Low-Pressure Injection Molding of Ceramic Micro Devices
Using Sub-Micron and Nano Scaled Powders
M. Müller, W. Bauer, H.-J. Ritzhaupt-Kleissl
Institut für Materialforschung 3, Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe, D
.
.
Abstract
The requirements for the manufacturing of ceramic micro parts are considered when applying the processing
technology of low-pressure injection molding (LPIM). For the preparation of the feedstocks 3 mol% yttria stabilized
zirconia (3YSZ) powders with different particle size were employed, ranging from the sub-µ (0.2-0.4 µm) to the nano
scale (10-100 nm). Resulting from the varying powder characteristics, significant changes in feedstock properties,
debinding, and sintering behavior were observed. Due to an increasing specific surface area the achievable feedstock
solid loading decreased from 52 vol.% for the sub-µ powder to 45 vol.% for the nano scaled material. Correspondingly
higher sintering shrinkages occurred for the feedstocks with lower solid loading, and owing to a higher binder content,
during debinding the tendency to shape deviation is enlarged. On the other hand smaller particle size allows reduced
sintering temperatures and leads to a smaller grain size. With nano scaled 3YSZ powder (synthesized by laser evaporation) dense samples with an average grain size of 0,26 µm were obtained at a sintering temperature of just 1300°C.
In spite of a promising microstructure, samples from the nano scaled powder do not exhibit the same level of mechanical strength, yet. Depending on surface roughness and edge geometry 3 point-micro-bending tests of the standard sub-µ powder show characteristic strength values σ0 of 2700 MPa and more. For specimen with the nano scaled
powder not more than 1780 MPa were detected. In the present state of research inhomogeneities, especially agglomerates, which are closely connected to the synthesis of the nano scaled powders, are considered to be the limitation
for improved mechanical properties.
Keywords: low-pressure injection molding, 3YSZ, nano scaled ceramic powder, mechanical properties
1. Introduction
Powder injection molding (PIM) is an advantageous
method for the fabrication of microstructured devices. It
allows near net shape and high-volume production of
complex-shaped ceramic parts with small dimensions
(µPIM). Low-pressure injection molding (LPIM) represents a less-common variant applied for ceramic injection molding. Due to markedly lower equipment and
tooling costs, it allows fast manufacturing of functional
models and therefore is an attractive alternative in the
field of product development or smaller production
numbers [1].
For the fabrication of micro parts it is of great interest to know, down to which component size LPIM can
be applied. It has been shown that molding of micro
parts with overall dimensions less than 100µm is possible [2]; micro details of such components measure in
the range of few microns. State-of-the-art powders for
high strength ceramics like 3YSZ lead to microstructures with a grain size of about 0.5 µm. As a result just
a limited number of grains are forming the details and
so the available surface quality is limited.
For present demands this seems to be acceptable,
but for further size reduction it is necessary to keep the
grain size small in relation to the dimension of the
smallest micro features. Consequently there must be a
shift of the raw materials from the approved submicrometer range into the nano scale.
Beside miniaturization, there are still more attractive features which justify the application of nano scaled
powders for structural ceramics. Advantages may arise
from improved mechanical properties, reduced surface
roughness, and lower sintering temperatures. As opposed to these potential advantages there are the
known disadvantages of highly dispersed powders.
Their behavior is increasingly determined by high surface to volume ratios. Correspondingly ambitious are
the attempts to prepare utilizable feedstocks. Particularly the achievable solid loadings are crucial, but also
an increased tendency to agglomeration of nano powders has to be considered.
To decide if the potential improvements of finer
powders justify the higher efforts which have to be
made, the processability as well as the resulting materials properties of one sub-µ zirconia powder and two
nano scaled qualities of the same composition (3 mol%
yttria stabilized ZrO2) are compared.
