Journal of Scientific & Industrial Research
Vol. 73, October 2014, pp. 667-670
Production and characterization of hybrid aluminum matrix composites reinforced
with boron carbide (B4C) and graphite
T Thirumalai*, R Subramanian, S Kumaran, S Dharmalingam and S S Ramakrishnan
Department of Metallurgical Engineering, PSG College of Technology, Coimbatore, India
Received 25 February 2013; revised 20 December 2013; accepted 5 August 2014
Aluminum matrix composites have been reinforced with Boron carbide (B 4C) and Graphite (Gr) for increasing
mechanical properties and wear resistance. Additions of Boron carbide (B4C) improves both strength and wear resistance of
composites, but addition of B4C alone in higher amounts makes the composite brittle and machining difficult. Thus, B 4C can
be advantageously used as a reinforcement to overcome the problem of strength reduction in Gr reinforced composites,
resulting in hybrid composites. Aluminum matrix composites reinforced with up to 12 wt % B4C and 3 wt % Gr particulates
are investigated in the present study. Hybrid composites exhibit better wear characteristics compared to aluminium alloy.
Wear tests were carried out with loads varying from 10 to 40 N and sliding distances of 500 and 1000 m with a constant
sliding speed of 1m per second. An interaction between load and sliding distance was observed in the composites which may
be attributed to the presence of Gr particulates. Decrease in wear with an increase in speed and vice versa were observed
with both load and sliding distance. Hardness of the composites measured using Vicker’s Hardness Tester indicated that
hardness increased with increasing percent of B4C reinforcement while addition of Gr imparted the lubrication effect in the
composites.
Keyword: Aluminum metal matrix composites (AMC), boron carbide, graphite, wear, stir casting, adhesive wear.
Introduction
Use of metal matrix composites in the fields of
aerospace, automotive and other engineering
applications is gaining momentum now days due to
their high strength to weight ratio, stiffness, hardness,
wear resistance, as well as thermal conductivity.
Combining low density metals with reinforcing
particles result in enhanced performance components
which can be used as substitutes for existing
monolithic materials. Among the various factors that
influence the hardness, wear, and other mechanical
properties of the AMC’s size of abrasives and per cent
weight of alumina play a significant role. Aluminium
alloys reinforced with ceramic particles such as SiC,
Al2O3, B4C, and graphite have been reported to posses
better hardness, wear properties and strength. Many
researchers have shown that, size of reinforcement
was the major parameter influencing the hardness of
composites followed by weight fraction of
reinforcement compared to other control factors.
Reinforcing hybrid composites with SiCp and
graphite increased the wear resistance but, the wear
rate decreased with increasing SiCp content1. Similar

*Author for Correspondence
Email: [email protected]
increase in hardness with increasing percent of
reinforcement have also been reported2,3. In addition,
sliding speed and applied load influence the friction
coefficient However, significant influence of sliding
distance on friction coefficient was observed.
However, holding temperature and time of alloy melt
showed limited effect on hardness4,5. Boron carbide is
a non-metallic reinforcement having a high hardness
coupled with high wear resistance and high melting
point thus possessing resistance to change due to the
addition of chemicals. Hence reinforcing the
aluminium composites with boron carbide particles
confers high specific strength, elastic modulus, good
wear resistance and thermal stability17. Wear studies
on Al-B4C composites with varying wt % of B4C
(5, 10 and 15 %) showed a linear increase in wear
resistance with increasing B4C content and the highest
wear resistance was observed at 15 per cent
reinforcement of B4C7. Fabrication of Al composites
with 40% SiC/5%Gr by squeeze casting revealed that
the addition of graphite led to a decrease in the
friction coefficient of composites but increased the
wear resistance by 170 to 340 times8. However,
increasing the Gr reinforcement beyond 5 %
decreased the fracture toughness due to the formation
of thick solid lubricant film which overrides the effect
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J SCI IND RES VOL 73 OCTOBER 2014
of fracture toughness9. Hence the present study was
taken up with the aim of fabricating Al–B4C–Gr
composite having improved wear resistance and
evaluate them for understanding the factors
contributing to the enhanced to wear resistance.
