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Paper   IPM / Nano-Sciences / 8229
School of Nano Science
  Title:   Computational Modelling of Tribological, Adhesion, Indentation and Fracture Processes in Nano-Scale Systems
  Author(s):  Hashem Rafii Tabar
  Status:   Published
  Journal: Handbook of Theoretical and Computational Nano-Technology
  No.:  March 2006
  Vol.:  Chapter 97
  Year:  2006
  Publisher(s):   American Scientific Publishers
  Supported by:  IPM
1. INTRODUCTION Nanoscience and nanotechnology [1, 2] form the key components of 21st-century science and technology. There is not an accepted definition of nanoscale science, but a definition over which a consensus can be built has been attempted. Accordingly, nanoscience has been defined as the study of structures, dynamics, and properties of systems in which one or more spatial dimensions is nanoscopic (i.e., 1�100 nm), thus resulting in dynamics and properties that are distinctly different (often in extraordinary and unexpected ways that can be favorably exploited) from both small-molecule systems and systems macroscopic in all dimensions [3]. The fundamental entities with which nanoscience is concerned are the physical and biological nanostructures that form the building blocks of inanimate and animate matter. Nanostructures operate on highly reduced time and energy scales. They are formed from a countable (finite) number of atoms and molecules, and their sizes are in the range between individual molecules and microstructures. Their characteristic feature is their high surface-to-volume ratio. Nanotechnology is concerned with constructing nanoscopic devices and components from the assemblies of individual nanostructures. Depending on the type of nanostructure involved, namely, soft or hard, nanoscience and nanotechnology have been categorized into soft (wet) and hard (dry) areas. The bottom-up approach to nanotechnology involves constructing nanostructures from below the nanoscale, atom by atom and molecule by molecule, by a precise positioning of these fundamental units at specified locations. This has led, for the first time in human history, to the possibility of designing and fabricating devices, components, and materials, including biomaterials, that exhibit totally different and novel physical, chemical, and biological properties, as compared to the behavior of single molecules and bulk phase materials. As a consequence, nanoscopic science and technology allow for the purposeful manipulation and structural transformation of condensed phases at their most elementary levels. Such manipulation offers the possibility of exercising a complete control over the properties and over the functioning of physical and biological matter at the atomic and molecular scales, implying that we can interrogate physical and biological matter atom by atom and induce predetermined property changes in them. The top-down approach to nanotechnology, however, provides a practical framework for the realization of the miniaturization program, initiated in the classic paper of Feynman [4]�one of the aims of which is to produce nanostructure-based low-dimensional quantum-scale devices starting from microscopic scales. Whatever strategy is followed to reach the nanoscale, there is no doubt that the resulting structures and processes that unfold at this scale have remarkable and unique properties. For example, nanostructured materials (i.e., materials composed of nanosized grains, or materials containing injected nanostructures, such as carbon nanotubes) have very different mechanical, thermal, electronic, and optical properties compared to their counterparts with microsized grains. For instance, it is known that nanostructured Fe, Cu, and Ni have electrical resistances, respectively, of 55corresponding coarse-grained polycrystalline samples [5]. On the experimental front, the enabling tools available for constructing, characterizing, joining, and manipulating individual nanostructures consist of various types of probe-based microscopes. The scanning tunneling microscope (STM), invented in 1982 [6], followed by the atomic force microscope (AFM), invented in 1986 [7], have provided invaluable tools to study and manipulate the surface morphology and crystal structure and electronic structure maps at material surfaces and of individual nanostructures. Probe-based microscopy techniques have been further extended and are collectively referred to as scanning probe Computational Modeling of Tribological, Adhesion, Indentation, and Fracture Processes 3 microscopy (SPM), making it possible for the emergence of new subfields within nanotechnology, such as nanoelectromechanical systems (NEMS), molecular electronics, nanomagnetism, nanooptics, nanomedicine, nanobiotechnology, and nanogenetics. Experimental nanoscience and nanotechnology have, over the last few years, been joined by the exciting fields of computational nanoscience and computational nanotechnology, based on numerical modeling and computer-based simulation, to simulate