Seiji Kajita

Deep bulk atoms in a solid cause friction

To analyze kinetic friction between solids on the atomic scale, a coupled-oscillator surface model including an infinite number of atomic layers is developed by a self-consistent scheme using a Greens function. The numerical approach shows that friction involves not only surface atoms and their interaction with an opposite surface but also bulk atoms in a solid. Energy transfer from kinetic energy of a sliding solid to low-frequency lattice vibration occurs in the presence of bulk atoms, and energy dissipation into the bulk system leads to friction.

A fundamental mechanism for carbon-film lubricity identified by means of ab initio molecular dynamics

Different hypotheses have been proposed to explain the mechanism for the extremely low friction coefficient of carbon coatings and its undesired dependence on air humidity. A decisive atomistic insight is still lacking because of the difficulties in monitoring what actually happens at the buried sliding interface. Here we perform large-scale ab initio molecular dynamics simulations of both undoped and silicon-doped carbon films sliding in the presence of water. We observe the tribologically-induced surface hydroxylation and subsequent formation of a thin film of water molecules bound to the OH-terminated surface by hydrogen bonds. The comparative analysis of silicon-incorporating and clean surfaces, suggests that this two-step process can be the key phenomenon to provide high slipperiness to the carbon coatings. The water layer is, in fact, expected to shelter the carbon surface from direct solid-on-solid contact and make any counter surface slide extremely easily on it. The present insight into the wettability of carbon-based films can be useful for designing new coatings for biomedical and energy-saving applications with environmental adaptability.

In Situ X-Ray Diffraction Study of Phase Transformation of Steel in Scuffing Process

Abstract We developed a novel in situ observation method associated with synchrotron radiation X-ray diffraction (XRD) that enables us to simultaneously monitor structural changes of materials, images at frictional interfaces, friction force and temperature with a time resolution on the order of tens of milliseconds. The in situ method was applied to study scuffing process of martensitic steel under a dry condition. The result shows that during scuffing, martensite to austenite phase transformation occurred with plastic flow. The generated austenite phase disappeared when the shear test was stopped. The austenite was present at a surface temperature lower than the nominal austenitisation temperature. After intermittent occurrences of the austenitisation with local plastic flow, the scuffing feature showed a larger amount of austenite, higher friction and greater plastic flow. The XRD spectra suggest that some metallurgical properties of the near-surface material of the steel may change at the scuffing-mode transition.

Mechanism of ultra low friction of multilayer graphene studied by coarse-grained molecular simulation

Coarse-grained Metropolis Monte Carlo Brownian Dynamics simulations are used to clarify the ultralow friction mechanism of a transfer film of multilayered graphene sheets. Each circular graphene sheet consists of 400 to 1,000,000 atoms confined between the upper and lower sliders and are allowed to move in 3 translational and 1 rotational directions due to thermal motion at 300 K. The sheet-sheet interaction energy is calculated by the sum of the pair potential of the sp2 carbons. The sliding simulations are done by moving the upper slider at a constant velocity. In the monolayer case, the friction force shows a stick-slip like curve and the average of the force is high. In the multilayer case, the friction force does not show any oscillation and the average of the force is very low. This is because the entire transfer film has an internal degree of freedom in the multilayer case and the lowest sheet of the layer is able to follow the equipotential surface of the lower slider.

Nanotribological Characterization of Lubricants between Smooth Iron Surfaces

We performed the resonance shear measurement (RSM) for evaluating the nanorheological and tribological properties of model lubricants, hexadecane and poly(α-olefin) (PAO), confined between iron surfaces. The twin-path surface forces apparatus (SFA) was used for determining the distance between the surfaces. The obtained resonance curves for the confined lubricants showed that the viscosity of the confined hexadecane and PAO increased due to liquid structuring when the surface separation (D) decreased to a value less than 24 and 20 nm, respectively. It was also determined that the iron surfaces were lubricated by the hexadecane when normal load (L) was less than 1.1 mN, while the confined hexadecane behaved almost solid-like and showed poor lubricity when L was greater than 1.1 mN. In contrast, PAO between the iron surfaces showed high lubricity even under the high load (L > 2 mN). The surface separation of hexadecane and PAO at a hard wall contact between the iron surfaces was determined to be 4.6 ± 0.5 and 5.0 ± 0.4 nm by applying the fringes of equal chromatic order (FECO) for half-transparent iron films deposited on mica surfaces as substrates. We also characterized hexadecane and PAO confined between mica surfaces for studying the effect of substrates on the confined lubricants.

