International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
Functionally Graded Materials: A Critical Review
Shailendra Kumar Bohidar1, Ritesh Sharma2, Prabhat Ranjan Mishra3
1,2,3Lecturer, Mechanical Engg. Department
Kirodimal Institute of Technology, Raigarh, Chhattisgarh, India
Abstract
With the development of new industries and
The paper will focus light on above
modern processes, many structures serve in
issues and each plays an important role
thermal environments, resulting in a new
within the FGMs literature and ultimately
class
influences on planning and development
of
composite
materials
called
functionally graded materials (FGMs).
practices. It
FGMs were initially designed as thermal
comprehensive contribution will be very
barrier materials for aerospace structural
beneficial
applications and fusion reactors. They are
interested in FGMs.
is
to
expected
everyone
that
involved
this
or
now developed for general use as structural
components in extremely high-temperature
Keywords: Functionally Graded Materials,
Processing techniques of FGMs
environments.FGMs are now recognized as
important composite materials throughout
1. Introduction
the world. Utilization of FGMs appears
to be one of the most efficient and
effective materials
in
Functionally graded materials (FGMs)
achieving
exhibit a continuous variation of material
sustainable development in Industries. This
properties which result from the non-
paper presents a critical review of the
homogenous microstructure. Due to the
existing literature of FGMs. This paper
unique graded materials properties, FGMs
explores following review strands:
have attracted a great amount of attention
from researchers in many fields, including
1. Introduction.
aerospace, biomaterials and engineering
2. Historical background.
among
3. Application Areas.
decades.Funtionally
4. Processing techniques of FGMs.
(FGMs) possess a position dependent
others
in
graded
the
past
materials
5. Conclusion and Future Scope.
Functionally Graded Materials: A Critical Review 289
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
microstructure, chemical composition or
thermal barrier (with outside temperature
atomic order, which may result in the
of 2000K and inside temperature of
continuous variation of material properties
1000K across
with position such as mechanical, electrical
thickness),
and thermal properties.
material
Composite material are a class of
advanced
material, made up of one or
less
came
called
up
than
with
Functionally
10
mm
a
novel
Graded
Material (FGM) [3,4].
Functionally Graded Material (FGM),
more materials
combined in solid states
a revolutionary material, belongs to a
with
physical
chemical
class of advanced materials with varying
properties. Composite material offers an
properties over a changing dimension
excellent
properties
[5,6]. Functionally graded materials occur
which are different from the individual
in nature as bones, teeth etc. [7], nature
parent materials and are also lighter in
designed this materials to meet their
weight. Wood is a composite material
expected service requirements. This idea is
from nature which consists of cellulose in a
emulated from nature to solve engineering
matrix of lignin [1].Composite materials
problem the same way artificial neural
will
network is used to emulate human brain.
distinct
combination
fail
conditions
under
through
and
of
extreme working
called
Functionally graded material, eliminates
delamination (separation of fibers from the
the sharp interfaces existing in composite
matrix) [2]. This can happen for example,
material
in high temperature application where two
initiated [2]. It replaces this sharp interface
metals
of
with a gradient interface which produces
expansion are used. To solve this problem,
smooth transition from one material to
researchers
mid
the next [3,4]. One unique characteristics
1980s,confronted with this challenge in an
of FGM is the ability to tailor a
hypersonic space plane project requiring a
material for specific application [5].
with
a
different
in
Japan
process
coefficient
in
the
which
is
where failure is
Fig.1.1 Traditional Composite Materials.
Functionally Graded Materials: A Critical Review 290
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
Fig. 1.2 Functionally Graded Materials.
Fig.1.3 Representation of Modern material Hierarchy
In the development of our society
ceramics, etc. The structure of development
and culture, materials have played an
of modern material is illustrated in Fig. 1.3.
essential role.
The scientific use of
These
available
materials
advantages that make them appropriate in
base
into
various
materials
possess
applications.
