Although titanium nitride (TiN) is among the most extensively studied and thoroughly characterized thin-film ceramic materials, detailed knowledge of relevant dislocation core structures is lacking. By high-resolution scanning transmission electron microscopy (STEM) of epitaxial single crystal (001)-oriented TiN films, we identify different dislocation types and their core structures. These include, besides the expected primary a/2{110}〈11–0〉 dislocation, Shockley partial dislocations a/6{111}〈112–〉 and sessile Lomer edge dislocations a/2{100}〈011〉. Density-functional theory and classical interatomic potential simulations complement STEM observations by recovering the atomic structure of the different dislocation types, estimating Peierls stresses, and providing insights on the chemical bonding nature at the core. The generated models of the dislocation cores suggest locally enhanced metal–metal bonding, weakened Ti-N bonds, and N vacancy-pinning that effectively reduces the mobilities of {110}〈11–0〉 and {111}〈112–〉 dislocations. Our findings underscore that the presence of different dislocation types and their effects on chemical bonding should be considered in the design and interpretations of nanoscale and macroscopic properties of TiN. © 2022 The Authors

Over the past decades, enormous effort has been dedicated to enhancing the hardness of refractory ceramic materials. Typically, however, an increase in hardness is accompanied by an increase in brittleness, which can result in intergranular decohesion when materials are exposed to high stresses. In order to avoid brittle failure, in addition to providing high strength, films should also be ductile, i.e., tough. However, fundamental progress in obtaining hard-yet-ductile ceramics has been slow since most toughening approaches are based on empirical trial-and-error methods focusing on increasing the strength and ductility extrinsically, with a limited focus on understanding thin-film toughness as an inherent physical property of the material. Thus, electronic structure investigations focusing on the origins of ductility vs. brittleness are essential in understanding the physics behind obtaining both high strength and high plastic strain in ceramics films. Here, we review recent progress in experimental validation of density functional theory predictions on toughness enhancement in hard ceramic films, by increasing the valence electron concentration, using examples from the V1-xWxN and V1-xMoxN alloy systems. © 2019 Elsevier B.V.

When it comes to providing the unusual combination of optical transparency, p-type conductivity, and relatively high mobility, Sn2+-based oxides are promising candidates. Epitaxial films of the simplest Sn2+ oxide, SnO, are grown in an adsorption-controlled regime at 380°C on Al2O3 substrates by molecular-beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A commensurately strained monolayer and an accompanying van der Waals gap is observed near the substrate interface, promoting layers with high structural perfection notwithstanding a large epitaxial lattice mismatch (-12%). The unintentionally doped films exhibit p-type conductivity with carrier concentration 2.5×1016cm-3 and mobility 2.4cm2V-1s-1 at room temperature. Additional physical properties are measured and linked to the Sn2+ valence state and corresponding lone-pair charge-density distribution. ©2019 American Physical Society.

We use a combined experimental and theoretical approach to study the rates of surface diffusion processes that govern early stages of thin Ag and Cu film morphological evolution on weakly-interacting amorphous carbon substrates. Films are deposited by magnetron sputtering, at temperatures T S between 298 and 413 K, and vapor arrival rates F in the range 0.08 to 5.38 monolayers/s. By employing in situ and real-time sheet-resistance and wafer-curvature measurements, we determine the nominal film thickness Θ at percolation (Θ perc ) and continuous film formation (Θ cont ) transition. Subsequently, we use the scaling behavior of Θ perc and Θ cont as a function of F and T s , to estimate, experimentally, the temperature-dependent diffusivity on the substrate surface, from which we calculate Ag and Cu surface migration energy barriers EDexp and attempt frequencies ν0exp. By critically comparing EDexp and ν0exp with literature data, as well as with results from our ab initio molecular dynamics simulations for single Ag and Cu adatom diffusion on graphite surfaces, we suggest that: (i) EDexp and ν0exp correspond to diffusion of multiatomic clusters, rather than to diffusion of monomers; and (ii) the mean size of mobile clusters during Ag growth is larger compared to that of Cu. The overall results of this work pave the way for studying growth dynamics in a wide range of technologically-relevant weakly-interacting film/substrate systems—including metals on 2D materials and oxides—which are building blocks in next-generation nanoelectronic, optoelectronic, and catalytic devices. © 2019, The Author(s).

Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the high- and low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials. © 2019 American Physical Society.

I present a development of the color-diffusion algorithm, used in non-equilibrium (accelerated) ab initio molecular dynamics simulations of point-defect migration in crystals [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)], to determine the temperature dependence of anion vacancy jump frequencies in rocksalt-structure (B1) TiN and VN characterized by quasiharmonic (TiN) vs. strongly anharmonic (VN) lattice dynamics. Over a temperature range [≈0.6·Tm < T < ≈0.9·Tm] relatively close to the materials melting points Tm, the simulations reveal that anion vacancy migration in TiN and VN exhibits an Arrhenius-like behavior, described by activation energies Ea TiN = 4.2 ± 0.3 eV and Ea VN = 3.1 ± 0.3 eV, and attempt frequencies νTiN = 8·1015±0.7 s−1 and νVN = 2·1017±0.8 s−1. A comparison of activation energies Ea extracted by Arrhenius linear regression at elevated temperatures with ab initio Ea0K values calculated at 0 Kelvin reveals that, while the nitrogen migration energy Ea TiN varies modestly with temperature {∆Ea TiN = [Ea(Tm) – Ea(0 K)]/Ea(0 K) ≈ 0.1}, the changes in Ea VN vs. T are considerable (∆Ea VN ≈ 1). The temperature-induced variations in vacancy migration energies and diffusivities are discussed in relation to the TiN and VN vibrational properties determined via ab initio molecular dynamics at different temperatures. It is argued that static 0-K calculations, which account for thermal expansion effects within the framework of quasiharmonic transition-state theory, accurately reproduce the finite-temperature mass transport properties of TiN. Conversely, the use of molecular dynamics simulations, which explicit treat lattice vibrations at any temperature of interest, is necessary to achieve reliable atomic diffusivities in B1 VN, a crystal phase dynamically stabilized by anharmonic vibrations [Mei et al., Phys. Rev. B 91, 054101 (2015)]. © 2019 Elsevier B.V.

