Analysis of the anisotropy, stoichiometry and polytypes in pyrolytic carbon and silicon carbide coatings

Highlights: ► We have compared the values of anisotropy of pyrolytic carbon measured by Raman spectroscopy and 2MGEM. ► The values of anisotropy depend on the sampling area. ► The values of diattenuation for SiC measured by 2MGEM were affected by the content of stacking faults. ► Raman spectroscopy can be used as a semi-quantitative tool for the characterisation of excess carbon and silicon in SiC. - Abstract: Silicon carbide (SiC) and pyrolytic carbon (PyC) coatings in tristructural isotropic (TRISO) coated fuel particles were characterised by a combination of 2-modulator generalised ellipsometry microscopy (2-MGEM), Raman spectroscopy and transmission electron microscopy. We compared the values of anisotropy obtained from 2-MGEM and Raman spectroscopy to investigate the effect of sampling area and microstructure. No linear correlation in anisotropy was found between these two techniques despite both sampling areas of 2–5 μm. The largest deviations were observed for highly anisotropic samples with optical anisotropy factors (OPTAFs) above 1.1. For medium and low anisotropy samples (OPTAF < 1.1) the relationship is close to linear. The limited use of only the average value of diattenuation does not give an accurate representation of the characteristics of the coatings as significant areas can exist with considerably higher diattenuations that could increase the probability of failure under neutron irradiation. We also observe a change in the diattenuation of SiC due to the presence of stacking faults as confirmed by Raman spectroscopy. Raman spectroscopy was also used to perform semiquantitative analysis of the Si and carbon excess in SiC in four TRISO particles.