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Effect of repeated laser surface treatments on shear bond strength between zirconia and veneering ceramic
RESEARCH AND EDUCATION
Effect of repeated laser surface treatments on shear bond strength between zirconia and veneering ceramic Adil O. Abdullah, BDS, HDD, MSc,a Hui Yu, BSc, MSc, PhD,b Sarah Pollington, BDS, MSc, PhD,c Fenik K. Muhammed, BDS, MSc,d Sun Xudong, BSc, MSc, PhD,e and Yi Liu, BDS, MSc, PhDf Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) has been used for dental applications including multiunit fixed dental prostheses, crowns, onlays, inlays, and implantsupported restorations because of its inertness, esthetics, and biocompatibility.1,2 Despite these advantages, ceramic restorations have higher failure rates than metal-ceramic restorations because of delamination between the zirconia framework and the veneering ceramic.3 Factors that lead to delamination include a thermal expansion mismatch between materials,4 inadequate veneering thickness,5 incorrect techniques for application of the veneering ceramics,6 and residual stress in the veneer exerted by the cooling cycle during the veneering procedure.7,8
ABSTRACT Statement of problem. Delamination failure may occur between ceramic frameworks and veneering ceramics, shortening the lifetime of fixed dental prostheses in load-bearing areas. Purpose. The purpose of this in vitro study was to compare the effect of different repeating CO2 laser treatment methods and conventional approaches on the shear bond strength of zirconia frameworks and veneering ceramics. Material and methods. Zirconia disks (N=110) were prepared and divided into 5 groups: milling without surface treatment (group M), airborne-particle abrasion (group APA), single laser treatment (group LX1), 2 laser treatments (group LX2), and 3 laser treatments (group LX3). The specimens in the first 2 groups were treated before the framework was coated using the spraying technique. Specimens in the remaining groups were coated with veneering ceramic using the spraying process, and then subjected to laser treatment. Surface roughness and topography, interface properties, phase transformation, shear bond strength, and fracture modes were investigated. Outcomes were analyzed using a profilometer, a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD), a stereomicroscope, and a universal testing machine for mechanical testing. Results. The XRD showed that phase transformation from tetragonal to monoclinic occurred after airborne-particle abrasion. This phenomenon was not observed in laser-treated specimens. Groups LX2 and LX3 had the highest surface roughness values, 1.18 ±0.23 mm and 1.21 ±0.22 mm, among all groups, and group LX3 had the highest shear bond strength values for unaged and aged conditions, 32.08 ±2.45 MPa and 31.43 ±2.07 MPa. The mixed-fracture mode was the most common type of fracture observed. Conclusions. The results indicated that the shear bond strength between the zirconia framework and veneering ceramic was higher after laser surface treatments than after milling alone or after airborne-particle abrasion. Laser treatment methods, particularly LX2 and LX3, could be considered reliable approaches for zirconia surface treatment. (J Prosthet Dent 2019;-:---)
This research was funded by the Liaoning Province Natural Science with grant # 20180550420, and Liaoning Province Key Research and Development Guidance Program grant # 2019JH8/10300015. a Graduate student, Dental Research Center, School and Hospital of Stomatology, China Medical University, Shenyang, PR China; and Assistant Lecturer, Prosthodontics Department, Erbil Polytechnic University, Erbil, Kurdistan Region Government, Iraq. b Lecturer, School of Environmental and Chemical Engineering, Dalian University, Dalian, PR China. c Clinical Lecturer, Restorative Department, School of Clinical Dentistry, The University of Sheffield, Sheffield, United Kingdom. d Graduate student, Dental Research Center, School of Stomatology, China Medical University, Shenyang, PR China. e Professor, School of Environmental and Chemical Engineering, Dalian University, Dalian, PR China. f Professor, School of Stomatology, China Medical University, Shenyang, PR China.
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Table 1. Ceramic materials used
Clinical Implications Enhancing shear bond strength between the zirconia framework and veneering ceramic with laser treatment could improve the longevity of zirconia-based prostheses.
Techniques such as airborne-particle abrasion, grinding, laser treatment, fine-brush painting, and airbrush spraying have been used for the surface treatment of zirconia.6,9-11 However, airborne-particle abrasion and grinding may result in phase transformation from tetragonal to monoclinic (t/m), leading to volumetric changes in the crystalline lattice structure of the zirconia.12 Laser technology has been used as an alternative approach to improve the surface of ceramic materials.13 Lasers used for ceramic surface treatment include carbon dioxide (CO2), neodymium-doped yttrium aluminum garnet (Nd:YAG), and erbium-doped yttrium aluminum garnet (Er:YAG).13 The CO2 laser has been recommended as an appropriate method for surface treatment because it creates high surface roughness and provides satisfactory shear bond strength values.14-17 A study evaluated the effect of different CO2 laser output powers on a zirconia framework and reported that surface damage caused by high laser output power may be more severe than that caused by low output power.18 However, low output power might not provide adequate surface roughness, leading to insufficient shear bond strength values.18 The adhesion between the veneering ceramic and the zirconia framework have been studied.13,16 The selection of material and the method of achieving the optimal bond between the 2 materials remain controversial.6,16 Studies on the effect of laser treatment on the shear bond strength of zirconia are limited.13,14,16-19 Thus, investigating the relationship between surface roughness, shear bond strength, and phase transformation, particularly t/m, is necessary. The purpose of this in vitro study was to assess the effect of different laser treatment methods on the shear bond strength of veneering ceramic to a zirconia framework. The research hypothesis was that laser treatment would improve the shear bond strength between the zirconia framework and the veneering ceramic. MATERIAL AND METHODS The compositions of the materials used in the study are presented in Table 1. Zenostar T Y-TZP disk-shaped specimens (N=110) were milled using a computer-aided design and computer aided-manufacturing (CAD-CAM) system (imes-icore GmbH). All the disks were sintered in
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Chemical Composition
Material
Material Type
Zenostar T
Translucent, yttriastabilized tetragonal zirconia polycrystal (Y-TZP), zirconia framework
ZrO2, HfO2, Y2O3,Y2O2, aluminum oxide, other oxides
Wieland Dental + Technik GmbH & Co KG
Manufacturer
IPS e.max Ceram
Fluorapatite glass ceramic
SiO2 60%-65%, Al2O3 8%-12%, Na2O 6%-9%, K2O 6%-8%, ZnO 2%-3%, CaO; P2O5; F 2%-6%, other oxides 2%8.5%, and pigments 0.1%-1.5%
Ivoclar Vivadent AG
