20110405 SS-Al2O3 FGM

Fabrication of crack-free SUS316L/Al2O3 functionally graded
materials by spark plasma sintering
M. Radwan • M. Nygren • K. Flodstro¨m •
S. Esmaelzadeh
Received: 15 October 2010 / Accepted: 5 April 2011 / Published online: 12 April 2011
Ó Springer Science+Business Media, LLC 2011
Abstract This paper describes the preparation of crack-
free stainless steel 316L/alumina functionally graded
material (SUS316L/Al2O3 FGM) by spark plasma sintering
(SPS). The sintering of individual powders and their
composites was first studied to find an optimum SPS con-
dition to get both materials and their mixtures highly dense.
Sintering of cylindrical-shaped FGM pellets (/20 mm)
with various numbers of interlayers of 1, 2, 3, 4, and 9 was
then carried out where the kind of and location of related
cracking were carefully examined. The FGMs faced the
problem of cracking mainly in the radial direction near
the Al2O3-rich layers regardless of the number of layers or
the gradient compositional profile. An yttrium-stabilized
zirconia (3Y-ZrO2) was added thereafter to the gradient on
the expense of Al2O3 content (up to 50 vol.%) and no
radial cracking was found. Defects-free FGM pellets were
successfully produced by interposing 19 interlayers
between the SUS316L and Al2O3 ends. The Vickers
hardness profile was determined throughout the cross-sec-
tion of sintered FGM and it was varying steadily through
the gradient from 2.0 GPa at the SUS316L surface to
14.4 GPa at the Al2O3 surface.
Introduction
The concept of functionally graded materials (FGMs) is an
ideal material design solution in engineering applications
that require innovative materials with hybrid properties
and/or functions which cannot be realized in a single
homogeneous material or composite, or that need a solution
to the problem of thermo-mechanical property mismatch at
the interface in a junction between different materials. The
general idea of graded materials was first proposed in 1972
[1, 2], while systematic development of the concept is
owed to the research programs in Japan in the middle of
1980s [3, 4]. In its simplest architecture, a FGM is a
structure or part that consists of one material on one side, a
second material on the other side, and an intermediate
region of graded composites whose composition changes
gradually from one material to the other. Two dissimilar
materials can thus be bonded in a structure where the
properties change gradually with the location. This gradient
is designed to obtain a thermal residual stress relaxation
through the smooth variation of the compositions, mor-
phologies, and properties along the structure. FGMs can be
fabricated by various processing methods such as physical–
chemical vapor depositions, thin sheet lamination, plasma
spraying, sol–gel methods, self-propagating high-tempera-
ture synthesis (SHS), powder metallurgy routes, and spark
plasma sintering (SPS) [5, 6]. The materials have been
successfully used in a variety of low- and high-technology
applications in transport systems, energy conversion sys-
tems, cutting and drilling tools, machine parts, semicon-
ductors, optics, and bio-systems [7–10].
The structures of metal/ceramic FGMs have received
much interest although ceramic/ceramic, metal/metal, and
polymer-based FGMs are also fabricated [6]. One of the
first metal/ceramic FGMs which was developed for
M. Radwan (&) Á M. Nygren Á S. Esmaelzadeh
Department of Material and Environmental Chemistry,
Arrhenius Laboratory, Stockholm University,
106 91 Stockholm, Sweden
e-mail: mohamed.radwan@mmk.su.se
M. Radwan Á K. Flodstro¨m Á S. Esmaelzadeh
Diamorph AB, Roslagstullsbacken 11,
106 91 Stockholm, Sweden
123
J Mater Sci (2011) 46:5807–5814
DOI 10.1007/s10853-011-5536-2

Japanese aerospace vehicle components was the austenitic
stainless steel 304/zirconia (SUS304/PSZ) FGM as a
thermal barrier material to bear the extreme heat flow
during re-entry with a function to withstand the tempera-
ture differences in the order of 1000 K in structural com-
ponents of about 10 mm thickness or less [7]. Similar
metal/ceramic FGMs have been fabricated for other areas
of applications, e.g., Ni/ZrO2, Cu/TiB2, Ni/TiC, WC/Co,
and Ti/HAp based FGMs [6, 7, 9]. Modeling to develop a
stainless steel/alumina (SUS/Al2O3) FGM system has been
studied over the last years by Grujicic and Zhao [11],
Matsuno et al. [12], Shinagawa [13], Pietrzak et al. [14],
and Liu et al. [15] as potential structural components
for applications in high wear resistance ceramic—heat
resisting stainless steel joints and tritium penetration bar-
riers (TPB). The effects of material properties, specimen
geometry, and gradient compositional profile on the
residual stress distributions during the cooling stage and
the crack formation especially in the Al2O3-rich region
have been well demonstrated. The SUS/Al2O3 FGM
system encounters two major problems; the large thermo-
mechanical property mismatch between the end compo-
nents and the brittleness of Al2O3. According to numerical
calculations of the residual stress concentration in some
critical Al2O3-rich regions within the FGM specimen, these
stresses could easily be in the same range as the bending or
tensile strength of A2O3 (250–275 MPa) [11, 14, 15],
which would make it difficult to fabricate FGM specimens
without material defects. Still, there has been a need for
systematic experimental studies to get close insight into
this material combination and to find a viable procedure
that could control the problem of residual stress-induced
cracking.
