3−報文−Phase Separation and Hydrothermal Degradation of 3 mol％
Y2O3 − ZrO2 Ceramics.
Isao YAMASHITA
Koji TSUKUMA

Phase separation and hydrothermal degradation of 3 mol% Y 2O3−stabilized tetragonal ZrO2 polycrystal (3Y−
TZP) have been studied by means of x−ray diffractometry. The Rietveld analysis revealed that the crystal
structure of the sintered body remarkably changes at high temperatures. The sintered body underwent phaseseparation into high and low yttrium regions with increasing sintering temperatures. The tetragonal phase was
assigned to both regions. The high yttrium region was analyzed on the basis of a tetragonal phase model ofc/a〜＝1. The amount of monoclinic phase induced by hydrothermal degradation increased with the increase ofdegradation time. The aging kinetics of the tetragonal to monoclinic phase transition was analyzed using
Johnson−Mehl−Avrami equation. The fraction of a transformable phase evaluated by the equation was in good
agreement with the fraction of low yttrium region derived from the Rietveld analysis. Quantitative analysis
showed that the low yttrium region selectively transforms into monoclinic phase under the hydrothermal
condition, while the high yttrium region remains unchanged. The fraction of 3Y−TZP ceramics transformable
under hydrothermal condition was governed by the amount of low yttrium region. Thus, the formation of low
yttrium region due to phase separation is responsible for the hydrothermal degradation of the sintered bodies.

TZP by means of either the formation of such

１．Introduction

compound as Y(OH)3 3) or the direct leaching 4). The

Numerous studies have been made on Y 2 O 3 −other is the termination of Zr−O−Zr bonds near thestabilized tetragonal ZrO2 polycrystal (Y-TZP) in thesurface by attack of H 2 O molecules. Sato et al . 5)

past decades. Especially, 3 mol％ Y2O3 doped systems

reported that the termination of Zr−O−Zr bonds leads to(3Y-TZP) have been studied extensively because of its

the reduction of strain near the surface. The reduction

high toughness and strength1). The tetragonalof lattice strain reduces the Gibbs free energy gap

structure in sintered body is closely related to thebetween the tetragonal and monoclinic phases.

mechanical property, because the enhanced toughness

Yoshimura et al. 6) suggested that the termination ofand strength are attributed to the stress-induced phase

Zr−O−Zr bonds allows OH− ion to penetrate into the2)transition of tetragonal phase .

tetragonal lattice and consequently to reduce an energy

The tetragonal to monoclinic phase transition isaccelerated under hydrothermal conditions. Thisbarrier of nucleation for the tetragonal to monoclinic
phase transition.

phenomenon is called the hydrothermal degradation of

Although the tetragonal to monoclinic phasetetragonal zirconia. Although many attempts have beentransition (so−called martensitic phase transition) plays

made to explain the hydrothermal degradation, thean important role not only for the high toughness

mechanism is still controversial. So far, two majormechanism but also for the hydrothermal degradation

theories have been proposed to explain the degradation

mechanism, only few studies have so far been made on

mechanism. One is the drawing out of yttrium from Y−

the crystal structure of the sintered body.

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TOSOH Research ＆ Technology Review Vol.50（2006）

Recently, the heterogeneity caused by yttriummonoclinic phases, respectively. In order to avoid local

segregation and cubic formation has been reported forminima, calculation was carried out with the conjection

7−10)

.

direction method through refinement. The split

reported that the enrichment ofpseudo-Voigt function was used for the profile function.

yttrium exclusively occurs along the grain boundary

Zr4＋, Y3＋ and O2− ions were used as chemical species.

and the solute dragging effect of yttrium plays an

Through the refinement, occupancy of atom was fixedimportant role for the grain-growth mechanism.

to (Y 2O 3 ) 0.03 (ZrO 2) 0.97 in all phases. In the case of

3Y−TZP ceramics using high−resolution TEM
Matsui et al.

8−10)

In this study, the structural changes of the sinteredmonoclinic phase, the linear constrain condition wasbody were analyzed using the Rietveld analysis as aapplied for Zr and Y with respect to atomic positions.

function of sintering temperature. The aging kinetics of

The linear constrain conditions for isotropic

the tetragonal to monoclinic phase transition was also

displacement parameters were also applied for Zr and Y

analyzed with an equation for nucleation-growthin all phases. For the degradation samples, themechanism. The correlation between the phase

prefered orientation (−102) was used for monoclinic

separated state of the sintered body and thephase 15).

hydrothermal degradation phenomena have been
discussed.

