Innovative Computer Modeling of Materials (ICMM)

For several decades atomistic computer simulations have made significant contributions to materials research by providing direct access to atomic-scale mechanisms which cannot be observed experimentally. Furthermore, atomistic simulations have been indispensable tools to develop and test predictive models based on the fundamental understanding of atomic-level processes that underlie materials processes and properties. However, even with the striking advances in computer technology, there still exists a disparity of many orders of magnitude in length and time scales between the behaviors of atoms comprising the materials system and processes of technological interest. In order to overcome the scale disparity problems in length and time, two classes of multiscale methods have been developed. Spatial multiscale methods focus on extending the size of the simulated system. One leading spatial multiscale method is quasicontinuum (QC), developed by Tadmor, where material bodies that have atomic-scale defects or deformation are treated fully atomistically while the other bodies are treated by the continuum approximation within a finite element method (FEM) framework. On the other hand temporal multiscale methods seek to extend the time scales accessible in conventional molecular dynamics (MD) simulations, which are limited to sub-microseconds. Increasing this short time scale is of vital importance because the mechanisms observed in MD simulations performed at rates several orders of magnitude faster than actual experiments can be completely different, leading to erroneous comparisons. Several methods to extend the MD time scale have also been proposed, including hyperdynamics, the parallel replica method, and temperature-accelerated dynamics. Recently, co-working with Tadmor and Voter, the originator of all three acceleration methods mentioned above, I developed a multiscale method which can extend both length scales and time scales, called hyper-QC? Hyper-QC simultaneously spans multiple length scales and extends the accessible time scale in atomistic simulations. It combines quasicontinuum (QC) and hyperdynamics, two leading spatial and temporal multiscale methods within a single framework. QC extends the system size by retaining atomic resolution only in regions of interest and adopting a continuum finite element method (FEM) approximation elsewhere, while hyperdynamics accelerates time in atomistic simulations using ideas from statistical mechanics.

Despite a long history of investigation dating back to the time of Leonardo da Vinci, friction remains poorly understood. In particular the fundamental understanding of friction based on atomistic processes is a significant challenge. In recent years, friction on a micro/nano scale has been a growing concern as nanotechnology, particularly nano/micro-electromechanical systems (NEMS/MEMS), advances. In MEMS development, preventing wear and adhesion is one of the most critical issues. At present it is possible to measure the friction force acting on a single-asperity contact at the nanometer scale due to the invention of AFM. One interesting and promising phenomenon which has been observed in recent AFM experiments is superlubricity, dry sliding with ultra-low kinetic friction. Moreover, it has been shown that many systems investigated by AFM exhibit the dependence of friction force on temperature and sliding velocity, which is negligible in macroscopic systems in general. Friction and wear of biomaterials are also investigated by AFM. The key challenge lying in the friction simulation is to reduce the sliding velocities. Although in recent years attempts have been made to reduce the time scales using accelerated MD schemes, all these fully atomistic MD simulations have modeled only the apex of the AFM tip and the near-surface region. The hyper-QC method will make it possible to simulate more realistic systems at the sliding velocities close to the experimental values. Specifically I plan to model AFM experiments to investigate (1) velocity and temperature-dependent friction, and (2) nanoscale wear. First, investigating the physical origin of the dependence of atomic-scale friction force on sliding velocity and temperature, which has been the subject of many recent AFM experiments, will provide the opportunity to broaden our fundamental understanding of friction. Moreover, nanoscale wear is of technological importance because MEMS devices with sliding interfaces have short lifetimes due to excessive wear. Many AFM experiments have revealed that Archard's wear law, which states that the wear volume is proportional to the normal force and the sliding distance, but independent of the sliding velocity, does not apply to wear at the nanometer scale. Instead, wear models in this regime can be based on thermally-activated processes within the framework of transition state theory. Thus, the hyper-QC method, which is designed to accelerate thermally-activated processes, is ideally suited for the study of nanoscale wear. The short-term goals of this study are to establish the explicit relationship between wear volume and controlling parameters such as normal force, temperature, sliding distance, and sliding velocity and to reveal the underlying physical mechanisms. The more challenging goal is to discover mechanisms to reduce the nanoscale wear so as to substantially increase the lifetime of MEMS devices.

Graphene is a promising 2D material for its excellent physical, thermal, mechanical and electrical properties. Moreover, because of its low resistance to sliding friction originating from weak intra-planar bonding, graphene can be used as an ideal lubricant for both macroscopic systems and small-length scale devices such as NEMS/MEMS. Recently, interests in other 2D materials such as h-BN (hexagonal boron nitride), MoS2 are also rapidly increasing. In this project we propose to study the properties of a novel material system consisting of graphene and h-BN using accelerated molecular dynamics methodology, in particular hyperdynamics, which can extend the time scales of conventional MD methods by several orders of magnitude. In particular, we will pursue the following specific aims: (1) Investigation of the dependence of friction/wear on temperature, sliding velocity and loading Force; (2) Investigation of the dependence on commensurate/incommensurate interfaces between graphene and h-BN; (3) Investigation of the effects of crystalline defects such as vacancy, dislocation, multi-grain, etc.; (4) Investigation of particular stacking, (i) simple hexagonal (A-A), (ii) Bernal stacking (A-B), and (iii) rhombohedral stacking (A-B-C).

As the size of mechanical systems of technological interest decreases, e.g. MEMS, a detailed knowledge and thorough understanding of underlying atomic scale deformation/failure mechanisms of materials on small length scales have become indispensable to the design of such systems. However, our understanding of the mechanisms of defect formation in crystalline materials such as dislocation nucleation still remains unclear, in particular on small length scales. Nanoindentation is the small-length scale counterpart to the conventional indentation test on macroscopic length scales used to measure hardness. Although the plastic deformation observed in macroscopic indentation tests is well-understood, the deformation mechanisms in nanoindentation tests are an ongoing area of research. Nanoindentation has been widely investigated using fully-atomistic schemes such as MD. However, due to the short time scale problem, most MD simulations are performed at indentation rates that are many orders of magnitude faster than those used in actual experiments. Hyper-QC enables the simulation at the indentation rates up to three orders of magnitude lower than that of the conventional MD scheme.