Processing and Characterization of Zr-, Hf- and Ta-Based Ultra High Temperature Ceramics

The fabrication of MB2-SiC and MB2-MC-SiC (M=Zr, Hf, Ta) Ultra High Temperature Ceramics (UHTCs) is investigated in this work by combining Self-propagating High-temperature Synthesis (SHS) and Spark Plasma Sintering (SPS). Zr, Hf or Ta, B4C, Si, and graphite powders are first reacted by SHS to successfully form in-situ the desired composites. For the case of the Tabased systems, a 20 min ball milling treatment is also required to mechanically activate the SHS reactions. The resulting powders are subsequently consolidated by SPS at 1800 °C and P=20 MPa, thus obtaining products with densities greater than 96% within 30 min of total processing time. Hardness, fracture toughness, and oxidation resistance of the resulting dense UHTCs are among the best when compared to the corresponding values reported in the literature relative to analogous products synthesized by alternative, more energy-consuming and less rapid methods. Thermogravimetric analysis results evidenced the beneficial effect of SiC on the oxidation resistance of the composite materials, while the presence of transition metal carbides appears to be inconvenient from this point of view. This is because, they rapidly oxidize at high temperature to form MxOy and carbon oxides which lead to an increase in sample porosity thus enhancing product oxidation.