Materials Science and Engineering C 33 (2013) 140–144
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Evaluation of the bond strength of a low-fusing porcelain to cast Ti–24Nb–4Zr–7.9Sn alloy Jiang Wu 1, Jian Zhou 1, Wen Zhao, Bo Gao ⁎ School of Stomatology, Fourth Military Medical University, Xi'an 710032, China
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 20 July 2012 Accepted 10 August 2012 Available online 21 August 2012 Keywords: Ti–24Nb–4Zr–7.9Sn Metal–ceramic restoration Bond strength Coefficient of thermal expansion
a b s t r a c t The purpose of this study was to compare the bonding characteristics of titanium porcelain Duceratin bonded to Ti–24Nb–4Zr–7.9Sn (TNZS) alloy and commercial pure titanium (cp Ti). The bond strengths between porcelain and TNZS were tested by a three-point flexural device. The same tests for the cp Ti were used as for the control. Coefficient of thermal expansion (CTE) of TNZS was evaluated with a push-rod dilatometer. Interfacial characterization was carried out by X-ray energy-dispersive spectrometry (EDS) analysis operating in line scan mode. Additionally, microstructure characterizations of TNZS and cp Ti after debonding fracture were analyzed by scanning electron microscope (SEM) and EDS. The porcelain bond strength of TNZS alloy was 31.51 MPa, showing a significant increase relatively to that of cp Ti (23.89 MPa) (Pb 0.05). Mean CTE values of TNZS alloy was 9.51×10−6/°C exceeding the porcelain by 0.81×10−6/°C, attesting to a better mechanical performance. Interfacial characterization showed the mutual diffusion of Ti, Si, O and Sn along the TNZS–ceramic interface. Both SEM and EDS results revealed that fracture modes of TNZS specimens exhibited a mixed mode of cohesive and adhesive failures. The results demonstrated that TNZS could be a good alternative for the metal–ceramic restoration in the future. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Porcelain-fused-to-metal crown combining the advantages of the strength of metal and the esthetics of porcelain is widely used [1,2]. During recent years, pure titanium and its alloy have become attractive dental restorative material for their excellent characteristics such as excellent biocompatibility, corrosion resistance, high strength and low cost [3,4]. However, some inherent problems in titanium–porcelain system compared to the traditional nickel–chromium dental alloy (Ni–Cr) system are still hindering its application. Contrary to the Ni–Cr casting process, titanium casting procedure shows some serious difficulties related mainly with its high melting point and its great chemical reactivity at high temperatures [5]. Special machine and protection gas should be applied to avoid forming a non-adherent layer, which will decrease the metal–ceramic bond strength [6,7]. Therefore, developing new titanium alloys was considered as a good option to improve the metal–ceramic bond strength [8,9], which could exhibit solid solution hardening and have lower fusion temperatures and better ductility than cp Ti. Another key factor of bond failure between titanium–porcelain is commonly due to the stress caused by the mismatch of the coefficient
⁎ Corresponding author at: 145 Changlexi Street, School of Stomatology, Fourth Military Medical University, Xi'an 710032, China. Tel.: +86 29 84773770; fax: +86 29 84776469. E-mail address: [email protected] (B. Gao). 1 Jiang Wu and Jian Zhou contributed equally as the first author to the article. 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.020
thermal expansion (CTE) of titanium and ceramic, that may affect the flexural bond strength of the titanium ceramic system [10]. Optimum adherence between the porcelain and metal is the main prerequisite for clinical success. Therefore, studies have focused on the titanium– porcelain bond [11,12]. Currently, great attention has been paid to the new titanium alloys, equipped with lower melting points, improved castability, better intraoral tarnish resistance, improved esthetics achieved by porcelain veneering, and increased metal–ceramic bond strength [8,9]. In addition, CTE of titanium could be influenced by the alloying additions either through dissolution in solid state, or by the formation of new phases, with some effects on metal–ceramic bond strength [13,14]. Among the various titanium alloys, β-titanium alloys have attracted much attention in dentistry for its relative low fusion temperature, improved ductility and excellent biocompatibility. Recently, a novel β-titanium alloy, Ti–24Nb–4Zr–7.9Sn (wt.%) (TNZS), has been developed by Institute of Metal Research Chinese Academy of Science (PCT/CN2004/ 001352). Comparing to the pure titanium, TNZS alloy exhibited solid solution hardening, relative lower fusion temperatures, better ductility, improved corrosion behavior and biocompatibility. The promising results illustrated that TNZS alloy had the potential for use in dental applications [15–17]. But for future dental clinical use, TNZS must be able to bond to dental porcelains for the esthetic demands of dentistry. The aim of the present study was to evaluate the bond strength of experimental TNZS alloys to dental porcelain. The bonding interface between metal and porcelain substrates after the bending test was also observed using a scanning electron microscope (SEM) with energy-
