journal of the mechanical behavior of biomedical materials 46 (2015) 197–204

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Research Paper

The effect of graded glass–zirconia structure on the bond between core and veneer in layered zirconia restorations Ruoyu Liua,1, Ting Sunb,1, Yanli Zhanga, Yaokun Zhanga, Danyu Jiangc,n, Longquan Shaoa,nn a

Department of Stomatology, Nanfang Hospital, Southern Medical University, Guangzhou 510515, China The 1st Affiliated Hospital of Jinan University, Guangzhou 510630, China c State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Shanghai 200050, China b

art i cle i nfo

ab st rac t

Article history:

Objective: The aim of this study was to test the hypothesis that a graded glass–zirconia

Received 2 December 2014

structure can strengthen the core–veneer bond in layered zirconia materials.

Received in revised form

Methods: A graded glass–zirconia structure was fabricated by infiltrating glass composi-

11 February 2015

tions developed in our laboratory into a presintered yttria tetrahedral zirconia polycrystal

Accepted 19 February 2015

(Y-TZP) substrate by the action of capillary forces. The wettability of the infiltrated glass

Available online 5 March 2015

and Y-TZP substrate was investigated by the sessile drop technique. The microstructures

Keywords:

of the graded glass–zirconia structure were examined by scanning electron microscopy

Dental ceramic

(SEM). The phase structure characterization in the graded glass–zirconia structure were

Functionally graded structure

identified by X-ray diffraction (XRD) analysis. The elastic modulus and hardness of the

Bonding properties

graded glass–zirconia structure were evaluated from nanoindentations. Further, the shear

Glass-infiltrated

bond strength (SBS) of the graded glass–zirconia structure and veneering porcelain was

Zirconia

also evaluated. Results: SEM images confirmed the formation of the graded glass–zirconia structure. Glass frits wet the Y-TZP substrate at 1200 1C with a contact angle of 43.21. Only a small amount of t–m transformation was observed in as-infiltrated Y-TZP specimens. Nanoindentation studies of the glass–zirconia graded structure showed that the elastic modulus and hardness of the surface glass layer were higher than those of the dense Y-TZP layer. The mean SBS values for the graded glass–zirconia structure and veneering porcelain (24.3570.40 MPa) were statistically higher than those of zirconia and veneering porcelain (9.2270.20 MPa) (Po0.05).

n

Corresponding author. Tel.: þ86 13817329659. Correspondence to: Nanfang Hospital, Southern Medical University, Guangzhou 510515, China. Tel.: þ86 15989283921. E-mail addresses: [email protected] (D. Jiang), [email protected] (L. Shao). 1 These authors contributed equally to this work and should be considered co-first authors.

nn

http://dx.doi.org/10.1016/j.jmbbm.2015.02.017 1751-6161/& 2015 Elsevier Ltd. All rights reserved.

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journal of the mechanical behavior of biomedical materials 46 (2015) 197 –204

Conclusions: A graded glass–zirconia structure can be fabricated by the glass infiltration/ densification technique, and this structure exhibits a strong core–veneer bond. & 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Owing to its good chemical properties, dimensional stability, and high mechanical strength and toughness, zirconia is currently widely utilized for fabricating prosthetic devices (Ozkurt and Kazazoğlu, 2010). Core materials of zirconia are usually added with translucent veneering porcelain for achieving better esthetics. However, the fracture rates of layered zirconia restorations have been reported to be significantly higher than those of metal ceramic restorations (Etman and Woolford, 2010). Fractures have been observed to originate from weak points, the veneer, or the core/veneer interface, resulting in chipping or delamination of the veneer and formation of cracks extending through the core materials (Rekow et al., 2011; Mochales et al., 2011). It has been reported that chipping and fracture result from chemical and physical mismatch, as well as inferior wetting of the zirconia core and veneering porcelain (Aboushelib et al., 2006). Therefore, in order to reduce clinical failures, considerable effort has been focused on increasing the bonding strength between veneering porcelain and zirconia. Tyszblat fabricated a ceramic/glass composite with high mechanical strength and low sintering shrinkage via the infiltration of lanthanum borosilicate glass into porous alumina (Tyszblat, 1987). This technique was then adopted by VITA Zahnfabrik and was commercialized under the brand name In-Ceram after several years of research and development (Sorensen et al., 1998). However, the mechanical characteristics of In-Ceram alumina materials were reported to be inferior to those of metal–ceramic materials (Sailer et al., 2007). Ceramics are brittle and therefore highly subject to premature failure from occlusal loading. In order to counter this brittleness, grading the material composition with a lower modulus at the external surfaces considered to be a good choice (Huang et al., 2007). Preliminary studies have demonstrated the feasibility of such a process, by infiltrating the surfaces of ceramic plates with an appropriate silicate glass (Zhang and Kim, 2009; Zhang and Ma, 2009). Zhang fabricated a functionally graded G/Z/G (glass/ zirconia/glass) structure – with Young’s modulus and hardness increasing from the surface to interior in accordance with a power law – by infiltrating glass compositions with a coefficient of thermal expansion and Poisson’s ratio similar to those of zirconia into porous Y-TZP (Zhang and Kim, 2009; Zhang and Ma, 2009). A low-modulus glassy surface dissipates surface bending

