Available online at www.sciencedirect.comWear 264 (2008)
4759Synergic effect of reinforcement and heat treatment on the two
body abrasivewear of an AlSi alloy under varying loads and abrasive
sizesS. Das, D.P. Mondal, S. Sawla, N. RamakrishnanRegional
Research Laboratory (CSIR), Bhopal 462026, IndiaReceived 18 January
2006; received in revised form 19 January 2007; accepted 24 January
2007Available online 27 February 2007AbstractIn the present study,
an attempt was made to examine the synergic effect of SiC particle
reinforcement and heat treatment on the two bodyabrasive wear
behavior of an AlSi alloy (BS: LM13) under varying loads and
abrasive sizes. Silicon carbide particles with size 5080 m
werereinforced in AlSi alloy, in varying concentration (10 wt% and
15 wt%), by solidication process (vortex technique) and the
composite melt wassolidied by gravity casting in a cast iron die.
The alloy and composites were solution treated at 495C for 8 h,
quenched in water and aged at175C for 6 h and cooled in air. Two
body abrasive wear behaviour of cast and heat-treated alloy and
composite, was examined against abrasives ofdifferent sizes (40 m,
60 m and 80 m), at varying applied loads (1 N, 3 N, 5 N and 7 N),
up to a sliding distance of 108 m. It has been noted thatthe alloy
suffers from higher wear rate than that of composites either in
cast or heat-treated conditions, irrespective of applied load and
abrasivesize. Further, in most of the cases, the wear rate of
composite decreases with increase in SiC particle content. Efforts
were made to correlate wearbehavior of Al alloy and composites in
terms of mechanical properties, microstructural characteristics,
applied load and abrasive size through anempirical equation. 2007
Elsevier B.V. All rights reserved.Keywords: Al alloy; Al alloy
composites; Abrasive wear; Heat treatment; Abrasive size1.
IntroductionAl-alloy matrix composites (AMCs) containing hard
disper-soidsaregainingimmenseindustrial importancebecauseoftheir
excellent combinationof physical, mechanical
andtri-bologicalpropertiesoverbasealloys[1]. Theseincludehighwear
and seizure resistance, high specic strength and stiffness,improved
high temperature strength, controlled thermal expan-sion coefcient
and high damping capacity [18]. It is reportedthat
Al-alloyreinforcedwith10 wt%SiCparticlecompositeprovides comparable
mechanical properties but better
thermalconductivityandspecicheatthanthecastirons[5,7]. Asaresult,
frictional heating of these composites are noted to be sig-nicantly
less than that of cast irons [7]. This leads to the use ofthese
composites in several automobile and engineering com-ponents where
wear, tear and seizure are the major problems
inadditiontotheweightsaving.SomeofthesecomponentsareCorresponding
author. Tel.: +91 755 2488562; fax: +91 755 2587042.E-mail address:
[email protected] (S. Das).pistons, brake drums, cylinder heads,
connecting rods and
driveshaftsforautomobilesectorsandimpellers,agitators,turbineblade,
valves, pumpinlet, vortexnderformarineandmin-ing sectors [911]. In
recent years, fabrication of several suchcomponents from AMCs,
reaches to the commercial productionstage [712]. Most of the
aforesaid components are subjected todifferent kinds of wear and
tear related failure. In this context, itis required to
characterize Al-composites in terms of wear underdifferent
experimental conditions.Sliding wear behavior of AMCs has been
studied by manyinvestigators [1316], however, limitedattempts have
beenmadetostudythe abrasive wear characteristics of
Al-composites[1727]. According to these reports, wear and seizure
resistanceof AMCs is considerably higher than that of the base
alloys. Thisis primarily attributed to the fact that the hard
dispersoids
(rein-forcingphase)protectthesurfacefromthedestructiveactionoftheabrasivesbyreducingthedepthofpenetrationoftheabrasives
(hard asperities of the counter surface) and the
con-tactbetweentheabrasiveandthematrix.
Ontheotherhand,fewinvestigatorshavereportedatransitionofabrasivewearbehaviour
of composites which was dependent on abrasive size0043-1648/$ see
front matter 2007 Elsevier B.V. All rights
reserved.doi:10.1016/j.wear.2007.01.03948 S. Das et al. / Wear 264
(2008)
4759andappliedload[19,21,24,28].Accordingtotheseinvestiga-tors,
aboveacritical valueofloadandabrasivesize,
AMCsexhibithigherwearratethanthealloys[2430].
Undersuchcircumstances, the depth and width of wear track become
largerthan the size of dispersoid (i.e., h/d and/orw/d is greater
thanunity; where h andw are the average depth and width of
weargroove, respectively, and d is the average diameter of the
rein-forcing phase in the composite). This leads to the scooping
offofthedispersoidfromthesurfaceofthecompositessamples[19,21,24].
