Processing and Sintering of Ultrafine MgO-ZrO2 ... - Wiley Online Library

Processing and Sintering of Ultrafine MgO-Zr02 and (Mg0,Y20$-Zr02 Powders Michael J. Readey,*.*Ran-Rong Lee,*#+ John W. Halloran,*.*and Arthur H. HeueP Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44lO6

Chemical coprecipitation was used to prod~~ce ultrafine and easily sinterrMc MsogcpbiliVa and (Mg0,Y203)stabilized Zr02 porrders. The sintering behavior is very sensitive to post-precipitation washing because Yhard" agglomerates form when the precipitated gels are washed with water, whereas "soft" agglomerates form when they are washed with ethanol. The soft agglomerates pack oniformly, resnlting in hshrinkage of powder compacts to neartheoretical density. The hard agglomerates result in compacts which have regions ofl d i z e d densification and a signifiint fraction of residual porosity. [Key nords: zirconi., pnmClsiO& sintering, agglome!mte&eoprrcipit.tion.] I. Introduction

T"

E advent of strong and tough Zrorbased ceramics has generated considerable scientific and technological interest in this class of structural ceramics.' Fine-grained sintered ceramics can now be produced using commercially available ultrafine powders, which can be sintered to full density at relatively low temperatures. Several wet chemical methods have been developed for preparation of such high-purity, homogeneous, and sinterable ceramic powders, metal alkoxide,23 citrate,' sol-gel misphere: and chloride6-' processes have all been used successfully for stabilized ZrO2's. The chloride process for Y203-stabilized ZrO2 was developed by Haberko6' and involves coprecipitation of Zr and Y hydroxides* from an aqueou~ sol~tionOf Z r c 1 4 and YCl3 using NH40H. Although the chloride synthesis process produces ultrafine ( 4 3 nm) sinterable powdersp a critical parameter that affects final density is the state of -on of the powders. Haberko,' and more recently Kosmac? found that the final washing medium greatly affects the state of powder agglamerati and hence the final density. In thiswork,we show that chloride coprecipitation can be extended to MgO-stabilized and (Mg0,Y203)-stabilized ZTO~, and that tbe state of Bsglomtration af these powders also dependpan the final Washing medium.

cause of their high solubility in water. Dilute solutions (0.15M total cation concentration) of (i) MgC12.6H20 and ZrC14 (corresponding to 9 mol% MgO in the final oxide) and (ii) MgCl2.6H20, YC13-6H20,and ZKl4(corresponding to 8.4 mol% MgO and 1.2 mol% Y203in the final oxide) were ppnred by slowly adding the salts to constantly stirred distilled water. Each chloride solution was then added dmpwise to a vigorouSly stirred 6M NH40H solution, which producd a white, gelatinous precipitate. The pH was maintained above 10 at all times to ensure COmpItte reaction (as determined by titration studiesm3). The volume of NH40H solution added was such that the OH- concentration was 50 times greater than ntcessary for complete reaction. Plastic Nalgene" containers were used throughout the precipitation process to minimize possible silica contamination. The resulting gels were washed with distilkd water until there was no indication of residual Cl- (qualitatively determined? adding a few drops of thewash effluent to aAgNo3 solution 3. (Chlorine ions are well-known to impede demification in Zrodontainiog To investigate the effects of washing, each a-freegel was separated into two batches. one batch was further washed with water, while the seumd was washed several times with 10096 ethanol. After washing, the rtmaining liquid was re moved by v~cuumfiltration, and the geb were dried at llVC for l3 h. The driedgelwascrushed in amortar and pestle and calcined at 400",600",or8oo"cfor l h to ddermiae the crystallization tem rature, as the dried gels are known to be amorphous.Ghe resulting powders were ball-milled in d h d for 12 h using N ~ ~ C bottks IE and Z r o z medie, and then finally dried. ~forsinteriJlgshditswae~bycddp#rsing at 70 MPa in a hardened stetl die, using 1 wt96 stearic acid in isopPpyl alcohd as a die lubricant. Mletswere dried for several hours at ll0T to rcmovc residual alcobol, and fired in air between 900"and 1400°C. F i t i m e s w e r e genexally 24 m h to 1 h.

