The influence of grain size on low-temperature degradation of dental zirconia

The influence of grain size on low-temperature degradationof dental zirconia

Lubica Hallmann,1 Albert Mehl,2 Peter Ulmer,3 Eric Reusser,3 Johannes Stadler,4

Renato Zenobi,4 Bogna Stawarczyk,1 Mutlu Ozcan,1 Christoph H. F. Hammerle1

1Clinic of Fixed and Removable Prosthodontics and Dental Material Science, Center of Dental Medicine,

University of Zurich, Zurich, Switzerland2Clinic of Preventive Dentistry, Periodontology and Cariology, Computer Assisted Restorations,

Center of Dental Medicine, University of Zurich, Zurich, Switzerland3Institute of Geochemistry and Petrology, ETH Zurich, Switzerland4Department of Chemistry and Applied Biosciences, ETH Zurich, Switzerland

Received 13 July 2011; revised 29 August 2011; accepted 6 September 2011

Published online 25 November 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.31969

Abstract: The purpose of this study was to evaluate the

influence of grain size and air abrasion on low-temperature

degradation (LTD) of yttria stabilized tetragonal zirconia poly-

crystalline (Y-TZP). Disc-shaped specimens were sintered at

1350, 1450, and 1600�C. Air abrasion was performed with dif-

ferent abrasive particles. The specimens were stored for 2 h

at 134�C under 2.3 bar water vapor pressure. All specimens

were characterized by X-ray powder diffraction analysis

(XRD), Raman spectroscopy, X-ray photoelectron spectros-

copy (XPS), atomic force microscopy (AFM), and field emis-

sion scanning electron microscopy (FESEM). Y-TZP sintered

at a temperature of 1350�C did not undergo the t–m phase

transformation during accelerated aging. The diffusion-con-

trolled t–m phase transformation initiated with the specimens

sintered at 1450�C. This transformation was remarkable for

the specimens sintered at 1600�C. X-ray photoelectron spec-

troscopy (XPS) measurements did not confirm the generation

of Zr-OH and Y-OH bonds. No increase of yttrium concentra-

tion on the grain boundaries of Y-TZP was detected, which

could be responsible for the destabilization of dental zirconia

ceramics. A slight increase of diffusion-controlled t–m phase

transformations was observed for all abraded specimens sin-

tered at 1350 and 1450�C. The size of abrasive particles did

not play a crucial role on LTD of Y-TZP. The retardation of

diffusion-controlled t–m phase transformation was evident

for all abraded specimens sintered at 1600�C by comparison

to non-abraded specimens. Conclusion: The LTD of Y-TZP

can be suppressed when the sintering temperature is set

between 1350 and 1450�C. VC 2011 Wiley Periodicals, Inc. J Biomed

Mater Res Part B: Appl Biomater 100B: 447–456, 2012.

Key Words: dental zirconia (Y-TZP), degradation, grain size,

air abrasion, tensile stress

How to cite this article: Hallmann L, Mehl A, Ulmer P, Reusser E, Stadler J, Zenobi R, Stawarczyk B, Ozcan M, Hammerle CHF. 2012. Theinfluence of grain size on low-temperature degradation of dental zirconia. J Biomed Mater Res Part B 2012:100B:447–456.

INTRODUCTION

Dental zirconia ceramic, consisting of 3 mol % yttria stabi-lized tetragonal polycrystalline zirconia (Y-TZP), has excel-lent mechanical, biomedical, aesthetic properties with itssemitranslucent and radiopacity features.1–3 The develop-ment of CAD/CAM systems, the increase in aestheticdemand and, worries about metallic hypersensitivity haveincreased the application of Y-TZP in dentistry. Y-TZP hasextraordinary strength and toughness due to the transfor-mation from the tetragonal to the monoclinic polymorphwhen external stress is applied.4,5 For these reasons, Y-TZPis used to substitute the traditional metal-based fixed dentalprosthesis (FDPs). Zirconia full-ceramic copings, FDP frame-works, and implant suprastructures are introduced toreplace the traditionally used metals.4 However, the failure

of a number of zirconia hip joints has drawn the attentionof researchers to the behavior of Y-TZP ceramics in humidenvironments. Fractures are often associated with a largeamount of monoclinic phase created during tetragonal–monoclinic phase transformation in the human body.6,7 Thistransformation reduces the mechanical properties of Y-TZPand is known as low-temperature degradation (LTD).6 Thespontaneous transformation from the tetragonal into mono-clinic structure initiates on the surface of Y-TZP via a stresscorrosion type mechanism and proceeds to the bulk of theceramic.4,7 The principal mechanism of LTD is diffusion-con-trolled, while the t–m modification transformation, oncenucleated, is martensitic.4 The transformation proceedsmost rapidly at temperatures of 200–300�C.8 The destabili-zation of the tetragonal phase is accompanied by the

Correspondence to: L. Hallmann; e-mail: [email protected]