a furnace (Austromat 674; Dekema Dental-Keramiköfen GmbH) in accordance with the manufacturer’s instructions. The size of each specimen was set at 10 mm in diameter and 5 mm in thickness, and the surface of the specimens was polished using a polishing kit (Zenostar T; Wieland Dental + Technik GmbH & Co KG). All specimens were ultrasonically cleaned in an ethanol solution using a digital ultrasonic cleaner for 10 minutes and dried using an electric oven for 5 minutes before treatment with the surface coating. The specimens were divided into 5 groups, as summarized in Table 2. The surface treatment of the specimens (N=110) was conducted using different approaches. The specimens in the control group (n=22) underwent milling without surface treatment (group M). Specimens in the airborneparticle abrasion group (group APA; n=22) were subjected to an abrasion process using 50-mm alumina particles with an abrasion machine (model S-606 Refo, Dental Sand Blaster; Serica Dental Supply). Each specimen underwent abrasion for 10 seconds at a pressure of 0.25 MPa, and the working distance was set at 15 mm. The direction of the airborne-particle abrasion process was perpendicular to the surface of the specimens. After the various surface treatments, the zirconia specimens were coated with the veneering ceramic. The ceramic powder was weighed on a digital analytical balance (Entris; Sartorius AG) and then mixed with the corresponding liquid following the manufacturer’s instructions. A minimagnetic stirrer was used to achieve a uniform mixture. The specimens were positioned on a flat surface and sprayed using a mini-airbrush sprayer (HS08AC-Skc; Ningbo Haosheng Pneumatic Machinery Co, Ltd) to allow uniform deposition of the veneering ceramic on the top surface of the zirconia. The main spraying parameters are presented in Table 3. Specimens in groups LX1, LX2, and LX3 were transferred to a flat XeY laser table and subjected to laser treatment. A CO2 laser (model JL-K6040; Julong Co, Ltd) was used to treat specimens in the 3 laser treatment groups (n=22). The laser treatment approaches for surface treatment of specimens are presented in Table 2. Laser Abdullah et al
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Table 2. Classification and description of experimental groups
Table 3. Coating parameters for airbrush spraying
Group
n
M (milling only)
22
Specimens not subjected to surface treatment, left as milled
Mixing ratio of veneering ceramic to corresponding liquid
APA (airborne-particle abrasion)
22
Specimens subjected to abrasion
Mixing speed and duration (using mini magnetic stirrer)
LX1 (single laser treatment)
22
Specimens subjected to 1 laser treatment
Distance between zirconia top surface and airbrush nozzle tip
LX2 (2 laser treatments)
22
Specimens subjected to 2 laser treatments; after first treatment, the specimens were rotated 90 degrees in clockwise direction and then second treatment performed
Nozzle diameter
0.2 mm
Working pressure
0.25 MPa
LX3 (3 laser treatments)
22
Specimens subjected to 3 laser treatments; after first and second treatments, specimens were rotated 90 degrees in clockwise direction and then next treatment performed
Drying time at ambient room temperature
Description
Coating Parameter
Value
Spray time
1:1.5 350 rpm for 10 min 15 cm
10 s 5 min
rpm, revolutions per minute.
Table 4. Typical CO2 laser treatment parameters and properties
parameters and properties, which were similar for all 3 laser treatments, are shown in Table 4. The time interval between each rotation and new treatment was 10 seconds. After the first laser treatment, specimens were left at room temperature to cool slowly. Surface roughness (Ra) values of the treated framework surfaces were determined by using a profilometer (Surtronic 25; Taylor Hobson Ltd). Recalibration for each specimen was conducted using a standard framework specimen (6 mm) that was provided by the manufacturer. A cut-off length of 0.08 mm was determined, and the specimens were positioned on a flat surface for the measurement of surface roughness. Three readings were recorded for each specimen, and the mean values were calculated. An X-ray diffractometer (SmartLab; Rigaku Corp) using CuKa radiation (l) of 1.5406 Å at 200 mA and 40 kV was used to identify phase transformations. Diffraction data were collected within the 2q range of 20 degrees to 90 degrees at a step size of 0.02 degrees and a step time of 8 minutes. The peaks were used to identify phase transformations. In each group, 2 specimens were selected and used for surface coating investigation (1 specimen for the interface and 1 for the top surface) with a scanning electron microscope (SEM) (Zeiss Ultra Plus; Carl Zeiss Microscopy GmbH) equipped with energy dispersive spectroscopy (EDS). The interfaces of specimens were studied after the specimens had been sectioned into the desired shapes using a slow-speed saw (model 5410; Sherline Products) with water as the coolant. The specimens were polished using SiC grit sizes #600 and #800 on a flat surface under running water. A sputter coater (JS-1600; Beijing HTCY Technology Co, Ltd) was used for gold sputtering. SEM images, spot EDS spectra, and line EDS spectra of specimens were recorded and analyzed. The specimens were divided into unaged (n=10) and aged (n=10) groups. Specimens were cleaned ultrasonically in ethanol solution for 15 minutes and then dried. A silicone mold was fabricated for the preparation of veneering ceramics in a cylindrical shape. Veneering Abdullah et al
Laser Parameter Laser treatment speed Output power Distance between laser tube and zirconia Space between scanned lines Laser treatment duration Laser tube diameter
Value 35 mm s-1
Laser Property Wavelength
Value 10.6 mm
20 W
Frequency
50 Hz
18 mm
Spot size Intensity
0.2 mm 0 to 1.6 ×106 W/cm2
0.25 mm 65 s
Pulse length Pulse energy Feed speed
10 to 50 ms 150 W 0 to 300 mm s-1
8 mm
ceramic powder and corresponding liquid were mixed following manufacturer’s directions and condensed into the silicone mold through a premade circular hole. The cylinder produced was 5 mm in diameter and 3 mm in height and was centered on the zirconia framework. A lubricating agent (Wagnersil S200; Wagner Dental GmbH & Co KG) was used around the hole to limit adhesion to the silicone mold. Gentle vibration and tissue paper were used to remove excess liquid during condensation of the veneering ceramic slurry. The manufacturer’s firing protocol for veneering ceramic material was followed. A second layer of the veneering ceramic slurry was added and refired to compensate for shrinkage of the veneering ceramic after the first firing cycle. The shear bond test was performed for aged and unaged specimens. The aged specimens were stored in water at 37 C for 1 month. A semicircular metal jig connected to a universal testing machine (model E44; MTS Systems Corp) was used to conduct the shear bond test with specimens secured on the machine. The tip of the metal jig was positioned close to the interface between the veneering ceramic and the zirconia framework at a crosshead speed of 1 mm/min until failure, and failure forces were recorded in megapascal (MPa). The fracture patterns were examined visually and under a stereomicroscope (model SZ61; Olympus Corp) to identify different fracture modes, defined as adhesive, cohesive, or mixed. The fracture mode values were recorded as percentages.
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Table 5. Summary of surface roughness and shear bond strength values Group Ra (mm) Unaged SBS (MPa) Aged SBS (MPa) 0.73 ±0.20a
27.1 ±2.13a
26.47 ±3.19a
APA
0.79 ±0.21a
27.9 ±2.73a
27.27 ±2.09a
LX1
1.15 ±0.21
b
b
30.46 ±2.03b
LX2
1.18 ±0.23b
31.58 ±2.54b
30.93 ±2.10b
LX3
1.21 ±0.22b
32.08 ±2.45b
31.43 ±2.07b
31.09 ±2.31
Ra (surface roughness); SBS, shear bond strength. Values marked with different letters indicate statistically significant differences among groups (P<.001).
