high-temperature deformation of al2o3/y-tzp particulate laminates

Acta Materialia 52 (2004) 4685–4693

www.actamat-journals.com

High-temperature deformation of Al2O3/Y-TZP particulate laminates

Jue Wang a, Eric M. Taleff a,b, Desiderio Kovar a,b,*

a Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, USAb Department of Mechanical Engineering, The University of Texas at Austin, 1 University Station C2200, Austin, TX 78712, USA

Received 23 April 2004; received in revised form 18 June 2004; accepted 21 June 2004

Available online 22 July 2004

Abstract

Al2O3/Y-TZP particulate laminates with varying compositions and ratios of layer thickness were fabricated by tapecasting, lam-

ination, and sintering. The resulting particulate laminates were tested in compression at a temperature of 1350 �C over strain rates

from 1.00·10�5 to 3.16·10�4 s�1. Microstructural changes during testing were observed to be minor. Stress exponents were meas-

ured to be approximately two and are consistent with previous data for particulate composites. Using parameters determined from

particulate composites, the behaviors of the particulate laminate composites are accurately predicted using a constrained isostrain

model without additional fitting parameters.

� 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: High-temperature deformation; Compression test; Laminates

1. Introduction

In the past decade, laminated ceramic composites

have emerged as promising candidates for use in struc-

tural applications [1–3]. By controlling microstructure,

many properties of laminated ceramic composites can

be tailored to be superior to their monolithic counter-

parts [4,5]. For example, greatly enhanced fracture

toughness has been observed in Al2O3/Ce-TZP lami-

nates compared to either monolithic Al2O3 or Ce-TZP [6]. Although, the deformation behaviors of some

laminated metal composites have been explored [7–9],

there have been only a few studies of the creep of

laminated ceramic composites [10–12] and, as yet, de-

formation behavior of laminates is not well under-

stood. In contrast, the high-temperature behavior of

Al2O3/Y-TZP particulate composites has been exten-

1359-6454/$30.00 � 2004 Acta Materialia Inc. Published by Elsevier Ltd. A

doi:10.1016/j.actamat.2004.06.034

* Corresponding author. Tel.: +1-512-471-6271; fax: +1-512-471-

7681.

E-mail address: [email protected] (D. Kovar).

sively studied [13–17], because they exhibit remarkably

high tensile elongations of up to 625% [17].In the present work, Al2O3/Y-TZP particulate com-

posites with different compositions were used as starting

materials to build up a novel layered architecture, i.e.

particulate laminates. The objective of this work is to

study the high-temperature behavior of Al2O3/Y-TZP

particulate laminates in compression as a function of

composition, specimen orientation, and layer thickness

ratios. The data obtained for particulate laminates arecompared to existing theoretical models and to data

for particulate composites.

2. Experimental procedure

2.1. Processing

Fine-grained yttria-stabilized tetragonal zirconia pow-

der (3Y-TZP, Tosoh, Tokyo, Japan) and high-purity

ll rights reserved.

mailto:[email protected]

4686 J. Wang et al. / Acta Materialia 52 (2004) 4685–4693

alumina powder (AKP-50, 99.99% purity, Sumitomo

Chemical Co. Ltd., Tokyo, Japan) were used as raw

materials. Ceramic powders were ball-milled in solvents

with a dispersant, a plasticizer, and a polymer binder.

Tapecasting was performed using a doctor blade on a

glass substrate. After drying, tapes with a length of 47mm, a width of 23 mm, and a thickness of 90 lm were

stacked and then laminated at 120 �C within a metallic

die to form a billet. The polymeric binder within the

billet was subsequently pyrolyzed by heating slowly in

flowing air, and the billets were then pressureless

sintered at 1450 �C for 1 h. Details of the tapecasting,

laminating and sintering processes can be found else-

where [15].Tapecasting slurries were prepared with solids con-

tents of 20, 40, 50, 60, and 80 vol% Al2O3, with the bal-

ance Y-TZP. Tapes of two different compositions were

stacked in alternating layers to form symmetric particu-

late laminates. Compositions in this study are desig-

nated by the volume fraction of Al2O3; for example,

20A contained 20 vol% Al2O3 and 80 vol% Y-TZP.

