Search

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 (Cu ad ) migrate among favored fourfold-hollow surface sites by passing across atop-Ti metastable positions. For increasing temperatures, however, Cu ad 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 Cu ad diffusivity D can be expressed by a fairly Arrhenius-like behavior D ( T ) = 8.26(×2 ±1 ) × 10 −4  cm 2  s −1 exp[(−0.04 ± 0.01 eV)/( k B T )] over the entire investigated temperature range (100 ≤  T  ≤ 1000 K). AIMD simulations also reveal that the condensation of Cu adatoms into Cu x>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. [ABSTRACT FROM AUTHOR]

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) V 0.5 Mo 0.5 N 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 V 0.5 Mo 0.5 N, as well as reference B1 TiN, subject to <001> and <110> tensile deformation. AIMD simulations reveal that V 0.5 Mo 0.5 N 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 V 0.5 Mo 0.5 N 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. [ABSTRACT FROM AUTHOR]

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 M 1 M 2 C alloys, for which M 1 = Ti or V and M 2 = 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 1 1 ¯ 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • High-resolution scanning transmission electron microscopy was used to image dislocation core structures in TiN. • Classical interatomic potential simulations complement the high-resolution images by confirming the atomic structure of the different dislocation types. • Density-functional theory bonding analyses suggest enhanced metal–metal bonding at the cores compared to defect-free regions, locally weakening the directional Ti-N bonds. • Complementary Peierls stress calculations predict substantial N vacancy-pinning effects at the dislocation core. 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} 〈 1 1 – 0 〉 dislocation, Shockley partial dislocations a/6{111} 〈 11 2 – 〉 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} 〈 1 1 – 0 〉 and {111} 〈 11 2 – 〉 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. [ABSTRACT FROM AUTHOR]

We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti 0.5 W 0.5 N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti 0.5 W 0.5 N, 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 Ti 0.5 W 0.5 N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [111]-oriented Ti 0.5 W 0.5 N 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 Ti 0.5 W 0.5 N 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 V 0.5 W 0.5 N and V 0.5 Mo 0.5 N epitaxial layers. [ABSTRACT FROM AUTHOR]

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 E des of 1.37eV and an attempt frequency A des =3.4×1013 s−1. E des is essentially equal to the N adatom surface diffusion barrier, Es =1.39eV, while As is only three to four times larger than A des, 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 ~3Å which rapidly increase to ~2eV at a separation of 1.5Å. We obtain good qualitative and quantitative agreement with the above results using the modified embedded atom method potential to perform classical molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Available information concerning the elastic moduli of refractory carbides at temperatures (T) of relevance for practical applications is sparse and/or inconsistent. Ab initio molecular dynamics (AIMD) simulations at T = 300, 600, 900, and 1200 K are carried out to determine the temperature-dependences of the elastic constants of rocksalt-structure (B1) TiC, ZrC, HfC, VC, TaC compounds, as well as high-entropy (Ti,Zr,Hf,Ta,W)C and (V,Nb,Ta,Mo,W)C. The second-order elastic constants are calculated by least-square fitting of the analytical expressions of stress/strain relationships to simulation results obtained from three tensile and three shear deformation modes. Sound-velocity measurements are employed to validate AIMD values of bulk, shear, and elastic moduli of single-phase B1 (Ti,Zr,Hf,Ta,W)C and (V,Nb,Ta,Mo,W)C at ambient conditions. In comparison with the predictions of previous ab initio calculations – where the extrapolation of finite-temperature elastic properties accounted for thermal expansion while neglecting intrinsic vibrational effects – AIMD simulations produce a softening of shear elastic moduli with T in closer agreement with experiments. The results show that TaC is the system which exhibits the highest elastic resistances to tensile and shear deformation up to 1200 K, and indicate the (V,Nb,Ta,Mo,W)C system as candidate for applications that require superior toughness at room as well as elevated temperatures. [Display omitted] • The elastic constants of binary and multicomponent transition-metal carbides decrease monotonically with temperature. • The explicit treatment of lattice vibrations allows reproducing experimental trends in the shear moduli with temperature. • Empirical criteria indicate that the ductility of carbides improves with increasing temperature and electron concentration. • Tantalum carbide exhibits the highest elastic resistances to both tensile and shear deformation up to 1200 K. [ABSTRACT FROM AUTHOR]

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 V 1-x W x N and V 1-x Mo x N alloy systems. [ABSTRACT FROM AUTHOR]

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. • Classical MD is used to model TiN deposition on misoriented grains. • Misoriented grains are reoriented in the early stages of deposition. • The grain reorientation coincides with an increase in the film N/Ti ratio. • The resulting film is epitaxial and stoichiometric. [ABSTRACT FROM AUTHOR]

