Evaluation of Experimental Coating to Improve the Zirconia-Veneering Ceramic Bond Strength Jay D. Matani, BDS, MDS,1 Mohit Kheur, BDS, MDS,1 Shantanu Subhashchandra Jambhekar, BDS, MDS,2 Parag Bhargava, BTech, PhD,3 & Aditya Londhe, BTech, MTech3 1
Department of Prosthodontics and Implantology, M A Rangoonwala College of Dental Sciences and Research Centre, Azam Campus, Pune, India Terna Dental College, Nerul, Navi, Mumbai, India 3 I.I.T., Mumbai, India 2
Keywords All-ceramic; zirconia; delamination; shear bond strength. Correspondence Jay D. Matani, Department of Prosthodontics, M A Rangoonwala College of Dental Sciences and Research Centre, Azam Campus, Pune, Maharashtra 411001, India. E-mail: [email protected] The authors deny any conflicts of interest. Accepted November 16, 2013 doi: 10.1111/jopr.12176
Abstract Purpose: To evaluate the shear bond strength (SBS) between zirconia and veneering ceramic following different surface treatments of zirconia. The efficacy of an experimental zirconia coating to improve the bond strength was also evaluated. Materials and Methods: Zirconia strips were fabricated and were divided into four groups as per their surface treatment: polished (control), airborne-particle abrasion, laser irradiation, and application of the experimental coating. The surface roughness and the residual monoclinic content were evaluated before and after the respective surface treatments. A scanning electron microscope (SEM) analysis of the experimental surfaces was performed. All specimens were subjected to shear force in a universal testing machine. The SBS values were analyzed with one-way ANOVA followed by Bonferroni post hoc for groupwise comparisons. The fractured specimens were examined to observe the failure mode. Results: The SBS (29.17 MPa) and roughness values (0.80) of the experimental coating group were the highest among the groups. The residual monoclinic content was minimal (0.32) when compared to the remaining test groups. SEM analysis revealed a homogenous surface well adhered to an undamaged zirconia base. The other test groups showed destruction of the zirconia surface. The analysis of failure following bond strength testing showed entirely cohesive failures in the veneering ceramic in all study groups. Conclusion: The experimental zirconia surface coating is a simple technique to increase the microroughness of the zirconia surface, and thereby improve the SBS to the veneering ceramic. It results in the least monoclinic content and produces no structural damage to the zirconia substructure.
An increasing demand for esthetics and recent advances in technology have contributed to the rise in popularity of allceramic restorations. Various studies have documented longterm success of different all-ceramic systems for single-unit restorations.1,2 However, many all-ceramic systems were confined to fabricating three-unit fixed dental prostheses (FDPs) that replaced a missing anterior tooth up to the second premolar due to their strength limitations and the brittle nature of ceramics.3 Zirconia is a recent introduction to the family of dental ceramics. Its superior mechanical properties make it the material of choice for the substructure without any limitations with respect to the size of the restoration.4 Zirconia occurs in three forms, monoclinic, cubic, and tetragonal. Pure zirconia at room temperature is monoclinic and sta-
ble up to 1170°C. Above this temperature, it transforms itself into the tetragonal and then further into the cubic phase at 2370°C. Pure zirconia can be alloyed with stabilizing oxides.5 This would allow the retention of tetragonal structure at room temperature and therefore the control of the stress-induced tetragonal → monoclinic transformation. This efficiently reduces crack propagation and results in high toughness. Progresses in tetragonal–monoclinic surface transformation may have unfavorable effects on mechanical behavior.6 One of the most frequently occurring technical complications for core-veneered zirconia restorations is the chipping of veneering ceramic or delamination of the veneer from its core.7,8 Long-term (5 years) evaluation of zirconia restorations has shown good success rates.9 The incidence of chipping has been reported to be 15% after 24 months,10 13% after
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37.2 months,11 25% after 31.2 months,12 and 15.2% after 35.1 months.13 This concurs with the results obtained in a review.8 For zirconia restorations, clinical success and reliability depends on the mechanical integrity and the bond strength of the interface between the veneering ceramic and the ceramic core. Many variables, such as the surface finish of the ceramic core, residual stresses generated by mismatch in the coefficient of thermal expansion (CTE) of the ceramics, development of flaws and structural defects at the core/veneer interface, and the wetting properties and volumetric shrinkage of the veneer, may affect the core/veneer bond strength.14 The individual and combined effects of such variables can influence the bond strength and the properties of the zirconia core, and thereby the clinical success of restorations. The use of different surface treatments relies on increasing the surface area present for bonding and hence establishing durable restorations; however, despite these applications, clinical complications in the form of chipping or delamination still persist. Zirconia-veneering ceramic bond strength is determined by various factors explained earlier.15-18 These factors influence the properties of the zirconia core as well. Previous investigations have focused on different chemomechanical treatments of the zirconia surface to optimize the bonding to the overlying veneering ceramic.16,17,19-23 These treatments, along with grinding of zirconia surfaces, lead to stresses at a depth of a few microns on the zirconia surface. This results in the formation of a superficial monoclinic layer, which is not suitable for veneering. The two crystalline structures of zirconia exhibit different CTEs; however, in a routine clinical/laboratory situation, at times excessive grinding and adjusting of the zirconia core cannot be avoided. Various authors have suggested airborne-particle abrasion of the zirconia surface prior to veneering.16,19-21,24 Kim et al suggested that airborne-particle abrasion is a better surface treatment when compared to the application of a liner.16 However, some authors have reported that use of air abrasion is not beneficial.17,18 The literature is inconclusive about the effects of laser on zirconia.25 The effect of laser on previously airborne particle abraded zirconia surfaces is also not known. The different surface treatments suggested have been “subtractive” techniques of increasing surface roughness and are known to damage the superficial tetragonal surfaces. The use of an additive technique that incorporates a submicron thick porous layer onto the surface of the zirconia to increase the surface roughness and also to improve the zirconia-veneering ceramic bond strength could be a potentially beneficial technique. The aim of this research is to evaluate the effect of the experimental technique in producing a roughened zirconia surface and to investigate its effect on the bond strength between zirconia and veneering ceramic.
