Bond strength and Raman analysis of the zirconia-feldspathic porcelain interface Carla Müller Ramos, DDS, MS,a Paulo Francisco Cesar, DDS, MS, PhD,b Rafael Francisco Lia Mondelli, DDS, MS, PhD,c Americo Sheitiro Tabata, MS, PhD,d Juliete de Souza Santos, DDS,e and Ana Flávia Sanches Borges, DDS, MS, PhDf Bauru School of Dentistry, Faculty of Science, University of São Paulo (USP), Bauru, SP, Brazil Statement of problem. Zirconia has the best mechanical properties of the available ceramic systems. However, the stability of the zirconiaefeldspathic porcelain interface may be jeopardized by the presence of the chipping and debonding of the feldspathic porcelain. Purpose. The purpose of this study is to evaluate the shear bond strength of 3 cold isostatic pressed zirconia materials and a feldspathic veneer by analyzing their interface with microeRaman spectroscopy. Material and methods. The test groups were experimental zirconia, Zirkonzahn zirconia, and Schuetz zirconia. Blocks of partially sintered zirconia were cut into disks (n¼20) and then veneered with a feldspathic porcelain. Half of the specimens from each group (n¼10) were incubated in 37 C water for 24 hours, and the other half were thermocycled. All the specimens were then subjected to shear testing. The fractured areas were analyzed with optical stereomicroscopy and classified as adhesive, cohesive, or an adhesive-cohesive failure. Spectral patterns were examined to detect bands related to the zirconia and feldspathic porcelain phases. The shear strength data were submitted to 2-way ANOVA. Results. No significant differences in shear bond strength were observed among the 3 groups, regardless of whether or not the specimens were thermocycled. Adhesive failures were the most prevalent types of failure (70%). Raman spectra were clearly distinguished for all the materials, which showed the presence of tetragonal and monoclinic phases. Conclusions. The controlled production of the experimental zirconia did not influence the results of the bond strength. Raman analysis suggested a process of interdiffusion by the presence of peaks associated with the zirconia and feldspathic ceramics. (J Prosthet Dent 2014;-:---)
Clinical Implications The success of the zirconia-feldspathic veneer combination could be limited by chipping or debonding of the feldspathic porcelain. An evaluation of the shear bond strength between a new zirconia and a feldspathic porcelain, and an analysis of the interface with Raman spectroscopy can be used to improve the understanding of the zirconiaefeldspathic ceramic relationship.
Supported by FAPESP 2011/18061-0. a
Doctoral student, Department of Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry. Professor, Department of Biomaterials and Oral Biology, Bauru School of Dentistry. c Professor, Department of Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry. d Professor, Department of Physics, Faculty of Science, São Paulo State University. e Graduate student, Bauru School of Dentistry. f Professor, Department of Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry. b
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Volume Yttria-stabilized tetragonal zirconia has been increasingly used in dentistry to replace metal substructures because of its strength,1-3 fracture toughness,4,5 chemical stability, and biocompatibility.6-8 Feldspathic porcelain veneers are applied to zirconia, not only for esthetic reasons but also because they are stable in combination with yttriastabilized tetragonal zirconia9-11 and are brittle and of limited tensile strength when used alone. However, clinical failures, such as the chipping and debonding of the feldspathic veneer, have been reported as one of the most common concerns in this type of restorative combination of materials.12 The mechanical strength provided by the zirconiaefeldspathic veneer combination may be limited by the fragile feldspathic component with its relatively low fracture toughness.13-16 Feldspathic ceramic chipping and fracture are common clinical problems.17 The rates of clinical failure of the zirconiae feldspathic veneer range from 13% to 15% over a period of between 2 and 5 years.9,10,18 When failures occur, they may be either cohesive within the feldspathic layer (chipping) or adhesive at the zirconia-veneer interface, which results in the delamination of the veneer layer.19,20 According to Komine et al,21 fractures in the feldspathic veneer usually progress from the area subjected to higher tensile stress toward the interface. When this phenomenon occurs, the stresses in the feldspathic layer may or may not reach the interface with the zirconia substructure.22 Cohesive failures are associated with the use of liners or bonding agents, the presence of pores within the feldspathic porcelain layer, or the location of the restoration inside the oral cavity.23-25 Moreover, adhesive failures between the feldspathic veneer and the zirconia substructure may be explained, in part, by the high contact angle between these 2 materials,26 differences in coefficients of thermal expansion (CTE),2,27 poor thermal conductivity associated with the poor thermal diffusivity,28 or the surface state of the zirconia substructure.21
Delamination failures are more frequent19 and may be associated with the residual stress29,30 promoted by temperature changes at the substructureveneer interface.24,31 The strength of the bond at the substructure-veneer interface32,33 and the resistance of the bonding layer itself may be affected by extensive thermal residual stress,34 a problem that may then develop during the cooling phase after sintering.22,35,36 Moreover, the residual stresses may be generated by differences in the CTE between the 2 materials.22 The bond strength of feldspathic-veneering ceramic and the zirconia interface has not been extensively studied.37 The aim of this study was to evaluate the shear bond strength of 3 cold isostatic pressed zirconia materials and the feldspathic veneer by analyzing their interface with microe Raman spectroscopy.38-40 The null hypotheses were that no difference would be found in the shear bond strength of 3 zirconia materials and that thermocycling would not influence zirconiae feldspathic ceramic bond strength.
MATERIAL AND METHODS Experimental zirconia (EP) blocks were obtained with isostatic pressing by
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applying pressure uniformly in all directions. A flexible elastomeric membrane was used as a matrix. The elastomeric matrix cavity was filled with EP powder (see Table I for composition) and sealed with a metal plate. The outer surfaces of the matrix were pressed by a fluid that transmits pressure toward the matrix, which resulted in powder compaction (Fig. 1). The pressure used for each block was 120 MPa for 5 seconds at 25 C and at 60% humidity. Blocks of partially sintered zirconia from the 3 groups (dimensions 15.5 19 39 mm) were milled in a pantograph system (Zirkonzahn GmbH) to form disks (11 mm in diameter and 2.7 mm in thickness). After sintering at 1530 C for 5 hours and 52 minutes, as recommended by the manufacturers, all the disks were polished with silicon carbide and airborne-particle abraded with 50 mm aluminum oxide at a standard distance of 10 mm.15 The specimens were cleaned by immersing them in acetone in an ultrasound machine (Ultrasonic Cleaner; Unique) for 10 minutes. A thin layer of opaque ceramic dentin (Opaker; Zirkonzahn GmbH) was applied onto all the zirconia disks, followed by firing (EDG
Materials used in study, Composition and Coefficient of thermal expansion (CTE)
Table I.