In this study, the possibility to prepare stainless steel/
Al2O3 FGMs has been evaluated. The experiments focused
first on sintering the mono materials of austenitic stainless
steel 316L (SUS316L) and Al2O3 as well as their com-
posites in order to explore optimum sintering parameters to
get the materials highly dense. Thereafter, sintering of
pellet-shaped FGMs has been carried out with careful
observation of the associated cracking problems. The
compositions of the graded interlayers were determined
using a modified rule-of-mixture power law equation
(proposed by the first author based on the basic formula
described in Refs. [11, 14, 15]) where the local volume
fraction of SUS316L, Vi, in each interlayer was calculated
as follows:
Vi ¼ 1 À i=n þ 1ð ÞPÂ Ã
ð1Þ
where i is the number of each layer, n is the total number of
interlayers, and P is a material concentration exponent
which shows how the concentration of the metal powder
gradually changes through the n interlayers. As will be
shown below, P and n were here varied, but no completely
crack-free FGM could be produced. In order to reduce the
thermal mismatch between the end components, the alu-
mina in the interlayers was replaced by a mixture of Al2O3
and yttrium-stabilized zirconia, ZrO2(3Y). Zirconia has
intermediate thermo-elastic properties between those of
SUS316L and Al2O3, implying that it can lower the
residual stresses, and simultaneously acts as an enforce-
ment component to strengthen the graded region.
The FGM pellets were fabricated by SPS, also called
field assisted sintering technology (FAST), which is a rel-
atively new sintering technique that allows consolidation of
numerous advanced materials such as gradient materials,
nano-structured materials, and composite materials [16].
The SPS process is a pressure assisted sintering technique
and the pressure mold, referred to as die, is heated by
allowing a pulsed DC current to pass through the mold,
implying that very rapid heating rates can be applied. It is
well known that short holding times and low sintering
temperatures can be used [17].
Experimental procedure
The materials used in this study were stainless steel type
Micro-MeltÒ
316L (Carpenter Powder Products Inc, USA),
ultrapure a-alumina powder (TM-DAR, Taimei Chemicals
Co., Ltd., Japan), and 3 mol.% Y2O3-doped zirconia
powder (TZ-3Y, Tosoh Corporation, Japan). The main
characteristics of the three powders are shown in Table 1.
Sinterability studies of the stainless steel and Al2O3
powders as well as their composites (20, 40, 60, and
80 wt% SUS316L) were first carried out to optimize the
SPS sintering parameters. Thereafter, a series of sintering
tests for pellet-shaped FGMs were performed. Appropriate
amounts of powders for the composites and the FGM in-
terlayers were manually mixed thoroughly to assure good
homogeneity. The sintering experiments were carried out
in vacuum in a SPS unit (SPS-5.40 MK-VI system, SPS
Syntex Inc, Japan). Cylindrical graphite dies (outer diam-
eter 50 mm, inner diameter 20.8 mm, height 50 mm, with
punches of the size /20 9 30 mm) were used. A graphite
sheet was used as a protecting layer between the inner wall
of the die and the powder. For the FGMs, the mixtures of
the layers were loaded in order, layer by layer into the
graphite die, beginning with the pure alumina. The com-
positions of the graded interlayers were determined as
described in Eq. 1.
Prior to the sintering, the samples were uniaxially
pressed for 30 s using a pressure of 60 MPa. After sinter-
ing, the dense pellets were polished by SiC paper to
remove the graphite residues. The relative densities of
sintered specimens, q, were measured in deionized water
5808 J Mater Sci (2011) 46:5807–5814
123

by the Archimedes method (European Standard EN 993-1)
and calculated as follows:
q ¼
m1
m3 À m2
:qw ð2Þ
where m1 is the weight of the sample determined in air, m2
is the weight of the sample completely immersed and sat-
urated in water, m3 is the weight of the saturated sample in
air, and qw is the density of deionized water defined for the
specific temperature.