Materials and methods A total of 48 presintered rectangular zirconia specimens (15 mm length, 5 mm width, and 5 mm height) were cut from presintered zirconia blocks (Lot No. 0019481; Metoxit, Thayngen, Switzerland) using a new carborundum disk for each specimen. Thirty six specimens were sintered in a furnace (Zirkonofen 600 2
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V2; Zirkonzahn Worldwide, Gais, South Tyrol, Italy) according to the manufacturer’s recommendations of 8 hours with a 2-hour holding time at 1500°C. The 36 sintered and 12 presintered specimens were successively wet polished for 15 seconds on one surface using silicon carbide (SiC) papers of #400, #600, and #1000. The specimens were manually polished to obtain experimental surfaces of a similar finish. The surface roughness of the specimens was analyzed at two locations on each specimen using a surface roughness measuring tester (Surftest SJ-210; Mitutoyo Corporation, Kanagawa, Japan). The testing was done as per ISO 1997 standards (traverse speed—0.5 mm/s, cutoff value of 0.8, 5× number of sampling length). The arithmetic mean was calculated for each specimen. Raman spectroscopy (Model: Ramnor HG-2S Spectrometer; Jobin-Yvon, Longjumeau, France) was employed to assess the phase of the zirconia specimens. The specimens were evaluated using an argon laser at 514.5 nm wavelength. The specimens were run under 15 mW power, with a 20× long working distance (LWD) objective, 1800 grit/mm grating, 100 µm slit, with an acquisition time of 30 seconds. The residual monoclinic volume content, Vm , of the zirconia specimens was then calculated using the Katagiri method, where It and Im represent the intensities of bands identified by their apexes.15 190 0.5 I180 m + Im Vm = 190 2.2I150 + 0.5 I180 t m + Im The 36 sintered specimens were divided equally into three groups depending on surface treatments received: 12 specimens were left untreated as polished specimens. This was the control group (group I). Twelve sintered and polished zirconia specimens were subjected to airborne-particle abrasion. This was done using 80 µm Al2 O3 particles for 10 seconds at 3 bar pressure with the experimental surface held 10 mm from the nozzle of the unit (group II). Twelve sintered and polished zirconia specimens were subjected to airborne-particle abrasion using the protocol described above. Following steam cleaning and cleaning with acetone, the experimental surface was coated with graphite using graphite leads. The specimens were irradiated with an Er:YAG laser (Fotona Fedalis III Plus; Fotonad.d., Ljubljana, Slovenia) for 15 seconds at a power setting of 2 W, with an energy intensity of 200 mJ at a 10 Hz frequency (group III). Intermittent cooling with air and water spray was used throughout the irradiation. For the experimental group (group IV), 12 presintered and polished zirconia specimens were coated with the experimental slurry. Experimental glass slurry was prepared using 3 vol% 3-Yttria partially stabilized zirconia mixed with 0.1 ml of Darvan 821 A (Ammonium Dispersing Agent; R.T. Vanderbilt Co., Norwalk, CT) solution per 5 gm of powder. To obtain 10 ml of the experimental coating solution, 7.2 ml of experimental slurry was mixed with 2.8 ml of water. The solution was applied on the experimental surface using a micropipette. The specimens were allowed to dry for 10 minutes and were sintered at 1500°C for 8 hours with a holding time of 2 hours as recommended by the manufacturer. Surface roughness and the zirconia specimen phases were evaluated again in the same manner as described above following surface treatments. A scanning electron microscope (SEM) analysis of the experimental surface of two random specimens
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Table 1 Surface roughness of various groups
Mean surface roughness (µm) (s.d.)
Control group
Airborne particle abrasion group
Laser irradiation group
Experimental coating group
0.044A (0.005)
0.597B (0.06)
0.659BC (0.059)
0.799C (0.284)
Significant difference was observed for groups with different superscript letters.
of every group was conducted (Hitachi S-3400 N; Hitachi HighTechnologies Europe GmbH, Krefeld, Germany). The experimental surfaces from each group were mounted and were gold sputter coated at room temperature. The images were captured at 500× and 5000× magnification at an accelerating SEM voltage of 15 kV. These specimens were not evaluated further for the zirconia-veneering ceramic bond strength. Each zirconia specimen was manually layered following manufacturer’s recommendations using IPS e.max Ceram Type 1, Class 2 (Lot No. P76936; Ivoclar Vivadent AG, Schaan, Liechtenstein) to a dimension of 3 × 3 mm2 . The veneered zirconia specimens were evaluated for the shear bond strength (SBS). The 10 zirconia specimens from each group were embedded in an autopolymerizing acrylic resin block (DPI Cold Cure; Dental Products India, Mumbai, India) to be placed in the mount of the universal testing machine. An SBS test was performed to evaluate the bond strength between the zirconia and the veneering ceramic. A 100 kg load cell was used, and the crosshead speed of the unit was kept at 3 mm/min. The force required to cause shear bond failure was recorded. SBS was calculated using the formula: SBS (MPa) = Force (N) / Area (mm2 ). The zirconia specimens were evaluated for the mode of fracture using a stereomicroscope at a 50× magnification. The fractured specimens were classified as complete cohesive failure and interfacial failure as per the mode of fracture. Statistical analysis was done with SPSS v.17 (SPSS Inc., Chicago, IL). Data were compared by applying specific statistical tests to find out the statistical significance of the results. Since the data were continuous, parametric tests were used for analysis. Mean and standard deviation (SD) were calculated. Paired sample t-test and one-way ANOVA test or multiple group comparisons followed by Bonferroni post hoc for groupwise comparisons were employed.
