Point defects stabilise cubic Mo-N and Ta-N

By Dean Williams on September 6, 2016

Does ordering of vacancies and their type play any role on stability of MoN and TaN? The authors of this recent Open Access article in Journal of Physics D investigate:

Modern tools often comprise many pieces, each with a specific task. For example, surface protection in a form of hard thin films, called coatings, of transition metal nitrides has proven a very successful concept for cutting tool applications. Such coatings are often synthesised by physical vapour deposition (PVD) techniques which operate away from thermodynamic equilibrium. Consequently, this leads to a high amount of point defects and/or stabilisation of metastable phases.

In this work we focused on two binary nitrides, MoN and TaN, and investigate the impact of various point defects on the stability of their metastable cubic variant with a rocksalt structure. To do so, we employed quantum mechanical calculations within the framework of Density Functional Theory (DFT) as implemented in a widely used software, Vienna Ab initio Simulation Package (VASP).

Our calculations clearly demonstrate that defected cubic structures exhibit lower energy of formation, E_f, than corresponding perfect materials. Already this is an anomaly. By definition, defects should be something unwanted, and hence are intuitively expected to increase energy of the system. The structures with the lowest E_f contain about 10 at.% defects. These are Ta vacancies in the case of TaN (hence leading to Ta_1-xN compositions). On the other hand, both N and Mo vacancies lower the energy of formation of MoN by similar amounts. Moreover, this holds true even in the case when both are present at the same time, hence forming so-called Schottky defects.

Apart from the vacancy type, we have also investigated the impact of their ordering in the structure. Surprisingly, both disordered and partially ordered Ta vacancies exhibit almost the same E_f. The same is true also for N vacancies in TaN, whereas MoN strongly prefers disordered vacancies irrespective of their type.

Figure 1: Formation energy of metal (left panels) and N (right panels) vacancies in c-MoN (upper panels) and c-TaN (lower panels), as a function of vacancy concentration and the degree of order characterised by
a parameter α (for detailed explanation please see our paper). Image taken from Nikola Koutná et al 2016 J. Phys. D: Appl. Phys. 49 375303, © IOP Publishing, All Rights Reserved.

To summarize, MoN and TaN had been proposed as candidate materials to design novel coating with a reduced coefficient of friction and improved thermal stability. When synthesised by PVD to
form a cubic structure, they are prone to be highly off-stoichiometric in composition. However, the type and ordering of the energetically most preferable vacancies is significantly different in both materials, making them both exciting and difficult to understand and characterise. In our long-term research we focus on nitrides, oxides, and borides, but we also study carbon-based nanoparticles and various intermetallic systems. Our particular strength is to efficiently combine theoretical predictions with experimental synthesis and characterisation in order to gain insights beyond experimental resolution, to discover novel design concepts, and to preselect materials with desired properties for experimental confirmation.

About the authors

Nikola Koutná is a PhD student reading Condensed Matter Physics at the Masaryk University in Brno, Czech Republic, where she has recentlty received her master’s degree in Physics (2016) and a bachelor’s degree in Mathematics (2015). Besides, she collaborates with the group of Professor Mayrhofer in Vienna. Her research interests include ab initio modelling of point defects in nitrides and oxides, currently focusing on the Mo-N, Ta-N, and Mo-Ta-N systems. Her work further relates to discussing thermodynamics and mechanical properties of materials.

David Holec graduated in 2005 in Mathematics and Physics from the Masaryk University in Brno, Czech Republic. In 2008, he received a PhD degree in Materials Science from the University of Cambridge, UK. After that he was appointed as a postdoctoral research fellow and later as a University Assistant at the Dept. of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Austria. His passion are all sorts of materials modelling, with a major expertise in DFT. He has published more than 65 peer-reviewed papers, 2 books and 2 book chapters.

Ondřej Svoboda has received a BSc degree in Mechanical Engineering from the Brno University of Technology (BUT), Czech Republic, in 2016. Currently, he is reading a Master programme Applied Sciences in Engineering at BUT. His research interest include ab initio modelling of mechanical properties ordered and disordered intermetallic phases in the Fe-Al system.

Fedor F. Klimashin graduated in 2012 in Materials Science (TU Bergakademie Freiberg, Freiberg, Germany) and Physical Chemistry (National University of Science and Technology “MISIS”, Moscow, Russia). In 2016, he received a PhD degree in Materials Science from TU Wien, Austria, where he subsequently started his postdoctoral research. His focuses primarily on the in-depth understanding and tuning of vacancies in various nitride materials, using PVD synthesis and various material characterisation methods.

Paul H. Mayrhofer is University Professor of Materials Science at the Institute of Materials Science and Technology, Technische Universiaet Wien, TU Wien, since 2012. Paul is also Guest Professor at the Central South University, Changsha, Hunan (China). He received a Ph.D. in 2001 and Habilitation in 2005 in Materials Science at the University of Leoben. His research activities focus on the development and characterization of vapor phase deposited nanostructured materials by a combination of computational and experimental material science. He has pioneered age hardening within hard ceramic thin films based on ternary nitrides and borides.