This article was downloaded by: [Oklahoma St University Tulsa] On: 04 January 2012, At: 15:22 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Phase Transitions Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gpht20 Residual stress delaying phase transformation in Y-TZP bio- restorations Masoud Allahkarami a & Jay C. Hanan a a Mechanical and Aerospace Engineering, Oklahoma State University, Tulsa, OK, USA Available online: 10 Nov 2011 To cite this article: Masoud Allahkarami & Jay C. Hanan (2012): Residual stress delaying phase transformation in Y-TZP bio-restorations, Phase Transitions, 85:1-2, 169-178 To link to this article: http://dx.doi.org/10.1080/01411594.2011.625347 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and- conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
Transcript
This article was downloaded by: [Oklahoma St University Tulsa]On: 04 January 2012, At: 15:22Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Phase TransitionsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/gpht20
Residual stress delaying phasetransformation in Y-TZP bio-restorationsMasoud Allahkarami a & Jay C. Hanan aa Mechanical and Aerospace Engineering, Oklahoma StateUniversity, Tulsa, OK, USA
Available online: 10 Nov 2011
To cite this article: Masoud Allahkarami & Jay C. Hanan (2012): Residual stress delaying phasetransformation in Y-TZP bio-restorations, Phase Transitions, 85:1-2, 169-178
To link to this article: http://dx.doi.org/10.1080/01411594.2011.625347
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representationthat the contents will be complete or accurate or up to date. The accuracy of anyinstructions, formulae, and drug doses should be independently verified with primarysources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand, or costs or damages whatsoever or howsoever caused arising directly orindirectly in connection with or arising out of the use of this material.
Residual stress delaying phase transformation in Y-TZP bio-restorations
Masoud Allahkarami and Jay C. Hanan*
Mechanical and Aerospace Engineering, Oklahoma State University, Tulsa, OK, USA
(Received 3 May 2011; final version received 16 September 2011)
Engineering favorable residual stress for the complex geometry of bi-layerporcelain-zirconia crowns potentially prevents crack initiation and improves themechanical performance and lifetime of the dental restoration. In addition toexternal load, the stress field depends on initial residual stress before loading.Residual stress is the result of factors such as the thermal expansion mismatch oflayers and compliance anisotropy of zirconia grains in the process of sintering andcooling. Stress induced phase transformation in zirconia extensively relaxes theresidual stress and changes the stress state. The objective of this study is toinvestigate the coupling between tetragonal to monoclinic phase transformationsand residual stress. Residual stress, on the surface of the sectioned single load tofailure crown, at 23 points starting from the pure tetragonal and ending at a fullymonoclinic region were measured using the micro X-ray diffraction sin2 method.An important observation is the significant range in measured residual stress froma compressive stress of �400MPa up to tensile stress of 400MPa and up to 100%tetragonal to monoclinic phase transformation.
There is a growing interest for all ceramic restorations as replacement for metal alloys [1].All-ceramic restorations have both the required high strength mechanical properties andappealing esthetics. However, they often fail early and unexpectedly, typically within thefirst 5 years of service [2,3]. Fatigue tests on a modified Y-TZP core, designed with reducedheight of the proximal walls and occlusal surface, reveals improved all-ceramic crownreliability [4]. There are several other design factors that can be changed. Furtheroptimization is achievable through understanding and engineering residual stresses. Forexample, accounting for stresses from phase transformations, base-veneer layerthicknesses [5,6], and the manufacturing process [7].
