1. Journal of Materials Processing Technology 190 (2007) 243–250
Investigations in the compaction and sintering of large ceramic parts
Peng Chen ∗ , Gap-Yong Kim, Jun Ni
Department of Mechanical Engineering, University of Michigan, 1210 HH Dow Bldg., 2350 Hayward St,
Ann Arbor, MI 48109, USA
Received 11 September 2006; received in revised form 12 February 2007; accepted 22 February 2007
Abstract
In this study, a large ceramic part was successfully compacted and sintered using uniaxial die compaction technique. The effects of die design,
compaction pressure, lubrication, sintering procedure and part orientation in the oven on the P/M part quality were investigated and the preferred
process conditions were discussed and concluded. The main quality issues encountered were cracking and distortion. A finite element model for
the powder compaction process was also developed and validated. Based on the model, the relationship between the cracking location and the
density distribution predicted from finite element analysis (FEA) was discussed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Uniaxial die compaction; Sintering; Lubrication; Die design
1. Introduction tions of the parts. This density distribution mainly originates
from particle–particle and die wall–particle friction, which cause
Ceramics have become increasingly important in modern quality issues such as cracking and warpage during the sinter-
industry due to their good mechanical and physical properties ing. Especially, when a large part size is considered, the poor
[1]. Ceramic parts are generally produced by combination of control over the density uniformity of the green part becomes
compaction and sintering. However, cracks and distortions have significant and results in part failures. In general, the dimen-
been recognized as the most significant concerns, and often limit sions of the sintered compacts are in the range of 10–100 mm
the application of uniaxial die compaction technology to produce [4].
large-scale ceramic parts. Cracks in powder metallurgy (P/M) The modeling methodology for powder compaction can be
components primarily originate from the compaction prior to classified into three categories based on the length scale [5]:
the sintering. Although the cracks may not become evident until (1) continuum models; (2) multi-particle models (discrete ele-
the sintering has occurred, the root cause is most likely the poor ment models and particle dynamics models) [6–8]; and (3)
interparticle bonding obtained prior to the sintering [1–4]. Usu- atomistic/molecular dynamics models [5,9]. Continuum mod-
ally, micro-cracks that are invisible during the compaction are els assume that a response of a powder packing is continuous
carried over and enlarged during the sintering. Another common and similar to that of a solid material. Hence, they are attrac-
challenge in sintering is the dimensional control of the sintered tive in practical applications and have been employed in this
products. Warpage could occur as a result of green density gra- study. There are two types of continuum models. The first is
dient, friction drag (caused by the support material), gravity, and the micromechanical model [10,11], which provides a way to
temperature gradient [4]. derive macroscopic model parameters from information on a
Uniaxial die compaction is the simplest form of consolida- smaller particle-level length scale. The micro-to-macro tran-
tion process that has been extensively used to densify powder sition is performed usually by homogenization techniques.
materials. One disadvantage of this technique, however, is the The other is the empirical (phenomenological) model [12–14],
variation of the pressed density that can occur at different loca- which is formulated using several ‘material’ functions that
describe the response of a specific porous material to the
stress.
∗ Corresponding author. Tel.: +1 734 615 7445 The objective of the study is to investigate the effects of pro-
E-mail address: pengchen@umich.edu (P. Chen). cess parameters to obtain a crack-free ring shaped part, which
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2007.02.039

2. 244 P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250
Fig. 1. (a) Instron 1136; (b) zirconia disk compaction die set; (c) section view of the tooling.
has a final dimension of 90 mm after uniaxial compaction and relatively slow speed. Finally, the base plate was replaced by an ejection ring,
sintering with minimum distortion. and the green part was pushed out. The die wall was lubricated with mineral oil,
which was recommended by the powder vendor.
