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Contents

ArticlesTribocorrosion 1

Tribology 2

Friction 6

Wear 17

Turbine 24

Turbine blade 30

Water turbine 34

Tungsten carbide 42

Titanium carbide 48

ReferencesArticle Sources and Contributors 50

Image Sources, Licenses and Contributors 52

Article LicensesLicense 53


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Tribocorrosion 1

TribocorrosionTribocorrosion is a material degradation process due to the combined effect of corrosion and wear.[1] The nametribocorrosion expresses the underlying disciplines of tribology and corrosion. Tribology is concerned with the studyof friction, lubrication and wear (its name comes from the Greek "tribo" meaning to rub) and corrosion is concernedwith the chemical and electrochemical interactions between a material, normally a metal, and its environment. As afield of research tribocorrosion is relatively new, but tribocorrosion phenomena have been around ever sincemachines and installations are being used.

Wear is a mechanical material degradation process occurring on rubbing or impacting surfaces, while corrosioninvolves chemical or electrochemical reactions of the material. Corrosion may accelerate wear and wear mayaccelerate corrosion.[2] One then speaks of corrosion accelerated wear or wear accelerated corrosion. Both thesephenomena, as well as fretting corrosion (which results from small amplitude oscillations between contactingsurfaces) fall into the broader category of tribocorrosion. Erosion-corrosion is another tribocorrosion phenomenoninvolving mechanical and chemical effects: impacting particles or fluids erode a solid surface by abrasion, chippingor fatigue while simultaneously the surface corrodes.[3]

Phenomena in different engineering fieldsTribocorrosion occurs in many engineering fields. It reduces the life-time of pipes, valves and pumps, of wasteincinerators, of mining equipment or of medical implants, and it can affect the safety of nuclear reactors or of transport systems. On the other hand, tribocorrosion phenomena can also be applied to good use, for example in thechemical-mechanical planarization of wafers in the electronics industry[4] or in metal grinding and cutting inpresence of aqueous emulsions. Keeping this in mind, we may define tribocorrosion in a more general wayindependently of the notion of usefulness or damage or of the particular type of mechanical interaction:Tribocorrosion concerns the irreversible transformation of materials or of their function as a result of simultaneous

mechanical and chemical/electrochemical interactions between surfaces in relative motion.

BiotribocorrosionBiotribocorrosion covers the science of surface transformations resulting from the interactions of mechanical loadingand chemical/electrochemical reactions that occur between elements of a tribological system exposed to biologicalenvironments.[5]It has been studied for aritificial joint prostheses. It is important to understand material degradationprocesses for joint implants to achieve longer service life and better safety issues for such devices.

Passive metalsWhile tribocorrosion phenomena may affect many materials, they are most critical for metals, especially thenormally corrosion resistant so-called passive metals. The vast majority of corrosion resistant metals and alloys usedin engineering (stainless steels, titanium, aluminium etc.) fall into this category. These metals are thermodynamicallyunstable in the presence of oxygen or water and they derive their corrosion resistance from the presence at thesurface of a thin oxide film, called thepassive film , which acts as a protective barrier between the metal and itsenvironment.[6] Passive films are usually just a few atomic layers thick. Nevertheless, they can provide excellentcorrosion protection because if damaged accidentally they spontaneously self-heal by metal oxidation. However,when a metal surface is subjected to severe rubbing or to a stream of impacting particles the passive film damagebecomes continuous and extensive. The self-healing process may no longer be effective and in addition it requires ahigh rate of metal oxidation. In other words, the underlying metal will strongly corrode before the protective passivefilm is reformed, if at all. In such a case, the total material loss due to tribocorrosion will be much higher than thesum of wear and corrosion one would measure in experiments with the same metal where only wear or only
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Tribocorrosion 2

corrosion takes place. The example illustrates the fact that the rate of tribocorrosion is not simply the addition of therate of wear and the rate of corrosion but it is strongly affected by synergistic and antagonistic effects betweenmechanical and chemical mechanisms. To study such effects in the laboratory, one most often uses mechanical weartesting rigs which are equipped with an electrochemical cell.[7] This permits one to control independently themechanical and chemical parameters. For example, by imposing a given potential to the rubbing metal one cansimulate the oxidation potential of the environment and in addition, under certain conditions, the current flow is ameasure of the instantaneous corrosion rate. For a deeper understanding tribocorrosion experiments aresupplemented by detailed microscopic and analytical studies of the contacting surfaces.

At high temperatures, the more rapid generation of oxide due to a combination of temperature and tribological actionduring sliding wear can generate potentially wear resistant oxide layers known as 'glazes'. Under such circumstances,tribocorrosion can be used potentially in a beneficial way.

References[1][1] D. Landolt, Electrochemical and materials aspects of tribocorrosion systems, J. Physics D: Appl. Phys. 39, 1-7 (2006)[2][2] S. W. Watson, F. J. Friedersdorf, B. W. Madsen, S. D. Cramer, Wear 181-183, (1995) 476-484

[3][3] K. Sasaki, G.T. Burstein, Philosophical Magazine Letters, 80 (2000) 489-493[4][4] S. Tagella, A.K. Skder, A. Kumar,J. Electrochem. Soc. 151 (2004) G205[5] Y.Yan, Biotribocorrosion an appraisal of the time dependence of wear and corrosion interactions Part II: Surface analysis. Journal of

Physics D: Applied Physics. 39(2006) p.3206-3212[6][6] D. Landolt, Corrosion and Surface Chemistry of Metals, EPFL Press, Lausanne Switzerland 2007 p.227-274[7][7] S. Mischler, P. Ponthiaux, WEAR, 248 (2001) 211-225

TribologyTribology is the science and engineering of interacting surfaces in relative motion. It includes the study and

application of the principles of friction, lubrication and wear. Tribology is a branch of mechanical engineering andmaterials science.

EtymologyThe wordtribology derives from the Greek root- of the verb ,tribo , "I rub" in classic Greek; and thesuffix-logy from -,-logia "study of", "knowledge of".

It was coined by the British physicist David Tabor,[1]and also by Peter Jost in 1964, a lubrication expert who noticedthe problems with increasing friction on machines, and started the new discipline of tribology.[2]

FundamentalsThe tribological interactions of a solid surface's exposed face with interfacing materials and environment may resultin loss of material from the surface. The process leading to loss of material is known as "wear". Major types of wearinclude abrasion, friction (adhesion and cohesion), erosion, and corrosion. Wear can be minimized by modifying thesurface properties of solids by one or more of "surface engineering" processes (also called surface finishing) or byuse of lubricants (for frictional or adhesive wear).

Estimated direct and consequential annual loss to industries in the USA due to wear is approximately 1-2% of GDP.(Heinz, 1987). Engineered surfaces extend the working life of both original and recycled and resurfaced equipment,thus saving large sums of money and leading to conservation of material, energy and the environment.Methodologies to minimize wear include systematic approaches to diagnose the wear and to prescribe appropriatesolutions. Important methods include:
http://en.wikipedia.org/w/index.php?title=Lubricanthttp://en.wikipedia.org/w/index.php?title=Surface_finishinghttp://en.wikipedia.org/w/index.php?title=Surface_engineeringhttp://en.wikipedia.org/w/index.php?title=Corrosionhttp://en.wikipedia.org/w/index.php?title=Erosionhttp://en.wikipedia.org/w/index.php?title=Cohesion_%28chemistry%29http://en.wikipedia.org/w/index.php?title=Adhesionhttp://en.wikipedia.org/w/index.php?title=Abrasion_%28mechanical%29http://en.wikipedia.org/w/index.php?title=Peter_Josthttp://en.wikipedia.org/w/index.php?title=David_Taborhttp://en.wiktionary.org/wiki/-logiahttp://localhost/var/www/apps/conversion/tmp/scratch_8/%EE%BF%80http://en.wikipedia.org/w/index.php?title=-logyhttp://en.wiktionary.org/wiki/tribo-http://localhost/var/www/apps/conversion/tmp/scratch_8/%EE%BF%80http://en.wikipedia.org/w/index.php?title=Ancient_Greekhttp://en.wikipedia.org/w/index.php?title=Materials_sciencehttp://en.wikipedia.org/w/index.php?title=Mechanical_engineeringhttp://en.wikipedia.org/w/index.php?title=Lubricationhttp://en.wikipedia.org/w/index.php?title=Motion_%28physics%29http://en.wikipedia.org/w/index.php?title=Glaze_%28metallurgy%29


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Tribology 4

between them wore at a faster rate.

Stribeck curve

The "Stribeck curve" or "Stribeck Hersey curve" (named after Richard Stribeck,[9][10][11]who heavily documentedand established examples of it, and Mayo D. Hersey[12][13]), used to categorize the friction properties between two

surfaces, was developed in the first half of the 20th century. The research of Professor Richard Stribeck (1861 1950)was performed in Berlin at the Royal Prussian Technical Testing Institute (MPA, now BAM). Similar work waspreviously performed around 1885 by Prof. Adolf Martens (1850 1914) at the same Institute and in the mid-1870sby Dr. Robert H. Thurston[14][15]at the Stevens Institute of Technology in the U.S. Prof. Dr. Thurston was thereforeclose to establishing theStribeck curve, but he presented noStribeck-like graphs, as he evidently did not fullybelieve in the relevance of this dependency. Since that time theStribeck-curve has been a classic teaching elementin tribology classes.[4]

The graphs of friction force reported by Stribeck stem from a carefully conducted, wide-ranging series of experiments on journal bearings. Stribeck systematically studied the variation of friction between two liquidlubricated surfaces.[3] His results were presented on 5 December 1901 during a public session of the railway society

and published on 6 September 1902. They clearly showed the minimum value of friction as the demarcation betweenfull fluid-film lubrication and some solid asperity interactions. Stribeck studied different bearing materials and aspectratios D/L from 1:1 to 1:2. The maximum sliding speed was 4 m/s and the geometrical contact pressure was limitedto 5 MPa. (These operating conditions were related to railway wagon journal bearings.)

