High-Temperature Tensile Strength of Er2O3-Doped ZrO2 SingleCrystals

Jose Y. Pastor and Javier LLorca*,w

Department of Materials Science, Polytechnic University of Madrid, E. T. S. de Ingenieros de Caminos, 28040 Madrid,Spain

Pedro Poza

Departamento de Ciencia e Ingenierıa de los Materiales, Universidad Rey Juan Carlos, Escuela Superior de CienciasExperimentales y Technologıa C/Tulipan, s/n 28993 Mostoles, Spain

Jose J. Quispe, A. R. de Arellano Lopez,* and Julian Martınez-Fernandez*

Department of Physics of Condensed Matter, University of Seville, 41080 Seville, Spain

Ali Sayir*

NASA Glenn Research Center, Cleveland, Ohio 44135-3191

Victor M. Orera*

Instituto de Ciencia de Materiales de Aragon, C.S.I.C.-Universidad de Zaragoza, 50009 Zaragoza, Spain

The deformation and fracture mechanisms in tension were stud-ied in single-crystal Er2O3-doped ZrO2 monofilaments proc-essed by the laser-heated floating zone method. Tensile testswere carried out between 251 and 14001C at different loadingrates and the dominant deformation and fracture mechanismswere determined from the shape of the stress–strain curves, themorphology of the fracture surfaces, and the evidence providedby monofilaments deformed at high temperature and broken atambient temperature. The tensile strength presented a minimumat 6001–8001C and it was controlled by the slow growth of acrack from the surface. This mechanism was also dominant insome monofilaments tested at 10001C and above, while othersshowed extensive plastic deformation before fracture at thesetemperatures. The strength of plastically deformed monofila-ments was significantly higher than those which failed by slowcrack growth due to the marked strain hardening capacityof this material.

I. Introduction

ZIRCONIA-based ceramics have high thermal expansion coef-ficient and very low thermal conductivity as compared with

other ceramic oxides. The unmatched combination of theseproperties provides thermal insulation with minimal problemsassociated with metal–ceramic thermal expansion mismatch andtherefore they are attractive for high-temperature engine com-ponents. The strength and toughness can be increased throughtransformation toughening at ambient temperature, while thehigh-temperature deformation resistance can be significantly

improved by the addition of Y2O3 through solid solution andprecipitation hardening.1,2 Further increases in the high-temper-ature creep resistance have been reported in partially stabilized 5mol% Er2O3-doped ZrO2 (Er-PSZ) monofilaments processed bythe laser-heated floating zone method.3 Basically, the monofil-ament microstructure consisted of a fine distribution of tetrago-nal variants with dimensions under 10 nm generated due to thefast cooling associated with the processing technique. Initialyielding, which occurred when the dislocations were able toshear the tetragonal domains, was followed by an acceleratedwork hardening as a result of the formation of dislocation pile-ups and the absence of mechanisms to release the stresses at thepile-up front.3

Partially stabilized Er-PSZ monofilaments with nanometrictetragonal domains present excellent properties (in terms ofcreep resistance and thermal conductivity) for high-temperaturestructural applications. However, these merits have been some-how hampered by recent results on the environmental suscepti-bility of Er-PSZ at intermediate temperatures (up to 8001C) orin presence of water at ambient temperature.4 It was found thatthe average flexure strength decreased markedly with tempera-ture and it was lower in water than in air. Moreover, the flexurestrength of the monofilaments tested in water or at 4001–8001Cincreased with the loading rate, and this behavior was accom-panied by significant changes in the morphology of the fracturesurfaces, which showed evidence of slow crack growth (SCG).These previous results raise the question of the environmentalsusceptibility of the Er-PSZ single crystals, particularly at ele-vated temperature, because it is well known that the mechanicalperformance at high temperature of some oxide single crystals,such as Al2O3, has been hindered by the occurrence of SCGdue to stress-corrosion cracking5 or thermally activated bondrupture.6,7

This investigation was aimed at elucidating the environmentalsusceptibility of Er-PSZ single crystals in the temperature range6001–14001C. It should be noted that the material as well as thetesting configuration differed from those used by previous re-searchers in this kind of studies. The monofilaments processedby the laser-heated floating zone method were single crystals

Journal

J. Am. Ceram. Soc., 89 [7] 2140–2146 (2006)

DOI: 10.1111/j.1551-2916.2006.00995.x

r 2006 The American Ceramic Society

2140

R. Hay—contributing editor

Supported by the Spanish Ministries of Science and Technology and Education andScience through grants MAT2000-1533 and MAT2003-6085 and by the Comunidad deMadrid through the program ESTRUMAT-CM (MAT-0077).

