The word tribology is derived from the Greek word, "tribos," which means "rubbing." Tribology covers the science of friction, lubrication and wear. Virtually any machine ever invented consists of components that must slide or roll relative to other components. Resistance to sliding and rolling caused by friction results in significant loss of energy. In fact, it has been estimated that as much as 33 percent of energy produced worldwide is used to overcome friction.
Friction and wear have many undesirable effects on individuals and society that are not widely known. Transportation vehicles have hundreds of moving parts that are subject to friction. Auto-mobiles generally wear out after about 100,000 miles of operation. Largely because of innovations to reduce friction and improve lubricants, the average lifetime (14 years in 2000) and miles of operation continue to increase. Since it requires as much energy to produce an automobile as it does to operate it for 100,000 miles, extending the life of an automobile would save considerable energy. The same applies to the steam and natural gas turbines that supply electricity and the home appliances that consume that electricity.
Friction and wear is even more important for national security. Downtime of military hardware as parts wear out and lower output power of military engines due to high friction can contribute to decreased effectiveness of the military and increases the costs of keeping highly specialized equipment in operation.
Wear of medical devices and biomaterials can affect quality of life. Wear of tooth fillings, artificial joints and heart valves can be inconvenient, costly (more frequent replacement) or even life-threatening (premature breakdowns). Wear of components can also cause accidents. Worn brakes and tires can cause automobile accidents, worn electrical cords can result in electrocution and fires and worn out seals can lead to radiation leaks at nuclear power plants.
In the last few decades of the twentieth century the field of tribology has undergone a great surge in interest from industry and the scientific community. This is likely due to greater interest in conserving energy and natural resources. By lowering friction we conserve energy, and by lowering wear we increase product life, which conserves raw materials and the energy needed to turn raw materials into useful technology. However, there are many challenges to be overcome in the study of tribology.
Tribology is a surface phenomenon, and surfaces are extremely complex. As seen in Figure 1, when surfaces are sufficiently magnified we can see they are not flat, but consist of a series of peaks and valleys. In addition to being geometrically complex, surfaces are also chemically complex. Surfaces typically react with oxygen in the air to form a thin oxide film on the original surface. A thin gaseous film then generally forms on top of the oxide film. These films, which are usually so thin that they are transparent to the eye, significantly affect friction and wear.
Besides surface topography, the hardness of the sliding materials, thickness and properties of oxide films, temperature, and type of lubricant all affect tribology. These factors overlap many fields of study, including physics, chemistry, material science and mechanical engineering. A complete understanding of tribology will require scientists and engineers from these diverse disciplines to work together. Traditionally, interdisciplinary research in tribology has not been done due to researchers being unwilling to cross the boundaries of their own discipline.
Surfaces are not easily observed during sliding. Tribologists study surfaces before and after sliding to determine chemical and geometric changes that occurred, and to infer how these changes came about and how they affected friction and wear. Because of the microscopic nature of tribology research, and the difficulty in duplicating real world conditions, it is not easy to observe and determine the cause of wear.
HISTORY OF LUBRICANTS
The most common method of minimizing friction and wear is through lubrication. The first recorded use of a lubricant was in ancient Egypt in 2400 B.C.E. Records show that they would pour a lubricant in front of a sledge being used to move a stone statue, which weighed tens of tons. The lubricant used was probably either water or animal fat. Various lubricants derived from animals and vegetables were used for thousands of years. Some examples of lubricants derived from animals are sperm oil, whale oil, and lard oil. Sperm oil was taken from a cavity in the head of sperm whales, whale oil from whale blubber, and lard from pig fat. Examples of vegetable lubricants are olive oil, groundnut oil and castor oil.
Lubricants began to receive significantly more attention from the industrial and scientific community in the mid-1800s with the introduction of mineral oils as lubricants. These proved to be effective lubricants; the demand for their use in machinery led to the development of many oil companies.
One common application for oils is the lubrication of automotive engines. When automotive oils were first introduced, their viscosities were classified as light, medium, medium heavy, heavy and extra heavy. This method of classification was too subjective and led to some engines being improperly lubricated. To remedy this, the Society of Automotive Engineers in 1912 introduced a quantitative numbering system for automotive viscosity. These numbers range from zero to fifty with lower numbers indicating lower viscosities. A general trend observed in oils is that as temperature increases, viscosity decreases. If viscosity becomes too small, the oil will be ineffective; if it is too high, the oil will not flow properly. Multigrade oils are now available that behave as lower viscosity oil at lower temperatures and higher viscosity oil at higher temperatures. For instance, a 10W-30 oil would have the viscosity of a 10-grade oil at low temperatures and the viscosity of a 30-grade oil at high temperatures. The use of multigrade oils minimizes change in viscosity with temperature.
In addition to viscosity grade, automotive oils also have a quality designation based on the API Engine Service classification system. This system was developed through the combined efforts of the American Society of Testing Materials (ASTM), the Society of Automotive Engineers (SAE), the American Petroleum Institute (API), automotive manufacturers and the United States military. API ratings describe the oil's ability to provide resistance to factors such as wear and corrosion. The original API rating was termed SA. This type of oil is suitable only for engines operated in very mild conditions. Oils with an API rating of SB are of higher quality than SA; SC has a higher API rating than SB, and so on.
