Study on the interface structure of transition metal nitride thin films by advanced electron microscopy

Multilayer coatings have excellent hardness and toughness, which largely depends on their microscopic interface structure. Therefore, characterizing the interface of materials on the atomic scale is very important for a comprehensive understanding of the mechanical properties of multilayer coatings. This thesis is to study the interface characteristics of transition metal nitrides (TMNs) multilayer hard coatings and their interface-related phenomena by a spherical aberration-corrected transmission electron microscopy. The first part of this thesis studies the interface effect on the metastable phase stability. HRTEM studies reveal that the different growth orientation exhibits a dissimilar capability to stabilize the metastable phase. Contrary to the <111> orientation, in both <110> and <100> orientations, several unusually highly mismatched cubic-CrN/wurtzite-AlN interface structures form as soon as wurtzite-AlN is present. DFT studies suggest that the larger critical thickness of the AlN layers in <100> and <110> orientation is allowed by the lower surface energy and higher cubic/wurtzite interfacial energy. These findings enrich the metastable-phase stabilization mechanism in multilayer and further offer a pathway for the design of high-quality superlattice coating. The second part of this thesis studies the phenomenon of interface intermixing driven by plastic deformation. Using the atomic-resolution electron microscopy and spectroscopy, together with theoretical calculations (MD and DFT simulation), a nanoindentation-induced large-scale alloying (forming a solid solution) at the TiN/AlN superlattice interfaces is first revealed. The alloying substantially reduces the interface density and leads to a sharp drop in the dislocation density, consequently reducing the achievable strength. These findings rationalize the mechanism responsible for the currently not well-understood inverse Hall-Petch effect in superlattice coatings with a relatively small bilayer period. In the final part, the TMN CrN twinning at the heterophase interface and the deformation mechanism of the twin interface have been studied in detail. A high density of rock-salt CrN twins with ?3{112} incoherent twin boundaries (ITB) was found in the {111}||{0002} textured film. It has been proved that the high density of twins is related to the existence of the wurtzite {0002} interface terrace. Based on the HRTEM observations and atomic-model analyses, supplemented with theoretical calculations, several nucleation modes of twins with ?3 {112} ITB and ?3 {111} CTB (coherent twin boundary) are proposed. Simultaneously, the migration behavior of CrN CTB is further studied in this thesis via in-situ atomic-resolution TEM. It is found that CTB migration is associated with a boundary structure alternating from an N-terminated to Cr-terminated, involving Cr and N atom respective motion, i.e., asynchronous CTB migration. Local strain analysis and DFT simulations further reveal the dynamic and thermodynamic mechanism of such asynchronous migration. These findings uncover an atomic-scale dynamic process of defect nucleation and CTB migration in a binary system, which provides new insight into the atomic-scale interface deformation mechanism in TMNs.