*Member, American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: jllorca@mater.upm.es

Manuscript No. 21185. Received November 25, 2005; approved February 1, 2006.

without grain boundaries and the mechanical behavior wasstudied through tension tests at stress rates spanning over twoorders of magnitude between 251 and 14001C. The results of themechanical tests, together with the analysis of the fracture sur-faces, demonstrated that SCG controls the tensile strength ofthis material at intermediate temperatures, while SCG and plas-tic deformation coexisted above 10001C, and the mechanical re-sponse of the monofilament depended on the dominantmechanism.

II. Material and Experimental Techniques

ZrO2 monofilaments doped with 5 mol% of Er2O3 were grownat NASA Glenn Research center using the laser-heated floatingzone method. An Y2O3–ZrO2 single crystal oriented with the/111S axis in the growth direction was used as a seed to initiatethe growth. Monofilaments of up to 20 cm in length were grownat 38 cm/h. The average diameter was around 170 mm, althoughthere were significant variations among the monofilaments(150–230 mm) and along each monofilament. The monofila-ments were single crystalline and their microstructure wasformed by an intepenetrating network of three tetragonal crys-tal variants with the /111S axis parallel to the monofilamentaxis and size under 10 nm. Additional information about themicrostructure could be obtained from previous publications3,4

and it will not be repeated here.Tensile tests of the monofilaments were carried out in a servo-

hydraulic mechanical testing machine (Model 8501, Instron Lim-ited, High Wycombe, UK). For the ambient temperature tests,the monofilament ends were glued with cyanoacrylate andmounted on perforated cardboard cards, which were connectedby mechanical grips to the mechanical testing machine. At thebeginning of the test, the sides of the cardboard cards were cut,so the load was carried by the monofilaments. For the high-temperature tests, the monofilament ends were deposited in alongitudinal groove (whose radius was close to the monofilamentradius) machined in an aluminum plate and gripped by screwinganother aluminum plate on top.z The aluminum plates wereconnected to the load cell and the actuator through two univer-sal joints to minimize bending and torsion stresses and the cen-tral length of the monofilaments was placed in a clamp furnace.

Tests at constant stress rates of 0.2 and 23.5 MPa/s were car-ried out at 251 and 14001C. The monofilament gage length atroom temperature was 25 mm, equal to the central hot zone inthe furnace. Tests at a constant cross-head velocity of 500 mm/min were performed at various temperatures between 251 and14001C. The monofilament gage length and the central hot zoneof the furnace in these tests was 19 mm, and the variation inmonofilament length during the high-temperature tests wasmeasured with a LVDT outside of the furnace. Only those testsin which the monofilaments failed between the grips (and withinthe furnace in the high temperature tests) were considered valid.

The fracture surface of the samples was examined using thescanning electron microscope (SEM, Model 6300, JEOL,Tokyo, Japan) to locate the origin of fracture and assess thedeformation and fracture micromechanisms. The true area ofthe monofilament at the fracture section was measured usingSEM to compute the failure stress. Raman measurements wereperformed at room temperature in a backscattering geometry onthe fracture and the lateral surfaces of the broken monofila-ments (near and far away the fracture section) to ascertain thepresence of monoclinic ZrO2 after high-temperature exposureand testing. The measurements were carried out with an opticalspectrometer (Model XY, DILOR, Lille, France) equipped witha diode array multichannel detector. Experiments were carriedout using an optical microscope and 400 mm field aperture in theimage focal plane to analyze a region of about 1 mm in diameterand 4 mm in depth. To avoid the intense Er31 emission from the4S3/2 level, Raman spectra were recorded using either the 457.9

nm excitation line of an Ar1-ion laser (model INNOVA 200,Coherent, Palo Alto, CA) or a dye laser at 575.6 nm.

