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.
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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)