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

In this work, (Zr0.5Ta0.5) B-2 and (Zr0.5Hf0.5) B-2 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 degrees 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)B-2 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 m(1/2), respectively. On the other hand, (Zr0.5Hf0.5) B-2 shows higher oxidation resistance in flowing and stagnant air at elevated temperatures.