The occurrence of surface cracks in the sintered FGM
pellets was examined visually and through optical
microscopy (Olympus SZx12 model, Olympus Optical Co.
Ltd., Japan). The microstructure of the sintered samples
was recorded in a field emission scanning electron
microscopy (FE-SEM, JSM-7000F, JEOL, Japan). One
successful FGM sample was cut and the cross-section
surface was polished with diamond slurries down to 1 lm.
A Vickers diamond indenter (Vickers-Armstrongs Engi-
neers Ltd., UK) was used to determine the hardness of the
individual interlayers using a load of 98.1 N. The sizes of
the indentations’ diagonals were determined using SEM
and the Vickers hardness was calculated according to the
following equation:
Hv ¼ 0:0018544 P=d2
À Á
ð3Þ
where Hv is the Vickers hardness (GPa), P is the load (N),
and d is the average length of the two diagonals of the
indentations (mm).
Results and discussion
Sinterability of stainless steel 316L and Al2O3 powders
In this set of experiments, the focus was to explore an
optimum SPS condition at which both SUS316L and Al2O3
could be made fully dense. Stainless steel can be sintered to
high relative density by SPS in the temperature range of
900–1000 °C [18, 19], while the minimum reported SPS
temperature to obtain fully dense Al2O3 was 1150 °C [20].
At first, the predetermined pressure was applied at room
temperature and the samples were heated from room
temperature to 600 °C in 3 min and then heated to the
predetermined sintering temperature. The samples were
thereafter held at this temperature for a certain holding
time before turning off the power. Disc-shaped samples
with a size of 20 mm in diameter and 4 mm in thickness
were sintered.
In the case of stainless steel, boron nitride (BN) powder
was placed as an insulating layer below and above the
stainless steel powder in order to direct the current to flow
mainly in the graphite die (not through the stainless steel
sample) thus avoiding local heating effects. Figure 1 shows
the effect of sintering temperature, holding time, and
applied pressure on the relative density of sintered SUS316
specimens. A density of [98.5% can be obtained at
1000–1100 °C with 5–10 min holding time under 50–75
MPa pressure. Above 1100 °C, the SUS316L starts melt-
ing. The SPS temperature was monitored and regulated by
an optical pyrometer focused on a small hole on the surface
of the die, however, as is well known, the actual temper-
ature of the sample is slightly higher than the measured
one.
The sintering of Al2O3 was then carried out at 1100 °C
under a pressure of 75 MPa and different holding times.
The effect of holding time on the relative density of sin-
tered Al2O3 samples is given in Fig. 2. Nearly fully dense
Al2O3 samples (98.6%) could be produced at 1100 °C
using a holding time of 30 min.
Sinterability of SUS316L–Al2O3 composites
A series of various compositions in steps of 20 wt%
SUS316L were sintered at 1100 °C for 30 min under
75 MPa. Figure 3 shows the relative densities of the
composites plotted as a function of the stainless steel
content. Initially the relative density decreases with the
increasing content of SUS316L, but around 40 wt%
SUS316L (or 25 vol.% SUS316L) it gradually increases
with increasing SUS316L. The microstructure of the
composite containing 25 vol.% S316L is shown in Fig. 4.
Figure 4a and b shows the microstructures of a polished
surface and a polished cross-section, respectively. The
content of stainless steel particles seems to be slightly
Table 1 Main characteristics of the starting powders used
Powder Name Average
particle
size (lm)
Density
(g/cc)
CTE (10-6
/°C) Chemical analysis (wt%)
Stainless steel 316L Micro-MeltÒ
316L 22 7.95 18.5 (649 °C) 0.03 C, 0.03 S, 0.045 P, 1.0 Si, 2.0 Mn,
2.0–3.0 Mo, 10.0–14.0 Ni, 16.0–18.0
Cr, 61.9–68.89 Fe
Alumina Taimicron TM-DAR 0.1 3.98 4 99.99% Al2O3
Zirconia Tosoh-Zirconia – 6.05 10 Contains 3 mol.% Y2O3
J Mater Sci (2011) 46:5807–5814 5809
123

higher at the surface than in the cross-section, and larger
islands of Al2O3 are found in the latter section. Figure 4c
shows the features of the Al2O3 matrix near a stainless steel
particle, where residual stresses occur upon cooling due to
the large mismatch in thermal expansion coefficients.
Micro-porosity as well as some voids in the Al2O3 matrix
and the stainless steel particles, respectively, can be
observed.
