J. SZYMANSKA et al.: RHEOLOGICAL PROPERTIES OF ALUMINA CERAMIC SLURRIES FOR CERAMIC ...
735–738
RHEOLOGICAL PROPERTIES OF ALUMINA CERAMIC
SLURRIES FOR CERAMIC SHELL-MOULD FABRICATION
REOLO[KE LASTNOSTI GO[^E IZ GLINICE ZA IZDELAVO
KERAMI^NIH KALUPOV
Joanna Szymañska, Pawe³ Wiœniewski, Marcin Ma³ek, Jaros³aw Mizera
Warsaw University of Technology, Faculty of Materials Science and Engineering, Woloska Street 141, 02-507 Warsaw, Poland
joanna.szymanska.pl@gmail.com
Prejem rokopisa – received: 2015-07-01; sprejem za objavo – accepted for publication: 2015-09-15
doi:10.17222/mit.2015.188
This research is about the properties of ceramic slurries prepared from hydrous nano-alumina-based binder and a corundum
matrix used for fabricating the prime coat of ceramic shell moulds. Solid-state alumina powders with different granulations were
used. The modification of the technological properties of the prepared slurries was based on additions of a polyacrylic binder
with different amounts of polymer with respect to the alumina for different powder ratios. The slurries were prepared and tested
in a mechanical mixer. During the slurry preparation (within 96 h), the plate weight, Zahn cup 4# viscosity and dynamic
viscosity were controlled. The morphology and chemical properties of corundum powders and polymer were characterized with
SEM and powder-grain-size distribution. The obtained results of the corundum-based ceramic slurries indicate that the
application of a polymeric binder with various concentrations based on nano-alumina oxides causes different properties in
comparison to the other commonly used binders.
Keywords: ceramic slurries, investment casting, shell moulds, alumina powder
Raziskava obsega lastnosti kerami~nih go{~, pripravljenih iz nanoglini~nega veziva na vodni osnovi in korundne osnove,
uporabljenih za prvo prevleko pri izdelavi kerami~nih tankostenskih form. Uporabljen je bil prah glinice v trdnem stanju, z
razli~no zrnatostjo. Spreminjanje tehnolo{kih lastnosti pripravljene go{~e je temeljilo na dodatku poliakrilnega veziva z razli~no
vsebnostjo polimerov, glede na glinico pri razli~nih razmerjih praha. Go{~e so bile pripravljene in preizku{ene v mehanskem
me{alniku. Med pripravo go{~e (v okviru 96 h), je bila kontrolirana te`a plo{~, viskoznost 4# z Zahn potopnim viskozimetrom
in dinami~na viskoznost. Morfologija in kemijske lastnosti korundnih prahov in polimerov so bile dolo~ene s SEM in z
razporeditvijo velikosti zrn. Dobljeni rezultati kerami~nih go{~ na osnovi korunda ka`ejo, da se z uporabo polimernega veziva in
razli~ne koncentracije nanoglinice, dose`e razli~ne lastnosti v primerjavi s standardno uporabljanimi vezivi.
Klju~ne besede: kerami~na go{~a, precizijsko litje, tankostenska forma, glinica v prahu
1 INTRODUCTION
The investment-casting process is commonly applied
in the manufacturing of the materials for the aviation,
energy and military industries. The limiting components
(flight safety parts) such as aircraft turbine blades
characterized by complicated shapes are cast with the
Bridgman method.1 A commonly applied technique is
the lost-wax processing including the use of ceramic
shells. It determines the precise shape, dimensional
accuracy, appropriate structure and metallurgic purity of
designed parts. So far, ceramic shell moulds were
fabricated on the basis of colloidal silica. However, the
presence of SiO2 in the prime coat during the Ni- or
Co-superalloy casting causes a reaction with the liquid
metal at a high temperature, inducing an oxidation of the
reactive metal such as Hf. Such an adverse phenomenon
reduces the quality of the properties of cast parts,
affecting its exploitation time.2
The basic components for a ceramic slurry are
binders and fillers in the form of ceramic powders and
supportive materials. A commonly used binder is hydro-
lyzed tetraethylorthosilicate together with organic com-
pounds of silicon.3 However, pure ethyl silicate does not
have the binding capacity. Water-based binders dry more
slowly than alcohol-based ones. Consequently, there is a
time elongation enabling the control of the surface
smoothness, permeability, strength and dimensional
stability of the model.4,5
A proper selection of powder for ceramic shell
moulds and their parameters such as the kind, shape and
size of particles affect the final characteristics of the cast
elements.6 Ceramic powders present a thermal resistance,
a slight thermal expansion and a lack of polymorphic
transitions.
Deflocculants, softeners and surfactants mainly deter-
mine the rheological properties of ceramic slurries.3
It was found that a nano-Al2O3-based binder does not
react with Ni-alloy components. Moreover, such a binder
demonstrates a higher melting point and a larger surface
area than other inorganic solvents. It is also characterized
by an improved dispersion of the particles in water, thus
allowing the control of rheological properties by
preventing the sedimentation of heavy particles in a cera-
mic slurry.6,7 This is why such a binder can be applied
instead of the colloidal silica-based binder.
