Monoclinic Zirconia Bodies by Thermoplastic Ceramic Extrusion

Article abstract of DOI:10.1111/j.1551-2916.2004.00358.x

Two ultrafine, undoped ZrO₂ powders with median primary particle sizes of 9 and 25 nm were used to prepare ceramic suspensions for thermoplastic extrusion. The organic vehicle consisted of an industrial-grade poly(ethylene-co-vinyl acetate) (EVA) or polyethylene (PE-HD) and decanoic acid as a dispersing agent. The powder volume loadings achieved were 44% and 52% by volume for the two powders, respectively. The amount of dispersant needed was calculated from a new model based on available chemisorption sites on the powder surface. Mixing and extrusion were conducted using a conventional modular plastic processing unit. Green bodies were dewaxed up to 450°C in an inert atmosphere and sintered to full density in air at 1060° and 1100°C, respectively. Analysis of the ceramic phase content and the microstructure of the bodies is presented.

Full text of DOI:10.1111/j.1551-2916.2004.00358.x

J. Am. Ceram. Soc., 87 [3] 358–64 (2004)
journal
Monoclinic Zirconia Bodies by Thermoplastic Ceramic Extrusion
†
‡
,‡
,†
Gerd Scheying, Ingrid W u¨ hrl, Ulrich Eisele,* and Ralf Riedel*
Department of Materials Science, Darmstadt University of Technology, 64287 Darmstadt, Germany
Corporate Materials Research, Robert Bosch GmbH (FV/FLW), 70049 Stuttgart, Germany
Two ultrafine, undoped ZrO powders with median primary
polar powder surface–nonpolar organic vehicle interface. Dispers-
ing agents are widely used in ceramic-forming technology to
overcome described effects and many are commercially available.
2
particle sizes of 9 and 25 nm were used to prepare ceramic
suspensions for thermoplastic extrusion. The organic vehicle
consisted of an industrial-grade poly(ethylene-co-vinyl acetate)
7
A new way of calculating the amount of required dispersing aid
is presented here. The model allows the exact determination of the
amount of dispersant needed based on the specific amount of
molecular dispersant chemisorbed on the powder surface. Beyond
this it takes into account that a certain dispersing aid/vehicle
(polymer or high molecular wax) ratio is needed for melt flow
(
EVA) or polyethylene (PE-HD) and decanoic acid as a dis-
persing agent. The powder volume loadings achieved were
4% and 52% by volume for the two powders, respectively.
4
The amount of dispersant needed was calculated from a new
model based on available chemisorption sites on the powder
surface. Mixing and extrusion were conducted using a conven-
tional modular plastic processing unit. Green bodies were
dewaxed up to 450°C in an inert atmosphere and sintered to
full density in air at 1060° and 1100°C, respectively. Analysis
of the ceramic phase content and the microstructure of the
bodies is presented.
8
reasons of a given ceramic suspension.
CIM using a thermoplastic vehicle is already widely used as a
standard process in ceramic green forming like tape casting or
9
pressing. Extrusion, on the other hand, is mostly done with
aqueous systems. This paper describes thermoplastic extrusion of
nanoscale powders where mixing and forming have been con-
ducted using industrial thermoplast processing equipment de-
signed for blending and shaping polymers, albeit using higher
temperature and torque.
I. Introduction
Although pore diameters of the prepared nanostructured speci-
men are well below 10 nm, debinding is possible without disrupt-
ing samples having a wall thickness of 5 mm. This is in contrast to
prior ascertainments of many authors who claim that debinding
from pores smaller than ϳ100 nm is only feasible in “infinite
ENERAL thermodynamic and kinetic considerations of ceramic
G
sintering processes demand the use of ultrafine (i.e.,
nanoscale) powders to lower the sintering temperature significant-
1
ly. In the case of pure monoclinic ZrO (m-ZrO ), grain sizes in
2
2
a low nanometer range are necessary for pressureless sintering to
full density. The monoclinic-to-tetragonal phase transition of ZrO2
at about 1150°C induces microcracking on cooling due to the
associated specific volume change. Therefore, the sintering tem-
perature must be kept below this point.
10–13
time,” i.e., impossible without damaging samples.
