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Freeze Granulation for the Processing of Silicon Nitride Ceramics
O. Lyckfeldt1, K. Rundgren2 and M. Sjöstedt3
1Swedish Ceramic Institute, P.O. Box 5403, SE-402 29 Göteborg, Sweden
2Permascand AB, P.O. Box 42, SE-840 10 Ljungaverk, Sweden
3PowderPro HB, Skogsängsvägen 19, SE-422 47 Hisings Backa, Sweden
Keywords: Freeze granulation, Pressing, Silicon Nitride
Abstract. Freeze granulation (LS-2, PowderPro HB, Sweden) has been demonstrated as a
favourable alternative to conventional granulation methods (spray drying, sieve granulation etc) in
the production of granules for the pressing of high-performance ceramic powders. Freeze
granulation/freeze drying prevents the migration of pressing aids or particle fines to the granule
surface, as is the case in spray drying. This ensures granule homogeneity and an easy breakdown of
granules during pressing. This, in turn, results in defect minimisation and optimal conditions for the
sintering and the development of the desired material properties. In this study silicon nitride
materials have been produced using freeze granulation, pressing and sintering to validate the
performance. Materials with competitive properties were manufactured based on medium-cost,
direct-nitrided powders (SicoNide P95, Permascand AB, Sweden), various pressing and sintering
aid compositions as well as various pressing and sintering schedules. MgO vs Fe2O3 as sintering
aid, PEG vs PVA as binder and higher pressure at the initial uniaxial pressing were found to
promote the sintering performance.
Introduction
When using powders in the submicron range, it is necessary to granulate the powder to enable
proper tool filling in the pressing of advanced ceramics. Pressing aids (binder and plasticizer) are
also required to achieve adequate pressing performance and sufficient handling strength of pressed
specimens. Large-scale granulation of ceramic powders normally takes place by preparing a
suspension and spraying it through a drop-atomising nozzle into a chamber with dry air, so called
spray drying. A commonly used technique on a smaller scale is sieve granulation in which a wet
powder mix is forced through sieves to create clusters or granules. Both these techniques have the
disadvantage of liquid transport during drying. This can cause migration of fines as well as of
pressing aids to the periphery of the granules making them hard to disintegrate (crush) at pressing,
thus causing strength-limiting defects in the sintered material. The use of water as liquid medium
often also creates hard bonding between the ceramic particles at drying which accentuates the
disintegration problems.
An alternative granulation technique for ceramic powders that has been developed at the
Swedish Ceramic Institute is denoted freeze granulation [1,2]. When spraying the powder
suspension similarly to spray drying, but in this case into a container with liquid nitrogen, freezing
of the suspension drops (granules) occurs instantaneously (Fig. 1). Subsequently, the frozen
granules are freeze dried, which means that the liquid is sublimated without liquid transport and risk
of migration phenomena (Fig. 2). In this way, spherical and homogeneous granules with excellent
flow properties and pressing performance are produced.
This paper exemplifies the uses of freeze granulation for producing high-performance silicon
nitride ceramics based on direct-nitrided Si3N4 powders (SicoNide P95L and P95H, Permascand
AB, Sweden) and water processing [3,4].
Key Engineering Materials Vols. 264-268 (2004) pp. 281-284
online at http://www.scientific.net
© (2004) Trans Tech Publications, Switzerland
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the
written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 129.16.146.44-26/02/07,10:47:33)
Slip Droplet
Freeze granulation
No shrinkage: density and
homogeneity retained
Sublimation
Shrinkage during drying
increases granule density
Void formation
Spray drying
Migration of binder
and small particles
to surface
Formation of
dried shell
Liquid transport
Fig. 1. Schematic illustration of the freeze-
granulation process.
Fig. 2. Comparison between freeze granulation
and spray drying regarding the drying effects.
Material and Experimental
All material used in this work are listed in Table 1. Slips with 40 vol% powder were prepared by
planetary milling using Si3N4 balls and linings. No dispersant was used owing to the ability of pH
adjustment to about 10 by the Si3N4 powders, which results in an efficient electrostatic stabilisation.
