Tribology

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Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. Tribology is highly interdisciplinary. It draws on many academic fields, including physics, chemistry, materials science, biology and engineering. People who work in the field of tribology are referred to as tribologists.

The word tribology derives from the Greek root ????- of the verb ?????, tribo, "I rub" in classic Greek, and the suffix -logy from -?????, -logia "study of", "knowledge of". Peter Jost coined the word in 1966, in the eponymous report which highlighted the cost of friction, wear and corrosion to the UK economy.

Video Tribology

History

Early History

Despite the relatively recent naming of the field of tribology, quantitative studies of friction can be traced as far back as 1493, when Leonardo da Vinci first noted the two fundamental 'laws' of friction. According to da Vinci, frictional resistance was the same for two different objects of the same weight but making contact over different widths and lengths. He also observed that the force needed to overcome friction doubles as weight doubles. However, da Vinci's findings remained unpublished in his notebooks.

The two fundamental 'laws' of friction were first published (in 1699) by Guillaume Amontons, with whose name they are now usually associated, they state that:

1. the force of friction acting between two sliding surfaces is proportional to the load pressing the surfaces together
2. the force of friction is independent of the apparent area of contact between the two surfaces.

Although not universally applicable, these simple statements hold for a surprisingly wide range of systems. These laws were further developed by Charles-Augustin de Coulomb (in 1785), who noticed that sliding (kinetic) friction is independent of the sliding velocity.

In 1798, Charles Hatchett and Henry Cavendish carried out the first reliable test on frictional wear. In a study commissioned by the Privy Council of the UK, they used a simple reciprocating machine to evaluate the wear rate of gold coins. They found that coins with grit between them wore at a faster rate compared to self-mated coins. In 1953, John. F Archard developed the Archard equation which describes sliding wear and is based on the theory of asperity contact.

Other pioneers of tribology research are Australian physicist Frank Philip Bowden and British physicist David Tabor, both of Cavendish Laboratory. Together they wrote the seminal textbook The Friction and Lubrication of Solids (Part I originally published in 1950 and Part II in 1964). Michael J. Neale was another leader in the field during the mid-to-late 1900's. He specialized in solving problems in machine design by applying his knowledge of tribology. Neale was respected as an educator with a gift for integrating theoretical work with his own practical experience to produce easy-to-understand design guides. The Tribology Handbook, which he first edited in 1973 and updated in 1995, is still used around the world and forms the basis of numerous training courses for engineering designers.

Duncan Dowson surveyed the history of tribology in his 1997 book History of Tribology (2nd edition). This covers developments from prehistory, through early civilizations (Mesopotamia, ancient Egypt) and highlights the key developments up to the end of the twentieth century.

The Jost Report

The term tribology became widely used following 'The Jost Report', published in 1966. The report highlighted the huge cost of friction, wear and corrosion to the UK economy (1.1-1.4% of GDP). As a result, the UK government established several national to address tribological problems. Since then the term has diffused into the international community, with many specialists now identifying as 'tribologists'.

Maps Tribology

Significance

Despite considerable research since the Jost Report, the global impact of friction and wear on energy consumption, economic expenditure, and carbon dioxide emissions are still considerable. In 2017, Kenneth Holmberg and Ali Erdemir attempted to quantify their impact worldwide. They considered the four main energy consuming sectors: transport, manufacturing, power generation, and residential. The following were concluded:

• In total, ~23% of the world's energy consumption originates from tribological contacts. Of that, 20% is to overcome friction and 3% to remanufacture worn parts and spare equipment due to wear and wear-related.
• By taking advantage of the new technologies for friction reduction and wear protection, energy losses due to friction and wear in vehicles, machinery and other equipment worldwide could be reduced by 40% in the long term (15 years) and 18% in the short term (8 years). On a global scale, these savings would amount to 1.4% of GDP annually and 8.7% of total energy consumption in the long term.
• The largest short term energy savings are envisioned in transport (25%) and in power generation (20%) while the potential savings in the manufacturing and residential sectors are estimated to be ~10%. In the longer term, savings would be 55%, 40%, 25%, and 20%, respectively.
• Implementing advanced tribological technologies can also reduce global carbon dioxide emissions by as much as 1,460 metric tons of carbon dioxide equivalent (MtCO₂) and result in 450,000 million Euros cost savings in the short term. In the long term, the reduction could be as large as 3,140 MtCO₂ and the cost savings 970,000 million Euros.

