In-situ nitriding of CoTi powders for TiN shell formation via diffusion-controlled mechanism

초록

Designing core-shell structured powders with precisely controlled nitride layers remains a challenge in advanced materials engineering. This work introduces a novel route for fabricating titanium nitride (TiN) shells on mechanically alloyed amorphous CoTi powders through diffusion-controlled nitriding. This process reveals a bidirectional elemental transport mechanism that governs shell growth and core phase evolution. The CoTi system was selected due to the strong affinity between titanium and nitrogen, as well as its tendency to form Co2Ti and Co3Ti intermetallics. Co and Ti powders were first alloyed via high-energy ball milling, then exposed to isothermal nitriding (1073-1273 K, 1-5 h) in an environment with a controlled PN2/PAr ratio of 0.1. Inward nitrogen diffusion coupled with outward titanium migration created a TiN shell, as confirmed by SEM, EDS, and XRD. A parabolic growth relationship between shell thickness and the square root of time indicated diffusion-limited kinetics, while Arrhenius analysis yielded an activation energy of 88 kJ/mol for titanium migration. The formation of Co2Ti and Co3Ti beneath the shell arose from titanium depletion at elevated temperatures. This study offers a mechanistic framework for TiN shell formation on alloyed powders, paving the way for surface-engineered particles with coherent ceramic-metal interfaces. The findings highlight the potential of durable core-shell powders structural and composite applications.

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