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Dry sliding wear behaviour of aluminium/alumina/graphite hybrid metal
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DOI: 10.1108/00368791211262499
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Industrial Lubrication and Tribology
Emerald Article: Dry sliding wear behaviour of aluminium/alumina/graphite
hybrid metal matrix composites
N. Radhika, R. Subramanian, S. Venkat Prasat, B. Anandavel
Article information:
To cite this document: N. Radhika, R. Subramanian, S. Venkat Prasat, B. Anandavel, (2012),"Dry sliding wear behaviour of
aluminium/alumina/graphite hybrid metal matrix composites", Industrial Lubrication and Tribology, Vol. 64 Iss: 6 pp. 359 - 366
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Dry sliding wear behaviour of aluminium/
alumina/graphite hybrid metal matrix
composites
N. Radhika
Department of Mechanical Engineering, Amrita School of Engineering, Coimbatore, India
R. Subramanian
Department of Metallurgical Engineering, PSG College of Technology, Coimbatore, India
S. Venkat Prasat
Department of Mechanical Engineering, Amrita School of Engineering, Coimbatore, India, and
B. Anandavel
Department of Metallurgical Engineering, PSG College of Technology, Coimbatore, India
Abstract
Purpose – Recent trends in material science show a considerable interest in the manufacturing of metal matrix composites to meet the stringent
demands of lightweight, high strength and corrosion resistance. Aluminium is the popular matrix metal currently in vogue that can be reinforced with
ceramic materials such as particulates to meet the desired property. The purpose of this paper is to fabricate hybrid metal matrix composites to improve
the dry sliding wear resistance and to study of the effect of sliding speed, load and reinforcement (alumina and graphite) on wear properties, as well as
its contact friction.
Design/methodology/approach – The present study addresses the dry sliding wear behaviour of Al-Si10Mg alloy reinforced with 3, 6 and 9 wt% of
alumina along with 3 wt% of graphite. Stir casting method was used to fabricate the composites. Mechanical properties such as hardness and tensile
strength have been evaluated. A pin-on-disc wear test apparatus was used to evaluate the wear rate and coefficient of friction by varying the loads of
20, 30 and 40 N, sliding speeds of 1.5 m/s, 2.5 m/s and 3.5 m/s at a constant sliding distance of 2100 m.
Findings – Mechanical properties of hybrid metal matrix composites (HMMCs) have shown significant improvement. The wear rate and coefficient of
friction for alloy and composites decreased with increase in sliding speed and increased with increase in applied load. Temperature rise during wearing
process for monolithic alloy was larger than that of HMMCs and Al/9% Al2O3/3% Gr composite showing the minimum temperature rise.The worn
surfaces of the composites were investigated using scanning electron microscope.
Practical implications – The paper shows that aluminium composites can improve strength and wear resistance.
Originality/value – HMMCs has proven to be useful in improving the dry sliding wear resistance.