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Applications

Transport and manufacturing tribology

Historically, tribology research concentrated on the design and effective lubrication of machine components, particularly for bearings. However, the study of tribology extends into most aspects of modern technology and any system where one material slides over another can be affected by complex tribological interactions.

Traditionally, tribology research in the transport industry focused on reliability, ensuring the safe, continuous operation of machine components. Nowadays, due to an increased focus on energy consumption, efficiency has become increasingly important and thus lubricants have become progressively more complex and sophisticated in order to improve this. Tribology also plays an important role in manufacturing. For example, in metal-forming operations, friction increases tool wear and the power required to work a piece. This results in increased costs due to more frequent tool replacement, loss of tolerance as tool dimensions shift, and greater forces required to shape a piece.

The use of lubricants which minimize direct surface contact reduces tool wear and power requirements. It is also necessary to know the effects of manufacturing, all manufacturing methods leave a unique system fingerprint (i.e. surface topography) which will influence the tribocontact (e.g. lubricant film formation).

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Tribology Research

Research Fields

Tribology research ranges from macro to nano scales, in areas as diverse as the movement of continental plates and glaciers to the locomotion of animals and insects. Tribology research is traditionally concentrated on transport and manufacturing sectors, but this has considerably diversified. Tribology research can be loosely divided into the following fields (with some overlap):

• Classical tribology is concerned with friction and wear in machine elements (such as rolling-element bearings, gears, plain bearings, brakes, clutches, wheels and fluid bearings) as well as manufacturing processes (such as metal forming).
• Biotribology studies friction, wear and lubrication in biological systems. The field is gaining importance as human lifetime expectancy increases. Human hip and knee joints are typical biotribology systems.
• Green tribology aims to minimize the environmental impact of tribological systems along their entire lifecycle. In particular, green tribology aims to reduce tribological losses (e.g., friction and wear) using technologies with minimal environmental impact. This is in contrast to traditional tribology, where the means of reducing tribological losses are not holistically evaluated.
• Geotribology studies friction, wear, and lubrication of geological systems, such as glaciers and faults.
• Nanotribology studies tribological phenomena at nanoscopic scales. The field is becoming increasingly important as devices become smaller (e.g. micro/nanoelectromechanical systems, MEMS/NEMS), and research has been aided by the invention of Atomic Force Microscopy.
• Tribotronics aims to integrate active control loops into tribological systems in order to increase machine efficiency and lifetime.
• Computational tribology aims to model the behavior of tribological systems through multiphysics simulations, combining disciplines such as contact mechanics, fracture mechanics and computational fluid dynamics.
• Space tribology studies tribological systems that can operate under the harsh environmental conditions of outer space. In particular, this requires lubricants with low vapor pressure that can withstand extreme temperature fluctuations.
• Open system tribology studies tribological systems that are exposed to and affected by the natural environment.

Recently, intensive studies of superlubricity (phenomenon of vanishing friction) have sparked due to increasing demand for energy savings. Furthermore, the development of new materials, such as graphene and ionic liquids, allows for fundamentally new approaches to solve tribological problems.

Research Societies

There are now numerous national and international societies, including: the Society for Tribologists and Lubrication Engineers (STLE) in the USA, the Institution of Mechanical Engineers' and Institute of Physics (IMechE Tribology Group, IOP Tribology Group) in the UK, the German Society for Tribology (Gesellschaft für Tribologie), the Malaysian Tribology Society (MYTRIBOS), the Japanese Society of Tribologists (JAST), the Tribology Society of India (TSI), the Chinese Mechanical Engineering Society (Chinese Tribology Institute) and the International Tribology Council (International Tribology Council).

Research Approach

Tribology research is mostly empirical, which can be explained by the vast number of parameters that influence friction and wear in tribological contacts. Thus, most research fields rely heavily on the use of standardized tribometers and test procedures as well component-level test rigs.

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Fundamental Concepts

Tribosystem

The concept of tribosystems is used to provide a detailed assessment of relevant inputs, outputs and losses to tribological systems. Knowledge of these parameters allows tribologists to devise test procedures for tribological systems.

Tribofilm

Tribofilms are thin films that form on tribologically stressed surfaces. They play an important role in reducing friction and wear in tribological systems.

Stribeck Curve

The Stribeck Curve shows how friction in fluid-lubricated contacts is a non-linear function of lubricant viscosity, entrainment velocity and contact load.

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See also

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References

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External links

Source of the article : Wikipedia