The results were statistically analyzed using statistical software (IBM SPSS Statistics, v20.0; IBM Corp). The KolmogoroveSmirnov test for normality was used to check the distribution of surface roughness and shear bond strength values. Homogeneity of variances was investigated using the Levene test. One-way ANOVA was used to compare surface roughness and shear bond strength mean values between groups. The ANOVA post hoc Tukey HSD test was performed for multiple comparisons. The Pearson test was used to analyze the correlation between surface roughness and shear bond strength values. The chi-square test was used to compare mean values of fracture modes (a=.05). RESULTS Surface roughness and shear bond strength values for the study groups are presented in Table 5. The surface roughness values indicated a statistically significant difference among all study groups (P<.001). However, the outcomes were not statistically significant between groups M and APA (P>.05) or among the laser treatment groups (P>.05). Groups LX3 and LX2 showed the highest surface roughness values, 1.18 ±0.23 mm and 1.21 ±0.22 mm, among all groups. The lowest surface roughness value of 0.73 ±0.20 mm was observed in group M. The top surface of specimens showed different roughness patterns after surface treatments. Group M specimens showed grooves, whereas group APA specimens had flake-like defects in certain areas. The lasertreated specimens showed microholes and irregularities within the deposited veneering ceramic on the zirconia framework. Strong adhesion was detected at the interface between the deposited veneering ceramic material and the zirconia framework after laser treatment, with holes within the veneering ceramic layer in groups M and APA. In addition, irregularities and microcracks were observed at the interfaces between veneering ceramic and the zirconia framework in the study groups. XRD analyses did not reveal phase transformation after laser treatment in groups LX1, LX2, and LX3 or in control specimens (group M). Prominent peaks representing the tetragonal (t) phase were found in these groups (Fig. 1). However, phase transformation from tetragonal to monoclinic (t/m) was detected in the
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Tetragonal
Monoclinic
60
80
-
(c)
Intensity (a.u.)
M
-
(b)
(a) 20
30
40
50
70
90
2θ (deg.) Figure 1. X-ray diffractometry graph showing zirconia crystallographic peaks. Zirconia framework did not present phase transformation after milling only (a) and laser treatments (b). Airborne-particle abrasion showed phase transformation from tetragonal to monoclinic (c).
airborne-particle abrasion group. The major peak of the tetragonal (t) phase was detected at 30.170 degrees, and other (t) phases were found at 34.742 degrees, 35.278 degrees, and 50.212 degrees. A major monoclinic (m) peak was found at 29.960 degrees. Table 5 presents the shear bond strength of each group under unaged and aged conditions. A statistically significant difference was found among the study groups (P<.001). The highest mean values of shear bond strength were recorded in group LX3 at 32.08 ±2.45 MPa and in group LX2 at 31.58 ±2.23 MPa. The minimum value was detected in group M at 27.1 ±2.13 MPa, followed by group APA at 27.9 ±2.73 MPa. Group M presented the lowest shear bond strength value, whereas groups LX3 and LX2 showed the highest shear bond strength, followed by group LX1 and group APA. The Pearson test demonstrated a positive significant correlation between surface roughness and shear bond strength values (r=0.9597, R2=0.9211, P<.001). The highest percentage of adhesive fracture mode was recorded in group M, followed by groups APA and LX1. The lowest percentages were observed in groups LX2 and LX3. Group LX2 showed the highest percentage of cohesive fracture mode, followed by group LX3. The minimum percentage of cohesive fracture mode was detected in groups M and APA. The mixed fracture mode was a common fracture pattern observed in all studied groups. The chi-square test did not show a statistically significant difference among the investigated groups (P>.05). Typical fracture mode patterns are presented in Table 6. Spot EDS analyses detected prominent peaks of silicon, aluminum, and zirconia. The presence of the first 2 Abdullah et al
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Table 6. Summary of fracture mode patterns Fracture Mode (%) Group*
Adhesive
Cohesive
M
40
10
Mixed 50
APA
35
10
55
LX1
35
15
50
LX2
30
25
45
LX3
30
20
50
*Chi-square test did not show statistically significant difference in fracture mode patterns among groups (P>.05).
elements indicated coverage of the framework surface by veneering ceramic, whereas the prominent zirconia peak indicated that the surface of the framework had delaminated from the veneering ceramic. Line EDS analyses did not show changes in the elemental compositions of specimens subjected to different surface treatment methods. DISCUSSION The research hypothesis was accepted because laser treatment improved shear bond strength. The present study indicates that laser treatment applied 2 or 3 times could increase surface roughness and provide satisfactory shear bond strength values. The results are consistent with those of earlier observations.6,16 Consistent with the findings of previous studies,16,20 the XRD analyses showed that airborne-particle abrasion induced crystallographic transformation of the zirconia from t/m. However, laser treatment groups and the milling-only group did not show such changes. Therefore, the structural integrity of zirconia after laser treatment is a promising indicator for longevity of fusion between the veneering ceramic and the zirconia framework. In this study, different top-surface topography was observed after milling only, airborne-particle abrasion, and laser treatment. Microholes were detected within coated veneering ceramic after laser treatment which should promote adhesion between veneering ceramic and zirconia by providing micromechanical retention. Microcracks were detected on the zirconia framework. These defects may have been due to the repeating firing cycle performed to fabricate the veneering ceramic cylinder on the zirconia for the shear bond strength test. In addition, the framework surface could also have been under internal pressure during the load application of shear bond strength testing. The results are consistent with those of a previous study.21 EDS analyses revealed different elemental compositions after zirconia frameworks were coated with the veneering ceramic. The failure to detect changes in elemental compositions may reflect the conservative approach adopted in this study. Previous studies have found that bond strength may depend primarily on the