All ceramic restorations in modern dentistry are mostly made of machined alumina orzirconia cores veneered with porcelain to the required shape before a heat treatmentprocess. Sintering involves firing the two layered structure at a temperature that the veneerfuses to the core. In zirconia-porcelain restorations, sintering the zircoina layer joins grainsstrongly and reduces porosity. This densification occurs by atomic diffusion in the solid
state phase. On the other hand, sintering of porcelain is achieved by formation of a largevolume fraction of liquid phase in which matter transport occurs mainly by viscous flow.The viscous flow remains as an amorphous glass after cooling. Crowns fabricated by thismethod aim for favorable high toughness and crack resistance of zirconia and appealingesthetics and biocompatibility of porcelain. Although the porcelain composition can becustomized to reduce its thermal expansion coefficient mismatch with zirconia [8–10] oreven with the metal bases [11], generally there is a residual stress at the interface of the twolayers [12] and other critical locations of the complex geometry crown. This residual stresscreation is avoidable, but requires some changes to the current practice. Since the thermalexpansion of components are a function of temperature and cooling rates and there aremultiple firing processes involved, including a finite element analysis based method [13–15]and analytical modeling [16,17] are necessary. High energy synchrotron beam radiationhas been implemented to measure residual stress tensors in ziconia core crowns usingpolychromatic Laue micro diffraction [18,19] and biaxial residual stresses were measuredusing the sin2 method with monochromatic micro X-ray diffraction [12]. Laboratorymicro X-ray diffraction is a well established technique and has been utilized widely forindustrial application [20,21]. Stress measurements using X-ray diffraction techniques aregenerally limited to crystalline materials, but they have the advantage of providingsimultaneous precise information on stress and phases. In this article, using this advantageof diffraction, the method and results of biaxial residual stress measurement usinglaboratory micro X-ray diffraction equipped with a two dimensional area detector ispresented.
In this present work, advances in focusing micro X-ray lenses that provide highintensity point size beams down to a few micrometers diameter, and two dimensional (2D)detectors, accurate motorized XYZ axis stages in a variety of configurations, along with alaser video auto z-alignment system were adapted to fulfill the requirement of reasonablemeasurement time and accuracy.
2. Experimental
2.1. Material
A clinically available ceramic crown made of tetragonal zirconia stabilized by the additionof 3mol % yttrium oxide (3Y-TZP) core and porcelain veneer was selected for this study.To create a stress induced phase region, the required failure load (�500N) was applied toone of the crown caps using a 1.9mm tungsten carbide ball indenter with a custom loadframe [22]. At failure, only the porcelain veneer was chipped off without failure of thesupporting zirconia core layer. The impacted crown was sectioned in two halfs by watercooled low speed diamond saw before mounting it onto an aluminum plate with highstrength epoxy glue. Polishing was not implemented to avoid potential damage to thesection.
2.2. Method
To find the best line for residual stress measurements, initially an X-ray micro-diffractionmapping global search was done using the Bruker D8 Discover XRD2 micro-diffractionsystem equipped with a Hi-Star 2D area detector and General area Diffraction DetectionSystem (GADDS). The maximum detector distance (299.5mm) by this system was selectedbecause frames collected at higher detector distance have more strain resolution and here
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peaks were better distinguished although their intensity decreases. At 300mm, the detectorsimultaneously covers the area of 20� in 2� and 20� in with a 0.02� pixel seperation.Two intervals for 2� at each diffraction point (X, Y) were considered, first a frame from18� to 38� for phase analysis and then a frame interval in high 2� from 63� to 83� wasselected for sin2 bi axial stress measurements.
Mapping X-ray diffraction was performed by collecting 186 frames at 2� between 18�
and 38� using Cu-K� radiation and tube parameters of 40 kV/40mA, with a 0.1mmdiameter collimator, in reflection mode. A fixed X-ray beam incident angle (�1¼ 14�), adetector angle of �2¼ 14�, with 120 s/frame exposure times, and 100 mm separation step inboth X and Y translation stage directions was used.
Residual stresses for a selected line were measured by the same system at 23 pointsalong a line scan using a mono capillary with a 50 mm diameter focused micro X-ray beamalong the line which connects two sides of the zirconia layer. Based on collecting frames atvarious times to find the best signal to noise ratio, a 45minute exposure was selected perframe.
2.3. Peak selection for residual stress measurement
Since there is a larger peak shift D(2�) �, and less chance of a displacement error for higher2�, it is appropriate to find a peak at high 2� angles. The selected peak is better to havesufficient intensity and be separated from other peaks. Figure 1(a) shows the typical wholelabeled XRD pattern of stabilized zirconia and a selected high 2� angle (004) ring forresidual stress measurements. Figure 1(b) illustrates that a frame’s background intensitydecreases at high 2� angles, which improves the signal gain.