stress, rendering the graded zirconia–glass material superior in flexural capacity relative to monolithic zirconia (Zhang and Ma, 2009). The graded G/Z/G structure exhibited high strength, endurance, resistance to chipping fracture, and fatigue resistance (Zhang and Ma, 2010; Zhang et al., 2010, 2012). Graded zirconia blanks were further processed into anatomic restorations by the CAD/CAM technique; these restorations showed superior optical properties to monolithic zirconia restorations (Zhang et al., 2012). However, the translucency of the infiltrated products was not as high as that of layered zirconia restorations (Zhang et al., 2012). Thus, it can be said that layered zirconia materials are irreplaceable unless the zirconia can be made much more translucent. The abovementioned reports provided new insights into the design of functionally graded structures that can be highly beneficial for designing next-generation allceramic restorations. Thus, we hypothesized that a graded glass– zirconia structure may strengthen the core–veneer bond and enhance the durability of dental restorations. The Schmitz–Schulmeyer test method (Schmitz and Schulmeyer, 1975), a planar interface shear bond test, is based on minimal experimental variables and has considered to be a reliably well-suited test set up for metal ceramic bond strength method (Hammad and Talic, 1996). Therefore, the method was then transferred for the application of all-ceramic systems (Luthardt et al., 1999) and chosen for the present study. The purpose of this study was to evaluate the core–veneer bond of layered zirconia restorations by utilizing a glass– zirconia functionally graded material. In this study, a graded glass–zirconia structure was fabricated by infiltrating glass compositions into a presintered Y-TZP substrate by the action of capillary forces. The wettability of the infiltrated glass and Y-TZP substrate, as well as the microstructure and physical and mechanical properties of the graded glass– zirconia structure were also investigated.

2.

Materials and methods

2.1.

Preparation of specimens

2.1.1.

Preparation of silicate glass compositions

A new family of glass – La2O3–SiO2–B2O3–BaO–Al2O3–ZrO2–Y2O3– TiO2–CaO – was selected in this study. The main components

Table 1 – Chemical composition and coefficient of thermal expansion (CTE) of glass and Y-TZP. Material Glass compositions Y-TZP Vita VM9

Manufacturer

Main components [wt%]

CTE [10  6 K  1]

Tosoh, Tokyo, Japan

La2O3 20.0; SiO2 20.0; B2O3 15.0; BaO 15.0; Al2O3 10.0; ZrO2 5.0; Y2O3 5.0; TiO2 4.0; CaO 4.0 Y2O3 5.18, ZrO2 94.82

9.2 (Zhu and Yu, 2005) 10.5 (Zhang and Kim, 2009) 9.1–9.2 (Kim et al., 2006)

Vita Zahnfabrik, Bad Säckingen, Germany

Confidential

journal of the mechanical behavior of biomedical materials 46 (2015) 197 –204

199

and percentages (41 wt%) of the infiltrating glass are listed in Table 1. Desired oxides were mixed uniformly and melted in a zirconia crucible at 1300 1C for 2 h; they were subsequently quenched in water and ball-milled into 200-mesh powders.

scanning electron microscopy (SEM; TM3000 Hitachi, Tokyo, Japan). The cross-sections of specimens were etched with dilute hydrofluoric acid (10% solution in water) for 30 min, and were then gold coated for SEM examination.

