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

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 (Al_1-xCr_x)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.

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.

Acknowledgment

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.

References

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

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 V_0.5Mo_0.5N/MgO(001) [3] and V_0.6W_0.4N/MgO(001) [4] alloys, were grown by dual-target reactive magnetron sputtering, together with VN/MgO(001) and TiN/MgO(001) reference samples. The V_0.5Mo_0.5N 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 V_0.5Mo_0.5N films do not crack. No ordering on the cation sublattice is observed in the V_0.5Mo_0.5N films, however, the onset of W ordering on adjacent {111} planes of the metal sublattice, is observed in V_0.6W_0.4N 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 V_0.5Mo_0.5N, V_0.5W_0.5N, Ti_0.5Mo_0.5N and Ti_0.5W_0.5N 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-t_2g metallic states, which allows the selective response to tensile/shearing stresses, and explains the enhanced toughness confirmed experimentally for V_0.5Mo_0.5N 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.

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.