Spark-plasma-sintered BaTiO3/Al2O3 nanocomposites

Guo-Dong Zhan, Joshua Kuntz, Julin Wan, Javier Garay, Amiya K. Mukherjee *

Department of Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, CA 95616-5294, USA

Received 18 March 2002; received in revised form 12 September 2002

Abstract

Using spark plasma sintering (SPS), BaTiO3/Al2O3 nanocomposites were successfully consolidated to more than 99% of

theoretical density at a sintering temperature as low as 1150 8C in only 3 min. The processing methods for these dense

nanocomposites where the retained grain size of alumina matrix was in the nanometer level were developed. The maximum volume

content of BaTiO3 in the nanocrystalline matrix for toughening was around 15 vol.%. A significant increase in fracture toughness up

to 5.36 MPa1/2 has been achieved in the 7.5 vol.% BaTiO3/Al2O3 nanocomposite. The toughening mechanism might be related to

ferroelastic domain switching of ferroelectric phase in these nanocomposites.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Alumina nanocomposite; Spark plasma sintering; Toughening

1. Introduction

The sintering of nanocrystalline ceramics is an excit-

ing theme in materials research because such bulk

nanocrystalline ceramics exhibit novel properties and

functions. Much progress has been made in consolidat-

ing nanocrystalline powders by a number of consolida-

tion methods during the past several years. However,

these studies have highlighted the problem of consoli-

dating these nanopowders into full dense ceramics

without excessive grain growth [1�/6]. Therefore, search-

ing for a new processing technique that requires shorter

duration could be the ideal choice. Spark plasma

sintering (SPS), a fast consolidation technique that can

enhance sintering kinetics and reduce the time available

for grain growth, has been used in the present study. It is

a pressure-assisted sintering method based on the short-

lived generation of high-temperature spark plasma at

the interfaces between powder particles. The basic

configuration of an SPS system consists of a sintering

die with a uniaxial pressurization mechanism, specially

designed punch electrodes, vacuum chamber with va-

cuum atmosphere control, a DC-pulse generator, and

control units. During SPS processing, the powder

surfaces are cleaned and activated, and the material is

transferred at both the micro and macro levels. Thus, a

high quality sintered compact is obtained at a lower

temperature and in a shorter time than conventional

sintering [7].

On the other hand, nanocrystalline ceramics do not

appear to possess high fracture toughness, as was

anticipated [8]. Therefore, research on processing fully

dense bulk nanocomposites that retain nanocrystalline

grain size in matrix and possess moderate fracture

toughness as well, is still a challenging problem. Domain

switching as a toughening mechanism has been recog-

nized in ferroelectric materials where either an applied

compressive stress or electrical field led to domain

switching [9]. This behavior has been demonstrated by

the facts that anisotropic fracture toughness was ob-

served in these poled materials and the fracture tough-

ness depends on the volume fraction of domains that are

aligned favorably in front of the crack tip [10�/13]. R -

curve behavior due to stress-induced ferroelastic domain

switching was also found in BaTiO3 [14]. Moreover, the

contribution to toughening due to domain switching in

zirconia was almost three times that of the intrinsic

toughness [15]. Recently, a new approach for toughen-

ing of ceramics has been proposed and investigated

where piezoelectric and ferroelectric second phases were

incorporated into the ceramic matrix as toughening* Corresponding author. Fax: �/1-530-752-9554.

E-mail address: akmukerjee@ucdavis.edu (A.K. Mukherjee).

Materials Science and Engineering A356 (2003) 443�/446

www.elsevier.com/locate/msea

0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-5093(02)00812-2

phase and energy dissipation by the piezoelectric effect

was suggested as a new toughening mechanism [16,17].

In the present study, BaTiO3 was selected as a model

ferroelectric toughening second phase for the presentstudy because it has been extensively characterized with

regards to its ferroelectric and ferroelastic properties.

This paper will report the microstructure, mechanical

properties, and toughening mechanisms in BaTiO3/

Al2O3 nanocomposites consolidated by this novel pro-

cessing technique.

2. Experimental procedures

Cubic BaTiO3 nanopowders were provided by Cabot

Corporation, which were prepared by hydrothermal

reaction of barium hydroxide (Ba(OH)2) with titanium

hydroxide (Ti(OH)4). The average particle size is 60 nm.

