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.