Friction, lubrication, wear, and adhesion are all phenomena familiar from everyday experience. We experience friction when we go for a walk on ordinary ground—we do not expect to fall flat on our faces, as we might when walking on ice. Wear is particularly well known because it leads to catastrophic failure and represents one of the most costly problems facing industry today. These phenomena are part of the field of tribology—the science of interacting surfaces in relative motion. A macroscopic understanding of tribology is crucial for the design and engineering of mechanical components. As dimensional tolerances of these components approach the nanometer scale and novel new materials are used in their fabrication, macroscopic understanding must give way to a microscopic atomic-scale understanding. This fundamental microscopic understanding and its practical application remain limited.

With the advent of surface proximity probes, however, those limitations are rapidly disappearing. Researchers are now able to test and develop a microscopic understanding of such tribological phenomena as boundary-layer lubrication and asperity-surface interactions—ideas that underlie our current macroscopic understanding. This emerging field is known as nanotribology or molecular tribology, the focus of this issue of the MRS Bulletin. The new experimental probes include the surface force apparatus for measuring interfacial forces as a function of surface separation and, more recently, rheological properties of molecularly thin layers; a quartz-crystal microbalance for measuring the atomic-scale friction of adsorbed monolayer films; and the atomic force microscope (AFM) for measuring nanomechanical properties of surfaces, atomicscale friction of clean surfaces, and frictional properties of surfaces coated with molecular films.

Advances in our understanding of the phenomena of adhesion, friction, and lubrication are facilitated by the recent development of new tools that allow the study of contacts in close-to-ideal conditions. These new tools are the surface force spparatus (SFA) and the atomic force microscope (AFM). The first was developed by Israelachvili in the 1970s. In this device, contact between two atomically flat surfaces of mica occurs over an area of several micrometers in diameter after the mica sheets, glued onto two perpendicular cylindrical lenses, are compressed. Force, area of contact, and separation distance can be controlled at the atomic scale. The second device, the AFM, was developed by Binnig et al. in 1986. The sharp tip of the AFM is a convenient idealization of a single asperity. In addition, the AFM can be used to image the surface in the weak repulsive or in the attractive modes so that minimum perturbation is introduced by the imaging process itself. These two devices have the necessary sensitivity to allow the application of forces weak enough not to dislodge atoms from their sites during contact. The order of magnitude of the force that can lead to the rupture of chemical bonds is a convenient figure to keep in mind in this context. A simple estimate of this force is obtained by considering a bond-length increase of 1 Å as leading to dissociation. For a bond energy of ≈1 eV, Fb ≈ 1 eV/1 Å ≈ 1 × 10−9 N.

Many attempts have been made in recent centuries to investigate friction, adhesion, lubrication, and wear. Most of the experimental approaches and theories were based on macroscopic experiments, such as tensile and indentation tests. For a long time, only the bulk properties of the materials were considered.

Late in this century a new term was created combining all of the above-mentioned properties which deal with the science of interacting material interfaces in relative motion: tribology. The state of the art of science today reveals that processing in nature depends strongly on interfaces that cannot be described only by bulk properties. Tribologists realize they must study the sliding surfaces by analytical surface-science tools. With the surface force apparatus developed by J.N. Israelachvili and D. Tabor, we have a surface analysis tool that provides new insight into the field of macroscopic sliding contact of lubricated systems.

After Amontons' laws were established as a first attempt to describe sliding friction analytically, theories were advanced over the course of this century. A classic discipline was developed: contact mechanics. More quantitative treatments of friction were developed by various authors. The energy dissipation in most processes in tribology induced the theorists to consider the sliding bodies as spring models creating phonon-phonon interactions. And with modern computer facilities, they started to perform computational experiments whenever classical experiments could not provide information on the submicron scale.

Understanding the atomistic mechanisms, energetics, structure, and dynamics underlying the interactions and physical processes that occur when two materials are brought together, separated, or rub against each other (hence the term tribology, from the Greek tribos, meaning to rub) is fundamentally important to many basic and applied problems. Examples include adhesion, capillarity, contact formation, surface deformation, elastic and plastic response characteristics, hardness, micro- and nanoindentation, friction, lubrication, wear, fracture, atomic-scale probing, and modifications and manipulations of materials surfaces. These considerations have for over a century motivated extensive theoretical and experimental research into the above phenomena and their technological consequences.

