tribology

Tribology is the science and technology of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. The word "tribology" derives from the Greek τρίβω ("tribo") meaning "(Ι) rub" (root τριβ-), and λόγος ("logos") meaning 'principle or logic'.

Applications

The study of tribology is commonly applied in bearing design but extends into almost all other aspects of modern technology, even to such unlikely areas as hair conditioners and cosmetics such as lipstick, powders and lipgloss.

Any product where one material slides or rubs over another is affected by complex tribological interactions, whether lubricated like hip implants and other artificial prosthesis or unlubricated as in high temperature sliding wear in which conventional lubricants can not be used but in which the formation of compacted oxide layer glazes have been observed to protect against wear.

Tribology plays an important role in manufacturing. 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 are required to shape a piece. A layer of lubricant which eliminates surface contact virtually eliminates tool wear and decreases needed power by one third.

Origins

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Tribological experiments suggested by Leonardo da Vinci

Historically, Leonardo da Vinci (1452-1519) was the first to enunciate two laws of friction. According to da Vinci, the frictional resistance was the same for two different objects of the same weight but making contacts over different widths and lengths. He also observed that the force needed to overcome friction is doubled when the weight is doubled. Similar observations were made by Charles-Augustin de Coulomb (1736-1806). The first reliable test on frictional wear was carried out by Charles Hatchett (1760- 1820) using a simple reciprocating machine to evaluate wear on gold coins. He found that compared to self-mated coins, coins with grits between them wore at a faster rate. The deciphering of da Vinci's work took several centuries, before the development of this branch of science, today called "tribology".

The term became widely used following The Jost Report in 1966, in which huge sums of money were reported to have been lost in the UK annually due to the consequences of friction, wear and corrosion. As a result several national centres for tribology were created in the UK. Since then the term has diffused into the international engineering field and many specialists now claim to be tribologists.

There are now numerous national and international societies, such as the Society for Tribologists and Lubrication Engineers (STLE) in the USA and the Institution of Mechanical Engineers' Tribology Group (IMechE Tribology Group) in the UK or the German Society for Tribology (Gesellschaft für Tribologie, www.gft-ev.de).

Most technical universities have a group working on tribology, often as part of their mechanical engineering departments. The limitations in tribological interactions are however no longer mainly determined by mechanical designs, but rather by material limitations so the discipline of tribology now counts at least as many materials engineers, physicists and chemists as it does mechanical engineers.

Stribeck curve

Long before the term “tribology” was coined in 1966, phenomena in the fields of friction and lubrication were named after relevant researchers. The research of Professor Richard Stribeck (1861 – 1950) was performed in Berlin at the Royal Prussian Technical Testing Institute (MPA, now BAM (www.bam.de)). Similar work were previously performed around 1885 by Prof. Adolf Martens (1850-1914) at the same Institute and in the mid 1870s by Dr. Robert H. Thurston ^[1][2] at the Stevens Institute of Technology in the U.S.. Prof. Dr. Thurston was therefore close to establish the “Stribeck”-curve, but he presented no “Stribeck”-like graphs, as he obviously did not fully believed in the relevance of this dependency. In the beginning 1920s, based on the results of Prof. Stribeck, the friction regimes for sliding lubricated surfaces are traditionally broadly categorised into (i) solid/boundary friction, (ii) mixed friction, and (iii) fluid friction, on the basis of the “Stribeck curve”, shown schematically in Figure 1. The “Stribeck”-curve is a classic teaching element in tribology classes ^[3].

As starting point Prof. Stribeck investigated the basic properties of sliding and roller bearings. Stribeck systematically studied the variation of friction between two liquid lubricated surfaces as function of a dimensionless lubrication parameter ηN/P (which is equivalent to the parameter vη/FN in Fig 1), where η is the dynamic viscosity, N the speed (e.g. revolutions per minute of a journal) and P the load projected on to the geometrical surface ^[4]. His results were presented on 5 December 1901 during a public session of the railway society and published on 6 September 1902.

The graphs of friction force reported by Stribeck stem from a carefully conducted, wide-ranging series of experiments on journal bearings. They clearly showed the minimum value of friction as the demarcation between full fluid-film lubrication and some solid asperity interactions. Stribeck studied different bearing materials and aspect ratios D/L from 1:1 to 1:2. The maximum sliding speed was 4 m/s and the geometrical contact pressure was limited to 5 MPa. These operating conditions were related to railway wagon journal bearings. Figure 2 presents one example from the original results published by Stribeck which best fit the classical “Stribeck” curve.

