Molecular dynamics simulation of VN thin films under indentation
Graphical abstract
Introduction
Transition metal nitrides, such as vanadium nitride (VN), titanium nitride (TiN), and niobium nitride (NbN), have attracted a great deal of interest due to their excellent physical and mechanical properties. In particular, their nano-multilayered coatings, which consist of alternative superposition of two different transition metal nitrides, have received considerable attention due to the extraordinary resistance against wear, super-hardness and thermal stability. For example, a hardness of over 50 GPa has been obtained for TiN/VN [1] and TiN/NbN [2] multilayers with a modulation periods of 5–10 nm, which is much larger than that of either of its bulk constituents. Several mechanisms have been proposed for the super-hard phenomenon, for example, the increase in the resistance against dislocation motion with the decrease of the modulation period [3], [4], the Hall–Petch effect [5], the interfacial misfit strain effect [6], and the supermodulus effect [7], etc. Among these mechanisms, the influence of dislocation motion has been thought to be one of the main factors in enhancing significantly the hardness of the nano-multilayered coatings [8]. However, whether and how dislocations are generated and migrate in nano-multilayered coatings remain unclear.
To clarify the mechanism for the superhardness of the nano-multilayered coatings, it is requisite to understand the deformation behavior of each of their constituents and uncover their microstructures, even in the atomic scale. First-principles calculation, in principle, is a powerful tool to probe fundamental physical and mechanical properties of materials at the atomic scale. It has been applied to investigate the elastic properties of transition metal nitrides [9], [10], and the interfacial properties of multilayered coatings (e.g. TiN/VN [11], [12], TiN/AlN [13], [14] and CrN/TiN [15]). In addition, first-principles calculation has also been used to investigate the uniaxial tension behaviors of transition metal nitrides along different orientations [16] because it can help us to gain an insight into the mechanisms of inelastic deformation and failure. However, these calculations are usually limited to a small supercell, which may not be sufficient to identify defects and their evolution. Nano-multilayers are often subjected to complicated stresses, e.g. indentation and scratch, which is of extreme challenge to identify whether slips or twins occur when first-principles calculations are used alone.
Molecular dynamics (MD) simulation, as a semi-empirical method, can be applied to investigate the behavior of materials on a much larger scale. It has been applied to investigate tension [17], nanoindentation[18], deposition [19], [20], surface energy [21], [22] and stacking fault energy [23], [24], etc. In our previous work, we performed simulations of the responses of VN layers with perfect lattice under uniaxial tension, where no dislocation or slip was found [25]. As the mechanical properties of ceramic materials may strongly depend on the states of stress, it is necessary to find the deformation and failure mechanisms under more general states of stress, such as compression or indentation. On the other hand, considering the difficulties in the identification of the inelastic deformation and failure mechanisms in the three dimensional (3D) cases, two dimensional (2D) indentation is performed in this article to provide more intuitionistic atomic structures.
The response of a 2D VN (0 0 1) film subjected to an indentation at 0 K with a columnar diamond indenter. The microstructures and their evolution at different stages are systematically analyzed, and the generalized stacking-fault energies as a function of displacement along the 1 1 0 direction in various planes are also calculated. To further verify the validity of the results, the simulations of indentations with various parameters are also performed.
Section snippets
Interatomic potential
The indentation system contains VN film and a diamond indenter, therefore, six interatomic potentials, V–V, N–N, V–N, C–V, C–N and C–C, should be considered. We chose the modified embedded atom method (MEAM) potential [26], [27], [28], [29] for N–N and V–V. The parameters of the MEAM) potentials for the two single elements [27], [28] are listed in Table 1. The atomic interaction between V and N was described with a binary MEAM potential [25], the parameters of which are given in Table 2. The
Deformation of VN films under indentation at 0 K
To gain an insight into the deformation mechanism and rule out the effects by the vibration of atoms, we performed the MD calculations for the indentation of VN (0 0 1) films. The specimen had a lengths of 206 Å, 16.48 Å and 123.6 Å along x ([1 0 0]), y ([0 1 0]), and z ([0 0 1]) directions, respectively. The specimen was first optimized before indentation using the conjugate gradient (CG) algorithm to achieve a stable configuration with the minimum equilibrium energy. All atoms were kept at 0 K by
Conclusions
The indentations on VN (0 0 1) thin films with a columnar diamond indenter were simulated with molecular dynamics. The modified embedded atom method potential was employed to describe the interaction between V and N, and Lennard-Jones potential was used to describe the interaction between the indenter and the film. We analyzed systematically the microstructures and their evolution during the indentation with the centro-symmetry parameters and the atomic configurations of the slices of the VN
Acknowledgements
The authors acknowledge the financial supports from National Natural Science Foundation of China (grant nos. 11332013 and 11272364), the Chongqing Graduate Student Research Innovation Project (grant no. CYB15029), the Scientific Research (B) (grant no. 15H04114), the Challenging Exploratory Research (grant no. 15K14117), the JSPS and CAS under Japan-China Scientific Cooperation Program, the Shorai Foundation for Science and Technology, and National Natural Science Foundation of Chongqing (grant
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