Ultra-high temperature ceramics are suitable structural ceramics for applications under high heat fluxes at temperature that can exceed 1600?C. Future hypersonic vehicles have a potential use temperature above 2000?C and require oxidation resistant materials. The ceramics object of this study, namely ZrB2, HfB2, HfC and TaC, possess a unique combination of properties including high melting point temperature (ZrB2: 3245?C, HfB2: 3250?C, HfC: 3890?C, TaC:3985?C), high hardness and strength, good oxidation resistance and high thermal conductivity. Ceramics based on borides and carbides of Zr, Hf and Ta were hot pressed at 1750?C-1900?C to full density thanks to the addition of 15 vol% of TaSi2. TaSi2 was selected to promote the densification, due to its high melting point (2200?C), its ductility at the sintering temperature and its capability to provide increased oxidation resistance. The microstructure of the composites was analyzed by X-ray diffraction, scanning and transmission electron microscopy in order to investigate the densification mechanisms occurring during sintering. In the boride-based composites the formation of (Ta,Me)B2 solid solution growing epitaxially on the matrix with low-angle grain boundary was observed. The chemistry of the triple points suggest that cation transfer is an active process and the passage through a liquid phase is also strongly hypothesized. The secondary phases identified were SiC, Ta5Si3 and Ta-oxides. Concerning the carbide-based materials, a higher solubility between Ta and Hf was observed both in the carbide grains and in the silicide. Also in these systems, Ta-rich solid solutions were observed surrounding the matrix. The microstructure evolution is discussed with respect to the chemistry of the elements involved, the phase diagrams and the thermodynamics
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