|Year : 2012 | Volume
| Issue : 4 | Page : 509-513
|The effect of thermal and mechanical cycling on bond strength of a ceramic to nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) alloys
M Vojdani1, S Shaghaghian2, A Khaledi1, S Adibi3
1 Biomaterial Research Center, Shiraz University of Medical Sciences, Shiraz, Iran
2 Department of Community Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran
3 Department of Radiology, Shiraz University of Medical Sciences, Shiraz, Iran
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|Date of Web Publication||20-Dec-2012|
| Abstract|| |
Purpose: This study evaluates the effect of thermo-mechanical cycling (TMC) on the bond strength of a ceramic to three cobalt-chromium (Co-Cr) and two nickel-chromium (Ni-Cr) alloys.
Materials and Methods: One hundred metal-ceramic specimens were prepared. While half of the specimens from each metal-ceramic combination (n = 10) were tested after storage in water at 37°C for 24 hours, the other half were subjected to TMC before testing. The bond strength was evaluated by the flexural strength test according to ISO 9693:1999 (E) recommendations.
Results: TMC decreased the bond strength of the tested metal-ceramic systems as compared to the water storage (control groups) (P=0.04). Although metal alloys were significantly different from each other in their bond strength with porcelain (P<0.001), the effect of TMC on the various metal-ceramic systems was not significantly different (P=0.99).
Conclusion: It may be concluded that base metal-ceramic bond strength is affected by aging and the effect is relatively the same for all the tested porcelain-metal systems.
Keywords: Bond strength, mechanical cycling, metal-ceramic, thermal cycling
|How to cite this article:|
Vojdani M, Shaghaghian S, Khaledi A, Adibi S. The effect of thermal and mechanical cycling on bond strength of a ceramic to nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) alloys. Indian J Dent Res 2012;23:509-13
Porcelain fused to metal (PFM) fixed and removable partial dentures are popular and still represent the majority of tooth-colored restorations. ,,
|How to cite this URL:|
Vojdani M, Shaghaghian S, Khaledi A, Adibi S. The effect of thermal and mechanical cycling on bond strength of a ceramic to nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) alloys. Indian J Dent Res [serial online] 2012 [cited 2019 Oct 22];23:509-13. Available from: http://www.ijdr.in/text.asp?2012/23/4/509/104960
Although many studies have focused on the development and improvement of all-ceramic systems, metal-ceramic fixed prostheses are still of great importance in the dental clinic because of their versatility, their viablity insituations where other therapeutic modalities are not, and their reasonable cost. ,,
A common indication for PFM restorations is in definitive metal overlay removable partial dentures (ORPD) construction. Sometimes extensive oral rehabilitation can only be achieved with a cast ORPD.  The veneers could be composite materials or ceramic. Some advantages of ceramic in comparison to composite veneers include higher bond strength, greater durability of the restoration, and predictable aesthetics. ,
The use of base metal alloys for the construction of ceramometal restorations became more popular, especially in developing countries, after the cost of gold alloys increased considerably in the 1970s.  The disadvantages of base metal alloys include higher melting temperatures and difficult handling and finishing in comparison to noble alloys. ,, But these nonprecious metals are superior to gold-based metals in several respects, including low density, hardness, elasticity modulus, and tensile strength. , The use of base metal alloys have made possible high-quality treatment for a large number of patients with limited financial means. , Currently, the most used base metal alloys for metal-ceramic restorations are nickel-chromium (Ni-Cr) materials. However, the potential health problems associated with beryllium and nickel have led to development of cobalt-chromium (Co-Cr) alloys. ,,,
The success of PFM restorations depends on the bond strength and integrity of the adhesion between ceramic and metal, the most fragile area and therefore the most likely site for crack initiation. ,,, There should be chemical and thermal compatibility between dental alloys and ceramics to allow optimum adhesion at the interface during porcelain sintering and also when the restoration is in service. ,,,, Bonding is based on the difference of the thermal coefficient between metal and ceramic material, mechanical interlocking, Van der Waal forces, and chemical bonding. It seems that chemical bonding is the principle mechanism of interaction between ceramic and metal. ,
There are several tests for measuring bond strength. These tests can be classified as shear, flexure fatigue, torsion, tensile, and pull-through tests. ,, Metal-ceramic bond strength test should be quantifiable, reproducible, and easy to perform.  Recently, the International Standards Organization (ISO) has recommended the three-point flexure test. , However, if these tests do not involve fatigue processes, the results remain optimistic to a certain extent. The oral environment can induce physiochemical changes in dental materials. But many in vitro experiments are performed using static mechanical tests that do not address the aggressive oral environment. , Therefore, tests based on thermal and mechanical cycling (i.e., TMC) procedures, by simulating the oral conditions before the evaluation of bond strength of porcelain to metal, provide more valid information for clinical purposes.