Results Average surface roughness values and the SD for each group are recorded and summarized in Table 1. All groups had a statistically higher roughness value as compared to the control group (p < 0.0001). The experimental coating group (group IV) had the highest surface roughness values (Table 1) and was statistically higher when compared to the control and airborne abrasion groups (p < 0.0001 and 0.006, respectively). The surface roughness was higher numerically when compared to the laser group (Table 1) but was not significant (p = 0.146). The SEM images showed the morphological differences in the zirconia surfaces following different surface treatments (Figs 1–5). SEM evaluation was done at 500× and 5000×. On
Table 2 Mean residual monoclinic content Airborne Laser Experimental Control particle irradiation coating group abrasion group group group Vm (Pretreatment) Vm (Posttreatment)
0.316 0.316
0.316 0.363
0.316 0.359
0.316 0.320
Table 3 Comparison of shear bond strength
Mean shear bond strength (MPa) (s.d.)
Control
Airborne particle abrasion group
Laser irradiation group
Experimental coating group
24A (7)
27B (2.5)
27.6B (3)
29B (7.5)
Significant difference was observed for groups with different superscript letters.
the polished surface (control group), linear grooves are visible. These grooves appear to have been formed due to the polishing process with the SiC paper (Fig 1). Sandblasting resulted in a rougher surface due to microcracks as compared to the control group. Small voids were evident in a rough surface. Loss of material can be appreciated. Nodules appeared on the surface (Fig 2). The laser-irradiated specimens appeared similar to the air-abraded specimens; however, they showed less zirconia surface damage. The voids had reduced in size and frequency compared to the air-abraded specimens but ejection of material was also seen (Fig 3). The experimental group showed uniform roughness with no damage to the surface. A linear pattern of roughness was visible. A homogenous pattern of zirconia surface was visible. The sintering process burned out the liquid content of the slurry. This resulted in smaller troughs between the powder particles, which were tightly held. Defects were absent, and no surface damage was present (Fig 4). A crosssection view of these specimens showed that the experimental coating was firmly adherent to the zirconia specimen (Fig 5). The mean residual monoclinic content (Vm ) was calculated for each group (Table 2). All groups showed similar values. The control group presented the least residual monoclinic content. All groups showed a marginal increase as compared to the control group. The airborne particle abraded specimens reported the highest Vm values. SBS values for each group were evaluated, and the mean and SD were calculated (Table 3). The experimental group showed the highest SBS values; followed by the airborne abrasion group
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Figure 1 SEM photomicrographs (500× and 5000×) of the control group.
Figure 2 SEM photomicrographs (500× and 5000×) of the airborne-particle abrasion group.
and the laser group. The control group showed the lowest SBS values; however, a comparison between the groups revealed no statistical difference between the experimental groups. All groups had statistically higher SBS values when compared to the control group (p < 0.0001). All specimens of the control group showed interfacial failure between the veneering ceramic and the zirconia surface. The specimens in the remaining groups presented with cohesive failure within the veneering ceramic.
Discussion The purpose of this study was to evaluate the efficacy of different surface treatments on the surface roughness of zirconia core and subsequently the bond strength of the zirconia/veneering ceramic interface. As compared to the other ceramics, zirconia exhibits the highest stability as a framework material.7 However, the literature has suggested that chipping or debonding of the veneering ceramic from the zirconia core is a common 4
complication.7,10-13 Hence, this study was undertaken to attempt to enhance the zirconia/veneering ceramic bond strength. Airborne particle abrasion was carried out using 80 µm particles, as coarser particles used previously have been shown to damage the zirconia surface.15,26 Alumina was used in this study rather than SiC, as the latter has also been shown to cause damage to the zirconia surface.15 A 3-bar pressure and 10-second application time were kept, similar to previously reported studies.21,27,28 Er:YAG laser was used for the laser group. The optical penetration depth of Er:YAG laser being only a few µm is an advantage for the surface treatment of dental ceramics, since structural modifications are restricted to the superficial layer.29 The settings of the laser unit were in accordance with previous studies.25,30 A 400 mJ intensity used in a previous study29 showed charring of the zirconia specimens, and hence 200 mJ was used in the present study. The absorption of laser energy by zirconia is known to be compromised, since it is free of water and has a white opaque color. Therefore, in this study, the zirconia specimens were coated with graphite to increase the absorption of the Er:YAG laser energy.29 This coating was
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Figure 3 SEM photomicrographs (500× and 5000×) of the laser group.
