Sintering and mechanical properties of ZrB2-TaSi2 and HfB2-TaSi2 ceramic composites

Fully dense fine-grained ZrB2- and HfB2-based composites contaning 15 vol% TaSi2 were produced by hot pressing at 1850-1900°C. Gas formation and mass loss, which occurred during sintering in both systems, were in agreement with thermodynamic predictions. In both composites, the presence of a solid solution formed by the diffusion of tantalum into the boride matrix was observed. The HfB2-based composite was harder (22 GPa), stiffer (528 GPa), and tougher (4.1M Pa .m1/2) than the ZrB2-based composite. Although the room-temperature flexural strength of the ZrB2-based composite (830 MPa) was higher than that of the HfB2-based composite (700 MPa), the opposite was true at 1200°C and 1500°C. Contrary to the significant strength decrease observed for the ZrB2-based materials at elevated temperature, the HfB2 composite retained ~86% of its room temperature strength up to 1500°C (~600 MPa).