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Two Fe85Si15 amorphous alloy models were created based on molecular dynamics with different cooling rates of 1010 K/s (slower model) and 1012 K/s (faster model) and examined the effects of atomic structure on the mechanical properties and shear band (SB) propagation behavior, which determines the shear processing quality. Voronoi analysis of the short-range ordered structures (SRO) revealed that the slower model has more full icosahedral SROs than the faster model, and Young’s modulus and tensile strength were 11 % and 14 % higher than those of the faster model, respectively. Indentation calculations presuming crack propagation during shear processing were then performed on both models. Only in the case of the slower model, the icosahedral SRO in the SB changed to intermediate structures, increasing the distorted body-centered cubic (BCC) structure. The SB in the faster model spread out isotropically from the indenter, whereas that in the slower model propagated in the indent direction. These results indicate that intermediate and distorted BCC structures in the SB provide directionality to the SB propagation and suggest that the slower model, in which cracks propagate toward the shear direction of the material and break it in a straight line, may produce a higher-quality surface in shear processing.

Abstract Wear of contact materials results in energy loss and device failure. Conventionally, wear is described by empirical laws such as the Archard's law; however, the fundamental physical and chemical origins of the empirical law have long been elusive, and moreover empirical wear laws do not always hold for nanoscale contact, collaboratively hindering the development of high‐durable tribosystems. Here, a non‐empirical and robustly applicable wear law for nanoscale contact situations is proposed. The proposed wear law successfully unveils why the nanoscale wear behaviors do not obey the description by Archard's law in all cases although still obey it in certain experiments. The robustness and applicability of the proposed wear law is validated by atomistic simulations. This work affords a way to calculate wear at nanoscale contact robustly and theoretically, and will contribute to developing design principles for wear reduction.

We theoretically investigated influences of film deposition condition on a friction of diamond-like carbon (DLC) film using our developed molecular dynamics method. The method can deal with chemical reactions on the basis of our original stochastic equation. DLC slab model was firstly constructed, and carbon atoms were deposited onto its surface with various kinetic energies (1, 10 and 30 eV) to mimic physical vapor deposition process. Lower sp3 carbon concentration and density were obtained for the 1 eV case and “subplantation” phenomenon was found for each model. The dynamical friction of the deposited DLC film/iron oxide combination was also simulated. The friction coefficient for the deposited film with kinetic energy of 1 eV was much lower than that of 10 and 30 eV cases due to the friction-induced structural change. The simulations provided us an important insight on DLC film preparation to achieve low friction regime.

We developed a novel ultra-accelerated quantum chemical molecular dynamics simulator and applied it to adsorption dynamics of a diphenyldisulphide molecule on Fe(001) surface. As a result, we observed formation of two Fe-S bonds among the diphenyldisulphide molecule and the Fe(001) surface. In the adsorption state of the molecule on the Fe surface, it was found that two phenyl groups of the molecule were faced parallel to the Fe surface. From the electronic structure analysis, it was clarified that the parallel configuration was induced by the interaction of the 3d atomic orbitals of Fe atoms with the 2p atomic orbitals of carbon atoms as well the formation of Fe-S bonds.

We investigated the decomposition reaction mechanism of a molybdenum dithiocarbamate (MoDTC) molecule using computational chemistry methods: density functional theory and hybrid quantum chemical/classical molecular dynamics methods. The density functional theory results showed that the linkage isomerization reaction of the MoDTC preferentially takes place than its direct decomposition reaction. During a hybrid quantum chemical/classical molecular dynamics simulation, the linkage isomer of MoDTC was observed and subsequently bond weakening of its Mo-O was also observed in the polyalphaolefine phase. From these results, we proposed a new decomposition reaction pathway of the MoDTC molecule: it first forms its linkage isomer as the intermediate in the engine oil phase then decomposes into molybdenum disulfide and monothiocarbamic acid molecules on the rubbing nascent surfaces.

The effects of a cross-linking layer on the wear resistance of polymer brush were investigated by using molecular dynamics-based sliding simulations. We found that a cross-linking layer improved wear resistance. The cross-linking layer suppressed the interpenetration of polymer chains on the counter surface and thus lowered the frictional force and wear. The degrees of interpenetration decreased as the cross-linking layer closed to the tip of the chain. A cross-linking layer in the tip of the polymer chains was thus found to improve wear resistance most effectively. [ABSTRACT FROM AUTHOR]

Research is presented describing the density functional study of the clusters of precious metal to determine adsorption properties.

