Al2O3/Ni functionally graded materials (FGM) obtained by centrifugal-slip casting method
J Therm Anal Calorim
Al2O3/Ni functionally graded materials (FGM) obtained by centrifugal-slip casting method
Justyna Zygmuntowicz 0 1 2
Paulina Wiecin´ ska 0 1 2
Aleksandra Miazga 0 1 2
Katarzyna Konopka 0 1 2
Waldemar Kaszuwara 0 1 2
0 Faculty of Chemistry, Warsaw University of Technology , 3 Noakowskiego St, 00-664 Warsaw , Poland
1 Faculty of Materials Science and Engineering, Warsaw University of Technology , 141 Woloska St, 02-507 Warsaw , Poland
2 & Justyna Zygmuntowicz
The fabrication and the characterization of Al2O3/Ni composites with a gradient distribution of the Ni particles are reported. The composites have been obtained by centrifugal-slip casting and subsequent sintering and had the shape of a hollow cylinder. TG/DTA analysis was done for the nickel powder with the addition of the dispersant used in centrifugal-slip casting as well as for the composite green body. The measurements were performed in two atmospheres: argon and the mixture of argon and hydrogen (1:1). Additionally, coupling the thermobalance with the mass spectrometer allowed to determine type of gases released from the samples during thermal treatment. The morphology and chemical composition of the produced composites were analyzed using a scanning electron microscope equipped with an EDS detector. Interface between alumina and nickel was described. Moreover, the X-ray diffraction was made. The stereological analysis confirmed that the nickel particles are distributed in the composite in a gradient way.
Centrifugal-slip casting; Composite; Reductive atmosphere; Mass spectrometry
Functionally graded materials (FGM) are a novel group of
materials characterized by continuous spatial distribution
of two or more components. Because of special properties
of materials from this group, they are becoming more
widely studied [
]. In particular, ceramic–metal FGM
composites, for example Al2O3/Ni, have advantageous
properties such as effective thermal stress relaxation, high
hardness, high fracture toughness and good corrosion
resistance. They can be used in many applications ranging
from aerospace to chemical industry [
methods have been proposed to obtain gradient distribution of
metallic particles in a ceramic matrix, for example slip
casting with magnetic field or the powder metallurgical
process applied for: Al2O3/Fe, SiC-AlN/Mo [
have obtained FGM samples by centrifugal-slip casting
which is an innovative, relatively simple and effective
shaping method . A combination of classical slip casting
with centrifugal force for alumina/nickel system allows to
receive variable distribution of the metallic particles in a
composite material. The gradient is obtained due to the
different speeds in liquid medium of particles with
different density [
]. The technology of the preparation of
composites with a zonal distribution of the metallic phase
by centrifugal-slip casting can be used for producing
composite sleeve-shaped parts with a metallic phase
concentration gradient (intended, e.g., for transporting a toxic
medium) or parts with increased mechanical strength. The
prepared suspensions for centrifugal-slip casting process
contain the mixture of ceramic and metallic powders,
solvent and deflocculants. It is important that used additives
are non-toxic and harmless to the environment during the
sintering process. For this reason, as a solvent in most cases
the distilled water is used [
]. In aqueous media, nickel
powder can undergo hydroxylation and the thin nickel
oxide layer present on its surface can be converted to
Ni(OH)2 or NiO(OH) [
]. In the case of the application of
metallic powder with micrometric size, the amount of
formed compounds is minimal and should be reduced
during the sintering process. It is worth to underline that
the resulting sintered bodies have a high relative density
and additionally strong connection between the
components of the composite is observed. The quality of the
fabricated elements strongly depends on sintering process.
In the case of alumina-nickel composites, it is preferable to
use a reductive atmosphere in order to prevent oxidation of
the nickel surface, remove the compounds formed on the
nickel particles surface and prevent its reaction with
alumina leading to the formation of nickel aluminate spinel
]. Moreover, it is significant that organic additives are
completely burned out and will not affect the
microstructure of the fabricated composite materials. In the literature,
there are several researches on the sintering of
ceramicbased materials in air or protective gases such as argon
]. There is little research on the analysis of the sintering
process conducted in reducing atmosphere. For this reason,
DTA/TG analysis gives important information about
thermal degradation of additives used in shaping process as
well as about the thermal behavior of samples in the inert
and reductive atmospheres [
]. Furthermore, coupling
mass spectrometer with thermobalance allows to detect
type of gases released from samples. These studies allow to
effectively plan the sintering process of the material.