The pure a-Al2O3 nanopowder used in the present study

had an average particle size of �/50 nm (obtained from

Baikowski International, Charlotte, NC) and surfacearea of 30 m2 g�1. The gas condensation synthesized g-

Al2O3 with an average particle size of 32 nm was

obtained from Nanophase Technologies Corporation

(Darien, IL 60651). The BaTiO3 powders at different

volume contents were mixed with the a-Al2O3 nano-

powder for 24 h in ethanol using zirconia ball media. A

high-energy ball-milling method was used to prepare the

starting g-Al2O3 nanopowders. This is that the g-Al2O3

and BaTiO3 powders are high-energy ball-milled with a

WC ball and vial set for 24 h. In order to prevent severe

powder agglomeration one weight percent polyvinyl

alcohol (PVA), a dry milling agent, was added. The

PVA is removed after ball-milling through a 350 8Cheat treatment in vacuum. SPS was carried out under

vacuum in a Dr. Sinter 1050 SPS apparatus (Sumitomo

Coal Mining Co., Japan). The powder mixtures wereplaced into a graphite die (20 mm in inner diameter) and

cold-pressed at 200 MPa to green-body with �/57% of

theoretical density. The SPS processing parameters used

in the present study were as follows: (1) an applied

pressure of 63 MPa, (2) the heating rate of 200 8Cmin�1 from 600 8C to the desired temperatures, (3) the

pulse duration time of 12 ms and the interval between

pulse of 2 ms, and (4) the pulse current of �/2000 A anda voltage of 10 V. The temperature was monitored with

an optical pyrometer that was focused on the ‘non-

through’ hole (0.5 mm in diameter and 2 mm in depth)

of the graphite die. The final densities of the sintered

compacts were determined by the Archimedes’ method

with deionized water as the immersion medium. The

theoretical densities of the specimens were calculated

according to the rule of mixtures. The microstructuralobservation and microanalysis were carried out using an

FEI XL30-SFEG high-resolution scanning electron

microscopy with a resolution better than 2 nm and

magnification over 600 k�/. Grain sizes were estimated

from high-resolution SEM of fractured surfaces. Frac-

ture toughness (KIC) was measured by indentation

techniques. Indentation tests were performed on aWilson Tukon hardness tester with a diamond Vickers

indenter. The indentation parameters for fracture

toughness (KIC) were a 1.5 kg load with a dwell of 15

s. The following equation, proposed by Antis et al., [18]

was used for the calculation:

KIC�0:016

�E

Hv

�1=2� P

c3=2

�(1)

where E , Hv, P and c represent Young’s modulus,

Vickers hardness, the applied indentation load, and the

half-length of the radial crack, respectively.

3. Results and discussion

The relative densities for all the SPS materials aregiven in Table 1. It can be noted that all the materials

could be consolidated by SPS at 1150 8C only for 3 min

to get almost full density. This is quite different from the

pressureless-sintered BaTiO3/Al2O3 composites where

the sintering temperatures were higher than 1450 8Cbut the maximum bulk density obtained was just 92% of

the theoretical density of alumina [16]. The microstruc-

tures of the fractured surface in the pure alumina andBaTiO3/a-Al2O3 nanocomposites in the present study

are shown in Fig. 1. It is very interesting to note that the

pure a-Al2O3 consolidated by SPS exhibited a mixture

of fracture modes (Fig. 1(a)). This is different from the

conventionally sintered monolithic alumina exhibiting

intergranular fracture. However, the fracture modes are

mainly intergranular in these BaTiO3/Al2O3 nanocom-

posites, as shown in Fig. 1(b) to (e) for 5, 7.5, 10, and 15vol.% BaTiO3/Al2O3 nanocomposites, respectively. It is

obvious that the microstructures consisted of nanoscale

grain sizes in these sintered nanocomposites. It can also

be noted that a dramatic grain growth occurred for the

pure BaTiO3 with grain size up to 15 mm (Fig. 1(f)).

These results demonstrate the effectiveness of SPS over

conventional method in obtaining nanocrystalline alu-

mina matrix nanocomposites at quite lower tempera-tures and shorter sintering duration resulting in high

density and nanosized grain size. Moreover, it is very

interesting to note that the grain size for the 7.5

vol.%BaTiO3/g-Al2O3 nanocomposite through high-en-

ergy ball-milling was as small as 190 nm. It is much finer

than 7.5 vol.%BaTiO3/a-Al2O3 nanocomposite without

high-energy ball-milling, suggesting that high-energy

ball-milling procedure can lead to more refined struc-ture.

Table 1 also summarizes the fracture toughness for

the present materials. In comparison to other alumina

G.-D. Zhan et al. / Materials Science and Engineering A356 (2003) 443�/446444

nanocomposites [8] and pure alumina in the present

work, a significant improvement in fracture toughness

was observed in the present nanocomposite materials.

Fig. 2 shows the relationship between toughness and

BaTiO3 volume contents. It can be seen that the fracture

toughness increases with increasing BaTiO3 content and

Table 1

Physical and fracture toughness of BaTiO3/Al2O3 nanocomposites consolidated by SPS at 1150 8C per 3 min

Material Relative density (%) Mean grain size (nm) Fracture toughness (MPa m1/2)

Pure a-Al2O3 99.8 3499/10 3.309/0.14

5 vol.%BaTiO3/a-Al2O3 99.5 3689/19 4.749/0.31

7.5 vol.%BaTiO3/a-Al2O3 99.6 2569/13 5.369/0.38

7.5 vol.%BaTiO3/g-Al2O3 99.2 1909/15 5.269/0.34

10 vol.%BaTiO3/a-Al2O3 99.8 2819/13 4.989/0.13

15 vol.%BaTiO3/a-Al2O3 99.9 3269/18 4.349/0.39

Pure BaTiO3 99.9 15 6359/1969

Fig. 1. High-resolution scanning electron micrographs of fractured surfaces for; (a) pure a-Al2O3, (b) 5 vol.%BaTiO3/Al2O3, (c) 7.5 vol.%BaTiO3/

Al2O3 (d) 10 vol.%BaTiO3/Al2O3, (e) 15 vol.%BaTiO3/Al2O3, and (f) pure BaTiO3 nanocomposites consolidated by spark-plasma-sintering at

1150 8C for 3 min.

