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Shear bond strengths between alumina–toughened zirconia cores and veneering ceramics and their susceptibility to aging
402
Asian Pacific Journal of Tropical Medicine (2012)402-405
Contents lists available at ScienceDirect
Asian Pacific Journal of Tropical Medicine journal homepage:www.elsevier.com/locate/apjtm
Document heading
doi:
Shear bond strengths between alumina-toughened zirconia cores and veneering ceramics and their susceptibility to aging Yong-Qi Zhao1, Li Jiang1, Jing-Chao Zhang2, Yun-Mao Liao1, Jun-Jun Lu1, Wei Li1* State Key Laboratory of Oral Diseases, Sichuan University, Chengdu 610041, P.R. China Hainan Stomatological Hospital, Haikou, 570100, P.R. China
1 2
ARTICLE INFO
ABSTRACT
Article history: Received 15 February 2012 Received in revised form 15 March 2012 Accepted 15 April 2012 Available online 20 May 2012
Objective: To evaluate the shear bond strength (SBS) between alumina-toughened zirconia (ATZ) cores and veneering ceramics, investigate the effect of aging in artificial saliva on SBS and compare it with that of yttria-stabilized tetragonal zirconia polycrystals(Y-TZP). Methods: Bars of ATZ and Y-TZP were layered with veneering ceramics in accordance to the recommendation of the manufacturer. Half of each group (n = 10) was aged at 134 曟 (under 2 bar pressure) in an autoclave for 48 h. Subsequently, all specimens were subjected to shear force in a universal testing machine. The interface and fractured surface of the specimens were evaluated using scanning electron microscopy and X-ray energy dispersive spectroscopy. Results: The initial mean SBS values in MPa暲SD were 28.9暲8.0 for ATZ and 26.2暲7.6 for Y-TZP. After aging, the mean SBS values for ATZ and Y-TZP were 22.9暲4.9 MPa and 22.8暲6.9 MPa, respectively. Neither the differences between the SBS values of the ATZ and Y-TZP groups nor the influence of aging on all groups were statistically significant. Conclusions: The SBS between the ATZ core and the veneering ceramics was not affected by aging. The SBS of ATZ to veneering ceramics was not significantly different compared with that of Y-TZP.
Keywords: Shear bond strength Zirconia Alumina Veneering ceramics Aging
1. Introduction Recently, yttria-stabilized tetragonal zirconia polycrystals (Y-TZP ) has been introduced in the field of dentistry because of its superior aesthetic potential compared with that of metal-ceramic constructs. Y-TZP offers high flexural strength; however, its poor anti-aging properties lead to surface and strength degradation with grain pullout and microcracking[1-3]. Composite materials can improve the reliability and lifespan of Y-TZP by providing higher fracture toughness and mechanical strength. A composite material processed with zirconia and alumina is reported to have outstanding mechanical and tribological properties[4]. *: Corresponding author: *Corresponding author: Wei Li, Prof., Ph D, State Key
Laboratory of Oral Diseases, Sichuan University, Chengdu 610041, P.R. China. Tel: 86-28-85503494 Fax: 86-28-85503494 E-mail: [email protected] Foundation project: It is supported by National High-Tech Research and Development Program of China (863 Program) (No. 2006AA03Z440).
Alumina-toughened zirconia (ATZ) processed with 80 wt% Y-TZP and 20 wt% alumina (Tosoh Co., Tokyo, Japan) has a bending strength of up to 1 900 MPa. The addition of alumina increases the hydrothermal stability of the tetragonal phase because it could work as a barrier for the propagation of phase transformation into the bulk[5,6]. The addition of alumina to zirconia clearly hinders aging, or at least reduces drastically its kinetics[4,5]. Thus, ATZ has been recommended as a material for fixed partial denture frameworks[3]. Limited information is available on the bond quality of veneering ceramics to ATZ. Considering the optical opacity of ATZ and Y-TZP ceramic, they are typically covered with veneering ceramic, usually feldspathic types, with aesthetic characteristics similar to those of natural tooth substances[7]. The bond strength between the framework and the veneer, which depends on various factors, such as the strength of the chemical bonds, the mechanical interlocking, and the extent of compressive stress in the veneer caused by a thermal mismatch between the framework and the veneering ceramic, might affect the success of restorations[8-10]. The
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mechanical integrity and adhesion of the veneering ceramic to the ceramic substructure have proven to be key factors for the successful performance of veneer/core bilayered restorations, so the initial bond strength and reliability after aging obtained from in vitro investigations can provide useful information for the behavior and predictability of allceramic systems in clinical applications[11,12]. The current study aimed to evaluate the SBS of hipped Y-TZP and ATZ core ceramics and their corresponding veneering ceramics. In addition, the effect of aging in artificial saliva on SBS was investigated. Fractured surfaces were microscopically analyzed to determine the characteristics of bond failure. The null hypothesis of the current study was that the bond strength of ATZ to veneering ceramics is not different from that of Y-TZP, and would not be affected by aging. 2. Materials and methods
Figure 1. The shear jig used in the current study.
