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RD2 micro-diffraction analysis of the interface between-TZP and veneering porcelain: Role of application methods

ichael J. Tholeya, Christoph Bertholdb, Michael V. Swainc,∗, Norbert Thiela

Research and Development Department VITA Zahnfabrik, Bad Saeckingen, GermanyDepartment of Geo Sciences, University of Tuebingen, GermanyDepartment of Oral Sciences, School of Dentistry, PO Box 647, Dunedin, New Zealand

r t i c l e i n f o

rticle history:

eceived 3 June 2009

eceived in revised form

7 November 2009

ccepted 1 February 2010

eywords:

irconia

-TZP

nterface

ll-ceramic

a b s t r a c t

Objectives. The metastability of the tetragonal crystal structure of yttria partial stabilized

zirconia polycrystalline (Y-TZP) ceramics is a basis of concern for dental restorations. Reac-

tions between the porcelain and the Y-TZP framework may result in a reduction of the

stability of the zirconia and interface bonding caused by a transformation from tetragonal

to monoclinic crystalline structure during veneering.

Methods. XRD2 micro-diffraction measurements were carried out on tapered veneered cross-

sections of the interface area to generate locally resolved information of the phase content

in this region. To get a high intensity X-ray beam for short measurement times a focussing

polycapillary with a spot size of app. 50 �m was used to evaluate the porcelain zirconia

interface.

Results. Under almost all conditions the phase composition of zirconia grains at the interface

revealed both the monoclinic and tetragonal structure. These observations indicate that

-ray diffraction

eneering material

destabilization of the tetragonal phase of zirconia occurs at the interface during veneering

with porcelain.

Significance. These results and their relevance to the long-term stability of the interface adhe-

sion between zirconia and veneering porcelain as well as the tetragonal to monoclinic crystal

transformations at the interface are discussed.

emy

© 2010 Acad

. Introduction

irconia holds an exclusive place amongst dental restora-ive materials compared with other oxide ceramics, such

s alumina, because of its excellent mechanical propertiess a consequence of transformation toughening that wasdentified in the mid-1970s [1]. Pure zirconia can exist inhree different crystal structures depending on tempera-

∗ Corresponding author. Tel.: +64 3 479 4196; fax: +64 3 479 7078.E-mail address: michael.swain@stonebow.otago.ac.nz (M.V. Swain).URL: http://www.otago.ac.nz (M.V. Swain).

109-5641/$ – see front matter © 2010 Academy of Dental Materials. Puoi:10.1016/j.dental.2010.02.002

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

ture. At room temperature up to 1170 ◦C, the symmetry ismonoclinic, the structure is tetragonal between 1170 and2370 ◦C and cubic above 2370 ◦C up to the melting point[2]. The transformation from tetragonal (t) to monoclinic(m) during a cooling process is accompanied by a volume

increase (approximately 4%) and shear distortion, sufficientto cause catastrophic failure. Alloying pure zirconia withstabilizing oxides such as Y2O3 allows the preservationof the meta-stable tetragonal structure at room tempera-

blished by Elsevier Ltd. All rights reserved.

mailto:michael.swain@stonebow.otago.ac.nzdx.doi.org/10.1016/j.dental.2010.02.002

546 d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 545–552

Table 1 – Preparation methods of the Y-TZP frameworks.

No. Liquid medium Porcelain Firing process

VVV

1. VITA VM liquid2. VITA VM liquid3. No

ture and therefore the potential to enable stress-inducedt → m transformation, which can enhance resistance to crackextension leading to higher toughness compared to alumina[1,3,4].

As a consequence of the metastability of tetragonal zir-conia, stress-generating surface treatments, such as grindingor sandblasting, are able to trigger the t → m transformationwith the associated volume increase leading to the forma-tion of surface compressive stresses, thereby increasing theflexural strength. However such metastability of the materialalso increases the susceptibility to aging [5]. The low tem-perature degradation (LTD) of zirconia is a well-documentedphenomenon, dependent upon the presence of moisture andmodest heat [6–11]. The consequences of this aging pro-cess are multiple and include surface degradation with grainpullout and micro-cracking as well as strength degradation.Although LTD has been shown to be associated with a seriesof orthopedic hip prostheses failures in 2001 and despite awell established definition of the conditions under which LTDoccurs, there is currently no clear relationship between LTDand failure predictability when zirconia is used as a dentalbio-ceramic [12].

