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veneer interfacial toughness evaluated by fracture mechanics method
JJOD-2279; No. of Pages 8 journal of dentistry xxx (2014) xxx–xxx
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Effect of zirconia surface treatment on zirconia/ veneer interfacial toughness evaluated by fracture mechanics method Gaoqi Wang a, Song Zhang a,*, Cuirong Bian b, Hui Kong b a Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education), School of Mechanical Engineering, Shandong University, Jinan 250061, PR China b Department of Prosthodontics, Qilu Hospital of Shandong University, Jinan 250012, PR China
article info
abstract
Article history:
Objectives: The aim of this study was to evaluate the effect of the airborne-particle abrasion
Received 17 February 2014
and liner application on the interfacial toughness between veneering porcelain and zirconia
Received in revised form
core by means of a fracture mechanics test.
31 March 2014
Methods: Beam-shaped zirconia specimens were sectioned and divided into 4 groups
Accepted 5 April 2014
according to different surface treatments as follows: Group C (control): no treatment; Group
Available online xxx
L: application of liner; Group A: airborne-particle abrasion with Al2O3 (sandblasting); and Group AL: airborne-particle abrasion and application of liner. The zirconia surfaces before
Keywords:
and after sandblasting were observed and analyzed by SEM and white light interferometer.
Bilayer
Specimens of each pretreated group were veneered with 3 core/veneer thickness ratios of
Airborne-particle abrasion
2:3, 1:1, and 3:2, corresponding to 3 phase angles respectively. Fracture mechanics test was
Liner
performed on each specimen, the energy release rate G and phase angle c were calculated to
Interfacial toughness
characterize interfacial toughness. The experimental data were analyzed statistically using three-way ANOVA and the Tukey’s HSD test. The surfaces of fractured specimens were examined by SEM and EDX. Results: At each phase angle, the interfaces with no treatment had higher mean G values than that of other groups. All the specimens showed mixed failure mode with residual veneer or liner on the zirconia surfaces. Conclusions: The toughness of zirconia/veneer interface with no treatment is significantly higher than that of interfaces subjected to liner application and airborne-particle abrasion. Clinical significance: Liner application and airborne-particle abrasion seem to reduce zirconia/veneer interfacial toughness. Therefore, the two surface treatment methods should be applied with caution. # 2014 Elsevier Ltd. All rights reserved.
* Corresponding author at: School of Mechanical Engineering, Shandong University, 17923 Jingshi Road, Jinan 250061, PR China. Tel.: +86 531 88392746; fax: +86 531 88392746. E-mail addresses: [email protected], [email protected] (S. Zhang). http://dx.doi.org/10.1016/j.jdent.2014.04.005 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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1.
Introduction
All-ceramic restorations have become suitable alternatives to metal restorations in recent decades because of their excellent biocompatible properties and high aesthetic performance, which is attributed to the veneering porcelain bonded on the ceramic substrate.1,2 However, delamination and chipping of the veneer are two common failure modes of ceramic/veneer prosthesis and have a high incidence rate of 6%–25% over 2–5 years,2–8 which is significantly higher than that of metal/ veneer restorations.5,9–11 Delamination failures in all-ceramic restorations either originate from the veneer and propagate to the interface or originate from the ceramic/veneer interface.12–14 Voids and flaws inevitably exist at the interface, and crack may initiate from these voids and flaws due to stress concentration under a certain loading. In vitro studies have reported that ceramic/veneer interface has lower bond strength and fracture toughness compared with metal/veneer interface.15–17 Therefore, sometimes delamination between ceramic and veneer is partly related to the poor bond strength and toughness of the interface. And zirconia/veneer interface is an important and weak link in the all-ceramic system. In order to improve zirconia/veneer interfacial adhesion, zirconia surface was treated with different methods, such as airborne-particle abrasion,18–24 liner application,18–20,25,26 polishing,18,27 grinding,19 acid etching,22 laser etching,24 and silica coating.18 The purpose of surface treatment is to clean the zirconia surface, to increase the surface roughness, or to promote high surface energy and better wettability, and thereby to improve interfacial adhesion.24 Some dental zirconia manufacturers recommended airborne-particle abrasion and liner application as routine pre-treatment methods. The interfacial adhesion and the failure mode were significantly affected by airborne-particle abrasion or application of liner material. However, it is still a controversial issue whether the two methods should be carried out to enhance the adhesion between zirconia framework and veneer.2 Kim et al.20 observed that airborne-particle abrasion with 110 mm Al2O3 under a pressure of 0.4 MPa could improve bond strength. This positive effect of sandblasting in increasing veneer to zirconia bond strength was also confirmed in the previous studies.24,28 However, Fischer et. al18 found that sandblasting was not a necessary surface pretreatment to enhance bond strength; as well, some studies considered it decreased bond strength because it might initiate surface defects that can become stress concentration sources.19,21,27 The effect of liner has been studied by shear, tensile and other tests.18–21,25,26 Conflicting viewpoints also exist as to whether the liner is useful for the bonding. The test methods in these studies—most prevalently the shear test and tensile test—evaluated interfacial adhesion in terms of bond strength. Specimens after bond strength test often showed cohesive fracture patterns within the veneer layer, which meant that the results did not represent the true bond strength of the interface. Interfacial toughness is another important property that evaluates the adhesion of the interface, representing the resistance of a material to crack propagation. Recently, a fracture mechanics test was performed and the negative influence of liner on the interfacial
toughness was indicated.15 The fracture mechanics test proposed by Charalambides et al.29 was proved to be an effective method of measuring interfacial toughness, which can produce relatively reliable data.15,16,29–31 However, few studies have been conducted to evaluate the interfacial adhesion between veneer and zirconia with different surface treatments using this test configuration. The purpose of the present paper was to evaluate the effect of airborne-particle abrasion on the zirconia/veneer interfacial toughness with the fracture mechanics method, and to compare the results with the previous liner applied and nontreated specimens. Fracture mechanics parameters (energy release rate G and phase angle c), which represented interfacial toughness, were calculated from experimental data by means of finite element analysis (FEA). The null hypothesis was that airborne-particle abrasion on zirconia would improve the zirconia/veneer interfacial toughness compared with no treatment and liner application.