Materiali in tehnologije / Materials and technology 50 (2016) 5, 735–738 735
UDK 67.017:666.122.3:666.7 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)735(2016)
The main aim of the following research was to
examine and define the properties of a nano-aluminum-
oxide-based binder and a corundum matrix using a
polymeric binder with various concentrations.
2 MATERIALS AND EXPERIMENTAL
METHODS
The subject of this research was a powder of Al2O3
with granulation of 0–30 and 200 mesh (Treibacher)
characterized by the average size of 11.79 and 45.00 μm,
respectively. Solvent, binder and a hydrous polymer were
dispersed in colloidal Al2O3 with a particle size of 16 nm
(Imerys, Evonik). The additive material applied to
modify the rheological properties of the slurry was a
poly acrylic polymer (Imerys, Evonik).
Ceramic slurries with a solid phase content of 72.5 %
by weight and polymer amounts of (6, 10, 15) % mass
fractions with respect to the alumina for different powder
ratios of 35:65 and 65:35 (200:030 mesh) were prepared
in a mechanical mixer within 96 h with a speed of
160 min–1. During the slurry preparation, the pH (with
the use of a pH meter), plate weight and Zahn cup 4#
viscosity were checked every 24 h. These measurements
are fundamental for the investment-casting industry.
After 96 h of mixing, rheological properties such as
dynamic viscosity were also defined with a Brookfield
DV-II rheometer with the spindle rotating in a speed
range of 1–200–1 min–1. All the measurements were
taken in an air-conditioned lab at 21 °C.
To characterize the morphology of the corundum
powders and the polymer, SEM images were taken with
a Hitachi SU70 scanning electron microscope and a BSE
detector at a voltage of 5 kV. A particle-size test was
done using a Horiba LA-950 laser diffraction device
(Hitachi, Japan).
The plate test was based on immersing the plate
(7.5×7.5 cm) in the moulding mass and estimating its
weight after 120 s.
3 RESULTS AND DISCUSSION
The morphology of the powder based on the SEM
analysis of #200 and #0–30 indicated typical structures
of molten powders with angular-shaped particles.
The obtained results shown on Figure 1 prove that
the lowest plate values correspond to 6 % of mass frac-
tions of the polymer content for a powder ratio of 35:65.
The largest ones were noticed for the slurries with 6 and
15 % of mass fractions of the polymer content at a pro-
portion of 65:35. The highest plate stability was obtained
for the slurries with 15 % of mass fractions of polymer
addition (65:35) and 10 % of mass fractions of polymer
content (35:65). The values of the plate weight con-
trolled on the last days of the measurements were in a
range of 1.7–2.4 g. The measurements of the plate weight
revealed a correlation with the polymer content: a 6 % of
mass fraction of the polymer addition resulted in the
highest weight value, equal to 2.40 g; this value was
slightly lower in the case of a 15 % of mass fraction of
the polymer content and the lowest for a 10 % of mass
fraction of the polymer amount.
Zahn Cup 4# measurements showed (Figure 2) the
lowest values (13–15 s) for the slurry with the 6 % of
mass fraction of the polymer at the 35:65 ratio. The
highest viscosity was noticed for the slurry with the
15 % of mass fraction at a powder ratio of 65:32. In this
case, there was also a rapid viscosity change from 32 s
(noticed on the first day) to 21 s after 96 h. The viscosity
was stable during the whole ceramic-slurry preparation
process for 6 (at 65:35) and 10 % of of mass fractions (at
35:65).
The obtained results shown on Figure 3 indicate sta-
bility of all the measured slurries within the measure-
ment time. The lowest values were noticed for the
slurries with 10 % of mass fractions of the polymer
content at the 65:35 ratio and for 6 of % of mass
fractions of the polymer addition at the 35:65 powder
proportion. The thickness values estimated after 96 h
oscillated from 0.12 to 16 mm. Moreover, the slurries
J. SZYMAÑSKA et al.: RHEOLOGICAL PROPERTIES OF ALUMINA CERAMIC SLURRIES FOR CERAMIC ...
736 Materiali in tehnologije / Materials and technology 50 (2016) 5, 735–738
Figure 2: Relation between Zahn cup 4# viscosity and stirring time
for ceramic slurries with 72.5 % of mass fractions of solid content for
different polymer contents at 35:65 and 65:35 powder ratios (200:030)
Slika 2: Razmerje med Zahn viskoznostjo 4# in ~asom me{anja kera-
mi~ne go{~e z 72,5 % trdnega masnega dele`a, pri razli~ni vsebnosti
polimera in razmerju prahov 35:65 in 65:35, mre`a (200:030)
Figure 1: Relation between plate weight and stirring time for ceramic
slurries with solid-content mass fraction of 72.5 % for different
polymer contents at 35:65 and 65:35 powder ratios (200:030)
Slika 1: Razmerje med te`o plo{~e in ~asom me{anja go{~e z
vsebnostjo 72,5 % trdnega masnega dele`a, pri razli~nih vsebnostih
polimera in razmerju prahov 35:65 in 65:35 (200:030)
with 6 and 10 % of mass fractions of the polymer
content were characterized as similar according to the
viscosity level.