2.1. Preparation of zirconia core Y-TZP specimens were prepared from as-received Y-TZP powder containing 3 mol% of yttrium oxide (TZ3 YB-E , Tosoh Co., Tokyo, Japan). The ATZ composite ceramic consisted of 80 wt% 3Y-TZP and 20 wt% alumina (TZ-3Y20AB, Tosoh Co., Tokyo, Japan). Bend bars were uniaxially pressed at 50 MPa and then isostatically coldpressed at 200 MPa. The specimens were then sintered
in a high-temperature furnace (Nabertherm, Germany) according to the protocols. The specimens were heated from room temperature to 1 500 曟 at a rate of 200 曟 per hour. The specimens were held at this temperature for two hours, and then naturally cooled to room temperature. Each core ceramic specimen produced had a length of (17.0±0.2) mm, width of (4.0±0.1) mm, and height of (2.0±0.1) mm. 2.2. Preparation of the core veneer specimens Prior to veneering, all core specimens were cleaned with 70 % ethanol by wiping their surfaces with cotton, and subsequently cleaned for 5 min in an ultrasonic bath with ethanol. On the prepared face of each core specimen, a 3-mm layer of veneering ceramic (Vita VM9, Vita, Bad S 覿ckingen, Germany) was bonded at one end (Figure 1). A separable resin mold was used to layer the ceramic. Ceramic powder and an appropriate amount of the respective liquid were mixed to a slurry and placed into the mold. Excess liquid was removed using a tissue. Only the base dentine was layered. Firing was performed in a ceramic oven (ProgramatX 1, Ivoclar, Liechtenstein) according to the recommendations of the manufacturers (Table 1). Table 1 Firing schedules of veneering ceramics. Procedure
Cleaning firing Base dentine
Pre-drying Temperature(曟) Time(min) 500 3 500 2
Heating time (min) 6.00 8.11
2.3. Aging Artificial saliva was prepared according to the recipe in
ISO/TR 10271[13,14]. The pH of the solution was adjusted to ~5 with 0.5 mol/L natrium hydroxydatum and 30% acetic acid. Prior to shear bond testing, half of each group (n = 15) was subjected to aging at 134 曟 (under 2 bar pressure) for 48 h in an autoclave that has artificial saliva. The remaining specimens (n = 15) were stored dry at room temperature.
2.4. Shear bond strength test
Each specimen was tightened in a metal holder in a universal testing machine (Instron5565, USA) with a span of 15 mm and a crosshead rate of 0.5 mm per minute at room temperature (Figure 1). SBS (MPa) were calculated by dividing the failure load (N) by the bonding area (mm2)[11]. 2.5. Scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) analyses of the core-veneer interface
After the shear bond test, the fractured surfaces and their interfaces were evaluated using SEM (Model S-4500, Hitachi, Tokyo, Japan). The chemical structure at the interface was analyzed using EDS (INCAPentaFET-x3, Oxford). 2.6. Statistical analysis
Data analysis was performed with SPSS11.0 statistical software. The differences were compared by using the Student's t test. All P values were two-tailed with statistical significance indicated by a value of P<0.05.