Sintering of SUS316L/Al2O3 FGMs
Based on the above experimental results, FGM pellets with
1, 2, 3, 4, and 9 interlayers (i.e., n = 1, 2, 3, 4, or 9) having
a linear compositional profile (P = 1) were sintered. The
SPS parameters were: pressure 75 MPa, heating rate 100°/
min, sintering temperature 1100 °C, and holding time
30 min. We found various interfacial defects in the sintered
FGMs such as delamination or fracture of alumina, or
radial cracking in gradient layers as seen in Fig. 5. In these
samples, the FGM containing 9 interlayers exhibited fewer
defects than the others.
The effect of composition profile through the interlayers
was studied by varying the value of the exponent P in
Eq. 1, setting P to 0.5, 0.75, 1.5, 1.8, 2.0, 3.0, and 4.0,
while keeping n = 9. For a given value of P, the volume
fractions of SUS316L and Al2O3 within each of the 9 in-
terlayers were calculated from Eq. 1. In all samples, the
thickness of the stainless steel layer was 2.0 mm, the
thickness of the Al2O3 layer was 1.0 mm and the thickness
of each interlayer was 1.0 mm. The results showed that the
cracking in the FGM pellets can be minimized, as shown in
Fig. 6. However, radial cracks were always found, located
mainly at the interfaces close to the composition of
30 vol.% SUS316L–70 vol.% Al2O3, i.e., in the composi-
tional area where the SUS316L–Al2O3 composite exhibited
lowest density, see Fig. 3. Microstructural observation of
the cross-section of one sintered FGM pellet with P = 1
and n = 9 is shown in Fig. 7. Radial cracks at the edge and
60
65
70
75
80
85
90
95
100
700 800 900 1000 1100
Relativedensity,%
SPS temperature, °C
(a)
• Pressure = 50 MPa
• Heating rate = 50 ºC/min
• Holding time = 5 min
92
94
96
98
100
0 2 4 6 8 10
Relativedensity,%
Holding time, min
(b)
• Temperature = 1050 ºC
• Pressure = 50 MPa
92
94
96
98
100
0 25 50 75
Relativedensity,%
SPS pressure, MPa
(c)
• Temperature = 1050 ºC
• Holding time = 5 min
Fig. 1 Relative density of SPSed SUS316L specimens as function of
temperature (a), holding time (b), and applied pressure (c)
65
70
75
80
85
90
95
100
0 5 10 15 20 25 30
Relativedensity,%
Holding time, min
Fig. 2 Relative density as a function of holding time for Al2O3
SPSed at 1100 °C using a heating rate of 100°/min and a pressure of
75 MPa
75
80
85
90
95
100
0 20 40 60 80 100
Relativedensity,%
SUS316L content, wt%
11.1
vol%
25.0
vol%
42.9
vol%
66.7
vol%
Fig. 3 Relative density of SPSed samples plotted versus the compo-
sition of sintered SUS316L–Al2O3 composites
5810 J Mater Sci (2011) 46:5807–5814
123

in the center of the sintered specimen were observed. The
crack width was larger near the edge than at the center of
the sample which is in agreement with the modeling of
residual stresses in Ref. [11].
The magnitudes of the residual stresses throughout the
FGM will depend on the extent of thermal strains that
occur both on a microstructure-level (at the particle–par-
ticle interface) and on a macrostructure-level (between
Fig. 4 SEM micrographs of 25 vol.%SUS316L–75 vol.%Al2O3 composite; a polished surface, b polished cross-section, and c features of Al2O3
matrix near a SUS316L particle
Fig. 5 Examples of cracking in sintered FGM specimens with various interlayers; a having 1 interlayer (the Al2O3 layer piled off), b having 4
interlayers, and c having 9 interlayers
Fig. 6 Optical photographs show radial cracking in SPSed FGM pellets having various P values
J Mater Sci (2011) 46:5807–5814 5811
123

adjacent layers) during the cooling as can be generally
described by the following basic equation:
r ¼ E Da DT ð4Þ
where r is the residual thermal stress (MPa), E is the
Young’s modulus (MPa), Da is the thermal expansion
mismatch (cm/cm/°C), and DT is the difference between
the sintering temperature and room temperature (°C).
According to Eq. 4, the best solution to reduce the
residual thermal stresses, r, lies in minimizing the thermal
expansion mismatch, Da, between the phases and the sin-
tering temperature, meanwhile improving the mechanical
toughness of the matrices especially in the composition
range where the maximum thermal stresses arise.
