High-temperature reactive synthesis of the Zr–Ta multiboride with a supercomposite structure

This study reports the formation of a ceramic composite consisting only of boride phases with an artificially created hierarchical superstructure. Such a composite was formed during the reaction-driven consolidation process using a mixture of ZrB₂, tantalum, and amorphous boron powders. The homogeneity of the reaction among these powders at 1900°C allowed the forming of a highly reproducible and repetitive superstructure where Ta₃B₄ forms a chain-like mesh that entraps the ZrB₂, ZrB, TaB, and (Zr,Ta)B₂ phases. The multiboride ceramic composite exhibited extremely high hardness: 28.6 ± 3.2 or 22.6 ± 0.6 GPa. These were estimated using 98- and 196-N loads, respectively. At 2000°C, the multiboride composite showed a strength of nearly 400 MPa and fractured in an elastic manner at the loading rate of 2.5 mm/min. This level of strength is usually observed for the bulk zirconium diboride at room temperature. The main factors leading to the improvement of the mechanical properties were considered to be the formation of the (Zr,Ta)B₂ solid solution as well the composite structure.