Alumina powder (Taimicron TM-DAR, Taimei Chemicals
Co., Japan) with a mean particles size of 133 ± 30 nm and
density 3.96 g cm-3, and nickel powder (Sigma-Aldrich,
Poland) with a mean particles size of 3 lm and density
8.9 g cm-3 were used for the preparation of the
suspensions. The purity of both powders was 99.99%.
Diammonium hydrocitrate (DAC, puriss, POCh, Poland) and citric
acid (CA, C99.5% Sigma-Aldrich, Poland) were used as
dispersants in the ceramic slurries in an amount 0.3 and
0.1 mass% with respect to powders content, respectively.
The selection of dispersant was made based on previous
The aqueous ceramic slurries containing 50 vol% of the
solid phase and including 10 vol% of the nickel particles
were made. The slurries were prepared by adding the
alumina and nickel powders to the water with deflocculants
and milling the mixture in a planetary ball mill with
rotational speed of 300 rpm for 60 min. The aqueous
suspensions were poured into a gypsum mold with the inner
diameter of 20 mm. Then, the tubular mold was centrifuged
in the radial direction with speed of 1000 rpm for 3 h. After
the centrifugation, the sample together with the gypsum
mold was removed from the metal mold and was dried in
the vertical position in a dryer at 25 C for 24 h. The
dimensions of the gypsum mold were: the outer radius
-40 mm, the thickness -10 mm, the length -60 mm and
the inner radius -20 mm. The dried sample can be easily
removed from the gypsum mold, thanks to the drying
shrinkage. Then, the sample was sintered at 1400 C in H2/
Ar atmosphere (Ar of 80 vol% and balance H2). During the
sintering, the heating and cooling rate were 5 C min-1.
This process allowed to obtain composites in the shape of a
hollow cylinder with gradient concentration of the nickel
particles. Drying shrinkage in the case of centrifugal-slip
casting is very low, negligible and therefore has not been
determined. The sintering linear shrinkage equaled 12.84%.
The thermal analysis has been done for the nickel
powder with the addition of the dispersants DAC and CA
used in the preparation of Al2O3/Ni composites as well as
for the obtained Al2O3/Ni green body. The purpose of the
addition of the dispersants was to create the conditions
similar to those in the suspensions used to prepare
composite samples by centrifugal-slip casting. DTA/TG
measurements were carried out by using Netzsch STA 449C
coupled with Quadrupole Mass Spectrometer Netzsch
QMS 403C. The heating rate was 10 C min-1, and the
final temperature was 550 C. The measurements were
performed in two atmospheres: argon and the mixture of
argon and hydrogen (1:1), and the total gas flow was
100 mL min-1. The construction of the apparatus did not
allow to perform the measurements with the high
concentration of the hydrogen to higher temperatures due to the
possible corrosion of platinum thermocouples. Mass
spectrometer was set to detect m/z values in the range of
10–300. The research could show the possible presence of
the oxidized form on the surface of nickel powder and the
influence of the metal on the thermal characteristic of the
Al2O3/Ni green body.
Samples for microstructure analysis were cut using a
diamond wheel by precision cut-off machine Secotom 15
(Struers). The observed surface was prepared by using
standard metallographic methods (grinding and polishing
with diamond paste up to 1 lm). The microstructure of the
cross section of sintered samples was examined by the
scanning electron microscopes Hitachi S-3500N and
SU70. The elemental distribution of nickel, aluminum and
oxygen in the cross section of the samples was analyzed by
energy-dispersive X-ray spectroscopy (EDS). The EDS
scan was taken at 15 kV.
The interface between alumina and nickel was observed
on SEM/STEM HITACHI S 5500.
The analysis of the phase composition of obtained
materials was performed using X-ray diffractometer
Ni – Ar
Ni – H2/Ar (1:1)
Rigaku MiniFlex II. CuKa1.54 was used with the value of
the angle 10 –80 and a step size of 1 . The analyses were
performed at the cross section of samples. For the
identification of the component phases, crystallographic database
ICCDD-PDF4-2014 has been used. The XRD patterns
were recorded for two samples for reproducible results.