Furthermore, it isevident fromtheliteraturethatthe wearing surface
and the subsurface undergo plastic
defor-mation,andthisdeformationbecomesmoreseverewhentheabrasive
size is coarser and the applied load is higher [20,29].As a result
of such plastic deformation, the hard ceramic
dis-persoidgetsfracturedandfragmentedintonerones, and/ordebonded
from the matrix [2329]. In due course, these frag-mented particles
come out from the specimen surface. Thus theabrasive wear behavior
of composite depends on the materialcharacteristics like shape,
size, distribution and volume fractionof the dispersoids and
experimental parameters like applied loadand abrasive size
[2131].Ithasbeenreportedthatthewearresistanceofcompositeincreases
with increase in volume fraction and size of the dis-persoids
[2429]. One of the prime factors of the improvementin wear
resistance is increased in hardness of the Al-alloy due tothe
addition of hard dispersoids [2431] and more protection ofthe
matrix fromthe destructive action of the abrasive as the meanfree
path between the SiC particles is reduced with increase involume
fraction of SiC particle [32]. Several investigators havealso
proposed that wear resistance of a material not only dependson its
hardness and strength but also on its ductility and tough-ness
[33,34]. The reinforcement of Al2O3 particles in aluminumalloy
enhances the abrasive wear of the matrix. The reinforce-ment of 16
m Al2O3 particle strengthens the aluminum matrixand enhances the
wear resistance. However, the reinforcement ofcoarse particle (66
m) shows higher wear resistance. The oper-ating wear mechanism is
mainly consists of plastic deformationof the matrix material
[35].Thehardnessaswell astoughnessof acompositemate-rial depends
signicantly on the matrix microstructure, size anddistribution of
the dispersoids and the interfacial bonding char-acteristics [32].
The hardness of the composite increases withincrease in the volume
fraction of the dispersoid but at the sametime its toughness
decreases. Additionally, the casting defectsmay increase or the
possibility of clustering of dispersoid parti-cle may be more as
one increases the dispersoid content.
Thesemayreducethewearresistanceofthecomposite[30]. Thustheoverall
improvement inwearresistanceofthealloy,
duetoadditionofmoredispersoidmaynotbesohighespeciallyatseverewearingconditions(i.e.,
athigherappliedloadandcoarser abrasive size).The mechanical and
tribological properties of the compos-ite alsodependuponthe
matrixmicrostructure; hence, thepropertiesof
compositecanbeimprovedbyheat treatment.During heat treatment, the
matrix of composite behaves almostsimilar tothat of
thebasealloyandthedispersoidremainsunchanged. Attempts have been
made to improve strength, hard-ness and toughness of AlSi alloy and
AlSi alloy reinforcedwith hard particle composites by altering the
matrix microstruc-turethroughheat treatment [36,37].
Studiesonslidingwearcharacteristics reported that the wear and
seizure resistance ofthe alloy or composite are improved by heat
treatment [36,37].However, it is interesting to examine the
abrasive wear behaviorof AlSi alloy reinforced with hard particles
after heat treatment.In the present study, an attempt has been made
to examinethesynergiceffect ofparticlecontent andheat treatment
ontheabrasivewearbehaviorofanAlSialloyandAlSiSiCcomposite under
varying applied loads and abrasive sizes.2. Experimental2.1.
MaterialsThe AlSi (British Standard: LM13) alloy and LM13
alloycontaining 10 wt% and 15 wt% SiC particle have been
selectedinthepresentinvestigation. TheLM13alloynominallycon-tains
11.95% Si, 1.0% Mg, 1.5% Ni, 1.0% Cu, 0.8% Fe, 0.6%Mn and balance
is Al. Silicon carbide particles of size 5080 mhave been
incorporated into the AlSi alloy melt in varying con-centration (10
wt% and 15 wt% of the matrix alloy) by vortextechnique. The
composite melt was solidied in a cast iron diein the form of
circular disc of diameter 100 mm and thickness5 mm. The Al-alloy
melt was also cast in the same die. The
castalloyandcompositesweresolutionizedat495Cfor6 handquenched in
water. The samples were tempered at 175C for6 h followed by
air-cooling.2.2. Microstructural examinationFor microstructural
observations, samples were mechanicallypolished using standard
metallographic techniques, etched withKellers reagent and observed
in a JEOL 5600 scanning electronmicroscope (SEM). Prior to
SEMexamination, the samples weresputtered with gold.2.3. Two-body
abrasive wear testingHigh stress (two-body) abrasive wear tests of
Al alloy andAlalloySiCcompositeswereconductedon40 mm35 mm5
mmrectangular specimens using Suga made AbrasionTester (Model:
NUSI, Japan). Aschematic viewof the apparatusisshowninFig. 1.
Emerypaper embeddedwithSiCparti-cles (size: 40 m, 60 m and 80 m)
was used as the abrasivemedium. These emery papers were cut into
sizes and xed onthewheel(50
mmdiameter),whichrotatesagainstthespeci-mensurface.
Thespecimenoverthewheel wasxedwithalocking arrangement and load on
the specimen was applied withthe help of cantilever mechanism. The
specimen was subjectedto reciprocating motion against the wheel on
which the abrasivemedia is rigidly xed. The wheel makes one
complete revolutionafter each 400 cycles (corresponds to 26 m of
sliding distance)ofreciprocatingmotionofthespecimen.
Theabrasivepaperon the wheel was changed after an interval of each
400 cyclesso that the specimen surface always makes contact with
freshS. Das et al. / Wear 264 (2008) 4759 49Fig. 1. Schematic view
of Suga made abrasion tester (1) specimen stage; (2)specimen guide;
(3) abrasion wheel: (4) specimen press; (5) metal tting; (6)double
number detector; (7) motor; (8) xed weight; (9) weight; (10)
weightscale; (11) lock lever.abrasives. The tests were conducted at
different loads (i.e., 1 N,3 N, 5 N and 7 N) and at varying
abrasive sizes (40 m, 60 mand 80 m) up to a sliding distance of 108
m. The wear rate ofspecimen was calculated from weight loss
measurements. Thespecimens were ultrasonically cleaned prior to and
after eachinterval of the wear tests.2.4. SEM examination of wear
surface and subsurfaceIn order to understand the mechanism of
material
removal,thewearsurfaceandsubsurfacewereexaminedinSEM.Forsubsurface
examination, the worn samples were cut transversely,mounted,
polished, etched with Kellers reagent and observedwith SEM.2.5.