Although several publications describe the use of nanopowders
14–16
for production of ceramic bodies,
to our knowledge there is
no published work about a similar industry compliant and eco-
nomic process like the one outlined here.
Many industry compliant production routes for nanoscale ce-
ramic powders with output rates of several kilograms per day have
2
,3
been developed recently. The undoped ZrO powders used here
2
II. Experimental Procedure
were manufactured via flame pyrolysis and a controlled growth
reaction under hydrothermal conditions. Both powders contain a
substantial amount of tetragonal phase which is thermodynami-
(1) Materials
Details of the ceramic powders and organic materials are given
in Tables I and II, respectively. MZ1 is commercially produced by
Degussa-Huels (Hanau), and MZ2 is a ceramic powder supplied by
the Institut fuer Neue Materialien (Saarbruecken). MZ1 is manu-
factured by the Aerosil™ process where ZrCl (instead of SiCl ) is
4
cally stable under ambient conditions due to particle size.
Besides the starting grain size, a homogeneous dispersion of
primary particles and a sufficient volume loading of powder in the
ceramic green body are necessary prerequisites for densification at
4
4
5
a given low temperature. Volume loadings of at least 50% should
injected into an oxyhydrogen flame to be pyrolyzed to small
17
be achieved.
crystalline ZrO2 grains (instead of amorphous SiO2). MZ2 is
produced by a hydrothermal growth process from organic zirco-
nium compounds. Details of this powder preparation are given
Most attempts to incorporate powders with high specific surface
areas (SBET), like the material used here, into an organic vehicle
have failed because of powder–vehicle interactions. Since both
phases are initially incompatible, segregation or even separation is
observed during mixing, or when specimens are reheated for
1
8
elsewhere. Both powders contain a substantial amount of tet-
ragonal phase because of their small particle size.
Poly(ethylene-co-vinyl acetate) (EVA), polyethylene (PE-HD),
and decanoic acid (98%) (DA) are of industrial grade supplied by
BASF AG, Hoechst AG, and Aldrich, respectively. According to
the manufacturer, EVA contains 12 wt% vinyl acetate. All other
data shown in Table II were determined experimentally.
6
dewaxing. This is generally a problem for almost every existing
I-W. Chen—contributing editor
(2) Pretreatment of Powders, Surface Modification,
Compounding, and Extrusion
Both powders were washed before compounding. In the case of
MZ1, washing with NH3 solution at pH 10 reduced the acidic
chloride content, due to residues from production from ZrCl , from
0.4 wt% down to Ͻ100 ppm. MZ2 was washed with concentrated
Manuscript No. 188424. Received September 1, 2000; approved October 15, 2003.
Based in part on the dissertation submitted by G. Scheying for the doctoral degree
in materials science, Darmstadt University of Technology, Darmstadt, Germany,
1
999.
Supported by German Federal Grant (bmbϩf) under Grant No. 03M2744A5.
4
*
Member, American Ceramic Society.
Darmstadt University of Technology.
Robert Bosch GmbH.
†
‡
H O solution (35%) to reduce the carbon residues from the
2
2
3
58

March 2004
Monoclinic Zirconia Bodies by Thermoplastic Ceramic Extrusion
359
Table I. Details of Powders
the samples throughout the whole pyrolysis. The furnace was
equipped with a flame ionization detector (FID; Model VE 7,
J.U.M. Engineering, Karlsfeld) for gaseous hydrocarbon emission
detection. The gas flow rate was determined by the demand of the
FID (180 L/min) and kept constant. All extruded bars were placed
on a stainless steel support.
Ave particle
size by
S
BET
2
PZC
(pH)
LOI
Tetragonal
Type
Grade
TEM (nm)
(m /g)
(wt%)
content (%)
MZ1 VP-ZrO2
MZ2 INMZ
25
8
55
120
7.6
7.9
3
7
75
83
Thermomechanical analysis (TMA) in static mode and macro-
scopic thermogravimetric analysis (TGA), up to sample weights of
5
5
g, were conducted using a Perkin-Elmer DMA 7 and a B a¨ hr STA
02 S, respectively. The final temperature in TGA experiments
manufacturing process from 1.5 wt% to Ͻ200 ppm. Both powders
were subsequently washed with deionized water, precipitated at
the point of zero charge (PZC) by the use of HNO3 or NH3
solutions, and freeze-dried. The washed powders were stored in
polyethylene boxes and no additional treatment, like intense drying
or purifying, was applied to the powders or the organics before
mixing.
was 800°C as there was no additional weight loss after that.
