18 research outputs found

    Simulation of Mechanical Deformation and Tribology of Nano-Thin Amorphous Hydrogenated Carbon (a:Ch) Films Using Molecular Dynamics

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    Molecular dynamics computer simulations are used to study the effect of substrate temperature on microstructure of deposited amorphous hydrogenated carbon (a:CH) films. A transition from dense diamond- like films to porous graphite-like films is observed between substrate temperatures of 400 and 600 K for a deposition energy of 20 eV. The dense a:CH film grown at 300 K and 20 eV has a hardness ({similar_to}50 GPa) about half that of a pure carbon (a:C) film grown under the same conditions

    Molecular Dynamics Simulation of Mechanical Deformation of Ultra-Thin Amorphous Carbon Films

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    Amorphous carbon films approximately 20nm thick are used throughout the computer industry as protective coatings on magnetic storage disks. The structure and function of this family of materials at the atomic level is poorly understood. Recently. we simulated the growth of a:C and a:CH films 1 to 5 nm thick using Brenner`s bond-order potential model with added torsional energy terms. The microstructure shows a propensity towards graphitic structures at low deposition energy (20eV). In this paper we present simulations of the evolution of this microstructure for the dense 20eV films during a simulated indentation by a hard diamond tip. We also simulate sliding, the tip across the surface to study dynamical processes like friction, energy transport and microstructure evolution during sliding

    High pressure diamond-like liquid carbon

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    We report density-functional based molecular dynamics simulations, that show that, with increasing pressure, liquid carbon undergoes a gradual transformation from a liquid with local three-fold coordination to a 'diamond-like' liquid. We demonstrate that this unusual structural change is well reproduced by an empirical bond order potential with isotropic long range interactions, supplemented by torsional terms. In contrast, state-of-the-art short-range bond-order potentials do not reproduce this diamond structure. This suggests that a correct description of long-range interactions is crucial for a unified description of the solid and liquid phases of carbon.Comment: 4 pages, 5 figure

    Structural transitions and nonmonotonic relaxation processes in liquid metals

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    Structural transitions in melts as well as their dynamics are considered. It is supposed that liquid represents the solution of relatively stable solid-like locally favored structures (LFS) in the surrounding of disordered normal-liquid structures. Within the framework of this approach the step changes of liquid Co viscosity are considered as liquid-liquid transitions. It is supposed that this sort of transitions represents the cooperative medium-range bond ordering, and corresponds to the transition of the "Newtonian fluid" to the "structured fluid". It is shown that relaxation processes with oscillating-like time behavior (ω∼10−2\omega \sim 10^{-2}~s−1s^{-1}) of viscosity are possibly close to this point

    A Simple Model of Liquid-liquid Phase Transitions

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    In recent years, a second fluid-fluid phase transition has been reported in several materials at pressures far above the usual liquid-gas phase transition. In this paper, we introduce a new model of this behavior based on the Lennard-Jones interaction with a modification to mimic the different kinds of short-range orientational order in complex materials. We have done Monte Carlo studies of this model that clearly demonstrate the existence of a second first-order fluid-fluid phase transition between high- and low-density liquid phases

    Intra-molecular coupling as a mechanism for a liquid-liquid phase transition

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    We study a model for water with a tunable intra-molecular interaction JσJ_\sigma, using mean field theory and off-lattice Monte Carlo simulations. For all Jσ≥0J_\sigma\geq 0, the model displays a temperature of maximum density.For a finite intra-molecular interaction Jσ>0J_\sigma > 0,our calculations support the presence of a liquid-liquid phase transition with a possible liquid-liquid critical point for water, likely pre-empted by inevitable freezing. For J=0 the liquid-liquid critical point disappears at T=0.Comment: 8 pages, 4 figure
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