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dispersive spectrometry (EDS). In addition, the CTE values of the TNZS alloys and cp Ti were also evaluated. 2. Materials and methods 2.1. Specimen preparation 40 rectangular wax (Shanghai Medical Instrument Co., Shanghai, China) patterns (25.0 mm× 3.0 mm× 0.5 mm) were invested with magnesia based investment material (Dentaurum, Ispringen, Germany) and divided into 2 groups randomly. Then, one group was casted with TNZS (Institute of Metal Research Chinese Academy of Science, China) using a two chamber inert-gas vacuum pressure casting machine (Fourth Military Medical University, Xi'an, China). The other group was casted with cp Ti (Northwest Institute for Non-Ferrous Metal Research, China). Both groups of castings were sandblasted with 120 μm alumina oxide particles (Al2O3) for the removal of the investment material. The porosity of all specimens was examined radiographically and those with internal pores were excluded from the study. For porcelain application, a low-fusing porcelain, Duceratin (Ducera Dental-GmbH, Rosbach, Germany), was selected. According to the manufacturer's instructions, all specimens were sandblasted with 120 μm alumina at a pressure of 2 mbar in the central area of their surfaces and ultrasonically cleaned with distilled water for 10 min. A thin layer of SiO2-based bonding agent (Duceratin Kiss, Ducera Dental-GmbH, Rosbach, Germany) was painted with a short bristle brush on the 8 mm ×3 mm porcelain-bearing area in the central portion of the respective specimens. The bonding agent was dried (600°C) at the muffle entrance of the furnace (Multimat 99, Dentsply Intl., U.S.A.) and fired (750°C). Porcelain was added to the metal specimens to the dimensions of 8.0 mm in length, 3.0 mm in width, and 1.0 mm in thickness in the central portion of each metal strip. The entire process followed the manufacturer's instructions. To minimize the effect of handling variations, all the metal–ceramic specimens were prepared by one dental technician. 2.2. Evaluation of the bond strength According to International Organization for Standardization (ISO) standard 9693 [18], a three-point flexural device in a universal testing machine (AGS-10kNG, Shimadzu, Japan) was used to test specimens' bond strength. The specimens were placed on a custom-made loading jig with rounded supporting rods (Ø1.8 mm) 20 mm apart and loaded in the center with a rounded loading rod with the porcelain layer facing down (Fig. 1). A compressive load was applied at the midline of the metal strip by a rounded loading rod at a cross-head speed of 1.5 mm/min until a sudden drop in load occurred in the load deflection curve, indicating the bond failure. The porcelain bond strength was calculated according to the formula in ISO 9693. In this standard, the porcelain bond strength was determined by: bond strength= F × k. where F is the debonding load (N), and k is a coefficient calculated from the ISO specification that is dependent on the modulus of elasticity of the alloy and height of each specimen. In the current study, k values were 5.66 and 4.71 for TNZS and Ti specimens, respectively. 15 specimens were prepared for each group for comparison. 2.3. Microstructure characterization of the bond fracture Scanning electron microscope (SEM, HITACHI, S3400, Japan) was applied to characterize the type and morphology of the fracture in representative specimens selected from each combination in which there was a complete separation between porcelain and metal after the three-point bending test. The elemental analysis of the failed surfaces at the metal–porcelain interface was characterized by energy dispersive spectrometry (EDS, INCA Energy, Oxford Instruments, Oxford, UK).
Fig. 1. Three point bending of a metal–ceramic specimen according to ISO 9693:1999. The loading was applied on the metallic strip with a crosshead speed of 1.5 mm/min while the distance between the supporting points is 20 mm.
2.4. Interfacial characterization One specimen from each group in Section 2.1 Specimen preparation was embedded in an acrylic resin (GC, Japan). After 24 h storage in room temperature, they were gradually ground with silicon carbide papers (2400 grit) under continuous water cooling in the grinder/ polishing machine (Buehler UK Ltd., UK). The specimens were then ultrasonically cleaned for 10 min in a water bath and sputter-coated with a gold–palladium layer (JEOL, JFC-1100, Tokyo, Japan). The elemental distribution across the metal–ceramic interface was determined by using line scan energy dispersive spectrometry (EDS) analysis (INCA Energy, Oxford Instruments, Oxford, UK). 2.5. Coefficient of thermal expansion Thermal expansion of TNZS alloy (five specimens per group) was measured with a NETZSCH DIL 402C dilatometer (NETZSCH, Germany) at a heating rate of 5 °C/min from 25 °C to the target temperature of 500 °C under normal pressure. For each specimen, the coefficient of thermal expansion (CTE) was determined between 25 °C and 500 °C from the plotted curve of expansion versus temperature. 2.6. Statistical analysis Where applicable, the bond strength is expressed as mean ±standard deviation. Data was submitted to the Shapiro–Wilk test and statistical analyses were performed using the Student's t-test for the purpose of multiple comparisons with SPSS 10.0 software. Differences were considered statistically significant at Pb 0.05. 3. Results 3.1. Porcelain bond strength The porcelain bond strengths of cp Ti and TNZS were evaluated by the three-point bending test. The mean values and standard deviation of the two metal–porcelain groups and ISO standard are presented in Fig. 2. The mean values for the TNZS, cp Ti and ISO standard are 31.51 MPa, 23.89 MPa and 25 MPa, respectively. The bond strength for the cp Ti is very close to the value of previous reported literatures [19,20]. Thus, the values determined in the present study are within reasonable expectations. The Shapiro–Wilk test of normality revealed that data presented a normal and homogeneous distribution, enabling a parametric analysis. The statistical significance of the data determined
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Fig. 2. The mean porcelain bond strength values after the three-point bending test.
