Estimate of polytype fractions and dislocation density in SiC before and after sintering in Si3N4 matrix
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A quantitative characterization of polytype fractions and dislocation morphology and density is presented for two α-SiC powders. The tools were X-ray diffraction (XRD) and etch-pit analysis carried out before and after hot-isostatic-press (HIP) sintering in an Si3N4 matrix at 2050 °C under 180 MPa. Results are compared with data from transmission electron microscopy and electron diffraction previously obtained on the same powders. To avoid overlapping of the major XRD peaks with that of the Si3N4 matrix and to make possible the observation of the Si plane during etch-pit analysis in the powders after sintering, a chemical etching procedure to separate nitride and carbide phases without damage was developed. The morphology and density of pits and dislocations were analysed to get quantitative information about the crystal structures of the SiC crystallites and their modifications after the HIP cycle. The polytype fractions were found to be unchanged after sintering. It was also determined that polytypes 6H, 4H and 15R generally share part of the surface in a single crystallite rather than existing as single crystallites themselves, the 15R polytype generally being a hosted structure by a 6H or 4H matrix. A high density of dislocations (1013–1014cm−2) was found in both the SiC powders after HIP sintering compared with the raw materials.
KeywordsTransmission Electron Microscopy Carbide Nitride Dislocation Density Electron Diffraction
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