Heating rate Firing temperature (曟/min) (曟) 33 700 55 950
Holding time (min) 5 1
Vacuum holding time (min) 0.00 8.11
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3. Results
Shear bond strength (Mpa)
The SBS results before and after aging are shown in Figure 2. Neither the differences between the SBS values of the ATZ and Y-TZP groups nor the influence of aging on all groups were statistically significant. The SBS of ATZ (28.9暲8.0) to veneering ceramics was higher than that of hipped Y-TZP (26.2暲7.6). The SEM images of the core-veneering ceramic sectioned areas showed that they were combined closely (Figure 3). In all the specimens, fracture began at the coreceramic interface and proceeded into the veneering ceramic. All groups showed a combination of cohesive and adhesive fracture modes (Figure 4). Figure 5 showed that the silicon in the interface between the TAP and VM9 linearly declined, whereas the silicon content in ATZ decreased beyond the average. Thus, veneering porcelain diffusion in the surface of ATZ may be better than that in TZP. After aging in artificial saliva for 48 h, the SEM images of the Y-TZP veneer and ATZ veneer surfaces were blurred (Figure 6).
20
VM9-TZP
30 20
10
Interface
10 0
0
10
20
30
40
ATZ-VM9
50
0
Interface 0
10
20
30
40
Figure 5. SEM image and EDS analysis of silicon on the fractured surface between the core-veneering ceramic (The above diagram is the EDS image and below is its respective SEM image).
pre-aging post-aging
40 35 30 25 20 15 10
5 0
Y-TZP
ATZ
Figure 2. Mean SBS of Y-TZP and ATZ before and after aging.
Figure 3. Sectioned surface morphology of the core and veneering ceramic interface (×500 magnification).
Figure 4. Fractured surface morphology showing combined fracture modes, namely, cohesion in the veneering ceramic and adhesion at the interface (×500 magnification).
Figure 6. SEM images of Y-TZP veneer (left) and ATZ veneer (right) specimens after 48 h of hydrothermal aging at 134 曟 in artificial saliva.
4. Discussion
The current study demonstrated that there was no significant difference between the SBS of ATZ to veneering ceramics compared with that of Y-TZP. Many factors may cause the production of inferior core-veneer bond strength. To date, no accepted mechanism has been reported to explain the nature of the bonding between cores and veneering porcelain. The coefficient of thermal expansion (CTE) seems to play a crucial role in the phenomenon[15-17]. SBS can be compromised by residual stresses because of CTE mismatch between the veneer and core ceramic[18]. Efforts have been made by dental manufacturers to develop ceramic cores and low-fusing veneering ceramics with similar CTE to generate acceptable levels of residual stress within a multilayer all-ceramic composite[11]. The CTE of ATZ (80 wt% 3Y-TZP and 20 wt% alumina) is significantly lower (10.1×10-6/K) than that of Y-TZP (10.5×10-6/K). According to the recommendations of the manufacturers, the CTE (25 曟 to 500 曟) of feldspathic veneering ceramic (VM9) is set to 9.0×10-6/ K to 9.2×10-6/K. From the metal ceramic bilayers, the recommended mean difference between the alloy and ceramic CTEs (from room temperature to 600 曟) is from 0.5×10-6/K to 1.0×10-6/K[19,20]. In the present study, the CTE mismatch between ATZ and veneer was closer to the recommended mean than that of Y-TZP. Thus, the SBS of ATZ to veneering ceramics is higher than that of Y-TZP. Considering that the veneering porcelain contains alumina and zirconia, and that the core ceramic does not contain silicon oxide, the spectrum of silicon dioxide EDS was used
Yong-Qi Zhao et al./Asian Pacific Journal of Tropical Medicine (2012)402-405