Keywords Mechanical properties of materials, Composite materials, Metals, Wear resistance, Hybrid metal matrix composites, Alumina, Graphite,
Stir casting
Paper type Research paper
phase having good stiffness and hardness (Song and Han,
1997). Miyajima and Iwai (2003) showed that particulate
reinforcement are most beneficial for improving the wear
resistance of MMC. The degree of improvement of wear
resistance primarily depends on the type, size and distribution
of the reinforcing phase as well as the manufacturing technique
of the composite. Ramesh and Safiulla (2007) indicated
improved dry sliding wear resistance of Al6061 based
composites with different reinforcements like SiC, Al2O3 and
CeO2. Reinforcement of hard particles in Al matrix protects the
matrix surface against destructive action of the abrasive during
the wear process. Uniform dispersion of reinforcement in the
matrix is very important. Singla et al. (2009) have discussed
the development of aluminium based silicon carbide particulate
metal matrix composite. A two step mixing method of stir
casting technique was used to obtain uniform dispersion of
reinforcement. Experiments were conducted by varying weight
fraction of SiC (5, 10, 15, 20, 25 and 30 per cent) and found an
increasing trend of hardness and impact strength with increase
in weight per centage of SiC. The size of the particle is also a very
1. Introduction
Metal matrix composites (MMCs) are one of the important
developments in the field of engineering materials. Present work
is focused on development of composites which would best fit
the present day need of light weight, high strength to weight
ratio and good wear properties. In recent years, there is a great
deal of interest in particulate reinforced MMCs, and in
particular those based on existing aluminium alloys (Goni et al.,
2000; Ibrahim et al., 1991). The improvement in mechanical
properties of the MMC with graphite reinforcement could be
achieved by adding a second reinforcement, also a non-metallic
The current issue and full text archive of this journal is available at
www.emeraldinsight.com/0036-8792.htm
Industrial Lubrication and Tribology
64/6 (2012) 359– 366
q Emerald Group Publishing Limited [ISSN 0036-8792]
[DOI 10.1108/00368791211262499]
359
Dry sliding wear behaviour of aluminium/alumina/graphite HMMCs
Industrial Lubrication and Tribology
N. Radhika, R. Subramanian, S. Venkat Prasat and B. Anandavel
Volume 64 · Number 6 · 2012 · 359 –366
important contributor to the wear behaviour of the MMCs.
Wang and Hutchings (1989) mentioned that the wear resistance
of the composites increases with decrease in the abrasive particle
size. Lee et al. (1992) studied the effect of sintered porosity,
volume fraction and particle size of SiC on abrasive wear
resistance of SiC reinforced Al6061 alloy. Their results show
that the beneficial effect of hard SiC addition on wear résistance,
the wear rates decreased as the amount of SiC increased.
Further, it was observed that for the composite containing the
same amount of SiC, the wear rates decreased with increasing
particle size.
Many researchers have investigated Al MMC with ceramic
reinforcement like silicon carbide but (Zhao et al., 2006)
investigated the friction and wear properties of aluminium
metal matrix composite reinforced with titanium diboride
(TiB2) and revealed that TiB2/Al composite exhibited a
frictional coefficient value six times lower than SiC/Al
composite. Roy et al. (2005) studied the tribiological
properties of Ti-aluminide-Al based MMC by varying per cent
volume fraction from 10 to 40 per cent and they concluded that
20 per cent reinforced MMC exhibited low coefficient of
friction under unlubricated condition which has five times lower
wear volume than the base Al alloy. Unlu (2008) investigated
the tribological and mechanical properties of Al2O3-SiC
reinforced Al composite fabricated by casting and powder
metallurgy method. Tribological properties were investigated
with 10 N and 50 rpm on a pin-on-disc wear tester at dry
condition. Their results showed that the tribological and tensile
strength of the cast specimen was about 1.5-2 times better than
powder metallurgy specimens but had similar compressive
strength and hardness value. Sahin and Murphy (1998) studied
the effect of sliding speed and microstructure on dry sliding
wear and frictional properties of Al2014 alloy reinforced with
unidirectional boron fibers. Tests were conducted on
composites with fiber orientation parallel and normal to the
sliding direction. Test results showed that normally oriented
fibers exhibited better wear resistance than the parallel oriented
fibers. The frictional coefficient of parallel oriented fibers was
also found to be lower than the normally oriented fibers and in
general, frictional coefficient decreased as the sliding speed
increased. Rodriguez et al. (2007) studied the dry sliding
behaviour of aluminium-lithium alloys reinforced with SiC
particles. They carried out wear tests at different pressures
(6.3-50 MPa) and temperature range of 20-3508C. Worn
specimens and debris were examined using SEM and EDAX
technique and it was observed that the presence of mechanically
mixed layer (MMLs) on the wear surface with varying
morphology and thickness influenced the wear rate.
