Understanding reaction sequences and mechanisms during synthesis of nanocrystalline Ti₂N and TiN via magnetically controlled ball milling of Ti in nitrogen

Mechanically induced reactive synthesis of TiN via magnetically controlled ball milling of titanium under nitrogen gas was investigated using X-ray diffraction, advanced electron microscopy and Raman spectroscopy. Ball milling of titanium powder with nitrogen gas was performed in Uni-Ball-Mill, with external temperature of the vial and initial pressures of the nitrogen gas monitored, while the milled samples were taken out periodically, both before and after detection of an exothermic ignition point. Before ignition, nitrogen-enriched Ti, small proportions of TiN and very minor amounts of Ti₂N are formed, in addition to the heavily deformed Ti. Raman spectroscopy revealed the pre-ignition products to include off-stoichiometric nitrides (TiN_x) and oxynitride skin (TiO_xN_y). The formation of the new TiN and Ti₂N products before ignition was attributed to the diffusion of highly polarized active N atoms into the mechanically activated clean surfaces of Ti, followed by local reaction. This local reaction is likely promoted by numerous cycles of induced local temperature rise and rapid quenching, large surface area and accumulation of deformation defects. After the exothermic ignition, there was rapid nucleation of new TiN crystals, and simultaneous growth of the pre-existing TiN and the newly formed TiN crystals. This understanding explains the reaction pathways leading to the formation of small proportions of TiN and very minor amounts of Ti₂N before ignition and the thin-plated TiN after ignition.