Several years ago, when my boss came to my office and said that he wanted me to manage the tribology program at Argonne National Laboratory, I was mystified. My first response was, “What is tribology?” In the years since, the cards have been somewhat reversed. The initial response of person after person when I mention my position as Tribology Section Manager of the Materials and Components Technology Division is to look blank and pose the same question to me. Having somewhat of a playful nature, I often simply try to drop the term very nonchalantly and continue speaking, taking great delight in the puzzled looks my approach engenders. When somone eventually injects the inevitable inquiry, I give a rather stilted answer, such as:

“Tribology is the science and art of interacting surfaces in relative motion with respect to one another.”

Since this succeeds in erasing only a portion of their curiosity, I eventually have pity on them and say, “It's really the rather mundane areas of friction, lubrication, and wear.” I have often pondered and discussed with colleagues whether we did ourselves any favor by coining the word in the first place, but it does serve a useful purpose in at least attempting to obtain a degree of helpful communication between people engaged in the various aspects of this broad area of endeavor.

Molecular tribology—the study of tribology at atomic and molecular scales—constitutes a new frontier of tribology research. In a major surge of activity, experimental methods have recently been developed to measure dynamic interfacial forces in shear. Building partly on earlier, somewhat neglected friction studies, striking new findings have been obtained. The new methods include the surface forces apparatus for measuring adhesion and static interfacial forces as a function of surface separation, new molecular tribometers for measuring friction in shear, atomic force microscopy, use of UHV tribometers, and the quartz-crystal microbalance. Theoretical calculations and molecular dynamics simulations are also emerging for friction in dry and lubricated sliding.

On the scientific side, appreciation is growing that scientific understanding of these systems, so complex and so far from equilibrium, is possible. Tribology is becoming recognized as an area with many exciting and useful surface science opportunities.

The engineering significance is that while tribology design and tribology-based applications are rooted in our economic life, too often the technologies and formulations are empirically derived. One tends to take friction, wear, and tear for granted. A scientific understanding is needed so that better design can emerge by rational extension.

This review seeks to bring out the excitement of new developments. The reader is referred to the original literature for full accounts.

In the context of this article, adhesion, friction, and wear are defined as follows: Adhesion is the resistance against separation of two bodies in a direction normal to the common interface. Friction is the resistance against motion of the contacting bodies in the direction of the common interface. Wear is the gradual change of geometry and loss of material due to the different contact processes that may occur in moving contacts.

Bowden and Tabor have shown convincingly that adhesion, friction and wear are interrelated at least to some extent. This article will review some of the current arguments that support this notion.

If surfaces are not separated by a viscous film, both friction and wear have their origin in the contact phenomena occurring at the roughness asperities of the sliding bodies.

Within one contact cycle, three primary events can be distinguished:

1. In order to obtain a (normal) load-carrying capacity, the contacting asperities must be compressed or indented (Figure 1), similar to a Brinell hardness test.

2. At the beginning of the contact cycle, the asperities interfere in the direction of sliding (Figure 2). Thus during the first half of the contact cycle, frontal loading of the asperities occurs (Figure 2b).

Particularly if the resulting deformation is plastic (as it will usually be for metals, see Reference 1), the net effect of this contributes to friction, the so-called “ploughing” effect.

Friction is often a nuisance, but it can be useful too. Brakes, clutches, and tires rely on it, of course, though the inevitable fractional heat remains a problem. Other applications use frictional heat: friction cutting and welding, skiing, skating, and curling. The damage to magnetic disks caused by head-disk contact and the striking of matches are also examples.

This article illustrates a framework where the thermal aspects of friction can be analyzed in an informative way. It uses a unified approach to the calculation of flash and bulk heating, and a helpful diagram—the frictional temperature map—to display the results. The method is approximate, but the approximations have been carefully chosen and calibrated to give precision adequate to most tasks, and the gain in simplicity is great.

The symbols used in this article are defined in Table I.

When two contacting solids 1 and 2, pressed together by a normal force F, slide at a relative velocity ν and with coefficient of friction ü, heat is generated at the surface where they meet. The heat generated, q, per unit of nominal contact area, An, per second is

The heat flows into the two solids, partitioned between them in a way that depends on their geometry and thermal properties. Figure 1 shows one geometry commonly used for laboratory tests: the pin-on-disk configuration. The pin is identified by the subscript 1, the disk by subscript 2. Solid 1 can have properties which differ from those of solid 2.

In unlubricated sliding contact, essentially all the mechanical work done to overcome friction is converted into heat produced in the vicinity of real contacts. The amount of frictional heat flux q is proportional to the friction coefficient ü, the normal force F, and the sliding velocity ν, but is inversely proportional to the nominal contact area An (e.g., q = (ü × F × ν)/An). The real areas of contact, being much smaller than the nominal contact area, give rise to much higher local heat fluxes in the vicinity of asperity contacts. Because the frictional heat flux enters the contacting bodies through these regions (or locations known as “hot spots”), their local temperatures (referred to as “flash temperature”) can be much higher than the overall or “bulk” surface temperature, as discussed in References 1-3.

Previous studies have demonstrated that frictional heat can profoundly affect the friction and wear behavior of both metallic and ceramic materials. In most steels and nonoxide ceramics, frictional heat was found to foster oxidation. The occurrence of phase transformations on or near the sliding surfaces was also cited in the literature for certain steels and ZrO2-based ceramics.

Except for SiC, BeO, and AlN, most ceramics have significantly lower thermal conductivity than do metals. When in sliding contact, ceramics cannot dissipate frictional heat generated at sliding interfaces as effectively as most metallic alloys. Large temperature gradients can often develop between areas of real contact and surrounding regions, thus creating high thermal stresses.

Lubrication may be defined as any method used to achieve control of friction and wear of interacting surfaces in relative motion under load. Gases, liquids, and solids have been used successfully as lubricants. To prevent surface contact, liquids and gases provide a film under hydrodynamic pressure to support the load.

When the load is high and/or the speed is low, the hydrodynamic or hydrostatic pressure may not be sufficient and the surfaces come into close contact. The amount and the extent of the surface contact depends on many factors: surface roughness, fluid film pressure, normal load, hardness of the materials, etc. When the surfaces come into close contact, many of the asperities undergo elastic deformation. The condition is generally referred to as elasto-hydrodynamic lubrication (EHL). EHL theories are well-developed. They describe and predict the surface temperatures, fluid film thickness, and hydrodynamic pressures. Contact pressure increases beyond the EHL conditions causes asperities to deform plastically and thinning of the fluid film. When the average fluid film thickness falls below the average surface roughness, the interaction between the contacting surfaces becomes the dominant factor in supporting the load.

This condition is referred to as the boundary lubrication (BL) regime. Theories for BL are not well-developed and the detailed processes are not understood. The classical view of boundary lubrication postulates the formation of a surface chemical film which is easily sheared and protects the surface.