Al content dependence of reaction behaviors and mechanism in combustion synthesis of TiB2–AlN-based composites

The TiB2–AlN and TiB2–AlN–TiN ceramic matrix composites were successfully produced by self-propagating high-temperature synthesis (SHS) reactions from the Al–Ti–BN compacts through the manipulation of the Al reactant content. The effects of the Al content on the adiabatic and practical maximum combustion temperatures, wave velocity, phase composition and microstructure of the synthesized products, and reaction mechanism were investigated in detail. Thermodynamically and practically, the reaction was most exothermic for the Al content being 35.5 wt.% in the reactants to produce only the dual TiB2–AlN phases. Kinetically, the reaction commenced with the formation of Al3Ti, and then proceeded with the interactions of the Al–Ti intermetallics/melts with BN to yield the AlN and TiB2 phases. The substantial reaction between the elemental Ti and BN into formation of TiN took place only in the low Al content samples. On the other hand, the presence of considerable amount of the TiN grains effectively inhibited the abnormal growth of the TiB2 grains, leading to very fine microstructures; whereas, the grain growth of TiB2 and AlN as well as their inhomogeneous distribution was enhanced by the increase in the Al reactant, while TiN was completely inhibited from development.