2.
Materials and methods
2.1.
Zirconia preparation
Interfaces with four different treatments were compared: Group C, no treatment; Group L, application of liner; Group A, airborne-particle abrasion or sandblasting with Al2O3; and Group AL, airborne-particle abrasion and application of liner (Fig. 1). The fracture toughness tests on interfaces with no treatment (Group C) and application of liner (Group A) have been performed and described in a previous work.21 Therefore, in the present study, the same tests were performed on Group A and Group AL, and the experimental results in the 4 situations were compared together. 40 zirconia beams were sectioned from Y-TZP zirconia presintered blocks (Cercon Zirconia; Dentsply DeguDent GmbH, Hanau-Wolfgang, Germany) by a diamond saw. Then they were sintered according to the manufacturer’s recommendations. Among the 40 beams, 10 beams were randomly chosen to conduct surface observation, 5 of which were in advance sandblasted on the bonding areas with 110 mm alumina particles under a pressure of 0.4 MPa for 10 s at a distance of 10 mm and a direction perpendicular to the surface. The remaining 30 beams were also sandblasted, and used for fracture mechanics test. All the specimens were cleaned in a sonic bath filled with ethanol for 5 min and gently air-dried.
2.2.
Surface quality evaluation
The 5 non-sandblasted and 5 sandblasted zirconia specimens were sputtered with carbon layers on surfaces, and they were observed under a scanning electron microscope (SEM, QUANTA FEG 250; FEI; USA). Due to the carbon-sputtering, the 10 specimens were not used in the follow-up tests. The surface quality of the remaining 30 zirconia specimens were evaluated by a white light interferometer (Wyko NT9300, Veeco Inc., Plainview, NY, USA) before and after sandblasting respectively to obtain surface topography and surface roughness.
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Fig. 1 – Overall experimental design. Groups A and AL are prepared and tested, and results are compared with previews results of Groups C and L.15 There are totally 12 groups with 5 specimens in each group (n = 5).
2.3.
Veneering and specimen preparation
After surface evaluation, the 30 specimens were evenly divided into Group A and Group AL. A liner (IPS e.max Ceram ZirLiner; Ivoclar Vivadent AG, Schaan, Liechtenstein) was applied to specimens of Group AL using a brush. The specimens of Group AL were fired at 960 8C 1 min. All specimens in Group A and Group AL were veneered with IPS e.max Ceram (Ivoclar Vivadent AG, Schaan, Liechtenstein), and they were fired strictly according to the recommended multi-steps-firing procedure (fired at 750 8C 1 min for 3 times) to simulate industrial processes applied in dentistry. Each of the 2 groups was evenly subdivided into 3 subgroups with veneer thicknesses of 2.25 mm, 1.5 mm, and 1.0 mm, corresponding to core/veneer thickness ratios of 2:3, 1:1, and 3:2, respectively. The different thickness ratios were aimed at creating different phase angle c, which is defined as the ratio between sliding and opening modes at an interfacial crack tip. The different zirconia/veneer thickness ratios led to different stress distributions of the interface, consequently changing
the ratio of shear/tensile stresses.15,32 All surfaces of each specimen were polished with aluminium oxide sandpapers (Imperial microfinishing film; 3M Corp) with sequentially finer grit size (40 mm, 20 mm, and 9 mm) to the final dimensions. A 0.7 mm deep notch was machined at the centre of the veneer surface of each specimen with a diamond saw blade (Fig. 2).
2.4.
Fracture mechanics test
Each specimen was loaded to failure at a crosshead speed of 0.1 mm/min using a material test machine (Instron 8801; Instron Corp, Canton, MA, USA) in standard four-point bending mode with an outer span of 30 mm, and inner span of 15 mm (Fig. 2), and the load–displacement curve was recorded. The load nearly kept steady when the crack extended between the inner rollers. For each specimen, the load in this steady-region was used to calculate the interfacial fracture parameters, including energy release rate G and phase angle c. After the test, fractured specimens were examined by SEM to characterize the fracture pattern (cohesive, adhesive, or combination). The elemental composition of the fractured zirconia surface from each group was analyzed using energy dispersive X-ray spectroscopy (EDX; XMAX50, Oxford Instruments, Abingdon, England). Three-way ANOVA was performed on G values considering three factors (airborne-particle abrasion, liner application, and phase angle) and their interaction. Then, multiple comparisons were made with Tukey’s Honestly Significant Difference (HSD) test. Both statistical analyses were made using the Statistical Package for the Social Sciences (SPSS v.17.0, Inc., Chicago, IL, USA). Statistical significances were set at the 0.05 probability level.
2.5.
Calculation of interfacial fracture parameters
Equations provided by Charalambides et al.29 were used to calculate the energy release rate G: Fig. 2 – Specimen design and fracture mechanics test configuration. Thickness of zirconia is h2 = 1.5 mm, and thickness of veneer is h1 = 1, 1.5, or 2.25 mm.
G¼
hðP2 l2 ð1 n21 ÞÞ E1 b2 h3
(1)
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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where P is the steady region load, l is the distance between inner and outer rollers on the same side, E1 and v1 are Young’s modulus and Poisson’s ratio of zirconia (210 GPa and 0.31)15 respectively, b and h are width and total thickness of the specimen, respectively. The non-dimensional parameter h can be calculated by: # " 3 1 l h¼ 2 ðh1 =hÞ3 ðh2 =hÞ3 þ lðh1 =hÞ3 þ 3lðh1 h2 =h2 Þ=ðh2 =h þ lh1 =hÞ
equations.32 The stress intensity factors of the crack tip K1 and K2 were linearly extrapolated from the stress intensity factors of the 10 nodes, and phase angle was obtained by K2 (4) c ¼ arc tan K1
3.
Results
3.1.
