David Holec

Decomposition pathways in age hardening of Ti-Al-N films

The ability to increase the thermal stability of protective coatings under work load gives rise to scientific and industrial interest in age hardening of complex nitride coating systems such as ceramic-like Ti1−xAlxN. However, the decomposition pathway of these systems from single-phase cubic to the thermodynamically stable binary nitrides (cubic TiN and wurtzite AlN), which are essential for age hardening, are not yet fully understood. In particular, the role of decomposition kinetics still requires more detailed investigation. In the present work, the combined effect of annealing time and temperature upon the nano-structural development of Ti0.46Al0.54N thin films is studied, with a thermal exposure of either 1 min or 120 min in 100 °C steps from 500 °C to 1400 °C. The impact of chemical changes at the atomic scale on the development of micro-strain and mechanical properties is studied by post-annealing investigations using X-ray diffraction, nanoindentation, 3D-atom probe tomography and high-resolution...

Trends in the elastic response of binary early transition metal nitrides

Motivated by an increasing demand for coherent data that can be used for selecting materials with properties tailored for specific application requirements, we studied elastic response of nine binary early transition metal nitrides (ScN, TiN, VN, YN, ZrN, NbN, LaN, HfN, and TaN) and AlN. In particular, single-crystal elastic constants, Youngs modulus in different crystallographic directions, polycrystalline values of shear and Youngs moduli, and the elastic anisotropy factor were calculated. Additionally, we provide estimates of the third order elastic constants for the ten binary nitrides.

Phase stability and decomposition products of Ti–Al–Ta–N thin films

Ab initio calculations of quaternary alloys were used to predict the phase stability of (Ti1−xAlx)1−yTayN. Experimental observation of a dual phase structure containing wurtzite AlN and cubic Ti1−yTayN after thermal decomposition of sputter deposited thin films by vacuum annealing to 1400 °C is in excellent agreement with the calculated phase stabilities of the investigated compositions. We found positive mixing enthalpies for Ti1−xAlxN and Al1−yTayN, with maximum values of 0.25 eV/atom and 0.30 eV/atom, respectively, but negative values for Ti1−yTayN over the whole composition range. The difference in lattice parameters obtained from experiments and ab initio calculations is within ∼1%.

Surface energies of AlN allotropes from first principles.

In this letter we present first-principles calculations of the surface energies of rock-salt (B1), zinc-blende (B3) and wurtzite (B4) AlN allotropes. Of several low-index facets, the highest energies are obtained for monoatomic surfaces (i.e. of only either Al or N atoms): γ{111}B1=410meV/Å2,γ{100}B3=346meV/Å2,γ{111}B3=360meV/Å2 and γ{0001}B4=365meV/Å2. The difference between Al- and N-terminated surfaces in these cases is less then 20 meV/Å2. The stoichiometric facets have energies lower by 100 meV/Å2 or more. The obtained trends could be rationalized by a simple nearest-neighbour broken-bond model.

Alloying-related trends from first principles: An application to the Ti–Al–X–N system

Tailoring and improving material properties by alloying is a long-known and used concept. Recent research has demonstrated the potential of ab initio calculations in understanding the material properties at the nanoscale. Here, we present a systematic overview of alloying trends when early transition metals (Y, Zr, Nb, Hf, and Ta) are added in the Ti1− x Al x N system, routinely used as a protective hard coating. The alloy lattice parameters tend to be larger than the corresponding linearised Vegards estimation, with the largest deviation more than 2.5% obtained for Y0.5 Al 0.5N. The chemical strengthening is most pronounced for Ta and Nb, although also causing smallest elastic distortions of the lattice due to their atomic radii being comparable with Ti and Al. This is further supported by the analysis of the electronic density of states. Finally, mixing enthalpy as a measure of the driving force for decomposition into the stable constituents is enhanced by adding Y, Zr, and Nb, suggesting that the onset of spinodal decomposition will appear in these cases for lower thermal loads than for Hf and Ta alloyed Ti1− x Al x N.

