Composite Materials
After a first observation they were “re-discovered” at the
beginning of the 90s and immediately started to cover a fundamental role in
several scientific branches, from Physics to Chemistry, going through Medicine
and Biology. In the Material Science and Engineering field a large number of
studies have been conducted on carbon nanotubes, as a consequence of their
extraordinary physical, technological and mechanical properties. For what
concerns this research, they have been largely investigated as reinforcement
materials in composites. Since 1991 a large number of attempts have been
conducted, trying to exploit the outstanding potential of this carbonaceous
material, in order to improve the properties of several matrices. The most
important application is the production of polymer matrices composites (PMCs),
but in last decades an increasing number of metal matrix ones (MMCs) have been
presented and recently also ceramic matrix (CMCs) applications have been
attempted. Despite massive efforts focused on CNTs-composites, the potential of
employing this reinforcement materials has not yet been fully exploited. This
lack is substantially due to the difficulties associated with the dispersion of
entangled carbon nanotubes during processing and poor interfacial interaction
between CNTs and matrix materials. The dispersion states of the nanotubes
involve complicated phenomena, since the carbon nanotubes are produced in
bundles or bundle aggregations. The states are affected by at least two
competitive interactions: (1) the interactions of van der Waals forces, among
carbon nanotube threads, and (2) the interactions between carbon nanotube
threads and dispersion medium. The characteristics of single carbon nanotube
and of bundles of carbon nanotubes are completely different. CNTs must be
uniformly dispersed to the level of isolated tubes individually wetted by the
matrix. This is necessary in order to achieve efficient load transfer to the
reinforcement network. This also results in a more uniform stress distribution
and minimises the presence of stress concentration centres.
Because of these reasons the very first aspect of this work
has been the study of the dispersion state of nanotubes. The aim of the
experiments was not only to obtain a good dispersion and distribution of the
CNTs, but also to evaluate their dispersion grade. Indeed, due to their
nanosize and to their carbonaceous nature, few simple experimental techniques
result suitable for this purpose. In addition, the objective was also to use
dispersion techniques which do not imply the utilisation of 7 high amount of
solvents, reagents or surfactants, and so the aim was to develop a process as
simple, low costs and as environmental friendly as possible.
The second part of the work consisted in the application of
the carbon nanotubes to the production of new materials for technological
applications, with improved mechanical properties. Three composite materials
with different matrices have been designed, developed and produced: a polymer
matrix composite, a ceramic matrix and a metal matrix one.
Also in these cases the aim of the work was the tuning of
simple and possibly low costs production systems.
For PMCs a polyvinyl butyral matrix has been used and the
composites were obtained by a deeply studied technique in the research group:
the tape casting technology. The same approach was also used in the case of
CMCs: tape casted silicon carbide matrix composites reinforced by carbon
nanotubes have been produced. Finally a third matrix has been experimented:
MMCs were investigated starting from pure aluminium powders. For Al matrix
composites a particular technique was used: the sintering was obtained starting
from a powder metallurgy approach and exploiting electric current and pressure
(Electric Current Assisted Sintering approach).
For all the three different composite materials, after the
development of the production route and the preparation of several specimens, a
characterization step followed. The materials were characterized in terms of
physical properties, morphology and microstructure, and mechanical behaviour.
Composite materials are gaining wide spread acceptance, due
to their characteristic behavior and high strength to weight ratio. Of these
aluminium metal matrix composites are finding increased applications, because
of their improved mechanical (such as hardness, young’s modulus, yield
strength, and ultimate tensile strength due to the presence of micro-sized
reinforcement particles into matrix material), physical and tribological
properties and also the thermal conductivity of composite material are good. A
metal matrix composite (MMCs) consists of a metallic alloy matrix (such as
aluminium, magnesium, and titanium). Typically reinforced with a ceramic phase
in the form of particles, platelets, whiskers, short fibers and continuously
aligned fibers. Aluminium alloy reinforced with various particulate ceramics
particles.
Fabrication of casting is done by stir casting processes
among all the different processes.
Tungsten carbide is used as reinforcement material in
aluminium alloy as matrix.
Aluminium alloy are used in vast number of applications
mostly used in automobile filed (such as engine cylinders, drive shafts,
pistons, and brake rotors), industrial applications, aerospace applications and
it is also useful in defence applications like torpedoes, manufacture of
missile bodies, because of their low density, high strength to weight ratio,
high thermal conductivity, high elastic modulus, and good structural rigidity.
Aluminium alloy reinforced with various particulate ceramics particles are
universally known as aluminium matrix composites (AMCs).
A composite may be defined as a structural material system
that consists of two or more combined constituent’s material that are combined
at a macroscopic level and are not soluble in each other. One constituent is
called the reinforcing phase and the one in which it is embedded is called the
matrix. The bulk material forms the continuous phase that is the matrix (e.g.
metals, polymers, etc) while the other acts as the discontinuous phase that is
the reinforcements (e.g. fibers, whiskers, particulates, etc). While the
reinforcing material usually carries the major amount of load, the matrix
enable the load transfer by holding them together.
Metal matrix composites (MMCs) are important class of
material with non metallic reinforcement incorporated in metal matrices.
Aluminium (Al) alloy matrices mainly 2024, 5052, 6061, and 7071 have been
widely used as matrix materials
The present investigation has been focused on tungsten
carbide (WC) particulate composite formation by utilization of low grade
powdered graphite (Gr) by its dispersion into aluminium matrix by stir casting
method. The objective is to form the reinforcing phase within the metallic
matrix by reaction of graphite with aluminium in the metallic melt.
In this investigation aluminium 2024 (Al 2024) is the base
material and reinforced with tungsten carbide (WC) and graphite (Gr) is used
(Al 2024/WC-Graphite).
No comments:
Post a Comment