A temperature dependency transition from mild to severe
wear has been observed and measured friction coefficients
are minimum at temperatures higher than 208C. Serdar Osman
and Buytoz (2007) studied the relationship between thermal
and sliding wear behaviour of Al6061/alumina MMC’s. It was
observed that the increase in alumina content decreased both
thermal conductivity and friction coefficient and hence
increased the transition load and transition temperature for
mild to severe wear during sliding wear test. In automobile
sector, Al composites are used for making various components
such as brake drum, cylinder liners and cylinder block.
Uyyuru et al. (2007) studied the tribological behaviour of stir
cast Aluminium/SiC particulate reinforced MMC against
automobile brake pad material using a pin-on-disc apparatus
and concluded that the formation of tribolayer played
a significant role in wear behaviour of MMC. Basavarajappa
et al. (2007) studied the tribological behaviour of Al2219
reinforced with SiC and graphite particles. Influence of
parameters like normal load, sliding speed and sliding
distance on dry sliding wear were analysed by employing
orthogonal array and analysis of variance technique. Their
results also showed that graphite particles are effective in
increasing dry sliding wear resistance of Al/SiC composite.
Much research work has been devoted to develop MMC and
investigation of their mechanical and tribological properties.
The friction and wear properties of composites depend on the
amount, size, shape and distribution of hard or soft particles
filled in matrix. Hard particles increase the strength and wear
resistance of composites, but decrease their ductility whereas
the soft particles acting as lubricant decrease the coefficient of
friction.
There is a growing interest at the international level in
manufacturing hybrid metal matrix composites (HMMC’s).
HMMC’s possess combined properties of its reinforcements
and exhibit improved mechanical and tribological properties.
Fu et al. (2004) made a wear behaviour study among Saffil/Al,
Saffil/Al2O3/Al and Saffil/SiC/Al with 20 per cent volume
fraction of reinforcement. The results showed that under dry
sliding condition, Saffil/SiC/Al exhibited better wear
resistance compared to other compositions.
In the present study, an attempt has been made to fabricate
HMMC to improve the dry sliding wear resistance. This
included the study of the effect of sliding speed, load and
reinforcement (alumina and graphite) on wear properties as
well as its contact friction.
2. Experimental procedure
2.1 Material selection
The metal matrix material selected for the present study is
Al-Si10Mg alloy since the mechanical properties can be tailored
through heat treatment process and also possesses excellent
strength at elevated temperature. The reinforcements were
alumina particles of average size 15-20 mm with varying volume
fraction (3, 6 and 9 wt.%) along with 3 wt.% graphite particles
of average size 50-70 mm. The average density of Al, alumina
(Al203) and graphite (Gr) were 2.68, 2.71 and 2.2 g/cc,
respectively. Alumina being hard and brittle in nature gets
accommodated in soft ductile aluminium base matrix,
enhancing the overall stiffness and strength of the
HMMC.The chemical composition of matrix alloy is given
in Table I.
2.2 Composite preparation
The composite specimens were prepared by stir casting
technique. In this method, the matrix material was heated
above its melting temperature and the preheated
reinforcements were mixed to this matrix material by stainless
steel stirrer rotating at 350 rpm to create a vortex, in order to
ensure uniform mixing of reinforcements with the molten alloy.
Degassing agent (hexachloro ethane) was added to reduce gas
porosities during casting processes. About 1.5 per cent
magnesium by weight was added to the molten metal while
mixing to obtain good wettability. The molten metal was then
poured into a permanent cast iron mould of diameter 14 mm
and length 100 mm. The die was released after 2 min and the
cast specimens were taken out.
