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Effect of alternative sonochemical treatment on zirconia surface and bond strength with veneering ceramic
Materials Science & Engineering B 252 (2020) 114473
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Effect of alternative sonochemical treatment on zirconia surface and bond strength with veneering ceramic
T
Natália Almeida Bastosa, Paulo Noronha Lisboa-Filhob, Americo Tabatac, Paulo Afonso Silveira Francisconia, Adilson Yoshio Furusea, Heitor Marques Honóriod, ⁎ Ana Flávia Sanches Borgesa, a
Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of Sao Paulo, Bauru, Brazil School of Sciences, Sao Paulo State University, Bauru, Brazil c Department of Physics, School of Science, Sao Paulo State University, Bauru, Brazil d Department of Pediatrics Dentistry, Orthodontics and Public Health, Bauru School of Dentistry, University of Sao Paulo, Bauru, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Y-TZP ceramic Air abrasion, dental Ceramics Microscopy Spectrum analysis, Raman
The morphological characterization of sonochemical treatment on bond strength of Y-TZP/veneer ceramic was evaluated. The surface topography and roughness were analyzed by Confocal Laser Scanning Microscopy, Atomic Force Microscopy and Scanning Electron Microscopy. The phase transformation was identified through Micro-Raman Spectroscopy. The specimens were subjected to the Shear Bond Strength (SBS) test. Failure modes were classified and examined by Fractographic analysis. No differences were observed between the groups regarding surface roughness. The C (control) and AOX (sandblasting) groups presented the highest median SBS values, which were significantly different from S40 (sonication with 40% nominal power) which showed the lowest median SBS value. All specimens exhibited phase transformation. Most specimens exhibited mixed failures. Although the alternative sonochemical treatment did not lead to sufficient bond strength at Y-TZP/veneer ceramic, it caused less damage to the ceramic surface than sandblasting. Therefore, only liner use seemed to be better than adding the sonochemical treatment.
1. Introduction In the last 10–15 years, there have been great improvements in the study and use of ceramic materials for dental applications. This is especially due to the increasing demand for long-term, aesthetic, and high-performance implants and restorations [1,2]. For these reasons, Yttria-stabilized tetragonal zirconia ceramic (Y-TZP) is used for allceramic, fixed dental restorations because of its high flexural strength (900–1200 MPa), superior biocompatibility, and aesthetic qualities (metal-free substructure) [3]. Although Y-TZP has superior mechanical properties, its clinical performance is significantly affected by the deficient bond strength between Y-TZP and veneer ceramic [4]. Therefore, Y-TZP/veneering ceramic bilayers are prone to chipping and/or delamination of the weaker material [5,6]. The poor adhesion between Y-TZP and veneering ceramic is due to the chemical inactivity of zirconia because there is a nonpolar covalent bond and no inherent glass in the matrix [7]. The chemical bonding, mechanical interlocking, wettability, and degree of compressive or
tensile stress on the veneering ceramic are factors that affect the bond between both materials [8]. Clinical studies show failure rates of zirconia and veneering ceramic, after 2 and 5 years, range from 13% to 15% [9–11]. When failure occurs, it can be cohesive inside the veneer layer (chipping), adhesive at the Y-TZP/veneering ceramic interface (delamination), or mixed failure when the combination of these two failure modes occur [11,12]. In order to improve the bond between Y-TZP and veneering ceramic, surface treatment of the Y-TZP surface can be performed by various methods such as sandblasting [7,13], grinding [14], liner application [15,16], acid etching [17], polishing [7,18], silica coating [7], and laser etching [19]. Among these, sandblasting is the most used and recommended treatment by Y-TZP manufacturers because it provides an effective increase in surface roughness. This leads to an improved micromechanical retention with the veneering ceramic [20–22]. However, the greater roughness that is obtained by sandblasting does not necessarily improve the bond strength between Y-TZP and veneering ceramic [23]. It is known that the topographic quality of a surface can
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Corresponding author at: Al. Dr. Otávio Pinheiro Brisolla 9-75 Vila Universitária, Bauru, SP, Brazil. E-mail addresses: [email protected] (P.N. Lisboa-Filho), [email protected] (A. Tabata), [email protected] (P.A.S. Francisconi), [email protected] (H.M. Honório), [email protected] (A.F.S. Borges). https://doi.org/10.1016/j.mseb.2019.114473 Received 10 April 2018; Received in revised form 12 August 2019; Accepted 17 November 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
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also modify the elastic modulus of the material and, consequently, influence load distributions [24]. Furthermore, rougher surfaces have a higher stress concentration, which can weaken the bonding interface [25] and promote an earlier phase transformation, from tetragonal to monoclinic, at the Y-TZP surface [26,27]. In restorative dentistry, liners are materials used as a thin coating (usually 0.5 mm) on the surface of a cavity preparation. The ceramic liners are composed of SiO2, Al2O3, Na2O, K2O, CaO, P2O5 and F. This composition is similar to that of veneer ceramics, which promote an effective bond between these materials. Liners can be applied as a layer between Y-TZP and veneering ceramic to mask the substructure color and increase the wettability of the zirconia surface [15]. Studies have shown that the use of a liner increased the bond strength of Y-TZP and veneering ceramic when compared with other surface treatments [15,28], while other studies have shown that the liner decreased the bond strength [29,30]. There is no consensus in the literature of the best surface treatment to increase the bond strength between Y-TZP and veneer ceramic. Alternative sonochemical treatment can be used to nano-modify surfaces and has been used to improve the surface functionality of ceramic oxides, polymers, and metals [31]. The effects of this treatment are derived from acoustic cavitation by the implosive collapse of bubbles, resulting in conversion of the surface and kinetic energy of liquid motion into heat and chemical energy [32]. The advantages of this treatment for alteration of materials are: the potential performance chemical and physical at high temperature, highly non-equilibrium physicochemical processes, and no need to use other reagents [31]. Some literature indicates that Sialon (a ceramic material including wt.% 88.3 Si3N4, 4.0 Al2O3, 7.7 Y2O3) and ZrO2 ceramics can provide advantages under cavitation when compared with alumina ceramics [33]. The aim of this study was to evaluate the morphological characterization and the effectiveness of sonochemical treatment on bond strength of Y-TZP to veneer ceramic. The null hypothesis tested was that this alternative treatment would improve the bond strength of YTZP to veneering ceramic.
roughening treatments groups (AOX, S40, S70), the liner was applied after each treatment. The specimens of the AOX group were first submitted to a final sintering and the sandblasting treatment was carried out with 50 μm alumina particles, under a pressure of 0.4 MPa for 10 s, perpendicular to the surface, at a distance of 10 mm, and using an airborne-particle abrasion device (Micro-Jet, BIOART, Sao Carlos, SP, Brazil) [34–36]. The two remaining groups were sonochemically treated with lowpower sound waves (S40) and high-power sound waves (S70), both producing acoustic cavitation on the surface of the specimens [31], through an ultrasonic liquid processor (Sonics Vibracell VCX-750, Sonics & Materials Inc., Newtown, CT, USA). The sonochemical treatment was performed for 15 min for both powers (40 and 70% nominal power), with the Y-TZP discs fixed to a device in order to standardize their centered position at the bottom of a beaker filled with deionized water (Fig. 1). The specimens were sintered according to the manufacturer’s instructions, as shown in Table 2. The final dimensions obtained were a 10.5 mm diameter and 2.8 mm thickness.