For laminates, the layer containing more Y-TZP is des-ignated as the soft layer; the layer containing more

Al2O3 is designated as the hard layer. The ratios of soft

layer to hard layer thicknesses varied from 1:4 to 4:1.

For 1:1, both layers were 450 lm thick prior to sintering;

for 1:2 (or 2:1) and 1:4 (or 4:1), the thinner layer was 450

lm and the thicker layer was twice and four times as

thick, respectively, prior to sintering. The total thickness

of each green billet was about 8 mm. After sintering, theshrinkage of the layer thickness was approximately 22%

for all the compositions. The particulate laminates are

designated by their constituent layers and ratios of layer

thicknesses, e.g. 20A/40A (1:1) consisted of 20A and

40A layers with a layer-thicknesses ratio of 1:1. A sum-

mary of all the particulate laminates fabricated for this

study is given in Table 1. Some compositions, indicated

with an asterisk, were fabricated but cracked duringprocessing due to differential sintering rates and thermal

expansion mismatch between layers [18] and, therefore,

were not tested.

Table 1

Compositions of the particulate laminates used in this study are shown

1:1 1:2 2:1 1:4 4:1

Particulate laminates

20A/40A 20A/40A 20A/40A

20A/50A 20A/50A 20A/50A

20A/60A 20A/60A 20A/60A 20A/60A 20A/60A

20A/80A 20A/80A 20A/80A* 20A/80A* 20A/80A*

40A/80A 40A/80A 40A/80A

50A/80A 50A/80A 50A/80A

60A/80A 60A/80A 60A/80A

40A/60A 40A/60A

Asterisks indicate compositions that fractured during processing.

2.2. Microstructure analysis and mechanical testing

The density of each billet was measured using the Ar-

chimedes method, according to ASTM C373-88 [19],

with water as the immersion medium. Microstructures

of the particulate laminates were characterized using ascanning electron microscope (SEM). Specimens for

SEM observation were polished to a final grit size of 1

lm using diamond abrasives and were then thermally

etched at 1370 �C for 20 min to reveal grain structure

[15]. Mean grain sizes (�d) of both Al2O3 and Y-TZP

were calculated from measurements of equivalent circu-

lar diameter (D), using the following expression [20]:

�d ¼ 1:27�PN

i¼1DN

; ð1Þ

where N is the number of measured grains. The mor-

phology of particulate laminates before and after testing

was observed using an optical microscope.

The sintered laminates were cut and ground into rec-tangular bars 6 mm·4 mm·4 mm with the largest di-

mension either perpendicular or parallel to the layer

interfaces. Compression strain-rate-change (SRC) tests

[15] were performed at 1350 �C in air using a split-tube

furnace with MoSi2 heating elements. The applied uni-

axial, compressive stress was either perpendicular or

parallel to the layer interfaces. Fig. 1 illustrates these

testing configurations, which are referred to as the iso-stress orientation, Fig. 1(b), and the isostrain orienta-

tion, Fig. 1(c) [21].

The compression SRC tests consisted of nine steps in

engineering strain rate, defined as the ratio of displace-

ment rate to the sample length at the beginning of each

step. A prestrain of approximately 2% was initially ap-

plied at a strain rate of 1.00·10�4 s�1 in order to ensure

mating between the sample and compression platens andto stabilize the sample microstructures. Following the

prestrain step, seven steps with strain rates from

1.00·10�5 to 3.16·10�4 s�1 were applied. The strain

Fig. 1. Illustrations of testing orientation for: (a) particulate compos-

ites, (b) particulate laminates, isostress orientation and (c) particulate

laminates, isostrain orientation are shown.

