Preparation of borides in Nb–B and Cr–B systems by combustion synthesis involving borothermic reduction of Nb2O5 and Cr2O3

An experimental study on the preparation of metal borides in the Nb–B and Cr–B systems was conducted by self-propagating high-temperature synthesis (SHS) involving the reduction of Nb2O5 and Cr2O3 by amorphous boron. The starting stoichiometry of the reactant compact was shown to make a great impact on the combustion behavior and the phase composition of the final product. For the powder compacts of Nb2O5 and boron, self-sustaining combustion was performed under a molar ratio of B/Nb2O5 between 5 and 10, but complete reduction of Nb2O5 was achieved when B/Nb2O5 ≥ 8. Partial reduction of Nb2O5 caused a decrease in the combustion temperature and velocity, and was responsible for the presence of NbO2 in the final products. For the samples with stoichiometry of 6 ≤ B/Nb2O5 ≤ 8, three boride phases NbB, Nb3B4, and NbB2 were synthesized. An increase in the boron content up to B/Nb2O5 = 8.5–10 resulted in not only full reduction of Nb2O5, but also formation of single-phase NbB2. On the other hand, the SHS process involving Cr2O3 and boron was feasible for the powder compacts of 4 ≤ B/Cr2O3 ≤ 9, wherein the highest combustion temperature and the fastest reaction front were observed in the compact with B/Cr2O3 = 6. During combustion Cr2O3 was fully reduced, leading to the formation of three borides Cr5B3, CrB, and CrB2 in either monolithic or composite form. With a boron content more than the stoichiometric amount, the powder compacts of B/Cr2O3 = 4, 5, and 9 yielded single-phase Cr5B3, CrB, and CrB2, respectively.