VC 2011 WILEY PERIODICALS, INC. 447

formation of micro- and macrocracks and is dependent onthe concentration of the stabilizer, and the grain size of ce-ramic.6–10,11

Y-TZP undergoes a stress-induced volume-expansionphase transformation during air abrasion. Flaws or micro-cracks can thereby be introduced in the Y-TZP surface.12,13

The pre-existing monoclinic phase on the air-abraded Y-TZPsurface hinders the propagation of the diffusion-controlledtransformation during accelerated aging.4

The aging of zirconia is a serious problem for the ortho-pedic community.4 In conventional dental applications, thezirconia core or framework is not in contact with the oralenvironment and hard dental tissues due to veneering andluting materials and the problem seems not to be predomi-nant.4 However, the luting resin could absorb water via den-tal tubules and the zirconia core and framework exposed towater may suffer from aging degradation.4 Additionally,more and more fully ceramic zirconia restorations areinserted, where zirconia surfaces are directly opposed tothe intraoral environment.

Several theoretical causes have been proposed to explainthe mechanism of LTD. Sato et al. suggested the chemicalreaction of water with Zr-O-Zr bonds at the pre-existing sur-face flaws.9 This reaction is associated with the formation ofZr-OH bonds, releasing the strain that stabilizes the tetragonalphase, thus promoting the t–m phase transformation.9 Thephase transformation starts at the surface and propagatesinto the interior.9 According to Lange et al., the water reactswith Y2O3 at the surface of the zirconia and clusters ofY(OH)3 are formed, leading to the depletion of stabilizersfrom the grains and the spontaneous t–m transformationtakes places in the grains.10 Yoshimura et al. explained thedegradation of Y-TZP with the annihilation of oxygen vacan-cies and about 60% of these vacancies are occupied by OH–

anions.14 The OH– anions would migrate faster than O2–

because of less charge and similar size.14 These authors didnot accept the depletion of yttrium as the result of Y(OH)3formation or the dissolution of Y3þ during hydrothermaltreatment. According to Hughes et al., the generation of O2– isresponsible for the rapid annealing of surface oxygen vacan-cies.15 The penetration of O2– into the lattice destabilizes thet-phase and the transformation or nucleation of the m-phasedomains occurs.15 Oxygen vacancies play an important role inthe stabilization of the metastable tetragonal and cubic struc-ture.16 Fabris et al. employed a self-consistent tight-bindingmodel to constrain that the stabilization of the cubic and tet-ragonal structure could be achieved by introducing oxygenvacancies into the zirconia lattice.17 Guo related the LTD withannihilation of oxygen vacancies that stabilize the metastabletetragonal phase through the OH– group.18 Chevalier proposesthat the filling of oxygen vacancies with O2– and not OH– ionsmay be responsible for low-temperature degradation of zirco-nia.6 The O2– ions originate from the dissociation of water onthe surface of Y-TZP ceramic.6

The processes of low-temperature degradation (LTD)can effectively be simulated at 134�C, under a water vaporpressure of 2 bar. One hour of this treatment correspondsroughly to 4 years of aging in human environment.7

Therefore, the purpose of the present study was toevaluate the effect of grain size, flaws, and pre-existingmonoclinic phase created during air abrasion on the low-temperature degradation (LTD) of dental Y-TZP ceramicunder above-mentioned conditions. One of the most impor-tant factors on LTD mechanism of dental zirconia is tensilestrain, which is grain size dependant. An additional goal ofthis study was to test the idea that flaws, micro-cracks, andpre-existing monoclinic phase play a certain role on the LTDof Y-TZP, but the most important factor is the critical grainsize. An attempt was made to ascertain the proper mecha-nism of LTD during the accelerated aging of Y-TZP ceramic.The working hypothesis was that the resulting LTD of Y-TZPceramic under hydrothermal condition is not primarily con-trolled by the concentration gradient of yttrium on grainboundaries and/or the Zr-OH and Y-OH bonds formationresulting from OH– anion diffusion, but critically depends onthe species that diffuse rapidly in the lattice of Y-TZPceramic.

MATERIALS AND METHODS

Specimen preparationY-TZP ceramic blanks (Cercon base, DeguDent, Hanau, Germany)(ZrO2, 5% Y2O3, < 2% HfO2, < 1% Al2O3 þ SiO2) were cutwith a cutting machine (Precision Diamond Wire Saw, Well,Switzerland) into discs (diameter: 25 mm, thickness: 3 mm)under copious water (n ¼ 216). The specimens weresintered in a programmable oven (Cercon heat, DeguDent,the sinter temperature was set to 1350�C for 2 h). Otherspecimens were sintered at 1450 and 1600�C for 2 hwith heating and cooling rates of 10�C/min (Nabertherm,Switzerland). The specimens were divided randomly into sixmain groups and each main group was divided into foursubgroups (see Table I). The specimens were abraded withdifferent air-borne particles (50 and 110 lm alumina,30 and 110 lm silica-coated alumina particles) at a pres-sure of 2.5 bar for 15 s/cm2 from a distance of 10 mm(LEMAT NT4, Wassermann, Germany).

