FGM Fabrication

8/20/2019 FGM Fabrication

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FGM fabrication

In this chapter the description of the procedure to manufacture the functionally graded (FG)

materials is presented. In this research, FGM specimens were manufactured from the

mixture of ceramics and polymers. Alumina (Al2!) is used for the ceramic constituent."poxy#resin is used for the polymer constituent. $ased on the extensi%e literature re%iew in

&hapter 2, it was found that, among three effecti%e techni'ues used for manufacturing

FGMs namely, thermal spraying, powder metallurgy and infiltration techni'ues, the

ceramic#polymer FGMs can only e manufactured appropriately y using the infiltration

techni'ue. *herefore, a multi#step se'uential infiltration techni'ue is used to faricate FG

eam specimens made from alumina and epoxy#resin in this research. In this chapter, the

information in relation to FGM farication will e pro%ided in detail as follows+

.- pecimen farication using multi#step se'uential infiltration techni'ue

.2 Microstructure analysis

.! Alternati%e techni'ue of specimen farication

5.1 Specimen Fabrication using multi-step sequential infiltration

techniqueAlumina#epoxy composite eams were manufactured using a multi#step se'uential

infiltration techni'ue. *he earliest de%elopment of the techni'ue was proposed y &ichoc/i

et al. (&ichoc/i et al., -001) to produce graded composite materials.

*he /ey principle of the infiltration techni'ue is to ma/e an alumina piece containing a

graded networ/ of porosity which will e infiltrated y epoxy in the final step, in order to

otain alumina#epoxy composite specimens. $y using this techni'ue, the composite

specimens can e represented in the form of interpenetrating#networ/ (I3) structuredcomposites which contain two phases of materials that are alumina and epoxy phases. In

se%eral pre%ious in%estigations (&lar/, -002, rielipp et al., -00, 4ange et al., -005), it

was affirmed that the infiltration techni'ue is an effecti%e methodology that can e used for

producing graded composite materials where their material compositions are continuous.

&omposite materials ha%ing the interpenetrating networ/ structures are different from

traditional fire#matrix and particle#matrix composite materials in terms of compositional

distriution and pattern which then lead to different eha%iour when they are su6ected to

mechanical loads. *herefore, this chapter will gi%e o%er%iews of the production process for

ma/ing graded composite eams.

*he uni'ue ad%antage of the infiltration techni'ue is that it is suitale for ma/ing graded

composites with a wide range of material constituents. As mentioned in &hapter 2 of theliterature re%iew, for example, 3eurand et al. (3eurand et al., 2552) successfully

employed the infiltration techni'ue to produce ceramic#metal graded composites which

were used for in%estigating residual stress analysis. *o otain graded composites which

ha%e a large disparity in elastic properties, ceramic#polymer could e used as the material

constituents. *hus, in the in%estigations of *ilroo/ et al. (*ilroo/ et al., 2557), the

alumina#epoxy graded composite system in which material compositions were %aried along

an axial direction, was produced using the infiltration techni'ue for the purpose of

in%estigating crac/ propagation. In this research, the material compositions etween

alumina and epoxy of the FG eam specimens will e graded across the thic/ness direction

of the specimens. *he outline of the faricating process is illustrated in Fig. .-.


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Fig. 5.1 chematic of faricating process of graded composite specimen

5.1.1 Foam preparation

According to the schematic of the production process, it egan with the foam preparation

process. *his is to construct a porous networ/ structure inside the specimens. *he networ/

structure was achie%ed y using open#celled polyurethane foam (8) as an imprint for such

a structure. *he polyurethane foam used in this preparation was commercial foam whose

a%erage cell si9e was approximately 5.1 mm. *he 8 foam was cut to the re'uired si9es(length -55 mm and readth -2 mm), then the foam pieces were sliced to three different

thic/nesses (h), 2 mm, : mm and 7 mm. *here were many ways to slice the foam pieces in

order to otain a series of %ery thin foams. For instance, in the foam preparation process of

(*ilroo/, 255), the 8 foam was immersed with water and fro9en to retain shape efore

it was cut to si9e with a andsaw. *his method of cutting the foam did not yield a perfect

result ecause the cut foam pieces were wet and re'uired thawing and drying later. 8sually,

the drying process of the wet foam pieces too/ around -2 hrs to dry in an o%en at : 5&.