by SPSS 10.0 software is shown in Table 1. It can be seen that bond strength of the TNZS is significantly higher than that of cp Ti (P b 0.05). 3.2. Microstructure characterization of the bond fracture The SEM images of the cp Ti and TNZS surface after debonding test are showed in Figs. 3 and 4. The cp Ti surface exhibited the least amount of retained porcelain on the metal surface after debonding (Fig. 3), suggesting the occurrence of a predominantly adhesive failure, and corroborating the lower bond strength values obtained. However, more traces of retained porcelain were observed on TNZS specimen's surface (Fig. 4), which indicates the higher bond strength. EDS results of the selected regions (marked a, b and c) of the fracture surfaces of the cp Ti and TNZS are given in Tables 2 and 3. For cp Ti, regions a, b and c in Fig. 3, contained higher amounts of Ti and O, but very low amounts of Al and Si. The results indicated that very few amounts of porcelain remained on the debonding surface of cp Ti. For TNZS, based on EDS analysis, the remaining traces (red arrow) of region c in Fig. 4 containing higher amounts of O (54.15%), Al (3.21%), and silicon (Si) (32.26%), should be porcelain, while the other regions (a and b), rich in Nb (28.21% and 23.77%) and Ti (63.05% and 43.29%) should be the metallaurgical bond with the substrate. From the SEM-EDS examination, failure of the TNZS specimens can be attributed to the mixed mode, i.e., a combination of adhesive and cohesive failures. However, this phenomenon was not obvious on cp Ti specimens.
Fig. 3. SEM images of the fracture surfaces of cp Ti–porcelain specimen after porcelain debonding. There was almost no visual observation of retained porcelain on the debonding surface.
the Duceratin Kiss, according to the manufacturer, is 8.7 × 10 −6/°C. Thus, the CTE of 9.51 × 10 −6/°C for TNZS alloy exceeds the difference between them by 0.81 × 10 −6/°C. 4. Discussion Porcelain bond strength is important for the longevity of metal– ceramic restorations. In the current study, the coefficient of thermal expansion, interfacial characterization, low-fusing porcelain bond strength and microstructure characterization of the bond fracture of TNZS were evaluated, whereas cp Ti was used for comparison. It was noticed that, for the TNZS, a significant improvement in porcelain bond strength could be obtained. Moreover, several crucial factors that influence the bond strength of TNZS were discussed. Firstly, it is known that the success of the porcelain bonded to metal is closely correlated with its ability to withstand the combination of residual, thermal and applied mechanical stresses [21]. The residual stress is one of the main factors which affect the fracture resistance of the system. It can be varied by thermal expansion mismatch between the porcelain and the metal substrate. Moreover, the mismatch of coefficient of thermal expansion (CTE) between metal and porcelain significantly affected the flexural bond strength of the ceramic–metal systems, which may contribute to failure. It is now widely accepted that the alloy should
3.3. Interfacial characterization Fig. 5 shows the results from the line scan EDS analysis of the materials tested. For cp Ti (Fig. 5a) and TNZS samples (Fig. 5b), Ti depicted a progressive reduction from metal to bonding agent, while Si and O showed the inverse behavior. Additionally, for TNZS, Sn also showed the rather stable diffusion from metal to bonding agent; but content level is low. 3.4. Coefficient of thermal expansion Mean CTE values of TNZS alloy and the difference from porcelain Δα (× 10 −6/°C) are presented in Table 4. For this study, the CTE of Table 1 Results of statistical analysis of bond strength of bond strength tests. Bond strength test
Mean
SD
t
P
TNZS–porcelain cp Ti–porcelain
31.51 23.89
2.31 1.56
10
b0.05
SD standard deviation.
Fig. 4. SEM images of the fracture surfaces of TNZS–porcelain specimen after porcelain debonding. A large amount of retained porcelain traces were found on the debonding surface (red arrow).
