Preparation and characterization of Al2O3–LaNbO4composites

Zhili Zhang, Lang Zhou *, Yougen Hu, Le Jiang

School of Chemistry and Materials Science, Nanchang University, Nanchang 330047, China

Received 25 February 2002; accepted 11 June 2002

Abstract

Lanthanum orthoniobate (LaNbO4) has been synthesized through a new coprecipitation approach, and added to

alumina to produce the composites by hot-press sintering. Fracture toughness of alumina can be significantly improved

by appropriate addition of LaNbO4, though the addition also greatly enhances its grain growth.

� 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Structural ceramics; Fracture toughness; Lanthanum orthoniobate

1. Introduction

Lanthanum orthoniobate (LaNbO4), among aseries of earth orthoniobates of similar structureand property, is a ferroelastic phase of monoclinicstructure at ambient temperature [1]. MonoclinicLaNbO4 is heavily domained in two orientations,and the domain boundaries are highly mobile.Under an external stress, the domain boundariesmay move, or in other words, domain switch mayoccur, resulting in a macro strain, which tends torelieve the stress. Upon removal of external stress,the domains switch backward, and the corre-sponding strain decrease or disappear. The term‘rubber-like behavior’ has been used to describethis phenomenon in single crystal LaNbO4 [2–6]. Itis expected that such a phase may be used totoughen structural ceramics. We attempt to con-

firm and develop such a toughening method by thestress-induced domain switching effect, which isindependent and hence addable to other knowntoughening effects. The present paper reports ourwork from synthesizing LaNbO4 crystalline pow-ders, preparing the Al2O3–LaNbO4 composites, tostructural and mechanical characterization of thecomposites. For synthesis, coprecipitation methodhas the advantage of achieving higher purity, finerparticle size and higher activity in sintering overhigh temperature solid reaction method. In thepresent work we explored a route of coprecipita-tion with locally available chemicals, although so-lid reaction synthesis of LaNbO4 is relatively easy.

2. Experimental procedures

2.1. Coprecipitation synthesis of LaNbO4 powders

The commonly employed process for coprecip-itation of LaNbO4 is through reaction of NbCl5

Scripta Materialia 47 (2002) 637–641

www.actamat-journals.com

*Corresponding author. Fax: +86-7918304441.

E-mail address: lzhou@ncu.edu.cn (L. Zhou).

1359-6462/02/$ - see front matter � 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

PII: S1359-6462 (02 )00258-0

with LaCl3 [7]. The present alternative new processwas pursued for sake of availability and economyof raw materials from local source. The major re-action involved is

K3NbO4 þ LaCl3 ¼ LaNbO4 # þ3KCl

First, K3NbO4 was prepared by an eutectic re-action between Nb2O5 powders (99.9 wt%) andK2CO3 powders (99.5 wt%). Aqueous LaCl3 solu-tion was prepared by dissolving La2ðCO3Þ3 � 6H2O(99.9 wt%) in dilute hydrochloric acid. Then, LaCl3solution was slowly poured into water solution ofK3NbO4 while stirring, until the molar ratio ofLaCl3 to K3NbO4 exceeded one. White flocculeprecipitates were found to form during the mixing.The precipitate was then filtered and washed indistilled water and ethanol. The obtained powderswere crystallized by calcination at 1000 �C for 1 h.

2.2. Preparation of Al2O3–LaNbO4 composites

Commercially available a-Al2O3 powders (99.5wt%, �2.9 lm, from Shanghai Chemistry, China)were mixed with 3.4%, 7.0% and 14.4 vol% of thecalcined LaNbO4 powders respectively. The mixedpowders were ball milled in water for 4 h andsieved. Specimens of £50 mm discs were formedby die-pressing under 7.8 MPa, and then firedunder 25 MPa in a hot-pressing graphite mold at1470 �C for 2 h.

2.3. Characterization

The structures of the coprecipitated LaNbO4precursors, the calcined powders, and the sin-tered Al2O3–LaNbO4 composites were analyzedby X-ray diffraction (XRD) with a diffractometerof Rigaku’s BMX–3B, using Cu Ka emission.Microstructures and fractography of the sampleswere examined with a scanning electron micro-scope (SEM) of model Hitachi S-570. Domainstructure in the sintered polycrystalline LaNbO4was examined with a transmission electron mi-croscope (TEM) of model Hitachi 800.The fracture toughness KIC was measured by

the three-point bending tests according to ASTM399-74. Specimens were cut from the sintered discs,and then ground and polished to remove micro-

cracks and defects in surface layers. The finalspecimen is a rectangular bar of 3� 4� 30 mm3,with edges ground to R0.5 mm. Preset cracks ofless than 0.25 mm wide were cut on bottom surfaceof the specimen. A universal mechanical tester wasused. A span of 24 mm and a crosshead speed of0.05 mm/min were adopted. In each test, themaximum load prior to fracture was recorded forcalculation of KIC.