*herefore, in this farication, the 8 foam was cut and sliced y using a hot#wire cutter

that can e considered as a etter way, without re'uiring any process of free9ing, thawing

and drying.

*he foam pieces with different thic/nesses were placed in a uniaxial hot press andcompressed to the same constant thic/ness of - mm, producing a series of foam pieces of


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different density. In order to retain the compressed shape, a heat treatment cycle had to e

applied during this compressing of foam. A schematic of the foam compression process is

shown in Fig. .2. As seen in Fig. .2, a pressure of !55 /a was applied to compress the

foam pieces. *he temperature was ramped up to -15 5& with the increasing rate of -

5&;min, and was held for - hour efore letting the temperature cool down naturally to room

temperature. *he foam was then remo%ed from the hot press. 8ne%enness at the edges dueto thermal stress was found which was remo%ed efore eing utilised in the suse'uent

stages.

*he uniaxial hot press that was used in this foam preparation process is shown in Fig. .!.

It is seen that the rass spacer containing the foam pieces ha%ing different thic/nesses was

positioned etween the top and ottom platforms. $oth platforms were set to heat up to -15

5& using a temperature control ox. *he pressure of the hot press was controlled at !55 /a

y an hydraulic system. *he spacer under the mentioned pressure and temperature was held

for - hour in order to ensure that the foam pieces inside had retained the new shapes. *he

spacer was then allowed to cool down naturally efore remo%al of the compressed foam

pieces.


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5.1.2 Slip casting and drying processes

*o manufacture graded composite specimens, a slip casting process was used to create

alumina phases in specimens. A useful and appropriate optimum slip formula for the

infiltration techni'ue was determined y 3eurand et al. (3eurand et al., -000).

*herefore, the alumina slip used to manufacture the specimens in this research was

produced using the same formula as proposed in (3eurand et al., -000, *ilroo/, 255).


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A %ery good mixture of all ingredients (the li'uid components) with the alumina powder

according to the formula can e achie%ed y the aid of a magnetic stirrer (I"& magnetic

stirrer) and an ultrasonic ath (83I3I&). All of the chemical ingredients and alumina

powder were weighed se'uentially corresponding to the formula and mixed together in a

ea/er. A teflon#coated magnetic ar was added to the ea/er, which was then placed on

the magnetic stirrer. *he magnetic stirrer was operated for -5 minutes efore the ea/er

was mo%ed to the ultrasonic ath for -5 minutes. Dowe%er, one cycle of mixing, was found

to e insufficient to otain a good mixture. *herefore, the mixing process with the magnetic

stirrer and the ultrasonic ath was repeated again. *he mixing process was carried out at

least ! times in order to produce the strongest mixture of all ingredients. It was found that

the mixing process led to a reduction of large agglomerates and produced good dispersion.

*he alumina slip was prepared one day efore casting for ageing purposes. $efore the

alumina slip was used for casting in the following day, it was re#mixed or re#dispersed y

the magnetic stirrer and the ultrasonic ath again.

In the slip casting process, the casting mould, which consists of pieces of erspex, was used

to create the graded composite specimens. *he erspex was made from plaster of aris. *he

pieces of the erspex were fitted together to the dimension of the compressed foam (length

-55 mm and readth -2 mm). *hey were placed on a laster of aris loc/ in a %acuum

chamer, as shown in Fig. .. *he enefit of the laster of aris loc/ is that it can asor

moisture from the alumina slip and let the specimen dry appropriately. *o minimise the

prolem of adhesion etween the surfaces of the cast specimen to the mould walls, the

erspex pieces were lined with *eflon tape. *he filter paper (A?A3*"&) was placed

etween the compressed foam and the plaster of aris loc/. *his is done in order to a%oid

the li/elihood of contamination of the alumina slip y particles or solule ions from the

plaster. *o clearly understand the slip casting set up, the cross#section of apparatus isillustrated in Fig. ..