Material
Code Manufacturer
Composition
CTE*
Zirconia Experimental
EP
Marília
Zr(hf)O2, 94.7%; Y2O3, 5.25%; SiO2, 0.020%; Fe2O3, 0.010%; Na2O, 0.010%; CL, 0.020%; TiO2, 0.010%
10
Zirkonzahn
ZK
Zirkonzahn GmbH
ZrO2, 91.5%; HfO2, 3.0%; Y2O3, 5%; SiO2, 0.020%; Fe2O3, 0.0050%; Na2O, 0.01%
10.5
Shuetz
SH
SchuetzDental GmbH
Zr (O2), 1%; Y2O3, >4%; Al2O3 97%
9.7
PO
Zirkonzahn GmbH
SiO2, Al2O3, P2O5, K2O, Na2O, CaO, F, TiO2 > 97%
9.7
Feldspathic ceramic Ceramic Dentin A3 Ceramic Dentin Opaker
*Coefficient of thermal expansion (CTE) in 10-6K-1 between 25 C and 500 C.
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1 Isostatic pressing. (A) Matrix cavity was filled with experimental zirconia (EP). (B) Isostatic pressure in all directions. (C) Compacted powder. (D) EP block. Equipamentos) at 900 C for 30 minutes. A custom-designed metallic device (5 mm in diameter and 5 mm in height) was constructed for the application of the feldspathic-veneer ceramic (Ceramic Dentin A3; Zirkonzahn GmbH) (Fig. 2). The feldspathic porcelain was mixed with modeling liquid (ICE; Zirkonzahn GmbH), placed in the device, and manually condensed with a Ward condenser number 4 (SSWhite; Duflex). The device was removed, and the specimens with the feldspathic veneer were sintered at 820 C for 30 minutes, according to the manufacturer’s instructions. After sintering, the specimens were embedded with acrylic resin (JET; Classico) in a polyvinyl chloride (PVC) cylinder of 10 mm in diameter. For the shear bond strength test, the specimen-size calculation was
performed based on a pilot study when considering an a of .05 and a power of .80. The calculations revealed that 10 specimens per group would be needed to detect the postulated effect size. Therefore, 10 specimens from each group (n¼10) were stored for 24 hours in water at 37 C, and 10 were thermocycled (500 cycles; 30 seconds at 5 C and 30 seconds at 55 C, with a 10second dwell time) for approximately 10 hours.32 Thereafter, all specimens were subjected to a shear test41-43 with a universal testing machine (EMIC DL500; EMIC Test Equipment and Systems Solutions) with a load cell of 50 Kgf and a special mechanical testing device (Fig. 3) with metallic ribbon at a speed of 0.5 mm/min until fracture. This device was adapted from that developed by Sinhoreti et al44 to minimize bending stress. A mechanical
2 A, Custom-designed metal device. B, Feldspathic porcelain on zirconia disk.
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support was placed opposite the specimen to avoid any movement at the zirconiaefeldspathic porcelain interface. The analysis of the zirconiae feldspathic porcelain interface was performed for all specimens with optical microscopy (Discovery V8 Stereo; Carl Zeiss Microimaging GmbH) at 32 magnification. Each specimen was classified according to the type of failure observed as follows: adhesive failure at the zirconia and feldspathic porcelain interface, cohesive failure of the feldspathic porcelain veneer, and when both types of failure were present. Specimens from each group that showed a remaining feldspathic veneer were cut perpendicularly, and the interface was examined with microRaman spectroscopy. The microRaman measurements were carried out at room temperature in backscattering geometry by means of a JobinYvon micro-Raman system (model T64000; Horiba Group) at the Physics Institute of the State University of São Paulo (USP). The 514.5 nm (2.41 eV) radiation of an argon ion laser (Spectra Physic Inc) was used for excitation. The beam was focused with a microscope at 500 magnification with a laser spot of approximately 5 mm in diameter. No polarization analyzer was used for the scattered beam. To avoid any thermal damage, the laser power was kept as low as 10 mW. The Raman spectra were analyzed by means of a double subtractive monochromator, with a focal length of 0.64 m and equipped
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with a diffraction grating with 1800 grooves/mm. The slit width was set to 200 mm, which provided a spectral resolution of approximately 2 cm1. A line scan was recorded on the sectioned specimen that crossed from the zirconia to the feldspathic ceramic with a charge-coupled device camera (Spectra One; Horiba Group), which yielded 5 spectra from each group with 8 mm of distance between them. Shear bond strength data were analyzed by 2-way ANOVA followed by the pairwise multiple comparison Tukey test (a¼.05). The failure types were classified with optical microscopy and calculated as a percentage for each group. Differences in the wavenumbers and the broadening bands of the Raman spectra were compared among groups.
3 Schematic illustration of device to measure shear bond strength. 22.00 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00
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EP-24 ZK-24 SH-24 EP-CT ZK-CT SH-CT 20.18 ±1.82 17.75 ±1.52 18.81 ±1.82 17.75 ±1.52 15.33 ±1.52 18.81 ±1.60 MPa
SD
4 Shear bond strength between feldspathic porcelain and ZrO2. EP, experimental zirconia; ZK, Zirkonzahn zirconia; SH, Shuetz zirconia after storage for 24 hours in water (-24); after thermocycling (-CT). Columns show mean values of 10 measurements (MPa) standard deviation (SD). 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%
EP-24
ZK-24
SH-24
EP-CT
ZK-CT
SH-CT
20%
10%
10%
30%
10%
10%
Cohesive 10%
0%
10%
10%
30%
20%
Adhesive 70%
90%
80%
60%
60%
70%
Mixed
5 Percentage of failure types. EP, experimental zirconia; ZK, Zirkonzahn zirconia; SH, Schuetz zirconia after storage for 24 hours in water (-24) and after thermocycling (-CT).