360
Dry sliding wear behaviour of aluminium/alumina/graphite HMMCs
Industrial Lubrication and Tribology
N. Radhika, R. Subramanian, S. Venkat Prasat and B. Anandavel
Volume 64 · Number 6 · 2012 · 359 –366
Table I Composition of Al-Si10Mg
Chemical composition
Percentage
Cu
Mg
Si
Fe
Mn
Ni
Zn
Lead
tin
Ti
Al
0.1 max
0.2-0.6
10.0-13.0
0.6 max
0.3-0.7
0.1 max
0.1 max
0.1 max
0.05 max
0.2 max
Balance
Figure 1 Pin-on-disc apparatus
2.3 Mechanical properties of composites
2.3a. Microhardness
Hardness measurements were carried out using Vickers
hardness tester for both base metal and hybrid metal matrix
composite specimens. Specimens of 15 mm length were cut
from the composite bar and the surfaces were polished using
emery papers. The load was applied for 20 s depending on the
intensity of the load and material being tested. Care is taken
to ensure flat surface for proper holding and for the even
application of load. It was observed that micro hardness of Al/
9%Al2O3/3% graphite HMMCs is higher than unreinforced
alloy and it increased with increase in content of
reinforcement. Improvement in the hardness of the
composites with increased content of reinforcement can be
mainly attributed to the higher hardness of the alumina.
2.3b. Tensile properties
The tensile property measurements were carried out using
universal testing machine as per ASTM standard procedure.
The rate of loading for all the samples was 3 mm/min. The
gauge length was 20 mm and all the results were average of
five measurements. The tensile strength is maximum for Al/
9%Al2O3/3%Gr composite. The result of hardness and tensile
strength are given in Table II.
re-weighed to determine the volumetric wear rate. By converting
the wear mass loss to wear volumetric loss and dividing by sliding
distance, the volumetric wear rate was obtained.
The wear test parameters:
.
Weight per centage of alumina (3, 6 and 9 per cent).
.
Weight per centage of graphite (3 per cent).
.
Applied load (20,30 and 40 N).
.
Sliding speed (1.5, 2.5 and 3.5 m/s).
.
Sliding distance (2,100 m).
2.4 Wear testing of composite
A pin-on-disc test apparatus was used to investigate the dry
sliding wear characteristics of composite specimens. Specimen
pins of 10 mm diameter and 30 mm length were machined
from the composite bar and then polished metalographically.
Each specimen is thoroughly cleaned by acetone solution,
dried, and then accurately weighed using a single pan
electronic weighing machine with an accuracy of 0.0001 g.
Figure 1 shows the pin-on-disc apparatus used for wear
studies. During the test, the pin is held pressed on the surface
of a hardened EN32 steel disc (67 HRC) by applying load
that act as counter weight and balances the pin. All the
specimens followed a same track of 110 mm diameter with a
tangential force. The LVDT (load cell) on the lever arm helps
determine the wear at any point of time by monitoring the
movement of the arm. Once the surface of contact wears out,
the load pushes the arm to remain in contact with the disc and
the movement of the arm generates a signal. The LVDT
which is connected to the computer receives the signal and
consolidates the data from the load cell and the friction
coefficient is recorded. At the end of each test, the specimen
is removed, cleaned with acetone to remove any debris and
3. Results and discussions
3.1 Wear behaviour of HMMCs
The effects of sliding speed and applied load with per centage
of reinforcements on dry sliding wear behaviour of different
Al/Al2O3/graphite composites are discussed here.