The XRD study was performed on the radius of
The physical properties and the density of sintered
bodies were determined by the Archimedes method
according to the PN-76/E-06307. The relative densities of
composite bodies were calculated using the rule of mixture.
The values 3.96 and 8.9 g cm-3 as the theoretical densities
of alumina and nickel, respectively, were used.
Quantitative description of the microstructure was made
on the basis of SEM images of randomly selected areas on
Ni – H2/Ar (1:1)
the samples using computer image analysis applying the
program Micrometer [
]. This method allows obtaining
information about the actual size and distribution of
metallic phases in the sample. SEM analysis of images
included: image processing, measurements and
interpretation of obtained results. Microstructure observations were
performed using magnification 10009. The average values
were calculated from measurement of 50 images. The
nickel particles were described by parameter d2—diameter
of circle of the same surface as the surface of the analyzed
grain [lm]. Based on the results of stereological analysis,
average values of shape factors characterizing metal
Metal particles Zone III Zone II Zone I
particles have been determined: elongation (a = dmax/d2),
surface development (R = p/(p d2)) and convexity
(W = p/pC), where dmax—maximum diameter of particle
projection [lm], p—perimeter of particle [lm],
pC—Cauchy perimeter [lm] [
Results and discussion
Figure 1 presents DTA/TG/DTG curves of nickel powder
with the addition of the dispersants DAC and CA. The total
mass loss in the case of the sample measured in H2/Ar (1:1)
atmosphere was 0.33%, while for sample measured only in
Ar—0.20%. The mass loss can be ascribed to the
decomposition of the dispersants from the powder surface. On the
other hand, the higher mass loss in the case of the second
sample may indicate on the presence of the oxidized form
on the surface of the nickel powder which has been reduced
in hydrogen. Additionally, the changes on the TG curve in
the case of the sample measured in H2/Ar are observed
since the beginning of the measurement, while for the
sample measured in Ar, no changes on TG are visible till
ca. 250 C. More information can be gained from the
results from the mass spectrometer coupled with the
thermobalance, shown in Fig. 3 and discussed later in the text.
Fig. 6 The areas selected to EDS measurements
Figure 2 presents DTA/TG/DTG curves of Al2O3/Ni
green body in which the concentration of the nickel was 10
vol%. Samples have been obtained by centrifugal-slip
casting. The total mass loss was ca. 0.70% both for the
sample measured in Ar and H2/Ar (1:1). It indicates that in
the case of the composite material no significant influence
of the nickel on the thermal behavior of the material is
observed. The mass loss indicates on the thermal
decomposition of the organic additives and sample dehydration.
Mass spectrometer has detected six m/z values: 15, 16,
17, 18, 32 and 44. Masses 17 and 18 can be ascribed to
H2O, and mass 44 corresponds to CO2. The presence of O2
released from the samples could be confirmed by the
increase of the intensities of masses 32 and 16, on the other
hand, masses 15 and 16 having similar intensity can
indicate on the presence of CH4. Analyzing the data obtained
for nickel measured in argon (Fig. 3a), it can be concluded
that the only gaseous product which is released from the
sample is CO2 which comes from the decomposition of the
dispersants. The situation is different in the case of the
measurement carried out in the reductive atmosphere that is
H2/Ar (1:1). The increase of the intensities of masses 15
and 16 may indicate on methane which comes from the
decomposition of organic dispersants, while the increase of
the intensities of masses 17 and 18 can be ascribed to the
dehydration of the sample. There is also the possibility that
the nickel oxide which can be located on the nickel surface
has been reduced to some substances, but due to the
reaction with other gases the final compounds present in
the gaseous mixture are H2O, CH4 and CO2 (Fig. 3b). The
similar conclusions can be drawn for the Al2O3/Ni samples,
except that the increase of the masses 17 and 18 is observed
for the measurements carried out both in argon and the
mixture of hydrogen and argon. This can be explained by
the higher amount of water present in the green bodies as a
result of the shaping process (Fig. 3c, d). The most
important similarity comparing the samples measured in Ar
and H2/Ar is that in the case of the measurements carried
out in the inert atmosphere predominant gaseous product is
CO2, while in the reductive atmosphere it is probably CH4.
