FUNCTIONALLY GRADED MATERIALS
DEFINITION
Functionally graded materials (FGM) are composite materials
which are designed to present a particular spatial variation of
their properties.
This is usually achieved by forming a compound of two
components whose volume fraction is changed across a certain
direction.
ORIGIN/MOTIVATION
The “first” FGM was developed in Japan in 1984-85 as the
result of a spaceplane project.
Although the concept of FGM is recent, many materials that fit
the description have existed for decades.
Some FGM also occur naturally:
• Bones and teeth
• Seashells
ORIGIN/MOTIVATION
Better adherence of a protective layer (against corrosion,
for instance)
Minimization of interfacial stresses between different
materials (e.g. due to temperature variation)
Increase in local fracture thoughness
Relocation of maximum stresses on a load bearing
component
ORIGIN/MOTIVATION
FGMs allow better customization and tailoring of materials for
specific tasks
Stiffer at clamped
end gradation in Y direction
Softer at clamped end
Material
More variety in material selection for engineering design
CLASSIFICATION OF FGMS
FGMs may be compositionally or micro-structurally graded
The gradient is established through a transition function
(usually volume fraction as a function of one or more
spatial coordinates)
FGMs come in several types, depending on their
constituents (e.g. ceramic-metal, metal-metal…)
GRADATION
Continuous
Stepped
Ceramic-Metal
TiC-Ni, Mullite-Mo, Al-AlB2
Metal-Metal/Intermetallic
W-Cu, W-Mo, Al-Al3Fe
Metal-Polymer
Al-Polycarbonate
Others
Single material (variation in porosity)
Glass - Ceramic
Ceramic - Ceramic
MODELING OF FGMS
Some researchers decided upon a basic unit to describe
FGMs
The maxel represents the smallest entity in which the
composition of a continuously graded FGM can be defined
Pure Component A
% Component A
% Component B
Pure Component B
It is the equivalent of the build resolution in rapid prototyping processes
(quantitized by voxels – hence maxel = material voxel)
MODELING OF FGMS
1) Assume a preset
variation
MODELING OF FGMS
2) Linear rule of mixtures (function of local volume
fraction)
In general, applicable only to metal-metal FGMs, may be used
as a first approximation for different compositions
3) Halpin-Tsai
More complex, takes into account the aspect ratio of the
inclusions (s)
MODELING OF FGMS
4) More:
Mori-Tanaka
Empirical rule of mixtures
…
Other properties such as the Poisson ratio and
thermal expansion coefficient follow similar trends.
Hardness and fracture thoughness of the
resulting material are more difficult to predict and some
examples will be given further ahead.
TUNGSTEN-COPPER
Tungsten surface:
Copper surface:
Hard, refractory
Good electric
material
and thermal
conductivity
TITANIUM CARBIDE-NICKEL
Maximum
Peak in hardness
fractureand
thoughness
flexure strength
is achieved
due for
to metal
30 wt.%
phase
Ni. The
changing
metalits
phase
surrounds
behavior from
the TiC
dispersive
particlestoand
connective
hence acts as a toughening phase.
MULLITE (AL6SI2O13) MOLYBDENUM
Smoother variation favors resistance to
thermal shock (Vf Mo = 1 – (x /L) p)
ALUMINUMPOLYCARBONATE
This type of materials is being researched for its
special properties of full wave transmission on one side
(Al) and full dissipation on the other, making it suitable for
NDT (Non-Destructive Testing) probes.
OPTIMAL DESIGN
As mentioned earlier, FGMs lend themselves well to
being optimized for various performance measures. An
example:
OPTIMAL DESIGN
Multiobjective optimization – generation of a
Pareto front using genetic algorithms
Experie
nce aquired from
research into
FGMs has
produced its
results.
Knowle
dge has been
gained on which
transition
functions are
best suited for
specific tasks
and material
types.
AEROSPACE APPLICATIONS
Ceramic-metal FGMs are particularly suited for
thermal barriers in space vehicles.
They have the added advantage that the metal
side can be bolted onto the airframe rather than bonded
as are the ceramic tiles used in the Orbiter.
Other possible uses include combustion chamber
insulation in ramjet or scramjet engines
FUEL CELL TECHNOLOGY
Creating a porosity gradient in the electrodes, the
efficiency of the reaction can be maximized
NUCLEAR FUSION REACTORS
Modification to heat exchangers in tokamak fusion
reactors
Reduction of interfacial stresses → prevention of
delamination effects → increase in lifetime
JET
SOLIDIFICATION
- Solid freeform process
- Ultimately requires
sintering of the resulting
green body
PRESSURE
FILTRATION
- Versatile process that can be
adapted to produce both axial and
radial gradients
- Also requires sintering
DIRECTIONAL SOLIDIFICATION
- Melt processing (no
sintering step required)
- Only axial gradients
- Material processed
at low speed on which the
shape of the transition
function is heavily dependant
upon
SINTERING
- Using a conventional oven, microwave or
laser beam
- External pressure may or may not
be applied
PROCESSING METHODS
CONCLUSIONS
Functionally graded materials are still a very
recent area of research (and thus very active)
Current research is mostly focused on
uncovering the complex nature of fracture mechanics due
to material nonhomogeneity as well as in
developing/improving forming processes so that the
target gradient is achieved with precision