Indian Journal of Dental Research

: 2012  |  Volume : 23  |  Issue : 1  |  Page : 11--14

Evaluation of flexural bond strength of porcelain to used nickel-chromium alloy in various percentages

VNV Madhav1, TV Padmanabhan2, R Subramnian3,  
1 Department of Prosthodontics, Sinhgad Dental College and Hospital, Pune, Maharashtra, India
2 Department of Prosthodontics, Sri Ramachandra Dental College and Hospital, Chennai, India
3 Department of Prosthodontics, Noorul-Islam College of Dental Sciences, Kerala, India

Correspondence Address:
VNV Madhav
Department of Prosthodontics, Sinhgad Dental College and Hospital, Pune, Maharashtra


Aim: The aim of this in vitro study was to evaluate the flexural bond strength of porcelain to combinations of used and new nickel-chromium alloy in various proportions. Materials and Methods: Used and new nickel-chromium bonding alloys were combined in various proportions (groups I to V; 10 samples per group) and their flexural bond strengths with porcelain were compared. A three-point loading system was used for the application of load. Load was applied at a constant speed of 0.5 mm/minute and the load required to fracture the porcelain was recorded for each specimen. Statistical Analysis Used: (a) Analysis of variance (ANOVA) and (b) Duncan«SQ»s multiple range tests. Results: The best bond strength values were seen when 100% new alloy was used. According to the findings of this study, there was no adverse effects noted with up to 75% recast metal, but serious changes were found in the bond strength values when 100% old metal was used. Conclusions: The following conclusions were drawn from the study
  1. Fresh nickel-chromium alloy shows the greatest porcelain adherence.
  2. There is no significant change in bond strength of ceramic to alloy with up to 75% of used nickel-chromium alloy.
  3. At least 25%- of new alloy should be added when recycled nickel-chromium alloy is being used for metal ceramic restorations.

How to cite this article:
Madhav V, Padmanabhan T V, Subramnian R. Evaluation of flexural bond strength of porcelain to used nickel-chromium alloy in various percentages.Indian J Dent Res 2012;23:11-14

How to cite this URL:
Madhav V, Padmanabhan T V, Subramnian R. Evaluation of flexural bond strength of porcelain to used nickel-chromium alloy in various percentages. Indian J Dent Res [serial online] 2012 [cited 2021 May 6 ];23:11-14
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Full Text

The non-precious alloys introduced in dentistry 30-40 years ago were so inexpensive that new ingots were melted and cast for each case, and the excess metal was discarded. When using these inexpensive non-precious alloys, technicians used all-new metal for each casting instead of mixing new metal with the leftovers from previously melted ingots. However, with the increase in the costs of the non-precious metals, it is now economically more reasonable to reuse them in combination with new metal, as is the practice when using precious metal alloys.

As an economy measure, excess gold (buttons and sprues) has routinely been recast in combination with new metal to produce clinically acceptable castings. Although recasting of base metal alloys is not as common, consistent results have been reported with the addition of new metal to previously used metal in recasting of a cobalt-chromium alloy. Nelson et al.[1] and Hesby et al.[2] studied the effect of recasting a nickel-chromium alloy routinely used for fixed partial denture castings through four generations. They found no significant alterations in the physical properties of the alloy after multiple castings. Presswood [3] tested the chemical composition of recast base metal alloys up to four generations and found that the composition of the alloy remained constant. However, none of these studies examined the effects of recasting on the flexural bond strength of porcelain to nickel-chromium alloy. Therefore, this study was designed to evaluate how using new metal in combination with used metal in various proportions affected the porcelain-metal flexural bond strength.

 Materials and Methods

In this study of bond strength of recast metal to porcelain, a nickel-chromium alloy was used (Yamachi Dental Mfg., Co. Japan); the alloy comprised Ni: 60%, Cr: 20%, Co: 10%, Mo: 5%, and others: 5%). Low-fusing feldspathic porcelain (Noritake Inc., Japan) was selected to be applied over the metal. For fabrication of patterns, a pattern resin (GC Corporation, Japan) was utilized since it produces more rigid patterns than inlay wax.

