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Year : 2011  |  Volume : 22  |  Issue : 6  |  Page : 877-878
Oxidation heat treatment affecting metal-ceramic bonding

1 Department of Prosthodontics, Dr. Z A Dental College, Aligarh Muslim University, Aligarh, India
2 Department of Prosthodontics, ITS Dental College, Muradnagar, Ghaziabad, India

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Date of Submission14-Oct-2010
Date of Decision20-Apr-2011
Date of Acceptance04-Aug-2011
Date of Web Publication5-Apr-2012


Aim of the Study: There is dearth of literature regarding the effects of oxidation heat treatment (OHT) as surface pretreatments on bond strength of base metal alloys and porcelain.
Materials and Methods: Forty-five bar specimens of each two commercially available base metal alloys Mealloy and Wirorn-99 (were fabricated. Dimensions of each specimen were 15.0 × 2.0 × 0.5 mm (according to the ISO 6872-1984). According to the surface pretreatments the samples of the two groups were categorized into three subgroups: With OHT only, with sandblasting only (with Al 2 O 3 of 110 μm) and with OHT and sandblasting. Application of commercially available Duceram porcelain in thickness of 2.00 mm was applied over the surface of metal with the pretreatments. Samples were then placed under SEM for EDX examination to evaluate ionic changes that occurred at the metal-ceramic interface. Flexural bond strength of each sample was calculated under Universal Testing Machine.
Results: The one-way ANOVA indicated no significant influence of either metal type (P=0.811) or any surface pretreatment (P=0.757) on the metal-ceramic bond strength.
Conclusion: OHT resulted in the increase in amount of oxides at the metal-ceramic interface. However, neither metal type nor surface pretreatments affected bond strength.

Keywords: Base metal, ceramic, oxidation heat treatment

How to cite this article:
Rathi S, Parkash H, Chittaranjan B, Bhargava A. Oxidation heat treatment affecting metal-ceramic bonding. Indian J Dent Res 2011;22:877-8

How to cite this URL:
Rathi S, Parkash H, Chittaranjan B, Bhargava A. Oxidation heat treatment affecting metal-ceramic bonding. Indian J Dent Res [serial online] 2011 [cited 2019 Nov 14];22:877-8. Available from:
Metal-ceramic restorations became popular in dentistry as they combine the natural esthetics of a brittle material such as porcelain with the durability and fit of a metal substructure. [1],[2] Restoration by providing internal support and places the external surface of porcelain in compression.

One of the primary requisite for bond formation between base metal alloys and porcelain is the oxide layer. The oxide layer is formed on the metal surface during the oxidation prior ceramic application. [1],[2],[3] Oxidation heat treatment (OHT) (degassing, outgasing and preoxidatiton) of the metal is used to remove the entrapped gases, eliminate surface contaminants and form the metal oxide layer. This treatment may, however, release stresses and cause distortion of the framework. [4] To achieve an optimum oxide layer and to remove surface contaminants the OHT of the metal is done, normally at 960-980°C temperature, depending upon the manufacturer's instructions. [5] Properties of the oxide layer such as oxide color, thickness and strength vary widely by alloy type and are critical to the strength and esthetics of the porcelain-metal bond. Almost all oxides are brittle, and therefore the thickness of the oxide layer should be minimized to avoid failure of the porcelain-metal bond within this layer. Alloys based on nickel and cobalt commonly form thick oxides, and it is common laboratory practice to remove some of the oxides before porcelain application. [6] On the other hand, alloys based on gold or palladium form thinner oxide layers because of the nobility of these metals. [6]

The various cast-finishing process affecting metal-ceramic bond strength are sandblasting, bonding agent and etching procedures. The purpose of the present study was to determine the difference in the effect of the following surface pretreatments viz., OHT or degassing and/or microabrasion over the metal substructure prior to the application of porcelain.

   Materials and Methods Top

Forty-five bar specimens each of two commercially available nickel-chromium (Ni-Cr) alloys, Mealloy (Dentsply, Surrey, UK) and Wiron-99 (Bego,Bremen, Germany), beryllium free, were fabricated in this study. Dental porcelain (Duceram, Hanau, Dentsply) was selected for the porcelain application. Based on the surface pretreatments the samples of the two groups (Group I and II) were categorized in three sub-groups: With OHT only, with sandblasting only and with OHT and sandblasting. The research was divided into two parts. Part I consisted of the scanning electron microscope (SEM) and energy-dispersive X-rays analysis (EDX) (Quanta-200 FEG, the Netherlands) for evaluation of the changes in chemical composition at the metal-ceramic interface after various surface pretreatments. [7],[8],[9] Part II was a three-point bending test for the bond strength between the porcelain and base metal alloys. [7],[10],[11],[12],[13]