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existence of silicon and oxygen on a smooth ceramic surface and may be a function of the composition and surface chemistry of ceramics.22 The results of the study are consistent with those of previous studies, provided that silicon and oxygen were the predominant elements on the flat framework surface.6,16 However, the specimens with mixed fracture mode demonstrated prominent silicon and oxygen peaks in their veneering ceramic areas and a prominent zirconia peak in their delaminated areas. These characteristics may reflect increased bond strength, which may be attributed to the variation in the elemental compositions and microstructure of the veneering ceramic and zirconia. An interlocking mechanism could also play a primary role in maximizing bond strength through the micromechanical interaction and chemical bonding of the treated zirconia surface.6 The presence of high amounts of silicon could provide a basis for this chemical integration.6 This study showed that different fracture mode patterns developed after laser treatment. Groups LX2 and LX3 showed the highest percentage of cohesive fracture mode among all groups. The highest percentage of adhesive fracture mode was recorded in the control group (group M). However, specimens subjected to 2 and 3 repetitions of laser treatment showed a minimal percentage of adhesive fracture. Existing high percentages of cohesive fracture mode in specimens that received 2 and 3 cycles of laser treatment may be attributed to changes in physical properties. The changes may be because of the appropriate fusion between the 2 materials. The Pearson test showed a significant positive correlation between surface roughness and shear bond strength, and this relation may be because of the 90-degree clockwise rotation of each specimen, in particular LX2 and LX3, which optimized the treated surface area. The results of this study show that laser treatment applied 2 or 3 times could be considered an alternative method to milling only and airborne-particle abrasion and may prevent the zirconia framework from undergoing delamination. Laser treatment can simplify the zirconia coating and yield a rougher framework surface that provides satisfactory shear bond strength. Limitations of the current study include the fact that laser treatment was not performed more than 3 times and that a thermocycling test was not conducted. Also, the fracture strength of treated zirconia after surface treatment was not measured. The current study did not evaluate the wetting property and the coefficient of thermal expansion. These aspects should be incorporated into future studies. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn:
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1. Laser treatment applied 2 or 3 times improved bond strength between veneering ceramic and zirconia and could be considered an acceptable technique for zirconia surface treatment. 2. The structural integrity of zirconia was preserved after laser and milling techniques. 3. Airborne-particle abrasion did not preserve the structural integrity of zirconia and resulted in phase transformation. 4. The single laser treatment provided satisfactory results but was inferior to laser treatment applied 2 or 3 times. 5. The fracture surface of the specimens revealed a mainly mixed fracture mode. REFERENCES 1. Schünemann FH, Galárraga Vinueza ME, Magini R, Fredel M, Silva F, Souza JCM, et al. Zirconia surface modifications for implant dentistry. Mater Sci Eng C Mater Biol Appl 2019;98:1294-305. 2. Liu D, Matinlinna JP, Tsoi JK, Pow EH, Miyazaki T, Shibata Y, et al. A new modified laser pretreatment for porcelain zirconia bonding. Dent Mater 2013;29:559-65. 3. Diniz AC, Nascimento RM, Souza JC, Henriques BB, Carreiro AF. Fracture and shear bond strength analyses of different dental veneering ceramics to zirconia. Mater Sci Eng C Mater Biol Appl 2014;38:79-84. 4. Steiner PJ, Kelly JR, Giuseppetti AA. Compatibility of ceramic-ceramic systems for fixed prosthodontics. Int J Prosthodont 1997;10:375-80. 5. Hammad IA, Talic YF. Designs of bond strength tests for metal ceramic complexes: review of the literature. J Prosthet Dent 1996;75:602-8. 6. Muhammed F, Pollington S, Sun X, Abdullah A, Liu Y. Novel coatings on zirconia for improved bonding with veneer ceramics. Coatings 2018;8:363. 7. Kim HJ, Lim HP, Park YJ, Vang MS. Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105: 315-22. 8. Reginato VF, Kemmoku DT, Caldas RA, Bacchi A, Pfeifer CS, Consani RLX. Characterization of residual stresses in veneering ceramics for prostheses with zirconia framework. Braz Dent J 2018;29:347-53. 9. Dapieve KS, Guilardi LSF, Silvestri T, Rippe MP, Pereira GKR, Valandro LF. Mechanical performance of Y-TZP monolithic ceramic after grinding and aging: survival estimates and fatigue strength. J Mech Behav Biomed Mater 2018;87:288-95. 10. Soltaninejad F, Valian A, Moezizadeh M, Khatiri M, Razaghi H, Nojehdehian H. Nd:YAG laser treatment of bioglass-coated zirconia surface and its effect on bond strength and phase transformation. J Adhes Dent 2018;20:1-9.
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11. Caravaca CF, Flamant Q, Anglada M, Gremillard L, Chevalier J. Impact of sandblasting on the mechanical properties and aging resistance of alumina and zirconia based ceramics. J Mech Behav Biomed 2018;38: 915-25. 12. Chevalier J, Gremillard L, Virkar Anil V, Clarke David R. The tetragonalmonoclinic transformation in zirconia: lessons learned and future trends. J Am Ceram Soc 2009;92:1901-20. 13. Arami S, Tabatabae MH, Namdar SF, Chiniforush N. Effects of different lasers and particle abrasion on surface characteristics of zirconia ceramics. J Dent (Tehran) 2014;11:233-41. 14. Shukla PP, Lawrence J. Evaluation of fracture toughness of ZrO2 and Si3N4 engineering ceramics following CO2 and fibre laser surface treatment. Opt Laser Eng 2011;49:229-39. 15. Tzanakakis E-GC, Tzoutzas IG, Koidis PT. Is there a potential for durable adhesion to zirconia restorations? A systematic review. J Prosthet Dent 2016;115:9-19. 16. Abdullah AO, Hui Y, Sun X, Pollington S, Muhammed FK, Liu Y. Effects of different surface treatments on the shear bond strength of veneering ceramic materials to zirconia. J Adv Prosthodont 2019;11:65-74. 17. Abdullah AO, Muhammed FK, Yu H, Pollington S, Xudong S, Liu Y. The impact of laser scanning on zirconia coating and shear bond strength using veneer ceramic material. Dent Mater J 2019;38:452-63. 18. Ural C, KalyoncuoGlu E, Balkaya V. The effect of different power outputs of carbon dioxide laser on bonding between zirconia ceramic surface and resin cement. Acta Odontol Scand 2012;70:541-6. 19. Martins FV, Mattos CT, Cordeiro WJB, Fonseca EM. Evaluation of zirconia surface roughness after aluminum oxide airborne-particle abrasion and the erbium-yag, neodymium-doped YAG, or CO2 lasers: a systematic review and meta analysis. J Prosthet Dent 2019;121:895-903.e2. 20. Tada K, Sato T, Yoshinari M. Influence of surface treatment on bond strength of veneering ceramics fused to zirconia. Dent Mater J 2012;31: 287-96. 21. Kim JW, Covel NS, Guess PC, Rekow ED, Zhang Y. Concerns of hydrothermal degradation in CAD-CAM zirconia. J Dent Res 2010;89:91-5. 22. Hooshmand T, Daw R, van Noort R, Short RD. XPS analysis of the surface of leucite-reinforced feldspathic ceramics. Dent Mater 2001;17:1-6. Corresponding author: Dr Yi Liu School of Stomatology China Medical University Shenyang, Liaoning Province, 110002 PR CHINA Email: [email protected] Acknowledgments Adil O. Abdullah would like to express his heartfelt gratitude to the Chinese Government Scholarship Council (CSC) for financial support throughout the study. The authors would also like to acknowledge Northeastern University and Qing Mei Dental Laboratory Technology Co, Ltd in Shenyang, PR China for providing help for conducting the experimental work. Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.10.007