After air abrasion, all specimens were first air-blown ata pressure of 3 bar for 1 min and then ultrasonicallycleaned in 99.8% isopropanol (Merck, Darmstadt, Germany)for 20 min (Brasonic Ultrasonic Cleaner, Danbury) at afrequency of 42 kHz.

The acceleration aging of specimens was performedunder autoclave conditions of 134�C and a water vaporpressure of 2.3 bar for a duration of 2 h (GETINGE industriv5 SE-471 31 Skarhamn, Sweden).

Ceramic veneering simulation was performed in an oven(Austromat M, Dekema, Freilassing, Deutschland) at 830�Cfor 1 min. The heating rate was set to 55�C/min. Additionalceramic veneering simulations were conducted at 820, 810,and 800�C. The holding time for these temperature treat-ments was 1 min and the heating rate was 45�C/min.

Phase transformationTetragonal (t)–monoclinic (m) phase transformations of Y-TZP ceramics were analyzed with X-ray powder diffractiontechnique (Bruker, AXS D8 Advance) using Cu Ka1 X-rays.

448 HALLMANN ET AL. LOW-TEMPERATURE DEGRADATION

The diffraction profiles were acquired from 25.5 to 29� (2h)with a step size of 0.001� and counting time of 2 s/step,and from 25 to 37� (2h) with a step size of 0.006� andcounting time of 2 s/step.

Raman spectroscopy was additionally employed to inves-tigate the t–m phase transformation. High-resolution AFMand confocal Raman measurements were conducted on acombined confocal Raman system and AFM instrument(Spectra Ntegra upright, NTMDT, Russia) equipped with a100 � 0.7 NA objective (Mitutoyo, Japan), a 632.8 nm HeNelaser (up to 3 mW), a 100 � 100 � 10 lm piezo scanner,and non-contact cantilever-based AFM probes (ATEC-NC,NanoSensors, Switzerland) for atomic force microscopy.

Surface elemental compositionThe surface chemistry of the specimens was investigated bymeans of X-ray photoelectron spectroscopy (XPS). The anal-yses were performed with a PHI Quantera SXM (ULVAC-PHI,Chanhassen, MN) spectrometer. All XPS spectra werecollected at an emission angle of 45� using a beam size of200 lm with a power of 50 W in the constant–analyzer–energy (CAE) mode. High-resolution spectra were acquiredusing a pass energy of 55 eV and a step size of 0.1 eV (full-width-at-half-maximum (FWHM) of the peak height for Ag3d5/2 0 0.64 eV). Survey spectra were collected with a passenergy of 280 eV and a step size of 1 eV. A high-perform-

ance, floating-column ion gun, and an electron neutralizerwere used for charge compensation. The residual pressurein the analysis chamber was below 5 � 10–7 Pa. The spec-trometer was calibrated according to ISO 15472:2001 withan accuracy of 60.1 eV.

Surface topographyThe topography of the Y-TZP ceramic surface before and af-ter aging was investigated using AFM and FESEM. Theimages were acquired using a BRUKER N8 RADOS scanningprobe microscope as follows:

• Aged specimens which were sintered at 1350 and1600�C: N8 RADOS equipped with NANOS 806, 80 � 80lm2 maximal scan area, 6 lm Z-range, active vibrationisolation, acoustic enclosure.

• Sintered specimens at 1350 and 1600�C: N8 NEOSequipped with NANOS 806, (80 � 80 lm2) maximal scanarea, 6 lm Z-range.

• Measurement Parameters:* Dynamic (intermittent contact) mode.* Free amplitude was varied between 10 and 170 nm.* Damping set point was varied 58 and 70%.* Scan speed: 0.5 line per second.

The phase images were acquired simultaneously withthe topography images.

TABLE I. Preparation of Specimens (72 Groups With Each n ¼ 3 Specimens)

Specimens Treatment

Y-TZP sintered at 1350,1450, and 1600�C

Main group aSubgroups:a0 control specimensa00 aged specimensa0 00 aged specimens þ first ceramic veneering stimulationa00 00 aged specimens þ fourth ceramic veneering stimulations

Main group bSubgroups:b0 abraded specimens with 50 lm alumina particlesb00 abraded specimens þ agingb0 00 abraded and aged specimens þ first ceramic veneering stimulationb00 00 abraded and aged specimens þ fourth ceramic veneering stimulations

Main group cSubgroups:c0 abraded specimens with 110 lm alumina particlesc00, c0 00, c00 00 the specimens are treated as specimens of main group b

Main group dSubgroups:d0 abraded specimens with 30 lm silica-coated alumina particlesd00, d0 00, c00 00; the specimens are treated as specimens of main Group b

Main group eSubgroups:e0 abraded specimens with 110 lm silica-coated alumina particlese00, e0 00, c00 00 the specimens are treated as specimens of main group b

Main group fSubgroups:f0 abraded specimens with 110 lm alumina particles followed by 110 lm silica-coated

alumina particles (Rocatec system)f00, f0 00, f00 00 the specimens are treated as the specimens of main group b

Ceramic veneering simulation was performed at 830�C with heating rate of 55�C/min and 820, 810, and 800�C with heating rate of 45�C/min.

