The implementation of technique for full structural optimizations of complex periodic systems in the DFT-PAW package VASP, including the volume and shape of the unit cell and the internal coordinates of the atoms, together with a correction that allows an appropriate modeling of London dispersion forces, as given by the DFT-D2 approach of Grimme [Grimme, S. J. Comp. Chem. 2006, 27, 1787], is reported. Dispersion corrections are calculated not only for the forces acting on the atoms, but also for the stresses on the unit cell. This permits a simultaneous optimization of all degrees of freedom. Benchmark results on a series of prototype systems are presented and compared to results obtained by other methods and experimental data. It is demonstrated that the computationally inexpensive DFT-D2 scheme yields reasonable predictions for the structure, bulk moduli, and cohesive energies of weakly bonded materials.

Improved description of the structure of molecular and layered crystals: ab initio DFT calculations with van der Waals corrections.

A review of Raman spectroscopy of vanadium oxides

Progress in the design and exploration of hard coatings with high temperature adaptive behavior in tribological contacts is reviewed. When coupled with most recent surface engineering strategies for high temperature contact thermal management, this progress opens a huge opportunity for adaptive coating applications on machine parts, where oils and coolants are commonly used. The adaptive mechanisms discussed here include metal diffusion and formation of lubricant phases at worn surfaces, thermally- and mechanically-induced phase transitions in hexagonal solids, contact surface tribo-chemical evolutions to form phases with low melting point, formation of easy to shear solid oxides, and others. All of these adaptive mechanisms are combined in nanocomposite coatings with synergistic self-adaptation of surface structure and chemistry to lubricate from ambient temperatures to 1000 °C and provide surface chemical and structural reversibility during temperature cycling to maintain low friction coefficients. The review also highlights emerging surface adaptive concepts, where advances with ab initio modeling of intrinsically layered solids point to new compositions for thermally stable, easy to shear ceramic coatings, load- and temperature-adaptive surfaces with arrays of compliant carbon and boron nitride nanotubes as well as low friction two-dimensional structures. Approaches for self-regulation of coating thermal conductivity, heat flow, and thermal spike mitigations are discussed in the context of surface structure evolution and phase transitions. Future progress is linked to the development of in situ exploration techniques, capable of identifying adaptive surface chemistry and structural evolutions in broad temperature regimes. When combined with predictive modeling, such approaches drastically accelerate adaptive coating developments. The review identifies opportunities, strategies, and challenges for designs and applications of hard coatings with high temperature adaptive lubrication and contact thermal management.

Hard coatings with high temperature adaptive lubrication and contact thermal management: review

The current review aims to provide an overview on the research performed with vanadium containing nitride hard coatings. Such coatings were synthesised with the objective of reducing the friction at high temperature via self-adaptation of the coating while still providing a high level of wear resistance. The lubricating effect is based on the formation of vanadium oxides with weakly bonded lattice planes and low melting temperature. The review focuses on aspects regarding the synthesis, structure and properties of these coatings and also includes a discussion on possible future developments and further improvements of the coating design.

Vanadium containing self-adaptive low-friction hard coatings for high-temperature applications: A review

This article provides an overview of the latest research developments on binary and ternary oxide coatings that have the potential to be used as solid lubricants at elevated temperatures. The review focuses on understanding the major mechanisms that lead to a reduction in friction and/or wear in high temperature lubricious oxides. Changes in the structural, chemical, and electronic properties of these oxides as a function of temperature will be correlated to their mechanical and tribological performance using a range of experimental tools in addition to modeling based on ab initio calculations and molecular dynamics simulation methods. This review also includes a discussion of the industrial applications of these coatings as well as of potential improvements to the coating design and other anticipated future developments.

Lubricious oxide coatings for extreme temperature applications: A review

VO, V2O3, VO2, V6O13, V4O9, V3O7 and V2O5 have been investigated in terms of structure, bulk modulus B and elastic constant C44 using ab initio calculations. The C44 values for V6O13, V4O9, V3O7 and V2O5 are significantly lower than those for V2O3 and VO2. As the V valency is increased from 3 to 5, C44 decreases by 83%, whereas the bulk modulus decreases by 61%, leading to an increase in the B/C44 ratio from 1.4 to 3.4. This is consistent with calculated decohesion energies for cleavage in VO2 and V2O5. When cleaving V2O5, decohesion energies are considerably lower than those of VO2. This behaviour may be understood based on V valency induced changes in the crystal and electronic structure as well as in the chemical bonding. As the V valency is increased, the bond strength decreases. The phases with a V valency >4 exhibit low C44 values, large anisotropy and possess weak ionic bonding between the layers. The formation of easily plastically deformable structures is enabled by the screened Coulomb potential. The largest distance and therefore weakest bond strength is observed for V2O5 in the (002) plane.

http://iopscience.iop.org/article/10.1088/0953-8984/21/14/145404/pdf

Ab initio calculations of the structure and mechanical properties of vanadium oxides

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 ...

Structure and mechanical properties of TiAlN–WNx thin films

Transition metal oxide Magneli phases are traditionally described as crystallographic shear structures. Some of these phases feature a layered crystal structure based on deformed metal–oxygen octahedra. Here, we describe the correlation between structure, decohesion energies and elastic properties of WO3, ReO3, MoO2, VO2, V2O5 and TiO2 using ab initio calculations. Decohesion energies and elastic constants C44 are inversely proportional to the original distance between the cleaved layers and correspond to the screened Coulomb potential. This can be understood based on the electronic structure. The bond strength decreases rapidly as the distance is increased, resulting in weak coupling between the layers, which in turn causes the formation of easily plastically deformable structures.

https://iopscience.iop.org/article/10.1088/0953-8984/22/29/292203/pdf

Coulomb-potential-dependent decohesion of Magnéli phases

The new spin ½ V4+ barium oxovanadate BaV4O9 was synthesized and studied by means of single-crystal X-ray diffraction. Its room-temperature structure is monoclinic, space group P2/c. We discuss the temperature evolution of the crystal structure and thermal expansion tensor of the material between 293 and 100 K.

Synthesis and structural characterization of BaV4O9

The tribarium dilithium divanadate tetrachloride Ba(3)Li(2)V(2)O(7)Cl(4) is a new vanadate with a channel structure and the first known vanadate containing both Ba and Li atoms. The structure contains four non-equivalent Ba(2+) sites (two with m and two with 2/m site symmetry), two Li(+) sites, two nonmagnetic V(5+) sites, five O(2-) sites (three with m site symmetry) and four Cl(-) sites (m site symmetry). One type of Li atom lies in LiO(4) tetrahedra (m site symmetry) and shares corners with VO(4) tetrahedra to form eight-tetrahedron Li(3)V(5)O(24) rings and six-tetrahedron Li(2)V(4)O(18) rings; these rings are linked within porous layers parallel to the ab plane and contain Ba(2+) and Cl(-) ions. The other Li atoms are located on inversion centres and form isolated chains of face-sharing LiCl(6) octahedra.

Thomas Reeswinkel

Papers

Ab initio calculations of the structure and mechanical properties of vanadium oxides

Structure and mechanical properties of TiAlN–WNx thin films

Coulomb-potential-dependent decohesion of Magnéli phases

Synthesis and structural characterization of BaV4O9

Ba3Li2V2O7Cl4, a new vanadate with a channel structure.