A Brief Introduction to FGM

Functionally Graded Materials – FGM

Definition of ‘FGM’

A Functionally Graded Material or FGM is a two-component “composite” characterized by a compositional gradient from one component to other. FGMs can be natural or they can be manufactured using various techniques like particulate processing, preform processing, layer processing, melt processing, etc. The materials can be designed for specific functions and applications.

FGMs vs. Composites

Although FGMs are loosely termed as “composites”, their properties vary considerably from the traditional composites. Traditional composites are homogeneous mixtures, and they therefore involve a compromise between the desirable properties of the component materials. Since significant proportions of an FGM contain the pure form of each component, the need for compromise is eliminated. The properties of both components can be fully utilized. For example, the toughness of a metal can be mated with the refractoriness of a ceramic, without any compromise in the toughness of the metal side or the refractoriness of the ceramic side. The illustration shown alongside shows the transition between a hard material and a soft one in eight discrete layers for a typical FGM.

History of FGM

The advent of the FGM concept was back in 1984 in Japan during the Spaceplane Project. The idea was to develop a thermal barrier capable of withstanding surface temperatures of about 2000 K and a temperature gradient of 1000 K across a cross section less than 10 mm thick. Since 1984, FGM thin films have been comprehensively researched, and are almost a commercial reality now.

Natural FGMs

The concept of FGM may be considered as a direct derivative from nature and natural systems where such materials have developed by evolution. The human body itself contains many parts which have evolved and transformed them selves according to their environment (like our teeth and bones which have a tough “bony” exterior and a soft pulpy interior). Another such natural application of FGMs can be found in almost all type of plants where the branches and twigs form a natural hard shell-like covering on the outside (mainly composed of dead tissues) for protection, prevention of moisture-loss, etc. The bamboo grass is an example of such an application; typically, the bamboo plant shows a change of

Fig 1: Transition between a hard and a soft material

A Brief Introduction to FGM

microstructure from the core to the outer surface to withstand external loads while keeping the internal activities like the flow of water and nutrition unhindered. The most common occurence FGMs in animals can be found in their skeletal structures (like bone and teeth, nail, shells, etc.). Although it is difficult to distinguish between biological structural systems, from a macroscopic viewpoint it can be said that there are three ways for a biological system to sustain external loads:

1. By changing the microstructure (i.e. by moderating the thickness and/or the shape), e.g. – Bamboo, Mollusk shell.

2. By changing the size and/or shape of a body, e.g. – the trunk of a tree and the stem of a plant, the shaft of a leaf or a feather.

3. By combining the first two, e.g. – human and animal bone, the spicule of a seaurchin, a rat tooth.

Applications of FGMs

The FGMs produced as a result of the natural evolution processes are time tested and are really the ultimate functional materials possible. Science has always tried to follow nature’s path. Engineering materials being produced these days are specially tailor-made for specific purposes like thermal shielding, corrosion resistance, etc. The mechanical and thermal response of materials with spatial gradients in composition and microstructure is of considerable interest in numerous technological areas such as tribology, optoelectronics, biomechanics, nanotechnology and high temperature technology.  The graded transition in composition across an interface of two materials (for instance, metal and ceramics or polymer) can essentially reduce the thermal stresses and stress concentration at intersection with free surfaces.  Similarly the stress

Fig 2: The mechanical properties of the humerus bone of a rhinoceros

Fig 3: The cross-section of a tooth

Fig 4: A bio-implant (an artificial hip)

A Brief Introduction to FGM

intensity factor at the crack tip can be altered by varying the gradient properties across the interface.  The ceramic-metal FGMs exhibit higher fracture resistance parameters resulting in higher toughness due to crack bridging in a graded volume fraction.  Varying thermal expansion in graded layers induces residual stress and affects the crack growth mode.  In fact, the interface bonding is much improved by providing smooth composition variation when traversing the interface.  The interest in graded materials focused primarily on the control of thermal stresses in elements exposed to high temperatures (to 1600C), for instance in gas turbine blades, aerospace structures, solid-oxide fuel cells, energy conversion systems using thermoelectric or thermionic materials (thermal barrier coatings, TBC). Subsequent applications include fusion and fast-breeder reactors as a first-wall composite material, piezoelectric and thermoelectric devices, high density magnetic recording media, in optical applications as graded refractive index materials, in audio-video discs, in bioengineering as dental and orthopedic implants, in structures as fire retardant doors and penetration resistant materials for armour plates and bullet-proof vests. 