Energy-dispersive X-rays analysis (EDX) - A total of 90 plastic strips of 16.0 × 2.5 × 1.0 mm thickness were prepared. Sprue of 3.0-mm diameter and 4.00-mm length was attached at the center of the prepared pattern and invested with phosphate bonded investment for casting. A two-stage programmed burnout process was followed i.e., the investment was allowed to bench set for 1 hour and then was placed in a burnout furnace. The casting was carried out in the induction casting machine using fresh nickel-chromium based Mealloy and Wiron-99 alloys for each casting. Thus a total of 90 cast specimens i.e., 45 samples of Mealloy and 45 samples of Wiron-99 were prepared. The cast strips were measured with calipers and were adjusted to dimensions of 15 × 2.0 × 0.5 mm (According to ISO: 6872-1984). [14] The standardized specimens were airabraded and ultrasonically cleaned and divided into groups for different surface pretreatments. Fifteen samples of each alloy underwent OHT under vacuum at 960°C for 1 minute, before the application of ceramic. Another 15 samples of each alloy were directly sandblasted using 110- μm aluminum oxide particles and steam cleaned, followed by porcelain application. The remaining 15 samples of each alloy underwent OHT, under vacuum at 960°C for 1 minute and were also sandblasted using 110-μm aluminum oxide abrasion particles, followed by steam cleaning and porcelain application.

Duceram porcelain was applied to the test strips in a conventional manner. A thin layer of opaque porcelain was applied to an area 9.0-mm long located in the centre of the metal strip. The metal strips with the wash opaque porcelain were vacuum fired at 950°C for 18 minutes. The application of second layer of paste opaque was done in the uniform thickness (till the first marking present on the jig) and fired at 940°C for 16 minutes. Body porcelain was built upto 2 mm (including the thickness of the opaque porcelain) using a custom-made measuring jig [Figure 1] on the opaque porcelain. The body porcelain was condensed by vibration and was fired at 930°C for 15 minutes, using vacuum press system (Lectra press, UGIN Dentaire, Korea).
Figure 1: Customized jig and prepared samples

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After the completion of the metal surface preparation and porcelain application, the samples [Figure 1] were placed under SEM for EDX (Quanta-200 FEG, the Netherlands) examination. The specimens were sputtered with carbon in a low vacuum chamber for 5-15 minutes and were then transferred to the SEM and EDX for evaluating the chemical composition at metal-ceramic interface. Five elements for each alloy were selected to compare the chemical composition at the interface zone i.e., Oxygen (O), Aluminum (Al), Silicon (Si), Chromium (Cr) and Nickel (Ni) [Figure 2]. [7] The results of composition were obtained on a graph and table, the values obtained in the Table for different specimen was compared to evaluate the changes in their composition.
Figure 2: EDX graph and fractured site under SEM

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Bending test - The flexural bond strength of each sample was tested using a three-point bending device and was measured with a Universal Testing Machine (WDW-5E, Times Shijin Group, China). The samples were subjected to compressive load on the free metal surface. A 200 lb (907.20 N) load cell with a crosshead speed of 0.01 in/min was used to test the samples.

The hitting plunger of the universal testing machine was directed at the center of the free metal surface with defined cross-head speed and the load was applied till drop in the graph was obtained [Figure 3]. The flexural strength on the samples was obtained with the help of the stress-strain curve obtained on the digital monitor attached to the machine. The first point of a sudden drop in the load curve was considered as the bond strength. [8] The values of the flexural bond strength of the two groups were compared using the Student 't'-test. In order to determine the correlation between OHT and bond strength, one-way analysis of variance (ANOVA) and Bonferroni statistical analysis with (P<0.001) as significant, was carried out to compare amongst sub-groups.
Figure 3: Specimen placed under universal testing machine

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   Results Top

The one-way ANOVA indicated no significant influence of either metal type (P=0.811) or any surface pretreatment (P=0.757) [Table 1] and [Table 2]. Values for the tensile bond strength of both the base metal alloys Mealloy and Wiron-99 averaged (SD) from 13.2 (0.002) to 14.5 (0.002) Pa, respectively.
Table 1: Values obtained for chemical composition (by Wt%) and flexural strength (in Pa) for Group I and II

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Table 2: Mean values for ionic concentration (by Wt%) and bond strength (in Pa)

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The concentration values of Al, Si, Cr and Ni did not show any significant difference in their ionic concentration. Only the concentration of oxygen showed a significant change with the variance of surface pre-treatments. And the highest values of oxygen concentration were observed in the subgroup with OHT, i.e., for the specimens of Mealloy, Oxygen concentration averaged 22.299 (4.789) wt%; while for the specimens of Wiron-99, value of Oxygen concentration was 23.175 (2.883) wt%. For the subgroup with microabrasion only as surface pretreatment the values for oxygen concentration were 15.219 (2.419) and 14.260 (1.946) wt%, respectively; and the subgroup with OHT and microabrasion the values for oxygen concentration were 19.861 (3.739) and 20.856 (2.615) wt%, respectively.

