Modelling and Simulation in Materials Science and Engineering

doi: 10.1088/0965-0393/18/3/030201pmid: N/A

Computer simulations of tribological phenomena, such as they occur when two solid bodies are slid past one another, or squeezed into each other, have become increasingly more realistic over the last decade. This progress is important because many different scenarios lead to similar behaviour. For example, the number of processes that can lead to stick slip motion is incredibly large. If we want to suppress this irregular motion in a given system, instabilities leading to undesired dynamics must be revealed. These processes, however, cannot be identified with toy models. Of course, realistic simulations demand a heavy computational overhead, which can be alleviated with the help of new methods, such as improved model potentials, mapping schemes from the atomistic scale to continuum descriptions, or even concurrent multi-scale techniques.This special issue consists of papers dealing with material-specific simulations of friction and contact mechanics. It includes contributions unravelling how seemingly small chemical changes can affect flow boundary conditions near a surface [Kong et al], atomic and multi-scale methods incorporating heat transport and assessing its relevance to dislocation formation during contact formation [Crill et al], the effect that the geometry of an atomic force microscope tip has on contact area and friction when indented into a self-assembled monolayer [Knippenberg et al], methods to bridge the time-scale gap allowing one to access the small velocities used in atomic force microscopy [Kim and Falk], and first-principles calculations estimating the thermodynamic conditions required to achieve low friction of diamond surfaces [Guo and Qi]. In addition, this special issue contains work on hybrid force field and tight binding simulations on lubricant additives in sliding contacts [Ondera et al], discrete dislocation simulations on the plastic response of nanoimprinted surfaces [Zhang et al], and the nanotribology of water [Lorenz et al], as well as structural and dynamic properties of various liquids [Leng et al] in confinement.All papers were peer-reviewed following the standard procedure established by the Editorial Board of Modelling and Simulation in Materials Science and Engineering.Martin H Müser and Judith A HarrisonGuest Editors

Crill, J W; Ji, X; Irving, D L; Brenner, D W; Padgett, C W

doi: 10.1088/0965-0393/18/3/034001pmid: N/A

A coarse graining method that introduces Joule heating and improves heat transport in a classical molecular dynamics simulation is reviewed, and two example sets of simulations, opening of goldgold nano-asperity contacts and nano-asperity sliding at loaded copperaluminum interfaces are discussed. For the gold contact, dislocations nucleate from the edges of where the asperity contacts the substrates and move along the close-packed planes, resulting in stacking faults that form two subsurface Thompson tetrahedra. For a null voltage, a nanowire with a diameter much smaller than the initial contact area is created when the two tetrahedra are completed, and as the wire yields the partial dislocations retreat to the surface. Opening with Joule heating enhances dislocation mobility and intransient subsurface plasticity. Constant current simulations show melting and boiling of the nanowires depending on the voltage cap. Sliding of an aluminum asperity on copper with a null voltage shows dislocation formation in the copper and aluminum, while heating from an applied voltage eliminates damage in the copper. Sliding with a copper asperity enhances plastic damage in the copper substrate compared with the aluminum asperity, while Joule heating enhances aluminum pile-up in front of the copper asperity due to plowing.

Knippenberg, M Todd; Mikulski, Paul T; Harrison, Judith A

doi: 10.1088/0965-0393/18/3/034002pmid: N/A

Experimental techniques that utilize atomic force microscopy are routinely used to examine tribological properties of tipsample interactions. While analysis of data obtained with these methods provides values for macroscale properties, such as interfacial shear strength, understanding nanoscale properties, such as contact radius, requires an atomic-scale approach. Molecular dynamics simulations provide the ability to numerically analyze the nanoscale origins of a wide-range of material and tribological properties. In this paper, the sliding contact between a self-assembled monolayer (SAM) and two countersurfaces (a nominally flat, amorphous carbon surface and a nearly spherical fullerene tip) is compared. By examining contact forces between the tip and monolayer atoms, large differences in monolayer behavior that occur due to tip geometry can be elucidated. The structure factor reveals that the fullerene tip creates a more disordered monolayer than the amorphous counterface. Friction forces were also studied using the atomic-level contact forces, which show that the depth at which the fullerene tip affects the SAMs substrate is much deeper than the amorphous counterface. The distribution of contact forces that contribute to friction and load were studied and show a difference in behavior between the two countersurfaces. Finally, while there are a large number of atoms that have a non-zero load during sliding, a smaller subset of 32 atoms carries 96% of the load. Using this subset of atoms to compute contact radius reveals a greater agreement with the continuum mechanics models than using all atoms with a non-zero load. This paper highlights how computer simulations can yield insight into tribological interactions at the atomic scale.