III. Constant Stress Rate Tests

The direct observation of crack nucleation and growth in mono-filaments is a daunting task, and it was decided to study theenvironmental susceptibility at 251 and 14001C by carrying outtensile tests at different stress rates. The tensile strengths of themonofilaments are plotted as a function of the stress rate inFig. 1. The results obtained in the tests performed under dis-placement control were also included in the ambient tempera-ture data, the stress rate being computed as the average strainrate (obtained from the cross-head velocity) times the monofil-ament elastic modulus (approximately 182 GPa). This is a rea-sonable approximation because the compliance of the load trainand frame was negligible in comparison with that of the mono-filament.

Although the scatter was very large, the tensile strength wasnot significantly modified with the stress rate at 251 and 14001C,in opposition to the expected behavior if SCG was controllingthe fracture strength. The topography of the fracture surfacescorresponding to the ambient temperature tests was rough(Fig. 2(a)), and the fracture origin could be found by tracingthe ridgelines back to the monofilament surface (Fig. 2(b)). Thefracture was nucleated at surface defects of a few microns indepth. In a few cases, these defects were associated with astrange particle located just below the fracture surface. The par-ticle composition (as determined by energy-dispersive X-raymicroanalysis) was identical to that of the rest of the mono-filament, and it may be originated by the lack of fusion duringprocessing. The fracture surfaces of the monofilaments tested atambient temperature did not contain any evidence of SCG, andthe mirror–mist–hackle structure of the fracture surfaces sug-gested a brittle fracture nucleated from surface defects related toprocessing, typical of ceramics and glasses (Fig. 2(a)).

The fracture surfaces of the monofilaments tested at 14001Cwere smoother and the mirror–mist–hackle structure was evi-dent (Fig. 3). The mirror zone (of few tenths of microns in di-ameter) was embedded in the mist region, which occupied mostthe fracture surface. A semi-elliptical defect was found at thecenter of the mirror zone. The observation of the both halves ofthe fracture surface showed that the initial defect and the crackwere in different planes, and this fact, together with the ‘‘per-fect’’ semi-elliptical shape of the defect, indicates that grew fromthe monofilament surface by an SCG mechanism.

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zTensile tests at room temperature using this gripping system were linear elastic untilfracture and showed that the movement of the monofilament within the grips was impeded.

July 2006 High-Temperature Tensile Strength Er-PSZ 2141

IV. Constant Displacement Rate Tests

(1) Tensile Tests

The monofilaments were tested in tension under displacementcontrol at 251, 6001, 8001, 10001, 12001, and 14001C. Fourteentests were carried out at 251C and between three and six validtests were carried out at each higher temperature. The strain wascomputed from the monofilament elongation during the test andthe length of hot zone in the furnace (19 mm) by assuming thatthe elastic deformation of the monofilament outside of the hotregion in the furnace was negligible in comparison with the in-elastic deformation within the furnace. This assumption is per-fectly applicable to these monofilaments, which showed a markednon-linear deformation even at intermediate temperatures.

Representative stress–strain curves of the behavior obtainedat different temperatures are plotted in Fig. 4. While the mono-filaments tested at ambient temperature presented a linear elasticbehavior until fracture, non-linear deformations were observedin all the tests carried out at 6001C and above. The monofila-ments tested at 6001C showed little non-linear deformation. Theslope of the stress–strain curve in the non-linear region was lowand the failure stress was also low (250–300 MPa). Similar re-sults were obtained at 8001C, although the amount of non-lineardeformation increased: however, the monofilaments tested at10001C and above presented two different behaviors. One setbehaved as those tested at lower temperatures: the slope of thestress–strain curve in the non-linear region was low and theirstrength was also limited (o400MPa) and comparable with thatof the monofilaments tested at 6001 and 8001C. Another set ofmonofilaments presented higher strain-to-failure and theirstress–strain curves displayed an accelerated hardening in the

non-linear regime. The strength of these monofilaments was sig-nificantly higher (from 500 up to 1400 MPa) particularly at10001 and 12001C.