2. Materials and methods
2.3. Phase analysis
2.1. Sample preparation
Y-TZP discs (n = 10) were evaluated with a Jobin Yvon microRaman system (T64000, Groupe Horiba—Longjumeau, France). An argon ion laser excitation source (Spectra Physic, Inc., California, USA) was used with a radiation of 514.5 nm (2.41 eV), and the beam was focused by 500x microscope magnification. The laser power was maintained at 10 mW to avoid thermal damage. The Raman spectra analyses were performed using a double subtractive monochromator with a focal distance of 0.64 M and equipped with a diffraction grating with 1,800 grooves/mm. This provided a spectral resolution of 2 cm with a slit width of 200 µm. Spectra were collected with a CCD camera (Spectra One—Groupe Horiba, Longjumeau, France). All specimens were evaluated using the average of 3 measurements for each disc.
2.2. Morphological characterization The discs (n = 10) of each group were analyzed at one site (400 µm of scanning area) by confocal laser scanning microscopy (CLSM) (DCM 3D Model, Leica Microsystems, Wetzlar, Germany) in order to evaluate the surface roughness (Ra) in nm and surface topography. An atomic force microscope (AFM) (XE-70, Park Systems, Tokyo, Japan) was used to obtain 3-dimensional images (n = 1). The measurements were performed in non-contact mode with an adjusted cantilever at a distance around 5.8 nm (set point), and the scan area was 4 × 4 mm in the center of the specimens. A morphological analysis of the zirconia surface was performed in a sample of each group with a scanning electron microscope (JSM 5600LV 353, JEOL, Tokyo, Japan). The analyzing procedures were carried out with x1000 magnification.
Discs were obtained from pre-sintered Y-TZP blocks (15.5 mm width × 19 mm length × 39 mm height) (IPS e.max ZirCAD, Ivoclar Vivadent, Schaan, Liechtenstein). They were milled from a cylindrical shape with a 12.5 mm diameter and 39 mm height. After that, each cylinder was cut using an Isomet 1000 cutter (Buehler, Lake Bluff, IL, USA) and diamond disc (series 15LC Diamond no. 11–4254, Buehler, Lake Bluff, IL, USA) at a 275 rpm low speed under water cooling. The specimens were subjected to a polishing machine (EXACT, Nordestedt, Schleswing-Holstein, Germany) with #1000 and #1200 sandpapers (Polishing paper K2000, EXACT, Nordestedt, SchleswingHolstein, Germany), followed by a sequence of treatments with felt discs of medium, fine, and extra-fine granulations with diamond paste (Polishing paper K2000, EXACT, Nordestedt, Schleswing- Holstein, Germany). The specimens were cleaned by double cycle soaking in 100% ethanol and distilled water in an ultrasonic cleaning (USC 700—Unique Industry and Trade of Electronic Products Ltda, Sao Paulo, SP, Brazil) for 10 min. Relevant information on the tested materials is presented in Table 1. The 40 pre-sintered Y-TZP discs (12.5 mm diameter and 3.5 mm thickness, before sintering) were randomly divided into 4 groups (n = 10) according to the intended surface treatment: application of the liner only (C), sandblasting with 50-µm aluminum particles (AOX), presintered sonication with 40% nominal power (S40), and pre-sintered sonication with 70% nominal power (S70). In the C group, after the final sintering of the specimens, no roughening treatment was performed before liner application. In the
2.4. Veneering ceramic application To build the veneering ceramic layer, the IPS e.max Ceram and Ceram Dentin and Build-Up system was used (Ivoclar Vivadent AG, Schaan, Liechtenstein), which applied a ZirLiner layer (ZirLiner, Ivoclar Vivadent AG, Schaan, Liechtenstein) on the surface of all of the Y-TZP discs. Thereafter, the specimens were taken to be fired, following a temperature control protocol (Table 2). Due to the limited heat conductivity of zirconium oxide, a firing wash is required. This wash was a mixture of the required IPS Ceram Dentin or Deep Dentin with IPS e.max Ceram Build-Up liquids. The firing wash was conducted to obtain a homogeneous connection with the ZirLiner layer. After that, the standard firing parameters for the material were followed. A custom-designed metallic device (5 mm in diameter and 5 mm in 2
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Table 1 Materials used in this study and their classification, manufacturer, composition, and coefficient of thermal expansion (CTE). Material/Code
Classification
Manufacturer
Composition (% by weight)
CTE
IPS e.max Zircad**/EZ IPS e.max Ceram/EC IPS e.max Ceram ZirLiner
Y-TZP ceramic Veneer ceramic Liner
Ivoclar Vivadent, Schaan, Liechtenstein Ivoclar Vivadent, Schaan, Liechtenstein Ivoclar Vivadent, Schaan, Liechtenstein
ZrO2, 87%; Y2O3, 4%; HfO2, 1%; Al2O3, 1% SiO2, 60%; Al2O3, 8%; Na2O, 6%; K2O, 6%; ZnO, 8%; CaO, P2O5, F, 2% SiO2, 50%; Al2O3, 16%; Na2O, 6%; K2O, 4%; CaO, P2O5, F, 2.5%
10.75 9.5 9.8
*Coefficient of thermal expansion (CTE) in 10−6 K−1 between 100 °C and 500 °C. **Based on manufacturer’s information by technical profile.
2.6. Failure mode and Fractographic analysis The analysis of failure mode and Fractographic was performed for all the specimens with a digital microscope DinoLite AM313T (AnMoElectronics Corporation, Dung-Da Road, Taiwan) at 32x magnification. Each specimen was classified according to the type of failure: (a) adhesive failure at the Y-TZP/veneering ceramic interface, (b) cohesive failure of the ceramic veneer, or (c) mixed, meaning both types of failure were present. The types of failure were classified by digital microscopy, and their respective percentages were calculated. The Fractographic analysis of the bond surface fractured were used to detect the location of fracture origin and to determine the crack propagation. 2.7. Statistical analysis Data on SBS and roughness were calculated and statistically analyzed with Statistica software (Statsoft®, Tulsa, OK, USA). The assumptions that were made of the normal distribution and of the equality of variances were verified for all of the variables using the Shapiro-Wilk and Levene tests, respectively. As only the assumption of normality was satisfied, data were subjected to the Kruskal-Wallis test (α = 0.05), followed by the Dunn’s test (α = 0.05) for individual comparisons.
Fig. 1. Pre-sintering sample in deionized water for sonochemical treatment with a microtip. The Y-TZP discs were centered at the bottom of a beaker filled with deionized water.
Table 2 Recommended cycles for zirconia and liner firing.
3. Results
Heating schedule
IPS e.max ZirCAD™
IPS e.max Ceram ZirLienr
Standby temperature Drying time Heating rate Maximum firing temperature Holding time
403 °C 12 °C/min 65 °C/min 1050 °C 15 min
403 °C 40 °C/min 40 °C/min 960 °C 1 min
3.1. Morphological characterization The sandblasting and sonochemical surface treatments did not show a significant quantitative difference in increased surface roughness (p = 0.255). The median roughness (Ra) for each group was 121.98 nm (C), 119.95 nm (S40), 125.22 nm (S70), and 118.89 nm (AOX) as shown in Fig. 2. Representative images of CLSM (Fig. 3) and AFM (Fig. 4) of the zirconia surfaces show the micro and nanoscale morphological
height) was used for the application of the IPS e.max veneering ceramic (Ivoclar Vivadent AG, Schaan, Liechtenstein). The veneering ceramic powder was mixed with modeling liquid (Ivoclar Vivadent AG, Schaan, Liechtenstein) and placed in the device by manually condensed. The device was removed, and the specimens were sintered. The recommended firing procedures were utilized. After that, the specimens were ready for the shear bond strength testing. After sintering, each specimen was embedded in a polyvinyl chloride (PVC) cylinder (21 mm diameter and 25 mm height) using an acrylic resin (JET; Classic, Sao Paulo, SP, Brazil).