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Effect of repeated laser surface treatments on shear bond strength between zirconia and veneering ceramic Adil O. Abdullah, BDS, HDD, MSc,a Hui Yu, BSc, MSc, PhD,b Sarah Pollington, BDS, MSc, PhD,c Fenik K. Muhammed, BDS, MSc,d Sun Xudong, BSc, MSc, PhD,e and Yi Liu, BDS, MSc, PhDf Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) has been used for dental applications including multiunit fixed dental prostheses, crowns, onlays, inlays, and implantsupported restorations because of its inertness, esthetics, and biocompatibility.1,2 Despite these advantages, ceramic restorations have higher failure rates than metal-ceramic restorations because of delamination between the zirconia framework and the veneering ceramic.3 Factors that lead to delamination include a thermal expansion mismatch between materials,4 inadequate veneering thickness,5 incorrect techniques for application of the veneering ceramics,6 and residual stress in the veneer exerted by the cooling cycle during the veneering procedure.7,8
ABSTRACT Statement of problem. Delamination failure may occur between ceramic frameworks and veneering ceramics, shortening the lifetime of fixed dental prostheses in load-bearing areas. Purpose. The purpose of this in vitro study was to compare the effect of different repeating CO2 laser treatment methods and conventional approaches on the shear bond strength of zirconia frameworks and veneering ceramics. Material and methods. Zirconia disks (N=110) were prepared and divided into 5 groups: milling without surface treatment (group M), airborne-particle abrasion (group APA), single laser treatment (group LX1), 2 laser treatments (group LX2), and 3 laser treatments (group LX3). The specimens in the first 2 groups were treated before the framework was coated using the spraying technique. Specimens in the remaining groups were coated with veneering ceramic using the spraying process, and then subjected to laser treatment. Surface roughness and topography, interface properties, phase transformation, shear bond strength, and fracture modes were investigated. Outcomes were analyzed using a profilometer, a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD), a stereomicroscope, and a universal testing machine for mechanical testing. Results. The XRD showed that phase transformation from tetragonal to monoclinic occurred after airborne-particle abrasion. This phenomenon was not observed in laser-treated specimens. Groups LX2 and LX3 had the highest surface roughness values, 1.18 ±0.23 mm and 1.21 ±0.22 mm, among all groups, and group LX3 had the highest shear bond strength values for unaged and aged conditions, 32.08 ±2.45 MPa and 31.43 ±2.07 MPa. The mixed-fracture mode was the most common type of fracture observed. Conclusions. The results indicated that the shear bond strength between the zirconia framework and veneering ceramic was higher after laser surface treatments than after milling alone or after airborne-particle abrasion. Laser treatment methods, particularly LX2 and LX3, could be considered reliable approaches for zirconia surface treatment. (J Prosthet Dent 2019;-:---)
This research was funded by the Liaoning Province Natural Science with grant # 20180550420, and Liaoning Province Key Research and Development Guidance Program grant # 2019JH8/10300015. a Graduate student, Dental Research Center, School and Hospital of Stomatology, China Medical University, Shenyang, PR China; and Assistant Lecturer, Prosthodontics Department, Erbil Polytechnic University, Erbil, Kurdistan Region Government, Iraq. b Lecturer, School of Environmental and Chemical Engineering, Dalian University, Dalian, PR China. c Clinical Lecturer, Restorative Department, School of Clinical Dentistry, The University of Sheffield, Sheffield, United Kingdom. d Graduate student, Dental Research Center, School of Stomatology, China Medical University, Shenyang, PR China. e Professor, School of Environmental and Chemical Engineering, Dalian University, Dalian, PR China. f Professor, School of Stomatology, China Medical University, Shenyang, PR China.
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Table 1. Ceramic materials used
Clinical Implications Enhancing shear bond strength between the zirconia framework and veneering ceramic with laser treatment could improve the longevity of zirconia-based prostheses.
Techniques such as airborne-particle abrasion, grinding, laser treatment, fine-brush painting, and airbrush spraying have been used for the surface treatment of zirconia.6,9-11 However, airborne-particle abrasion and grinding may result in phase transformation from tetragonal to monoclinic (t/m), leading to volumetric changes in the crystalline lattice structure of the zirconia.12 Laser technology has been used as an alternative approach to improve the surface of ceramic materials.13 Lasers used for ceramic surface treatment include carbon dioxide (CO2), neodymium-doped yttrium aluminum garnet (Nd:YAG), and erbium-doped yttrium aluminum garnet (Er:YAG).13 The CO2 laser has been recommended as an appropriate method for surface treatment because it creates high surface roughness and provides satisfactory shear bond strength values.14-17 A study evaluated the effect of different CO2 laser output powers on a zirconia framework and reported that surface damage caused by high laser output power may be more severe than that caused by low output power.18 However, low output power might not provide adequate surface roughness, leading to insufficient shear bond strength values.18 The adhesion between the veneering ceramic and the zirconia framework have been studied.13,16 The selection of material and the method of achieving the optimal bond between the 2 materials remain controversial.6,16 Studies on the effect of laser treatment on the shear bond strength of zirconia are limited.13,14,16-19 Thus, investigating the relationship between surface roughness, shear bond strength, and phase transformation, particularly t/m, is necessary. The purpose of this in vitro study was to assess the effect of different laser treatment methods on the shear bond strength of veneering ceramic to a zirconia framework. The research hypothesis was that laser treatment would improve the shear bond strength between the zirconia framework and the veneering ceramic. MATERIAL AND METHODS The compositions of the materials used in the study are presented in Table 1. Zenostar T Y-TZP disk-shaped specimens (N=110) were milled using a computer-aided design and computer aided-manufacturing (CAD-CAM) system (imes-icore GmbH). All the disks were sintered in
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Chemical Composition
Material
Material Type
Zenostar T
Translucent, yttriastabilized tetragonal zirconia polycrystal (Y-TZP), zirconia framework
ZrO2, HfO2, Y2O3,Y2O2, aluminum oxide, other oxides
Wieland Dental + Technik GmbH & Co KG
Manufacturer
IPS e.max Ceram
Fluorapatite glass ceramic
SiO2 60%-65%, Al2O3 8%-12%, Na2O 6%-9%, K2O 6%-8%, ZnO 2%-3%, CaO; P2O5; F 2%-6%, other oxides 2%8.5%, and pigments 0.1%-1.5%
Ivoclar Vivadent AG