Figure 4 SEM photomicrographs (500× and 5000×) of the experimental coating group.
manually applied by rubbing a graphite lead on each specimen, by the same operator. In this study, all the material used for the different groups, their dimensions, and other parameters of the testing were kept the same, with the only difference being the different surface treatments of the zirconia cores. Hence, it is valid to assume that these surface treatments would have a role to play in the SBS measured for different groups studied. There was a statistically significant increase in the surface roughness for the airborne-abraded and laser-irradiated groups. All experimental groups had a statistically higher roughness value as compared to the control group (Table 1). Group IV, where the experimental coating was used, had the highest surface roughness values. This was statistically higher when compared to the control and airborne abrasion groups. In this group, the specimens were subjected to an application of coating at the presintered stage. During sintering, it was seen that the liquid content in the coating was burned out, leaving behind zirconia powders adherent to the experimental surface, thus resulting in high surface roughness values. Thus, the addition of zirconia coating onto presintered zirconia followed by sintering leaves a
highly rough layer on the zirconia surface. This was confirmed by SEM (Fig 4). It has been previously reported that the use of laser increases the surface roughness.29 However, the settings used for that study showed charring of the zirconia specimens and hence could have influenced the surface roughness. This study used laser application with a lower energy setting. Laser energy was applied to airborne particle abraded specimens to repair the damage done by the latter. No increase in roughness by airborne-particle abrasion followed by laser has been reported.30 The results of this study concur. The monoclinic content of the zirconia surface is critical for the zirconia/veneering ceramic bond strength. Compared to the control group, higher monoclinic content was seen for airborne abrasion and laser irradiation, although they were not significant. The experimental group showed similar values compared to the control group. All the groups demonstrated predominantly tetragonal zirconia. The results obtained for the amount of monoclinic phase reduction in this study were not in agreement with the much higher values reported previously.26 The residual monoclinic content reported in the previous study was
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Figure 5 SEM photomicrographs (500× and 5000×) cross-section view of the experimental coating.
higher to begin with as compared to that in all the experimental groups in this study.26 This could be due to the use of coarser aluminium oxide particles abraded for a longer duration at a higher pressure (110 µm Al2 O3 for 20 seconds at 5 MPa) in that study. A significant finding of this study is that the specimens irradiated with laser showed reduced monoclinic content as compared to the airborne particle abrasion group. The graphite coating on the zirconia produced some absorption of laser energy, thereby causing localized heat generation. There was also a reduction in the monoclinic content, which could be attributed to the heat produced by the laser; however, this cannot be confirmed with the present results. SEM analysis of the specimens revealed that the airborne particle abrasion produced a rough surface with irregularly shaped voids and areas of structural damage (Fig 2). The specimens in the laser irradiation group showed fewer voids and a slightly more controlled roughening (Fig 3). It appears that the application of laser energy produces a localized surface change in
Figure 6 Stereomicroscope photomicrographs (50×) depicting cohesive failure.
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the morphology of the zirconia surface and “heals” some of the damage produced by the airborne particle abrasion. An important observation of this phase of the study was the SEM analysis of the experimental group. These specimens showed a well-controlled deposition of a homogenously porous zirconia layer on the zirconia substrate. There was no evidence of surface voids, loss of material, or any surface damage (Fig 4). This zirconia microporous layer appeared to be well adhered to the underlying zirconia substrate with no gaping (Fig 5). When compared to the control group, all the remaining groups showed greater SBS values. The experimental coating technique showed the highest SBS values. It has been reported that airborne particle abrasion does not significantly improve the bond strength between zirconia and veneering ceramic.17,23 The results of this study also show that airborne particle abrasion only marginally improved the SBS. The specimens in the laser group also showed higher values of SBS and surface roughness when compared to the control and airborne particle abrasion groups and were not statistically significant (p = 1.000). The SBS of the experimental coating group was numerically higher compared to the control, airborne abrasion, and laser groups but was not statistically significant (p = 0.312, p = 1.000, and p = 1.000, respectively). Also, the effect of the laser in reduction of monoclinic content, though shown to be beneficial, does not produce a statistically significant increase in SBS. It is possible that the increase in roughness produced by the different surface treatments may not be a critical factor in the resulting SBS. The use of the experimental coating produced sintered zirconia specimens with a very homogenous, void-free, porous surface. This additive layer was firmly attached to the zirconia substrate underneath it. The technique produced the maximum surface roughness and the highest SBS. The role of development of such coating to be added to the zirconia at the presintered stage is an exciting area of future research. The mode of interfacial failure following SBS testing was analyzed. As in Aboushelib et al,20 the term “interfacial” failure
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was used instead of “adhesive,” because it classifies the failure according to the fracture initiation site, which is related to the weakest link in the structure, rather than classifying the failure based on structure at fracture surface. For example, a crack initiated at the interface could have propagated though the weakest layer due to an asymmetric stress distribution in the specimen. This could leave traces of ceramic attached to the interface and when examined for analysis, this will give a false classification of cohesive failure. All control group specimens presented an interfacial failure. All the experimental group specimens, however, demonstrated cohesive failure in the veneering ceramic (Fig 6).
Conclusions Within the limitations of this study, the following conclusions can be drawn: 1. All surface treatments of zirconia resulted in an increase in the bond strength to the overlying ceramic and resulted in a cohesive fracture of the veneering ceramic. There is a definite adhesive failure between the zirconia and veneering ceramic when the zirconia is not surface treated. 2. The experimental group had the highest surface roughness, followed by the laser irradiation and the airborne abrasion groups. 3. The monoclinic content was lowest for the control group. The highest monoclinic content was for the airborne abrasion group, followed by the laser irradiation and the experimental coating groups. 4. The use of the experimental coating produced the highest SBS values, followed by the laser group and the airborne abrasion group. 5. The use of the experimental coating produced the highest SBS values with maximum roughness, minimum voids, and a uniform homogenous surface texture. The monoclinic phase content for these specimens was similar to that for the control treatment. Coating of presintered zirconia with the experimental coating prior to sintering is a potentially useful technique to obtain a good bond with the overlying veneering ceramic.