3.2 Effect of sliding speed and reinforcement on wear
rate
The effect of sliding speed on wear rate of base alloy and
composites with weight per centage of reinforcements is shown
in Figures 2-4 for loads of 20, 30 and 40 N, respectively, at a
constant sliding distance of 2,100 m. It can be observed from
the graph that the wear rate of the composites as well as
unreinforced alloy decreases, as the sliding speed increases up to
3.5 m/s. This is due to the fact that at higher interfacial
temperature, the oxidation of aluminium alloy forms an oxide
layer thus preventing the sliding interfaces by decreasing the
wear rate. Alphas and Zhang (1992) observed that iron was
oxidized during wearing process and has been shown that oxide
layers, in particular iron layers generated during wear, act as
solid lubricants and help to reduce the wear rates. The wear rate
of the unreinforced alloy is more than that of the composites for
all the applied loads. The incorporation of Al2O3 in Al alloy
improves the dry sliding wear resistance in comparison to the
unreinforced alloy. This type of behaviour has been previously
observed by (Basavarajappa et al., 2006).They found that for
the composite Al2219/SiC/Gr, as sliding speed increases,
Table II Results of hardness and tensile strength
S. no. Composition
1
2
3
4
Al
Al/3% Al2O3/3% Gr
Al/6% Al2O3/3% Gr
Al/9% Al2O3/3% Gr
Tensile strength (MPa) Micro hardness
152.7
172.1
190
201
114
126
129
133
361
Dry sliding wear behaviour of aluminium/alumina/graphite HMMCs
Industrial Lubrication and Tribology
N. Radhika, R. Subramanian, S. Venkat Prasat and B. Anandavel
Volume 64 · Number 6 · 2012 · 359 –366
Figure 2 Variation of wear rate with sliding speed at a load of 20 N
Figure 5 Variation of wear rate with applied load at a sliding speed of
1.5 m/s
Al
Al/3Al2O3/3Gr
Al/6Al2O3/3Gr
Al/9Al2O3/3Gr
0.005
Al
Al/3Al2O3/3Gr
0.004
0.003
0.002
0.001
0
1
1.5
2
2.5
3
Sliding speed (m/s)
3.5
Al/6Al2O3/3Gr
0.01
Wear rate (mm3/m)
Wear rate (mm3/m)
0.006
4
Al/9Al2O3/3Gr
0.008
0.006
0.004
0.002
0
15
20
25
30
Load (N)
35
40
45
Figure 3 Variation of wear rate with sliding speed at a load of 30 N
0.006
0.005
Al
0.007
0.004
Wear rate (mm3/m)
Wear rate (mm3/m)
Figure 6 Variation of wear rate with applied load at a sliding speed of
2.5 m/s
Al
Al/3Al2O3/3Gr
Al/6Al2O3/3Gr
Al/9Al2O3/3Gr
0.003
0.002
0.001
0
1
1.5
2
2.5
3
Sliding speed (m/s)
3.5
4
Al/3Al2O3/3Gr
0.006
Al/6Al2O3/3Gr
0.005
Al/9Al2O3/3Gr
0.004
0.003
0.002
0.001
0
15
20
25
30
Load (N)
35
40
45
Figure 4 Variation of wear rate with sliding speed at a load of 40 N
Figure 7 Variation of wear rate with applied load at a sliding speed of
3.5 m/s
Al
Al/9Al2O3/3Gr
0.008
0.006
0.004
0.002
0
1
1.5
2
2.5
3
Sliding speed (m/s)
3.5
Al
Al/3Al2O3/3Gr
Al/6Al2O3/3Gr
Al/9Al2O3/3Gr
0.006
Al/6Al2O3/3Gr
0.01
Wear rate (mm3/m)
Wear rate (mm3/m)
Al/3Al2O3/3Gr
0.005
0.004
0.003
0.002
0.001
0
4
15
20
25
30
Load (N)
35
40
45
the pin increases with increase in the applied load (Sudarshan
and Surappa, 2008). The wear rate of the monolithic alloy is
more than the composite for all the sliding speeds. Among
the composites, Al/9%Al2O3/3%Gr HMMC exhibited the
highest wear resistance. The dry sliding wear resistance of the
composite increases with increasing the weight per centage of
reinforcements. This is due to the fact that increase in contact
area of alumina with steel counterface, hence the wear rate is
less in Al/9%Al2O3/3%Gr compared to other composites.
The addition of alumina particulates increases the wear
resistance of aluminium alloys (Serdar Osman and
Buytoz, 2007).
the wear rate decreases up to 4.5 m/s, the SiC in the
hybrid composites wear the counterface and iron oxides
were formed by oxidation of iron particles from the counterface.
