Phase Transformation Pathways of Titanium Nitride in Oxidation Process: Investigated by Environmental Scanning/Transmission Electron Microscopy

Titanium nitride (TiN), as a classic ceramic material, has been widely used in a variety of practical applications due to its unique physical and inert chemical properties. The oxidation behaviour of TiN is detrimental to both its engineering and functional applications. However, the intuitive recognition of the oxidation mechanism and corresponding structural failure is ambiguous so far. This work reports the determination of the grain size effects on their oxidation pathways of TiN via careful atomic structure characterization. Nano-sized TiN single crystal undergoes a two-step phase transformation, i.e. anatase and subsequently rutile TiO2, whereas micron-sized TiN single crystal or polycrystals directly convert to rutile TiO2 upon oxidation. Notably, numerous N2 nanobubbles generate in the micron-sized TiN oxidation process, which preferentially embedded inside of rutile TiO2 grain boundaries for single crystal and the oxide/nitride interface for polycrystals. The atomic structure of a nanobubble edge indicates it is generally epitaxial bonding with {101} and {200} of rutile TiO2. Besides, the oxidation induces great volume expansion in all types of TiN, but only micron-sized single crystals are fractured and pulverized dramatically, accompanying with rationally impairing the mechanical properties. The fundamental exploration of oxidation behaviour at atomic scale would help to guide rational structural/phase design of advanced ceramic materials. (a) Schematic diagram of the oxidation process of different sizes of titanium nitride. (b) Load depth curves of titanium nitride coatings before and after oxidation obtained by nanoindentation. (c) Detailed characterization of the bubble region in micron-sized TiN.