Mechanical testingThehardnessofthealloyandcomposites, inascast
andheat-treated conditions, is measured using a Vickers
hardnesstester at 2 Nload. Before hardness measurements, each
sample ispolished and its opposite sides are made perfectly
parallel. Yieldstrength, ultimate tensile strength and percentage
elongation
ofAl-alloyandAl-compositeweredeterminedfromtensiletestFig. 2. (a)
Microstructure of AlSi (LM13) alloy (A: aluminum, B: silicon). (b)
A higher magnication micrograph of (a) clearly depicts needle
shaped eutecticSi in Al matrix (B: silicon needle). (c) Typical
microstructure of heat-treated AlSi (LM13) alloy showing near
spherical Si particle in Al matrix. (d) A highermagnication
micrograph showing the spherical shaped eutectic Si (marked A) and
intermetallic phases (arrow marked).50 S. Das et al. / Wear 264
(2008) 4759data. Tensiletest
wasconductedinanINSTRON(UniversalTesting Machine) at a strain rate
of 104s1.3. Results3.1. MicrostructureFig. 2a shows a typical
microstructure of cast AlSi
(LM13)alloywhichdepictsdendritesofaluminium(markedA)andneedle
shaped eutectic silicon (marked B) in the interdendriticregions and
around the dendrites. Ahigher magnication micro-graph(Fig.
2b)clearlydepictsneedleshapedeutecticsilicon(marked B) in Al
matrix. The microstructure of heat-treatedLM13 alloy shows
fragmentation of needle-shaped eutectic sili-con (in cast
condition) into more or less spherical one (Fig. 2c). Ahigher
magnication micrograph (Fig. 2d) clearly shows
heat-treatedeutecticsilicon(markedA)andintermetallicphases(arrow
marked).Fig. 3a represents the microstructure of cast AlSi alloy
rein-forced with 10 wt%SiCp composite showing distribution of
SiCparticles in AlSi alloy matrix. A higher magnication
micro-graphofthecompositeshowsbondingbetweenSiCparticleandtheAlSi
alloymatrix(Fig. 3b). Thechangeinmatrixmicrostructure of composite
due to heat treatment is noted tobe similar to that observed in the
case of heat-treated alloy asshownin(Fig. 2b). Fig.
3cshowsthemicrostructureofcastAlSi15 wt% SiC composite. It also
indicates distribution ofSiC particles in Al matrix. The
distribution of SiC particle inAlSi alloy matrix is not affected by
the heat treatment. Fig. 3dshows the microstructure of
heat-treatedAlSiCcompositedepicts interface bonding between SiC and
AlSi alloy matrix.3.2. Mechanical propertiesThehardness,
ultimatetensilestrengthandelongationofLM13 alloy and LM13SiC
composites in cast and heat-treatedconditions are shown in Table 1.
It indicates that hardness andstrength of AlSiCcomposite are
2530%higher (depending onSiC content and heat treatment) than that
of alloy, respectively.But the ductility of composite is recorded
to be less than thatof the alloy. It is further noted from Table 1
that all the
abovementionedpropertiesareimprovedsignicantlyduetoheatFig. 3. (a)
Microstructure of Al10 wt% SiC composite in cast condition showing
distribution of SiC particle in Al matrix. (b) A higher magnication
micrographof (a) exhibits interface bonding between SiC particle
and Al matrix (A: SiC particle; B: AlSi alloy; C: eutectic silicon;
D: Al/SiC interface). (c) Microstructureof cast Al15 wt% SiC
composite showing distribution of SiC particle in Al matrix. (d)
Microstructure of heat-treated Al15 wt% SiC composite shows
interfacebonding between SiC particle and Al matrix.S. Das et al. /
Wear 264 (2008) 4759 51Table 1Mechanical properties of as cast and
heat-treated Al alloy and compositesMaterial Processing condition
Hardness (MPa) UTS (MPa) Elongation (%)LM13 As cast 132 180
1.0Heat-treated 146 210 3.0LM1310 wt% SiC As cast 151 210
1.0Heat-treated 155 220 1.0LM1315 wt% SiC As cast 159 220
1.0Heat-treated 173 230 1.0treatment. It is further notedthat
hardness andstrengthofcomposite increasedwithincrease inSiCpcontent
but theductility of composite is reduced considerably.3.3. Two body
abrasive wear behaviorThe wear rates of LM13 alloy and LM13SiC
composite
areplottedasafunctionofslidingdistancewhentheyaretestedagainst
different abrasivesizesunder varyingappliedloads.Fig. 4 represents
variation of the wear rate of alloy and compos-ite as a function of
sliding distance, when tested at 1 N load and40 m abrasive size. It
is noted that wear rate is almost invariantto the sliding distance
irrespective of the material. It has furtherbeen noted that the
wear rate of cast composites decreases withincrease in SiCp
content. However, after heat treatment, the wearrate of alloy and
composite were reduced. On the other hand, thewear rate of LM1315
wt% SiCp composite is noted to be
moreorlesssameinheat-treatedandincastconditions.Itmaybenoted that
the wear rate of LM1310 wt% SiCp composite andLM1315 wt% SiC
composite at 1 N load is more or less same.However, at an applied
load of 7 N, appreciable decrease in wearratewasobservedinLM1315
wt%SiCthanLM1310 wt%SiCcomposite. Thewearrateofheat-treatedLM1315
wt%SiCpcompositeisnotedtobe7.0 1011m3/m, whereasitis found to be 15
1011m3/m in LM1310 wt% SiCp com-posite. Additionally, when the load
is signicantly low and theabrasive size is ner, the abrasives are
unable to penetrate deepinto the specimen surface especially in the
case of composite. Insuch case, the abrasive primarily rubs on the
specimen surfacesFig. 4. Wear rate of Al alloy and composite as a
function of sliding distance(applied load: 1 N and abrasive size:
40 m).and leads to very less material removal in composites.
