A process has been developed to produce a crossed-lamellar-type microstructure in mullite combining tape casting, oriented lamination, and templated grain growth. Ceramic laminates were produced with aligned rod-like grains with the alignment direction varying from layer-to-layer with abrupt interfaces between layers. Other designed microstructures are also possible using this process

Faber, K. T.

Kaul, Vikram S.

Caltech Authors - Main

Journal of Ceramic Processing Research. Vol. 6, No. 3, pp. 218~222 (2005)
218
J O U R N A L O F
Ceramic
Processing Research
Synthetic crossed-lamellar microstructures in oxide ceramics
Vikram S. Kaul and K.T. Faber*
Department of Materials Science and Engineering, Robert R. McCormick School of Engineering and Applied Sciences,
Northwestern University, Evanston, IL, USA
A process has been developed to produce a crossed-lamellar-type microstructure in mullite combining tape casting, oriented
lamination, and templated grain growth. Ceramic laminates were produced with aligned rod-like grains with the alignment
direction varying from layer-to-layer with abrupt interfaces between layers. Other designed microstructures are also possible
using this process. 
Key words: mullite, tape casting, texture, templated grain growth, multilayer, x-ray diffraction.
Introduction
Many natural organisms produce ceramics such as
bone, tooth and shell that can sustain high stresses, like
those applied through impact or contact during wear
[1]. Nature provides complex and organized micro-
structures in these ceramics to create tougher and more
flaw-tolerant materials [2]. One specific example of an
organism that creates a natural ceramic material is the
giant pink Queen conch, Strombus gigas. The shell of
Strombus gigas is composed of over 99.9% aragonite
[2], arranged in a unique microstructure known as the
crossed-lamellar microstructure (Fig. 1). This hierarchical
microstructure consists of layers of ceramic lamellae
over several length scales. The lamellae in each layer
are aligned with the direction of alignment being
perpendicular to lamellae in adjacent layers [2]. Previ-
ous work has shown that, as a result of this ordered and
complex microstructure, this material exhibits enhanced
toughness and flaw tolerance [3]. The energy-dissipat-
ing mechanisms employed in this system include
multiple microcracking, crack bridging, crack blunting,
and grain pull out [4]. 
In an effort to take advantage of and replicate exist-
ing ceramic microstructural designs, a process has been
developed to create a modified version of the crossed-
lamellar microstructure in mullite. Mullite was the
chosen model ceramic material because of its aniso-
tropic grain growth properties and desirable strength
and creep resistance at high temperature [5]. Gönenli
and Messing have previously shown that rod-like
grains of mullite can be grown by seeding a mullite
matrix with aluminum borate whisker templates [6].
Although the process described here involves established
ceramic processing techniques of tape casting, oriented
lamination, and templated grain growth, this is the first
time that the three methods have been used in concert
to produce a crossed-lamellar microstructure. Tape
casting serves as a method for making thin ceramic
layers that can be stacked to form laminates. In
addition, shear forces created during tape casting can
be used to align anisotropic slurry particles in a single
direction [7]. This, coupled with templated grain growth,
provides a way to create a textured material with
aligned grains in a controllable direction. The process
is generic enough to fashion laminates with alignment
that can vary arbitrarily between layers. 
Experimental Procedure
Thin ceramic tapes used to produce the crossed-
lamellar microstructure in mullite were cast from aque-
ous slurries. The ceramic components of the laminates
were 82 vol.% 3Al
2
O
3
·2SiO
2
, (KM-101, Kyoritsu
Ceramics), 12 vol.% 9Al
2
O
3
·B
2
O
3 
whiskers (YS3a,
Shikoku Chemicals), and 6 vol.% TiO
2
 (Alfa Aesar).
TiO
2
 aids densification by serving as a liquid phase
former during sintering and enhances anisotropic grain