The effect of fatigue conditions on flexural strength of ceramics to titanium and gold alloys has been verified in several studies, ,,, But there has been no investigation of the combined effect of thermal and mechanical cycling on the bond strength of dental ceramics to base metal alloys to verify the effect of these variables on the longevity of the restoration. On the other hand, the bond strength of ceramics to base metal alloys has not been evaluated by the ISO recommended three-point bending test as yet. Therefore, the aim of this in vitro study was to determine the influence of aging on the bond strength of a ceramic to three Co-Cr and two Ni-Cr alloys by using the three-point flexure test.
The tested hypotheses were that TMC leads to weaker bond strength of the ceramic to base metal alloys, and that the effect of the fatigue condition would be similar for all the tested metal-ceramic systems.
| Materials and Methods|| |
The characteristics, composition, and manufacturers of the materials used in this study are shown in [Table 1].
Metal-ceramic specimens were prepared from each of the five base metal alloys. Specimen preparation was divided into two phases: fabrication of the metal strips and ceramic veneering. For obtaining metal strips, rectangular acrylic templates (Pattern Resin, GC America, Alsip, IL), 25±1 mm×3±0.1 mm×0.5±0.05 mm, were embedded into phosphate-bonded investment (Fujivest II, GC America, Alsip, IL) and preheated according to the manufacturer's recommendation. Castings of each alloy (n=20) were performed using a natural gas and oxygen torch with a centrifugal casting machine (Multicast, DeguDent Hanau, Germany). All molds were bench cooled after casting. Finally, the metal strips were divested and cleaned. The surface of the specimens that would receive the ceramic were air-borne-particle abraded, using 150 μm aluminum oxide at an angle of 45° for 10 seconds from a distance of approximately 2 cm, under 2-bar pressure. Then the metal bars were cleaned ultrasonically in isopropyl alcohol (Vitassonic II; Vita Zahnfabrik, Bad Sδckingen, Germany) for 5 minutes and allowed to dry at room temperature. An area of 8×3 mm was initially marked on the metal strips with a graphite pencil and then veneering ceramic was fired onto the metal frameworks in an oven (Vacumat 40; Vita Zahnfabrik, Bad Sδckingen, Germany). The sintering programs are listed in [Table 2].
Prior to glazing, excess porcelain was adjusted to the dimensions prescribed by ISO 9693, i.e., 8±0.1 mm×3±0.1 mm×1±0.1 mm. The specimens within each group were randomly divided into two subgroups (n=10) and subjected to either a combination of thermal and mechanical cycling or only storage in distilled water for 24 hours at 37°C (control group) prior to flexural strength test. Thermal cycling was performed for 3000 cycles between 5°C and 55°C in deionised water, with 10 seconds of immersion time. Mechanical cycling was carried out according to the procedure described by Oyafuso et al. The testing jig of the device used for this test (custom made at Shiraz University Dental School, Shiraz, Iran) was set for a support span of 20 mm. The tip of supports was rounded to a radius of 1 mm. An upper rod (tip diameter: 1.0 mm) was fixed on the plier, which induced a 10 N load at a frequency of 0.8 Hz for 20000 times on the center of the specimens. The device for testing was placed on the base of the whole assembly that contained a thermostat to allow testing in aqueous medium at a temperature of 37°C. The flexural strength test was accomplished according to ISO 9693: 1999 (E)  in a universal testing machine (Model 5581, Instron Corp, Canton, MA, USA). A center load at a cross-head speed of 1 mm/minute was applied and recorded up to failure. Specimens failing by cracks in the middle of the ceramic layer were replaced until ten appropriate specimens were obtained from each alloy. The load at fracture for each group was calculated as the mean of the ten values obtained. The bond strength (αb ) in MPa was determined using the following equation:
αb =K×F Fail
The coefficient K is a function of the thickness of the metal strip and the value of Young's modulus of the alloy, which was derived from the diagram shown in the ISO 9693: 1999 (E). Statistical analysis was performed using SPSS ® 17. The means of each group were analyzed by two-way analysis of variance (ANOVA), with the flexural strength test as the dependent variable and the ceramic-metal combinations and TMC as the independent factors. P<0.05 was considered to be statistically significant in all tests.
