Tribology

Tribology (from Greek Τρίβος “tribos”, friction, and λόγος “logos”, study or science) is the science that studies the behavior and interactions between surfaces that interact with each other with a relative motion. In it converge study topics included in different disciplines, such as friction phenomena, theory and technique of lubrication, study and use of lubricants, microscopic and macroscopic theories of wear, studies and use of wear-resistant materials.

In particular, friction comes into play in manufacturing processes and influences forces and powers involved, energy dissipation, and part quality. Wear modifies the surfaces and geometries of dies and tools, determining the consistency and quality of the geometries produced. Finally, lubrication in manufacturing processes is fundamental in both machining operations and machine operation.

The tribological system

For the optimal solution of a problem in the tribological field it is necessary to know all the quantities that interact and affect a tribological system (also called tribosystem). The influences exerted by the environment (dust, temperature and humidity) and the design factors (material, surface and geometry of the friction bodies) are just as important as the stress factors (speed, pressure stress and vibration) in selecting the intermediate substance (lubricant).

It is clear, therefore, that the dynamic behavior of machines is characterized by the phenomena that occur in the contact between solid members, or between solid members and fluids. The contacts between solids and fluids are mainly studied by disciplines such as fluid mechanics (hydraulics), thermofluid dynamics (technical physics and fluid machines) and other derived disciplines. Whereas, the study of surface interactions between solid members is one of the most important topics in machine mechanics , because:

  • friction is inherent in motion both when it enables the movement of machines (automobile wheels) and when it must be combated as a source of losses and high temperatures (gears);
  • lubrication causes the profound modification of friction and wear phenomena that would occur between dry surfaces;
  • wear is one of the main causes that make machines unusable, as are breakage and obsolescence.

Applications

  • Industrial tribology: it is used in the study of friction phenomena between mechanical parts in motion and in contact;
  • Space tribology: it is used to study tribological phenomena in boundary conditions. The problems that arise in space are of various kinds, from the impressive temperature ranges to low pressures, from the presence of dust and debris to solar radiation, from high vacuum to the absence of maintenance. All these conditions imply the need of some adjustments, especially in the lubrication systems, that just because of the thermal changes and low pressures, cannot be of traditional liquid type, but solid lubricants are preferred.
  • Computational tribology: it deals with the reproduction of tribological systems by means of simulations, able to combine together more physical disciplines.
  • Nanotribology: is the study of tribology at the nanoscale. This study has two main purposes: to explain at a fundamental level the laws of friction and to define the nanoscopic properties of the same, in order to support the nanotechnologies under development. The main idea was developed from the work of Tomlinson, considering the energy dissipation of friction on a microscopic scale as due to elastic waves arising from the vibrational motions of the material. This hypothesis was verified experimentally in the ’90s, thanks to the work of Jacqueline Krim, in particular thanks to an experiment conducted in 1991 on a monoatomic krypton film. In general, it came to distinguish two main mechanisms of atomic energy dissipation: the first is the dissipation due to phonon excitation, while the second depends on electronic excitations. While the former increases with the number of atomic layers, the latter affects only the most superficial layers of the material. Nanotribological studies are fundamental for the widespread applications of miniaturized systems, such as magnetic memories in computers and micro and nano machines, which are extremely sensitive to friction and wear.
  • Biotribology: is the application of tribology in the biological field. In the medical field tribology plays a fundamental role, as for example in the study of wear and rubbing of daily contact lenses, or the wear of prostheses and biomedical devices (such as pacemakers or stents); tribological phenomena that occur in the rubbing of teeth and their consequent wear; the longevity of screws and plates used in the repair of bone fractures and so on.
  • Geotribology: is a branch of tribology that deals with the study of friction and wear in geologic systems such as glaciers or seismic faults.
  • Tribotronics: is a field of application of tribology that deals with the integration of active control loops in machinery where tribological phenomena occur in order to improve the efficiency of the machinery itself.
  • Ecotribology: in 1966, the year the “Jost report” was published, the word “ecotribology” was used to refer primarily to the economic gain that could result from the development of tribological technologies. Today, we are faced with an insidious set of environmental problems, such as global warming and pollution, which is why the term ecotribology is used to represent the possibility not only of economic savings, but also of improved environmental conditions, thanks to new technologies and new materials used. Improvements in the field of tribology include the abolition of asbestos from vehicle brake systems, the replacement of certain refrigerants with less polluting ones and the constant control of lead used in bearings. The fields of research related to sustainable tribology are the most varied, ranging from tribomaterials for the control of friction and wear, to tribosystems for the transport sector, from new types of lubricants with less environmental impact, to the study of maintenance of machinery gears. All these fields of research aim at saving money and energy and reducing the environmental impact more and more. To do this, a number of important aspects must be taken into consideration, such as the choice of ecofriendly materials, the reuse of materials, optimized designs and the optimization of industrial processes. An important example of eco-tribological developments is the work of Shi and Huang, published in 2017, on materials with “self-healing” properties such as Hydroxypropyl Methylcellulose (HPMC). This material has a unique chemical structure capable of reducing the coefficient of friction and wear, can be used as a lubricant, and has a good ability to dissolve in certain solvents.

References

  1. Stachowiak G.W., How tribology has been helping us to advance and to survive, in Friction, vol. 5, n. 3.
  2. Bartz W.J., Ecotribology: Environmentally acceptable tribological practices, in Tribology International, vol. 39.
  3. Sasaki S., Environmentally friendly tribology (Eco-Tribology), in Journal of Mechanical Science and Technology.
  4. Green Tribology – SpringerLink. doi:10.1007/978-3-642-23681-5.
  5. Shi S. and Huang T, Self-Healing Materials for Ecotribology, in Materials.
  6. Glavatskih, Sergei; Höglund, Erik (2008). “Tribotronics—Towards active tribology”. Tribology International. 41 (9–10): 934–939. doi:10.1016/j.triboint.2007.03.001.
  7. Zhang, Chi; Wang, Zhong Lin (2016). “Tribotronics—A new field by coupling triboelectricity and semiconductor”. Nano Today. 11 (4): 521–536. doi:10.1016/j.nantod.2016.07.004.
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