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TribologyinMechanicalEngineeringMAE493N/593T
Dr.Konstantinos
A.
Sierros
WestVirginiaUniversity
Mechanical&AerospaceEngineering
ESBAnnex
263
[email protected]
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Project
listTeams:Names&Members
Projecttitle
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Wearofcoins
Eachgroup
must
submit
1short
report
(2
pages
max
including
figures,graphs,tablesetc)
Deadline:Thursday7October2010(duringclass)
Pleasetypeyourreportandsubmitahardcopy
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Wear
IntroductionDefinitionandmeasurementofwear
Classificationofwear
Mechanismsofwear
Seizure
Meltwear
Oxidationdominatedwear
Mechanicalwearprocesses
Fatigue
wear
in
rolling
contacts
Frettingandcorrosionwear
Erosivewear
Thirdbodiesandwear
Wear
by
abrasive
contaminants
Interfacialthird
bodies
Debrisanalysis
Furtherreading
http://www.engineering.leeds.ac.uk/ietsi/Research_Projects/Lubricant
surface.shtml
DLCcoatingweartrackinoillubricatedconditions
http://www.engineering.leeds.ac.uk/ietsi/Research_Projects/Lubricant-surface.shtmlhttp://www.engineering.leeds.ac.uk/ietsi/Research_Projects/Lubricant-surface.shtmlhttp://www.engineering.leeds.ac.uk/ietsi/Research_Projects/Lubricant-surface.shtmlhttp://www.engineering.leeds.ac.uk/ietsi/Research_Projects/Lubricant-surface.shtml
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Seizure
Atveryhighcontactpressures,grossseizureofthesurfacesoccur(junction
growth)
Theseizureregimecorrespondstocatastrophicgrowthofasperityjunctionsso
that; Areal
Aapparent
Whenseizureisreached;temperatureishighenoughsohardness
isreduced
while
the
local
resistance
to
deformation
is
decreased
due
to
the
strain
rate
increase
Seizureisindependentofslidingvelocity
Wearmechanisms
21
2~
/1 mp =
Setting1
/2 =0.9
andm= 0.9
Pressureisbetween
0.5and0.6
Frictionfactor constants
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WearmechanismsMeltwear
Localizedmeltingoftheuppermostlayerofthewearingsurface
Experimentallyhas
been
observed
for
steels
at
sliding
speeds
as
low
as
1m/s
Athighervelocities
isdecreasingandliquidmetal formsattheinterfacedueto
melting(hydrodynamiclubricatingfilm)
meltlubrication
situationmeltsmoreandmorematerial(highwearrate)while
is
low
Equivalencetosolidsslidingonsnow/ice
~
~~
U
pw
=
Wearrate Materialparameters
Whensliding
velocity
is
low
2nd
term
of
equation
islargeandwearrateissmall
Whenslidingvelocityishigh 1st
termissignificant
andmelt
wear
is
occurring
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WearmechanismsOxidationdominatedwear
Dependsontheabilityofthewearingmaterialtooxidize
Atlowspeedsweardebrisismetallic,athigherspeedsconsistsofironoxidessincetemperatureisraisingtoflashpoint(700oC)
Oxidefilmcanreducethewearrateattheinterface
ClosetoPQline,oxidefilmisthin,patchyandbrittleandweariscausedbyfilm
spallation(mild
oxidational
wear
regime)
Thisspallingaway,exposesmorefreshmetalthatoxidizesagain
Athighervelocities,thefilmismorethick
andtheoxidationbecomessevere
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Wearmechanisms
Oxidationdominatedwear
Mild andsevere refertooxidationandnotwearrate
Mildoxidationalwearisconcernedwithratekineticsoftheoxidationprocess
~
~
max
~
exp
U
p
R
QCw o
= Arrheniusrelationship
ActivationenergyforstaticoxidationofFe
FlashtemperaturetocauseoxidationMustbeestablished
experimentally
Gasconstant
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WearmechanismsOxidationdominatedwear
Severeoxidational
wear
Athighervelocities,highertemperaturesaregeneratedresultinginincreased
oxidationrateandsofteninganddeformationoftheoxidefilm
