A review on manufacturing technique for Functionally Graded Materials

Amit Joshi (Department of Mechanical Engineering, GBPEC, Pauri)

Abstract

Designing FGMs allows manipulation of many material properties or new functions. Various

production methods are available for manufacturing graded structures such as casting,

deposition, laser cladding, combustion synthesis and powder metallurgy. There is also an

increased interest in devising new material systems and versatile methods of fabrication. Due to

recent advancements in the method of processing FGMs, they have gain popularity due to better

properties variation, volume fraction variation, economical and easy manufacturability. This

paper reveals the various developments in the manufacturing processes.

Key words: FGM, Powder metallurgy, electrophoresis, electromagnetic separation, spark

plasma, laser assisted manufacturing, novel methods.

Introduction

Further development in science and technology will depend upon the development of new

materials that can withstand severe conditions. There are two approaches: (a) create a new

material that differs completely from advanced materials currently in use, (b) develop an

advanced material with new functions. Composites can fulfill these requirements, especially

composites which have new functions. Due to that reason, many special composites have been

designed to eliminate the macroscopic boundary in laminated type materials. These are the

motivations for the development of functionally graded materials (FGM). Functionally graded

material concept was first proposed at Sendai, Japan in 1984. FGM on its simplest structure

consist of one side and a second material on the other side and an intermediate layer whose

structure, composition and morphology vary smoothly from one material to the other at the

micron level. FGM are multifunctional materials used for producing components that require

functional performance which is variable within the component. This is basically termed as

engineering the transition in micro/nano structures thus enhancing the overall performance of the

component. FGMs have some advantages such as:

Provide multi-functionality

Provide ability to control deformation, dynamic response, wear, corrosion, etc and ability

to design for different complex environments.

Provide ability to remove stress concentrations.

Provide opportunities to take the benefits of different material systems (e.g. ceramic and

metals)

Human bone is a good example of FGM found in nature. It is a mix of Collagen (ductile protein

polymer ) and hydroxyapatite (brittle calcium phosphate ceramic). The yellow marrow consisits

of fat which contributes to the weight and the red marrow is where the formation of red blood

cells occur. A gradual increase in the pore distribution from the interior to the surface can pass

on properties such as shock resistance, thermal insulation, catalytic efficiency, and the relaxation

of the thermal stress. The distribution of the porosity affects the tensile strength and the Young’s

modulus.

Historical Background of FGM: Already in 1972, the usefulness of functionally graded

composites was recognized in theoretical papers by Bever and Duwez and Shen and Bever.

However, their work had only limited impact, probably due to lack of suitable production

methods for FGMs at that time. It took 15 more years until systematic research on manufacturing

processes for functionally graded materials was carried in the framework of a national research

program on FGMs in Japan. In the year 1987 a large national project was commenced in Japan,

entitled “Research on The Basic Technology for the Development of Functionally Gradient

Material for the Thermal Stress” aimed for development of superheat-resistant materials for the

propulsion system of an air-frame of space frame. Since then, a major part of the research work

on FGMs was dedicated to processing of these materials and a large variety of production

processing of these materials and a large variety of production methods has been developed.

1. Powder Metallurgy Technique: It is a process by which fine powdered materials

are blended, pressed into a desired shape (compacted), and then heated (sintered) in a

controlled atmosphere to bond the contacting surfaces of the particles and establish the

desired properties. The process, commonly designated as P/M, readily lends itself to the

mass production of small, intricate parts of high precision, often eliminating the need for

additional machining or finishing. There is little material waste; unusual materials or

mixtures can be utilized; and controlled degrees of porosity or permeability can be

produced. The properties of powder metallurgy products are highly dependent on the

characteristics of the metal (or material) powders are used. Some important properties

and characteristics include chemistry and purity, particle size, size distribution, particle

shape and the surface texture of the particles.

By powder metallurgy route namely three types of gradient can be processed [1]:

(i) Porosity and pore-size gradient: Porosity gradients may be created either by the

deposition of powder mixtures with different particle shape or varying the deposition

parameters including the use of space holders. Pore-size gradients in general are

produced by the variation of particle size. The different sintering behavior of the

powders requires special attention to manage the problems of part distortion but may

also be used to produce a porosity gradient.

(ii) Gradients in chemical composition of single phase materials: The deposition of

powders with a continuous change in the composition of the mixture during sintering

will lead to a single phase material with a smooth change in the distribution of

elements if the phase diagram shows solubility over the chosen range of composition.

(iii) Gradients of the volume content of phases and grain size gradients in two or

multiphase materials: The phase diagram of such a multi-component system is

characterized by no or limited solubility. Here grain growth of the soluble phase

during the heat treatment has to be taken into account, in particular if a liquid phase is

used for activated sintering. The microstructure of resulting FGM consists of two or

more phases with gradients in volume content or in grain size which were

predetermined during the deposition of the powders. The combination phases can

include metal-metal, metal-ceramic, and ceramic-ceramic systems. Microstructural

gradients can also be produced by controlling the phase equilibrium during liquid-

phase sintering with gradients of the components.

In general, step wise gradient and continuous gradient can be obtained by powder metallurgy

technique. For step wise gradient researchers has use dry P/M approach which involves

stacking of different layers, with various combination and orientation, of mixtures powders in

a die, usually done manually, of uniform thickness and distribution and then pre-compaction

is done to a desired level, after stacking of powder, cold isostatic pressuring (CIP) is done

followed by sintering process (Consolidation). Sintering of the powder is an important step as

gradient in the liquid phase can easily be destroyed so utmost care should be taken for

building the gradient. Most of the materials have different sintering behavior which is

affected by many other parameters like powder grain size, composition and porosity of the

mixture in use and also with the difference in coefficient of thermal expansion (CTE).

The various approaches used in wet P/M processes are centrifugal casting, wet powder

spraying, slip casting, gravity sedimentation, and slurry dipping process. Use of binders for

green body stabilization is generally needed in wet P/M technique which is then pull-up at

the time of sintering so as to make continuous gradient in the process.

Various materials being fabricated by Powder metallurgy technique:

Lot of combination of materials was successfully being fabricated by P/M technique.

Use of FGM in orthopedics and dentistry is reported in order to promote bone in-growth into

prosthetic implants. Hydoxyapatite (Ca10 (PO4)6(OH)2, (HA) is a bioactive material and reacts

with the living bone to give a strong bond that can resist high stresses. Various combination

of HA and other high toughness material were being investigated. Bishop et al. has used HA-

Ti combination FGM by P/M route. His study was mainly concentrated to the manufacturing

and macrostructure of FGM without detailed factors affecting the microstructure

characteristic and mechanical behaviour. It was Chu Chenglin et.al. [2] Who give a detailed

procedure of fabrication, analyzed microstructure, phase constitution, density and mechanical

properties and fracture behavior of this FGM. He by his experiment suggests that with the

increase in Ti content fracture behavior varies and can be controlled by Ti matrix. R. Roop

Kumar et.al [3] found success in fabrication of graded bioactive coating of HA/TiO2 by P/M

process and mechanical properties of HA-TiO2-Ti FGM was measured using a nano-

indenter. Coating of 230µm was successfully obtained by this coating technique. Fumio

Watari et.al [4] has successfully fabricated HA/Ti FGM by P/M and the specimen of this

FGM is implanted into femora of ten week old Wistar strain rats to evaluate their

biocompatibility. The test shows positive result of this implant with the help of EPMA

elemental mapping method. In the further development in this field Lydia Helena Wong et al.

[5] fabricate tricalcium phosphate and fluoroapatite composites by dry P/M technique. In its

test he found that this composite become porous so if implanted the graded FA/β-TCP would

become graded porous FA, which would be desirable for both bone ingrowth and bone

bonding. SivaPrasad Katkam et al. [6] successfully fabricate an FGM of HA and Ag2O using

a microwave processing by gradually changing the composition of calcium phosphates from

the surface to the interior. By X-ray powder diffraction study it was found that the sintered

samples contain tricalcium phosphate (β-TCP) and Ag gradually decreases with increasing

depth from the surface. He compare the FGMs formed by both microwave processing and by

conventional heat treatment and found that microwave processed FGM have more amount of

TCP and Ag on the surface (bioactive material) and thus have greater compositional control

in FGM fabrication by microwave processing. M.Bram et al. [7] introduces a prototype of an

acetabular cup made of titanium powders provided as biomedical implant in hip replacement.

The place holder method is attractive for the production of highly porous titanium implants.

Titanium is a well known implant material due to its biocompatibility. The open porosity

enhances the structural and functional bonding between implant surface and human bone.

Moreover this method of making highly porous functional parts with open porosity by

application of powder metallurgy can be used where the functional porosity is required. In

this method he uses titanium powder and the place holder material mixed in a volumetric

ratio of 26:74 and compact the mixture in a cylindrical die under 400MPa pressure and green

compact is machined by lathe to the spherical form of a cup taking into account the shrinkage

allowance followed by sintering in vacuum. The place holder must be removed at

temperatures below 150°C to avoid strong increase of impurities like oxygen, carbon or

nitrogen.

Molybdenum is a good electrical and thermal conductor whereas mullite is an electrical

insulator and has a very low thermal conductivity. FGM of Mullite and molybdenum having

a large number of possible applications in electronics and microelectronics, in thermal barrier

coatings or in electrical conductor/insulator components was made by researchers. A.P.