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Department of Prosthodontics and Implantology, M A Rangoonwala College of Dental Sciences and Research Centre, Azam Campus, Pune, India Terna Dental College, Nerul, Navi, Mumbai, India 3 I.I.T., Mumbai, India 2
Keywords All-ceramic; zirconia; delamination; shear bond strength. Correspondence Jay D. Matani, Department of Prosthodontics, M A Rangoonwala College of Dental Sciences and Research Centre, Azam Campus, Pune, Maharashtra 411001, India. E-mail: [email protected] The authors deny any conflicts of interest. Accepted November 16, 2013 doi: 10.1111/jopr.12176
Abstract Purpose: To evaluate the shear bond strength (SBS) between zirconia and veneering ceramic following different surface treatments of zirconia. The efficacy of an experimental zirconia coating to improve the bond strength was also evaluated. Materials and Methods: Zirconia strips were fabricated and were divided into four groups as per their surface treatment: polished (control), airborne-particle abrasion, laser irradiation, and application of the experimental coating. The surface roughness and the residual monoclinic content were evaluated before and after the respective surface treatments. A scanning electron microscope (SEM) analysis of the experimental surfaces was performed. All specimens were subjected to shear force in a universal testing machine. The SBS values were analyzed with one-way ANOVA followed by Bonferroni post hoc for groupwise comparisons. The fractured specimens were examined to observe the failure mode. Results: The SBS (29.17 MPa) and roughness values (0.80) of the experimental coating group were the highest among the groups. The residual monoclinic content was minimal (0.32) when compared to the remaining test groups. SEM analysis revealed a homogenous surface well adhered to an undamaged zirconia base. The other test groups showed destruction of the zirconia surface. The analysis of failure following bond strength testing showed entirely cohesive failures in the veneering ceramic in all study groups. Conclusion: The experimental zirconia surface coating is a simple technique to increase the microroughness of the zirconia surface, and thereby improve the SBS to the veneering ceramic. It results in the least monoclinic content and produces no structural damage to the zirconia substructure.
An increasing demand for esthetics and recent advances in technology have contributed to the rise in popularity of allceramic restorations. Various studies have documented longterm success of different all-ceramic systems for single-unit restorations.1,2 However, many all-ceramic systems were confined to fabricating three-unit fixed dental prostheses (FDPs) that replaced a missing anterior tooth up to the second premolar due to their strength limitations and the brittle nature of ceramics.3 Zirconia is a recent introduction to the family of dental ceramics. Its superior mechanical properties make it the material of choice for the substructure without any limitations with respect to the size of the restoration.4 Zirconia occurs in three forms, monoclinic, cubic, and tetragonal. Pure zirconia at room temperature is monoclinic and sta-
ble up to 1170°C. Above this temperature, it transforms itself into the tetragonal and then further into the cubic phase at 2370°C. Pure zirconia can be alloyed with stabilizing oxides.5 This would allow the retention of tetragonal structure at room temperature and therefore the control of the stress-induced tetragonal → monoclinic transformation. This efficiently reduces crack propagation and results in high toughness. Progresses in tetragonal–monoclinic surface transformation may have unfavorable effects on mechanical behavior.6 One of the most frequently occurring technical complications for core-veneered zirconia restorations is the chipping of veneering ceramic or delamination of the veneer from its core.7,8 Long-term (5 years) evaluation of zirconia restorations has shown good success rates.9 The incidence of chipping has been reported to be 15% after 24 months,10 13% after
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37.2 months,11 25% after 31.2 months,12 and 15.2% after 35.1 months.13 This concurs with the results obtained in a review.8 For zirconia restorations, clinical success and reliability depends on the mechanical integrity and the bond strength of the interface between the veneering ceramic and the ceramic core. Many variables, such as the surface finish of the ceramic core, residual stresses generated by mismatch in the coefficient of thermal expansion (CTE) of the ceramics, development of flaws and structural defects at the core/veneer interface, and the wetting properties and volumetric shrinkage of the veneer, may affect the core/veneer bond strength.14 The individual and combined effects of such variables can influence the bond strength and the properties of the zirconia core, and thereby the clinical success of restorations. The use of different surface treatments relies on increasing the surface area present for bonding and hence establishing durable restorations; however, despite these applications, clinical complications in the form of chipping or delamination still persist. Zirconia-veneering ceramic bond strength is determined by various factors explained earlier.15-18 These factors influence the properties of the zirconia core as well. Previous investigations have focused on different chemomechanical treatments of the zirconia surface to optimize the bonding to the overlying veneering ceramic.16,17,19-23 These treatments, along with grinding of zirconia surfaces, lead to stresses at a depth of a few microns on the zirconia surface. This results in the formation of a superficial monoclinic layer, which is not suitable for veneering. The two crystalline structures of zirconia exhibit different CTEs; however, in a routine clinical/laboratory situation, at times excessive grinding and adjusting of the zirconia core cannot be avoided. Various authors have suggested airborne-particle abrasion of the zirconia surface prior to veneering.16,19-21,24 Kim et al suggested that airborne-particle abrasion is a better surface treatment when compared to the application of a liner.16 However, some authors have reported that use of air abrasion is not beneficial.17,18 The literature is inconclusive about the effects of laser on zirconia.25 The effect of laser on previously airborne particle abraded zirconia surfaces is also not known. The different surface treatments suggested have been “subtractive” techniques of increasing surface roughness and are known to damage the superficial tetragonal surfaces. The use of an additive technique that incorporates a submicron thick porous layer onto the surface of the zirconia to increase the surface roughness and also to improve the zirconia-veneering ceramic bond strength could be a potentially beneficial technique. The aim of this research is to evaluate the effect of the experimental technique in producing a roughened zirconia surface and to investigate its effect on the bond strength between zirconia and veneering ceramic.
Materials and methods A total of 48 presintered rectangular zirconia specimens (15 mm length, 5 mm width, and 5 mm height) were cut from presintered zirconia blocks (Lot No. 0019481; Metoxit, Thayngen, Switzerland) using a new carborundum disk for each specimen. Thirty six specimens were sintered in a furnace (Zirkonofen 600 2
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V2; Zirkonzahn Worldwide, Gais, South Tyrol, Italy) according to the manufacturer’s recommendations of 8 hours with a 2-hour holding time at 1500°C. The 36 sintered and 12 presintered specimens were successively wet polished for 15 seconds on one surface using silicon carbide (SiC) papers of #400, #600, and #1000. The specimens were manually polished to obtain experimental surfaces of a similar finish. The surface roughness of the specimens was analyzed at two locations on each specimen using a surface roughness measuring tester (Surftest SJ-210; Mitutoyo Corporation, Kanagawa, Japan). The testing was done as per ISO 1997 standards (traverse speed—0.5 mm/s, cutoff value of 0.8, 5× number of sampling length). The arithmetic mean was calculated for each specimen. Raman spectroscopy (Model: Ramnor HG-2S Spectrometer; Jobin-Yvon, Longjumeau, France) was employed to assess the phase of the zirconia specimens. The specimens were evaluated using an argon laser at 514.5 nm wavelength. The specimens were run under 15 mW power, with a 20× long working distance (LWD) objective, 1800 grit/mm grating, 100 µm slit, with an acquisition time of 30 seconds. The residual monoclinic volume content, Vm , of the zirconia specimens was then calculated using the Katagiri method, where It and Im represent the intensities of bands identified by their apexes.15 190 0.5 I180 m + Im Vm = 190 2.2I150 + 0.5 I180 t m + Im The 36 sintered specimens were divided equally into three groups depending on surface treatments received: 12 specimens were left untreated as polished specimens. This was the control group (group I). Twelve sintered and polished zirconia specimens were subjected to airborne-particle abrasion. This was done using 80 µm Al2 O3 particles for 10 seconds at 3 bar pressure with the experimental surface held 10 mm from the nozzle of the unit (group II). Twelve sintered and polished zirconia specimens were subjected to airborne-particle abrasion using the protocol described above. Following steam cleaning and cleaning with acetone, the experimental surface was coated with graphite using graphite leads. The specimens were irradiated with an Er:YAG laser (Fotona Fedalis III Plus; Fotonad.d., Ljubljana, Slovenia) for 15 seconds at a power setting of 2 W, with an energy intensity of 200 mJ at a 10 Hz frequency (group III). Intermittent cooling with air and water spray was used throughout the irradiation. For the experimental group (group IV), 12 presintered and polished zirconia specimens were coated with the experimental slurry. Experimental glass slurry was prepared using 3 vol% 3-Yttria partially stabilized zirconia mixed with 0.1 ml of Darvan 821 A (Ammonium Dispersing Agent; R.T. Vanderbilt Co., Norwalk, CT) solution per 5 gm of powder. To obtain 10 ml of the experimental coating solution, 7.2 ml of experimental slurry was mixed with 2.8 ml of water. The solution was applied on the experimental surface using a micropipette. The specimens were allowed to dry for 10 minutes and were sintered at 1500°C for 8 hours with a holding time of 2 hours as recommended by the manufacturer. Surface roughness and the zirconia specimen phases were evaluated again in the same manner as described above following surface treatments. A scanning electron microscope (SEM) analysis of the experimental surface of two random specimens
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Table 1 Surface roughness of various groups
Mean surface roughness (µm) (s.d.)
Control group
Airborne particle abrasion group
Laser irradiation group
Experimental coating group
0.044A (0.005)
0.597B (0.06)
0.659BC (0.059)
0.799C (0.284)
Significant difference was observed for groups with different superscript letters.
of every group was conducted (Hitachi S-3400 N; Hitachi HighTechnologies Europe GmbH, Krefeld, Germany). The experimental surfaces from each group were mounted and were gold sputter coated at room temperature. The images were captured at 500× and 5000× magnification at an accelerating SEM voltage of 15 kV. These specimens were not evaluated further for the zirconia-veneering ceramic bond strength. Each zirconia specimen was manually layered following manufacturer’s recommendations using IPS e.max Ceram Type 1, Class 2 (Lot No. P76936; Ivoclar Vivadent AG, Schaan, Liechtenstein) to a dimension of 3 × 3 mm2 . The veneered zirconia specimens were evaluated for the shear bond strength (SBS). The 10 zirconia specimens from each group were embedded in an autopolymerizing acrylic resin block (DPI Cold Cure; Dental Products India, Mumbai, India) to be placed in the mount of the universal testing machine. An SBS test was performed to evaluate the bond strength between the zirconia and the veneering ceramic. A 100 kg load cell was used, and the crosshead speed of the unit was kept at 3 mm/min. The force required to cause shear bond failure was recorded. SBS was calculated using the formula: SBS (MPa) = Force (N) / Area (mm2 ). The zirconia specimens were evaluated for the mode of fracture using a stereomicroscope at a 50× magnification. The fractured specimens were classified as complete cohesive failure and interfacial failure as per the mode of fracture. Statistical analysis was done with SPSS v.17 (SPSS Inc., Chicago, IL). Data were compared by applying specific statistical tests to find out the statistical significance of the results. Since the data were continuous, parametric tests were used for analysis. Mean and standard deviation (SD) were calculated. Paired sample t-test and one-way ANOVA test or multiple group comparisons followed by Bonferroni post hoc for groupwise comparisons were employed.
Results Average surface roughness values and the SD for each group are recorded and summarized in Table 1. All groups had a statistically higher roughness value as compared to the control group (p < 0.0001). The experimental coating group (group IV) had the highest surface roughness values (Table 1) and was statistically higher when compared to the control and airborne abrasion groups (p < 0.0001 and 0.006, respectively). The surface roughness was higher numerically when compared to the laser group (Table 1) but was not significant (p = 0.146). The SEM images showed the morphological differences in the zirconia surfaces following different surface treatments (Figs 1–5). SEM evaluation was done at 500× and 5000×. On
Table 2 Mean residual monoclinic content Airborne Laser Experimental Control particle irradiation coating group abrasion group group group Vm (Pretreatment) Vm (Posttreatment)
0.316 0.316
0.316 0.363
0.316 0.359
0.316 0.320
Table 3 Comparison of shear bond strength
Mean shear bond strength (MPa) (s.d.)
Control
Airborne particle abrasion group
Laser irradiation group
Experimental coating group
24A (7)
27B (2.5)
27.6B (3)
29B (7.5)
Significant difference was observed for groups with different superscript letters.
the polished surface (control group), linear grooves are visible. These grooves appear to have been formed due to the polishing process with the SiC paper (Fig 1). Sandblasting resulted in a rougher surface due to microcracks as compared to the control group. Small voids were evident in a rough surface. Loss of material can be appreciated. Nodules appeared on the surface (Fig 2). The laser-irradiated specimens appeared similar to the air-abraded specimens; however, they showed less zirconia surface damage. The voids had reduced in size and frequency compared to the air-abraded specimens but ejection of material was also seen (Fig 3). The experimental group showed uniform roughness with no damage to the surface. A linear pattern of roughness was visible. A homogenous pattern of zirconia surface was visible. The sintering process burned out the liquid content of the slurry. This resulted in smaller troughs between the powder particles, which were tightly held. Defects were absent, and no surface damage was present (Fig 4). A crosssection view of these specimens showed that the experimental coating was firmly adherent to the zirconia specimen (Fig 5). The mean residual monoclinic content (Vm ) was calculated for each group (Table 2). All groups showed similar values. The control group presented the least residual monoclinic content. All groups showed a marginal increase as compared to the control group. The airborne particle abraded specimens reported the highest Vm values. SBS values for each group were evaluated, and the mean and SD were calculated (Table 3). The experimental group showed the highest SBS values; followed by the airborne abrasion group
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Figure 1 SEM photomicrographs (500× and 5000×) of the control group.
Figure 2 SEM photomicrographs (500× and 5000×) of the airborne-particle abrasion group.
and the laser group. The control group showed the lowest SBS values; however, a comparison between the groups revealed no statistical difference between the experimental groups. All groups had statistically higher SBS values when compared to the control group (p < 0.0001). All specimens of the control group showed interfacial failure between the veneering ceramic and the zirconia surface. The specimens in the remaining groups presented with cohesive failure within the veneering ceramic.
Discussion The purpose of this study was to evaluate the efficacy of different surface treatments on the surface roughness of zirconia core and subsequently the bond strength of the zirconia/veneering ceramic interface. As compared to the other ceramics, zirconia exhibits the highest stability as a framework material.7 However, the literature has suggested that chipping or debonding of the veneering ceramic from the zirconia core is a common 4
complication.7,10-13 Hence, this study was undertaken to attempt to enhance the zirconia/veneering ceramic bond strength. Airborne particle abrasion was carried out using 80 µm particles, as coarser particles used previously have been shown to damage the zirconia surface.15,26 Alumina was used in this study rather than SiC, as the latter has also been shown to cause damage to the zirconia surface.15 A 3-bar pressure and 10-second application time were kept, similar to previously reported studies.21,27,28 Er:YAG laser was used for the laser group. The optical penetration depth of Er:YAG laser being only a few µm is an advantage for the surface treatment of dental ceramics, since structural modifications are restricted to the superficial layer.29 The settings of the laser unit were in accordance with previous studies.25,30 A 400 mJ intensity used in a previous study29 showed charring of the zirconia specimens, and hence 200 mJ was used in the present study. The absorption of laser energy by zirconia is known to be compromised, since it is free of water and has a white opaque color. Therefore, in this study, the zirconia specimens were coated with graphite to increase the absorption of the Er:YAG laser energy.29 This coating was
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Figure 3 SEM photomicrographs (500× and 5000×) of the laser group.