2. Low-pressure injection molding
2.1. Processing
Fig. 1 shows a schematic flow sheet for the fabrication of ceramic components via injection molding technique with two alternative options available. As the
name implies, the main difference between high pressure injection molding (HPIM) and LPIM lies in the
pressure regime the process is carried out. For LPIM it
is the range of 0.1-1 MPa, whereas HPIM requires 50
MPa and more. To withstand these severe conditions
mold inserts for HPIM have to be manufactured out of
wear resistant materials (typically steel), e.g. by micro
milling, electro discharge machining (EDM) or laser
beam machining (LBM) as a negative cavity of the micro part.
In contrast, for LPIM even soft molds (for example
from silicone rubber) can be applied. For the preparation of the also negatively shaped soft molds a master
model is required. It is one of the most important advantages of LPIM that due to this copying process the
master model can be made (nearly) of any available
Multi-material Micro Manufacture
W. Menz & S. Dimov
(C) 200X. Published by Elsevier Ltd.
1
material. Hence, from the several rapid prototyping
techniques for micro parts those with the highest resolution can be chosen, which presently is microstereolithography [3,4]. But soft molds from silicone
offer additionally interesting advantages as their manufacturing is simple, fast, and inexpensive. High mechanical elasticity and low adhesion forces facilitate
demolding of complex shaped parts. Even undercuts
can be realized to a certain degree in one single tool.
The limit of soft tools is, that only moderate injection
pressures can be applied, otherwise deformation of the
mold inserts and of the molded part would occur.
This is desirable as high solid loadings minimize sintering shrinkage, which always is a source of deformation,
anisotropy, and generally an obstacle to meet narrow
tolerances. As indicated in Fig. 2 the solid loading of
the feedstock is in the range of 45 to 65 vol%; this
strongly depends on the powder particle size, size distribution, morphology and chemical composition of the
starting material.
Fig. 2. Process flow feedstock preparation.
For the homogeneity of the feedstocks it is essential to separate the more or less aggregated powder
particles as far as possible. To ease their handling,
especially fine-scaled powders generally are supplied
as granules. These aggregates which result from the
spray-drying process consist of weakly adhering particles and can be redispersed by moderate mechanical
agitation like stirring or ultrasonication. But with decreasing powder grain size the efforts to achieve isolated primary particles are increasing significantly. In
the low-viscous LPIM feedstocks high shear stresses
can be obtained for example by a fast turning dissolver
stirrer with a vane carrying disc. Homogenization and
deagglomeration alternatively can be achieved by roller
milling or ball milling. To avoid trapping of air bubbles
during the stirring process, evacuation of the heated
container is advisable.
3. Experimental Details
Fig. 1. Process flow micro powder injection molding.
3.1. Starting materials
To realize complete mold filling even with the lowest injection pressure, feedstock viscosity has to be
reduced clearly, i.e. for several orders of magnitude
compared to typical HPIM feedstocks. This cannot be
achieved by means of decreasing feedstock’s solid
content as this parameter should be kept as high as
possible. Instead of that it is necessary to adapt the
nature of the binder system. Long-chained thermoplastic polymers like low-density-polyethylene or polypropylene which are typical HPIM binder components,
have to be replaced by compounds with lower melting
temperatures, i.e. with shorter chain-length.
2.2. Preparation of the Feedstock
Binder systems on paraffin wax basis became
widely accepted in LPIM. They offer a beneficial melting
range, are inexpensive and non toxic. The debinding
behavior is uncomplicated as they decompose without
boiling. Molten paraffin waxes exhibit low viscosities
and thus relatively high solid loadings can be achieved.
Three 3YSZ powders with different particle size
were used for the investigations. The first one is the
commercially available TZ-3YS-E zirconia powder, from
Tosoh company, Japan, which is explicitly recommended for injection molding [5]. It is supplied in the
form of spray dried granules containing primary particles between 0.2 and 0.4µm; in the following this quality is termed sub-µ powder.
