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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 5  |  Page : 610-615
Comparative evaluation of effect of laser on shear bond strength of ceramic bonded with two base metal alloys: An in-vitro study


1 Department of Prosthodontics, Chettinad Dental College and Research Institute, Chennai, Tamil Nadu, India
2 Departments of Prosthodontics, SRM Dental College, Ramapuram, Chennai, Tamil Nadu, India

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Date of Submission01-Mar-2013
Date of Decision12-Mar-2013
Date of Acceptance01-May-2013
Date of Web Publication21-Dec-2013
 

   Abstract 

The most common clinical failure in metal ceramic restoration is at the ceramo-metal interface. For the clinical longevity, metal-ceramic prostheses must have satisfactory bond strength between metal and ceramic.
Aim
and Objective: The aim of this study is to evaluate the effect of Laser etching on shear bond strength between base metal alloys and ceramic.
Materials and Methods: A total of 60 specimens were made (Base 5 mm diameter and 1 mm thickness, step with 4 mm diameter and 4 mm in length). They were divided into three groups. Group A-control, Group B-sand blasting, and Group C-laser etching. The Surface morphology, surface roughness, and wettability of the specimens were observed under scanning electron microscope (SEM) Ceramic application was carried out layer by layer for an optimal height of 4 mm. The shear bond strength test was performed using a universal testing machine and the nature of the fracture was examined under SEM.
Results: The mean shear bond strength values for laser etched (Group C) Nickel-chromium (Ni-Cr) alloy bonded with ceramic was (49.12 ± 7.12 MPa) and ceramic bonded with Cobalt-chromium (Co-Cr) was (50.04 ± 4.27 MPa), sand blasted (Group B) Ni-Cr alloy bonded with ceramic was (26.00 ± 5.22 MPa), and ceramic bonded with Co-Cr was 24.54 ± 4.78 MPa. The SEM image after debonding showed 10% of adhesive failure and 70% cohesive failure and 20% of both adhesive and cohesive failure for Laser etching. However, there was no significant difference in the values of shear bond strength between the two base metal alloys in Group C.
Conclusion: The s hear bond strength between ceramic bonded with Ni-Cr alloys using the Laser etching as surface treatment was 49.12 ± 7.12 MPa and for Co-Cr alloys 50.04 ± 4.27 MPa. Laser surface treatment produces an excellent surface roughness and achieved good shear bond strength values and aid in achieving a better bond strength between metals and ceramic.

Keywords: Laser etching, metal-ceramic, shear bond strength

How to cite this article:
Deepak K, Ahila S C, Muthukumar B, Vasanthkumar M. Comparative evaluation of effect of laser on shear bond strength of ceramic bonded with two base metal alloys: An in-vitro study. Indian J Dent Res 2013;24:610-5

How to cite this URL:
Deepak K, Ahila S C, Muthukumar B, Vasanthkumar M. Comparative evaluation of effect of laser on shear bond strength of ceramic bonded with two base metal alloys: An in-vitro study. Indian J Dent Res [serial online] 2013 [cited 2019 Sep 17];24:610-5. Available from: http://www.ijdr.in/text.asp?2013/24/5/610/123396
Although in modern dentistry, we use metal-free restorations, when clinically analyzed, metal-ceramic restorations are still being the most frequently used restorative material for making fixed partial dentures (FPDs) and single crowns as these restorations have excellent clinical performance, low cost when compared to metal-free restorations. These restorations are used in fixed or conventional dental prosthesis for many years, joining the natural esthetic of a fragile material as ceramics, with the durability and marginal adaptation of the cast metal. Due to the high-cost of precious alloys in the 1970s and progress made in ceramic technology, the use of basic metal alloys as infrastructure materials for FPDs increased considerably. [1],[2]

Ceramic fracture at the interface between metal and porcelain is one of the most common clinical failures. [3] For the clinical longevity, metal-ceramic prostheses must have satisfactory bond strength of the metal structure to Ceramic. The longevity of these restorations depends on the formation of a stable adhesive layer between two layers. The adhesion mechanism between the metals and ceramic has not been completely defined in the literatures, but it is believed that oxidation of metal and inter diffusion of ions are responsible for the bonding between metal and ceramic. [4]