ThegeLsand~pawdaswaecltamincdwithboth 0and X-raydifflaction (XRD)to ascertain the particle size. particle cry* tallinity, and the artent of tmnsmkim elcctrm micmapy

betwcar9oo"d14o(w=. o p t i c a l m i ~ w a s u s e d t o o b a e s v t t h oe f~ the two types ofcompacts. Rue size d i s t r i i in gretn and sintered canpa3swae dctarmned byanalysisofnitm

-

M. Sacks-contributing editor

gen adsorption and desorption isotherms using a commacial instrument." m.Rcalb

The binary Mgo-zro2 and ternary ( W . Y 2 0 3 ) - ~ 2

& andpawdersbehavedi&nticallywithrespedtopoceas-

1499

1500

Vol. 73, No. 6

burnal of the American Ceramic Society -Readey et al.

ing and sintering characteristics. Therefore, the results will be discussed in terms of the general powder attributes. (1) Crystallization

The morphology of the dried gel is shown in Fig. 1. The diffuse ring pattern in the inset selected area diffraction pattern (SADP) indicates that the hydroxide precipitates are amorphous. The lack of well-defined peaks in the XRD pattern (Fig. 2) confirms this result. The powder remains amorphous after calcining at 400"C, but begins to crystallize to the tetragonal structure near 600°C (Fig. 2). Increasing the calcining temperature to 800°C results only in a narrowing of the diffraction peaks, indicating an increase in crystallite size. There was no evidence of cubicphase formation during calcining, perhaps because the calcining time was too long to retain any cubic phase that may have formed on initial crystallization. (2) Powder Characteristics Calcining the ethanol-washed gel at 600°C produces the crystallites shown in the TEM micrograph in Fig. 3. Individual -10-nm crystallitesare readily discernible.Analysis of the XRD pattern using the Scherrer f o r m ~ l asuggested '~ a particle size of 8.2 nm, in close agreement with the TEM result. The structure of the powder appears "open" and relatively nonagglomerated. The well-defined rings in the accompanying selected area diffraction pattern (SADP)indicate that the powder is fully crystalline, consistent with the XRD result. In comparison, the water-washed gel powder calcined at 600°C appears dense and less electron-transparent (Fig. 4), suggesting that this powder is more agglomerated. In fact, in regions of lower particle density, crystallites actually appear bonded together. The SADP indicates that these powders are also crystalline. (3) Dilatometry Figure 5 shows the sintering behavior of powder compacts derived from ethanol-washed gels. The green densities of the compact were 4 0 % of theoretical and achieved final density within minutes for nearly all sintering temperatures; longer sintering times produced very little change in final density. The highest density achieved was -99% of theoretical, obtained at 1400°C. Densification clearly proceeds very quickly in these ultrafine powders. Compacts formed from water-washed powders had a green density of 36.5% of theoretical, less than the ethanol-washed powders. The dilatometry results are similar to those observed in ethanol-washed powders, except that the final densities achieved at any temperature were always lower, with a

Fig. 1. TEM microgra h showing the morphology of the dried gel. Inset selected area dikraction attern indicates that the MgOZrOz gel precipitates are amorplous.

1. Moot

I

0

*

30

I

.

40

l

.

'

*

60

~

.

70

*

I

*

28-Angle (degrees)

Fig. 2. X-ray diffraction atterns of the dried MgO-ZrOz gel. and el calcined for 1 at W ,W ,and 800°C (CuKa radiation!.

