET), Tsinghua University , Beijing 100084 , China 4 Commissariat a l'Energie Atomique (CEA), CEA/Cadarache , 13108 St. Paul lez Durance , France The elastic modulus and hardness of different silicon carbide (SiC) coatings in tristructural-isotropic (TRISO) fuel particles were measured by in situ high temperature nanoindentation up to 500 C. Three samples fabricated by different research institutions were compared. Due to varied fabrication parameters the samples exhibited different grain sizes and one contained some visible porosity. However, irrespective of the microstructural features in each case the hardness was found to be very similar in the three coatings around 35 GPa at room temperature. Compared with the significantly coarser grained bulk CVD SiC, the drop in hardness with temperature was less pronounced for TRISO particles, suggesting that the presence of grain boundaries impeded plastic deformation. The elastic modulus differed for the three TRISO coatings with room temperature values ranging from 340 to 400 GPa. With increasing measurement temperature the elastic modulus showed a continuous decrease. - Silicon carbide (SiC) is an important technical ceramic that is widely applied due to its high hardness and temperature stability. Over the past years there have been extensive efforts to develop SiC for the nuclear environment. Thus in the future, SiC-based composites could replace metal fuel cladding or fissile material is diluted within an inert SiC matrix to form a new type of fuel [1, 2]. The high temperature reactor (HTR) concept foresees the application of a fully ceramic fuel compact in which all fissionable material is completely encapsulated within composite tristructural-isotropic (TRISO) fuel particles consisting of successive layers of pyrolytic carbon (PyC) and SiC around the spherical kernel. In particular, the integrity of the SiC coating is crucial to ensure full retention of all radiotoxic compounds within the fuel in-service as well as during final disposal. Such SiC coatings are being produced by a fluidised bed chemical vapour deposition (FBCVD) process that can achieve dense, pure, and homogeneous coating layers. By optimising the coating parameters (temperature, precursor concentration), the desired microstructural characteristics can be obtained. Thus coatings vary in grain size and shape, texture, presence of residual porosity or amount of co-deposited second-phase free silicon or carbon. Even though our understanding of the relationship between deposition conditions and microstructural characteristics vastly improved over the past decades, some aspects in obtaining the best mechanical performance are still not clear. One crucial shortcoming is that the mechanical properties are usually assessed at ambient conditions, whereas a wide variety of applications use SiC in an elevated temperature environment. Using the high temperature nanoindentation technique it is possible to measure the mechanical properties of thin coatings in situ up to several hundred degrees celsius. Numerous publications have addressed the technical issues that arise when conducting reliable measurements at higher temperatures, but the data on SiC are still scarce [3, 4]. A short communication of our previous study reported the effect of neutron irradiation on a similar SiC specimen [5]. It was found that neutron irradiation at 1000 C had caused some irradiation hardening, but no sizeable impact on the elastic modulus was measured. However, that study included only one SiC sample and thus no conclusions regarding the role of morphological features on the mechanical properties could be drawn. In addition, even though a few different research institutions have successfully produced TRISO particle fuel, few comparison studies have been published. The different sizes of the custom-built coating facilities and the variation in the fabrication parameters result in differing microstructural features, but the impact on the mechanical properties has not been identified yet. Here, we want to fill the aforementioned gaps by evaluating the influence of the microstructural characteristics on the elastic modulus and hardness up to 500 C of three different SiC coatings fabricated by different research institutions. In addition to the high temperature nanoindentation tests, extensive microstructural characteri (...truncated)


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Nadia Rohbeck, Dimitrios Tsivoulas, Ian P. Shapiro, Ping Xiao, Steven Knol, Jean-Michel Escleine, Marc Perez, Bing Liu. Comparison study of silicon carbide coatings produced at different deposition conditions with use of high temperature nanoindentation, Journal of Materials Science, 2017, pp. 1868-1882, Volume 52, Issue 4, DOI: 10.1007/s10853-016-0476-5