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The results of the shear bond strength tests for the specimens after 24 hours of storage in water and after thermocycling are shown in Figure 4. According to 2-way ANOVA, the interaction between the material and thermocycling was not significant. The shear bond strength values obtained for the EP were not significantly different from the Zirkonzahn zirconia (ZK) and Shuetz zirconia, either before or after thermocycling. On average (SD), the EP showed a shear bond strength of 20.18 1.82 MPa compared with 17.75 1.52 MPa for the ZK and 18.81 1.82 MPa for the Shuetz zirconia after storage in water for 24 hours. After thermocycling for 24 hours, the EP showed 17.75 1.52 MPa compared with 15.33 1.52 MPa for the ZK and 18.81 1.60 MPa for the Shuetz zirconia. The effect of thermocycling on the shear bond strength was not statistically significant for any group. The most common type of failure was adhesive (Fig. 5). Representative images of the failure types are shown in Figure 6. The peaks used in this study corresponded to the main bands related to the zirconia and feldspathic porcelain components.3 The wavelengths of bands of 5 spectra from each group were
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6 Failure types by optical microscopy (32 magnification). A, Adhesive. B, Cohesive. C, Mixed. 80
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EP Zirconia
Raman signal (counts/s)
analyzed. In all images, the 2 upper spectra are related to the zirconia, the middle spectrum (red) was recorded from the exact visible transition interface, and the 2 lower spectra are related to the feldspathic porcelain. In the EP group, strong peaks were found at approximately 260, 320, 464, and 642 cm1 (Fig. 7). In the ZK group, strong peaks were found at approximately 260, 320, 465, and 640 cm-1 (Fig. 8). In the Shuetz zirconia group, strong peaks also were found at approximately 260, 320, 465, and 640 cm1 (Fig. 9). Additional weak features were observed for all zirconia groups at approximately 145 cm1 (Figs. 7-9). The feldspathic porcelain showed strong peaks at approximately 256, 320, 460, and 640 cm1 (Figs. 7-9). The spectra of each group were composed of 924 data points in a spectral region between approximately 100 and 700 cm1. The mean spectra of the groups had the same baseline; however, they were dissociated to
60
40
20
100
200
300
400
500
600
700
–1
Raman shifts (cm )
7 Experimental zirconia (EP) strong peaks. improve the visualization of each spectrum (Figs. 7-9).
DISCUSSION The 2 null hypotheses were accepted. No differences in shear bond strength
were found among the groups, regardless of thermocycling. The bond strength between the feldspathic porcelain and the zirconia substructure can determine the longevity of dental restorations in the oral environment.13,14 According to Yamamoto,45 an isolated analysis of
6
Volume ZK Zirconia
Raman intensity (counts/s)
120 100 80 60 40 20 100
200
300
400
500
600
700
–1
Raman shift (cm )
8 Zirkonzahn zirconia (ZK) strong peaks. SH Zirconia
Raman signal (counts/s)
90
60
30
0 100
200
300
400
Raman shifts
500
600
700
(cm–1)
9 Schuetz zirconia (SH) strong peaks.
each ceramic is critical to predict the success of the bilayer. Therefore, a bilayer that involves 2 ceramics is more clinically realistic and behaves differently from each isolated ceramic. The bond strength between the feldspathic ceramic and zirconia can be influenced by certain factors, such as the feldspathic porcelain sintering, the zirconia surface treatment, and thermal variations, which in the present study were identical and, therefore, did not influence the results. The feldspathic porcelain application was carried out carefully, with the same condensation force by a specific device. So, all specimens showed standardized volume
and shape of the feldspathic porcelain. A thin layer of the liner applied onto the feldspathic porcelain surface (Ceramik Opaker) at a temperature of 920 C is recommended by the manufacturer and is thought to increase the molecular interaction between the 2 materials. Aboushelib et al24 showed that the liner application significantly improves the bond strength of the zirconia coree feldspathic porcelain veneer and reduces interfacial failures.11,14 The zirconia surface treatment recommended by the manufacturer was followed by airborne-particle abrasion with 50 mm aluminum oxide. This procedure has been controversial. The surface
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treatment associated with liner reduce chances of delamination of the feldspathic ceramic as shown x-ray diffraction experiments.24 The airborne-particle abrasion with 125 mm aluminum oxide recommended by the manufacturer before applying the veneering ceramic changed the structure of the tetragonal zirconia to monoclinic (9.5% and a depth of 27 mm); the feldspathic porcelain application reversed this phase transformation.27 Despite the controversy that involves zirconia surface treatment with airborne-particle abrasion before applying veneering ceramic, it may increase the wetting of the zirconia surface by increasing the free surface energy, which is measured by the contact angle between the liquid (feldspathic ceramic) and the surface to be bonded (zirconia).26 Polycrystalline ceramics under wet and cyclic loading conditions are most susceptible to subcritical crack growth.8,11,46 The current study showed that the EP yielded bond strength results comparable with other zirconia products tested after thermocycling in another study.37 With respect to the feldspathic porcelain, its bond strength is related to its crystal structure20 and is influenced by factors such as the CTE, thickness,18 and cooling rate.35,36 The maximum stresses developed are a combination of different stresses induced by CTE and tempering residual stress in both ceramics.22 The poor thermal conductivity associated with the poor thermal diffusivity of the ceramics results in a difference of temperature in the zirconiaefeldsphatic porcelain interface.28 The importance of the CTE of ceramics has been widely discussed in the literature27 because the difference in the values between 2 materials with different compositions, such as feldspathic ceramic and zirconia, can lead to residual stresses and decrease the bond strength between them. Manufacturers should provide similar CTEs for ceramics to be used in multilayers. In this study, zirconia and feldspathic ceramics had values considered similar to each other, but the small difference
