Hot pressing of hafnium diboride aided by different sinter additives

Experiments were conducted by hot-pressing to densify HfB₂ commercial powders using HfSi₂ (5 vol%) or B₄C (7 vol%) as sinter additives. The former was very effective as sintering enhancer: 15 min at 1,600 °C was a sufficient condition to achieve near full density. The occurrence of a liquid phase sintering, which substantially enhanced densification during hot-pressing, was found out. The HfB₂–B₄C powder mixture, hot-pressed at 1,900 °C for 40 min, achieved a relative density of 94%. Aside the key role of sintering enhancer, B₄C also allowed the development of a uniform microstructure, preventing an excessive growth of the diboride matrix (2 μm mean size). On the contrary, thanks to the liquid phase that sustained substantial transfer of mass during heating, the HfSi₂-doped composition had an uneven and more obvious grain growth (4 μm mean size), with faceted diboride grains up to 10 μm.