Sintering was done in a muffle furnace (Heraeus KS120) in air
atmosphere. Dilatometry was conducted using a B a¨ hr Dilatometer
8
02.
Analysis of phase content and particle size by line broadening
of powder and specimen samples was probed by X-ray diffraction
Siemens X-ray Diffractometer D 500), and several nitrogen
(
The interaction of the dispersing agent decanoic acid (DA) with
the powder surface was characterized initially by modifying the
powder surface through chemisorption of DA. This was done by
suspending the powder in industrial-grade octane (C H ), con-
desorption measurements for evaluation of SBET and pore volume
distribution (pvd) were taken using an ASAP 2010 (Micromerit-
ics). Rietveld software TOPAS-R was used to determine crystal
size and phase content by fundamental approach. The microstruc-
ture of sintered specimens was characterized by atomic force
microscopy (AFM, Park Scientific Instruments) in “error mode” to
enhance edge contrast. A Philips XL 30 FEG high-resolution
scanning electron microscope (HSEM) was used to examine
fracture surfaces of sintered bodies. Optical microscopy up to a
magnification of 1000ϫ was conducted in addition, using several
microscope setups.
8
18
taining enough DA to react with all available surface hy-
1
9,20
droxyls.
ϳ125°C) under reflux conditions until the deposition of water in
The suspensions were then heated until boiling
(
the stripper had stopped. After the modification reaction the
powders were dried for 2 h under vacuum at ϳ70°C to remove
residual octane and physisorbed DA. FT-IR spectra were recorded
from 200 mg of powder samples compacted with KBr using a
Bruker IFS 88 spectrometer.
Compounding (e.g., kneading) of the components was con-
ducted on a modular plastic processing unit (Plasti-Corder PL
III. Results and Discussion
2
000, Brabender oHG, Duisburg) using the discontinuous knead-
(
1) Powder Characterization, Surface Modification, and
Suspension Formulation
Transmission electron micrographs (TEM) of the powders are
shown in Figs. 1 and 2. Quantitative phase analysis shows the
powders being mainly tetragonal due to their small particle size
ing module W 350, equipped with roller paddles.
Compounding was done by adding all ingredients alternatingly
into the mixing chamber at a starting temperature of 80°C (100°C
in the case of PE-HD) and 30 rpm rotation speed, until a filling
3
level of 0.85% (ϳ300 cm or 1 kg of compound) was reached, then
(Table I).
homogenized by a special rotation speed program as described
later. After compounding, the suspensions were granulated in a
rotor beater mill, aided by cooling of the suspension with liquid
nitrogen to increase brittleness, i.e., to avoid smearing of the
mixtures at the milling blades.
Modification of powder surfaces changed the behavior of the
material from hydrophilic to hydrophobic. While MZ1 and MZ2
could easily be suspended in water, MZ1-DA and MZ2-DA (the
powders after modification) floated when added to water. After
The same propulsion unit as used for mixing, equipped with a
single screw extruder module, was used for extrusion. The tem-
peratures were generally 200°–210°–220°C for the barrel and
2
20°C for the separately heated dies. The extruder had a tap-water-
cooled feeding zone and was equipped with a compressionless
worm, usually used for high-throughput thermoplast extrusion.
The forming tools were of circular, ring, and rectangular cross
sections. The respective dies had a round shape of 8 mm diameter,
a ring shape of 14 mm diameter at 3 mm wall thickness, and 4
mm ϫ 5 mm and 1 mm ϫ 10 mm rectangular dimensions.
Extrusion pressure perpendicular to flow direction was measured
with a pressure gauge in each die. Extrusion screw diameter was
1
9 mm at an L/D ratio of 25 and rotation speed at extrusion was
generally 40 rpm except for extrusion through the ring die where
the rotation speed was 25 rpm.
