ICMCTF2014 Session B7: Computational Design and Experimental Development of Functional Thin Films

Friday, May 2, 2014 8:00 AM in Room Royal Palm 4-6

Friday Morning

Time Period FrM Sessions | Abstract Timeline | Topic B Sessions | Time Periods | Topics | ICMCTF2014 Schedule

Start Invited? Item
8:00 AM B7-1 Ab-initio Simulation of Vacancy Formation in Ti0.5Al0.5N Alloy: From the Diverse Local Environments Towards Self-diffusion
Ferenc Tasnádi, Igor Abrikosov (Linköping University, IFM, Sweden); Magnus Odén (Linköping University, IFM, Nanostructured Materials, Sweden)

Vacancies are common point defects in crystalline materials. Vacancy-mediated atomic transport, i.e. self-diffusion has fundamental importance in phase transformations, nucleations or even in the mechanical properties of hard coatings through spinodal decomposition, like in TiAlN alloys. Self-diffusion is determined by three major parameters: the (i) equilibrium vacancy concentration, the (ii) vacancy migration energy barrier and the (iii) vibrational entropy, what is connected to the jump-rate. Although self-diffusion is a fundamental physical phenomenon, it is purely studied for disordered alloys. Vacancy formation is believed to have a reasonably local, short-ranged environmental dependence. This fact has been used to develop a local cluster expansion technique [1] and applied recently in Al-Li, LixO2 alloys [1,2] and for hydrogen diffusion in Pd-Cu alloys [3].

Here we present how to perform a local environment analysis of metal (Ti, Al) and nitrogen (N) vacancy formations in Ti0.5Al0.5N using the Special Quasirandom Structure model [4] with the corresponding correction techniques. We underline the complexity of the problem and estimate the equilibrium vacancy formation energy and vacancy concentration in Ti0.5Al0.5N. Furthermore, we show that in non-equilibrium conditions – for example, during segregation, spinodal decomposition – vacancies are more favorable in some particular local environments, what can be selected by counting the number of first and second neighboring Al/Ti atoms around the vacant site, or by short-range order parameters.

[1] A. Van der Ven and G. Ceder, Phys. Rev. B 71, 054102 (2005).

[2] A. Van der Ven et al., Phys. Rev. B 58, 2975 (1998).

[3] L. Qin and C. Jiang, International Journal of Hydrogen Energy 37, 12760 (2012). [4] A. Zunger et al., Phys. Rev. Lett. 65, 353 (1990).

8:40 AM B7-3 Room-Temperature Plasticity in ZrC: Role of Crystal Anisotropy
Sara Kiani, Suneel Kodambaka, Christian Ratsch (University of California, Los Angeles, US); Andrew Minor (University of California, Berkeley; National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, US); Jenn-Ming Yang (University of California, Los Angeles, US)
NaCl structure Group IV and V transition-metal carbides are hard, stiff, and high-melting solids with excellent wear, abrasion, and corrosion resistances, and are commonly used in advanced high-temperature structural applications. In this presentation, we report results obtained from in situ transmission electron microscopy (TEM) studies and density functional theory calculations of uniaxial compression of ZrC(100) and ZrC(111) single crystals. In situ TEM observations show that dislocation motion and tangling lead to plastic deformation in ZrC(111), while slip along {110}<1-10> is dominant in ZrC(100). We find that the yield strengths of ZrC crystals increase with decreasing size. Interestingly, yield strengths of uniaxially compressed ZrC(111) crystals are lower than those of ZrC(100), unexpected for NaCl-structured compounds. Based upon the density-functional theory calculations, we attribute the orientation-dependent yield strengths to relatively lower energy barrier for shear along {110}<1-10> compared to {110}<1-10>. Our results provide important insights into the effects of crystal size and orientation on room-temperature plasticity. We expect that similar phenomena are likely to exist in other cubic-structured transition-metal carbides and nitrides.
9:00 AM B7-4 Ab Initio Guided Design of Corundum Type (Al1-x-yCrxMy)2O3 Thin Films
Christian Koller (Vienna University of Technology, Austria); Jürgen Ramm (Oerlikon Balzers Coating AG, Liechtenstein); Szilárd Kolozsvári (Plansee Composite Materials GmbH, Germany); David Holec (Montanuniversität Leoben, Austria); Jörg Paulitsch, Paul Heinz Mayrhofer (Vienna University of Technology, Austria)

