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TribologyinMechanicalEngineering
MAE493N/593T
Dr.Konstantinos
A.
Sierros
WestVirginiaUniversity
Mechanical&AerospaceEngineering
ESBAnnex
263
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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.aspx8/8/2019 MAE 493N 593T Lec14
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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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Elastohydrodynamiclubricationonthemicroscale
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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Elastohydrodynamiclubricationonthemicroscale
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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Lubricationofsoftsurfaces
Whenonesurfaceiscoatedwithasoftelasticmaterialtheinteractionofthesurfacesandthelubricantbecomemorecomplicated Naturalexamplesofthosesituationsareanimaljoints
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Lubricationofsoftsurfaces
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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Lubricationofsoftsurfaces
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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Transitionfromonelubricationregimetoanother
Twolubricatedsteelsurfacesslidingunderisothermalconditions
Region
I:Coefficient
of
friction
is
low
(
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Transitionfromonelubricationregimetoanother
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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Testmethodsinboundarylubrication
Fourballtests
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Testmethodsinboundarylubrication
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
Lubricationofsoftsurfaces
Breakdownof
lubrication:
Scuffing
TheBlok
temperaturehypothesis
Testmethodsinboundarylubrication
Solidlubricants