Wear denotes the progressive " loss of material from the surface of a solid body, caused by mechanical causes, i. H. Contact and relative movement of a solid, liquid or gaseous counter-body, ie the loss of mass (surface removal) of a material surface due to abrasive, rolling, hitting, scratching, chemical and thermal stress. In common parlance, wear is also equated with other types of wear .
This mostly undesirable change in the surface occurs, for example, on bearings , clutches , gears , nozzles and brakes , as well as wear on cutting tools . You also have to change the tires of cars regularly due to tire wear . Wear changes the geometry of components so that they change their function. This results in component damage and the associated failure of machines and devices. Reducing wear and tear is therefore an essential way of increasing the service life of machines and devices and thus saving costs and raw materials . On the other hand, an attempt is made to limit the unavoidable wear to components that are easy to replace, which are summarized under the term wear part .
The wear behavior can often be predicted. However, the procedures for determining the lifespan must be distinguished from obsolescence .
The study of friction and wear as well as lubrication is tribology .
- Base body (material, shape, surface)
- Intermediate substance (type, particle size, etc.)
- Counter body (material, shape, surface)
- Load (size, time course)
- Type of movement (sliding, rolling, pushing)
- surrounding atmosphere (e.g. air, protective gas, vacuum)
- Temperature (altitude, time course)
Wear is always a system property , not a property of the components involved : There are always (at least) two components in interaction , often an intermediate medium ( fluid ). In contrast to this, material fatigue and chemical aging are possible without involving a second component.
Wear is mainly determined by four different wear mechanisms:
Adhesive wear occurs when there is insufficient lubrication. If components that are in contact are tightly on top of each other with high surface pressure , the contact surfaces adhere to each other as a result of adhesion (also: attachment force ). When sliding, surface layer particles are then sheared off. This creates holes and flake-like material particles that often stick to the sliding surface of the harder partner. This wear mechanism is called adhesive wear and adhesive wear .
Protective oxidation layers are broken through as a result of the adhesion through the high local pressure on individual surface roughness hills. A local micro cold weld is created . As a result of the work hardening, the material does not then tear in the area between the two welds (fusion zone), but in the adjacent areas.
If hard particles of a lubricant or roughness peaks of one of the friction partners penetrate the surface layer, scratches and micro-cutting occur . This wear is called abrasive wear , furrow wear or erosion wear - the latter can also be caused by liquids. The loss of material caused by abrasion is called abrasion .
To avoid abrasive (i.e. scraping) wear, lubricants should be monitored and replaced if necessary. In principle, abrasive wear can be prevented during the design of a tribological system. Instead of metallic pairings, metal-plastic or metal-ceramic pairings should be preferred. In principle, a favorable ratio (e.g. hard carbides in a tough intermediate material) between strength and toughness should be aimed for with metallic pairings .
Abrasive wear and tear plays a special role in systems in which media are conveyed that contain angular, hard particles. For example, abrasive wear plays a role in pipelines and pumps through which water with suspended matter (sand), plaster and concrete ( aggregates ) or filled plastic compounds ( fillers ), for example in potting systems , are to be pumped. In these cases, abrasive wear is a major cause of the shortening of the service life of components through which flow has passed.
Abrasive wear can be determined with a mechanical test method using the so-called Taber Abraser in accordance with ISO 9352, ASTM D 1044 or DIN EN standard 438 - 2.6. Here , wheels with sandpaper are pressed with a defined pressure against the rotating surface of the test specimen . The measured variable is usually the loss of mass of the test specimen after a certain number of revolutions.
In the event that abrasive solids are suspended in liquids and cause wear, one speaks of hydroabrasion .
Surface distress is a wear mechanism that is caused by changing or swelling mechanical stresses. The result is a disruption of the surface, i. This means that microcracks develop and grow in the material layers close to the surface. Surface distress occurs in rolling bearings , for example, due to constant rolling over. This wear, also known as rolling wear, creates dimples or pitting . Since tensile stresses in the surface promote surface distress, compressive stresses can be introduced into the surface as a countermeasure. Suitable processes are nitriding , oxidizing or shot peening of the surfaces.
The formation of intermediate layers, e.g. B. Oxide layers resulting from chemical reactions and their destruction by movement of the components are called tribo-oxidation or reaction layer wear . It almost always occurs together with adhesive wear. This wear mechanism, which arises as a result of a chemical reaction and mechanical destruction of the reaction layer , is a tribochemical reaction. An example of tribo-oxidation is fretting corrosion .
Wear and tear in electrical and electronic equipment
Many components are subject to wear and tear, such as electronic components such as electrolytic capacitors , rechargeable batteries or incandescent lamps and gas discharge lamps. The choice of problematic materials as well as complex manufacturing techniques and their poor control lead to serious quality differences in components of the same type, which are often, but not always, reflected in the price. Furthermore, there are component parameters such as stress and temperature load, which can have a major influence on the service life and can be more or less exhausted when dimensioning by the developer.
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- K.-H. Zum Gahr: Microstructure and wear of materials. Elsevier, Amsterdam 1987, ISBN 0-444-42754-6 .
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- Society for Tribology: Worksheet 7: Tribology - Definitions, Terms, Examination. August 2002, accessed October 3, 2017 .