Resistance to Low-Temperature Degradation of EquimolarYO1.5–TaO2.5 Stabilized Tetragonal ZrO2 Ceramics in Air

Yang Shen and David R. Clarkew

School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138

A narrow range of composition exists along the ZrO2–YTaO4

quasi-binary in which the tetragonal phase can be retained oncooling. Ceramics within this region, corresponding to equimo-lar YO1.5 and TaO2.5 stabilizer concentrations, have beensubject to accelerated testing for their susceptibility to mois-ture-induced low-temperature degradation (LTD) by annealing inair at 2001C. No low-temperature transformation from tetrago-nal to monoclinic phase was evident even after 400 h in the te-tragonal YO1.5–TaO2.5–ZrO2 ceramics, while 50% of tetragonalphase in a sintered 3 mol% Y2O3-doped ZrO2 transformed intomonoclinic after the same long-term annealing. The results notonly demonstrate the superiority in LTD of equimolar yttria–tantala-doped zirconia (ZrO2) but have important implicationsfor the proposed mechanisms of LTD in polycrystalline ZrO2.

I. Introduction

THE long-term aging of (Y2O3)-stabilized tetragonal zirconia(ZrO2) at low temperatures (2001–4001C) in the presence of

moisture and the subsequent transformation of the tetragonalphase to the monoclinic phase has been the subject of consid-erable investigation for many years since the phenomenon wasfirst identified.1–4 Accompanying the tetragonal to monoclinictransformation, there is usually a marked decrease in mechan-ical strength. Together, the transformation and the associatedmechanical degradation have come to be referred to as low-temperature degradation (LTD).

Initially a laboratory curiosity, the phenomenon has becomeof critical importance in a number of applications of ZrO2, es-pecially in the cases where long-term stability is critical. Prob-ably the most well known was the use of 3 mol% Y2O3-dopedZrO2 (6YSZ) as prosthetic devices, principally in hip implants,that were eventually withdrawn from the market after manyfailed as a result of LTD in the body of recipients.5 Less wellknown, is that Y2O3-stabilized ZrO2 thermal barrier coatingsare susceptible to the same degradation after long exposures atvery high temperatures even though they are typically depositedin a metastable tetragonal-prime state. Recent studies showedthat electron beam-physical vapor deposited coatings of8.6 mol% YO1.5–ZrO2 (8YSZ) very slowly transformed tomonoclinic at low temperatures after the exposure to prolongedannealing at 14251C, a higher temperature than coatings areexposed to in gas turbine engines.6 The same LTD behavior hasalso been observed in the plasma-sprayed TBCs.7 Although thedetailed mechanism by which moisture facilitates the transfor-mation in Y2O3-stabilized ZrO2 materials remains elusive, thereis a consensus that the transformation occurs most rapidly in anarrow temperature range, 1501–4001C,3,8–11 and that the trans-formation is isothermal in character.

Among possible alternative ZrO2 materials for applicationssuch as prosthetic devices, dental fixtures and thermal barriercoatings, materials in the YO1.5–TaO2.5–ZrO2 system are con-sidered to be promising. The strong interactions between Y31

and Ta51 ions12 increase the solubility of YTaO4 in the tetrag-onal ZrO2 and stabilize it up to 15001C.13 In solid solution withtetragonal ZrO2, the activities of Y31 and Ta51 are mutuallyreduced by their interactions rendering both enhanced hot-cor-rosion resistance14 and increased resistance to corrosion of mol-ten deposits15 compared with 8YSZ. An unusual feature of theYO1.5–TaO2.5–ZrO2 system pointed out by Kim and Tien,13 andshown in Fig. 1, is the existence of an extended solid solutiontetragonal phase field for equal concentrations of Y31 and Ta51

ions. These tetragonal compositions do not contain significantconcentrations of vacancies and hence are not stabilized by ox-ygen vacancies as are the Y2O3-stabilized ZrO2s. Furthermore,Kim and Tien suggest that there is a narrow range of compo-sition over which the tetragonal was ‘‘nontransformable’’ oncooling to room temperature and subsequent grinding. Thisrange is shown heavily shaded in Fig. 1. These compositionsare also unusually tough, although not quite as tough astransformation toughened 6YSZ, with fracture energies of40–50 J/m2.11,15 Little is known, however, about the behaviorof these ‘‘nontransformable’’ compositions when subject to low-temperature annealing in air. In this contribution, a series ofsamples of YO1.5–TaO2.5–ZrO2 system along the ZrO2–YTaO4

quasi-binary were sintered into dense bodies and subject toannealing in laboratory air at 2001C. The phase transformationbehavior after different times was characterized by Raman spec-troscopy. Previous studies of the time–temperature–transforma-tion behavior of Y2O3-stabilized ZrO2 have shown that this testtemperature is in the range of temperature where the moisture-enhanced transformation is most rapid.3 For direct comparison,a dense pellet of 3 mol%Y2O3-doped ZrO2 (6 mol% YO1.5) wassubjected to the same heat treatment.

