Atomic scale tribology

Most atomic-scale tribological simulations use force fields (FFs) to describe the interactions between atoms. A huge amount of literature exists regarding the development and use of FFs, and we will not attempt to cover this vast topic here. Instead, we will point out aspects of FFs and their use that are relevant to tribological simulations. The reader interested in a more general discussion of FFs is directed to the chapters by Bowen and Allinger58 and Dinur and Flagler59 in volume 2 of this series and by Landis et al. in volume 6.60... 

Tribology is the branch of science and engineering of surfaces in relative motion. Included are issues of friction, wear, and lubrication of surfaces. Modem technology has enabled the study of these characteristics in a number of different ways. These studies have given rise to a new branch atomic-scale tribology. This branch deals with issues and processes from atomic/molecular scale to microscale. These... 

Friction can now be probed at the atomic scale by means of atomic force microscopy (AFM) (see Section VIII-2) and the surface forces apparatus (see Section VI-4) these approaches are leading to new interpretations of friction [1,1 a,lb]. The subject of friction and its related aspects are known as tribology, the study of surfaces in relative motion, from the Greek root tribos meaning mbbing. 

C. M. Mate, Atomic Scale Friction, in Handbook of MicrolNano Tribology, B. Bhushan, ed., CRC Press, 1995. 

Bhushan, Ed., CRC Press, Boca Raton, FL, 1999, pp. 525—597. Atomic-Scale Simulations of Tribological and Related Phenomena. 

For the reader interested in shear and friction studies using the SFA, an excellent overview of the advances in this area up to 1998 is given by Kumacheva [51]. The recent developments of the tribological SFA have allowed for the study of tribology at the molecular and atomic scale [20]. 

Tribology. These papers included M. D. Perry and J. A. Harrison, Molecular Dynamics Studies of the Frictional Properties of Hydrocarbon Materials Langmuir 12 (1996) 4552-4556 E. Manias, G. Hadziiannou and G. ten Brinke, Inhomogeneities in Sheared Ultrathin Lubricating Fluids, ibid 4587-4593 U. Landman, W. D. Luedtke and J. Gao, Atomic-scale Issues in Tribology Interfacial Junctions and Nano-elastohydrodynamics, ibid 4514-4528. 

R.W. Caipick and M. Salmeron, Scratching the Surface Fundamental Investigations of Tribology with Atomic Force Microscopy,in Chem. Rev. 97 (1997) 1163. (Review relevant technical aspects using AFM for nanotiibology, results on bare interfaces and model lubricant films (SAM s and LB films), aimed at atomic-scale understanding of processes such as friction, the onset of wear, nanometer-scale elasticity, plasticity ind adhesion.)... 

We are currently in the midst of a new revolution in tribology, driven by (a) the advent of experimental techniques that allow controlled friction measurements at atomic scales and (b) computers that allow the complex dynamics in atomic scale contacts to be analyzed. This new line of study, dubbed nanotribology, is playing a central role in the quest to build robust machines with nanometer-scale moving parts and is poised in turn to beneht from the resulting advances in nanotechnology. 

The three major new atomic-scale experimental methods developed in the last decade are the quartz crystal microbalance (QCM) [2 4], atomic and friction force microscopes (AFM/FFM) [5,6], and the surface force apparatus (SEA) [7,7a,8]. These new tools reveal complementary information about tribology at the nanometer scale. The QCM measures dissipation as an adsorbed him of submonolayer to several monolayer thickness slides over a substrate. AFM and FFM explore the interactions between a surface and a tip whose radius of curvature is 10 100 nm [9]. The number of atoms in the contact ranges from a few to a few thousand. Larger radii of curvature and contacts have been examined by gluing spheres to an AFM cantilever [10,11]. SEA experiments measure shear forces in even larger-diameter ( 10 pm) contacts, but with angstrom-scale control of the thickness of lubricating hlms. 

G.M. McClelland and S.R. Cohen. Tribology at the Atomic Scale. In R. Vanselow and R. Howe, editors. Chemistry and Physics of Solid Surfaces VIII. Springer Series in Surface Sciences, Volume 22. Springer-Verlag, Berlin, 1990, p. 419. 

After 30 years of continuing investigation, the adsorption properties of the noble ses on metal and semiconductor surfaces have recently attracted renewed interest. On the one hand, some fundamental aspects have come within the reach of modem experimental and theoretical techniques, sueh as the very nature of physisorption and the noble gas - substrate interaction, the possibility to study growth and surface kinetics at the atomic scale, and the recent interest in nanoscale surface friction and related tribological issues, where noble gas adlayers serve as model systems [99P]. On the other hand, noble gas adsorption is being used as a non-destmctive and quantitative surface analytical tool as, for instance, in photoemission of adsorbed xenon (PAX) [97W] and for titration analysis of heterogeneous surfaees based on the site specificity of the interaction strength [96S, 98W]. 

Harrison, J. A. Stuart, S. J. Brenner, D. W. In Handbook of Micro/Nanotribology, 2nd edition, Ed. Bhushan, B. CRC Press Boca Raton, FL, 1999 Chapter 11 "Atomic-Scale Simulation of Tribological and Related Phenomena", pp. 525-594 and references therein. 

In spite of the strong economic importance of friction and wear and the resulting scientific effort, our understanding of the fundamental processes is still rudimentary. This results from the complexity of these topics. In addition, this complexity demands a multi-disciplinary approach to tribology. In recent years the development of new experimental methods such as the surface forces apparatus, the atomic force microscope, and the quartz microbalance made it possible to study friction and lubrication at the molecular scale. However, this new wealth of information does not alter the fact, that there are no fundamental equations to describe wear or calculate friction coefficients. Engineers still have to rely largely on their empirical knowledge and their extensive experience. 

Friction is the tangential resistance offered to the sliding of one solid over another, due to dry friction. Friction is an apparently simple phenomenon with very complex mechanisms that take place on a variety of length scales, from atomic to nano and up. The study of friction is part of the engineering-scientific discipline of tribology,3 which is the scientific study of friction, wear, and lubrication (6). It was Leonardo da Vinci (1452-1519) who discovered the first two laws of friction, namely, that the area of contact has no effect on friction and that friction is proportional to the load. These two laws were rediscovered later by Guillaume Amontons (1663-1705), and later Charles-Augustin Coulomb (1736-1806), added the third law ... 

The results obtained by the EQCM contain information relevant to the understanding of phenomena in the area of nano-tribology, where techniques such as surface force apparatus and atomic force microscopy are used. In both cases the results carry information regarding the properties of a nano-scale layer of liquid at the interface. 

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