Abstract:Niobium boride powders having NbB, NbB₂ and Nb₃B₄ phases in various amounts and single phase NbB powders were successfully synthesized by using powder metallurgy methods from related metal oxide raw materials in the presence of a strong reducing agent. Nb₂O₅, B₂O₃ and Mg powder blends were milled at room temperature by a high-energy ball mill for different time. Subsequently, undesired MgO phase was removed from the milled powders by HCl leaching to constitute NbB-NbB₂-Nb₃B₄ as final products and they were subjected to an annealing process at 1500 °C for 4 h to observe probable boride transformation. Characterization was carried out by XRD, DSC, PSA, SEM/EDX, TEM and VSM. The effects of milling time (up to 9 h) on the formation, microstructure and thermal behavior of the final products were investigated. Reduction reaction took place after milling stoichiometric powder blends for 2 h. Nano-sized NbB-NbB₂-Nb₃B₄ powders in high purity were obtained in the absence of any secondary phase and any impurity via mechanochemistry by milling for 5 h and leaching with 4 mol/L HCl. After annealing, pure and nano-sized NbB-NbB₂-Nb₃B₄ powders transformed to a single NbB phase without leaving behind NbB₂ and Nb₃B₄ phases.

Key words: niobium boride powders; powder metallurgy; mechanochemical synthesis; annealing; microstructure