| Results|| |
The ceramometal bond strength of five base metal alloy-porcelain systems were determined by the three-point flexure test. The mean and 95% confidence interval of bond strength values are illustrated in [Figure 1]. According to this figure, with TMC, all five metal-ceramic combinations showed a slight loss of bond strength compared to the control groups. The results of two-way analysis of variance (ANOVA) for the experimental conditions are presented in [Table 3]. According to this table, TMC had a slight but statistically significant effect on the bond strength of the base metal alloys to dental ceramic (P=0.04). Although the metal alloys were significantly different from each other in regard to their bond strength with the porcelain (P<0.001), the interaction between the metal-ceramic systems and TMC was not statistically significant (P=0.99). Therefore, TMC had relatively similar effect on all the tested base metal-ceramic systems.
|Figure 1: The mean (and 95% confidence intervals) of the bond strength values (MPa) of different metal-ceramic combinations, with and without thermo-mechanical cycling C = Ceramic, F = Flexicast, A = 4All, R = Regular, V = Vera PDN, W = Wiron Light, TMC = thermo-mechanical cycling, MPa = mega Pascal|
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|Table 3: Results of two-way analysis of variance for the experimental conditions|
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| Discussion|| |
This study evaluated the effect of thermo-mechanical fatigue conditions on bond strength of a ceramic to metalic infrastructures cast in Ni-Cr or Co-Cr alloys. Based on the results of this study, the hypotheses could be proven that metal-ceramic bond strength is affected by aging conditions, and that the effect of aging is relatively the same for all the tested metal-ceramic systems.
Metal-ceramic fixed and removable partial dentures are still popular and represent the majority of indirect tooth-colored restorations in the dental clinic. ,,, The noble alloys have gradually been replaced by base metal ones because of the low sag resistance and the high cost of the former. ,,,,,, Ni-Cr alloys with and without berylium, are the alloys most commonly used in ceramo-metal restorations. However, the use of Ni-Cr materials have been under constant investigation due to the presence of toxic and allergenic elements in them and even the carcinogenic potential of these alloys. , In the presence of acidic solutions the release of nickel is modified. The Cr-Co alloys were developed as an alternative to Ni-Cr substructures without endangering the physical properties of PFM systems. , However Cr-Co alloys have higher hardness than Ni-Cr alloys, which means that finishing restorations made with the former may be more difficult. ,
The results of the current study show satisfactory bond strength between Co-Cr and Ni-Cr alloys and veneering porcelain, which is consistent with the findings of previous studies. ,,, In the study by Melo et al. shear bond strength of the interface formed by two Ni-Cr and two Cr-Co alloys with a dental porcelain revealed no statistically significant differences.  Salazar et al. did not show statistically significant differences between the shear bond strengths of a specific ceramic to Cr-Co and Ni-Cr alloys submitted to thermocycling.  Joias et al. concluded that Cr-Co alloys provide higher shear bond strength to a veneering ceramic than gold-palladium (Au-Pd) metal alloy.  The results from another study showed higher shear strength for Co-Cr alloys as compared to a gold and Ni-Cr alloy combined with esthetic materials.  However, the effect of both thermal and mechanical cycling on the base metal-ceramic interface has not been studied by the three-point flexure test as yet.