Based on the shrinkage ratios from the preliminary experiments, the dimen-
2. Experimental and simulation procedure sions of the die for the ring component was determined as shown in Fig. 2. A
previous attempt with a one-piece die design similar to the disk shaped part
2.1. Experimental setup and procedures as shown in Fig. 1 but with a diameter of 114 mm resulted in green parts with
cracks due to excessive friction during ejection process, which caused a large
Two types of experimental setup have been designed and fabricated for this density gradient in the green compact. Therefore, a split die design was adopted
study. To acquire a final part dimension of 90 mm, green part dimensions need to reduce the friction during ejection as shown in Fig. 2. The Container was
to be larger than 110 mm, which is rather large for a part made by uniaxial designed, so it may be split into two halves to facilitate the ejection process. The
compaction. Since the final target part dimensions were larger than those of two symmetric pieces were assembled by eight screws and two aligning pins.
a typical ceramic powder compaction and sintering, a preliminary experiment The core rod was designed to produce a ring shaped part, which was assembled
was performed to compact a cylindrical part with a diameter of 46.4 mm to by two screws and an aligning pin. All the tooling components were made from
investigate appropriate range of process parameters. Based on the results from A2 tool steel, which were heat treated to the hardness of HRC 55 and were
this preliminary experiment, a die for the larger ring shaped part was designed precision ground.
and fabricated. The detailed experimental setup for the compaction and sintering The experiment cycle requires assembly and disassembly of the die com-
of both sizes are described below. ponents. First, the Dividable Container is assembled. Next, core rod and base
A commercial zirconia powder (YSZ, Inframat Advanced Materials, LLC) plate are placed in the container, and then 380 g of zirconia powder is poured
stabilized by 3 mol% Y2 O3 was used for all the experiments conducted in this into the container. The Punch is placed on top of the powder, which is guided by
study. The particles have a mean particle size of 0.5 m. the core rod and the container during the compaction. A constant punch speed is
The setup for the preliminary experiment is shown in Fig. 1, which includes retained by Instron 1136 material testing system. After holding the punch at the
a punch, a base plate, and a die. The components were made by precision milling final position until the load is stabilized, it is released at a relatively lower speed.
and turning process from A2 tool steel. 50 g of zirconia powder was poured into After the compaction, the Dividable Container is disassembled, and the base
the assembled die/punch set. The setup was then placed in the Instron 1136 plate is detached from the core rod to eject the compact. Finally, the ejected part
material testing system for compaction, which had a single acting upper punch. is sintered in an oven. Three sintering procedures (Fig. 3) were used in this study.
The compaction process was achieved by a constant punch speed. The punch Procedure II has a higher sintering temperature than Procedure I. Procedure III
was held at the final position until the force stabilized and was released at a has a very slow heating speed than those of Procedure I and II.
Fig. 2. (a) Zirconia ring compaction die set; (b) section view of the tooling.

3. P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250 245
Table 1
Material properties of zirconia powder [14]
Full density Initial density Particle size β α
6.08 (g/cm3 ) 1.885 (g/cm3 ) 0.53 m (Nominal) 54.3◦ 0.03
E ν d R
206 GPa 0.31 1.53 MPa 0.835
Fig. 3. Illustration of the sintering procedures.
2.2. Material modeling for compaction simulation
In order to simulate the zirconia powder compaction process, a finite element
analysis (FEA) model was developed utilizing the material property data from
Fig. 5. Densification behavior of the zirconia powder [14].