Thefilmflowsandspreadsoutovercoolerregionsofthesurface
Asperity
melting
is
away
of
redistributing
the
heat
input
in
a more
uniform
way
Factorof
moltenoxide
lostfrom
surface
+=~
~
~~
U
ppfw m
materialparameters
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WearmechanismsMechanicalwearprocesses
Atslidingvelocitiesbelow0.1m/ssurfaceheatingisnegligible
andthe
frictionalforce
is
principally
deforming,
shearing
and
removing
material
in
the
formofsmallparticles(weardebris)
InthisregimeArchardequationisobeyed;
HWw = Archard equation
Sincevelocitydoesnotplayamajorrole,wecanusethe2nd
formofwearmaps
whichinclude
asurface
topography
parameter
h
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WearmechanismsMechanicalwearprocessesRunningin
Whenlubricatedmachinecomponentsruntogetherforthefirsttime,their
loadcarrying
capacity
is
much
less
than
would
be
ifthey
had
been
running
togetherforaninitialperiodunderalightload
Improvementinconformity,topographyandfrictionalcompatibilityiscalled
runningin
During running
in
wearrateisinitiallyquitehighbutassurfacesbecomesmootherandconformthewearratefalls
Runninginisassociatedwithadhesionandabrasion
Whenrunninginiscomplete,steadystatewearfollowsandthen
fatigue
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Wearmechanisms
Mechanical
wear
processes
Adhesive
wear
Adhesivewearmodelsarebasedonthenotionthattouchingasperities
adheretogetherandplasticshearingofthejunctionsleadsthesoftasperity
tipstodetachandformwearfragments
Tearing
of
macroscopic
chunks
of
material
is
called
galling
Scuffingisusedtodescribetheonsetofadhesivewearbetweenlubricated
surfacesduetolubricantfilmfailure
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Wearmechanisms
MechanicalwearprocessesAdhesivewear(Archard,1953)
Single
asperity
junction
of
diameter
2
ShearingofjunctionABshouldoccuralongpath2ratherthanpath1(original
interface).ThisistrueespeciallywhenplaneABisinclinedw.r.t.tothesliding
direction.
A
transfer
wear
particle
forms
(assume
that
particle
is
hemisphere
of
radius
) Volumeofparticleis2/33 foraslidingdistanceof2
Ifwedivideby2
wegetthevolumeperunitslidingdistanceequalto1/32
W h i
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WearmechanismsMechanicalwearprocessesAdhesivewear(Archard,1953)
Sincetheasperitydeformsplasticallywecanrelate
toWi
usinghardnessH
H
W
H
Ww i
33
1
3
2 ===
2= HWi
Summingup
all
such
wear
events
we
have;
Where;
Wistotalloadcarriedbythetwosurfaces
Apredicted
value
of
Kfor
the
junction
as
a
wholeis1/3
However,theassumptionthatevery
junction
contributesto
wear
volume
gives
unrealistic
valuesofK (lookattable5.2)
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Wearmechanisms
MechanicalwearprocessesAdhesivewear(Archard,1953)
Sowe
can
suppose
that
Krepresents
the
probability
of
generating
awear
fragment
ThereforeKmustalwaysbelessthan1
SuchsimplemodelscannotpredicttherangeofKvaluesthatareobservedin
practice
The
state
of
debris
particles
and
their
shape
when
they
where
first
produced
and
wheretheyarecollectedmustbecarefullyexaminedmicroscopicallyandanalyzed
chemically
Wear debris
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Weardebris
Thispictureshowsatypicalwearscarobtainedunderlinearslidingconditions(a=90).
Mostofthedebrisaretrappedandcompactedwithinacentralrolloriented
perpendiculartotheslidingdirection.
Alimited
amount
of
debris
is,
however,
displaced
from
the
contact
by
an
extrusion
processoccurringattheendsoftheroll.
PMMAsurface/steelball
FromA.Chateauminois webpage
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Summary
Wearmechanisms
Seizure
Meltwear
Oxidation
dominated
wearMechanicalwearprocesses
Adhesivewear