Tomsia et al. [8] fabricate mullite/Mo FGM cylinders by pressure slip casting. In his work he

processed FGM by dipping the powders in ethyl alcohol slurry and advised not to use water

based slurries which make a homogeneous mullite/Mo composite instead of FGM. In the

mean time Bartlome et al. [9] again used the same combination with varying the grain size of

molybdenum, but the focus of this work was to evaluate the processing, mechanical

properties and microstructural properties. R Sivakumar et al. [10] developed a new modified

technique of centrifugal molding to produce mullite-molybdenum FGM. Here he used

alcohol based slips of binder and dispersant and a faster drying rates was employed for

graded cylinder in partial vacuum to avoid drying cracks. In this process of centrifugal

molding technique graded specimen are prepared by instant mix rather than mixing of pre-

mixed composite powder. He claimed to have a uniform gradation of Mo along radial

direction by this process. Gang Jin [11] also synthesizes this Mullite/Mo FGM by pulsed

electric current sintering (PECS) technique. In which the layered powder is sintered by a

pulsed electric current sintering apparatus at 150°C/min rate up-to 1500° and 20MPa

pressure. The thermo-mechanical properties of mullite/Mo system were checked and found to

have a gradient distribution. Mullite/Mo FGM was fabricated by Shuheng Chen et al.[12] by

powder stacking process followed by vacuum sintering at 1500°C. Author claimed that the

mullite/Mo FGM has well densified graded distribution with absence of new phase.

Ceramic-metal and ceramic-ceramic FGMs show promise for hypothesized applications such

as thermoelectric converters, graded solid oxide fuel cells, graded piezoelectrics, heat sinks

for fusion reactors, electrically insulating joints, and thermal barrier skins for light weight

space-planes. Zirconia/stainless steel continuous graded material was fabricated by S.Lopez

Esteban et.al [13] which can be used in electromechanical sensors, wearing sensors. Here

pressure slip casting is used for gradation in the structure. In his work author measures the

Vickers hardness and dielectric constant at various points in the gradient and through which

he found the percolation threshold (metal/insulator) location. Jingchuan Zhu et.al [14]

developed ZrO2-NiCr FGM by powder metallurgy. He found that cold forming and sintering

behavior of FGM strongly depends on the compositional variation. The shrinkage of ZrO2 is

more at the high NiCr alloy matrix. This shrinkage difference should be controlled to develop

perfect FGM. Vickers hardness change with the composition change but the bending strength

does not change monotonically. J. Ma et.al [15] has developed a Ti/TiO2 FGM and studied

the effect of particle size and SiC additive on the sintering of the FGM. SiC as a sintering aid

has been shown to provide a greater enhancement in the sinterability of the TiB2 layer and

resulted in a great improvement in both the flexural strength and toughness of the FGM. The

starting particle size improves the densification of the TiB2 ceramics and hence of the overall

FGM system. Al-(Al3Ti + Al3Ni) FGMs were fabricated by Yoshimi Watanabe and Tatsuru

Nakamura [16] by centrifugal method. Detail observation on the orientation of Al3Ti platelets

and the size of Al3Ni granular particles were performed. The wear resistance of the FGM was

measured and the density of FGM is found smaller than that of Al-Al3Ni FGMs so these can

be used for transportation of parts subjected to harsh tribological conditions. Centrifugal

casting method was employed by R. Rodriguez-Castro [17] for the fabrication of Al

A359/SiCp where different casting rotational speeds were utilized while keeping all other

casting conditions constant. He found that with the increase in rotational speed volume

fraction profiles vary and high concentration were obtained at the outer circumference for

maximum angular speed and the concentration of particles was homogeneous due in part to

engulfment promoted by the high relative velocity between the solidification front and the

particles, and also due to the elevated cooling rates developed in this case. But with varying

the rotational speed the microporosity also get increased. In his study he found that the

matrix structure is modified in the reinforced region as a consequence of solidification taking

place rapidly and in very restricted spaces and also due to breaking of dendrites by the

combined loading action of accelerated particles and loaded dendrites. Quench modification

of the eutectic structure was observed, going from a fully modified structure at the opposite

side of the casting. In continuation with this FGM in his other paper R. Rodriguez-Castro et

al. studied the mechanical behavior including tensile and fracture properties of the same

FGM Al A359/SiCp composite. The effect of SiC particulate reinforcement on strengthening

of A359 Al alloy was limited up to a certain volume fraction. At the high cooling rate zone

(0.2-0.3 volume fraction of SiC) an improvement in tensile strength is seen due to uniform

distribution of reinforcement. Fracture toughness of the FGM composite was low for small

crack length due to limited dissipation of energy on contrary to this with longer crack lengths

the material surrounding the crack tip was more able to plastically deform thus fracture

toughness of the composite get increased. J.Q.Li [19] and his co-author fabricate YSZ-NiCr

functionally graded materials interlayer by hot pressing process. He found that the relative

density of the interlayer decreases as the YSZ content increases in the interlayer. By

conducting thermal cycling test the YSZ reach layer shows cracks after 2 thermal cycles

which can propagate anywhere in this region independently. By increasing the thickness of

interlayer the cracks generated are retired to a greater extent even after 10 thermal cycles.

When the interlayer thickness was increased to 2 mm no spallation was found in the joint

even after 30 thermal cycles and shear strength is also retained. O. Eso and his coauthor [20]

succeeded to manufacture WC-Co FGM by liquid phase sintering and discussed about the

various factors, which can be used to influence the migration of liquid during sintering like

gradients in grain size, carbon and cobalt content and sintering time. Author has explained

the effect of these factors based on the roles of capillary force and phase reactions. He found

that liquid phase cobalt migrates in the direction of carbon diffusion during liquid phase

sintering, which will lead to a continuous cobalt gradient in the final sintered microstructure

while maintaining Stoichiometric carbon content of sintered products. Al/SiC FGM were

produced by powder metallurgy route, exhibit microstructures with homogneneous

distribution of fragmented and clustered SiC particles in Al matrix by Mandira Bhattacharya

et al. [21] and had studied microstructure, porosity content and microhardness along with

effective flexural strength, thermal fatigue behavior and thermal shock resistance.

Ceramic materials are difficult to machine so in order to enhance the machinability of

materials we can use FGM as one of the solution. Ruigang Wang et al. [22] has developed a

new concept for manufacturing FGM of such ceramics by using Si3N4/h-BN and

Al2O3/LaPO4 material combination. Here in this process they employed layers of different

materials on the core layer in different proportion on either side of the core layer. So by using

this processing technique they were able to fabricate the above two FGMs and thereby

enhancing the machinability and fracture toughness of ceramics.

FGMs can also be used in Plasma Facing Components (PFCs) in nuclear reactors where they

are imposed to very high heat and high neutron flux generated by plasma. Various

combinations were tried by researchers for a suitable material combination for PFCs. Chang-

Chun Ge et al. [23] fabricate three combinations for PFCs of SiC/C, B4C/Cu and W/Cu by

three different processing technologies and some primary results of plasma relevant

characteristic of these materials were reported. Combination of SiC/Cu material for PFCs

was reported by Yun-Han Ling et al [24]. The author has tested this material under plasma

condition and found that the chemical sputtering and thermal desorption is less as compared

to the materials used earlier. Ultra high pressure is employed in order to prevent macro

diffusion between SiC and Cu. A.H. Wu et al. [25] has fabricated SiC/C FGM by hot

pressing and then simultaneous step sintering of SiC and C so as to get FGM without micro-

pores and high thermal conductivity as compared to SiC/C prepared by CVI and CVD. So it

can be employed as first wall materials for fusion reactors. He also checked their samples for

thermal shock and fatigue and observed that they did not detect any micro-crack and flaw

under hundred times plasma erosion conditions. W/Cu FGM was manufactured by Wei-ping

Shen et al. [26] for plasma facing components. FGM mock-ups were manufactured by

resistance sintering under ultrahigh pressure or three times hot pressing. W/Cu FGM was also

successfully fabricated by Zhang-Jian Zhou et al. [27] by a novel one step method named

“resistance sintering under ultra-high pressure” without the addition of any sintering additive.

The density observed of this FGM was 97% of theoretical density. A special experimental

set-up is employed to fabricate the W/Cu FGM in which the green compacts of FGM having

five layered W/Cu were quasi isostatic loaded followed by alternating current flow to the

sample to heat the sample by Joule heating. In another study Bin-Bin Liu et al. [28] fabricate

W-Cu graded heat sink materials with nearly full density by particle size adjustment and

solid state hot press when the parameters like sintering temperature of 1060°C, pressure of 85

MPa and holding time of 3 hr were maintained.

Defect free sintering of powders by injection molding technique was introduced by D.F.

Heaney et al. [29] on his research paper where the dilatometer study of various alloys is

studied to predict their compatibility during sintering. This was accomplished by comparing

the individual shrinkage versus temperature behavior of the candidate system. Based on his

study two materials 316L stainless steel/ Boron and Fe-10Cr steel/B were fabricated by

injection molding followed by sintering in vacuum. In his research he observed that for better

sintering of two materials by PIM components require one material to mimic the

densification behavior of the other material also the net shrinkage of the compacts after

sintering should be equal.

Piezoelectric actuators and sensors have novel applications for micromechanical systems

(MEMS) and smart material systems, especially in the medical and aerospace industries.

Piezoelectric composites with functional gradients are novel devices which can successfully

overcome the inherent structural defect in conventional piezoelectric bending-type actuators

resulting from the use of epoxy binder. The FGM piezoelectric device is a monolithic

structure, which has no mechanically weak junction or interface and also substantially

reduces discontinuous stress during operation. Thus longevity and reliability are enhanced. In

this regard Denger Jin [30] has developed a PZT/ZnO/PZT FGM piezoelectric bimorph

composite, in which ZnO ceramic was used as an internal electrode and also as a component

to modify the distribution of functional gradients. The author had claimed that it is a

piezoelectric device promising large displacement, high reliability and long longevity. Kenta

Takagi et al. [31] fabricate PZT/Pt piezoelectric composite FGM in which seven layers with

a centre symmetric composition profile, in which the compositions from the central layer to

the surface layer were stepwise, changed. He found that the piezoelectric constants decrease

monotonously but the dielectric constant remarkably increases as the Pt content increases.

