Wear From Wikipedia, the free encyclopedia
For other uses, see Wear (disambiguation). (disambiguation). [hide hide]] •
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e Mechanical failure modes
Buckling Corrosion Corrosion fatigue Creep Fatigue Fouling Fracture Hydrogen embrittlement Impact echanical overload !tress corrosion cracking "hermal shock Wear
#ielding
Wear is related to interactions between surfaces and specifically the removal and def ormation of material on a surface as a result of mechanical action of the opposite surface.[1] In materials science, science, wear is is erosion or sideways displacement of material from its "derivative" and original position on a solid surface surface performed performed by the action of another surface. Wear of metals of metals occurs by the plastic displacement of surface and near-surface material and by the detachment of particles that form wear debris. he si!e of the g enerated particles may vary from millimeter range down to an ion range range..[] his process may occur by contact with other metals, nonmetallic solids, flowing li#uids, or solid particles or li#uid droplets entrained in flowing gasses.[$] Wear can also be defined as a process where interaction between two surfaces or bounding faces of solids within the wor%ing environment results in dimensional loss of one s olid, with or without any actual decoupling and a nd loss of material. &spects of the wor%ing environment which affect wear include loads and feat ures such as unidirectional sliding, reciprocating, rolling, and impact loads, speed, temperature, but also different types of counter-bodies such as solid, li#uid li#uid or or gas gas and and type of contact ranging between single phase or multiphase, in which the last multiphase may combine li#uid with solid particles and gas bubbles. [']
(ear sproc%et for a bicycle where the left is unused and the right has obvious wear from cloc%wise rotation.
Contents [hide] •
1)tages of wear
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ypes .1 &dhesive wear wear
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.1. . 1.1 1*riccohesity
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. &brasive wear wear
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.$)urface fatigue
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.'*retting wear
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.+rosive wear
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.orrosion and o/idation wear
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$esting and valuation
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')ee also
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+(eferences o
+.10eneral references *urther reading
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Stages of wear[edit edit]] nder normal mechanical and practical procedures, the wear-rate normally changes through three different stages2ref.'34 •
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5rimary stage or early run-in period, where surfaces adapt to each other and the wearrate might vary between high and a nd low. )econdary stage or mid-age process, where a steady rate of ageing is in motion. 6ost of the components operational life is comprised in this stage.
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ertiary stage or old-age period, where the components are sub7ected to rapid failure due ertiary to a high rate of ageing.
he secondary stage is shortened with increasing severity of environmental conditions such as higher temperatures, strain rates, stress and sliding velocities etc. 8ote that, wear rate is strongl y influenced by the operating conditions. )pecifically, normal loads and sliding speeds play a pivotal role in determining wear rate. In addition, tribo-chemical reaction is also important in order to understand the wear behavior. 9ifferent o/ide layers are developed during the sliding motion. he layers are originated from comple/ interaction among surface, lubricants, and environmental molecules. In general, a single plot, namely wear map. demonstrating wear rate under different loading condition is used for operation. his graph also represents dominating wear modes under different loading conditions 2ref. 1$3. In e/plicit 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. )urface engineering and engineering and treatments are used to [+][] ] minimi!e wear and e/tend the components wor%ing life. [+][
edit]] Types[edit he study of the processes of wear is part of the discipline of tribology tribology.. he comple/ nature of wear has delayed its investigations and resulted in isolated studies towards specific wear mechanisms or processes.[:] )ome commonly referred to wear mechanisms 2or processes3 include4 1. Adhesive wear . Abrasive wear $. Surface fatigue '. Fretting wear +. Erosive wear . Corrosion and oxidation wear & number of different different wear processes are also commonly encountered and presented in the ['] literature. Impact-, cavitation-, diffusive- and corrosive- wear are all such e/amples. hese wear mechanisms, however, do not necessarily act independently and wear mechanisms are not mutually e/clusive.[;] "Industrial Wear" are commonly described as incidence of multiple wear mechanisms occurring in unison. ¬her way to describe "Industrial Wear" is to define clear distinctions in how different friction mechanisms operate, for e/ample distinguish between mechanisms with high or low energy density. Wear mechanisms and