These results are summarized in Fig. 5, where the tensilestrength of the monofilaments is plotted as a function of the testtemperature from 251 to 14001C. Despite of the large experi-mental scatter, it is evident that the monofilament strength un-derwent a severe degradation at intermediate temperatures. Themonofilaments with the lowest strength (open circles) alwayspresented low ductility and strain hardening in the non-linearregime. The monofilaments with higher strength at elevatedtemperature (solid circles) showed accelerated hardening in thenon-linear regime and higher ductility. Moreover, the scatter inthe tensile strength was significantly reduced at 6001 and 8001C(as compared with room and higher temperatures). This behav-ior was already reported in this material when tested in three-point bending and attributed to the development of subcriticalcrack growth.4

The analysis of the fracture surfaces in the SEM demonstrat-ed that the fracture mechanisms were different in each set ofmonofilaments. Those with low strength and strain-to-failurepresented flat fracture surfaces, as the ones shown in Figs. 6 and 7,corresponding to monofilaments broken at 6001 and 12001C,respectively. The general morphology of these fracture surfaceswas very similar to that depicted in Fig. 3 for the monofilamenttested at 14001C under constant stress rate. Crack nucleationoccurred at small defects at the monofilament surface, and thefracture surface near these initial defects was completely smooth.River patterns pointed toward the fracture origin developed as

Fig. 3. Matching halves of the fracture surface of a monofilament bro-ken at 14001C under a constant stress rate of 26 MPa/s. The regionswithin the circles show the semi-elliptical defect which led to the mono-filament fracture. This defect is slightly depressed with respect to thecrack plane in (a) and protuberates from the crack plane in (b).

Fig. 2. Fracture surfaces of the monofilaments broken at 251C underan average stress rate of 60 MPa/s. (a) Overall aspect of the rough frac-ture surface. (b) Detail of the fracture origin, showing the mirror–mist–hackle structure around a surface defect.

2142 Journal of the American Ceramic Society—Pastor et al. Vol. 89, No. 7

the crack propagated and were clearly visible in the mono-filament broken at 6001C (Figs. 6(a) and (b)). The fracture sur-faces of the monofilaments with high strength and acceleratedhardening presented different features (Fig. 8). A small flatregion was found at the fracture origin but the crack path wasrough, and the most significant feature observed at high mag-nification was a globular structure with an average size ofaround 100–200 nm.

(2) Interrupted Tensile Tests

The different morphology of the fracture surfaces in the mono-filaments and the marked reduction in strength at relatively lowtemperatures (6001–8001C) pointed to the development of SCG.The non-linear deformation of the monofilaments below 10001Ccannot be attributed to the plastic deformation of tetragonalZrO2 but to the nucleation and growth of cracks from the sur-face. To check this extreme, three monofilaments were deformedat 8001C up to �1% of strain (within the non-linear region),unloaded, cooled to room temperature, and deformed untilfracture.

The average strength of these monofilaments was 0.8270.35GPa, significantly lower than that of the pristine ones in Fig. 5(1.2770.37 GPa) and the corresponding fracture surfaces of twoof them are shown in Fig. 9. The first one (Fig. 9(a)), whichfailed at 0.83 GPa, showed a crack with a few mm in depth and asurface trace of �100 mm. Secondary cracks were observed nearthe crack tip at higher magnification (Fig. 9(b)). They could becreated during the final monofilament fracture at ambient tem-perature but it might be also a result of the environmental deg-radation during high-temperature deformation. SCG wasevident in the fracture surface shown in Fig. 9(c), which showed

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Fig. 5. Tensile strength of the monofilaments at as a function of thetemperature.

Fig. 6. Fracture surface of a monofilament broken at 6001C. (a) Gen-eral view. (b) Crack nucleation region at the surface. The fracturestrength was 0.31 GPa.

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July 2006 High-Temperature Tensile Strength Er-PSZ 2143

two regions at different heights. The left one corresponded to theSCG region, and the right one to the fracture surface created atambient temperature. The subcritical crack propagated across asignificant fraction of the monofilament section during defor-mation at 8001C, being responsible for the non-linearity in thestress–strain curve. Moreover, the presence of this defectreduced the ambient temperature strength to 0.46 GPa.