2.5. Shear bond strength test For this test, the specimens were stored for 24 h in water at 37 °C, following ISO TR 11405, Type I. After this step, all specimens were subjected to shear mechanical testing by a Kratos 5002 universal biomechanical test machine (Kratos Dynamômetros, Sao Paulo, Brazil), with a load cell of 500 kgf, in a special device with a metal strip, at a speed of 0.5 mm/min until fracture occurred. This device [37] was adapted from Sinhoreti et al. [38] in order to minimize bending forces during the test [39].
Fig. 2. Effect of surface treatment on the roughness of Y-TZP ceramic surfaces. Vertical bars indicate the inner quartile range, vertical lines are the maximum and minimum values, and the horizontal line represents the median. * Outlier (S70 = 87.08; AOX = 240.27). 3
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Fig. 3. Surface topography represented by 3D images of the selected groups: C (A), S40 (B), S70 (C), and AOX (D).
The Fig. 5 shows the different morphological surfaces after surface treatments on zirconia. In overview, in the C group (Fig. 5A), the surface is smoothest compared with other groups. S40 and S70 showed a similar surface with the C group, with small irregularities (Fig. 5B, C). The surface of AOX (Fig. 5D) group appeared more irregular which demonstrates the removal some of the surface material by the highimpact of alumina particles.
differences, respectively. The red color represents a vertical (z-axis) size of around 400 nm and the dark blue color represents −400 nm. From Fig. 3, it is possible to compare the surface morphologies. In microscale, all groups showed a similar pattern of surface roughness. The peaks and valleys ranged between 400 nm and − 400 nm in all groups (Fig. 3A–D). Some scratches were founded only in C and S40 groups due to polishing finishing (Fig. 3A and B). AFM was used to investigate the surface morphology in the nanoscale. Besides differences in roughness, we observed the grain size, topological changes in the nanostructure, and grain damage due to surface treatments (Fig. 4). It was observed that only sandblasting treatment promoted a damaged on the grain of the Y-TZP surfaces (Fig. 4D).
3.2. Phase analysis Fig. 6 shows the Raman spectra of each group, reporting the bands of the monoclinic and tetragonal phases. Previous literature has reported the major bands that represent each crystalline phase of Y-TZP. 4
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Fig. 4. AFM images of Y-TZP: C (A), S40 (B), S70 (C), and AOX (D).
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Fig. 7. Shear bond strength (SBS) of the veneering ceramics after surface treatment. The vertical bars indicate the inner quartile range, vertical lines represent the maximum and minimum values, and the horizontal line represents the median.
Fig. 5. SEM examination (magnification x1000) on zirconia surfaces after treatments: (A) C; (B) S40; (C) S70; (D) AOX.
Fig. 8. Results of the multiple Dunn’s test comparisons for shear bond strength. Intervals that do not contain zero have corresponding medians that are significantly different.
3.4. Failure mode analysis Fig. 6. Raman analysis of the four groups studied showing the Monoclinic (M) and Tetragonal (T) phases.
Fig. 10 shows the failure modes and the distribution of these modes for each group. Most specimens in all groups exhibited a mixed mode of failures (67.5%), and the remaining specimens showed adhesive failures. In the Fig. 11 is it possible to verify the three types of failure founded in the present study. Fig. 11-A represents an adhesive failure, without the presence of any residual part of veneer ceramic. Fig. 11-B characterizes a cohesive failure of veneer ceramic, where the bond strength on interface between Y-TZP and veneer ceramic was higher than the inner strength of the ceramic. Fig. 10-C denotes a mixed failure, where there is a combination of the adhesive and cohesive failures at the same time. Images of Fractographic analysis are described in Fig. 12. All groups showed similar fracture patterns, predominantly mixed failure mode. The hackle and arrest lines were founded in Fig. 12A-D that indicates the direction of fracture propagation.
The tetragonal phase is characterized by strong peaks at ~144–147, 262, 320, 466, 635–637 cm−1, while the monoclinic phase presents peaks at ~189, 304, 345–349, 379–383, 638–647 cm−1 [40,41]. The peaks of bands for all groups were similar, as follow: ~151 (t), 266 (t), 322 (t), 467 (t), 645 (m). All groups showed characteristic bands of the tetragonal (t) and monoclinic (m) phases, suggesting that there was a phase transformation (t → m). 3.3. Shear bond strength test The lowest median SBS (MPa) was obtained for the S40 group (15.43 MPa), which was lower than the C (29.9 MPa), AOX (25.0 MPa), and S70 groups (18.49 MPa) (Fig. 7). The Dunn’s test results showed a significant difference (p = 0.008) in the shear bond strengths between the C and S40 as well as the AOX and S40 groups. Also, the C and AOX groups exhibited a higher median strength than S40. This is shown in Fig. 8, wherein intervals that do not contain 0 signify that the corresponding medians are significantly different. The highest values of SBS of group C can be explained due to the bond interaction between Y-TZP and liner. A schematic representation to highlight and explain this possible chemical bond of this both materials can be founded on Fig. 9.
4. Discussion The null hypothesis tested in this study was rejected. The alternative sonochemical treatment does not improve the bond strength between YTZP and veneering ceramic. Despite the altered morphology that was observed for the sonochemical treatment, this change was not sufficient to improve the bond strength of Y-TZP to the veneer ceramic. Therefore, there are likely other factors that determine the zirconia 6
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Fig. 9. Schematic representation of the oxides presents in liner and zirconia that can flow and chemically interact with each other.
Fig. 10. Percentage of failure modes in each group (%) (n = 40). Fig. 12. Fractographic analysis of groups C (A), S40 (B), S70 (C) and AOX (D) (32x magnification). All groups showed arrest and hackle lines.
bond strength. This study did not show statistical quantitative differences among the studied groups for surface roughness, which was be confirmed by CLSM and AFM analysis. The average surface roughness was statistically similar in all groups on the microscale, but we detected some differences for the high-energy treatments in the nanoscale (Fig. 4). These results agree with a related study, wherein surface roughening with 110 µm Al2O3 (2.4 bar) sandblasting had no significant influence on the core veneer bond strength in comparison to the non-sandblasted groups [42]. Although there was no statistical difference in surface roughness analysis, the average highest values were found for the AOX group (Fig. 2), which can be confirmed by SEM images (Fig. 5D), which is more irregular pattern was observed compared to the other groups. Therefore, it is possible that there is no relationship between increased bond strength and the surface roughness alteration. The effect of sandblasting on increasing the SBS of Y-TZP/veneering ceramic was not confirmed in the current study. This could be explained by the fact that sandblasting affects the Y-TZP mechanical strength and probably the bonding capacity of the material [7,21]. This is due to the CTE difference between the tetragonal (10.8 ppm/K) and monoclinic (7.5 ppm/K) zirconia, as the monoclinic is considerably lower [43]. A modified nano-surface can be obtained by sonochemical therapy, whereby sound waves result in acoustic cavitation that is produced by the implosive collapse of bubbles. This cavitation potentially modifies the treated ceramic surface. Although sonochemical
treatment is innovative, it did not increase the zirconia/veneer SBS when compared with other treatments. Although no previous studies have used sonochemical treatment for zirconia, other studies have used this treatment to improve the surface functionality of ceramic oxides, polymers, and metals [31]. Therefore, studies with different nominal powers of sound waves should be performed in order to more thoroughly investigate the effect of this treatment on ceramics. The resultant surface from each treatment presented different surface characteristics. For AOX, the surface appeared rough and irregular, with well-defined, micro-sized elevations and depressions, possibly created by the high-impact of alumina particles. Alternatively, there was more regularity and homogeneity of the surfaces for the S40, S70, and C groups (Figs. 3 and 4). Even though sandblasting caused a significant change on the surfaces, the bond strength between Y-TZP and the veneering ceramic did not increase when compared with the groups treated with sonochemical treatment (S40 and S70). These results are in agreement with a related study wherein there was no influence of sandblasting on the shear strength for all zirconia and corresponding veneering ceramics that were investigated [42]. The application of a liner only (group C) increased the zirconia/ veneer SBS, probably due to the sufficient wetting of the liner over the zirconia surface [36]. These results are in agreement with other studies
Fig. 11. Failure modes by optical microscopy. (A) Adhesive, (B) Cohesive, and (C) Mixed (magnification 32×). 7
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geometry that allowed for measurements of shear bond strength. We believe that the results of this study will help to define a surface treatment protocol for zirconia. Nevertheless, more clinical and in vitro studies with sonochemical treatment of different nominal powers are needed to confirm the validity of these results.