Figure 1. (a) X-ray diffraction pattern of tetragonal zirconia and selected (004) peak and (b)background intensity decreases vs. 2�.
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Using Bragg’s law for �¼ 1.54178 A and given 2�¼ 73.065� and a 004 ring,d004¼ 1.295 A.
1
d2hkl¼
h2 þ k2
a2þ
l2
c2!
1
d2004¼
02 þ 02
3:606702
þ42
5:180202! d004 ¼ 1:2951 A
The peak shift is expressed in degrees [23],
Dð2�Þ� ¼ �2180
�
1þ �
E�� tan � sin
2ð Þ
) �� ¼Dð2�Þ��
�360 tan � sin2ð Þ
E
1þ �
Dð2�Þ� ¼ �2180
�
1þ v
E�� tan � sin
2ð Þ
) �� ¼Dð2�Þ��
�360 tan � sin2ð Þ
E
1þ v
For a given maximum sensitivity of 0.01�, E¼ 193MPa, �¼ 0.22, minimum �� as afunction of 2� and can be obtained. For a typical 2�¼ 73.065� and ¼ 12� a residualstress with magnitude of �1MPa is measurable.
3. Results and discussion
Since the main objective of this experiment is to investigate the relation between the stressstate and the degree of tetragonal to monoclinic phase transformation in zirconia, findingthe line between two points on the complex cracked geometry of a crown that starts fromtetragonal phase and ends in monoclinic phase is critical. The area suspected to have stressinduced phase transformation was mapped to find the best line for residual stressmeasurement, shown in Figure 2(a). The areas under three monoclinic related peaks werenormalized to the maximum area for all collected frames, illustrated in Figure 2(b). Tovalidate this map, the area under a normalized tetragonal peak is mapped in Figure 2(c).From both maps, the best location with the desired property of monoclinic phase in one
Figure 2. (a) Optical microscope photo of a sectioned failed crown. The box outlines the location ofthe X-ray map, (b) high resolution X-ray diffraction map of the total area under (011,110, and 101)monoclinic peaks, and (c) map of the area under the (101) tetragonal peak.
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side and tetragonal on the other side across the thickness of the zirconia layer is apparent.Without the map, it would be difficult to find such a path, as the tetragonal phase may justpartially transform to monoclinic under the impact load.
Along the line that connects the two sides of the zirconia layer, 23 points were selectedfor a line scan using a 50 mm monocapillary optic (also indicated in Figure 6a).Two intervals for 2� at each point were considered, one frame from 18� to 38� and a framefrom 63� to 83�. Frames at the first interval were collected for phase analysis, because atthis interval both monoclinic and tetragonal peaks are bright and have enough separation.The second interval at high 2� was selected for sin2 stress measurements.
The frame in Figure 3(a) shows only the tetragonal crystal structure while Figure 3(b)shows just the monoclinic structure corresponding to the point 1 and 23, labeled onFigure 6(a). Frames collected in between the pure tetragonal and monoclinic phases, wereintegrated to yield intensity versus 2� diagrams. Diffraction patterns across the selectedline for all measurement points (illustrated in Figure 3c), reveal a gradual phase transition.Similarly, the tetragonal to monoclinic phase transition is confirmed at high 2� frames(shown in Figure 4).
The mapping process was repeated on a sectioned crown without impact. It did notshow monoclinic phase. This suggests that the phase transition discussed above wascreated by impact damage. The external load created a stress field that finally caused thefailure of the veneered layer while the zirconia did not fail but transformed. The gradient
Figure 3. (a) Tetragonal 2-D diffraction image (2�¼ 18� to 38�). (b) Monoclinic 2-D diffractionimage. (c) Integrated rings giving intensity versus 2� diagrams, starting at the zirconia porcelaininterface and proceeding across the zirconia layer.
