J. ZYGMUNTOWICZ et al.: METAL PARTICLES SIZE INFLUENCE ON GRADED STRUCTURE IN COMPOSITE Al2O3-Ni
537–541
METAL PARTICLES SIZE INFLUENCE ON GRADED STRUCTURE
IN COMPOSITE Al2O3-Ni
VPLIV VELIKOSTI KOVINSKIH DELCEV NA GRADIENTNO
STRUKTURO KOMPOZITA Al2O3-Ni
Justyna Zygmuntowicz, Aleksandra Miazga, Katarzyna Konopka,
Waldemar Kaszuwara
University of Technology, Faculty of Materials Science and Engineering, Woloska St. 141, 02-507 Warsaw, Poland
justyna.zygmuntowicz@inmat.pw.edu.pl
Prejem rokopisa – received: 2015-06-15; sprejem za objavo – accepted for publication: 2015-07-28
doi:10.17222/mit.2015.120
The aim of this study was to investigate the effect of the nickel particle size on the changes in metallic phase content in the
graded structure in the Al2O3-Ni composites. Centrifugal slip casting was chosen as the method of composite fabrication. This
method allows the creation of a graded distribution of Ni particles in the hollow cylinder composite sample. Functional graded
materials were prepared in the vertical rotation axis. In the experiments the following powders were used: -Al2O3 TM-DAR
from Taimei Chemicals (Japan) of an average particle size 0.133 μm and density 3.96 g/cm3 and Ni powders from
Sigma-Aldrich of average particle sizes 3 μm and 8.5 μm. Aqueous slurries containing alumina (50 % of volume fractions of solid
phase volume content) and nickel powders (10 % of volume fractions) were tested. Deflocculates diammonium citrate (p.a.,
Aldrich) and citric acid (p.a., POCH Gliwice) were also added. Final sintering was conducted on all the specimens at 1400 °C in
a reducing atmosphere (N2/H2). The obtained samples were characterized by X-ray diffraction (XRD) and scanning electron
microscopy (SEM). In addition, quantitative analyses of the Ni particles distribution were made. In the fabricated samples the
graded structure were characterized by zones with different Ni particles concentration. The size of the Ni particles influences the
width of these zones. Vickers indentation was used to determine the hardness of the materials.
Keywords: functionally graded material (FGM), centrifugal slip casting (CSC), Al2O3-Ni system
Namen {tudije je bil preiskati vpliv velikosti delcev niklja na spreminjanje vsebnosti kovinske faze v gradientni strukturi
kompozita Al2O3-Ni. Centrifugalno oblikovalno ulivanje je bilo izbrano kot metoda za izdelavo kompozita. Ta metoda omogo~a
stopenjsko razporeditev delcev Ni v votlem cilindri~nem kompozitnem vzorcu. Funkcionalno razporejen material je bil izdelan
na vertikalni rotacijski osi. Za preizkuse so bili uporabljeni naslednji prahovi: -Al2O3 TM-DAR iz Taimei Chemicals (Japan) s
povpre~no velikostjo delcev 0.133 μm in gostoto 3.96 g/cm3 ter prah Ni iz Sigma-Aldrich, s povpre~no velikostjo delcev 3 μm in
8.5 μm. Preizku{ena je bila na vodi osnovana me{anica (z vsebnostjo 50 % volumenskega dele`a trdne faze), ki je vsebovala
prah glinice in niklja (10 % volumenskega dele`a). Uporabljeni deflokulant je bil sestavljen iz diamonium citrata (p.a., Aldrich)
in citronske kisline (p.a., POCH Gliwice). Kon~no sintranje je bilo pri vseh vzorcih na 1400 °C, v reduktivni atmosferi (N2/H2).
Dobljeni vzorci so bili pregledani z rentgensko difrakcijo (XRD) in vrsti~no elektronsko mikroskopijo (SEM). Poleg tega je bila
izvedena tudi kvantitativna analiza razporeditve Ni delcev. V vzorcih je bila analizirana gradientna struktura po podro~jih z
razli~no koncentracijo Ni delcev. Velikost Ni delcev vpliva na {irino teh podro~ij. Dolo~ena je bila trdota materiala po Vickersu.
Klju~ne besede: funkcionalno gradientni material (FGM), centrifugalno oblikovalno ulivanje (CSC), Al2O3-Ni sistem
1 INTRODUCTION
Novel ceramic-metal composites should have a com-
bination of properties such as good strength, high
hardness together with high fracture toughness, wear and
thermal resistance as well as chemical inertness, among
others. Such demands may be fulfilled by functional gra-
ded materials (FGM). These materials are characterized
by a variation in composition and structure gradually
over volume, resulting in corresponding changes in the
chemical and physical properties of the composite.1,2
The concept of graded materials was shown for the
first time in 1971 in an article entitled "Preliminary work
on Functionally Graded Materials".3 These materials can
be prepared by a variety of methods. Currently, among
the most popular techniques for producing FGM are
powder technology methods, inter alia: dry powder com-
paction,3,4 tape casting,1,3,5–7 self-propagating high – tem-
perature synthesis – SHS,7,8 slip casting and filtra-
tion.1,9–13 However, other in–situ techniques such as:
spray forming,14,15 centrifugal casting1,16–19 and the depo-
sition methods of Electrophoretic Deposition (EDP)20–26
and Pulsed Laser Deposition (PLD)27,28 have also gained
broad attention.