Commercial submicron or micron-sized Al2O3 powder
is usually sintered by SPS in the temperature range of
1400–1700 °C [21]. In this work, a commercial nano-sized
Al2O3 powder with high sinterability was chosen so that
using the sintering parameters given above (1100 °C/
30 min/75 MPa) yielded a compact with a relative density
of 98.6%. 1100 °C is the critical maximum temperature, as
the SUS316L starts melting above this temperature. The
DT value given in Eq. 4 is thus reduced to *1100 °C.
Fig. 7 SEM micrographs of the
radial crack in SPSed SUS316L/
Al2O3 FGM sample; a at edge,
and b in the center (n = 9 and
P = 1)
Fig. 8 Optical photographs
showing a the sintered
SUS316L/Al2O3 FGM sample,
b a top-view of the Al2O3 layer,
and c cross-section of the
multilayer structure
Fig. 9 The Vickers hardness proﬁle throughout the cross-section of
crack-free SUS316L/Al2O3 FGM pellet
5812 J Mater Sci (2011) 46:5807–5814
123

The thermal expansion coefficient of Al2O3 (aAl2O3
% 6
Â10À6
=
C) is much lower than that of SUS316L
(aSUS316L 18 9 10-6
/°C). Reducing the thermal
expansion mismatch, Da, and achieving reinforcement of
the Al2O3 layers were accomplished further by the addition
of adequate amounts of ZrO2(3Y) into the SUS316L–
Al2O3 composite layers. ZrO2(3Y) has an intermediate
coefficient of thermal expansion (aZrO2
% 10 Â 10À6
=
C),
large bending strength (*900 MPa) and high fracture
toughness (6–10 MPa m1/2
). In the ceramic-rich region, the
addition of ZrO2(3Y) reduces the thermal expansion mis-
match and improves the mechanical properties of the
composite layers. Thus, in a set of experiments, Al2O3 was
replaced by 25, 30, 40, 45, and 50 vol.% of ZrO2(3Y),
respectively, in all interlayers. The results showed that the
radial cracking decreased while increasing the zirconia
content in the interlayers. For ZrO2(3Y) contents C40
vol.%, no radial cracks were found. However, tiny axial
cracking developed in the pure alumina layer during pol-
ishing of the sample. This problem was solved by
increasing the number of interlayers to 19 layers. Photos of
the FGM pellet, the pure Al2O3 layer, and the cross-section
of the gradient are given in Fig. 8. No delamination or
internal cracking could be observed. The relative density of
that sintered FGM pellet was *95% as measured by the
Archimedes method.
The Vickers hardness profile along the cross-section of
the successful crack-free FGM is plotted as a function of
the distance in Fig. 9. The hardness values increase stea-
dily with decreasing stainless steel content. Figure 10
shows the crack patterns generated by the Vickers’
indentations on a monolithic Al2O3 sample and in the 19th
layer (adjacent to pure Al2O3 layer) close to the 18th layer.
The SEM images show that the length of the cracks parallel
to the interface, for the composite layer, was markedly
shorter than those normal to the interface, while, in the pure
Al2O3 the size of the cracks from the four corners was very
similar. This behavior is a direct evidence of the existence
of a compressive stress in the direction normal to the
interface. Under fairly high compressive stresses,
ZrO2(3Y) may withstand a transformation from a tetrago-
nal to a monoclinic modification which might enhance the
fracture toughness of the relevant layer according to a
transformation toughening mechanism as previously illus-
trated by other research groups [22–24].
Conclusions
A highly dense, crack-free, stainless steel type 316L, and
alumina FGM (SUS316L/Al2O3 FGM) was fabricated
successfully from powders by a simple SPS process. The
densification of mono materials as well as their composite
mixtures was first conducted, in order to get a good
understanding of the material system. The successful FGM
specimen, 20 mm in diameter and 21 mm high, consisted
of 19 graded interlayers having various fractions of
SUS316L, Al2O3, and yttrium-stabilized zirconia (ZrO2
(3Y)), placed between the pure SUS316L and Al2O3 end
surfaces. The success to fabricate crack-free SUS316L/
Al2O3 FGM pellets is believed to be attributed to the fol-
lowing reasons: (1) using a nano-Al2O3 powder that can be
sintered at a low temperature by SPS, (2) adding ZrO2(3Y)
into the graded interlayers as a toughening phase, and (3)
interposing a sufficient number of interlayers between the
pure SUS316L and Al2O3 layers to decrease the differences
in the thermal expansion mismatches between the adjacent
layers.
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indentation tests using a load of
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FGM
J Mater Sci (2011) 46:5807–5814 5813
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