Figure 4 SEM photomicrographs (500× and 5000×) of the experimental coating group.
manually applied by rubbing a graphite lead on each specimen, by the same operator. In this study, all the material used for the different groups, their dimensions, and other parameters of the testing were kept the same, with the only difference being the different surface treatments of the zirconia cores. Hence, it is valid to assume that these surface treatments would have a role to play in the SBS measured for different groups studied. There was a statistically significant increase in the surface roughness for the airborne-abraded and laser-irradiated groups. All experimental groups had a statistically higher roughness value as compared to the control group (Table 1). Group IV, where the experimental coating was used, had the highest surface roughness values. This was statistically higher when compared to the control and airborne abrasion groups. In this group, the specimens were subjected to an application of coating at the presintered stage. During sintering, it was seen that the liquid content in the coating was burned out, leaving behind zirconia powders adherent to the experimental surface, thus resulting in high surface roughness values. Thus, the addition of zirconia coating onto presintered zirconia followed by sintering leaves a
highly rough layer on the zirconia surface. This was confirmed by SEM (Fig 4). It has been previously reported that the use of laser increases the surface roughness.29 However, the settings used for that study showed charring of the zirconia specimens and hence could have influenced the surface roughness. This study used laser application with a lower energy setting. Laser energy was applied to airborne particle abraded specimens to repair the damage done by the latter. No increase in roughness by airborne-particle abrasion followed by laser has been reported.30 The results of this study concur. The monoclinic content of the zirconia surface is critical for the zirconia/veneering ceramic bond strength. Compared to the control group, higher monoclinic content was seen for airborne abrasion and laser irradiation, although they were not significant. The experimental group showed similar values compared to the control group. All the groups demonstrated predominantly tetragonal zirconia. The results obtained for the amount of monoclinic phase reduction in this study were not in agreement with the much higher values reported previously.26 The residual monoclinic content reported in the previous study was
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Figure 5 SEM photomicrographs (500× and 5000×) cross-section view of the experimental coating.
higher to begin with as compared to that in all the experimental groups in this study.26 This could be due to the use of coarser aluminium oxide particles abraded for a longer duration at a higher pressure (110 µm Al2 O3 for 20 seconds at 5 MPa) in that study. A significant finding of this study is that the specimens irradiated with laser showed reduced monoclinic content as compared to the airborne particle abrasion group. The graphite coating on the zirconia produced some absorption of laser energy, thereby causing localized heat generation. There was also a reduction in the monoclinic content, which could be attributed to the heat produced by the laser; however, this cannot be confirmed with the present results. SEM analysis of the specimens revealed that the airborne particle abrasion produced a rough surface with irregularly shaped voids and areas of structural damage (Fig 2). The specimens in the laser irradiation group showed fewer voids and a slightly more controlled roughening (Fig 3). It appears that the application of laser energy produces a localized surface change in
Figure 6 Stereomicroscope photomicrographs (50×) depicting cohesive failure.
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the morphology of the zirconia surface and “heals” some of the damage produced by the airborne particle abrasion. An important observation of this phase of the study was the SEM analysis of the experimental group. These specimens showed a well-controlled deposition of a homogenously porous zirconia layer on the zirconia substrate. There was no evidence of surface voids, loss of material, or any surface damage (Fig 4). This zirconia microporous layer appeared to be well adhered to the underlying zirconia substrate with no gaping (Fig 5). When compared to the control group, all the remaining groups showed greater SBS values. The experimental coating technique showed the highest SBS values. It has been reported that airborne particle abrasion does not significantly improve the bond strength between zirconia and veneering ceramic.17,23 The results of this study also show that airborne particle abrasion only marginally improved the SBS. The specimens in the laser group also showed higher values of SBS and surface roughness when compared to the control and airborne particle abrasion groups and were not statistically significant (p = 1.000). The SBS of the experimental coating group was numerically higher compared to the control, airborne abrasion, and laser groups but was not statistically significant (p = 0.312, p = 1.000, and p = 1.000, respectively). Also, the effect of the laser in reduction of monoclinic content, though shown to be beneficial, does not produce a statistically significant increase in SBS. It is possible that the increase in roughness produced by the different surface treatments may not be a critical factor in the resulting SBS. The use of the experimental coating produced sintered zirconia specimens with a very homogenous, void-free, porous surface. This additive layer was firmly attached to the zirconia substrate underneath it. The technique produced the maximum surface roughness and the highest SBS. The role of development of such coating to be added to the zirconia at the presintered stage is an exciting area of future research. The mode of interfacial failure following SBS testing was analyzed. As in Aboushelib et al,20 the term “interfacial” failure
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was used instead of “adhesive,” because it classifies the failure according to the fracture initiation site, which is related to the weakest link in the structure, rather than classifying the failure based on structure at fracture surface. For example, a crack initiated at the interface could have propagated though the weakest layer due to an asymmetric stress distribution in the specimen. This could leave traces of ceramic attached to the interface and when examined for analysis, this will give a false classification of cohesive failure. All control group specimens presented an interfacial failure. All the experimental group specimens, however, demonstrated cohesive failure in the veneering ceramic (Fig 6).
Conclusions Within the limitations of this study, the following conclusions can be drawn: 1. All surface treatments of zirconia resulted in an increase in the bond strength to the overlying ceramic and resulted in a cohesive fracture of the veneering ceramic. There is a definite adhesive failure between the zirconia and veneering ceramic when the zirconia is not surface treated. 2. The experimental group had the highest surface roughness, followed by the laser irradiation and the airborne abrasion groups. 3. The monoclinic content was lowest for the control group. The highest monoclinic content was for the airborne abrasion group, followed by the laser irradiation and the experimental coating groups. 4. The use of the experimental coating produced the highest SBS values, followed by the laser group and the airborne abrasion group. 5. The use of the experimental coating produced the highest SBS values with maximum roughness, minimum voids, and a uniform homogenous surface texture. The monoclinic phase content for these specimens was similar to that for the control treatment. Coating of presintered zirconia with the experimental coating prior to sintering is a potentially useful technique to obtain a good bond with the overlying veneering ceramic.
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