Bulk equiatomic (Hf-Ta-Zr-Ti)C and (Hf-Ta-Zr-Nb)C high entropy Ultra-High Temperature Ceramic (UHTC) carbide compositions were fabricated by ball milling and Spark Plasma Sintering (SPS). It was found that the lattice parameter mismatch of the component monocarbides is a key factor for predicting single phase solid solution formation. The processing route was further optimised for the (Hf-Ta-Zr-Nb)C composition to produce a high purity, single phase, homogeneous, bulk high entropy material (99% density); revealing a vast new compositional space for the exploration of new UHTCs. One sample was observed to chemically decompose; indicating the presence of a miscibility gap. While this suggests the system is not thermodynamically stable to room temperature, it does reveal further potential for the development of new in situ formed UHTC nanocomposites. The optimised material was subjected to nanoindentation testing and directly compared to the constituent mono/binary carbides, revealing a significantly enhanced hardness (36.1 ± 1.6 GPa,) compared to the hardest monocarbide (HfC, 31.5 ± 1.3 GPa) and the binary (Hf-Ta)C (32.9 ± 1.8 GPa).

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Different microstructures in Si3N4 ceramics containing Y2O3and Al2O3 as sintering additives were prepared by two‐step sintering. Pull‐out and elastic bridging were most frequently observed as the toughening mechanisms in samples with fine‐grained microstructures having needlelike β‐Si3N4grains with diameters of <1 μm. Crack deflection was the main toughening mechanism observed in samples with coarse‐grained microstructures having grains with diameters of >1 μm. The values of fracture toughness were varied from 6.1 to 8.2 MPa·m1/2 with respect to the microstructural characteristics, characterized by the volume fraction of needlelike grains and their diameter.

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A High Entropy (Hf-Ta-Zr-Nb)C Ultra-High Temperature Ceramic (UHTC) was fabricated by ball milling and Spark Plasma Sintering (SPS) with a density of 99%. The microstructure characteristics were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) in combination with electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). Atomic structure and local chemical disorder was determined by means of scanning transmission electron microscopy (STEM) in conjunction with energy dispersive X-ray spectroscopy (EDS). According to the results, high purity, dense and homogeneous high entropy carbide with Fm-3m crystal structure was successfully produced. The grain size ranged from approximately 5 µm to 25 µm with average grain size of 12 µm. Chemical analyses proved that all grains had the same chemical composition at the micro as well as on the nano/atomic level without any detectable segregation. The approximately 1.5 nm thin amorphous grain boundary phase contained impurities that came from the starting powders and the ball milling process.

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