Adhesive wear [edit edit]] &dhesive wear can be found between surfaces during frictional frictional contact contact and generally refers to unwanted displacement and attachment of wear debris and material compounds from one surface to another. wo wo separate mechanisms operate between the surfaces. Friccohesity[edit edit]] *riccohesity defines actual changes in cohesive forces and their reproduction in form of %inetic or frictional forces in li#uid when the clustering of the nano-particles scatter in medium for ma%ing smaller cluster or aggregates of different nanometer levels. he friccohesity basically is originated out of similar and dissimilar dipolar vectori!ation in li#uid states. )ince similar dipoles
li%e water, water dipole develop their own patterns of stabili!ation so are called similar vectors leading to have specific force of attraction among them at surface. )imilarly, the dissimilar vectors do have another patterns p atterns of optimi!ation so their e/pression different with similar forces noted as cohesive forces along with e/ohesive 2e/trovert3forces leading to have distribution. 1. &dhesive wear is is caused by relative motion, "direct contact" and plastic deformation which create wear debris and material transfer from one surface to another. In case of thermodynamic systems the friccohesity plays a master role and quantitatively explain a dissolution work to be undertaken or materialied by putting cohesive energy of solvent into distributing energy of mixtures allowing solute molecules to be distributed among the solvent phase with higher entropy! "he solvent with higher cohesive forces gain higher potential energy and lea st entropy! . ohesive ohesive adhesive adhesive forces, holds holds two surfaces surfaces together together even though though they are separated separated by a measurable distance, with or without any actual transfer of material. he above description and distinction between "Adhesive "Adhesive wear " and its ounterpart "cohesive "cohesive adhesive forces" forces" are #uite common. sually cohesive surface forces and adhesive energy potentials between surfaces are e /amined as a special field in physics ph ysics departments. he adhesive wear and material transfer due to direct contact and plastic deformation are e/amined in engineering science and in industrial research. wo aligned surfaces may always cause material transfer and due to overlaps and symbiotic relations between relative motional "wear" and "chemical" cohesive attraction, the wearcategori!ation have been a source for discussion. onse#uently, the definitions and nomenclature must evolve with the latest science and empiric observations. 0enerally, adhesive wear occurs when two bodies slide over or are pressed into each other, which promote material transfer. his can be described as plastic deformation of very s mall fragments within the surface layers. he asperities asperities or or microscopic high points or surface or surface roughness found roughness found on each surface, define the severity on how fragments of o/ides are pulled off and adds to the other surface, partly due to strong adhesive forces between atoms[1] but also due to accumulation of energy in the plastic !one between the asperities during relative motion. •
)6 micrograph of adhesive wear 2transferred materials3 on +1== steel sample sliding against &l alloy. 2>ellow arrow indicate sliding direction3
he outcome can be a growing roughening and creation of protrusions 2i.e., lumps3 above the original surface, in industrial manufacturing referred to as galling galling,, which eventually breaches the o/idi!ed surface layer and connects to the underlying bul% material which enhance the possibility for a stronger adhesion[?] and plastic flow around the lump. he geometry and the nominal sliding velocity of the lump defines how the flowing material will be transported and accelerated around the lump which is critical to define contact pressure and developed temperature during sliding. he mathematical function for acceleration of flowing material is thereby defined by the lumps surface contour. It@s clear, given these prere#uisites, that contact pressure and developed temperature is highly dependent on the lumps geometry. *low of material e/hibits an increase in energy density, because initial phase transformation and displacement of material demand acceleration of material and high pressure. Aow pressure is not compatible with plastic flow, only after deceleration may the flowing material
be e/posed to low pressure and #uic%ly cooled. In other words, you can@t deform a solid material using direct contact without applying a high pressure and somewhere along the process must acceleration and deceleration ta%e place, i.e., high pressure must be applied on all sides of the deformed material. *lowing material will immediately e/hibit energy loss and reduced ability to flow due to phase transformation, if e7ected from high pressure into low pressure. his ability withholds the high pressure and energy density in the contact !one and decreases the amount of energy or friction force needed for further advancement when the sliding continues and partly e/plain the difference between the static and sliding coefficient of friction 2 friction 2# #3 if the main fracture mechanisms are e#ual to the previous. &dhesive wear is a common fault factor in industrial applications such as sheet metal forming 2)6*3 forming 2)6*3 and commonly encountered in con7unction with lubricant failures and are often referred to as welding wear or galling due to the e/hibited surface characteristics, phase transition and plastic flow followed by cooling. he 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". 6ost solids will adhere on contact to some e/tent. Bowever, o/idation films, lubricants and contaminants naturally occurring generally suppress adhesion.[1=] and spontaneous e/othermic chemical reactions between surfaces generally produce a substance with low energy status in the absorbed species.[?]