previous study in literature [14]. It is necessary to recognize the major physical
phenomena that occur during the compaction of ceramic particles. The com-
paction process can be divided into three main distinctive stages. In the early
deformation. If the stress state is such that Eq. (1) is satisfied, the material fails
stages of compaction, particles are rearranged (we will refer to this as stage 0
in shearing. At high hydrostatic pressures, the yield surface is described by a
compaction). As compaction force is further exerted onto the powders, the rel-
cap surface, Fc :
ative density (RD, which is defined as the ratio of the density of the compact to
the full density of the material) increases, and compaction is accommodated by
R×q 2
elastic deformation of the particles (stage 1 compaction). At higher pressure, the Fc (q, p) = (p − pa )2 +
compact structure will breakdown with a small amount of particle rearrangement 1 + α − α/cos(β)
(stage 2 compaction). −R × (d + pa × tan(β) = 0 (2)
In this study, we employed the modified Drucker–Prager/Cap (DPC) model,
which has been widely used in powder metallurgy and ceramic industry (Fig. 4). R is a material parameter that controls the shape of the cap. pa is an
It is a phenomenological model that has been adapted from soil mechanics. The evolution parameter that represents the volumetric inelastic strain driven hard-
model is attractive in compaction modeling because it contains features that ening/softening, which is related to hydrostatic compression yield stress (pb ).
are in accordance with the physical response of particulate compacts [12,15]. The parameters pa and R may be obtained from compaction experiments. The
The DPC model at low hydrostatic pressure is a shear failure model, similar to parameter α does not have a physical meaning, but ensures a smooth transition
those used in granular flow, which reflects the dependence of the strength on between the cap and the shear failure regions for numerical robustness. Typi-
the confining pressure. This enables the model to predict the strength in tension cally, a small value (α = 0.01–0.05) is used to avoid the situation of α = 0, which
to be smaller than the strength in compression, a concept which is common for will form a sharp corner at the intersection of Fc and FS . This may lead to
rocks, brittle materials, and pressed powder compacts. In its simplest form, it numerical problems [16]. The geometric representation of the complete yield
is represented by a straight line in the p–q plane, which is also known as the locus is represented in the p–q plane as a limiting curve F(q, p, RD) = 0 in Fig. 4.
Mohr–oulomb shear failure line, FS : This form is consistent with the DPC model implemented in the finite element
package ABAQUS, which was used for this study.
FS (q, p) = q − d − p × tan(β) = 0 (1) The FEA results were compared with the medium size compaction exper-
iments (Ø = 46.4 mm). The material properties used for the simulation are
where d and β are cohesion and internal friction angle, respectively. If the stress summarized in Table 1. These values were adopted from the experimental work
state is such that the corresponding Mises equivalent stress (q) and hydrostatic of Kim et al. [14] (3 mol% Y2 O3 stabilized zirconia powder, HSY-3.0, Daiichi-
pressure (p) result in a value of F(q, p) < 0, then the stress causes only elastic Kigenso Kagaku Kogyo Co. Ltd., Japan). Fig. 5 shows the densification behavior
of this powder.
Considering the geometric symmetry of the process, only an axisymmet-
ric section of the compact was simulated using the commercial FEA software,
ABAQUS v6.5. The tooling was represented by rigid elements, whereas the
material mesh for the powder consisted of an array of 4-node bilinear axisym-
metric quadrilateral elements with reduced integration (CAX4R).
