Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.14, pp.1337-1344, 2011 Printed in the USA. All rights reserved
Development of Aluminium Based Hybrid Metal Matrix
Composites for Heavy Duty Applications
M. Asif*, K. Chandra, P.S. Misra
Department of Metallurgical and Materials Engineering
Indian Institute of Technology Roorkee, Roorkee – 247667 (INDIA)
*Corresponding author:
Mobile No +91-9411982614
The present study deals with the investigation of dry sliding wear behavior of aluminium
alloy based composites, reinforced with silicon carbide particles and
solid lubricants such as
graphite/antimony tri sulphide (Sb
). The first one of the composites (binary) consists of Al.
with 20% Silicon Carbide particles (SiCp) only. The other composite has SiCp and solid
lubricants: Graphite + Sb
(hybrid composite) at solid state
Both composites are
fabricated through P/M route using “Hot powder perform forging technology”. The density
and hardness are measured by usual methods. The pin-on-disc dry wear tests to measure the
tribological properties are conducted for one hour at different parameters namely load: 30,
50 and 80N and speed: 5, 7 and 9m/s. The tested samples are examined using scanning
electron microscope (SEM) for the characterization of microstructure and tribolayer on worn
surface of composites. The results reveal that wear rate of hybrid composite is lower than
that of binary composite. The wear rate decreased with the increasing load and increased
with increasing speed. The results of the proposed composites are compared with iron based
metal matrix composites (FM01N, FM02) at corresponding values of test parameters. These
iron based metal matrix composites are also fabricated by P/M route using ‘Hot powder
perform forging technology’. The comparative study reveals that the proposed composites
have lower friction coefficient, less temperature rise and low noise level; however they have
little higher wear rate. It is concluded that the hybrid composite has acceptable level of
tribological characteristics with blacky and smooth worn surface.
Keywords: Al-MMC, Powder Metallurgy, Powder Forging, Heavy Duty applications.
Discontinuous reinforced aluminum metal matrix composites (DRAMMCs) are a class of
composite materials having desirable properties like low density, high specific stiffness, high
specific strength, controlled co-efficient of thermal expansion, increased fatigue resistance
and superior dimensional stability at elevated temperatures etc. [1, 2]. These materials have
emerged as the important class of advanced materials giving engineers the opportunity to
tailor the material properties according to their needs. Essentially these materials differ from
1338 M. Asif, K. Chandra, P.S. Misra Vol.10, No.14
the conventional engineering materials from the viewpoint of homogeneity. In composites,
controlled distribution of one or more reinforcement materials in continuous metal matrix
phase is possible. Large majority of these composite materials are metallic materials
reinforced with high strength, high modulus and brittle ceramic phases which can be either
continuous in the form of fiber, discontinuous in the form of whisker, platelets or particulate
reinforcements embedded in a ductile metallic matrix. In the last two decades, wear
performances of DRMMCs reinforced with various reinforcements ranging from very soft
materials like graphite, talc etc., to high hardened ceramic particulates like SiCp, Al
[3–6] have been reported to be superior to their respective unreinforced alloys. A good
number of studies have been done on the Al/SiCp [4–6] and Al/Graphite [3, 7] individually.
Traditionally, the external lubricant plays an important role in wear behavior. Though the
wear behavior of Al/SiCp is good, the addition of natural lubricant like graphite enhances the
self-lubricating capacity of the composites, which is essential in some of the applications
where lubrication needs to be applied periodically, especially for wear parts which are
difficult to access. Solid lubricant contained in the composite can be released automatically
during the wear process and reduces the wear. The incorporation of graphite alone will give
the desired solutions but various researchers have identified both experimentally and
mathematically that the incorporation of graphite will reduce the mechanical properties of the
composites [7], which is undesirable for the component used in structural and high-elevated
temperature applications. The wear behavior of Al-alloy reinforced with SiCp-Graphite
particles has been documented. As a result, combination of high-hardened SiC particles and
soft graphite were used in the present work to investigate the wear behavior of the Al-alloy
incorporated with SiCp and graphite particles for comparison with corresponding value of
iron based metal matrix composites (Fe MMCs) [8].