The reason why the form of the friction curve for liquid lubricated surfaces was later attributed to Stribeck, althoughboth Thurston and Martens achieved their results considerably earlier, (Martens even in the same organizationroughly 15 years before), may be because Stribeck published in the most important technical journal in Germany atthat time, Zeitschrift des Vereins Deutscher Ingenieure (VDI, Journal of German Mechanical Engineers). Martenspublished his resultsonly in the official journal of the Royal Prussian Technical Testing Institute, which has nowbecome BAM. The VDI journal, as one of the most important journals for engineers, provided wide access to thesedata and later colleagues rationalized the results into the three classical friction regimes. Thurston however, did nothave the experimental means to record a continuous graph of the coefficient of friction but only measured thefriction at discrete points; this may be the reason why the minimum in the coefficient of friction was not discoveredby him. Instead, Thurston's data did not indicate such a pronounced minimum of friction for a liquid lubricated journal bearing as was demonstrated by the graphs of Martens and Stribeck.

Jost Report

The termtribology became widely used following The Jost Report in 1966, in which huge sums of money werereported to have been lost in the UK annually due to the consequences of friction, wear and corrosion. As a result

several national centres for tribology were created in the UK. Since then the term has diffused into the internationalengineering field and many specialists now claim to be tribologists.

There are now numerous national and international societies, such as theSociety for Tribologists and Lubrication Engineers (STLE[16]) in the USA and the Institution of Mechanical Engineers' Tribology Group (IMechE TribologyGroup[17]) in the UK or the German Society for Tribology (Gesellschaft fr Tribologie, www.gft-ev.de).Most technical universities have a group working on tribology, often as part of their mechanical engineeringdepartments. The limitations in tribological interactions are however no longer mainly determined by mechanicaldesigns, but rather by material limitations so the discipline of tribology now counts at least as many materialsengineers, physicists and chemists as it does mechanical engineers.
http://en.wikipedia.org/w/index.php?title=Mechanical_engineeringhttp://www.imeche.org/knowledge/industries/tribology/overviewhttp://en.wikipedia.org/w/index.php?title=Institution_of_Mechanical_Engineershttp://www.stle.org/http://en.wikipedia.org/w/index.php?title=United_Kingdomhttp://en.wikipedia.org/w/index.php?title=The_Jost_Reporthttp://en.wikipedia.org/w/index.php?title=Royal_Prussian_Technical_Testing_Institutehttp://en.wikipedia.org/w/index.php?title=Zeitschrift_des_Vereins_Deutscher_Ingenieurehttp://en.wikipedia.org/w/index.php?title=Journal_bearinghttp://en.wikipedia.org/w/index.php?title=Stevens_Institute_of_Technologyhttp://en.wikipedia.org/w/index.php?title=Richard_Stribeck


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Tribology 5

ApplicationsThe study of tribology is commonly applied in bearing design but extends into almost all other aspects of moderntechnology, even to such unlikely areas as hair conditioners and cosmetics such as lipstick, powders and lipgloss.Any product where one material slides or rubs over another is affected by complex tribological interactions, whetherlubricated like hip implants and other artificial prostheses, or unlubricated as in high temperature sliding wear inwhich conventional lubricants cannot be used but in which the formation of compacted oxide layer glazes have beenobserved to protect against wear.

Tribology plays an important role in manufacturing. In metal-forming operations, friction increases tool wear and thepower required to work a piece. This results in increased costs due to more frequent tool replacement, loss of tolerance as tool dimensions shift, and greater forces required to shape a piece. The use of lubricants which minimizedirect surface contact reduces tool wear and power requirements.

References[1] Field, J. (2008). "David Tabor. 23 October 1913 -- 26 November 2005". Biographical Memoirs of Fellows of the Royal Society 54: 425.

doi:10.1098/rsbm.2007.0031.[2] Mitchell, Luke (November 2012). Ward, Jacob. ed. "The Fiction of Nonfriction". Popular Science . No. 5281 (November 2012): 40.[3][3] R. Stribeck, Die wesentlichen Eigenschaften der Gleit- und Rollenlager (The basic properties of sliding and rolling bearings), Zeitschrift des

Vereins Deutscher Ingenieure, 2002, Nr. 36, Band 46, p. 1341-1348, p. 1432-1438 and 1463-1470[4][4] H. Czichos, K.-H. Habig, Tribologie-Handbuch (Tribology handbook), Vieweg Verlag, Wiesbaden, 2nd edition, 2003, ISBN 3-528-16354-2[5][5] Duncan Dowson, History of Tribology, Second Edition, Professional Engineering Publishing, 1997, ISBN 1-86058-070-X[6] Palaci, Ismal (2007), Atomic Force Microscopy Studies of Nanotribology and Nanomechanics. p. 52. (http:/ / biblion. epfl. ch/ EPFL/ theses/

2007/ 3905/ EPFL_TH3905. pdf)[7] Armstrong-Hlouvry, Brian (1991).Control of machines with friction (http:/ / books. google. com/ ?id=0zk_zI3xACgC& pg=PA10). USA:

Springer. pp. 10. ISBN 0-7923-9133-0. .[8] van Beek, Anton. "History of Science Friction" (http:/ / www. tribology-abc. com/ abc/ history. htm). tribology-abc.com. . Retrieved

2011-03-24.[9][9] Stribeck, R. (1901), Kugellager fr beliebige Belastungen (Ball Bearings for any Stress), Zeitschrift des Vereins Deutscher Ingenieure 45.[10][10] Stribeck, R. (1902), Die wesentlichen Eigenschaften der Gleit- und Rollenlager (Characteristics of Plain and Roller Bearings), Zeit. des VDI

46.[11] Jacobson, Bo (2003), The Stribeck memorial lecture. (http:/ / www. sciencedirect. com/ science?_ob=ArticleURL&

_udi=B6V57-497HBKM-4& _user=10& _coverDate=11/ 30/ 2003& _rdoc=1& _fmt=high& _orig=search& _sort=d& _docanchor=&view=c& _searchStrId=1433619808& _rerunOrigin=google& _acct=C000050221& _version=1& _urlVersion=0& _userid=10&md5=634f43c7420d0709195dbd4ac5e8a719)

[12][12] Hersey, M. D. (1914), The Laws of Lubrication of Horizontal Journal Bearings, J. Wash. Acad. Sci., 4, 542-552.[13] Biography of Mayo D. Hersey (http:/ / www. nndb. com/ people/ 221/ 000169711/ )[14][14] Robert H. Thurston, Friction and lubrication - Determination of the laws and co-fficients of friction by new methods and with new

apparatus, Trbner and Co., Ludgate Hill, London, 1879[15] Robert H. Thurston, A treatise on friction and lost work in machinery and millwork, John Wiley&Sons, New York, 1894, fifth edition[16] http:/ / www. stle. org

[17] http:/ / www. imeche. org/ knowledge/ industries/ tribology/ overview

Bibliography Surface Wear Analysis, Treatment, and Prevention: R. Chattopadhyay, published by ASM-International,

Materials Park, OH, 2001, ISBN 0-87170-702-0. Advanced Thermally Assisted Surface Engineering Processes: Ramnarayan Chattopadhyay, Kluwer Academic

Publishers, MA (now Springer, NY), 2004. DeGarmo, E. Paul, J T. Black, and Ronald A. Kohser. Materials and Processes in Manufacturing . Upper Saddle

River, New Jersey: Prentice Hall, 1997. ISBN 0-02-328621-0 Zum Gahr, Karl-Heinz (1987). Microstructure and Wear of Materials . Tribology Series, 10. Elsevier.

