Toshihide Ohmori

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 Cold Forging Oil Containing Phosphorus Type EP Additives

The galling prevention property of oils containing oleyl acid phosphate (OLAP) and/or phosphoric acid has been investigated. A synergistic effect was observed with the oil containing these two additives. Heat-treatment was found to effectively enhance the galling prevention property of the oil. Surface analyses showed more phosphate formed on workpiece surfaces after the tests, using the oils with superior galling prevention property. Oil analyses suggested that OLAP, phosphoric acid and water associated to form molecular aggregates, and that the association between OLAP and phosphoric acid was promoted by heat-treatment. An oil with the highest, chemical reactivity and galling prevention property was obtained by the addition of the two additives to the base oil, followed fry heat-treatment at 120°C for one hour with stirring. The pretreatment of the workpieces in this oil further increased the galling prevention property due to the effect of phosphate films preformed on the. workpiece surfaces. Presented...

TOF-SIMS study on the adsorption behavior of mixtures of a phosphite and a friction modifier onto ferrous material

The adsorption behavior of a mixture of lubricant additives onto ferrous materials has been investigated by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Tri-n-butylphosphite (TBPi) and di-n-butylphosphite (DBPi) were selected as model compounds of phosphate-type additives and palmitic acid (PA), oleyl amine (OA) and glycerol mono-oleate (GM) were selected as model compounds of friction modifiers. The mixtures of one of the phosphites and one of the friction modifiers were dissolved in a paraffinic mineral oil for the lubrication tests. By TOF-SIMS analysis of the friction surfaces, the following conclusions were obtained. Adsorption of PA and GM was inhibited in the presence of phosphites, but the adsorption of OA was not inhibited by the phosphites. On the other hand, the reaction of TBPis was inhibited in the presence of OA, but for other mixed systems no inhibition of the reaction of phosphites by a friction modifier was observed. Except for OA mixed systems, the amount of reaction product of the phosphites was found to be correlated to the lubrication properties for all other mixture systems.

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.

Molecular dynamics simulations of elasto-hydrodynamic lubrication and boundary lubrication for automotive tribology

Friction control of machine elements on a molecular level is a challenging subject in vehicle technology. We describe the molecular dynamics studies of friction in two significant lubrication regimes. As a case of elastohydrodynamic lubrication, we introduce the mechanism of momentum transfer related to the molecular structure of the hydrocarbon fluids, phase transition of the fluids under high pressure, and a submicron thickness simulation of the oil film using a tera-flops computer. For boundary lubrication, the dynamic behavior of water molecules on hydrophilic and hydrophobic silicon surfaces under a shear condition is studied. The dynamic structure of the hydrogen bond network on the hydrophilic surface is related to the low friction of the diamond-like carbon containing silicon (DLC-Si) coating.

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.

All-Atom Molecular Dynamics Simulation of Submicron Thickness EHL Oil Film

All-atom molecular dynamics simulations of an elastohydrodynamic lubricating oil film have been performed to study the effect of the oil film thickness (large spatial scale; thickness: 430 nm, MD time: 25 ns) and the effect of pressure (long time scale; thickness: 10 nm, MD time: 50 ns, external pressure: 0.1 to 8.0 GPa). Fluid layers of n-hexane are confined between two solid Fe plates by a constant normal force. Traction simulations are performed by applying a relative sliding motion to the Fe plates. In a long spatial scale simulation, the mean traction coefficient was 0.03, which is comparable to the experimental value of 0.02. In a long time scale simulation, a transition of the traction behavior is observed around 0.5 GPa to 1.0 GPa which corresponds to a change from the viscoelastic region to the plastic-elastic region which have been experimentally observed. This phase transition is related to a suppressed fluctuation of the molecular motion.© 2007 ASME

Evaluation of galling prevention properties of cold-forging oils by ball penetration test

Abstract A ball penetration test has been devised as a new laboratory-scale cold-forging test for determining the galling prevention properties of lubricants. The galling prevention properties of cold-forging oils were clearly classified by this test. After the tests, various reaction products were detected on the workpiece surfaces. The oils with higher reactivity exhibited superior galling prevention properties. The order of performance as determined by the ball penetration test correlated well with that determined by the backward can extrusion test, which is generally recognized as a standard cold-forging test. Moreover, the ball penetration test requires no tedious test procedures, in contrast with conventional cold-forging tests. Judging from the results obtained, the ball penetration test is effective as a means of evaluating the galling prevention capacity of cold-forging oils adequately and easily.

Newly Developed Friction Tester for in situ Soft X-Ray Absorption Measurements of Frictional Engine-Oil/Metals Interfaces

A novel friction tester has been developed to clarify the friction mechanism between engine oil and metals by in situ/operando measurements of the X-ray absorption near edge structure (XANES) using the total electron yield (TEY) method. The tester can perform frictional motion with engine oil and metals in a vacuum chamber under a vacuum of 10-7 Torr during TEY-XANES measurements in BL-6.3.2 at the Advanced Light Source (ALS). From in situ XANES measurements in the C K and Fe L regions, the organic molecule layers at the oil/metals interfaces have been successfully observed.

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