Materials and Methods
Al–B4C–Gr and Al composites were fabricated
usin0g LM25 as matrix alloy by stir casting method.
The composition of LM25 alloy and that of al alloyB4C-Gr particulate composites are given in Table 1
and 2. Hybrid composites (Al–B4C–Gr) were
prepared by adding 3, 6, 9 and 12% B4C by weight
along with 3% Gr reinforcement.
Fabrication of Composites
Stir-casting process was used to fabricate the
AMC’s by physically mixing matrix alloy Aluminium
(LM25) with graphite and B4C. The stir-casting setup
consisted of a motor to drive the steel stirrer. The
matrix alloy was melted in a graphite crucible in an
electric furnace under argon atmosphere. Weighed
quantity of aluminium alloy was superheated to 750ºC
and brought to a semi-solid state by lowering the
temperature gradually below the liquidus temperature.
Magnesium (3 wt.%) was added to the molten metal
to improve wettability of reinforcements with
aluminium. At this stage mixture of Boron carbide
(B4C) and graphite (Gr) particulates, preheated to
200ºC, were introduced into the melt and the slurry
was continuously stirred by rotating the impeller at a
speed of 400 rpm for uniform dispersion of
partculates in the melt. Stirring was continued for
about 2-3 minutes to ensure good wetting and then the
composite melt was degassed using hexachloroethane.
The melt was then superheated above the liquidus
temperature and finally poured into a cast iron
permanent mould to obtain cylindrical samples of
Table 1Chemical composition of the matrix alloy
Aluminium(LM25)
Chemical composition (weight %)
Si
7.1
Mg
0.3
Fe
0.3
Cu
Zn
Ni
< 0.012 0.004 0.002
Mn
0.28
Al
Rest
Table 2Details of reinforcements
S. No
1.
2.
3.
Type of
Reinforcement level (%)
reinforcement
B4C
Gr
Total
3.0
3.0
6.0
6.0
3.0
9.0
9.0
3.0
12.0
12.0
3.0
15.0
Size
25–5 Micron
25–75 Micron
15 mm diameter and 75 mm length. Unreinforced
matrix alloy specimens were also cast for comparison
purpose.
Characterization of Composites
Wear test specimens of 15 mm length and 6 mm
diameter were machined form the cast samples.
Specimen’s surface was polished with abrasive paper
(600 grade) and followed by fine polishing with grade
1000 paper. Dry sliding wear tests were carried out as
per ASTM G99-95a standard using pin-on-disc
equipment. The disc was made of EN31 steel with
surface roughness, Ra 0.1 and a hardness of 65 Rc.
Test pins were cleaned with acetone and weighed
before and after testing (accuracy of 0.0001 g) to
determine the weight loss of the specimens. The
flatness of test sample is maintained so that entire
surface of the Pin is in contact with disc. The wear
test were carried out with loads varying between 10 to
40 N , for sliding distances of 500 and 1000 m at a
constant sliding speed of 1m per second. The hardness
of the composites is measured using varying Vicker’s
Hardness Tester.
Results and Discussions
Hardness is an indicator of the resistance to plastic
deformation and an important mechanical property
often considered in the design of automotive
components. Compsoite with 12 wt % B4C 3 wt%
graphite reinforcement showed a significant increase
in hardness. Higher strength of boron carbide
(9.30 Mohs) might have contributed to the increased
strength of hybrid composites. Similar results have
been observed in 6061Al matrix reinforced with B4C
particles by the researchers5,6. Measured hardness
values correlate well with other properties like
strength and wear resistance (Table 3). An increase in
wear resistance was also observed in the present
investigation which might be attributed to the
instability developed at longer sliding distances in the
tribolayer. Among the composites containing different
Table 3Hardness of Al alloy and hybrid composites
S. No
1.