Becauseof such low wear rate, the exact contribution from SiCp
cannotclearlybeunderstood. Fig. 5clearlyindicates theeffect ofSiCp
content and heat treatment on the abrasive wear behaviouras a
function of sliding distance of alloy and compositesunder higher
load (7 N) and coarser abrasives (80 m). It
maybenotedfromthisgurethat thecompositeexhibitshigherwear
resistance than the alloy and the wear rate of compositereduces
with increase in SiCp content irrespective of cast andheat-treated
conditions. However, it is interesting to note that thewear rate of
LM1310 wt% SiCp composite is marginally lessthan that of the alloy.
The heat-treated alloy exhibits even lesswear rate than the cast
composite reinforced with 10 wt% SiCparticles.It is evident
fromFigs. 4 and 5 that the wear rate of each of thematerial is
increasing with increasing in abrasive size and load.However, the
wear rate is not following any specic functionalrelationship with
abrasive size irrespective of material. It is notedthat the wear
rate of the alloy is higher than those of composites.The wear rate
of composite containing 15 wt% SiCp is noted tobe less than that of
composite containing 10 wt% SiCp underheat-treated condition.
Similar type of plots at loads of 3 N, 5 Nand 7 N are shown in
Figs. 68, respectively. It is evident fromFig. 8 that in most of
the cases, the wear rate is almost
invarianttotheslidingdistanceirrespectiveofthematerialswhentheabrasive
size is 40 m. Fig. 9 shows the effect of abrasive sizeon the wear
rate of LM13 alloy and LM13SiC composite atan applied load of 5 N.
It is evident from Fig. 9 that in most ofFig. 5. Wear rate of Al
alloy and composite as a function of sliding distance(applied load:
7 N and abrasive size: 80 m).52 S. Das et al. / Wear 264 (2008)
4759Fig. 6. Wear rate of Al alloy and composite as a function of
sliding distance(applied load: 3 N and abrasive size: 40 m).Fig. 7.
Wear rate of Al alloy and composite as a function of sliding
distance(applied load: 5 N and abrasive size: 40 m).Fig. 8. Wear
rate of Al alloy and composite as a function of sliding
distance(applied load: 7 N and abrasive size: 40 m).Fig. 9. Wear
rate of Al alloy and composite as a function of abrasive size
(appliedload: 5 N).the cases the wear rate is almost invariant to
the abrasive sizeirrespective of the materials, when the abrasive
size is less than60 m. However, the wear rate increases rapidly if
the abrasivesize is coarser than 60 m.Fig. 10 shows the variation
of the wear rate of materials withappliedload, whentestedagainst
abrasiveof size60 m. Itisnotedthat thewearrateofcast alloyandcast
compositesisincreaseswithappliedloadanddoesnot followanyspe-cic
functional relationship except for cast LM13 alloy and castLM1310
wt%SiCcomposite which follows a liner relationshipwith applied
load. It may be noted further that at lower appliedload (3 N), the
wear rate of composites varies marginally withheat treatment. But
at the higher applied load (5 N) the wearrate of composite reduced
signicantly due to heat treatment andincrease in SiCp content. This
gure (Fig. 10) clearly demon-strates that the wear of alloy
decreases after heat treatment andreinforcement of SiC
particle.Fig. 10. WearrateofAl
alloyandcompositeasafunctionofappliedload(abrasive size: 60 m).S.
Das et al. / Wear 264 (2008) 4759 534. Discussion4.1. Materials and
mechanical propertiesIntheheat treatment of AlSi (LM13) alloy,
theneedle-shaped eutectic silicon is fragmented into more or less
sphericalone duringsolutiontreatment andne
precipitationonthematrixtakesplacewhileageingtreatment.
Thisisattributedtoreductioninsurfaceenergyof
theneedleshapedsiliconparticles while changing into near spherical
ones as the surfacearea to volume ratio is the minimum for
spherical shape.
Thisreductioninsurfaceenergyactsasthedrivingforceforsuchtransformation.
Again, the concentration of Si per unit volumeis more at the sharp
edges of the needle shaped particle, whichled to the existence of
concentration gradient between the sharpedges and the wider portion
of the Si needle. As a result,
thesiliconfromthesharpedgesisdissolvedanddiffusedtothewider region
and in due course of time, it turns into near spher-ical shape.
Here, the heat energy applied during heat treatmentacts as the
activation energy for such kind of transformation. Inaddition,
because of the same reasons, there is a possibility ofcoarsening of
spherical silicon particles with aging time. Hence,it is expected
that the spherical silicon particles may be coarserwhen they are
aged for longer duration. Further more, duringaging, precipitation
of alloying elements like Cu, Mg, Ni and Femay take place which
might have caused further strengtheningof theAl-alloy.
Thecompositeexhibits morehardness andstrength than that of alloy.
This is attributed to the fact that byaddition of SiCparticles in
the alloy generates more dislocationsbecause of thermal
mismatchstress [38,39]. Suchstress isoriginated due to differences
in thermal expansion coefcientof the matrix and the SiC particles
[38,39] and makes the matrixplastically constrained and caused
higher dislocation strength-ening of the matrix. The hard SiC
particle also acts as barriertoplasticdeformationof thematrixalloy,
whichresults intriaxial interaction stress at the interface. This
interaction stressalso results in higher strength of the composite.