Improved toughness is a central goal in the development of wear-resistant refractory ceramic coatings. Extensive theoretical and experimental research has revealed that NaCl-structure VMoN alloys exhibit surprisingly high ductility combined with high hardness and toughness. However, during operation, protective coatings inevitably oxidize, a problem that may compromise material properties and performance. Here, the authors explore the role of oxidation in altering VMoN properties. Density functional theory and theoretical intrinsic hardness models are used to investigate the mechanical behavior of cubic V0.5Mo0.5N1-xOx solid solutions as a function of the oxygen concentration x. Elastic constant and intrinsic hardness calculations show that oxidation does not degrade the mechanical properties of V0.5Mo0.5N. Electronic structure analyses indicate that the presence of oxygen reduces the covalent bond character, which slightly lowers the alloy strength and intrinsic hardness. Nevertheless, the character of metallic d-d states, which are crucial for allowing plastic deformation and enhancing toughness, remains unaffected. Overall, the authors' results suggest that VMoNO oxynitrides, with oxygen concentrations as high as 50%, possess high intrinsic hardness, while still being ductile. © 2019 Author(s).

We present a modified embedded atom method (MEAM) semi-empirical force-field model for the Ti 1-x Al x N (0 ≤ x ≤ 1) alloy system. The MEAM parameters, determined via an adaptive simulated-annealing (ASA) minimization scheme, optimize the model's predictions with respect to 0 K equilibrium volumes, elastic constants, cohesive energies, enthalpies of mixing, and point-defect formation energies, for a set of ≈40 elemental, binary, and ternary Ti-Al-N structures and configurations. Subsequently, the reliability of the model is thoroughly verified against known finite-temperature thermodynamic and kinetic properties of key binary Ti-N and Al-N phases, as well as properties of Ti 1-x Al x N (0 < x < 1) alloys. The successful outcome of the validation underscores the transferability of our model, opening the way for large-scale molecular dynamics simulations of, e.g., phase evolution, interfacial processes, and mechanical response in Ti-Al-N-based alloys, superlattices, and nanostructures. © 2019 by the authors.

We present a modified embedded atom method (MEAM) semi-empirical force-field model for the Ti1-xAlxN (0 ≤ x ≤ 1) alloy system. The MEAM parameters, determined via an adaptive simulated-annealing (ASA) minimization scheme, optimize the model's predictions with respect to 0 K equilibrium volumes, elastic constants, cohesive energies, enthalpies of mixing, and point-defect formation energies, for a set of ≈40 elemental, binary, and ternary Ti-Al-N structures and configurations. Subsequently, the reliability of the model is thoroughly verified against known finite-temperature thermodynamic and kinetic properties of key binary Ti-N and Al-N phases, as well as properties of Ti1-xAlxN (0 < x < 1) alloys. The successful outcome of the validation underscores the transferability of our model, opening the way for large-scale molecular dynamics simulations of, e.g., phase evolution, interfacial processes, and mechanical response in Ti-Al-N-based alloys, superlattices, and nanostructures. © 2019 by the authors.

Recent theoretical investigations [A. B. Belonoshko et al. Nat. Geosci. 10, 312 (2017)1752-089410.1038/ngeo2892] revealed the occurrence of the concerted migration of several atoms in bcc Fe at inner-core temperatures and pressures. Here, we combine first-principles and semiempirical atomistic simulations to show that a diffusion mechanism analogous to the one predicted for bcc iron at extreme conditions is also operative and of relevance for the high-temperature bcc phase of pure Ti at ambient pressure. The mechanism entails a rapid collective movement of numerous (from two to dozens) neighbors along tangled closed-loop paths in defect-free crystal regions. We argue that this phenomenon closely resembles the diffusion behavior of superionics and liquid metals. Furthermore, we suggest that concerted migration is the atomistic manifestation of vanishingly small ω-mode phonon frequencies previously detected via neutron scattering and the mechanism underlying anomalously large and markedly non-Arrhenius self-diffusivities characteristic of bcc Ti. © 2019 American Physical Society.

We perform large-scale molecular dynamics simulations of TiN deposition at 1200 K on TiN substrates consisting of under-stoichiometric (N/Ti = 0.86) misoriented grains. The energy of incoming Ti atoms is 2 eV and that of incoming N atoms is 10 eV. The simulations show that misoriented grains are reoriented during the early stages of growth, after which the film grows 001 epitaxially and is nearly stoichiometric. The grain reorientation coincides with an increase in film N/Ti ratio. As the grains reorient, additional nitrogen can no longer be accommodated, and the film composition becomes stoichiometric as the overlayer grows epitaxially. © 2019 Elsevier B.V.