ISBN 0-444-42754-6. Heshmat, Hooshang.Tribology of Interface Layers . CRC Press. ISBN 978-0-8247-5832-5.
http://en.wikipedia.org/w/index.php?title=Hooshang_Heshmathttp://en.wikipedia.org/w/index.php?title=Elsevierhttp://www.imeche.org/knowledge/industries/tribology/overviewhttp://www.stle.org/http://www.nndb.com/people/221/000169711/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V57-497HBKM-4&_user=10&_coverDate=11%2F30%2F2003&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1433619808&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=634f43c7420d0709195dbd4ac5e8a719http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V57-497HBKM-4&_user=10&_coverDate=11%2F30%2F2003&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1433619808&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=634f43c7420d0709195dbd4ac5e8a719http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V57-497HBKM-4&_user=10&_coverDate=11%2F30%2F2003&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1433619808&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=634f43c7420d0709195dbd4ac5e8a719http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V57-497HBKM-4&_user=10&_coverDate=11%2F30%2F2003&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1433619808&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=634f43c7420d0709195dbd4ac5e8a719http://www.tribology-abc.com/abc/history.htmhttp://books.google.com/?id=0zk_zI3xACgC&pg=PA10http://biblion.epfl.ch/EPFL/theses/2007/3905/EPFL_TH3905.pdfhttp://biblion.epfl.ch/EPFL/theses/2007/3905/EPFL_TH3905.pdfhttp://en.wikipedia.org/w/index.php?title=Popular_Sciencehttp://en.wikipedia.org/w/index.php?title=Biographical_Memoirs_of_Fellows_of_the_Royal_Societyhttp://en.wikipedia.org/w/index.php?title=David_Taborhttp://en.wikipedia.org/w/index.php?title=Compacted_oxide_layer_glazehttp://en.wikipedia.org/w/index.php?title=Powder_%28substance%29http://en.wikipedia.org/w/index.php?title=Lipstickhttp://en.wikipedia.org/w/index.php?title=Hair_conditionerhttp://en.wikipedia.org/w/index.php?title=Bearing_%28mechanical%29


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Tribology 6

Litt, Fred. "Starting from Scratch: Tribology Basics Volume I" (http:/ / www. stle.org/ assets/ document/ Starting_from_Scratch. pdf). STLE. Retrieved 2010-06-10.

External links Tribology NL (http:/ / www. tribology-abc. com/ default. htm) an overview of tribology topics targeted at

mechanical engineers. (http:/ / rtec-instruments. com/ tribometer. htm) Rtec Instrument- Tribology studies instrument manufacturer IET Tribology Network (http:/ / kn.theiet. org/ communities/ tribology/ index. cfm?origin-wiki) IET Tribology Network (http:/ / kn.theiet. org/ communities/ tribology/ index. cfm?origin-wiki) Tribology: Friction, Wear, and Lubrication (http:/ / web. mit.edu/ professional/ short-programs/ courses/

designing_tribological_systems. html) a short course offered at the Massachusetts Institute of Technology

FrictionFriction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding againsteach other. There are several types of friction: Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided intostatic

friction ("stiction") between non-moving surfaces, andkinetic friction between moving surfaces. Fluid friction describes the friction between layers within a viscous fluid that are moving relative to each

other.[1][2]

Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces.[3][4][5]

Skin friction is a component of drag, the force resisting the motion of a solid body through a fluid. Internal friction is the force resisting motion between the elements making up a solid material while it undergoes

deformation.[2]When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energyinto heat. This property can have dramatic consequences, as illustrated by the use of friction created by rubbingpieces of wood together to start a fire. Kinetic energy is converted to heat whenever motion with friction occurs, forexample when a viscous fluid is stirred. Another important consequence of many types of friction can be wear,which may lead to performance degradation and/or damage to components. Friction is a component of the science of tribology.

Friction is not itself a fundamental force but arises from fundamental electromagnetic forces between the chargedparticles constituting the two contacting surfaces. The complexity of these interactions makes the calculation of

friction from first principles impossible and necessitates the use of empirical methods for analysis and thedevelopment of theory.

HistoryThe classic rules of sliding friction were discovered by Leonardo da Vinci (1452 1519), but remained unpublishedin his notebooks.[6][7][8]They were rediscovered by Guillaume Amontons (1699). Amontons presented the nature of friction in terms of surface irregularities and the force required to raise the weight pressing the surfaces together.This view was further elaborated by Belidor (representation of rough surfaces with spherical asperities, 1737)[6]andLeonhard Euler (1750) who derived the angle of repose of a weight on an inclined plane and first distinguishedbetween static and kinetic friction.[9]A different explanation was provided by Desaguliers (1725), who demonstratedthe strong cohesion forces between lead spheres of which a small cap is cut off and which were then brought intocontact with each other.
http://en.wikipedia.org/w/index.php?title=Angle_of_reposehttp://en.wikipedia.org/w/index.php?title=Leonhard_Eulerhttp://en.wikipedia.org/w/index.php?title=Guillaume_Amontonshttp://en.wikipedia.org/w/index.php?title=Leonardo_da_Vincihttp://en.wikipedia.org/w/index.php?title=Empirical_methodhttp://en.wikipedia.org/w/index.php?title=First_principlehttp://en.wikipedia.org/w/index.php?title=Electromagnetic_forcehttp://en.wikipedia.org/w/index.php?title=Fundamental_forcehttp://en.wikipedia.org/w/index.php?title=Viscous_flowhttp://en.wikipedia.org/w/index.php?title=Heathttp://en.wikipedia.org/w/index.php?title=Kinetic_energyhttp://en.wikipedia.org/w/index.php?title=Deformation_%28mechanics%29http://en.wikipedia.org/w/index.php?title=Drag_%28physics%29http://en.wikipedia.org/w/index.php?title=Viscoushttp://en.wikipedia.org/w/index.php?title=Stictionhttp://en.wikipedia.org/w/index.php?title=Surfacehttp://en.wikipedia.org/w/index.php?title=Forcehttp://web.mit.edu/professional/short-programs/courses/designing_tribological_systems.htmlhttp://web.mit.edu/professional/short-programs/courses/designing_tribological_systems.htmlhttp://kn.theiet.org/communities/tribology/index.cfm?origin-wikihttp://kn.theiet.org/communities/tribology/index.cfm?origin-wikihttp://rtec-instruments.com/tribometer.htmhttp://www.tribology-abc.com/default.htmhttp://en.wikipedia.org/w/index.php?title=STLEhttp://www.stle.org/assets/document/Starting_from_Scratch.pdfhttp://www.stle.org/assets/document/Starting_from_Scratch.pdf


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The understanding of friction was further developed by Charles-Augustin de Coulomb (1785). Coulomb investigatedthe influence of four main factors on friction: the nature of the materials in contact and their surface coatings; theextent of the surface area; the normal pressure (or load); and the length of time that the surfaces remained in contact(time of repose).[6]Coulomb further considered the influence of sliding velocity, temperature and humidity, in orderto decide between the different explanations on the nature of friction that had been proposed. The distinctionbetween static and dynamic friction is made in Coulomb's friction law (see below), although this distinction wasalready drawn by Johann Andreas von Segner in 1758.[6] The effect of the time of repose was explained byMusschenbroek (1762) by considering the surfaces of fibrous materials, with fibers meshing together, which takes afinite time in which the friction increases.John Leslie (1766 1832) noted a weakness in the views of Amontons and Coulomb. If friction arises from a weightbeing drawn up the inclined plane of successive asperities, why isn't it balanced then through descending theopposite slope? Leslie was equally skeptical about the role of adhesion proposed by Desaguliers, which should onthe whole have the same tendency to accelerate as to retard the motion.[6] In his view friction should be seen as atime-dependent process of flattening, pressing down asperities, which creates new obstacles in what were cavitiesbefore.

Arthur Morrin (1833) developed the concept of sliding versus rolling friction. Osborne Reynolds (1866) derived theequation of viscous flow. This completed the classic empirical model of friction (static, kinetic, and fluid) commonlyused today in engineering.[7]

The focus of research during the last century has been to understand the physical mechanisms behind friction. F.Phillip Bowden and David Tabor (1950) showed that at a microscopic level, the actual area of contact betweensurfaces is a very small fraction of the apparent area.[8] This actual area of contact, caused by "asperities"(roughness) increases with pressure, explaining the proportionality between normal force and frictional force. Thedevelopment of the atomic force microscope (1986) has recently enabled scientists to study friction at the atomicscale.[7]

Laws of dry frictionThe elementary properties of sliding (kinetic) friction were discovered by experiment in the 15th to 18th centuriesand were expressed as three empirical laws: Amontons' First Law : The force of friction is directly proportional to the applied load. Amontons' Second Law : The force of friction is independent of the apparent area of contact. Coulomb's Law of Friction : Kinetic friction is independent of the sliding velocity.

Dry friction

Dry friction resists relative lateral motion of two solid surfaces in contact. The two regimes of dry friction are 'staticfriction' ("stiction") between non-moving surfaces, andkinetic friction (sometimes called sliding friction or dynamicfriction) between moving surfaces.

Coulomb friction, named after Charles-Augustin de Coulomb, is an approximate model used to calculate the force of dry friction. It is governed by the equation:

where

is the force of friction exerted by each surface on the other. It is parallel to the surface, in a direction oppositeto the net applied force.

is the coefficient of friction, which is an empirical property of the contacting materials, is the normal force exerted by each surface on the other, directed perpendicular (normal) to the surface.
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The Coulomb friction may take any value from zero up to , and the direction of the frictional force againsta surface is opposite to the motion that surface would experience in the absence of friction. Thus, in the static case,the frictional force is exactly what it must be in order to prevent motion between the surfaces; it balances the netforce tending to cause such motion. In this case, rather than providing an estimate of the actual frictional force, theCoulomb approximation provides a threshold value for this force, above which motion would commence. This

maximum force is known as traction.The force of friction is always exerted in a direction that opposes movement (for kinetic friction) or potentialmovement (for static friction) between the two surfaces. For example, a curling stone sliding along the iceexperiences a kinetic force slowing it down. For an example of potential movement, the drive wheels of anaccelerating car experience a frictional force pointing forward; if they did not, the wheels would spin, and the rubberwould slide backwards along the pavement. Note that it is not the direction of movement of the vehicle they oppose,it is the direction of (potential) sliding between tire and road.