Surface topography and roughness
(2) where h1 and h2 are thicknesses of zirconia and veneer, respectively. l can be determined by: l¼
ð1 n22 ÞE1 ð1 n21 ÞE2
(3)
where E2 and v2 are Young’s modulus and Poisson’s ratio of the veneer (70 GPa and 0.27),15 respectively. Displacement extrapolation method based on finite element analysis (FEA) was used to calculate the phase angle c. As described in the previous work,15 two-dimensional FEA models with the same geometry as specimens were established in the FEA programme ANSYS. The steady-region load of each specimen was applied on corresponding model. The displacements of 10 nodes behind the crack tip were extracted from finite element results after static analysis. The interfacial stress intensity factors of the 10 nodes were calculated from the x and y components of the displacements by corresponding
Representative SEM images of the zirconia surfaces are presented in Fig. 3, which showed a great variation in surface topography between the two kinds of surfaces. They were full of apparent machining scratches made by diamond saw on the surface with no treatment (Fig. 3A and B); whereas the scratches almost disappeared after sandblasting (Fig. 3C and D), which possibly resulted from the high velocity impact of Al2O3 particles. However, the impact of the particles also generated rough surface with elevated and depression area and new irregular traces (Fig. 3D). The mean Ra (average roughness) of the zirconia surfaces before airborne-particle abrasion was 0.33 0.06 mm and mean Rt (total height of difference) was 4.88 0.63 mm. The mean Ra and Rt of the sandblasted zirconia surfaces were 0.73 0.13 mm and 6.65 0.97 mm, respectively. Apparently, the Ra and Rt of the surfaces experienced airborne-particle
Fig. 3 – SEM images of zirconia surface before airborne-particle abrasion ((A) 100T magnification, and (B) 1000T magnification) and after ((C) 100T magnification, and (D) 1000T magnification) airborne-particle abrasion, showing a great variation in surface topography between the two kinds of surfaces. Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Fig. 4 – Typical three-dimensional images of surface topography of (A) non-sandblasted surface with Ra = 0.29 mm and Rt = 4.77 mm; (B) sandblasted surface with Ra = 0.65 mm and Rt = 6.50 mm.
abrasion were significantly higher than that of surfaces with no treatment. Examples of the three-dimensional topography of a non-sandblasted surface with Ra = 0.29 mm and Rt = 4.77 mm, and a sandblasted surface with Ra = 0.65 mm and Rt = 6.50 mm are shown in Fig. 4.
3.2.
Crack path and failure modes
For each specimen, the crack originated from the notch tip, propagated to the interface, and extended along the interface. The fractured zirconia surfaces were observed by SEM at 100, 1000, and 5000 magnifications, which revealed mixed cohesive/adhesive failure modes for all specimens. The typical surfaces of different specimens are illustrated in Fig. 5. Apparently, very thin residual layers of veneer (Fig. 5A–C and G–I) or liner (Fig. 5D–F and J–L) were remained on zirconia surfaces, and zirconia substructures were partially exposed. The surface of zirconia in Group C had the largest area of residual (Fig. 5B and C) layer within the 4 groups. Moreover, the areas of residual layers on the sandblasted surfaces (Fig. 5H and K) were obviously smaller than that on the non-sandblasted surfaces (Fig. 5B and E). EDX analysis was performed on the fractured surfaces where the zirconia substrates were exposed (black boxes in Fig. 5). In each analysis, porcelain components, such as silicon, aluminium, sodium, and potassium were not found on the exposed zirconia surface.
3.3.
Interfacial fracture toughness
The means of the steady-region loads (F values) and the G values and their standard deviations are presented in Tables 1 and 2, respectively. Three-way ANOVA revealed that G value was significantly affected by three factors: the application of airborne-particle abrasion, the application of liner, and phase angle (P < 0.01). There were no significant interactions between the three factors (P = 0.574), and there were no significant interactions between liner application and phase angle (P = 0.053). However, significant interactions between airborne-particle abrasion and liner application (P < 0.01), and between airborne-particle abrasion and phase angle (P < 0.01) were noted.
At almost every phase angle, the 2 sandblasted groups (Group A and Group AL) had significantly lower G values compared with the 2 non-sandblasted groups (Group C and Group L) (P < 0.05). For non-sandblasted specimens, interfaces without liner (Group C) had significantly higher G values than liner applied interfaces (Group L) (P < 0.05). Within the 4 groups (C, L, A, and AL), the Group C showed the highest G values (P < 0.05).
4.
Discussion
Based on the results of this study, the null hypothesis that airborne-particle abrasion would improve the zirconia/veneer interfacial toughness was rejected, because the results in Table 2 demonstrated that airborne-particle abrasion on zircnoia surface resulted in significantly lower zirconia/veneer interfacial toughness compared with non-treated and linerapplied specimens. The test configuration in the present study is an effective method to measure the interfacial toughness. Specimens tested with the fracture mechanics method often failed in adhesive mode at the interface, so the method can evaluate the real interfacial adhesive property. In addition, the data obtained by the fracture mechanics test have small coefficients of variation (4.1–12.7% for F values in the present study, as shown in Table 1), indicating 5 specimens per group to be sufficient to test for statistical differences. As far as fracture mechanics is concerned, the energy release rate G is an energy criterion that represents the threshold value over which the crack will initiate and propagate. G is a function of phase angle c, and G(c) of an interface is defined as interfacial toughness. It is easier for a crack to initiate and propagate at a low-toughness interface than a high-toughness interface. Consequently, the results in Table 2 indicate that among the three kinds of interfaces, the non-treated interface has the highest interfacial toughness, whereas the sandblasted interface has the lowest interfacial toughness. In other words, a sandblasted interface has a higher probability to fracture than a liner applied or a nontreated interface under the same condition.
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Fig. 5 – Typical surface topography of Group C,15 L,15 A, and AL at 100T, 1000T, and 5000T magnifications. Very thin residual layers are remained on zirconia surfaces. EDX spectrometry analyses are performed on areas in black boxes.
The effect of liner application has been evaluated by the fracture mechanics method recently.15 The application of liner reduced the zirconia/veneer interfacial toughness, which might be due to the poor wetting of the liner over zirconia surface.