Metal organic chemical vapor deposition (MOCVD) of group III nitrides on graphene heterostructures offers new opportunities for the development of flexible optoelectronic devices and for the stabilization of conceptually-new two-dimensional materials. However, the MOCVD of group III nitrides is regulated by an intricate interplay of gas-phase and surface reactions that are beyond the resolution of experimental techniques. We use density-functional ab initio molecular dynamics (AIMD) with van der Waals corrections to identify atomistic pathways and associated electronic mechanisms driving precursor/surface reactions during metal organic vapor phase epitaxy at elevated temperatures of aluminum nitride on graphene, considered here as model case study. The results presented provide plausible interpretations of atomistic and electronic processes responsible for delivery of Al, C adatoms, and C-Al, CHx, AlNH2 admolecules on pristine graphene via precursor/surface reactions. In addition, the simulations reveal C adatom permeation across defect-free graphene, as well as exchange of C monomers with graphene carbon atoms, for which we obtain rates of ∼0.3 THz at typical experimental temperatures (1500 K), and extract activation energies Eexca = 0.28 ± 0.13 eV and attempt frequencies Aexc = 2.1 (×1.7±1) THz via Arrhenius linear regression. The results demonstrate that AIMD simulations enable understanding complex precursor/surface reaction mechanisms, and thus propose AIMD to become an indispensable routine prediction-tool toward more effective exploitation of chemical precursors and better control of MOCVD processes during synthesis of functional materials. © 2018 the Owner Societies.

Density-functional ab initio molecular dynamics (AIMD) simulations are carried out to determine Cu adatom and admolecule transport properties as a function of temperature, as well as atomistic processes leading to formation of Cu/TiN(0 0 1) islands at 350 K. At very low temperatures T ≤ 200 K, Cu adatoms (Cuad) migrate among favored fourfold-hollow surface sites by passing across atop-Ti metastable positions. For increasing temperatures, however, Cuad transport becomes progressively more isotropic, and switches continuously from normal- to super-diffusive with mean-square displacement dependencies on time that alternate between linear and exponential. Despite that, the Cuad diffusivity D can be expressed by a fairly Arrhenius-like behavior D(T) = 8.26(×2±1) × 10−4 cm2 s−1 exp[(−0.04 ± 0.01 eV)/(kBT)] over the entire investigated temperature range (100 ≤ T ≤ 1000 K). AIMD simulations also reveal that the condensation of Cu adatoms into Cux>1 adspecies is kinetically hindered by long-range (>5.5 Å) adatom/adatom repulsion. During Cu island nucleation, all Cu atoms occupy atop-N positions indicating favored Cu(0 0 1)/TiN(0 0 1) epitaxial growth. Nevertheless, Cu agglomerates formed by five, or more, atoms tend to arrange in 3D structures, which maximize intracluster bonds while minimizing film/substrate interactions. Results here presented provide insights for understanding the properties of weakly-interacting metal/substrate interface systems in general. © 2018 Elsevier B.V.

There are several approaches to the description of van der Waals (vdW) forces within density functional theory. While they are generally found to improve the structural and energetic properties of those materials dominated by weak dispersion forces, it is not known how they behave when the material is subject to an external pressure. This could be an issue when considering the pressure-induced structural phase transitions, which are currently attracting great attention following the discovery of an ultrahard phase formed by the compression of graphite at room temperature. In order to model this transition, the functional must be capable of simultaneously describing both strong covalent bonds and weak dispersion interactions as an isotropic pressure is applied. Here, we report on the ability of several dispersion-correction functionals to describe the energetic, structural, and elastic properties of graphite and diamond, when subjected to an isotropic pressure. Almost all of the tested vdW corrections provide an improved description of both graphite and diamond compared to the local density approximation. The relative error does not change significantly as pressure is applied, and in some cases even decreases. We therefore conclude that the use of dispersion-corrected exchange-correlation functionals, which have been neglected to date, will improve the accuracy and reliability of theoretical investigations into the pressure-induced phase transition of graphite. © 2018 American Physical Society.

We carried out density-functional ab initio molecular dynamics (AIMD) simulations of Ti adatom (Tiad) migration on, and descent from, square TiN(100) epitaxial islands on TiN(001) at temperatures (T) ranging from 1200 to 2400 K. Adatom-descent energy barriers determined via ab initio nudged-elastic-band calculations at 0 Kelvin suggest that Ti interlayer transport on TiN(001) occurs essentially exclusively via direct hopping onto a lower layer. However, AIMD simulations reveal comparable rates for Tiad descent via direct hopping vs push-out/exchange with a Ti island-edge atom for T≥1500K. We demonstrate that this effect is due to surface vibrations, which yield considerably lower activation energies at finite temperatures by significantly modifying the adatom push-out/exchange reaction pathway. © 2018 American Physical Society.

Transition-metal (TM) carbides are an important class of hard, protective coating materials; however, their brittleness often limits potential applications. We use density functional theory to investigate the possibility of improving ductility by forming pseudobinary cubic M1M2C alloys, for which M1 = Ti or V and M2 = W or Mo. The alloying elements are chosen based on previous results showing improved ductility of the corresponding pseudobinary nitride alloys with respect to their parent compounds. While commonly-used empirical criteria do not indicate enhanced ductility in the carbide alloys, calculated stress/strain curves along known slip systems, supported by electronic structure analyses, indicate ductile behavior for VMoC. As VMoC layers are sheared along the 11¯0 direction on {111} planes, the stress initially increases linearly up to a yield point where the accumulated stress is partially dissipated. With further increase in strain, the stress increases again until fracture occurs. A similar mechanical behavior is observed for the corresponding TM nitride VMoN, known to be a ductile ceramic material [1]. Thus, our results show that VMoC is a TM carbide alloy which may be both hard and ductile, i.e. tough. © 2017 Acta Materialia Inc.