Ceramic-metal composites with a gradient concen-
tration of the metal particles are an example of FGM
materials. Typical scheme of ceramic-metal FGM com-
posites is shown in Figure 1. Such composites can be
used as functional materials, and also as structural
materials in the aerospace industry.
The principal advantage of ceramic matrix compo-
sites with metal particle concentration gradients is an
increase of the fracture toughness with respect to the
ceramic matrix.29-31 The literature data indicate that the
particle size and amount of the metal phase essentially
affect crack propagation.32 Modification of the particle
Materiali in tehnologije / Materials and technology 50 (2016) 4, 537–541 537
UDK 66.017:669.71:669.24 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(4)537(2016)
size of metal in the composite enables precise control of
the material properties.
In the present work, Al2O3-Ni composites with a
concentration gradient of the metal particles were
fabricated using centrifugal slip casting. This method
allows fabrication of a graded distribution of Ni particles
in a hollow cylinder composite sample. The aim of this
study was to investigate the effect of the nickel particles
size on the metallic phase content in graded Al2O3-Ni
composites.
2 EXPERIMENTAL PART
2.1 Materials and methods
In the tests the following powders were used:
-Al2O3 TM-DAR from Taimei Chemicals (Japan) of an
average particle size D50 = 0.133 μm and density 3.96
g/cm3 and Ni powders from Sigma-Aldrich of average
particle sizes D50 = 3 μm and D50 = 8.5 μm. For both Ni
particle sizes a series of composite samples were
prepared. Aqueous slurries containing alumina (with 50
% of volume fractions content of solid phase) and nickel
powder (10 % of volume fractions with respect to total
volume) were tested. Deflocculates diammonium citrate
(p.a., Aldrich) and citric acid (p.a., POCH Gliwice) were
also added. Figure 2 shows the scanning electron micro-
scopy images of -Al2O3 and the two Ni powders.
The components were homogenized in a planetary
mill with a rotation speed of 300 min–1 for 90 min.
Afterwards, the air absorbed on the particle surfaces was
removed in a THINKY ARE-250 Mixer and Degassing
Machine for 15 min at a speed of 900 min–1. The equip-
ment allows the removal of bubbles above 1μm. The
ceramic water-based slurries were cast into thick-walled
tubes using a plaster mold. A stirrer with a vertical
rotation axis was used in the centrifugal slip casting
process. The process parameters were first chosen by set
of trials. The dimensions of the fabricated tubes are as
follows: the outer radius is 20 mm, the length 40 mm and
thickness 18 mm. Thereafter, the samples were dried and
removed from the plaster mold. The final step was sint-
ering which was conducted on all specimens at 1400 °C
in a reducing atmosphere (N2/H2).
An X-ray Rigaku MiniFlex X-ray diffractometer II
was used to study the structure of the composites. The
data were recorded using the "step-scanning" method in
the 2 mode with Cu-K1.54 radiation.
The Al2O3-Ni composites microstructures were
characterized using a SEM HITACHI SU-70 scanning
J. ZYGMUNTOWICZ et al.: METAL PARTICLES SIZE INFLUENCE ON GRADED STRUCTURE IN COMPOSITE Al2O3-Ni
538 Materiali in tehnologije / Materials and technology 50 (2016) 4, 537–541
Figure 2: Electron micrographs of: a) -Al2O3, b) Ni powder, (D50 =
3 μm), c) Ni powder (D50 = 8.5 μm)
Slika 2: SEM posnetki: a) -Al2O3, b) prah Ni (D50 = 3 μm), c) prah
Ni (D50 = 8.5 μm)
Figure 1: Schema of ceramic-metal FGM composite, dark-grey –
metal particles
Slika 1: Shema kompozita keramika-kovina FGM, temno sivo – delci
kovine
electron microscope. Quantitative analysis of the graded
region microstructures from SEM images was carried out
using the Micrometer computer image analysis pro-
gram.33
The hardness of the microstructures from outer sam-
ple to inner sample were measured with a Vickers hard-
ness tester. The hardness measurements were made
under a load of 49.03 N.