a furnace (Austromat 674; Dekema Dental-Keramiköfen GmbH) in accordance with the manufacturer’s instructions. The size of each specimen was set at 10 mm in diameter and 5 mm in thickness, and the surface of the specimens was polished using a polishing kit (Zenostar T; Wieland Dental + Technik GmbH & Co KG). All specimens were ultrasonically cleaned in an ethanol solution using a digital ultrasonic cleaner for 10 minutes and dried using an electric oven for 5 minutes before treatment with the surface coating. The specimens were divided into 5 groups, as summarized in Table 2. The surface treatment of the specimens (N=110) was conducted using different approaches. The specimens in the control group (n=22) underwent milling without surface treatment (group M). Specimens in the airborneparticle abrasion group (group APA; n=22) were subjected to an abrasion process using 50-mm alumina particles with an abrasion machine (model S-606 Refo, Dental Sand Blaster; Serica Dental Supply). Each specimen underwent abrasion for 10 seconds at a pressure of 0.25 MPa, and the working distance was set at 15 mm. The direction of the airborne-particle abrasion process was perpendicular to the surface of the specimens. After the various surface treatments, the zirconia specimens were coated with the veneering ceramic. The ceramic powder was weighed on a digital analytical balance (Entris; Sartorius AG) and then mixed with the corresponding liquid following the manufacturer’s instructions. A minimagnetic stirrer was used to achieve a uniform mixture. The specimens were positioned on a flat surface and sprayed using a mini-airbrush sprayer (HS08AC-Skc; Ningbo Haosheng Pneumatic Machinery Co, Ltd) to allow uniform deposition of the veneering ceramic on the top surface of the zirconia. The main spraying parameters are presented in Table 3. Specimens in groups LX1, LX2, and LX3 were transferred to a flat XeY laser table and subjected to laser treatment. A CO2 laser (model JL-K6040; Julong Co, Ltd) was used to treat specimens in the 3 laser treatment groups (n=22). The laser treatment approaches for surface treatment of specimens are presented in Table 2. Laser Abdullah et al
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Table 2. Classification and description of experimental groups
Table 3. Coating parameters for airbrush spraying
Group
n
M (milling only)
22
Specimens not subjected to surface treatment, left as milled
Mixing ratio of veneering ceramic to corresponding liquid
APA (airborne-particle abrasion)
22
Specimens subjected to abrasion
Mixing speed and duration (using mini magnetic stirrer)
LX1 (single laser treatment)
22
Specimens subjected to 1 laser treatment
Distance between zirconia top surface and airbrush nozzle tip
LX2 (2 laser treatments)
22
Specimens subjected to 2 laser treatments; after first treatment, the specimens were rotated 90 degrees in clockwise direction and then second treatment performed
Nozzle diameter
0.2 mm
Working pressure
0.25 MPa
LX3 (3 laser treatments)
22
Specimens subjected to 3 laser treatments; after first and second treatments, specimens were rotated 90 degrees in clockwise direction and then next treatment performed
Drying time at ambient room temperature
Description
Coating Parameter
Value
Spray time
1:1.5 350 rpm for 10 min 15 cm
10 s 5 min
rpm, revolutions per minute.
Table 4. Typical CO2 laser treatment parameters and properties
parameters and properties, which were similar for all 3 laser treatments, are shown in Table 4. The time interval between each rotation and new treatment was 10 seconds. After the first laser treatment, specimens were left at room temperature to cool slowly. Surface roughness (Ra) values of the treated framework surfaces were determined by using a profilometer (Surtronic 25; Taylor Hobson Ltd). Recalibration for each specimen was conducted using a standard framework specimen (6 mm) that was provided by the manufacturer. A cut-off length of 0.08 mm was determined, and the specimens were positioned on a flat surface for the measurement of surface roughness. Three readings were recorded for each specimen, and the mean values were calculated. An X-ray diffractometer (SmartLab; Rigaku Corp) using CuKa radiation (l) of 1.5406 Å at 200 mA and 40 kV was used to identify phase transformations. Diffraction data were collected within the 2q range of 20 degrees to 90 degrees at a step size of 0.02 degrees and a step time of 8 minutes. The peaks were used to identify phase transformations. In each group, 2 specimens were selected and used for surface coating investigation (1 specimen for the interface and 1 for the top surface) with a scanning electron microscope (SEM) (Zeiss Ultra Plus; Carl Zeiss Microscopy GmbH) equipped with energy dispersive spectroscopy (EDS). The interfaces of specimens were studied after the specimens had been sectioned into the desired shapes using a slow-speed saw (model 5410; Sherline Products) with water as the coolant. The specimens were polished using SiC grit sizes #600 and #800 on a flat surface under running water. A sputter coater (JS-1600; Beijing HTCY Technology Co, Ltd) was used for gold sputtering. SEM images, spot EDS spectra, and line EDS spectra of specimens were recorded and analyzed. The specimens were divided into unaged (n=10) and aged (n=10) groups. Specimens were cleaned ultrasonically in ethanol solution for 15 minutes and then dried. A silicone mold was fabricated for the preparation of veneering ceramics in a cylindrical shape. Veneering Abdullah et al
Laser Parameter Laser treatment speed Output power Distance between laser tube and zirconia Space between scanned lines Laser treatment duration Laser tube diameter
Value 35 mm s-1
Laser Property Wavelength
Value 10.6 mm
20 W
Frequency
50 Hz
18 mm
Spot size Intensity
0.2 mm 0 to 1.6 ×106 W/cm2
0.25 mm 65 s
Pulse length Pulse energy Feed speed
10 to 50 ms 150 W 0 to 300 mm s-1
8 mm
ceramic powder and corresponding liquid were mixed following manufacturer’s directions and condensed into the silicone mold through a premade circular hole. The cylinder produced was 5 mm in diameter and 3 mm in height and was centered on the zirconia framework. A lubricating agent (Wagnersil S200; Wagner Dental GmbH & Co KG) was used around the hole to limit adhesion to the silicone mold. Gentle vibration and tissue paper were used to remove excess liquid during condensation of the veneering ceramic slurry. The manufacturer’s firing protocol for veneering ceramic material was followed. A second layer of the veneering ceramic slurry was added and refired to compensate for shrinkage of the veneering ceramic after the first firing cycle. The shear bond test was performed for aged and unaged specimens. The aged specimens were stored in water at 37 C for 1 month. A semicircular metal jig connected to a universal testing machine (model E44; MTS Systems Corp) was used to conduct the shear bond test with specimens secured on the machine. The tip of the metal jig was positioned close to the interface between the veneering ceramic and the zirconia framework at a crosshead speed of 1 mm/min until failure, and failure forces were recorded in megapascal (MPa). The fracture patterns were examined visually and under a stereomicroscope (model SZ61; Olympus Corp) to identify different fracture modes, defined as adhesive, cohesive, or mixed. The fracture mode values were recorded as percentages.
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Table 5. Summary of surface roughness and shear bond strength values Group Ra (mm) Unaged SBS (MPa) Aged SBS (MPa) 0.73 ±0.20a
27.1 ±2.13a
26.47 ±3.19a
APA
0.79 ±0.21a
27.9 ±2.73a
27.27 ±2.09a
LX1
1.15 ±0.21
b
b
30.46 ±2.03b
LX2
1.18 ±0.23b
31.58 ±2.54b
30.93 ±2.10b
LX3
1.21 ±0.22b
32.08 ±2.45b
31.43 ±2.07b
31.09 ±2.31
Ra (surface roughness); SBS, shear bond strength. Values marked with different letters indicate statistically significant differences among groups (P<.001).