Structural stability and thermodynamics of CrN magnetic phases from ab initio calculations and experiment

The dynamical and thermodynamic phase stabilities of the stoichiometric compound CrN including different structural and magnetic configurations are comprehensively investigated using a first-principles density-functional-theory (DFT) plus U approach in conjunction with experimental measurements of the thermal expansion. Comparing DFT and DFT+U results with experimental data reveals that the treatment of electron correlations using methods beyond standard DFT is crucial. The non-magnetic face-centered cubic B1-CrN phase is both, elastically and dynamically unstable, even under high pressure, while CrN phases with non-zero local magnetic moments are predicted to be dynamically stable within the framework of the DFT+U scheme. Furthermore, the impact of different treatments for the exchange-correlation (xc)-functional is investigated by carrying out all computations employing the local density approximation and generalized gradient approximation. To address finite-temperature properties, both, magnetic and vibrational contributions to the free energy have been computed employing our recently developed spin-space averaging method. The calculated phase transition temperature between low-temperature antiferromagnetic and high-temperature paramagnetic (PM) CrN variants is in excellent agreement with experimental values and reveals the strong impact of the choice of the xc-functional. The temperature-dependent linear thermal expansion coefficient of CrN is experimentally determined by the wafer curvature method from a reactive magnetron sputter deposited single-phase B1-CrN thin film with dense film morphology. A good agreement is found between experimental and ab initio calculated linear thermal expansion coefficients of PM B1-CrN. Other thermodynamic properties, such as the specific heat capacity, have been computed as well and compared to previous experimental data.

First-principles study of elastic properties of cubic Cr1−xAlxN alloys

The elastic properties of paramagnetic cubic B1 (c-) Cr1−xAlxN ternary alloys are studied using stress-strain and energy-strain methods within the framework of density functional theory. A strong compositional dependence of the elastic properties is predicted. Youngs modulus, E, and shear modulus, G, exhibit the same compositional trends as experimentally measured hardness values (i.e., increasing with Al content), while bulk modulus, B, remains almost constant. The isotropic elastic response in the c-Cr1−xAlxN is predicted for concentrations around x = 0.50. Brittle behavior and directional bonding characteristics are predominant in the c-Cr1−xAlxN coatings in the whole composition range and become more pronounced with increasing Al content.

Elastic constants and critical thicknesses of ScGaN and ScAlN

Elastic constants of hexagonal ScxGa1−xN and ScxAl1−xN up to x = 0.375 were calculated using a stress-strain approach. C11, C33, C44, and C66 decreased while C12 and C13 increased slightly with increasing x. The biaxial [0001] Poisson ratios increased from 0.21 for GaN to 0.38 for Sc0.375Ga0.625 N and from 0.22 for AlN to 0.40 for Sc0.375Al0.625N, due to greater u values, in-plane bond lengths and bond ionicities. Subsequently, critical thicknesses for stress relaxation were calculated for ScxAl1−xN/AlN, ScxGa1−xN/GaN, and ScxAl1−xN/GaN heterostructures using an energy balance model. These range from 2 nm for Sc0.375Al0.625N/AlN and Sc0.375Ga0.625N/GaN to infinity for lattice-matched Sc0.18Al0.82N/GaN.

Theory-Guided Materials Design of Multi-Phase Ti-Nb Alloys with Bone-Matching Elastic Properties

We present a scale-bridging approach for modeling the integral elastic response of polycrystalline composite that is based on a multi-disciplinary combination of (i) parameter-free first-principles calculations of thermodynamic phase stability and single-crystal elastic stiffness; and (ii) homogenization schemes developed for polycrystalline aggregates and composites. The modeling is used as a theory-guided bottom-up materials design strategy and applied to Ti-Nb alloys as promising candidates for biomedical implant applications. The theoretical results (i) show an excellent agreement with experimental data and (ii) reveal a decisive influence of the multi-phase character of the polycrystalline composites on their integral elastic properties. The study shows that the results based on the density functional theory calculations at the atomistic level can be directly used for predictions at the macroscopic scale, effectively scale-jumping several orders of magnitude without using any empirical parameters.

Electronic origin of structure and mechanical properties in Y and Nb alloyed Ti–Al–N thin films

Abstract Ti1–xAlxN thin films are industrially well established protective coatings, whose beneficial mechanical properties are mainly based on the formation of a metastable microstructure and local composition during film synthesis. Alloying of a transition metal (TM) to Ti1–xAlxN is a promising approach to reach yet higher oxidation and corrosion resistance in high-temperature environments, while maintaining a high intrinsic hardness and elasticity, being essential for a good wear performance. In order to study the effect of alloying with Y and Nb on the structure and mechanical properties of the industrially preferred cubic (c) Ti1–xAlxN system, quaternary Ti1–x–zAlxYzN and Ti1–x–zAlxNbzN films were deposited by means of plasma-assisted reactive magnetron sputtering and investigated using X-ray diffraction, transmission electron microscopy and nanoindentation. It is shown that Y addition to c-Ti0.42Al0.58N changes its structure towards single phase wurtzite (w) Ti0.36Al0.55Y0.09N, with deteriorated mec...