II. Experimental Procedure

Samples of three different compositions, B, C, and D, in the‘‘nontransformable’’ range as well as one on either side of theregion on the phase diagram, as shown in Fig. 1 and listed inTable I were made. The powders were synthesized by reversecoprecipitation method using zirconium oxychloride, yttriumnitrate, and tantalum chloride precursor solutions. First, solu-tions of ZrOCl2 and Y(NO3)3 in deionized (DI) water and so-lutions of TaCl5 in ethanol were prepared. They were thenmixed together just before the precipitation step to prevent thehydrolysis of the TaCl5 in the water (all chemicals used in thisexperiment were from Sigma-Aldrich with purity of 99.99%;Milwaukee, WI). Once mixed, precipitation was initiated byslowly adding the solution, drop by drop, into ammonia hy-droxide solution (the pH value was controlled to be410 duringprecipitation) with vigorous stirring (as no attempt was made tomake nanoparticles, the exact solution compositions are notimportant). The precipitates were then washed twice, first withDI water and then with ethanol, and dried over night at 701C.The precipitants were then calcined at 9501C for 2 h to produce

P. Becher—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: clarke@seas.harvard.edu

Manuscript No. 26455. Received June 26, 2009; approved January 13, 2010.

Journal

J. Am. Ceram. Soc., 93 [7] 2024–2027 (2010)

DOI: 10.1111/j.1551-2916.2010.03665.x

r 2010 The American Ceramic Society

2024

a molecularly mixed oxide. The resultant powders were groundwith a mortar and pestle and passed through ao325 mesh sieve.Solid disk pellets were then made by cold, uniaxial pressing thesieved powder at 100 MPa and then sintering for 5 h at 15001C.For comparison, a pellet of 6YSZ (Tosoh, Tokyo, Japan) waspressed and sintered at the same condition.

X-ray diffraction (XRD) measurements were performed witha Philips Xpert powder diffractometer (Almelo, the Nether-lands). In addition, lattice parameters for the tetragonal sampleswere determined by slow scans of XRD at 711–761 so as tocompare with values in the literature.

The samples were annealed at 2001C in laboratory air fordifferent times. Raman spectroscopy was used as a complemen-tary tool to XRD to monitor the progress of the low-tempera-ture transformation of the tetragonal phase to the monoclinicphase. The Raman spectra were excited with an Argon ion laserof 488 nm (Coherent, Santa Clara, CA) and collected with amicroscope-based Raman spectroscopy system (T64000 triplemonochromator, Jobin-Yvon, Edison, NJ). In the ZrO2 con-taining both tetragonal and monoclinic phases, the fraction ofmonoclinic phase was quantified from the area, I, of the Ramanpeaks characteristic of the monoclinic phase at 182 and191 cm�1 and the tetragonal peaks at 148 and 263 cm�1

(Fig. 2).16 The empirical relationship used was:

fm ¼I182m þ I191m

0:97ðI148t þ I263t Þ þ I182m þ I191m

(1)

where fm is the fraction of monoclinic phase and the subscriptsm and t refer to the monoclinic and tetragonal phases, respec-

tively. Raman spectroscopy rather than XRD was used to mon-itor the phase transformation since it typically probes a largervolume of material on account of the greater penetration depthof light than X-rays.

III. Results

In accord with the results of Kim and Tien,13 the ZrO2-richcomposition A just outside of the ‘‘nontransformable’’ regiontransformed to monoclinic on cooling. This transformation oc-curred even when the material was rapidly cooled to room tem-perature. The other compositions, both the single-phasecompositions B, C, and D as well as the two-phase composi-tion E, remained tetragonal. The Raman spectra for the threesamples in the nontransformable zone and the two-phase com-position are shown in Fig. 2 together with the spectrum of themonoclinic composition. The spectra from the single-phasecompositions are similar to those reported with broad peaks in-stead of relatively sharp peaks in the range 250–400 cm�1. Theorigin of this broadening is unknown but is attributed to a com-bination of extensive cation site disorder associated with themixing of Zr, Ta, and Y on the cation lattice and the possiblepreferential association of Y31 and Ta51 ions. The latter hasbeen observed by X-ray absorption spectroscopy for Y31 andNb51 ions.12 After annealing for 400 h at 2001C in air, theRaman spectra for the three single-phase samples B, C, D andthe two-phase sample E remained unchanged from their origi-nal, as-sintered state. For the 6YSZ material, there was an in-crease in the concentration of monoclinic phase with time ofexposure and a corresponding decrease in tetragonal phase, asshown in Fig. 3. The fractions of monoclinic phase as a functionof time were plotted in Fig. 4, both for the 6YSZ and the com-position C. Clearly, the single phase Y–Ta–Zr–O samples showedextreme resistance to the LTD compared with the 6YSZ ceramics.The transformation kinetics followed the well known Avrami–Johnson–Mehl–Kolmogorov (AJMK) relation:3,17