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· T m < T < ≈0.9· T m ] relatively close to the materials melting points T m , the simulations reveal that anion vacancy migration in TiN and VN exhibits an Arrhenius-like behavior, described by activation energies E a TiN = 4.2 ± 0.3 eV and E a 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 E a extracted by Arrhenius linear regression at elevated temperatures with ab initio E a 0K values calculated at 0 Kelvin reveals that, while the nitrogen migration energy E a TiN varies modestly with temperature {∆ E a TiN = [ E a (T m) – E a (0 K)]/ E a (0 K) ≈ 0.1}, the changes in E a VN vs. T are considerable (∆ E a 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)]. • Developed method for nonequilibrium (accelerated) ab initio molecular dynamics. • Diffusivities in B1 VN, dynamically stable >250 K, determined by molecular dynamics. • The N vacancy migration energy in B1 TiN increases with temperature. • Lattice dynamics linked to mass transport properties. • Argued reliability of quasiharmonic transition-state theory vs. molecular dynamics. [ABSTRACT FROM AUTHOR]

The defect structures forming during high-temperature decomposition of Ti 1-x Al x N y films were investigated through high-resolution scanning transmission electron microscopy. After annealing to 950 °C, misfit edge dislocations and a / 6 〈 112 〉 { 111 } partial dislocations permeate the interface between TiN-rich and AlN-rich domains to accommodate lattice misfits during spinodal decomposition. The stacking fault energy associated with the partial dislocations decreases with increasing Al content, which facilitates the coherent cubic to wurtzite structure transition of AlN-rich domains. The wurtzite AlN-rich structure is recovered when every third cubic {111} plane is shifted by a / 6 along the [211] direction. After annealing to 1100 °C, a temperature where coarsening dominates the microstructure evolution, we observe intersections of stacking faults, which form sessile locks at the interface of the TiN- and AlN-rich domains. These observed defect structures facilitate the formation of semicoherent interfaces and contribute to hardening in Ti 1-x Al x N y. [Display omitted] [ABSTRACT FROM AUTHOR]

High hardness and toughness are generally considered mutually exclusive properties for single-crystal ceramics. Combining experiments and ab initio molecular dynamics (AIMD) atomistic simulations at room temperature, we demonstrate that both the hardness and toughness of single-crystal NaCl-structure VN x /MgO(001) thin films are simultaneously enhanced through the incorporation of anion vacancies. Nanoindentation results show that VN 0.8 , here considered as representative understoichiometric VN x system, is ≈20% harder, as well as more resistant to fracture than stoichiometric VN samples. AIMD modeling of VN and VN 0.8 supercells subjected to [001] and [110] elongation reveal that the tensile strengths of the two materials are similar. Nevertheless, while the stoichiometric VN phase cleaves in a brittle manner at tensile yield points, the understoichiometric compound activates transformation-toughening mechanisms that dissipate accumulated stresses. AIMD simulations also show that VN 0.8 exhibits an initially greater resistance to both { 110 } 〈 1 1 ¯ 0 〉 and { 111 } 〈 1 1 ¯ 0 〉 shear deformation than VN. However, for progressively increasing shear strains, the VN 0.8 mechanical behavior gradually evolves from harder to more ductile than VN. The transition is mediated by anion vacancies, which facilitate { 110 } 〈 1 1 ¯ 0 〉 and { 111 } 〈 1 1 ¯ 0 〉 lattice slip by reducing activation shear stresses by as much as 35%. Electronic-structure analyses show that the two-regime hard/tough mechanical response of VN 0.8 primarily stems from its intrinsic ability to transfer d electrons between 2nd-neighbor and 4th-neighbor (i.e., across vacancy sites) V–V metallic states. Our work offers a route for electronic-structure design of hard materials in which a plastic mechanical response is triggered with loading. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Traditional age hardening mechanisms in refractory ceramics consist of precipitation of fine particles. These processes are vital for widespread wear-resistant coating applications. Here, we report novel Guinier-Preston zone hardening, previously only known to operate in soft light-metal alloys, taking place in refractory ceramics like multicomponent nitrides. The added superhardening, discovered in thin films of Ti-Al-W-N upon high temperature annealing, comes from the formation of atomic-plane-thick W disks populating {111} planes of the cubic matrix, as observed by atomically resolved high resolution scanning transmission electron microscopy and corroborated by ab initio calculations and molecular dynamics simulations. Guinier-Preston zone hardening concurrent with spinodal decomposition is projected to exist in a range of other ceramic solid solutions and thus provides a new approach for the development of advanced materials with outstanding mechanical properties and higher operational temperature range for the future demanding applications. [Display omitted] [ABSTRACT FROM AUTHOR]