to detect mutual penetration of the interface between the core and veneer ceramic. The result shows that the silicon in the interface between the TAP and VM9 linearly declined, whereas the silicon content in ATZ decreased beyond the average. Thus, veneering porcelain diffusion in the surface of ATZ may be better than that in TZP. The failure mode observed for the Y-TZP-veneer and ATZ-veneer was mainly combined to show adhesion at the interface and cohesion in the veneering ceramic. This combination can be interpreted in two ways. First, crack deflection could be a consequence of the superior ability of Y-TZP to resist crack propagation. Second, the interlaminar crack deflection could correlate with the relatively poor bond of the zirconia core to the veneering ceramic. The clinical implication of this finding is that the investigated all-ceramic system could have a tendency to produce chipped-off fractures of the veneering ceramic and delaminations rather than catastrophic failure of the core structure[11,21]. Oral fluids facilitate stress corrosion of the ceramic materials, resulting in slow crack growth and finally leading to failed ceramic restorations in complex situations of the oral cavity[22,23]. In vitro aging sensitivity of the SBS of the bilayered specimens was therefore assessed by exposure in an autoclave[2]. In the present study, exposure to 48 h of aging at 134 曟 had no influence on the SBS of all groups investigated. Comparative studies on the bond strength of core and veneering ceramics after exposure to aging in artificial saliva are not available currently[11]. After aging in artificial saliva for 48 h, the SEM images of the Y-TZP veneer and ATZ veneer surfaces were blurred. The putative hydroxyapatite sediment remained on the sample, which was attributed to sample aging in artificial saliva containing large amounts of calcium and phosphorus ions. During the aging process, calcium and phosphorus ions in artificial saliva are continuously deposited on the ceramic surface. The sediment may only affect the surface of the ceramic. No effect on the internal structure was observed; thus, the SBS of Y-TZP or ATZ to veneering ceramics was not affected. In the present study, the shape of the samples was very different from that used in clinical application. Therefore, the results are not directly comparable to clinical situations. In conclusion, considering the limitation of the present study, the bond strength of Ce-TZP/A to veneering ceramics is concluded to be similar to that of Y-TZP and would not be affected by aging. Conflict of interest statement We declare that we have no conflict of interest. References [1] C attani-Lorente M, Scherrer SS, Ammann P, Jobin M, Wiskott HWA.Low temperature degradation of a Y-TZP dental ceramic. Acta Biomaterialia 2011; 7(2): 858-865. [2] Lughi V, Sergo V. Low temperature degradation -aging- of zirconia: A critical review of the relevant aspects in dentistry. Dental Materials 2010; 26(8): 807-820. [3] N evarez-Rascon A, Aguilar-Elguezabal A, Orrantia E, Bocanegra-Bernal MH. On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content. Intl J Refractory Metals & Hard Materials 2009; 27(6): 962-970.
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[4] C hevalier J. What future for zirconia as a biomaterial? Biomaterials 2006; 27(4): 535-543. [5] Schneider J, Begand S, Kriegel R, Kaps C, Glien W, Oberbach T. Low-temperature aging behavior of alumina-toughened zirconia. J Am Ceramic Soc 2008; 91(11): 3613-3618. [6] Tsubakino H, Nozato R, Hamamoto M. Effect of alumina addition on the tetragonal-to-monoclinic phase transformation in zirconia-3 mol% yttria. J Am Ceramic Soc 1991; 74(2): 440-443. [7] 毤ilo M, Gjerdet NR, Tvinnereim HM. The firing procedure influences properties of a zirconia core ceramic. Dental Materials 2008; 24(4): 471-475. [8] Fischer J, Stawarzcyk B, Trottmann A, H覿mmerle CHF. Impact of thermal misfit on shear strength of veneering ceramic/zirconia composites. Dental Materials 2009; 25(4): 419-423. [9] Al-Shehri SA, Mohammed H, Wilson CA. Influence of lamination on the flexural strength of a dental castable glass ceramic. J Prosthetic Dent 1996; 76(1): 23-28. [10] Isgr侔 G, Pallav P, van der Zel JM, Feilzer AJ. The influence of the veneering porcelain and different surface treatments on the biaxial flexural strength of a heat-pressed ceramic. J Prosthetic Dent 2003; 90(5): 465-473. [11] G uess P C, Kulis A, Witkowski S, Wolkewitz M, Zhang Y, Strub JR.Shear bond strengths between different zirconia cores and veneering ceramics and their susceptibility to thermocycling. Dental Materials 2008; 24(11): 1556-1567. [12] R aigrodski AJ, Chiche GJ, Potiket N, Hochstedler JL, Mohamed SE, Billiot S, et al. The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prostheses: A prospective clinical pilot study. J Prosthetic Dent 2006; 96(4): 237-244. [13] Marino CEB, Mascaro LH. EIS characterization of a Ti-dental implant in artificial saliva media: dissolution process of the oxide barrier. J Electroanalytical Chem 2004; 568(1-2): 115-120. [14] ISO/TR 11405: Guidance on testing of adhesion to tooth structure, in dental materials. 