the mechanical, thermal, and electronic properties of nanoscale structures, and nanoscale processes that unfold in nanoscopic systems, particularly in metallic and semiconducting nanosystems. The aim of these simulations can be broadly stated as revealing the atomistic and subatomistic origins of the energetics, dynamics, transport, mechanical, thermodynamical, and electronic properties of nanostructures and nanoscale processes, and how to exploit this knowledge in designing and testing real nanoscopic functional devices. Broadly speaking, simulation-based nanoscience employs two approaches. The first approach employs highly advanced many-body quantum mechanical�based concepts and methods, such as the density-functional theory (DFT) of atoms and molecules, for an ab initio investigation of the properties of nanosystems, particularly their electronic transport properties, composed of several tens of to at most several hundred atoms. The second approach is based on the use of highly advanced classical statistical mechanics-based models and methods, such as the molecular dynamics (MD) and Monte Carlo (MC) simulation methods, for modeling the properties of nanoscale structures and processes, particularly their mechanical properties, composed of several thousand to several billion atoms. For these classical mechanics-based simulations, interatomic potentials, from which the forces experienced by individual atoms are derived, play an all-important and very crucial role. The more accurate these potentials are, the closer are the simulation results to the data obtained experimentally, and the closer these simulation results reflect the actual properties of the nanoscale systems and processes. A great deal of effort and time has, therefore, been spent over the years to develop and test highly accurate interatomic potentials for the description of different classes of materials. Some of the state-of-the-art potentials for the description of the metallic and covalently bonding systems are many-body potentials, with which very accurate materials properties have been obtained. This review is concerned with the emerging fields of numerical modeling and computerbased simulations applied to the highly complex, mechanically induced processes that can appear in real nanoscopic devices, components, and systems. We will specifically focus on the simulations dealing with the phenomena observed in nanocontacts between surfaces, such as nanoindentation, nanoscale wear, and nanoscale friction. We will also consider the simulations concerned with the all-important field of crack propagation in nanoscopic and macroscopic structures. The organization of this review is as follows. In Section 2 we consider the principal computational modeling tools that are available for both computational and experimental nanoscientists and nanotechnologists for numerically simulating the complex many-body systems and processes involving a rather large number of atoms and molecules. These tools have already been employed, with a good deal of success, in modeling the nanomechanics of the abovementioned processes. There are other tools, such as those constructed within the quantummechanical framework, that can be effectively employed in very elegant and sophisticated modeling studies, such as in a multi-scale approach, involving some aspects of these processes. A combination of these tools has the potential to apply to practically any modeling of nanoscopic structures and processes involving a significant number of atoms. In this review, we consider the classical- mechanical-based tools only, as these have been primarily employed in the modeling studies that we shall review. They include the powerful classical MD simulation method and its realization in, for instance, a canonical ensemble, and the methods of MC simulation. The quantum-mechanical based tools that include several ab initio DFT-based MD methods are not considered here, and we refer the interested reader to an easy-to-follow introduction in Ref. [8]. In Section 3, pertinent state-of-the-art many-body interatomic potentials that have been employed in modeling studies covered in this review are presented and discussed. I have attempted to provide as comprehensive a presentation as possible for ease of reference for future use, particularly for those researchers coming from the experimental side 4 Computational Modeling of Tribological, Adhesion, Indentation, and Fracture Processes of the subject. From Section 4 onward, I have considered the modeling simulations concerned with the abovementioned processes. I have tried to be as up to date as possible with the research materials covered. Obviously, the selection has been personal and in no way makes a claim to be either exhaustive or to be a value judgment on the quality of those works that have somehow not been included in this study. I have also included some key experimental investigations where I thought they might help clarify a modeling issue.

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