The main drawback of this FGM is that the Pt is a novel material and is very costly.

Y.Gelbstein et al. [32] summarizes the design considerations, preparation techniques and

characterization methods of graded p-type Pb1-xSnxTe samples with equi-wide layers and with

an optimal Z envelope. In this research the potential for increasing the thermoelectric

efficiency, using functionally graded materials based on Pb1-xSnxTe was explored by

understanding the characteristics and the mechanisms of the annealing processes. The

researcher claimed that the thermoelectric properties of this FGM are comparable to those of

single crystals.

Transition metal silicides have been the focus of extensive investigations for high

temperature applications including structural materials for turbine blades, engines and other

applications at temperature exceeding 1500K. For industrial use a toughness of about 10MPa

m1/2 is required and for turbine applications a fracture toughness of 15-20 MPam1/2 is desired.

Various attempt have been reported to fabricate FGM using high melting point silicides of

niobium. Ellen M. Heian et al. [33] synthesize Nb5Si3/Nb functionally graded composites

using elemental powders of niobium and silicon in Stoichiometric ratio. They used a custom

build apparatus where the powder layers are subjected to uniaxial pressure in argon

atmosphere and AC electric current was passed through the die and sample to heat the

sample until a chemical reaction is initiated. The author has claimed the enhancement in

fracture toughness with increase in content of Nb.

Particulate metal matrix composite (PMMC) with a functionally graded characteristic are

used in many field. In an attempt Jian Zhang et al. [34] synthesize Al/Mg2Si in-situ

composite by centrifugal casting with Mg2Si reinforcements in both the inside and the

outside wall of the tube specimen so as to get high inside wear-resistance. The density of

Mg2Si is less than Aluminum matrix so with this combination the hardness of the material

gets enhanced at the cost of reduced ductility at outer region. Li Sun et al. [35] has studied

the various processing parameters for the fabrication of alumina/zirconia FGM. In this

research Li Sun has varied various parameters such as the powder characteristics, compaction

load, interface profile, and temperature ramp/soak profiles in order to determine the optimum

parameters for eliminating cracks and decreasing camber. The sintering kinetics of the

chosen powder mixture was investigated under optimized experimental parameters and the

phenomenological constitutive models to describe the relationship between the densification

rate and the relative density were determined and compared.

Yoshimi Watanabe and Yasuyoshi Fukui [36] done microscopic study of Al-base

intermetallic compound-dispersed functionally graded materials fabricated by centrifugal

method and observed the microscopic features by several analytical techniques. Here three

methods of fabricating FGMs was introduced those are Al/Al3Ti FGM fabricated by the

centrifugal solid particle method, Al/Al3Ni and Al/Al2Cu FGMs fabricated by the centrifugal

in-situ method and Al/(Al3Ti+Al3Ni) hybrid FGM made by the simultaneous combination of

both solid-particle and in-situ methods. He emphasize on the formation mechanism of graded

composition by the centrifugal method as a practical FGM fabrication method. Yoshimi

Watanabe and Shin Oike [37] employs in situ Al-Al2Cu FGMs fabricated by the centrifugal

in situ method using hypoeutectic, eutectic and hypereutectic Al-Cu alloys and origin of the

graded composition in FGMs is discussed. J.Q.Li et al. [38] conceptualize a new 5 layered

FGM of YSZ-NiCrAl using the Al2O3-NiCrAl composite with 25, 52.2 and 75 vol% Al2O3

as interlayers. The Al2O3-NiCrAl composite was fabricated by mixing NiO, Al and Cr

powders and then reactive hot pressing. No cracking was found in the FGM after 10 thermal

cycles upto 1000°C. Alumina-Nickel FGM was fabricated by Michael L.Pines et al. [39] by

pressureless sintering and porosity and shrinkage were measured and models were developed

to understand both the nature of the porosity during sintering and the sintering behavior of

the FGM. Al-Cu-Fe ternary alloy was casted by centrifugal in situ method to fabricate a new

type of functionally graded material (fiber dispersed FGM) by Watanabe et al. [40] having

four different phases of Al, Al2Cu, Al7Cu2Fe (ω) and Al13Fe4 phases. The shape of ω phase

was fiber (needle) and is increasing towards the outer region of the ring and its orientation

and aspect ratio varied in the rings in a gradually graded manner. Al-Al3Ni in situ

intermetallic composite was fabricated by T.P.D. Rajan et al. [42] by vertical centrifugal

casting. In his experiment Al-Ni alloys with varying percentage of nickel 10-40% were

employed and he finds that by using Al-20wt% Ni it gives the best microstructure.

2. Electrophoretic deposition: The potential of the electrophoretic deposition

(EPD) technique for the realization of unique microstructure and novel (and complex)

material combinations in a variety of morphologies and dimensions is being increasingly

appreciated by material scientists. Although the basic phenomena involved in EPD are

well known and have been the subject of extensive theoretical and experimental research

there is general agreement in the scientist community that further R&D work needs to be

done to develop a full, quantitative understanding of the fundamental mechanism of EPD

to optimize the working parameters for a broader use of EPD in materials processing.

EPD is a combination of two processes, electrophoresis and deposition[42].

Electrophoresis is the motion of charged particle in a suspension under the influence

of an electric field gradient. It was discovered by the Indian scientist G.M.Bose in the

1740s in a liquid-siphon experiment. The Russian Scientist Reuss, was the first to

observe electric field-induced motion of clay particles in the water. The second process is

deposition, i.e., the coagulation of particles into a dense mass. In Electrophoretic

deposition, powder particles are charged in suspension and are deposited on an electrode.

The suspension rate of particles is varied during the process and the sintering of various

layers of varying composition occurs at different temperatures.

EPD needs a stable suspension with particles charged so as to respond to an applied

electric field. Therefore only electrostatic and electosteric stabilization are appropriate for

suspensions to be electrophoretically deposited. The oxide particles which are present in

the surface of material are usually covered by amphoteric hydroxyl groups. These when

suspended in a electrolyte suffer reaction with the H+/or OH- of a suspension liquid,

depending on the suspension pH[42]. The reactions are as follows:

MOH(surface) + OH- = MO-(surface) + H2O

MOH(surface) + H+ = MOH2+

(surface)

According to these equations, there is an intermediate pH where the number of positively

and negatively charged surface groups are equal thus resulting in zero surface charge.

The profile can be controlled precisely by controlling the deposition current density,

second component flow rate, suspension concentration etc.

The setup for deposition of material is shown in figure:

(a) EPD setup (b) Detail of the

deposition cell

In typical EPD deposition there are two electrodes one positively charged and other negatively

charged. The suspension of one ceramic/metal is put in the deposition cell than by varying the

suspension of other ceramic/metal we can deposit an FGM either at the two electrodes which

depends on the pH of suspension. EPD is a cheap and simple technique to fabricate complicated

ceramic shapes.

Various materials being fabricated by Electrophoretic deposition technique:

Lot of work in EPD has been done and lot of research is still going on. Sarkar and Nicholsan [43] are the

first to demonstrate the EPD can be successfully use for fabrication of laminated and functionally graded

materials. C. Kawai and S. Wakamatsu [44] fabricate C/C based composite by having one side rich with

C/C composite and the other with C/SiC side. The author has claimed to achieve a specific strength and

oxidation resistance of the FGM approximately 1.8 times and equal to those of the C/SiC composite.

Partho Sarkar et al. [42] synthesize Al2O3/YSZ, Al2O3/MoSi2, Al2O3/Ni and YSZ/Ni FGMS by constant

current Electrophoretic deposition. Here author cited suspension stability by famous DLVO theory and

proposed an alternative mechanism for deposition based on EPD mechanism and lyosphere

distortion/thinning. WC-Co FGM was produced by S.Put [45] by using EPD from a suspension of hard

metal powder in acetone, with variable cobalt content. Dense FGM plates with closed porosity and a Co

gradient from 4 up to 17 wt% were obtained in this process by pressure-less sintering at 1290°C and

1340°C. One important observation in this process is that when this FGM was pressure-less sintered at

1400°C for one hr in order to get dense FGM the gradation get disappeared and a homogeneous but fully

dense material is obtained. In another experiment Put et al. [46] produce a ZrO2-WC graded FGM plate of

35×35 mm, with a WC gradient from 0 wt% up to 55 wt% by EPD. After hot pressing the thickness of

plate was 2mm with a gradient of 1mm width. Author has developed an kinematic model in order to

predict the gradient profile based on Hamaker’s equation. Further continuation S. put et al. [47]

developed an FGM with an outer layer of pure Al2O3, a central homogeneous Al2O3/ZrO2 composite layer

and intermediate continuously graded layers by means of EPD from a suspension of acetone and

butylamine and subsequent sintering in air. Moreover, a symmetrically graded Al2O3/ZrO2/Al2O3 FGM

disk with a diameter of 25mm and a thickness of 4mm were also engineered, containing Al2O3-rich edges

and a ZrO2-Al2O3 core. Ceramic-metal WC-Co FGM disk of 25mm diameter and thickness of 4mm after

sintering was also produced with a cobalt gradient from 4 to 17wt%.