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between them may also have been responsible for the stress generated at the interface. When the feldspathic porcelain is subjected to high temperatures, a phase change occurs at the structural level, counteracted by the tensile stress of the crystalline transformation of zirconia, which leads to an increased volume expansion that, even if as low as 4% in volume, could secondarily develop tensile stress in the feldspathic porcelain.29,30 Altogether, this can cause dimensional differences that influence the bond between the feldspathic porcelain and zirconia.32,34 Bond strength in terms of nominal stress values can be questionable because of the heterogeneous stress distribution and also the occurrence of cohesive failures.43 Results of the present study show that the failures were mostly of the adhesive type for all groups, which indicate that the interface between the feldspathic porcelain and zirconia is often compromised. Slow cooling during the final veneering of dental restorations with zirconia frameworks reduces the temperature gradients and residual stresses within the porcelain layer, one possible cause for chipping.22 Shear and microtensile bond strength tests are routinely used to measure the bond strength between feldspathic porcelain and zirconia.15,41,42 The choice of a shear test with stainless-steel tape is based on a study by Sinhoreti et al44 that showed that stainless-steel tape provided sliding between 2 tested surfaces. The use of stainless-steel tape produced smaller tensile and compression forces on the interface than those obtained from other tests when using a chisel and orthodontic wires. The use of the chisel indicated by International Organization for Standardization (ISO) TR 1140547 promoted a cleavage stress that was initially concentrated on the subsuperficial layer of the covered material. The tests that used orthodontic wire caused a flexion stress and consequently tensile and compressive stresses on the axial loading regions, which are concentrated diametrically in opposed
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7 and perpendicular directions to the interface evaluated. The support adapted to the upper face of the stainless-steel tape to minimize any possible bending stresses and cleavage (Fig. 3). This could cause higher adhesive failures because the tension distribution is more homogeneous along the interface during the shear test.48 In the present study, the failures were mostly adhesive for all groups (Fig. 5), which is in agreement with the expected results for this test.44 The characterization of the interface was performed with microeRaman spectroscopy and the results are based on spectra analyses. The bands are a set of wavenumbers that contain a main wavenumber related to the peak of the band. The current study identified 5 wavenumbers of the peaks (approximately 260, 320, 464, 642, and 147 cm1) (Figs. 7-9), similar to the frequencies of the vibration that corresponds to the 6 Raman active bands of the tetragonal zirconia.3 According to some studies,3,38,49 the phase transformations can be identified because the 3 main phases (monoclinic, tetragonal, and cubic) exhibit changes in the zirconium oxide bond angles and lengths.3,4,38,49 The main broad bands shown in the present study were at approximately 260, 464, and 642 cm1 (Figs. 7-9). They are characteristic of the tetragonal or cubic phases.3 Different chemical stabilizers4,5 and crystal sizes5 probably caused few spectral variations at the tetragonalcubic phases of this study (approximately 260, 464, and 642 cm1) (Figs. 6-8). The feldspathic porcelain spectra shown in the present study are between approximately 100 and 700 cm1, similar to the frequencies of vibration that correspond to the feldspathic porcelain between approximately 300 and 700 cm1 previously described.3 In the feldspathic porcelain, the presence of more symmetrical peaks was observed, which suggests localized vibrations from a well-ordered crystalline structure, which can be explained by the composition of silicon oxide (SiO2).39 The
strong peaks were found at approximately 256, 320, 460, and 640 cm1, and similar peaks were found in a study of glass-based SiO2.40 The 580 cm1 vibration corresponds to either SieO stretching or to Si-O-Si bending.39 The aluminum and potassium chemical elements embedded in the matrix glass (SiO2) are responsible for changing the properties of the crystalline grid,50 with stronger and more defined peaks. The interface analysis illustrated by the red line (Figs. 7-9) shows the transition among the different materials with characteristic bands. Analysis of this line suggests a process of interdiffusion by the presence of peaks associated with zirconia and feldspathic ceramics. Some chemical elements such as silicon, aluminum, sodium, and potassium can diffuse the zirconium dioxide layer, gradually decreasing to a 8- to 10-mm depth.24 The presence of defects in the crystalline solid can facilitate the movement of atoms, which characterizes the interdiffusion.3 Although the bond mechanism between the zirconia and feldspathic porcelain is still unknown,47 one hypothesis is that the interaction begins by a compressive stress during feldspathic porcelain firing.2 The different CTE of the 2 materials induces interior compressive stresses near the substructure and the periphery of the zirconia grains, which could be the main site of the feldspathic porcelain interaction.3
CONCLUSIONS Based on the results and limitations of this study, the controlled production of EP did not influence the results of the bond strength. Raman analysis suggests a process of interdiffusion by the presence of peaks associated with zirconia and feldspathic ceramics.
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18. Swain MV. Unstable cracking (chipping) of veneering feldspathic ceramic on all-ceramic dental crowns and fixed partial dentures. Acta Biomater 2009;5:1668-77. 19. Aboushelib MN, Feilzer AJ, Kleverlaan CJ. Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach. Dent Mater 2009;25:383-91. 20. Choi BK, Han JS, Yang JH, Lee JB, Kim SH. Shear bond strength of veneering porcelain to zirconia and metal cores. J Adv Prosthodont 2009;1:129-35. 21. Komine F, Saito A, Kobayashi K, Koizuka M, Koizumi H, Matsumura H. Effect of cooling rate on shear bond strength of veneering porcelain to a zirconia ceramic material. J Oral Sci 2010;52:647-52. 22. Tholey MJ, Swain MV, Thiel N. Thermal gradients and residual stresses in veneered YTZP frameworks. Dent Mater 2011;27: 1102-10. 23. Al-Dohan HM, Yaman P, Dennison JB, Razzoog ME, Lang BR. Shear strength of core-veneer interface in bi-layered ceramics. J Prosthet Dent 2004;91:349-55. 24. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Part II: zirconia veneering ceramics. Dent Mater 2006;22:857-63. 25. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Evaluation of a high fracture toughness composite ceramic for dental applications. J Prosthodont 2008;17:538-44. 26. Baier RE. Principles of adhesion. Oper Dent 1992;5:1-9. 27. de Kler M, de Jager N, Meegdes M, van der Zel JM. Influence of thermal expansion mismatch and fatigue loading on phase changes in porcelain veneered Y-TZP zirconia discs. J Oral Rehabil 2007;34:841-7. 28. Bonfante EA, Rafferty B, Zavanelli RA, Silva NR, Rekow ED, Thompson VP, et al. Thermal/mechanical simulation and laboratory fatigue testing of an alternative yttria tetragonal zirconia polycrystal core-veneer all-ceramic layered crown design. Eur J Oral Sci 2010;118:202-9. 29. Mainjot AK, Schajer GS, Vanheusden AJ, Sadoun MJ. Influence of veneer thickness on residual stress profile in veneering ceramic: measurement by hole-drilling. Dent Mater 2012;28:160-7. 30. Mainjot AK, Douillard T, Gremillard L, Sadoun MJ, Chevalier J. 3D-characterization of the veneer-zirconia interface using FIB nano-tomography. Dent Mater 2013;29: 157-65. 31. Fischer J, Stawarzcyk B, Trottmann A, Hammerle CH. Impact of thermal misfit on shear strength of veneering ceramic/zirconia composites. Dent Mater 2009;25:419-23. 32. Mackert JR Jr, Butts MB, Fairhurst CW. The effect of the leucite transformation on dental porcelain expansion. Dent Mater 1986;2: 32-6. 33. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Effect of zirconia type on its bond strength with different veneer ceramics. J Prosthodont 2008;17:401-8.