Sophisticated alloying concepts are of utmost importance for application oriented coating development in order to obtain specifically tailored and optimised material properties allowing for extended application ranges. Recent studies on borides, nitrides, or oxides have proven the effectiveness of combining first principle calculations with experimental developments in obtaining an atomistic-to-macroscopic understanding of high performance materials. In this work we describe the impact of several selected alloying elements (M) on the quasi-binary system of (Al1-xCrx)2O3 with focus on their capability to promote the desired corundum type α phase.

Based on ab initio calculated energies of formation for three different crystallographic structures (α, cubic B1-like, and cubic γ) it was investigated if elements such as B, Si, Hf, Ta, or Y promote the formation of metastable cubic phases instead of the desired α phase. The findings are compared with coatings synthesised by reactive cathodic arc evaporation. In the case of Fe alloying, for example, our predictions reveal no explicit impact on the phase stability sequence α, B1-like, and γ, whereas experiments suggest an increased amount of α phase fractions. Detailed analysis of the binding characteristics and structural defect sensibility provides an atomistic understanding of the sometimes observed discrepancy between calculations and experiments. This advanced approach allows for a knowledge-based development of high performance Al-Cr-based oxide coatings.

10:00 AM B7-7 Molecular Dynamics Study of the Growth of Various Crystalline Phases of Metal Oxides
Jiri Houska (University of West Bohemia, Czech Republic); Stanislav Mráz, Jochen Schneider (RWTH Aachen University, Germany)

Thin films of crystalline metal oxides are of high interest due to a wide range of functional properties. Because of different properties exhibited by individual phases, it is necessary to define pathways for preparation of desired phases. We study the growth of individual phases using atom-by-atom molecular dynamics (MD) simulations (>=3000 deposited atoms per simulation). We focus on the effect of intrinsic process parameters such as particle energy, ion fraction in the particle flux, growth temperature and growth template. We report a methodological progress: while most of the structures of various materials obtained (predicted) previously by classical MD are amorphous, we study the growth of multiple competing crystalline phases described by a single interaction potential. We identify which interaction potentials allow such simulations and which do not.

In the case of TiO2, experiments indicate that the deposition of rutile requires higher temperatures and/or energies compared to anatase. However, MD simulations [1] allow us to disentangle crystal nucleation and crystal growth, and show that the growth of (previously nucleated) rutile can take place in a wider range of process parameters compared to anatase.

In the case of Al2O3, MD simulations [2] allow us to identify that an energy of 50-70 eV (at 300 K) or 30-40 eV (at 800 K) is ideal for the growth of (previously nucleated) alpha-Al2O3. We show that all atom impacts have to be considered separately, i.e. all arriving atoms have to have the correct energy (correct average energy per atom is insufficient).

In all cases, the crystal growth is supported (the amorphization is slowed down) if the crystal (column) is sufficiently wide compared to the thermal spike size. In such a case, the undamaged crystal cells support "healing" of the damaged neighboring cells.

Phenomena observed experimentally are in agreement with the MD results. Collectively, the results provide an insight into the complex relationships between process parameters and deposited film structures. Consequently, they allow one to tailor the synthesis pathways for the production of metal oxide thin films, considering conditions for nucleation and different conditions for growth.


European Regional Development Fund through project "NTIS - New Technologies for Information Society", European Centre of Excellence, CZ.1.05/1.1.00/02.0090, and Alexander von Humboldt Foundation through fellowship No. 1140782.