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Table 2 EDS analyses of the compositions of the selected regions in Fig. 3 (wt.%). Region
Element
cp Ti a cp Ti b cp Ti c
Ti
C
O
Al
Si
84.02 67.49 69.33
4.15 2.77 10.75
5.89 16.46 19.21
5.21 4.04 0.34
0.73 9.24 0.38
have a higher thermal expansion coefficient than that of the porcelain (a positive expansion coefficient mismatch) in order to produce compressive stresses in the porcelain on cooling. In order to be compatible, the difference in the CTE between metal and porcelain should be equal to or less than 1 × 10 −6/°C at given temperature [22]. In this study, the CTE of 9.51× 10−6/°C for TNZS alloy exceeds that of porcelain (Duceratin Kiss 8.7 × 10−6/°C) by 0.81× 10−6/°C, which indicated a suitable mismatch of CTE. Whereas according to the previous research, the CTE of 10.1 × 10 −6/°C for cp Ti exceeds the difference between them by 1.4× 10−6/°C [20]. In comparing to cp Ti, TNZS alloy showed ideal thermal compatibility and it can also be speculated that TNZS alloy has the potential to overcome the mismatch in the CTE with lowfusing porcelains. Line scan analysis provided interesting information on the mutual elemental diffusion at the metal–ceramic interface. For 2 kinds of samples, with SiO2-based bonding agent, Ti decreased while the porcelain elements (Si and O) exhibited inverse distribution from metal to porcelain and it is in accordance with the results of previous research [19]. On the basis of the researches [23,24], this finding denotes that during the treatment of the ceramic layer there occurs diffusion of the components and possibly a chemical reaction at the interface between the metal substrate–intermediate coatings–porcelain. In another study [25], it was also suggested that the titanium–ceramic adhesion involved a chemical reaction between titanium and silica, and the formation of a new Ti5Si3O phase. A proposed mechanism is that Ti reduces SiO2 at the interface producing Ti2O3 or TiO and the free Si reacts with Ti to form the Ti complex compound Ti5Si3O [26]. Moreover, it is also found that obtaining chemical bonding is possible because of the diffusion of porcelain components into the titanium oxides during heating [24]. Suansuwan and Swain [8] observed the diffusion of Si and O across the interface. However, the nature of chemical bonding still requires further research. In the current study, a SiO2-based bonding agent was mixed with an aqueous solution producing slurry that was applied and fired on Ti or TNZS surface. Therefore, a possible mechanism for lower bond strength of cp Ti could be that Ti has enough time to be oxidized before the particles sintered providing a protective layer [19]. Moreover, for diffusion of Sn of TNZS at the interface, it has been demonstrated that β-Sn (metallic Sn) may play an important role in combining with oxygen and metal substrate, which can be attributed to the following structure: porcelain–O–1/2Sn 4+–1/2Sn 0–Ti [27,28]. And it may be partially responsible for the increased metal–ceramic bond strength. In this study, superior porcelain bond strengths of the TNZS metal–porcelain system were obtained. SEM images after debonding test showed the different failure modes of cp Ti–porcelain and TNZS alloy–porcelain. In general, failure modes of the metal–porcelain system can be classified as: (1) adhesive: between the metal and the porcelain, (2) cohesive: entirely within the porcelain, and (3) mixed Table 3 EDS analyses of the compositions of the selected regions in Fig. 4 (wt.%). Region
TNZS a TNZS b TNZS c
Fig. 5. Line scan EDS analysis demonstrating the variation of each element from cp Ti (a) or TNZS alloy (b) toward the bonding agent. The horizontal line denotes the directions of analysis. Line scans have been expanded in y-axis for the sake of clarity and thus should not be used for quantitative comparisons among elements.
mode: combination of adhesive and cohesive failures. In our results, based on SEM-EDS analyses, there was almost no visual observation of retained porcelain on cp Ti specimen's surface. More specifically, SEM/EDS analysis between cp Ti and porcelain indicated that metal fracture sites consisted mainly of titanium with interspersed regions of Al and Si, which were most likely due to residual porcelain. Few amounts of Al and Si proved that the phenomenon of cp Ti on the debonding surface should be that of adhesive mode and suggested that the lower mechanical bond strength values were obtained.
Table 4 Coefficients of thermal expansion for TNZS and the difference from the porcelain Δα (×10−6 K−1, 25–500 °C).
Element Ti
O
K
Na
Nb
Sn
Al
Si
63.05 43.29 –
– 19.82 54.15
– – 6.56
– – 3.82
28.21 23.77 –
8.74 6.79 –
– 4.73 3.21
– 1.60 32.26
TNZS Porcelain (Duceratin)
CTE α (×10−6/°C)
Difference in Δα (×10−6/°C)
9.51 8.7
0.81 –
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Almost the same results were acquired by Triantafillos [11] and suggested that the fracture should be located between the bonding agent and cp Ti surface. But for TNZS, SEM-EDS analyses showed many traces of retained porcelain remaining on the fracture surfaces of the TNZS specimens, which further demonstrated better porcelain bond strength of the TNZS alloy after debonding test. Based on SEM-EDS analyses, cp Ti specimen after debonding test exhibited metal oxide–porcelain failures (adhesive mode). But TNZS specimen after porcelain debonding test can be attributed to the mixed mode. O'Brien [29] has concluded that the overproduction of oxide, causing a sandwich effect between metal and porcelain, should be mainly responsible for the adhesive mode of fracture. And the high reactivity of titanium may lead to the over production of an oxide layer although SiO2-based bonding agent was applied. Moreover, low adhesion of cp Ti–porcelain can also be due to stress accumulation in the inwardly growing oxide scale at high temperature and long oxidation time, which prevented the formation of protective scale on the oxide surface [30,31]. Generally speaking, for the TNZS alloy, a significant improvement in strength was obtained. The mean value was superior to that of the cp Ti under the same conditions, and the SEM-EDS examination of the porcelain-debonded surfaces demonstrated the better porcelain bond strength of the TNZS alloy. With the improved mechanical properties and porcelain bond strength of TNZS alloy, it is highly expected that TNZS could be a good alternative for the metal–ceramic restoration in the future. 5. Conclusion In the study, TNZS alloy was shown to have a significantly improved porcelain bond strength, comparing to cp Ti. It was also concluded that the difference in the CTE between TNZS alloy and Duceratin porcelain is less than 1 × 10−6/°C, verifying the data obtained by bond strength testing. It can be anticipated that the potential of TNZS alloy could be a good alternative in the fabrication of substrate alloy for the metal–ceramic restoration in the future.