1994. [15] Z arone F, Russo S, Sorrentino R. From porcelain-fused-to-metal to zirconia: Clinical and experimental considerations. Dental Materials 2011; 27(1): 83-96. [16] Aboushelib MN, Kleverlaan CJ, Feilzer AJ.Microtensile bond strength of different components of core veneered all-ceramic restorations: Part II: Zirconia veneering ceramics. Dental Materials 2006; 22(9): 857-863. [17] Aboushelib MN, Kleverlaan CJ, Feilzer AJ.Effect of zirconia type on its bond strength with different veneer ceramics. J Prosthodontics 2008; 17(5): 401-408. [18] A boushelib MN, de Jager N, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Dental Materials 2005; 21(10): 984-991. [19] Lopes SCPV, Rollo JM, Leal MB, Bezzon OL. Correlation between metal-ceramic bond strength and coefficient of linear thermal expansion difference. J Appl Oral Sci 2009; 17(2): 122-128. [20] Anusavice KJ. Dental casting and soldering alloys. In: Phillips science of dental materials. 11th ed. St. Louis: WB Saunders Co; 2003, p. 563-620. [21] White SN, Miklus VG, McLaren EA, Lang LA, Caputo AA. Flexural strength of a layered zirconia and porcelain dental allceramic system. J Prosthetic Dent 2005; 94(2): 125-131. [22] P eterson IM, Wuttiphan S, Lawn BR, Kenneth C. Role of microstructure on contact damage and strength degradation of micaceous glass-ceramics. Dental Materials 1998; 14(1): 80-89. [23] Z hang Y, Song J-K, Lawn BR. Deep-penetrating conical cracks in brittle layers from hydraulic cyclic contact. J Biomed Materials Res Part B: Appl Biomaterials 2005; 73B(1): 186-193.
Asian Pacific Journal of Tropical Medicine (2012)402-405
Contents lists available at ScienceDirect
Asian Pacific Journal of Tropical Medicine journal homepage:www.elsevier.com/locate/apjtm
Document heading
doi:
Shear bond strengths between alumina-toughened zirconia cores and veneering ceramics and their susceptibility to aging Yong-Qi Zhao1, Li Jiang1, Jing-Chao Zhang2, Yun-Mao Liao1, Jun-Jun Lu1, Wei Li1* State Key Laboratory of Oral Diseases, Sichuan University, Chengdu 610041, P.R. China Hainan Stomatological Hospital, Haikou, 570100, P.R. China
1 2
ARTICLE INFO
ABSTRACT
Article history: Received 15 February 2012 Received in revised form 15 March 2012 Accepted 15 April 2012 Available online 20 May 2012
Objective: To evaluate the shear bond strength (SBS) between alumina-toughened zirconia (ATZ) cores and veneering ceramics, investigate the effect of aging in artificial saliva on SBS and compare it with that of yttria-stabilized tetragonal zirconia polycrystals(Y-TZP). Methods: Bars of ATZ and Y-TZP were layered with veneering ceramics in accordance to the recommendation of the manufacturer. Half of each group (n = 10) was aged at 134 曟 (under 2 bar pressure) in an autoclave for 48 h. Subsequently, all specimens were subjected to shear force in a universal testing machine. The interface and fractured surface of the specimens were evaluated using scanning electron microscopy and X-ray energy dispersive spectroscopy. Results: The initial mean SBS values in MPa暲SD were 28.9暲8.0 for ATZ and 26.2暲7.6 for Y-TZP. After aging, the mean SBS values for ATZ and Y-TZP were 22.9暲4.9 MPa and 22.8暲6.9 MPa, respectively. Neither the differences between the SBS values of the ATZ and Y-TZP groups nor the influence of aging on all groups were statistically significant. Conclusions: The SBS between the ATZ core and the veneering ceramics was not affected by aging. The SBS of ATZ to veneering ceramics was not significantly different compared with that of Y-TZP.