Normal force

Free-body diagram for a block on a ramp.Arrows are vectors indicating directions andmagnitudes of forces. N is the normal force,mg is the force of gravity, and F

f is the force

of friction.

The normal force is defined as the net force compressing two parallelsurfaces together; and its direction is perpendicular to the surfaces. In thesimple case of a mass resting on a horizontal surface, the only componentof the normal force is the force due to gravity, where . In thiscase, the magnitude of the friction force is the product of the mass of theobject, the acceleration due to gravity, and the coefficient of friction.However, the coefficient of friction is not a function of mass or volume; itdepends only on the material. For instance, a large aluminum block has thesame coefficient of friction as a small aluminum block. However, themagnitude of the friction force itself depends on the normal force, andhence on the mass of the block.

If anobject is on a level surface and the force tending to cause it to slideis horizontal, the normal force between the object and the surface is justits weight, which is equal to its mass multiplied by the acceleration due toearth's gravity,g. If theobject is on a tilted surface such as an inclinedplane, the normal force is less, because less of the force of gravity isperpendicular to the face of the plane. Therefore, the normal force, andultimately the frictional force, is determined using vector analysis, usuallyvia a free body diagram. Depending on the situation, the calculation of thenormal force may include forces other than gravity.

Coefficient of friction

The coefficient of friction (COF), often symbolized by the Greek letter ,is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. Thecoefficient of friction depends on the materials used; for example, ice onsteel has a low coefficient of friction, while rubber on pavement has a highcoefficient of friction. Coefficients of friction range from near zero to greater than one. Under good conditions, forexample, a tire on concrete may have a coefficient of friction of 1.7.

For surfaces at rest relative to each other , where is thecoefficient of static friction . This is usuallylarger than its kinetic counterpart.
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For surfaces in relative motion , where is thecoefficient of kinetic friction . The Coulomb friction isequal to , and the frictional force on each surface is exerted in the direction opposite to its motion relative to theother surface.It was Arthur-Jules Morin who introduced the term and demonstrated the utility of the coefficient of friction.[6] Thecoefficient of friction is an empirical measurement it has to be measured experimentally, and cannot be found

through calculations. Rougher surfaces tend to have higher effective values. Both static and kinetic coefficients of friction depend on the pair of surfaces in contact; for a given pair of surfaces, the coefficient of static friction isusually larger than that of kinetic friction; in some sets the two coefficients are equal, such as teflon-on-teflon.Most dry materials in combination have friction coefficient values between 0.3 and 0.6. Values outside this range arerarer, but teflon, for example, can have a coefficient as low as 0.04. A value of zero would mean no friction at all, anelusive property even magnetic levitation vehicles have drag. Rubber in contact with other surfaces can yieldfriction coefficients from 1 to 2. Occasionally it is maintained that is always < 1, but this is not true. While in mostrelevant applications < 1, a value above 1 merely implies that the force required to slide an object along the surfaceis greater than the normal force of the surface on the object. For example, silicone rubber or acrylic rubber-coatedsurfaces have a coefficient of friction that can be substantially larger than 1.

While it is often stated that the COF is a "material property," it is better categorized as a "system property." Unliketrue material properties (such as conductivity, dielectric constant, yield strength), the COF for any two materialsdepends on system variables like temperature, velocity, atmosphere and also what are now popularly described asaging and deaging times; as well as on geometric properties of the interface between the materials. For example, acopper pin sliding against a thick copper plate can have a COF that varies from 0.6 at low speeds (metal slidingagainst metal) to below 0.2 at high speeds when the copper surface begins to melt due to frictional heating. The latterspeed, of course, does not determine the COF uniquely; if the pin diameter is increased so that the frictional heatingis removed rapidly, the temperature drops, the pin remains solid and the COF rises to that of a 'low speed' test.

Negative coefficient of friction

As of 2012, a single study has demonstrated the potential for a negative coefficient of friction, meaning that adecrease in force leads to an increase in friction. This contradicts the everyday experience that an increase of normalforce improves friction.[10] This was reported in the journal Nature in October 2012 and involved the frictionencountered by an atomic force microscope stylus when dragged across a graphene sheet in the presence of graphene-adsorbed oxygen.[10]

Approximate coefficients of friction

Materials Static friction,

Dry and clean Lubricated

Aluminium Steel 0.61

Copper Steel 0.53

Brass Steel 0.51

Cast iron Copper 1.05

Cast iron Zinc 0.85

Concrete (wet) Rubber 0.30

Concrete (dry) Rubber 1.0

Concrete Wood 0.62[11]

Copper Glass 0.68

Glass Glass 0.94
http://en.wikipedia.org/w/index.php?title=Copperhttp://en.wikipedia.org/w/index.php?title=Atmospherehttp://en.wikipedia.org/w/index.php?title=Velocityhttp://en.wikipedia.org/w/index.php?title=Temperaturehttp://en.wikipedia.org/w/index.php?title=Acrylic_rubberhttp://en.wikipedia.org/w/index.php?title=Silicone_rubberhttp://en.wikipedia.org/w/index.php?title=Maglev_trainhttp://en.wikipedia.org/w/index.php?title=Magnetic_levitationhttp://en.wikipedia.org/w/index.php?title=Polytetrafluoroethylenehttp://en.wikipedia.org/w/index.php?title=Experimenthttp://en.wikipedia.org/w/index.php?title=Measurementhttp://en.wikipedia.org/w/index.php?title=Empiricalhttp://en.wikipedia.org/w/index.php?title=Arthur_Morin


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Metal Wood 0.2 0.6[11] 0.2 (wet)[11]

Polyethene Steel 0.2[12] 0.2[12]

Steel Steel 0.80[12] 0.16[12]

Steel PTFE 0.04[12] 0.04[12]

PTFE PTFE 0.04[12] 0.04[12]

Wood Wood 0.25 0.5[11] 0.2 (wet)[11]

An AlMgB14-TiB2 composite has an approximate coefficient of friction of 0.02 in water-glycol-basedlubricants,[13][14]and 0.04 0.05 when dry.[15] Under certain conditions, some materials have even lower frictioncoefficients. An example is (highly ordered pyrolytic) graphite, which can have a friction coefficient below 0.01.[16]

This ultralow-friction regime is called superlubricity.

Static friction

Static friction is friction between two or more solid objects that are not moving relative to each other. For example,static friction can prevent an object from sliding down a sloped surface. The coefficient of static friction, typicallydenoted as s, is usually higher than the coefficient of kinetic friction.The static friction force must be overcome by an applied force before an object can move. The maximum possiblefriction force between two surfaces before sliding begins is the product of the coefficient of static friction and thenormal force: . When there is no sliding occurring, the friction force can have any value from zero up to

. Any force smaller than attempting to slide one surface over the other is opposed by a frictional forceof equal magnitude and opposite direction. Any force larger than overcomes the force of static friction andcauses sliding to occur. The instant sliding occurs, static friction is no longer applicable the friction between the

two surfaces is then called kinetic friction.An example of static friction is the force that prevents a car wheel from slipping as it rolls on the ground. Eventhough the wheel is in motion, the patch of the tire in contact with the ground is stationary relative to the ground, soit is static rather than kinetic friction.The maximum value of static friction, when motion is impending, is sometimes referred to aslimiting friction ,[17]

although this term is not used universally.[1]It is also known as traction.

Kinetic friction

Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sledon the ground). The coefficient of kinetic friction is typically denoted as

k, and is usually less than the coefficient of

static friction for the same materials.[18][19]However, Richard Feynman comments that "with dry metals it is veryhard to show any difference."[20]

New models are beginning to show how kinetic friction can be greater than static friction.[21]Kinetic friction is nowunderstood, in many cases, to be primarily caused by chemical bonding between the surfaces, rather thaninterlocking asperities;[22] however, in many other cases roughness effects are dominant, for example in rubber toroad friction.[21] Surface roughness and contact area, however, do affect kinetic friction for micro- and nano-scaleobjects where surface area forces dominate inertial forces.[23]
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Angle of friction

For certain applications it is more useful to define static friction in terms of the maximum angle before which one of the items will begin sliding. This is called theangle of friction or friction angle . It is defined as:

where is the angle from vertical and s is the static coefficient of friction between the objects.[24]This formula can

also be used to calculate s from empirical measurements of the friction angle.

Friction at the atomic level

Determining the forces required to move atoms past each other is a challenge in designing nanomachines. In 2008scientists for the first time were able to move a single atom across a surface, and measure the forces required. Usingultrahigh vacuum and nearly-zero temperature (5 K), a modified atomic force microscope was used to drag a cobaltatom, and a carbon monoxide molecule, across surfaces of copper and platinum.[25]

Limitations of the Coulomb model

The Coulomb approximation mathematically follows from the assumptions that surfaces are in atomically closecontact only over a small fraction of their overall area, that this contact area is proportional to the normal force (untilsaturation, which takes place when all area is in atomic contact), and that frictional force is proportional to theapplied normal force, independently of the contact area (you can see the experiments on friction from Leonardo daVinci). Such reasoning aside, however, the approximation is fundamentally an empirical construction. It is a rule of thumb describing the approximate outcome of an extremely complicated physical interaction. The strength of theapproximation is its simplicity and versatility though in general the relationship between normal force andfrictional force is not exactly linear (and so the frictional force is not entirely independent of the contact area of thesurfaces), the Coulomb approximation is an adequate representation of friction for the analysis of many physicalsystems.