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Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Evaluation of the bond strength of a low-fusing porcelain to cast Ti–24Nb–4Zr–7.9Sn alloy Jiang Wu 1, Jian Zhou 1, Wen Zhao, Bo Gao ⁎ School of Stomatology, Fourth Military Medical University, Xi'an 710032, China
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 20 July 2012 Accepted 10 August 2012 Available online 21 August 2012 Keywords: Ti–24Nb–4Zr–7.9Sn Metal–ceramic restoration Bond strength Coefficient of thermal expansion
a b s t r a c t The purpose of this study was to compare the bonding characteristics of titanium porcelain Duceratin bonded to Ti–24Nb–4Zr–7.9Sn (TNZS) alloy and commercial pure titanium (cp Ti). The bond strengths between porcelain and TNZS were tested by a three-point flexural device. The same tests for the cp Ti were used as for the control. Coefficient of thermal expansion (CTE) of TNZS was evaluated with a push-rod dilatometer. Interfacial characterization was carried out by X-ray energy-dispersive spectrometry (EDS) analysis operating in line scan mode. Additionally, microstructure characterizations of TNZS and cp Ti after debonding fracture were analyzed by scanning electron microscope (SEM) and EDS. The porcelain bond strength of TNZS alloy was 31.51 MPa, showing a significant increase relatively to that of cp Ti (23.89 MPa) (Pb 0.05). Mean CTE values of TNZS alloy was 9.51×10−6/°C exceeding the porcelain by 0.81×10−6/°C, attesting to a better mechanical performance. Interfacial characterization showed the mutual diffusion of Ti, Si, O and Sn along the TNZS–ceramic interface. Both SEM and EDS results revealed that fracture modes of TNZS specimens exhibited a mixed mode of cohesive and adhesive failures. The results demonstrated that TNZS could be a good alternative for the metal–ceramic restoration in the future. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Porcelain-fused-to-metal crown combining the advantages of the strength of metal and the esthetics of porcelain is widely used [1,2]. During recent years, pure titanium and its alloy have become attractive dental restorative material for their excellent characteristics such as excellent biocompatibility, corrosion resistance, high strength and low cost [3,4]. However, some inherent problems in titanium–porcelain system compared to the traditional nickel–chromium dental alloy (Ni–Cr) system are still hindering its application. Contrary to the Ni–Cr casting process, titanium casting procedure shows some serious difficulties related mainly with its high melting point and its great chemical reactivity at high temperatures [5]. Special machine and protection gas should be applied to avoid forming a non-adherent layer, which will decrease the metal–ceramic bond strength [6,7]. Therefore, developing new titanium alloys was considered as a good option to improve the metal–ceramic bond strength [8,9], which could exhibit solid solution hardening and have lower fusion temperatures and better ductility than cp Ti. Another key factor of bond failure between titanium–porcelain is commonly due to the stress caused by the mismatch of the coefficient
⁎ Corresponding author at: 145 Changlexi Street, School of Stomatology, Fourth Military Medical University, Xi'an 710032, China. Tel.: +86 29 84773770; fax: +86 29 84776469. E-mail address: [email protected] (B. Gao). 1 Jiang Wu and Jian Zhou contributed equally as the first author to the article. 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.020
thermal expansion (CTE) of titanium and ceramic, that may affect the flexural bond strength of the titanium ceramic system [10]. Optimum adherence between the porcelain and metal is the main prerequisite for clinical success. Therefore, studies have focused on the titanium– porcelain bond [11,12]. Currently, great attention has been paid to the new titanium alloys, equipped with lower melting points, improved castability, better intraoral tarnish resistance, improved esthetics achieved by porcelain veneering, and increased metal–ceramic bond strength [8,9]. In addition, CTE of titanium could be influenced by the alloying additions either through dissolution in solid state, or by the formation of new phases, with some effects on metal–ceramic bond strength [13,14]. Among the various titanium alloys, β-titanium alloys have attracted much attention in dentistry for its relative low fusion temperature, improved ductility and excellent biocompatibility. Recently, a novel β-titanium alloy, Ti–24Nb–4Zr–7.9Sn (wt.%) (TNZS), has been developed by Institute of Metal Research Chinese Academy of Science (PCT/CN2004/ 001352). Comparing to the pure titanium, TNZS alloy exhibited solid solution hardening, relative lower fusion temperatures, better ductility, improved corrosion behavior and biocompatibility. The promising results illustrated that TNZS alloy had the potential for use in dental applications [15–17]. But for future dental clinical use, TNZS must be able to bond to dental porcelains for the esthetic demands of dentistry. The aim of the present study was to evaluate the bond strength of experimental TNZS alloys to dental porcelain. The bonding interface between metal and porcelain substrates after the bending test was also observed using a scanning electron microscope (SEM) with energy-