2.1.2.

2.5.

Fabrication of Y-TZP substrate

Five Y-TZP substrate specimens were fabricated as cylinders (thickness: 4.0 mm, diameter: 4.0 cm). Yttria-stabilized zirconia powders (5.18 wt% Y2O3, TZ-3Y-E grade; Tosoh, Tokyo, Japan) were compressed under a uniaxial pressure of 150 MPa for 2 min and were then sintered at 1350 1C for 2 h in a muffle furnace (Qiantong, Shanghai, China).

2.2.

Wetting test

The sessile drop technique (Jong et al., 1982) was used to investigate the wettability of the Y-TZP substrate and infiltrated glass. The abovementioned glass powder was compressed into a cylindrical shape (height: 8.070.1 mm, diameter: 870.1 mm). The compressed glass was then placed on the middle of the sintered Y-TZP substrate specimen (prepared in Section 2.1.2) and heated in a muffle furnace. A digital camera (Sony, Tokyo, Japan) was used to record photographs of the wetting process from 1100 1C to 1200 1C at intervals of 10 1C. A glass drop was deposited on the Y-TZP surface and imaged by collecting one image per 2 min. The contact angle (θ) of a glass frit on a Y-TZP substrate specimen at 1200 1C was then calculated with a contact angle meter (KSVCAM100 KSV, Instruments LTD, Finland). Determination of contact angle was based on the YoungLaplace equation, yielding the contact angles on both sides of the droplet.

2.3.

Preparation of functionally graded material layer

Five Y-TZP substrate specimens (thickness: 4.0 mm, diameter: 1.0 cm) were presintered at 1200 1C for 2 h in air, thereby forming a porous structure for glass infiltration. The glass was reduced to a frit and applied, in slurry form, onto the top surface of the presintered Y-TZP porous substrate specimens. The coated specimens were then infiltrated at 1350 1C for 2 h to produce a glass–zirconia graded structure. The molten glass infiltrated the porous Y-TZP substrate by the action of capillary forces at a high temperature (Fig. 1). Glass infiltration and densification were carried out simultaneously.

2.4.

Microstructure

The cross-sectional surface morphology of the Y-TZP substrate specimens after glass infiltration was observed by

Phase structure characterization of the non-infiltrated and as-infiltrated specimens were performed by XRD (DX 2700, Dandongfangyuan, China) carried out using Cu Kα radiation at a scanning rate 21 min  1 and a step size of 0.021. Then volume fraction of monoclinic phase of the non-infiltrated and as-infiltrated specimens were calculated using the method of Garvie and Nicholson (1972).

2.6.

Nanoindentation test

The elastic modulus and hardness of the residual glass layer, graded glass–zirconia interior, and Y-TZP layer were measured using nanoindentations (Nano Indenter, G200, Agilent, USA). Measurements were carried out on the polished (1μm finish) cross-sections of the fabricated G/G–Z/Z (glass/glass– zirconia/zirconia) structure at a maximum load of 50 mN to produce a penetration depth of 0.3 μm in all regions. Nanoindentations were made from the surface residual glass layer, through the graded glass–zirconia layer to the Y-TZP layer with a step size of 100 μm. The elastic modulus and hardness for each indentation were determined from load–displacement curves by using the well-documented method of Oliver and Pharr (1992, 2004). Elastic modulus and hardness at each point was calculated from the unloading portions of the indentation load-displacement records according to a routine software protocol (Oliver and Pharr, 1992).The actual modulus of our graded material, E, was computed using the relationship proposed by Oliver and Pharr (1992):   ð1Þ 1=Er ¼  1 ν2 =E þ 1 νi 2 =Ei where ν is Poisson’s ratio of the ceramic material, and Ei and νi are the elastic modulus and Poisson’s ratio of the diamond indenter used, respectively. Here we use ν ¼0.3 for Y-TZP and for the graded glass–zirconia composite, Ei ¼1040 GPa and vi ¼0.07 for the diamond indenter. The hardness, H, was determined from the formula below: H ¼ Pmax =A

ð2Þ

where Pmax is the peak indentation load and A is the projected area of hardness impression (Oliver and Pharr, 1992). Then, asymptotic growth curves for the gradient elastic modulus and hardness of the functionally graded structure were fitted using Origin Software (Origin 8.0, Origin Lab, USA).

