Mechanisms of Self-Sustained Reaction in Mechanically Induced nanocomposites: Titanium Nitride and Boron

We report the results from the investigation of the mechanisms of self-sustained reaction in mechanically induced TiN/B nanocomposites. The exothermic displacement reaction, 3B + TiN → BN + TiB2, with an adiabatic reaction temperature of 1905 K, was initiated through two different means: localized thermal heating and mechanical stimulation by high-energy ball milling. Comprehensive studies strongly indicate that the chemical interaction in the system involves three main stages. Solid-state mass transfer, likely the substitutional diffusion of B into a nitrogen deficient TiNx crystal lattice, is responsible for the formation of TiB2 during the first stage of the reaction (1350−1500 K) under ambient pressure. Thermal gravimetric analysis coupled with mass spectrometry demonstrated the evolution of gas phase nitrogen from TiN during an intermediate stage of the reaction in the temperature range of 1500−1800 K. Electron microscopy results show the formation of h-BN along the pores present in the initial composite, thus indicating reaction of gaseous nitrogen with boron during the final reaction stage. In addition, propagation characteristics of an oscillatory spin combustion regime in the TiN−3B system were analyzed using high-speed infrared imaging, which suggest a strongly nonlinear spin combustion mode.