The two Si3N4 powders, P95L and P95H, have broad particle size distributions with a d50 of 1.3 and
0.9, and specific surface areas of about 6 and about 10 m2/g, respectively.
Table 1. Ceramic powders and organic additives used in this study.
Material Label Producer
Si3N4 SicoNide P95L and P95H Permascand AB, Sweden
Sintering aids
Y2O3
Al2O3
MgO
Fe2O3
Grade C
AKP 30 and AKP 50
Analytic grade
Chemical grade
HC Starck GmbH, Germany
Sumitomo Corp., Japan
Merck KGaA, Germany
Panreac Quimica SA, Spain
Pressing aids
PVA (Mw 31000)
PEG 20000
PEG 400
Mowiol 4–88
Carbowax 20M
Clariant GmbH, Germany
Union Carbide, Belgium
Merck-Schuchardt, Germany
Together with 4 wt% Y2O3, 2 wt% Al2O3 and 0.25 wt% MgO or 0.37 wt% Fe2O3 resulting in
the same total volume of sintering aid, the P95L or P95H was milled for 24 h or 48 h at 100 rpm.
Pressing aids composed of 6 vol% PVA or PEG 20000 and 1.5 vol% PEG 400 based on solids were
added and freeze granulation took place in a lab-scale granulator (LS-2, PowderPro HB, Sweden).
Frozen granules were freeze dried in a Lyovac GT2 (Leybold AB, Sweden) and characterised by
flow test, measurement of tap density and studies in a LOM.
Uniaxial pressing of small quadratic specimens (14.5 × 14.5 × 10 mm) and large discs (∅ = 65
mm, h = 7 mm) was conducted with various pressures (10 to 80 MPa). All specimens were finally
isostatically pressed at 300 MPa. The organic material was removed in nitrogen with 1ºC/min up to
500ºC and a dwell time of 30 min. Sintering was carried out with a gas-pressure sintering furnace
(FPW 250/300, FCT, Germany) using 0.5–3 h at 1800ºC and 1 MPa N2 (g) followed by 0.5–3 h at
1900ºC and 2 MPa N2 (g). The specimens were placed in a Si3N4 powder bed in a Si3N4 crucible.
Material evaluations took place by density measurements using the water intrusion method
according to Archimedes’ principle. Specimens were polished and etched for SEM studies (JEOX-
Euro Ceramics VIII282
8600, JEOL, Japan). Hardness and fracture toughness were measured by Vickers indentation and
bars were machined from the large sintered discs for flexural strength tests.
Results and Discussion
Fig. 3 illustrates the viscosity profiles utilising the different pressing aids. Using PVA as binder
gave considerably higher viscosity than PEG but did not critically disturb the granulation
performance. In general, continuous shear-thinning behaviour ensures proper flow and atomisation
in the spray nozzle. However, also viscoelastic properties, for example, displayed by the degree of
elasticity at high frequency in oscillatory shear measurements, is considered important as an
indicator of possible elongational flow and inaccurate drop formation. Fig. 4 shows an image of the
spherical and homogeneous granules in the size range of 50–400 µm obtained using the PVA slip.
The measured flowability (160 g/50 s, ISO 4490) and tap density (0.73 g/cm3, EN 23923-2) also
indicated suitable properties for pressing operations.
0.01
0.1
1
1 10 100 1000
Shear rate (s-1)
Viscosity (Pa s)
Slip with PVA/PEG
Slip with PEG/PEG
Fig. 3. Viscosity profiles of Si3N4 slips for
granulation containing PVA/PEG or PEG/PEG.
Fig. 4. LOM image of granules produced by
freeze granulation with PVA/PEG.
Manual measurements showed pressed densities (after debinding) in the range of 59–61% of
the theoretical. With PVA/PEG the pressed density reached 1–1.5% higher values than those
obtained with PEG/PEG. The compaction was also promoted (0.5–1%) by higher initial uniaxial
pressure, although the same subsequent isostatic pressure was used.