2.7.

Fig. 1 – Mechanism of glass infiltration process.

X-ray diffraction (XRD) analysis

Shear bond strength (SBS) test

Thirty Y-TZP substrate specimens were prepared as identical bar shaped (5.0 mm length, 5.4 mm width and 13.0 mm height) and divided into two groups – a test group and a control group – each containing 15 specimens. For the test group, the Y-TZP substrate specimens were presintered and then infiltrated with glass compositions. The specimens were prepared by the aforementioned process. For the control

200

journal of the mechanical behavior of biomedical materials 46 (2015) 197 –204

group, the Y-TZP substrates were sintered to full density. Thereafter, veneering porcelain Vita VM9(Vita Zahnfabrik, Bad Säckingen, Germany) was built up to all specimens to the final dimension(4.0 mm length, 5.4 mm width and 3.0 mm height) by a metal mold according to the Schmitz–Schulmeyer method (Schmitz and Schulmeyer, 1975) (Fig. 2). Firng steps followed the exact procedure recommended by the manufacturers (Table 2). The specimens were then subjected to the SBS test. Each specimen was tightened in a metal holder in a universal testing machine (Instron 022, Instron Limited, UK). The load was applied parallel to the long axis of the specimen through a wedge at the core–veneer interface at a crosshead speed of 5 mm/min until delamination of the veneering porcelain occurred. SBS (MPa) was then calculated by dividing the failure load (N) by the bonding area (mm2).

2.8.

Statistical analysis

The SBS values were statistically analyzed using the SPSS Program (SPSS 13.0, SPSS Inc., Chicago, USA). The difference between the SBS values of the two groups was calculated by an independent-sample t-test. The level of significance was set at 0.05.

3.

Results

3.1.

Wetting test

Typical photographs of the wetting process of a glass frit on a sintered Y-TZP substrate at different temperatures are shown in Fig. 3. The contact angle increased primarily but then decreased with an increase in the temperature. The glass frits almost completely wet the Y-TZP substrate at 1200 1C with a contact angle of 43.21.

Fig. 2 – Design and dimensions of Schmitz–Schulmeyer specimen. Arrows indicate load application during shear bond testing.

3.2.

Visual observation

The G/G–Z/Z structure with a thickness of 470.1 mm was consequently obtained, and included a thin, surface residual glass layer (0.3 mm70.1 mm), a graded glass–zirconia interior (2 mm70.1 mm), and a dense Y-TZP layer(1.7 mm70.1 mm).

3.3.

Microstructure

SEM images of the cross-section of the graded structure are shown in Fig. 4. The graded layer contained a relatively high glass content (dark phase in Fig. 4A) at the residual glass layer interface, and it gradually transformed to a dense Y-TZP layer (light phase in Fig. 4A) owing to the higher atomic weight of Y-TZP than that of the glass phase. The SEM image shown in Fig. 4B depicts that after etching for 30 min, a threedimensional network morphology consisting of traces of residual glass, glass-coated zirconia grains, and intergranular voids was formed, which created a surface morphology ideal for increasing the core–veneer bond strength.

3.4.

XRD analysis

Fig. 5A and B shows the XRD analysis of the surface of noninfiltrated and as-infiltrated Y-TZP specimens, respectively. In contrast to the XRD analysis results for the sintered Y-TZP specimen, the main crystalline phase of zirconia, i.e., tetragonal zirconia (t-ZrO2), was maintained even after glass infiltration in the as-infiltrated Y-TZP specimen. The asinfiltrated specimen contained 19.7% monoclinic phase which is higher than non-infiltrated specimen (13.4%). The peaks of monoclinic phase increased after glass-infiltrating. Only a small amount of t–m transformation was observed in as-infiltrated Y-TZP specimens.

