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Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g.,
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
) or
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...
(e.g.,
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
). The study of wear and related processes is referred to as
tribology Tribology is the science and engineering of interacting surfaces in relative Motion (physics), motion. It includes the study and application of the principles of friction, lubrication and wear. Tribology is highly interdisciplinary, drawing on m ...
. Wear in
machine element Machine element or hardware refers to an elementary component of a machine. These elements consist of three basic types: # '' structural components'' such as frame members, bearings, axles, splines, fasteners, seals, and lubricants, # '' mech ...
s, together with other processes such as
fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
and creep, causes functional surfaces to degrade, eventually leading to material failure or loss of functionality. Thus, wear has large economic relevance as first outlined in the Jost Report. Abrasive wear alone has been estimated to cost 1-4% of the gross national product of industrialized nations. Wear of
metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
occurs by plastic displacement of surface and near-surface material and by detachment of particles that form wear debris. The particle size may vary from millimeters to
nanometers 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
. This process may occur by contact with other metals, nonmetallic solids, flowing liquids, solid particles or liquid droplets entrained in flowing gasses. The wear rate is affected by factors such as type of loading (e.g., impact, static, dynamic), type of
motion In physics, motion is the phenomenon in which an object changes its position with respect to time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed and frame of reference to an observer and mea ...
(e.g., sliding,
rolling Rolling is a type of motion that combines rotation (commonly, of an axially symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that, if ideal conditions exist, the two are in contact ...
),
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
, and
lubrication Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology. Lubrication mechanisms such as fluid-lubric ...
, in particular by the process of deposition and wearing out of the boundary lubrication layer. Depending on the
tribosystem A tribosystem is a tribological system that consists of at least two contacting bodies and any environmental factor that affects their interaction. Tribologists study tribological systems in detail, and devise tribological test procedures. Defin ...
, different wear types and wear mechanisms can be observed.


Wear types and mechanisms

Types of wear are identified by relative
motion In physics, motion is the phenomenon in which an object changes its position with respect to time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed and frame of reference to an observer and mea ...
, the nature of disturbance at the worn surface or "mechanism", and whether it effects a self regenerative or base layer. Wear mechanisms are the physical disturbance. For example, the mechanism of adhesive wear is
adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be ...
. 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.Williams, J. A. (2005). "Wear and wear particles - Some fundamentals." Tribology International 38(10): 863-870


Adhesive wear

Adhesive wear can be found between surfaces during
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
al contact and generally refers to unwanted displacement and attachment of wear debris and material compounds from one surface to another. Two adhesive wear types can be distinguished: #Adhesive wear is caused by relative motion, "direct contact" and plastic deformation which create wear debris and material transfer from one surface to another. #Cohesive adhesive forces, holds two surfaces together even though they are separated by a measurable distance, with or without any actual transfer of material. Generally, adhesive wear occurs when two bodies slide over or are pressed into each other, which promote material transfer. This can be described as plastic deformation of very small fragments within the surface layers. The
asperities In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness" (from the Latin ''asper''—"rough"), has implications (for example) in physics and seismology. Smooth surfaces, even those polished to a mirror finish, a ...
or microscopic high points ( surface roughness) found on each surface affect the severity of how fragments of oxides are pulled off and added to the other surface, partly due to strong adhesive forces between atoms, but also due to accumulation of energy in the
plastic zone Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics t ...
between the asperities during relative motion. The type of mechanism and the amplitude of surface attraction varies between different materials but are amplified by an increase in the density of "surface energy". Most solids will adhere on contact to some extent. However, oxidation films, lubricants and contaminants naturally occurring generally suppress adhesion,Stachowiak, G. W., and A. W. Batchelor (2005). Engineering Tribology. Burlington, Elsevier Butterworth-Heinemann and spontaneous exothermic chemical reactions between surfaces generally produce a substance with low energy status in the absorbed species.Glaeser, W. A., Ed. (1993). Adhesive wear can lead to an increase in roughness and the creation of protrusions (i.e., lumps) above the original surface. In industrial manufacturing, this is referred to as
galling Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is cau ...
, which eventually breaches the oxidized surface layer and connects to the underlying bulk material, enhancing the possibility for a stronger adhesion and plastic flow around the lump. A simple model for the wear volume for adhesive wear, V, can be described by: V = K\frac where W is the load, K is the wear coefficient, L is the sliding distance, and H_v is the hardness.


Abrasive wear

Abrasive wear occurs when a hard rough surface slides across a softer surface.Rabinowicz, E. (1995). Friction and Wear of Materials. New York, John Wiley and Sons.
ASTM International ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, ...
defines it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface. Abrasive wear is commonly classified according to the type of contact and the contact environment.ASM Handbook Committee (2002). ASM Handbook. Friction, Lubrication and Wear Technology. U.S.A., ASM International. Volume 18. 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 remove material from the opposite surface. The common analogy is that of material being removed or displaced by a cutting or plowing operation. 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 removed. Three commonly identified mechanisms of abrasive wear are: # Plowing # Cutting # 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
spall Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ba ...
ing. Abrasive wear can be measured as loss of mass by the Taber Abrasion Test according to ISO 9352 or ASTM D 4060. The wear volume for single-abrasive wear, V, can be described by: V = \alpha\beta\frac = K\frac where W is the load, \alpha is the shape factor of an asperity (typically ~ 0.1), \beta is the degrees of wear by an asperity (typically 0.1 to 1.0), K is the wear coefficient, L is the sliding distance, and H_v is the hardness.


Surface fatigue

Surface fatigue is a process in which the surface of a material is weakened by cyclic loading, which is one type of general material fatigue. Fatigue wear is produced when the wear particles are detached by cyclic crack growth of microcracks on the surface. These microcracks are either superficial cracks or subsurface cracks.


