To extend the service life of vehicles and parts constituting vehicles like eAxles, finding new organic oil additives that prevent the wear of steel surfaces involved in mechanical contacts while being environmentally friendly is essential. To achieve this goal, computational approaches based on non-reactive molecular dynamics for predicting the ease with which organic additives adsorb on steel surfaces were considered. It allowed the analysis of how the molecular structure of additives affects adsorption, enabling the search for additive molecules that suppress the wear in an effective way. In this short review, the research that has been conducted so far to improve additive adsorption on steel surfaces was briefly summarized. Then, our contribution to this research field is introduced with adsorption properties of selected amine-based organic additives.

View full abstract

In the automotive industry, reducing carbon dioxide emissions and achieving a circular economy, such as materials recycling, are strongly necessary. Numerous methods have been proposed for modeling the friction interface in automotive applications to improve friction loss and wear resistance. In particular, atomistic simulation methods are expected to promote the understanding of friction mechanisms, chemical processes, and wear. Molecular dynamics (MD) simulations require interatomic potentials as force fields, which are developed for modeling many-body interactions and chemical reaction processes. In this article, we discuss the application of molecular dynamics methods to model automotive sliding surfaces and the strategy of force field development.

View full abstract

Effective viscosity is used to evaluate the performance of lubricants. It is influenced by many factors, such as intrinsic lubricant molecular interactions, lubricant-wall interactions, the thickness of the lubricant film, and shear rate. Non-equilibrium molecular dynamics simulations have been employed to predict the effective viscosity, but the convergence of the effective viscosity in these simulations sometimes requires a long time. Here, we review a machine learning approach to analyze trajectories from non-equilibrium molecular dynamics simulations to achieve feature extraction. We explain how the extracted features can predict the effective viscosity and provide molecular insights into the changes in effective viscosity. Furthermore, we review the correspondence between molecular orientation and changes in effective viscosity.

View full abstract

The friction between solid and liquid is characterized by the two parameters concerning the Navier boundary condition (BC), which states that the shear stress on the solid wall is proportional to the liquid slip velocity relative to the solid. One is the proportionality constant, and the other is the position of the solid-liquid interface on which the Navier BC should be applied. To identify these two parameters by molecular dynamics (MD) simulation, there are two classes of methods: the methods by nonequilibrium MD (NEMD) simulation and by equilibrium MD (EMD) simulation. The NEMD methods are straightforward, applying shear to the liquid and measuring the wall shear stress and the liquid slip velocity, but have some limitation to obtain results being independent of the liquid slip velocity. The EMD methods are free of such limitation, while they are less straightforward and have known pitfalls to be circumvented. We describe both NEMD and EMD methods with some examples and supporting theories in this article.

View full abstract

Recent experimental studies of boundary lubrication of nanostructured steel surfaces revealed a significant reduction in friction coefficients, suggesting a new mechanism that the enhanced adsorption of lubricant molecules onto grain boundaries should contribute to the improvement of tribological properties in steels. Being motivated by such experiments, molecular dynamics simulations based on the coarse-graining method have been attempted aiming to prove the hypothesis and obtain insights to nanoscale dynamics of lubricant molecules on nanograined metal surfaces. Critical normal stress to cause oil film delamination or transition from shear-flow (SF) to stick-slip (SS) behaviors, which can be interpreted as the indicator of resistance against solid-solid contact, was found to increase with increasing strength of lubricant adsorption onto grain boundaries. When the separation of opposing metal surfaces was kept constant, the SF and SS states were found to coexist, associated with alternate transition between the two states. Moreover, nanoscale surface roughness can hinder the oil film delamination, implying the contribution to reduction in friction coefficients. Despite such new implications in line with experimental observations, the findings must of course be understood with the caveat that the simulations were based on ambitious assumptions.

View full abstract

A mesoscale friction calculation model based on a particle method has been developed, fundamentally incorporating constitutive equations for continuum (macroscale perspective) elasto-plastic deformation and interfacial interactions that account for atomistic interactions (nano-micro scale perspective). This model enables detailed simulations of friction, wear, adhesion, and heat generation. The methods and results are explained in detail, and its application to actual material calculations and real-space scale computations using high-performance computing infrastructure (HPCI) is discussed.

View full abstract

Even now, nearly 40 years after joining the Society of Tribologists, I have been able to continue my research into DLC tribology for nearly 30 years. The main reason for this is that I have been blessed with “wonderful people around me,” as many researchers who have created new technology say. I would like to write about how I made the decision to continue my research, based on my relationships with benefactors who gave me valuable words, advice, and opportunities at important times in my research.

View full abstract

[in Japanese]

View full abstract

[in Japanese]

View full abstract