The holding time was 1 min for each temperature.

ORIGINAL RESEARCH REPORT

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | FEB 2012 VOL 100B, ISSUE 2 449

FESEM experiments were carried out with a Carl ZeissSupra 50 VP FESEM (Oberkochen, Germany). The accelera-tion voltage of the cathode was 5–30 kV. All specimenswere palladium gold coated before FESEM examinations.

RESULTS

Phase identificationFigure 1 displays the X-ray diffraction pattern (XRD) of Y-TZP sintered at various temperatures (1350, 1450, and1600�C) over the range 27.5–29� (2h) before and afteraging. The profiles are zoomed in this range because thet–m phase transformation can visually be identified. Forspecimens sintered at 1350�C, the t–m phase transformationafter aging for 2 h at 134�C and under water vapor pres-sure of 2.3 bar, was not observed [Figure 1(a), plot a00],whereas the specimens sintered at 1450 and 1600�Cshowed the t–m transformation [Figure 1(b,c), plots a00].After air abrasion with different air-borne particles, adetectable monoclinic peak with orientation m(11-1) wasdiscernable [Figure 1(a–f), plots; b0–f0]. The intensity of thispeak was increased after accelerated aging for abradedspecimens [Figure 1(a–f), plots b00–f00]. In contrast, the inten-sity of these peaks decreased after the ceramic veneeringsimulation at 830�C for 1 min. [Figure 1(b–f), plots a0 00–f0 00].The highest extent of t–m phase transformation wasobserved for Y-TZP sintered at 1600�C. A retardation of the

t–m phase transformation was evident for all the abradedspecimens sintered at 1600�C in comparison with non-abraded specimens [Figure 1(c,f)].

Figure 2 reveals the effect of low-temperature degrada-tion (LTD) at 134�C under a water vapor pressure of 2.3bar of Y-TZP ceramic sintered at different temperatures overa range of diffraction angles from 25 to 39� (2h). The X-raydiffraction patterns of non-aged and aged specimens sin-tered at 1350�C are similar and clearly indicate the missingof a t–m phase transformation [Figure 2(a)]. The effects ofaccelerated aging on the t–m phase transformation for Y-TZP ceramic sintered at 1450 and 1600�C are shown in Fig-ure 2(c,e). Compared to the non-aged Y-TZP specimens, theaged Y-TZP specimens sintered at 1600�C contains the high-est concentration of the monoclinic phase. The reverse m-tphase transformation took place after ceramic veneeringsimulation. The intensity of the m(11-1) peak in this casedecreased accompanied by an increase of the t(111) peakintensity. However, a small proportion of monoclinic phaseseems to be present in both cases, but for the specimen sin-tered at 1600�C the amount of non-transformed monoclinicphase was higher than for the specimen sintered at 1450�C[Figure 2(c,e) and plots a0, a00, and a0 00]. After air abrasion,the monoclinic peak m(11-1) and a remarkable t(111) peakbroadening accompanied by the reversed intensity of thetetragonal t(002) and t(020) peaks were observed in all

FIGURE 1. XRD patterns of Y–TZP specimens before and after aging at the range of 27.5–29� (2y). [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]


XRD patterns. After accelerated aging, the intensity of m(11-1) peak was only slightly increased for the abraded speci-mens sintered at 1350�C followed by the specimens sin-tered at 1450�C, whereas the specimens sintered at 1600�Cresulted significantly higher t–m phase transformation [Fig-ure 2(b,d,f,g)]. The intensity of the m(11-1) peak andthe broadening of the t(111) peak were reduced for allspecimens after the firths ceramic veneering stimulation.After that no intensity change of the m(11-1) and t(111)

peaks was evident after the fourth ceramic veneeringstimulation.

Figure 3 presents Raman spectra of Y-TZP ceramic. It isevident from the Raman spectra that major bands arelocated at 141, 255, 315, 462, and 641 cm–1. The peak at635 cm–1 has a shoulder at 603 cm–1. These peaks are char-acteristic for the tetragonal phase of Y-TZP ceramic.19–22 Thepeaks at 175 and 373 cm–1 are characteristic for a mono-clinic structure.23–25 The weak peak at 550 cm–1 likewise is

FIGURE 2. XRD pattern of Y–TZP specimens before and after aging at the range of 25–37� (2y). [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]



attributed to the monoclinic phase (Table II). Comparing plots3a, b and c reveals that the distribution of the monoclinicphase for aged Y-TZP sintered at 1600�C was not homogenous.

