92 research outputs found
Screened empirical bond-order potentials for Si-C
Typical empirical bond-order potentials are short ranged and give ductile
instead of brittle behavior for materials such as crystalline silicon or
diamond. Screening functions can be used to increase the range of these
potentials. We outline a general procedure to combine screening functions with
bond-order potentials that does not require to refit any of the potential's
properties. We use this approach to modify Tersoff's [Phys. Rev. B 39, 5566
(1989)], Erhart & Albe's [Phys. Rev. B 71, 35211 (2005)] and Kumagai et al.'s
[Comp. Mater. Sci. 39, 457 (2007)] Si, C and Si-C potentials. The resulting
potential formulations correctly reproduce brittle materials response, and give
an improved description of amorphous phases
Atomistic Insights Into Lubricated Tungsten/Diamond Sliding Contacts
The reinforcement of coatings with diamond particles results in superior tribological performance in automotive applications. In addition to improving the coating's bulk properties, sliding of diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond-reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribochemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C–O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication, and ex situ chemical analysis by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and electron energy loss spectroscopy (EELS) provides evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations
Magnetization models for particle-based simulations of magnetorheological fluids
In this study, three-dimensional particle-based simulations are used to model
magnetorheological fluids. The numerical model of the MRF is implemented in the framework
of the Discrete Element Method (DEM) and takes into account the coupling of the
magnetic dipoles, the hydrodynamic drag forces and steric forces between particles. To
accurately treat the magnetic interaction between particles, the magnetic field at the
particles’ position is computed and an appropriate magnetization model is implemented.
DEM simulations with different volume fractions of the MRF are carried out and the
resulting magnetization curves are put in comparison with experimental data
Magnetization models for particle-based simulations of magnetorheological fluids
In this study, three-dimensional particle-based simulations are used to model
magnetorheological fluids. The numerical model of the MRF is implemented in the framework
of the Discrete Element Method (DEM) and takes into account the coupling of the
magnetic dipoles, the hydrodynamic drag forces and steric forces between particles. To
accurately treat the magnetic interaction between particles, the magnetic field at the
particles’ position is computed and an appropriate magnetization model is implemented.
DEM simulations with different volume fractions of the MRF are carried out and the
resulting magnetization curves are put in comparison with experimental data
Humidity-dependent lubrication of highly loaded contacts by graphite and a structural transition to turbostratic carbon
Graphite represents a promising material for solid lubrication of highly loaded tribological contacts under extreme environmental conditions. At low loads, graphite’s lubricity depends on humidity. The adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies. To explore the humidity dependence and validate the adsorption model for high loads, a commercial graphite solid lubricant is studied using microtribometry. Even at 1 GPa contact pressure, a high and low friction regime is observed - depending on humidity. Transmission electron microscopy reveals transformation of the polycrystalline graphite lubricant into turbostratic carbon after high and even after low load (50 MPa) sliding. Quantum molecular dynamics simulations relate high friction and wear to cold welding and shear-induced formation of turbostratic carbon, while low friction originates in molecular water films on surfaces. In this work, a generalized adsorption model including turbostratic carbon formation is suggested
Symmetry and Electronic Structure of Noble Metal Nanoparticles and the Role of Relativity
High resolution photoelectron spectra of cold mass selected Cu_n-, Ag_n- and
Au_n- with n =53-58 have been measured at a photon energy of 6.42 eV. The
observed electron density of states is not the expected simple electron shell
structure, but seems to be strongly influenced by electron-lattice
interactions. Only Cu55- and Ag55- exhibit highly degenerate states. This is a
direct consequence of their icosahedral symmetry, as is confirmed by density
functional theory calculations. Neighboring sizes exhibit perturbed electronic
structures, as they are formed by removal or addition of atoms to the
icosahedron and therefore have lower symmetries. Gold clusters in the same size
range show completely different spectra with almost no degeneracy, which
indicates that they have structures of much lower symmetry. This behaviour is
related to strong relativistic bonding effects in gold, as demonstrated by ab
initio calculations for Au55-.Comment: 10 pages, 3 figure
Atomistic insights into lubricated tungsten/diamond sliding contacts
The reinforcement of coatings with diamond particles results in superior tribological performance for automotive applications. In addition to improving the coating’s bulk properties, sliding of diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribo-chemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C-O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication and ex situ chemical analysis by means of XPS, AES and EELS provide evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations
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