The results were statistically analyzed using statistical software (IBM SPSS Statistics, v20.0; IBM Corp). The KolmogoroveSmirnov test for normality was used to check the distribution of surface roughness and shear bond strength values. Homogeneity of variances was investigated using the Levene test. One-way ANOVA was used to compare surface roughness and shear bond strength mean values between groups. The ANOVA post hoc Tukey HSD test was performed for multiple comparisons. The Pearson test was used to analyze the correlation between surface roughness and shear bond strength values. The chi-square test was used to compare mean values of fracture modes (a=.05). RESULTS Surface roughness and shear bond strength values for the study groups are presented in Table 5. The surface roughness values indicated a statistically significant difference among all study groups (P<.001). However, the outcomes were not statistically significant between groups M and APA (P>.05) or among the laser treatment groups (P>.05). Groups LX3 and LX2 showed the highest surface roughness values, 1.18 ±0.23 mm and 1.21 ±0.22 mm, among all groups. The lowest surface roughness value of 0.73 ±0.20 mm was observed in group M. The top surface of specimens showed different roughness patterns after surface treatments. Group M specimens showed grooves, whereas group APA specimens had flake-like defects in certain areas. The lasertreated specimens showed microholes and irregularities within the deposited veneering ceramic on the zirconia framework. Strong adhesion was detected at the interface between the deposited veneering ceramic material and the zirconia framework after laser treatment, with holes within the veneering ceramic layer in groups M and APA. In addition, irregularities and microcracks were observed at the interfaces between veneering ceramic and the zirconia framework in the study groups. XRD analyses did not reveal phase transformation after laser treatment in groups LX1, LX2, and LX3 or in control specimens (group M). Prominent peaks representing the tetragonal (t) phase were found in these groups (Fig. 1). However, phase transformation from tetragonal to monoclinic (t/m) was detected in the
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Tetragonal
Monoclinic
60
80
-
(c)
Intensity (a.u.)
M
-
(b)
(a) 20
30
40
50
70
90
2θ (deg.) Figure 1. X-ray diffractometry graph showing zirconia crystallographic peaks. Zirconia framework did not present phase transformation after milling only (a) and laser treatments (b). Airborne-particle abrasion showed phase transformation from tetragonal to monoclinic (c).
airborne-particle abrasion group. The major peak of the tetragonal (t) phase was detected at 30.170 degrees, and other (t) phases were found at 34.742 degrees, 35.278 degrees, and 50.212 degrees. A major monoclinic (m) peak was found at 29.960 degrees. Table 5 presents the shear bond strength of each group under unaged and aged conditions. A statistically significant difference was found among the study groups (P<.001). The highest mean values of shear bond strength were recorded in group LX3 at 32.08 ±2.45 MPa and in group LX2 at 31.58 ±2.23 MPa. The minimum value was detected in group M at 27.1 ±2.13 MPa, followed by group APA at 27.9 ±2.73 MPa. Group M presented the lowest shear bond strength value, whereas groups LX3 and LX2 showed the highest shear bond strength, followed by group LX1 and group APA. The Pearson test demonstrated a positive significant correlation between surface roughness and shear bond strength values (r=0.9597, R2=0.9211, P<.001). The highest percentage of adhesive fracture mode was recorded in group M, followed by groups APA and LX1. The lowest percentages were observed in groups LX2 and LX3. Group LX2 showed the highest percentage of cohesive fracture mode, followed by group LX3. The minimum percentage of cohesive fracture mode was detected in groups M and APA. The mixed fracture mode was a common fracture pattern observed in all studied groups. The chi-square test did not show a statistically significant difference among the investigated groups (P>.05). Typical fracture mode patterns are presented in Table 6. Spot EDS analyses detected prominent peaks of silicon, aluminum, and zirconia. The presence of the first 2 Abdullah et al
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Table 6. Summary of fracture mode patterns Fracture Mode (%) Group*
Adhesive
Cohesive
M
40
10
Mixed 50
APA
35
10
55
LX1
35
15
50
LX2
30
25
45
LX3
30
20
50
*Chi-square test did not show statistically significant difference in fracture mode patterns among groups (P>.05).
elements indicated coverage of the framework surface by veneering ceramic, whereas the prominent zirconia peak indicated that the surface of the framework had delaminated from the veneering ceramic. Line EDS analyses did not show changes in the elemental compositions of specimens subjected to different surface treatment methods. DISCUSSION The research hypothesis was accepted because laser treatment improved shear bond strength. The present study indicates that laser treatment applied 2 or 3 times could increase surface roughness and provide satisfactory shear bond strength values. The results are consistent with those of earlier observations.6,16 Consistent with the findings of previous studies,16,20 the XRD analyses showed that airborne-particle abrasion induced crystallographic transformation of the zirconia from t/m. However, laser treatment groups and the milling-only group did not show such changes. Therefore, the structural integrity of zirconia after laser treatment is a promising indicator for longevity of fusion between the veneering ceramic and the zirconia framework. In this study, different top-surface topography was observed after milling only, airborne-particle abrasion, and laser treatment. Microholes were detected within coated veneering ceramic after laser treatment which should promote adhesion between veneering ceramic and zirconia by providing micromechanical retention. Microcracks were detected on the zirconia framework. These defects may have been due to the repeating firing cycle performed to fabricate the veneering ceramic cylinder on the zirconia for the shear bond strength test. In addition, the framework surface could also have been under internal pressure during the load application of shear bond strength testing. The results are consistent with those of a previous study.21 EDS analyses revealed different elemental compositions after zirconia frameworks were coated with the veneering ceramic. The failure to detect changes in elemental compositions may reflect the conservative approach adopted in this study. Previous studies have found that bond strength may depend primarily on the
