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.

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.

Peter Jost coined the word in 1966,[1] in the eponymous report which highlighted the cost of friction, wear and corrosion to the UK economy.

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.

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,[14] 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.

This term is used to describe all those dissipative phenomena, capable of producing heat and of opposing the relative motion between two surfaces.

The force of rolling friction depends, therefore, on the small deformations suffered by the supporting surface and by the wheel itself, and can be expressed as

Plastic, elastic, or rupture deformations can be observed: The energy that is dissipated during the phenomenon is transformed into heat, thus increasing the temperature of the surfaces in contact.

If at this point the two surfaces are sliding between them, a resistance to shear stress t is observed, given by the presence of adhesive bonds, which were created precisely because of the plastic deformations, and therefore the frictional force will be given by

To obtain even more realistic considerations, the phenomenon of the third body should also be considered, i.e., the presence of foreign materials, such as moisture, oxides or lubricants, between the two solids in contact.

The simplest and most immediate method for evaluating the friction coefficient of two surfaces is the use of an inclined plane on which a block of material is made to slide.

This allows us to state that the coefficient of friction can be calculated very easily, by means of the tangent of the angle in which the block begins to slip.

Today there are digital machines such as the "Friction Tester" which allows, by means of a software support, to insert all the desired variables.

Of course, some lubricants are more suitable than others, depending on the type of use they are intended for: air and water, for example, are readily available, but the former can only be used under limited load and speed conditions, while the second can contribute to the wear of materials.

From what has been said, it is possible to state that the force F, necessary to cause a rolling motion in a fluid contained between two plates, is proportional to the area of the two surfaces and to the speed gradient:

To conclude we want to underline that it is possible to divide the fluids into two types according to their viscosity: Temperature and pressure are two fundamental factors to evaluate when choosing a lubricant instead of another.

Wear plays a fundamental role in tribological studies, since it causes changes in the shape of the components used in the construction of machinery (for example).

Erroneously we tend to imagine wear in a direct correlation with friction, in reality these two phenomena can not be easily connected.

[21] If a shearing force is applied in the contact area, it may be possible to detach a small part of the weaker material, due to its adhesion to the harder surface.

To reduce this type of wear, therefore, it is good to try to decrease both the contact forces and the thermal cycling, that is the frequency with which different temperatures intervene.

For optimal results it is also good to eliminate, as much as possible, impurities between surfaces, local defects and inclusions of foreign materials in the bodies involved.

Clearly this type of wear can be reduced by trying to create an 'ad hoc' environment, free of pollutants and sensible to minimal thermal changes.

In fact, the oxides that are created, contribute to decrease the coefficient of friction between the surfaces, or, being in many cases harder than the metal to which they belong, can be used as excellent abrasives.

The rubbing wear occurs in systems subject to more or less intense vibrations, which cause relative movements between the surfaces in contact within the order of nanometers.

Instead, to express the volume of wear V it is possible to use the Holm equation where W / H represents the real contact area, l the length of the distance traveled and k and

[24] Traditionally, tribology research in the transport industry focused on reliability, ensuring the safe, continuous operation of machine components.

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 can be loosely divided into the following fields (with some overlap): Recently, intensive studies of superlubricity (phenomenon of vanishing friction) have sparked due to increasing demand for energy savings.