3.5.

Nanoindentation study

The dependence of elastic modulus and hardness gradations on the depth (from the glass layer to the Y-TZP layer) of the graded glass–zirconia structure is shown in Fig. 6A and B, respectively. The specimens were prepared from the G/G–Z/Z structure. The residual glass layer (labeled as zone I in Fig. 6A and B) possessed an elastic modulus value of E¼121.8726.5 GPa (mean7S.D., n¼ 3) and a hardness value of H¼10.172.5 GPa (mean7S.D., n¼ 3). For the graded glass–zirconia layer (marked as zone II in Fig. 6A and B), the elastic modulus and hardness varied from E¼ 204.7 GPa and H¼13.4 GPa near the residual glass/graded layer interface to E¼ 256.2 GPa and H¼ 16.4 GPa near the graded layer/Y-TZP layer boundary. The Y-TZP layer (identified as zone III in Fig. 6A and B) exhibited elastic modulus and hardness values of E¼ 279.472.8 GPa (mean7S.D., n¼ 3) and H¼ 21.371.1 GPa (mean7S.D., n¼ 3), respectively. It should be noted here that the n values quoted in parentheses represent the number of indents made in the residual glass layer and Y-TZP layer on the graded glass–zirconia structure specimens. The asymptotic growth curves are shown in Fig. 6A. With an increase in the infiltration depth, the elastic modulus and hardness of the graded structure increased. Further, the elastic modulus and hardness increased rapidly in the primary stage. The increase continued to level off through a depth of 2400 μm.

journal of the mechanical behavior of biomedical materials 46 (2015) 197 –204

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Table 2 – Veneering procedures. Veneering ceramic

Core materials used for 1. firing

Core materials used for 2. firing

Glaze firing

Furnace manufacturer

Vita VM9

Effect bonder

Dentin 2 M 3 modeling liquid vita VM9

One layer

Multimat MCII (Biodent, Quebec, Canada)

Fig. 3 – A wetting process of glass frit on a Y-TZP substrate from 1100 1C to 1200 1C. Fig. 3B Schematic diagram of wetting process.

Fig. 4 – A SEM image showing microstructural features of graded structure of a cross-section of glass-infiltrated zirconia (magnification:  5000). Fig. 4B SEM image of a graded layer surface treated with dilute HF (10% solution in water) for 30 min (magnification:  15,000).

Fig. 5 – A XRD pattern of sintered Y-TZP surface without glass infiltration. Fig. 5B XRD pattern of as-infiltrated glass–zirconia surface.

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Fig. 6 – A Variation of elastic modulus (E) as a function of depth (z) of functionally graded structure. Fig. 6B Variation of hardness (H) as a function of depth (z) of functionally graded structure. Three distinctive zones are apparent: a surface residual glass layer (I), a graded glass–zirconia layer (II), and a dense Y-TZP layer (III).

3.6.

SBS test

The SBS values for the specimens in the two test groups are listed in Table 3. The mean SBS values were 24.3570.40 MPa for the as-infiltrated group and 9.2270.20 MPa for the noninfiltrated group (control group). The SBS values for graded zirconia and veneering porcelain were statistically higher than those for the control group (Po0.05). The fractured modes of two groups could be classified as cohesive failures in the veneer porcelain and a combination of adhesive and cohesive failures.

4.