f ¼ f1ð1� expð�ðktÞnÞÞ (2)

where fN is the fraction of the phase at equilibrium, t is the time, nis a parameter reflecting the dimensionality of the mechanism thatcontrols the transformation, and k is a parameter correlated withthe rate of the transformation. The fit of the AJMK equation tothe data for the volume fraction of monoclinic phase is shown by

Table I. Compositions Studied

Samples Compositions

A Y0.135Ta0.135Zr0.73O2

B Y0.138Ta0.147Zr0.715O2.004

C Y0.155Ta0.160Zr0.685O2.002

D Y0.17Ta0.185Zr0.645O2.007

E Y0.2Ta0.2Zr0.6O2

6YSZ Y0.06Zr0.94O1.97

160 240 320 400 480 560 640 720

E

Raman shift (cm–1)

Inte

nsity

(a.

u.)

400 hrs @ 200°Cm

C

D

B

A

Fig. 2. Raman spectra of the samples after annealing in air at 2001C for400 h. The spectra of all the samples remained unchanged. The compo-sition A, at the edge of the ‘‘nontransformable’’ region, transformed tomonoclinic on cooling after sintering.

F

O

ZrO2

YT = YTaO4 ; YT3 = YTa3O9

mol

e %

YO 1.

5

0 0

20

20

30

30

40

40

50

50

mole %

TaO2.5

t

1010

t+F+

YT

t+O+

YT

O+

YT+

YT3

% anion vacancies0510

C0(t/m)

% cation vacancies

6YSZ

ABC

E

Non-Transformable Zone

D

Fig. 1. Isothermal section of the YO1.5–TaO2.5–ZrO2 phase diagram at15001C.1,13,15,20 The compositions studied are indicated by the labeledcircles. The compositional range of the ‘‘nontransformable zone’’ alongthe ZrO2–YTaO4 quasi-binary is also indicated. The vacancy concen-trations and the T0 line are also labeled.

July 2010 Resistance to Low-Temperature Degradation 2025

the red line through the data in Fig. 4. The values for the param-eters n and k compare with those found by Chambers and Clarke18

using Raman spectroscopy (n50.9 and k50.09) whereas largervalues have been determined from XRD studies of the transfor-mation, for instance, those in Chevalier et al.3 The difference isattributed to the fact that the Raman measurements probe a con-siderably greater depth into the ZrO2 samples than does XRD andhence the volume of material probed is much larger because of thelarger penetration depth of visible light than X-rays.

IV. Discussion

The results presented in this work clearly show that the com-positions along the ZrO2–YTaO4 quasi-binary, sometimes re-ferred to as the ‘‘nontransformable tetragonal’’ compositions,not only remain tetragonal on cooling to room temperature—asdo most Y2O3-stabilized ZrO2—but unlike many of these latter

materials they are also resistant to transformation in the pres-ence of moisture at intermediate temperatures. So, althoughthey are less tough (B40–50 J/m2) than the transformationtoughened Y2O3-stabilized ZrO2 the toughness, which is stillhigher than most ceramics, can be utilized in moist atmospheres.As mentioned earlier, the temperature chosen for this test wasclose to the most stringent developed in the LTD degradation ofY2O3-stabilized ZrO2

3,8 and used in accelerated testing of med-ical ceramics.19

While the results themselves may be of practical importancethey also have important consequences for the viability of themechanisms proposed to explain the LTD phenomenon. Else-where, we have argued that it is a necessary but not sufficientcondition that the degradation temperature is below the T0 ormartensite temperature.11 Kim and Tien have determined themartensite temperatures for a range of compositions along thesingle phase ZrO2–YTaO4 binary but not, unfortunately as faras our composition A. Their measurements show that the mar-tensite temperature decreases from that of pure ZrO2 withdecreasing ZrO2 concentration, falling to B5001C at 78 mol%ZrO2