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34. Isgro G, Wang H, Kleverlaan CJ, Feilzer AJ. The effects of thermal mismatch and fabrication procedures on the deflection of layered all-ceramic discs. Dent Mater 2005;21:649-55. 35. Gostemeyer G, Jendras M, Dittmer MP, Bach FW, Stiesch M, Kohorst P. Influence of cooling rate on zirconia/veneer interfacial adhesion. Acta Biomater 2010;6:4532-8. 36. Mainjot AK, Schajer GS, Vanheusden AJ, Sadoun MJ. Influence of cooling rate on residual stress profile in veneering ceramic: measurement by hole-drilling. Dent Mater 2011;27:906-14. 37. Guess PC, Kulis A, Witkowski S, Wolkewitz M, Zhang Y, Strub JR. Shear bond strengths between different zirconia cores and veneering ceramics and their susceptibility to thermocycling. Dent Mater 2008;24: 1556-67. 38. Ishigame M, Sakurai T. Temperature dependence of the Raman spectra of ZrO2. J Am Ceram Soc 1977;60:367-9. 39. Jestel NL, Morris MD, O’Brien WJ. Depthresolved Raman microprobe examination of a commercial dental porcelain exposed to a simulated oral environment. Dent Mater 1998;14:375-82. 40. Kailer A, Nickel KG, Gogotsi YG. Raman microspectroscopy of nanocrystalline and amorphous phases in hardness indentations. J Raman Spectrosc 1999;30:939-46. 41. Dundar M, Ozcan M, Gokce B, Comlekoglu E, Leite F, Valandro LF. Comparison of two bond strength testing methodologies for bilayered all-ceramics. Dent Mater 2007;23:630-6. 42. Pittayachawan P, McDonald A, Petrie A, Knowles JC. The biaxial flexural strength and fatigue property of Lava Y-TZP dental ceramic. Dent Mater 2007;23: 1018-29. 43. Braga RR, Meira JB, Boaro LC, Xavier TA. Adhesion to tooth structure: a critical review of ‘macro’ test methods. Dent Mater 2010;26:38-49. 44. Sinhoreti MA, Consani S, De Goes MF, Sobrinho LC, Knowles JC. Influence of loading types on the shear strength of the dentin-resin interface bonding. J Mater Sci Mater Med 2001;12:39-44. 45. Yamamoto M. Metal-ceramic. Principles and methods of Makoto Yamamoto. Chicago: Quintessence; 1985. p. 15-202. 46. Salazar Marocho SM, Studart AR, Bottino MA, Bona AD. Mechanical strength and subcritical crack growth under wet cyclic loading of glass-infiltrated dental ceramics. Dent Mater 2010;26:483-90. 47. International Organization for Standardization. ISO TR 11405. Dental materials: guidance on testing of adhesion to tooth structure 1994. Available at: http://www.iso.org/iso/ home/store/catalogue_ics/catalogue_detail_ ics.htm?csnumber¼19347&ICS1¼11&ICS2¼ 060&ICS3¼10. Accessed March 19, 2014. 48. Anusavice KJ, Dehoff PH, Fairhurst CW. Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. J Dent Res 1980;59:608-13.
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49. Bouvier P, Gupta HC, Lucazeau G. Zone center phonon frequencies in tetragonal zirconia: lattice dynamical study and new assignment proposition. J Phys Chem Solids 2001;62:873-9. 50. Music S, Zivko-Babic J, Mehulic K, Ristic M, Popovic S, Furic K. Microstructure of Lucite glasseceramics for dental use. Mater Lett 1996;26:195-9.
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9 Corresponding author: Dr Ana Flávia Sanches Borges Bauru School of Dentistry, University of São Paulo Al. Octávio Pinheiro Brisola, 9-75 Vila Universitária 17012-901, Bauru, SP BRAZIL E-mail: [email protected]
Acknowledgments The authors thank Renato Murback, Heitor Marques Honório, and Adolfo Coelho for their collaboration on this study. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications The success of the zirconia-feldspathic veneer combination could be limited by chipping or debonding of the feldspathic porcelain. An evaluation of the shear bond strength between a new zirconia and a feldspathic porcelain, and an analysis of the interface with Raman spectroscopy can be used to improve the understanding of the zirconiaefeldspathic ceramic relationship.
Supported by FAPESP 2011/18061-0. a
Doctoral student, Department of Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry. Professor, Department of Biomaterials and Oral Biology, Bauru School of Dentistry. c Professor, Department of Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry. d Professor, Department of Physics, Faculty of Science, São Paulo State University. e Graduate student, Bauru School of Dentistry. f Professor, Department of Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry. b
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Volume Yttria-stabilized tetragonal zirconia has been increasingly used in dentistry to replace metal substructures because of its strength,1-3 fracture toughness,4,5 chemical stability, and biocompatibility.6-8 Feldspathic porcelain veneers are applied to zirconia, not only for esthetic reasons but also because they are stable in combination with yttriastabilized tetragonal zirconia9-11 and are brittle and of limited tensile strength when used alone. However, clinical failures, such as the chipping and debonding of the feldspathic veneer, have been reported as one of the most common concerns in this type of restorative combination of materials.12 The mechanical strength provided by the zirconiaefeldspathic veneer combination may be limited by the fragile feldspathic component with its relatively low fracture toughness.13-16 Feldspathic ceramic chipping and fracture are common clinical problems.17 The rates of clinical failure of the zirconiae feldspathic veneer range from 13% to 15% over a period of between 2 and 5 years.9,10,18 When failures occur, they may be either cohesive within the feldspathic layer (chipping) or adhesive at the zirconia-veneer interface, which results in the delamination of the veneer layer.19,20 According to Komine et al,21 fractures in the feldspathic veneer usually progress from the area subjected to higher tensile stress toward the interface. When this phenomenon occurs, the stresses in the feldspathic layer may or may not reach the interface with the zirconia substructure.22 Cohesive failures are associated with the use of liners or bonding agents, the presence of pores within the feldspathic porcelain layer, or the location of the restoration inside the oral cavity.23-25 Moreover, adhesive failures between the feldspathic veneer and the zirconia substructure may be explained, in part, by the high contact angle between these 2 materials,26 differences in coefficients of thermal expansion (CTE),2,27 poor thermal conductivity associated with the poor thermal diffusivity,28 or the surface state of the zirconia substructure.21
Delamination failures are more frequent19 and may be associated with the residual stress29,30 promoted by temperature changes at the substructureveneer interface.24,31 The strength of the bond at the substructure-veneer interface32,33 and the resistance of the bonding layer itself may be affected by extensive thermal residual stress,34 a problem that may then develop during the cooling phase after sintering.22,35,36 Moreover, the residual stresses may be generated by differences in the CTE between the 2 materials.22 The bond strength of feldspathic-veneering ceramic and the zirconia interface has not been extensively studied.37 The aim of this study was to evaluate the shear bond strength of 3 cold isostatic pressed zirconia materials and the feldspathic veneer by analyzing their interface with microe Raman spectroscopy.38-40 The null hypotheses were that no difference would be found in the shear bond strength of 3 zirconia materials and that thermocycling would not influence zirconiae feldspathic ceramic bond strength.