J. Wang et al. / Acta Materialia 52 (2004) 4685–4693 4687

rate of 1.00·10�4 s�1 was repeated in the final step of

the series to check for repeatability of the measurements.

Engineering strain and stress were obtained from load–

displacement curves by assuming that the change in

displacement of the crosshead corresponded to the

reduction in the height of the specimen, after compensa-tion for elastic deflection. True stress, true strain, and

true strain-rate were derived from engineering stress,

strain, and strain rate under the assumptions of uniform

deformation and volume conservation. Due to the small

strain within each step, the true strain-rate was within

2% of the engineering strain-rate for each step.

Fig. 2. Scanning electron micrographs of particulate laminates at

different magnifications are shown for: (a) 60A before testing; (b) 20A/

60A (1:1) after testing at 1350 �C. An optical micrograph is shown for

(c) 20A/60A (1:1) before testing. The arrows indicate the orientation of

applied stress in (b).

3. Results

3.1. Microstructure

An example of the microstructure of a typical Al2O3/

Y-TZP particulate composite, prior to testing, is shown

in Fig. 2(a). The grains with the lighter shading are

Y-TZP and the grains with the darker shading areAl2O3. Both Al2O3 and Y-TZP phases are generally

well-dispersed and have equiaxed grain shapes. In a pre-

vious study of particulate composites [15], it was found

that the mean grain size of Al2O3 ranged from 0.35 to

0.46 lm, and the mean grain size of Y-TZP varied from

0.32 to 0.22 lm, with the Al2O3 grain size increasing as

the volume fraction of Al2O3 in the Al2O3/Y-TZP par-

ticulate composites increased [15].A SEM micrograph of a representative 20A/60A (1:1)

particulate laminate after compression SRC testing is

shown in Fig. 2(b), where the darker layers are 60A.

An optical micrograph of the 20A/60A (1:1) laminate

before testing is shown in Fig. 2(c), where the layers with

the lighter shading are 60A. From these micrographs, it

can be seen that the interfaces between the layers of par-

ticulate composites with different compositions arestraight and remain bonded during testing. Previously,

we have shown that, within the test temperature and

range of strain rates used in the current study, no statis-

tically significant changes in grain size or grain shape, as

measured by SEM microscopy, occurred during com-

pression SRC testing of particulate composites contain-

ing from 20 to 80 vol% Al2O3 [15]. Observations of the

microstructures in the particulate laminates tested underthe same conditions indicate that they also have no

significant change in grain size or shape during SRC

testing.

The relative densities of selected laminates are shown

in Table 2 before and after testing. The densities prior to

testing range from 95% to 99%. The highest densities are

obtained in materials for which the composition differ-

ence between layers is small (e.g. 20A/40A and 40A/60A). These results are consistent with previous studies

on similar Al2O3/Y-TZP laminates, which found that

large composition differences result in differential sinter-

ing rates between the layers and low density [18]. Com-

paring the densities before and after testing (Table 2), it

is observed that only a slight (1%) increase in density oc-

curred during compression testing.

3.2. Deformation behavior

The dimensions of all the (1:1), (1:2), and (2:1) speci-

mens were measured after SRC testing. The macroscopic

Table 2

Relative densities of Al2O3/Y-TZP particulate laminates before and after testing (in the isostrain orientation) are shown

Laminates Relative densities (%)

2:1 1:1 1:2 1:4

Before After Before After Before After Before After

20A/40A 98.9 99.1 99.1 99.1 98.4 99.2

20A/50A 97.6 98.5 97.9 98.9 99.1 99.4

20A/60A 97.3 98.0 97.3 98.3 98.6 99.0 97.3 97.8

20A/80A 95.8 96.4 95.9 96.6

40A/60A 98.4 99.2 97.1 98.1


strains calculated from the final specimen dimensions

in the directions perpendicular to the loading axis were

equal, indicating that the macroscopic, in-plain plastic

strains were isotropic. Measurements of strains after

SRC testing revealed no significant differences between

the strains in the two directions perpendicular to the ap-

plied stress for specimens tested in either the isostress or

isostrain orientations, indicating that the in-plane defor-mations are approximately isotropic. Fig. 3 shows data