1

maximum density of only 85% of theoretical at 1200°C (Fig. 6). No increase in final density was achieved upon increasing the temperature to 1400°C. (4) Microstructure

Optical micrographs of ethanol- and water-washed powder compacts sintered for 1 h at loOO", 12W, and 1400°C are shown in Figs. 7 and 8. The differences due to the washing treatment are readily apparent. As already noted, the ethanolwashed powder compacts sinter uniformly to near-theoretical density at 1400°C (Fig. 7).

Fig. 3. TEM micrograph of ethanol-washed MgO-ZrOz gels after calcining at 600°C for 1 h.

Processing and Sintering of Ultrafine MgO-ZrO2 and (MgO, Y203)-ZrO2 Powders

June 1990

0

10

20

30

1501

40

50

60

Time (minutes)

Fig. 6. Theoretical density as a function of sintering time at various temperatures for

the water-washed (Mg0,Yz03)-Zr02 gels.

I\ Fig. 4. TEM micrograph of water-washed MgO-ZrOz gels after calcining at 600°C for 1 h.

In contrast, the water-washed powder compacts sinter very heterogeneously; at 1000°C, local regions of high density have formed (the light areas in Fig. 8(A)). At 1200”C, the dense regions continue to grow both in number and in size (up to several hundred micrometers), as the surrounding matrix begins to densify (Fig. 8(B)). At 1400°C, the matrix continues to densify, but still contains large pores (Fig. 8(C)). Thus the major effect of ethanol washing appears to be to prevent pref erential densification of localized regions, which occurs in water-washed compacts, and which prevents complete densification. The micropore distributions (i.e., the intra-agglomerate porosity) in green and sintered compacts from the nitrogen adsorption/desorption isotherms are shown in Figs. 9(A) and (B). Micropore distributions (see Figs. 3 and 4) in the waterwashed and ethanol-washed green compacts are similar, the latter having a slightly higher density of larger micropores (in the size range 15 to 25 nm). This difference is clearly not important in the sintering behavior, as virtually all micropores of 25 nm and smaller are eliminated in both types of sintered compacts. The inter-agglomerate porosity is also not implicated in the poor densification of the water-washed powders, as 50-nm pores between agglomerates also sinter readily (see Figs. 7 and 8). Pores larger than -50 nm, arising from the jagged gel fragments in the ball-milled water-washed samples, are the major reason for the difference in sintered density between the two types of powders and result in the -10- to 30-pm pores in the water-washed samples shown in Fig. 8.

....................

1200’C

...

I 0

10

20

30

40

50

60

Time (minutes)

Fig. 5. Theoretical density as a function of sintering time at various temperatures for

the ethanol-washed (MgO.Y203)-ZrO2 gels.

Discussion

Although the size distribution of the crystallites of the calcined powders are similar for both washing treatments, the shrinkage behavior and final densities for these two materials are quite different. The water-washed powder clearly contains “hard” agglomerates, whereas the ethanol-washed powder is comprised of “soft” agglomerates. During compaction, the soft agglomerates within the ethanol-washed powders are easily broken down in the die to produce uniformly packed compacts which sinter homogeneously to near-theoretical density and a small and uniform grain size. The agglomerates in the water-washed powders do not completely break down during compaction, as indicated by the lower green density, and regions of higher-than-average green density exist within the agglomerates. As densification occurs, these densely packed regions densify first, pulling away from the surrounding regions and leaving large pores, which prevent complete densification. These results confirm the work of Fthodes16 and Lange,” who demonstrated that nonuniform packing from agglomerates can lead to localized and inhomogeneous densification. It is useful to consider three size scales-primary, secondary, and tertiary. The primary particles are -10 nm in diameter and pack into secondary agglomerates which contain 6- to 8-nm intra-agglomerate pores. These secondary agglomerates are 50 to 100 nm in diameter in alcohol-washed powders (Fig. 3), and 100 to 200 nm in diameter in waterwashed samples. Secondary pores, -50 nm in diameter, form in both powders when these secondary agglomerates are packed together. Finally, tertiary fragments, -100 p m in diameter, are present in the water-washed gels, but are absent in the “fluffy” alcohol-washed gels. These give rise to 10- to 30-pm pores after sintering. The effect of the washing medium on the agglomerate structure is clearly profound, and there are several possible reasons why hard incompressible agglomerates can form from water-washed gels. From capillary pressure arguments, it might be that the surface tension of the washing liquid is a critical component in the development of agglomerates. The surface tension of water is nearly 3 times that of ethanol, and the capillary pressure would thus be 3 times greater than in the ethanol system, thus leading to hard agglomerates in water-washed gels. However, our particles are on the order of 10 nm in size, and the capillary pressure developed in both systems is extremely high. It thus does not seem plausible to appeal to surface tension arguments. Another possible cause for agglomerate formation is the solubility of the particles in water and ethanol. There is inevitably some dissolution of hydroxides in water or ethanol, and as suspensions dry, the solute may reprecipitate between particles, essentially bonding the particles and forming a hard