Erratum to: Insights into the Tribochemistry of Silicon-doped Carbon-Based Films by Ab Initio Analysis of Water–Surface Interactions

Diamond and diamond-like carbon are used as coating materials for numerous applications, ranging from biomedicine to tribology. Recently, it has been shown that the hydrophilicity of the carbon films can be enhanced by silicon doping, which highly improves their biocompatibility and frictional performances. Despite the relevance of these properties for applications, a microscopic understanding on the effects of silicon is still lacking. Here, we apply ab initio calculations to study the interaction of water molecules with Si-incorporating C(001) surfaces. We find that the presence of Si dopants considerably increases the energy gain for water chemisorption and decreases the energy barrier for water dissociation by more than 50 %. We provide a physical rational for the phenomenon by analyzing the electronic charge displacements occurring upon adsorption. We also show that once hydroxylated, the surface is able to bind further water molecules much strongly than the clean surface via hydrogen bond networks. This two-step process is consistent with and can explain the enhanced hydrophilic character observed in carbon-based films doped by silicon.

A Universal 3D Voxel Descriptor for Solid-State Material Informatics with Deep Convolutional Neural Networks

Material informatics (MI) is a promising approach to liberate us from the time-consuming Edisonian (trial and error) process for material discoveries, driven by machine-learning algorithms. Several descriptors, which are encoded material features to feed computers, were proposed in the last few decades. Especially to solid systems, however, their insufficient representations of three dimensionality of field quantities such as electron distributions and local potentials have critically hindered broad and practical successes of the solid-state MI. We develop a simple, generic 3D voxel descriptor that compacts any field quantities, in such a suitable way to implement convolutional neural networks (CNNs). We examine the 3D voxel descriptor encoded from the electron distribution by a regression test with 680 oxides data. The present scheme outperforms other existing descriptors in the prediction of Hartree energies that are significantly relevant to the long-wavelength distribution of the valence electrons. The results indicate that this scheme can forecast any functionals of field quantities just by learning sufficient amount of data, if there is an explicit correlation between the target properties and field quantities. This 3D descriptor opens a way to import prominent CNNs-based algorithms of supervised, semi-supervised and reinforcement learnings into the solid-state MI.

Direct Observation of Surface Transition During Scuffing in Dry Condition

This study investigates surface changes during scuffing in a dry condition. In the test a ball-on-disc apparatus was used, in which a rotating sapphire disc was loaded to a stationary steel ball. The contact area was directly observed and recorded by a digital camera attached to a microscope during the test. The variations in frictional force were synchronously measured with the capturing of images of the camera. After the test, the hardness of the scuffed steel ball were measured at different points in the contact area. The direct observation of the contact area shows that areas of macro plastic flow appeared from the trailing side of the contact area with a dramatic increase in frictional force. The macro plastic flow areas were changed, resulting in a dramatic expansion of the contact area. During the dramatic expansion, the friction coefficient kept a high constant value of about 0.4. The hardness distributions of the scuffed steel ball showed that the hardness was smaller at the tailing side of the contact area, in which macro plastic flows started, than that at the leading side. On the other hand the temperature rise calculated by a simple temperature estimation model was insufficient to cause the hardness reduction.Copyright

Synchronous Observation With Friction Measurement Prior to and During Occurrence of Scuffing

In the current work, scuffing phenomenon was investigated by a direct observation of a contact area. A ball-on-disc test rig was used, which produced a point contact area between a rotating sapphire disc and a stationary steel ball. Sequence of the friction and photograph in the contact area was synchronously obtained prior to and during occurrences of scuffing. Experiments were conducted in a dry condition and a lubricated condition with hexadecane. In the lubricated condition, wear debris accumulated in the inlet zone entered suddenly into the contact area to sharply increase the friction coefficient. On the other hand, macro plastic flow occurred in the whole contact area in the dry condition during a rapid increase in friction coefficient.Copyright