from a representative SRC test for the 20A/40A (1:2)

material tested in the isostress orientation. The total true

strain for this test is e=0.115. After a brief transient at

every rate change, a reasonably steady-state stress is

achieved. It is also apparent that for the repeated steps

at a strain rate of 1.00·10�4 s�1, the steady-state flow

stress at a given rate is nearly constant over the rangeof strains imposed in any single SRC test. Behaviors

similar to those shown in Fig. 3 were observed for all

the particulate laminates tested.

Fig. 4 shows data accumulated from SRC tests at

1350 �C for all of the (1:1) laminates as plots of true

strain-rate against true stress on log–log scales. Figs.

4(a) and (c) contain data for specimens tested in the iso-

stress orientation and Figs. 4(b) and (d) contain data forspecimens tested in the isostrain orientation. From these

plots, it is apparent that the flow stresses for a given ma-

terial are very similar in both testing orientations. The

Fig. 3. A plot of true stress versus true strain is shown with SRC test

data from a 20A/40A (1:2) specimen in the isostress orientation. The

dashed line indicates the average flow stress at a strain rate of

1.00·10�4 s�1.

slope in Fig. 4 is equal to the stress exponent, n, from

the phenomenological equation for creep, which can

be written at a constant temperature as [22]

r ¼ K _e1=n; ð2Þwhere K is a constant for a given material. For all these

materials, the stress exponents are approximately equal

to 2 over the range of measured strain rates. A slight

negative curvature is apparent in all of the data, suggest-

ing that a transition occurs to lower values of stress ex-

ponent as strain rate increases which is consistent with

previous studies [23,24]. Similar behavior is observed

for all the laminates tested and has been observed previ-ously in Al2O3/Y-TZP particulate composites [15].

The influence of layer composition on the high-tem-

perature deformation of particulate laminates can be as-

sessed from Fig. 4. In Figs. 4(a) and (b), the composition

of the hard layer is fixed at 80A and the composition

of the soft layer is varied. As the fraction of Al2O3 in

the soft layer is increased, the resistance to deforma-

tion in the particulate laminates increases slightly. InFigs. 4(c) and (d), the composition of the soft layer is

fixed at 20A and the composition of the hard layer is

varied. In this case, the resistance to deformation in

the particulate laminates increases significantly as the

volume fraction of Al2O3 in the hard layer is increased.

Values of strain rate at a stress of 50MPa, interpolated

from the data in Figs. 4(a)–(d), are shown in Fig. 5, as

a function of total Al2O3 volume fraction for all of the(1:1) laminates in both the isostress and isostrain orien-

tations. Also shown in Fig. 5 are values of strain rate for

Al2O3/Y-TZP particulate composites with similar grain

sizes at a stress of 50 MPa, interpolated from previous

work (see Fig. 4 in [15]). The solid line and dashed line

are trend lines for the particulate composites and partic-

ulate laminates, respectively. The values of strain rate in

Fig. 5 indicate that the creep rates in the Al2O3/Y-TZPparticulate laminates decrease with increasing Al2O3

content, consistent with observations of the Al2O3/Y-

TZP particulate composites [15]. Moreover, comparing

particulate laminates and particulate composites with

the same volume fraction of Al2O3 indicates that the

particulate laminates creep more slowly than do the par-

ticulate composites for Al2O3 volume fractions up to

Fig. 4. The influence of layer composition on deformation response is shown for: (a) isostress and (b) isostrain orientations for which the

composition of the hard layer is held constant and the composition of the soft layer is varied; and (c) isostress and (d) isostrain orientations for which

the composition of the soft layer is held constant and the composition of the hard layer is varied.