３．Results and discussion
The x−ray diffraction data of the sintered body were２．Experimental

analyzed by the Rietveld method. According toCommercially available 3Y−TZP powder of a specific

Ohmichi et al . 16), the cubic phase in 3Y−TZP was

surface area 16 m 2/g (TZ−3Y grade, Tosoh, Tokyo,

detected as a function of sintering temperatures. Thus,

Japan) was used as the starting material. The powder

the cubic and tetragonal models were first examined for

was synthesized by the hydrolysis method. The 3Y−

the data from the samples sintered at 1500 ℃. The

TZP powder was pressed uniaxially into a disk at 70

reliability factors−the weighted profile, Bragg intensity,

MPa and then isostatically cold−pressed at 200 MPa.

and goodness−of−fit indicator−were Rwp＝6.66 ％, RI＝

The green compacts were sintered at 1100, 1200, 1300,

1.92 ％ for the cubic phase, R I ＝2.22 ％ for the

1400 and 1500 ℃ in air for 2 h with a constant heating

tetragonal phase, S ＝1.61. A small discrepancy

rate of 100 ℃/h. The bulk density of the sinteredattributable to the cubic structure was observed aroundbodies was measured by the Archimedes method. The

the (400) cubic reflection. Recent studies on Y−TZP

hydrothermal degradation experiments were carried

showed the existence of a tetragonal phase with c/a〜
＝1

out on the samples sintered at 1500 ℃ at 140 ℃ using a(so−called pseudo−cubic structure) 17, 18). Thus, wepressure-resistant stainless vessel for a period of 3 to 48

made the refinement using the two−tetragonal (namedh.

t1, t2) model.
The x− ray diffractometry was made at room

Fig. 1 illustrates the profile fit and difference patterns

temperature using a powder diffractometer systemof the revised Rietveld analysis of the sample sintered(Model MXP, MAC Science Co. Ltd., Tokyo, Japan)

at 1500 ℃. The region of (400) tetra, (004)tetra reflections

with the Cu Ka radiation source, under 40 KV and 30

is enlarged as an inset. The two−tetragonal model gave

mA. The diffraction data were collected in the area 2θmuch less reliable factor than the cubic and tetragonal= 20 to 90 ° with a step of 0.04 °. Intensity wasmodel. The reliable factors are Rwp＝6.25, RI＝1.89 for

integrated for 20 sec at each step. Measurements weret1 phase, RI＝1.78 for t2 phase, S＝1.51. As the two−

made on an as-sintered surface of the sintered body.

tetragonal model gave much less reliable factors thanThe diffraction data were analyzed by the Rietveld
method with a program Rietan−2000

11), where thestructural parameters reported by Yashima et al.
Morinaga et al.

13), and Howard et al.

14)

12)

,

the tetragonal and cubic model, we made therefinement for all samples using the two−tetragonalmodel.

were used as

The results obtained by the Rietveld analysis based

the initial parameters for the tetragonal, cubic andon the two-tetragonal phase model (t1, t2) and the bulk

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東ソー研究・技術報告 第50巻（2006）

5

60000
3000

Rwp ＝6.25％

2000

50000

S ＝1.51

Intensity

1000

40000

0

t1

Intensity

t2

30000

70.0 70.5 71.0 71.5 72.0 72.5

73.0 73.5

74.0 74.5 75.0 75.5 76.0 76.5

77.0 77.5 78.0

2θ［degree］

20000

10000

0

t1
t2

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

2θ［ degree］

Fig.１

Table１Results of x-ray diffractometry and revised Rietveld analysis of the sample sintered at 1500 ℃.
Dotted line: data obtained. Solid ilne: calculated intensity. Bars: angular position of possible
Bragg reflection of t1, t2 phase. Plots at bottom: ( I calc- I obs) difference. The region of (400)tetra,(004)tetra reflections is enlarged as an inset.

Bulk densities and results of Rietveld analysis of 3Y-TZP ceramics with two-tetragonal model ( P4 2 /nmc ).

Sintering Density
Phase
temp.(℃) (g/cm3)

wt％

Y2O3*
(mol％)a (nm)

0.36046(1)
2.46
76.3
t1
0.36248(5)
6.77
23.7
t2
0.36082(3)
2.75
85.4
t1
1400
6.06
0.36236(6)
6.80
14.6
t2
0.36080(3)
3.02
92.1
t1
1300
5.38
0.3621(1)
6.67
7.9
t2
0.36083(4)
3.17
90.8
t1
1200
4.20
0.3621(1)
6.67
9.2
t2
0.36074(4)
3.21
91.2
t1
1000
3.38
0.3621(1)
6.67
8.8
t2
*:calculated from eq.1 (see text).
**:calculated from eq.1 and fraction of t1, t2 phase.
1500

6.07

c (nm)

Atomiccoordinates z

100・U Zr,Y
(nm2)

