CHAPTER atmospheric and marine conditions along with high







A detailed discussion on materials for
composites, production process as well as equipment for evaluating the
properties of the aluminium alloy metal matrix and hybrid composites is given
in this following section.


The materials selected for producing
aluminium alloy metal matrix and hybrid composites are described in the
following section.

3.2.1 Matrix Material

In this study, A16061T6 aluminium alloy
(Table 3.1) with the density of 2700 Kg/m3 was used as the matrix
material. Al6061 alloy exhibits excellent resistance to corrosion under both
ordinary atmospheric and marine conditions along with high strength and hardness.


Table 3.1 Chemical composition of Al6061 aluminium
alloy used (wt %)























3.2.2 Reinforcements

Generally, the following requirements
are applicable for selection of reinforcement material: low density,
compatibility with matrix alloy, chemical compatibility, thermal stability,
high compression and tensile strength and economics efficiency.



In metal matrix composites,
reinforcement like alumina, silicon carbide strengthens the metal matrix both
extrinsically through load transfer to the ceramic reinforcement, and
intrinsically by increasing the dislocation density. The interaction between
the particular reinforcement and the metallic matrix is the basis for the
enhanced physical and mechanical properties associated with metal matrix

Silicon carbide also known as
carborundum, is a compound of silicon and carbon with chemical formula (SiC).
Alumina (Al2O3) is one of the most cost effective and
widely used materials in the family of engineering ceramics.

In this study, Silicon Carbide (SiC) and
aluminium oxide (Al2O3) particles (Figure 3.1 (a)) were
used as reinforcement with an average particle size are 37 µm. Aluminium oxide
possesses very low reactivity in molten metal and is relatively cheap. The
density of silicon carbide (SiC) and alumina (Al2O3) are
3.2 gm/cc and 3.7 gm/cc respectively.

Many ceramic particles Al2O3,
SiC, TiC, SiO2 and graphite are added in the Al6061 composites to
form composites. Among this SiC are commonly and commercially used
reinforcements. This is because SiC has advantages over other ceramic reinforcements
such as thermal conductivity, density, relative cost and corrosion resistance.

In hybrid metal
matrix composites, a soft reinforcement like graphite contribute to low wear
rate, friction and anti-seizing properties, graphite is a soft, slippery,
greyish-black substance. It has a metallic luster and is opaque to light.
Density of graphite is 2.3 gm/cc. Special attention has been given to relative
amount of solid lubricants in the metal matrix composites, since it affects the
mechanical and tribological properties significantly.










Figure 3.1 Reinforcement material



In the present investigation Al-6061T6
alloy was chosen as the base matrix which is reinforced with silicon carbide
and alumina having size of 37um. Silicon carbide and alumina being hard and brittle
in nature, gets accommodated in soft ductile aluminium base matrix enhancing
the overall stiffness and strength of the metal matrix composites (MMCs). In
order to achieve high level of mechanical properties in the composite, a good
interfacial bonding between the dispersed phase and the liquid matrix has to be
obtained. To increase the wettability of the liquid metal to 2 % by weight,
cerium is added. The silicon carbide and alumina is preheated at 500o C
for one hour before mixing it to the molten metal. Care was taken to maintain
an optimum casting parameter such as stirring speed (350 rpm), stirring time
(5-10 min.) and pouring temperature (700o C). The molten metal was
poured into green silica sand mould of diameter 14 mm and length 120 mm. And
after cooling the samples required for tribological testing and are prepared by
different machining processes.

In the present work, Al6061T6  MMCs reinforced  with 5 wt%, 10wt%, 15wt%, 20 wt%, 25wt%,
30wt%, 35 wt% and 40wt%, silicon carbide were produced. The same procedure was
repeated for hybrid metal matrix composite with 15 wt% of SiC and alumina each.
The photograph of cast composites and machined were pin in shown in Fig 3.2.











Figure 3.2 Composite casting and
wear test pins.



Density of composites is determined
using top loading electronic balance. According to the Archimedean principle, a
solid body immersed in a liquid apparently loses as much of its own weight of
the liquid it has displaced. This makes it possible to determine the unknown
value. The density of the solid body is determined by using a liquid of known

Microstructure of the composite
specimens was carried out using optical microscope. The specimens were
metallographically polished to obtain an average roughness value of 0.8 m.

The micrographs of the polished
specimens were recorded with different magnifications. Microhardness values
were measured at various locations in composite specimen employing microhardness
tester which demand indenter at a load of 100 gm. The average of five readings
was taken as the hardness of composites.

Morphology of worn surface of the
composite specimen is carried out using optical micrograph.



The technique used for studying dry
sliding wear of composites is described in the following section.

3.5.1 Pin-on-disc

Dry sliding wear behaviour of composites
were studied using a pin-on-disc apparatus (DUCOM make), Figure3.3 shows the
arrangement of pin-on-disc apparatus. The disc material was made of EN-32 steel
with a hardness of 65 HRC.

The pin specimen is pressed against disc
at a specified load usually by means of an arm and attached weights. The
apparatus has a friction force measuring system, for example, a load cell, that
allows the coefficient of friction to be determined.









Figure 3.3 pin-on-disc apparatus

3.5.2 Dry Sliding Wear
Test procedure

The dry sliding wear tests were carried
out at room temperature (30oC ± 3oC, RH 55 % ± 5 %) under
dry sliding condition in accordance the ASTM G99-95 standard. Cylindrical pins
of 10 mm diameter and 50 mm long were machined from composite casting and
metallographically polished (Figure 3.2). Immediately prior to testing, it was cleaned
and dried using acetone to remove all dirt and foreign matter from the

The following equations (3.1-3.4) are
used for calculating volume loss, wear rate, specific wear rate and coefficient
of friction.

Volume loss =

 ×1000 mm3                                 – (3.1)

Wear rate =    

 mm3/m                                          – (3.2)

Specific wear rate =   

  mm3/ N-m                      – (3.3)

Coefficient of Friction =       

                               – (3.4)


Where m1 is the mass of the
specimen before the wear test, m2 is the mass of the specimen after
the wear test, ? is the density of the composite in gm/cm3, V is the
volume loss in mm3, L is the applied load in Newton, and D is the
sliding distance in meter. The coefficient of friction is calculated by the
ratio between tangential forces (FT) and the normal force (FN).
The tangential force is obtained from the load cell fitted in the pin-on-disc
apparatus. The measured tangential forces measured only during the steady state



The same pin-on-disc type apparatus was
employed to evaluate the high stress abrasive wear characteristics of
composites. The disc was covered with commercial SiC emery sheet was fastened
to a rotating disc. In order to encounter fresh abrasive material, the specimen
was also moved against the parallel surfaces of the rotational steel disc.

Mass loss of the specimen were measured
before and after the wear test using electronic weighting balance (accuracy
0.0001g) were repeated with additional specimens to obtain sufficient data for
significant results.




Continuous improvements
in testing and manufacturing process to obtain improvement of properties at
lower cost remains at the forefront of efforts to expand the importance of
metal matrix composites. During materials development and testing, every effort
was made to ensure quality of the composites as well as reliability and
repeatability of the test method adopted. The test methods were carried out in
particular wear test methods with reference to common occurrences of friction
and wear in machinery to understand and solve existing or expected wear
problems leading to significant cost reduction.