Fig. 5. The dependence of strain rate on volume fraction of Al2O3

is shown. The solid line indicates the trend for particulate compos-

ites [15] and the dashed line indicates the trend for particulate

laminates.


0.6, i.e. the particulate laminates appear to be stronger

than the particulate composites.

Fig. 6 shows SRC data for 20A/60A particulate lam-

inates with different layer thickness ratios as plots of thetrue strain-rate against true stress on log–log scales.

Figs. 6(a) and (b) are for specimens tested in the isostress

and isostrain orientations, respectively. Data for 20A

and 60A particulate composites are shown in Fig. 6

for comparison [15]. As expected, the flow stresses for

the 20A/60A laminates lie between the flow stress of

20A and 60A particulate composites at any given strain

rate. There is little difference between the data for spec-imens tested in the isostress and isostrain orientations.

4. Discussion

4.1. Microstructure

Prior to testing, the particulate laminates used in thecurrent study exhibited lower densities than the particu-

late composites tested previously [15]. As a result, many

of the specimens exhibited a slight increase (61%) in

density during compression testing. To evaluate the in-

fluence of changes in density that occurred during the

deformation, the strain attributed to densification was

calculated and found to be less than 1%. Compared to

the total engineering strain (�11%) during testing, thestrain from densification is relatively small and was,

therefore, neglected. The flow stress at a given strain rate

is nearly constant over the strains used in testing (see

Fig. 3), further confirming that densification and other

Fig. 6. The influence of thickness ratios on deformation response of 20A/60A particulate laminates is shown for: (a) isostress orientation, (b)

isostrain orientation. Data from particulate composites are shown for comparison [15].


changes in microstructure do not strongly influence the

deformation behavior of these particulate laminates.

4.2. Stress exponents

Previous studies of Al2O3/Y-TZP particulate compos-

ites suggest that stress exponents are approximately 2

for composites with high Al2O3 contents and high strain

rates and approximately 3 at low Al2O3 contents and

low strain rates [15]. In addition, Jimenez-Melendo et

al. [11] have reported stress exponents of approximately

2 over similar stress and temperature ranges for Al2O3/

Y-TZP hybrid laminates consisting of alternating layersof Y-TZP and Al2O3/Y-TZP. The present results for

particulate laminates are similar, with a stress exponent

of approximately 2 at high strain rates and a transition

to higher values as strain rate decreases. This suggests

that there is likely a common deformation mechanism

between Al2O3/Y-TZP particulate composites and par-

ticulate laminates.

4.3. Models for deformation behavior

French et al. [21] proposed that the creep of particu-

late composites could be analyzed using either an iso-

stress or an isostrain model. The isostress model

assumes that the average stress in the composite, rc, isequal to that in each component of the composite, r1and r2 for a two-component composite, i.e. rc=r1=r2.The average strain rate in the composite, _ec, is then given

by

_ec ¼ V 1 _e1 þ V 2 _e2; ð3Þwhere V1 and _e1 are the volume fraction and strain rate

of component one and V2 and _e2 are the volume fraction

and strain rate of component two. The isostrain model,on the other hand, requires that the strain in each com-

ponent of the composite equal the average composite

strain, i.e. ec= e1= e2. Thus, the strain rates must also

be equal, i.e. _ec ¼ _e1 ¼ _e2. The average stress in the com-

posite can be represented for the isostrain case as

rc ¼ V 1r1 þ V 2r2: ð4ÞAssuming that each component obeys Eq. (2), the iso-

strain model predicts the composite flow stress to be

rc ¼ V 1K1 _e1=n1c þ V 2K2 _e

1=n2c ; ð5Þ

where Ki ¼ EiðAiÞ1=niðdi=biÞpi=ni expðQci=niRT Þ for eachcomponent i, where E is the dynamic, unrelaxed

Young�s modulus, A is a material constant, d is the grain

size, b is the magnitude of the Burgers vector, p is the

grain-size exponent, n is the stress exponent, Qc is the ac-

tivation energy for creep, R is the universal gas constant,

and T is the absolute temperature [22].