When the surfaces are conjoined, Coulomb friction becomes a very poor approximation (for example, adhesive taperesists sliding even when there is no normal force, or a negative normal force). In this case, the frictional force maydepend strongly on the area of contact. Some drag racing tires are adhesive in this way. However, despite thecomplexity of the fundamental physics behind friction, the relationships are accurate enough to be useful in manyapplications.

Numerical simulation of the Coulomb model

Despite being a simplified model of friction, the Coulomb model is useful in many numerical simulation applicationssuch as multibody systems and granular material. Even its most simple expression encapsulates the fundamentaleffects of sticking and sliding which are required in many applied cases, although specific algorithms have to bedesigned in order to efficiently numerically integrate mechanical systems with Coulomb friction and bilateral and/orunilateral contact.[26][27][28][29][30]Some quite nonlinear effects, such as the so-called Painlev paradoxes, may beencountered with Coulomb friction.[31]
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Dry friction and instabilities

A physical model of the so-called 'Ziegler column', atwo-degree-of-freedom system, exhibiting flutter instability as

induced by dry friction. Watch the complete movie[32]for moredetails.

Dry friction can induce several types of instabilities inmechanical systems, which display a stable behaviourin the absence of friction.[33] For instance,friction-related dynamical instabilities are thought to be

responsible of brake squeal and of the 'song' of a glassharp,[34][35]phenomena which involve stick and slip,modelled as a drop of friction coefficient withvelocity.[36]

A connection between dry friction and flutter instabilityin a simple mechanical system has been discovered.[37]

Fluid friction

Fluid friction occurs between layers within a fluid thatare moving relative to each other. This internalresistance to flow is described byviscosity . In everydayterms viscosity is "thickness". Thus, water is "thin",having a lower viscosity, while honey is "thick", havinga higher viscosity. Put simply, the less viscous the fluidis, the greater its ease of movement.

All real fluids (except superfluids) have some resistance to stress and therefore are viscous, but a fluid which has noresistance to shear stress is known as an ideal fluid or inviscid fluid.

Lubricated frictionLubricated friction is a case of fluid friction where a fluid separates two solid surfaces. Lubrication is a techniqueemployed to reduce wear of one or both surfaces in close proximity moving relative to each another by interposing asubstance called a lubricant between the surfaces.

In most cases the applied load is carried by pressure generated within the fluid due to the frictional viscous resistanceto motion of the lubricating fluid between the surfaces. Adequate lubrication allows smooth continuous operation of equipment, with only mild wear, and without excessive stresses or seizures at bearings. When lubrication breaksdown, metal or other components can rub destructively over each other, causing heat and possibly damage or failure.

Skin frictionSkin friction arises from the friction of the fluid against the "skin" of the object that is moving through it. Skinfriction arises from the interaction between the fluid and the skin of the body, and is directly related to the area of thesurface of the body that is in contact with the fluid. Skin friction follows the drag equation and rises with the squareof the velocity.

Skin friction is caused by viscous drag in the boundary layer around the object. There are two ways to decrease skinfriction: the first is to shape the moving body so that smooth flow is possible, like an airfoil. The second method is todecrease the length and cross-section of the moving object as much as is practicable.
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Internal frictionInternal friction is the force resisting motion between the elements making up a solid material while it undergoesplastic deformation.Plastic deformation in solids is an irreversible change in the internal molecular structure of an object. This changemay be due to either (or both) an applied force or a change in temperature. The change of an object's shape is calledstrain. The force causing it is called stress. Stress does not necessarily cause permanent change. As deformationoccurs, internal forces oppose the applied force. If the applied stress is not too large these opposing forces maycompletely resist the applied force, allowing the object to assume a new equilibrium state and to return to its originalshape when the force is removed. This is what is known in the literature as elastic deformation (or elasticity). Largerforces in excess of the elastic limit may cause a permanent (irreversible) deformation of the object. This is what isknown as plastic deformation.

Other types of friction

Rolling resistance

Rolling resistance is the force that resists the rolling of a wheel or other circular object along a surface caused bydeformations in the object and/or surface. Generally the force of rolling resistance is less than that associated withkinetic friction.[38] Typical values for the coefficient of rolling resistance are 0.001.[39] One of the most commonexamples of rolling resistance is the movement of motor vehicle tires on a road, a process which generates heat andsound as by-products.[40]

Triboelectric effect

Rubbing dissimilar materials against one another can cause a build-up of electrostatic charge, which can behazardous if flammable gases or vapours are present. When the static build-up discharges, explosions can be caused

by ignition of the flammable mixture.

Belt friction

Belt friction is a physical property observed from the forces acting on a belt wrapped around a pulley, when one endis being pulled. The resulting tension, which acts on both ends of the belt, can be modeled by the belt frictionequation.

In practice, the theoretical tension acting on the belt or rope calculated by the belt friction equation can be comparedto the maximum tension the belt can support. This helps a designer of such a rig to know how many times the belt orrope must be wrapped around the pulley to prevent it from slipping. Mountain climbers and sailing crews

demonstrate a standard knowledge of belt friction when accomplishing basic tasks.

Reducing friction

Devices

Devices such as wheels, ball bearings, roller bearings, and air cushion or other types of fluid bearings can changesliding friction into a much smaller type of rolling friction.Many thermoplastic materials such as nylon, HDPE and PTFE are commonly used in low friction bearings. They areespecially useful because the coefficient of friction falls with increasing imposed load. For improved wear resistance,very high molecular weight grades are usually specified for heavy duty or critical bearings.
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Lubricants

A common way to reduce friction is by using a lubricant, such as oil, water, or grease, which is placed between thetwo surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is calledtribology. Lubricant technology is when lubricants are mixed with the application of science, especially to industrialor commercial objectives.

Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of frictionbetween two sliding objects, approaching zero levels. A very small amount of frictional energy would still bedissipated.Lubricants to overcome friction need not always be thin, turbulent fluids or powdery solids such as graphite and talc;acoustic lubrication actually uses sound as a lubricant.Another way to reduce friction between two parts is to superimpose micro-scale vibration to one of the parts. Thiscan be sinusoidal vibration as used in ultrasound-assisted cutting or vibration noise, known as dither.

Energy of friction

According to the law of conservation of energy, no energy is destroyed due to friction, though it may be lost to thesystem of concern. Energy is transformed from other forms into heat. A sliding hockey puck comes to rest becausefriction converts its kinetic energy into heat. Since heat quickly dissipates, many early philosophers, includingAristotle, wrongly concluded that moving objects lose energy without a driving force.

When an object is pushed along a surface, the energy converted to heat is given by:

where

is the normal force,

is the coefficient of kinetic friction,is the coordinate along which the object transverses.

Energy lost to a system as a result of friction is a classic example of thermodynamic irreversibility.

Work of friction

In the reference frame of the interface between two surfaces, static friction doesno work, because there is neverdisplacement between the surfaces. In the same reference frame, kinetic friction is always in the direction oppositethe motion, and doesnegative work.[41]However, friction can do positive work in certain frames of reference. Onecan see this by placing a heavy box on a rug, then pulling on the rug quickly. In this case, the box slides backwards

relative to the rug, but moves forward relative to the frame of reference in which the floor is stationary. Thus, thekinetic friction between the box and rug accelerates the box in the same direction that the box moves, doing positivework.[42]

The work done by friction can translate into deformation, wear, and heat that can affect the contact surface properties(even the coefficient of friction between the surfaces). This can be beneficial as in polishing. The work of friction isused to mix and join materials such as in the process of friction welding. Excessive erosion or wear of matingsurfaces occur when work due frictional forces rise to unacceptable levels. Harder corrosion particles caught betweenmating surfaces (fretting) exacerbates wear of frictional forces. Bearing seizure or failure may result from excessivewear due to work of friction. As surfaces are worn by work due to friction, fit and surface finish of an object maydegrade until it no longer functions properly.[43]
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ApplicationsFriction is an important factor in many engineering disciplines.

Transportation

Rail adhesion refers to the grip wheels of a train have on the rails, see Frictional contact mechanics. Road slipperiness is an important design and safety factor for automobiles

Split friction is a particularly dangerous condition arising due to varying friction on either side of a car. Road texture affects the interaction of tires and the driving surface.

Measurement

A tribometer is an instrument that measures friction on a surface. A profilograph is a device used to measure pavement surface roughness.

Household usage

Friction is used to ignite matchstick (Friction between the head of a matchstick and the rubbing surface of thematch box).

Instances where friction is advantageous Friction between meteorite and air helps to increase its temperature and ignite it, protecting the living things on

Earth. Finger prints and palm prints on hand help to hold up objects easily in human and ape hands. Foot prints increase friction and helps to stand up. Automobile brakes inherently rely on friction, slowing a vehicle by converting its kinetic energy into heat.

Incidentally, dispersing this large amount of heat safely is one technical challenge in designing brake systems.