3. Results and discussions
3.1. Compaction of ceramic disk (Ø = 46.4 mm)
3.1.1. Compaction results and discussion
As summarized in Table 2, different process parameter levels
Fig. 4. The Drucker–Prager/Cap (DPC) model [5]. were investigated. The green part quality was evaluated in terms

4. 246 P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250
Table 2
Experimental conditions for the compaction (Ø 46.4 mm disk)
Case Pressure (MPa) Pressing speed (mm/s) Releasing speed (mm/s) Ejection speed (mm/s) Green Part
1 92.30 0.085 0.004 0.008 Horizontal crack
2 134.5 0.042 0.021 Manual Horizontal crack
3 118.7 0.042 0.008 Manual Horizontal crack
4 65.93 0.042 0.008 2.117 Crack free
5 (3 repeats) 65.93 0.042 0.008 4.233 Crack free
6 (3 repeats) 52.74 0.042 0.008 4.223 Crack free
of the occurrence of cracks. It was found that the most important tion as shown in Fig. 7. In general, the simulation results agree
process parameter was the compaction pressure. The effects of well with the experiment. The underestimation of the load at
pressing, releasing and ejection speed on cracking were rela- the initial loading stage is most likely due to the inaccuracy of
tively small. As long as the compaction pressure was less than the material modeling at low densities. As explained in [17], the
or around 65.9 MPa, cracks did not occur. Typically, a crack material parameters for DPC model at low densities are usu-
is initiated by the existence of a sharp density gradient. When ally not obtainable from the material testing experiment: the
the compaction force is reduced, the density gradient of the lower the density, the more measurement noise in the experi-
green part also decreases correspondingly. Therefore, a smaller ment. In addition, the powders used in the experiment (Inframat
compression pressure helps to avoid cracks [2]. Fig. 6 shows a Advanced Materials, LLC) and simulation (Daiichi-Kigenso
successful and an unsuccessful case from the compaction: one Kagaku Kogyo Co. Ltd.) were from different sources, which may
has a horizontal crack (case #2), and the other is free of crack have contributed to the different loading characteristic, although
(case #5). the powders were the same grade. The green part height from
the simulation (11.866 mm) agreed well with that of the actual
3.1.2. FEA results and discussion part (11.557 mm), and the final density of the green part from
The loading curve obtained from the experiment case #5 from the simulation (2.554 g/cm3 , at the top surface) also matched the
Table 2 was compared with the loading curve from the simula- actual part density (2.575 g/cm3 ).
Furthermore, the model was utilized to study the relationship
between the density distribution and the crack formation. Thus,
a crack-free case (case #5) and a case with cracks (case #2) were
simulated. Fig. 8 shows the relative density distribution of the
simulated parts after ejection. Following observations have been
made from Fig. 8.
1. The highest density in a crack-free case (case #5) occurred
at the upper corner of the compact, which agreed with the
common practice in the sense that the upper corner of the
part experiences the highest compaction pressure [14]. On
Fig. 6. Compacted part: (a) Case #2; (b) Case #5. Fig. 7. Loading curve comparison (Case #5).

5. P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250 247
Fig. 8. Density distribution after ejection: (a) Case #5; (b) Case #2.
the other hand, the highest density for the case #2 (which Fig. 9. Density distribution after ejection (ring compaction with a pressure of
had cracks in the experiment) occurred below the upper 40.49 MPa).
surface.
2. Case #2 has a sharper density gradient near the upper corner The simulation results indicated the migration of the loca-
region, and the corresponding relative density curves exhibit tion of the highest density region of a compact from the top
a sharp distribution. This indicates a sudden change of density surface to below the surface as the pressure was increased. As
in a localized area, while the relative density curves of case the pressure increased, cracks developed under the top surface
#5 are smoother and corresponds to a more uniform density as shown in the actual part (Fig. 6a case #2). The location of the
distribution. crack corresponds to the highest density area in Fig. 8b. When
Table 3
Zirconia ring compaction experiment conditions
Case Pressure (MPa) Green part before ejection Lubrication of the base plate surface Separation between the compact
and lower portion of core rod and the base plate
Ring 1 40.49 Crack free Mineral oil A blade was used for the
separation, which resulted in a
bad compact surface condition
Ring 2 40.49 Crack free Mineral oil
Ring 3 40.49 Crack free Wax
Ring 4 40.49 Crack free Aluminum foil Easy separation, but the foil was
embedded into the compact,
elimination of the foil resulted in
a very bad compact surface
condition and cracks, Fig. 9 a)
Ring 5 40.49 Crack free Coolube 5500 metalworking fluid Failed, cracks
Ring 6 28.92 Crack free Water based graphite particle Successful
lubricant (Lubrodal F705 ALX)
Ring 7 57.84 Crack free Oil based graphite particle lubricant Successful, Fig. 9 b)
(Lubrodal Hykogeen Conc HI)
Ring 8 40.49 Crack free Oil based graphite particle lubricant Successful
(Lubrodal Hykogeen Conc HI)
Ring 9 52.05 Crack free Oil based graphite particle lubricant Successful
(Lubrodal Hykogeen Conc HI)
Ring 10 58.3 Crack free Oil based graphite particle lubricant Successful
(Lubrodal Hykogeen Conc HI)
Ring 11 63.62 Crack free Oil based Graphite Particle lubricant Successful
(Lubrodal Hykogeen Conc HI)
Ring 12 72.88 Crack free Oil based graphite particle lubricant Successful
(Lubrodal Hykogeen Conc HI)

6. 248 P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250
the high-density region is located on the top surface, a density
gradient is created from top to bottom as indicated in Fig. 8,
case #5. In contrast, as the pressure increased, the high-density
region shifted under the top surface, and the density gradient was
created such that it caused tensile internal stress at the location
as indicated in Fig. 8, case #2. It is speculated that the cracks
initiate from these highly dense regions to relieve the internal
stress built up from the density gradient.