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of phase transformation from zero to 100 percent that was observed is understood if we
consider the role of residual stress.In order to measure residual stress at each point, four frames with different sample tilt
value were collected to cover 60� of angle. Using a sequential list, the system
automatically located each predetermined point to the instrument center of diffraction
using a motorized five axis (X, Y, Z, (tilt), � (rotation)) sample stage. Final height
adjustments in the Z-direction were made by an auto video-laser system before each data
collection. Figure 5(a), illustrates a typical frame with a segmented region that involves
(004) and (220) rings. The integrated intensity for each segment was fitted by two Pearson
VII functions using a Matlab program.The (004) tetragonal zirconia peak at 73.065� was selected for stress calculation using
the sin2 method. The range was segmented into smaller integration segments of a 1� range. The residual stress for every scan point was evaluated from sin2 versus curves
using the following equation [23],
�� ¼Slope
d ¼ 0
E
1þ �
Based on the slope and the intercept d ¼0 the residual stresses were determined. It is
convenient to use d ¼0 as stress free reference value, which typically leads to an error of
Figure 4. (a) Tetragonal 2-D diffraction image (2�¼ 63� to 83�). (b) Monoclinic 2-D diffractionimage. (c) Intensity versus 2� diagrams across the thickness, from integrated rings.
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only 0.1% [23]. Similarly, residual stress for the monoclinic phase was calculated using the(231) peak.
Figure 6(b) shows combined residual stress measurement results with tetragonal tomonoclinic phase transition ratio in percent.
Initially the tetragonal phase starts at �400MPa, compressive stresses (point 1).Considering both the phase transformation ratio with measured residual stress results,reveals that the transformation ratio increases along with gradually changing residualstress from compressive to tensile residual stress (point 7). The maximum measuredresidual stress was around þ400MPa, and after this point, extensive phase transformation(up to 100% for the final point) along with residual stress relaxation down to an average ofzero was observed. This mechanism of phase transformation resists crack growth in thezirconia layer.
The dental crown zirconia-porcelain bi-layer has complex geometry and only some partof the porcelain layer was chipped off by impact load. Even if somehow all porcelain layerswere removed, still some residual stress might remain, because residual stress has beencreated between the grains in the polycrystalline microstructure of zirconia during coolingfrom high temperature. Removing the layers may release the macro scale residual stressbut still grain-to-grain residual stress will remain in the poly crystalline microstructure ofzirconia. The X-ray diffraction method traditionally measures the average of many grains’residual stress.
The observed residual stress profile is likely a remnant from the original stress profile.An initial compressive residual stress state would have competed with an applied tensile
Figure 5. (a) Typical segmented diffraction frame. (b) Integrated intensity profile, fitted by twoPearson VII functions for a 1� highlighted segment.
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stress state from the indentation load. This competition can delay or prevent transfor-mation. Where there was less compressive residual stress, more tetragonal zirconiatransformed. This observation needs confirmation by observing initial stress states beforeindentation.
4. Conclusion
The overall state of residual stress is an important factor in failure and lifetime of ceramicdental restorations. Novel crowns could be designed with an understanding of residualstress critical role. A method of combined phase transformation and residual stressmeasurement were adapted for dental crown studies. Stabilized tetragonal zirconia,
Figure 6. (a) 23 exposure points, starting at the interface between zirconia and porcelain (point 1),proceeding across the zirconia layer away from the interface (point 23). (b) Combined phasetransition and residual stress along the line scan.
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currently a dominant material for dental restoration, is meta-stable and transforms tomonoclinic under critical stress. Current results couple phase transformation and residual
stress measurements. This is an important factor in design, because this transformationunder critical load retards failure. A significant range of residual stress from a compressive
stress of �400MPa up to tensile stress of 400MPa and up to 100% tetragonal tomonoclinic phase transformation were observed for an impacted crown under a single loadto failure. Knowledge on phase transformation coupled with residual stress states, layer
thickness, and the manufacturing process opens up the keys to designing longer life dentalrestorations.
Acknowledgments
We gratefully acknowledge the assistance of researchers at NYU including Paulo Coelho and NelsonDa Silva. Partial research support was provided by the Oklahoma Health Research award (projectnumber HR07-134), from the Oklahoma Center for the Advancement of Science and Technology(OCAST).
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