MATERIAL AND METHODS Experimental zirconia (EP) blocks were obtained with isostatic pressing by
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applying pressure uniformly in all directions. A flexible elastomeric membrane was used as a matrix. The elastomeric matrix cavity was filled with EP powder (see Table I for composition) and sealed with a metal plate. The outer surfaces of the matrix were pressed by a fluid that transmits pressure toward the matrix, which resulted in powder compaction (Fig. 1). The pressure used for each block was 120 MPa for 5 seconds at 25 C and at 60% humidity. Blocks of partially sintered zirconia from the 3 groups (dimensions 15.5 19 39 mm) were milled in a pantograph system (Zirkonzahn GmbH) to form disks (11 mm in diameter and 2.7 mm in thickness). After sintering at 1530 C for 5 hours and 52 minutes, as recommended by the manufacturers, all the disks were polished with silicon carbide and airborne-particle abraded with 50 mm aluminum oxide at a standard distance of 10 mm.15 The specimens were cleaned by immersing them in acetone in an ultrasound machine (Ultrasonic Cleaner; Unique) for 10 minutes. A thin layer of opaque ceramic dentin (Opaker; Zirkonzahn GmbH) was applied onto all the zirconia disks, followed by firing (EDG
Materials used in study, Composition and Coefficient of thermal expansion (CTE)
Table I.
Material
Code Manufacturer
Composition
CTE*
Zirconia Experimental
EP
Marília
Zr(hf)O2, 94.7%; Y2O3, 5.25%; SiO2, 0.020%; Fe2O3, 0.010%; Na2O, 0.010%; CL, 0.020%; TiO2, 0.010%
10
Zirkonzahn
ZK
Zirkonzahn GmbH
ZrO2, 91.5%; HfO2, 3.0%; Y2O3, 5%; SiO2, 0.020%; Fe2O3, 0.0050%; Na2O, 0.01%
10.5
Shuetz
SH
SchuetzDental GmbH
Zr (O2), 1%; Y2O3, >4%; Al2O3 97%
9.7
PO
Zirkonzahn GmbH
SiO2, Al2O3, P2O5, K2O, Na2O, CaO, F, TiO2 > 97%
9.7
Feldspathic ceramic Ceramic Dentin A3 Ceramic Dentin Opaker
*Coefficient of thermal expansion (CTE) in 10-6K-1 between 25 C and 500 C.
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1 Isostatic pressing. (A) Matrix cavity was filled with experimental zirconia (EP). (B) Isostatic pressure in all directions. (C) Compacted powder. (D) EP block. Equipamentos) at 900 C for 30 minutes. A custom-designed metallic device (5 mm in diameter and 5 mm in height) was constructed for the application of the feldspathic-veneer ceramic (Ceramic Dentin A3; Zirkonzahn GmbH) (Fig. 2). The feldspathic porcelain was mixed with modeling liquid (ICE; Zirkonzahn GmbH), placed in the device, and manually condensed with a Ward condenser number 4 (SSWhite; Duflex). The device was removed, and the specimens with the feldspathic veneer were sintered at 820 C for 30 minutes, according to the manufacturer’s instructions. After sintering, the specimens were embedded with acrylic resin (JET; Classico) in a polyvinyl chloride (PVC) cylinder of 10 mm in diameter. For the shear bond strength test, the specimen-size calculation was
performed based on a pilot study when considering an a of .05 and a power of .80. The calculations revealed that 10 specimens per group would be needed to detect the postulated effect size. Therefore, 10 specimens from each group (n¼10) were stored for 24 hours in water at 37 C, and 10 were thermocycled (500 cycles; 30 seconds at 5 C and 30 seconds at 55 C, with a 10second dwell time) for approximately 10 hours.32 Thereafter, all specimens were subjected to a shear test41-43 with a universal testing machine (EMIC DL500; EMIC Test Equipment and Systems Solutions) with a load cell of 50 Kgf and a special mechanical testing device (Fig. 3) with metallic ribbon at a speed of 0.5 mm/min until fracture. This device was adapted from that developed by Sinhoreti et al44 to minimize bending stress. A mechanical
2 A, Custom-designed metal device. B, Feldspathic porcelain on zirconia disk.
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support was placed opposite the specimen to avoid any movement at the zirconiaefeldspathic porcelain interface. The analysis of the zirconiae feldspathic porcelain interface was performed for all specimens with optical microscopy (Discovery V8 Stereo; Carl Zeiss Microimaging GmbH) at 32 magnification. Each specimen was classified according to the type of failure observed as follows: adhesive failure at the zirconia and feldspathic porcelain interface, cohesive failure of the feldspathic porcelain veneer, and when both types of failure were present. Specimens from each group that showed a remaining feldspathic veneer were cut perpendicularly, and the interface was examined with microRaman spectroscopy. The microRaman measurements were carried out at room temperature in backscattering geometry by means of a JobinYvon micro-Raman system (model T64000; Horiba Group) at the Physics Institute of the State University of São Paulo (USP). The 514.5 nm (2.41 eV) radiation of an argon ion laser (Spectra Physic Inc) was used for excitation. The beam was focused with a microscope at 500 magnification with a laser spot of approximately 5 mm in diameter. No polarization analyzer was used for the scattered beam. To avoid any thermal damage, the laser power was kept as low as 10 mW. The Raman spectra were analyzed by means of a double subtractive monochromator, with a focal length of 0.64 m and equipped
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with a diffraction grating with 1800 grooves/mm. The slit width was set to 200 mm, which provided a spectral resolution of approximately 2 cm1. A line scan was recorded on the sectioned specimen that crossed from the zirconia to the feldspathic ceramic with a charge-coupled device camera (Spectra One; Horiba Group), which yielded 5 spectra from each group with 8 mm of distance between them. Shear bond strength data were analyzed by 2-way ANOVA followed by the pairwise multiple comparison Tukey test (a¼.05). The failure types were classified with optical microscopy and calculated as a percentage for each group. Differences in the wavenumbers and the broadening bands of the Raman spectra were compared among groups.
