Dr
Miroslav Černý
(CEITEC, Brno University of Technology)
The ever more demanding application conditions call for new systems with properties over-performing current materials. Multilayer architecture is one of perspective concept to reach these goals. For example, it has been shown that TiN/VN superlattices with bi-layer period in a range of several nanometers yield hardness values higher that either of the constituent materials [1]. This motivated us to aim on detail exploring the interface impact on the ideal strength of nitride multilayers consisting of various combinations of cubic VN, TiN and AlN.
Epitaxial superlattices having a common $(100)$ plane as the interface were loaded along the $[100]$ direction. Brittle cleavage along the $(100)$ planes was simulated by splitting the system into two rigid blocks to obtain cleavage parameters such as cleavage energy, critical stress, and critical length. The total energies of such systems were calculated using Density Functional Theory as implemented in the Vienna Ab initio Simulation Package (VASP) [2].
Values of the cleavage energy and stress were found to oscillate in VN layers [3]. These values oscillate within a wide range without any damping. However, only minor alternations are evident in the TiN part. Moreover, with increasing distance from the VN/TiN interface, the oscillations inside TiN tend to level out to the value of single-phased TiN (the horizontal dotted line). Similar effects were also obtained for other systems, namely AlN/TiN and AlN/VN. We have been able to link the oscillatory behavior to interplanar distances and charge accumulation.
Summary
First-principles study of cleavage characteristics of nitride multilayers was performed for brittle cleavage of AlN/VN, AlN/TiN and VN/TiN systems. In agreement with previous studies we predict an interface-induced toughening of VN in VN/TiN system (as compared to bulk values), and a similar effect for the VN/AlN system. However, a more detailed analysis revealed that the theoretical critical stress for brittle cleavage largely oscillates (even below the critical stress for the bulk) with the distance from the interface inside the VN layer, a phenomenon not present (or remarkably reduced) in TiN and AlN layers. The oscillating values for critical stress well correlate with the same behavior of interplanar distances and charge density. The origin of these unexpected properties was pinpointed to structural instabilities of cubic VN.
References
[1] U. Helmersson et al., J. Appl. Phys. 62, 481 (1987).
[2] G. Kresse and J. Hafner, Phys. Rev. B 48, 13115 (1993).
[3] P. Řehák, M. Černý, and D. Holec, Surf. Coat. Technol. 325, 410, (2017).
Dr
Miroslav Černý
(CEITEC, Brno University of Technology)
Dr
David Holec
(Montanuniversitat Leoben)
Dr
Petr Řehák
(CEITEC, Brno University of Technology)