Hard refractory transition-metal nitrides possess unique combinations of outstanding mechanical and physical properties, but are typically brittle. Recent experimental results demonstrated that single-crystal NaCl-structure (B1) V0.5Mo0.5N pseudobinary solid solutions are both hard (∼20 GPa) and ductile; that is, they exhibit toughness, which is unusual for ceramics. However, key atomic-scale mechanisms underlying this inherent toughness are unknown. Here, I carry out density-functional ab initio molecular dynamics (AIMD) simulations at room temperature to identify atomistic processes and associated changes in the electronic structure which control strength, plasticity, and fracture in V0.5Mo0.5N, as well as reference B1 TiN, subject to <001> and <110> tensile deformation. AIMD simulations reveal that V0.5Mo0.5N is considerably tougher than TiN owing to its ability to (i) isotropically redistribute mechanical stresses within the elastic regime, (ii) dissipate the accumulated strain energy by activating local structural transformations beyond the yield point. In direct contrast, TiN breaks in brittle manner when applied stresses reach its tensile strength. Charge transfer maps show that the adaptive mechanical response of V0.5Mo0.5N originates from highly populated d-d metallic-states, which allow for counterbalancing the destabilization induced via tensile deformation by enabling formation of new chemical bonds. The high ionic character and electron-localization in TiN precludes the possibility of modifying bonding geometries to accommodate the accumulated stresses, thus suddenly causing material's fracture for relatively low strain values. © 2018 Acta Materialia Inc.

Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200 K, a temperature within the optimal range for epitaxial TiN growth, with an incident N-to-Ti flux ratio of four, are carried out using incident N energies EN = 2 and 10 eV and incident Ti energy ETi = 2 eV. To further highlight the effect of EN, we grow a bilayer film with EN = 2 eV initially and then switch to EN = 10 eV. As-deposited layers are analyzed as a function of composition, island-size distribution, island-edge orientation, and vacancy formation. Results show that growth with EN = 2 eV results in films that are globally overstoichiometric with islands bounded by N-terminated polar 110 edges, whereas films grown with EN = 10 eV are flatter and closer to stoichiometric. However, EN = 10 eV layers exhibit local N deficiency leading to the formation of isolated 111-oriented islands. Films grown by changing the incident energy from 2 to 10 eV during growth are more compact than those grown entirely with EN = 2 eV and exhibit greatly reduced concentrations of upper-layer adatoms, admolecules, and small clusters. Islands with 110 edges formed during growth with EN = 2 eV transform to islands with 100 edges as EN is switched to 10 eV. © 2017 Author(s).

We use the color diffusion (CD) algorithm in nonequilibrium (accelerated) ab initio molecular dynamics simulations to determine Ti monovacancy jump frequencies in NaCl-structure titanium nitride (TiN), at temperatures ranging from 2200 to 3000 K. Our results show that the CD method extended beyond the linear-fitting rate-versus-force regime [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)10.1103/PhysRevB.93.094305] can efficiently determine metal vacancy migration rates in TiN, despite the low mobilities of lattice defects in this type of ceramic compound. We propose a computational method based on gamma-distribution statistics, which provides unambiguous definition of nonequilibrium and equilibrium (extrapolated) vacancy jump rates with corresponding statistical uncertainties. The acceleration-factor achieved in our implementation of nonequilibrium molecular dynamics increases dramatically for decreasing temperatures from 500 for T close to the melting point Tm, up to 33 000 for T≈0.7Tm. © 2017 American Physical Society.

We combine experiments and ab initio density functional theory calculations to investigate the evolution in structural and mechanical properties of TaAlN coatings as a function of the annealing temperature T. Formation of coherent cubic TaN- and AlN-rich nanometer-size domains, occurring during the initial stage of thermally induced phase separation within cubic NaCl-type (B1) TaAlN solid solutions, yields a monotonic increase in hardness from 29 GPa (as deposited coatings) up to a maximum of 35 GPa (+17%) reached after annealing at 1000 °C. Further thermal treatment at T > 1000 °C leads to the transformation of metastable cubic domains into stable hexagonal TaNx and wurtzite AlN phases, thus resulting in hardness reductions. A comparison of our results with those reported in the literature reveals that TaAlN coatings are at least as hard while considerably less stiff (lower elastic moduli) than TiAlN coatings, thus indicating a substantial increase in toughness achieved upon replacing Ti with Ta in the host lattice. Present findings suggest that cubic TaAlN solid solutions are promising candidates for applications in protective coatings possessing both high-temperature hardness and toughness. © 2017 Author(s).

We use density-functional ab initio molecular dynamics to investigate the kinetics of N/VN(001) surface reactions at temperatures ranging from 1600 to 2300 K. N adatoms (Nad) on VN(001) favor epitaxial atop-V positions and diffuse among them by transiting through 4-fold hollow (FFH) sites, at which they are surrounded by two V and two N surface atoms. After several atop-V → FFH → atop-V jumps, isolated N adatoms bond strongly with an underlying N surface (Nsurf) atom. Frequent Nad/Nsurf pair exchange reactions lead to N2 desorption, which results in the formation of an anion surface vacancy. N vacancies rapidly migrate via in-plane 〈110〉 jumps and act as efficient catalysts for the dissociative chemisorption of incident N2 molecules. During exposure of VN(001) to incident atomic N gas atoms, Nad/Nad recombination and desorption is never observed, despite a continuously high N monomer surface coverage. Instead, N2 desorption is always initiated by a N adatom removing a N surface atom or by energetic N gas atoms colliding with Nad or Nsurf atoms. Similarities and differences between N/VN(001) vs. previous N/TiN(001) results, discussed on the basis of temperature-dependent ab initio electronic structures and chemical bonding, provide insights for controlling the reactivity of NaCl-structure transition-metal nitride (001) surfaces via electron-concentration tuning. © 2016 American Chemical Society.