Surface elemental compositionThe survey XPS spectra [Figure 4(a)] revealed that the ele-mental composition of the Y-TZP ceramic surface did notchange after accelerated aging at a temperature of 134�Cunder a water pressure of 2.3 bar for 2 h. The C 1s peak ofall the specimens resulted from an overlaying contaminatinghydrocarbon layer, which was unavoidable during XPS mea-surement [Figure 4(b)]. Figure 4(c) compares the Zr 3dspectra obtained before and after accelerated aging for theY-TZP sintered at three different temperatures. The bindingenergies of Zr 3d3/2 and Zr 3d5/2 were 183.2–183.6 eV and180.8–182.2 eV, respectively (Table III). The binding energyof the Zr 3d core was shifted for 0.3 eV to negative valuesfor aged specimens sintered at 1600�C. The Y 3d spectraare shown in Figure 4(d). The binding energy of Y 3d3/2and Y 3d5/2 varied from 158.0 to 158.4 eV and 156.0 to156.5 eV, respectively. The Y 3d core peaks were shifted for0.4 eV to negative values for the samples sintered at1600�C. A positive shift of binding energy for Zr 3d and Y

3d core level was not observed. Such a positive shift wouldbe indicative for the formation of M-OH bonds. No remark-able change of the binding energy for the aluminium ionwas detected from the Al 2p spectra [Figure 4(e)]. Figure4(f) shows the energy binding of the O 1s core level beforeand after aging. The peak at low binding energy is assignedto oxygen in the metal-O-metal bond. The shoulder at higherbinding energy 530.4–531 eV (Table III) could be assignedto the OH bond, absorbed water, or oxygen–carbon bonds.

Surface topographyFigure 5 shows the AFM images of surface morphology of Y-TZP ceramic, which was sintered at 1350 and 1600�Cbefore and after aging. No change of the surface morphologyand/or roughness of non-aged and aged Y-TZP ceramic, sin-tered at 1350�C, was not observable. The roughness fornon-aged and aged Y-TZP ceramic surface was 10.37 and8.97 nm, respectively. Figure 5(a0–d0) presents AFM imagesof the surface morphology of Y-TZP ceramic sintered at1600�C, indicating that a change of grain topographyoccurred as the grain surfaces were not flat after acceler-ated aging. The monoclinic phase precipitated as dispersedlenses in the tetragonal matrix phase. The roughness of Y-TZP ceramic surface increased after accelerated aging to9.75 and 15.3 nm for non-aged and aged Y-TZP ceramicsurfaces, respectively.

Figure 6 displays the FESEM images of non-aged andaged Y-TZP ceramics, which were sintered at different tem-peratures. The average grain size was small (about 0.21lm) and not homogenous for the specimens sintered at1350�C. The same applied to the specimens sintered at1450 and 1600�C, with a grain size of 0.3 and 0.72 lm,respectively. After accelerated aging, the grains of specimenssintered at temperature1350 and 1450�C appeared not tobe uplifted in contrast to the specimen sintered at 1600�C.The surface of aged Y-TZP sintered at 1600�C was damagedas a result of volume increasing during t–m phase transfor-mation and holes were created [Figure 6(f)]. Figure 6(h)shows the precipitation of the monoclinic phase as dis-persed lenses. The EDS measurements did not reveal anychange in the yttrium concentration before and after accel-erated aging of Y-TZP ceramic sintered at different tempera-tures. Screening of elemental composition for grain surface

FIGURE 3. Raman spectra of Y-TZP specimens before and after aging.

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

TABLE II. Wave Number (in cm�1) of Raman Vibration Bands for the Tetragonal and Monoclinic Structure of Y-TZP Ceramics

Specimens Tetragonal Phase

Sintered specimen at 1350�C 141 254 314 458 601 sh 637Aged specimen (sintered at 1350�C) 141 254 314 458 601 sh 637Sintered specimen at 1450�C 144 258 318 462 604 sh 641Aged specimen (sintered at 1450�C) 143 257 316 462 604 sh 640Sintered specimen at 1600�C 144 257 318 462 604 sh 640(a) Aged specimen (sintered at 1600�C) 142 256 317 460 604 sh 639(b) Aged specimen (sintered at 1600�C) 144 258 319 464 604 sh 640(c) Aged specimen (sintered at 1600�C) 141 256 323 466 634

Specimens Monoclinic Phase(b) Aged specimen (sintered at 1600�C 175 187 377(c) Aged specimen (sintered at 1600�C) 175 187 374 549


and boundary concentration changes showed a homogenousdistribution of yttrium across the surface and no concentra-tion gradient in the grain-boundary region was observed(Table IV).