Oxygen is not observed as the gas reaching QMS detector.
Area 1 Area 2 Area 3
The XRD pattern of the prepared samples is given in
Fig. 4. The pattern corresponds to a sample sintered at
1400 C. The X-ray diffraction patterns showed no
reflections other than those due to alumina and nickel.
There was no nickel aluminate spinel phase (NiAl2O4) in
the samples after sintering. The absence of the NiAl2O4
spinel phase peaks indicates that nickel has not reacted
with alumina into nickel aluminate spinel. The reductive
atmosphere (H2/Ar) used during the sintering allowed to
avoid the formation of the NiAl2O4 spinel phase which
frequently appears in such processes [
The results of the measurements of selected physical
properties for sintered samples are shown in Table 1. It can
be noticed that samples are described by relative density
equal to 99.33%. It means that the prepared material is
characterized by high degree of packing of the grains in the
sintered body. Such high value of relative density was
achieved through the use of innovative manufacturing
method. The use of centrifugal force allowed to obtain a
high density in the green state which facilitated the process
of sintering. The linear shrinkage of the sintered samples
The typical microstructure of the Al2O3/Ni composite
with different zones of the metal particles concentration is
presented in Fig. 5. In the microstructure, the areas with
dark contrast represent the Al2O3 matrix and with light
contrast represent the Ni particles. The results of the
observation of the microstructure showed that the obtained
samples are characterized by a gradient distribution of the
metallic phase throughout the surface of the sample. The
changes in microstructure are represented by three zones:
from the outer surface toward inner side of the hollow
cylinder. It was found that the outer part of the graded
region was formed as a result of removing water by the
capillary forces active in the gypsum mold. The maximum
concentration of the metal particles was observed in the
central region of the composites. This part of the sample
was formed due to the centrifugal acceleration combined
with the capillary action. The metal content decreases
continuously from a maximum value to zero in the inner
part of samples. The own results of research showed the
characteristic sharp transitions between zones in obtained
Furthermore, by using the EDS technique, the gradient
distribution of Ni particles was confirmed. Figure 6 shows
the areas selected to EDS measurements. The research
consists in finding the chemical composition in three
regions along the gradation direction. The results of the
concentration of nickel, alumina and oxygen in composite
have been collected in Table 2. Deficit of oxygen in all
zones is due to the large error of measurement of light
elements like oxygen what is typical for EDS method.
The microstructure of the Al2O3/Ni interface is
presented in Fig. 7. The observation confirmed that at the
phase boundary there are no cracks and other defects.
Moreover, there are no new phases at interfaces in the
Figure 8 shows the histograms of nickel particles size
distribution. It was found that histograms have unimodal
character for each zone. The analysis of histograms showed
in each zone the maximum frequency of metallic particles
incidence with an average size 2–3 lm. This value is
close to the size of starting nickel powder. It can be
concluded that in the obtained composite there are no
The stereological analysis indicated that the metal
particles in all zones had the similar oval shape. This is
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 4
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 01 11 12 13 14 14
evidenced by the values of shape parameters (the curvature
of grain boundary, convexity presented in Table 3). These
values are close to one.
Fabrication of Al2O3/Ni gradient material by
centrifugalslip casting was done in this study. The microstructure of
the outer surface revealed the presence of particles as result
of removing water by the capillary forces active in the
gypsum mold. The maximum concentration of the metallic
particles was observed in the middle part of sample due to
the centrifugal acceleration combined with the capillary
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 4
1 1 1 1 1 1
action. In the inner part of samples, it was observed that the
nickel content decreases continuously from a maximum
value to zero.
Thermal analysis carried out in the inert atmosphere
that is argon and in the reductive atmosphere that is the
mixture of hydrogen and argon (1:1) revealed the
differences in the total mass loss of the nickel powder what may
indicate on the presence of the oxidized formed on the
nickel surface. The data obtained from the mass
spectrometer coupled with the thermobalance reveal that the
predominant gaseous products released from the samples
during thermal treatment are H2O, CO2 and probably
CH4 in the case of the measurements carried out in
Acknowledgements The results presented in this paper were
obtained within the project from the Polish National Science Centre
(NCN), Agreement No. 2013/11/B/ST8/0029.
Open Access This article is distributed under the terms of the
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