Densification and Mechanical Behavior of HfC and HfB2 Fabricated by Spark Plasma Sintering

Hafnium diboride (HfB₂)- and hafnium carbide (HfC)-based materials containing MoSi₂ as sintering aid in the volumetric range 1%–9% were densified by spark plasma sintering at temperatures between 1750° and 1950°C. Fully dense samples were obtained with an initial MoSi₂ content of 3 and 9 vol% at 1750°–1800°C. When the doping level was reduced, it was necessary to raise the sintering temperature in order to obtain samples with densities higher than 97%. Undoped powders had to be sintered at 2100°–2200°C. For doped materials, fine microstructures were obtained when the thermal treatment was lower than 1850°C. Silicon carbide formation was observed in both carbide- and boride-based materials. Nanoindentation hardness values were in the range of 25–28 GPa and were independent of the starting composition. The nanoindentation Young's modulus and the fracture toughness of the HfB₂-based materials were higher than those of the HfC-based materials. The flexural strength of the HfB₂-based material with 9 vol% of MoSi₂ was higher at 1500°C than at room temperature.