growth of mullite [8]. Densification to 92% of theore-
tical density is necessary before templated grain growth
can occur [9]. The ceramic components made up 38
*Corresponding author: 
Tel : +1-847-491-2444
Fax: +1-847-491-7820
E-mail: k-faber@northwestem.edu
Fig. 1. Schematic of the crossed-lamellar microstructure seen in
the shell of Strombus gigas, from Reference 4 (used with
permission).
Synthetic crossed-lamellar microstructures in oxide ceramics 219
vol.% of the tape casting slurry. The non-ceramic
components were 36 vol.% deionized water, 1 vol.%
Darvan C dispersant (Vanderbilt Company), 5 vol.%
polypropylene glycol plasticizer (Fluka), and 20 vol.%
rhoplex b60-a polyacrylic emulsion binder (Rohm and
Haas). The relative volume fractions of solvent, binder,
and ceramic powder were carefully chosen to give
optimal viscosity for casting and to maintain whisker
alignment across the plane as well as through the
thickness of the tape. The dispersant, binder, and
plasticizer were chosen based on the work of Ushifusa
and Cima [10]. 
Slurry components along with 10 mm alumina
milling media were combined and ball milled for 24
hours, followed by de-airing prior to casting. The
mullite tapes were manually cast with a modified
doctor blade on a glass substrate covered with a
polypropylene film. Flow of the slurry behind the blade
was divided into 75 channels across the ~14 cm tape
width by a flow divider. This was intended to increase
the torque tending to align the aluminum borate
whisker templates, which is proportional to n
2
 where n
is the number of channels [11]. The tape casting set up
is shown in Figure 2a. Tapes were cast at 6 cm s
−1
 with
a blade gap of 460 µm. Following casting, tapes were
dried in air for approximately 1 hour. Dried tape
thickness prior to lamination was ~300 µm. 
Dried tapes were sectioned at +45
o
 and –45
o
 to the
casting direction. Sections were then stacked with alter-
nating ± 45
o
 orientations as shown in Figure 2b. Each
sample was composed of 20 layers. The tape stacks
were then uniaxially pressed at room temperature with
~40 MPa pressure. Following the lamination and press-
ing, the organic components in the green laminates
were burned out using heating rates and temperature
holds determined from thermogravimetric analysis (TGA).
Samples were heated in a vented furnace to 220
o
C at
3.5 K minute
−1
, held at 220
o
C for one hour, and further
heated to 475
o
C at 0.5 K minute
−1
. Samples were then
heated to 1000
o
C at 3.5 K minute
−1
, held at 1000
o
C for
2 hours, and cooled to room temperature at 10 K
minute
−1
. The final heating step was necessary to
ensure that the samples would be robust enough to
handle following organic burnout. The laminates were
then heated at 10 K minute
−1
 to 1650
o
C and sintered
for 6 hours to allow for densification and anisotropic
grain growth. Sintered samples were cooled to room
temperature at 10 K minute
−1
.
Scanning electron microscopy and optical microscopy
(for greater contrast) were used to characterize mor-
phological texture and to assess grain alignment in the
samples in the casting plane and cross-section. Samples
for microscopy were polished and thermally etched in
air at 1550
o
C for 12 minutes. X-ray diffraction was
also used to characterize crystallographic texture and
grain alignment. A Cu-Kα X-ray source with a 2 mm
slit size was used for diffraction. Density of sintered
materials was measured using Archimedes’ principle
[12]. Samples were also fractured in air at 1350
o
C
using a silicon carbide three-point bend fixture in a
universal material tester at a rate of 0.4 mm/minute.
Fracture surfaces were examined using the scanning
electron microscope to observe crack propagation in
relation to grain orientation.
Results and Discussion
Micrographs of mullite with aligned rod-like grains
created by templated and untemplated mullite are
shown in Figure 3. The templated mullite consists of
long aligned rod-like grains of up to 100 µm in length
while the untemplated mullite consists of fine equiaxed
grains and elongated grains of up to 15 µm in length.
The sintered mullite, templated and untemplated, was