We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti0.5W0.5N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti0.5W0.5N, as estimated by stress-strain results and resistance to layer glide, are comparable to that of the parent binary TiN, while ductility is considerably enhanced. After an initial elastic response to an applied load, the pseudobinary alloy deforms plastically, thus releasing accumulated mechanical stress. In contrast, stress continues to increase linearly with strain in TiN. Layer glide in Ti0.5W0.5N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [111]-oriented Ti0.5W0.5N layers characterized by high local metal-sublattice ordering exhibit low resistance to slip along <110> directions due to energetically favored formation of (111) hexagonal stacking faults. This is consistent with the positive formation energy of <111>-ordered Ti0.5W0.5N with respect to mixing of cubic-B1 TiN and hexagonal WC-structure WN. In the cubic pseudobinary alloy, slip occurs parallel, as well as orthogonal, to the resolved applied stress at the interface between layers with the lowest friction. We suggest that analogous structural metastability (mixing cubic and hexagonal TM nitride binary phases) and electronic (high valence electron concentration) effects are responsible for the enhanced toughness recently demonstrated experimentally for cubic single-crystal pseudobinary V0.5W0.5N and V0.5Mo0.5N epitaxial layers. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We revisit the color-diffusion algorithm [Aeberhard, Phys. Rev. Lett. 108, 095901 (2012)10.1103/PhysRevLett.108.095901] in non equilibrium ab initio molecular dynamics (NE-AIMD) and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored atom) is accelerated toward the neighboring defect site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate kNE increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed kNE(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures and reaches a factor of 4 orders of magnitude at the lowest temperature considered in the present study. © 2016 American Physical Society.

VNx/MgO(001) films, ∼300 nm thick, with x ranging from 1.00 (stoichiometric) to 0.80 are grown by magnetically-unbalanced reactive magnetron sputter deposition in mixed N2/Ar atmospheres. The combination of lattice-resolution cross-sectional electron microscopy with X-ray diffraction ω - 2θ, φ-scans, pole figures, and high resolution reciprocal space maps show that VNx layers are epitaxial single crystals which grow cube-on-cube with respect to their substrates: (001) VNx ∥(001)MgO and [100] VNx∥[100]MgO. VNx(001) relaxed lattice parameters a0(x) decrease linearly from 0.4134 (x = 1.00) to 0.4098 nm (x = 0.80), in agreement with density functional theory (DFT) calculations. Near-stoichiometric VNx layers (0.95 ≲ x ≤ 1.0) are fully relaxed during growth, while films with lower x values are partially strained as a result of increased anion vacancies impeding dislocation glide. VNx complex dielectric functions ϵ(ω) are determined between 0.7 and 4.5 eV using variable-angle spectroscopic ellipsometry and valence states are probed via ultraviolet photoelectron spectroscopy (UPS) in concert with DFT calculations. VN(001) UPS spectra exhibit a feature at binding energies ranging from the Fermi level to 3 eV, together with two peaks deeper in the valence band. These results are consistent with electronic densities of states computed by scaling Kohn-Sham electronic eigenvalues to account for many-body interactions. Imaginary VN(001) dielectric functions ϵ(ω) determined by ellipsometry also agree with theoretical values obtained within the random-phase approximation using scaled eigenvalues. Analyses of optical matrix element calculations reveal that VNx dielectric responses are controlled by the phase space for interband transitions; band-structure analyses indicate that ϵ2(ω) spectral features in the infrared-visible range arise primarily from the combination of intraband and d-d transitions, while features at higher energies result primarily from p-d interband transitions. The combined nanostructural and spectroscopic analyses establish that, surprisingly, N vacancies are essentially non-interacting in high-quality epitaxial VNx containing vacancy concentrations up to ∼1022 cm-3 (x = 0.80). © 2016 The Royal Society of Chemistry.

Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200 K are carried out using incident flux ratios N/Ti = 1, 2, and 4. The films are analyzed as a function of composition, island size distribution, island edge orientation, and vacancy formation. Results show that N/Ti = 1 films are globally understoichiometric with dispersed Ti-rich surface regions which serve as traps to nucleate 111-oriented islands, leading to local epitaxial breakdown. Films grown with N/Ti = 2 are approximately stoichiometric and the growth mode is closer to layer-by-layer, while N/Ti = 4 films are stoichiometric with N-rich surfaces. As N/Ti is increased from 1 to 4, island edges are increasingly polar, i.e., 110-oriented, and N-terminated to accommodate the excess N flux, some of which is lost by reflection of incident N atoms. N vacancies are produced in the surface layer during film deposition with N/Ti = 1 due to the formation and subsequent desorption of N2 molecules composed of a N adatom and a N surface atom, as well as itinerant Ti adatoms pulling up N surface atoms. The N vacancy concentration is significantly reduced as N/Ti is increased to 2; with N/Ti = 4, Ti vacancies dominate. Overall, our results show that an insufficient N/Ti ratio leads to surface roughening via nucleation of small dispersed 111 islands, whereas high N/Ti ratios result in surface roughening due to more rapid upper-layer nucleation and mound formation. The growth mode of N/Ti = 2 films, which have smoother surfaces, is closer to layer-by-layer. © 2016 American Vacuum Society.

We use molecular dynamics (MD) based on the modified embedded atom method (MEAM) to determine diffusion coefficients and migration pathways for Ti and N adatoms (Tiad and Nad) on TiN(111). The reliability of the classical model-potential is verified by comparison with density functional theory (DFT) results at 0 K. MD simulations carried out at temperatures between 600 and 1800 K show that both Tiad and Nad favor fcc surface sites and migrate among them by passing through metastable hcp positions. We find that Nad species are considerably more mobile than Tiad on TiN(111); contrary to our previous results on TiN(001). In addition, we show that lattice vibrations at finite temperatures strongly modify the potential energy landscape and result in smaller adatom migration energies, Ea = 1.03 for Tiad and 0.61 eV for Nad, compared to 0 K values Ea0K = 1.55 (Tiad) and 0.79 eV (Nad). We also demonstrate that the inclusion of dipole corrections, neglected in previous DFT calculations, is necessary in order to obtain the correct formation energies for polar surfaces such as TiN(111). © 2016 Elsevier B.V. All rights reserve.