Discussion

Using glass powder developed in our laboratory, we successfully fabricated a graded glass–zirconia structure for use in dental restorations. Nanoindentation studies of the graded glass–zirconia structure revealed that its elastic modulus and

hardness increase from the surface glass layer to the Y-TZP layer. The data obtained in this study showed that the graded glass–zirconia structure had markedly superior bond strength than veneering porcelain. The principal requirements for obtaining reasonable infiltration of glass were as follows: good wettability and interlayer diffusion with zirconia to obtain a strong bond, an average coefficient of thermal expansion (CTE) lower than that of zirconia to minimize residual stress, low viscosity conducive to infiltration at the experimental temperature, and chemical compatibility with zirconia to avoid adverse chemical degradation in a reactive environment (Saied et al., 2011). Good wettability with zirconia was the predominant factor for the infiltration process. A smaller contact angle (o901) resulted in good wettability between the solution and substrates. Fig. 2 shows that the contact angle was 43.21 at 1200 1C, indicating the good wettability of the infiltrated glass and Y-TZP substrate. In this study, the CTE of the glass compositions was lower than that of Y-TZP (Table 1), which was beneficial for the formation of a compressive stress field around grain boundaries during cooling. Such stress fields exerted an opposing force to crack growth, offering resistance to crack propagation (Diego et al., 2007). For fabricating a graded structure, Y-TZP substrates were primarily presintered at 1200 1C for 2 h, producing a porous template for glass infiltration. Glass infiltration and densification were then carried out at 1350 1C for 2 h. This ensured the controllability of the glass infiltration depth by manipulating the porosity of the infiltrating structures (Kingery et al., 1976). Moreover, the grain growth and/or destabilization of the tetragonal zirconia phase (Piascik et al., 2006), which are known to be deleterious for the hydrothermal stability of Y-TZP in the body, associated with the postsintering heat treatment were prevented (Fig. 5). It is possible that glass, both in the external surface glass layer and in the graded glass–ceramic layer, may undergo a compositional change and/or crystallization during the infiltration process. This can change the CTE of glass and ceramic, introducing residual stresses in the graded material and hence altering its strength (Zhang et al., 2012). The addition of ZrO2 (5 wt%) and Y2O3 (5 wt%) into the glass compositions prevented any chemical reaction between the compositions and zirconia, resulting in a minor t–m phase transition during glass infiltration (Fig. 5). Therefore, it was unlikely that significant residual stresses developed in the graded materials. Nanoindentation studies of the graded glass–zirconia structure showed that its elastic modulus and hardness increased from the surface glass layer to the dense Y-TZP layer (Fig. 6). The mechanical properties of functionally graded materials vary with position according to a power law or linear function (Zhang and Ma, 2009). The asymptotic growth curves (Fig. 6) indicated that the glass composition content decreased with an increase in the infiltration depth. Further, the glass composition content remained fairly steady till a depth of 2400 μm, indicating that the infiltration depth was limited to this value. The thermal and mechanical properties of the glassinfiltrated graded layer can be matched more closely to that of the veneering porcelain, thus reducing the possibilities of catastrophic flexure-induced radial cracks in the veneering porcelain on a compliant interface support (Chai and Lawn

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journal of the mechanical behavior of biomedical materials 46 (2015) 197 –204

Table 3 – T-test results for SBS of two groups (x7s). Group

n

SBS values (mean7S.D., MPa)