On statistical analysis by the Student 't'-test, no significant difference was observed in the mean oxygen concentration values for the two different base metal alloys. For the samples of Mealloy the mean value of Oxygen concentration [Table 3] was 19.126 (4.735) wt%; and for the samples of Wiron-99 the Oxygen concentration values was 19.430 (4.540) wt%.
Table 3: Comparison between the bond strength and amount of oxygen concentration in Group I (Mealloy and Deuceram) and Group II (Wiron-99 and Deuceram)

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On the statistical analysis, no significant difference was observed in tensile bond strength for the various surface pretreatments for both the alloys. The mean bond strength value for the subgroup with OHT, for Mealloy was 14.2 (0.003) Pa and for Wiron-99 was 13.6 (0.002) Pa; for the subgroup with microabrasion only as surface pretreatment was 13.4 (0.002) and 14.2 (0.003) Pa respectively; and for the subgroup with OHT and microabrasion both as surface pretreatment was 14.1 (0.002) and 14.3 (0.002) Pa, respectively.

According to the Student 't '-test, no significant difference was observed in the tensile bond strength amongst the different type of base metal alloys. The mean bond strength value for the samples of Mealloy was 13.6 (0.002) Pa and Wiron-99 was 13.5 (0.002) Pa, respectively.

The mode of failure was evaluated under Scanning Electron Microscope (SEM). It was found that out of ninety samples, only 06 samples showed the cohesive type of failure i.e. the fracture line was observed within the layer of porcelain and the fracture line was present approximately at the centre of the strip, perpendicular to the long axis of the sample [Figure 2]. On further increasing the load complete debonding of the porcelain from the metal strip occurred in all the samples. None of the samples showed the fracture of the metal strip. Rest 84 samples showed completed debonding of the porcelain from the metal strip at the junction of the opaque (porcelain) and the metal.

   Discussion Top

The study was undertaken to evaluate the OHT as a pretreatment alternative prior to the application of porcelain over base metal alloys affecting their bond strength. Elemental concentration profile across interfacial reaction zones between the base-metal and porcelain was studied under SEM and EDX. [7],[8],[9] Bond strength between the base-metal alloy and porcelain was evaluated through the three-point bending test (as per the recommendations by the American Dental Association Council on Dental Materials and Devices). [7],[10],[11],[12],[13],[14],[15],[16] The specimens are relatively easy to fabricate with minimum number of variables and the data obtained from the study is comparable with the other published values using the same testing method. In this study, the effect of OHT on the bond strength was determined by comparing different surface pretreatments i.e. "with OHT"; "without OHT" and "with OHT and sandblasting" prior to the application of porcelain. [7],[17] The values of tensile bond strength of the two groups were compared using the Student 't'-test and one-way ANOVA. For all the samples P<0.001 was considered significant.

In the study the various elements observed at the metal-ceramic interface were O, Al, Si, Cr and Ni. Although allergy to Ni have been reported for stomatology system. [18] It was observed that the mean oxygen concentration value was highest and significant in the group with surface pretreatments as OHT; followed by the group with OHT and microabrasion and the least values were found in the group without OHT.

However, there was no significant difference observed for the tensile bond strength amongst all the groups with different surface pretreatments in both the types of base-metal alloys.

The experimental results have invalidated the hypothesis that OHT increases the bond strength between the base-metal alloys and porcelain. [3] The test data indicated that there was neither any significant influence of the various surface pretreatments (P=0.757) nor any significant difference with the different metal types (P=0.811). The findings of the present study are in accordance with the study conducted by Wu, Moser, Jamenson and Malone. [7] They studied the line scan evaluation of boundary phase changes between metal and ceramic through SEM and EDX, concluded that "the presence of Nickel, Chromium and Aluminum ions at the porcelain-metal interface and OHT has no significant effect on the porcelain-metal bond strength". And also the result for the present study is in favor of the findings by Abreu, Loza, Elias, Mukhopadhyay and Rueggeberg. [14] They studied the effect of various surface pretreatment, including OHT on the various ceramometal types and concluded that "neither metal type nor surface pretreatments affects bond strength".

