Tribology and Nanotribology

(a sub-field of tribology involving contact geometries)

Nanotribology and who coined the term

On Thursday, January 21, 1999, the following information came from Dr. Jacqueline Krim, Professor of Physics at North Carolina State University, Raleigh, North Carolina:

"Thank you for your inquiry. Yes, I coined the term nanotribology in a paper I wrote in 1991, entitled, 'Nanotribology of a Kr [krypton] monolayer: A Quartz Crystal Microbalance Study of Atomic-Scale Friction', J. Krim, D. Solina and R. Chiarello, PRL, 66, (1991) p. 181-184."

"I would define nanotribology as the sub-field of tribology involving contact geometries which are well-characterized at atomic length or time scales. These tend to be on the order of nanometers and nanoseconds."

—J. Krim

As the need to conserve both energy and raw materials becomes increasingly urgent, our rush to understand basic frictional processes can only be expected to accelerate.

—Jacqueline Krim, Professor of Physics

Other definitions of nanotribology

Micro/nanotribology as a field is concerned with experimental and theoretical investigations of processes ranging from atomic and molecular scales to the microscale, occurring during adhesion, friction, wear, and thin-film lubrication at sliding surfaces.

This involves determination of the chemical, physical and mechanical properties of the surfaces undergoing relative motion at length scales of the order of nanometers. Interaction between rubbing surfaces occurs at asperities [roughness of surfaces] at which the local pressure and temperatures can be very high.

These conditions can lead to formation of tribochemical films with the unusual properties necessary for efficient wear protection. The nanomechanical properties of these films are being investigated by interfacial force microscopy (IFM) which is capable of determining the elastic constants and anelastic behavior of the films in boundary layer lubrication.

Tribology is the study of friction, lubrication and wear. Nanotribology is roughly defined as the study of these same phenomena down to the nN and nanometer force and length scales.

Many industrial processes require a detailed understanding of tribology at the nanometer scale. The development of lubricants in the automobile industry depends on the adhesion of nanometer layers (mono layers) to a material surface. Assembly of components can depend critically on the adhesion of materials at the nanometer length scale.

There are a number of traditional tools for characterizing friction, lubrication and wear. One of the most common characterization tool is the tribometer having several configurations; such as, pin-on-disk, ball on flat, and flat on flat, etc. New characterization techniques are required to understand tribology at the nanometer scale.

The atomic force microscope is now being routinely applied for studying nanoscale tribology. The natural extension of this microscope for tribology applications is derived from the motion of a nanometer-sized stylus in the atomic force microscope (AFM) over a surface. Although traditional tribology testing is not done with an AFM, many new types of applications are possible.

Examples of the application of an AFM to tribology include:

  • Direct three-dimensional visualization of wear tracks, or scars on a surface.
  • Measurement of the thickness of solid and liquid lubricants having nanometer or even monolayer thickness.
  • Measurement of frictional forces at the nanometer scale.
  • Surface characterization of morphology, texture, and roughness.
  • Evaluation of mechanical properties such as hardness and elasticity, and plastic deformation at the nanometer scale.

Modern tribology another Leonardo da Vinci revelation?

Modern tribology began some 500 years ago, when Leonardo da Vinci deduced the laws governing the motion of a rectangular block sliding over a planar surface. Hundreds of years later, in 1699, the French physicist Guillaume Amontons published the first formal account of the classical, macroscopic friction laws.

He found that the frictional force that resists the sliding motion between two interfaces is directly proportional to the perpendicular force that squeezes the surfaces together. In addition, the frictional force is independent of the apparent area of contact. A brick standing on its end, for example, experiences the same friction as when it is laid flat.

Charles Augustin de Coulomb later proposed a third law of macroscopic friction, which states that at ordinary sliding speeds the frictional force is independent of velocity.

The functioning of friction

These classical laws of friction hold for a remarkably wide range of materials, but they are equally remarkable in terms of how difficult it is to derive them from fundamental atomic or molecular principles.

It is reminiscent of the situation in thermodynamics before statistical mechanics came to the rescue. The roughness of a surface was ruled out as a possible mechanism for most types of friction by the 1970s, and was replaced by the notion that the atoms in two materials may bond together and resist sliding as the materials are pressed into contact.

Unfortunately, this "adhesive bonding" view of friction, which was promoted by Philip Bowden and David Tabor of Cambridge University in the 1960s, does not make any predictions about the magnitude of the frictional force or the mechanism of energy dissipation that gives rise to it.

Today, there are perhaps a hundred physicists and other scientists worldwide who working in the field of nanotribology. This situation has come about largely because of the availability of new experimental and theoretical techniques in the 1970s and 1980s, which gave rise to a renaissance in experiments exploring the microscopic origins of friction.

Devices such as the quartz crystal microbalance and the lateral force microscope, for example, can measure the friction due to a single contacting interface. This procedure is vastly simpler to study than that of macroscopic objects, where friction reflects the collective behavior of a multitude of contacts.

In 2004 Ernst Meyer and co-workers at the University of Basel used a lateral force microscope to observe the transition between stick-slip and continuous sliding for the first time.

This instrument, which is common in nanotribology research, is a modification of an atomic force microscope (AFM) and consists of a sharp tip mounted on a flexible cantilever. As the tip is dragged over the surface of a sample, the cantilever is deflected by an amount that depends on the friction between the tip and the substrate.

The future for greater control of friction

The potential for nanotechnology to transform civilization as we know it is breathtaking, and the nanomechanical systems of the future will all require new atomic lubrication schemes to overcome the debilitating effects of friction. In order for this impending revolution to be fully realized, we need a fundamental understanding of friction at the atomic to meso-scale.

In addition, the tribological considerations of these systems will be an integral aspect of the system design, rather than the "after the fact" application of lubricants that is so common at the macro-scale.

Finally, the dreams of a nanotechnological revolution will only be realized by training both existing and future scientists and engineers in nano-scale phenomena; an area that is largely absent, for instance, in the present curriculum of universities.

—The last three sections were compiled from
"Friction at the nano-scale", February, 2005, by Jacqueline Krim,
Department of Physics, North Carolina State University,
Raleigh, North Carolina, as seen in "Physics Web.Org"
and in an article, "A Little Friction" by Jim Wilson
in Popular Mechanics, August 12, 2003.

Pointing to a page about tribology The list of tribology words.

Pointing to a page about tribology The presentation about tribology and its applications.

Additional topics are available at Nanotechnology: Index of Articles.