It
numerous
inorganic and organic compounds has made
potential
includes
a
the path for developing the advanced
potential reduction of in-plane and through-
polymers, engineering alloys, structural
Functionally Graded Materials: A Critical Review 291
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
the thickness transverse stresses, improved
concepts applied to impart new properties
thermal properties, high toughness, etc. [8]
and functions of materials were proposed
2. Historical Background:
by continuous control of the microstructure,
The original idea of compositional and
structural
gradient
in
material
also the design of such materials was
initially
introduced.
The
term
of
microstructure was first proposed for
functionally graded materials was coined
composites
materials
for these gradient composites and materials
in1972.Bever in 1972 studied various
for more accurate description and grammar
gradient composites, investigated the global
in 1986.In 1987,the famous research plan
material properties and reviewed potential
of FGMs,”fundamental studies on the
applications of graded composites.Shen in
relaxation of Thermal stress by Tailoring
1972 reported
that the gradation of
Graded Structures” was pioneered in the
polymeric material might be induced by the
thermal barrier for a space plane in Japan.
variation of the chemical nature of the
The capabilities of withstanding a surface
monomers, the molecular constitution of
temperature of 17000C and a temperature
the polymers and the supramolecular
gradient of 10000C across only a 10mm
structure
section were achieved by FGMs as a
and
or
polymeric
morphology
of
the
polymers[9]. The effective properties, such
thermal
as
and
development of this research project were
possible
spread worldwide via papers, media and
chemical,mechanical,biomedical
transport
properties
and
applications, including gasoline tank and
damping
materials
barrier.
The
results
and
international conferences.
were
Due to their unique graded material
considered.However,the design, fabrication
properties, FGMs attracted great amounts
and evaluation of this gradient structure
of researcher interest, FGMs are potentially
was not studied.
widely applicable in numerous fields. In
Until 1985, the use of continuous
addition to the aforementioned thermal
texture control was presented to improve
barriers, coatings and joints in aerospace,
the adhesion strength and minimize the
FGMs have also been developed for other
thermal stress in the ceramic coatings and
novel
joints being developed for a reusable rocket
biomaterials, biomedical implants such as
engine (Niino et.al., 1986).More general
artificial bones and dental implants are
applications.
In
the
field
of
Functionally Graded Materials: A Critical Review 292
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
classic examples.Tampieri et.al. in 2011
Advances in automation technology during
attempted to produce the porosity graded
the last decades have rendered constitutive
hydroxyapatite (HAP) ceramics, which not
gradation processes technologically and
only good and fast bone in growth but also
economically viable. In
withstand early physiological stress as an
processes a sharp interface between two
implant to replace natural bone. Many other
materials is converted into a gradient by
researchers agreed and reported that FGMs
material transport. Segregating processes
could provide the implant a suitable
start with a macroscopically homogeneous
stiffness
physiological
material which is converted into a graded
loading and that the graded porosity
material by material transport caused by an
structure could enhance the mechanical
external field (for example a gravitational
property of the implant to optimize the
or
materials response to external loading
segregating processes produce continuous
(backer and Bolton, 1997, Pompe et.al.
gradients, but have limitations concerning
2003, Wang et.al. 2012).
the types of gradients which can be
to
endure
the
electric
field).
homogenizing
Homogenizing
and
produced. Usually drying and sintering or
solidification follows the gradation step.
These consolidation processes need to be
adapted to FGMs: processing conditions
should be chosen in such a way that the
3. Processing Techniques of FGMs:
The manufacturing process of a
gradient is not destroyed or altered in an
FGM can usually be divided in building the
uncontrolled fashion. Attention also has to
spatially
structure
be paid to uneven shrinkage of FGMs
(“gradation”) and transformation of this
during free sintering. Since the sintering
structure
material
behavior is influenced by porosity, particle
(“consolidation”). Gradation processes can
size and shape and composition of the
be
constitutive,
powder mixture, these problems must be
homogenizing and segregating processes.
handled for each materials combination and
Constitutive processes are based on a
type of gradient individually referring to
stepwise build-up of the graded structure
the existing knowledge about the sintering
from precursor materials or powders.
mechanisms
inhomogeneous
into
classified
a
into
bulk
[10].