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dispersive spectrometry (EDS). In addition, the CTE values of the TNZS alloys and cp Ti were also evaluated. 2. Materials and methods 2.1. Specimen preparation 40 rectangular wax (Shanghai Medical Instrument Co., Shanghai, China) patterns (25.0 mm× 3.0 mm× 0.5 mm) were invested with magnesia based investment material (Dentaurum, Ispringen, Germany) and divided into 2 groups randomly. Then, one group was casted with TNZS (Institute of Metal Research Chinese Academy of Science, China) using a two chamber inert-gas vacuum pressure casting machine (Fourth Military Medical University, Xi'an, China). The other group was casted with cp Ti (Northwest Institute for Non-Ferrous Metal Research, China). Both groups of castings were sandblasted with 120 μm alumina oxide particles (Al2O3) for the removal of the investment material. The porosity of all specimens was examined radiographically and those with internal pores were excluded from the study. For porcelain application, a low-fusing porcelain, Duceratin (Ducera Dental-GmbH, Rosbach, Germany), was selected. According to the manufacturer's instructions, all specimens were sandblasted with 120 μm alumina at a pressure of 2 mbar in the central area of their surfaces and ultrasonically cleaned with distilled water for 10 min. A thin layer of SiO2-based bonding agent (Duceratin Kiss, Ducera Dental-GmbH, Rosbach, Germany) was painted with a short bristle brush on the 8 mm ×3 mm porcelain-bearing area in the central portion of the respective specimens. The bonding agent was dried (600°C) at the muffle entrance of the furnace (Multimat 99, Dentsply Intl., U.S.A.) and fired (750°C). Porcelain was added to the metal specimens to the dimensions of 8.0 mm in length, 3.0 mm in width, and 1.0 mm in thickness in the central portion of each metal strip. The entire process followed the manufacturer's instructions. To minimize the effect of handling variations, all the metal–ceramic specimens were prepared by one dental technician. 2.2. Evaluation of the bond strength According to International Organization for Standardization (ISO) standard 9693 [18], a three-point flexural device in a universal testing machine (AGS-10kNG, Shimadzu, Japan) was used to test specimens' bond strength. The specimens were placed on a custom-made loading jig with rounded supporting rods (Ø1.8 mm) 20 mm apart and loaded in the center with a rounded loading rod with the porcelain layer facing down (Fig. 1). A compressive load was applied at the midline of the metal strip by a rounded loading rod at a cross-head speed of 1.5 mm/min until a sudden drop in load occurred in the load deflection curve, indicating the bond failure. The porcelain bond strength was calculated according to the formula in ISO 9693. In this standard, the porcelain bond strength was determined by: bond strength= F × k. where F is the debonding load (N), and k is a coefficient calculated from the ISO specification that is dependent on the modulus of elasticity of the alloy and height of each specimen. In the current study, k values were 5.66 and 4.71 for TNZS and Ti specimens, respectively. 15 specimens were prepared for each group for comparison. 2.3. Microstructure characterization of the bond fracture Scanning electron microscope (SEM, HITACHI, S3400, Japan) was applied to characterize the type and morphology of the fracture in representative specimens selected from each combination in which there was a complete separation between porcelain and metal after the three-point bending test. The elemental analysis of the failed surfaces at the metal–porcelain interface was characterized by energy dispersive spectrometry (EDS, INCA Energy, Oxford Instruments, Oxford, UK).
Fig. 1. Three point bending of a metal–ceramic specimen according to ISO 9693:1999. The loading was applied on the metallic strip with a crosshead speed of 1.5 mm/min while the distance between the supporting points is 20 mm.
2.4. Interfacial characterization One specimen from each group in Section 2.1 Specimen preparation was embedded in an acrylic resin (GC, Japan). After 24 h storage in room temperature, they were gradually ground with silicon carbide papers (2400 grit) under continuous water cooling in the grinder/ polishing machine (Buehler UK Ltd., UK). The specimens were then ultrasonically cleaned for 10 min in a water bath and sputter-coated with a gold–palladium layer (JEOL, JFC-1100, Tokyo, Japan). The elemental distribution across the metal–ceramic interface was determined by using line scan energy dispersive spectrometry (EDS) analysis (INCA Energy, Oxford Instruments, Oxford, UK). 2.5. Coefficient of thermal expansion Thermal expansion of TNZS alloy (five specimens per group) was measured with a NETZSCH DIL 402C dilatometer (NETZSCH, Germany) at a heating rate of 5 °C/min from 25 °C to the target temperature of 500 °C under normal pressure. For each specimen, the coefficient of thermal expansion (CTE) was determined between 25 °C and 500 °C from the plotted curve of expansion versus temperature. 2.6. Statistical analysis Where applicable, the bond strength is expressed as mean ±standard deviation. Data was submitted to the Shapiro–Wilk test and statistical analyses were performed using the Student's t-test for the purpose of multiple comparisons with SPSS 10.0 software. Differences were considered statistically significant at Pb 0.05. 3. Results 3.1. Porcelain bond strength The porcelain bond strengths of cp Ti and TNZS were evaluated by the three-point bending test. The mean values and standard deviation of the two metal–porcelain groups and ISO standard are presented in Fig. 2. The mean values for the TNZS, cp Ti and ISO standard are 31.51 MPa, 23.89 MPa and 25 MPa, respectively. The bond strength for the cp Ti is very close to the value of previous reported literatures [19,20]. Thus, the values determined in the present study are within reasonable expectations. The Shapiro–Wilk test of normality revealed that data presented a normal and homogeneous distribution, enabling a parametric analysis. The statistical significance of the data determined
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Fig. 2. The mean porcelain bond strength values after the three-point bending test.
