Fabrication of fully dense UHTC by combining SHS and SPS

The combination of the Self-propagating High-temperature Synthesis (SHS) technique and the Spark Plasma Sintering (SPS) technology is adopted in this work for the fabrication of fully dense MB2-SiC and MB2-MC-SiC (M = Zr, Hf, Ta) Ultra High Temperature Ceramics (UHTCs). Specifically, Zr, Hf or Ta, B4C, Si, and graphite reactants are first converted to the desired composites by SHS. For the case of the lowexothermic Ta-based compositions, a preliminary 20 min ball milling treatment of the starting reactants is required to activate the corresponding synthesis reactions. When the resulting powders are then subjected to consolidation by SPS, it is found that products with relative densities greater than 96% can be obtained for all systems investigated within 30 min of total processing time, when setting the dwell temperature to 1800 °C and the mechanical pressure to 20 MPa. Hardness, fracture toughness, and oxidation resistance characteristics of the resulting dense UHTCs are comparable to, or superior than, those relative to similar products synthesized by alternative, less rapid, processing routes. Moreover, it is found that the ternary composites display relatively low resistance to oxidation as a consequence of the lower SiC content in the composite, in comparison with the binary systems, as well as to the presence of transition metal carbides. Indeed, although the latter ones are potentially able to increase mechanical and resistance to ablation properties, they tend to oxidize rapidly to form MO2 and COx, so that the resulting porosity make the material bulk more sensitive to oxidation.