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Abstract The improvement of comprehensive properties including hardness, toughness, wear, and corrosion resistance in the transition-metal carbides/nitrides (TMC(N)) films, especially avoiding the trade-off between the hardness and toughness, is strongly required for various applications. Herein, we provide a new strategy, by activating solid-state dewetting during layered deposition, to accomplish the self-assembly of ordered TaC@Ta core–shell-like nanocomposite film consisting of TaC nanocrystallines encapsulated with thin pseudocrystal Ta tissues (∼1.5 nm). The novel core−shell-like structure can simultaneously achieve superhardness (∼45.1 GPa) mainly dominated by the Orowan strengthening mechanism and high toughness attributed to the indenter-induced phase transformation from the pseudocrystal to body-centered cubic α-Ta, together with drastically enhanced wear and corrosion resistance. Furthermore, thin pseudocrystal Ta encapsulated layer (∼1.5 nm) in the TaC@Ta core–shell-like structure helps for promoting the formation of lubricious TaO x Magnéli phase during sliding, thereby further dropping the coefficient of friction relative to TaC monolayer. Apparently, solid-state dewetting may provide a new route to construct ordered TMC(N)@TM core–shell-like nanocomposite structure capable of combining superhardness, high toughness, low friction, and superior wear and corrosion resistance. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

A stoichiometry-driven phase transition from rocksalt to “nitrogen-rich” structure exists in group-IVB transition metal nitride films. As this phase transition is critical in controlling the film properties it has attracted numerous studies. However, researchers are still divided with regard to the structural identity of this “nitrogen-rich” phase, not to mention detailed exploration of the phase transition mechanisms. In this study, we confirmed that the “nitrogen-rich” phase in hafnium nitride (HfN x ) films had a cubic Th 3 P 4 structure of space group symmetry of I- 43 d (220), namely c -Hf 3 N 4 . The confirmation was obtained by combining the first-principle calculations with a series of experiments: Selected Area Electron Diffraction, High Resolution Transmission Electron Microscopy, Raman, Gracing Incident X-ray Diffraction and X-ray Photoelectron Spectroscopy. The mechanisms of the phase transition were elucidated through calculations on enthalpy of formation ( EOF ). The experimental results agree well with the theoretical calculations. We conclude that with increasing nitrogen, phase transition takes place from rocksalt ( δ -HfN) to c -Hf 3 N 4 through three stages of structural evolution: δ -HfN (containing Hf vacancies) → mixture of ( δ -HfN + c -Hf 3 N 4 ) → c -Hf 3 N 4 . The driving force of the phase transition is energy minimization. The three stages of structural evolution are explained by comparing the EOF of the δ -HfN and c -Hf 3 N 4 phases. As the phase transition takes place, the hafnium nitride film morphs from a conductive and opaque metal into an insulating and transparent semiconductor. [ABSTRACT FROM AUTHOR]