by SPSS 10.0 software is shown in Table 1. It can be seen that bond strength of the TNZS is significantly higher than that of cp Ti (P b 0.05). 3.2. Microstructure characterization of the bond fracture The SEM images of the cp Ti and TNZS surface after debonding test are showed in Figs. 3 and 4. The cp Ti surface exhibited the least amount of retained porcelain on the metal surface after debonding (Fig. 3), suggesting the occurrence of a predominantly adhesive failure, and corroborating the lower bond strength values obtained. However, more traces of retained porcelain were observed on TNZS specimen's surface (Fig. 4), which indicates the higher bond strength. EDS results of the selected regions (marked a, b and c) of the fracture surfaces of the cp Ti and TNZS are given in Tables 2 and 3. For cp Ti, regions a, b and c in Fig. 3, contained higher amounts of Ti and O, but very low amounts of Al and Si. The results indicated that very few amounts of porcelain remained on the debonding surface of cp Ti. For TNZS, based on EDS analysis, the remaining traces (red arrow) of region c in Fig. 4 containing higher amounts of O (54.15%), Al (3.21%), and silicon (Si) (32.26%), should be porcelain, while the other regions (a and b), rich in Nb (28.21% and 23.77%) and Ti (63.05% and 43.29%) should be the metallaurgical bond with the substrate. From the SEM-EDS examination, failure of the TNZS specimens can be attributed to the mixed mode, i.e., a combination of adhesive and cohesive failures. However, this phenomenon was not obvious on cp Ti specimens.
Fig. 3. SEM images of the fracture surfaces of cp Ti–porcelain specimen after porcelain debonding. There was almost no visual observation of retained porcelain on the debonding surface.
the Duceratin Kiss, according to the manufacturer, is 8.7 × 10 −6/°C. Thus, the CTE of 9.51 × 10 −6/°C for TNZS alloy exceeds the difference between them by 0.81 × 10 −6/°C. 4. Discussion Porcelain bond strength is important for the longevity of metal– ceramic restorations. In the current study, the coefficient of thermal expansion, interfacial characterization, low-fusing porcelain bond strength and microstructure characterization of the bond fracture of TNZS were evaluated, whereas cp Ti was used for comparison. It was noticed that, for the TNZS, a significant improvement in porcelain bond strength could be obtained. Moreover, several crucial factors that influence the bond strength of TNZS were discussed. Firstly, it is known that the success of the porcelain bonded to metal is closely correlated with its ability to withstand the combination of residual, thermal and applied mechanical stresses [21]. The residual stress is one of the main factors which affect the fracture resistance of the system. It can be varied by thermal expansion mismatch between the porcelain and the metal substrate. Moreover, the mismatch of coefficient of thermal expansion (CTE) between metal and porcelain significantly affected the flexural bond strength of the ceramic–metal systems, which may contribute to failure. It is now widely accepted that the alloy should
3.3. Interfacial characterization Fig. 5 shows the results from the line scan EDS analysis of the materials tested. For cp Ti (Fig. 5a) and TNZS samples (Fig. 5b), Ti depicted a progressive reduction from metal to bonding agent, while Si and O showed the inverse behavior. Additionally, for TNZS, Sn also showed the rather stable diffusion from metal to bonding agent; but content level is low. 3.4. Coefficient of thermal expansion Mean CTE values of TNZS alloy and the difference from porcelain Δα (× 10 −6/°C) are presented in Table 4. For this study, the CTE of Table 1 Results of statistical analysis of bond strength of bond strength tests. Bond strength test
Mean
SD
t
P
TNZS–porcelain cp Ti–porcelain
31.51 23.89
2.31 1.56
10
b0.05
SD standard deviation.
Fig. 4. SEM images of the fracture surfaces of TNZS–porcelain specimen after porcelain debonding. A large amount of retained porcelain traces were found on the debonding surface (red arrow).
