Tribology

Summary

All physical materials interact where their surfaces interface. The interaction is characterized by friction, abrasion, and heat generation. These effects have harmful effects in every instance. Tribology, the study of those effects, works to eliminate negative effects and to find positive ways to harness them. Tribological effects, primarily friction, play a role in every mechanical aspect of existence. They are of particular significance in the ultrasmall devices of nanotechnology and in the biomechanics of living systems. The development of scanning probe microscopy has made it possible to understand tribology at the atomic scale.

Definition and Basic Principles

The word “tribology” means “study of rubbing.” In every practical sense, it applies to the study of the interactions of physical matter at an interface (that is, where one surface contacts another). These interactions are characterized by friction, abrasion, and the generation of heat, all of which affect the subsequent behavior of the surfaces and the dimensional characteristics of the material.

Friction can be described as the resistance to relative lateral motion between two surfaces, while abrasion describes the deformation and forcible removal of material from one surface by material of another surface. Both of these effects facilitate the release of energy, altering the physical surface structure at the atomic and molecular levels of the materials. The energy released by the alteration of surface structure by friction and abrasion becomes perceptible as heat conducted through the mass of the material.

Tribology examines and quantifies the relationships of friction, abrasion, and heat as they relate to the physical performance of mechanical devices. An especially significant field of research in tribology is the study of the qualities of lubrication, as lubricating materials are used to counteract tribological effects. At the same time, the lubricating materials themselves become active contributors to tribological effects, and their study seeks to identify and quantify their corresponding effects.

Background and History

Friction is one of the oldest known technological effects. Archaeologists have unearthed implements dating from the Paleolithic (early Stone Age) fitted with pieces of antler or bone to act as antifriction bearings. Chariots found in ancient Egyptian tombs dating from about five thousand years ago contained the residue of animal fats in the axle-bearing surfaces of their wheels, indicating that the Egyptians of the time understood the value of lubrication. Tomb paintings also indicate using lubrication was an essential component in the movement of large stone blocks used in construction. The physical concept of the coefficient of friction was deduced by Renaissance artist and thinker Leonardo da Vinci in the fifteenth century. Still, it remained generally unknown because his notebooks were only published centuries later.

The rules of friction were rediscovered in 1699 by French physicist and inventor Guillaume Amontons and were later verified by French physicist Charles-Augustin de Coulomb. These rules acquired great significance with the mechanization developed during the Industrial Revolution of the eighteenth century. In modern times, the precision with which mechanical devices are built demands that the effects of friction, abrasion, and heat be fully understood from the atomic scale upward so that their detrimental effects can be minimized or eliminated.

How It Works

Friction. The classical view of tribology is focused on studying the causes and effects of friction. The simplest explanation comes from the view that no matter how smooth a surface may appear to be, as the scale of resolution becomes ever smaller, even the smoothest of surfaces becomes more and more irregular.

This process is exemplified by examining a billiard ball, a hard and extremely smooth spherical object. If one could expand the scale of the billiard ball to the size of the planet, maintaining the surface irregularities to scale, then the surface of the billiard ball would be covered with bumps and ridges higher than Mount Everest and depressions deeper than the Mariana Trench. In the simplest of terms, friction results from the binding of the irregularities of one surface with those of another.

However, the processes of friction are much more complicated than this simple example. Since the development of the current atomic theory and quantum mechanics, it is now known that many other effects play a role in the causes of friction. Researchers of tribological phenomena are only now beginning to understand the details of the process at the atomic level, where tribology begins. This new understanding has been made possible by the development of scanning probe microscopy and, particularly, of the atomic force microscope.

Scanning probe microscopes allow examination of surfaces at the atomic scale, with resolutions as fine as 10 picometers (10⁻¹¹ meters). Even the most cursory examination of a surface image from a scanning probe microscope reveals that at the atomic level, an assumed perfectly smooth surface consists instead of a series of bumps and depressions reminiscent of what one would observe in a layer of golf balls, marbles, or any other spherical object. Additionally, magnetism, electronic interactions, quantum effects, such as van der Waals forces, and the chemical nature of the material are important components of friction at the atomic level. The accumulation of effects from the atomic level to the normal size of the object determines how friction is generated between material objects.

The basic principles of friction are deduced from empirical observation. First, the frictional force that resists the sliding of one surface against another is directly proportional to the normal load between them. In other words, the more pressure that is mutually exerted against the two surfaces, the harder it is to slide them across each other. Second, the amount of frictional force does not depend on the size of the area of contact between the two surfaces. This can be examined simply and cursorily by sliding an irregularly shaped object across a tabletop, using different surface areas each time. Third, once the sliding motion has begun, the frictional force is independent of the velocity. Sliding two surfaces against each other at a high velocity requires the same force as it does at a low velocity.