2.1 Disc
EN-32 steel disc has been used as brake rotor / brake drum with hardness of 60HRC,
machined from a commercial passenger car brake rotor. The inner diameter, the outer
diameter and thickness are 180, 140 and 15mm respectively. The surface is machined to an
average roughness value of 1.5 µm (measured by P-6 stylus profile meter) which is same as
the roughness value of sliding surface of the actual commercial brake rotor. The composition
of the steel disc material is shown in Table 1.
TABLE 1 Composition of EN-32 steel disc
C Cr Si Mn P S Mg Fe
Percentage 0.45 0.25 0.27 0.65 0.04 0.04 .25 Balance
2.2 Test Pins:
The chemical formulation for preparation of aluminum metal matrix composites (Al-MMCs)
is shown in Table 2. The Al-MMCs were manufactured through powder metallurgy route
using “Hot Powder Preform Forging Technology” developed by Lenin et. al [8]. Proposed
composites were fabricated by using of Al-alloy as matrix reinforced with silicon carbide
particles (size 34µm) and solid lubricants (graphite and Sb
). The composition of powders
(wt %) was mixed into two stages by mechanical alloying, followed by attrition and conical
ball milling for two respectively and mixture filled in designed capsule. The capsule was
Vol.10, No.14 Development of Aluminium Based Hybrid Metal 1339
heated up to 450
C and soaked for one hour in tubular furnace. At this temperature, it was
forged in channel die without application of inert gas or vacuum by friction screw press of
100 ton capacity. After forging, slabs were prepared by using of machining processes. They
were reheated to 530
C in muffle furnace and repressed in hydraulic press at 20 bar.
Homogenization process was done at 530
C for six hours.
TABLE 2 Chemical formulation of Al- MMCs (by wt. %)
Samples Nature of
(45 µm)
Al alloy
Al01 binary 20 -------- ------- 80
Al02 hybrid 10 6 3 81
3.1. Determination of Density (Archimedes Principle)
The basic method of determining the density of the specimen by measuring the ratio of mass
and volume was used. In this paper the density of the specimens was estimated by
Archimedes’ principle is shown in Table 3.
TABLE 3 Physical and Mechanical properties of Al-MMC
Test Pins Density(ρ)
Al01 2.80 89
Al02 2.73 83
3.2. Brinell Hardness Test
Test material was indented with a 10 mm diameter hardened steel ball subjected to a load of
15.25 kg applied for 10 to 15 seconds. The diameter of the indentation left in the test material
is measured with a low powered microscope. Brinell harness number is calculated by
dividing the load applied by the surface area of the indentation. The diameter of the
impression is the average of two readings at right angles and the use of a Brinell hardness
number table can simplify the determination of Brinell hardness structures. Brinell hardness
number of test pins is shown in Table 3.
3.3. Wear Testing of the Composites
A pin-on-disc test apparatus was used to investigate the dry sliding wear characteristics of Al-
MMCs (Al01 and Al02) as per ASTM G99-95 standards. The wear specimens (30x7x7mm)
were machined forged slabs, and polished for metallographic examination. The tests were
conducted for one hour at different loads: 30, 50 and 80N and sliding speeds: 5, 7 and 9m/s at
room temperature. The initial weight of the specimen was measured in a single pan electronic
weighing machine with a least count of 0.0001g. During the test the pin was pressed against
the counter part rotating against EN-32 steel disc by applying the load. A strain–gauged
friction-detecting arm holds and loads the pin specimen vertically into a rotating hardened
1340 M. Asif, K. Chandra, P.S. Misra Vol.10, No.14
steel disc. The frictional traction experienced by the pin during sliding is measured
continuously by PC-based data-logging system. After running through a fixed time period,
the specimen were removed, cleaned with acetone, dried and weighed to determine the
weight loss due to wear. The difference in the weight measured before and after the test gives
the wear of the specimen. The wear rates were determined using the weight loss method. The
frictional forces are recorded by mechanical transducer/load cell. Although the frictional
forces vary with sliding time, an average value for the analysis was considered. Each test was
conducted for a constant load and speed for one hour and similar tests were carried out for
different speeds and loads to investigate the tribological behavior. Scanning Electron
micrographs (SEM) of the prepared Al-MMC samples are shown in Fig.1. The distribution of
particles throughout the matrix is found to be fairly uniform for both binary and hybrid
(a) (b)
Fig.1. SEM micrographs of (a) Binary and (b) Hybrid MMC.