References[1] Beer, Ferdinand P.; E. Russel Johnston, Jr. (1996).Vector Mechanics for Engineers (Sixth ed.). McGraw-Hill. p. 397. ISBN 0-07-297688-8.[2] Meriam, J. L.; L. G. Kraige (2002). Engineering Mechanics (fifth ed.). John Wiley & Sons. p. 328. ISBN 0-471-60293-0.[3] Ruina, Andy; Rudra Pratap (2002). Introduction to Statics and Dynamics (http:/ / ruina. tam. cornell. edu/ Book/ RuinaPratapNoProblems.

pdf). Oxford University Press. p. 713. .[4] Hibbeler, R. C. (2007). Engineering Mechanics (Eleventh ed.). Pearson, Prentice Hall. p. 393. ISBN 0-13-127146-6.[5] Soutas-Little, Robert W.; Inman, Balint (2008). Engineering Mechanics . Thomson. p. 329. ISBN 0-495-29610-4.[6] Dowson, Duncan (1997). History of Tribology, 2nd Edition . Professional Engineering Publishing. ISBN 1-86058-070-X.[7] Armstrong-Hlouvry, Brian (1991).Control of machines with friction (http:/ / books. google. com/ ?id=0zk_zI3xACgC& pg=PA10). USA:

Springer. pp. 10. ISBN 0-7923-9133-0. .[8] van Beek, Anton. "History of Science Friction" (http:/ / www. tribology-abc. com/ abc/ history. htm). tribology-abc.com. . Retrieved2011-03-24.

[9] "Leonhard Euler" (http:/ / www. nano-world. org/ frictionmodule/ content/ 0200makroreibung/ 0400historisch/ 0300euler/ ?lang=en). Friction Module . Nano World website (http:/ / www. nano-world. org/ ). 2002. . Retrieved 2011-03-25.

[10] Deng, Zhao; et al. (October 14, 2012), "Adhesion-dependent negative friction coefficient on chemically modified graphite at the nanoscale"(http:/ / www.nature.com/ nmat/ journal/ vaop/ ncurrent/ full/ nmat3452. html), Nature , doi:10.1038/nmat3452, , retrieved November 18,2012, Lay summary (http:/ / www. rdmag. com/ news/ 2012/ 10/ nanoscale-graphite-can-turn-friction-upside-down) R&D Magazine(October 17, 2012)

[11] Engineers: Handbook Friction Coefficients (http:/ / www. engineershandbook. com/ Tables/ frictioncoefficients. htm)[12] "The Engineering Toolbox: Friction and Coefficients of Friction" (http:/ / www. engineeringtoolbox. com/ friction-coefficients-d_778.

html). . Retrieved 2008-11-23.[13] Kurt Kleiner (2008-11-21). "Material slicker than Teflon discovered by accident" (http:/ / www. newscientist. com/ article/

dn16102-material-slicker-than-teflon-discovered-by-accident. html). . Retrieved 2008-12-25.[14] Higdon, C.; Cook, B.; Harringa, J.; Russell, A.; Goldsmith, J.; Qu, J.; Blau, P. (2011). "Friction and wear mechanisms in AlMgB14-TiB2

nanocoatings".Wear 271 (9 10): 2111. doi:10.1016/j.wear.2010.11.044.
http://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.htmlhttp://www.newscientist.com/article/dn16102-material-slicker-than-teflon-discovered-by-accident.htmlhttp://www.engineeringtoolbox.com/friction-coefficients-d_778.htmlhttp://www.engineeringtoolbox.com/friction-coefficients-d_778.htmlhttp://www.engineershandbook.com/Tables/frictioncoefficients.htmhttp://en.wikipedia.org/w/index.php?title=R%26D_Magazinehttp://www.rdmag.com/news/2012/10/nanoscale-graphite-can-turn-friction-upside-downhttp://en.wikipedia.org/w/index.php?title=Nature_%28journal%29http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3452.htmlhttp://www.nano-world.org/http://www.nano-world.org/frictionmodule/content/0200makroreibung/0400historisch/0300euler/?lang=enhttp://www.tribology-abc.com/abc/history.htmhttp://books.google.com/?id=0zk_zI3xACgC&pg=PA10http://ruina.tam.cornell.edu/Book/RuinaPratapNoProblems.pdfhttp://ruina.tam.cornell.edu/Book/RuinaPratapNoProblems.pdfhttp://en.wikipedia.org/w/index.php?title=Ferdinand_Beerhttp://en.wikipedia.org/w/index.php?title=Vehicle_brakehttp://en.wikipedia.org/w/index.php?title=Meteoritehttp://en.wikipedia.org/w/index.php?title=Profilographhttp://en.wikipedia.org/w/index.php?title=Tribometerhttp://en.wikipedia.org/w/index.php?title=Texture_%28roads%29http://en.wikipedia.org/w/index.php?title=Split_frictionhttp://en.wikipedia.org/w/index.php?title=Road_slipperinesshttp://en.wikipedia.org/w/index.php?title=Frictional_contact_mechanicshttp://en.wikipedia.org/w/index.php?title=Rail_adhesion


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[15] Tian, Y.; Bastawros, A. F.; Lo, C. C. H.; Constant, A. P.; Russell, A. M.; Cook, B. A. (2003). "Superhard self-lubricating AlMgB[sub 14]films for microelectromechanical devices". Applied Physics Letters 83 (14): 2781. Bibcode 2003ApPhL..83.2781T. doi:10.1063/1.1615677.

[16] Martin Dienwiebel, Gertjan S. Verhoeven, Namboodiri Pradeep, Joost W. M. Frenken, Jennifer A. Heimberg, and Henny W. Zandbergen(2004). "Superlubricity of Graphite" (http:/ / www. physics. leidenuniv. nl/ sections/ cm/ ip/ group/ PDF/ Phys. rev. lett/ 2004/ 92(2004)12601.pdf). Phys. Rev. Lett. 92 (12): 126101. Bibcode 2004PhRvL..92l6101D. doi:10.1103/PhysRevLett.92.126101. .

[17] Bhavikatti, S. S.; K. G. Rajashekarappa (1994). Engineering Mechanics (http:/ / books. google. com/ ?id=4wkLl4NvmWAC& pg=PA112&lpg=PA112). New Age International. p. 112. ISBN 978-81-224-0617-7. . Retrieved 2007-10-21.

[18] Sheppard and Tongue (2005).Statics: Analysis and Design of Systems in Equilibrium . Wiley and Sons. p. 618. ISBN 0-471-37299-4. "Ingeneral, for given contacting surfaces, k < s"

[19] Meriam and Kraige (2002). Engineering Mechanics: Statics . Wiley and Sons. p. 330. ISBN 0-471-40646-5. "Kinetic friction force is usuallysomewhat less than the maximum static friction force."

[20] Feynman, Leighton, and Sands (1964). "The Feynman Lectures on Physics, Vol. I, p. 12-5" (http:/ / hyperphysics. phy-astr. gsu. edu/ hbase/ frict2. html). Addison-Wesley. . Retrieved 2009-10-16.

[21] Persson, B. N.; Volokitin, A. I (2002). "Theory of rubber friction: Nonstationary sliding" (http:/ / juwel. fz-juelich. de:8080/ dspace/ bitstream/ 2128/ 1343/ 1/ 17249. pdf). Physical Review B 65 (13): 134106. Bibcode 2002PhRvB..65m4106P.doi:10.1103/PhysRevB.65.134106. .

[22] Beatty, William J.. "Recurring science misconceptions in K-6 textbooks" (http:/ / amasci. com/ miscon/ miscon4. html#fric). . Retrieved2007-06-08.

[23] B. N. J. Persson (2000).Sliding friction: physical principles and applications (http:/ / books. google. com/ books?id=1jb-nZMnRGYC).

Springer. ISBN 978-3-540-67192-3. . Retrieved 12 December 2011.[24] Nichols, Edward Leamington; Franklin, William Suddards (1898).The Elements of Physics (http:/ / books. google. com/ ?id=8IlCAAAAIAAJ).1. Macmillan. p. 101. .

[25] Ternes, Markus; Lutz, Christopher P.; Hirjibehedin, Cyrus F.; Giessibl, Franz J.; Heinrich, Andreas J. (2008-02-22). "The Force Needed toMove an Atom on a Surface".Science 319 (5866): 1066 1069. Bibcode 2008Sci...319.1066T. doi:10.1126/science.1150288.PMID 18292336.

[26] Haslinger, J.; Nedlec, J.C. (1983). "Approximation of the Signorini problem with friction, obeying the Coulomb law". Mathematical Methods in the Applied Sciences 5: 422 437. Bibcode 1983MMAS....5..422H. doi:10.1002/mma.1670050127.

[27] Alart, P.; Curnier, A. (1991). "A mixed formulation for frictional contact problems prone to Newton like solution method".Computer Methods in Applied Mechanics and Engineering 92 (3): 353 375. Bibcode 1991CMAME..92..353A. doi:10.1016/0045-7825(91)90022-X.

[28] Acary, V.; Cadoux, F.; Lemarchal, C.; Malick, J. (2011). "A formulation of the linear discrete Coulomb friction problem via convexoptimization". Journal of Applied Mathematics and Mechanics / Zeitschrift fr Angewandte Mathematik und Mechanik 91 (2): 155 175.doi:10.1002/zamm.201000073.

