Tribology at the Nanoscale
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The moving contact of two solid surfaces results in friction and wear. Annually, friction and wear are estimated to cost the United States 6% of the gross national product, or over half a billion dollars per year. Friction is a familiar concept that many people assume is well understood. Students often learn in high school or introductory college physics that friction can be described by a simple expression − Ff = µFN, where Ff is the magnitude of frictional force, μ is the (constant) coefficient of friction, and FN is the magnitude of the normal force between two surfaces—and that there is a different coefficient of friction for initial motion (i.e., beginning at rest) and for two surfaces already moving (sliding friction), with tabulations of the coefficients of friction available for various pairs of surfaces. However, these relatively simple concepts—which often fail in practice, because the phenomenological coefficient of friction turns out to be a function of the details of the preparation of the two surfaces in contact, the operating conditions, the sliding velocity, and so on—hide the fact that friction is a very complicated phenomenon, little understood fundamentally, and dependent on processes taking place at the nanoscale.
Lubrication—defined by Webster's dictionary as the “application of a substance capable of reducing friction, heat, and wear when introduced as a film between solid surfaces”—is necessary to reduce damage to the moving surfaces and to enable reliable operation. Fundamentally, the phenomena of friction, wear, and lubrication involve molecular mechanisms occurring on a nanometer scale, and hence a good understanding of lubricant behavior on this scale is critical to developing new technologies for reduction of loss due to friction. Practical examples are many and range from applications at the leading edge of lubrication technology [microelectromechanical systems (MEMS) and next generation magnetic disk drives] to the superficially more mundane area of automotive lubrication (where the distances between asperities in moving metal surfaces can be in the range of nanometers or less).
In recent years, the goal of understanding lubrication at the nanoscale, and the related concepts of friction and wear, has resulted in the emergence of the field of nanotribology as both an experimental and a theoretical discipline. Experimentally, the development of versions of the surface force apparatus that permitted the controlled oscillatory sliding motion of two surfaces (typically mica) at measurable gaps of the order of nanometers opened up the possibility of studying friction and lubrication at the nanoscale. Similarly, around the same time, the ever-increasing availability of computing power made it possible to perform molecular-level simulations of the tribology of nanoconfined systems. For more details, several recent reviews of various aspects are available. In this article, we will focus on a relatively small set of nanotribological experiments and related theoretical calculations. However, before doing so, we briefly comment on some applications of nanotribology to provide some perspective on the range of situations encountered.