that showed better results only with liner application [44–48]. The liner wettability over Y-TZP depends of the composition of the veneer and the liner, the morphology of the zirconia surface, and the surface energy of the core material [49]. This energy provides free atoms for chemical bonding between the ceramic materials [50]. The grain borders of YTZP, which have higher surface area interfaces [51], are composed of Y2O3 (4%), HfO2 (1%), and Al2O3 (1%) (Table 1). Al2O3 (16%) was also present in the liner, which suggests that these other oxides present in both materials can flow and chemically bond with each other (Fig. 9). In addition, the mechanism for adhesion between zirconia and veneering ceramic has not been fully explained, but the literature has reported the existence of a fusion between the zirconia and veneering ceramic, resulting in the diffusion of elements from each material at the bonded interface [39,55]. On the other hand, one study reported that there is no evidence of chemical bonding between liner/veneer and zirconia/liner, because dispersive X-ray spectrometry (EDX) analysis of the fractured zirconia showed no porcelain components from the liner on the surfaces of all specimens [36]. In this way, more study in this field should be performed to better understand the chemical bond between zirconia/liner/veneer ceramic. To investigate the types of fractures, microscopy is the most important technique for identifying the location of crack onset, size, and spread [52]. Regarding the type of failure, most specimens in all groups exhibited the mixed failure mode, while the remaining specimens exhibited adhesive failures. Few cohesive failures were observed, and none of the specimens fractured within the zirconia. Most mixed fractures that were observed began as an adhesive-type at the load application side and terminated as a cohesive-type on the opposite side [53]. Yoon et al. [54] observed, with the same sandblasting parameters, a higher percentage of mixed failures which agrees well with the observations in this study. In addition, Fractographic analysis data showed in all groups that fractures started from the tension surface and propagated to the compression surface for all groups. Raman analysis showed that all groups presented a crystallographic transition. The preparation of the specimens is likely to have caused this result, as the liner group (without roughening treatment) also showed identifiable monoclinic peaks. Some studies have reported the effect of sandblasting on the mechanical properties and bonding to zirconia [2,56]. It was observed that this process could lead to a phase transformation (t → m) of the surface grains, due to the formation of a compressive layer on the zirconia surface [57]. This resulted in surface flaws and voids, which could impair the longevity and clinical performance of zirconia-based restorations [58]. The clinical implication of this finding is that the previously investigated all-ceramic systems, instead of presenting catastrophic failure of the core, have a tendency to exhibit chip-off fractures and delamination of the veneering ceramic [42]. This study suggests that the sole application of a liner onto Y-TZP as a pretreatment to the zirconia substrate, has the highest bond strength between the zirconia and veneering ceramic. This treatment could be an alternative to sandblasting, as it is not so aggressive to the Y-TZP surface and it is, therefore, less harmful to the longevity of the ceramic restoration. The gold standard for SBS between the core of Y-TZP and its corresponding veneering ceramics was not reached [42]. The high rates of veneering ceramics chipping that are observed in clinical studies [59,60] may be explained by the interaction between both materials, their compositions, and processing. Clinical trials are still required to evaluate the long-term behavior of the interface of Y-TZP and veneering ceramics. According to results of this study, roughening treatments seem unnecessary to provide adequate bond strength at Y-TZP/veneering ceramic interface, since the recommended manufacturer protocol was the most effective. However, a limitation of this study was that specimen design and dimensions were not representative of the shape of dental restorations. This was done intentionally in order to provide a
5. Conclusions This study shows that sandblasting, although it leads to a higher surface roughness, does not influence the bond strength between Y-TZP and veneering ceramic. The sonochemical treatment with low-power sound waves led to a lower bond strength, but both high and low-power sound waves caused less damage to the ceramic surface than sandblasting. Liner application was the most effective treatment when considering bond strength, although all of the groups led to a phase transformation of the Y-TZP surface. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank Alcides Urias da Costa for his assistance with the study methodology and Nair Cristina for performing the statistical analyses. This study was supported by the Sao Paulo Research Foundation (FAPESP) #2011/18061-0. Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations References [1] S. Abou-Ayash, M. Strasding, G. Rücker, W. Att, Impact of prosthetic material on mid- and long-term outcome of dental implants supporting single crowns and fixed partial dentures: a systematic review and meta-analysis, Eur. J. Oral Implantol. 10 (1) (2017) 47–65. [2] S. Bhargava, H. Doi, R. Kondo, H. Aoki, T. Hanawa, S. Kasugai, Effect of sandblasting on the mechanical properties of Y-TZP zirconia, Biomed. Mater. Eng. 22 (6) (2012) 383–398. [3] P.F. Manicone, P.R. Iommetti, L. Raffaelli, An overview of zirconia ceramics: basic properties and clinical applications, J. Dent. 35 (11) (2007) 819–826. [4] A. Della Bona, J.R. Kelly, The clinical success of all-ceramic restorations, J. Am. Dent. Assoc. 139 (2008) S8–S13. [5] A.J. Raigrodski, A. Yu, G.J. Chiche, J.L. Hochstedler, L.A. Mancl, S.E. Mohamed, Clinical efficacy of veneered zirconium dioxide-based posterior partial fixed dental prostheses: five-year results, J. Prosthet. Dent. 108 (4) (2012) 214–222. [6] S.B. Bitencourt, D.M. Dos Santos, E.V.F. da Silva, V.A.R. Barão, E.C. Rangel, C. da Cruz, G.M. de Souza, M.C. Goiato, A.A. Pesqueira, Characterisation of a new plasma-enhanced film to improve shear bond strength between zirconia and veneering ceramic, Mater. Sci. Eng. C 1 (92) (2018) 196–205. [7] J. Fischer, P. Grohmann, B. Stawarczyk, Effect of zirconia surface treatments on the shear strength of zirconia/veneering ceramic composites, Dent. Mater. 27 (3) (2008) 448–454. [8] I. Sailer, B.E. Pjetursson, M. Zwahlen, C.H. Hämmerle, A systematic review of the survival and complication rates of all-ceramic and metal–ceramic reconstructions after an observation period of at least 3 years. Part II: fixed dental prostheses, Clin. Oral. Implants. Res. 18 (2007) 86–96. [9] I. Sailer, N.A. Makarov, D.S. Thoma, M. Zwahlen, B.E. Pjetursson, All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review of the survival and complication rates. Part I: single crowns (SCs), Dent. Mater. 31 (6) (2015) 603–623. [10] M.V. Swain, Unstable cracking (chipping) of veneering ceramic on all-ceramic dental crowns and fixed partial dentures, Acta. Biomater. 5 (5) (2009) 1668–1677. [11] M.N. Aboushelib, A.J. Feilzer, C.J. Kleverlaan, Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach, Dent. Mater. 25 (3) (2009) 383–391. [12] S.W. Pharr, E.C. Teixeira, R. Verrett, J.R. Piascik, Influence of veneering fabrication techniques and gas-phase fluorination on bond strength between zirconia and
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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Effect of alternative sonochemical treatment on zirconia surface and bond strength with veneering ceramic
T
Natália Almeida Bastosa, Paulo Noronha Lisboa-Filhob, Americo Tabatac, Paulo Afonso Silveira Francisconia, Adilson Yoshio Furusea, Heitor Marques Honóriod, ⁎ Ana Flávia Sanches Borgesa, a
Department of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry, University of Sao Paulo, Bauru, Brazil School of Sciences, Sao Paulo State University, Bauru, Brazil c Department of Physics, School of Science, Sao Paulo State University, Bauru, Brazil d Department of Pediatrics Dentistry, Orthodontics and Public Health, Bauru School of Dentistry, University of Sao Paulo, Bauru, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Y-TZP ceramic Air abrasion, dental Ceramics Microscopy Spectrum analysis, Raman
The morphological characterization of sonochemical treatment on bond strength of Y-TZP/veneer ceramic was evaluated. The surface topography and roughness were analyzed by Confocal Laser Scanning Microscopy, Atomic Force Microscopy and Scanning Electron Microscopy. The phase transformation was identified through Micro-Raman Spectroscopy. The specimens were subjected to the Shear Bond Strength (SBS) test. Failure modes were classified and examined by Fractographic analysis. No differences were observed between the groups regarding surface roughness. The C (control) and AOX (sandblasting) groups presented the highest median SBS values, which were significantly different from S40 (sonication with 40% nominal power) which showed the lowest median SBS value. All specimens exhibited phase transformation. Most specimens exhibited mixed failures. Although the alternative sonochemical treatment did not lead to sufficient bond strength at Y-TZP/veneer ceramic, it caused less damage to the ceramic surface than sandblasting. Therefore, only liner use seemed to be better than adding the sonochemical treatment.