The bonding of ceramic to a base metal alloy is achieved by several factors such as chemical bonding, mechanical bonding, van der Waals forces, and a slight mismatching in co-efficient of thermal expansion between porcelain and metal. Among those factors, a mechanical bonding depended on the surface roughness of the metal casting alloy. Therefore, interfacial bond strength should be changed by the pre-surface treatment of the casting alloy before ceramic application. [5],[6] To improve the mechanical bonding between metal and ceramic, several methods had been introduced. Those methods include sandblasting with airborne particles, acid etching, applications of bonding agent, laser sintering, and laser etching etc. [7],[8],[9],[10] Among those methods, laser etching is a surface treatment, which makes easier and control of micro-topography because of its depth of optical penetration depending on the material irradiated and provides more surface roughness and a stable surface morphology. [11] Hence, this study was carried out to evaluate the effect of laser etching on two different base metal alloys Nickel-chromium and Cobalt-chromium (Ni-Cr and Co-Cr) on shear bond strength between metal and ceramic.


   Materials and Methods Top


The specimens were milled in a mechanical lathe (Dynamic Machine Tools, India) according to International Organization for Standardization ISO 9693 Technical report (TR) (11406) [12],[13] with a dimension of base 5 mm diameter and 1 mm thickness and a step with 4 mm diameter and 4 mm length in Ni-Cr (Ceralloy NI, Dentalloy International Private Ltd., India.) and Co-Cr (Ceralloy NI, Dentalloy International Private Ltd, India.) alloys [Figure 1].
Figure 1: Line diagram

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A total of 60 specimens were made and were divided into 3 groups (n = 20). The specimens without any surface treatment were considered as a control (Group A). The specimens were surface treated with Al 2 O 3 (sandblasting) 50 ΅m at a pressure of 60 psi for 30 s (Micro-Blaster; China) were considered as Group B, specimen's surface treated with laser etching was considered as Group C. The laser surface treatment was carried out in a custom made pulsed Nd: YAG laser system [Figure 2], which was connected to a computer numerical control (CNC) milling machine (Q-Switched, Jinan MORN Technology Co., Ltd., Korea.) The metal surfaces of the specimens were irradiated by the linear movement of a glass fibre of the Nd: YAG lasers at a power setting of 2kW, representing energy and frequency levels of 120 mj with 50 Hz frequency at a depth of 20 μ in 2 mm space interval. A CNC x-y table controls the movement of the sample. The surface indentation and surface roughness obtained were uniform. After surface treatment of all specimens, they were cleaned with an ultrasonic cleaner (Sonic Clean, Transkit System, Australia) and dried with absorbent paper then the micro-structural analysis was performed using the scanning electron microscope (SEM) (SEM, Quanta 200 F). All specimens were transferred to a silicon nitride firing tray (Ivoclar, Leischtenstein) and placed in the ceramic furnace (Ivoclar, Programat P100) for firing. Initial oxidation was performed by raising the temperature to 980΀C with 140΀C rise in temperature per minute to remove the combustible contaminants such as oils and gases from the metal surface, also to oxidize the metal surface. According to the manufacturer instruction, the samples were coated with two thin layers of opaque paste on the metal specimens using a fine sable brush, and then these samples were kept in the ceramic furnace for first firing. Then, the body ceramic powder and liquid (IPS d. Sign System Ivoclar, Leischtenstein) were mixed to less viscous consistency and applied to get a height of 4 mm. Ceramic was fired with vacuum to eliminate air left after condensation procedure and to create denser and more esthetic appearance [Figure 3].
Figure 2: Laser unit

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Figure 3: Ceramic bonded specimen

