ICMCTF2016 Session B7: Computational Design and Experimental Development of Functional Thin Films

The Modified Embedded Atom Method (MEAM) interatomic potential is used within the classical Molecular Dynamics (MD) framework to perform simulations of important model materials such as TiN, in order to understand the processes which control TiN growth modes on a fundamental level. We report the results of large-scale simulations of TiN/TiN(001) deposition using a TiN MEAM parameterization which reproduces experimentally-observed surface diffusion trends, correctly accounts for Ehrlich barriers at island step edges [1], [2], and has been shown to give results in good qualitative and quantitative agreement with Ab Initio MD based on Density Functional Theory [3], [4]. We deposit 85% of a monolayer of TiN on 100x100 atom TiN(001) substrates maintained at 1200 K, at a rate of 1 Ti atom per 50 ps, for total simulation times of 212.5 ns. We use N/Ti flux ratios of 1, 2, and 4, and incident N energies of 2 and 10 eV, to probe the effects of N₂ partial pressure and substrate bias on TiN(001) growth modes. We observe nucleation of Ti_xN_y molecules; N₂ desorption; formation, growth and coalescence of mixed <100>, <110>, and <111> faceted islands; as well as intra- and interlayer mass transport mechanisms. For N/Ti flux ratios of 1 at 2 eV incidence energy, films exhibit Ti-rich surface regions which serve as traps to nucleate higher layers, leading to multilayer growth. Increasing the N/Ti flux ratio shifts the growth mode to layer-by-layer and modifies the overall film composition from under- to over-stoichiometric. As the N content of films is increased, N-terminated <110>-oriented island edges become increasingly dominant and the substrate vacancy concentration changes from being N- to Ti-dominated. We discuss the implications of these results on thin film growth and process tailoring.

[1] D. G. Sangiovanni, D. Edström, L. Hultman, V. Chirita, I. Petrov, and J. E. Greene, “Dynamics of Ti, N, and TiNx (x=1–3) admolecule transport on TiN(001) surfaces,” Phys. Rev. B, vol. 86, no. 15, p. 155443, 2012.

[2] D. Edström, D. G. Sangiovanni, L. Hultman, V. Chirita, I. Petrov, and J. E. Greene, “Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands,” Thin Solid Films, vol. 558, pp. 37–46, 2014.

[3] D. G. Sangiovanni, D. Edström, L. Hultman, I. Petrov, J. E. Greene, and V. Chirita, “Ab initio and classical molecular dynamics simulations of N₂ desorption from TiN(001) surfaces,” Surf. Sci., vol. 624, pp. 25–31, 2014.

[4] D. G. Sangiovanni, D. Edström, L. Hultman, I. Petrov, J. E. Greene, and V. Chirita, “Ti adatom diffusion on TiN(001): Ab initio and classical molecular dynamics simulations,” Surf. Sci., vol. 627, pp. 34-41, 2014.

Ti_(1-x)Al_xN coatings are of great interest in the cutting tool industry as hard and wear resistant coatings. It is possible to grow thermodynamically metastable c-Ti_(1-x)Al_xN (B1 structure) for a wide range of compositions (typically x < 0.75) with a low temperature (~ 400 °C) coating synthesis, e.g. arc deposition. During cutting, the coating is subjected to elevated temperatures (900-1200°C) and c-TiAlN tend to spinodal decompose and form alternating coherent domains of c-TiN and c-AlN. Accompanying this decomposition is a well-known age hardening. Despite the experimental efforts spent on investigating TiAlN there is a need for a detailed description of the rapid microstructure evolution, its impact on mechanical properties at high temperature such as elastic properties, and gain understanding of the underlying contributions to the observed age hardening.

In order to address these issues, we simulate the microstructure, stress and strain evolution, and Young’s modulus evolution during spinodal decomposition for a range of global compositions (x = 0.3, 0.5, 0.67, 0.75) in 3D at 900°C. The microstructure is analyzed by the auto correlation function revealing a refinement of the structure as well as an increased morphological anisotropy for higher Al content samples. The refinement is explained by the critical wavelength variation and the increased anisotropy is discussed in terms of spatial variation in elastic properties with composition.

The microstructure simulations are based on the Cahn Hilliard partial differential equation (PDE), extended to include the elastic energy contribution. The finite element program package FiPy [1] parallelized on a supercomputer is used to solve the PDE equations. The enthalpy of mixing [2], the elastic compliance tensor [3], and the lattice parameter, are determined by first-principles density-functional theory (DFT) calculations over the entire compositional range of Ti_(1-x)Al_xN and used as input parameters.

[1] J.E. Guyer, D. Wheeler, J. A Warren, FiPy: Partial Differential Equations with Python, Comput. Sci. Eng. 11 (2009) 6–15. doi:10.1109/MCSE.2009.52.

[2] B. Alling, A. Ruban, A. Karimi, L. Hultman, I. Abrikosov, Unified cluster expansion method applied to the configurational thermodynamics of cubic Ti_(1-x)Al_xN, Phys. Rev. B. 83 (2011) 104203. doi:10.1103/PhysRevB.83.104203.

[3] F. Tasnádi, I. A. Abrikosov, L. Rogström, J. Almer, M.P. Johansson, M. Odén, Significant elastic anisotropy in Ti_(1-x)Al_xN alloys, Appl. Phys. Lett. 97 (2010) 231902. doi:10.1063/1.3524502.

Here we show that cubic-structured Mo–Al–N coatings have a high potential to be used as wear protection for various high-demanding applications. We have found that within this material system, nitrogen vacancies have a significant and determining role for the stabilization of the cubic structure – a necessary prerequisite for high thermal stability and strength. While for solid solutions without N-vacancies the wurtzite-type structure is always energetically preferred, nitrogen vacancies favor the formation of the cubic phase. These ab initio predictions were experimentally verified with the help of magnetron sputtered Mo_1-xAl_xN_y coatings. We could show for the first time, that the cubic phase can be maintained up to the very high Al-content of Al/(Mo+Al) ≈ 0.57. The significantly increased Al-content within the cubic phase for our sputtered Mo_1-xAl_xN_y coatings suggests for an increased thermal stability and also helps to increase their hardness up to 38 GPa. (Further addition of Al results in a substantial softening to ~22 GPa as a consequence of the evolving wurtzite-type structure.) The study shows the extremely important role of vacancies, here nitrogen vacancies, which are often neglected.