Fabrication of Dense Zr-, Hf- and Ta-based Ultra High Temperature Ceramics by Combining Self-Propagating High-Temperature Synthesis and Spark Plasma Sintering

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 powders are first reacted by SHS to successfully form the desired composites. For the case of the Ta based composites, a 20 min ball milling treatment of the starting reactants is required to mechanochemically activate the corresponding synthesis reactions. The resulting powders are then subjected to consolidation by SPS. In particular, by setting a dwell temperature level equal to 1800 degrees C, a mechanical pressure P=20 MPa, and a non isothermal heating time t(h) = 10 min, products with relative densities greater than 96% can be obtained for all systems investigated within 30 min of total processing time. The characteristics of the resulting dense UHTCs, i.e. hardness, fracture toughness, and oxidation resistance, are similar to, and in some cases superior than, those related to analogous products synthesized by alternative, less rapid, methods. 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 MC. In fact, although the latter ones are potentially able to increase the resistance to ablation of the composites, they oxidize rapidly to form MO2 and carbon oxides which lead to sample porosity increase thus enhancing product oxidation.