Formation of chromium borides by combustion synthesis involving borothermic and aluminothermic reduction of Cr2O3

Chromium borides of various phases were fabricated through combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) by adopting the powder compacts of Cr2O3 + xB (with x = 4–9) and Cr2O3 + 2Al + yB (with y = 1–8). Because aluminothermic reduction of Cr2O3 is more exothermic than borothermic reduction, the reaction temperature and combustion front velocity of the Al-added samples are much higher than those of the Cr2O3–B samples. In agreement with the composition dependence of reaction exothermicity, the fastest combustion wave was observed in the compact with x = 6 for the Cr2O3–B sample and y = 4 for the Cr2O3–Al–B sample. According to the XRD analysis, Cr5B3, CrB, and CrB2 were produced in the monolithic form respectively from the Cr2O3–B samples of x = 4, 5, and 9, or in the composite form from samples of other stoichiometries. On the other hand, five different borides were identified in Al2O3-added products. Among them, Cr2B, CrB, and CrB2 were yielded as the sole boride compound from the Al-added samples of y = 1.2, 3, and 7 or 8, respectively. Cr5B3 and Cr3B4 were produced along with CrB as the secondary phase. Based upon experimental evidence, it was found that an excess amount of boron in the reactant mixture was required to facilitate the formation of chromium borides.