Keywords: Shear bond strength Zirconia Alumina Veneering ceramics Aging
1. Introduction Recently, yttria-stabilized tetragonal zirconia polycrystals (Y-TZP ) has been introduced in the field of dentistry because of its superior aesthetic potential compared with that of metal-ceramic constructs. Y-TZP offers high flexural strength; however, its poor anti-aging properties lead to surface and strength degradation with grain pullout and microcracking[1-3]. Composite materials can improve the reliability and lifespan of Y-TZP by providing higher fracture toughness and mechanical strength. A composite material processed with zirconia and alumina is reported to have outstanding mechanical and tribological properties[4]. *: Corresponding author: *Corresponding author: Wei Li, Prof., Ph D, State Key
Laboratory of Oral Diseases, Sichuan University, Chengdu 610041, P.R. China. Tel: 86-28-85503494 Fax: 86-28-85503494 E-mail: [email protected] Foundation project: It is supported by National High-Tech Research and Development Program of China (863 Program) (No. 2006AA03Z440).
Alumina-toughened zirconia (ATZ) processed with 80 wt% Y-TZP and 20 wt% alumina (Tosoh Co., Tokyo, Japan) has a bending strength of up to 1 900 MPa. The addition of alumina increases the hydrothermal stability of the tetragonal phase because it could work as a barrier for the propagation of phase transformation into the bulk[5,6]. The addition of alumina to zirconia clearly hinders aging, or at least reduces drastically its kinetics[4,5]. Thus, ATZ has been recommended as a material for fixed partial denture frameworks[3]. Limited information is available on the bond quality of veneering ceramics to ATZ. Considering the optical opacity of ATZ and Y-TZP ceramic, they are typically covered with veneering ceramic, usually feldspathic types, with aesthetic characteristics similar to those of natural tooth substances[7]. The bond strength between the framework and the veneer, which depends on various factors, such as the strength of the chemical bonds, the mechanical interlocking, and the extent of compressive stress in the veneer caused by a thermal mismatch between the framework and the veneering ceramic, might affect the success of restorations[8-10]. The
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mechanical integrity and adhesion of the veneering ceramic to the ceramic substructure have proven to be key factors for the successful performance of veneer/core bilayered restorations, so the initial bond strength and reliability after aging obtained from in vitro investigations can provide useful information for the behavior and predictability of allceramic systems in clinical applications[11,12]. The current study aimed to evaluate the SBS of hipped Y-TZP and ATZ core ceramics and their corresponding veneering ceramics. In addition, the effect of aging in artificial saliva on SBS was investigated. Fractured surfaces were microscopically analyzed to determine the characteristics of bond failure. The null hypothesis of the current study was that the bond strength of ATZ to veneering ceramics is not different from that of Y-TZP, and would not be affected by aging. 2. Materials and methods
Figure 1. The shear jig used in the current study.
2.1. Preparation of zirconia core Y-TZP specimens were prepared from as-received Y-TZP powder containing 3 mol% of yttrium oxide (TZ3 YB-E , Tosoh Co., Tokyo, Japan). The ATZ composite ceramic consisted of 80 wt% 3Y-TZP and 20 wt% alumina (TZ-3Y20AB, Tosoh Co., Tokyo, Japan). Bend bars were uniaxially pressed at 50 MPa and then isostatically coldpressed at 200 MPa. The specimens were then sintered
in a high-temperature furnace (Nabertherm, Germany) according to the protocols. The specimens were heated from room temperature to 1 500 曟 at a rate of 200 曟 per hour. The specimens were held at this temperature for two hours, and then naturally cooled to room temperature. Each core ceramic specimen produced had a length of (17.0±0.2) mm, width of (4.0±0.1) mm, and height of (2.0±0.1) mm. 2.2. Preparation of the core veneer specimens Prior to veneering, all core specimens were cleaned with 70 % ethanol by wiping their surfaces with cotton, and subsequently cleaned for 5 min in an ultrasonic bath with ethanol. On the prepared face of each core specimen, a 3-mm layer of veneering ceramic (Vita VM9, Vita, Bad S 覿ckingen, Germany) was bonded at one end (Figure 1). A separable resin mold was used to layer the ceramic. Ceramic powder and an appropriate amount of the respective liquid were mixed to a slurry and placed into the mold. Excess liquid was removed using a tissue. Only the base dentine was layered. Firing was performed in a ceramic oven (ProgramatX 1, Ivoclar, Liechtenstein) according to the recommendations of the manufacturers (Table 1). Table 1 Firing schedules of veneering ceramics. Procedure
Cleaning firing Base dentine
Pre-drying Temperature(曟) Time(min) 500 3 500 2
Heating time (min) 6.00 8.11
2.3. Aging Artificial saliva was prepared according to the recipe in
ISO/TR 10271[13,14]. The pH of the solution was adjusted to ~5 with 0.5 mol/L natrium hydroxydatum and 30% acetic acid. Prior to shear bond testing, half of each group (n = 15) was subjected to aging at 134 曟 (under 2 bar pressure) for 48 h in an autoclave that has artificial saliva. The remaining specimens (n = 15) were stored dry at room temperature.