DISCUSSION

Low-temperature degradation (LTD) of dental zirconia byannealing in water vapor is dependent on several factorssuch as crystal structure, grain size, the amount of dopant,residual stress. In this study, the stress-induced t–m phasetransformation of Y-TZP started at a sintering temperatureof 1450�C. The highest transformation rate observed for Y-TZP ceramic sintered at 1600�C can be explained byincreased tensile stresses, as larger grain sizes correspondto a larger tensile stress (this work will be soon submitted).The grains of Y-TZP sintered at 1350�C were more sphericalthan the grains of Y-TZP sintered at 1450�C, while the grainsof Y-TZP sintered at 1600�C were flattened and enlarged.The stress distribution is more uniform for the sphericalgrains than for the flat grains. The tensile and compressivestresses are present in the sintered ceramic, but the ratios

of these stresses depend of the sintering temperature. Theslight increasing of the extent of the t–m phase transforma-tion observed for aged ceramic sintered at 1450�C impliesthat this ratio was changed in favor of tensile stress in com-parison to the ceramic sintered at 1350�C.

Some groups of Y-TZP ceramic surfaces in this studywere abraded before accelerated aging. The aim was toinvestigate the effect of micro-cracks that were introducedon the Y-TZP ceramic surface during air abrasion. There arestudies which relate the propagation of the t–m phasetransformation to the creation of micro-cracks during accel-erated aging.26,27 Micro-cracks served as preferential pathsfor water diffusion inside the bulk of the Y-TZP ceramic.28

The air abrasion induced the t–m phase transformation,plastic deformations, flaws, grooves, cracks, and impingingof air-borne particles on the surface of the ceramic. Thecompressive stresses created during the air abrasion serveto inhibit micro-crack extension.4 Our study showed thatthe air abrasion was responsible for the retardation of diffu-sion controlled t–m phase transformation for Y-TZP ceramicsintered at 1600�C. The slight increasing of the t–m phase

FIGURE 4. XPS spectra of Y-TZP ceramic surfaces before and after aging. [Color figure can be viewed in the online issue, which is available at


TABLE III. Binding Energies (in eV) of Zr 3d, Y 3d, Al 2p, and O 1s Core Levels

Specimens Zr 3d3/2 Zr 3d5/2 Y 3d3/2 Y 3d5/2 Al 2p O 1s O 1s

Sintered specimen at 1350�C 183.2 180.8 158.0 156.0 73.0 528.6 530.0Aged sintered specimen at 1350�C 183.2 180.8 158.0 156.0 73.1 528.8 530.5Sintered specimen at 1450�C 183.5 181.2 158.2 156.2 73.2 528.9 530.8Aged sintered specimen at 1450�C 183.4 181.1 158.0 156.0 73.1 528.7 531.0Sintered specimen at 1600�C 183.6 181.2 158.5 156.5 73.5 529.1 531.2Aged sintered specimen at 1600�C 183.3 180.9 158.1 156.2 73.5 528.8 531.2



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transformation for aged Y-TZP sintered at 1350 and 1450�Cafter air abrasion in comparison with non-abraded Y-TZPceramic can be explained by the different stresses whichpredominate in these ceramics before and after air abrasion.After air abrasion, this ratio changed in favor of tensilestresses, which were responsible for contrasting behaviorsof sintered and abraded ceramics sintered at these tempera-tures. The grain size of abrasive particles did not play a cru-cial role on LTD of Y-TZP sintered at different temperatures.

The binding energy shift of a core-electron is related tothe change of the chemical environment of the element. Thenegative shift of the Zr 3d binding energy for the specimensintered at 1600�C after aging in this study indicates achange of the chemical state for the zirconium ion. Bindingenergy reduction of a metal ion is an indication for anincreasingly covalent bond formation,29 whereas the forma-tion of a Zr-OH bond shows a higher binding energy thanthat of ZrO2 for the same oxidation state.30 It may thereforebe suggested that the partial reduction of Zr4þ to Zr3þ tookplace and the conversion of ZrO2 to suboxides ZrOx

occurred during aging process.31 The lack of a positive shiftfor the zirconium and yttrium ions shown in this study indi-cated that Zr-OH and Y-OH bonds were not introduced intothe Y-TZP lattice. The lack of concentration gradients for yt-trium ions at the grain boundaries does not support thetheory of LTD occurring through the depletion of yttriumions from the lattice of Y-TZP ceramic.

The theoretical atomistic study of Kushima et al. con-ducted to explain the mechanism of oxygen anion transportin anion conducing ceramic reveled that the biaxial tensilelattice stress is a very important factor for the migration ofoxygen vacancies by the hopping mechanism.28 The energybarrier of migrating vacancies is affected by the migrationspace, cation-oxygen bond strength, and additional factors.The increase of cation–cation distances and the weakness ofthe cation–oxygen bond strength decrease the migrationenergy barrier of vacancies. The oxygen ions migrate afterthe same mechanism as vacancies. These authors found thatthe macroscopic oxygen diffusivity increases exponentiallyin yttria stabilized zirconia up to a critical value of biaxialtensile stress, especially at lower temperatures. Beyond thiscritical value of lattice stress, the migration of oxygendecreases due to local relaxation.28 The higher t–m phasetransformation rate during accelerated aging observed forsintered Y-TZP at 1600�C in this study confirms theircalculations.