Abrasion. Abrasion is friction to the extreme, resulting in the deformation and displacement of material from one or both interacting surfaces. Abrasion is not the result of matter passing across the surface of other matter. Rather, abrasion results from matter physically passing through the same space occupied by other matter. The harder or tougher of the two materials will correspondingly force the other into a new relative position to the point of separating from the main mass.

Material abraded from one surface can also transfer to the other surface in a chemical and physical sense. Research on friction between Teflon and aluminum surfaces, for example, has revealed the formation of a certain amount of aluminum trifluoride on the aluminum surface, a condition made possible only by a chemical reaction between the aluminum metal and heat-induced breakdown products of the perfluorinated chemical structure of the Teflon surface.

Heat. Heat is the third major component of tribological effects, easily examined by rubbing one's hands together briskly, first dry and then wet. The heat produced through friction can be intense, leading to dimensional changes that aggravate both friction and abrasion and perhaps lead to the failure of the mechanism.

Applications and Products

Tribological effects can have both positive and negative effects, both of which are crucial to the functioning of modern machinery. The applied science of tribology is a multi-aspect study that seeks first to identify the genesis of tribological effects and then to identify how negative effects can be reduced or eliminated and positive effects used or enhanced.

Friction and wear (abrasion) occur simultaneously in all physical systems. Environmental erosion and skeletal joints obey the same principles of tribology as do steel bearings and internal combustion engines. Examples of positive applications of friction include braking and clutching systems; the drive wheels of trains, cars, and other vehicles; and bolts, nuts, and other devices whose proper functioning depends on the application of friction. Positive wear or abrasion includes such diverse applications as pencils, pens, and other writing or drawing materials; various machining and polishing techniques; and even a morning shave. Negative friction includes the resulting dimensional changes and physical damage that occur with internal combustion engines, gears, cams, bearings, and seals, and even such minor inconveniences as getting stuck halfway down a playground slide.

Lubrication and Lubricants. The essential principle of lubrication is simply to add a third material to a system to lower the coefficient of friction between them as much as possible. In the worst-case scenario, lubrication can have disastrous results, such as when water-soaked soil slides under the force of gravity to create an avalanche or mudslide. Controlled lubrication, on the other hand, is essential to the long-term functioning of machines and other mobile structures, including biological and biomechanical systems.

Lubrication is a complex system in its own right because a lubricating material interacts somewhat differently with the other materials in the system. For example, in a system in which oil is used between steel and aluminum components, the oil molecules will have a different level of adhesion and adsorption to the aluminum surface than they will to the steel surface, resulting in a dynamic movement of material within the oil that may affect how the system functions over time.

Lubricating materials come in various forms and viscosities, ranging from plain dry air to microgranulated solid particles, such as graphite powder. Typically, the selection of a lubricant depends on the amount of pressure it must bear in application. Teflon is unique in this regard because it is a material that becomes more slippery as the pressure it bears increases; typical lubricants tend to lose their lubricating properties as the pressure placed on them increases. There are as many possible lubricating materials as there are materials and material combinations, presenting an impossible challenge to tribological research. The vast majority of common lubricants fall into a few general classes: liquids and semiliquids (such as oils and greases) and solid lubricants (such as graphite). Within these classes, there are hundreds of variations.

A special class of liquid lubricants function as abrasive carriers as they lubricate. Such lubricants are used almost exclusively in deep boring operations, such as petroleum and natural gas recovery. The extreme pressures encountered during deep well boring in rock formations demand water as the lubricant while simultaneously transporting abrasive and abraded material at the drill head to assist the boring process. Various cutting fluids and honing oils used in machine-shop operations for fine grinding and polishing procedures serve a similar function.

Tribological Research and Control Devices. At the lowest end of this technology are grease guns and oil cans for the crude application of lubricants. At the highest end are scanning probe microscopes, enabling researchers to examine the causes and processes of friction at the atomic level. Between the two ends are numerous specially designed devices that test and measure the properties and capabilities of lubricating materials and machine components under operating conditions likely to be encountered in the working environments of those devices. In most cases, such as for a synthetic oil blend in standard roller bearings, this is an almost trivial exercise. In other cases, the working environment is extreme, ranging from the deep ocean floor to deep space, demanding that the materials and designs function flawlessly the first time and for the device's lifetime.

Social Context and Future Prospects

In society, tribology plays a sizeable economic role, both positively and negatively. Tribological effects are integral parts of the physical world. A world in which friction did not function would be a grand failure at a basic level, given that friction and frictional wear have, for example, enabled humans to walk upright and write meaningful information on materials, whose production, in turn, was possible only because of frictional processes. Modern devices with moving parts call for more ways to defeat the adverse effects of friction, abrasion, and heat.

In the twenty-first century, innovations in tribology continued. Artificial intelligence and machine learning have allowed for the creation of predictive models that indicate possible issues and increase efficiency in design, preventing future problems. New materials, including aluminum metal matrix composites and spray-on gradient coatings, have increased the lifespan of many product components.

Bibliography

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