In addition to liquid lubricants, solid lubricants such as graphite and grease have also been used for several centuries. Graphite was mined in the sixteenth century in Cumberland, England. It was originally called "black-lead" and was used in writing instruments. It is still used in "lead" pencils. There are numerous reports of graphite being used to lubricate machinery and silk looms in the nineteenth century. A common modern application for graphite is in the lubrication of door locks.
A specification for grease was given in 1812 by Henry Hardacre as follows: "one hundred weight of plumbago (graphite) to four hundred weight of pork lard or beef suet, mutton suet, tallow, oil, goose grease or any other kind of grease or greasy substance, but pork lard is the best, which must be well mixed together, so as to appear to the sight to be only one substance." Grease is used in many industrial applications including lubrication of gears and bearings.
CONSERVATION OF LUBRICANTS AND ENERGY
Most lubricating oils and greases use mineral oil as their base component. However, oil is a natural resource with a finite supply. Thus, it is imperative that measures such as recycling and extending the life of lubricants be taken to conserve the world's supply of oil and energy. There are approximately 2.5 billion gallons of waste oil generated each year in the United States. Only 83 million gallons are refined and reused, while one billion gallons are burned as fuel and another billion gallons or more are released into the environment. In response to these statistics, the United States Congress encouraged recycling of used oil through the Energy Policy and Conservation Act of 1975 and Resource Conservation and Recovery Act of 1976. The United States Environmental Protection Agency has stated that used oil is an environmental hazard, making the need for recycling even more critical.
Extending the interval between oil changes in automobiles and machinery is another way to conserve lubricants. Oil life is limited by depletion of oil additives, overheating, chemical contamination, and contamination of the oil by foreign particles. Using improved additives and filtering of particles, oil life can be considerably extended.
HISTORY OF FRICTION
DaVinci is well known for his famous paintings, such as the Mona Lisa, but his genius had amazing breadth. The scientific study of friction began with his research in the later part of the fifteenth century. He performed experiments in which he measured the force required to slide wood blocks over a wooden table. One of his most important findings was that the force F, required to produce sliding, increases in proportion with the applied load, L. DaVinci also defined what is today called the coefficient of friction, f, between two sliding objects as: where F is friction force and L is normal load, as shown in Figure 2.
DaVinci's experiments on friction also formed the basis for what today are called the first two laws of friction:
- The force of friction, F, is directly proportional to the applied load, L.
- The force of friction is independent of the apparent area of contact.
In general, a larger coefficient of friction between sliding bodies will require a larger force to produce sliding and, hence, more energy will be consumed. The most common method to reduce frictional energy losses is to lubricate the sliding surfaces. The reduction in coefficient of friction due to lubrication can be dramatic; typical values for dry sliding can range from 0.2 to 0.3, while typical values for lubricated sliding can range from 0.03 to 0.12.
The phenomenon of frictional heating is well known; for example, starting fires through the use of friction has been common since prehistoric times. However, frictional heating is generally a detrimental phenomenon. The temperature rise on sliding surfaces that occurs due to frictional heating generally produces a decrease in surface hardness, causing the surfaces to wear more easily, thus decreasing product life.
From an energy consumption point of view, we desire friction to be as small as possible. However, there are specific applications where friction should not be minimized. One common example is the brakes used in automobiles. If friction is too low, automobiles will not stop in a reasonable distance. However, if the friction is too high, a jerky ride will be produced. Other examples where high friction is desirable are shoe soles, nails, screws, and belt drives.
It is interesting to note that although friction has been studied for hundreds of years, there is no universal agreement on the fundamental mechanisms of how friction is produced. The two most popular theories are interlocking and adhesion. The interlocking theory states that as two rough surfaces slide over each other, the peaks on one surface "interlock" with peaks on the adjacent surface, producing resistance to sliding. The adhesion theory suggests that as the sliding surfaces contact each other, adhesive bonds form. These molecular bonds must be broken to continue sliding, which results in increased friction. The cost of not understanding the fundamentals of friction is high. Estimates have shown that as much as 0.5 percent of industrial countries' gross national products are wasted because we do not know enough about minimizing frictional losses during sliding.
FUTURE ADVANCES IN LUBRICATION
The transportation industries are being challenged with increasingly stringent government regulations that demand improved vehicle fuel economy and reduced emissions. Engine oils with greater ability to conserve energy are becoming more important as a way to lower engine friction and thus conserve natural resources. One specific approach under development is to produce engine oils with a friction-reducing additive such as molybdenum dialkyldithiocarbamate which has showed promise in developmental engine tests.
Another approach being used to conserve energy is to increase combustion temperatures in engines. This results in increased engine efficiency that produces energy savings. Combustion temperatures may eventually become so high that liquid lubricants break down and become ineffective. A possible approach for lubrication under these extreme temperatures is vapor phase lubrication, which refers to the deposition of lubricant from an atmosphere of vaporized lubricant. The vaporized lubricant is delivered in a carrier gas, such as nitrogen, to the component to be lubricated where it reacts chemically to form a solid lubricant film. These films are effective at much higher temperatures than liquid-based lubricants. Other approaches that can be used in components that operate in high operating temperatures are the use of gas bearings or magnetic bearings.
Gary C. Barber Barbara Oakley
See also: Automobile Performance; Materials
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