There is a debate32 about the species that are responsi-ble for stress induced t–m phase transformation of Y-TZP ce-ramic. The backward phase transformation m-t observedduring the first ceramic veneering simulation provides infor-mation about these species. This transformation took placevery fast at the beginning and thereafter the reversible reac-tion is retarded. The full reversible reaction took place athigher temperatures only. This implies that the species that

FIGURE 6. FESEM images of the specimens sintered at 1350, 1450, and 1600�C before and after aging.

TABLE IV. Elemental Compositions in wt % of Non-aged and Aged Y-TZP Ceramic in Terms of Zr and Y

Zr (wt %) Y (wt %)

Grain Boundary Grain Boundary

Sintered zirconia at 1350�C 78.55 78.70 7.08 7.12Aged zirconia (sintered at 1350�C) 78.37 77.97 6.85 6.91Sintered zirconia at 1450�C 78.63 78.48 7.04 7.35Aged zirconia (sintered at 1450�C) 79.35 79.27 7.12 7.21Sintered zirconia at 1600�C 78.45 78.31 7.47 7.78Aged zirconia (sintered at 1600�C) 78.69 78.96 8.53 7.78



are involved in this transformation moved very fast at firstand after that, due to the alteration of the lattice stress, themigration barrier energy increased. The surface of Y-TZPserves as a catalyst for the dissociation of water into O2–

and Hþ ions. Hþ ions, therefore, probably play a crucial roleon LTD of Y-TZP. For a full understanding of the LTD mecha-nism, additional theoretical calculations at an atomistic scaleare required. The biaxial tensile stress is one of the factorswhich control the diffusion of vacancies and oxygen into thelattice of Y-TZP. The role of hydrogen on the LTD mecha-nisms is not yet clear and request complementary measure-ments which are beyond the scope of this article.

CONCLUSION

1. The degradation of Y-TZP ceramic at the condition ofaccelerated aging is strongly dependent on the grainsizes.

2. When the grain size has achieved the critical value, whichis about 0.3 lm, the t–m phase transformation occurred.

3. Y-TZP sintered at 1600�C resulted the highest rate of deg-radation (average grain size was about 0.72 lm).

4. Flaws and cracks which were induced on the surface ofY-TZP through the air abrasion process were not an im-portant factor for the LTD. The size of air-borne particleswas not a crucial factor on the LTD. The retardation ofLTD caused by air abrasion was observed for sinteredspecimens at 1600�C.

5. The formation of Y-OH and Zr-OH bonds was notobserved. The concentration gradient of yttrium ions onthe surface of grains or on the grain boundaries did nottake place.

6. A stable Y-TZP can be achieved from a sintering tempera-ture ranging between 1350 and 1450�C.

ACKNOWLEDGMENTS

The authors thank Prof. N. Spencer and Mr. F. Mangolini fromthe Surface & Science Technology, Department of ETH Zurichfor XPS measurements, Dr. Igor Nemeth Bruker Nano GmbHBerlin for AFM measurements, Dr. L Volkl of Degudent for theY-TZP blanks, Prof. Gauckler from the Nonmetallic InorganicMaterials, Swiss Federal Institute of Technology, Departmentof Materials of ETH Zurich for his advice.

REFERENCES1. Lung KYC, Kukk E, Hagerth T, Matinlinna PJ. Surface modification

of silica-coated zirconia by chemical treatments. Appl Surf Sci

2010;257:1228–1235.

2. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomate-

rials 1999;20:1–25.

3. Sato H, Ban S, Hashiguchi M, Yamasaki Y. Effect of autoclave

treatment on bonding strength of dental zirconia ceramics to

resin cements. J Ceram Soc Japan 2010;118:508–511.

4. Kosmac T, Oblak C, Marion L. The effects of dental grinding and

sandblasting on ageing and fatigue behavior of dental zirconia (Y-

TZP) ceramics. J Euro Ceram Soc 2008;28:1085–1090.

5. Moser ME, Keller AB, Lienemann P, Hug P. Surface analytical

study of hydrothermally treated zirconia ceramics. Fresenius

J Anal Chem 1993;346:255–260.

6. Chevalier J, Gremillard L, Virkar VA, Clarke RD. The tetragonal-

monoclinic transformation in zirconia:Lessons learned and future

trends. J Am Ceram Soc 2009;92:1901–1920.

7. Gremillard L, Chevalier J, Epicier T, Deville S, Fantozzi G. Model-

ing the ageing kinetics of zirconia ceramics. J Euro Ceram Soc

2004;24:3483–3489.

8. Chevalier J, Cales B, Drouin MJ. Low-temperature aging of Y-TZP

ceramics. J Am Ceram Soc 1999;82:2150–2154.