3 Schematic illustration of device to measure shear bond strength. 22.00 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00
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EP-24 ZK-24 SH-24 EP-CT ZK-CT SH-CT 20.18 ±1.82 17.75 ±1.52 18.81 ±1.82 17.75 ±1.52 15.33 ±1.52 18.81 ±1.60 MPa
SD
4 Shear bond strength between feldspathic porcelain and ZrO2. EP, experimental zirconia; ZK, Zirkonzahn zirconia; SH, Shuetz zirconia after storage for 24 hours in water (-24); after thermocycling (-CT). Columns show mean values of 10 measurements (MPa) standard deviation (SD). 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%
EP-24
ZK-24
SH-24
EP-CT
ZK-CT
SH-CT
20%
10%
10%
30%
10%
10%
Cohesive 10%
0%
10%
10%
30%
20%
Adhesive 70%
90%
80%
60%
60%
70%
Mixed
5 Percentage of failure types. EP, experimental zirconia; ZK, Zirkonzahn zirconia; SH, Schuetz zirconia after storage for 24 hours in water (-24) and after thermocycling (-CT).
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The results of the shear bond strength tests for the specimens after 24 hours of storage in water and after thermocycling are shown in Figure 4. According to 2-way ANOVA, the interaction between the material and thermocycling was not significant. The shear bond strength values obtained for the EP were not significantly different from the Zirkonzahn zirconia (ZK) and Shuetz zirconia, either before or after thermocycling. On average (SD), the EP showed a shear bond strength of 20.18 1.82 MPa compared with 17.75 1.52 MPa for the ZK and 18.81 1.82 MPa for the Shuetz zirconia after storage in water for 24 hours. After thermocycling for 24 hours, the EP showed 17.75 1.52 MPa compared with 15.33 1.52 MPa for the ZK and 18.81 1.60 MPa for the Shuetz zirconia. The effect of thermocycling on the shear bond strength was not statistically significant for any group. The most common type of failure was adhesive (Fig. 5). Representative images of the failure types are shown in Figure 6. The peaks used in this study corresponded to the main bands related to the zirconia and feldspathic porcelain components.3 The wavelengths of bands of 5 spectra from each group were
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6 Failure types by optical microscopy (32 magnification). A, Adhesive. B, Cohesive. C, Mixed. 80
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EP Zirconia
Raman signal (counts/s)
analyzed. In all images, the 2 upper spectra are related to the zirconia, the middle spectrum (red) was recorded from the exact visible transition interface, and the 2 lower spectra are related to the feldspathic porcelain. In the EP group, strong peaks were found at approximately 260, 320, 464, and 642 cm1 (Fig. 7). In the ZK group, strong peaks were found at approximately 260, 320, 465, and 640 cm-1 (Fig. 8). In the Shuetz zirconia group, strong peaks also were found at approximately 260, 320, 465, and 640 cm1 (Fig. 9). Additional weak features were observed for all zirconia groups at approximately 145 cm1 (Figs. 7-9). The feldspathic porcelain showed strong peaks at approximately 256, 320, 460, and 640 cm1 (Figs. 7-9). The spectra of each group were composed of 924 data points in a spectral region between approximately 100 and 700 cm1. The mean spectra of the groups had the same baseline; however, they were dissociated to
60
40
20
100
200
300
400
500
600
700
–1
Raman shifts (cm )
7 Experimental zirconia (EP) strong peaks. improve the visualization of each spectrum (Figs. 7-9).
DISCUSSION The 2 null hypotheses were accepted. No differences in shear bond strength
were found among the groups, regardless of thermocycling. The bond strength between the feldspathic porcelain and the zirconia substructure can determine the longevity of dental restorations in the oral environment.13,14 According to Yamamoto,45 an isolated analysis of
6
Volume ZK Zirconia
Raman intensity (counts/s)
120 100 80 60 40 20 100
200
300
400
500
600
700
–1
Raman shift (cm )
8 Zirkonzahn zirconia (ZK) strong peaks. SH Zirconia
Raman signal (counts/s)
90
60
30
0 100
200
300
400
Raman shifts
500
600
700
(cm–1)