3.3 Effect of load and reinforcement on wear rate
The effect of applied load on wear rate of base alloy and
composites with weight per centage of reinforcements is shown
in Figures 5-7 for sliding speeds of 1.5, 2.5 and 3.5 m/s,
respectively, at constant sliding distance of 2,100 m. It is evident
from the graph that the wear rate of the unreinforced alloy and
composites increase with increase in applied load. This is
because, the temperature at the interface between the disc and
362
Dry sliding wear behaviour of aluminium/alumina/graphite HMMCs
Industrial Lubrication and Tribology
N. Radhika, R. Subramanian, S. Venkat Prasat and B. Anandavel
Volume 64 · Number 6 · 2012 · 359 –366
3.4 Coefficient of friction measurement
The coefficient of friction is a measure of friction between the
surfaces in contact. The force of friction is always exerted in a
direction that opposes movement or potential movement
between the two surfaces. It is the ratio between the frictional
force and reaction force. The frictional force during sliding is
to be the power function of applied load and the sliding speed
at a particular temperature. Figures 8-11 show a plot between
the frictional coefficients and sliding speeds. The graphs
clearly indicate that the coefficient of friction decreases
slightly with increasing sliding speed for all composites as
well as monolithic alloy regardless of applied load. Also, the
addition of graphite particles reduces the heat generated due
Figure 11 Variation of frictional coefficient with sliding speed for Al/
9Al2O3/3Gr
Coefficient of friction
0.5
Coefficient of friction
Figure 8 Variation of frictional coefficient with sliding speed for Al
alloy
Load=40N
0.4
0.3
0
1
0.5
Load = 20 N
0.3
Load = 30 N
Load = 40 N
1
2
Sliding speed (m/s)
3
4
0.5
0.4
Load = 20 N
Load = 30 N
Load = 40 N
0.2
0
1
2
Sliding speed (m/s)
3
4
4
Figure 12 Temperature rise of composites with applied loads at a
sliding speed of 1.5 m/s
Figure 10 Variation of frictional coefficient with sliding speed for Al/
6Al2O3/3Gr
Al
Al/3Al2O3/3Gr
Temp rise (Degree celsius)
Load=20N
0.5
3
3.5 Study of temperature rise during wearing process
Thermal behaviour of the HMMC’s during wearing process is
studied by measuring the temperature rise during the wear
process. The temperature was measured using a thermocouple
inserted into a hole drilled in the samples. Difference in intial and
final temperatures gives the temperature rise for that sample.
From the data obtained, tempature rise versus load graph
(Figures 12-14)is plotted and it is observed that the temperature
rise of wear sample increases as load increases. Frictional heat
was generated during the wear process and the temperature of
the worn samples was also increased, especially at the contact
zone between the samples and counterpats (Long yan et al.,
2002). In their study, the temperature rise of worn surface was up
to 608C. So the increasing temperature raised the friction
coefficient and the wear rate. Temperature rise for monolithic
alloy is larger than that of HMMCs. Al/9%Al2O3/3% Gr
Figure 9 Variation of frictional coefficient with sliding speed for Al/
3Al2O3/3Gr
0.3
2
Sliding speed (m/s)
to friction by its intrinsic lubricity. The friction coefficient
decreased with increase in alumina content and a similar
observation was reported by (Deuis et al., 1997). At low sliding
speeds, the friction is independent of the relative surface velocity
but at higher sliding speeds the coefficient of friction decreases.
It is also observed that coefficient of friction increases as load
increases from 20 to 40 N.