After preparing the casting mould, the series of compressed foam pieces were stac/ed into

the mould. *he highest density foam piece was placed in the mould first, followed y the

lighter ones. ?ue to the density of the foam pieces eing much lighter than that of the

alumina slip, the foam pieces would normally float in the slip causing the prolem of

discontinuity etween ad6acent layers. *o sol%e this prolem, there were se%eral attempts to

glue the foam pieces together in order to ensure phase connecti%ity across the stepinterfaces.

8sing glue to stic/ the foam pieces together was one of many attempts to create

the connecti%ity howe%er, the glue tended to cause warping of the foam, producing

delamination. Another attempt was to use a heat treatment with slight pressure to construct

the foam piece connection. Dowe%er, these foam 6oining techni'ues presented ao%e werenot effecti%e in producing a suitale ond. *hus a etter way to 6oin the foam pieces was to


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place the foam pieces so they were restrained slightly y the mould walls or the erspex

pieces without using glue and then apply a little pressure when placing them into the

mould. &onse'uently, good connecti%ity etween the ad6acent layers can e otained.

lip casting was performed in the %acuum chamer (4A$"&) y placing the mould

assemly into the chamer. At the eginning stage of the slip casting process, the alumina

slip was filled into the foam pieces at a slow filling rate and using a small amount of theslip otherwise, the foam pieces would ha%e floated on the surface. *o enhance infiltration

of the slip casting process after the slip was poured o%er the foam pieces the pressure of the

chamer was lowered to around -55 mar that led to air ules lea%ing the foam.


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ceramic phase. *he temperature profile used for the whole process of foam pyrolysis is

illustrated in Fig. . with the details of temperature increment. For example, starting with

the room temperature, the following temperature was used+ 2 5& to 2! 5& at !55&;hour

held for - hour (foam urnout), 2! 5& to :55 5& at !55&;hour held for - hour (inderurnout),

:55 5& to 155 5& at :55&;hour held for - hour (ash urnout). *he specimen was

allowed to cool down naturally to room temperature efore it was heated up again for sintering the ceramic phase at -:55 5&, held for - hour. *he temperature profile of sintering

process is shown in Fig. .1. *he furnace, (&eramic engineering furnace manufacturer,

ydney, Australia) as shown in Fig. .0, was used for oth urning out foam and sintering

ceramic processes. It was found that sintering reduces the dimensions of the specimen in

e%ery direction, and lea%es an alumina specimen with layers of graduated porosities.


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5.1.4 Polymer infiltration

*he alumina specimen ha%ing layers of graduated porosities was ready for polymer

infiltration, using a %acuum chamer, in order to otain an alumina#epoxy composite

specimen. *he alumina specimen was placed into a silicone mould which was suitale for

this process ecause the specimen can e remo%ed easily when polymer is cured. "poxy

resin ("pofix, truers, ?enmar/) was chosen to fill the graduated porosities inside the

alumina specimen under %arying pressures and was then cured at room temperature. $y

%arying the pressure, air ules were forced to lea%e the specimen and epoxy resin could

infiltrate the graduated porosities more easily. *he cycles of %arying pressure within a

range of -55 to -555 mar were re'uired until ensuring the amount of air ules was

minimised. *he next step was to lea%e the epoxy resin cured at room temperature for

around 2: hours. After curing, excess epoxy was remo%ed y polishing with coarse

sandpaper. Finally, the surfaces of specimen were ground and polished with diamond paste.

5.2 Microstructure Analysis*he graded composite eam specimens made from alumina#epoxy which were produced

using the ao%e procedure were selected randomly to section across the thic/ness direction

to examine the 'uality of the graded area. *hen the sectioned specimen was mounted y

epoxy resin efore grinding and polishing with sandpapers and diamond paste, respecti%ely.

For polishing with the diamond paste, the specimen was first polished using an automatic

polisher (4eco #5). *he used diamond paste has a diamond particle si9e of ! m which

was reduced to - m at the final step.

Images of the polished surfaces, otained y optical microscopy with a 3i/on 255

microscope and digital camera, were used to characterise the pure alumina and epoxy

phases o%er the graded region. ne such cross#section image was shown in Fig. .-5. A

measurement of areas, using the areal techni'ue, in the cross#section yielded the

percentages of material compositions in each graded region. *he top and ottom surfaces

were referred to as the pure alumina and epoxy resin layers, respecti%ely.