greater than 94% dense, based upon the theoretical
density of mullite.
*
 Due to the shear forces created
during tape casting and the division of slurry flow, the
long axes of the anisotropic aluminum borate whisker
templates are aligned parallel to the casting direction
[7]. 
Upon sintering, the aluminum borate whiskers
encourage anisotropic grain growth within the mullite
matrix through templated grain growth. The templates
grow to consume the surrounding matrix and aniso-
tropic rod-like mullite grains nucleate and grow to
form a highly textured microstructure [9]. Due to the
alignment of the aluminum borate whiskers during tape
casting, the rod-like mullite grains in the sintered
material are all aligned parallel to the tape casting
direction. The whisker templates in the mullite matrix
Fig. 2. Diagram of the a) tape casting setup showing aligned
aluminum borate whisker templates in the mullite tape, and b) the
sectioning of green tape with aligned whisker templates and ±45
o
stacking.
*
The percent density may be underestimated due to the presence of a
lower density intergranular phase.
220 Vikram S. Kaul and K.T. Faber
tend to decompose to Al
2
O
3
 and B
2
O
3 
[5]. Al
2
O
3
 is
soluble in mullite, while B
2
O
3
 and TiO
2
 tend to form
glassy grain boundary phases observable by transmi-
ssion electron microscopy [13, 14]. 
The process developed allows control of grain align-
ment between laminate layers so that the direction of
alignment can change abruptly from one layer to the
next. Sintering of green laminates with alternating
± 45
o
 template alignment produces a material with a
microstructure similar to that of the middle layer of the
crossed-lamellar microstructure seen in Fig. 1. In the
sintered mullite, each layer is analogous to a first-order
lamella and the individual rod-like mullite grains are
analogous to second-order lamellae in the crossed-
lamellar microstructure. Cross-sectioning this material
at 45
o
 shows layers in the laminate material with
alternating alignment of the rod-like mullite grains at 0
o
and 90
o
 (Fig. 4a). It is clear from Fig. 4a that alignment
and texture is maintained through the thickness of each
layer and that sharp interfaces exist between each layer,
similar to the crossed-lamellar microstructure of Strombus
gigas. Closer inspection of a layer interface shown in
Figure 4b shows no delamination between layers. 
In addition to morphological texture seen in optical
and scanning electron micrographs, X-ray diffraction
reveals clear crystallographic texture and serves as
another confirmation of grain alignment. Growth of the
mullite grains is along the <001> direction as templated
by the aluminum borate whiskers. The grain faces con-
sist of {110} and {111} planes [15]. X-ray diffraction
scans of the tape casting plane, and of the cross-section
parallel to the direction of grain alignment are shown in
Fig. 5 along with a scan of an untemplated sample. Fig.
5a shows a large peak corresponding to the (110)
reflection and virtually no indication of the (001) and
(002) reflections, showing the long axes of the mullite
grains are in the tape casting plane. Figure 5b reveals
Fig. 3. Sintered microstructures in the tape casting plane of a)
templated and b) untemplated mullite laminates by optical and
scanning electron microscopy, respectively.
Fig. 4. a) Optical micrograph cross-section of a mullite laminate
showing alternate grain alignment. Symbols represent the
direction of grain alignment in each layer. b) SEM micrograph of
the interface region showing the abrupt change of grain alignment. 
Synthetic crossed-lamellar microstructures in oxide ceramics 221
the complement, as expected. These scans indicate that
the preferred growth direction is <001> and that grains
are well aligned in each layer.
Crack propagation in the mullite laminates can be
observed by examining the fracture surface shown in
Fig. 6. Samples were fractured at 1350
o
C because
mullite exhibits transgranular fracture at room temper-
ature, but a mix of intergranular and transgranular
fracture at high temperatures [16]. Figure 6 shows a
change in crack path at a layer interface as the direction
