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 degrees and 1950 degrees C. Fully dense samples were obtained with an initial MoSi2 content of 3 and 9 vol% at 1750 degrees-1800 degrees 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 degrees-2200 degrees C. For doped materials, fine microstructures were obtained when the thermal treatment was lower than 1850 degrees 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 degrees C than at room temperature.