Investigations in the compaction and sintering of large ceramic parts

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 Ceramics have become increasingly important in modern industry due to their good mechanical and physical properties [1]. Ceramic parts are generally produced by combination of compaction and sintering. However, cracks and distortions have been recognized as the most significant concerns, and often limit the application of uniaxial die compaction technology to produce large-scale ceramic parts. Cracks in powder metallurgy (P/M) components primarily originate from the compaction prior to the sintering. Although the cracks may not become evident until the sintering has occurred, the root cause is most likely the poor interparticle bonding obtained prior to the sintering [1–4]. Usually, micro-cracks that are invisible during the compaction are carried over and enlarged during the sintering. Another common challenge in sintering is the dimensional control of the sintered products. Warpage could occur as a result of green density gradient, friction drag (caused by the support material), gravity, and temperature gradient [4]. Uniaxial die compaction is the simplest form of consolidation process that has been extensively used to densify powder materials. One disadvantage of this technique, however, is the variation of the pressed density that can occur at different loca-

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tions of the parts. This density distribution mainly originates from particle–particle and die wall–particle friction, which cause quality issues such as cracking and warpage during the sintering. Especially, when a large part size is considered, the poor control over the density uniformity of the green part becomes significant and results in part failures. In general, the dimensions of the sintered compacts are in the range of 10–100 mm [4]. The modeling methodology for powder compaction can be classified into three categories based on the length scale [5]: (1) continuum models; (2) multi-particle models (discrete element models and particle dynamics models) [6–8]; and (3) atomistic/molecular dynamics models [5,9]. Continuum models assume that a response of a powder packing is continuous and similar to that of a solid material. Hence, they are attractive in practical applications and have been employed in this study. There are two types of continuum models. The first is the micromechanical model [10,11], which provides a way to derive macroscopic model parameters from information on a smaller particle-level length scale. The micro-to-macro transition is performed usually by homogenization techniques. The other is the empirical (phenomenological) model [12–14], which is formulated using several ‘material’ functions that describe the response of a specific porous material to the stress. The objective of the study is to investigate the effects of process parameters to obtain a crack-free ring shaped part, which

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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 sintering with minimum distortion. 2. Experimental and simulation procedure 2.1. Experimental setup and procedures Two types of experimental setup have been designed and fabricated for this study. To acquire a final part dimension of 90 mm, green part dimensions need to be larger than 110 mm, which is rather large for a part made by uniaxial compaction. Since the final target part dimensions were larger than those of a typical ceramic powder compaction and sintering, a preliminary experiment was performed to compact a cylindrical part with a diameter of 46.4 mm to investigate appropriate range of process parameters. Based on the results from this preliminary experiment, a die for the larger ring shaped part was designed and fabricated. The detailed experimental setup for the compaction and sintering of both sizes are described below. A commercial zirconia powder (YSZ, Inframat Advanced Materials, LLC) stabilized by 3 mol% Y2 O3 was used for all the experiments conducted in this study. The particles have a mean particle size of 0.5 ␮m. The setup for the preliminary experiment is shown in Fig. 1, which includes a punch, a base plate, and a die. The components were made by precision milling and turning process from A2 tool steel. 50 g of zirconia powder was poured into the assembled die/punch set. The setup was then placed in the Instron 1136 material testing system for compaction, which had a single acting upper punch. The compaction process was achieved by a constant punch speed. The punch was held at the final position until the force stabilized and was released at a

relatively slow speed. Finally, the base plate was replaced by an ejection ring, 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 dimensions 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 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 density gradient in the green compact. Therefore, a split die design was adopted to reduce the friction during ejection as shown in Fig. 2. The Container was designed, so it may be split into two halves to facilitate the ejection process. The two symmetric pieces were assembled by eight screws and two aligning pins. The core rod was designed to produce a ring shaped part, which was assembled by two screws and an aligning pin. All the tooling components were made from A2 tool steel, which were heat treated to the hardness of HRC 55 and were precision ground. The experiment cycle requires assembly and disassembly of the die components. First, the Dividable Container is assembled. Next, core rod and base plate are placed in the container, and then 380 g of zirconia powder is poured into the container. The Punch is placed on top of the powder, which is guided by the core rod and the container during the compaction. A constant punch speed is retained by Instron 1136 material testing system. After holding the punch at the final position until the load is stabilized, it is released at a relatively lower speed. After the compaction, the Dividable Container is disassembled, and the base plate is detached from the core rod to eject the compact. Finally, the ejected part is sintered in an oven. Three sintering procedures (Fig. 3) were used in this study. Procedure II has a higher sintering temperature than Procedure I. Procedure III 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.