(3) Binder Pyrolysis, Sintering, and Structure Analysis
Debinding was conducted in flowing oxygen-free nitrogen or
argon (5.0 purity) in a quartz tube furnace under a controlled
heating rate. The transparent quartz tube allowed observation of
Table II. Details of Organic Materials
Density
Melt range
(°C)
MFI 190/2.16
(g/10 min)
M៮
w
(g/mol)
3
Type
Grade
(g/cm )
EVA
V3510K
0.89
0.94
0.93
87–92
115–120
31–32
4.6
0.2
–
170 500
323 000
–
PE-HD GM5050
DA
D165-3
Fig. 1. Transmission electron micrograph of MZ1.

3
60
Journal of the American Ceramic Society—Scheying et al.
Vol. 87, No. 3
Fig. 3. FT-IR spectrum of MZ1-DA.
Ϫ
carboxylate groups (–COO ), and represent the respective sym-
metric and asymmetric stretching oscillations.
{
{
4
ϩ
Ϫ
O Zr–OH ϩ R–COOH O3 O Zr
}
OOC–R ϩ H O (2)
2
}
where R is C H in the case of DA.
9
19
The partial positively charged zirconium atoms act as Lewis
acid centers and originally dangling bonds (“perpendicular to the
2
2
surface”) are saturated with hydroxyls from ambient water.
Assuming that, according to Eq. (2), each hydroxyl-saturated Zr
atom can adsorb one molecule of DA, then aDA should be equal to
the amount of surface hydroxyls present. Other authors report
Fig. 2. Transmission electron micrograph of MZ2.
2
23
about 12 ␮mol/m of hydroxyl on ZrO surfaces. It is concluded
2
2
that the amount of roughly 5 ␮mol/m of DA equals one
stirring, the powder precipitated as lumps. After some time the
lumps decayed, and the buildup of a floating oil-like film on the
water surface was observed. This can be interpreted as evidence
that the surface modification is a reversible process. Preliminary
experiments showed that both modified powders are easily dis-
persed in molten paraffin wax, a completely nonpolar material.
The specific amount of DA chemisorbed to the powder surface,
monolayer also taking into account that the carboxylate group
needs somewhat more space than a hydroxyl group. Also, a single
carboxylate can be bound to two Zr sites at the surface since
charging of surface atoms is variable. Natural surfaces exhibit
structure elements like terraces, steps, and various other disrup-
tions which quantitatively determine the charge of surface atoms.
From the FT-IR spectrum it can furthermore be claimed that
␦ϩ
aDA, was independent of DA added to octane suspension and about
almost no residual or physisorbed DA is present since there is only
2
Ϫ1
5
␮mol/m for both powders (Table III).
a small absorption shoulder at ϳ1700 cm
.
To use those findings for extrusion suspension formulation it is
assumed that the amount of dispersant needed for an optimum of
phase compatibility equals the amount of DA to completely coat
the powder surface with a monolayer of a C10 carboxylate. This
was determined from preliminary experiments: mixing modified
powder with nonpolar media like paraffin wax and no segregation
could be observed.
x
aDA
ϭ
(1)
͑
1 Ϫ x͒MSBET
where aDA represents the specific amount of DA on the powder
surface, x is the percentage weight loss measured by TGA, and M
is the molecular weight of DA (172.3 g/mol). SBET was determined
from unmodified samples by nitrogen desorption before modifica-
tion. This method of powder surface modification with carboxylic
acids has been described before as a processing aid for powder
For a judgment on processability of a thermoplastic composi-
tion the following estimate has been drawn up. As a target for
ceramic technology we set a powder volume fraction of 0.5. To
ensure a reasonable flow behavior, the fraction of dispersant within
the organics should not exceed 50%. Inserting the densities of
2
1
compaction and other applications.