FGM can also be used as a substitute to coating where the coating thickness is of range 1-150µm which

limits the coated material life subjected to wearing like in cutting tools and moreover there is a sharp

interface between coating and substrate, often causing delamination during operation also thermal stress

induced during the operation is itself a problem. These problems can be addressed by using FGM, in

which the thermal stresses can be reduced and the coating adherence can be improved by providing a

continuously gradual transition in microstructure between the two elastically dissimilar materials. Stijn

Put et al. [48] produce an functionally graded hard-metals with a continuously graded symmetrical

profile. An FGM of WC-Co-Ti(C,N) with a gradient in cobalt and Ti(C,N) by means of electrophoresis

and subsequent cold isostatic pressing and liquid phase sintering is reported. For creating a symmetric

gradient profile an EPD model is analyzed and based on that experiment is done which has excellent

correlation with the prediction of gradation. The Vickers hardness and fracture toughness is also

measured of the sample which was found to increase continuously from 16.1GPa in the bulk material to

18.1GPa on the edge while fracture toughness reduces along the same direction from 8.8 to 6.6 MPam 1/2.

Author suggests that EPD model and colloidal shaping technique can be used to engineer the slope, width

and composition of the gradient by changing the EPD processing parameters and concentration of the

suspension.

By EPD evolution of gases like hydrogen is a problem also the thickness of gradient is very small to

overcome such difficulty J Tabellion and R. Clasen uses ion permeable membrane thus dividing the

electrophoresis chamber into two chambers that contain the suspension and another fluid, respectively.

Deposition occurs at the membrane whereas the ions can pass the pores of the membrane so that

recombination of the ions and generation of gas bubble occurs at the electrodes with no gas bubbles

incorporated into the deposit. Based on this a in house EPD camber was used by the researcher and silica

glass and zirconia components were successfully manufactured having complex shapes like tubes or

structured parts.

G. Anne et al. [50] measured the strength and residual stresses of functionally graded Al 2O3/ZrO2 discs

prepared earlier by S.Put and his coauthors [47] also they studied the mechanism of that. In their work

they find the increase in biaxial strength from 288MPa for pure Al2O3 discs to 513MPa for FGM also an

interesting thing which he get is that there is not a considerable effect on the compressive stress of FGM

by hot isostatic pressing but it get drastically increased upon surface material removal by grinding.

Zeolites have good gas adsorption capability. Zeolites have been well recognized for their unique ability

to absorb and separate gases, catalyse chemical reactions and selectively exchange cations in solutions.

These properties result from their uniform, well defined pore structure which promotes the adsorbents

chemical interactions that occur at discrete sites within their lattice. These chemical interactions either

generate or require heat, thus the application of zeolites for either endothermic or exothermic processes is

often limited by heat transfer into or from these insulator like alumino-silicates. FGM provide a new and

unique approach in managing heat transfer and maintaining temperature uniformity in either adsorptive or

catalytic applications of these molecular sieves. X. Wang et al. [51] conceptualize a FGM of 3A/5A

zeolites by electrophoretic deposition using acetone and n-butylamine.

For preparing biocompatible materials EPD process was successfully utilized in order to get a coating of

HA-silica-chitosan of graded composition and laminates by K.Grandfield and I.Zhitomirsky [52].

Coatings of 100µm were successfully deposited with varying silica content.

In another work E. Garcia-Lecina et al. [53] Ni/SiC functionally graded coatings have been obtained by

electrochemical deposition of silicon carbide particles from nickel Watts baths with different

concentrations of SiC particles in solution. Electoacustic technique was employed to get zeta potential

and particle size from which effect of the concentration of SiC particles in solution on the amount of SiC

deposition was investigated. Higher the percentage of SiC particles in the electrolyte the higher the

percentage of SiC particles embedded in the nickel matrix, also SiC particles change the texture from

(100) to a mixed preferred orientation through (111) and (211) axes. Wear resistance also get increased

when incorporating SiC particles in the nickel matrix.

3. Spark Plasma Sintering: Sintering as an art had origins that are thousands of years old. The

formation of bricks by heating clay bodies in an open pit fire is one of the earliest examples of sintering

practiced by ancient civilizations of Mesopotamia. The ancient Indians sintered metals and ceramics as

early as 3000BC. Understanding the basic phenomena and the important parameters governing sintering

has led to investigation on means to activate the process. The objective of these investigations was to

enhance mass transport to either make possible the sintering of extremely refractory materials or to lower

the temperature of consolidation. One of the methods of activating the sintering process involves the use

of electrical current. Although the recent widespread use of this approach has been generated by the

availability of commercially built devices, its origin is much older. Spark plasma sintering (SPS) method

enables the manufacture of dense materials in a short period at low sintering temperatures. SPS utilizes

the high energy of a high temperature plasma (discharge plasma) generated when pulse-formed electric

energy is applied to the space among fine particles. Then this heat treated at varying temperature and

pressure to give graded material.

Schematic Diagram of Spark Plasma Sintering Technique

SPS process utilizes pulsed high DC current along with uniaxial pressure to consolidate powders.

The sintering of powders under the simultaneous influence of a current and pressure is done in

SPS process. Powders are placed in a die (typically graphite) and heating is affected by passing a

current (typically pulsed DC) through the die and the sample (if latter is conducting) while a

pressure is applied on the powder. The characteristics which govern the SPS process are therefore

(a) high heating rate, (b) the application of a pressure, and (c) the effect of the current.

Various materials being fabricated by Spark Plasma Sintering

Hongo Huo et al.[54] fabricated functionally graded hydroxyapatite/yttria stabilized tetragonal zirconia

composites by spark plasma sintering in order to get multilayered FGM. The HA/Yttria stabilized TZP

composite exhibited much improved mechanical properties compared to the pure HA ceramics also with

increasing the sintering temperature higher mechanical properties are achieved. The bending strength of

the FGM nearly doubled to 200MPa as compared to pure HA ceramics when sintered at 1200°C. Yu –

Peng Lu et.al [55] reported an FGM with two layers of HA top coating/HA+TiO2 bond coat

composite coating on Ti fabricated by plasma spraying. FGMs surface and interface

microstructure are studied before and after heat treatment. The author has claimed to probe into

the toughening and strengthening mechanism of TiO2 in the bond coat and to evaluate the effect

of the bond coat on the stress level of the HA top coating. H.Mishina et al. [56] fabricate

ZrO2/AISI316L functionally graded materials for joint prosthese. The powders are prepared by mixing

ZrO2 (3Y-PSZ) with 10-50vol% of AISI316L granules. In this experiment attempt were made to evaluate

the fabricated ZrO2/AISI316L FGMs to optimize the properties of the ductile metals and the highly

biocompatible ceramics while giving a high resistance to wear. The author suggests the thickness of FGM

layers should be more than 2mm so as to increase the fracture toughness and the wear resistance of the

FGM for better mechanical and biotribological properties.

The Al2O3-Ti3SiC2 FGMs composite has been synthesized by SPS method by Luo Yongming [57]. The

presence of the Ti3SiC2 phase in the matrix has a profound influence on the properties of the composite.

The percolation threshold of this FGM was observed at 20-30wt% Ti3SiC2 and due to it an increase in the

electrical conductivity from 10-4 to 105 Sm-1 followed by reduction in hardness was observed. Haibo Feng

et al. [58] successfully synthesized six layered Ti-Tib-TiB2 FGM composite rapidly at relatively low

sintering temperature by spark plasma sintering. In these layers, the synthesized TiB were needle shape

and short agglomerated whiskers, and the volume proportion of agglomerated whiskers increased and the

size of the needle shape TiB decreased with the increasing of target TiB volume fraction.

Manchang Gui et al. [59] developed an Al-SiC FGM by plasma spraying. In his research thermal

conductivity of the composites sprayed with Al powders containing 55 and 75vol% SiC particles were

measured along with the mechanism of thermal transfer in the composites was discussed, and numerical

analyses were conducted based on theories of Maxwell and thermal boundary resistance. In this study

they found that interfacial gaps at the interface of Al/SiC and boundary of aluminum grains, porosity and

Fe impurity are three factors to degrade the thermal conductivity. Long ball milling time for feedstock

preparation and small SiC size cause further degradation of thermal conductivity and these factors can be

addressed by using hot process to improve the thermal conductivity of the composites.

Gurusamy Shanmugavelayutham and Akira Kobayashi [60] employ versatility of image analysis methods

for microstructural quantification for Thermal barrier coatings deposited with partially stabilized zirconia

(PSZ), alumina and zirconia –alumina composite coatings by gas tunnel type plasma spraying. In gas

tunnel type plasma spraying torch spraying powder is fed inside plasma falme in axial direction from

centre electrode of plasma gun. The coating was formed on the substrate transverse at the sparaying

distance L. The thickness of all the coatings and the distribution of porosity along the cross section were

measured by image analysis. In plasma spraying, the microstructure of the coatings is strongly dependent

on the processing conditions. With increase in the alumina mixing ratio, the Vickers hadness of the high

hardness layer became large, which was changed from Hv = 1083 to 1371 at the same time the porosity in

hardness layer was decreased which was changed from 33.7% to 7.65%. Here only nontransferable

tetragonal phase of ZrO2 were observed.

Dustin M. Hulbert et al. [61] developed B4C-Al FGM by spark plasma sintering and after that the boron

carbide compacts with large precipitous density gradients were melt infiltrated with pure Aluminum at

1180°C for 10min in a 10-3 Pa to 10-4Pa vacuum. Prior to melt infiltration, X-ray diffraction data indicates

that the B4C compacts have some microstructural defects present and after melt infiltration some of the

defects are dissipated and small amounts of Al3BC were present.

4. Electromagnetic Separation Method:

Application of electromagnetic force to material processing, so called Electromagnetic Separation

Method (EPM) has been recognized as a cutting edge technology, especially in the field of advanced

material processing. Because of the different conductivities between the primary phase (low electric

conductivity) and the metal melt, electromagnetic force (EMF) scarcely acts on the primary phase.

Thus, the primary phase is moved in the direction opposite to that of the EFM when the melted metal

with primary phase solidifies under an electromagnetic field. Archimedes force will act on the

primary particles because of the difference in the electrical conductivity between the primary particles

and the melt and its direction is opposite to the direction of EMF induced in the primary melt.