[29] De Saxc, G.; Feng, Z.-Q. (1998). "The bipotential method: A constructive approach to design the complete contact law with friction andimproved numerical algorithms". Mathematical and Computer Modelling 28 (4): 225 245. doi:10.1016/S0895-7177(98)00119-8.

[30] Simo, J.C.; Laursen, T.A. (1992). "An augmented lagrangian treatment of contact problems involving friction".Computers and Structures42 (2): 97 116. doi:10.1016/0045-7949(92)90540-G.

[31] Acary, V.; Brogliato, B. (2008). Numerical Methods for Nonsmooth Dynamical Systems. Applications in Mechanics and Electronics . 35.Springer Verlag Heidelberg.

[32] http:/ / www. ing. unitn. it/ ~bigoni/ flutter. html[33] Bigoni, D.. Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability . Cambridge University Press, 2012.

ISBN 9781107025417.[34] J. Flint and J. Hultn (2002). "Lining-deformation-induced modal coupling as squeal generator in a distributed parameter disk brake model".

J. Sound and Vibration 254 : 1 21. Bibcode 2002JSV...254....1F. doi:10.1006/jsvi.2001.4052.[35] M. Krger, M.Neubauer, and K. Popp (2008). "Experimental investigation on the avoidance of self-excited vibrations". Phil. Trans. R. Soc.

A 366 (1866): 785 810. Bibcode 2008RSPTA.366..785K. doi:10.1098/rsta.2007.2127. PMID 17947204.[36] Rice and Ruina; Ruina, A. L. (1983). "Stability of Steady Frictional Slipping" (http:/ / ruina. tam. cornell. edu/ research/ topics/

friction_and_fracture/ stability_steady. pdf). Journal of Applied Mechanics 50 (2): 343 349. Bibcode 1983JAM....50..343R.doi:10.1115/1.3167042. .

[37] D. Bigoni and G. Noselli (2011). "Experimental evidence of flutter and divergence instabilities induced by dry friction" (http:/ / www. ing.unitn. it/ ~bigoni). Journal of the Mechanics and Physics of Solids 59 (10): 2208 2226. Bibcode 2011JMPSo..59.2208B.doi:10.1016/j.jmps.2011.05.007. .

[38] Benjamin Silliman, Principles of Physics, Or Natural Philosophy , Ivison, Blakeman, Taylor & company publishers (1871)[39] Hans-Jrgen Butt, Karlheinz Graf, Michael Kappl, Physics and Chemistry of Interfaces , Wiley, ISBN 3-527-40413-9 (2006)[40] Hogan, C. Michael (1973). "Analysis of highway noise".Water, Air, and Soil Pollution 2 (3): 387. doi:10.1007/BF00159677.[41] Den Hartog, J. P. (1961). Mechanics (http:/ / books. google. com/ ?id=WRXrtu44W9UC). Courier Dover Publications. p. 142.

ISBN 0-486-60754-2. .[42] Leonard, William J (2000). Minds-on Physics (http:/ / books. google. com/ ?id=t_AKvmza5s8C& pg=PA603). Kendall/Hunt. p. 603.

ISBN 0-7872-3932-1. .
http://books.google.com/?id=t_AKvmza5s8C&pg=PA603http://books.google.com/?id=WRXrtu44W9UChttp://www.ing.unitn.it/~bigonihttp://www.ing.unitn.it/~bigonihttp://ruina.tam.cornell.edu/research/topics/friction_and_fracture/stability_steady.pdfhttp://ruina.tam.cornell.edu/research/topics/friction_and_fracture/stability_steady.pdfhttp://www.ing.unitn.it/~bigoni/flutter.htmlhttp://en.wikipedia.org/w/index.php?title=Springer_Science%2BBusiness_Mediahttp://en.wikipedia.org/w/index.php?title=Science_%28magazine%29http://books.google.com/?id=8IlCAAAAIAAJhttp://books.google.com/?id=8IlCAAAAIAAJhttp://books.google.com/books?id=1jb-nZMnRGYChttp://amasci.com/miscon/miscon4.html#frichttp://juwel.fz-juelich.de:8080/dspace/bitstream/2128/1343/1/17249.pdfhttp://juwel.fz-juelich.de:8080/dspace/bitstream/2128/1343/1/17249.pdfhttp://hyperphysics.phy-astr.gsu.edu/hbase/frict2.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/frict2.htmlhttp://books.google.com/?id=4wkLl4NvmWAC&pg=PA112&lpg=PA112http://books.google.com/?id=4wkLl4NvmWAC&pg=PA112&lpg=PA112http://www.physics.leidenuniv.nl/sections/cm/ip/group/PDF/Phys.rev.lett/2004/92(2004)12601.pdfhttp://www.physics.leidenuniv.nl/sections/cm/ip/group/PDF/Phys.rev.lett/2004/92(2004)12601.pdf


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[43] Bayer, Raymond George (2004). Mechanical wear (http:/ / books. google. com/ ?id=Q64Kq2HlyucC& pg=PA3& lpg=PA3& dq=Physical+wear+ is+ associated+ with+ friction). CRC Press. pp. 1, 2. ISBN 0-8247-4620-1. . Retrieved 2008-07-07.

External links Coefficients of Friction (http:/ / www. roymech. co. uk/ Useful_Tables/ Tribology/ co_of_frict.htm) tables of

coefficients, plus many links Physclips: Mechanics with animations and video clips (http:/ / www.physclips. unsw. edu.au/ ) from theUniversity of New South Wales

CRC Handbook of Chemistry & Physics Values for Coefficient of Friction (http:/ / books.google. com/ books?id=WDll8hA006AC& pg=PT2503& lpg=PT2503)

Characteristic Phenomena in Conveyor Chain (http:/ / chain-guide.com/ basics/ 2-3-1-coefficient-of-friction.html)

Atomic-scale Friction Research and Education Synergy Hub (AFRESH) (http:/ / nsfafresh. org) an EngineeringVirtual Organization for the atomic-scale friction community to share, archive, link, and discuss data, knowledgeand tools related to atomic-scale friction.

WearIn materials science,wear is erosion or sideways displacement of material from its "derivative" and original positionon a solid surface performed by the action of another surface.

Wear is related to interactions between surfaces and more specifically the removal and deformation of material on asurface as a result of mechanical action of the opposite surface.[1]The need for relative motion between two surfacesand initial mechanical contact between asperities is an important distinction between mechanical wear compared toother processes with similar outcomes.[2]

The definition of wear may include loss of dimension from plastic deformation if it is originated at the interfacebetween two sliding surf aces.However, plastic deformation such as yield stress is excluded from the wear definition if it doesn't incorporates arelative sliding motion and contact against another surface despite the possibility for material removal, because itthen lacks the relative sliding action of another surface.Impact wear is in reality a short sliding motion where two solid bodies interact at an exceptional short time interval.Previously due to the fast execution, the contact found in impact wear was referred to as an impulse contact by thenomenclature. Impulse can be described as a mathematical model of a synthesised average on the energy transportbetween two travelling solids in opposite converging contact.Cavitation wear is a form of wear where the erosive medium or counter-body is a fluid.Corrosion may be included in wear phenomenons, but the damage is amplified and performed by chemical reactionsrather than mechanical action.Wear can also be defined as a process where interaction between two surfaces or bounding faces of solids within theworking environment results in dimensional loss of one solid, with or without any actual decoupling and loss of material. Aspects of the working environment which affect wear include loads and features such as unidirectionalsliding, reciprocating, rolling, and impact loads, speed, temperature, but also different types of counter-bodies suchas solid, liquid or gas and type of contact ranging between single phase or multiphase, in which the last multiphasemay combine liquid with solid particles and gas bubbles.
http://en.wikipedia.org/w/index.php?title=Liquidhttp://en.wikipedia.org/w/index.php?title=Phase_%28matter%29http://en.wikipedia.org/w/index.php?title=Gashttp://en.wikipedia.org/w/index.php?title=Liquidhttp://en.wikipedia.org/w/index.php?title=Corrosionhttp://en.wikipedia.org/w/index.php?title=Cavitationhttp://en.wikipedia.org/w/index.php?title=Impulse_%28physics%29http://en.wikipedia.org/w/index.php?title=Yield_stresshttp://en.wikipedia.org/w/index.php?title=Plastic_deformationhttp://en.wikipedia.org/w/index.php?title=Asperity_%28materials_science%29http://en.wikipedia.org/w/index.php?title=Surfacehttp://en.wikipedia.org/w/index.php?title=Solidhttp://en.wikipedia.org/w/index.php?title=Erosionhttp://en.wikipedia.org/w/index.php?title=Materials_sciencehttp://nsfafresh.org/http://chain-guide.com/basics/2-3-1-coefficient-of-friction.htmlhttp://chain-guide.com/basics/2-3-1-coefficient-of-friction.htmlhttp://books.google.com/books?id=WDll8hA006AC&pg=PT2503&lpg=PT2503http://books.google.com/books?id=WDll8hA006AC&pg=PT2503&lpg=PT2503http://www.physclips.unsw.edu.au/http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htmhttp://books.google.com/?id=Q64Kq2HlyucC&pg=PA3&lpg=PA3&dq=Physical+wear+is+associated+with+frictionhttp://books.google.com/?id=Q64Kq2HlyucC&pg=PA3&lpg=PA3&dq=Physical+wear+is+associated+with+friction


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Rear sprocket for a bicycle where the left isunused and the right has obvious wear from

clockwise rotation.