4.1 Effect of Load and Sliding Speed on Wear Characteristics:
Figs. 2(a) and (b) show the variation of wear rate (weight loss/min) with varying load and
sliding speed for binary and hybrid composites. A drastic increment in wear rate of binary
composite was observed with load increased from 50-80N. The wear rate shows little
variation with increasing sliding speed over the load range 30-50N and for higher load range
50-80N, slight reduction is observed with increased in sliding speed from 07 to 09m/s (Fig. 2
(a). For hybrid composite, it is almost stable and minimum at sliding speed 05m/s and
increased with increasing of speed from 05 to 09m/s (30-50N) and decreased for 50-80N
(Fig. 2(b)). It has been concluded that wear resistance is improved with incorporation of SiC-
particles with solid lubricants than that of binary Al-alloy.
(a) (b)
Fig. 2 Effect of load on wear rate of composites for different speeds: 05, 07 and 09m/s.
Vol.10, No.14 Development of Aluminium Based Hybrid Metal 1341
4.2 Effect of Load and Sliding Speed on Coefficient of Friction (COF):
Figs. 3(a) and (b) show the variation of coefficient of friction with varying the load and
sliding speed. The COF of binary composite decreased linearly with applied load (30-80N)
and speed (05-09m/s) (Fig. 3 (a)). It is observed that, the value of COF is high at low loads
and tends to decease with increasing load, and nature of decline of COF with various sliding
speeds is similar. For hybrid composite, the COF at low speed (05m/s) is higher with reduced
sensitivity at lower load range (30-50N) (Fig. 3 (b)). At speed (07m/s), COF deceased with
increased load from 30 to 50N and becomes stable for higher applied load range (50-80N). It
is minimum at high speed (09m/s) and decreased with increased applied load from 30-80N. It
has been concluded that coefficient of friction is stabilized with incorporation of solid
lubricants for various sliding speeds and applied loads.
Fig. 3 Effect of load on coefficient of friction at different speeds: 05, 07 and 09m/s.
4.3 Effect of Load and Sliding Speed on Temperature Rise at Mating Surface
The temperature rise of binary composite at mating surface is increased over lower load range
(30- 50N) for sliding speeds; 05 and 07m/s and decreased for sliding speed (09m/s) (Fig.
4(a)). Temperature rise of hybrid composite increased with sliding speed from 05m/s to
09m/s and stabilized at high speed (09m/s) for load range 30-80N (Fig. 4(b)). At speed
(05m/s) it increased for load range 30-50N and decreased for load range 50-80N. It has been
concluded that temperature rise for hybrid composite is more than temperature rise for binary
(a) (b)
Fig. 4 Effect of load on temperature rise for different speeds: 05, 07 and 09m/s.
1342 M. Asif, K. Chandra, P.S. Misra Vol.10, No.14
4.4 Effect of Load and Sliding Speed on Noise Level
From Figs. 5(a) and (b), it has been found that incorporation of complementary solid
lubricants, the noise level is suppressed with reduced dampening and vibration. It is increased
with load at sliding speeds; 07 and 09m/s and decreased for speed; 05m/s. It has been
observed that at high sliding speed, the nose level is lower with slight variation for hybrid
composites (Fig. 5(b)) than that of binary composites (Fig. 5(a)). It is concluded that solid
lubricant incorporation in composites plays an important role in suppressing the noise level.
Fig. 5 Effect of load on noise level of composites for different velocities: 05, 07 and 9m/s.
4.5 Comparative Study of Al-Based Composites with Fe-Based Composites:
Wear rate, coefficient of friction, temperature rise and noise level of aluminum based
composites have been compared to corresponding values of iron based composites [8] at
different loads for a sliding speed; 09m/s. It has been found that the wear rate of aluminum
based composites (Al01, Al02) is more than that of iron based composites (FM01N, FM02N).
The variation of wear rate at high load for Al based composites is of opposite nature to the Fe-
based composites Fig. 6 (a). The coefficient of friction for Al-based composites is lower than
that of coefficient of friction for Fe based composites at lower as well as higher loads, but
nature of variation with load is similar Fig. 6 (b). The temperature rise at mating surface is
much lower in case of Al-based composites than iron based composites Fig. 6 (c). The nature
of temperature rise with load is not similar for both types of composites. For composites:
Al02 and FM01N, it is increased with load and decreased for composites: Al01 and FM02N.