Obtaining high hardness combined with enhanced toughness represents one of the current challenges in material design of hard ceramic protective coatings. In this work, we combine experimental and ab initio density functional theory (DFT) analysis of the mechanical properties of Ti-Al-Nb-N coatings to validate the results of previous theoretical investigations predicting enhanced toughness in TiAlN-based systems highly alloyed (>25 at. %) with nitrides of pentavalent VB group elements Nb, Ta, and V. As-deposited Ti1-x-yAlxNbyN coatings (y = 0 ÷ 0.61) exhibit single phase cubic sodium chloride (B1) structure identified as TiAl(Nb)N solid solutions. The highest hardness, ∼32.5 ± 2 GPa, and the highest Young's modulus, ∼442 GPa, are obtained in Nb-free Ti0.46Al0.54N exhibiting pronounced 111 growth-orientation. Additions of Nb in the coatings promote texture evolution toward 200. Nanoindentation measurements demonstrate that alloying TiAlN with NbN yields significantly decreased elastic stiffness, from 442 to ∼358 ÷ 389 GPa, while the hardness remains approximately constant (between 28 ± 2 and 31 ± 3 GPa) for all Nb contents. DFT calculations and electronic structure analyses reveal that alloying dramatically reduces shear resistances due to enhanced d-d second-neighbor metallic bonding while retaining strong metal-N bonds which change from being primarily ionic (TiAlN) to more covalent (TiAlNbN) in nature. Overall, Nb substitutions are found to improve ductility of TiAlN-based alloys at the cost of slight losses in hardness, equating to enhanced toughness. © 2016 Acta Materialia Inc.

Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below Tc=250K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I?(Tc-T)1/2. Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function αtr2F(ω), the product of the phonon density of states F(ω) and the transport electron/phonon coupling strength αtr2(ω), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T>Tc. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general. © 2015 American Physical Society.

We use ab initio and classical molecular dynamics (AIMD and CMD) based on the modified embedded-atom method (MEAM) potential to simulate diffusion of N vacancy and N self-interstitial point defects in B1 TiN. TiN MEAM parameters are optimized to obtain CMD nitrogen point-defect jump rates in agreement with AIMD predictions, as well as an excellent description of TiNx(∼0.7<x≤1) elastic, thermal, and structural properties. We determine N dilute-point-defect diffusion pathways, activation energies, attempt frequencies, and diffusion coefficients as a function of temperature. In addition, the MD simulations presented in this paper reveal an unanticipated atomistic process, which controls the spontaneous formation of N self-interstitial/N vacancy (NI/NV) pairs (Frenkel pairs), in defect-free TiN. This entails that the N lattice atom leaves its bulk position and bonds to a neighboring N lattice atom. In most cases, Frenkel-pair NI and NV recombine within a fraction of ns; ∼50% of these processes result in the exchange of two nitrogen lattice atoms (N-NExc). Occasionally, however, Frenkel-pair N-interstitial atoms permanently escape from the anion vacancy site, thus producing unpaired NI and NV point defects. © 2015 American Physical Society.

It has been shown both experimentally and by density functional theory calculations that the primary diffusing species during the epitaxial growth of TiN/TiN(001) are Ti and N adatoms together with TiN<inf>x</inf> complexes (x = 1, 2, 3), in which the dominant N-containing admolecule species depends upon the incident N/Ti flux ratio. Here, we employ classical molecular dynamics (CMD) simulations to probe the dynamics of TiN<inf>x</inf> (x = 1-3) admolecules on 8 × 8 atom square, single-atom-high TiN islands on TiN(001), as well as pathways for descent over island edges. The simulations are carried out at 1000 K, a reasonable epitaxial growth temperature. We find that despite their lower mobility on infinite TiN(001) terraces, both TiN and TiN<inf>2</inf> admolecules funnel toward descending steps and are incorporated into island edges more rapidly than Ti adatoms. On islands, TiN diffuses primarily via concerted translations, but rotation is the preferred diffusion mechanism on infinite terraces. TiN<inf>2</inf> migration is initiated primarily by rotation about one of the N admolecule atoms anchored at an epitaxial site. TiN admolecules descend from islands by direct hopping over edges and by edge exchange reactions, while TiN<inf>2</inf> trimers descend exclusively by hopping. In contrast, TiN<inf>3</inf> admolecules are essentially stationary and serve as initiators for local island growth. Ti adatoms are the fastest diffusing species on infinite TiN(001) terraces, but on small TiN/TiN(001) islands, TiN dimers provide more efficient mass transport. The overall results reveal the effect of the N/Ti precursor flux ratio on TiN(001) surface morphological evolution and growth modes. © 2015 Elsevier B.V.

Ab initio molecular dynamics simulations based on density functional theory show that N adatoms are chemisorbed in threefold sites close to a N surface atom and between the two diagonally opposed neighboring Ti surface atoms on TiN(001). The most probable N adatom reaction pathway, even in the presence of nearby N adatoms, is for the N adatom and N surface atom pair to first undergo several exchange reactions and then desorb as a N2 molecule, resulting in a surface anion vacancy, with an activation barrier Edes of 1.37 eV and an attempt frequency Ades = 3.4 × 10 13 s- 1. Edes is essentially equal to the N adatom surface diffusion barrier, Es = 1.39 eV, while As is only three to four times larger than Ades, indicating that isolated N adatoms migrate for only short distances prior to N2 desorption. The probability of N2 desorption via recombination of N adatoms on TiN(001) is much lower due to repulsive adatom/adatom interactions at separations less than ~ 3 Å which rapidly increase to ~ 2 eV at a separation of 1.5 Å. We obtain good qualitative and quantitative agreement with the above results using the modified embedded atom method potential to perform classical molecular dynamics simulations. © 2014 Elsevier B.V.