V. Discussion

(1) Deformation and Fracture Mechanisms

The tensile tests at constant stress rate (Fig. 1) showed a markedreduction in the tensile strength of the Er-PSZ monofilamentsbetween 251 and 14001C and these results are very similar tothose reported by McClellan et al.8 on cubic ZrO2 monofila-ments doped with 21 mol% of Y2O3: the average tensile strengthof 1.5 GPa at ambient temperature dropped to 0.5 GPa at14001C, and the scatter was also reduced at elevated tempera-ture. The first explanation for this reduction in strength withtemperature comes from the corresponding fracture surfaces inFigs. 2 and 3. While the ambient temperature strength was con-trolled by processing-related flaws at the monofilament surface(which were also responsible for the large experimental scatter),high-temperature strength was dictated by the SCG of a penny-shaped crack until attained a critical size (Fig. 3). This is inagreement with the evidence of SCG in tetragonal and cubicY2O3-doped ZrO2

9,10 as well as in Al2O36,7 at intermediate and

elevated temperatures.More experimental evidence of SCG in Er-PSZ monofila-

ments was provided by Ridruejo et al.4 who carried out thethree-point bend tests at intermediate temperatures (6001–

8001C) in Er-PSZ monofilaments. They showed that SCGoccurs in this material, and the fracture surfaces of the mono-filaments tested in bending at 6001–8001C were smooth withsimilar features to those found in the monofilaments tested intension at 6001C (Fig. 6). So, the marked reduction in monofil-ament tensile strength in the range 6001–8001C was due to SCG.Moreover, the non-linear deformation detected in the stress–strain curves at these temperatures could not be related to plasticdeformation but to the increase in monofilament compliance asa result of crack growth, as it was confirmed by the interruptedtensile tests.

The tensile stress–strain curves and the fractographic obser-vations at 10001C showed the presence of two different mono-filament populations. Low-strength monofilaments (200–350MPa) failed by the nucleation and growth of subcritical cracks,the stress–strain curves and the fracture surfaces being analo-gous to those reported at lower temperatures, although the non-linear deformation was more marked than at 6001–8001C. Thislatter observation supports the possibility that plastic deforma-tion also contributed to the total strain in the tests at 10001C andabove which failed by SCG. High-strength monofilaments failedfrom processing-related flaws at the surface, as indicated by theappearance of the corresponding fracture surfaces (Fig. 8), andthe fractographic analysis did not show any evidence of SCG.Moreover, the stress–strain curves were very similar to thosemeasured in compression on the same material.3 This is shownin Fig. 10 which includes the stress–strain curves in tension andcompression (the latter taken from Martınez-Fernandez et al.3)of Er-PSZ single crystals at 14001C. The slightly lower yieldstress of the samples tested in compression was due to the dif-ferences in strain rate between tension (4.4� 10�4 s�1) and com-pression (2.5� 10�5 s�1) tests. The stress increased rapidly after

Fig. 7. Fracture surface of a monofilament broken at 12001C. (a) Gen-eral view. (b) Crack nucleation region at the surface. The fracturestrength was 0.28 GPa.

Fig. 8. Fracture surface of a monofilament broken at 12001C. (a) Cracknucleation region at the surface. (b) Globular structure. The fracturestrength was 1.4 GPa.

2144 Journal of the American Ceramic Society—Pastor et al. Vol. 89, No. 7

the initial yielding in tension and compression as a result of ac-celerated hardening due to the interaction between dislocationssliding along different planes in the {100} /011S slip system.3

As a result of the strong hardening, the tensile strengths of themonofilaments were similar to those obtained at ambient tem-perature, and the coexistence of both fracture and deformationmechanisms at 10001C and above (SCG and plasticity) was re-sponsible for the large scatter in the tensile strength, which didnot occur at intermediate temperatures, where the strength wasdominated by SCG.

It should be finally noted that the tensile tests carried out at14001C failed to detect any dependence of the tensile strength

with stress rate (Fig. 1) but this result, which in principle con-tradicts the hypothesis that SCG was controlling the fracturestrength, could be explained by the large experimental scatterand the limited number of tests performed.

