Nowadays, 500 PFLOPS super computer is launched, which is about 50 times faster than the previous flagship machine in Japan in 10 years before. This means that the simulation size for molecular simulation is extended 3.7 times larger in each 3 directions. Since the multi-scale nature of the phenomena is essential for the tribological phenomena, not only the machine power dependent but the efficient modeling to grab the phenomena is important. Here we describe the analyses using all-atom and coarse grained molecular simulation methods to reveal tribological phenomena. For the solid friction, the origin of layered materials is cleared using coarse-grained simulations and all-atom simulations. The heat escape motion on the commensurate surface between transfer layer and the base material is essential, and the environmental effect is also important. For the boundary lubrication, the simulation of very beginning physical adsorption of oiliness additive molecules revealed that the effect of base oil is different which is understood as “chain matching”. Molecular dynamics can treat not only physical but chemical adsorption process. The simulation of adsorption process for anti-copper-corrosion additives revealed that the selective adsorption of additives on newly formed surface is due to the charge transfer from the metal surface to additive molecules. The large-scale all-atom dynamics simulation for elastohydrodynamic lubrication oil film revealed the origin of boundary slip, and saturation of traction coefficient. Now the authors are extending multi-physics method to treat tribology of complex fluids such as viscosity index improver. At last we discuss about the micron-sale bottom up simulation for solid friction and wear. We show recent approach to attack multi-physics and large scale phenomena such as occurrence of wear from molecular level, using smoothed particle hydrodynamics simulator.
For soft materials based on functional organic compounds, it is of fundamental importance to understand the structure and dynamics at the interfaces between different matters. Since the intermolecular interactions strongly affect the phenomena at the soft interfaces between molecular assemblies of organic compounds and inorganic solids or liquids, additional molecular-level knowledge is needed to elucidate the mechanism of these phenomena. The molecular dynamics simulation provides one of the useful methods for studying phenomena that occur at the soft interfaces. In this paper, we showed the structural and dynamical properties of ideal liquid crystal monolayers and functional aqueous-liquid crystal monolayers investigated by means of all-atom molecular dynamics simulations.
This review article presents the results of molecular dynamics simulations of sliding processes and sliding experiments using a friction force microscope to clarify the sliding friction phenomena between a diamond slider and a copper specimen in the air and to reduce the gap between the experiments and molecular dynamics simulations. In the simulation model, both the adhesion effects obtained through the nanocontact experiments in the air and the effects of the oxide film generated in the uppermost specimen surface were considered. From the comparisons between the simulation and the experimental results, it was confirmed that they have good similarities in the friction coefficients. From the simulation, the breaking mechanism of the oxide film was also clarified by observing the slip deformation generated in the copper specimen.
Owing to their extraordinary combination of mechanical properties, thermal conductivities and chemical inertness, carbon hard coatings, such as nanocrystalline diamond and diamond-like amorphous carbon, have recently drawn attention for superlubricity applications (μ < 0.01) in a broad range of engineering systems. Yet, despite many efforts to understand phenomena occurring at buried sliding interfaces, they still remain elusive. Atomistic simulations of friction between two surfaces in relative motion have been of increasingly importance to gain insights into the physical and chemical origin of friction and wear. They can be also utilized for designing custom-tailored lubricants and superlubricious interfaces. This article reviews state-of-the-art results of atomistic simulations in tribology of diamond and diamond-like amorphous carbon surfaces in both dry friction and boundary lubrication with water and organic friction modifiers such as fatty acids and glycols.
Nowadays, molecular dynamics (MD) approach is widely used to investigate the tribochemical reactions. Recent reactive MD simulations that can handle large-scale simulation model have elucidated the effects of the tribochemical reactions on the tribological properties, whereas first-principles MD (FPMD) simulations that can consider electronic structure enable us to understand the tribochemical reaction through the knowledge of the electronic structure of sliding materials, lubricants and additives. However, not only the individual study but also the collaborative study by reactive MD, FPMD, and friction experiment is important for more precise design of sliding materials, lubricants, and additives. In this paper, we introduced the representative work in which the collaboration of reactive MD, FPMD and friction experiment elucidated the superlow friction mechanism.
The establishment of effective lubrication with nanometer-thick liquid films is crucial for the development of advanced miniaturized mechanical systems. Complementary to experiments, molecular dynamics (MD) simulations provide atomic-level insights into the structure and movement of nanometer-thick liquid films. However, conventional all-atom MD simulations are computationally expensive for studying systems over extended length and time scales. Coarse graining, which eliminates some degrees of freedom by grouping several atoms to one bead, is one effective method to reduce computational cost. In this review, the procedure of coarse graining and some points to note for accurate modeling of lubrication phenomena are presented through a concrete example of coarse-grained MD simulations of confined shear of nanometer-thick polar liquid lubricant films at the head-disk interface of magnetic disk drives.
Friction reduction always attracted attention, and improvement has been continued while it was said that the automotive piston ring relatively had a big frictional force in the part which constituted an engine, and fuel efficiency regulation continued being strengthened. Generally, the lubrication state that frictional force is the smallest is Hydrodynamic lubrication, and naturally it is similar for the low friction of the piston ring. However, as for the main reason, the recent piston ring tends to avoid the formation of the hydrodynamic lubrication oil film thickness to a contact face from consideration to oil consumption. We think that such a status is off the basic theory of the piston ring greatly. This document reports, returning to the origin once again, that the principle the function of the piston ring expresses and the difference of performance on the piston ring in recent years.
It is well known that variations in the frictional coefficients between thread surfaces and bearing surfaces cause tightening error on the torque control method. Recent years, the special lubricants for bolt tightening have been developed to reduce the variations of the frictional coefficients. However, lubricants alone cannot reduce the variation in the frictional coefficients. In this article, I introduce the improvement of the tightening accuracy in the bolt tightening that we have been researching on. The squareness error of bearing surface on the bolts must be reduce less than 0.2° in order to improve the tightening accuracy. Then polyisobutylene is effective for reduction of variation in friction coefficients between thread surfaces and bearing surfaces.
Ball bearings on the main shaft of rocket engine turbopumps which supply cryogenic propellants to the main combustion chamber are critical elements of the entire propulsion system of a rocket. A self-lubricating ball bearing with a retainer made of glass-cloth-polytetrafluoroethylene (PTFE) laminate has been used in turbopumps developed in Japan. In the operation of the turbopump, the bearing heat generation is possible to cause sudden temperature rises of bearing elements which finally result in bearing seizure. Therefore, it is important to predict the accurate bearing heat generation under various operating conditions. In this research, the bearing heat generation operated in cryogenic hydrogen was experimentally investigated under the various operating conditions where the rotational speed and the bearing coolant condition were changed. In addition, the bearing heat generation was compared with that theoretically predicted on a numerical model of mechanical losses. It was finally clarified that the bearing heat generation is influenced dominantly by the friction loss on balls and the drag loss on an inner race.