G.-D. Zhan et al. / Materials Science and Engineering A356 (2003) 443�/446 445

reachs a maximum at 7.5 vol.%. More than 1.6 times

increase in fracture toughness over the pure nanocrystal-line alumina has been achieved in the 7.5 vol.%BaTiO3/

Al2O3 nanocomposite, suggesting that adding ferro-

electric phase into nanocrystalline alumina is effective

for toughening. In regard to the particle toughening, it

could be ruled out due to the fact that grain size for the

second phase is in the nano-region for the present study.

Thus, the contributions to toughening by crack bridging

and crack deflection due to the second phase is likely tobe very small. Therefore, there may be a toughening

effect related to stress-induced domain switching tough-

ening of the ferroelectric second phase [19,20].

4. Conclusions

SPS to almost theoretical density at a quite lowtemperature of 1150 8C for only 3 min could success-

fully consolidate BaTiO3/Al2O3 nanocomposites. The

7.5 vol.%BaTiO3/g-Al2O3 nanocomposite with a mean

gain size of alumina matrix as small as 190 nm could be

obtained through high-energy ball-milling process.

Fracture toughness depends on the contents of BaTiO3

in the nanocrystalline alumina matrix. The optimum

contents of BaTiO3 are less than 10 vol.%. A significant

increase in fracture toughness up to 5.36 MPa m1/2 was

achieved in the 7.5 vol.%BaTiO3/Al2O3 nanocomposite.

Acknowledgements

This work was supported by a grant (#G-DAAD 19-

00-1-0185) from US Army Research Office with Dr

William Mullins as the Program Manager.

References

[1] H. Gleiter, Prog. Mater. Sci. 33 (1989) 223.

[2] R.S. Mishra, J.A. Schneider, J.F. Shackelford, A.K. Mukherjee,

NanoStruct. Mater. 5 (1995) 525.

[3] E.J. Gonzalez, B. Hockey, G.J. Piermarini, Mater. Manufacturing

Processes 11 (1996) 951.

[4] M.J. Mayo, NanoStruct. Mater. 9 (1997) 717.

[5] S. Bhaduri, S.B. Bhaduri, NanoStruct. Mater. 8 (1997) 755.

[6] S.-C. Liao, Y.-J. Chen, B.H. Kear, W.E. Mayo, NanoStruct.

Mater. 10 (1998) 1063.

[7] M. Tokita, J. Soc. Powder Technol. Jpn. 30 (1993) 790�/804.

[8] R.S. Mishra, C.E. Lesher, A.K. Mukherjee, in: D.L. Bourell

(Ed.), Synthesis and Processing of Nanocrystalline Powder, TMS,

1996, p. 173.

[9] K. Mehta, A.V. Virkar, J. Am. Ceram. Soc. 73 (1990) 567.

[10] Z. Zhang, R. Raj, J. Am. Ceram. Soc. 78 (1995) 3363.

[11] C.S. Lynch, Acta Mater. 46 (1998) 599.

[12] R.A. Pferner, G. Thurn, F. Aldinger, Mater. Chem. Phys. 61

(1999) 24.

[13] M.J. Busche, K.J. Hsia, Scripta Mater. 44 (2001) 207.

[14] F. Meschke, A. Kolleck, G.A. Schneider, J. Eur. Ceram. Soc. 17

(1997) 1143.

[15] A.V. Virkar, R.L.K. Matsumoto, J. Am. Ceram. Soc. 69 (1986) c-

224.

[16] B. Yang, X.M. Chen, Mater. Lett. 33 (1997) 237.

[17] G.-D. Zhan, J. Kuntz, J. Wan, J. Garay, A.K. Mukherjee, Scripta

Mater. (revised, 2001).

[18] G.R. Antis, P. Chantikul, B.R. Lawn, D.B. Marshall, J. Am.

Ceram. Soc. 64 (1981) 533.

[19] G. Winfield, F. Azough, R. Freer, Ferroelectrics 133 (1992) 181.

[20] G.H. Haertling, J. Am. Ceram. Soc. 82 (1999) 797.

Fig. 2. Relationship between fracture toughness and BaTiO3 contents

in spark-plasma-sintered BaTiO3/Al2O3 nanocomposites.

G.-D. Zhan et al. / Materials Science and Engineering A356 (2003) 443�/446446