Characterization of SiC coatings on HTGR fuel particles. Final report

Technical Report · Thu Jan 01 04:00:00 EST 1981

DOI:https://doi.org/10.2172/6943439· OSTI ID:6943439

Lauf, R. J.; Braski, D. N.

Fuel particles for the HTGR contain a layer of pyrolytic silicon carbide to act as a pressure vessel and fission product barrier. The SiC is deposited by thermal decomposition of methyltrichlorosilane (CH/sub 3/SiCl/sub 3/ or MTS) in an excess of hydrogen. Coatings deposited at temperatures from 1500 to 1700/sup 0/C and coating rates from 0.4 to 1.2 ..mu..m/min have been studied by transmission electron microscopy, small-angle x-ray scattering, x-ray diffraction, optical microscopy, density measurements, scanning electron microscopy, and microhardness measurements. Transmission electron microscopy has the necessary resolution to provide crystallographic information on small coating defects. Major defects were voids, stacking faults, and dislocations. Small-angle x-ray scattering was used to measure the void size distribution; voids were generally from 20 to 80 nm in diameter. X-ray diffraction indicates that SiC coatings are predominantly cubic ..beta..-SiC. However, the high stacking fault density in some coatings can give rise to both x-ray and electron diffraction effects. In some small areas the faulted structure resembles one or more polytypes of ..cap alpha..-SiC. The evidence indicates though that this is a consequence of rapid growth and not a two-phase ..cap alpha.. + ..beta.. mixture in the thermodynamic sense. Reflected-light microscopy can be used quantitatively to measure average grain size and shape but cannot resolve most coating defects. Density measurement can be used to differentiate between coatings that exhibit extremes in performance but cannot be used to detect small variations in quality. Scanning electron microscopy shows that deposition variables affect coating surface morphology, but these features are difficult to quantify and do not bear a simple relationship to internal coating defects. Microhardness was a very insensitive indicator of coating quality. 15 figures.

• Research Organization: Oak Ridge National Lab., TN (USA)
• DOE Contract Number: W-7405-ENG-26
• OSTI ID: 6943439
• Report Number(s): ORNL/TM-7571
• Country of Publication: United States
• Language: English

Related Subjects

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS
210300* -- Power Reactors
Nonbreeding
Graphite Moderated
CARBIDES
CARBON COMPOUNDS
CHEMICAL COATING
CHEMICAL VAPOR DEPOSITION
COATED FUEL PARTICLES
COATINGS
COHERENT SCATTERING
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
CRYSTALLOGRAPHY
DEPOSITION
DIFFRACTION
DISLOCATIONS
ELECTRON MICROSCOPY
FUEL PARTICLES
GAS COOLED REACTORS
GRAPHITE MODERATED REACTORS
HARDNESS
HTGR TYPE REACTORS
LINE DEFECTS
MECHANICAL PROPERTIES
MICROHARDNESS
MICROSCOPY
OPTICAL MICROSCOPY
REACTORS
SCATTERING
SILICON CARBIDES
SILICON COMPOUNDS
STACKING FAULTS
SURFACE COATING
VOIDS
X-RAY DIFFRACTION