The authors report the growth and mechanical properties of epitaxial B1 NaCl-structure V1-xWxN/MgO(001) thin films with 0 ≤ x ≤ 0.60. The Gibbs free energy of mixing, calculated using density functional theory (DFT), reveals that cubic V1-xWxN solid solutions with 0 ≤ x 0.7 are stable against spinodal decomposition and separation into the equilibrium cubic-VN and hexagonal-WN binary phases. The authors show experimentally that alloying VN with WN leads to a monotonic increase in relaxed lattice parameters, enhanced nanoindentation hardnesses, and reduced elastic moduli. Calculated V1-xWxN lattice parameters and elastic moduli (obtained from calculated C11, C12, and C 44 elastic constants) are in good agreement with experimental results. The observed increase in alloy hardness, with a corresponding decrease in the elastic modulus at higher x values, combined with DFT-calculated decreases in shear to bulk moduli ratios, and increased Cauchy pressures (C 12-C44) with increasing x reveal a trend toward increased toughness. © 2014 American Vacuum Society.

Improved toughness is one of the central goals in the development of wear-resistant coatings. Previous studies of toughness in transition metal nitride alloys have addressed the effects of chemical composition in these compounds. Herein, we use density functional theory to study the effects of various metal sublattice configurations, ranging from fully ordered to fully disordered, on the mechanical properties of VM2N and TiM2N (M2 = W, Mo) ternary alloys. Results show that all alloys display high incompressibility, indicating strong Me-N bonds. Disordered atomic arrangements yield lower values of bulk moduli and C11 elastic constants, as well as higher values of C44 elastic constants, compared to ordered structures. We attribute the low C44 values of ordered structures to the formation of fully-bonding states perpendicular to the applied stress. We find that the ductility of these compounds is primarily an effect of the increased valence electron concentration induced upon alloying. © 2014 Elsevier B.V.

Ab initio and classical molecular dynamics (AIMD and CMD) simulations reveal that Ti adatoms on TiN(001) surfaces migrate between neighboring fourfold hollow sites primarily along in-plane b100N channels. b100N and b110N single jumps, as well as b100N double jump rates, obtained directly from MD runs as a function of temperature, are used to determine diffusion activation energies Ea, and attempt frequencies A, for the three preferred Ti adatom migration pathways on TiN(001). From transition rates Aexp[-Ea/ (kBT)], we determine adatom surface distribution probabilities as a function of time, which are used to calculate adatom diffusion coefficients Ds(T). AIMD and CMD predictions are consistent and complementary. Using CMD, we investigate the effect on the adatom jump rate of varying the phonon wavelength degrees of freedom by progressively increasing the supercell size. We find that long-wavelength phonons significantly contribute to increasing adatom mobilities at temperatures ≤600 K, but not at higher temperatures. Finally, by directly tracking the Ti adatom mean-square displacement during CMD runs, we find that Ti adatom jumps are highly correlated on TiN(001), an effect that yields lower Ds values (Ds corr) than those estimated from uncorrelated transition probabilities. The temperature-dependent diffusion coefficient is D s corr (T)=(4.5 × 10-4 cm2 s-1) exp[.0.55 eV/ (kBT)]. © 2014 Elsevier B.V. All rights reserved.

We use classical molecular dynamics and the modified embedded atom method to determine residence times and descent pathways of Ti and N adatoms on square, single-atom-high, TiN islands on TiN(001). Simulations are carried out at 1000 K, which is within the optimal range for TiN(001) epitaxial growth. Results show that the frequency of descent events, and overall adatom residence times, depend strongly on both the TiN(001) diffusion barrier for each species as well as the adatom island-edge location immediately prior to descent. Ti adatoms, with a low diffusion barrier, rapidly move toward the island periphery, via funneling, where they diffuse along upper island edges. The primary descent mechanism for Ti adatoms is via push-out/exchange with Ti island-edge atoms, a process in which the adatom replaces an island edge atom by moving down while pushing the edge atom out onto the terrace to occupy an epitaxial position along the island edge. Double push-out events are also observed for Ti adatoms descending at N corner positions. N adatoms, with a considerably higher diffusion barrier on TiN(001), require much longer times to reach island edges and, consequently, have significantly longer residence times. N adatoms are found to descend onto the terrace by direct hopping over island edges and corner atoms, as well as by concerted push-out/exchange with N atoms adjacent to Ti corners. For both adspecies, we also observe several complex adatom/island interactions, before and after descent onto the terrace, including two instances of Ti island-atom ascent onto the island surface. © 2014 Elsevier B.V. All rights reserved.