Heat treatmentresults in formation of spherical Si particles and
precipitation ofother alloying elements (Figs. 2c and 3b) in terms
of complexintermetallic compounds. This leads to higher strength
aswell as higher toughness of the alloy and composite after
heattreatment.4.2. Effect of reinforcement on the abrasive wearIn
two-body abrasive wear, the abrasive particles in the abra-sive
media make contact with the specimen surface and penetrateinside
the specimen surface when load is applied. The
penetratedabrasiveissubjectedtoreciprocatingmotionoverthespeci-mensurfaceandthusresultinginformationofweargroovesonthesurfaceofthetest
specimen. Thesegroovesaregen-eratedduetooworremoval
ofmaterialseitherbycuttingor ploughing action. The depth and width
of the grooves andtheir number on the test specimen surface depend
on the shape,sizeandrakeangleoftheabrasives,
specimensurfacechar-acteristicssuchassurfaceroughnessandthestabilityoftheasperities
or hard protrusions, and the hardness of the specimensurface
[3138]. The hardness of the composites is noted to
behigherthanthatofthealloyandthehardnessincreaseswithincrease in
SiCp content (Table 1). Additionally, the hard
SiCparticlesincompositeactlikehardprotrusionoverthespec-imensurface.
Asaresult,
lessnumberofabrasivescomeincontactwiththematrixalloyandhencetheirdepthofpene-tration
is reduced. The abrasive particles, which penetrate
intothecompositesurface, interact
withtheSiCparticlesduringreciprocating motion. During this process,
the abrasive particlesmayeither scoopoff fromtheabrasivemedia,
asthebond-ingbetweenSiCandAl matrixisstrongerthanthebondingof
abrasive particles in the cloth or paper, or get degraded
bytheSiCparticles. Fig. 11ashowsatypical SEMmicrographof emerypaper
(particlesize: 80 m) beforewear test. Itshows SiC particles with
sharp edges embedded on the paper.Fig. 11b shows a SEM micrograph
of the emery paper (parti-cle size : 80 m) after the wear test (
load: 5 N and distance:81 m).
ItclearlydepictsthedegradationofSiCparticlesandsmearingoff
nefragmentedparticles ontheemerypaper.However insomeinstances,
scoopingoff (markedA) andbreaking (arrow marked) of the SiC
particle are also observed(Fig. 11c). Asamatter of thesefacts,
thecuttingefciencyortheseverityofthedestructiveactionofabrasivemediaisreducedsubstantiallywhentheymoveagainst
thecompositesurface and nally resulting in signicantly lower wear
rate ofthe composite materials (Figs. 49). The number of SiC
parti-cles on the composite increases with increasing SiCp
content,andthehardnessofcompositeisalsoincreasedduetoaddi-tion of
more SiC. Furthermore, inter-particle distances are alsoreduced due
to increase in SiC content. These, in turn,
decreasethewearrateofcompositewithincreaseinSiCcontent.Theimprovement
inwearresistanceofthealloyduetoadditionof SiC particles is shown in
Figs. 49. It is clearly noted
thatthewearresistanceofthealloyisimprovedby20%duetoaddition of
SiCand other experimental conditions. The improve-ment is
reducedwithincreaseinappliedloadandabrasivesize.It is
awell-establishedphenomenonthat
duringabrasivewearaportionofeffectivestresscausedplasticdeformationof
the specimen surface [33,34]. The hard ceramic dispersoidscarry the
major portion of this stress and hence the area
aroundtheseceramicparticlesundergoesmoreplasticdeformation.As the
matrix in composite is plastically constrained
andtheSiCparticlesarebrittleinnature, therearepossibilitiesof
either debondingor fracturingof SiCparticles at severewear
conditions (higher load and coarser abrasive size). Theseparticles,
asaconsequence, scoopedoff fromthespecimensurface and may lead to
increased wear rate. However, there isa possibility of picking up
of ner fragmented SiC particles ordegraded abrasives in the wear
groove which may improve wearresistance of the matrix so long they
are remained intact on theweargrooves.
Theplasticdeformationbecomessignicantlyhigh when wear is occurring
under the condition of highapplied load and coarser abrasive size.
Due to signicantly highplastic deformation, the lateral and
longitudinal cracks on thespecimen surface are generated. As a
result, at higher appliedload and coarser abrasive size, the
possibility of scooping off54 S. Das et al. / Wear 264 (2008)
4759Fig. 11. (a) SEMmicrograph of as received emery paper (abrasive
size: 80 m).(b) SEMmicrograph of emery paper (abrasive size: 80 m)
after wear test (load:5 N, distance: 81 m).
(c)SEMmicrographdepictingdegraded, scoopingoff(marked A) and
breaking (arrow marked) of SiC abrasive particles in
emerypaper.hard SiCparticles becomes higher and the improvement in
wearresistance of the composite over the base alloy is relatively
lessas compared to that obtained at lower load and ner
abrasivesize.4.3. Effect of heat treatmentThe abrasive wear
behaviour of a material depends on fac-tors like abrasive size and
applied load as the depth of cuttingand over all cutting efciency
of the abrasive media
increaseswithincreaseinabrasivesizeandappliedload. Astherela-tive
hardness of the specimen surface with respect to abrasiveincreases,
thedepthofpenetrationofabrasivedecreasesandcausinglesssurfacedamage.
