Densification of ZrB 2 –TaSi 2 and HfB 2 –TaSi 2 Ultra‐High‐Temperature Ceramic Composites

The microstructure of fully dense hot‐pressed ultra‐high‐temperature ceramics (UHTCs), namely ZrB 2 and HfB 2 containing 15 vol% of TaSi 2 , was characterized by X‐ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). ZrB 2 and HfB 2 grains displayed a core–shell structure: the core was constituted by the original MB 2 grain and the shell by a (M, Ta)B 2 solid solution, which grew epitaxially on the core. The compositional misfit between core–shell was accommodated by low‐angle grain boundaries and dislocations pile‐up, especially pronounced in the ZrB 2 ‐based composite, where a higher amount of Ta entered the boride lattice. Ta 5 Si 3 , Ta 4.8 Si 3 C 0.3 , and Ta 5 SiB 2 , with Zr or Hf impurities, were detected at the triple points and wetting of the grain boundaries by a Ta–Si–B–C–O amorphous phase was observed. Based on the new microstructural features detected by TEM, thermodynamic calculations and the available phase diagrams, a densification mechanism for ZrB 2 and HfB 2 with addition of TaSi 2 is proposed. The microstructures of the UHTC composites presented here are compared with composites sintered with the addition of MoSi 2 in the same amount.