3. Results and discussion

3.1. Characterization of the synthesized powders

XRD spectra of the as-precipitated and calcinedpowder products are shown in Fig. 1. As can beseen, while the as-coprecipitated powders are com-pletely amorphous, they crystallize well in the 1000�C calcination and form the LaNbO4 crystallinephase. Compared with the solid-phase reactionsynthesis [8], the temperature required is about 300�C lower. The average particle size was measuredwith a laser granularity meter, and it was foundthat D50¼ 0.91, D10¼ 0.87 and D90¼ 1.07 lm.

3.2. Phase constitution and microstructure of theAl2O3–LaNbO4 composites

Fig. 2 shows the XRD spectra of the sinteredAl2O3–LaNbO4 composites of various LaNbO4

Fig. 1. XRD spectra of the synthesized LaNbO4 powders, along

with the standard JCPD spectrum for LaNbO4.

638 Z. Zhang et al. / Scripta Materialia 47 (2002) 637–641

proportions. Presence of monoclinic LaNbO4 in a-Al2O3 is confirmed, while no XRD evidence of anythird phase can be found in the spectra. A back-scattered electron image of the composite with 3.4vol% addition of LaNbO4 is shown in Fig. 3. In aback-scattered electron image the brightness rep-resents magnitude of atomic weight, so the brightwhite spots in Fig. 3 are believed to be the LaNbO4phase, while the dark matrix is Al2O3. This isconfirmed by EDAX spot analysis. The small grayarea labeled ‘S’ is found to contain 2–3% at Laapart from Al and O, and is believed to be analumina-based solution.

The average grain sizes of the sintered samplesare measured on micrographs and listed in Table 1along with their densities and mechanical proper-ties. As can be seen, Grain size of the aluminamatrix increases remarkably with addition ofLaNbO4 up to 14.4 vol%. Besides, addition of 3.4vol% LaNbO4 results in a slightly denser materialas compared with the pure alumina, while nofurther significant improvement in density is foundin the samples with increased addition of LaNbO4.In a zirconia-based ceramics, remarkable in-

crease of grain size and the associated decrease inKIC, by addition of YNbO4, has also been re-ported, yet no explanation was given [13]. Usually,it is expected that second phase will hinder growthof matrix grains. However, the effect may be re-versed if a liquid phase forms with addition of thesecond phase. Because the liquid phase will greatlyfacilitate diffusion process at grain boundaries andhence raise the mobility of the boundary. This isprobably the case here. Morphology of theLaNbO4 phase in the composite as shown in Fig. 3suggests that it may have partially experiencedliquid state. The increase in density with additionof LaNbO4 may also be attributed to this.

3.3. Mechanical properties

The results of the mechanical tests are summa-rized in Table 1. Each value of KIC is an average offive samples. As can be seen, addition of 3.4 vol%LaNbO4 significantly raises both the fracturetoughness and hardness. But further addition ofLaNbO4 results in decrease of the both. While thereduction in hardness can be reasonably explainedby presence of excessive softer LaNbO4 phase, thedecrease in KIC is probably caused by the drastic

Fig. 2. XRD spectra of the composites prepared by hot-press

sintering at 1470 �C: (1) Al2O3–3.4%LaNbO4; (2) Al2O3–7.0%LaNbO4; (3) Al2O3–14.4%LaNbO4.

Fig. 3. A SEM back scattered electron image of the Al2O3–

3.4%LaNbO4 composite.

Table 1

Structural and mechanical properties of the Al2O3–LaNbO4composites

LaNbO4(vol%)

Relative

density

(%)

Average

grain size

(lm)

Fracture

toughness, KIC

(MPam1=2)

Vickers

hardness

(GPa)

0 97.1 4.1 3.12 0.29 17.3

3.4 97.8 7.5 5.71 0.20 25.1

7.0 97.9 11.2 4.33 0.32 20.4

14.4 98.1 14.0 3.19 0.55 15.1

Z. Zhang et al. / Scripta Materialia 47 (2002) 637–641 639

increase of grain size. The results suggest that thepresence of LaNbO4 phase does have a tougheningeffect, while the associated increase in grain sizeproduces a negative effect on KIC. The balancebetween the both effects results in a maximum KIC

at 3.4 vol% addition of LaNbO4. It should benoted that although the increased density con-tributed to the improvement of KIC by LaNbO4addition, it alone cannot account for the largeincrement observed.Strengthening of ceramics by addition of