1. Introduction In the last 10–15 years, there have been great improvements in the study and use of ceramic materials for dental applications. This is especially due to the increasing demand for long-term, aesthetic, and high-performance implants and restorations [1,2]. For these reasons, Yttria-stabilized tetragonal zirconia ceramic (Y-TZP) is used for allceramic, fixed dental restorations because of its high flexural strength (900–1200 MPa), superior biocompatibility, and aesthetic qualities (metal-free substructure) [3]. Although Y-TZP has superior mechanical properties, its clinical performance is significantly affected by the deficient bond strength between Y-TZP and veneer ceramic [4]. Therefore, Y-TZP/veneering ceramic bilayers are prone to chipping and/or delamination of the weaker material [5,6]. The poor adhesion between Y-TZP and veneering ceramic is due to the chemical inactivity of zirconia because there is a nonpolar covalent bond and no inherent glass in the matrix [7]. The chemical bonding, mechanical interlocking, wettability, and degree of compressive or
tensile stress on the veneering ceramic are factors that affect the bond between both materials [8]. Clinical studies show failure rates of zirconia and veneering ceramic, after 2 and 5 years, range from 13% to 15% [9–11]. When failure occurs, it can be cohesive inside the veneer layer (chipping), adhesive at the Y-TZP/veneering ceramic interface (delamination), or mixed failure when the combination of these two failure modes occur [11,12]. In order to improve the bond between Y-TZP and veneering ceramic, surface treatment of the Y-TZP surface can be performed by various methods such as sandblasting [7,13], grinding [14], liner application [15,16], acid etching [17], polishing [7,18], silica coating [7], and laser etching [19]. Among these, sandblasting is the most used and recommended treatment by Y-TZP manufacturers because it provides an effective increase in surface roughness. This leads to an improved micromechanical retention with the veneering ceramic [20–22]. However, the greater roughness that is obtained by sandblasting does not necessarily improve the bond strength between Y-TZP and veneering ceramic [23]. It is known that the topographic quality of a surface can
⁎
Corresponding author at: Al. Dr. Otávio Pinheiro Brisolla 9-75 Vila Universitária, Bauru, SP, Brazil. E-mail addresses: [email protected] (P.N. Lisboa-Filho), [email protected] (A. Tabata), [email protected] (P.A.S. Francisconi), [email protected] (H.M. Honório), [email protected] (A.F.S. Borges). https://doi.org/10.1016/j.mseb.2019.114473 Received 10 April 2018; Received in revised form 12 August 2019; Accepted 17 November 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
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also modify the elastic modulus of the material and, consequently, influence load distributions [24]. Furthermore, rougher surfaces have a higher stress concentration, which can weaken the bonding interface [25] and promote an earlier phase transformation, from tetragonal to monoclinic, at the Y-TZP surface [26,27]. In restorative dentistry, liners are materials used as a thin coating (usually 0.5 mm) on the surface of a cavity preparation. The ceramic liners are composed of SiO2, Al2O3, Na2O, K2O, CaO, P2O5 and F. This composition is similar to that of veneer ceramics, which promote an effective bond between these materials. Liners can be applied as a layer between Y-TZP and veneering ceramic to mask the substructure color and increase the wettability of the zirconia surface [15]. Studies have shown that the use of a liner increased the bond strength of Y-TZP and veneering ceramic when compared with other surface treatments [15,28], while other studies have shown that the liner decreased the bond strength [29,30]. There is no consensus in the literature of the best surface treatment to increase the bond strength between Y-TZP and veneer ceramic. Alternative sonochemical treatment can be used to nano-modify surfaces and has been used to improve the surface functionality of ceramic oxides, polymers, and metals [31]. The effects of this treatment are derived from acoustic cavitation by the implosive collapse of bubbles, resulting in conversion of the surface and kinetic energy of liquid motion into heat and chemical energy [32]. The advantages of this treatment for alteration of materials are: the potential performance chemical and physical at high temperature, highly non-equilibrium physicochemical processes, and no need to use other reagents [31]. Some literature indicates that Sialon (a ceramic material including wt.% 88.3 Si3N4, 4.0 Al2O3, 7.7 Y2O3) and ZrO2 ceramics can provide advantages under cavitation when compared with alumina ceramics [33]. The aim of this study was to evaluate the morphological characterization and the effectiveness of sonochemical treatment on bond strength of Y-TZP to veneer ceramic. The null hypothesis tested was that this alternative treatment would improve the bond strength of YTZP to veneering ceramic.
roughening treatments groups (AOX, S40, S70), the liner was applied after each treatment. The specimens of the AOX group were first submitted to a final sintering and the sandblasting treatment was carried out with 50 μm alumina particles, under a pressure of 0.4 MPa for 10 s, perpendicular to the surface, at a distance of 10 mm, and using an airborne-particle abrasion device (Micro-Jet, BIOART, Sao Carlos, SP, Brazil) [34–36]. The two remaining groups were sonochemically treated with lowpower sound waves (S40) and high-power sound waves (S70), both producing acoustic cavitation on the surface of the specimens [31], through an ultrasonic liquid processor (Sonics Vibracell VCX-750, Sonics & Materials Inc., Newtown, CT, USA). The sonochemical treatment was performed for 15 min for both powers (40 and 70% nominal power), with the Y-TZP discs fixed to a device in order to standardize their centered position at the bottom of a beaker filled with deionized water (Fig. 1). The specimens were sintered according to the manufacturer’s instructions, as shown in Table 2. The final dimensions obtained were a 10.5 mm diameter and 2.8 mm thickness.