However, in the present study the sandblasted specimens had lower interfacial toughness than the liner applied specimens. Airborne-particle abrasion is one of the most commonly used methods for surface treatment. However, there still exist
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Table 1 – Means of the steady-region loads (F value) of different groups. The F values obtained by the fracture mechanics test have small coefficients of variation (4.1–12.7%). Thickness ratio
Phase angle (deg)
F (N) (SD) [15]
C 2:3 1:1 3:2
40.4 42.4 45.0
L
45.5 (2.1) 55.5 (3.3) 68.0 (2.8)
[15]
42.5 (2.1) 49.3 (3.3) 59.8 (3.7)
A
AL
36.9 (4.7) 48.3 (2.6) 51.5 (3.1)
41.8 (3.5) 46.0 (4.1) 52.3 (3.6)
Table 2 – Means of G value of different groups. Tukey test is performed between groups. G (J/m2) (SD)
Phase angle (deg)
40.4 42.4 45.0
C[15]
L[15]
A
AL
7.8 (0.6) Aa 10.2 (1.2) Ba 12.2 (1.0) Ca
6.8 (0.7) Aa 8.0 (1.1) ABb 9.4 (1.2) Bb
5.2 (1.3) Ab 7.7 (0.8) Bb 7.0 (0.8) ABc
6.6 (1.1)Aa,b 7.0 (1.3) Ab 7.2 (1.0) Ac
Mean values represented with same superscript uppercase letters (column) or lowercase letters (row) are not significantly different (P > 0.05).
contradictory points about whether it should be carried out to improve the zirconia/veneer adhesion. Airborne-particle abrasion can change surface roughness, increase bonding area, and may change wettablility and induce defects and microcracks.19–24,27,28 Different test methods and materials are sensitive to different factors, resulting in the different conclusions. The positive results of airborne-particle abrasion on the adhesion were mainly interpreted as the increasing surface roughness and bonding area, the improvement of wettability behaviour, and the better chemical bonding by removing organic contaminants.22,24,28 Whereas the negative effects of it were usually explained to be the surface defects and stress concentration caused by impact of particles.19,21,27 In the present study, 110 mm alumina particles and a pressure of 0.4 MPa were used to examine the effect of airborne-particle abrasion with the fracture mechanics method to make a comparison with previous studies using other test methods. Interfaces with airborne-particle abrasion had lower fracture toughness than liner-applied and non-treated interfaces, similar results were obtained in previous studies.19,21,27,31 In terms of fracture mechanics, the difficulty of crack formation depends significantly on the size and shape of flaws and voids. Sandblasting acts as a critical factor in the development of surface flaws and voids (Fig. 3), which decreases interfacial fracture toughness, consequently would impair the clinical performance and reliability of zirconiabased restorations clinically. The residual layers on the surfaces of sandblasted groups had apparently smaller areas than that of non-sandblasted surfaces, whereas there was no significant difference between the two sandblasted groups (Fig. 5). This indicates that airborne-particle abrasion is not useful for the bonding. Liner applied zirconia surfaces also showed smaller residual areas than the non-treated specimens, indicating liner application is also harmful for the bonding. Moreover, there were apparent microspaces between the edges of residual layers and zirconia for the liner applied and sandblasted specimens, whereas no microspace was found at non-treated interfaces. It is related to poor wettability between zirconia and liner, and between zirconia and veneer. The wettability is considered to involve
the composition of veneer and liner, the morphology of zirconia surface, and the surface energy of the core material.33 The surface roughness of zirconia became significantly larger after sandblasting, which may be the critical factor of the poor wettability. In the EDX analysis of the fractured zirconia, porcelain components were not found on the exposed zirconia surfaces in all the specimens. Therefore, there is no evidence of chemical bonding between zirconia/veneer and zirconia/ liner. The initial zirconia/veneer interfacial toughness was investigated in the present paper. In future, it has great clinical significance to consider the effect of cyclic loading on the interfacial adhesion, which is necessary to simulate the cyclic nature of clinical conditions. Moreover, more efforts would be concentrated on the improvement of bond strength and interfacial toughness by various methods such as surface treatment, surface design, and variation of compositions of materials.
5.
Conclusions
The present study evaluated the fracture toughness of zirconia/veneer interfaces with different surface treatments on zirconia. Within the limitations of this study, the following conclusions are drawn: liner application and airborne-particle abrasion on zirconia surface reduce the zirconia/veneer interfacial toughness for the current material combination. The poor wettability due to airborne-particle abrasion and liner application, and the surface flaws and voids due to airborne-particle abrasion are considered to be the main reasons of the experimental results.
Acknowledgements This research is based upon work supported by the Independent Innovation Foundation of Shandong University (Grant No. 2012JC032), and Scientific and Technological Planning Project of Shandong Province (Grant No. 2010GDD20211)
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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17. Choi BK, Han JS, Yang JH, Lee JB, Kim SH. Shear bond strength of veneering porcelain to zirconia and metal cores. The Journal of Advanced Prosthodontics 2009;1:129–35. 18. Fischer J, Grohmann P, Stawarczyk B. Effect of zirconia surface treatments on the shear strength of zirconia/ veneering ceramic composites. Dental Materials Journal 2008;27:448–54. 19. Mosharraf R, Rismanchian M, Savabi O, Ashtiani AH. Influence of surface modification techniques on shear bond strength between different zirconia cores and veneering ceramics. Journal of Advanced Prosthodontics 2011;3:221–8. 20. Kim HJ, Lim HP, Park YJ, Vang MS. Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. Journal of Prosthetic Dentistry 2011;105:315–22. 21. Harding AB, Norling BK, Teixeira EC. The effect of surface treatment of the interfacial surface on fatigue-related microtensile bond strength of milled zirconia to veneering porcelain. Journal of Prosthodontics 2012;21:346–52. 22. Komine F, Fushiki R, Koizuka M, Taguchi K, Kamio S, Matsumura H. Effect of surface treatment on bond strength between an indirect composite material and a zirconia framework. Journal of Oral Science 2012;54:39–46. 23. Gomes AL, Castillo-Oyagu¨e R, Lynch CD, Montero J, Albaladejo A. Influence of sandblasting granulometry and resin cement composition on microtensile bond strength to zirconia ceramic for dental prosthetic frameworks. Journal of Dentistry 2013;41:31–41. 24. Liu D, Matinlinna JP, Tsoi JK, Pow EH, Miyazaki T, Shibata Y, et al. A new modified laser pretreatment for porcelain zirconia bonding. Dental Materials 2013;29:559–65. 25. Aboushelib MN, De Jager N, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Dental Materials 2005;21:984–91. 26. Tada K, Sato T, Yoshinari M. Influence of surface treatment on bond strength of veneering ceramics fused to zirconia. Dental Materials Journal 2012;31:287–96. 27. Kim ST, Cho HJ, Lee YK, Choi SH, Moon HS. Bond strength of Y-TZP-zirconia ceramics subjected to various surface roughening methods and layering porcelain. Surface and Interface Analysis 2010;42:576–80. 28. Nakamura T, Wakabayashi K, Zaima C, Nishida H, Kinuta S, Yatani H. Tensile bond strength between tooth-colored porcelain and sandblasted zirconia framework. Journal of Prosthodontic Research 2009;53:116–9. 29. Charalambides PG, Lund J, Evans AG, McMeeking RM. A test specimen for determining the fracture resistance of bimaterial interfaces. Journal of Applied Mechanics 1989;56:77– 82. 30. Go¨stemeyer G, Jendras M, Borchers L, Bach FW, Stiesch M, Kohorst P. Effect of thermal expansion mismatch on the YTZP/veneer interfacial adhesion determined by strain energy release rate. Journal of Prosthodontic Research 2012;56:93–101. 31. Elsaka SE. Influence of surface treatments on the surface properties of different zirconia cores and adhesion of zirconia-veneering ceramic systems. Dental Materials 2013;29:E239–E251. 32. Kuang JH, Chen LS. A displacement extrapolation method for two-dimensional mixed-mode crack problems. Engineering Fracture Mechanics 1993;46:735–41. 33. Nosonovsky M, Bhushan B. Multiscale friction mechanisms and hierarchical surfaces in nano-and bio-tribology. Materials Science and Engineering: R: Reports 2007;58:162–93.
