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Wear
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In materials science, wear is the erosion of material from a solid surface by the action of another substance. The study of the processes of wear is part of the discipline of tribology.
There are five principal wear processes:
- Adhesive wear
- Abrasive wear
- Surface fatigue
- Fretting wear
- Erosion wear
The definition of wear does not include loss of dimension from plastic deformation, although wear has occurred despite no material removal.
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Encyclopedia
In materials science, wear is the erosion of material from a solid surface by the action of another substance. The study of the processes of wear is part of the discipline of tribology.
There are five principal wear processes:
- Adhesive wear
- Abrasive wear
- Surface fatigue
- Fretting wear
- Erosion wear
The definition of wear does not include loss of dimension from plastic deformation, although wear has occurred despite no material removal. This definition also fails to include impact wear, where there is no sliding motion, cavitation, where the counterbody is a fluid, and corrosion, where the damage is due to chemical rather than mechanical action.
Wear can also be defined as a process in which interaction of the surfaces or bounding faces of a solid with its working environment results in dimensional loss of the solid, with or without loss of material.
Aspects of the working environment which affect wear include loads (such as unidirectional sliding, reciprocating, rolling, and impact loads), speed, temperature, type of counterbody (solid, liquid, or gas), and type of contact (single phase or multiphase, in which the phases involved can be liquid plus solid particles plus gas bubbles).
In the results of standard wear tests (such as those formulated by the respective subcommittees of ASTM Committee G-2), the loss of material during wear is expressed in terms of volume. The volume loss gives a truer picture than weight loss, particularly when comparing the wear resistance properties of materials with large differences in density. For example, a weight loss of 14 g in a sample of tungsten carbide + cobalt (density = 14000 kg/m³) and a weight loss of 2.7 g in a similar sample of aluminium alloy (density = 2700 kg/m³) both result in the same level of wear (1 cm³) when expressed as a volume loss.
The working life of an engineering component is over when dimensional losses exceed the specified tolerance limits. Wear, along with other aging processes such as fatigue, creep, and fracture toughness, causes progressive degradation of materials with time, leading to failure of material at an advanced age. Under normal operating parameters, the property changes during usage normally occur in three different stages as follows:-
- Primary or early stage or run-in period, where rate of change can be high.
- Secondary or mid-age process where a steady rate of aging process is maintained. Most of the useful or working life of the component is comprised in this stage.
- Tertiary or old-age stage, where a high rate of aging leads to rapid failure.
With increasing severity of environmental conditions such as higher temperatures, strain rates, stress and sliding velocities, the secondary stage is shortened and the primary stage tends to merge with the tertiary stage, thus drastically reducing the working life. Surface engineering processes are used to minimize wear and extend working life of material.
Adhesive wear
Adhesive wear is also known as scoring, galling, or seizing. It occurs when two solid surfaces slide over one another under pressure. Surface projections, or asperities, are plastically deformed and eventually welded together by the high local pressure. As sliding continues, these bonds are broken, producing cavities on the surface, projections on the second surface, and frequently tiny, abrasive particles, all of which contribute to future wear of surfaces.
Abrasive wear
When material is removed by contact with hard particles, abrasive wear occurs. The particles either may be present at the surface of a second material (two-body wear) or may exist as loose particles between two surfaces (three-body wear).
Abrasive wear can be measured as loss of mass by the Taber Abrasion Test according to ISO 9352 or ASTM D 1044.
Surface fatigue
Surface fatigue is a process by which the surface of a material is weakened by cyclic loading, which is one type of general material fatigue.
Fretting wear
Fretting wear is the repeated cyclical rubbing between two surfaces, which is known as fretting, over a period of time which will remove material from one or both surfaces in contact. It occurs typically in bearings, although most bearings have their surfaces hardened to resist the problem. Another problem occurs when cracks in either surface are created, known as fretting fatigue. It is the more serious of the two phenomena because it can lead to catastrophic failure of the bearing. An associated problem occurs when the small particles removed by wear are oxidised in air. The oxides are usually harder than the underlying metal, so wear accelerates as the harder particles abrade the metal surfaces further. Fretting corrosion acts in the same way, especially when water is present. Unprotected bearings on large structures like bridges can suffer serious degradation in behaviour, especially when salt is used during winter to deice the highways carried by the bridges. The problem of fretting corrosion was involved in the Silver Bridge tragedy and the Mianus River Bridge accident.
See also
Further reading
- Bowden, Tabor: Friction and Lubrication of Solids (Oxford:Clarendon Press 1950)
- Rabinowicz, Ernest: Friction and Wear of Materials. Wiley-Interscience, 1995, ISBN-10: 0471830844.
- Kleis I. and Kulu P.: Solid Particle Erosion. Springer-Verlag, London, 2008, 206 pp.
- Zum Gahr K.-H.: Microstructure and wear of materials, Elsevier, Amsterdam, 1987, 560 S.
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