2.
3.
4.
5.
Reinforcement
Al
Al/3B/3 Gr (C1)
Al/6B/3Gr (C2)
Al/9B/3Gr(C3)
Al/12B/3 Gr(C4)
Average VHN
(load-100)
% of
Improvement
87
110
119
128
140
26.46%
36.78%
47.13%
60.92%
THIRUMALAI et al: PRODUCTION AND CHARCTERIZATION OF HYBRID MATRIX COMPOSITES WITH B4C & GRAPHITE
amounts of B4C (3, 6, 9 and 12%), composites with
12 wt 5 B4C showed optimal properties. A similar
increase in wear resistance was also observed in nano
composites11,12 and in fine grained boron carbide and
B4C-TiB2 composites5. A drastic increase in wear rate
of A356 Al–10SiC–4Gr hybrid composites with
sliding distance was observed immediately after the
removal of tribolayers3,4,9. The amount of wear was
considerably lower for Al–B4C–Gr hybrid composites
than unreinforced Al as seen from figure 1- 4. Similar
observations indicating an increased wear loss with
increasing load and sliding distance for both Al–B4C–Gr
hybrid composites and Al have been reported by
several researchers1,11,12. Fig. 1 and 2 depict the effect
of per cent B4C reinforcement and load on the wear of
Al–B4C-Gr hybrid composites and Al. The amounts
of wear loss as well as its trend at a sliding speed of
1m/s for various load and reinforcement levels are
similar for both the sliding distance of 500 m and
1000 m. It is also evident from the plot that the wear
loss increases with increasing sliding distance and
load irrespective of the amount of reinforcement in
both Al and hybrid composites and are found to be the
predominant factors affecting wear [Fig. 2, 3, 4].
Addition of increasing levels of B4C particles along
with Gr (3%) in the Al matrix increases the strength
by offering more resistance to stress and increased the
hardness of the Al hybrid composites in the present
investigation. Similar improvement for the addition of
B4C and Gr individually was reported by many
researchers2,4,5,6,7,8. Reinforcement, sliding speed, load
and sliding distance significantly influences the wear
and notable interactions exist among sliding speed,
load and sliding distance in Al–B4C–Gr hybrid
composites. The distribution of boron carbide and
Fig. 3Sliding Distance 1000 m
Fig. 1Sliding Distance 500 m
Fig. 2Sliding Distance 500 m
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Fig. 4Sliding Distance 1000 m
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J SCI IND RES VOL 73 OCTOBER 2014
graphite particles in the hybrid Al composite was
studied using optical microscopy and was found to be
nearly uniform. Optical micrographs of Al alloy and
Al-B4C-Gr hybrid composite revealed spherical B4C
particles and slightly angular graphite particles along
with dendritic aluminium particles in a eutectic
matrix. The distributions of reinforced particles were
observed to be randomly oriented.
Conclusion
The Al–B4C–Gr hybrid composites were prepared
successfully by stir casting method and reinforcement
of boron carbide increased the strength of composites.
Optimal per cent reinforcement can be of 12% for any
value of sliding distance, speed and load within the
range considered in this investigation. Al–B4C–Gr
hybrid composites are better substitutes the Al alloy
owing to improved hardness and wear resistance as a
result of the addition B4C–Gr particulates to Al.
Analysis shows that, increase in per cent
reinforcement reduces the wear up to 12%.
Acknowledgements
The authors are grateful to the Principal, Head of the
Department and faculties of Dept. of Metallurgical
Engineering, PSG College of Technology, Coimbatore,
for helping in carrying out the work and allowing
utilization of facilities available. Thanks are also due to
the Dr.P.C.Angelo, PSG College of Technology,
Coimbatore and Dr.B.Ravishankar, NIT, Trichy for
their help and constructive suggestions rendered during
the study.
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