3Y-TZP is now widely used in dentistry for the fabrica-tion of dental restorations, mostly processed by machining ofpartially sintered blanks followed by sintering at high temper-ature. The mechanical properties of 3Y-TZP strongly dependon its grain size [13–15]. Above a critical size, Y-TZP is less sta-ble and more vulnerable to spontaneous t → m transformationthan smaller grain sizes (

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 545–552 547

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completely tetragonal and had no observable monoclinic peak.The GADDS frame in Fig. 2 shows only the tetragonal crystal

structure on the surface of the Y-TZP sample. In all samplesreflections no monoclinic Baddeleyite structure of Zr02 was

Fig. 2 – Typical GADDS frame of the diffraction pattern

ig. 1 – Measurement points on the samples, on the left a sccross the interface while on the right hand side the elliptic

ollimators to generate a small analysis spot on the sam-le surfaces. The use of such pinhole collimators howeverecreases the intensity of the X-ray beam on the sample dra-atically, which results in long measurement times. In the last

ecade pinhole collimators have been replaced by monocapil-ary optics and in the last few years by focussing polycapillary

icro-lenses in order to focus the generated X-ray beam on theample. This has increased the localized intensity on the sam-le by orders of magnitude depending on the X-ray source andype of optics in comparison to a standard pinhole collimator19].

The X-ray micro-diffractometer (�-XRD2) used in this studyas a modification of typical powder diffractometer withfocussing micro-lens to achieve a micrometer-sized X-ray

eam and a 2-dimensional detector system (BRUKER-HiStar),hich covers app. 30◦ in 2� and app. 30◦ � at the same time [19].he advantage of such a focussing polycapillary micro-lens

nstead of the common pinhole collimator or a monocapil-ary is the short measurement time required, down to a feweconds, due to the high brilliance of the X-ray beam on theample combined with a spot size currently down to approx-mately 50 �m diameter FWHM. A general disadvantage ofmall spot sizes in powder diffraction setups is the potentiallyoor crystallite statistics in the analyzed volume depending onhe crystallite size. In addition and to avoid this disadvantagehe additional 2-dimensional HiStar-detector provides directssessment of texture effects and crystallite size in the ana-yzed sample.

Before commencing the �-XRD2-analysis of the interfaceegion various sample preparation methods of the taper sec-ions were examined by locally scanning the surface withhe micro-diffractometer. This indicated that the cutting andolishing preparation [18] did not generate significant t → mransformation of the zirconia grains on the surface. Mono-linic peaks and broadening of the tetragonal/cubic peak werenly observed on the ground surface of the prepared area andot on the polished area in the vicinity of the veneered inter-

ace with the relatively low spatial resolution of the X-rayiffraction system used in the former study [18].

All three preparation procedures were scanned with theicro-diffractometer in the manner shown in Fig. 1. The

eneered part and the polished zirconia surfaces were mea-

ured at 8 different positions in steps of 50 �m across thenterface with a measurement time of 120 s per pattern usinghe 50 �m micro-lens and Co K� (radiation with 30 kV/30 mAetting) with a fixed incident angle of 10◦ to the flat interface.

atic illustration vertically through the tapered sampleanned surface locations are shown.

Due to this incident angle the measurement spot has an ellip-tical geometry with a length of app. 400 �m and a width of app.50 �m. Three of the scanning positions are shown as an exam-ple for the working area in Fig. 1 where the middle positionfocuses on the location directly at the interface between Y-TZPand porcelain and the others on the adjacent positions. Theschematic on the left hand side vertically through the taperedinterface sample shows positions of the spots indicated bythe elliptical areas on the right hand side. No etching processsuch as HF content gel was used to ensure that other potentialinfluences on the framework material were minimized.

3. Results

The zirconia surfaces were scanned, as described above,before the veneering process to ensure that the zirconia was

observed for untreated Y-TZP with a solid angle ofincidence of 10◦. The major peak is associated with the 1 0 0tetragonal/cubic peaks while the less intense peaks to theright are of the 2 2 0 series of tetragonal and cubic peaks.