13 (this composition is labeled with a star on the phase di-agram of Fig. 1). More recent estimates by Pitek and Levi15 in-dicate that the T0 temperature continues to decrease as the ZrO2

concentration decreases further. The compositional dependenceof the T0 line estimated is shown in Fig. 1 and intersects theregion of ‘‘nontransformability’’ close to our composition A. So,it is likely that the reason that the ‘‘nontransformable’’ compo-sitions along the binary between points B and D do not trans-form in the presence of moisture is that there is no driving forcefor them to transform: the transformation temperature is belowboth room temperature and the LTD testing temperature (as theequilibrium ZrO2 phase in the two-phase material, E, has acomposition close to that of D, it is likely to have a similar low-temperature transformation temperature). To test the hypothe-sis that the T0 line must be lower than room temperature overthe ‘‘nontransformable’’ range and decrease with decreasingZrO2 content, samples of the compositions, B, C, and D werecooled in liquid nitrogen, their Raman spectra monitored in situand then warmed back to room temperature. Compositions Cand D remained tetragonal throughout the cooling and warm-ing cycle. In contrast, composition B partially transformed tomonoclinic on cooling, as shown in Fig. 5, and only partially

0.0

0.2

0.4

0.6

0.8

1.0

1 10 100 1000

6YSZC - ZrYTaO

Time (hour)

Vol

ume

Fra

ctio

n of

Mon

oclin

ic P

hase

k=0.0016n=0.96

Fig. 4. Monoclinic concentration as a function of annealing time at2001C in air for 6YSZ and binary composition C (the data for binarycompositions B, D, and E are indistinguishable from that of C, are notshown for the sake of clarity). The line through the 6YSZ data is a fit tothe Avrami–Johnson–Mehl–Kolmogorov relationship (Eq. (2) in thetext) with the constants indicated.

160 240 320 400 480 560 640 720

Raman Shift (cm–1)

Inte

nsity

(a.

u.)

0 hrs

200 hrs

300 hrs

400 hrs

m

tt

m 6YSZ @ 200°C

tetragonalmonoclinic

Fig. 3. Raman spectra of the 6YSZ sample after being annealing in airat 2001C for different times showing that the tetragonal phase slowlytransforms to the monoclinic phase. The peaks of tetragonal (solid) andmonoclinic (dash) were labeled. The characteristics peaks of monoclinicand tetragonal phase used to quantify the relative content of monoclinicphase were also indicated.

Raman shift (cm–1)

160 240 320 400 480 560 640 720

Inte

nsity

(a.

u.) cooled with

liquid nitrogen

Cooled to room temperature

m

m reheated to room temperature

Fig. 5. Raman spectrum from binary composition B after annealing at2001C in air for 400 h and cooling to room temperature (top), then oncooling with liquid nitrogen (middle) and then warming back to roomtemperature (bottom). Cooling with liquid nitrogen results in partialtransformation of the ‘‘nontransformable’’ tetragonal to the monoclinicphase which then incompletely transforms back to tetragonal on return-ing to room temperature. The most distinctive monoclinic peaks, thedoublet at 181 and 192 cm�1 are indicated by the arrows, m.

2026 Journal of the American Ceramic Society—Shen and Clarke Vol. 93, No. 7

transformed back on returning to room temperature. The pre-cise temperature at which the transformation to monoclinic oc-curred could not be determined but taken together with thenontransformability of compositions C and D, suggests that theT0 line is between room temperature and 77 K for compositionB, and falling below 77 K for composition C. An alternativeexplanation for the lack of transformation might be if the grainsize were sufficiently small that the tetragonal phase was stabi-lized by a surface energy contribution. However, the grains wereseveral microns in size in all the compositions studied.

We are thus led to conclude that the ‘‘nontransformable’’compositions along the single phase ZrO2–YTaO4 binary regionare insensitive to LTD in moisture and that this is a consequenceof their transformation temperatures being lower than roomtemperature and any temperature for viable moisture sensitivity.

V. Conclusions

Long term, low-temperature annealing in laboratory air was con-ducted on sintered ‘‘nontransformable’’ YO1.5–TaO2.5–ZrO2

ceramics at 2001C for up to 400 h in air. No phase transforma-tion from tetragonal to monoclinic was observed. In contrast, thesame heat treatment caused transformation of B50% of thetetragonal phase in a 6YSZ. A sintered, two-phase ceramic con-taining the nontransformable ZrO2 and YTaO4 was also immuneto LTD. The absence of any transformation is attributed to thetransformation temperature being below the test temperature.The superior resistance to LTD makes the YO1.5–TaO2.5–ZrO2

ceramics promising candidates for a variety of applications, rang-ing from the next generation TBC materials to dental implants.

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