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 degrees-1900 degrees 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 (4.1 M Pa . m(1/2)) 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 degrees and 1500 degrees C. Contrary to the significant strength decrease observed for the ZrB2-based materials at elevated temperature, the HfB2 composite retained similar to 86% of its room temperature strength up to 1500 degrees C (similar to 600 MPa).