The apparatus used in the present study namely SEM and EDX were inadequate to quantify the thickness of the oxide layer at the interface. Thus, the role of minimum thickness of the oxide layer required for the optimum porcelain-metal bond strength could not be established. There is further need to study various metal alloy types namely high noble, noble and base metal alloys on similar grounds for understanding the influence of alloys constituents on bond strength.

Clinical significance

From the present study we may draw an inference that there is no significant influence of degassing cycle or OHT, as surface pretreatments prior to the application of porcelain on the metal-ceramic bond strength only when while using the selected Ni-Cr alloys under identical conditions. [14] Avoiding the degassing cycle will prevent excessive formation of oxides which may prevent the discoloration of the veneering porcelain. [14] And also, structural deformation of the metal substructure can be prevented by avoiding its repeated firing through OHT, thus preventing release of residual stress from metal. [4],[5],[7]

   Conclusion Top

The conclusion of the present study may be drawn that there is neither any significant effect of OHT on the metal-ceramic bonding nor microabrasion of the metal substructure affects metal-ceramic bonding.

   Acknowledgment Top

This investigation was supported in part by Indian Institute of Technology, Rourkee, Uttaranchal, India and in part by Institute of Technical Studies Engineering college, Greater Noida, U.P., India.

   References Top

1.Annusavice KJ. Phillip's science of dental materials. 10 th ed. Philadelphia: Saunders Publication; 1996. p. 256-330.  Back to cited text no. 1
2.Craig RG, Powers JM. Restorative dental materials. 11 th ed. Missouri: Mosby Publication; 2002. p. 163-84.  Back to cited text no. 2
3.Mc Lean JW. The science and art of dental ceramics. Vol. 11. Bridge Design and Laboratory Procedures in Dental Ceramics. Chicago: Quintessence Books; 1982. p. 221-81.  Back to cited text no. 3
4.Bryant RA, Nicholls JI. Measurement of distortions in fixed partial dentures resulting from degassing. J Prosthet Dent 1979;42:515-20.  Back to cited text no. 4
5.Dent RJ, Preston JD, Moffa JP, Caputo A. Effect of oxidation on ceramometal bond strength. J Prosthet Dent 1982;47:59-62.  Back to cited text no. 5
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8.Ringle RD, Mackert JR Jr, Fairhurst CW. An X-ray spectrometric technique for measuring porcelain-metal adherence. J Dent Res 1983;62:933-6.  Back to cited text no. 8
9.Baran GR. Phase changes in base metal alloys metal-porcelain interfaces. J Dent Res 1979;58:2095-104.  Back to cited text no. 9
10.Knap FJ, Ryge G. Study of bond strength of dental porcelain fused metal. J Dent Res 1966;45:1047-51.  Back to cited text no. 10
11.Wight TA, Bauman JC, Pelleu GB Jr. An evaluation of four variables affecting the bond strength of porcelain to nonprecious alloy. J Prosthet Dent 1977;37:570-7.  Back to cited text no. 11
12.Campbell SD. A comparative strength study of metal ceramic and all-ceramic esthetic materials: Modulus of rupture. J Prosthet Dent 1989;62:476-9.  Back to cited text no. 12
13.Rake PC, Goodcare CJ, Moore BK, Munoz CA. Effect of two opaquing techniques and two metal surface conditions on metal-ceramic bond strength. J Prosthet Dent 1995;74:8-17.  Back to cited text no. 13
14.Abreu A, Loza MA, Elias A, Mukhopadhyay S, Rueggeberg FA. Effect of metal type and surface treatment on in vitro tensile strength of copings cemented to minimally retentive preparations. J Prosthet Dent 2007;98:199-207.  Back to cited text no. 14
15.Gemalmaz D, Berksun S, Alkumru HN, Kasapoglu C. Thermal cycling distortion of porcelain fused to metal fixed partial dentures. J Prosthet Dent 1998;80:654-8.  Back to cited text no. 15
16.Ucar Y, Aksahin Z, Kurtoglu C. Metal ceramic bond after multiple castings of base metal alloy. J Prosthet Dent 2009;102:165-71.  Back to cited text no. 16
17.Coffey JP, Anusavice KJ, Dehoff PH, Lee RB, Hojjatie B. Influence of contraction mismatch and cooling rate on flexural of PFM systems. J Dent Res 1988;67:61-5.  Back to cited text no. 17
18.Mehuliæ K, Mehuliæ M, Kos P, Komar D, Katunariæ M. Investigation of contact allergy expressions in prosthodontic patients with oral diseases. Minerva Stomatol 2005;54:303-9.  Back to cited text no. 18

Correspondence Address:
Shraddha Rathi
Department of Prosthodontics, Dr. Z A Dental College, Aligarh Muslim University, Aligarh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0970-9290.94664

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