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
Available online at www.sciencedirect.com
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Effect of zirconia surface treatment on zirconia/ veneer interfacial toughness evaluated by fracture mechanics method Gaoqi Wang a, Song Zhang a,*, Cuirong Bian b, Hui Kong b a Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education), School of Mechanical Engineering, Shandong University, Jinan 250061, PR China b Department of Prosthodontics, Qilu Hospital of Shandong University, Jinan 250012, PR China
article info
abstract
Article history:
Objectives: The aim of this study was to evaluate the effect of the airborne-particle abrasion
Received 17 February 2014
and liner application on the interfacial toughness between veneering porcelain and zirconia
Received in revised form
core by means of a fracture mechanics test.
31 March 2014
Methods: Beam-shaped zirconia specimens were sectioned and divided into 4 groups
Accepted 5 April 2014
according to different surface treatments as follows: Group C (control): no treatment; Group
Available online xxx
L: application of liner; Group A: airborne-particle abrasion with Al2O3 (sandblasting); and Group AL: airborne-particle abrasion and application of liner. The zirconia surfaces before
Keywords:
and after sandblasting were observed and analyzed by SEM and white light interferometer.
Bilayer
Specimens of each pretreated group were veneered with 3 core/veneer thickness ratios of
Airborne-particle abrasion
2:3, 1:1, and 3:2, corresponding to 3 phase angles respectively. Fracture mechanics test was
Liner
performed on each specimen, the energy release rate G and phase angle c were calculated to
Interfacial toughness
characterize interfacial toughness. The experimental data were analyzed statistically using three-way ANOVA and the Tukey’s HSD test. The surfaces of fractured specimens were examined by SEM and EDX. Results: At each phase angle, the interfaces with no treatment had higher mean G values than that of other groups. All the specimens showed mixed failure mode with residual veneer or liner on the zirconia surfaces. Conclusions: The toughness of zirconia/veneer interface with no treatment is significantly higher than that of interfaces subjected to liner application and airborne-particle abrasion. Clinical significance: Liner application and airborne-particle abrasion seem to reduce zirconia/veneer interfacial toughness. Therefore, the two surface treatment methods should be applied with caution. # 2014 Elsevier Ltd. All rights reserved.
* Corresponding author at: School of Mechanical Engineering, Shandong University, 17923 Jingshi Road, Jinan 250061, PR China. Tel.: +86 531 88392746; fax: +86 531 88392746. E-mail addresses: [email protected], [email protected] (S. Zhang). http://dx.doi.org/10.1016/j.jdent.2014.04.005 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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1.
Introduction
All-ceramic restorations have become suitable alternatives to metal restorations in recent decades because of their excellent biocompatible properties and high aesthetic performance, which is attributed to the veneering porcelain bonded on the ceramic substrate.1,2 However, delamination and chipping of the veneer are two common failure modes of ceramic/veneer prosthesis and have a high incidence rate of 6%–25% over 2–5 years,2–8 which is significantly higher than that of metal/ veneer restorations.5,9–11 Delamination failures in all-ceramic restorations either originate from the veneer and propagate to the interface or originate from the ceramic/veneer interface.12–14 Voids and flaws inevitably exist at the interface, and crack may initiate from these voids and flaws due to stress concentration under a certain loading. In vitro studies have reported that ceramic/veneer interface has lower bond strength and fracture toughness compared with metal/veneer interface.15–17 Therefore, sometimes delamination between ceramic and veneer is partly related to the poor bond strength and toughness of the interface. And zirconia/veneer interface is an important and weak link in the all-ceramic system. In order to improve zirconia/veneer interfacial adhesion, zirconia surface was treated with different methods, such as airborne-particle abrasion,18–24 liner application,18–20,25,26 polishing,18,27 grinding,19 acid etching,22 laser etching,24 and silica coating.18 The purpose of surface treatment is to clean the zirconia surface, to increase the surface roughness, or to promote high surface energy and better wettability, and thereby to improve interfacial adhesion.24 Some dental zirconia manufacturers recommended airborne-particle abrasion and liner application as routine pre-treatment methods. The interfacial adhesion and the failure mode were significantly affected by airborne-particle abrasion or application of liner material. However, it is still a controversial issue whether the two methods should be carried out to enhance the adhesion between zirconia framework and veneer.2 Kim et al.20 observed that airborne-particle abrasion with 110 mm Al2O3 under a pressure of 0.4 MPa could improve bond strength. This positive effect of sandblasting in increasing veneer to zirconia bond strength was also confirmed in the previous studies.24,28 However, Fischer et. al18 found that sandblasting was not a necessary surface pretreatment to enhance bond strength; as well, some studies considered it decreased bond strength because it might initiate surface defects that can become stress concentration sources.19,21,27 The effect of liner has been studied by shear, tensile and other tests.18–21,25,26 Conflicting viewpoints also exist as to whether the liner is useful for the bonding. The test methods in these studies—most prevalently the shear test and tensile test—evaluated interfacial adhesion in terms of bond strength. Specimens after bond strength test often showed cohesive fracture patterns within the veneer layer, which meant that the results did not represent the true bond strength of the interface. Interfacial toughness is another important property that evaluates the adhesion of the interface, representing the resistance of a material to crack propagation. Recently, a fracture mechanics test was performed and the negative influence of liner on the interfacial
toughness was indicated.15 The fracture mechanics test proposed by Charalambides et al.29 was proved to be an effective method of measuring interfacial toughness, which can produce relatively reliable data.15,16,29–31 However, few studies have been conducted to evaluate the interfacial adhesion between veneer and zirconia with different surface treatments using this test configuration. The purpose of the present paper was to evaluate the effect of airborne-particle abrasion on the zirconia/veneer interfacial toughness with the fracture mechanics method, and to compare the results with the previous liner applied and nontreated specimens. Fracture mechanics parameters (energy release rate G and phase angle c), which represented interfacial toughness, were calculated from experimental data by means of finite element analysis (FEA). The null hypothesis was that airborne-particle abrasion on zirconia would improve the zirconia/veneer interfacial toughness compared with no treatment and liner application.