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The shear bond test was performed in a universal testing machine (UTM) (Instron 3382) [Figure 4] with a 10-KN load cell. A perpendicular load was applied at a distance of 0.5 mm from the metal-ceramic interface occurred, and the maximum load at fractured was expressed in MPa. Then, the shear bond strength values were statistically analyzed using the two-way analysis of variance (ANOVA) Tukey's post hoc multiple range test.
Figure 4: Shear bond test using UTM

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After fracture, SEM (SEM, Quanta 200 F) observation was performed to evaluate the nature of the fractured surfaces whether it is adhesive or cohesive failures [Figure 5].
Figure 5: Surface roughness and mode of failure under scanning electron microscope

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   Results Top


The results obtained from the studies were statistically analyzed using the two-way ANOVA and the multiple group comparison was performed using the Tukey's post hoc test. The mean bond strength and standard deviation of shear bond strength values between ceramic bonded with Ni-Cr and Co-Cr alloys are given in [Table 1]. There was a significant difference in shear bond strength with respect to surface treatments. The highest mean bond strength value and SD was found ceramic bonded with Ni-Cr alloy for Laser surface treatment (49.12 ± 7.12 MPa) and ceramic bonded with Co-Cr (50.04 ± 4.27 MPa), which is significantly greater (P < 0.05) than sandblast surface treatment of Ni-Cr alloy with ceramic (26.00 ± 5.22 MPa) and ceramic bonded with Co-Cr (24.54 ± 4.78 MPa).
Table 1: Mean and standard deviation of shear bond strength


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The results of two-way ANOVA of shear bond strength of Ni-Cr and Co-Cr alloys with ceramic after laser, where the P > 0.05. Hence, it is statistically significant [Table 2]. Tukey's honestly significant difference (HSD) test for multiple group comparison for Ni-Cr and Co-Cr alloys for various surface treatment showed the P < 0.05 and hence there was no significant difference in bond strength between Ni-Cr and Co-Cr alloys after various surface treatment [Table 3] and [Figure 6].
Table 2: Results of two-way ANOVA for shear bond strength


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Table 3: Tukey HSD test for multiple comparison


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Figure 6: Comparison of shear bond strength between Nickel-chromium and Cobalt-chromium alloy after various surface treatment

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[Table 4] showed the mode of failure for ceramic bonded with Ni-Cr and Co-Cr alloys after various surface treatments. The results of SEM analysis revealed that the maximum failure of (80%) occurred in adhesive type, which was in samples without any surface treatment Group-A (GA) for Ni-Cr alloys. The maximum failure of (60%) occurred in adhesive type of failure, which was in samples without any surface treatment for Co-Cr alloys. The samples showed cohesive failure 10% and 20% for Ni-Cr and Co-Cr salloys.
Table 4: Mode of failure


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The statistical analysis of this study showed that the shear bond strength between Ni-Cr and Co-Cr alloys with ceramic was more in laser treatment compared to sandblasting and control group. Mode of failure also is maximums in adhesive type that showed better bonding between metal and ceramic after laser treatment.


   Discussion Top


The clinical performance of metal ceramic FPDs is usually estimated by the mechanical strength tests where the adhesion of the specific ceramic to the metal substrate is tested. the nature of the metal ceramic union was studied extensively and it is fundamentally based on three mechanisms namely micro-mechanical retention, compressive adaption, chemical union as well as Van der Waals forces. [14] Laser processing is a new method of treating implant surface to produce a high degree of purity with adequate surface roughness, in comparison with other surface treatments. [15],[16]

According to the literature, [17] there is no test that can be considered as ideal for the evaluation of the bond strength between metal and porcelains. Several authors considered the shear test as the most adequate method to measure bond between the metal and ceramics [18],[19],[20],[21],[22],[23] However, other authors suggest the three point flexure test, four point flexure test [24] or biaxial flexure test [25] However, the ISO-recommended 3-point bending test method could make it difficult to interpret the bond strength measurement because of uneven stress distribution at the interface between metal and ceramic, compared to the shear bond strength measuring method. [26] The shear test used was highly reliable because it is based on minimal experimental variables and creates less residual stress at the metal-ceramic interface, where the oblique forces are also minimized. [27]