In this Communication, we use density functional theory (DFT) to examine the fracture properties of ceria (CeO2), which is a promising electrolyte material for lowering the working temperature of solid oxide fuel cells. We estimate the stress-strain curve by fitting the energy density calculated by DFT. The calculated Young'smodulus of 221.8 GPa is of the same order as the experimental value, whereas the fracture strength of 22.7 GPa is two orders of magnitude larger than the experimental value. Next, we combine DFT and Griffith theory to estimate the fracture strength as a function of a crack length. This method produces an estimated fracture strength of 0.467 GPa, which is of the same order as the experimental value. Therefore, the fracture strength is very sensitive to the crack length, whereas the Young's modulus is not. [ABSTRACT FROM AUTHOR]

Coarse-grained molecular dynamics simulations can model the deformation and fracture processes of the lamellar structure in polyethylene on a molecular scale; however, the simulations have not been performed due to the difficulty in building the structure and the limitations of small-scale simulations on the order of 104 beads. Thus, we propose a crystallization method for a large-scale lamellar structure on the order of 106 beads. The highly oriented lamellar structure is stretched in a coarse-grained molecular dynamics simulation. The stress and variation of crystallinity during stretching parallel and perpendicular to the crystal direction agree with experimental results, confirming the validity of our simulation results. During stretching parallel to the crystal direction, the amorphous layers crystallize and the crystalline layers fragment. We also find that the movement of the polymer chain ends from amorphous to crystalline layers, which is difficult to observe experimentally, increases the compression and generation of voids in the amorphous layers. [ABSTRACT FROM AUTHOR]

Copper–aluminate (Cu–Al2O3) with a spinel structure is an important catalyst in the abatement of pollution by nitrogen oxides (NOx), and its high-temperature phase, CuAl2O4, is considered to be an active component. In general, the Cu2+ ion in CuAl2O4 is believed to act as an active site in the NOx removal process. However, a theoretical description of adsorption on the Cu2+ site is still missing. In this study, the interaction of NH3, NO2 and NO molecules with the Cu2+ present on the CuAl2O4(100) surface has been investigated by using a periodic density functional approach. The results show that the adsorption of all the molecules on the Cu2+ site is energetically favourable, whereas NO is the most strongly bound with the adsorption site. The driving force of the adsorption is interpreted on the basis of charge transfer between the adsorbates and the (100) surface, and key information about the structural and energetic properties is also addressed. [ABSTRACT FROM AUTHOR]

We study fracture processes of amorphous and semicrystalline polymers with a coarse-grained molecular dynamics simulation. In the amorphous state, the stress caused by strain mainly arises from the loss of the attractive interaction in the voids. However, in semicrystalline polymers, the elongation of bonding is the dominant factor and it causes much more stress than that in an amorphous state. This is because growth of the voids is prevented by the amorphous regions and it is difficult to relax the folded polymers. [ABSTRACT FROM AUTHOR]

Different types of cationic species acting as Lewis acid sites as well as Na-counter ion in dealuminated zeolites are investigated, for the first time, using periodic density functional method. The energy of ammonia adsorption is taken as a measure for the acidity. The results reveal that the relative strength of the investigated active sites increases in the order: Na+ < Al(OH)2+ < AlO+ < ≡Si+. Moreover, the interaction of NH3 with tri-coordinated aluminum dislodged in the framework is less favorable than that with the neighboring silicon ion. [Copyright &y& Elsevier]

An accelerated quantum chemical molecular dynamical code “Colors-Excite” was used to investigate the photolysis of alkyl nitrites series, RONO (RᆖCH3 and C(CH3)3) on copper surfaces. Our calculations showed that the photo-dissociated processes are associated with the alkyl substituents of RONO when adsorbed on copper surfaces. For RᆖCH3, a two-step photolysis reaction occurred, yielding diverse intermediate products including RO radical, NO, and HNO, consistent with those reported in gas phase. While for RᆖC(CH3)3, only one-step photolysis reaction occurred and gave intermediate products of RO radical and NO. Consequently, pure RO species were achieved to adsorb on metal surfaces by removing the NO species in photolysis reaction. The detailed photo-dissociated behaviors of RONO on copper surfaces with different alkyl substituents which are uncovered by the present simulation can be extended to explain the diverse dissociative mechanism experimentally observed. The quantum chemical molecular dynamical code “Colors-Excite” is proved to be highly applicable to the photo-dissociations on metal surfaces. [Copyright &y& Elsevier]