2.4. Shear bond strength test
Each specimen was tightened in a metal holder in a universal testing machine (Instron5565, USA) with a span of 15 mm and a crosshead rate of 0.5 mm per minute at room temperature (Figure 1). SBS (MPa) were calculated by dividing the failure load (N) by the bonding area (mm2)[11]. 2.5. Scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) analyses of the core-veneer interface
After the shear bond test, the fractured surfaces and their interfaces were evaluated using SEM (Model S-4500, Hitachi, Tokyo, Japan). The chemical structure at the interface was analyzed using EDS (INCAPentaFET-x3, Oxford). 2.6. Statistical analysis
Data analysis was performed with SPSS11.0 statistical software. The differences were compared by using the Student's t test. All P values were two-tailed with statistical significance indicated by a value of P<0.05.
Heating rate Firing temperature (曟/min) (曟) 33 700 55 950
Holding time (min) 5 1
Vacuum holding time (min) 0.00 8.11
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3. Results
Shear bond strength (Mpa)
The SBS results before and after aging are shown in Figure 2. Neither the differences between the SBS values of the ATZ and Y-TZP groups nor the influence of aging on all groups were statistically significant. The SBS of ATZ (28.9暲8.0) to veneering ceramics was higher than that of hipped Y-TZP (26.2暲7.6). The SEM images of the core-veneering ceramic sectioned areas showed that they were combined closely (Figure 3). In all the specimens, fracture began at the coreceramic interface and proceeded into the veneering ceramic. All groups showed a combination of cohesive and adhesive fracture modes (Figure 4). Figure 5 showed that the silicon in the interface between the TAP and VM9 linearly declined, whereas the silicon content in ATZ decreased beyond the average. Thus, veneering porcelain diffusion in the surface of ATZ may be better than that in TZP. After aging in artificial saliva for 48 h, the SEM images of the Y-TZP veneer and ATZ veneer surfaces were blurred (Figure 6).
20
VM9-TZP
30 20
10
Interface
10 0
0
10
20
30
40
ATZ-VM9
50
0
Interface 0
10
20
30
40
Figure 5. SEM image and EDS analysis of silicon on the fractured surface between the core-veneering ceramic (The above diagram is the EDS image and below is its respective SEM image).
pre-aging post-aging
40 35 30 25 20 15 10
5 0
Y-TZP
ATZ
Figure 2. Mean SBS of Y-TZP and ATZ before and after aging.
Figure 3. Sectioned surface morphology of the core and veneering ceramic interface (×500 magnification).
Figure 4. Fractured surface morphology showing combined fracture modes, namely, cohesion in the veneering ceramic and adhesion at the interface (×500 magnification).
Figure 6. SEM images of Y-TZP veneer (left) and ATZ veneer (right) specimens after 48 h of hydrothermal aging at 134 曟 in artificial saliva.
