Wear

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Mechanical failure modes
Buckling
Corrosion
Creep
Fatigue
Fracture
Impact
Mechanical overload
Rupture
Thermal shock
Wear
Yielding
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This article may be too long to read and navigate comfortably. Please consider splitting content into sub-articles and using this article for a summary of the key points of the subject. (September 2009)

In materials science, wear is the erosion of material from a solid surface by the action of another surface. It is related to surface interactions and more specifically the removal of material from a surface as a result of mechanical action.[1] The need for mechanical action, in the form of contact due to relative motion, is an important distinction between mechanical wear and other processes with similar outcomes.[2]

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).

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[edit] Measurement

There is no specific standard for testing or measuring a materials wear resistance. This can be attributed to the complex nature of wear, in particular "industrial wear", and the difficulties associated with accurately simulating wear processes. A number of wear tests have been developed by committees in an attempt to standardise wear testing for specific applications. 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. Wear is one of a limited number of manners in which a material object losses its usefulness. The economic implications can be of enormous value to industry.

[edit] Stages of wear

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. [3][4]


[edit] Types of wear

The study of the processes of wear is part of the discipline of tribology. The complex nature of wear has delayed its investigations and resulted in isolated studies towards specific wear mechanisms or processes.[5] Some commonly referred to wear mechanisms (or processes) include:

  1. Adhesive wear
  2. Abrasive wear
  3. Surface fatigue
  4. Fretting wear
  5. Erosive wear

A number of different wear phenomena are also commonly encountered and represented in literature. Impact wear, cavitation wear, diffusive wear and corrosive wear are all such examples.

These wear mechanisms, however, do not necessarily act independently in many applications. Wear mechanisms are not mutually exclusive.[2] "Industrial Wear" is the term used to describe the incidence of multiple wear mechanisms occurring in unison. Wear mechanisms and/or sub-mechanisms frequently overlap and occur in a synergistic manner, producing a greater rate of wear than the sum of the individual wear mechanisms.

[edit] Adhesive wear

There are two types of adhesive friction.

  1. Cohesive adhesive forces, holds two surfaces together even though they are separated by a distance. (i.e., atom/atom or cluster interaction)
  2. Adhesive wear, material transfer from one surface to another cased by direct contact and plastic deformation.

Adhesive wear occurs when two bodies slides over each other, or are pressed into one another, which promote material transfer between the two surfaces.

However, material transfer is always present when to surfaces are aligned against each other for a certain amount of time and the wear-categorization and the cause for material transfer have been a source for discussion and argumentation amongst researchers around the world for quite some time and there are frequent misinterpretations, misunderstandings due to overlaps and symbiotic relations between mechanisms as previously mentioned.

The above description and distinction between "cohesive" adhesive forces and it´s counterpart, such as adhesive "wear" are quite common and usually goes for most researchers in engineering science and physics.


Having described the restriction on the subject wear, we can focus on what causes material transfer.

Adhesive wear can be described as plastic deformation of very small fragments within the surface layer when two surfaces slides against each other.

The asperities (i.e., microscopic high points) found on the mating surfaces will penetrate the opposing surface and develop a plastic zone around the penetrating asperity.

Dependent on the surface roughness and depth of penetration will the asperity cause damage on the oxide surface layer or even the underlying bulk material. In initial asperity/asperity contact, fragments of one surface are pulled off and adhere to the other, due to the strong adhesive forces between atoms.[1] It is thereby clear that physical-chemical adhesive interaction between the surfaces plays a role in the initial build up process but the energy absorbed in plastic deformation and movement is the main cause for material transfer and wear.


The outcome can be a growing roughening and creation of protrusions (i.e., lumps) above the original surface. If the lump grows to a certain hight it will penetrate deep,(several microns), down in to the bulk material and create a plastically flowing deformed volume around it. The geometry and the nominal sliding velocity of the lump defines how the flowing material will be transported, (accelerated) around the lump and is critical for defining the lumps contact pressure and developed temperature during sliding. The mathematical function or curve for acceleration of flowing material is thereby defined by the lumps surface contour.


Given these prerequisites it´s clear that contact pressure and developed temperature is highly dependent on the lumps geometry and it´s also clear that the flowing material exhibits an increase in energy content per volume unit because the initial acceleration doesn't allow low pressure. Only after deceleration can the flowing material be exposed to low pressure and be quickly cooled. In other words, you can´t deform a material white-out applying a high pressure. And somewhere along the process must acceleration and deceleration take place, i.e., high pressure must be applied on all sides of the deformed material. However, it is possible that the needed tight enclosure exhibits small gaps or openings but flowing material will immediately exhibit energy loss and reduced ability to flow if ejected. In other words the thigh enclosure can be withhold and the compressive pressure gives potential for further deformation when the sliding continues. Some researchers might argue that instantaneous low or negative pressure drives the deformation but these two features are not possible without a strong attraction or the corresponding high pressure in the initial process described above.


Adhesive wear is the most common form of wear and is commonly encountered in conjunction with lubricant failures. It is commonly referred to as welding wear due to the exhibited surface characteristics. In engineering science, adhesive wear is usually referred to as galling and is common in sheet metal forming (SMF) and other industrial applications.