3
3
An FT-IR-spectrum recorded from surface-modified powder
sample MZ1-DA after vacuum drying is shown in Fig. 3 (the
spectrum for MZ2 is similar). All organic acids with carboxyl
groups show, independent of the state of matter, at least one sharp
powder (5.8 g/cm ) and organics (0.9 g/cm ), we find for the
volume fraction of dispersant within the organics, Fdisp, the
correlation sketched in Fig. 4. Both criteria cannot be kept in
2
conjunction for powders above 80–100 m /g. Keeping the flow
Ϫ1
oscillation band at ϳ1700 cm . The absence of this typical band
(Fig. 3), together with the observed deposition of water in the
stripper of the reflux cooler, indicates a saponification according to
reaction Eq. (2). The apparent bands in Fig. 3 at 1466 and 1541
Ϫ1
cm are to be interpreted as distinct evidence for the presence of
Table III. Amount of DA Chemisorbed
(by Thermogravimetric Analysis)
2
Sample
Powder
TGA (wt%)
a
DA (␮mol/m )
MZ1-DA
MZ2-DA
MZ1
MZ2
4.5
10.1
5.0
5.4
Fig. 4. Fraction of dispersant within organics needed as a function of
specific surface area (SBET).

March 2004
Monoclinic Zirconia Bodies by Thermoplastic Ceramic Extrusion
361
Fig. 5. Maximum possible powder volume fraction (pvf) as function of
specific surface area (SBET) assuming a coverage of 4.5 ␮mol/m DA.
2
Fig. 6. Pore size distribution of powders.
proof of homogeneity and purity of the suspension since entrap-
ment of air or any other material inclusions would reduce this
effect.
behavior criterion, the maximum possible powder volume fraction
drops below 50% as shown in Fig. 5 for a powder surface coverage
of 4.5 ␮mol/m . Based on these estimates, the compound formu-
2
Optical analysis of fracture surfaces of the rectangular cords
showed a circular core with a diameter of about half the sample
edge length. It became clear that this is due to cooling and
accompanied shrinkage of the cords after passing the die into
ambient condition. The accompanying stresses frequently resulted
in cutlike ruptures of the cords when reheated. This could be
avoided when extruding into a heated tube where cooling could be
controlled and adjusted. The symptom can be explained by the
poor heat conductivity of the green bodies and the relatively high
shrinkage which is described later by thermomechanical analysis
lations given in Table IV were created.
(2) Mixing, Suspension Homogeneity, and Extrusion
All components (i.e., ZrO2 powder, DA, and polymer) were
alternately filled into the kneading chamber in small quantities and
rolled around for at least 60 min, after all material had been poured
in. Rotational speed was varied during the process to ensure good
dispersion and homogeneity. The powder volume fraction, pvf, of
all compositions was verified by TGA of granulated samples of
each suspension after compounding (Table V).
To analyze the quality of dispersion, measurements of BET
surface area and pore size distibutions (psd) of the vehicle-free
samples (after TGA analysis) were made. It was found that SBET
did not change from powder to vehicle-free compound samples.
However, the psd changed dramatically, observable when compar-
ing the graphs in Figs. 6 and 12.
Pore diameters of about half of the median primary particle size
can theoretically be achieved at a pvf of ϳ0.5. The pore size
distribution is a good indicator of homogeneity of the green body,
i.e., an indication of quality of primary particle arrangement and
deagglomeration. It can be seen that a nearly ideal psd could be
achieved.
After granulation of the suspensions, extrusion was conducted.
The cords were extruded onto a conveyor belt after synchronizing
velocities. Experiments of controlled cooling of some of the cords
were performed by extrusion into a heated ceramic tube.
(TMA). Irregular shrinkage leads to microscopic segregation of the
“
core” from the “shell” within the cords and the dividing line
becomes visible.
Preliminary debinding experiments showed that debinding in air
leads to damage of all samples. Very flat heating ramps like 1 K/h
and also fast ramps of 200 K/h had been tested in addition to
heating ramps adjusted to the rate of weight loss (“rate-controlled
debinding”) but all failed. It was obvious that oxygen-containing
atmosphere damaged the samples by oxidizing their surfaces. In
the case of KN1, the damage was assumed to be caused by
O -catalyzed cross-linking of the polymer in surface regions of the
2
specimens up to 300°C. When production of volatile pyrolysis
products started at about 380°C, the flaking of the sample surface
and of the whole specimen in a later stage was caused by vapor
pressure from inside the bodies against the cross-linked polymer
surface which had a much lower gas permeability than thermo-
plastic polymers. In the case of KN2 and KN3, the damage looked
the same although the vehicle consisted of a different polymer.