9 Schuetz zirconia (SH) strong peaks.
each ceramic is critical to predict the success of the bilayer. Therefore, a bilayer that involves 2 ceramics is more clinically realistic and behaves differently from each isolated ceramic. The bond strength between the feldspathic ceramic and zirconia can be influenced by certain factors, such as the feldspathic porcelain sintering, the zirconia surface treatment, and thermal variations, which in the present study were identical and, therefore, did not influence the results. The feldspathic porcelain application was carried out carefully, with the same condensation force by a specific device. So, all specimens showed standardized volume
and shape of the feldspathic porcelain. A thin layer of the liner applied onto the feldspathic porcelain surface (Ceramik Opaker) at a temperature of 920 C is recommended by the manufacturer and is thought to increase the molecular interaction between the 2 materials. Aboushelib et al24 showed that the liner application significantly improves the bond strength of the zirconia coree feldspathic porcelain veneer and reduces interfacial failures.11,14 The zirconia surface treatment recommended by the manufacturer was followed by airborne-particle abrasion with 50 mm aluminum oxide. This procedure has been controversial. The surface
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treatment associated with liner reduce chances of delamination of the feldspathic ceramic as shown x-ray diffraction experiments.24 The airborne-particle abrasion with 125 mm aluminum oxide recommended by the manufacturer before applying the veneering ceramic changed the structure of the tetragonal zirconia to monoclinic (9.5% and a depth of 27 mm); the feldspathic porcelain application reversed this phase transformation.27 Despite the controversy that involves zirconia surface treatment with airborne-particle abrasion before applying veneering ceramic, it may increase the wetting of the zirconia surface by increasing the free surface energy, which is measured by the contact angle between the liquid (feldspathic ceramic) and the surface to be bonded (zirconia).26 Polycrystalline ceramics under wet and cyclic loading conditions are most susceptible to subcritical crack growth.8,11,46 The current study showed that the EP yielded bond strength results comparable with other zirconia products tested after thermocycling in another study.37 With respect to the feldspathic porcelain, its bond strength is related to its crystal structure20 and is influenced by factors such as the CTE, thickness,18 and cooling rate.35,36 The maximum stresses developed are a combination of different stresses induced by CTE and tempering residual stress in both ceramics.22 The poor thermal conductivity associated with the poor thermal diffusivity of the ceramics results in a difference of temperature in the zirconiaefeldsphatic porcelain interface.28 The importance of the CTE of ceramics has been widely discussed in the literature27 because the difference in the values between 2 materials with different compositions, such as feldspathic ceramic and zirconia, can lead to residual stresses and decrease the bond strength between them. Manufacturers should provide similar CTEs for ceramics to be used in multilayers. In this study, zirconia and feldspathic ceramics had values considered similar to each other, but the small difference
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between them may also have been responsible for the stress generated at the interface. When the feldspathic porcelain is subjected to high temperatures, a phase change occurs at the structural level, counteracted by the tensile stress of the crystalline transformation of zirconia, which leads to an increased volume expansion that, even if as low as 4% in volume, could secondarily develop tensile stress in the feldspathic porcelain.29,30 Altogether, this can cause dimensional differences that influence the bond between the feldspathic porcelain and zirconia.32,34 Bond strength in terms of nominal stress values can be questionable because of the heterogeneous stress distribution and also the occurrence of cohesive failures.43 Results of the present study show that the failures were mostly of the adhesive type for all groups, which indicate that the interface between the feldspathic porcelain and zirconia is often compromised. Slow cooling during the final veneering of dental restorations with zirconia frameworks reduces the temperature gradients and residual stresses within the porcelain layer, one possible cause for chipping.22 Shear and microtensile bond strength tests are routinely used to measure the bond strength between feldspathic porcelain and zirconia.15,41,42 The choice of a shear test with stainless-steel tape is based on a study by Sinhoreti et al44 that showed that stainless-steel tape provided sliding between 2 tested surfaces. The use of stainless-steel tape produced smaller tensile and compression forces on the interface than those obtained from other tests when using a chisel and orthodontic wires. The use of the chisel indicated by International Organization for Standardization (ISO) TR 1140547 promoted a cleavage stress that was initially concentrated on the subsuperficial layer of the covered material. The tests that used orthodontic wire caused a flexion stress and consequently tensile and compressive stresses on the axial loading regions, which are concentrated diametrically in opposed
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7 and perpendicular directions to the interface evaluated. The support adapted to the upper face of the stainless-steel tape to minimize any possible bending stresses and cleavage (Fig. 3). This could cause higher adhesive failures because the tension distribution is more homogeneous along the interface during the shear test.48 In the present study, the failures were mostly adhesive for all groups (Fig. 5), which is in agreement with the expected results for this test.44 The characterization of the interface was performed with microeRaman spectroscopy and the results are based on spectra analyses. The bands are a set of wavenumbers that contain a main wavenumber related to the peak of the band. The current study identified 5 wavenumbers of the peaks (approximately 260, 320, 464, 642, and 147 cm1) (Figs. 7-9), similar to the frequencies of the vibration that corresponds to the 6 Raman active bands of the tetragonal zirconia.3 According to some studies,3,38,49 the phase transformations can be identified because the 3 main phases (monoclinic, tetragonal, and cubic) exhibit changes in the zirconium oxide bond angles and lengths.3,4,38,49 The main broad bands shown in the present study were at approximately 260, 464, and 642 cm1 (Figs. 7-9). They are characteristic of the tetragonal or cubic phases.3 Different chemical stabilizers4,5 and crystal sizes5 probably caused few spectral variations at the tetragonalcubic phases of this study (approximately 260, 464, and 642 cm1) (Figs. 6-8). The feldspathic porcelain spectra shown in the present study are between approximately 100 and 700 cm1, similar to the frequencies of vibration that correspond to the feldspathic porcelain between approximately 300 and 700 cm1 previously described.3 In the feldspathic porcelain, the presence of more symmetrical peaks was observed, which suggests localized vibrations from a well-ordered crystalline structure, which can be explained by the composition of silicon oxide (SiO2).39 The
strong peaks were found at approximately 256, 320, 460, and 640 cm1, and similar peaks were found in a study of glass-based SiO2.40 The 580 cm1 vibration corresponds to either SieO stretching or to Si-O-Si bending.39 The aluminum and potassium chemical elements embedded in the matrix glass (SiO2) are responsible for changing the properties of the crystalline grid,50 with stronger and more defined peaks. The interface analysis illustrated by the red line (Figs. 7-9) shows the transition among the different materials with characteristic bands. Analysis of this line suggests a process of interdiffusion by the presence of peaks associated with zirconia and feldspathic ceramics. Some chemical elements such as silicon, aluminum, sodium, and potassium can diffuse the zirconium dioxide layer, gradually decreasing to a 8- to 10-mm depth.24 The presence of defects in the crystalline solid can facilitate the movement of atoms, which characterizes the interdiffusion.3 Although the bond mechanism between the zirconia and feldspathic porcelain is still unknown,47 one hypothesis is that the interaction begins by a compressive stress during feldspathic porcelain firing.2 The different CTE of the 2 materials induces interior compressive stresses near the substructure and the periphery of the zirconia grains, which could be the main site of the feldspathic porcelain interaction.3
CONCLUSIONS Based on the results and limitations of this study, the controlled production of EP did not influence the results of the bond strength. Raman analysis suggests a process of interdiffusion by the presence of peaks associated with zirconia and feldspathic ceramics.
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9 Corresponding author: Dr Ana Flávia Sanches Borges Bauru School of Dentistry, University of São Paulo Al. Octávio Pinheiro Brisola, 9-75 Vila Universitária 17012-901, Bauru, SP BRAZIL E-mail: [email protected]
Acknowledgments The authors thank Renato Murback, Heitor Marques Honório, and Adolfo Coelho for their collaboration on this study. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