LaNbO4 has been reported previously [8], wherethe concerned composite was tetragonal zirconia-based and contained impurity of SiO2. LaNbO4phase was found to grow into needle-like shapeafter sintering, and crack wake bridging by theseLaNbO4 needles was proposed to account for thestrengthening associated with LaNbO4 addition.In the present case, however, no such needle-likeor fibrous features are seen for the LaNbO4 phasein alumina.The ferro-elasticity of single crystalline LaNbO4

and its ‘‘rubber-like behavior’’ have been well es-tablished, and domain switch has been identifiedas the mechanism responsible for such behavior[2–6]. A domained structure is essential here. Inorder to confirm that the domain structure existsin the sintered ceramics as well, we prepared by ionmilling a thin foil of the LaNbO4 sintered underthe same condition as that for the composites, andcarried out a TEM examination. Pure LaNbO4rather than the composites of concern was useddue to extreme difficulty in preparation of the thinfoil with the embedded LaNbO4 phase. Fig. 4shows a bright field image obtained, which revealsa heavily domained structure inside the grains ofthe ceramics. We expect that the same structurewill exist in the LaNbO4 phase embedded in thecomposites, which undergoes the same sinteringprocess.To further confirm that in sintered compact of

polycrystalline LaNbO4, domain switch can read-ily occur at ambient temperature, we compared theXRD spectra of an annealed LaNbO4 ceramicspecimen before and after surface grinding re-spectively. As can be seen from Fig. 5, the relativeintensity between (0 4 0) and (2 0 0) peaks is greatlychanged by surface grinding. Because the process

is at room temperature, the only possible reasonfor the change is domain switch in the surface layerin response to the shear stress applied in grinding.We propose that during a cracking process, in theLaNbO4 phase, domain switch in response tostress concentration may take place and the asso-ciated strain serves to relieve stress concentrationahead of the crack tip, and hence increases thetoughness.

Fig. 4. A TEM bright field image of the sintered polycrystalline

LaNbO4.

Fig. 5. The (0 4 0) and (2 0 0) peaks of XRD of a hot-press

sintered LaNbO4 specimen after 1000 �C annealing and thensurface grinding respectively.

640 Z. Zhang et al. / Scripta Materialia 47 (2002) 637–641

Ferroelastic domain switching has also beenobserved in tetragonal zirconia, and many exper-imental evidences suggest that it may be an effec-tive mechanism for toughening of zirconia andrelated materials [9–13]. A recent study showedthat even a ferroelectric phase can significantlytoughen alumina matrix, and that stress-inducedmotion of ferroelectric domains may be respon-sible for the effect [14]. LaNbO4 exhibits mostprofound ferroelasticity and domain switch-ability[2–6]. It is reasonable to expect that the role ofstress-induced domain switch in toughening maywell extend to LaNbO4.

4. Conclusions

LaNbO4 amorphous precursors were preparedby coprecipitation approach, using K3NbO4 andLaCl3 as the reagents. In hot press sinteringof Al2O3–LaNbO4 mixture at 1470 �C, most ofthe added LaNbO4 remains stable. Addition ofLaNbO4 greatly enhances growth of aluminagrains in sintering. Fracture toughness of aluminacan be significantly improved by appropriate ad-dition of LaNbO4. Sintered polycrystallineLaNbO4 has domain structures within its grains,and domain switch readily occurs at ambienttemperature. It is postulated that the observedtoughening effect is mainly attributed to stress-induced domain switch in the embedded ferro-elastic LaNbO4 phase.

Acknowledgements

We gratefully acknowledge the support of thepresent work by the Natural Science Foundationof Jiangxi Province, China, and the OpeningFoundation of the State Key Laboratory for NewCeramics and Fine Processes at Tsinghua Uni-versity, China, under grants numbered 9950030and KF0001 respectively.

References

[1] Brixner LH, Whitney JF, Zamsteg FC, Jones GA. Mat Res

Bull 1977;12:17.

[2] Tsunekawa S, Takei H. J Phys Soc Japan 1976;40:1523.

[3] Tsunekawa S, Hara K, Nishitani R, Kasuya A, Fukuda T.

Mater Trans JIM 1995;36:1188.

[4] Li J, Huang CM, Xu G, Wayman CM. Mater Lett 1994;

21:105.

[5] Li J, Wayman CM. J Am Ceram Soc 1996;79:1642.

[6] Li J, Wayman CM. Acta Mater 1995;43:3893.

[7] Maschio S, Bachiorrini A, Dimonte R, Montanaro L.

J Mater Sci 1995;30:5433.

[8] Maschio S, Pezzotti G, Sbaizero O. J Euro Ceram Soc

1998;18:1779.

[9] Virkar AV, Matsumoto RLK. J Am Ceram Soc

1986;69:C224.

[10] Chan CJ, Lange FF, Ruhle M, Jue JF, Vikar AV. J Amer

Ceram Soc 1991;74:807.

[11] Srinivasan GV, Jue JF, Virkar AV. J Amer Ceram Soc

1989;72:2098.

[12] Lankford J, Page RA, Rabenberg L. J Mater Sci 1988;

23:4144.

[13] Yuh SD, Lai YC, Chou CC, Lee HY. J Mater Sci 2001;

36:2303.

[14] Liu YG, Jia DC, Zhou Y. Ceram Int 2002;28:111.

Z. Zhang et al. / Scripta Materialia 47 (2002) 637–641 641