4. Discussion
The current study demonstrated that there was no significant difference between the SBS of ATZ to veneering ceramics compared with that of Y-TZP. Many factors may cause the production of inferior core-veneer bond strength. To date, no accepted mechanism has been reported to explain the nature of the bonding between cores and veneering porcelain. The coefficient of thermal expansion (CTE) seems to play a crucial role in the phenomenon[15-17]. SBS can be compromised by residual stresses because of CTE mismatch between the veneer and core ceramic[18]. Efforts have been made by dental manufacturers to develop ceramic cores and low-fusing veneering ceramics with similar CTE to generate acceptable levels of residual stress within a multilayer all-ceramic composite[11]. The CTE of ATZ (80 wt% 3Y-TZP and 20 wt% alumina) is significantly lower (10.1×10-6/K) than that of Y-TZP (10.5×10-6/K). According to the recommendations of the manufacturers, the CTE (25 曟 to 500 曟) of feldspathic veneering ceramic (VM9) is set to 9.0×10-6/ K to 9.2×10-6/K. From the metal ceramic bilayers, the recommended mean difference between the alloy and ceramic CTEs (from room temperature to 600 曟) is from 0.5×10-6/K to 1.0×10-6/K[19,20]. In the present study, the CTE mismatch between ATZ and veneer was closer to the recommended mean than that of Y-TZP. Thus, the SBS of ATZ to veneering ceramics is higher than that of Y-TZP. Considering that the veneering porcelain contains alumina and zirconia, and that the core ceramic does not contain silicon oxide, the spectrum of silicon dioxide EDS was used
Yong-Qi Zhao et al./Asian Pacific Journal of Tropical Medicine (2012)402-405
to detect mutual penetration of the interface between the core and veneer ceramic. The result shows that the silicon in the interface between the TAP and VM9 linearly declined, whereas the silicon content in ATZ decreased beyond the average. Thus, veneering porcelain diffusion in the surface of ATZ may be better than that in TZP. The failure mode observed for the Y-TZP-veneer and ATZ-veneer was mainly combined to show adhesion at the interface and cohesion in the veneering ceramic. This combination can be interpreted in two ways. First, crack deflection could be a consequence of the superior ability of Y-TZP to resist crack propagation. Second, the interlaminar crack deflection could correlate with the relatively poor bond of the zirconia core to the veneering ceramic. The clinical implication of this finding is that the investigated all-ceramic system could have a tendency to produce chipped-off fractures of the veneering ceramic and delaminations rather than catastrophic failure of the core structure[11,21]. Oral fluids facilitate stress corrosion of the ceramic materials, resulting in slow crack growth and finally leading to failed ceramic restorations in complex situations of the oral cavity[22,23]. In vitro aging sensitivity of the SBS of the bilayered specimens was therefore assessed by exposure in an autoclave[2]. In the present study, exposure to 48 h of aging at 134 曟 had no influence on the SBS of all groups investigated. Comparative studies on the bond strength of core and veneering ceramics after exposure to aging in artificial saliva are not available currently[11]. After aging in artificial saliva for 48 h, the SEM images of the Y-TZP veneer and ATZ veneer surfaces were blurred. The putative hydroxyapatite sediment remained on the sample, which was attributed to sample aging in artificial saliva containing large amounts of calcium and phosphorus ions. During the aging process, calcium and phosphorus ions in artificial saliva are continuously deposited on the ceramic surface. The sediment may only affect the surface of the ceramic. No effect on the internal structure was observed; thus, the SBS of Y-TZP or ATZ to veneering ceramics was not affected. In the present study, the shape of the samples was very different from that used in clinical application. Therefore, the results are not directly comparable to clinical situations. In conclusion, considering the limitation of the present study, the bond strength of Ce-TZP/A to veneering ceramics is concluded to be similar to that of Y-TZP and would not be affected by aging. Conflict of interest statement We declare that we have no conflict of interest. References [1] C attani-Lorente M, Scherrer SS, Ammann P, Jobin M, Wiskott HWA.Low temperature degradation of a Y-TZP dental ceramic. Acta Biomaterialia 2011; 7(2): 858-865. [2] Lughi V, Sergo V. Low temperature degradation -aging- of zirconia: A critical review of the relevant aspects in dentistry. Dental Materials 2010; 26(8): 807-820. [3] N evarez-Rascon A, Aguilar-Elguezabal A, Orrantia E, Bocanegra-Bernal MH. On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content. Intl J Refractory Metals & Hard Materials 2009; 27(6): 962-970.
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