Explorations of materials systems and phenomena in the nanoscale regime often require experimental probes and theoretical and computational methods that allow investigations with refined spatial, as well as temporal, resolution. Consequently, until recently most theoretical approaches to the above issues, with a few exceptions, have been anchored in continuum elasticity and contact mechanics. Experimental observations and measurements of surface forces and the consequent materials response to such interactions have been macroscopic in nature.

Hydrodynamics and elastohydrodynamics have been successful in describing lubrication by micron-thick films. However, these continuum theories begin to break down as film thicknesses become comparable to molecular dimensions. An increasing number of applications require an understanding of lubricants in such severely confined geometries. Examples include lubrication of nanoscale bearings in micromachinery or high-density magnetic disk drives, as well as asperity interactions in macroscopic bearings that operate in the mixed lubrication regime.

Development of new experimental and theoretical techniques for studying thin lubricant films has paralleled the growing interest in their properties. The surface force apparatus (SFA) allows normal and shear forces to be measured between atomically flat solid surfaces while their separation is determined to within 0.1 nm using interferometry or capacitance. The contact area in the SFA is typically 100 μm across, much larger than the separation between solid walls. The atomic force microscope (AFM) can be used to explore friction in lubricated contacts whose diameter is comparable to the separation (5 nm). This allows spatial resolution of the frictional force on a molecular scale. Quartz-crystal oscillators have been used to determine the frictional forces between a surface and an adsorbed film of one or more monolayers. Theoretical advances have been aided by the advent of supercomputers that allow thin films to be simulated at the molecular level using molecular dynamics. These new experimental and theoretical techniques reveal a sequence of dramatic changes in the static and dynamic properties of fluid films as their thickness approaches molecular scales.

Friction, or the resistance to motion of two bodies in contact, and the related phenomenon of wear are two of the more costly problems facing industry today. Despite their importance, a fundamental understanding of friction and wear, especially at the atomic scale, has remained elusive. This is rapidly changing, however, as new scientific instrumentation has been developed that allows, for the first time, the study of friction at the atomic scale. These pioneering efforts have led to the emergence of a rapidly growing field called nanotribology, the subject of this issue of the MRS Bulletin. Some of the contributing techniques include the surface force apparatus, which has been used to study the rheology of molecularly thin liquid layers, a quartz-crystal microbalance, which has been used to measure the sliding friction of molecularly thin adsorbed films, and the atomic force microscope (AFM), which has been used to measure the frictional force between a sharp tip (possibly a single asperity) and a flat surface during sliding. In addition to providing a vast amount of information related to friction on the atomic scale, these innovative experiments have provided the necessary data to test the validity of older theoretical models and have stimulated new theoretical work. For instance, atomic-scale friction has been investigated theoretically using analytic models, first principles calculations, and molecular dynamics simulations.

The precision machining of metal surfaces and the ductile-regime grinding of ceramic surfaces are examples of fundamental cutting processes used in fabricating high-tolerance parts. Components with dimensional tolerances of a few tens of nanometers are currently being produced by direct machining with single-point diamond tools. Despite the ability to fabricate these parts, little is understood of the basic deformation mechanisms that determine how material is removed and deformed, how a tool-tip interacts with a workpiece, how induced surface and subsurface damage occurs, and how cutting tools wear.

The key to solving these problems is a fundamental understanding of basic tribological processes such as surface indentation and scraping. Indentation experiments measure the mechanical response of a surface, the onset of plastic deformation, and material hardness. Macroscopic hardness measurements have been shown to correlate well with observed tensile yield strengths. Microscopic indentation studies, where the indentation size is smaller than the material grain size, show new and interesting phenomena. In the pioneering work of Gane and Bowden, no permanent penetration occurred until a critical load was achieved. They related this critical yielding to the theoretical shear strength in the metal, the strength required to create dislocations. Yielding of this sort has since been observed by many investigators.