Collection of used metal

We collected used metal, in the form of sprues and buttons, over a 1-month period. Care was taken not to mix two different types of alloys. The recovered castings were scrubbed and sandblasted and kept ready for reuse.

Used metal and new metal were combined in different proportions for use in the study, as follows:

Group I: New alloy - 100%, used alloy - 0%Group II: New alloy - 75%, used alloy - 25%Group III: New alloy - 50%, used alloy - 50%Group IV: New alloy - 25%, used alloy - 75%Group V: New alloy - 0%, used alloy - 100%

Fabrication of the metal framework

The pattern had a length of 30 mm and a width of 10 mm. The area was divided into three chambers of equal dimensions of 10 mm 2 each [Figure 1]; the middle portion was 1.5 mm in height and the wall of the parts on either side was raised to 3 mm. Thus, a depression of 1.5 mm was created in the center portion for the ceramic buildup.{Figure 1}

Spruing the pattern

Padilla and Rudd in a study using non-precious metals, have sucessfully produced buttonless castings. [4] The procedure reduces the risk of alteration of the composition of the alloy to be reused. The use of preformed wax sprue formers ensures uniform size and weight, which allows one to accurately determine the amount of alloy required for a casting. Padilla and Rudd suggested the following formula to determine the amount of alloy needed for buttonless casting:

(A-B) × C = D, where

A = Weight of the sprued pattern; B = average weight of the button portion of the preformed wax sprue; C = specific gravity of the alloy; and D = amount of metal to be used.

With this formula, the values obtained were as follows:

A = 1650 mg

B = 920 mg

C = 13.5 gm/cm 3

D = 9.855 gm of alloy per pattern

Investing, burnout, casting procedure, and finishing procedure were done according to the standard protocol, and the castings were retrieved.

Application of porcelain

The frameworks of all the groups, (i.e., groups I-V, with ten specimens per group), received 0.3 mm thickness of opaque porcelain. As recommended by the manufacturer, the opaque porcelain (Noritake Co., Japan) was built up in two layers to achieve the best possible bonding. The first layer was approximately 0.1 mm thick and the second layer was approximately 0.2 mm thick. After firing, body porcelain was built up to 1.2 mm and fired. A firing cycle of 6 minutes at 980°C was selected in the porcelain furnace. The thickness of the porcelain was measured using Iwanson wax callipers, which gives readings in one-tenths of a millimeter.

A three-point loading system was used for the application of load as recommended. Load was applied at a constant speed of 0.5 mm/minute. Failure of a specimen was indicated by a sharp cracking sound accompanied by a sudden change in the digital signal. The amount of load required to induce failure in each specimen was recorded in Newtons. It was then converted to flexural strength (σ) using the following formula:


σ = flexural strength; P = applied load; I = outer span length; w = specimen width; and h = specimen height.

The outer span length was obtained by measuring the distance between the two supports on which the specimen was placed. It was found to be 25 mm. The width of the specimen was 10 mm and the height 3 mm.


The minimum load required to fracture the porcelain was recorded for each specimen. These values were statistically analyzed using analysis of variance (ANOVA) and Duncan's multiple range tests as appropriate [Table 1] and [Table 2]. Duncan's multiple range test showed that the mean bond strengths in groups I-IV were significantly higher than the mean strength in group V (P<.05) [Figure 2].{Figure 2}{Table 1}{Table 2}


The aim of this study was to evaluate the effect of using new metal in combination with used metal in various proportions on the porcelain-metal flexural bond strength.