Density measurements of sintered Si3N4 specimens showed that full densification could be
reached independent of the pressing and/or sintering aid composition, if a sufficient time (≥ 1 h)
was used at the initial dwell at a lower temperature and pressure (1800ºC and 1 MPa). Wet milling
of the Si3N4 powders, 24 h with P95H and 48 h with P95L, eliminated the coarse fractions of
particles and enhanced the sinterability. Specimens with Fe2O3 required longer initial dwell (2 h)
than specimens with MgO included in the sintering aid composition. 1–2 h at the second dwell
(1900ºC and 2 MPa) were required for both compositions.
Although specimens with PVA/PEG showed higher pressed density, specimens with PEG/PEG
appeared to densify more rapidly during sintering. It was proposed that the higher carbon residues
expected from PVA tended to retard the sintering performance. Pressing with higher uniaxial
pressure resulting in a higher degree of compaction also promoted the sintering performance and
full density was reached in a shorter time. At those sintering schedules that resulted in full
densification for materials with PVA and/or pressed with low uniaxial pressure, specimens with
PEG and/or pressed with high uniaxial pressing displayed coarser microstructure owing to a faster
densification. Figs. 5 and 6 show examples of the typical bimodal and essentially pore-free
microstructures achieved with the two sintering aid compositions. The coarser grain-structure
appearance in Fig. 5 can be attributed the MgO content that promoted sintering and allowed time
Key Engineering Materials Vols. 264-268 283
for the Ostwald ripening. Overall, the results showed that many factors influence the sintering of
Si3N4 materials and that the sintering schedule has to be adapted to achieve desirable
microstructure.
Fig. 5. SEM image of specimen based on 48 h
milled P95L, MgO and PVA sintered with 2 h at
each dwell.
Fig. 6. SEM image of specimen based on 24 h
milled P95H, Fe2O3 and PEG sintered with 2 h
at each dwell.
Regarding mechanical properties, specimens with MgO yielded higher Vickers hardness, about
15 GPa, compared to 13 GPa for specimens with Fe2O3. The fracture toughness was in the range of
6–7 MPa m-1 for both types, with higher values for the materials with coarser and more pronounced
bimodal grain structures. 4-point bending strength at optimised sintering was in the range of 800
MPa, which is considered to be competitive for Si3N4 materials based on the low-to-medium cost
powders and processing routes used.
Summary and Conclusions
Freeze granulation has been shown to be a very suitable method for preparing powders for pressing
and sintering of Si3N4 materials. Spherical and free-flowing granules with optimal homogeneity
owing to the freeze-drying concept made it possible to detect effects of other factors on the sintering
performance of pressed specimens. Together with Y2O3 and Al2O3, small amount of MgO favoured
sintering more than Fe2O3 as sintering aid. PVA as binder favoured compaction at pressing but
tended to retard sintering compared to PEG owing to higher carbon residues. Higher pressure at the
initial uniaxial pressing favoured the degree of compaction and the sintering performance
independent of the fact that the same pressure was used in the subsequent isostatic pressing. Si3N4
materials with competitive properties were produced by adapting the gas-pressure sintering
schedule to the specific properties of the specimens, caused by the composition and/or processing.
Typically, Vickers hardness of 13–15 GPa, fracture toughness of 6–7 MPa m-1 and 4-point bending
strength of 800 MPa were achieved.
References
[1] B. Nyberg, E. Carlström and R. Carlsson: Euro-Ceramics II, Vol. 1 (Deutsche Keramische
Gesellschaft, 1993) p 447-451.
[2] N. Uchida, T. Hiranami, S Tanaka and K. Uematsu: Am. Ceram. Soc. Bull., 81, [2] (2002) p 57-
60.
[3] K. Rundgren and O. Lyckfeldt: Ceram. Eng. Sci. Proc., 23 [3] (2002) p 3–10.
[4] O. Lyckfeldt, D. Käck and K. Rundgren: In print, Ceram. Eng. Sci. Proc. (2003).
Euro Ceramics VIII284
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