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existence of silicon and oxygen on a smooth ceramic surface and may be a function of the composition and surface chemistry of ceramics.22 The results of the study are consistent with those of previous studies, provided that silicon and oxygen were the predominant elements on the flat framework surface.6,16 However, the specimens with mixed fracture mode demonstrated prominent silicon and oxygen peaks in their veneering ceramic areas and a prominent zirconia peak in their delaminated areas. These characteristics may reflect increased bond strength, which may be attributed to the variation in the elemental compositions and microstructure of the veneering ceramic and zirconia. An interlocking mechanism could also play a primary role in maximizing bond strength through the micromechanical interaction and chemical bonding of the treated zirconia surface.6 The presence of high amounts of silicon could provide a basis for this chemical integration.6 This study showed that different fracture mode patterns developed after laser treatment. Groups LX2 and LX3 showed the highest percentage of cohesive fracture mode among all groups. The highest percentage of adhesive fracture mode was recorded in the control group (group M). However, specimens subjected to 2 and 3 repetitions of laser treatment showed a minimal percentage of adhesive fracture. Existing high percentages of cohesive fracture mode in specimens that received 2 and 3 cycles of laser treatment may be attributed to changes in physical properties. The changes may be because of the appropriate fusion between the 2 materials. The Pearson test showed a significant positive correlation between surface roughness and shear bond strength, and this relation may be because of the 90-degree clockwise rotation of each specimen, in particular LX2 and LX3, which optimized the treated surface area. The results of this study show that laser treatment applied 2 or 3 times could be considered an alternative method to milling only and airborne-particle abrasion and may prevent the zirconia framework from undergoing delamination. Laser treatment can simplify the zirconia coating and yield a rougher framework surface that provides satisfactory shear bond strength. Limitations of the current study include the fact that laser treatment was not performed more than 3 times and that a thermocycling test was not conducted. Also, the fracture strength of treated zirconia after surface treatment was not measured. The current study did not evaluate the wetting property and the coefficient of thermal expansion. These aspects should be incorporated into future studies. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn:
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1. Laser treatment applied 2 or 3 times improved bond strength between veneering ceramic and zirconia and could be considered an acceptable technique for zirconia surface treatment. 2. The structural integrity of zirconia was preserved after laser and milling techniques. 3. Airborne-particle abrasion did not preserve the structural integrity of zirconia and resulted in phase transformation. 4. The single laser treatment provided satisfactory results but was inferior to laser treatment applied 2 or 3 times. 5. The fracture surface of the specimens revealed a mainly mixed fracture mode. REFERENCES 1. Schünemann FH, Galárraga Vinueza ME, Magini R, Fredel M, Silva F, Souza JCM, et al. Zirconia surface modifications for implant dentistry. Mater Sci Eng C Mater Biol Appl 2019;98:1294-305. 2. Liu D, Matinlinna JP, Tsoi JK, Pow EH, Miyazaki T, Shibata Y, et al. A new modified laser pretreatment for porcelain zirconia bonding. Dent Mater 2013;29:559-65. 3. Diniz AC, Nascimento RM, Souza JC, Henriques BB, Carreiro AF. Fracture and shear bond strength analyses of different dental veneering ceramics to zirconia. Mater Sci Eng C Mater Biol Appl 2014;38:79-84. 4. Steiner PJ, Kelly JR, Giuseppetti AA. Compatibility of ceramic-ceramic systems for fixed prosthodontics. Int J Prosthodont 1997;10:375-80. 5. Hammad IA, Talic YF. Designs of bond strength tests for metal ceramic complexes: review of the literature. J Prosthet Dent 1996;75:602-8. 6. Muhammed F, Pollington S, Sun X, Abdullah A, Liu Y. Novel coatings on zirconia for improved bonding with veneer ceramics. Coatings 2018;8:363. 7. Kim HJ, Lim HP, Park YJ, Vang MS. Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105: 315-22. 8. Reginato VF, Kemmoku DT, Caldas RA, Bacchi A, Pfeifer CS, Consani RLX. Characterization of residual stresses in veneering ceramics for prostheses with zirconia framework. Braz Dent J 2018;29:347-53. 9. Dapieve KS, Guilardi LSF, Silvestri T, Rippe MP, Pereira GKR, Valandro LF. Mechanical performance of Y-TZP monolithic ceramic after grinding and aging: survival estimates and fatigue strength. J Mech Behav Biomed Mater 2018;87:288-95. 10. Soltaninejad F, Valian A, Moezizadeh M, Khatiri M, Razaghi H, Nojehdehian H. Nd:YAG laser treatment of bioglass-coated zirconia surface and its effect on bond strength and phase transformation. J Adhes Dent 2018;20:1-9.
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11. Caravaca CF, Flamant Q, Anglada M, Gremillard L, Chevalier J. Impact of sandblasting on the mechanical properties and aging resistance of alumina and zirconia based ceramics. J Mech Behav Biomed 2018;38: 915-25. 12. Chevalier J, Gremillard L, Virkar Anil V, Clarke David R. The tetragonalmonoclinic transformation in zirconia: lessons learned and future trends. J Am Ceram Soc 2009;92:1901-20. 13. Arami S, Tabatabae MH, Namdar SF, Chiniforush N. Effects of different lasers and particle abrasion on surface characteristics of zirconia ceramics. J Dent (Tehran) 2014;11:233-41. 14. Shukla PP, Lawrence J. Evaluation of fracture toughness of ZrO2 and Si3N4 engineering ceramics following CO2 and fibre laser surface treatment. Opt Laser Eng 2011;49:229-39. 15. Tzanakakis E-GC, Tzoutzas IG, Koidis PT. Is there a potential for durable adhesion to zirconia restorations? A systematic review. J Prosthet Dent 2016;115:9-19. 16. Abdullah AO, Hui Y, Sun X, Pollington S, Muhammed FK, Liu Y. Effects of different surface treatments on the shear bond strength of veneering ceramic materials to zirconia. J Adv Prosthodont 2019;11:65-74. 17. Abdullah AO, Muhammed FK, Yu H, Pollington S, Xudong S, Liu Y. The impact of laser scanning on zirconia coating and shear bond strength using veneer ceramic material. Dent Mater J 2019;38:452-63. 18. Ural C, KalyoncuoGlu E, Balkaya V. The effect of different power outputs of carbon dioxide laser on bonding between zirconia ceramic surface and resin cement. Acta Odontol Scand 2012;70:541-6. 19. Martins FV, Mattos CT, Cordeiro WJB, Fonseca EM. Evaluation of zirconia surface roughness after aluminum oxide airborne-particle abrasion and the erbium-yag, neodymium-doped YAG, or CO2 lasers: a systematic review and meta analysis. J Prosthet Dent 2019;121:895-903.e2. 20. Tada K, Sato T, Yoshinari M. Influence of surface treatment on bond strength of veneering ceramics fused to zirconia. Dent Mater J 2012;31: 287-96. 21. Kim JW, Covel NS, Guess PC, Rekow ED, Zhang Y. Concerns of hydrothermal degradation in CAD-CAM zirconia. J Dent Res 2010;89:91-5. 22. Hooshmand T, Daw R, van Noort R, Short RD. XPS analysis of the surface of leucite-reinforced feldspathic ceramics. Dent Mater 2001;17:1-6. Corresponding author: Dr Yi Liu School of Stomatology China Medical University Shenyang, Liaoning Province, 110002 PR CHINA Email: [email protected] Acknowledgments Adil O. Abdullah would like to express his heartfelt gratitude to the Chinese Government Scholarship Council (CSC) for financial support throughout the study. The authors would also like to acknowledge Northeastern University and Qing Mei Dental Laboratory Technology Co, Ltd in Shenyang, PR China for providing help for conducting the experimental work. Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.10.007
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