To determine whether existing models could be used

to describe the behavior of particulate laminates, the be-havior of particulate composites with compositions

from 20A to 80A were first examined. Previously, it

was shown that the isostress model did not fit the exper-

imental data well for Al2O3/Y-TZP particulate compos-

ites. Although the isostrain model yielded better fits to

the data, physically unrealistic values of the fitting pa-

rameters were observed [15]. The deformation behaviors

of these particulate composites were, however, accurate-ly modeled using a constrained isostrain model for

which the stress exponent was constrained to a single

value, i.e. n1=n2, and differences in grain size were ac-

counted for. This model predicts

rc ¼ ðV 1K1 þ V 2K2Þ_e1=2c ; ð6Þwhere the stress exponent for Al2O3 and Y-TZP were

both taken to be 2, i.e. n1=n2=2, [15].

To model the behavior of Al2O3/Y-TZP particulatelaminates, the laminates are considered to consist of lay-

ers with two compositions, each of which are particulate

composites. The particulate composites, which all exhibit

n�2 within compositions ranging from 20 to 80 vol%

Al2O3, have been shown to closely obey Eq. (6) [15]. Be-

Table 3

The properties of Al2O3/Y-TZP particulate composites are shown

(T=1350 �C, r=50 MPa, n=2) from [15]

Particulate composites _e ðs�1Þ K (s�2 MPa)

20A 2.73·10�4 3026.1

40A 1.66·10�4 3880.8

50A 1.00·10�4 5000.0

60A 5.51·10�5 6735.9

80A 2.77·10�5 9500.1


cause the particulate laminate composites also exhibit

n�2, it is hypothesized that their behaviors may also

obey Eq. (6), when each particulate composite is consid-

ered a component of the laminate composite, i.e. one

particulate composite layer provides V1 and K1 andthe other provides V2 and K2. Because the values of

K1 and K2 were previously determined [15] for particu-

late composites within the range of compositions used

in the laminate composites, Eq. (6) can be used to directly

predict the behavior of each particulate laminate com-

posite with no additional fitting parameters required.

The isostress model, Eq. (3), can be evaluated in a sim-

Fig. 7. The dependence of strain rate on the volume fraction of the hard layer

laminates. The solid symbols represent the data for the 20A and 60A particu

particulate laminates.

ilar manner for comparisons with predictions from the

constrained isostrain model, Eq. (6). The data for partic-

ulate composites used in these models are summarized in

Table 3 [15].

Predictions of the isostress and the constrained iso-

strain models are shown in Figs. 7(a)–(d) as dashedand solid lines, respectively. It is apparent that when

the composition differences between the layers are small

(e.g. 20A/40A and 60A/80A), the differences between the

predictions of the isostress and the constrained isostrain

models are small, as shown in Figs. 7(a) and (d). When

the composition differences between the layers are larger

(e.g. 20A/60A and 40A/80A), the differences between the

predictions are large, as seen in Figs. 7(b) and (c). InFigs. 7(a) and (d) the differences between the two models

are within the range of experimental scatter. However,

in Figs. 7(b) and (c) it is clear that the constrained iso-

strain model fits the experimental data best both when

specimens are tested in the isostress orientation and

when tested in the isostrain orientation.

To further examine the applicability of the con-

strained isostrain model to Al2O3/Y-TZP particulate

is shown for: (a) 20A/40A; (b) 20A/60A; (c) 40A/80A; and (d) 60A/80A

late composites [15], and the open symbols correspond to the data for

Fig. 8. The flow stresses at three different strain rates are shown for

20A/60A particulate laminates. The solid lines are predictions from the

constrained isostrain model. The solid symbols are data for 20A and

60A particulate composites [15], and the open symbols are data for



laminates, flow stresses are predicted at three different

strain rates using Eq. (6) and compared to experimental

data. The values of K1 and K2 for particulate composites

at a flow stress of 50 MPa, as shown in Table 3, were

used to predict flow stresses of the particulate laminates,

and these predictions are shown as solid lines in Fig. 8.

