PREPARATION OF NANOCRYSTALLINE NBN AND (NB, AL)N POWDERS BY MECHANICAL ALLOYING UNDER NITROGEN ATMOSPHERE

Elemental niobium powder and Nb50Al50 elemental powder mixture were milled in a vibrational ball mill, Spex 8000, under nitrogen atmosphere, and the structures of milled powders and their sintered materials were examined by means of X-ray analysis, SEM and HRTEM observations. The niobium powder absorbed 50 at% of nitrogen during milling. Several types of nitrides were formed during milling, i.e., gamma-Nb4N3 (FCT), delta-NbN (FCC) and delta'-NbN (hexagonal) with increasing nitrogen content. The mean grain size of these nitrides were about 5 nm. After sintering the milled powder at 1573 K and above, the structure consisted of delta'-NbN and epsilon-NbN (hexagonal) phases, which were assumed to be formed during cooling from delta-NbN stable at the sintering temperature. The Nb50Al50 powder mixture absorbed also about 50 at% of nitrogen during milling and formed a solid solution nitride (Nb0.5Al0.5)N with NaCl type structure. By sintering the (Nb0.5Al0.5)N powder at 1573 K and above, the structure decomposed to stable phases of delta-NbN and AlN (hexagonal). In contrast to the case of the Nb-N system, the delta-NbN remained stable after furnace-cooling from sintering temperatures.