3.2. Compaction and sintering of a large scale ceramic
ring (Ø = 114 mm)
3.2.1. Compaction simulation results and analysis
The simulation tool was used to detect potential cracks that
can form in the ring shaped part prior to designing of an exper-
imental setup. The resulting density distribution of the ejected
part at a compaction pressure of 40.49 MPa is shown in Fig. 9.
The relative density distribution was smooth, and no potential
crack locations could be identified. This was proved later by the
experiment having the same condition (Case Ring 1 in Table 3).
Higher densities were found at the top surface where the moving
punch contacted the powder. The density distribution at the ring
inner perimeter is very close to that at the ring outer perimeter.
The design change from the disk shape to the ring shape also
reduced the required load due to the decreased contact area.
3.2.2. Compaction experiment results and discussion
Pressing speed and releasing speed of the punch also affect
the crack formation of the compacted parts. A too high punch
speed will result in a higher density at the contacting surface sus-
ceptible to cracks. Also, a too high releasing speed will discharge
Fig. 10. Green parts: (a) unsuccessful case (Ring 4); (b) successful case
the internal pressure too quickly and lead to cracks. Thus, based
(Ring 7).
on the process parameters used in Section 3.1, and a few trial-
and-errors from both simulations and experiments with the new
3.2.3. Sintering experiment results and analysis
compaction die set, the pressing speed and releasing speed was
Two biggest challenges encountered during the sintering are
selected to be 0.042 and 0.002 mm/s, respectively. As shown in
cracks and distortions. Cracks are mostly due to the non-uniform
Table 3, the pressure for experiments was selected in the range
density distribution induced from the compaction process and
of values used in the simulation. The parts showed no cracks
the temperature gradient [4]. The distortions that are commonly
after the compaction; however, a strong bond was formed at
observed in cylindrical parts are of a conical shape, which has
the interface of the powder compact-base plate and the powder
compact-lower portion of the core rod. Hence, the green parts
were frequently damaged during the separation process.
In order to successfully detach the powder compact from the
die, various lubrication and separation methods were evaluated
as summarized in Table 3. Mineral oil, which was suggested
by the vendor, only seemed to be effective for smaller parts as
seen in previous experiments. As the part size became larger
and interface area increased for the ring shape part, all the
compacts failed during ejection (Ring 1 and Ring 2). The wax
(Ring 3), aluminum foil (Ring 4), and metal working lubricant
(Ring 5) also failed to maintain the part intactness while ejecting
as shown in Fig. 10a. However, as demonstrated in cases Ring 6
through Ring 12, the graphite particle based lubrication greatly
improved the separation performance as shown in Fig. 10b. For
the unsuccessful attempts, a mechanical press was used to eject
the part, whereas the compact could be taken out by hands when
graphite lubrication was used. Fig. 11. Schematic view of the part orientations in the oven.