2.
Materials and methods
2.1.
Zirconia preparation
Interfaces with four different treatments were compared: Group C, no treatment; Group L, application of liner; Group A, airborne-particle abrasion or sandblasting with Al2O3; and Group AL, airborne-particle abrasion and application of liner (Fig. 1). The fracture toughness tests on interfaces with no treatment (Group C) and application of liner (Group A) have been performed and described in a previous work.21 Therefore, in the present study, the same tests were performed on Group A and Group AL, and the experimental results in the 4 situations were compared together. 40 zirconia beams were sectioned from Y-TZP zirconia presintered blocks (Cercon Zirconia; Dentsply DeguDent GmbH, Hanau-Wolfgang, Germany) by a diamond saw. Then they were sintered according to the manufacturer’s recommendations. Among the 40 beams, 10 beams were randomly chosen to conduct surface observation, 5 of which were in advance sandblasted on the bonding areas with 110 mm alumina particles under a pressure of 0.4 MPa for 10 s at a distance of 10 mm and a direction perpendicular to the surface. The remaining 30 beams were also sandblasted, and used for fracture mechanics test. All the specimens were cleaned in a sonic bath filled with ethanol for 5 min and gently air-dried.
2.2.
Surface quality evaluation
The 5 non-sandblasted and 5 sandblasted zirconia specimens were sputtered with carbon layers on surfaces, and they were observed under a scanning electron microscope (SEM, QUANTA FEG 250; FEI; USA). Due to the carbon-sputtering, the 10 specimens were not used in the follow-up tests. The surface quality of the remaining 30 zirconia specimens were evaluated by a white light interferometer (Wyko NT9300, Veeco Inc., Plainview, NY, USA) before and after sandblasting respectively to obtain surface topography and surface roughness.
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Fig. 1 – Overall experimental design. Groups A and AL are prepared and tested, and results are compared with previews results of Groups C and L.15 There are totally 12 groups with 5 specimens in each group (n = 5).
2.3.
Veneering and specimen preparation
After surface evaluation, the 30 specimens were evenly divided into Group A and Group AL. A liner (IPS e.max Ceram ZirLiner; Ivoclar Vivadent AG, Schaan, Liechtenstein) was applied to specimens of Group AL using a brush. The specimens of Group AL were fired at 960 8C 1 min. All specimens in Group A and Group AL were veneered with IPS e.max Ceram (Ivoclar Vivadent AG, Schaan, Liechtenstein), and they were fired strictly according to the recommended multi-steps-firing procedure (fired at 750 8C 1 min for 3 times) to simulate industrial processes applied in dentistry. Each of the 2 groups was evenly subdivided into 3 subgroups with veneer thicknesses of 2.25 mm, 1.5 mm, and 1.0 mm, corresponding to core/veneer thickness ratios of 2:3, 1:1, and 3:2, respectively. The different thickness ratios were aimed at creating different phase angle c, which is defined as the ratio between sliding and opening modes at an interfacial crack tip. The different zirconia/veneer thickness ratios led to different stress distributions of the interface, consequently changing
the ratio of shear/tensile stresses.15,32 All surfaces of each specimen were polished with aluminium oxide sandpapers (Imperial microfinishing film; 3M Corp) with sequentially finer grit size (40 mm, 20 mm, and 9 mm) to the final dimensions. A 0.7 mm deep notch was machined at the centre of the veneer surface of each specimen with a diamond saw blade (Fig. 2).
2.4.
Fracture mechanics test
Each specimen was loaded to failure at a crosshead speed of 0.1 mm/min using a material test machine (Instron 8801; Instron Corp, Canton, MA, USA) in standard four-point bending mode with an outer span of 30 mm, and inner span of 15 mm (Fig. 2), and the load–displacement curve was recorded. The load nearly kept steady when the crack extended between the inner rollers. For each specimen, the load in this steady-region was used to calculate the interfacial fracture parameters, including energy release rate G and phase angle c. After the test, fractured specimens were examined by SEM to characterize the fracture pattern (cohesive, adhesive, or combination). The elemental composition of the fractured zirconia surface from each group was analyzed using energy dispersive X-ray spectroscopy (EDX; XMAX50, Oxford Instruments, Abingdon, England). Three-way ANOVA was performed on G values considering three factors (airborne-particle abrasion, liner application, and phase angle) and their interaction. Then, multiple comparisons were made with Tukey’s Honestly Significant Difference (HSD) test. Both statistical analyses were made using the Statistical Package for the Social Sciences (SPSS v.17.0, Inc., Chicago, IL, USA). Statistical significances were set at the 0.05 probability level.