2. Materials and methods
2.3. Phase analysis
2.1. Sample preparation
Y-TZP discs (n = 10) were evaluated with a Jobin Yvon microRaman system (T64000, Groupe Horiba—Longjumeau, France). An argon ion laser excitation source (Spectra Physic, Inc., California, USA) was used with a radiation of 514.5 nm (2.41 eV), and the beam was focused by 500x microscope magnification. The laser power was maintained at 10 mW to avoid thermal damage. The Raman spectra analyses were performed using a double subtractive monochromator with a focal distance of 0.64 M and equipped with a diffraction grating with 1,800 grooves/mm. This provided a spectral resolution of 2 cm with a slit width of 200 µm. Spectra were collected with a CCD camera (Spectra One—Groupe Horiba, Longjumeau, France). All specimens were evaluated using the average of 3 measurements for each disc.
2.2. Morphological characterization The discs (n = 10) of each group were analyzed at one site (400 µm of scanning area) by confocal laser scanning microscopy (CLSM) (DCM 3D Model, Leica Microsystems, Wetzlar, Germany) in order to evaluate the surface roughness (Ra) in nm and surface topography. An atomic force microscope (AFM) (XE-70, Park Systems, Tokyo, Japan) was used to obtain 3-dimensional images (n = 1). The measurements were performed in non-contact mode with an adjusted cantilever at a distance around 5.8 nm (set point), and the scan area was 4 × 4 mm in the center of the specimens. A morphological analysis of the zirconia surface was performed in a sample of each group with a scanning electron microscope (JSM 5600LV 353, JEOL, Tokyo, Japan). The analyzing procedures were carried out with x1000 magnification.
Discs were obtained from pre-sintered Y-TZP blocks (15.5 mm width × 19 mm length × 39 mm height) (IPS e.max ZirCAD, Ivoclar Vivadent, Schaan, Liechtenstein). They were milled from a cylindrical shape with a 12.5 mm diameter and 39 mm height. After that, each cylinder was cut using an Isomet 1000 cutter (Buehler, Lake Bluff, IL, USA) and diamond disc (series 15LC Diamond no. 11–4254, Buehler, Lake Bluff, IL, USA) at a 275 rpm low speed under water cooling. The specimens were subjected to a polishing machine (EXACT, Nordestedt, Schleswing-Holstein, Germany) with #1000 and #1200 sandpapers (Polishing paper K2000, EXACT, Nordestedt, SchleswingHolstein, Germany), followed by a sequence of treatments with felt discs of medium, fine, and extra-fine granulations with diamond paste (Polishing paper K2000, EXACT, Nordestedt, Schleswing- Holstein, Germany). The specimens were cleaned by double cycle soaking in 100% ethanol and distilled water in an ultrasonic cleaning (USC 700—Unique Industry and Trade of Electronic Products Ltda, Sao Paulo, SP, Brazil) for 10 min. Relevant information on the tested materials is presented in Table 1. The 40 pre-sintered Y-TZP discs (12.5 mm diameter and 3.5 mm thickness, before sintering) were randomly divided into 4 groups (n = 10) according to the intended surface treatment: application of the liner only (C), sandblasting with 50-µm aluminum particles (AOX), presintered sonication with 40% nominal power (S40), and pre-sintered sonication with 70% nominal power (S70). In the C group, after the final sintering of the specimens, no roughening treatment was performed before liner application. In the
2.4. Veneering ceramic application To build the veneering ceramic layer, the IPS e.max Ceram and Ceram Dentin and Build-Up system was used (Ivoclar Vivadent AG, Schaan, Liechtenstein), which applied a ZirLiner layer (ZirLiner, Ivoclar Vivadent AG, Schaan, Liechtenstein) on the surface of all of the Y-TZP discs. Thereafter, the specimens were taken to be fired, following a temperature control protocol (Table 2). Due to the limited heat conductivity of zirconium oxide, a firing wash is required. This wash was a mixture of the required IPS Ceram Dentin or Deep Dentin with IPS e.max Ceram Build-Up liquids. The firing wash was conducted to obtain a homogeneous connection with the ZirLiner layer. After that, the standard firing parameters for the material were followed. A custom-designed metallic device (5 mm in diameter and 5 mm in 2
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Table 1 Materials used in this study and their classification, manufacturer, composition, and coefficient of thermal expansion (CTE). Material/Code
Classification
Manufacturer
Composition (% by weight)
CTE
IPS e.max Zircad**/EZ IPS e.max Ceram/EC IPS e.max Ceram ZirLiner
Y-TZP ceramic Veneer ceramic Liner
Ivoclar Vivadent, Schaan, Liechtenstein Ivoclar Vivadent, Schaan, Liechtenstein Ivoclar Vivadent, Schaan, Liechtenstein
ZrO2, 87%; Y2O3, 4%; HfO2, 1%; Al2O3, 1% SiO2, 60%; Al2O3, 8%; Na2O, 6%; K2O, 6%; ZnO, 8%; CaO, P2O5, F, 2% SiO2, 50%; Al2O3, 16%; Na2O, 6%; K2O, 4%; CaO, P2O5, F, 2.5%
10.75 9.5 9.8
*Coefficient of thermal expansion (CTE) in 10−6 K−1 between 100 °C and 500 °C. **Based on manufacturer’s information by technical profile.
2.6. Failure mode and Fractographic analysis The analysis of failure mode and Fractographic was performed for all the specimens with a digital microscope DinoLite AM313T (AnMoElectronics Corporation, Dung-Da Road, Taiwan) at 32x magnification. Each specimen was classified according to the type of failure: (a) adhesive failure at the Y-TZP/veneering ceramic interface, (b) cohesive failure of the ceramic veneer, or (c) mixed, meaning both types of failure were present. The types of failure were classified by digital microscopy, and their respective percentages were calculated. The Fractographic analysis of the bond surface fractured were used to detect the location of fracture origin and to determine the crack propagation. 2.7. Statistical analysis Data on SBS and roughness were calculated and statistically analyzed with Statistica software (Statsoft®, Tulsa, OK, USA). The assumptions that were made of the normal distribution and of the equality of variances were verified for all of the variables using the Shapiro-Wilk and Levene tests, respectively. As only the assumption of normality was satisfied, data were subjected to the Kruskal-Wallis test (α = 0.05), followed by the Dunn’s test (α = 0.05) for individual comparisons.
Fig. 1. Pre-sintering sample in deionized water for sonochemical treatment with a microtip. The Y-TZP discs were centered at the bottom of a beaker filled with deionized water.
Table 2 Recommended cycles for zirconia and liner firing.
3. Results
Heating schedule
IPS e.max ZirCAD™
IPS e.max Ceram ZirLienr
Standby temperature Drying time Heating rate Maximum firing temperature Holding time
403 °C 12 °C/min 65 °C/min 1050 °C 15 min
403 °C 40 °C/min 40 °C/min 960 °C 1 min
3.1. Morphological characterization The sandblasting and sonochemical surface treatments did not show a significant quantitative difference in increased surface roughness (p = 0.255). The median roughness (Ra) for each group was 121.98 nm (C), 119.95 nm (S40), 125.22 nm (S70), and 118.89 nm (AOX) as shown in Fig. 2. Representative images of CLSM (Fig. 3) and AFM (Fig. 4) of the zirconia surfaces show the micro and nanoscale morphological
height) was used for the application of the IPS e.max veneering ceramic (Ivoclar Vivadent AG, Schaan, Liechtenstein). The veneering ceramic powder was mixed with modeling liquid (Ivoclar Vivadent AG, Schaan, Liechtenstein) and placed in the device by manually condensed. The device was removed, and the specimens were sintered. The recommended firing procedures were utilized. After that, the specimens were ready for the shear bond strength testing. After sintering, each specimen was embedded in a polyvinyl chloride (PVC) cylinder (21 mm diameter and 25 mm height) using an acrylic resin (JET; Classic, Sao Paulo, SP, Brazil).