The tendency of contacting surfaces to adhere arises from the attractive forces that exist between the surface atoms of the two materials. The type and mechanism of attraction varies between different materials. Most solids will adhere on contact to some extent, however, oxidation films and contaminants naturally occurring, generally suppress adhesion.[6] Surfaces also generally have low energy states due to reacted and absorbed species.[7]

The mechanism of adhesive wear occurs due to contact possibly producing surface plastic flow, scraping off soft surface films or breaking up and removing oxide layers. This brings clean regions into contact and introduces the possibility of strong adhesion.[7] The removal of material, or wear, takes the form of small particles. These small particles are usually transferred to the other surface but may come off in loose form.

[edit] Abrasive wear

Abrasive wear occurs when a hard rough surface slides across a softer surface.[1] ASTM (American Society for Testing and Materials) define it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface.[8]

Abrasive wear is commonly classified according to the type of contact and the contact environment.[9] The type of contact determines the mode of abrasive wear. The two modes of abrasive wear are known as two-body and three-body abrasive wear. Two-body wear occurs when the grits, or hard particles, are rigidly mounted or adhere to a surface, when they remove the material from the surface. The common analogy is that of material being removed with sand paper. Three-body wear occurs when the particles are not constrained, and are free to roll and slide down a surface. The contact environment determines whether the wear is classified as open or closed. An open contact environment occurs when the surfaces are sufficiently displaced to be independent of one another

There are a number of factors which influence abrasive wear and hence the manner of material removal. Several different mechanisms have been proposed to describe the manner in which the material is remove. Three commonly identified mechanisms of abrasive wear are:

  1. Plowing
  2. Cutting
  3. Fragmentation

Plowing occurs when material is displaced to the side, away from the wear particles, resulting in the formation of grooves that do not involve direct material removal. The displaced material forms ridges adjacent to grooves, which may be removed by subsequent passage of abrasive particles. Cutting occurs when material is separated from the surface in the form of primary debris, or microchips, with little or no material displaced to the sides of the grooves. This mechanism closely resembles conventional machining. Fragmentation occurs when material is separated from a surface by a cutting process and the indenting abrasive causes localized fracture of the wear material. These cracks then freely propagate locally around the wear groove, resulting in additional material removal by spalling.[9]

Abrasive wear can be measured as loss of mass by the Taber Abrasion Test according to ISO 9352 or ASTM D 1044.

[edit] Surface fatigue

Main article: Fatigue (material)

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.

[edit] Fretting wear

Main article: Fretting

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.

[edit] Erosive wear

Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object.[6] The impacting particles gradually remove material from the surface through repeated deformations and cutting actions.[10] It is a widely encountered mechanism in industry. A common example is the erosive wear associated with the movement of slurries through piping and pumping equipment.

The rate of erosive wear is dependent upon a number of factors. The material characteristics of the particles, such as their shape, hardness, impact velocity and impingement angle are primary factors along with the properties of the surface being eroded. The impingement angle is one of the most important factors and is widely recognized in literature.[11] For ductile materials the maximum wear rate is found when the impingement angle is approximately 30o, whilst for non ductile materials the maximum wear rate occurs when the impingement angle is normal to the surface.[11]

[edit] See also

[edit] References

  1. ^ a b c Rabinowicz, E. (1995). Friction and Wear of Materials. New York, John Wiley and Sons.
  2. ^ a b Williams, J. A. (2005). "Wear and wear particles - Some fundamentals." Tribology International 38(10): 863-870
  3. ^ Chattopadhyay, R. (2001). Surface Wear - Analysis, Treatment, and Prevention. OH, USA: ASM-International. ISBN 0-87170-702-0. 
  4. ^ Chattopadhyay, R. (2004). Advanced Thermally Assisted Surface Engineering Processes. MA, USA: Kluwer Academic Publishers. ISBN 1-4020-7696-7. 
  5. ^ Jones, M., H., and D. Scott, Eds. (1983). Industrial Tribology: the practical aspects of friction, lubrication, and wear. New York, Elsevier Scientific Publishing Company.
  6. ^ a b Stachowiak, G. W., and A. W. Batchelor (2005). Engineering Tribology. Burlington, Elsevier Butterworth-Heinemann
  7. ^ a b Glaeser, W. A., Ed. (1993). Characterization of Tribological Materials. Materials Characterization Series. Boston, Butterworth-Heinemann.
  8. ^ Standard Terminology Relating to Wear and Erosion, Annual Book of Standards, Vol 03.02, ASTM, 1987, p 243-250
  9. ^ a b ASM Handbook Committee (2002). ASM Handbook. Friction, Lubrication and Wear Technology. U.S.A., ASM International. Volume 18.
  10. ^ Mamata, K. P. (2008). "A review on silt erosion in hydro turbines." Renewable & sustainable energy reviews 12(7): 1974.
  11. ^ a b Sinmazcelik, T. and I. Taskiran (2007). "Erosive wear behaviour of polyphenylenesulphide (PPS) composites." Materials in engineering 28(9): 2471-2477.

[edit] Further reading

  • Bowden, Tabor: Friction and Lubrication of Solids (Oxford:Clarendon Press 1950)
  • 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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