Debinding in O -free atmosphere leads to different results. The
PE-HD-binder containing samples show an appearance similar to
puff pastry” after reaching a temperature of ϳ400°C. This is in
correlation to the typical hydrocarbon (HC) emission diagram
shown in Fig. 8. The PE-HD samples revealed only one temper-
ature region of HC emission and accompanying significant weight
2
(
3) Green Body Characterization and Binder Pyrolysis
Before binder burnout the green bodies were inspected optically
“
for macroscopic defects. The appearance of the samples is shown
in Fig. 7, where the most characteristic feature—a distinct trans-
lucency—can be observed. This can be considered additional
Table IV. Composition of Suspensions
Sum of
organics
(wt%)
MZ1
(wt%)
MZ2
(wt%)
DA
(wt%)
EVA
(wt%)
PE-HD
(wt%)
Type
KN1
KN2
KN3
86.6
86.6
–
–
–
83.3
13.4
13.4
16.7
4
–
9.4
7.3
9.4
–
–
4
9.4
Table V. Analysis of Suspensions
†
†
pvf, calculated
(vol%)
Mixing time
(min)
TGA
(wt%)
pvf from TGA
Type
(vol%)
KN1
KN2
KN3
†
50.1
50.1
43.6
60
120
60
13.35
13.03
16.47
50.2
50.9
44.1
Fig. 7. Optical apperance of samples with dimensions of 1 mm ϫ 10 mm
and 4 mm ϫ 5 mm with rectangular cross sections. Heights of samples are
1 and 4 mm, respectively.
pvf ϭ powder volume fraction.

3
62
Journal of the American Ceramic Society—Scheying et al.
Vol. 87, No. 3
Fig. 8. Temperature and HC emission at pyrolysis of KN1 vehicle.
Fig. 10. Thermomechanical analysis (TMA) of KN2 in all dimensions
(each dimension separately measuerd). Heating rate is 2 K/min.
loss. On the other hand, samples from EVA (KN2/KN3) showed a
weight loss and HC emission in two steps, as shown in Fig. 9. This
indicates for KN1 that both the cracking reactions of the polymeric
vehicle PE-HD and the pyrolysis of chemisorbed DA are taking
place in the same temperature range, from 390° to 440°C. PE-HD
is cracked to lower molecular weight polyethylene waxes which
could be collected from colder sections of the quartz tube of the
furnace. The vapor pressure developed by the simultaneous crack-
ing of PE-HD and DA seem to be the reason for the damage of all
the KN1 bodies since this was not apparent in the samples from
EVA-containing suspensions. It is furthermore concluded that the
crack products of PE-HD are of higher molecular weight than
those from EVA since condensed waxes in the furnace were of
obviously higher viscosity. No further investigation of debinding
bodies from KN1 has been conducted.
However, for samples KN2/KN3 a failure mode not correlated
with weight loss or HC emission of the bodies persisted. The parts
delaminated at the core–shell line described before, or small
intrinsic crack initiation, leads to breaking of the specimens. No
swelling, blistering, or other shape-altering deformation was ob-
served. The failures were only noticeable when samples were
taken out of the furnace after debinding. The irregular appearance
of that damage could not be correlated to parameters like location
in the furnace or gas flow rate. Their occurrence was coincidental
and not reproducible when debinding different parts of the same
cord.
Special attention was given to adjust the heating ramps to
overcome the described sample damage. From observation of the
damage, HC emission, and weight loss, the furnace heating
program displayed in Fig. 11 was developed. This heating program
could be applied for debinding specimens up to a wall thickness of
6
mm with a very low scrap rate of about 5%. The heating ramps
had been generally adjusted to the HC emission rate and the
temperature range of the highest expansion rate from TGA
measurements. It has to be noted here that the amount of HC
emission depends on sample mass when comparing different
graphs. After the HC emission from the samples had stopped, the
bodies revealed an average carbon content of 1.3% for KN2 and
1
.6% for KN3. All bodies were almost black in color, indicating an
overall carbon layer from vehicle residues.