548 d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 545–552

Fig. 3 – X-ray scans across the interface area of the Wash-Dentin (method 1) prepared sample measured using a 50 �mmicro-lens, Co K�, 30 kV/30 mA, fixed incident angle 10◦, 120 s/frame.

Fig. 4 – X-ray scans across the interface area of the thin layer of porcelain coated zirconia (preparation procedure 1)measured using a 50 �m micro-lens, Co-K�, 30 kV/30 mA, fixed incident angle 10◦, 120 s/frame. An arrow at position 2correlates the intense single crystal reflection with the corresponding monoclinic (−1 1 1)-reflection in the XRD-pattern.

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 545–552 549

Fig. 5 – (a) Area a in the veneered part of preparationmethod no. 1; 500 �m monocapillary optic with 300 �mpinhole Co K�, 30 kV/30 mA, fixed incident angle 10◦,600 s/frame. (b) Diffraction pattern from Spot a showing thetetragonal diffraction rings and isolated spots caused bys

dwp

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t

Fig. 6 – (a) Area b in the veneered part of preparationmethod no. 2; 500 �m monocapillary optic with 300 �mpinhole Co K�, 30 kV/30 mA, fixed incident angle 10◦,600 s/frame. (b) Diffraction pattern from Spot b againhighlighting the presence of intense monoclinic single

ingle crystal (1 1 1)-reflections of the monoclinic structure.

etectable. After the phase composition of the Y-TZP surfacesas established they were veneered using the different sam-le preparation methods listed in Table 1.

The result of the �-XRD2-measurements from one typicalample prepared with a Wash-Dentin (preparation method no.) coating and prepared as schematically shown in Fig. 1 areresented in Fig. 3. The black arrow on the left side at positionidentifies the location of the interface of the taper section

etween the veneered zirconia grain surface and the polishedirconia. Scans of intensity versus 2� at 8 measurement pointsere performed, starting in the veneered surface, proceed-

ng across the interface region and ending in the substrateirconia.

Both measurements at position 1 (green coloured patterns)how only low intensities of both, monoclinic and tetrago-

al/cubic Zirconia due to the overlying veneering glass, whichbsorbs the X-ray beam.

At the locations 2 and 3 (red patterns) the intensities fromhe zirconia surface beneath the porcelain layer were sig-

crystal (1 1 1)-reflections of the monoclinic structure.

nificantly increasing, due to the decreasing thickness of theoverlying veneering. It should be pointed out that the observedmonoclinic (−1 1 1)-reflection which appears at app. 33◦ 2�seems to be significantly more intense than for the other loca-tions. Also in both green patterns (positions 1 and 2) there isa significantly intense monoclinic reflection at this position.

At positions 4–7 (black patterns) no porcelain is coveringthe zirconia substrate due to the geometry of the sample,therefore the information comes from the ground and pol-ished zirconia at areas below the interface. These areas arebeneath the veneered zirconia interface and so they providea basis for comparison with the diffraction patterns near the

interface. These results are similar to positions 2 and 3 (redpatterns). The X-ray patterns show broader tetragonal/cubicpeaks and weak and significantly broad monoclinic peaks at 2�33◦ in all of these areas. As such the influence of the veneering

550 d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 545–552

Fig. 7 – Interface area of the without liquid veneered zirconia (preparation procedure 3) measured using a 50 �m micro-lens,nly d

Co-K�, 30 kV/30 mA, fixed incident angle 10◦, 120 s/frame, o

observable.

process can be compared with the grinding/polishing prepa-ration of the taper section.

The right hand side of Fig. 3 presents the scan from 2� 32◦

to 37◦ at a higher magnification superimposing results fromall locations. The slight broadening in width of the knowncubic/tetragonal peak at about 2� 35◦ is evident for the blacktraces and an increase in intensity and sharpness of the mon-oclinic peak at 2� 33◦ for some of the green and red traces isalso obvious.

Another feature evident in Fig. 3 is the two reflections inthe range around 70◦ 2�, the cubic (3 1 1)/tetragonal (0 1 3 and1 2 1) peaks. Both show a switch of tetragonal peaks inten-sity (1 2 1 to 0 1 3), starting under the veneering (red patterns2, 3). This grinding induced intensity flip phenomenon ofthese tetragonal intensities is well known on zirconia surfaces[20].