The measurements of the ceramic-slurry dynamic
viscosity are shown on Figures 4 do 6 where the rela-
tionship between the shear rate and viscosity is pre-
sented. As seen on the diagrams, additions of different
concentrations of polymers to the ceramic slurries of
Al2O3, with two powder ratios, determine their viscosity.
The obtained results indicate that the application of 10 %
of mass fractions of polymer at the 35:65 powder ratio
causes the largest increase in the dynamic viscosity
where the maximum value is 763 MPa s. The most
effective was the addition of 6 % of mass fraction of
polymer at the 35:65 powder ratio resulting in the lowest
dynamic-viscosity value of 321.12 MPa s.
4 CONCLUSION
The Al2O3 powder characterized by irregularly
shaped particles with sharp edges demonstrates the
ability to agglomerate, resulting in a non-uniform par-
ticle-size distribution. The Zahn cup viscosity (7.35 s) is
slightly larger in comparison to water viscosity (5.83 s),
thus the Al2O3 particle dispersion in the binder is faci-
litated.
In addition, a relatively large content of the solid
phase in a slurry reduces the coat shrinkage during the
drying process and enhances its strength. The properties
of the coating surface may be improved by increasing the
plate weight. An addition of a poly acrylic polymer at
the lowest content to the alumina powders with various
granulation values allows a regulation of the rheological
properties of the ceramic slurry towards more effective
ceramic shell-mould fabrication.
The investigated slurries show standard features in
the investment-casting process on an industrial scale.
They are prospective for future shell-mould fabrication.
Acknowledgement
The financial support from the Structural Funds for
the Operational Programme Innovative Economy (IE
OP) provided by the European Regional Development
Fund – Project "Modern material technologies in aero-
J. SZYMAÑSKA et al.: RHEOLOGICAL PROPERTIES OF ALUMINA CERAMIC SLURRIES FOR CERAMIC ...
Materiali in tehnologije / Materials and technology 50 (2016) 5, 735–738 737
Figure 6: Relation between viscosity and shear rate of Al2O3 ceramic
slurries with 72.5 % of mass fractions of solid content for 15 % of
mass fractions of polymer content at two powder ratios, 35:65 and
65:35 (200:030 mesh)
Slika 6: Odvisnost med viskoznostjo in stri`no hitrostjo Al2O3
kerami~ne go{~e z 72,5 % masnim dele`em trdnega pri 15 % masnega
dele`a polimera, pri dveh razmerjih zrnatosti prahov 35:65 in 65:35,
mre`a (200:300)
Figure 4: Relation between viscosity and shear rate of Al2O3 ceramic
slurries with 72.5 % of mass fractions of solid content for 6 % of mass
fractions of polymer content at two powder ratios, 35:65 and 65:35
(200:030 mesh)
Slika 4: Odvisnost med viskoznostjo in stri`no hitrostjo Al2O3
kerami~ne go{~e z 72,5 % dele`em trdnega pri 6 % masnega dele`a
polimera, pri dveh razmerjih zrnatosti prahov 35:65 in 65:35, mre`a
(200:030)
Figure 5: Relation between viscosity and shear rate of Al2O3 ceramic
slurries with 72.5 % of mass fractions of solid content for 10 % of
mass fractions of polymer content at two powder ratios, 35:65 and
65:35 (200:030 mesh)
Slika 5: Odvisnost med viskoznostjo in stri`no hitrostjo Al2O3
kerami~ne go{~e z 72,5 % masnim dele`em trdnega pri 10 % masnega
dele`a polimera, pri dveh razmerjih zrnatosti prahov 35:65 in 65:35,
mre`a (200:030)
Figure 3: Coating thickness (H) dependence on time for the slurries
with 72.5 % of mass fractions of solid content for (6, 10, 15) % of
mass fractions of polymer content at two powder ratios, 35:65 and
65:35 (200:030 mesh)
Slika 3: Debelina nanosa (H) v odvisnosti od ~asa pri go{~i z 72, 5 %
masnim dele`em trdnega in pri vsebnosti (6, 10 , 15) % masnega
dele`a polimera, pri dveh razmerjih prahov 35:65 in 65:35, mre`a
(200:030)
space industry", Nr POIG.01.01.02-00-015/08-00, is
gratefully acknowledged.
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J. SZYMAÑSKA et al.: RHEOLOGICAL PROPERTIES OF ALUMINA CERAMIC SLURRIES FOR CERAMIC ...
738 Materiali in tehnologije / Materials and technology 50 (2016) 5, 735–738