FUNCTIONALLY GRADED MATERIALS

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.

DEFINITION

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)

Relocation of maximum stresses on a load bearing

component

Increase in local fracture thoughness

ORIGIN/MOTIVATION

FGMs allow better customization and tailoring of materials for

specific tasks

More variety in material selection for engineering design

Stiffer at clamped end Softer at clamped endMaterial gradation in Y direction

ORIGIN/MOTIVATION

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

Metal-Metal/Intermetallic

Metal-Polymer

Single material (variation in porosity)

W-Cu, W-Mo, Al-Al3Fe

TiC-Ni, Mullite-Mo, Al-AlB2

Al-Polycarbonate

Others

Glass - Ceramic

Ceramic - Ceramic

Pure Component B

Pure Component A

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

It is the equivalent of the build resolution in rapid prototyping processes

(quantitized by voxels – hence maxel = material voxel)

% Component A

% Component B

MODELING OF FGMS

1) Assume a preset variation

MODELING OF FGMS

3) Halpin-Tsai

More complex, takes into account the aspect ratio of the

inclusions (s)

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

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.

MODELING OF FGMS

TUNGSTEN-COPPER

Tungsten

surface:

Hard, refractory

material

Copper surface:

Good electric

and thermal

conductivity

TITANIUM CARBIDE-NICKEL

Peak in hardness and flexure strength due to metal phase changing its

behavior from dispersive to connective

Maximum fracture thoughness is achieved for 30 wt.% Ni. The metal phase

surrounds the TiC particles and hence acts as a toughening phase.

MULLITE (AL6SI2O13) -MOLYBDENUM

Smoother variation favors resistance to

thermal shock (Vf Mo = 1 – (x /L) p)

ALUMINUM-POLYCARBONATE

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.

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

OPTIMAL DESIGN

Experience aquired from research into FGMs has produced its results.

Knowledge has been gained on which transition functions are best suited for specific tasks and material types.

Ceramic-metal FGMs are particularly suited for

thermal barriers in space vehicles.

Other possible uses include combustion chamber

insulation in ramjet or scramjet engines

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.

AEROSPACE APPLICATIONS

Creating a porosity gradient in the electrodes, the

efficiency of the reaction can be maximized

FUEL CELL TECHNOLOGY

Modification to heat exchangers in tokamak fusion

reactors

Reduction of interfacial stresses → prevention of

delamination effects → increase in lifetime

NUCLEAR FUSION REACTORS

JET SOLIDIFICATION

- Versatile process that can be

adapted to produce both axial and

radial gradients

- Also requires sintering

- Solid freeform process

- Ultimately requires

sintering of the resulting

green body

PRESSURE FILTRATION

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

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

CONCLUSIONS