The development of the FGMs was primarily for the purpose of thermal insulations. For example, the Space Shuttle utilizes ceramic tiles as thermal protection from heat generated during re-entry into the Earth’s atmosphere. However, these tiles are laminated to the vehicle’s superstructure and are prone to cracking and debonding at the superstructure/tile interface due to abrupt transition between thermal expansion coefficients. In other words, the ceramic tile expands a different amount than the substructure it is protecting. The difference in expansion causes stress concentrations at the interface of the tile and superstructure which results in cracking or debonding. Figure 5 demonstrates the stress concentrations found in conventional thermal protection panels at the tile - superstructure interface. It also shows how an FGM can alleviate those stress concentrations by gradually changing material properties through-the-thickness of the material but still provide the thermal protection found in conventional thermal shielding.

A Brief Introduction to FGM

An FGM composed of ceramic on the outside surface and metal on the inside surface eliminates the abrupt transition between coefficients of thermal expansion, offers thermal/corrosion protection, and provides load carrying capability. This is possible because the material composition of an FGM changes gradually through-the-thickness; therefore, stress concentrations from abrupt changes in material properties (i.e., coefficients of thermal expansion) are eliminated.

Manufacturing techniques for making FGMs The concept of thin surface layers is closely related to

functionally graded materials.  In fact, all surface treatments and coatings aimed to increase wear and fracture resistance induce gradual or stepwise transition to bulk properties.  Well established techniques such as shot-peening, laser treatment, ion implantation, have been developed and extended to generate nanocrystalline surface coatings with grain sizes of order of few tens of nanometers.  New techniques include thermal spray, electrodeposition, electrophoretic deposition, chemical (CVD) and physical vapour deposition (PVD), ion beam assisted deposition (IBAD), etc.  The surface layer can be created with grain sizes varying smoothly from the surface to the bulk.  Also the gradients in porosity and density can be controlled to increase the damage resistance and reduce stress intensity factors at crack tips.  Recent experimental and theoretical works demonstrated that controlling gradients in thermal and mechanical properties provides a new potential for design of surfaces and interfaces with higher resistance to cracking and wear subjected to mechanical surface loading and thermal gradients.  The diverse applications include load-bearing engineering structures, protective coatings, bio-implants, and magnetic storage media. 

Fig 5: Thermal protection and induced thermal stresses

A Brief Introduction to FGM

There are several types of FGMs that exhibit exceptional multifunctional properties and multisectoral applications as shown below: 

Type Properties Processing Applications

Ceramic/metal bulk FGMs

Thermal stress relaxation; high heat resistance and wear resistance, high mechanical strength

Spark plasma sintering process

High efficiency engine components

Titanium (alloys) with graded density or porosity

Combination of good mechanical properties and light weight

Additive, layer-wise process: direct metal laser sintering (DMLS) of powders

Light weight structures for aircraft and space industry, implants

Toolsteels with C, V, Cr gradients; steels or Ni super alloys with ceramic (oxide, carbide) particle gradient

Combination of toughness and hardness or wear resistance

Additive, layer-wise process: 3D-printing with local material composition control (generating a green part of powdered material and sintering or infiltration)

Tools, medical instruments, implants, aircraft and space industry.

Functionally graded cemented carbide: titanium based surface ceramic layer, tough cemented carbide core and intermediate layer with graded composition

Wear resistance, breakage resistance, thermal crack resistance

sintering Cutting insert

Ferritic-austenitic combinations like 316L and 17-4PH; steel-ceramic combinations

Magnetic and non-magnetic; ductile and stiff and other

Co-injection moulding and co-sintering (building a graded interface)

Automotive industry, sensors; medical instruments

Precious metals like Pt, Ag (catalysis) and metal oxides like SnO2 (sensors)

High specific surface and strong gas-metal interaction; graded porosity

PVD based on sputter techniques and inert gas evaporation and

Gas sensors and catalytic active layers, low-temperature bonding for

A Brief Introduction to FGM

with graded porosity from bulk to nanometre scale

combines optimised contact on substrates (bulk side) and high functionality (nano-structured side)

condensation, with in-situ design of the deposed structures by controlling the process parameters

electronic connections

Conclusion

The manufacturing techniques currently used to produce FGMs are in general very expensive and have very high energy demands. For example, kevlar, the toughest material produced synthetically and used in bullet proof jackets, are produced by heating petroleum to very high temperatures and consequently subjecting it to immense pressures. Whereas, a simple spider, with no access to such heating techniques, can naturally produce a material in its stomach which is almost six times stronger than steel. Now, with the world on the threshhold of an insense energy crisis, one must realise that the existing processes are not sustainable. So, the emphasis should be on the development of more ecofriendly, sustainable processes with lesser energy demands.