| dc.description.abstract | Silicon carbide has excellent mechanical properties. Sintered or hot-pressed (HP) SiC is essen- tial for applications that require dense bodies. In this work, the microstructure of one solid state sintered SiC and one HP-SiC sample are studied. Both are made from SiC powder (Densitec, Saint-Gobain Lillesand) with carbon (C) and boron (B) additions, and the crucial roles of C and B in densification are debated. The microstructure determines the mechanical properties, and the aim of this work is to provide a full characterization of the samples, with a focus on secondary phases. Different imaging, spectroscopy and diffraction techniques within transmis- sion electron microscopy (TEM) are used. TEM specimens are prepared by wedge polishing with diamond lapping films, and optionally argon (Ar) ion milling. Morphologically, the two samples are similar. SiC grains are ∼ 2 μm in diameter, generally equiaxed and mainly of the polytype 6H. Scanning precession electron diffraction (SPED) was applied to SiC for the first time, and indexation of SPED data indicates that SiC grains are randomly oriented. Lat- tice imaging proves that SiC grain boundaries (GBs) are clean of any secondary or amorphous phases, which points to a solid state diffusion mechanism for densification. High angle annu- lar dark field scanning TEM imaging clearly shows intergranular secondary phases, commonly located at triple junctions. Energy dispersive X-ray spectroscopy-mapping showed that these are either i) pure C-phases or ii) B-C-phases. In addition, metal inclusions of a few hundred nanometer in size, are typically located at triple junctions and have different compositions of e.g. iron (Fe) and titanium (Ti). Electron diffraction shows that the C-phases have crystalline graphitic domains, and that some have a texture in the [0002]-direction. Lattice imaging re- veals sheet cross-sections with the appearance of fiber-like structures that has clear graphite (0002) lattice fringes running parallel to their longest direction. Narrower cross-sections (∼ 5 nm) are often bent and have relaxed d(0002)-spacings (∼ 3.5 Å). Boron carbide (B4C) grains are crystalline, on the same size scale as the SiC grains and are often found adjacent to C-phases. Electron energy loss spectroscopy (EELS) confirms that B-C-phases are B4C, and that C-phases consist of graphite. B was not detected neither in SiC grains nor on SiC GBs, probably since the B concentration is below the EELS detection limit. For future work, the novel SPED technique should be refined in terms of both acquisition of, treatment of and analysis of data. Specimen preparation routines should be improved, and use of complimentary techniques attempted, in order to investigate the distribution of B in SiC grains and in SiC GBs. Different techniques within TEM provide a complete characterization of sintered SiC, in terms of microstructure, crystal structure and phase distribution. | en |
