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 stoichionnetry 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. (C) 2009 Elsevier B.V. All rights reserved.