of grain alignment shifts from –45
o
 to +45
o
 across the
interface, similar to fracture in Strombus gigas [3].
The process developed here can be readily applied to
other ceramics where control of anisotropy is desirable.
In addition to mullite, templated grain growth has been
used to produce textured microstructures in many
ceramics including alumina [9, 17], silicon carbide [18],
silicon nitride [19], bismuth titanate [20], and barium
hexaferrite [21]. The control of whisker alignment
within layers through tape casting and between layers
through oriented lamination allows for the creation of
designed microstructures for heat conduction, electrical
conduction, or in piezoelectric applications where
anisotropy may be desired. 
Conclusions
By combining tape casting, oriented lamination, and
templated grain growth it is possible to produce a
ceramic material that exhibits a simplified version of
the crossed-lamellar microstructure. Mullite laminates
produced through this process exhibit a microstructure
of aligned rod-like grains with abrupt changes in the
direction of grain alignment at layer interfaces. Both
morphological and crystallographic texture was confirmed
through microscopy and X-ray diffraction. High
temperature fracture of this material results in tortuous
crack paths as crack propagation changes directions as
grain alignment shifts across the +45
o
/−45o interfaces.
The process developed here can be further extended to
ceramics for applications where control of anisotropy is
important. 
Fig. 5. X-ray diffraction scans of mullite laminates (a) of the tape casting plane, (b) the cross-section in the direction parallel to the long axis
of the grains, and (c) the tape casting plane of untemplated mullite.
Fig. 6. Mullite laminate fracture surface showing crack path
change at a layer interface (marked by the black lines). The
direction of grain alignment shifts from –45
o
 to +45
o
 across the
interface. The direction of crack propagation is from the bottom of
the figure to the top.
222 Vikram S. Kaul and K.T. Faber
References
1. M. Sarikaya, Proc. Nat. Acad. Sci. 96[25] (1999) 14183-
14185.
2. V.J. Laraia and A.H. Heuer, J. Am. Ceram. Soc. 72[11]
(1989) 2177-2179.
3. T.J. Hill, “Quantitative Fracture Analysis of a Biological
Ceramic Composite,” Doctoral Dissertation. University of
Florida (2001) p.55.
4. S. Kamat, X. Su, R. Ballarini, and A.H. Heuer, Let. to Nat.
405 (2000) 1036-1040.
5. I.A. Aksay, D.M. Dabbs, and M. Sarikaya, J. Am. Ceram.
Soc. 74[10] (1991) 2343-2358.
6. I.E. Gönenli and G. L. Messing, J. Euro. Ceram. Soc. 21
(2001) 2495-2501.
7. G.L Messing, F. Lang, and S.-I. Hirano, “Ceramic
Processing Science” (American Ceramic Society, 1998) p.
497.
8. S.H. Hong and G.L. Meesing, J. Am. Ceram. Soc. 82[4]
(1999) 867-872.
9. E. Suvaci and G.L. Messing, J. Am. Ceram. Soc. 83[8]
(2000) 2041-2048.
10. N. Ushifusa and M.J. Cima, J. Am. Ceram. Soc. 74[10]
(1991) 2443-2447.
11. D.S. Park and C.W. Kim, J. Mat. Sci. 34 (1999) 5827-
5832.
12. ASTM C373, “Standard Test Method for Water Absorp-
tion, Bulk Density, Apparent Porosity, Specific Gravity of
Fired Whiteware Products” (ASTM International, 1999).
13. T. Huang, M. Rahaman, B. Eldred, and P. Ownby, J. Mater.
Res. 16[11] (2001) 3223-3228.
14. H. Schneider, Ceram. Int. 13 (1987) 77-82.
15. T. Huang, M.N. Rahaman, T.-I. Mah, and T.A.
Parthasarathay, J. Am. Ceram. Soc. 83[1] (2000) 204-210.
16. R. Torrecillas, G. Fantozzi, S. de Aza, and J. Moya, Acta
Mater. 45[3] (1997) 897-906.
17. T. Carisey, I. Levin and D. Brandon, J. Euro. Ceram. Soc.
15[4] (1994) 283-289.
18. M. Sacks, G. Scheiffele, and G.J. Staab, J. Am. Ceram.
Soc. 79[6] (1996) 1611-1616.
19. K. Hirao, M. Ohashi, M. Brito, and S.J. Kanzaki, J. Am.
Ceram. Soc. 78[6] (1995) 1687-1690.
20. Y. Kan, P. Wang, Y. Li, Y. Cheng, and D. Yan, J. Euro.
Ceram. Soc. 23[12] (2003) 2163-2169.
21. D.B. Hovis and K.T. Faber, Scripta Mat. 44 (2001) 2525-
2529.


Synthetic crossed-lamellar microstructures in oxide ceramics

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Synthetic crossed-lamellar microstructures in oxide ceramics

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