Thisleadstothefactthatboththe alloy and composites exhibit minimum
wear rate after heattreatment. Similarly, the hardness of cast
composite is
improvedby1520%afterheattreatmentandthewearrateisreducedby 1570%
depending on test conditions. These facts
suggestthat,notonlythehardnessbutalsoothermechanicalproper-ties and
microstructural characteristics of alloy and compositecontrol
abrasive wear behaviour depending on the experimentalparameters.It
was mentioned that plastic deformation of the surface andsubsurface
is taking place during abrasive wear. In cast AlSialloy, needle
shaped eutectic silicon is randomly oriented. In fewregions, these
silicon particles are parallel to the wear groovesand in some other
regions they are placed perpendicular to thewear groove direction
at which the abrasive scratches the sur-face. Under this condition,
when the abrasive interacts with thesilicon needle oriented
perpendicular to the wear direction, theabrasive movement is
restricted until the Si needles are fractured.The fracturing of the
silicon needle is happened due to the inter-action between the
SiCparticles and load applied on the sample.Fig. 12 shows a SEM
micrograph of the subsurface of the AlSialloy clearly depicts the
highly deformed region (marked A),cracks (arrow marked) and
undeformed region (marked B). Itis observed that the sharp edged
silicon needles are fragmentedin to smaller size particles in the
subsurface deformed region.During abrasive wear, the effective
stress on the tip of abrasivemay be signicantly high and the depth
of penetration may
beconsiderablyhigherthanthediameterofthesiliconneedles.This in turn
causes fracturing of Si particles and is removed
asmicrocuttingchipsfromthespecimensurface. Additionally,the edges
of the needle shaped silicon particles may act as stressFig. 12.
SEM micrograph of subsurface of AlSi alloy depicting
fragmentationof eutectic silicon.S. Das et al. / Wear 264 (2008)
4759 55raiser andcausedmore surface cracks duringabrasive wear of
thecast specimens. Furthermore, the inter-silicon particle
distancemaybehigherincastconditions,andhence,eithertheabra-sive
particles may penetrate deeper into the specimen surfaceor more
number of abrasives penetrates into the worn surface.These facts
lead to considerably higher wear rate of cast alloyand composites
as compared to that in heat-treated
condition.Itmaybenotedthatthewearresistanceofheat-treatedalloyimproved
by 4070%over the as cast condition depending uponthe applied load
and abrasive size. The wear resistance of
heat-treatedcompositeimprovedby2070%overthecastone.
Itshowsthatheattreatmentalsosignicantlyreducedthewearrate of the
materials. However, the contribution to reduce wearrate due to
particle reinforcement and heat treatment is difcultto quantify as
each of these contributions are varying
dependingontestconditionsandmaterial.Inanutshell,itdemonstratesqualitativelythesynergiceffectofparticlereinforcementandheat
treatment.As the silicon particle become spherical, the points of
stressraiserisreducedconsiderablyandthepossibilityoffractureand
fragmentation of the silicon particle is reduced. The
siliconparticlesremainintact withintheAl-matrix.
Duringabrasivewear, a fraction of the matrix material is displaced
fromthe weargrooves and spread along the side of wear grooves in
the formofakes. Theseakymaterialsremainintactalongtheweartrack for
a longer duration or get smeared on the wear surfacebecause of
their higher ductility. Higher ductility of heat-treatedAlSi alloy
is primarily attributed to near spherical shape siliconparticles
and results in less surface and subsurface cracking. Theakes formed
due to frictional heating adhered and deformationon the wear
surface in due course is delaminated in the form oflong akes and
plates. As a matter of this fact, the heat-treatedmaterials are
exhibiting relatively less wear rate.4.4. Effect of load and
abrasive
sizeTheabrasivewearbehaviourofamaterialissignicantlyinuenced by the
combined actions of load and abrasive size.The efciency of material
removal by an abrasive media dependson the elastic and plastic
contact load [20], which varies withapplied load and abrasive size.
If the applied load is xed, thenthe effective stress on individual
abrasives increases with coarserabrasive particles, as the load is
shared by less number of abra-sives. When the abrasive particles
are ner in size, they makeonly elastic contact with the test
specimen surface, as the effec-tive stress inindividual abrasive is
less. As a result, these abrasiveparticles only support the applied
load without contributing
suf-cientmaterialremoval.However,athigherloadregime,theeffective
stress on each individual abrasive particles reach to
alevelwheretheabrasivesmakeplasticcontactwiththespec-imensurfaceandcausingmoresurfacedamageevenatnerabrasive
size. Thus the overall rate of material removal dependson the
extent of plastic contact of the abrasives with the specimen[30].
Further more, coarser size abrasives generally contain largenumber
of ows and hence may break easily [35]. As a result,cutting
efciency of the abrasive media is reduced. These bro-ken abrasives
are also picked up by the wear surface more easilyand protect the
specimen from destructive action of the abrasivemedia [35]. The
interaction of these facts causes very marginalvariation in wear
rate with abrasive size up to a size of 60 m(Fig. 9).It has been
reported that the abrasive wear is also associatedwith plastic
deformation of the surface and subsurface. Duringsuchkindofwear,
theabrasiveparticlesmakeweargrooveseither by cutting or ploughing
action over the specimen surface.It has been reported that the wear
rate depends onw/rratio,wherew is the groove width and r is the
radius of the spheri-cal tip of the abrasive particle [32]. The
radius of the abrasivetip is varying with increase in abrasive
size. But the width ofthe groove increases substantially with
increase in abrasive size.Thismaybeattributedtothefact that,
theeffectivestressesoneachindividualabrasivesincreasessubstantiallywhentheabrasives
become coarser in size, and make it more effective topenetrate
deeper into the specimen surface. In addition, degra-dation of
abrasive is reduced when the abrasive size becomescoarser. Because
of greater penetration, the surface and subsur-face is subjected to
more plastic deformation, which causes morenumber of cracks on the
surfaces. As a result, the wear rate ofmaterial increases rapidly
when abrasive size is increased from60 mto 80 min majority of the
cases. Fig. 13a shows the wearsurface of an AlSi alloy abraded
using emery paper of abrasivesize 80 m at an applied load of 7 N.