Schematic of Electromagnetic Separation Method

When the melt temperature is down below the liquidus temperature, primary particles will precipitate

from the parent melt and move in the direction of the resultant force to the one side of the sample.

With the temperature decreasing, primary particles precipitate continuously and regular distribution

of the primary particles precipitate continuously and regular distribution of the primary particles from

one side to the other is formed in the sample.

Various Materials fabricated by Electromagnetic separation method

Changjiang Song et al. [62] had produced in situ multi-layer FGMs using alloy with nominal

composition Al-22% Si-3.9% Ti-0.78%B by electromagnetic separation method. In this work three

layers of Al/Si/Ti, Al/Si eutectic structure and Al/Si/Ti are formed. The interfaces between the layers

are gradient transition in the microstructure from one layer to another in the in situ multilayer

composite samples by Electromagnetic Separation method. In continuation to its research work

Changjiang Song [63] and his fellow researchers fabricated Al/Mg2Si functionally graded materials

and study the effects of the process parameters of the particle volume fraction distribution by the

ESM process. In this experiment the author elaborate that the emf, the melt solidification rate and the

alloy composition greatly affects the particle volume fraction. They also investigate that FGM can

only be produced when the emf reaches beyond a critical value. In another attempt Changjiang Song

et al. [64] fabricated Al/Si FGM by electromagnetic separation method. Investigation results show

that the microstructure of the sample changes from Al-Si hypereutectic structure to Al-Si eutectic

structure to Al-Si hypoeutectic structure from one side of the sample to the other side. In continuation

to this in the same year author [65] also developed Al/Al3Ni FGM by ESM. The major axis of

primary Al3Ni particles tends to be perpendicular to the direction of the electro-particles Archimedes

force, however, in the opposite part, orientation arrangement of primary Al3Ni particles is random. In

another work Chang-Jiang Song et al. [66] successfully fabricate Al/Mg2Si FGM by electromagnetic

separation method. In this research the author get a difficulty in fabricating the FGM without other

additional elements due to the formation of primary Mg2Si particle clusters and as such a smooth

gradient distribution is not achieved. Addition of Ti can give rise to precipitation of Al3Ti phase and

Al-Si-Ti intermetallic compound before the precipitation of primary Mg2Si particles, which prevents

the formation of larger Mg2Si particle clusters and results in in-situ Al/Mg2Si FGMs with a smooth

gradient distribution of primary Mg2Si particles.

Xiaoling Peng et al. [67] use the concept of magnetic separation to prepare ferromagnetic-

nonmagnetic functionally graded material via slip casting. ZrO2-Ni FGM with a continuously

changing composition was successfully fabricated using this method.

Qiang Wang et al. [68] fabricated Mn-Sb composites with a gradient structure in morphology and

composition towards the surface by high magnetic field gradient. The volume fraction of the

segregated phases strongly depended on the product of the magnetic flux density and its gradient.

This process creates new opportunities for the fabrication of more complex bulk hybrid materials in

situ. Tie liu et al. [69] fabricate MnSb/Sb-MnSb FGM using a semi-solid forming process under

magnetic field gradient. Mn-Sb alloy with a hypoeutectic composition was used to produce particle-

dispersed composite. During the process, the alloy was heated to a temperature lower than the

liquidus temperature of the alloy, where the dispersed phase remained solid in a liquid matrix. The

author investigated that MnSb particles could be controlled by the combination of the field gradient

and holding time also the direction along which the volume fraction of the particles increased could

be controlled artificially by adjusting the field gradient direction.

5. Laser Assisted Manufacturing Technique:

Laser assisted manufacturing technique is applied for manufacturing functionally graded materials

coating. Various methods are conceptualized which employs laser and are discussed here:

(a)Laser Cladding: Laser cladding is a technique used to coat a substrate with a layer of other materials

to improve surface properties such as resistance to corrosion, erosion or wear. The process typically

involves blowing a powder mixture pneumatically into the laser melted zone. The processing parameters

such as laser power, transverse speed, and powder flow rate are controlled to achieve the required

thickness with minimum dilution from the substrate. The two main laser cladding (and alloying)

techniques are:

(i) Pre-Placed powder: here powder is preplaced on the substrate with a binding agent to stick the

powder into place until it is melted by the laser beam. The whole process is normally shrouded in

an inert gas such as argon. It has a small operating region of process parameters where low

dilution and a good fusion bond is possible.

(j) Blown powder: it has a much larger operating region. Powder is blown directly into a laser

generated melt-pool as it scans across the surface of the substrate. The powder melts and a clad

track is built up. Complete coverage is achieved by overlapping the tracks.

First research paper so far as with author knowledge is concern is reported by K. Mohammed Jasmin et

al. [70] for fabricating FGM of Al-SiC on a nickel alloy substrate (IN 625) having three successive tracks

were deposited to give a wide range of compositions progressing from inner to the outer regions. He uses

premixed aluminum and SiC powders. However, the accompanying changes in microstructure through

the 3mm thick FGM were not fully satisfactory. J.H. Abboud et al. [71] produced functionally graded

nickel-aluminide and iron-aluminide coatings via laser cladding. The composition of each clad layer was

tailor made. Compositional control was achieved by keeping the powder flow of Ni or Fe constant and

changing the flow rate of Al. In-situ alloying occurs in the laser generated melt pool and compositional

control is exercised via the powder feeder rates. Functionally graded coatings (up to several mm in total

height) were produced by superimposition of a series of layers of different compositions. In further

research Abboud et al. [72] produced another functionally gradient titanium-aluminide composite by laser

cladding. 2mm thick FGM with a progressive increase in the Al content layers from 0 to 35%Al (50 at

%Al) were reported by vertically overlapping two clad layers using powder mixture of selected

compositions.

Y.T. Pie and J. TH. M. DE Hosson [73] uses one step laser cladding process to manufacture AlSi40

functionally graded coatings consisting of silicon primary particle surrounded by α-Al dendritic halos.

Silicon exhibits a continuous increase in both size and volume fraction along with morphology changes

from bottom to top of the FGM tracks. In another research the author [74] studied the fivefold twining

and growth features of Sip with orientation imaging microscopy and electron microscopic examination. In

the FGM so produced of Al-40 wt% Si five-fold branched silicon particles were observed with maximum

particles in the upper region. These particles have grown from twinned decahedron nuclei. In another

research Pei et al. [75] produce SiCp/Ti6A14V FGM by laser melt injection where SiCp are injected just

behind the laser beam into the extended part of the laser melt pool by high beam scanning velocity. An

injection model based on finite element calculation is designed based on the temperature/viscosity field of

the laser pool for a deeper understanding of the mechanism of formation of FGM and based on the

calculations the experiment is conducted.

M. Riabkina-Fishman et al. [77] produced functionally graded tungsten carbide coatings on M2 high

speed tool steel by laser cladding with direct injection of WC powder into the melt pool. Single layer

coatings having four different types of structures with a wide alloying range of tungsten and carbon were

produced by varying laser beam power and beam transverse velocity. The author concluded that the

increase in the degree of alloying from layer to layer as well as the thickness of each layer can be varied

by changing the beam power, the beam travel velocity and the powder injection rate. With the increase in

tungsten content microhardness and wear resistance of the coating increases as compared to unalloyed

laser treated M2 steel. G. Pintsuk et al. [78] developed W/Cu FGM by both 3-D laser cladding and spark

plasma spraying in which he employs powders of Copper and tungsten along with a coating of copper.

Here for melting tungsten and copper powders different power of laser are employed owing to low

melting point of Cu as compared to W, thus for laser sintering of graded structure copper side is

assembled first.

(b) Laser Engineered Net Shaping (LENS): it is a technology that is gaining

importance and in early stages of commercialization. Its strength lies in the ability to fabricate fully dense

metal parts with good metallurgical properties at reasonable speeds. A high power laser is used to melt

metal powder supplied coaxially to the focus of the laser beam through a deposition head. The laser beam

typically travels through the centre of the head and is focused to a small spot by one or more lenses. The

X-Y table is moved in raster fashion to fabricate each layer of the object. Typically the head is moved up

vertically as each layer is completed. The laser beam may be delivered to the work by any convenient

means like right angle mirror or fiber optics. Metal powders are delivered and distributed around the

circumference of the head either by gravity, or by using an inert, pressurized carrier gas. Even in cases

where it’s not required for feeding, an inert shroud gas is typically used to shield the melt pool from

atmospheric oxygen for better control of properties, and to promote layer to layer adhesion by providing

better surface wetting. Most system uses powder feedstocks, but there has also been work done with

material provided as fine wires. In this case the material is fed off-axis to the beam. The building area is

usually contained within a chamber both to isolate the process from the ambient surroundings and to

shield the operators from possible exposure to fine powders and the laser beam. The laser power used

varies greatly, from a few hundred watts to 20KW or more, depending on the particular material, feed-

rate and other parameters. Objects fabricated are near net shape, but generally will require finish machine.

They are fully dense with good grain structure, and have properties similar to, or even better than the

intrinsic materials.

Weiping Liu and J.N. DuPont [78] fabricate crack free functionally graded TiC/Ti composite materials by

LENS with compositions changing from pure Ti to approximately 95 vol% TiC. Amit Bandyopadhyay et

al. [79] fabricate porous and functionally graded structures for load bearing implants. The author has

demonstrated that LENS can fabricate net shape, complex metallic implants with designed porosities up

to 70 vol% to reduce stress-shielding. The effective modulus of Ti, Ni-Ti, and other alloys was tailored to

suit the modulus of human cortical bone by introducing 12-42 vol% porosity. To minimize the wear

induced osteolysis, unitized structures with functionally graded Co-Cr-Mo coating on porous Ti6Al4V

were also made using LENS which showed high hardness with excellent bone cell-materials interactions.