Kim, W K; Falk, M L

doi: 10.1088/0965-0393/18/3/034003pmid: N/A

Accelerated molecular dynamics (MD) simulations are implemented to model the sliding process of atomic force microscope (AFM) experiments and to lower the sliding speeds below those in a conventional MD simulation. In this study the hyperdynamics method, originally devised to extend MD time scales for non-driven systems, is applied to the frictional sliding system. This technique is combined with a parallel algorithm that simultaneously simulates the system over a range of slider positions so that the overall acceleration rate is approximately the number of processors multiplied by the boost factor from the hyperdynamics method. The new methodologies are tested using two-dimensional and three-dimensional Lennard-Jones AFM models. Direct comparison with the results from conventional MD shows close agreement validating the methods. The acceleration rate achieved in this study is four orders of magnitude in 2D and three orders of magnitude in 3D.

Kong, Ling-Ti; Denniston, Colin; Müser, Martin H

doi: 10.1088/0965-0393/18/3/034004pmid: N/A

We study the slip-boundary conditions of short, linear paraffins and olefins confined between two sliding aluminum surfaces with molecular dynamics. Our simulations are based on a recently developed force field for the interaction between organic molecules and bulk aluminum. The lubricant molecules investigated all consist of six monomers but differ in the existence or location of merely one double bond. It turns out that this small change in the chemistry of the lubricant molecules can alter slip lengths quite dramatically, and is not strongly correlated with surface energies and bulk viscosity of the lubricant. For example, and -hexene have similar large negative slip length of 8, even though -hexene adheres twice as strongly to the surface as -hexene. Eliminating the double bond in -hexene reduces the surface energy by another factor of two, but increases from 8 to 120. These results and those of additional simulations based on unrealistic, albeit occasionally used model potentials, make us conclude that surface energies and/or molecular geometries alone are not reliable indicators for slip-boundary conditions. Instead, it is necessary to consider the full chemical detail. As a more encouraging result, we find that the bulk viscosity appears to describe the dissipation within the sheared fluid close to the wall quite well, despite significant ordering near the boundaries. Moreover, all our systems show a relatively weak dependence of the slip length on the normal pressure.

Lorenz, Christian D; Chandross, Michael; Lane, J Matthew D; Grest, Gary S

doi: 10.1088/0965-0393/18/3/034005pmid: N/A

We report the results of large-scale molecular dynamics simulations of water confined between alkylsilane Si(OH)3(CH2)10COOH self-assembled monolayers (SAMs) on an amorphous silica substrate. The structure and dynamics of the confined water are studied for applied pressures ranging from approximately 50 to 400 MPa. The viscosity and microscopic friction of the confined water are determined from steady-state shear simulations. We find that the viscosity of the water increases only slightly compared with bulk water under comparable pressures. There is no evidence of ice-like layers being formed near the COOH end groups of the SAMs. The microscopic friction coefficients could only be calculated at high shear rates due to the low viscosity of the water and are found to decrease with increasing amounts of water, similar to experiment.