7. P. Chen et al. / Journal of Materials Processing Technology 190 (2007) 243–250 249
Table 4
Sintering experiment conditions and results
Case Compaction pressure (MPa) Sintering procedure Orientation of the part in the oven Cracks
Ring 1 40.49 Procedure I Bottom up Yes
Ring 6 28.92 Procedure II Sideways Yes
Ring 7 57.84 Procedure III Bottom down No
Ring 9 52.05 Procedure III Bottom down No
Ring 10 58.30 Procedure III Bottom down No
Ring 11 63.62 Procedure III Bottom down No
Ring 12 72.88 Procedure III Bottom down No
Table 5
Shrinkage and the dimensional differences between top and bottom after sintering
Case Shrinkage in diameter Height after sintering (mm) Diameter difference (mm) Conical taper
Top Bottom
Ring 1 0.271 0.282 31.43 1.32 7.22E–02
Ring 6 0.292 0.283 32.36 1.04 4.20E–02
Ring 7 0.258 0.263 27.78 0.5 1.80E–02
Ring 9 0.267 0.278 31.84 0.9 2.83E–02
Ring 10 0.256 0.266 30.18 0.83 2.75E–02
Ring 11 0.254 0.259 29.93 0.62 2.07E–02
Ring 12 0.249 0.254 29.86 0.41 1.37E–02
been quantified by the amount of change in diameter over a unit (Ring 6 was orientated sideways in the oven so that the conical
length (conical taper) in this work. shape was purely due to shrinkage anisotropy). The other is the
Three sintering procedures (Fig. 3) and various part orienta- friction drag introduced by the support substrate, which restricts
tions (Fig. 11) in the oven were evaluated, and the results are the shrinkage of the bottom compared with the unrestricted top
summarized in Table 4. It was observed that the sintering curve portion (Fig. 12c).
had a significant effect on the crack formation. The parts sin- Table 5 shows detailed information regarding the shrinkage
tered using sintering Procedure III (Ring 7–Ring 12) are free of and the dimensional differences between top and bottom after
cracks while other parts which used sintering Procedure I and II sintering. Case Ring 1 in Fig. 12a demonstrates the combined
had cracks (Ring 1 and Ring 6). A slower sintering procedure effect of friction drag and shrinkage anisotropy. Since the bottom
helped to prevent cracks by minimizing the temperature gradient of the green compact was placed facing upwards, the distortion
[4]. from the density gradient and friction drag will multiply. Mea-
According to the classic sintering theory [4], there are two sured conical taper in this configuration is the largest, and the
contributors to the conical shape. The first is the non-uniform result is confirmed by the observation. Therefore, to reduce the
density distribution of the green part (shrinkage anisotropy). distortion by offsetting the distortion caused by the density gra-
Since the bottom of the green part has a lower density com- dient and the friction drag, the bottom of the green part was
pared with the top (Fig. 9), the bottom shrinks more than the placed facing down on the substrate. As confirmed by the con-
top does and results in a conical shape as shown in Fig. 12b ical taper measurement (Ring 7–Ring 12), the measured taper
significantly decreased. In addition, the effect of compaction
force on the distortion can be observed from Ring 9 through
Ring 12. A higher compaction force produced a denser green
part, which resulted in less shrinkage during the sintering pro-
cess, and therefore helps to reduce conical shape.
4. Conclusions
In this study, a large-scale ceramic part (Ø114 mm) was suc-
cessfully compacted and sintered using uniaxial die compaction
technique. The effects of die design, compaction pressure, lubri-
cation, sintering procedure, and part orientation in the oven on
the P/M part quality were investigated, and the preferred process
conditions were discussed. Furthermore, a FEA tool was utilized
to predict the location of a crack for a disk shaped part. On the
Fig. 12. Illustration of the effect of shrinkage anisotropy and friction drag: (a) basis of the quantitative and qualitative analysis made herein,
Ring 12; (b) Ring 6; (c) Ring 1. the following conclusions could be drawn: (1) A compaction

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