Using a combination of experiments and density functional theory (DFT), we demonstrate the first example of vacancy-induced toughening, in this case for epitaxial pseudobinary NaCl-structure substoichiometric V0.5Mo 0.5Nx alloys, with N concentrations 0.55 ≤ x ≤ 1.03, grown by reactive magnetron sputter deposition. The nanoindentation hardness H(x) increases with increasing vacancy concentration from 17 GPa with x = 1.03 to 26 GPa with x = 0.55, while the elastic modulus E(x) remains essentially constant at 370 GPa. Scanning electron micrographs of indented regions show ductile plastic flow giving rise to material pile-up, rather than cracks as commonly observed for hard, but brittle, transition-metal nitrides. The increase in alloy hardness with an elastic modulus that remains constant with decreasing x, combined with the observed material pile-up around nanoindents, DFT-calculated decrease in shear to bulk moduli ratios, and increased Cauchy pressures (C12-C44), reveals a trend toward vacancy-induced toughening. Moreover, DFT crystal orbital overlap population analyses are consistent with the above results. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Hardness is an essential property for a wide range of applications. However, hardness alone, typically accompanied by brittleness, is not sufficient to prevent failure in ceramic films exposed to high stresses. Using VN as a model system, we demonstrate with experiment and density functional theory (DFT) that refractory VMoN alloys exhibit not only enhanced hardness, but dramatically increased ductility. V0.5Mo0.5N hardness is 25% higher than that of VN. In addition, while nanoindented VN, as well as TiN reference samples, suffer from severe cracking typical of brittle ceramics, V0.5Mo0.5N films do not crack. Instead, they exhibit material pile-up around nanoindents, characteristic of plastic flow in ductile materials. Moreover, the wear resistance of V0.5Mo0.5N is considerably higher than that of VN. DFT results show that tuning the occupancy of d-t2g metallic bonding states in VMoN facilitates dislocation glide, and hence enhances toughness, via the formation of stronger metal/metal bonds along the slip direction and weaker metal/N bonds across the slip plane. © 2013 Author(s).

We use classical molecular dynamics and the modified embedded atom method formalism to investigate the dynamics of atomic-scale transport on a low-index model compound surface, TiN(001). Our simulations, totaling 0.25 μs for each case study, follow the pathways and migration kinetics of Ti and N adatoms, as well as TiN x complexes with x=1-3, which are known to contribute to the growth of TiN thin films by reactive deposition from Ti, N 2, and N precursors. The simulations are carried out at 1000 K, within the optimal range for TiN(001) epitaxial growth. We find Ti adatoms to be the highest-mobility species on TiN(001), with the primary migration path involving jumps of one nearest-neighbor distance d NN between adjacent fourfold hollow sites along in-plane 100 channels. Long jumps, 2d NN, are also observed, but at much lower frequency. N adatoms, which exhibit significantly lower migration rates than Ti, diffuse along in-plane 110 directions and, when they intersect other N atoms, associatively form N 2 molecules, which desorb at kinetic rates. As expected, TiN and TiN 3 complexes migrate at even lower rates with complex diffusion pathways involving rotations, translations, and rototranslations. TiN 2 trimers, however, are shown to have surprisingly high diffusion rates, above that of N adatoms and almost half that of Ti adatoms. TiN 3 motion is dominated by in-place rotation with negligible diffusion. © 2012 American Physical Society.

Improved 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. We use density functional theory calculations to investigate a number of (TiAl) 1 - xM xN thin films in the B1 structure, with 0.06 ≤ x ≤ 0.75, obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti 0.5Al 0.5 N. For (TiAl) 1 - xW xN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t 2g metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses. © 2012 Elsevier B.V. All rights reserved.

A combinatorial method was employed to grow TiAlN-WNx films by DC sputtering as well as by High Power Pulsed Magnetron Sputtering (HPPMS) where the W concentration was varied between 10-52. at.% and 7-54. at.%, respectively. Experiments were paired with ab initio calculations to investigate the correlation between composition, structure, and mechanical properties. During all depositions the time averaged power was kept constant. As the W concentration was increased, the lattice parameter of cubic TiAlN-WNx films first increased and then decreased for W concentrations above ≈. 29. at.% (DCMS) and ≈. 27. at.% (HPPMS) as the N concentration decreased. Calculations helped to attribute the increase to the substitution of Ti and Al by W and the decrease to the presence of N vacancies. Young's modulus and hardness were around 385-400. GPa and 29-31. GPa for DCMS and 430-480. GPa and 34-38. GPa for HPPMS, respectively, showing no significant trend as the W concentration was increased, whereas calculations showed a continuous decrease in Young's modulus from 440 to 325. GPa as the W concentration was increased from 0 to 37.5. at.%. The presence of N vacancies was shown to increase the calculated Young's modulus. Hence, the relatively constant values measured may be understood based on N vacancy formation as the W concentration was increased. HPPMS-deposited films exceed DCMS films in Young's modulus and hardness, which may be a consequence of the larger degree of ionization in the HPPMS plasma. It is reasonable to assume that especially the ionized film forming species may contribute towards film densification and N vacancy formation. © 2011 Elsevier B.V.

We use density functional theory calculations to explore the effects of alloying cubic TiN and VN with transition metals M = Nb, Ta, Mo or W at 50% concentrations. The ternary systems obtained are predicted to be supertough, as they are shown to be harder and significantly more ductile compared with reference binary systems. The primary electronic mechanism of this supertoughening effect is shown in a comprehensive electronic structure analysis of these compounds to be the increased valence electron concentration intrinsic to these ternary systems. Our investigations reveal the complex nature of chemical bonding in these compounds, which ultimately explains the observed selective response to stress. The findings presented in this paper thus offer a design route for the synthesis of supertough transition metal nitride alloys via valence electron concentration tuning. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Toughness, besides hardness, is one of the most important properties of wear-resistant coatings. We use ab initio density-functional theory calculations to investigate the mechanical properties of ternary metal nitrides Ti xM1-xN, with M=Mo and W, for x=0.5. Results show that Mo and W alloying significantly enhances the toughness of TiN. The electronic mechanism responsible for this improvement, as revealed by electronic structure calculations, stems from the changes in charge density induced by the additional transition-metal atom. This leads to the formation of a layered electronic arrangement, characterized by strong, respectively, weak, directional bonding, which enables a selective response to strain, respectively, shear, deformations of the structures and yields up to 60% decrease in C44 values. © 2010 The American Physical Society.