2.5. Shear bond strength test For this test, the specimens were stored for 24 h in water at 37 °C, following ISO TR 11405, Type I. After this step, all specimens were subjected to shear mechanical testing by a Kratos 5002 universal biomechanical test machine (Kratos Dynamômetros, Sao Paulo, Brazil), with a load cell of 500 kgf, in a special device with a metal strip, at a speed of 0.5 mm/min until fracture occurred. This device [37] was adapted from Sinhoreti et al. [38] in order to minimize bending forces during the test [39].
Fig. 2. Effect of surface treatment on the roughness of Y-TZP ceramic surfaces. Vertical bars indicate the inner quartile range, vertical lines are the maximum and minimum values, and the horizontal line represents the median. * Outlier (S70 = 87.08; AOX = 240.27). 3
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Fig. 3. Surface topography represented by 3D images of the selected groups: C (A), S40 (B), S70 (C), and AOX (D).
The Fig. 5 shows the different morphological surfaces after surface treatments on zirconia. In overview, in the C group (Fig. 5A), the surface is smoothest compared with other groups. S40 and S70 showed a similar surface with the C group, with small irregularities (Fig. 5B, C). The surface of AOX (Fig. 5D) group appeared more irregular which demonstrates the removal some of the surface material by the highimpact of alumina particles.
differences, respectively. The red color represents a vertical (z-axis) size of around 400 nm and the dark blue color represents −400 nm. From Fig. 3, it is possible to compare the surface morphologies. In microscale, all groups showed a similar pattern of surface roughness. The peaks and valleys ranged between 400 nm and − 400 nm in all groups (Fig. 3A–D). Some scratches were founded only in C and S40 groups due to polishing finishing (Fig. 3A and B). AFM was used to investigate the surface morphology in the nanoscale. Besides differences in roughness, we observed the grain size, topological changes in the nanostructure, and grain damage due to surface treatments (Fig. 4). It was observed that only sandblasting treatment promoted a damaged on the grain of the Y-TZP surfaces (Fig. 4D).
3.2. Phase analysis Fig. 6 shows the Raman spectra of each group, reporting the bands of the monoclinic and tetragonal phases. Previous literature has reported the major bands that represent each crystalline phase of Y-TZP. 4
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Fig. 4. AFM images of Y-TZP: C (A), S40 (B), S70 (C), and AOX (D).
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Fig. 7. Shear bond strength (SBS) of the veneering ceramics after surface treatment. The vertical bars indicate the inner quartile range, vertical lines represent the maximum and minimum values, and the horizontal line represents the median.
Fig. 5. SEM examination (magnification x1000) on zirconia surfaces after treatments: (A) C; (B) S40; (C) S70; (D) AOX.
Fig. 8. Results of the multiple Dunn’s test comparisons for shear bond strength. Intervals that do not contain zero have corresponding medians that are significantly different.
3.4. Failure mode analysis Fig. 6. Raman analysis of the four groups studied showing the Monoclinic (M) and Tetragonal (T) phases.
Fig. 10 shows the failure modes and the distribution of these modes for each group. Most specimens in all groups exhibited a mixed mode of failures (67.5%), and the remaining specimens showed adhesive failures. In the Fig. 11 is it possible to verify the three types of failure founded in the present study. Fig. 11-A represents an adhesive failure, without the presence of any residual part of veneer ceramic. Fig. 11-B characterizes a cohesive failure of veneer ceramic, where the bond strength on interface between Y-TZP and veneer ceramic was higher than the inner strength of the ceramic. Fig. 10-C denotes a mixed failure, where there is a combination of the adhesive and cohesive failures at the same time. Images of Fractographic analysis are described in Fig. 12. All groups showed similar fracture patterns, predominantly mixed failure mode. The hackle and arrest lines were founded in Fig. 12A-D that indicates the direction of fracture propagation.
The tetragonal phase is characterized by strong peaks at ~144–147, 262, 320, 466, 635–637 cm−1, while the monoclinic phase presents peaks at ~189, 304, 345–349, 379–383, 638–647 cm−1 [40,41]. The peaks of bands for all groups were similar, as follow: ~151 (t), 266 (t), 322 (t), 467 (t), 645 (m). All groups showed characteristic bands of the tetragonal (t) and monoclinic (m) phases, suggesting that there was a phase transformation (t → m). 3.3. Shear bond strength test The lowest median SBS (MPa) was obtained for the S40 group (15.43 MPa), which was lower than the C (29.9 MPa), AOX (25.0 MPa), and S70 groups (18.49 MPa) (Fig. 7). The Dunn’s test results showed a significant difference (p = 0.008) in the shear bond strengths between the C and S40 as well as the AOX and S40 groups. Also, the C and AOX groups exhibited a higher median strength than S40. This is shown in Fig. 8, wherein intervals that do not contain 0 signify that the corresponding medians are significantly different. The highest values of SBS of group C can be explained due to the bond interaction between Y-TZP and liner. A schematic representation to highlight and explain this possible chemical bond of this both materials can be founded on Fig. 9.
4. Discussion The null hypothesis tested in this study was rejected. The alternative sonochemical treatment does not improve the bond strength between YTZP and veneering ceramic. Despite the altered morphology that was observed for the sonochemical treatment, this change was not sufficient to improve the bond strength of Y-TZP to the veneer ceramic. Therefore, there are likely other factors that determine the zirconia 6
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Fig. 9. Schematic representation of the oxides presents in liner and zirconia that can flow and chemically interact with each other.
Fig. 10. Percentage of failure modes in each group (%) (n = 40). Fig. 12. Fractographic analysis of groups C (A), S40 (B), S70 (C) and AOX (D) (32x magnification). All groups showed arrest and hackle lines.
bond strength. This study did not show statistical quantitative differences among the studied groups for surface roughness, which was be confirmed by CLSM and AFM analysis. The average surface roughness was statistically similar in all groups on the microscale, but we detected some differences for the high-energy treatments in the nanoscale (Fig. 4). These results agree with a related study, wherein surface roughening with 110 µm Al2O3 (2.4 bar) sandblasting had no significant influence on the core veneer bond strength in comparison to the non-sandblasted groups [42]. Although there was no statistical difference in surface roughness analysis, the average highest values were found for the AOX group (Fig. 2), which can be confirmed by SEM images (Fig. 5D), which is more irregular pattern was observed compared to the other groups. Therefore, it is possible that there is no relationship between increased bond strength and the surface roughness alteration. The effect of sandblasting on increasing the SBS of Y-TZP/veneering ceramic was not confirmed in the current study. This could be explained by the fact that sandblasting affects the Y-TZP mechanical strength and probably the bonding capacity of the material [7,21]. This is due to the CTE difference between the tetragonal (10.8 ppm/K) and monoclinic (7.5 ppm/K) zirconia, as the monoclinic is considerably lower [43]. A modified nano-surface can be obtained by sonochemical therapy, whereby sound waves result in acoustic cavitation that is produced by the implosive collapse of bubbles. This cavitation potentially modifies the treated ceramic surface. Although sonochemical
treatment is innovative, it did not increase the zirconia/veneer SBS when compared with other treatments. Although no previous studies have used sonochemical treatment for zirconia, other studies have used this treatment to improve the surface functionality of ceramic oxides, polymers, and metals [31]. Therefore, studies with different nominal powers of sound waves should be performed in order to more thoroughly investigate the effect of this treatment on ceramics. The resultant surface from each treatment presented different surface characteristics. For AOX, the surface appeared rough and irregular, with well-defined, micro-sized elevations and depressions, possibly created by the high-impact of alumina particles. Alternatively, there was more regularity and homogeneity of the surfaces for the S40, S70, and C groups (Figs. 3 and 4). Even though sandblasting caused a significant change on the surfaces, the bond strength between Y-TZP and the veneering ceramic did not increase when compared with the groups treated with sonochemical treatment (S40 and S70). These results are in agreement with a related study wherein there was no influence of sandblasting on the shear strength for all zirconia and corresponding veneering ceramics that were investigated [42]. The application of a liner only (group C) increased the zirconia/ veneer SBS, probably due to the sufficient wetting of the liner over the zirconia surface [36]. These results are in agreement with other studies
Fig. 11. Failure modes by optical microscopy. (A) Adhesive, (B) Cohesive, and (C) Mixed (magnification 32×). 7
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geometry that allowed for measurements of shear bond strength. We believe that the results of this study will help to define a surface treatment protocol for zirconia. Nevertheless, more clinical and in vitro studies with sonochemical treatment of different nominal powers are needed to confirm the validity of these results.