Zhang, Yunhe; Van der Giessen, Erik; Nicola, Lucia

doi: 10.1088/0965-0393/18/3/034006pmid: N/A

Simulations of rough surface flattening are performed on thin metal films whose roughness is created by nanoimprinting flat single crystals. The imprinting is carried out by means of a rigid template with equal flat contacts at varying spacing. The imprinted surfaces are subsequently flattened by a rigid platen, while the change of roughness and surface profile is computed. Attention is focused mainly on comparing the response of the film surfaces with those of identical films cleared of the dislocations and residual stresses left by the imprinting process. The aim of these studies is to understand to what extent the loading history affects deformation and roughness during flattening. The limiting cases of sticking and frictionless contact between rough surface and platen are analyzed. Results show that when the asperities are flattened such that the contact area is up to about one third of the surface area, the loading history strongly affects the flattening. Specifically, the presence of initial dislocations facilitates the squeezing of asperities independently of the friction conditions of the contact. For larger contact areas, the initial conditions affect only sticking contacts, while frictionless contacts lead to a homogeneous flattening of the asperities due to yield of the metal film. In all cases studied the final surface profile obtained after flattening has little to no resemblance to the original imprinted surface.

Leng, Yongsheng; Lei, Yajie; Cummings, Peter T

doi: 10.1088/0965-0393/18/3/034007pmid: N/A

Aqueous hydration water confined between two mica surfaces and nonpolar liquid argon confined between two solid crystals have been comparably studied through molecular dynamics simulations. A liquidvapor molecular ensemble developed in previous studies (Leng 2008 J. Phys.: Condens. Matter 20 354017) has been used to investigate the solvation structures and diffusion dynamics of confined films. We find that water always tends to diffuse even under two-layer extreme confinement (D = 0.73nm), whereas liquid argon undergoes a spontaneous liquid-to-solid phase transition at an appreciable large distance (n = 9 layers) between the two crystal solids. Vacancy diffusion in the solid phase of argon is observed. We attribute this phase transition of argon to the tendency of argon molecules to form a close-packed structure to maximize the cohesion energy contributed from weak van der Waals attractions.

Guo, Haibo; Qi, Yue

doi: 10.1088/0965-0393/18/3/034008pmid: N/A

The adhesion and friction of both diamond and diamond-like carbon coatings can be dramatically changed by active gases in the environment, such as hydrogen, water vapor and humid air, due to tribochemical reactions. To understand the atmospheric effects and to predict the optimized environmental conditions (gas species, pressure and temperature), the tribochemical reactions on diamond surfaces are modeled from first principles thermodynamics. The results show that both H2 and a mixture of H2O plus O2 (such as humid air) can effectively achieve low adhesion and low friction with a fully H or OH passivated surface at very low partial pressures. Water vapor itself can passivate diamond (111) and (100) surfaces into half H and half OH terminated surfaces, but only at unrealistically high partial pressures. Even a trace amount of oxygen combined with water vapor can significantly reduce the water partial pressure for passivation. In all tribochemical reactions considered, the partial pressure required to reach low adhesion and low friction increases rapidly with temperature, and diamond (100) surface requires less partial pressures than (111) surface for surface passivation.

Onodera, T; Miura, R; Suzuki, A; Tsuboi, H; Hatakeyama, N; Endou, A; Takaba, H; Kubo, M; Miyamoto, A

doi: 10.1088/0965-0393/18/3/034009pmid: N/A

Tribology at the atomistic and molecular levels has been theoretically studied by a classical molecular dynamics (MD) method. However, this method inherently cannot simulate the tribochemical reaction dynamics because it does not consider the electrons in nature. Although the first-principles based MD method has recently been used for understanding the chemical reaction dynamics of several molecules in the tribology field, the method cannot simulate the tribochemical reaction dynamics of a large complex system including solid surfaces and interfaces due to its huge computation costs. On the other hand, we have developed a quantum chemical MD tribochemical simulator on the basis of a hybrid tight-binding quantum chemical/classical MD method. In the simulator, the central part of the chemical reaction dynamics is calculated by the tight-binding quantum chemical MD method, and the remaining part is calculated by the classical MD method. Therefore, the developed tribochemical simulator realizes the study on tribochemical reaction dynamics of a large complex system, which cannot be treated by using the conventional classical MD or the first-principles MD methods. In this paper, we review our developed quantum chemical MD tribochemical simulator and its application to the tribochemical reaction dynamics of a few lubricant additives.