F

P

As-infiltrated group Non-infiltrated group

15 15

24.3570.40 9.2270.20

4.97

o0.05

2000). Additionally, the graded structure eliminated the sharp core–veneer interface that would have led to chipping and delamination of the veneering porcelain (Sundh et al., 2005; Vult von Steyern et al., 2006). Good optical properties can be obtained by controlling the glass content on the Y-TZP surface. Moreover, the veneering porcelain applied on the surface of the graded structure could prevent any unwanted wear of opposing natural dentition, thus avoiding minor possibilities of crack propagation from low-modulus, low-toughness porcelain to high-modulus, high-toughness zirconia (Kim et al., 2006). It has been reported that the Schmitz–Schulmeyer test was a simple and reliable screening measurement to investigate shear bond strengths of metal and all-ceramic systems (Guess et al., 2008). The stresses during the shear tests were reported to be directed mainly at the interface resulting in a relatively uniform distribution of interfacial stresses (Anusavice et al., 1980) The method was therefore chosen for the present study and considered as an applicative test to measure the effective shear bond strength of all-ceramic systems. The results of the failure modes for the two groups indicated cohesive failures in the veneer porcelain or a combination of adhesive and cohesive failures. The mean SBS values for the asinfiltrated group (24.3570.40 MPa) were statistically higher than those for the non-infiltrated group (9.2270.20 MPa) (Po0.05). In a previous study, it was reported that the ideal bonding strength of veneering porcelain to a metal in metal–ceramic restorations reached 26.6 MPa (Guess et al., 2008). Thus, it could be inferred that the bond strength of veneering porcelain to the graded structure (24.3570.40 MPa) was very close to that of a metal–ceramic system that is generally considered as “gold standard.” The markedly superior bond strength between the graded glass–zirconia structure and veneering porcelain may be attributed to their good chemical compatibility. Further, interpenetrated, ionic interdiffusion may also have existed, indicating good chemical bonding (Chai and Lawn, 2000). Moreover, in layered zirconia structures, cracks are liable to propagate because of the relatively low toughness of veneering porcelain (Chai et al., 2011). The toughness of the core–veneer interface was invariably lower than that of the veneering porcelain itself (Kim et al., 2006; Guess et al., 2008). For a large contact-to-edge distance, cracks were likely to intersect this weak interface, causing premature delamination (Zhang et al., 2012). However, for the graded structure fabricated in this study, owing to the stronger bonding between the veneering porcelain layer and glass–zirconia graded layer, any cracks intersecting the stronger interface will penetrate the zirconia sublayer, thus resulting in much improved resistance to veneer chipping and fracture. The current research is different from previous studies (Zhang and Kim, 2009; Zhang and Ma, 2009, 2010; Zhang et al., 2010, 2012) in two aspects. First, in previous studies, the top and bottom surfaces of presintered Y-TZP were infiltrated at a high

temperature (1450 1C) for 2 h. In contrast, in the present study, we infiltrated only the bonding surface of Y-TZP by using a new family of glass composition, thereby improving both the aesthetics and bond strength at the interface between Y-TZP and veneering porcelain. Secondly, unlike previous studies in which G/Z/G structures were fabricated, we prepared a graded glass– zirconia layer with veneering porcelain (V/G/Z) structures. The mechanical and physical properties of the glass-infiltrated layer can be matched more closely to that of typical veneering porcelain, providing a strong interface support. A controlled modulus gradient of the structure was not obtained. Further studies are needed for expressing the modulus and hardness as a function of depth beneath the surface. This was the limitation of the present study. However, this study has provided practical guidelines for the enhancement of the core–veneer bond in layered zirconia materials.

5.

Conclusions

Within the limitations of this study, it can be concluded that a graded glass–zirconia structure exhibiting a strong core– veneer bond can be successfully fabricated by the glass infiltration/densification technique.

Acknowledgments This work was supported by the National Natural Science Foundation of China (31070857, 50973045, 51172283, 81400557), the Project on the Integration of Industry, Education and Research of Guangdong Province, China (2012B091000147), Key Innovation Foundation of SICCAS, and the Scientific Cultivation Foundation of the First Affiliated Hospital of Jinan University (2013205).

r e f e r e nc e s

Aboushelib, M.N., Kleverlaan, C.J., Feilzer, A.J., 2006. Microtensile bond strength of different components of core veneered allceramic restorations. Part II: Zirconia veneering ceramics. Dent. Mater. 22, 857–863. Anusavice, K.J., Dehoff, P.H., Fairhurst, C.W., 1980. Comparative evaluation of ceramic–metal bond tests using finite element stress analysis. J. Dent. Res. 59, 608–613. Chai, H., Lawn, B.R., 2000. Role of adhesive interlayer in transverse fracture of brittle layer structures. J. Mater. Res. 15, 1017–1024. Chai, H., Lee, J.J., Lawn, B.R., 2011. On the chipping and splitting of teeth. J. Mech. Behav. Biomed. Mater. 4, 315–321. Diego, A.A., Dos Santos, C., Landim, K.T., 2007. Characterization of ceramic powders used in the In-Ceram systems to fixed dental prosthesis. Mater. Res. 10, 47–51.

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The effect of graded glass-zirconia structure on the bond between core and veneer in layered zirconia restorations.

The aim of this study was to test the hypothesis that a graded glass-zirconia structure can strengthen the core-veneer bond in layered zirconia materi...
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