The noise level is lower for Al-based hybrid composite than that of Fe based composite at
higher load. It is slightly higher for Al-based binary composite at higher load applied Fig.6
(d). It has been concluded that the proposed Al-based composites with lower wear resistance
have better tribological characteristics than Fe- based composites.
It was found from Fig. 1(a), (b) that wear rate of hybrid composite during run-in period is
more than that of binary composite. The asperities of both the pin and counter face which are
in contact with each other are subjected to relative motion under the influence of applied load.
Initially both the surfaces are associated with a large number of sharp asperities and contact
between the two surfaces takes place primarily at these points. In the present case also, the
asperities on the pin are having a large number of reinforcements. Under the influence of
applied load and speed, the asperities in each surface come in contact with each other and they
are either plastically deformed or remain in elastic contact. As the asperities are very sharp in
nature, the effective stress on these sharp points may be more than the elastic stress and then
all these sharp asperities are plastically deformed at their contact points except the partially
projected points of the reinforcement. The plastically deformed surface will fill the valley of
Vol.10, No.14 Development of Aluminium Based Hybrid Metal 1343
the material both in pin and the counter face during the course of action and there is a
possibility of fracture of a few asperities on both the surfaces leading to very fine debris. The
asperities of the sliding pin surface come in contact with the steel disc surface and work
hardening of the matrix material takes place under the applied load and speed [9]. The SiC
particles, which are very strong in compression, are pushed back in to the soft Al alloy
initially instead of fracturing. During the initial run-in period in all the cases, the wear rate is
more because of the fact that the few of highly projected SiC broken particles from the pin
will act as debris and plough the surface particularly the Al-particles. In the resent
investigation a new observation has been marked that the wear rate during the run-in period is
so less and insensitive to sliding speeds. It shows the formation of strong adhesion of SiC
with matrix material in binary composite than that of hybrid composite. As a result it is
confirmed that incorporation of SiC and solid lubricants and applied technology play an
important role to improve the wear characteristics of composites in wear dominant
applications. In the case of hybrid composites the stress on the surface of the pin at high load
and speed (>50N and > 05m/s) is almost uniform and contact between the pin and the counter
face is intact. The initial wear is less in hybrid composites with the protecting layer, known as
tribolayer, formed in SiCp composite-graphite smears and forms a protecting layer such that
the wear rate is still less compared to SiCp composites. The decreasing trend of the wear rate
when sliding speed is increased is due to the formation of protective oxide film [Fe
] along
with the lubricating films of Graphite. At higher speed (>05m/s) due to increase in the
temperature, the surface of the material becomes smooth that promotes local yielding and the
wear mechanism changes from abrasion to delamination wear. Hence the graphitic composite
has lesser wear rate for a particular speed and load compared to all other classes of
composites under study.
(a) (b)
(c) (d)
Figs. 6 Comparison of proposed Al based composites with corresponding values of Fe-based
composites [8] at loads: 50 and 80N for sliding speed: 09m/s.
The following conclusions can be drawn from this work.
1. Incorporation of graphite particles in the aluminium matrix as a second reinforcement.
decreases the wear rates of the composite compared to SiCp reinforced composite.
1344 M. Asif, K. Chandra, P.S. Misra Vol.10, No.14
2. Coefficient of friction is stabilized with incorporation of solid lubricants in composition
of composite at solid state for various sliding speeds and applied loads.
3. Temperature rise for hybrid composite is more than that of temperature rise for binary
composite but it is significantly lower than iron based composites.
4. Incorporating solid ingradients in aluminium powder play an important role to reduced the
noise level.
5. Wear rate decreases as the sliding speed increases up to transition speed and load, due to
work hardening of the surface, formation of Iron oxide, crushing of the SiC particles and
smearing of Graphite.
6. Seizure occurred for wrought aluminium based alloys, but no seizure occurred for Al/
SiCp and Graphitic powder composites under the present study. Combination of abrasion,
delamination and adhesive nature of wear was observed.
7. Proposed aluminium based composites with lower wear resistance have better tribologocal
characteristics than iron based composites.
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