Effects of excess boron on combustion synthesis of alumina–tantalum boride composites

TaB- and TaB2–Al2O3 in situ composites were fabricated by thermite-incorporated combustion synthesis from the powder mixtures of different combinations, including Ta2O5–Al–B, Ta2O5–Al–B2O3–B, and Ta2O5–B2O3–Al. Effects of excess boron were studied on the combustion dynamics and phase constituents of final products. For the B2O3-containing samples, the reaction was less exothermic and aluminothermic reduction of Ta2O5 and B2O3 was less complete, resulting in the deficiency of boron and the presence of TaO2 and Ta. For the samples containing elemental boron, the occurrence of borothermic reduction of Ta2O5 also caused the loss of boron. Experimental evidence showed that boron in excess of the stoichiometric amount substantially enhanced the formation of tantalum borides, which in turn facilitated the reduction of Ta2O5 by Al. Consequently, the samples rich with boron in the molar proportions of Ta2O5:Al:B=3:10:9 and 3:10:16 (i.e., B/Ta=1.5 and 2.67) were found to be the optimum stoichiometries of producing TaB- and TaB2–Al2O3 composites through a self-sustaining combustion process.