In another research B. Vami Krishna and its co author [80] proposed novel design concepts coupled with

LENS method for fabricating metallic implants with three-dimensionally interconnected porosities down

to 70 vol%. It is shown that the porosities and mechanical properties of laser processed pure Ti structures

with interconnected porosity and/or novel internal architecture can be tailored by changing the LENS

process parameters. In another experiment B. Vami Krishna et al. [81] fabricated functionally graded

crack free coatings Co-Cr-Mo coating on Ti-6Al-4V alloy structures with metallurgical sound interface

using optimized LENS parameters with excellent reproducibility. Vitro biocompatibility test is conducted

and it shows that increasing the Co-Cr-Mo concentration in the coating reduced the live cell numbers

after 14 days of culture on the coating compared with base Ti-6Al-4V alloy, however, coated samples

always showed better bone cell proliferation than 100% Co-Cr-Mo alloy. Author suggested that 50% Co-

Cr-Mo alloy surface provides the best combination of wear resistance and biocompatibility.

(c)Selective Laser Sintering: It was developed in the late 1980’s as layer manufacturing process

that was used for rapid prototyping. Later on it also becomes a common technology to produce products

for long term use. SLS have the potential to be ideal technique to automatically build complex

components using material gradation. SLS creates objects directly from CAD models using a layer-by-

layer material deposition and consolidation approach. Thin layers of powders are successively deposited

and selectively fused using a computer-controlled scanning laser beam that scans patterns corresponding

to slices of the CAD model. With appropriately designed powder deposition systems, this approach

allows deposition systems; this approach allows successive layers but also within each layer, enabling the

manufacture of FGM components.

FGMs by SLS have also been reported previously. Jespon et al.[82] created one dimensional FGMS by

SLS processing of blends of tungsten carbide and cobalt powders. In this work, FGMs were created in the

plane of each layer by carefully placing different blends contiguously and scanning the entire layer using

a laser. Beal et al. [83] reported the SLS processing of one-dimensional FGMs using blends of H-13 tool

steel and cooper powders. In this work, five different powder blends were deposited in the plane of a

layer using a multi-container feed hooper and processed by an Nd:YAG pulsed laser. Filter elements from

metal –polymer powder compositions were fabricated by the LENS method by I.Shishkovsky et al. [84]

Here in his work permeability and porosity coefficients of synthesizing 3D parts were determined and

dependence on laser influence parameters and a polymer quantity are discussed. Haseung Chung and

Suman Das [85] produce functionally graded Nylon-11/silica nano-composites by selective laser

sintering. They developed SLS processing parameters for the different compositions by design of

experiments (DOE). They reported that particulate filled functionally graded polymer nano-composites

exhibiting a one-dimensional composition gradient can be successfully processed by SLS to produce

three-dimensional components with spatially varying mechanical properties in a single, uninterrupted

process run.

6. Novel New Concepts For Fabricating FGM: Various new methods were

conceptualized by researchers for synthesizing FGMs have their own peculiarity:

(a)Combustion synthesis: In 1967, Merzhanov and his cooperators raised the concept of SHS

and developed combustion synthesis. Combustion synthesis is a technique in which a material is

synthesized from a homogeneous reactant mixture of powders or a compact made of the reactant

powder mixture without further processing and involves two steps: mixing the reactant powder within

a certain stoichiometry and effecting the combustion synthesis reaction to produce the required

materials. Combustion synthesis or self propagating high temperature synthesis (SHS) methods are

known to be relatively advantageous due to lower energy consumption, shorter processing times and

better availability of starting materials. The ignition can be performed either locally at one point or by

heating the whole pellet up to the required ignition temperature of exothermic reaction.

The flow chart for manufacturing a FGM via SHS reaction coupled with simultaneous

densification

The control of the combustion temperature is an important issue in the combustion synthesis of FGMs.

The temperature must be sufficiently high everywhere to cause densification yet not so high to avoid

uncontrolled reactions. Because of the fast kinetics, it was reported that FGM fabrication by SHS is less

prone to homogenization. Therefore, the gradient structure is generally preserved.

(b) Impeller Dry Blending Technique: This novel method for manufacturing FGMs

was conceptualized by A.J. Ruys et al. [86] which offer the possibility of producing large bulk-FGMs of

a wide range of controllable continuous gradients and compositions. The impeller dry blending process

claims to offer large bulk FGMs with an easy control of continuous gradient and compositions. The

process of involves four steps: Feeding, blending, homogenization and deposition.

Feeding is done by two separate feed hoppers. Blending of the powders is done by using control gates

such that volume percentage of all the components is 100%. Homogenization of the blended powder mix

is done by impeller chamber. Deposition is done in the mold beneath the impeller. The uniqueness of

this process lies that controlled segregation and blending is achieved from this process. No binder is

required and the process is restricted to flat FGM parts. Author suggested use of hydrostatic shock

forming so as to get localized pressure more than 30GPa by explosive charge in order to get dense FGMs

where the constituents have a huge difference in their melting temperature such as in case of metal-

ceramic.

(c)Gradient Slurry Disintegration and Deposition: this innovative technique for

synthesizing free-standing, one dimensional FGM was given by M. Gupta, M. O. Lai and T. S. Srivatsan.

Here they successfully fabricate Al-Cu/SiC FGM by this technique. Here they use Al-Cu metals and

superheat them to 950°C to ensure wettability of the reinforcing phase in the alloy melt followed by SiC

addition which is preheated up-to 950°C and the mixture is stirred at 294 rpm to facilitate controlled

sedimentation of the SiC particulates. The slurry is then allowed to flow into a stream through a hole

drilled centrally in the crucible. The molten stream was disintegrated using argon gas jets and deposited

onto a metallic substrate.

Schematic of the gradient slurry disintegration and deposition technique

(d) Differential Temperature Aging Technique: This new concept was given by

S. Bakshi, K. Chattopadhyay and S. Kumar [88] of Indian Institute of Science, Banglore (India). In this

technique they devised to produce a FGM in an originally homogeneous, Al-4.6Cu alloy specimen

utilizing a differential heat treatment technique. Most of the processing routes for the preparation of FGM

involve creation of composition gradients in the material over a length scale so as to get the desired

property gradient. The required gradient in functional properties can also be achieved through a variation

in microstructure without altering the composition of the bulk. In his work Bakshi et al. achieved the

functional gradient by giving a differential aging treatment to an age-hardenable alloy. The differential

aging leads to the development of a gradient in microstructure and hence in properties. For preparation of

FGM they first solutionzed the alloy specimen in an furnance and then quenching is done in water then

the specimen are immediately transferred to the temperature gradient furnace with varying temperature

gradient of 6°C/mm in the range of 70°C to 170°C. The specimen is left for being differentially aged for

38 hours so as to get one side which is at 170°C will be peak aged whereas, for the same time period the

material remains under aged to various degrees for the aging temperatures. Therefore in principle, a

differencial temperature aging may lead to a microstructural as well as hardness gradient across the

specimen cross section.

Experimental set-up for preparing FGM by differential age hardening

(e)Cast-Decant-Cast (CDC) process: this novel technique for production of light weight

functionally gradient alloys was proposed by Michelle Scanlan, David J. Browne and Andrew Bates

[89]. The procedure involves the introduction of the first liquid alloy A into a mould allowing it to

partially solidify against the mould walls, then decanting any unsolidified material, leaving of alloy A

against the mould wall. Alloy B is then introduced into the mould in a superheated state, partially

remelting the inner layer of alloy A, thus mixing with it and forming a smooth gradient between alloy

A and B. The author suggested three techniques associated with the CDC process: turnover

technique- in which rectangular steel mould is fixed to a rotatable frame which enables the mould to

be inverted for the decanting step. Second technique is internal decant- here mould consists of a

reservoir, drainage channels, carbon rods and a steel core around which a functionally gradient

hollow cylinder will form. The carbon rods are used to plug the drainage channels to prevent alloy A

draining into the reservoir as a layer is forming. In the third method called low pressure technique the

molten materials are pressurized in an airtight vessel allowing it to rise up a tube into a mould. The

material is held under pressure in the mould until it is solidified.

Schematic apparatus for turnover technique Schematic apparatus for internal cast decant

Schematic low pressure sequence for Cast Decant Method

Conclusion

This paper gives a basic insight for FGMs beginners. The main focus of this paper is on the main

techniques for developing FGMs and the resent advancement in them. Effort is made to cover all

the aspects of manufacturing FGM from various conventional as well as new methods. All

important materials used to make graded materials are deliberately tried to be cover in this work.

References

1. Siddhartha, Amar Patnaik and Amba D. Bhatt; A Review on Functionally Graded Materials

Manufacturing Techniques; Journal of Reinforced Plastics and Composites, Vol.0, No. 00/2009.

2. Chu Chenglin, Zhu Jingchaun, Yin Zhongda and Wang Shidong; Hydeoxyapatite-Ti functionally

graded biomaterials fabricated by powder metallurgy; Materials Science and Engineering A271

(1999) 95-100.

3. R. Roop Kumar, M. Wang; Functionally graded bioactive coatings of hydroxyapatite/titanium

oxide composite system; Materials letters 55 (2002) 133-137.

4. Fumio Watari, Atsuro Yokoyama, Fuminori Saso, Motohiro Uo and Takao Kawasaki;

Fabrication and properties of functionally gradeddental coatings; Composites part B 28B (1997)

5-11.

5. Lyndia Helena Wong, Betty Tio and Xigeng Miao; Functionally graded tricalcium phosphate

/fluoroapatite composites; Materials Science and Engineering C 20 (2002) 111-115.