9. Sato T, Shimada M. Transformation of yttria-doped tetragonal ZrO2

polycrystals by annealing inwater. J AmCeramSoc 1985;68:356–359.

10. Lange FF, Dunlop LG, Davis IB. Degradation during aging of

transformation-toughened ZrO2-Y2O3 materials at 250�C. J Am

Ceram Soc 1986;69:237–240.

11. Kelly RJ, Denry I. Stabilized zirconia as a structural ceramic: an

overview. Dental Mater 2008;24:289–298.

12. Zhang Y, Lawn RB, Malament AK, Thompson PV, Rekow DE.

Damage accumulation and fatigue life of particle-abraded

ceramics. Int J Prosthodont 2006;19:442–448.

13. Scherrer S.S, Lorente CM, Vittecoq E, Mestral F, Griggs AJ, Wis-

kott AWH. Fatigue behavior in water of Y-TZP zirconia ceramics

after abrasion with 30 lm silica-coated alumina particles. Dental

Mater 2011;27:e28–e42.

14. Yoshimura M, Noma T, Kawabata K, Somiya S. Role of H2O on the

degradation process of Y-TZP. J Mater Sci Lett 1987;6:465–467.

15. Hughes EA, John SH, Kountouros P, Shubert H. Moisture sensi-

tive degradation in TiO2-Y2O3-ZrO2. J Euro Ceram Soc 1995;15:

1125–1134.

16. Kawata K, Maekawa H, Nemoto T, Yamamura T. Local structure anal-

ysis of YSZ by Y-89MAS-NMR. Solid State Ion 2006;177:1687–1690.

17. Fabris S, Paxton TA, FinnisWM. A stabilizationmechanism of zirconia

based on oxygen vacancies only. ActaMater 2002;50:5171–5178.

18. Guo X. On the degradation of zirconia ceramics during low-tem-

perature annealing in water or water vapour. J Phys Chem Solids

1999;60:539–546.

19. Li M, Feng Z, Ying P, Xin, Q, Li C. Phase transformation in the

surface region of zirconia and doped zirconia detected by UV

Raman spectroscopy. Phys Chem Chem Phys 2003;5:5326–5332.

20. Merle T, Guinebretiere R, Mirgorodsky A, Quintard P. Polarized

Raman spectra of tetragonal pure ZrO2 measured on epitaxial

films. Phys Rev B 2002;65:144302-1–144302-6.

21. Quintard EP, Barberis P, Mirgorodsky PA,MejeanMT. Comparative lat-

tice-dynamical study of the raman spectra of monoclinic and tetrago-

nal phases of zirconia and hafnia. J AmCeramSoc 2002;85:1745–1749.

22. Lopez FE, Escribano SV, Panizza M, Carnasciali MM, Busca G.

Vibrational and electronic spectroscopic properties of zirconia

powders. J Mater Chem 2001;11:1891–1897.

23. Daramola AD, Muthuvel M, Botte GG. Density functional theory

analysis of Raman frequency modes of monoclinic zirconium ox-

ide using Gaussian basis sets and isotopic substitution. J Phys

Chem B 2010;114:9323–9329.

24. Kim KB, Hamaguchi H. Mode assignments of the Raman spec-

trum of monoclinic zirconia by isotopic exchange technique. Phys

Status Solidi B 1997;203:557–563.

25. Maczka M, Lutz GTE, Verbeek JH, Oskam K, Meijerink A, Hanuza

J, Stuivinga M. Spectroscopic studies of dynamically compacted

monoclinic ZrO2. J Phys Chem Solids 1999;60:1909–1914.

26. Chevalier J. What future for zirconia as a biomaterial? Biomateri-

als 2006;27:535–543.

27. Lawson S. Environmental degradation of zirconia ceramics. J

Euro Ceram Soc 1995;15:485–502.

28. Kushima A, Yildiz B. Oxygen ion diffusivity in strained yttria stabi-

lized zirconia: where is the fastest strain? J Mater Chem 2010;20:

4809–4819.

29. Barr TL. Recent advances in x-ray photoelectron spectroscopy sty-

dies of oxides. J Vacuum Sci Technol A 1991;9:1793–1805.

30. Li SY, Wong CP, Mitchell RAK. XPS investigations of the interac-

tions of hydrogen with thin films of zirconium oxide II. Effect of

heating a 26 A thick film after treatment with a hydrogen plasma.

Appl Surf Sci 1995;89:263–269.

31. Kumari L, Li ZW, Xu MJ, Leblanc MR, Wang ZD,. Li Y, Guo H, Zhang J.Controlled hydrothermal synthesis of zirconium oxide nanostructuresand their optical properties. Crystal Growth Design 2009;9:3874–3880.

32. Duong T, Limarga MA, Clarke RD. Diffusion of water species in

yttria-stabilized zirconia. J Am Ceram Soc 2009;92:2731–2737.


The influence of grain size on low-temperature degradation of dental zirconia

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