Tribology

Tribology is the science and engineering of understanding friction, lubrication and wear phenomena for interacting surfaces in relative motion. It is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, materials science, mathematics, biology and engineering. The fundamental objects of study in tribology are tribosystems, which are physical systems of contacting surfaces. Subfields of tribology include biotribology, nanotribology and space tribology. It is also related to other areas such as the coupling of corrosion and tribology in tribocorrosion and the contact mechanics of how surfaces in contact deform.
Approximately 20% of the total energy expenditure of the world is due to the impact of friction and wear in the transportation, manufacturing, power generation, and residential sectors.

Etymology

The word tribology derives from the Greek root τριβ- of the verb wiktionary:τρίβω, tribo, "I rub" in classic Greek, and the suffix -logy from wiktionary:-λογία, -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.

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 Leonardo, 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 However, Leonardo's findings remained unpublished in his notebooks.
The two fundamental 'laws' of friction were first published by Guillaume Amontons, with whose name they are now usually associated. They state that:

the force of friction acting between two sliding surfaces is proportional to the load pressing the surfaces together
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, who noticed that static friction force may depend on the contact time and sliding friction may depend on sliding velocity, normal force and contact area.
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 1860, Theodor Reye proposed. In 1953, John Frederick 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 the Cavendish Laboratory at Cambridge University. Together they wrote the seminal textbook The Friction and Lubrication of Solids. Michael J. Neale was another leader in the field during the mid-to-late 1900s. 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. This covers developments from prehistory, through early civilizations 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. As a result, the UK government established several national centres to address tribological problems. Since then the term has diffused into the international community, with many specialists now identifying as "tribologists".

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 and 18% in the short term. 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 and in power generation 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 million tons of carbon dioxide equivalent 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.

Classical tribology covering such applications as ball bearings, gear drives, clutches, brakes, etc. was developed in the context of mechanical engineering. But in the last decades tribology expanded to qualitatively new fields of applications, in particular micro- and nanotechnology as well as biology and medicine.

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

s 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.

Physics

Friction

The word friction comes from the Latin "frictionem", which means rubbing. This term is used to describe all those dissipative phenomena, capable of producing heat and of opposing the relative motion between two surfaces. There are two main types of friction:
; Static friction: Which occurs between surfaces in a fixed state, or relatively stationary.
; Dynamic friction: Which occurs between surfaces in relative motion.
The study of friction phenomena is a predominantly empirical study and does not allow to reach precise results, but only to useful approximate conclusions. This inability to obtain a definite result is due to the extreme complexity of the phenomenon. If it is studied more closely it presents new elements, which, in turn, make the global description even more complex.

Laws of friction

All the theories and studies on friction can be simplified into three main laws, which are valid in most cases:
; First Law of Amontons: The frictional force is directly proportional to the normal load.
; Second Law of Amontons: Friction is independent of the apparent area of contact.
; Third Law of Coulomb: Dynamic friction is independent of the relative sliding speed.
Coulomb later found deviations from Amontons' laws in some cases. In systems with significant nonuniform stress fields, Amontons' laws are not satisfied macroscopically because local slip occurs before the entire system slides.

Static friction

Consider a block of a certain mass m, placed in a quiet position on a horizontal plane. If you want to move the block, an external force must be applied, in this way we observe a certain resistance to the motion given by a force equal to and opposite to the applied force, which is precisely the static frictional force.
By continuously increasing the applied force, we obtain a value such that the block starts instantly to move. At this point, also taking into account the first two friction laws stated above, it is possible to define the static friction force as a force equal in modulus to the minimum force required to cause the motion of the block, and the coefficient of static friction as the ratio of the static friction force. and the normal force at block, obtaining

Dynamic friction

Once the block has been put into motion, the block experiences a friction force with a lesser intensity than the static friction force. The friction force during relative motion is known as the dynamic friction force. In this case it is necessary to take into account not only the first two laws of Amontons, but also of the law of Coulomb, so as to be able to affirm that the relationship between dynamic friction force, coefficient of dynamic friction k and normal force N is the following:

Static and dynamic friction coefficient

At this point it is possible to summarize the main properties of the static friction coefficients and the dynamic one.
These coefficients are dimensionless quantities, given by the ratio between the intensity of the friction force and the intensity of the applied load, depending on the type of surfaces that are involved in a mutual contact, and in any case, the condition is always valid such that:.
Usually, the value of both coefficients does not exceed the unit and can be considered constant only within certain ranges of forces and velocities, outside of which there are extreme conditions that modify these coefficients and variables.
In systems with significant nonuniform stress fields, the macroscopic static friction coefficient depends on the external pressure, system size, or shape because local slip occurs before the system slides.
The following table shows the values of the static and dynamic friction coefficients for common materials:

Contact surfaces	Static friction	Dynamic friction
Wood–wood	0.25–0.5	0.2
Wood–cardboard	0.32	0.23
Ice–ice	0.1	0.02
Scioled wood ski–snow	0.04	0.04
Glass–glass	0.9–1.0	0.4
Steel–steel	0.6	0.6
Steel–steel	0.09	0.05
Steel–ice	0.1	0.05
Steel–ice	0.78	0.42
Steel–aluminum	0.61	0.47
Steel–brass	0.51	0.44
Steel–air	0.001	0.001
Steel–Teflon	0.04	0.04
Teflon–Teflon	0.04	0.04
Rubber–cement	1.0	0.8
Rubber–cement	0.7	0.5
Copper–steel	0.53	0.36
Copper–glass	0.68	0.53
Synovial joints	0.01	0.003