Flexural test using different loading systems has been considered. In a four-point system, specimen configuration dictates the location of the failure. The exact site and type of fracture is difficult to determine. Because of the complexity of stress distribution below the line of force application, this test may give misleading information concerning the effects of experimental variables on interface failure. A three-point loading system was used by Lorenzana et al.[5] The ADA (American Dental Association) also recommends a three-point loading system. However, Ihab [6] does not recommend this as there is the possibility of tensile failure occurring in a bend test. On the subject of the geometry of the specimen, Ihab suggests that flat planar specimens - either rectangular or circular ones - should be used for the flexural test. Such specimens provide precise control over the thickness of porcelain and the metal-porcelain interfacial surface area, which is critical for bond strength. They allow uniform distribution of interfacial stresses and also allow testing the effects of different textures for metal surfaces. This has been substantiated by Caputo et al.[7] in their study of bond strength using flat specimens. In a clinical situation wherein a three-unit fixed partial denture is given, the prosthesis is under a flexural load; the longer the span, the greater is the flexural load on the prosthesis. In this study to test the flexural bond strength of porcelain to different metal specimens we wished to simulate the clinical situation and therefore we used the three-point bend test.

The results of this study using different ratios of fresh nickel-chromium alloy with recast metal showed that porcelain adherence was maximum when 100% fresh nickel-chromium alloy was used (558.17 N/mm 2 ). With the use of 75% fresh alloy/25% recycled alloy and 50% fresh alloy/50% recycled alloy, the bond strength values decreased, but only negligibly (to 533.22 N/mm 2 when 75% fresh alloy/25% recycled alloy was used and to 531.32 n/ mm 2 when 50% fresh alloy/50% recycled alloy was used).

The 50% rule for the maximum amount of recast metal is an excellent guideline according to Rasmussen. [8] In this study, there was no adverse effects on bond strength noted even with 75% recast metal (469.60 N/mm 2 ) but serious changes were found in the bond strength values when 100% old metal was used (362.71 N/mm 2 ). The possible cause of decrease in the bond strengths according to Rasmussen's study was an increase in the frequency of interfacial voids as the percentage of recast metal increased. This is serious because failures initiated at a void are enhanced by stress concentration, and the increased frequency of voids raises the chance of a defect being in an area of high stress. An enlargement in the size of a failure-inducing void reduces the load required for failure. Consequently, the probability of failure is greater for a porcelain-metal restoration with more than 75% of recast metal. Another possible reason for failure of recast alloy may be due to excessive oxidation of the metal alloy which, according to Lacy, [9] can lead to fracture through the metal oxide itself. The metal oxide-porcelain bond remains strong, but the oxidized metal substrate separates from its veneer, leading to failure.

The strength of the chemical bond will be proportional to the strength of the individual metal-oxygen bond multiplied by the number of bonds formed across the interface. The latter component will depend upon the concentration of the oxidizable metal in the alloy, the degree of wetting of the metal, and the presence of impurities at the interface. Factors that inhibit direct chemical continuity between the porcelain and the metal will surely weaken the bond and thereby reduce the bond strength. Therefore, a detailed study related to interfacial voids, mode of failure, thickness of oxide layer, and amount of impurities at the interface of recast metal and their effects on the bond strength need to be done before a definitive inference can be made.

Research work is never complete unless the scope for future work in the field is mentioned and we therefore suggest that the effect of recasting on the biocompatibility of these alloy systems needs to be examined in a future study. Such a study could be further extended to identify the surface characteristics at the interface, the width of the various layers that are formed, and the frequency of interfacial voids and interactive complexes formed at various levels; this can be done by using x-ray microprobe analysis or an energy-dispersive x-ray analyzer and spectrometer (EDXS) in conjunction with scanning electron microscopy (SEM). It is also necessary to find out the thickness of the oxide layer on these alloy surfaces, which plays a part in porcelain adherence.


This study compared the effects of using combinations of fresh and used base metal alloys in various proportions on porcelain adherence to porcelain-compatible nickel-chromium alloy. Within the limitations of these experiments, the following conclusions were drawn:

Fresh nickel-chromium alloy shows the greatest porcelain adherence.There are no significant changes in bond strength to ceramic with up to 75% of used nickel-chromium alloy.At least 25% of new alloy should be added when recycled nickel-chromium alloy is used for metal ceramic restorations.


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