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Table 1 Material properties of zirconia powder [14] Full density 6.08

Initial density

(g/cm3 )

1.885

(g/cm3 )

Particle size

β

α

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 previous study in literature [14]. It is necessary to recognize the major physical phenomena that occur during the compaction of ceramic particles. The compaction process can be divided into three main distinctive stages. In the early stages of compaction, particles are rearranged (we will refer to this as stage 0 compaction). As compaction force is further exerted onto the powders, the relative 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 elastic deformation of the particles (stage 1 compaction). At higher pressure, the compact structure will breakdown with a small amount of particle rearrangement (stage 2 compaction). 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). It is a phenomenological model that has been adapted from soil mechanics. The model is attractive in compaction modeling because it contains features that are in accordance with the physical response of particulate compacts [12,15]. The DPC model at low hydrostatic pressure is a shear failure model, similar to those used in granular flow, which reflects the dependence of the strength on the confining pressure. This enables the model to predict the strength in tension to be smaller than the strength in compression, a concept which is common for rocks, brittle materials, and pressed powder compacts. In its simplest form, it is represented by a straight line in the p–q plane, which is also known as the Mohr–oulomb shear failure line, FS : FS (q, p) = q − d − p × tan(β) = 0

(1)

where d and β are cohesion and internal friction angle, respectively. If the stress state is such that the corresponding Mises equivalent stress (q) and hydrostatic pressure (p) result in a value of F(q, p) < 0, then the stress causes only elastic

Fig. 5. Densification behavior of the zirconia powder [14].

deformation. If the stress state is such that Eq. (1) is satisfied, the material fails in shearing. At high hydrostatic pressures, the yield surface is described by a cap surface, Fc :



Fc (q, p) =

(p − pa )2 +



R×q 1 + α − α/cos(β)

2

−R × (d + pa × tan(β) = 0

(2)

R is a material parameter that controls the shape of the cap. pa is an evolution parameter that represents the volumetric inelastic strain driven hardening/softening, which is related to hydrostatic compression yield stress (pb ). The parameters pa and R may be obtained from compaction experiments. The parameter α does not have a physical meaning, but ensures a smooth transition between the cap and the shear failure regions for numerical robustness. Typically, a small value (α = 0.01–0.05) is used to avoid the situation of α = 0, which will form a sharp corner at the intersection of Fc and FS . This may lead to numerical problems [16]. The geometric representation of the complete yield locus is represented in the p–q plane as a limiting curve F(q, p, RD) = 0 in Fig. 4. This form is consistent with the DPC model implemented in the finite element package ABAQUS, which was used for this study. The FEA results were compared with the medium size compaction experiments (Ø = 46.4 mm). The material properties used for the simulation are summarized in Table 1. These values were adopted from the experimental work of Kim et al. [14] (3 mol% Y2 O3 stabilized zirconia powder, HSY-3.0, DaiichiKigenso Kagaku Kogyo Co. Ltd., Japan). Fig. 5 shows the densification behavior of this powder. Considering the geometric symmetry of the process, only an axisymmetric 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 axisymmetric quadrilateral elements with reduced integration (CAX4R).

3. Results and discussions 3.1. Compaction of ceramic disk (Ø = 46.4 mm)

Fig. 4. The Drucker–Prager/Cap (DPC) model [5].

3.1.1. Compaction results and discussion As summarized in Table 2, different process parameter levels were investigated. The green part quality was evaluated in terms

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Table 2 Experimental conditions for the compaction (Ø 46.4 mm disk) Case 1 2 3 4 5 (3 repeats) 6 (3 repeats)

Pressure (MPa)

Pressing speed (mm/s)

Releasing speed (mm/s)

Ejection speed (mm/s)