The second material is produced in the laboratory
scale by a laser-evaporation process. As a raw material
Tosoh’s TZ3Y was exposed to the focus of a CO2 laser,
leading to evaporation and subsequent condensation of
particles in a carrier gas stream [6,7]. By this process,
which was developed and performed at Bergakademie
Freiberg and FSU Jena, spherical particles of reduced
diameter are obtained; this powder is termed in the
following as nano1. Another nano scaled starting material, processed by flame pyrolysis of metal organic precursors was investigated additionally. As shown in
Tab.1 this powder (termed nano2) reveals the highest
2
specific surface area and correspondingly the finest
average primary particle size.
Table 1: Properties of applied 3YSZ powders
Powder
BET surface
primary particle size
m²/g
nm
sub-µ
6.3
200-400
nano1
17.7
10-100
a
nano2
64
12
a
manufacturer information
3.2 Mechanical investigations
In the macroscopic dimensions 3YSZ is appreciated as engineering ceramic because of its excellent
mechanical strength. For the dimensioning of micro
components, which have to withstand high mechanical
load and wear, the knowledge of reliable mechanical
characteristics is of great importance. This is especially
true for ceramics as their strength is described by the
Weibull theory. Due to the Weibull size effect, micro
parts are expected to reveal superior strength values,
as with reduced component size the probability for a
critical defect is decreasing.
The investigation of the micro mechanical properties and the microstructural characterizations were performed within the collaborative research center SFB
499 by the Institute of Materials Science (IWK 1) at the
University of Karlsruhe [6]. For the 3Pt.-micro-bendingtests microbars with dimensions after sintering of about
3
200 x 200 x 1200 µm were produced by LPIM. To obtain enough samples under identical experimental conditions, arrays of 25 x 25 columns were manufactured
(see Fig. 3).
From a statistical analysis of the measured bending strength, the characteristic strength σ0 and the
Weibull exponent m were obtained. For each value of
σ0 and m at least 30 specimens were tested. Beside
measuring the dimensions for every micro bending bar,
following characterizations were made from selected
samples: microstructure, porosity, edge geometry and
surface roughness Rt.
-1
20 Pa*s (at a shear rate of 100 s ). The resulting solid
loadings and viscosities are summarized in Tab. 2.
As expected a decrease of achievable solid loading
is observed with decreasing particle size. For the nano2
powder a solid loading of only 30% was obtained. This
is definitely too low to mold components which could be
debindered and sintered without severe deformation.
Instead of that, feedstocks were prepared by blending
sub-µ and nano2 powder (ratio 5:1). The intention has
been, to examine if the finer fraction fills the space between the sub-µ particles and thus would enable higher
solid loadings and lower viscosities, respectively.
Table 2: Properties of paraffin based feedstocks
solid loading
shear viscosity
vol%
Pa*s
sub-µ
52
19
nano1
45
15
nano2
30
16
sub-µ + nano2
52
12
This effectively has been observed (see Tab. 2);
with 12 Pa*s a significantly lower viscosity was detected for the sub-µ + nano2 blend compared to the
pure sub-µ feedstock (19 Pa*s).
A solid loading of 45% which was achieved with
the nano1 powder also is quite high, taking into account
the small particle size of that material. Two reasons
might be responsible for this fact. As one can see in
Fig. 4 the laser evaporation process results in perfect
spherical particles and a broad size distribution. Owing
to the spherical shape a minimized surface area and
good flowability is provided. The broad size distribution
has a similar effect on the feedstock’s rheology as the
blending of different particle size discussed above.
Fig.4. Morphology of laser evaporated 3YSZ.
4.2. Mechanical properties
Fig. 3. Array of sintered 3YSZ bars for microbending
specimen supply.
4. Results and Discussion
4.1 Feedstock properties
To examine the maximum solid loading for the different
starting materials, feedstocks were prepared utilizing a
commercially available paraffin based binder / dispersant system (Siliplast LP65, Zschimmer & Schwarz,
Lahnstein, Germany). For acceptable moldability it is
necessary to keep the feedstock viscosity between 10-
Together with the Weibull parameters σ0 and m in
Tab. 3 microstructural and surface properties are summarized for 7 sets of micro bending samples, each obtained from one column array.