2.5.
Calculation of interfacial fracture parameters
Equations provided by Charalambides et al.29 were used to calculate the energy release rate G: Fig. 2 – Specimen design and fracture mechanics test configuration. Thickness of zirconia is h2 = 1.5 mm, and thickness of veneer is h1 = 1, 1.5, or 2.25 mm.
G¼
hðP2 l2 ð1 n21 ÞÞ E1 b2 h3
(1)
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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where P is the steady region load, l is the distance between inner and outer rollers on the same side, E1 and v1 are Young’s modulus and Poisson’s ratio of zirconia (210 GPa and 0.31)15 respectively, b and h are width and total thickness of the specimen, respectively. The non-dimensional parameter h can be calculated by: # " 3 1 l h¼ 2 ðh1 =hÞ3 ðh2 =hÞ3 þ lðh1 =hÞ3 þ 3lðh1 h2 =h2 Þ=ðh2 =h þ lh1 =hÞ
equations.32 The stress intensity factors of the crack tip K1 and K2 were linearly extrapolated from the stress intensity factors of the 10 nodes, and phase angle was obtained by K2 (4) c ¼ arc tan K1
3.
Results
3.1.
Surface topography and roughness
(2) where h1 and h2 are thicknesses of zirconia and veneer, respectively. l can be determined by: l¼
ð1 n22 ÞE1 ð1 n21 ÞE2
(3)
where E2 and v2 are Young’s modulus and Poisson’s ratio of the veneer (70 GPa and 0.27),15 respectively. Displacement extrapolation method based on finite element analysis (FEA) was used to calculate the phase angle c. As described in the previous work,15 two-dimensional FEA models with the same geometry as specimens were established in the FEA programme ANSYS. The steady-region load of each specimen was applied on corresponding model. The displacements of 10 nodes behind the crack tip were extracted from finite element results after static analysis. The interfacial stress intensity factors of the 10 nodes were calculated from the x and y components of the displacements by corresponding
Representative SEM images of the zirconia surfaces are presented in Fig. 3, which showed a great variation in surface topography between the two kinds of surfaces. They were full of apparent machining scratches made by diamond saw on the surface with no treatment (Fig. 3A and B); whereas the scratches almost disappeared after sandblasting (Fig. 3C and D), which possibly resulted from the high velocity impact of Al2O3 particles. However, the impact of the particles also generated rough surface with elevated and depression area and new irregular traces (Fig. 3D). The mean Ra (average roughness) of the zirconia surfaces before airborne-particle abrasion was 0.33 0.06 mm and mean Rt (total height of difference) was 4.88 0.63 mm. The mean Ra and Rt of the sandblasted zirconia surfaces were 0.73 0.13 mm and 6.65 0.97 mm, respectively. Apparently, the Ra and Rt of the surfaces experienced airborne-particle
Fig. 3 – SEM images of zirconia surface before airborne-particle abrasion ((A) 100T magnification, and (B) 1000T magnification) and after ((C) 100T magnification, and (D) 1000T magnification) airborne-particle abrasion, showing a great variation in surface topography between the two kinds of surfaces. Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Fig. 4 – Typical three-dimensional images of surface topography of (A) non-sandblasted surface with Ra = 0.29 mm and Rt = 4.77 mm; (B) sandblasted surface with Ra = 0.65 mm and Rt = 6.50 mm.
abrasion were significantly higher than that of surfaces with no treatment. Examples of the three-dimensional topography of a non-sandblasted surface with Ra = 0.29 mm and Rt = 4.77 mm, and a sandblasted surface with Ra = 0.65 mm and Rt = 6.50 mm are shown in Fig. 4.
3.2.
Crack path and failure modes
For each specimen, the crack originated from the notch tip, propagated to the interface, and extended along the interface. The fractured zirconia surfaces were observed by SEM at 100, 1000, and 5000 magnifications, which revealed mixed cohesive/adhesive failure modes for all specimens. The typical surfaces of different specimens are illustrated in Fig. 5. Apparently, very thin residual layers of veneer (Fig. 5A–C and G–I) or liner (Fig. 5D–F and J–L) were remained on zirconia surfaces, and zirconia substructures were partially exposed. The surface of zirconia in Group C had the largest area of residual (Fig. 5B and C) layer within the 4 groups. Moreover, the areas of residual layers on the sandblasted surfaces (Fig. 5H and K) were obviously smaller than that on the non-sandblasted surfaces (Fig. 5B and E). EDX analysis was performed on the fractured surfaces where the zirconia substrates were exposed (black boxes in Fig. 5). In each analysis, porcelain components, such as silicon, aluminium, sodium, and potassium were not found on the exposed zirconia surface.
3.3.
Interfacial fracture toughness
The means of the steady-region loads (F values) and the G values and their standard deviations are presented in Tables 1 and 2, respectively. Three-way ANOVA revealed that G value was significantly affected by three factors: the application of airborne-particle abrasion, the application of liner, and phase angle (P < 0.01). There were no significant interactions between the three factors (P = 0.574), and there were no significant interactions between liner application and phase angle (P = 0.053). However, significant interactions between airborne-particle abrasion and liner application (P < 0.01), and between airborne-particle abrasion and phase angle (P < 0.01) were noted.
At almost every phase angle, the 2 sandblasted groups (Group A and Group AL) had significantly lower G values compared with the 2 non-sandblasted groups (Group C and Group L) (P < 0.05). For non-sandblasted specimens, interfaces without liner (Group C) had significantly higher G values than liner applied interfaces (Group L) (P < 0.05). Within the 4 groups (C, L, A, and AL), the Group C showed the highest G values (P < 0.05).
4.
