Temperature-controlled molecular dynamics study on Velocity-Dependent Threshold Behavior of Nano-Friction - Search for Guiding Principles to Design Artificial Tribo-Materials -

In order to develop methods of designing artificial materials of desired tribological characteristics for nano-electromechanical systems (NEMS), a better atomistic understanding of wearless-frictional characteristics of mesoscopic solid materials is indispensable. We have searched for laws of wearless friction accompanying steady sliding motion between mesoscopic crystal lattices, by molecular dynamics (MD) simulations using simplified twoand three-dimensional models [1]. In these studies, several universal features of nano-friction were found such as a simple relationship between the threshold value of the sliding velocity and the lattice-constant ratio. The purpose of the present investigation is to carry out the systematic analyses of the nano-frictional characteristics under steady spatial distribution of the local quasi-temperature [1,2]. We applied a Nosé-Poincaré thermostat to the MD model of nano-friction, by extending the isothermal MD method using the thermostat to models with non-conservative extended systems [3]. By analyzing the dependence of the wearless frictional force on the applied load and the sliding velocity, we elucidated the above-mentioned universal features of nano-friction revealed in our previous studies. Furthermore, the spatial distribution of the local pseudo-heat flux was defined at the atomic scale and was associated with that of the local quasi-temperature. The results suggested that, unlike the wearless-frictional characteristics of a macroscopic solid system, those of a mesoscopic system reflect phonon modes of the lattices sliding relative to each other. Based on these results, we have been developing "Phonon-Band Engineering" approaches [4] to design artificial tribo-materials for NEMS by utilizing the up-to-date nano-fabrication technology. Moreover, we revealed unique nano-frictional characteristics common to lateral strained-layer superlattices, with the help of visualization of the spatial distribution of the local quasi-temperature. Further insights into the spatial selection rule of phonon-mode excitation underlying these unique frictional features were obtained by theoretical analyses. These results support the phonon mechanism proposed to explain the velocity-dependent threshold phenomena in our previous investigation [1]. This work is financially supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan.