Interface toughness of a zirconia-veneer system and the effect of a liner application Gaoqi Wang, BE,a Song Zhang, PhD,b Cuirong Bian, SMM, MD,c and Hui Kong, MDd School of Mechanical Engineering, Shandong University, Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education), Jinan, P.R. China; Qilu Hospital of Shandong University, Jinan, P.R. China Statement of problem. Chipping of veneering porcelain and delamination of a zirconia-veneer interface are 2 common clinical failure modes for zirconia-based restorations and may be partially due to weak interface bonding. The effect of liner on the bond strength of the interface has not been clearly identified. Purpose. The purpose of the research was to evaluate the interface toughness between the zirconia core and veneering porcelain by means of a fracture mechanics test and to assess the effect of liner on the bond strength of the interface. Material and methods. Thirty bilayered beam-shape specimens were prepared and divided into 2 groups according to liner application. The specimens in each group were subdivided into 3 subgroups in accordance with 3 different veneer thicknesses. A fracture mechanics test was used on each specimen, and the energy release rate, G, and phase angle, j, were calculated according to the experimental results. A video microscope was used to monitor the crack propagation, and a scanning electron microscope was used to identify the fracture mode after testing. Two-way ANOVA and the Tukey honestly significant difference test were performed to analyze the experimental data (a¼.05) . Results. At each phase angle, the interfaces without a liner had higher mean G values than the interfaces with a liner. Both of the interfaces showed mixed failure mode with thin layers of a veneer or a liner that remained on the zirconia surfaces. Conclusions. Liner application before veneering reduced the interface toughness between zirconia and veneer. (J Prosthet Dent 2014;-:---)
Clinical Implications Pretreatment of a zirconia core with a liner may negatively affect the bond strength between the zirconia and the veneer. Consequently, the liner should be applied with caution. Ceramic restorations have been widely used for decades because of their excellent esthetics, biocompatibility, and inertness. Chipping of the veneering
porcelain, which has incidence rates of 6% to 25% over 2 to 5 years,1-7 is one of the most common failure modes of zirconia-based bilayered restorations.
Other failure modes, such as the delamination of zirconia-veneer and framework fracture, have also been observed clinically.7-11 Delamination is
Supported by the Independent Innovation Foundation of Shandong University (grant 2012JC032), and Scientific and Technological Planning Project of Shandong Province (grant 2010GDD20211). a
Doctoral student, School of Mechanical Engineering, Shandong University, Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education). b Professor, School of Mechanical Engineering, Shandong University, Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education). c Chief Physician, Department of Prosthodontics, Qilu Hospital of Shandong University. d Resident, Department of Prosthodontics, Qilu Hospital of Shandong University.
Wang et al
2
Volume related to the weak interfacial adhesion and large difference in mechanical properties of the zirconia and veneer materials. Moreover, the rate of chipping of veneering ceramic and delamination of ceramic prostheses are significantly higher than that of metal ceramic prostheses.5,12,13 As a result, many studies have been carried out in an effort to improve the bond strength of the zirconia-veneer interface with different zirconia surface treatments. These include airborne-particle abrasion,14-19 liner application,14-17,20,21 polishing,14,22 grinding,15 acid etching,18 laser etching,19 and silica coating,14 among which airborne-particle abrasion and liner application are the 2 most commonly studied methods. The application of a liner on the surface of a zirconia core is advised by the manufacturers of some ceramic systems to improve the bond strength and to mask the opaqueness of zirconia. However, conflicting viewpoints exist as to whether a liner might improve the bond strength between zirconia and veneering ceramics. The positive effect of a liner was considered in a Schwickerath crack initiation test21 and a tensile test,20 in which the use of a liner between the zirconia and veneer doubled the microtensile bond strength. Results of other studies indicated the negative effect of a liner.15,16,23 Kim et al16 found that liner application significantly decreased the shear bond strength of the zirconia-veneer interface compared with airborne-particle abrasion. Tinschert et al23 stated that the application of a liner can significantly weaken the bond strength and increase the percentage of interfacial failure between the zirconia core and pressable veneer ceramics. In addition, 2 in vitro studies with the tensile method17 and the shear method14 showed that the application of liner had no effect on interface bond strength. Different test methods (the shear test and the tensile test are the most prevalent) have been used to evaluate coreveneer bond strength. However, both of these test methods often show cohesive fracture patterns within the veneer
layer,13,14,24-27 which means that the results do not represent the true bond strength of the interface. Moreover, nonuniform stress distribution may also be produced during the testing procedures, which makes the results unreliable. The fracture mechanics approach proposed by Charalambides et al28 has been used to measure the interface toughness of bimaterial structures29 and has been applied in dentistry to determine the bond strength in metal ceramic structures.30-32 However, few studies have applied the fracture mechanics method to test the interface toughness of the zirconia-veneer system.33 The objectives of this study were to evaluate the zirconia-veneer interface toughness (represented by the energy release rate, G) at a different phase angle, j, with the fracture mechanics method and to assess the effect of a liner on the bond strength of the interface. The null hypothesis was that the bond strength of the zirconia-veneer interface would be improved by liner application.
MATERIAL AND METHODS Specimen preparation The chemical components of the materials are listed in Table I. Thirty zirconia beams with the same dimensions (1.5 5 40 mm) were cut from presintered blocks (Cercon Zirconia; Dentsply DeguDent GmbH) with a diamond saw and sintered according to the manufacturer’s recommendations. The specimens were cleaned in a sonic bath filled with ethanol for 5 minutes and gently air-dried. They were divided
Table I.
-
Issue
-
into 2 groups according to the application of the liner (IPS e.max Ceram ZirLiner; Ivoclar Vivadent AG). After the liner was applied to 15 specimens, all 30 specimens were veneered with IPS e.max Ceram (Ivoclar Vivadent AG). Each of the 2 groups was subdivided into 3 groups with veneer thicknesses of 2.25, 1.5, and 1.0 mm, which corresponded to core-veneer thickness ratios of 2:3, 1:1, and 3:2, respectively. The different thickness ratios were aimed at creating different phase angle, j, which is defined as the ratio between the shear and tensile modes at the interfacial crack tip. The different zirconia-veneer thickness ratios led to different stress states at the interface, which changed the ratio of shear-tensile mode at the interfacial crack tip. All the specimens were fired according to the manufacturer’s instructions. All specimen surfaces were polished with aluminum oxide abrasive papers (Imperial microfinishing film; 3M Corp) with sequentially finer grit size (40, 20, and 9 mm) to the final dimensions. A 0.7-mm-long notch was machined at the center of the veneer surface of each specimen with a diamond saw blade (Fig. 1A). During machining, a micrometer (Chengdu Chengliang Tools Corp) was attached to the machine to obtain a notch with accurate dimensions.
Four-point bending test All specimens were loaded to failure at a crosshead speed of 0.1 mm/min with a test machine (Instron 8801; Instron Corp) in standard 4-point bending mode (Fig. 1B), and the load-displacement
Chemical composition of materials
Materials
Chemical Composition
Cercon Zirconia
ZrO2; Y2O3 (5%); Hf2O3
Clinical Implications Pretreatment of a zirconia core with a liner may negatively affect the bond strength between the zirconia and the veneer. Consequently, the liner should be applied with caution. Ceramic restorations have been widely used for decades because of their excellent esthetics, biocompatibility, and inertness. Chipping of the veneering
porcelain, which has incidence rates of 6% to 25% over 2 to 5 years,1-7 is one of the most common failure modes of zirconia-based bilayered restorations.
Other failure modes, such as the delamination of zirconia-veneer and framework fracture, have also been observed clinically.7-11 Delamination is
Supported by the Independent Innovation Foundation of Shandong University (grant 2012JC032), and Scientific and Technological Planning Project of Shandong Province (grant 2010GDD20211). a
Doctoral student, School of Mechanical Engineering, Shandong University, Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education). b Professor, School of Mechanical Engineering, Shandong University, Key Laboratory of High Efficiency and Clean Mechanical Manufacture (Ministry of Education). c Chief Physician, Department of Prosthodontics, Qilu Hospital of Shandong University. d Resident, Department of Prosthodontics, Qilu Hospital of Shandong University.
Wang et al
2
Volume related to the weak interfacial adhesion and large difference in mechanical properties of the zirconia and veneer materials. Moreover, the rate of chipping of veneering ceramic and delamination of ceramic prostheses are significantly higher than that of metal ceramic prostheses.5,12,13 As a result, many studies have been carried out in an effort to improve the bond strength of the zirconia-veneer interface with different zirconia surface treatments. These include airborne-particle abrasion,14-19 liner application,14-17,20,21 polishing,14,22 grinding,15 acid etching,18 laser etching,19 and silica coating,14 among which airborne-particle abrasion and liner application are the 2 most commonly studied methods. The application of a liner on the surface of a zirconia core is advised by the manufacturers of some ceramic systems to improve the bond strength and to mask the opaqueness of zirconia. However, conflicting viewpoints exist as to whether a liner might improve the bond strength between zirconia and veneering ceramics. The positive effect of a liner was considered in a Schwickerath crack initiation test21 and a tensile test,20 in which the use of a liner between the zirconia and veneer doubled the microtensile bond strength. Results of other studies indicated the negative effect of a liner.15,16,23 Kim et al16 found that liner application significantly decreased the shear bond strength of the zirconia-veneer interface compared with airborne-particle abrasion. Tinschert et al23 stated that the application of a liner can significantly weaken the bond strength and increase the percentage of interfacial failure between the zirconia core and pressable veneer ceramics. In addition, 2 in vitro studies with the tensile method17 and the shear method14 showed that the application of liner had no effect on interface bond strength. Different test methods (the shear test and the tensile test are the most prevalent) have been used to evaluate coreveneer bond strength. However, both of these test methods often show cohesive fracture patterns within the veneer
layer,13,14,24-27 which means that the results do not represent the true bond strength of the interface. Moreover, nonuniform stress distribution may also be produced during the testing procedures, which makes the results unreliable. The fracture mechanics approach proposed by Charalambides et al28 has been used to measure the interface toughness of bimaterial structures29 and has been applied in dentistry to determine the bond strength in metal ceramic structures.30-32 However, few studies have applied the fracture mechanics method to test the interface toughness of the zirconia-veneer system.33 The objectives of this study were to evaluate the zirconia-veneer interface toughness (represented by the energy release rate, G) at a different phase angle, j, with the fracture mechanics method and to assess the effect of a liner on the bond strength of the interface. The null hypothesis was that the bond strength of the zirconia-veneer interface would be improved by liner application.
MATERIAL AND METHODS Specimen preparation The chemical components of the materials are listed in Table I. Thirty zirconia beams with the same dimensions (1.5 5 40 mm) were cut from presintered blocks (Cercon Zirconia; Dentsply DeguDent GmbH) with a diamond saw and sintered according to the manufacturer’s recommendations. The specimens were cleaned in a sonic bath filled with ethanol for 5 minutes and gently air-dried. They were divided
Table I.
-
Issue
-
into 2 groups according to the application of the liner (IPS e.max Ceram ZirLiner; Ivoclar Vivadent AG). After the liner was applied to 15 specimens, all 30 specimens were veneered with IPS e.max Ceram (Ivoclar Vivadent AG). Each of the 2 groups was subdivided into 3 groups with veneer thicknesses of 2.25, 1.5, and 1.0 mm, which corresponded to core-veneer thickness ratios of 2:3, 1:1, and 3:2, respectively. The different thickness ratios were aimed at creating different phase angle, j, which is defined as the ratio between the shear and tensile modes at the interfacial crack tip. The different zirconia-veneer thickness ratios led to different stress states at the interface, which changed the ratio of shear-tensile mode at the interfacial crack tip. All the specimens were fired according to the manufacturer’s instructions. All specimen surfaces were polished with aluminum oxide abrasive papers (Imperial microfinishing film; 3M Corp) with sequentially finer grit size (40, 20, and 9 mm) to the final dimensions. A 0.7-mm-long notch was machined at the center of the veneer surface of each specimen with a diamond saw blade (Fig. 1A). During machining, a micrometer (Chengdu Chengliang Tools Corp) was attached to the machine to obtain a notch with accurate dimensions.
Four-point bending test All specimens were loaded to failure at a crosshead speed of 0.1 mm/min with a test machine (Instron 8801; Instron Corp) in standard 4-point bending mode (Fig. 1B), and the load-displacement
Chemical composition of materials
Materials
Chemical Composition
Cercon Zirconia
ZrO2; Y2O3 (5%); Hf2O3
