Wear

Wear designated after the 1997 withdrawn DIN 50320, the progressive loss of material from the surface of a solid body ( body ) caused by mechanical causes, ie Contact and relative movement of a solid, liquid or gaseous counter- body, ie the mass loss ( surface removal ) and has a fabric surface by rubbing, rolling, hitting, scratching, chemical and thermal stress. In common usage, wear and tear is equated with other types of wear.

This usually undesirable change of the surface occurs, for example, to bearings, couplings, gears, nozzles and brakes on, as well as cutting tool wear. Wear changes the geometry of components, so that they change their function. It follows by a component damage and the associated loss of machinery and equipment. The reduction of wear is therefore a significant opportunity to increase the service life of machinery and equipment and thus reduce costs and raw materials. On the other hand, tries to reduce the unavoidable wear on Easy to replace components that can be summed up under the term wear and tear.

The doctrine of friction, wear and lubrication is the tribology.

Factors

• Body (material, shape, surface )
• Precursor (type, particle size, and others)
• Counter body (material, shape, surface )
• Load (size, time course )
• Type of movement ( sliding, rolling, pushing )
• Surrounding atmosphere (e.g., air, inert gas, vacuum)
• Temperature (height, chronological sequence)

Wear is always a system property, not a property of the components involved: There are always ( at least) two components in interaction, often even an intermediate medium (fluid).

Mechanisms

Wear is mainly determined by four different wear mechanisms:

Adhesive wear

Adhesive wear occurs when a lack of lubrication. Lying to components in contact at high surface pressure firmly together, so the contact surfaces liable as a result of adhesion ( also: Appendix force ) to each other. When sliding Randschichtteilchen then be sheared. This gives rise to holes and scale-like particles that often remain stuck to the sliding surface of the harder Partners. This wear mechanism is called adhesive wear, or adhesive wear.

Example: Protective oxidation layers are breached as a result of adhesion by the high local pressure on individual Oberflächenrauheitshügeln. The result is a local Mikrokaltverschweißung. By the hardening of the material tears then not in the area between the two welds (fusion area ), but the side areas.

Abrasive wear

When hard particles of a lubricant or asperities of the friction partners penetrate into the skin, so it comes to scoring and micro-machining. It is called wear as abrasive wear, grooving wear or erosion wear - the latter can also be done by liquids.

To avoid abrasive (ie: abschabendem ) wear should be monitored lubricants and replaced if necessary. Principle abrasive wear can be prevented even in the construction of a tribological system. Instead of metallic combinations metal - plastic or metal-ceramic pairings should be preferred. In principle, with metallic pairings a favorable ratio (eg hard carbides in a tough intermediate material ) should be sought between strength and toughness.

Plays a special role abrasive wear in systems in which media are supported which include angular, hard particles. For example, playing abrasive wear in pipes and pumps a role through the water with suspended particles (sand ), plaster and concrete ( aggregates) or filled plastic materials ( fillers), as in Vergussanlagen, shall be promoted. In these cases, abrasive wear is a major cause of shortening the life of components energized.

Abrasive wear can be 1044 or DIN EN standard 438 with a mechanical test methods on the so-called Taber abrader according to ISO 9352, ASTM D - 2.6 are determined. This is provided with wheels with abrasive paper under a defined pressure to be pressed against the rotating surface of the sample body. The measured quantity is often the loss of mass of the specimen after a predetermined number of revolutions.

In the event that abrasive solids suspended in liquids and cause wear, one speaks of hydroabrasion.

The abrasion caused by the loss of material is called abrasion.

Surface distress

Surface fatigue is a wear mechanism that is caused by alternating or pulsating mechanical stresses. Consequence is a disruption of the surface, that is, it develop and grow micro-cracks in the superficial layers of material. Surface fatigue in rolling bearings occur, for example by the constant rolling over. This wear and tear, also called rolling wear, creates dimples or pitting. As tensile stress in the surface of the support surface disruption, may be incorporated as a countermeasure, compressive stresses in the surface. Suitable methods are nitriding, oxidizing or peening the surfaces.

Tribooxidation

The formation of intermediate layers, such as oxide layers, a result of chemical reaction and its destruction by movement of the components is known as tribo-oxidation or reaction layer wear. It almost always occurs together with adhesive wear. This wear mechanism, which arises due to chemical reaction and mechanical destruction of the reaction layer is a tribochemical reaction. An example of tribo-oxidation is fretting.