Thin
functionally
Functionally Graded Materials: A Critical Review 293
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
graded materials are usually in the form
of surface coatings, there are a wide range
of surface deposition processes to choose
from
depending
on
the service
requirement from the process.
Some of the fabrication methods for
producing
bulk
functionally
graded
materials are as follows:
3.1. Vapour Deposition Technique:
There
are
different
vapour
deposition
include:
sputter
types
3.2 Powder Metallurgy (PM)
of
techniques, they
deposition,
Powder
metallurgy
(PM)
technique is used to produce functionally
Chemical
graded material [13, 14] through three
Vapour Deposition (CVD) and Physical
basic steps namely: weighing and mixing of
Vapour Deposition (PVD). These vapour
powder according to the
deposition methods are used to deposit
spatial distribution as dictated by the
functionally graded surface coatings and
functional
they
but
ramming of the premixed-powders, and
they can only be used for depositing
finally sintering [15]. PM technique gives
thin surface coating. They are energy
rise to a stepwise structure. If continuous
intensive and produce poisonous gases as
structure is desired, then centrifugal method
their byproducts [12]. Other methods used
is used.
in producing functionally graded coating
3.3 Centrifugal Method
give excellent microstructure,
include:
plasma
spraying,
electrodeposition, electrophoretic,
requirement,
pre-designed
stacking
and
Centrifugal method is similar to
Ion
centrifugal casting where the force of
Beam Assisted Deposition (IBAD), Self-
gravity is used through spinning of the
Propagating High-temperature Synthesis
mould to form bulk
(SHS), etc[11]. All the above mentioned
material [16]. The graded
processes cannot be used to produce bulk
produced in this way because of the
FGM because they are generally slow
difference in material densities and the
and energy intensive, therefore they are
spinning of the mould. There are other
uneconomical to be used in producing
similar processes like centrifugal method
bulk FGM.
in the literature (e.g. Gravity method,
functionally graded
material
is
etc.). Although continuous grading can
Functionally Graded Materials: A Critical Review 294
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
be achieved using centrifugal method but
processes
only cylindrical shapes can be formed.
fabrication
Another problem of centrifugal method
materials [20]. Laser based SFF process
is
for FGM [21,22] include:
that there is limit to which type of
are
of
mostly
employed
functionally
in
graded
laser cladding
gradient can be produced [17]because the
based
method
gradient
natural
Laser
Sintering
density
DPrinting (3-DP) [25, 27], and Selective
is
formed
process (centrifugal
through
force
and
[21, 23-28],
(SLS)
[31,
Selective
32],
difference).
To solve these problems,
Laser
researchers
are
alternative
cladding based system and selective laser
manufacturing method known as solid
melting are capable of producing fully
freeform.
dense components. Solid freeform provide
3.4 Solid Freeform (SFF) Fabrication
manufacturing flexibility amongst other
Method
advantages
Solid
using
freeform
the
technology
is
characterized by poor surface finish making
manufacturing process that offers lots of
it necessary to carry out a secondary
advantages that include: higher speed of
finishing operation. There
production,
intensive,
research
efforts
in this direction to
maximum material utilization, ability to
improve
surface
finish,
produce
accuracy etc.There are other fabrication
complex
energy
shapes
an
but
Laser
additive
less
is
Melting (SLM) [24, 26].
