Effects of B4C and BN additions on formation of NbB2–Al2O3 composites from reduction-based combustion synthesis

Abstract

Formation of NbB2–Al2O3 composites was investigated by reduction-based combustion synthesis in the SHS mode. Amorphous boron, B4C, and BN were adopted as the source of boron in three reaction systems composed of Nb2O5–Al–B, Nb2O5–Al–B–B4C and Nb2O5–Al–B–BN. In addition, the effect of excess boron up to 30 at% was examined. For the B- and B/BN-based reaction schemes, the synthesis process involved not only aluminothermal but borothermal reduction of Nb2O5. The latter caused a significant loss of boron in the form of gaseous oxides, and therefore, their resulting products contained NbB, Nb3B4, and NbB2 even if the samples were formulated with boron of 30 atom.% in excess. Besides acting as a boron precursor, B4C was showed to effectively suppress the borothermal reaction for the B/B4C-adopted sample. Consequently, a small amount of excess boron of 10 atom.% was sufficient for the B/B4C-adopted sample to produce the NbB2–Al2O3 composite with trivial minor phases.