Sintering and Mechanical Properties of ZrB 2 –TaSi 2 and HfB 2 –TaSi 2 Ceramic Composites

Fully dense fine‐grained ZrB 2 ‐ and HfB 2 ‐based composites contaning 15 vol% TaSi 2 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 HfB 2 ‐based composite was harder (22 GPa), stiffer (528 GPa), and tougher (4.1M Pa·m 1/2 ) than the ZrB 2 ‐based composite. Although the room‐temperature flexural strength of the ZrB 2 ‐based composite (830 MPa) was higher than that of the HfB 2 ‐based composite (700 MPa), the opposite was true at 1200° and 1500°C. Contrary to the significant strength decrease observed for the ZrB 2 ‐based materials at elevated temperature, the HfB 2 composite retained ∼86% of its room temperature strength up to 1500°C (∼600 MPa).