Discussion
Based on the results of this study, the null hypothesis that airborne-particle abrasion would improve the zirconia/veneer interfacial toughness was rejected, because the results in Table 2 demonstrated that airborne-particle abrasion on zircnoia surface resulted in significantly lower zirconia/veneer interfacial toughness compared with non-treated and linerapplied specimens. The test configuration in the present study is an effective method to measure the interfacial toughness. Specimens tested with the fracture mechanics method often failed in adhesive mode at the interface, so the method can evaluate the real interfacial adhesive property. In addition, the data obtained by the fracture mechanics test have small coefficients of variation (4.1–12.7% for F values in the present study, as shown in Table 1), indicating 5 specimens per group to be sufficient to test for statistical differences. As far as fracture mechanics is concerned, the energy release rate G is an energy criterion that represents the threshold value over which the crack will initiate and propagate. G is a function of phase angle c, and G(c) of an interface is defined as interfacial toughness. It is easier for a crack to initiate and propagate at a low-toughness interface than a high-toughness interface. Consequently, the results in Table 2 indicate that among the three kinds of interfaces, the non-treated interface has the highest interfacial toughness, whereas the sandblasted interface has the lowest interfacial toughness. In other words, a sandblasted interface has a higher probability to fracture than a liner applied or a nontreated interface under the same condition.
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Fig. 5 – Typical surface topography of Group C,15 L,15 A, and AL at 100T, 1000T, and 5000T magnifications. Very thin residual layers are remained on zirconia surfaces. EDX spectrometry analyses are performed on areas in black boxes.
The effect of liner application has been evaluated by the fracture mechanics method recently.15 The application of liner reduced the zirconia/veneer interfacial toughness, which might be due to the poor wetting of the liner over zirconia surface.
However, in the present study the sandblasted specimens had lower interfacial toughness than the liner applied specimens. Airborne-particle abrasion is one of the most commonly used methods for surface treatment. However, there still exist
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Table 1 – Means of the steady-region loads (F value) of different groups. The F values obtained by the fracture mechanics test have small coefficients of variation (4.1–12.7%). Thickness ratio
Phase angle (deg)
F (N) (SD) [15]
C 2:3 1:1 3:2
40.4 42.4 45.0
L
45.5 (2.1) 55.5 (3.3) 68.0 (2.8)
[15]
42.5 (2.1) 49.3 (3.3) 59.8 (3.7)
A
AL
36.9 (4.7) 48.3 (2.6) 51.5 (3.1)
41.8 (3.5) 46.0 (4.1) 52.3 (3.6)
Table 2 – Means of G value of different groups. Tukey test is performed between groups. G (J/m2) (SD)
Phase angle (deg)
40.4 42.4 45.0
C[15]
L[15]
A
AL
7.8 (0.6) Aa 10.2 (1.2) Ba 12.2 (1.0) Ca
6.8 (0.7) Aa 8.0 (1.1) ABb 9.4 (1.2) Bb
5.2 (1.3) Ab 7.7 (0.8) Bb 7.0 (0.8) ABc
6.6 (1.1)Aa,b 7.0 (1.3) Ab 7.2 (1.0) Ac
Mean values represented with same superscript uppercase letters (column) or lowercase letters (row) are not significantly different (P > 0.05).
contradictory points about whether it should be carried out to improve the zirconia/veneer adhesion. Airborne-particle abrasion can change surface roughness, increase bonding area, and may change wettablility and induce defects and microcracks.19–24,27,28 Different test methods and materials are sensitive to different factors, resulting in the different conclusions. The positive results of airborne-particle abrasion on the adhesion were mainly interpreted as the increasing surface roughness and bonding area, the improvement of wettability behaviour, and the better chemical bonding by removing organic contaminants.22,24,28 Whereas the negative effects of it were usually explained to be the surface defects and stress concentration caused by impact of particles.19,21,27 In the present study, 110 mm alumina particles and a pressure of 0.4 MPa were used to examine the effect of airborne-particle abrasion with the fracture mechanics method to make a comparison with previous studies using other test methods. Interfaces with airborne-particle abrasion had lower fracture toughness than liner-applied and non-treated interfaces, similar results were obtained in previous studies.19,21,27,31 In terms of fracture mechanics, the difficulty of crack formation depends significantly on the size and shape of flaws and voids. Sandblasting acts as a critical factor in the development of surface flaws and voids (Fig. 3), which decreases interfacial fracture toughness, consequently would impair the clinical performance and reliability of zirconiabased restorations clinically. The residual layers on the surfaces of sandblasted groups had apparently smaller areas than that of non-sandblasted surfaces, whereas there was no significant difference between the two sandblasted groups (Fig. 5). This indicates that airborne-particle abrasion is not useful for the bonding. Liner applied zirconia surfaces also showed smaller residual areas than the non-treated specimens, indicating liner application is also harmful for the bonding. Moreover, there were apparent microspaces between the edges of residual layers and zirconia for the liner applied and sandblasted specimens, whereas no microspace was found at non-treated interfaces. It is related to poor wettability between zirconia and liner, and between zirconia and veneer. The wettability is considered to involve
the composition of veneer and liner, the morphology of zirconia surface, and the surface energy of the core material.33 The surface roughness of zirconia became significantly larger after sandblasting, which may be the critical factor of the poor wettability. In the EDX analysis of the fractured zirconia, porcelain components were not found on the exposed zirconia surfaces in all the specimens. Therefore, there is no evidence of chemical bonding between zirconia/veneer and zirconia/ liner. The initial zirconia/veneer interfacial toughness was investigated in the present paper. In future, it has great clinical significance to consider the effect of cyclic loading on the interfacial adhesion, which is necessary to simulate the cyclic nature of clinical conditions. Moreover, more efforts would be concentrated on the improvement of bond strength and interfacial toughness by various methods such as surface treatment, surface design, and variation of compositions of materials.
5.
Conclusions
The present study evaluated the fracture toughness of zirconia/veneer interfaces with different surface treatments on zirconia. Within the limitations of this study, the following conclusions are drawn: liner application and airborne-particle abrasion on zirconia surface reduce the zirconia/veneer interfacial toughness for the current material combination. The poor wettability due to airborne-particle abrasion and liner application, and the surface flaws and voids due to airborne-particle abrasion are considered to be the main reasons of the experimental results.
Acknowledgements This research is based upon work supported by the Independent Innovation Foundation of Shandong University (Grant No. 2012JC032), and Scientific and Technological Planning Project of Shandong Province (Grant No. 2010GDD20211)
Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005
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Please cite this article in press as: Wang G, et al. Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.04.005