Combustion synthesis of TiN-TiB2 composites in Ti/BN/N2 and Ti/BN/B reaction systems

An experimental study on in situ formation of TiN-TiB2 composites with a wide range of compositions by self-propagating high-temperature synthesis (SHS) was conducted in two reaction systems. Combustion of sample compacts made up of titanium (Ti) and boron nitride (BN) powders was initiated under nitrogen pressure for the synthesis of composites with TiN contents of 75-87.5 mol%. Experiments with reactant compacts from the Ti-BN-B powder blends were performed in argon (Ar) for the preparation of composites with 50-75 mol% of TiB2. Self-sustained reactions featuring a distinct combustion front were observed in both systems. Due to the involvement of solid and gaseous reagents in the former reaction system, sample porosity and dilution by TiN made a significant impact on the degree of nitridation. On the contrary, combustion synthesis in the latter test configuration was essentially governed by the reaction between solid constituents. Experimental results show direct formation of a composite with 75 mol% TiN in nitrogen of 1.48 MPa from an undiluted compact of 2Ti + BN at 60% TMD. For the preparation of composites with TiN contents as high as 83 and 87.5 mol%, complete conversion was achieved by 60% TMD TiN-diluted samples with starting compositions of 2.5Ti + BN + 0.5TiN and 3Ti + BN + TiN, respectively, at 1.14 MPa of nitrogen. Through the use of elemental boron as one of the reactants along with the implementation of SHS reactions in Ar, TiN-TiB2 composites with TiB2 mole fractions of 50-75% were obtained from the Ti-BN-B powder compacts at different stoichiometries. In addition, complete conversion was also observed on a 1.5Ti + BN compact which yielded the composite with a molar ratio TiN/TiB2 = 67/33 under an Ar environment.