Synthesis, sintering, mechanical properties, and oxidation behavior of (Zr0.5Me0.5)B2 (Me = Ta, Hf) solid solutions

In this work, (Zr0.5Ta0.5)B2 and (Zr0.5Hf0.5)B2 solid solutions are produced in dense form by coupling the Self-propagating High-temperature (SHS) and Spark Plasma Sintering (SPS) routes. A single boride phase solid solution is formed in both cases by SPS (1850 °C, 20 min) from the multiphasic ceramic powders preliminarily obtained by SHS. The use of small amounts of graphite during SPS is highly beneficial to eliminate oxide contaminants (from 14.5–16.0 wt% to 2.1–2.8 wt%), improve powder consolidation (from 87-90 % to 97.5–98 %), and make the operating conditions milder. Better mechanical properties are exhibited by the binary ceramics with respect to ZrB2 produced by SHS-SPS. The presence of Ta makes the performance of (Zr0.5Ta0.5)B2 superior compared to the Hf-containing system, with hardness, Young's modulus, and fracture toughness equal to 22.1 GPa, 636.9 GPa, and 2.46 MPa m1/2, respectively. On the other hand, (Zr0.5Hf0.5)B2 shows higher oxidation resistance in flowing and stagnant air at elevated temperatures.