6. Sivaprased Katakam, D. Siva Rama Krishna, T.S. Sampath Kumar; Microwave processing of

functionally graded bioactive materials; Materials Letter 57 (2003) 2716-2721.

7. M. Bram, A. Laptev, H.P. Buchkremer, D. Stover; Application of powder metallurgy for the

production of highly porous parts with open porosity; Materials Forum Volume 29- published

2005.

8. A. P. Tomsia, E. Saiz, H. Ishibashi, M. Diaz, J. Requena and J. S. Moya; Powder processing of

Mullite/Mo functionally graded materials; Journal of the European Ceramic Society 18 (1998)

1365-1371.

9. J.F. Bartolome, M.Diaz, J.Requena, J. S. Moya and A.P. Tomsia; Mullite/Molybdenum ceramic-

metal composites; Acta mater. Vol. 47, No. 14, pp.3891-3899, 1999.

10. R. Sivakumar, T. Nishikawa, S.Honda, H. Awaji, F.D. Gnanam; Processing of mullite-

molybdenum graded hollow cylinders by centrifugal molding technique; Journal of the European

Ceramic Society 23 (2003) 765-772.

11. Gang Jin, Makoto Takeuchi, Sawao Honda, Tadahiro Nishikawa and Hideo Awaji; Properties of

multilayered mullite/Mo functionally graded materials fabricated by powder metallurgy

processing; Materials chemistry and physics 89 (2005) 238-243.

12. Shuheng Chen, Linjiang Wang, Xiangli Xie and Ye Zhang; Preparation and characterization of

multilayered Mullite/Mo functionally gradient Materials; Key Engineering Materials Vols. 368-

372 (2008) pp 1866-1868.

13. S. Lopez-Esteban, J.F. Bartolome, C. Pecharroman and J. S. Moya; Zirconia/stainless-steel

continuous functionally graded material; Journal of the European Ceramic Society 22 (2002)

2799-2804.

14. Jingchaun Zhu, Zhonghong Lai, Zhongda Yin, Jaeho Jeon and Sooyoung Lee; Fabrication of

ZrO2-NiCr functionally graded material by powder metallurgy; Materials Chemistry and Physics

68 (2001) 130-135.

15. J. Ma and G. E. B. Tan; Processing and characterization of metal-ceramics functionally gradient

materials; Journal of Materials Processing Technology 113 (2001) 446-449.

16. Yoshimi Watanabe and Tatsuru Nakamura; Microstructures and wear resistances of hybrid Al-

(Al3Ti + Al3Ni) FGMs fabricated by a centrifugal method; Intermetallics 9 (2001) 33-43.

17. R. Rodriguez-Castro and M. H. Kelestemur; Processing and microstructural characterization of

functionally gradient AlA359/SiCp composite; Journal of Materials Science 37 (2002) 1813-1821.

18. R. Rodriguez-Castro, R. C. Wetherhold and M. H. Kelestemur; Microstructure and mechanical

behavior of functionally graded AlA359/SiCp composite; Materials Science and Engineering

A323 (2002) 445-456.

19. J. Q. Li, X. R. Zeng, J. N. Zeng, J. N. Tang and P. Xiao; Fabrication and thermal properties of a

YSZ-NiCr joint with an interlayer of YSZ-NiCr functionally graded material; Journal of the

European Ceramic Society 23 (2003) 1847-1853.

20. O. Eso, Z. Fang, A. Griffo; Liquid phase sintering of functionally graded WC-Co composites;

International Journal of Refractory Metals & Hard Materials 23 (2005) 233-241.

21. Mandira Bhattacharya, Amar Nath Kumar, Santosh Kapuria; Synthesis and characterization of

Al/SiC and Ni/Al2O3 functionally graded materials; Materials Science and Engineering A xxx

(2007) xxx-xxx.

22. Ruigang Wang, Wei Pan, Jian Chen, Minghao Fang, Mengning Jiang and Yongming Luo;

Graded machinable Si3N4/h-BN and Al2O3/LaPO4 ceramic composites; Materials and Design 23

(2002) 565-570.

23. Chang-Chun Ge, Jiang-Tao Li, Zhang-Jian Zhou, Wen-Bin Cao, Wei-Ping Shen, Ming-Xu Wang,

Nian-Man Zhang, Xiang Liu and Zheng-Yu Xu; Development of functionally graded plasma-

facing materials; Journal of Nuclear Materials 283-287 (2000) 1116-1120.

24. Yun-Han Ling, Jiang-Tao Li, Chang-Chun Ge and Xin-De Bai; Fabrication and evaluation of

SiC/Cu functionally graded material used for plasma facing components in a fusion reactor;

Journal of Nuclear Materials 303 (2002) 188-195.

25. A. H. Wu, W. B. Cao, C. C. Ge, J. F. Li and A. Kawasaki; Fabrication and characterization of

plasma facing SiC/C functionally graded composite material; Materials Chemistry and Physics 91

(2005) 545-550.

26. Wei-ping Shen, Qiang Li, Ke Chang, Zhang-jian Zhou and Chang-Chun Ge; Manufacturing and

testing W/Cu functionally graded material mock-ups for plasma facing components; Journal of

Nuclear Materials 367-370 (2007) 1449-1452.

27. Zhang-Jian Zhou, Juan Du, Shu-Xiang Song, Zhi-Hong Zhong and Chang-Chun Ge;

Microstructural characterization of W/Cu functionally graded materials produced by a one-step

resistance sintering method; Journal of Alloys and Compounds 428 (2007) 146-150.

28. Bin-Bin Liu, Jian-Xin Xie and Xuan-Hui Qu; Fabrication of W-Cu functionally graded materials

with high density by particle size adjustment and solid state hot press; Composites Science and

Technology 68 (2008) 1539-1547.

29. D. F. Heaney, P. Suri and R. M. German; Defect free sintering of two material powder injection

molded components; Journal of Materials Science 38 (2003) 4869-4874.

30. Denger Jin; Functionally graded PZT/ZnO piezoelectric composites; Journal of Materials science

letters 22, 2003, 971-974.

31. Kenta Takagi, Jing-Feng Li, Shohei Yokoyama and Ryuzo Watanabe; Fabrication and evaluation

of PZT/Pt piezoelectric composites and functionally graded actuators; Journal of the European

Ceramic Society 23 (2003) 1577-1583.

32. Y. Gelbstein, Z. Dashevsky and M. P. Dariel; Powder metallurgical processing of functionally

graded p-Pb1-xSnxTe materials for thermoelectric applications; Physica B 391 (2007) 256-265.

33. Ellen M. Heian, Jeffery C. Gibeling, Zuhair A. Munir; Synthesis and characterization of

Nb5Si3/Nb functionally graded composites; Materials Science and Engineering A 368 (2004) 168-

174.

34. Jian Zhang, Yu-Quing Wang, Ben-Lian Zhou and Xing-Qiang Wu; Functionally graded Al/Mg2Si

in situ composites, prepared by centrifugal casting; Journal of Materials Science Letters 17

(1998) 1677-1679.

35. Li Sun, Adam Sneller and Patrick Kwon; Fabrication of alumina/zirconia functionally graded

material: From optimization of processing parameters to phenomenological constitutive models;

Materials Science and Engineering A xxx (2007) xxx-xxx.

36. Yoshimi Watanbe and Yasuyoshi Fukui; Microscopic study of Al-base intermetallic compound-

dispersed functionally graded materials fabricated by a centrifugal method; Current Issues on

Multidisciplinary Microscopy Research and Education.

37. Yoshimi Watanabe and Shin Oike; Formation mechanism of graded composition in Al-Al 2Cu

functionally graded materials fabricated by a centrifugal in situ method.

38. J. Q. Li, K. M. Gu, J. N. Tang, S. H. Xie and Y. H. Zhuang; Al2O3-NiCrAl composites and

functionally gradient materials fabricated by reactive hot pressing; Materials Chemistry and

Physics 97 (2006) 31-36.

39. Michael L. Pines and Hugh A. Bruck; Presureless sintering of particle-reinforced metal-ceramic

composites for functionally graded materials: part I Porosity reduction models; Acta MAterialia

54 (2006) 1457-1465.

40. Yoshimi Watanabe, Masahito Kurahashi, I. S. Kim, Shinya Miyazaki, Shinji Kumai, A. Sato and

Shun-ichiro Tanaka; Fabrication of fiber-reinforced functionally graded materials by a centrifugal

in situ method from Al-Cu-Fe ternary alloy; Composites: Part A 37 (2006) 2186-2193.

41. T. P. D. Rajan, R. M. Pillai and B.C. Pai; Functionally graded Al-Al3Ni in situ intermetallic

composites: Fabrication and microstructural characterization; Journal of Alloys and Compounds

xxx (2006) xxx-xxx.

42. Partho Sarkar, Someswar Datta and Patrick S. Nicholson; Functionally graded ceramic/ceramic

and metal/ceramic composites by electrophoretic deposition; Compoistes Part B 28B (1997) 49-

56.

43. P. Sarkar, X. Huang and P.S. Nicholson; Structural Ceramic Microlaminates by Electrophoretic

Deposition; Jounral of American Ceramic Society 75 (1992) 2907-2909.

44. C. Kawai and S. Wakamatsu; Synthesis of a functionally gradient material based on C/C

composites using an electro-deposition method; Journal of Materials Science Letters 14 (1995)

467-469.

45. S. Put, J. Vleugels and O. Van der Biest; Functionally graded WC-Co materials produced by

electrophoretic deposition; Scripta Materialia 45 (2001) 1139-1145.

46. S. Put, G. Anne, J. Vleugels and O. Van der Biest; Functionally graded ZrO2-WC composites

processed by elcectrophoretic deposition; Key Engineering Materials Vols. 206-213 (2002) pp.

189-192.