Measurement

Today there exist a couple of standard test methods for different typesof wear to determine the amount of material removal during a specifiedtime period under well-defined conditions.The ASTM International Committee G-2 attempts to standardise weartesting for specific applications, which are periodically updated. TheSociety for Tribology and Lubrication Engineers (STLE) hasdocumented a large number of frictional wear and lubrication tests. Butall test methods have inbuilt limitations and do not give a true picturein every aspect.This can be attributed to the complex nature of wear, in particular"industrial wear", and the difficulties associated with accuratelysimulating wear processes.(ref4)

A standard result review for wear tests, defined by the ASTM International and respective subcommittees such asCommittee G-2, should be expressed as loss of material during wear in terms of volume. The volume loss gives atruer picture than weight loss, particularly when comparing the wear resistance properties of materials with largedifferences in density.For example, a weight loss of 14 g in a sample of tungsten carbide + cobalt (density = 14000 kg/m) and a weightloss of 2.7 g in a similar sample of aluminium alloy (density = 2700 kg/m) both result in the same level of wear(1 cm) when expressed as a volume loss. The inverse of volume loss can be used as a comparable index of wearresistance.Standard wear tests are only used for comparative material ranking of a specific test parameter as stipulated in thetest method. For more realistic values of material deterioration in industrial applications it is necessary to conduct

wear testing under conditions simulating the exact wear process.(ref4)The working life of an engineering component is expired when dimensional losses exceed the specified tolerancelimits. Wear, along with other ageing processes such as fatigue and creep in association with stress concentrationfactors such as fracture toughness causes materials to progressively degrade, eventually leading to material failure atan advanced age.Wear in industrial applications is one of a limited number of fault factors in which an object loses its usefulness andthe economic implication can be of enormous value to the industry.(ref 4)The Archard Equation is the classic wear prediction model - see Bisson[3]

Stages of wearUnder normal mechanical and practical procedures, the wear-rate normally changes through three differentstages(ref.4):

Primary stage or early run-in period, where surfaces adapt to each other and the wear-rate might vary betweenhigh and low.

Secondary stage or mid-age process, where a steady rate of ageing is in motion. Most of the componentsoperational life is comprised in this stage.

Tertiary stage or old-age period, where the components are subjected to rapid failure due to a high rate of ageing.

The secondary stage is shortened with increasing severity of environmental conditions such as higher temperatures,strain rates, stress and sliding velocities etc.Note that, wear rate is strongly influenced by the operating conditions. Specifically, normal loads and sliding speedsplay a pivotal role in determining wear rate. In addition, tribo-chemical reaction is also important in order to
http://en.wikipedia.org/w/index.php?title=Archard_Equationhttp://en.wikipedia.org/w/index.php?title=Fracture_toughnesshttp://en.wikipedia.org/w/index.php?title=Creep_%28deformation%29http://en.wikipedia.org/w/index.php?title=Fatigue_%28material%29http://en.wikipedia.org/w/index.php?title=Aluminium_alloyhttp://en.wikipedia.org/w/index.php?title=Cobalthttp://en.wikipedia.org/w/index.php?title=Densityhttp://en.wikipedia.org/w/index.php?title=ASTM_Internationalhttp://en.wikipedia.org/w/index.php?title=ASTM_Internationalhttp://en.wikipedia.org/w/index.php?title=File%3AK%C3%A6detandhjul.jpg


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understand the wear behavior. Different oxide layers are developed during the sliding motion. The layers areoriginated from complex interaction among surface, lubricants, and environmental molecules. In general, a singleplot, namely wear map. demonstrating wear rate under different loading condition is used for operation. This graphalso represents dominating wear modes under different loading conditions (ref. 13).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 frictionmechanisms. Surface engineering and treatments are used to minimize wear and extend the components working life.[4][5]

TypesThe study of the processes of wear is part of the discipline of tribology. The complex nature of wear has delayed itsinvestigations and resulted in isolated studies towards specific wear mechanisms or processes.[6] Some commonlyreferred to wear mechanisms (or processes) include:1.1. Adhesive wear2.2. Abrasive wear3.3. Surface fatigue4.4. Fretting wear5.5. Erosive wearA number of different wear phenomena are also commonly encountered and presented in the literature. Impact-,cavitation-, diffusive- and corrosive- wear are all such examples.These wear mechanisms, however, do not necessarily act independently and wear mechanisms are not mutuallyexclusive.[2] "Industrial Wear" are commonly described as incidence of multiple wear mechanisms occurring inunison. Another way to describe "Industrial Wear" is to define clear distinctions in how different frictionmechanisms operate, for example distinguish between mechanisms with high or low energy density. Wear

mechanisms and/or sub-mechanisms frequently overlap and occur in a synergistic manner, producing a greater rateof wear than the sum of the individual wear mechanisms.

Adhesive wear

Adhesive wear can be found between surfaces during frictional contact and generally refers to unwanteddisplacement and attachment of wear debris and material compounds from one surface to another. Two separatemechanisms operate between the surfaces. ==Friccohesity== defines actual changes in cohesive forces and theirreproduction in form of kinetic or frictional forces in liquid when the clustering of the nano-particles scatter inmedium for making smaller cluster or aggregates of different manometer levels.

1. Adhesive wear are caused by relative motion, "direct contact" and plastic deformation which create wear debrisand material transfer from one surface to another.

2.2. Cohesive adhesive forces, holds two surfaces together even though they are separated by a measurable distance,with or without any actual transfer of material.

The above description and distinction between "Adhesive wear " and its Counterpart "cohesive adhesive forces " arequite common. Usually cohesive surface forces and adhesive energy potentials between surfaces are examined as aspecial field in physics departments. The adhesive wear and material transfer due to direct contact and plasticdeformation are examined in engineering science and in industrial research.Two aligned surfaces may always cause material transfer and due to overlaps and symbiotic relations betweenrelative motionalwear and chemical cohesive attraction, the wear-categorization have been a source for

discussion. Consequently, the definitions and nomenclature must evolve with the latest science and empiricobservations.
http://en.wikipedia.org/w/index.php?title=Adhesive_wear_%28material%29http://en.wikipedia.org/w/index.php?title=Surface_engineering


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Generally, adhesive wear occurs when two bodies slide over or are pressed into each other, which promote materialtransfer. This can be described as plastic deformation of very small fragments within the surface layers. Theasperities or microscopic high points or surface roughness found on each surface, define the severity on howfragments of oxides are pulled off and adds to the other surface, partly due to strong adhesive forces betweenatoms[1]but also due to accumulation of energy in the plastic zone between the asperities during relative motion.

SEM micrograph of adhesive wear (transferred materials) on 52100 steel samplesliding against Al alloy. (Yellow arrow indicate sliding direction)

The outcome can be a growing roughening and creation of protrusions (i.e., lumps) above the original surface, inindustrial manufacturing referred to as galling, which eventually breaches the oxidized surface layer and connects tothe underlying bulk material which enhance the possibility for a stronger adhesion[7] and plastic flow around thelump.The geometry and the nominal sliding velocity of the lump defines how the flowing material will be transported andaccelerated around the lump which is critical to define contact pressure and developed temperature during sliding.The mathematical function for acceleration of flowing material is thereby defined by the lumps surface contour.It's clear, given these prerequisites, that contact pressure and developed temperature is highly dependent on thelumps geometry.Flow of material exhibits an increase in energy density, because initial phase transformation and displacement of

material demand acceleration of material and high pressure.Low pressure is not compatible with plastic flow, only after deceleration may the flowingmaterial be exposedto lowpressure and quickly cooled. In other words, you can't deform a solid material using direct contact without applyinga high pressure and somewhere along the process must acceleration and deceleration take place, i.e., high pressuremust be applied on all sides of the deformed material. Flowing material will immediately exhibit energy loss andreduced ability to flow due to phase transformation, if ejected from high pressure into low pressure. This abilitywithholds the high pressure and energy density in the contact zone and decreases the amount of energy or frictionforce needed for further advancement when the sliding continues and partly explain the difference between the staticand sliding coefficient of friction ( ) if the main fracture mechanisms are equal to the previous.Adhesive wear is a common fault factor in industrial applications such as sheet metal forming (SMF) and commonlyencountered in conjunction with lubricant failures and are often referred to as welding wear or galling due to theexhibited surface characteristics, phase transition and plastic flow followed by cooling.
http://en.wikipedia.org/w/index.php?title=Coefficient_of_frictionhttp://en.wikipedia.org/w/index.php?title=Sheet_metal_forminghttp://en.wikipedia.org/w/index.php?title=Sheet_metal_forminghttp://en.wikipedia.org/w/index.php?title=Coefficient_of_frictionhttp://en.wikipedia.org/w/index.php?title=Gallinghttp://en.wikipedia.org/w/index.php?title=File%3AAdhesive_wear_on_52100_steel_sample.jpghttp://en.wikipedia.org/w/index.php?title=Plastic_zonehttp://en.wikipedia.org/w/index.php?title=Surface_roughnesshttp://en.wikipedia.org/w/index.php?title=Asperity_%28materials_science%29


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The type of mechanism and the amplitude of surface attraction, varies between different materials but are amplifiedby an increase in the density of "surface energy". Most solids will adhere on contact to some ex

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