In this study, two commonly used base metal alloys Ni-Cr and Co-Cr were taken to compare and evaluate the bond strength between metal-ceramic restorations since they have shown excellent marginal integrity and absence of adverse reactions. These alloys allow quality efficient surface treatment because they have satisfactory mechanical properties such as hardness, elasticity and tensile strength. The basic data of the shear bond strength between Ceramic bonded to Ni-Cr and Co-Cr alloy were statistically analyzed. The mean bond strength of ceramic bonded to Ni-Cr alloy is Group A: 22.00 ± 1.14 MPa, Group B: 26.34 ± 5.22 MPa, and Group C: 49.12 ± 7.12 MPa. The mean bond strength of ceramic bonded to Co-Cr alloys is Group A: 21.46 ± 2.21, Group B: 24.54 ± 4.78 MPa, and Group C: 50.04 ± 4.27 MPa. In accordance with ISO standards 9693, an adequate bond between a metal alloy and ceramic occurs when shear bond strength is higher than 25 MPa, suggesting clinically acceptable values for this study. [12],[28] The results of the previous study stated that the tensile bond strength between the metal and ceramic was increased after surface treatment. [29] The shear bond strength of cast metal to porcelain was less than laser sintered titanium. [30]

Statistical analysis was performed using the two-way ANOVA among the groups. The calculated P < 0.05 and hence the results were significant. Multiple comparisons were performed using the Tukey's HSD post hoc test for Ni-Cr and Co-Cr alloys for various surface treatments. The shear bond strength between metal to ceramic was greater in Laser etched surface (Group C) than in sandblasted (Group B) and control group (Group A).

On SEM analysis, the majority of the specimens in the three groups showed adhesive failure at the metal ceramic interface, with small amounts of ceramic on the metal surface. This observation indicated that the mode of failure was primarily adhesive between the ceramic and metal. The results of this study showed that laser etching enhanced the metal ceramic bond strength more than sandblasting.

According to ISO/Technical specification (TS) 11405, [31] crosshead speed does not seem to influence bond strength values. Thermo and mechanical cycling and veneering method do not influence Yttria stabilized zirconia (Y-TZP) core/veneer interface bond strength. [32] However, the shear bond strength results may be inferior, when extended thermo cycling time is applied. [27]

The SEM evaluation in the present study showed that the Laser etching increased surface roughness of the base metal alloys when compared to sandblasting, which would allow greater micro-mechanical bonding, which was also evident by previous studies. As mentioned above, the shear bond strength in terms of nominal stress values is questionable due to the heterogeneous stress distribution and also due to the occurrence of cohesive failures both in the dental substrate and the resin composite. Defining categories for classification of failure modes of deboned specimens is a complicated task and in some instances, the limit between mixed and cohesive failure becomes merely subjective. Rather than an indication of strong bonding, cohesive failure is explained by the mechanics of the test and the brittleness of the materials involved. [33]

Successful long-term bonding requires proper technique and deep knowledge over dental materials as well as control over pre-treatment techniques. The most important individual factor in order to achieve the highest possible shear bond strength is to choose a reliable bonding system and standardization of the surface treatments.


   Conclusion Top


Within the limitations, the following conclusions were made. The shear bond strength between ceramic bonded with Ni-Cr alloys using the Laser etching as surface treatment was 49.12 ± 7.12 MPa and shear bond strength between ceramic bonded with Co-Cr alloys was 50.04 ± 4.27 MPa. Laser surface treatment produce an excellent surface roughness and achieved good shear bond strength values and aid in achieving a better bond strength between metals and ceramic. No significant difference in the bond strength was found between the Ni-Cr and Co-Cr alloys using Laser as surface treatment.


   Acknowledgment Top


I acknowledge Dr. K. Murugesan, Dr. A. Ponsekar and Dr. N. Gopichander, Professors, Department of Prosthodontics, SRM Dental College, Chennai - 89 for their valuable support for the study.

 
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Correspondence Address:
S C Ahila
Departments of Prosthodontics, SRM Dental College, Ramapuram, Chennai, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.123396

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