3 RESULTS AND DISCUSSION
Figure 3 shows typical examples of fabricated tubes
before and after sintering. Using the Archimedes
method, it was found that the average sintering shrinkage
was about 13 %. A tube having a post-sintering diameter
of 17 mm, inner radius 6.5 mm and 40 mm could be
fabricated successfully (i.e. without breakage of the cast
tubes during subsequent drying and sintering) with a
relatively high consolidation (> 98.8 % of relative
density). No damage in the form of cracks or voids on
the surface of samples were noticed. The X-ray data
from the surfaces and the cross-sections of composites
confirmed the presence of the two phases Ni and Al2O3.
Figure 4 presents an typical diffraction pattern.
In Figure 5 the graded distribution of metal particles
in Al2O3-Ni composites is shown. The grey area is Al2O3
and the bright area is Ni. Three zones of Ni particles of
the samples can be distinguished in the cross-section. A
quantitative analysis of the photomicrographs using the
Micrometer computer image analysis program33,34 yield-
ed the compositional profile variations shown in Figure
6. The measurements show that in area A, the nickel
particle content was equal to 12 % of volume fractions
per 1 mm width in both samples. Between areas A and B
there is a mild increase in nickel particles. In area B
there was a maximum of nickel particles in both
composites. In the case of the nickel powder with the
larger particle size (D50 = 8.5 μm) it was observed that
zone B is narrower than zone A. In area B there was a
maximum of nickel particles equal to about 28 % of vol-
ume fractions per 560 μm wide for the sample prepared
using Ni D50 = 8.5 μm powder. In contrast, the sample
prepared using a Ni powder (D50 = 3 μm) was contained
25 % of volume fractions per 840 μm. Then there was a
sharp decline in nickel particles. However, in area C
there was a mild decrease, down to 0 % of volume
fractions, in the percentage of nickel particles.
The motion of metal particles in a slurry under cen-
trifugal force can be determined by Stokes’ law.35
According to this law the velocity of the particles is
proportional to the square of the particles’ diameter.
Therefore the migration distance is greater in the case of
larger particles. For this reason the width of zone B is
smaller in the case of samples prepared with 8.5 μm Ni
than for composites with 3 μm Ni powder.
The hardness values measured from the outer to the
inner periphery are shown in Figure 7. It has been found
that the hardness profiles for both series (3 μm and 8.5
μm Ni powders) have similar behaviour (Figure 8). In
region A hardness values in the range 1000-1300 HV are
found for both series. Region A results from the removal
J. ZYGMUNTOWICZ et al.: METAL PARTICLES SIZE INFLUENCE ON GRADED STRUCTURE IN COMPOSITE Al2O3-Ni
Materiali in tehnologije / Materials and technology 50 (2016) 4, 537–541 539
Figure 5: SEM photo of cross-section of composite: a) the sample
prepared using a Ni powder (D50 = 3 μm) and b) the sample prepared
using a Ni powder (D50 = 8.5 μm)
Slika 5: SEM-posnetek preseka kompozita: a) vzorec, pripravljen z
uporabo prahu Ni (D50 = 3 μm) in b) vzorca, pripravljenega z uporabo
prahu Ni (D50 = 8,5 μm)
Figure 3: Views of composite sample prepared using a Ni powder
(D50 = 8.5 μm) before and after sintering
Slika 3: Izgled kompozitnega vzorca, pripravljenega z uporabo prahu
Ni (D50 = 8,5 μm), pred in po sintranju
Figure 4: Diffraction patterns of the sample prepared using a Ni
powder (D50 = 8.5 μm)
Slika 4: Rentgenogram vzorca, izdelanega z uporabo prahu Ni (D50 =
8,5 μm)
of fluid through capillary action in the plaster mold. In
Figure 7 a slightly lower hardness is observed in the area
between A and B due to the increase of nickel particles
in the composite. The maximum amount of nickel parti-
cles in the region B corresponds to the lowest hardness
values. This part of the sample was produced as a result
of centrifugal acceleration. As expected, in both series of
samples the maximum hardness values are observed in
region C, at the inner edge of the casting due to the
absence of nickel particles. The area C in both samples
corresponds to hardness values in the range 1800-1920
HV.
4 CONCLUSIONS
Al2O3-Ni FGM ceramic matrix composites with a
graded distribution of Ni particles have been successfully
produced by the centrifugal slip casting method.
Quantitative analysis of the graded microstructure in
the composites revealed that the graded zones depend on
the size of the starting metal particles in the slurry.
By changing the content of the metallic phase it is
possible to control the hardness profile. As a result of the
centrifugal slip casting method used, the packing of the
powder particles prevents grain growth.
Acknowledgments
The authors would like to thank Professor M. Szafran
and his Team from the Faculty of Chemistry of Warsaw
University of Technology for help in preparing the sam-
ples. The results presented in this paper were obtained as
part of the the Polish National Science Centre (NCN)
project No. 2013/11/B/ST8/0029.
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540 Materiali in tehnologije / Materials and technology 50 (2016) 4, 537–541
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