3-
and
design
are lots of
dimensional
freedom as parts are produced directly
methods
from CAD (e.g. AutoCAD) data [18].
materials; readers can refer to the review
SFF involves
five basic steps [19]:
studies by Kieback and Neubrand and
generation
CAD
the
Gasik, [17, 31]. These authors presented
software like AutoCAD, Solid edge etc,
comprehensive processing techniques of
conversion of the CAD data to Standard
functionally graded materials.
of
data
from
for
functionally
graded
Triangulation Language (STL) file, slicing
of the STL into two dimensional crosssection
profiles,
building
component layer by layer,
of
4. Application Area:
the
FGMs have great potential in
and lastly
applications where the operating conditions
removal and finishing. There are various
are
types of SFF technologies, laser based
shields, heat exchanger tubes, biomedical
severe,
including
spacecraft
heat
Functionally Graded Materials: A Critical Review 295
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
implants, flywheels, and plasma facings for
reactors, etc. For example, a discrete layer
fusion reactors, etc. Various combinations
of ceramic material is bonded to a metallic
of the ordinarily incompatible functions can
structure in a conventional thermal barrier
be implemented to create new materials for
coating for high temperature applications.
aerospace, chemical plants, nuclear energy
Fig.1.4 Various fields of Application of FGMs.
Typical applications of FGMs are as
Living tissues like bones and teeth
follows:
are characterized as functionally graded
* Smart Structures
materials from nature, to replace these
Functionally graded piezoelectric
materials.
tissues, a compatible material is needed that
will serve the purpose of the original bio-
Shape memory alloys.
tissue.
The
ideal
for
application
* Electronics and optoelectronics
material.FGM has found wide range of
transmission.
Computer circuit boards (PCB)
application
in
applications
functionally
this
* MEMS and sensors
Optical fibers for wave high speed
is
candidate
dental
for
and
teeth
graded
orthopedic
and
bone
replacement.
Cellular phone.
* Medicine:
* Biomaterials
Functionally Graded Materials: A Critical Review 296
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
Artificial bones, joist.
thermal barriers in Space vehicles.FGMs
Teeth
have the added advantage that the metal
Cancer prevention.
side can be bolted onto the airframe rather
than bonded as are the ceramic tiles used in
* Others
Baseballs cleats.
the orbiter. Other possible uses include
Razor blades.
combustion chamber insulation in ramjet or
Titanium watches.
scramjet engines.
* Applications of FGM as Thermal
* Power plant
Thermal barrier coatings.
barrier Coating
One of the salient applications of
Heat Exchanger tubes.
FGM is in thermal Barrier Coatings.
* Manufacturing
Machine Tools
Thermal
Forming and cutting tools.
widespread applications in automotive and
Metal
casting
and
forging
barrier
coatings
find
their
aircraft industries. They are specifically
processes.
designed to reduce heat loss from engine
* Aerospace and Aeronautics
exhaust
Exhaust
separates
wash
exhaust
gas
structure
from
that
aircraft
system
components
including
exhaust manifolds, turbocharger casings,
exhaust headers, down pipes and tail pipes.
structure for vehicles which have internally
exhausted engines, i.e., stealth aircraft and
UAVs with engines that don’t exhaust
directly to atmosphere. Hot, high speed
engine exhaust flows over the top surface
* Defense:
of exhaust wash structures which, in turn,
One
of
the
of
most
important
causes large deflections. An FGM patch
characteristics
functionally
graded
applied to the underside of the exhaust
material is the ability to inhibit crack
wash structure can be designed such that
propagation. This property makes it useful
thermally induced deflection of the FGM
in defense application as a penetration
patch is in a direction opposite to the
resistant materials used for armor plates
exhaust wash structure deflection. Ceramic
and bullet-proof vests.
metal FGMs are particularly suited for
* Energy:
Functionally Graded Materials: A Critical Review 297
International Journal of Research (IJR) Vol-1, Issue-7, August 2014 ISSN 2348-6848
FGM are used in energy conversion
[2]
S. S. Wang “Fracture mechanics
devices. They also provide thermal barrier
for
and used as protective coating on turbine
composite materials,”
blades in gas turbine engines.
Composite Materials, (1983), vol.
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in
Journal of
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5. Conclusion and Future Scope:
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