1502

burnal of the American Ceramic Society -Readey et al.

Vol. 73, No.6

Fig. 7. 0 tical micrograph of polished surface of ethanol-washed MgO-Zr& compact after sintering for 1 h at (A) looo", (B)12W,

and (C) 1400°C.

Fig. 8. Optical micro raph of Polished surface of water-washed MgO-Zr02 compact aker sintering for 1 h at (A)lOW, (B)12W, and (C) 1400°C.

agglomerate. If the solubility of the hydroxides is higher in water than in ethanol, this could be important. However, the solubilities of the hydroxides are fairly low for both water and ethanol, and again, this effect is not thought to dominate. Finally, surface chemistry arguments suggest that the agglomeration of water-washed powders involves hydrogen bonding of surface hydroxyl groups on the hydroxide precipitates. Once hydrogen-bonded, the water molecule can bridge the surface hydroxyl groups of neighboring precipitates

(Fig. lo), thus bonding the two precipitates. During drying and calcining, the hydroxide is converted to the oxide, which forms a solid neck between neighboring precipitates; i.e., a hard agglomerate forms. A different situation clearly must obtain for hydroxide precipitates in the presence of ethanol; there appears to be no tendency for bridging between neighboring precipitates. As discussed in the companion paper by Kaliszewski and Heuer," surface ethoxide groups form during ethanol wash-

June 1990

Processing and Sintering of Ultrafine MgO-ZrOz and (MgO, EO,)-ZrO, Powders -

-

- Green Cornpoct - Fired 140fi°C

0.003

-H

-f

H

H

L'm?d 7 . I Ethonol-Woshed Powders

i

1503

I

\

............ 0-H

..

........H'

.....,O

0

>

I

L Y

4 B

0.002

.

ZrlOHL *.

i

0.001

0.000 0

50

100

150

Pore Radius

200

250

(i)

0.005 0.004

.

I

Woter-Washed Powders

'

- Green Cornpoct - Fired 1400T

0.003 0.002 0.001

i

0.000 0

50

100

150

Pore Radius

H'

....... O - H ............ / H

\

200

250

(i)

(8) Fig. 9. M i c r o p o r e d i s t r i b u t i o n i n c a l c i n e d powder and f i r e d (MgO,Y203)-Zr02 compacts for (A) ethanol-washed powders and (B)water-washed samples.

ing. During drying and calcining, the ethoxide surface groups decompose, but there is no tendency for neck formation, and relatively nonagglomerated powder particles are thereby obtained.

V. Conclusions We have shown that ultrafine MgO-ZrOz and (Mg0,Y20+ Zr02 powders can be readily produced using chemical coprecipitation and that the all-important ethanol-washing treatment can be extended to these powders. Water-washed gels produce hard agglomerates which cannot be broken down during compaction, resulting in localized densification and significant residual porosity. Ethanol-washed gels are relatively nonagglomerated, and sinter uniformly to neartheoretical density.

.

10. S c h e m a t i c illustration of bridging of hydroxide part d e s by water molecules.

Fi

.