Fretting wear

Fretting wear is the repeated cyclical rubbing between two surfaces. Over a period of time fretting 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 oxidized 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 the during winter to deice the highways carried by the bridges. The problem of fretting corrosion was involved in the
Silver Bridge The Silver Bridge was an eyebar-chain suspension bridge built in 1928 and named for the color of its aluminum paint. The bridge carried U.S. Route 35 over the Ohio River, connecting Point Pleasant, West Virginia, and Gallipolis, Ohio. On Dec ...
tragedy and the Mianus River Bridge accident.


Erosive wear

Erosive wear can be defined as an extremely short sliding motion and is executed within a short time interval. Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object. The impacting particles gradually remove material from the surface through repeated deformations and cutting actions. It is a widely encountered mechanism in industry. Due to the nature of the conveying process, piping systems are prone to wear when abrasive particles have to be transported. 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.Sinmazcelik, T. and I. Taskiran (2007). "Erosive wear behaviour of polyphenylenesulphide (PPS) composites." Materials in engineering 28(9): 2471-2477. For ductile materials, the maximum wear rate is found when the impingement angle is approximately 30°, whilst for non-ductile materials the maximum wear rate occurs when the impingement angle is normal to the surface. A detailed theoretical analysis of dependency of the erosive wear on the inclination angle and material properties is provided in. For a given particle morphology, the erosion rate, E, can be fit with a power law dependence on velocity: E = kv^n where k is a constant, v is velocity, and n is a velocity exponent. n is typically between 2 - 2.5 for metals and 2.5 - 3 for ceramics.


Corrosion and oxidation wear

Corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
and
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
wear occurs both in lubricated and dry contacts. The fundamental cause are chemical reactions between the worn material and the corroding medium. Wear caused by a synergistic action of tribological stresses and corrosion is also called
tribocorrosion Tribocorrosion is a material degradation process due to the combined effect of corrosion and wear. The name tribocorrosion expresses the underlying disciplines of tribology and corrosion. Tribology is concerned with the study of friction, lubricati ...
.


Other Types of Wear

Other, less common types of wear are impact, cavitation, and diffusive wear.


Wear stages

Under nominal operation conditions, the wear rate normally changes in three different stages: *Primary stage or early run-in period, where surfaces adapt to each other and the wear-rate might vary between high and low. *Secondary stage or mid-age process, where steady wear can be observed. Most of the component's operational life is spent in this stage. *Tertiary stage or old-age period, where surfaces are subjected to rapid failure due to a high rate of wear. Note that the wear rate is strongly influenced by the operating conditions and the formation of
tribofilm Tribofilms (boundary lubricant films, boundary lubricating films, tribo-boundary films or boundary films) are films that form on tribologically stressed surfaces. Tribofilms are mostly solid surface films that result from a chemical reaction of lu ...
s. The secondary stage is shortened with increasing severity of environmental conditions, such as high temperatures, strain rates and stresses. So-called wear maps, demonstrating wear rate under different operation condition, are used to determine stable operation points for tribological contacts. Wear maps also show dominating wear modes under different loading conditions. In explicit wear tests simulating industrial conditions between metallic surfaces, there are no clear chronological distinction between different wear-stages due to big overlaps and symbiotic relations between various friction mechanisms.
Surface engineering Surface engineering is the sub-discipline of materials science which deals with the surface of solid matter. It has applications to chemistry, mechanical engineering, and electrical engineering (particularly in relation to semiconductor manufact ...
and treatments are used to minimize wear and extend the components working life.


Wear testing

Several standard test methods exist for different types of wear to determine the amount of material removal during a specified time period under well-defined conditions.
ASTM International ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, ...
Committee G-2 standardizes wear testing for specific applications, which are periodically updated. The Society for Tribology and Lubrication Engineers (STLE) has documented a large number of frictional, wear and lubrication tests. Standardized wear tests are used to create comparative material rankings for a specific set of test parameter as stipulated in the test description. To obtain more accurate predictions of wear in industrial applications it is necessary to conduct wear testing under conditions simulating the exact wear process. An attrition test is a test that is carried out to measure the resistance of a granular material to wear.


Modeling of wear

The Reye–Archard–Khrushchov wear law is the classic wear prediction model.


Measuring wear


Wear coefficient

The wear coefficient is a physical coefficient used to measure, characterize and correlate the wear of materials.


Lubricant analysis

Lubricant A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...
analysis is an alternative, indirect way of measuring wear. Here, wear is detected by the presence of wear particles in a liquid lubricant. To gain further insights into the nature of the particles, chemical (such as XRF, ICP-OES), structural (such as
ferrography Ferrography is a specialized type of oil analysis used to study particle wear on machine components through analysis of contaminants in lubricating oil. It can be used to predict and diagnose errors occurring on machinery. Ferrography is related to ...
) or optical analysis (such as
light microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...
) can be performed.


See also

* * * * — Equipment used to measure friction and wear * * * *


References


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 pp. * Jones J. R.:''Lubrication, Friction, and Wear'', NASA-SP-8063, 1971, 75 pp. A nice, free and good document availabl
here
* S. C. Lim. Recent Development in Wear Mechanism Maps. Trib. Intl. 1998; 31; 87–97. * H.C. Meng and K. C Ludema. Wear 1995; 183; 443–457. * R. Bosman and D. J. Schipper. Wear 2012; 280; 54–62. * M. W. Akram, K. Polychronopoulou, A. A. Polycarpou. Trib. Int.: 2013; 57;9 2–100. * P. J. Blau, Tribosystem Analysis - A Practical Approach to the Diagnosis of Wear Problems. CRC Press, 2016.


External links



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