Green Part

92.30 134.5 118.7 65.93 65.93 52.74

0.085 0.042 0.042 0.042 0.042 0.042

0.004 0.021 0.008 0.008 0.008 0.008

0.008 Manual Manual 2.117 4.233 4.223

Horizontal crack Horizontal crack Horizontal crack Crack free Crack free Crack free

of the occurrence of cracks. It was found that the most important process parameter was the compaction pressure. The effects of pressing, releasing and ejection speed on cracking were relatively small. As long as the compaction pressure was less than or around 65.9 MPa, cracks did not occur. Typically, a crack is initiated by the existence of a sharp density gradient. When the compaction force is reduced, the density gradient of the green part also decreases correspondingly. Therefore, a smaller compression pressure helps to avoid cracks [2]. Fig. 6 shows a successful and an unsuccessful case from the compaction: one has a horizontal crack (case #2), and the other is free of crack (case #5). 3.1.2. FEA results and discussion The loading curve obtained from the experiment case #5 from Table 2 was compared with the loading curve from the simula-

tion as shown in Fig. 7. In general, the simulation results agree well with the experiment. The underestimation of the load at the initial loading stage is most likely due to the inaccuracy of the material modeling at low densities. As explained in [17], the material parameters for DPC model at low densities are usually not obtainable from the material testing experiment: the lower the density, the more measurement noise in the experiment. In addition, the powders used in the experiment (Inframat Advanced Materials, LLC) and simulation (Daiichi-Kigenso Kagaku Kogyo Co. Ltd.) were from different sources, which may have contributed to the different loading characteristic, although the powders were the same grade. The green part height from the simulation (11.866 mm) agreed well with that of the actual part (11.557 mm), and the final density of the green part from the simulation (2.554 g/cm3 , at the top surface) also matched the 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).

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Fig. 8. Density distribution after ejection: (a) Case #5; (b) Case #2.

the other hand, the highest density for the case #2 (which had cracks in the experiment) occurred below the upper surface. 2. Case #2 has a sharper density gradient near the upper corner region, and the corresponding relative density curves exhibit a sharp distribution. This indicates a sudden change of density in a localized area, while the relative density curves of case #5 are smoother and corresponds to a more uniform density distribution.

Fig. 9. Density distribution after ejection (ring compaction with a pressure of 40.49 MPa).

The simulation results indicated the migration of the location of the highest density region of a compact from the top surface to below the surface as the pressure was increased. As the pressure increased, cracks developed under the top surface as shown in the actual part (Fig. 6a case #2). The location of the 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 and lower portion of core rod

Separation between the compact 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 Ring 3 Ring 4

40.49 40.49 40.49

Crack free Crack free Crack free

Mineral oil Wax Aluminum foil

Ring 5 Ring 6

40.49 28.92

Crack free Crack free

Ring 7

57.84

Crack free

Ring 8

40.49

Crack free

Ring 9

52.05

Crack free

Ring 10

58.3

Crack free

Ring 11

63.62

Crack free

Ring 12

72.88

Crack free

Coolube 5500 metalworking fluid Water based graphite particle lubricant (Lubrodal F705 ALX) Oil based graphite particle lubricant (Lubrodal Hykogeen Conc HI) Oil based graphite particle lubricant (Lubrodal Hykogeen Conc HI) Oil based graphite particle lubricant (Lubrodal Hykogeen Conc HI) Oil based graphite particle lubricant (Lubrodal Hykogeen Conc HI) Oil based Graphite Particle lubricant (Lubrodal Hykogeen Conc HI) Oil based graphite particle lubricant (Lubrodal Hykogeen Conc HI)

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) Failed, cracks Successful Successful, Fig. 9 b) Successful Successful Successful Successful Successful

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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 experimental 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 susceptible to cracks. Also, a too high releasing speed will discharge the internal pressure too quickly and lead to cracks. Thus, based on the process parameters used in Section 3.1, and a few trialand-errors from both simulations and experiments with the new compaction die set, the pressing speed and releasing speed was selected to be 0.042 and 0.002 mm/s, respectively. As shown in Table 3, the pressure for experiments was selected in the range of values used in the simulation. The parts showed no cracks after the compaction; however, a strong bond was formed at 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. 10. Green parts: (a) unsuccessful case (Ring 4); (b) successful case (Ring 7).

3.2.3. Sintering experiment results and analysis Two biggest challenges encountered during the sintering are cracks and distortions. Cracks are mostly due to the non-uniform density distribution induced from the compaction process and the temperature gradient [4]. The distortions that are commonly observed in cylindrical parts are of a conical shape, which has

Fig. 11. Schematic view of the part orientations in the oven.