100・UO(nm2)

c f /a f

0.51791(2)0.51550(1)0.51805(4)0.51530(1)0.51769(5)0.5151(2)0.51754(6)0.5151(3)0.51735(7)0.5151(3)0.209(1)0.214(5)0.211(1)0.200(5)0.212(1)0.20(1)0.210(1)0.22(2)0.212(2)0.21(2)0.0004(8)0.014(4)0.0017(9)0.013(6)0.0033(8)0.002(8)0.0007(8)0.017(9)0.0025(9)0.02(1)0.005(1)0.044(6)0.002(2)0.004(9)0.004(1)0.01(fixed)
0.001(1)0.01(fixed)
0.006(1)0.01(fixed)

1.01601.00561.01531.00551.01461.00581.0142
1.00581.01411.0058

total Y2O3**(mol％）

RI
(％)

3.48

1.80
1.79
1.67
1.62
1.25
1.30
1.52
1.36
1.09
0.96

3.34
3.31
3.49
3.52

R wp
(％)

S

6.25

1.51

6.98

1.77

7.12

1.67

6.33

1.59

6.37

1.63

densities of the sintered body are shown in Table 1.

The total Y 2O3 concentration was calculated fromThe fractional atomic coordinates of oxygen z andY2O3 concentration and weight fraction of each phase.

isotropic displacement parameters U are also given,

The value of total Y2O3 concentration was slightly larger

where the thermal displacement parameter of oxygen isthan that of the ideal one (3 mol％). As the equation 1

fixed in t2 phase for the samples sintered at 1100, 1200

was derived using the data of a metastable tetragonal

and 1300 ℃. The Y2O3 concentration of each tetragonalphase (so−called t' phase) that transformed from thephase was calculated from tetragonality (cf/af) using

high temperature cubic phase 19), a small discrepancy

the following equation 18), where af and cf are the lattice

may be observed.

parameters of a fluorite−type structure (af= √2 a, cf=c).

The tetragonality of t1 phase increased with theincrease of sintering temperatures, while theYO1.5 mol％＝cf
1.0223− a f
0.001319……（1）

tetragonality of t2 phase slightly decreased. The Y2O3
concentrations of t1 and t2 phases are plotted as a

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TOSOH Research ＆ Technology Review Vol.50（2006）

Fraction of tetragonal phase［wt％］ Fraction of monoclinic phase［wt％］61800
Tetra Tetragonal＋Cubic
gonal

T［℃］

1600

t1

Cubic
t2

1400

1200

1000

5

10

15

YO1.5［mol％］Fig.２

Phase separation diagram of 3Y−TZP ceramics.
Solid circles correspond to YO1.5 concentrationof t1 (low yttrium) and t2 (high yttrium) phases.
Solid line refers to equilibrium phase boundary
proposed by Scott20). Broken line referes toyttrium concentration of starting powder.

100
(a)

80
60
40

Aged at 140℃

20
0

0

due to the discrepancy mentioned above.).
As the crystal structure of the starting powder istetragonal phase with a uniform 3 mol％ Y 2 O 3

40

50

(b)

80

t1

60
40

t2

20
0

0

Fig.３

that the phase boundary is shifted by ＋0.5 mol％Y2O3

20
30
Aging Time［h］

100function of sintering temperature in Fig.2, where thephase diagram proposed by Scott 20) is also given (Note

10

10

20
30
Aging Time［h］

40

50

Kinetics of tetragonal to monoclinic phasetransition of the sample sintered at 1500℃
under hydrothermal conditions. Aging timedependence of fraction is plotted for monoclinic
phase in (a) and for tetragonal phases in (b).
Solid line corresponds to a curve fitted using
the Johnson−Mehl−Avrami equation (see text).

distribution, Fig.2 reflects the process of phaseseparation in the sintered body toward an equilibrium

for the Rietveld analysis, such microscopic

state. Phase separation is strongly promoted for theheterogeneity should be considered in further analysis.

samples sintered at 1400 and 1500 ℃. On the otherHydrothermal degradation experiments werehand, the sintering body is less separated at lower

performed on the sample sintered at 1500 ℃ and thetemperatures. According to the phase diagram, theresults are shown in Fig. 3. The fractions of t1, t2 andsintered body should be separated into the tetragonal

monoclinic phases induced by hydrothermaland cubic phase in an equilibrium state. However,degradation are plotted as a function of aging time. At

when equilibrium is not reached under the sintering

early stage of degradation, the monoclinic phaseconditions, the structure of the sintered body is freezed

increases with increasing aging time, while the low

in a nonequilibrium state.

