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TribologyinMechanicalEngineering

MAE493N/593T

Dr.Konstantinos

A.

Sierros

WestVirginiaUniversity

Mechanical&AerospaceEngineering

ESBAnnex

263

[email protected]

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Boundarylubricationandfriction

Introduction

Mechanismofboundarylubrication

Importanceofmolecularfilms

Effectoftemperatureonsurfacefilms

Elastohydrodynamiclubrication

Lubricationofsoftsurfaces

Breakdownoflubrication:Scuffing

TheBlok

temperaturehypothesis

Testmethodsinboundarylubrication

Solidlubricants

http://www.stle.org/resources/lubelearn/lubrication/default.aspx
http://www.stle.org/resources/lubelearn/lubrication/default.aspxhttp://www.stle.org/resources/lubelearn/lubrication/default.aspx

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Elastohydrodynamiclubricationonthemicroscale

If

>>1

the

film

is

thick

and

friction

will

depend

on

the

bulk

rheological

propertiesofthelubricantandtheeffectofsurfaceroughnessisnegligible

If1

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However,

it

is

not

very

clear

when

exactly

the

transition

from

hydrodynamic

to

boundarylubricationhappens

Inthemicroelastohydrodynamicregime,anyanalysismusttakeintoaccount

thestatistical

nature

of

the

involved

surfaces.

Thephysicalpropertiesofthelubricantshouldalsobeaccountedfor

Assurfacesarepushedclosertogether,smallvolumesoflubricant(which

becomesmore

viscous)

are

trapped

in

valleys

and

pockets

between

asperities

Asperitiesareflattenedbylocalstressfields

U

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Anabsolute

minimum

film

thickness

can

be

produced

Forheavilyloadedgearsandcamsisoftenaround0.5m

Experimentalworksuggeststhatunderhighpressuresthebehaviourof

lubricantsbecomesnonNewtonian

Atveryhighrangesofpressuresthefluidisbehavinglikeaverythinfilmofa

perfectlyplasticmaterial

Thismaterialflows

ataconstantshearstresswhosevalueisindependentofthe

shearstrain

rate

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http://tribo.kaist.ac.kr/zres1.htm

Optical

interferometry

Flowinlet

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Whenonesurfaceiscoatedwithasoftelasticmaterialtheinteractionofthesurfacesandthelubricantbecomemorecomplicated Naturalexamplesofthosesituationsareanimaljoints

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Hip

or

shoulder

can

be

represented

by

spherical

or

ellipsoidal

shapes

Kneeandfingerjointscanbemodeledascylindricalcontacts

Softandporousarticularcartilagecoversthebonesurface(afewmmthick)

Synovialfluid

separates

the

surfaces

(a

little

bit

more

viscous

than

water)

Surfaceofcartilageisrelativelyrough(peaktovalleyheight

oftheorderof1m)

Relativelysliding speedsofthetwosurfacesremainlow

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Conventionalhydrodynamicswouldpredictverysmallthicknesses (

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Breakdownoflubrication:scuffing

Itisverydifficulttoavoidinitiationofcatastrophicfailureoflubricantundersevereconditions

Thefirstsignoffailureinitiationisasuddenincreaseinnoiseandvibration

followedbyanincreaseinoperatingtemperature

Theincreaseintemperaturemayevencausesmokeandfumesgivenofffromthelubricant

Grossadhesionandmaterialtransferfromonesurfacetotheotheroccursatthis

stage

Obliterationof

the

surface

roughness

also

occurs

Scuffing

Localizeddamage

caused

by

the

occurrence

of

solid

phase

welding

between

sliding

surfaces,withoutlocalsurfacemelting

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TheBloktemperaturehypothesis

Theonsetofscuffinglimitstheoperationofanumberofmachineelementssuchasgears,cams,followers(inrollingandsliding)

Thereisstillnofullyreliablewayofpredictingwhetherscuffingwillbe

encounteredinaparticularsituation

Blok(1937)suggestedthatscuffingisinitiatedwhenthetotal

contact

temperaturereachessomecriticalvalue (assumingthatthelubricantischemically

inactive)

Totalcontacttemperature

Bulktemperatureof

machinecomponent

Instantaneous increase in

surface temperature

(flash temperature)

Canbemeasuredoratleast

estimatedwithout

difficulty

Canbeestimated

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Transitionfromonelubricationregimetoanother

Alternative

to

Stribeck

curve

to

represent

transition

of

lubrication

regimes

TheOECDInternationalResearchGrouponWearofMaterialstransitionmap

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Twolubricatedsteelsurfacesslidingunderisothermalconditions

Region

I:Coefficient

of

friction

is

low

(

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Region

III:

Conditions

are

very

severe

and

no

lubricant

film

can

prevent

metallic

contact

Rapidwearandhighfrictionprevail

Scuffing

TransitionfromregionIItoIIIdependsmainlyonchemicaleffectsandismuch

less

dependent

on

surface

roughness

or

sliding

speed

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Scuffing

Scuffing

of

the

contact

can

be

avoided

by;Loweringfriction

Loweringspeed

Loweringload

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Fourballtests

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Felexpinandvee

blocktest

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Solidlubricants

Coherentsurfacefilmofalowshear()materialwithathicknesscomparable

withthesizeofthesurfaceasperities

Thisfilmisinterposedbetweenthetwometalsurfaces(solidlubricant)

TrueareaofcontactA

willbecontrolledbyindentationhardnessHofthe

substrate

FrictionforcedependsontheproductofareaA

andthestressrequiredtoshear

thebondbetweenthefilmandtheopposingsolidsurface

H

Minimumcoefficientoffrictionwillbeprovidedbyrelativelysoftfilmsdeposited

onhard

substrates

Conceptof

solid

lubricants

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Solidlubricants

Organicfilms;

soaps

and

greases

Polymericmaterials

Lamellarsolids

Thinmetallicfilms

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Soapsandgreases

Thinlayermetallicsoapsareusedasadditivesinhydrocarbonmineraloils

(boundarylubrication)

Solidsoapcanbeusedasactivelubricantinitsownright

Agreaseconsistsofabasehydrocarbonoilwhichisthickenedtoalmostasolid

byaddingmetallicsoapandinorganicfillers

Soapsarepresentasmicelles.Fillersareusedtocontroloperatingtemperature

(silica)

http://www.hcc.mnscu.edu/chem/V.26/page_id_23713.html

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Polymericmaterials

PTFE

polytetrafuoroethylene Teflon

Lowtendencytointerfacialadhesion

Widelyusedasdryboundarylubricant

WhenrubbedagainstametalsurfaceathinPTFElayeristransferredonthe

metal

Verylowfrictioncoefficients(

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Lamellarsolids

Solidswithlamellar(platelike)crystalstructure Directionalanisotropy

Graphiteandmolybdenumdisulphide(MoS2

)

Strength

of

interatomic

bonding

between

successive

layers

is

very

much

weaker

thanthatwithintheplanesthemselves

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Thinmetallicfilms

Coatingorplatingthemetalsurfacewithathinlayerofalow

shearstrength

metaloralloy

Copperleadorleadbronzeautomotivebearingsarecoatedwithafewtensof

micronsleadtinalloy.(Providesboundarylubricationandcorrosionprotection)

Solidlubricants:Lead,tin,indium,silver,gold(asthinas0.1micron)

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Summary

Elastohydrodynamiclubrication


Breakdownof

lubrication:

Scuffing

TheBlok

temperaturehypothesis


Solidlubricants

Documents

MAE 493N 593T Lec14