(2) Mechanisms of Subcritical Crack Growth

Most of the explanations of SCG in ZrO2 assumed the des-tabilization of the tetragonal or cubic phase into the monoclinicone,11–13 and this hypothesis was supported by the presence ofmonoclinic ZrO2 detected by X-ray diffraction on the specimenssurfaces.9,10 This mechanism is, however, unlikely to developabove the critical temperature for the martensitic transforma-tion and, in fact, no traces of monoclinic ZrO2 were detected inthe monofilament lateral and fracture surface by Raman spec-troscopy before and after testing at all temperatures.

Another hypothesis is that SCG at intermediate temperatures(6001–8001C) may be related to the thermal residual stresses thatdevelop as a result of the radial and longitudinal thermal gra-dients upon monofilament solidification and the poor thermalconductivity of tetragonal ZrO2. They are maximum (and ten-sile) at the monofilament surface and decrease rapidly along theradius, and can be superposed to the stress concentrations as-sociated to the surface defects, leading to the slow growth of thecrack. This possibility was checked by Ridruejo et al.,4 whotested monofilaments at 8001C in the as-received condition andafter 2 h of annealing at 12001C, and found no difference in theflexure strength. Moreover, this scenario should promote SCGparticularly at ambient temperature, where the residual stressesare maxima, but not at high temperature, and this is contrary tothe experimental evidence.

The experimental results summed up in Fig. 5 are qualita-tively similar to those reported previously in single-crystalAl2O3, which also showed a minimum in strength around6001C.14,15 The strength degradation at low temperature wasattributed to stress corrosion cracking. Wiederhorn5 providedexperimental evidence of SCG in single-crystal Al2O3 at ambienttemperature in presence of moisture, the crack speed increasingwith the relative humidity. In this respect, Er2O3-doped ZrO2

showed the behavior because Ridruejo et al.4 also detected areduction in strength at ambient temperature in presence of wa-ter, and this mechanism could also be responsible for the SCGobserved above room temperature. However, it cannot be ruledout that SCG in Er2O3-doped ZrO2 at intermediate and hightemperatures may be due to other causes. For instance, New-

Fig. 9. Fracture surfaces of monofilaments broken at 251C after defor-mation at 8001C up to �1%. (a) General view of a monofilament bro-ken at 0.83 GPa. (b) Detail of the subcritical crack growth region at themonofilament surface. Secondary cracks (parallel to the monofilamentaxis) are marked with white arrows. (c) General view of a monofilamentbroken at 0.46 GPa.

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Fig. 10. Tension and compression stress–strain curves of Er-PSZ singlecrystals at 14001C. The strain rate in tension was �4.4� 10�4 and�2.5� 10�5 s�1 in compression.

July 2006 High-Temperature Tensile Strength Er-PSZ 2145

comb and Tressler6 have attributed the strength reduction ofsingle-crystal Al2O3 above 8001C to SCG due to thermally ac-tivated bond rupture associated with a lattice trapping mecha-nism. However, most of the samples in their study failed frominternal pores, and this was not the case here. Other authors16

found that the fracture strength minimum at intermediate tem-peratures (3001–9001C) in sapphire could be eliminated byMg21

and Ti41 doping, and this behavior led to the conclusion thatfracture was controlled by dislocation-assisted SCG. In princi-ple, a similar process could be responsible for SCG in single-crystal Er-PSZ at intermediate and elevated temperatures but nodefinitive conclusions can be drawn from the available experi-mental evidence.

VI. Conclusions

The tensile deformation of single-crystal partially stabilized 5mol% Er2O3-doped ZrO2 was studied between 251 and 14001C.The tensile strength presented a minimum at 6001–8001C, whichwas associated with the slow growth of cracks from the surface.SCG was also present at 10001C and above but it was not al-ways the dominant fracture mechanism and some monofila-ments tested in the range 10001–14001C presented higherductility (�3%–4%) as a result of the development of plasticdeformation. They failed from process-related flaws at the sur-face and their strength was significantly higher than those thatfailed by slow crack growth due to the marked strain hardeningcapacity of this material.

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