0.4
0
Coefficient of friction
Load=30N
0.2
0.2
Coefficient of friction
Load=20N
Load=30N
Load=40N
0.4
0.3
25
Al/6Al2O3/3Gr
Al/9Al2O3/3Gr
20
15
10
5
0
0.2
0
1
2
Sliding speed (m/s)
3
15
4
20
25
30
Load (N)
363
35
40
45
Dry sliding wear behaviour of aluminium/alumina/graphite HMMCs
Industrial Lubrication and Tribology
N. Radhika, R. Subramanian, S. Venkat Prasat and B. Anandavel
Volume 64 · Number 6 · 2012 · 359 –366
Temp rise (Degree celcius)
Figure 13 Temperature rise of composites with applied loads at a
sliding speed of 2.5 m/s
Figure 15 SEM of worn surface of Al alloy
35
30
25
20
Al
15
Al/3Al2O3/3Gr
10
Al/6Al2O3/3Gr
5
Al/9Al2O3/3Gr
0
15
20
25
30
Load (N)
35
40
45
Figure 14 Temperature rise of composites with applied loads at a
sliding speed of 3.5 m/s
Temp rise (Degree Celscius)
60
Notes: Load = 40 N; V = 3.5 m/s
50
Figure 16 SEM of worn surface of composite (Al/3%Al203/3%Gr)
40
Al
30
Al/3Al2O3/3Gr
20
Al/6Al2O3/3Gr
10
Al/9Al2O3/3Gr
0
15
20
25
30
35
40
45
Load (N)
composites show minimum temperature rise. The same trend is
observed for all loads and also for all sliding speeds. Addition of
alumina particles improve the thermal stability of the
composites. The study also revealed that the temperature rise
during wear process of the HMMC follows the same trend as that
of wear rate responses obtained for same load. This reaffirms the
fact that wear resistance is higher for the Al/9%Al2O3/3%Gr than
compositions and unreinforced aluminium alloy.
Notes: Load = 40 N; V = 3.5 m/s
Figure 17 SEM of worn surface of composite (Al/9%Al203/3%Gr)
4. SEM investigations of wear specimen
To understand the wear mechanism of unreinforced aluminium
alloy and its composites, the worn surfaces were examined by
scanning electron microscopy. During sliding, the entire surface of
the pin has contact with the surface of the steel disc and machine
marks on the steel disc can also be observed. Figures 15-17
show the micrographs of the worn surface of aluminium alloy and
composites at an applied load of 40 N, sliding speed of 3.5 m/s for
a sliding distance of 2,100 m. The micrograph of the aluminium
alloy shows that more material has been removed from the
surface of the pin during wearing. As the reinforcement of
alumina content increases, the depth and number of grooves on
the surface of the pin decreases, thus increasing the fatigue
resistance. The layer of materials removed as debris was less in Al/
9%Al2O3/3%Gr and comaparitively smooth worn surface could
be observed. The friction and wear rate in MMC are significantly
reduced compared with those in matrix alloys, as a result of
the incorporation of graphite particles (Rohatgi et al., 1992).
Notes: Load = 40 N; V = 3.5 m/s
364
Dry sliding wear behaviour of aluminium/alumina/graphite HMMCs
Industrial Lubrication and Tribology
N. Radhika, R. Subramanian, S. Venkat Prasat and B. Anandavel
Volume 64 · Number 6 · 2012 · 359 –366
Figures 18-19 show the worn surface of Al/9%Al2O3/3%Gr at a
sliding speed of 1.5 m/s, sliding distance of 2,100 m for an applied
load of 20 and 40 N. It is evident from the micrographs that at low
loads, the worn surfaces predominantly reveal grooves in the
sliding direction. Grooves are mainly formed by the reinforcing
particles. The alumina particles support the normal pressure on
the surface and act as effective abrasive element. Thus, at low
loads, the abrasion wear mechanism becomes dominant and the
extent of damage regions on the wear surface is also less due to the
lubricating phase of graphite. As the load increases, the grooves
become deeper and the induced stresses exceed the fracture
strength of the particles causing their fracture. The material
transfer from pin onto the disc also occurs due to the rubbing
action of the fractured alumina particles against steel disc. These
results in an increase in wear rate. A small quantity of plastic
deformation can also be observed. Thus, at high loads, the plastic
flow of the material is dominant. Roy et al. (2005) discussed that
the severity of plastic deformation is reduced with the
incorporation of harder reinforcement and for unreinforced Al,
extensive plastic deformation was observed to cause more wear.
The improvement in hardness due to harder aluminide
reinforcements cause increase in wear resistance. Figures 20-21
show the worn surface of Al/3%Al2O3/3%Gr at an applied load of
40 N, sliding distance of 2,100 m for sliding speed of 1.5 and
3.5 m/s. As the sliding speed increases, the number of grooves
also increases and the reinforcements are projecting out from the
pin surface due to ploughing action between counterface and pin.