As a result, two tetragonal

yttrium region (t1 phase) decreases. The t1 phase isregions of low (t1 phase with c/a＞1) and high yttrium

completely transformed into monoclinic phase during(t2 phase with c/a〜
＝1 ) concentrations are formed. It isdegradation. On the other hand, the t2 phase is stable

note worthy that the structure is assigned to theeven after long−time aging. It is clear that the〜1 even in the high yttrium
tetragonal phase of c/a＝tetragonal phase of low yttrium concentration isregion. Although a simple cubic phase has been

exclusively transformed into monoclinic phase underconsidered for the high yttrium region in 3Y−TZP

the hydrothermal conditions.
Yasuda et al. 21) obserevd a heterogeneous structure

ceramics, the nonequilibrium tetragonal phase is moreplausible in the case of usual sintering conditions.

attributed to the high and low yttrium concentration
8−10）

regions in 3Y−TZP ceramics, and the low yttrium

revealed that enrichment of yttrium exclusively occurs

region is selectively transformed into monoclinic phasealong the grain boundary (so−called segregation), andat early stage of hydrothermal degradation. Ohmichi etcontinuous distribution of yttrium was reported on 3Y−

al. 16) also revealed the formation of tetragonal phase of

TZP. Although simple two−tetragonal model was used

low Y 2 O 3 concentration. They reported that the

Recent studies using high−resolution TEM（ 6 ）東ソー研究・技術報告 第50巻（2006）

7

reduction of yttrium concentration in tetragonal phase

TZP ceramics is attributable to the formation of highpromotes the degradation of sintered body. In order toyttrium region. Under hydrothermal conditions, theclarify the correlation between the phase−separated

low yttrium region can be easily transformed into

structure and hydrothermal degradation mechanism,

monoclinic phase, while the high yttrium region

further consideration was made on the kinetics of theremains unchanged because of its resistance totetragonal to monoclinic phase transition.

transform. Hydrothermally transformable fraction in

It is known that the kinetics of tetragonal tomonoclinic phase transition is given by the Johnson−
22)

3Y−TZP ceramics is governed by the amount of low
yttrium region.

Thus, we may conclude that the. Thus, the time dependence

formation of the low yttrium region due to phaseof monoclinic fraction is analyzed using the following

separation is responsible for the hydrothermal

general equation.

degradation of 3Y−TZP ceramics.

Mehl−Avrami equation

f m＝b (1−exp(−at n))……（2）

where fm is fraction of monoclinic phase, t is aging time

４．Conclusion

and n is the dimension of growth of nucleation.

The crystal structure of 3Y−TZP ceramics has been

Parameter a is rate constant for phase transition.

studied using x-ray diffractometry. The sintered body

Parameter b is a value of the fraction when the phase

underwent phase separation into high and low yttriumtransition is complete under the hydrothermal

regions with the increase of sintering temperature.

conditions. These parameters are determined by the

Tetragonal phase was assigned both regions. The highnonlinear least square fitting method.

yttrium region was analyzed using a tetragonal phase

The values of parameters are a＝0.0397 h , b＝77.4〜1. The amount of monoclinic phasemodel with c/a＝％, n ＝1.70. The calculated results are shown by ainduced by hydrothermal degradation increased withsolid line in Fig. 3. If the transition preceeds via

increasing degradation time, while that of the low

homogeneous nucleation and growth mechanism, theyttrium region decreased.

value of parameter n should be 3 〜 4. However, thetransformable phase evaluated by the Johnson−Mehl−

value of parameter n is determined as n＝1.70. This

Avrami equation was in good agreement with thesuggests operation of highly heterogeneous nucleation

fraction of t1 phase (low yttrium region) determined by

and growth mechanism in the tetragonal to monoclinic

the Rietveld analysis. Quantitative analyses clearlyphase transition. Parameter a implies a rate ofshowed that the low yttrium region selectivelytransition. It has been considered that the stability oftransforms into monoclinic phase under hydrothermal

tetragonal phase is influenced by crystal size (grain

conditions, while the high yttrium region remains

−1

2)

23)

The fraction of. Therefore, theunchanged. Hydrothermally transformable fraction inparameter a is governed by the concentration of yttrium

3Y−TZP ceramics is governed by the amount of low

in transformable phase as well as grain size. Parameteryttrium region. Thus, the formation of low yttrium

b indicates a fraction of transformable tetragonal phase.

region due to phase separation is responsible for the

The value of transformable tetragonal phase ishydrothermal degradation of 3Y−TZP ceramics.

size)

and concentration of yttrium

determined as b ＝77.4 ％. This value is in good
agreement with the result from Rietveld analysis(fraction of t1 phase: 76.3 ％).
It is known that 3Y−TZP does not transform into

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著者

氏名 山 下著
勲

Isao YAMASHITA

者

氏名 津久間 孝 次
Koji TSUKUMA入社 平成15年 4 月 1 日

入社 昭和50年 4 月 1 日

所属 東京研究所

所属 東京研究所

新材料・無機分野

企画管理グループ

副主任研究員

主席研究員

（ 8 ）

78 (8), 2229−32 (1995).