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Fig. .-- illustrates the whole material composition at the graded region of the specimen.

*his includes information aout percentages of alumina and epoxy resin in each layer

across the graded region.

As shown in Fig. .--+ the cross#section image, a few pores were found due to imperfect

epoxy infiltration. It was also seen that the %ariation of material compositions throughout

the cross#section presented in the form of layer or step changes rather than a continuouschange o%er the section. $y using the areal techni'ue, one can otain the percentages of

alumina and epoxy resin in each layer in the graded region. It is clearly seen that the top

layer was for a pure alumina and followed with (5> alumina#!5> epoxy), (:5> alumina#

75> epoxy) and (25> alumina#15> epoxy) layers, whereas, the ottom layer was for a

pure epoxy resin layer. According to the material compositions or material %olume fraction

throughout the cross#section, these were plotted and illustrated in Fig. .-2. *he area underthe

step graph as shown in Fig. .-2 matches the percentage of alumina (V c) the proportion

of epoxy resin is indicated y the complementary area of the graph.


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*he process of slip casting was further in%estigated. A microstructure analysis displayed

the result of an imperfectly graded composite specimen when the alumina slip was filled

into the compressed foam pieces with a high filling rate and too much slip. *he

microstructure of the imperfect specimen that was made y a high rate of alumina slip

infiltration is shown in Fig. .-!. It can e seen that there are a numer of uffer layers of

alumina across the specimen (seen in white colour layers). *his is due to the compressed

foamHs density eing much lighter than that of the slip. *herefore, the foam pieces easily

float on the slip etween layers so that the uffer layers are constructed. *he uffer layers

cause the prolem of porosities occurring inside the specimen, especially at the middle 9one

of the specimen. It is ecause the uffer layers ha%e loc/ed or sealed a way for the epoxy

resin to come into the lower 9one. *his can e rectified y filling the alumina slip into thefoam pieces with a slow filling rate. *he foam pieces were allowed to asor all of the slip

in the pre%ious infiltration efore more slip was added to complete the slip casting process.


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5.3 Alternatie technique of specimen fabrication*he principles and ideas of powder metallurgy and multi#step se'uential infiltration

techni'ues as presented in &hapter 2 are comined to propose an alternati%e techni'ue to

produce layered graded specimens. *he main ad%antage of the comined techni'ue is that it

does not re'uire the processes of slip casting and drying therefore, time ta/en to

manufacture specimens is greatly reduced. *he comined techni'ue utilises the fact thatsmall particles of mothalls can e used to mix with the ceramic powder in each layer with

different compositions in order to produce graded porosities. *he mothalls are urnt out at

low temperature during the sintering process this lea%es the ceramic specimen ha%ing

graded porosities. After sintering, the ceramic specimen is solidified and ready for the

process of polymer infiltration, conse'uently, the graded composite specimen made from

ceramic and polymer can e otained.

*his proposed idea of a comined techni'ue can e pro%en y manufacturing specimens

from the new process. Mothalls are selected to mix with ceramic powder as they can e

urnt out completely at around 255 o&. *he production process of the specimens using the

comined techni'ue egan with grinding mothalls to small particle si9es whose diameter

are around 5.#-.5 mm. *he 00.00> purity alumina powder (*aimicron *M#?A=, *aimei&hemical &o 4td, Eapan) was mixed with the ground mothalls in different ottles which

were desired to ha%e different material compositions. In this farication, the fi%e layer

composite specimens were re'uired with the percentages of materials in each layer eing

(-55 %ol.> alumina#5 %ol.> mothall) (25 %ol.> alumaina#15 %ol.> mothall) (:5 %ol.>

alumina#75 %ol.> mothall) and (5 %ol.> alumina#!5 %ol.> mothall). Dowe%er, the last

layer contains pure epoxy which can e created in the final step of polymer infiltration.