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Table 2 EDS analyses of the compositions of the selected regions in Fig. 3 (wt.%). Region
Element
cp Ti a cp Ti b cp Ti c
Ti
C
O
Al
Si
84.02 67.49 69.33
4.15 2.77 10.75
5.89 16.46 19.21
5.21 4.04 0.34
0.73 9.24 0.38
have a higher thermal expansion coefficient than that of the porcelain (a positive expansion coefficient mismatch) in order to produce compressive stresses in the porcelain on cooling. In order to be compatible, the difference in the CTE between metal and porcelain should be equal to or less than 1 × 10 −6/°C at given temperature [22]. In this study, the CTE of 9.51× 10−6/°C for TNZS alloy exceeds that of porcelain (Duceratin Kiss 8.7 × 10−6/°C) by 0.81× 10−6/°C, which indicated a suitable mismatch of CTE. Whereas according to the previous research, the CTE of 10.1 × 10 −6/°C for cp Ti exceeds the difference between them by 1.4× 10−6/°C [20]. In comparing to cp Ti, TNZS alloy showed ideal thermal compatibility and it can also be speculated that TNZS alloy has the potential to overcome the mismatch in the CTE with lowfusing porcelains. Line scan analysis provided interesting information on the mutual elemental diffusion at the metal–ceramic interface. For 2 kinds of samples, with SiO2-based bonding agent, Ti decreased while the porcelain elements (Si and O) exhibited inverse distribution from metal to porcelain and it is in accordance with the results of previous research [19]. On the basis of the researches [23,24], this finding denotes that during the treatment of the ceramic layer there occurs diffusion of the components and possibly a chemical reaction at the interface between the metal substrate–intermediate coatings–porcelain. In another study [25], it was also suggested that the titanium–ceramic adhesion involved a chemical reaction between titanium and silica, and the formation of a new Ti5Si3O phase. A proposed mechanism is that Ti reduces SiO2 at the interface producing Ti2O3 or TiO and the free Si reacts with Ti to form the Ti complex compound Ti5Si3O [26]. Moreover, it is also found that obtaining chemical bonding is possible because of the diffusion of porcelain components into the titanium oxides during heating [24]. Suansuwan and Swain [8] observed the diffusion of Si and O across the interface. However, the nature of chemical bonding still requires further research. In the current study, a SiO2-based bonding agent was mixed with an aqueous solution producing slurry that was applied and fired on Ti or TNZS surface. Therefore, a possible mechanism for lower bond strength of cp Ti could be that Ti has enough time to be oxidized before the particles sintered providing a protective layer [19]. Moreover, for diffusion of Sn of TNZS at the interface, it has been demonstrated that β-Sn (metallic Sn) may play an important role in combining with oxygen and metal substrate, which can be attributed to the following structure: porcelain–O–1/2Sn 4+–1/2Sn 0–Ti [27,28]. And it may be partially responsible for the increased metal–ceramic bond strength. In this study, superior porcelain bond strengths of the TNZS metal–porcelain system were obtained. SEM images after debonding test showed the different failure modes of cp Ti–porcelain and TNZS alloy–porcelain. In general, failure modes of the metal–porcelain system can be classified as: (1) adhesive: between the metal and the porcelain, (2) cohesive: entirely within the porcelain, and (3) mixed Table 3 EDS analyses of the compositions of the selected regions in Fig. 4 (wt.%). Region
TNZS a TNZS b TNZS c
Fig. 5. Line scan EDS analysis demonstrating the variation of each element from cp Ti (a) or TNZS alloy (b) toward the bonding agent. The horizontal line denotes the directions of analysis. Line scans have been expanded in y-axis for the sake of clarity and thus should not be used for quantitative comparisons among elements.
mode: combination of adhesive and cohesive failures. In our results, based on SEM-EDS analyses, there was almost no visual observation of retained porcelain on cp Ti specimen's surface. More specifically, SEM/EDS analysis between cp Ti and porcelain indicated that metal fracture sites consisted mainly of titanium with interspersed regions of Al and Si, which were most likely due to residual porcelain. Few amounts of Al and Si proved that the phenomenon of cp Ti on the debonding surface should be that of adhesive mode and suggested that the lower mechanical bond strength values were obtained.
Table 4 Coefficients of thermal expansion for TNZS and the difference from the porcelain Δα (×10−6 K−1, 25–500 °C).
Element Ti
O
K
Na
Nb
Sn
Al
Si
63.05 43.29 –
– 19.82 54.15
– – 6.56
– – 3.82
28.21 23.77 –
8.74 6.79 –
– 4.73 3.21
– 1.60 32.26
TNZS Porcelain (Duceratin)
CTE α (×10−6/°C)
Difference in Δα (×10−6/°C)
9.51 8.7
0.81 –
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Almost the same results were acquired by Triantafillos [11] and suggested that the fracture should be located between the bonding agent and cp Ti surface. But for TNZS, SEM-EDS analyses showed many traces of retained porcelain remaining on the fracture surfaces of the TNZS specimens, which further demonstrated better porcelain bond strength of the TNZS alloy after debonding test. Based on SEM-EDS analyses, cp Ti specimen after debonding test exhibited metal oxide–porcelain failures (adhesive mode). But TNZS specimen after porcelain debonding test can be attributed to the mixed mode. O'Brien [29] has concluded that the overproduction of oxide, causing a sandwich effect between metal and porcelain, should be mainly responsible for the adhesive mode of fracture. And the high reactivity of titanium may lead to the over production of an oxide layer although SiO2-based bonding agent was applied. Moreover, low adhesion of cp Ti–porcelain can also be due to stress accumulation in the inwardly growing oxide scale at high temperature and long oxidation time, which prevented the formation of protective scale on the oxide surface [30,31]. Generally speaking, for the TNZS alloy, a significant improvement in strength was obtained. The mean value was superior to that of the cp Ti under the same conditions, and the SEM-EDS examination of the porcelain-debonded surfaces demonstrated the better porcelain bond strength of the TNZS alloy. With the improved mechanical properties and porcelain bond strength of TNZS alloy, it is highly expected that TNZS could be a good alternative for the metal–ceramic restoration in the future. 5. Conclusion In the study, TNZS alloy was shown to have a significantly improved porcelain bond strength, comparing to cp Ti. It was also concluded that the difference in the CTE between TNZS alloy and Duceratin porcelain is less than 1 × 10−6/°C, verifying the data obtained by bond strength testing. It can be anticipated that the potential of TNZS alloy could be a good alternative in the fabrication of substrate alloy for the metal–ceramic restoration in the future.
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