[1] J. Houska, S. Mraz and J.M. Schneider, J. Appl. Phys. 112, 073527 (2012)

[2] J. Houska, Surf. Coat. Technol., in print (2013)
10:20 AM B7-8 Lattice Ordering Effects on Toughness Enhancement in TiN and VN Thin Films Alloys
Daniel Edström, Davide Sangiovanni, Valeriu Chirita, Lars Hultman (Linköping University, IFM, Thin Film Physics Division, Sweden)

Enhanced toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions, in modern applications. In our previous Density Functional Theory (DFT) investigations, we predicted significant improvements in the hardness/ductility ratio of several pseudobinary B1 NaCl structure transition-metal nitride alloys, obtained by alloying TiN or VN with NbN, TaN, MoN and WN [1, 2]. The initial calculations, which were carried out on model, highly ordered configurations with Cu-Pt ordering on the cation sublattice, reveal that the electronic mechanism responsible for toughness enhancement stems from the high valence electron concentration (VEC) of these alloys, and ultimately allows a selective response to tetragonal and trigonal deformations.

Recently, these results have been validated experimentally. Single-crystal V0.5Mo0.5N/MgO(001) [3] and V0.6W0.4N/MgO(001) [4] alloys, were grown by dual-target reactive magnetron sputtering, together with VN/MgO(001) and TiN/MgO(001) reference samples. The V0.5Mo0.5N films exhibit hardness >50% higher than that of VN, and, in contrast to nanoindented VN and TiN reference samples, which suffer from severe cracking, the V0.5Mo0.5N films do not crack. No ordering on the cation sublattice is observed in the V0.5Mo0.5N films, however, the onset of W ordering on adjacent {111} planes of the metal sublattice, is observed in V0.6W0.4N alloys.

Here we present new DFT results, which address the issue of lattice ordering effects on the mechanical properties of these pseudobinary alloys. Our investigations concentrate on V0.5Mo0.5N, V0.5W0.5N, Ti0.5Mo0.5N and Ti0.5W0.5N alloys obtained by alloying TiN and VN with WN and MoN. Our calculations, carried out for structures with increasing levels of disorder, reveal that while the degree of electronic structure layering, i.e. the formation of alternating layers of high and low charge density upon shearing, becomes less pronounced in disordered configurations, the overall VEC effect is not affected. The essential feature in the disordered alloys, as initially predicted for highly ordered configurations, remains the increased occupancy of electronic d-t2g metallic states, which allows the selective response to tensile/shearing stresses, and explains the enhanced toughness confirmed experimentally for V0.5Mo0.5N films.

[1] D. G. Sangiovanni et. al. Phys. Rev. B 81 (2010) 104107.

[2] D. G. Sangiovanni et. al. Acta Mater. 59 (2011) 2121.

[3] H. Kindlund et. al. Appl. Phys. Lett. Materials, 1 (2013) 000000.

[4] H. Kindlun et. al. J. Vac. Sci. Technol. A 31 (2013) 040602.

10:40 AM B7-9 A Computational Approach to Designing Boron Based Coatings
Holger Euchner, Jörg Paulitsch, Paul Heinz Mayrhofer (Vienna University of Technology, Austria)

Computational materials science has proven to be an extremely useful tool for developing high performance materials tailor made for specific applications. Recent studies show, that first principle calculations allow to reliably predict crystal structure, phase stability or elastic properties of ceramic-like hard coatings. However, while structure optimization and determination of elastic properties are nowadays rather standard calculations, hardness still remains a quantity that is difficult to access computationally. Here we present a detailed ab initio study on hardness and elastic properties of transition-metal (TM) diborides, which due to their intrinsically high hardness values are highly acknowledged for a wide range of industrial applications. Based on a semi-empirical approach, mainly using the concept of bond strength, the hardness values of several binary TM diborides, like TiB2 or WB2, as well as ternary TM diborides, e.g. TiAlB2 and WAlB2, were evaluated and compared with experimental data obtained by nanoindentation on sputter deposited coatings.

Our results may indicate possible pathways for further advances in the design of tailored functional materials, based on atomistic scale simulations.

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