Dence, four ottles with different percentages of alumina powder and ground mothalls

were otained. *he alumina powder and ground mothalls with &M& onder powder were

then lended for e%ery ottle y using a all milling machine (tar Machinery ty 4td) as

shown in Fig. .-:. *he all milling machine was set to run for :# hours at 255 rpm in

order to otain a good mixture of the material compositions.


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After the materials in each ottle were mixed, they were rought to fill and form in a steel

die. Fig. .- (a) illustrates the steel die. Fig. .- () displays the press machine ("nerpac

-! series), which was operated at !5 /3 to compress materials in the steel die. For layered

graded specimens, it was noted that the mixed powders were stac/ed se'uentially layer y

layer with a layer#wise compositional distriution. *he first layer (at the ottom of the die)

was gi%en for pure alumina layer and followed with (25 %ol.> alumaina#15 %ol.>mothall) (:5 %ol.> alumina#75 %ol.> mothall) and (5 %ol.> alumina#!5 %ol.>

mothall), respecti%ely.

*he green compacted specimen was remo%ed from the die slowly with a small application

of force through the compressing ar of the die in order to a%oid crac/s. *he greenspecimen was then sintered using the high temperature furnace which is shown in Fig. .0.

*he increasing temperature profile of this sintering was used on the same principle as

presented in Fig. .1. ?uring the sintering, the mothalls were urnt out. After sintering,

one can otain the alumina specimen ha%ing graded porosities. *o complete the farication

process, the graded porosities inside the alumina specimen were infiltrated y epoxy#resin("pofix,

truers, ?enmar/) with the aid of %arying pressure in the %acuum chamer as

shown in Fig. .-7.


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*he layered graded specimen made from alumina and epoxy using the comined techni'ue

was finalised in a similar fashion to the infiltration techni'ue y grinding and polishing the

surfaces. *he specimen was then sectioned to examine the 'uality of material gradients

across the graded direction using optical microscopy. It was found that the specimen

otained %ia the comined techni'ue had much more porosity than that produced y the

multi#step se'uential infiltration techni'ue. *o show the comined techni'ue leading to

porosities inside the specimen, Fig. .- presents diagrams of layered graded specimen

farication %ia this techni'ue. *he diagrams can e used to explain the mechanisms of

graded construction in the specimen. *he process of layer preparation is shown in Fig. .-

(a). *he mixtures of ground mothalls and alumina powder are used to create layers

according to the desired layer#wise compositional distriution. *he sintering process leads

to mothalls eing urnt out and lea%es the alumina specimen with layers of graded

porosities. *he suse'uent step is to infiltrate epoxy into the graded porosities of thealumina

specimen as shown in Fig. .- (). It can e seen that the epoxy profile - cannot

pass into the following layer, ut the epoxy profile 2 can penetrate through the porosity

connection. Dowe%er, %ery few of the porosity connections are created inside the specimen

using the comined techni'ue. *herefore, there is much porosity left without epoxy

infiltration in the specimen as shown in Fig. .- ().


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?ue to few porosity connections, the final specimen produced from alumina and epoxyusing the comined techni'ue may ha%e a lot of porosities as seen in Fig. .-1. *he

specimen in this figure contains ! phases+ epoxy, alumina and porosity phases. nly the topand

ottom layers can e classified as the perfect layers which are the layers of pure epoxy

and pure alumina respecti%ely.

According to the diagrams of layered graded specimen farication, it can e concluded thatthe comined techni'ue which is de%eloped from the comination of ideas of powder

metallurgy and infiltration techni'ues is not suitale for manufacturing the layered graded


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specimen. *his is ecause the comined techni'ue cannot produce the interpenetratingnetwor/

(I3) structures which are %ery important to minimise the amount of porosity.

Dence, to produce the layered graded specimen made of alumina and epoxy, the multi#step

se'uential infiltration techni'ue as presented in ection .- is strongly recommended to e

used for producing such specimen. In Fig. .--, it is clearly seen that the interpenetratingnetwor/

(I3) structures are created in the specimen produced y the multi#step se'uentialinfiltration techni'ue and the numer of porosities is minimal. *hus, the layered graded

specimens faricated y the multi#step se'uential infiltration techni'ue are used for further

in%estigation of %iration testing in this research.

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FGM Fabrication