It clearly shows deep andwide wear grooves and formation of cracks
in the longitudinalas well as in transverse directions. Under the
same experimen-tal conditions,
thesubsurfaceexaminationindicatedseverelydeformed region with
transverse and longitudinal cracks (shownby arrow mark in Fig.
13b).Theplasticcontactoftheabrasivewiththespecimensur-face
increases with increase in applied load. This leads to moredepth of
penetration of the abrasives into the specimen surfaceand more
plastic deformation of the wear surface and subsur-face. The w/r
ratio may be increasing with increase in appliedload as r remains
constant for xed abrasive size. In case of com-posites, the SiC
particles act as barrier against the penetrationand movement of
abrasives when the applied load is less. As aresult w/r ratio may
not be changing signicantly with appliedload when the applied load
is in the lower range (3 N). Thisleads to either more or less wear
rate or slower rate of increasein wear rate with applied load when
the load is kept less than orequal to 3 N (Fig. 10). When the load
increases above 3 N, thesurface and subsurface is subjected to
severe plastic deforma-tion in addition to cutting and ploughing of
material. Under thesecircumstances a large number of silicon
particles and a few SiCparticles of composite get fractured or
fragmented and also thew/r ratio increases signicantly. It led to
debonding and scoop-ing off of SiC particles from the matrix, in
addition to removalof matrix material by cutting and ploughing
actions. Severe sur-face cracking also facilitates more surface
damage and reducesthe possibility of picking up of detached
abrasive particles andthe aky debris in the wearing surface. These
facts as a wholeleads to signicantly rapid increase in wear rate
especially ofcomposite at intermediate load level (35 N).At higher
applied load, severe plastic deformation of the sur-face and
subsurface causes heating of the wear surface which56 S. Das et al.
/ Wear 264 (2008) 4759Fig. 13. (a) SEM micrograph shows deep and
wide grooves and formation oflongitudinal and transverse cracks
(load: 7 N; abrasive size: 80 m). (b)
SEMmicrographshowingsubsurfacedepictingseveredeformationandformationofcracksinlongitudinal
andtransversedirections(A: deformedregion; B:undeformed
region).makes the alloy or the matrix of composite more ductile. As
aconsequence, major portion of the material from the wear sur-face
is removed by ploughing action. In this case, large akesare
generated and retained along the side of the wear track fora long
duration (Fig. 14). Due to heating, these akes are sub-jected to
severe plastic deformation and spread over the wearingsurface. In
due course, these are adhered to the worn surface andnally removed.
Additionally, the abrasives are degraded
(espe-ciallynerones)underhigherappliedload.Thusthecuttingefciency
of such abrasive is reduced. Fig. 15 shows the
SEMmicrographofemerypaper(size: 40 m)usedinanexperi-ment conducted
at a load of 7 Nand sliding distance of 108 m. Itclearlydepicts
degradedSiCparticles. Because of higher plastic-ity (due to
frictional heating) under higher applied load (>5
N),thematrixcanalsoholdSiCparticlesmoreeffectively. Thisfact
becomes more dominant especially when the tests are con-ducted
against ner abrasives. At coarser abrasive, the depth ofpenetration
of the abrasive particle is so high that matrix can-not hold the
SiC particles. Under such conditions these debrismay not be spread
over the matrix so effectively and removedFig. 14. SEM micrograph
showing formation of large size debris and retainedalong the wear
track.easily. All these facts led to slower rate of increase in
wear rateabove an applied load of 5 N especially for composite
materialswhen tested against ner abrasive size, but at coarser
abrasivethis effect is less dominating.Incaseofalloy,
theincreaseinwearratemaybealmostsame for the entire range of
applied load. At higher applied load,because of generation of high
heat, the alloy becomes softer thanthe composite and during
abrasive action; the softer alloy fromthe wear groove may also be
spreading over the wear surface.But the depth of cut is so high
that signicantly larger akes aregenerated and removed from the
alloy surface.When the abrasive size is coarser (60 m), the wear
rateofthealloyandcompositeisincreasingalmostlinearlywithapplied
load. This may be attributed to the fact, that the coarserabrasives
are stronger than the ner one and they experiencedhigher effective
stress even at lower applied load which has beenmentioned earlier.