The �-XRD2-results for the “normal” Wash-Dentin layer-ing technique (preparation method no. 1) coating and thethicker veneered specimen (preparation method no. 2) showsimilar results. It was impossible with the diffraction methodto recognize dissimilar effects between the zirconia samplesfrom these two preparation methods as anticipated from other

studies [18]. In both cases clear monoclinic zirconia peakswere detectable directly under the porcelain layer from thezirconia surface, but they were not stronger or more clearlyrecognizable from those associated with the preparation with

iffraction rings caused by the tetragonal ZrO2 are

the thicker layer (method no. 2) than the layering technique(method no. 1).

To understand the reason for the sharp and increasing(−1 1 1)-reflection of the monoclinic phase especially in theinterface region in Fig. 3 the related GADDS–frames from theHi-Star detector are shown in Fig. 4 for the 6 patterns whichare in the direct neighbourhood of the interface region.

The GADDS frame especially for position 2 shows a verystrong single crystal reflection which is the reason for themonoclinic (−1 1 1) reflection in the corresponding pattern(The arrow indicates this single crystal diffraction spot withinthe corresponding reflection pattern).

Higher magnification X-ray patterns provide a better appre-ciation of the structural phase change with the Wash-Dentincoating and the thicker layer of the porcelain on top of Y-TZP(preparation no. 1 and no. 2). Figs. 5 and 6 present azimuthalprojections from positions at similar distances from the inter-face region with preparation methods 1 and 2 within theveneered areas measured in this case with a 500 �m mono-capillary optic with a 300 �m exit pinhole to analyse a largersurface area for better statistics. Again in both cases similarsingle crystal reflection spots from the monoclinic phase are

detectable in the area under the porcelain independent of thepreparation method.

A clearly identifiable difference in the Y-TZP surface regionprepared in the absence of a liquid medium by the veneered

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 545–552 551

Table 2 – Monoclinic volume percentage from the analysis of Garvie and Nicholson [3].

Preparation Monoclinic V% @ red line position 2 Monoclinic V% @ black line position 5

pat

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Method 1 (wash-firing) 11.5%Method 2 (thick layer) 13%Method 3 (no liquid)

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2008;27(3):448–54.

552 d e n t a l m a t e r i a

achieve a 50 by 200 �m-sized localized beam had the advan-tage to enable phase stability of the near surface layer of thezirconia interface to be evaluated. This specially designed X-ray polycapillary micro-lens system also enabled detection ofthe zirconia through thin layers of the porcelain surface.

This structural t → m change at the interface creates local-ized residual stresses as a consequence of the volume dilationas well as the different coefficient of thermal expansion of theframework material. Such localized residual stresses at theinterface may weaken the porcelain to zirconia adhesion andmay have influenced the shear interfacial strength tests by Fis-cher et al. [24]. However to date there have been no publishedreports of clinical adhesion failure between porcelain and Y-TZP frameworks despite increasing observations of chippinginduced failures within the porcelain on such frameworks [25].

The localized stresses developed between the veneeringand the reliability of the dental restoration in the clinical situa-tion are not completely established and further investigationswith a focus on these topics need to be undertaken.

5. Conclusions

The present observations using the locally resolved �-XRD2-technique have clearly established that the porcelainveneering process, especially with a wet veneer during firing,results in a localized tetragonal to monoclinic structural trans-formation at the surface of the zirconia framework materialduring preparation of these all-ceramic dental restorations.

In the case of moist veneering porcelain a highly texturedor oriented monoclinic crystalline phase was observed at thezirconia/porcelain interface. In the absence of moisture withinthe veneering porcelain no transformation of the Y-TZP tetrag-onal phase was identified.

As a consequence of the findings in this study, it is stronglyrecommended to use a porcelain layering technique that isvery thin and as dry as possible for the initial layering applica-tion to prevent destabilization of the tetragonal crystals of theY-TZP framework at the interface, which will otherwise inducelocal mechanical stress into the overlying porcelain layer andtherefore could decrease the mechanical stability of the finalproduct.

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XRD2 micro-diffraction analysis of the interface between Y-TZP and veneering porcelain: Role of application methodsIntroductionMaterials and methodsResultsDiscussionConclusionsReferences