In

'H 0-H

'H-.

H

(4 0.006 1

0,

References 'K. Tsukuma, K. Ueda, K. Matsushita, and M.Shimada, "High-Temperature Strength and Fracture Toughness of Y203-Partially-Stabilized ZrOz/A1203 Composites," 1. Am. Cemm. Soc., 68 [2] C-56-C-58 (1985). 2K.S. Mazdiyasni, C.T. Lynch, and J.S. Smith, "Preparation of UltraHigh-Purity Submicron Refractory Oxides," 1. Am. Ceram. Soc., 48 [7] 372-75 (1965). 'K. S. Mazdiyasni, C.T. Lynch, and J. S. Smith, "Cubic Phase Stabilization of Translucent Yttria-Zirconia at Very Low Temperatures," 1. Am. Ceram. Soc., SO [lo] 532-36 (1967). 'C. Marcilly, P. Conty, and B. Delmon, "Preparation of Highly Dispersed Mixed Oxides and Oxide Solid Solutions by Hydrolysis of Amorphous Organic Precursors," 1. Am. Gram. &., 53 [l]56-57 (1970). W. A.C. G. Van de Graaf and A. J. Burggraaf, "Wet-Chemical Preparation of Zirconia Powders: Their Microstructure and Behavior"; pp. 744-65 in Advances in Ceramics, Vol. 12, Science and Technology of Zirconia 11. Edited by A. H. Heuer and L.W. Hobbs. American Ceramic Society, Columbus, OH, 1984. 6K. Haberko, A. Ciesla, and A. Pron, "Sintering Behavior of YttriaStabilized Zirconia Powders Prepared from Gels," Gmmurgicr Inr.. 1[3] 11116 (1975). 7K. Haberok, "Characteristics and Sintering Behavior of Zirconia Ultrafine Powders," Gramurgia Inf., 5 [4] 148-54 (1979). 8H.T. Rijnten, "Formation, Preparation, and Properties of Hydrous Zirconia"; Ch. 7 in Physical and Chemical Aspects of Adsorbates and Catalysts. Edited by B.G. Linson. Academic Press, New York, 1970. 9T.Kosmac, R. Gopala Krishnan, V. Kervasevec, and M. Kosmac, "Effect of Dewatering of Amorphous Hydrous Zirconia Precipitates on Tetragonal Zirconia Content in Calcined Powders," 1. fijs..,[Feb.] C1-43 (1986). 1°R. R. Lee; Ph.D. Thesis. Case Western Reserve University, Cleveland, OH, 1987. IIM. J. Readey; Ph.D. Thesis. Case Western Reserve University, Cleveland, OH, 1988. 12W.T.Lippencott, D.W. Meek, and F.H. Verhoek, Experimental General Chemistry; pp. 393-98 W.B. Saunders, Philadelphia, PA, 1974. I T . E. Scott and J. S. Reed, "Laundering Zirconia Powders,"Am. G r a m . SOC. BUN., 58 [6] 587-90 (1979). I'M. J. Readey and D.W. Readey, "Sintering of Zr02 in HCI Atmospheres," J Am. Cemm. Soc., 69 [7] 580-82 (1986). lSB.D. Cullity, Elements of X-Ray Diffraction; p. 102. Addison-Wesley, Reading, MA, 1978. 16W.H. Rhodes, "Agglomerate and Particle Size Effects on Sintering Yttria-Stabilized Zirconia,"J Am. Ceram. &., 64 [l] 19-22 (1981). 17F.F. Lange, "Sinterability of Agglomerated Powders," 1. Am. Ceram. Soc., 76 [2] 83-89 (1984). '"W. Porterfield, Inorganic Chemistry; pp. 263-68. Addison-Wesley, Reading, MA, 1984. 19M.S. Kaliszewski and A. H. Heuer, "Alcohol Interaction with Zr02 Powders,"1. Am. Ceram. Soc., 73 [6] 1504-509 (1990). 0