In comparing the findings of Thurston, Martens and Stribeck, the minimum in the coefficient of friction was found for two liquid-lubricated surfaces

1. by Stribeck as a function of speed at different contact pressures,
2. by Martens as a function of contact pressure for different speeds and for different viscosities of rape seed oil realized through temperature (See Figure 3).
3. by Thurston as function of speed at different pressures

Figure 3: Typical “Stribeck” curves obtained by Martens

The reason why the form of the friction curve for liquid lubricated surfaces was later assigned to Stribeck, although both Thurston and Martens achieved their results considerably earlier, Martens even in the same organization roughly 15 years earlier (see Figure 3), may be attributed to the fact that Stribeck published in the most important technical journal in Germany at that time. This was the Zeitschrift des Vereins Deutscher Ingenieure (VDI, Journal of German Mechanical Engineers). Martens published his results “only” in the official journal of the Royal Prussian Technical Testing Institute, which has now become BAM (www.bam.de). The VDI journal being one of the most important journals for engineers, provided wide access to these data, and later colleagues rationalized the results into the three classical friction regimes. Thurston however, did not have the experimental means to record a continuous graph of the coefficient of friction but only measure the friction at discrete points which may be the reason that the minimum in the coefficient of friction was not discovered by him. The data of Thurston showed on the other hand not such a pronounced minimum of friction for a liquid lubricated journal bearing as it can be seen in the graphs of Martens and Stribeck.

It has to be noted, that Stribeck’s extensive research ^[5][6] on ball bearing steels identified the metallurgy of the commonly used 100Cr6H (AISI 52100).

^[7] showing coefficient of friction as a function of “pressure” (water quenched, hardened steel shaft 99.6 mm diameter, running in bearing shell made from “Rotguß” (red brass, a Cu-Sn-Zn cast alloy; dotted lines for v= 1.0 m/s).

Fundamentals of Tribology

The tribological interactions of a solid surface's exposed face with interfacing materials and environment may result in loss of material from the surface. The process leading to loss of material is known as "wear". Major types of wear include abrasion, adhesion (friction), erosion, and corrosion. Estimated direct and consequential annual loss to industries in USA due to wear is approximately 1-2% of GDP. (Heinz, 1987). Wear can be minimized by modifying the surface properties of solids by one or more of "surface engineering" processes (also called surface finishing) or by use of lubricants (for frictional or adhesive wear).

Engineered surfaces extend the working life of both original and recycled and resurfaced equipments, thus saving large sums of money and leading to conservation of material, energy and the environment.

Methodologies to minimize wear include systematic approaches to diagnose the wear and to prescribe appropriate solution. Important ones include:

• Terotechnology in UK (Peter Jost , 1972), where a system approach of multidisciplinary engineering and management techniques is used to protect plant, equipment and machinery (assets) from degradation by improving performance in all the functional areas;
• Horst Czichos system approach (H. Czichos,1978) where appropriate material is selected by checking material properties against tribological requirements under operating environment
• Asset Management by Material Prognosis - a concept similar to terotechnology has been introduced recently by the US Military (DARPA) for upkeepment of key equipments in good health and start-ready condition for 24 hours. Good health monitoring system combined with appropriate medication at M&R stages have led to improved performance, reliability and extended life cycle of the assets, like advanced military hardwares and civil aircraft.

In recent years, micro- and nanotribology have been gaining ground. Frictional interactions in microscopically small components are becoming increasingly important for the development of new products in electronics, life sciences, chemistry, sensors and by extension for all modern technology.

See also

• Compacted oxide layer glaze
• Fretting
• Lubrication
• Oil analysis
• Surface engineering
• Surface science
• Tribometer
• Wear
• Oil additive
• Tribocorrosion

References

Bibliography

• Surface Wear – Analysis, Treatment, and Prevention: R. Chattopadhyay, published by ASM-International, Materials Park, OH, 2001, ISBN 0-87170-702-0.
• Advanced Thermally Assisted Surface Engineering Processes: Ramnarayan Chattopadhyay, Kluwer Academic Publishers, MA (now Springer, NY), 2004.
• DeGarmo, E. Paul, J T. Black, and Ronald A. Kohser. Materials and Processes in Manufacturing. Upper Saddle River, New Jersey: Prentice Hall, 1997. ISBN 0-02-328621-0
• Zum Gahr, Karl-Heinz (1987). Microstructure and Wear of Materials. Tribology Series, 10. Elsevier. ISBN 0444427546. 

External links

Wikimedia Commons has media related to: Tribology

• national Centre for Advanced Tribology at Southampton (nCATS)(UK) The main broad areas of research thrust of nCATS is the advanced tribology associated with: Transport, Environment, Energy, Quality of Life/Health Care and Tiny-Technologies. nCATS is the first multidisciplinary tribology research centre of its kind and aspires to solve next generation design issues, improving quality of life and enabling surface interactions to occur with minimal energy loss and impact on the environment.
• Tribology at Imperial College London The tribology group at Imperial College London is one of the world's largest university-based tribology research groups.
• [1] Society of Tribologists and Lubrication Engineers. STLE is the premier technical society serving the needs of more than 10,000 individuals and 150 companies and organizations that comprise the tribology and lubrication engineering business sector.
• What is Tribology? Introduction, Tribology Research Group at Sheffield University (UK)
• Tribology NL a comprehensive overview of tribology from a mechanical engineers point of view
• Tribology: Friction, Wear, and Lubrication - a short program at MIT
• Atomic-scale Friction Research and Education Synergy Hub (AFRESH) an Engineering Virtual Organization for the atomic-scale friction community to share, archive, link, and discuss data, knowledge and tools related to atomic-scale friction.