As a result, the surface is subjected to higherplastic deformation
even at lower applied load. The wear rate isprimarily depending on
the depth and width of the wear groovesFig. 15. SEM micrograph of
emery paper (size: 40 m) shows degraded SiCabrasive (load: 7 N and
sliding distance: 108 m).S. Das et al. / Wear 264 (2008) 4759
57Table 2Experimental and calculated (shown in bracket) values of
abrasive wear rate of Al alloy and composites at different applied
loadsMaterial Processing condition Wear rate of different load1 N 3
N 5 N 7 NLM13 As cast 7.80(7.71) 18.71(18.71) 25.65(28.2)
32.47(36.25)Heat-treated 4.50(5.58) 12.29(13.41) 15.60(20.39)
25.15(26.27)LM1310 wt% SiC As cast 4.90(5.2) 12.70(12.48)
19.30(18.85) 25.44(24.51)Heat-treated 2.76(4.05) 8.52(9.02)
19.61(14.71) 18.27(19.25)LM1315 wt% SiC As cast 3.70(3.98)
10.7(9.8) 17.40(16.46) 23.80(20.91)Heat-treated 1.46(3.14) 7.5(7.5)
9.85(11.30) 14.50(14.8)made by the abrasives and also on the
surface and subsurfacecracking. At coarser abrasive size and higher
applied load,
widthofweargroovesandsurfacecrackingareincreasedsteadily.Becauseofrelativelyhighereffectivestressandhigherleveloffrictionalheatingofthesurface,
thematerialremovalduetocuttingandploughingactionisincreased.Whileconsider-ing
the composite materials, it may be found that the fractureand
fragmentation of SiC particles are increased steadily withapplied
load. As a result, in majority of the cases the wear rate
isincreasing almost linearly with applied load. However, in
somecases composites exhibit comparable wear rate with that of
thealloy.4.5. Correlation of abrasive wear rate with
mechanicalproperties, microstructure, applied load and abrasive
sizeThe above discussionthus suggests that the wear behavior of
amaterial is governedbyseveral factors whichinclude experimen-tal
parameters like load and abrasive size; material propertiessuchas
hardness, strengthandductilityandmicrostructuralcharacteristics
like shape, size and volume fraction of the hardceramic dispersoid.
Assuming, uniform distribution of SiC par-ticle in metallic matrix,
good interfacial bonding between SiCpand matrix and toughness
proportional to the product of UTSandelongation; the wear rate (WR)
of the materials may be expressedby following empirical equation:WR
= K
d1 +D
mH0.55T0.30P0.80(1 V0.53f) (1)whereK is a constant which signies
the probability of wearparticle formation, H the hardness of a
material (in MPa), P theapplied load (in N), T the product of UTS
(in MPa) and elonga-tion (in percentage), Vf the volume fraction of
dispersoids, Dthesize of reinforcing particle (in m), d the size of
the abrasive (inm) and m is a constant. The calculated values of
the wear rate(shown in the bracket) using the above equation and
consider-ing K=270 1011are compared with the experimental
values(Table 2).The value of K and m may be different in different
materi-als. It may also vary with heat treatment, presence of
ceramicdispersoids and the experimental conditions. As in the
earlierdiscussions,
ithasbeennotedthattherateofincreaseinthewearrateofalloyorcompositedoesnotfollowanyspecicFig.
16. Effect of abrasive size on the probability of formation of wear
debris(K) of Al alloy and AlSiC composite.relationship with
abrasive size; one can express the equation forxed abrasive size.
For example, the abrasive wear rate of alloyor composite at an
abrasive size of 80 m can be expressed bythe following equation:WR
= 270 1011H0.55T0.30P0.80(1 V0.53f) (2)Because of the above facts,
in few cases signicant deviationfrom the experimental value is
noted. However, in most of theFig. 17. Effect of relative abrasive
size on the formation of wear debris (K) ofAl alloy and AlSiC
composite.58 S. Das et al. / Wear 264 (2008) 4759cases the
calculated values are in close approximate with experi-mental
values. This suggests the reliability of the above equationfor
predicting wear rate of Al-alloys and composites from mea-sured
tensile data, hardness at any applied load and abrasive size.The
value of K at different abrasive sizes are calculated usingEq. (1)
and the variation of K as a function of abrasive size isshown in
Fig. 16. It depicts that the formation of wear particle (Kvalue) is
more for larger abrasive size. It may also be noted thatthere is
sudden increase in the value of K when the abrasivesize is coarser
than 60 m. An attempt is also made to under-stand the effect of
relative abrasive size (size of abrasive/sizeof reinforcement) on
the formation of wear particle (K value).Fig. 17 shows the effect
of relative abrasive size on the value ofK. It indicates that when
the relative abrasive size is more than0.8, there is an abrupt
increase in the value of K.5. Conclusions(1) The wear rate of
composite is less than that of the alloy andit decreases with
increase in SiC content.(2) The hardness and strength of composite
are higher than
thatofalloyandtheyincreasewithincreaseinSiCcontent.Whereas reverse
trend is noted to be true for the ductilityof these materials. The
hardness and strength of compos-ite is noted to be more after heat
treatment. But in case ofalloy, the hardness and strength are noted
to be more whenthey are aged for 6 h. This may be attributed to the
fact thatcomposites are aged faster than the alloy.(3) The abrasive
wear rate (WR) of the materials is a functionof applied load (P),
hardness (H), strength and ductility (T)ofthematerials,
volumefractionoftheharddispersoids(Vf) and relative size of
abrasive with respect to size of thedispersoid. The wear rate can
be expressed by the followingtype of relations:WR = K
d1 +D
mH0.55T0.30P0.80(1
V0.53f)Thisequationclearlydemonstratestheeffectofsizeandvolumefractionofreinforcingphaseandthesizeoftheabrasive
particle on the wear rate of Al alloy and compos-ites. It suggests
that the wear rate of the composite will beincreasing with
increasing in size of reinforcing phase andthe composite may be
suffered from higher wear rate thanthe alloy if the abrasive size
is higher than that of reinforcingphase.(4) The wear rate of Al
alloy and Al composite is invariant to thesliding distance and
increases with increase in applied loadand abrasive size. The
effect of abrasive size is noted to beinsignicant when the abrasive
size is less than 60 m. Sud-den increase in the wear rate was
observed when the abrasivesizewasmorethan60 m. But thewear
rateincreasesalmost linearly with applied load.(5) Addition of SiC
particle and heat treatment provide compa-rable improvement in wear
resistance. Thus, their synergiceffect is noted to be quite
signicant for improvement thewear resistance of AlSi
alloy.AcknowledgementAuthors are grateful to Director, RRL, Bhopal,
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