(4) Sintering and Nanostructure Characterization
Sintering behavior was examined by monitoring the linear
shrinkage of vehicle-free samples up to a temperature of 1083°C
for KN3 and 1110°C for KN2. Both formulations led to a mean
density of 99% of theoretical after sintering. The vehicle-free
samples of KN2 and KN3 had almost the same density before
sintering, as was obvious when comparing the linear shrinkage of
1
5.2% and 15.3%, respectively. Density after debinding, i.e.,
before sintering, was calculated from shrinkage data as 61% for
both KN2 and KN3. This means that green bodies made of KN3
undergo a much higher shrinkage on debinding. Starting densities of
the corresponding suspensions were 50% and 44% (see Table V).
KN3 shows better densification performance with respect to
temperature and dwell time necessary for complete consolidation.
This is due to the smaller particle size and the smaller pore size
distribution of MZ2. The pore size distribution of the respective
vehicle-free parts is shown in Fig. 12.
After sintering, some of the bodies revealed the same damage as
described above: flaws or cracks with sharp, cutlike edges or
continuous breaks which made some of the samples fall apart. The
tubular specimen had the lowest scrap rate, which is most probably
due to the better stress relaxation ability of this geometry. Since all
failures after sintering looked the same as the damage occurring in
The cause of this crack-initiated sample rupture was investi-
gated using static TMA on samples of ϳ4 mm ϫ 5 mm ϫ 5 mm
with quad cross section, cut out from the respective cords. Figure
1
0 shows the average linear thermal expansion in all dimensions at
a heating rate of 2 K/min for KN2, measured separately. In
addition to the noticeable higher expansion in the z-direction at
about 300°C (this is the direction of extrusion flow) a much higher
expansion rate can be observed. This could be caused by relaxation
of incorporated anisotropic stresses and similar effects, which have
2
4,25
been reported by other authors.
Mechanical stress is brought
into the samples through the extrusion process. Pressures in the
dies were up to 70 MPa. Part of this stress could be lowered by
controlled cooling of the cords after extrusion, where a small linear
expansion of about 0.5%–1% in the z-direction was measured.
Fig. 9. Interrelation of HC emission and weight loss for KN2/KN3. Same
Fig. 11. Temperature program adjusted to HC emission and adapted to
heating rate as above.
overcome sample damage.

March 2004
Monoclinic Zirconia Bodies by Thermoplastic Ceramic Extrusion
363
Fig. 12. Pore size distribution of vehicle-free samples from KN2 and
KN3.
the debinding stage, it is presumed that a sort of “predamage” was
invisibly incorporated, like subsurface flaws. Some of the samples
fell apart with audible cracks while transporting them from the
quartz tube furnace to the sintering furnace. When special attention
was given to handling vehicle-free samples free from mechanical
and thermal shock, the given scrap rate of ϳ5% was not altered by
sintering.
Fig. 14. High-resolution scanning electron micrograph of a fracture
An AFM picture of a polished and thermally etched surface of
a KN2 sample is displayed in Fig. 13. Grains of different sizes with
a maximum diameter of ϳ70 nm can be observed. Figure 14 shows
an HSEM picture of a raw crack surface of KN3 after sintering.
The maximum grain size is about 40 nm. It is very interesting and
unusual that the grains in the sintered structure of KN2 hardly
grew despite the long dwell time at 1100°C. The material is purely
monoclinic.
surface of sintered KN3 body.
about 5% was recorded, this is the first time that bodies of different
shapes from monoclinic ZrO were produced in a simple process.
As a new approach to ceramic suspension formulation, the amount
of dispersant needed was calculated, based on generally applicable
equations. Contrary to current understanding, the polymeric vehi-
cle could be pyrolyzed from pores as small as 5 nm in less than
2
IV. Conclusions
2
5
4 h, through a wall thickness of 6 mm with a scrap rate of only
%.
An industry compliant process of fabricating dense ceramic
These advances in processing allow the manufacture of mono-
bodies of monoclinic ZrO from powders with surface areas up to
2
2
clinic ZrO
2
with a sintering temperature of 1100°C or below.
1
20 m /g has been presented. Only industrial-grade organic
chemicals and polymers, together with standard polymer mixing
and shaping equipment, were used to produce cords with various
cross sections by thermoplastic extrusion. Although a scrap rate of
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Ⅺ