that showed better results only with liner application [44–48]. The liner wettability over Y-TZP depends of the composition of the veneer and the liner, the morphology of the zirconia surface, and the surface energy of the core material [49]. This energy provides free atoms for chemical bonding between the ceramic materials [50]. The grain borders of YTZP, which have higher surface area interfaces [51], are composed of Y2O3 (4%), HfO2 (1%), and Al2O3 (1%) (Table 1). Al2O3 (16%) was also present in the liner, which suggests that these other oxides present in both materials can flow and chemically bond with each other (Fig. 9). In addition, the mechanism for adhesion between zirconia and veneering ceramic has not been fully explained, but the literature has reported the existence of a fusion between the zirconia and veneering ceramic, resulting in the diffusion of elements from each material at the bonded interface [39,55]. On the other hand, one study reported that there is no evidence of chemical bonding between liner/veneer and zirconia/liner, because dispersive X-ray spectrometry (EDX) analysis of the fractured zirconia showed no porcelain components from the liner on the surfaces of all specimens [36]. In this way, more study in this field should be performed to better understand the chemical bond between zirconia/liner/veneer ceramic. To investigate the types of fractures, microscopy is the most important technique for identifying the location of crack onset, size, and spread [52]. Regarding the type of failure, most specimens in all groups exhibited the mixed failure mode, while the remaining specimens exhibited adhesive failures. Few cohesive failures were observed, and none of the specimens fractured within the zirconia. Most mixed fractures that were observed began as an adhesive-type at the load application side and terminated as a cohesive-type on the opposite side [53]. Yoon et al. [54] observed, with the same sandblasting parameters, a higher percentage of mixed failures which agrees well with the observations in this study. In addition, Fractographic analysis data showed in all groups that fractures started from the tension surface and propagated to the compression surface for all groups. Raman analysis showed that all groups presented a crystallographic transition. The preparation of the specimens is likely to have caused this result, as the liner group (without roughening treatment) also showed identifiable monoclinic peaks. Some studies have reported the effect of sandblasting on the mechanical properties and bonding to zirconia [2,56]. It was observed that this process could lead to a phase transformation (t → m) of the surface grains, due to the formation of a compressive layer on the zirconia surface [57]. This resulted in surface flaws and voids, which could impair the longevity and clinical performance of zirconia-based restorations [58]. The clinical implication of this finding is that the previously investigated all-ceramic systems, instead of presenting catastrophic failure of the core, have a tendency to exhibit chip-off fractures and delamination of the veneering ceramic [42]. This study suggests that the sole application of a liner onto Y-TZP as a pretreatment to the zirconia substrate, has the highest bond strength between the zirconia and veneering ceramic. This treatment could be an alternative to sandblasting, as it is not so aggressive to the Y-TZP surface and it is, therefore, less harmful to the longevity of the ceramic restoration. The gold standard for SBS between the core of Y-TZP and its corresponding veneering ceramics was not reached [42]. The high rates of veneering ceramics chipping that are observed in clinical studies [59,60] may be explained by the interaction between both materials, their compositions, and processing. Clinical trials are still required to evaluate the long-term behavior of the interface of Y-TZP and veneering ceramics. According to results of this study, roughening treatments seem unnecessary to provide adequate bond strength at Y-TZP/veneering ceramic interface, since the recommended manufacturer protocol was the most effective. However, a limitation of this study was that specimen design and dimensions were not representative of the shape of dental restorations. This was done intentionally in order to provide a
5. Conclusions This study shows that sandblasting, although it leads to a higher surface roughness, does not influence the bond strength between Y-TZP and veneering ceramic. The sonochemical treatment with low-power sound waves led to a lower bond strength, but both high and low-power sound waves caused less damage to the ceramic surface than sandblasting. Liner application was the most effective treatment when considering bond strength, although all of the groups led to a phase transformation of the Y-TZP surface. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank Alcides Urias da Costa for his assistance with the study methodology and Nair Cristina for performing the statistical analyses. This study was supported by the Sao Paulo Research Foundation (FAPESP) #2011/18061-0. Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations References [1] S. Abou-Ayash, M. Strasding, G. Rücker, W. Att, Impact of prosthetic material on mid- and long-term outcome of dental implants supporting single crowns and fixed partial dentures: a systematic review and meta-analysis, Eur. J. Oral Implantol. 10 (1) (2017) 47–65. [2] S. Bhargava, H. Doi, R. Kondo, H. Aoki, T. Hanawa, S. Kasugai, Effect of sandblasting on the mechanical properties of Y-TZP zirconia, Biomed. Mater. Eng. 22 (6) (2012) 383–398. [3] P.F. Manicone, P.R. Iommetti, L. Raffaelli, An overview of zirconia ceramics: basic properties and clinical applications, J. Dent. 35 (11) (2007) 819–826. [4] A. Della Bona, J.R. Kelly, The clinical success of all-ceramic restorations, J. Am. Dent. Assoc. 139 (2008) S8–S13. [5] A.J. Raigrodski, A. Yu, G.J. Chiche, J.L. Hochstedler, L.A. Mancl, S.E. Mohamed, Clinical efficacy of veneered zirconium dioxide-based posterior partial fixed dental prostheses: five-year results, J. Prosthet. Dent. 108 (4) (2012) 214–222. [6] S.B. Bitencourt, D.M. Dos Santos, E.V.F. da Silva, V.A.R. Barão, E.C. Rangel, C. da Cruz, G.M. de Souza, M.C. Goiato, A.A. Pesqueira, Characterisation of a new plasma-enhanced film to improve shear bond strength between zirconia and veneering ceramic, Mater. Sci. Eng. C 1 (92) (2018) 196–205. [7] J. Fischer, P. Grohmann, B. Stawarczyk, Effect of zirconia surface treatments on the shear strength of zirconia/veneering ceramic composites, Dent. Mater. 27 (3) (2008) 448–454. [8] I. Sailer, B.E. Pjetursson, M. Zwahlen, C.H. Hämmerle, A systematic review of the survival and complication rates of all-ceramic and metal–ceramic reconstructions after an observation period of at least 3 years. Part II: fixed dental prostheses, Clin. Oral. Implants. Res. 18 (2007) 86–96. [9] I. Sailer, N.A. Makarov, D.S. Thoma, M. Zwahlen, B.E. Pjetursson, All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review of the survival and complication rates. Part I: single crowns (SCs), Dent. Mater. 31 (6) (2015) 603–623. [10] M.V. Swain, Unstable cracking (chipping) of veneering ceramic on all-ceramic dental crowns and fixed partial dentures, Acta. Biomater. 5 (5) (2009) 1668–1677. [11] M.N. Aboushelib, A.J. Feilzer, C.J. Kleverlaan, Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach, Dent. Mater. 25 (3) (2009) 383–391. [12] S.W. Pharr, E.C. Teixeira, R. Verrett, J.R. Piascik, Influence of veneering fabrication techniques and gas-phase fluorination on bond strength between zirconia and
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