More material is removed from the pin surface forming a MML.
Deuis et al. (1997) stated that on the worn surface of MMCs a
MML was present and this layer exhibited a hardness
approximately six times that of the bulk composite. So the
formation of work hardened layer between the pin surface and
steel disc reduces the wear rate as sliding speed increases. This
mechanicaly mixed layer will be more stable by increasing the
reinforcements. Venkataraman and Sundararajan (1996) also
stated that the presence of MML layer controlled the wear rate
and the wear debris indicates the presence of a substantial level of
iron, transferred from the disc material. These results show that
Figure 18 SEM of worn surface of composite (Al/9%Al203/3%Gr)
Figure 20 SEM of worn surface of composite (Al/3%Al203/3%Gr)
Notes: Load = 20 N; V = 1.5 m/s
Notes: Load = 40 N; V = 1.5 m/s
Figure 19 SEM of worn surface of composite (Al/9%Al203/3%Gr)
Figure 21 SEM of worn surface of composite (Al/3%Al203/3%Gr)
Notes: Load = 40 N; V = 1.5 m/s
Notes: Load = 40 N; V = 3.5 m/s
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Dry sliding wear behaviour of aluminium/alumina/graphite HMMCs
Industrial Lubrication and Tribology
N. Radhika, R. Subramanian, S. Venkat Prasat and B. Anandavel
Volume 64 · Number 6 · 2012 · 359 –366
the wear rate was found to increase with an increase in wear load
and decrease with an increase in wear speed (Seah et al., 1996).
Miyajima, T. and Iwai, Y. (2003), “Effects of reinforcements
on sliding wear behaviour of aluminium matrix
composites”, Wear, Vol. 255 Nos 1-6, pp. 606-16.
Ramesh, C.S. and Safiulla, M. (2007), “Wear behaviour of hot
extruded Al6061 based composites”, Wear, Vol. 263,
pp. 629-35.
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sliding wear behaviour of aluminium-lithium alloys reinforced
with SiC particles”, Wear, Vol. 262 Nos 3/4, pp. 292-300.
Rohatgi, P.K., Ray, S. and Liu, Y. (1992), “Tribological
properties of metal matrix-graphite particle composites”,
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Sahin, Y. and Murphy, S. (1998), “Effect of sliding speed and
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Seah, K.H.W., Sharma, S.C., Girish, B.M. and Lim, S.C. (1996),
“Wear characteristics of as-cast ZA-27/graphite particulate
composites”, J. Materials and Design, Vol. 17 No. 2, pp. 63-7.
Serdar Osman, Y. and Buytoz, S. (2007), “Relationship between
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Sudarshan and Surappa, M.K. (2008), “Dry sliding wear of
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Venkataraman, B. and Sundararajan, G. (1996), “The sliding
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5. Conclusion
Mechanical properties and dry sliding wear behaviour of HMMC
have been studied. Hardness and tensile strength of HMMCs has
improved. A pin-on-disc apparatus is used to study the wear
behaviour of HMMCs and unreinforced alloy by varying applied
loads, sliding speeds and weight per centage of reinforcements.
The results indicated that the wear rate and coefficient of friction
of HMMCs and alloy is increased with increase in applied loads
and and decreased with increase in sliding speeds. The wear rate
of HMMCs is lower than the unreinforced alloy in all applied
conditions. The increase in reinforcement reduces the wear rate
and Al/9%Al2O3/3%Gr has highest wear resistance compared to
unreinforced alloy. Temperature rise during wearing process is
also observed and found that the temperature rise of wear sample
increases as load increases. The wornout surfaces of the
specimens investigated using scanning electron microscopy
revealed that the extent of damage was more in alloy compared
to composites, also the depth and number of grooves increases as
sliding speed and load increases.
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Corresponding author
N. Radhika can be contacted at: rcn_kongu@yahoo.co.in
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