The longevity of all metal-ceramic restorations depends on durable adhesion of ceramic to the metal framework. The adhesion is primarily produced by an oxide layer, but this oxide layer should not be excessively thick. A thicker oxide layer occurs in nonprecious alloys with Ni and Co because they contain elements that easily form oxides during the initial step of oxidation. , Lenz et al. showed that preoxidation of some base metal alloys is disadvantageous for the bond.  On the other hand, it was observed that air-borne-particle abrasion, by improving surface energy, increases the wettability of porcelain. However, several studies have shown that the grain size of alumina has only little effect on bond strength. , So, in the present study no initial oxidation step was performed before opaque porcelain application, but the metal bars were sandblasted with aluminum oxide as a metal surface treatment.
To compare or quantify the bond strength between the ceramic and the metal, mechanical tests such as the three- or four-point bending test, biaxial flexural test, and shear test could be used. ,, Studies that applied the shear test with different methods and different alloys reported results varying from 15 to 97 MPa. ,,, This range of variation is high due mainly to the lack of a universal methodology for measuring of the bond strength of metal porcelain systems. ,, Therefore, the test selected for this study was the three-point flexure bond test, which has been recommended by the ISO guidelines as an international standard. Metal-ceramic systems pass ISO 9693 if at least four out of six specimens have a debonding strength of more than 25 MPa. ,
The search for an ideal metal-ceramic bond strength test continues because of deviations found between in vitro and in vivo performance of the restorations. , It is impossible to simulate intraoral conditions completely. The oral environment appears to have all the factors necessary for fatigue-related failures to occur in dental prosthesis. Therefore, the evaluation of initial bond strength cannot answer the question of whether a bonding system will show successful long-term stability despite intraoral mechanical, thermal, and hydrolytic loadings. ,,,
In this in vitro study we attempted to simulate the complexity of the oral mechanism and environment in terms of moisture, temperature, and flexural cyclic loading. The quantity of mechanical cycles and the temperature alterations used in this study were based on previous researches presented in the literature. ,, Since the aim of the present study was to evaluate the effect of mechanical cycling on bond strength, after the pilot study we decided to perform 20000 cycles with 10 N load in order to prevent crack formation prior to flexural tests. Some earlier studies that investigated the same thermal variations on bond strength did not find any influence of thermal cycling on adhesion of the ceramics to the metals. , Probably this factor would be more significant in more drastic conditions such as those employed in the study of Shimoe et al. Since the current study focused on the combined effect of thermal and mechanical cycling, extended thermal cycling was not performed. Whether an extended number of thermo-mechanical cycling or longer dwell time would create more stress to the metal-ceramic bond remains to be answered.
Although TMC influenced the bond strength values of the metal-ceramic combinations investigated in the present study, the bond strength of the control and the experimental specimens ranged above 25 MPa, the minimum value determined in ISO 9693.  Hence, it may be possible that the reduction of bond strength caused by aging is not critical for the clinical performance of the base metal-ceramic bond.
Further studies comparing the bond strength of other base metal-ceramic combinations are needed. In addition, longitudinal clinical trials are also required to investigate the behavior of these materials in PFM restorations in the clinical situation.
| Conclusion|| |
Based of the results, and within the limitations of this in vitro study, it may be concluded that:
- Thermo-mechanical fatigue condition decreases the bond strength of Ni-Cr and Co-Cr alloys to the ceramic.
- Neverthless, the tested metal-ceramic systems still show sufficient bond strength for clinical performance of the restorations.
- The effect of thermo-mechanical cycling was relatively the same for all the tested base metal-ceramic systems.
| Acknowledgment|| |
The authors would like to thank the office of Vice Chancellor of Research and Biomaterial Research Center at Shiraz University of Medical Sciences for providing financial support.
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Biomaterial Research Center, Shiraz University of Medical Sciences, Shiraz
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3]
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