The experimentally determined values of flow stress arealso shown in Fig. 8. Predictions and data are for strain

rates of 1.00·10�5, 1.00·10�4, and 3.16·10�4 s�1. The

solid symbols correspond to the data for particulate

composites from previous work (see Fig. 4 in [15]). Note

that there are no fitting parameters used in the predictive

equations since all the parameters in Eq. (6) are deter-

mined independently of the data in Fig. 8. Excellent

agreement is observed between data in both the isostressand isostrain orientations and the constrained isostrain

model. As shown in Fig. 8, the strength of the particu-

late laminates increases with increasing volume fraction

of the hard layer (i.e. 60A), especially at high strain rate,

e.g. 3.16·10�4 s�1.

These results also explain why the particulate lami-

nates appear to be stronger than particulate composites

with the same overall composition, as shown in Fig. 5.For a given stress, the strain rate does not vary linearly

with composition for materials such as Al2O3/Y-TZP

particulate composites that obey the constrained iso-

strain model [15]. Since the particulate laminates con-

sisting of layers A and B were shown to follow a

constrained isostrain model, and since the constituent

layers A and B themselves obey the constrained isostrain

model, the deviation of strain rate from a rule-of-mixtures with composition is further increased for the


As discussed by Jimenez-Melendo et al. [11], the dis-

crepancy between the isostress model and experimental

data for samples tested in the isostress orientation re-

sults from inter-layer constraint imposed by the hard

layer on the soft layer, which is not accounted for in

the isostress model. Because volume is conserved during

plastic deformation and because the layers remain

bonded without slipping at the interface, the strains in

each layer must be equal, except near free surfaces, where

this constraint is relaxed away from the layer interfaces.

Note that since the volume fraction of material near sur-

faces is relatively small for the test specimens used in thisstudy and the strains during SRC testing were also

small, the loss of constraint at free surfaces is a minor

effect and the measured deformation behavior is domi-

nated by material that is constrained. As a result, the be-

havior of specimens tested in both orientations should

be similar, as was observed. The slightly higher strength

of specimens tested in the isostrain orientation results

from this orientation having fewer free surfaces whereinter-layer constraint can be relaxed (2 for the isostrain

versus 4 for the isostress orientation). Jimenez-Melendo

et al. [11] also found that the isostrain model could be

used to predict the behavior Y-TZP/Al2O3+Y-TZP

hybrid laminates tested in the isostrain orientation.

Although they did not attempt to predict the behavior

of specimens tested in the isostress orientation, it is ap-

parent from visual inspection that the isostrain modelcould also be used to predict the behavior of their spec-

imens tested in the isostress orientation.

5. Conclusions

The high-temperature deformation behaviors of

Al2O3/Y-TZP particulate laminates with varying com-positions and ratios of layer thickness, fabricated by

tapecasting, lamination, and sintering, have been investi-

gated in compression at 1350 �C with a stable, fine-

grained microstructure. Experimental results show that

stress exponents of all the Al2O3/Y-TZP particulate lam-

inates are approximately 2, which is similar to that meas-

ured on particulate composites within the same range of

composition. Experimental measurements indicate thatlaminates in the isostrain orientation are very slightly

stronger than laminates in the isostress orientation. This

is a result of good layer bonding, which requires the

strains in each layer to be identical, except near free sur-

faces where constraint is relaxed; the difference in free sur-

face area where inter-layer constraint is relaxed between

the isostress and isostrain orientations results in their

slight difference in strength. The constrained isostrainmodel, with n1=n2=2, provides a good prediction of

the flow behavior in particulate laminates tested in both

the isostrain and isostress orientations.

Acknowledgement

This work has been supported by the Texas Ad-vanced Research Program under project #003658-

0426-1999.


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