edit]] Abrasive wear [edit &brasive wear occurs when a hard rough surface slides across a softer surface.[1] &)6 International 2formerly &merican )ociety for esting esting and 6aterials3 defines de fines it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface.[11] &brasive wear is commonly classified according to the type of contact and the contact environment.[1] he type of contact determines the mode of a brasive wear. he two modes of abrasive wear are %nown as two-body and three-body abrasive wear. wo-body wo-body wear occurs when the grits or hard particles p articles remove material from the opposite surface. he common ana logy is that of material being removed or displaced by a cutting or plowing operation. hree-body wear occurs when the particles are not constrained, and are free to roll and slide down a surface. he contact environment determines whether the wear is classified as open or closed. &n open contact environment occurs when the surfaces are sufficiently displaced to be independent of one another •
9eep @groove@ li%e surface indicates abrasive wear over cast iron 2yellow arrow indicate sliding direction3
here are a number of factors which influence abrasive wear and hence the manner of material removal. )everal different mechanisms have been proposed to describe the manner in which the material is removed. hree commonly identified mechanisms of abrasive wear are4 1. 5lowing . utting $. *r *rag agme ment ntat atio ion n 5lowing occurs when material is displaced to t he side, away from the wear particles, p articles, resulting in the formation of grooves that do not involve direct material removal. he displaced material forms ridges ad7acent to grooves, which may be removed by subse#uent passage of abrasive particles. utting occurs when material is separated from the surface in the form of primary debris, or microchips, with little or no material displaced to the sides of the grooves. his mechanism closely resembles conventional machining. *ragmentation occurs when material is separated from a surface by a cutting process and the indenting abrasive causes locali!ed fracture of the wear material. hese crac%s then freely propagate locally around the wear groove, resulting in additional material removal by spalling. [1]
&brasive wear can be measured as loss of mass by the aber aber &brasion est est according to I)C ?$+ or &)6 9 '==.
Surface fatigue [edit edit]] Main article: Fatigue (material) )urface fatigue is a process by which the surface of a material is wea%ened by cyclic loading, which is one type of general material fatigue. *atigue wear is produced when the wear particles are detached by cyclic crac% growth of microcrac%s on the surface. hese microcrac%s are either superficial crac%s or subsurface crac%s.
edit]] Fretting wear [edit Main article: Fretting *retting wear is the repeated cyclical rubbing between two surfaces. Cver 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. ¬her problem occurs when crac%s in either surface are created, %nown as fretting fatigue. It is the more serious of the two phenomena because it can lead to catastrophic failure of the bearing. &n associated problem occurs when the small particles removed by wear are o/idised in air. he o/ides are usually harder than the underlying metal, so wear accelerates as the harder particles abrade the metal surfaces further. *retting corrosion acts in the same way, especially when water is present. nprotected bearings on large structures li%e bridges can suffer serious degradation in behaviour, especially when salt is used the during winter to deice the highways carried by the bridges. he problem of fretting corrosion was involved in the )ilver Dridge tragedy Dridge tragedy and the 6ianus (iver Dridge accident. Dridge accident.
Erosive wear [edit edit]] rosive wear can be defined as an e/tremely short sliding motion and is e/ecuted within a short time interval. rosive wear is caused by the impact of particles of solid or li#uid against the surface of an ob7ect. ob7ect.[1=] he impacting particles gradually remove material from the surface through repeated deformations and cutting actions.[1$] It is a widely encountered mechanism in industry. 9ue to the nature of the conveying process, piping systems are prone to wear when abrasive particles have to be transported.[1'] he rate of erosive wear is dependent upon a number of factors. he 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. he impingement angle is one of the most important factors and is widely recogni!ed in literature. [1+] *or ductile materials the ma/imum wear rate is found when the impingement angle is appro/imately $=E, whilst for non ductile materials the ma/imum wear rate occurs when the impingement angle is normal to the surface.[1+]
Corrosion and oxidation wear [edit edit]] his %ind of wear occur in a variety of situations both in lubricated and unlubricated contacts. he fundamental cause of these forms of wear is chemical reaction between the worn material and the corroding medium. medium.[1] his %ind of wear is a mi/ture of corrosion, wear and the s ynergistic term of corrosion-wear which is also called tribocorrosion tribocorrosion..
edit]] Testing and Evaluation [edit )everal standard test methods e/ist for different types of wear to determine the amount of material removal during a specified time period under well-defined conditions. he &)6 he &)6 International ommittee International ommittee 0- attempts to standardise wear testing for specific applications, which are periodically updated. he )ociety for ribology and Aubrication ngineers 2)A3 has documented a large number of frictional wear and lubrication tests. Dut all test methods have inbuilt limitations and do not give a true picture in every aspect. his can be attributed to the comple/ nature of wear, in particular "industrial wear", and the
difficulties associated with accurately simulating wear processes.2ref'3 &n attrition test is test is a test is carried out to measure the resistance of a granular material to wear. & standard result review for wear wear tests, defined by the &)6 the &)6 International and International and respective subcommittees such as ommittee 0-, should be e/pressed as loss of material during wear in terms of volume. he volume loss gives a truer picture than weight loss, particularly when comparing the wear resistance properties of materials with large differences in density density.. *or e/ample, a weight loss of 1' g in a sample of tungsten carbide F carbide F cobalt cobalt 2density 2density G 1'=== %g
See also[edit edit]]