Set A, B, and C were made of the sub-µ powder
and sintered at 1500°C. Their characteristic strength
values reach from 1417 MPa for set A, up to 2690 MPa
for set C. Due to identical sintering conditions the grain
size exhibits no significant variation and therefore
should not be responsible for the observed discrepancy. However, bending strength is clearly increasing
with decreasing porosity, decreasing surface roughness, and increasing edge radius.
3
Table 3: Properties of micro bending specimen from sub-µ and nano scaled 3YSZ
Grain Size
Porosity
Edge Radius
Rt
Array Powder
TSint.
°C
µm
%
µm
µm
A
sub-µ
1500
0.47
1.58
3.7
2.7
σ0
MPa
1417
m
12.3
B
1500
0.46
0.84
6.3
1.2
1967
9.8
C
1500
0.45
0.14
14.0
1.1
2690
9.2
1500
0.54
1.14
2.9
1.7
1588
12.1
1500
0.54
0.43
4.7
1.5
1749
16.0
1400
0.32
0.11
5.3
2.4
1776
6.6
1300
0.26
0.05
9.7
2.1
1766
4.5
D
sub-µ + nano2
E
F
nano1
G
a
a
3-point bending strength from Tosoh data sheet: 1200 MPa [5]
It is difficult to decide which parameter is most critical, as they are assumed to be connected. An increase
of edge radius during the debinding stage probably will
cause simultaneously a reduction of surface roughness.
At the same time also pores which are located near the
surface, may vanish during those rearrangement processes.
Due to the high fraction of sub-µ powder arrays D
and E were also sintered at 1500°C. However, the average grain size was found to be slightly enlarged
(0.54µm). This is assumed to result from the higher
sintering activity of the nano powder fraction giving rise
to expressed grain growth at temperatures of 1500°C.
Porosity, edge radii, and surface roughness are always
somewhat superior for set E compared to D, consequently higher σ0 is observed. Also the highest value of
16 for parameter m is noteworthy and indicates low
scattering of the single values of bending strength.
The sample sets F and G are manufactured with
the pure nano1 powder and therefore sintering temperature could be reduced, resulting in a finer microstructure and smaller grain size of 0.26 and 0.32 µm.
Residual porosity was nearly not detectable. The edge
radii differ considerably, but this seems not to have an
influence on the characteristic strength, which is virtually equal with 1776 and 1766 MPa. Parameter m however, is clearly poorer than for the other samples. Although the grain size is reduced, the surface roughness
with Rt values of 2.1 –2.4 µm is relatively high and
more comparable to the specimen set of array A
(2.7µm), which stands for the lowest observed σ0 of
merely 1417 MPa.
5. Conclusions
Several items of the nano1 derived samples,
namely small grain size, very low porosity, and rounded
edges should give rise to outstanding micro mechanical
properties [8,9]. Obviously this is not the case and other
factors or the significance of the known features must
be reconsidered. One influencing factor might be the
presence of agglomerates, which were detected during
the investigation of fracture surfaces. Their role in the
failure behavior however, is not entirely understood yet,
but they might also contribute to the relatively rough
surfaces of F and G.
The highest characteristic strength measured with
set C is due to positive combination of several parameters concerning the surface quality of the micro bars. As
there is no chance for a surface finishing of micro parts
any efforts have to be made to improve the surface
quality during processing, to guarantee highest mechanical strength and reliability.
Acknowledgements
We want to thank H.-D. Kurland, J. Grabow, and G.
Staupendahl from the Friedrich–Schiller University in
Jena, and C. Oestreich and E. Müller the from Technical University in Freiberg for processing and supplying
the nano scaled powder. Also the extensive characterization of our samples by M. Auhorn, B. Kasanická and
their students at the University of Karlsruhe, is highly
appreciated. Financial support for this work by the
German Research Foundation (DFG) within the framework of the SFB 499 is gratefully acknowledged.
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