47. S. Put, J. Vleugels, G. Anne, O. Van der Biest; Functionally graded ceramic and ceramic-metal

composites shaped by electrophoretic deposition; Colloids and Surfaces A: Physicochem. Eng.

Aspects 222 (2003) 223-232.

48. Stijn Put, Jef Vleugels, Guy Anne and Omer Van der Biest; Functionally Graded Hardmetals with

a Continuously Graded Symmetrical Profile; Materials Science Forum Vols. 423-425 (2003) pp.

33-38.

49. J. Tabellion and R. Clasen; Electrophoretic deposition from aqueous suspensions for near-shape

manufacturing of advanced ceramics and glasses-applications; Journal of Materials Science 39

(2004) 803-811.

50. G. Anne, S. Hecht-Mijic, H. Richter, O. Van der Beist, J. Vleugels; Strength and residual stresses

of functionally graded Al2O3/ZrO2 discs prepared by electrophoretic deposition; Scripta

Materialia 54 (2006) 2053-2056.

51. X. Wang, E.A. Olevsky, E. Bruce, M.B. Stern and D. T. Hayhurst; Fabrication of functionally

graded 3A/5A zeolites by electrophoretic deposition; Institute of materials, Minerals and Mining,

2007.

52. K. Grandfield and I. Zhitomirsky; Electrophoretic deposition of composite hydroxyapatite-silica-

chitosan coatings; Materials Characterization 58 (2008) 61-67.

53. E. Garcia-Lecian, I. Garcia-Urrutia, J. A. Diez, M. Salvo, F. Smeacetto, G. Gautier, R. Seddon

and R. Martin; Electrochemical preparation and characterization of Ni/SiC compositionally

graded multilayered coatings; Electrochimica Acta 54 (2009) 2556-2562.

54. Hongbo Guo, Khiam Aik Khor, Yin Chiang Boey and Xigeng Miao; Laminated and functionally

graded hydroxyapatite/yttria stabilized tetragonal zirconia composite fabricated by spark plasma

sintering; Biomaterials 24 (2003) 667-675.

55. Yu-Peng Lu, Mu-Sen Li, Shi-Tong Li, Zhi-Gang Wang and Rui-Fu Zhu; Plasma-sprayed

hydroxyapatite + titania composite bond coat for hydroxyapatite coating on titanium substrate;

Biomaterials 25 (2004) 4393-4403.

56. H. Mishina, Y. Inumaru and K. Kaitoku; Fabrication of ZrO2/AISI316L functionally graded

materials for joint prosthese; Materials Science and Engineering A475 (2008) 141-147.

57. Luo Yongming, Pan Wei, Li Shuqin, Wang Ruigang and Li Jianqiang; A novel functionally

graded material in the Ti-Si-C system; Materials Science and Engineering A345 (2003) 99-105.

58. Haibo Feng, Oingchang Meng, Yu Zhou and Dechang Jia; Spark plasma sintering of functionally

graded material in the Ti-TiB2-B system; Materials Science and Engineering A397 (2005) 92-97.

59. Manchang Gui, Suk Bong Kang and Kwangjun Euh; Thermal conductivity of Al-SiCp composites

by plasma sintering; Scripta Materialia 52 (2005) 51-56.

60. Gurusamy Shanmugavelayutham and Akira Kobayashi; Mechanical properties and oxidation

behavior of plasma sprayed functionally graded zirconia-alumina thermal barrier coatings;

Materials Chemistry and Physics 103 (2007) 283-289.

61. Dustin M. Hulbert, Dongtao Jaing and Umberto Anselmi-Tamburini; Experiment and modeling

of spark plasma sintered, functionally graded boron carbide-aluminum composites; Materials

Science and Engineering Axxx(2007)xxx-xxx.

62. Changjiang Song, Zhenming Xu, Xiangyang Liu, Gaofei Liang and Jianguo Li; In situ multi-

layer functionally graded materials by Electromagnetic Separation Method; Materials Science and

Engineering A393 (2005) 164-169.

63. Chang-Jiang Song, Zhen-Ming Xu, Gaofei Laing and Jian-Guo Li; Study of in-situ Al/Mg2Si

functionally graded materials by electromagnetic separation method; Materials Science and

Engineering A424 (2006) 6-16.

64. Changjiang Song, Zhenming Xu and Jainguo Li; Structure of in situ Al/Si functionally graded

materials by electromagnetic separation method; Materials and Design 28 (2007) 1012-1015.

65. Chang-Jiang Song, Zhen-Ming Xu and Jian-Guo Li; In-situ Al/Al3Ni functionally graded

materials by electromagnetic separation method; Materials Science and Engineering A 445-446

(2007) 148-154.

66. Chang-Jian Song, Zhen-Ming Xu and Jain-Guo Li; Fabrication of in situ Al/Mg2Si functionally

graded materials by electromagnetic separation method; Composite: Part A 38 (2007) 427-433.

67. Xiaoling Peng, Mi Yan and Weitang Shi; A new approach for the preparation of functionally

graded materials via slip casting in a gradient magnetic field; Scripta Materialia 56 (2007) 907-

909.

68. Quian Wang, Tie Liu, Ao Gao, Chao Zhang, Chunjiang wang and Jicheng He; A novel method

for in situ formation of bulk layered composites with compositional gradients by magnetic field

gradient; Scripta Materialia 56 (2007) 1087-1090.

69. Tie Liu, Qiang Wang, Ao Gao, Chao Zhang, Chunjiang Wang and Jicheng He; Fabrication of

functionally graded materials by a semi-solid forming process under magnetic field gradients;

Scripta Materialia 57 (2007) 992-995.

70. K. Mohammed Jasmin, R. D. Rawlings and D. R. F. West; Metal-Ceramic functionally gradient

material produced by laser processing; Journal of Materials Science 28 (1993) 2820-2826.

71. J. H. Abboud, R. D. Rawlings and D. R. F. West; Functionally graded nickel-aluminide and iron-

aluminide coatings produced via laser cladding; Journal of Materials Science 30 (1995) 5931-

5938.

72. J. H. Abboud, D. R. F. West, R. D. Rawlings; Functionally gradient titanium-aluminide

composites produced by laser cladding; Journal of Materials Science 29 (1994) 3393-3398.

73. Y. T. Pei and J. H. M. DE Hosson; Functionally graded materials produced by Laser Cladding;

Acto Materialia 48 (2000) 2617-2624.

74. Y. T. Pei and J. H. M. DE Hosson; Five-flod branched Si particles in laser clad AlSi functionally

graded materials; Acta Materialia 49 (2001) 561-571.

75. Y. T. Pei, V. Ocelik and J. H. M. DE Hosson; SiCp/Ti6A14V functionally graded materials

produced by laser melt injection; Acta Materialia 50 (2002) 2035-2051.

76. M. Riabkina-Fishman, E. Rabkin, P. Levin, N. Frage, M. P. Dariel, A. Waisheit, R. Galun and B.

L. Mordike; Laser produced functionally graded tungsten carbide coatings on M2 high-speed tool

steel.

77. G. Pintsuk, S. E. Brunings, J. E. Doring, J. Linke, I. Smid and L.Xue; Develpoment of W/Cu-

functionally graded materials; Fusion Engineering and Design 66-68 (2003) 237-240.

78. Weiping Liu and J. N. DuPont; Fabrication of functionally graded TiC/Ti composites by laser

engineered net shaping; Scripta Materialia 48 (2003) 1337-1342.

79. Amit Bandyopadhyay, B. V. Krishna, Weichang Xue and Susmita Bose; Application of Laser

Engineered Net Shaping (LENS) to manufacture porous and functionally graded structures for

load bearing implants; J Mater Sci: Mater Med.

80. B. Vamsi Krishna, Susmita Bose and Amit Bandyopadhyay; Low stiffness porous Ti structures

for load bearing implants; Acta Biomaterialia 3 (2007) 997-1006.

81. B. Vamsi Krishna, Weichang Xue, Susmita Bose and Amit Bandyopadhyay; Functionally graded

Co-Cr-Mo coating on Ti-6Al-4v alloy structures; Acta Biamaterialia xxx (2007) xxx-xxx.

82. L.Jepson, J. J. Beaman, D. L. Bourell and K. L. Wood; Proceedings of the solid free form

fabrication symposium, The University of Texas at Austin, 1997, pp. 67-79

83. V. E. Beal, P. Erasenthiran, N. Hophinsan, P. Dickens and C. H. Ahrens; Proceedings of the solid

free form fabrication symposium, The University of Texas at Austin, 2004, pp .187-197.

84. I. Shishkovsky, V. Scherbakov and A. Petrov; Laser Synthesis of the functional graded filter

elements from metal-polymer powder compositions.

85. Haseung Chung and Suman Das; Processing and properties of glass bead particulate-filled

functionally graded Nylon-11 composites produced by selective laser sintering; Materials Science

and Engineering A437 (2006) 226-234.

86. A. J. Ruys, E. B. Popov, D. Sun, J. J. Rusell and C. C. J. Murray; Functionally graded

electrical/thermal ceramic systems; Journal of the European Ceramic Society 21 (2001) 2025-

2029.

87. M. Gupta, M. O. Lai and T. S. Srivatsan; Synthesis and Characterization of a free-standing, one

dimensional, Al-Cu/SiC- based functionally graded material; Journal of Materials Synthesis and

Processing, Vol 10, No.2, March 2001.

88. S. Bakshi, K. CHattopadhyay and S. Kumar; Studies on the mechanical behavior of a newely

developed Al-4.6Cu functionally graded material; Materials Forum Volume 29 (2005).

89. Michelle Scanlan, David J. Browne and Andrew Bates; New Casting route to novel functionally

gradient light alloys; Materials Science and Engineering A 413-414 (2005) 66-71.