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Table 4 Sintering experiment conditions and results Case Ring Ring Ring Ring Ring Ring Ring

1 6 7 9 10 11 12

Compaction pressure (MPa)

Sintering procedure

Orientation of the part in the oven

Cracks

40.49 28.92 57.84 52.05 58.30 63.62 72.88

Procedure I Procedure II Procedure III Procedure III Procedure III Procedure III Procedure III

Bottom up Sideways Bottom down Bottom down Bottom down Bottom down Bottom down

Yes Yes No No No No No

Table 5 Shrinkage and the dimensional differences between top and bottom after sintering Case

Ring Ring Ring Ring Ring Ring Ring

Shrinkage in diameter

1 6 7 9 10 11 12

Top

Bottom

0.271 0.292 0.258 0.267 0.256 0.254 0.249

0.282 0.283 0.263 0.278 0.266 0.259 0.254

Height after sintering (mm)

Diameter difference (mm)

Conical taper

31.43 32.36 27.78 31.84 30.18 29.93 29.86

1.32 1.04 0.5 0.9 0.83 0.62 0.41

7.22E–02 4.20E–02 1.80E–02 2.83E–02 2.75E–02 2.07E–02 1.37E–02

been quantified by the amount of change in diameter over a unit length (conical taper) in this work. Three sintering procedures (Fig. 3) and various part orientations (Fig. 11) in the oven were evaluated, and the results are summarized in Table 4. It was observed that the sintering curve had a significant effect on the crack formation. The parts sintered using sintering Procedure III (Ring 7–Ring 12) are free of cracks while other parts which used sintering Procedure I and II had cracks (Ring 1 and Ring 6). A slower sintering procedure helped to prevent cracks by minimizing the temperature gradient [4]. According to the classic sintering theory [4], there are two contributors to the conical shape. The first is the non-uniform density distribution of the green part (shrinkage anisotropy). Since the bottom of the green part has a lower density compared with the top (Fig. 9), the bottom shrinks more than the top does and results in a conical shape as shown in Fig. 12b

(Ring 6 was orientated sideways in the oven so that the conical shape was purely due to shrinkage anisotropy). The other is the friction drag introduced by the support substrate, which restricts the shrinkage of the bottom compared with the unrestricted top portion (Fig. 12c). Table 5 shows detailed information regarding the shrinkage and the dimensional differences between top and bottom after sintering. Case Ring 1 in Fig. 12a demonstrates the combined effect of friction drag and shrinkage anisotropy. Since the bottom of the green compact was placed facing upwards, the distortion from the density gradient and friction drag will multiply. Measured conical taper in this configuration is the largest, and the result is confirmed by the observation. Therefore, to reduce the distortion by offsetting the distortion caused by the density gradient and the friction drag, the bottom of the green part was placed facing down on the substrate. As confirmed by the conical 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 process, and therefore helps to reduce conical shape. 4. Conclusions

Fig. 12. Illustration of the effect of shrinkage anisotropy and friction drag: (a) Ring 12; (b) Ring 6; (c) Ring 1.

In this study, a large-scale ceramic part (Ø114 mm) 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. Furthermore, a FEA tool was utilized to predict the location of a crack for a disk shaped part. On the basis of the quantitative and qualitative analysis made herein, the following conclusions could be drawn: (1) A compaction

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pressure range of 30–75 MPa is preferred for the compaction of zirconia powder using uniaxial die compaction. The less the compaction force, the less tendency for cracking, but more tendency for warpage after sintering. (2) In the case of disk-shape ceramic compact (FEA simulation), crack occurred around the region where final density was the highest. The density distribution curve showed an abrupt density distribution in the cracking area. (3) The oil based graphite particle lubricant showed the best performance in the uniaxial compaction of zirconia powder. The friction between the powder and the tooling surface was reduced. Moreover, the tendency of zirconia powder sticking to the tooling surface was eliminated to ensure a smooth separation process. (4) A two-piece split die design reduced the possibility of cracking during ejection. (5) The conical shape distortion after sintering from the shrinkage anisotropy could be offset by the friction drag of the support substrate. (6) A slow multi-step sintering process (Procedure III) is preferred to prevent cracking. (7) A higher compaction pressure resulted in less distortion due to less shrinkage during sintering. Acknowledgement We greatly acknowledge the help from Jie Feng on conducting the experiments and financial support from Powerix Technologies. References [1] F. Klocke, Modern approaches for the production of ceramic components, J. Euro. Ceram. Soc. 17 (1997) 457–465. [2] R.M. German, Powder Metallurgy Science, second ed., Metal Powder Industries Federation, Princeton, 1994.

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