ORIGINAL RESEARCH


https://doi.org/10.5005/jp-journals-10019-1423
International Journal of Prosthodontics and Restorative Dentistry
Volume 13 | Issue 3 | Year 2023

Evaluation of the Effect of Varying Percentage of Recast Metal–Ceramic Alloy on Bond-strength and Oxide-layer Composition using SEM and EDX: An In Vitro Study


Ripul Pahwa1, Ishita Dureja2, Akshay Pahwa3, Ajai Gupta4, Bhupender Yadav5

1,2,4Department of Prosthodontics, Inderprastha Dental College & Hospital, Ghaziabad, Uttar Pradesh, India

3Department of Endodontics, Santosh Dental College, Ghaziabad, Uttar Pradesh, India

5Department of Prosthodontics, Faculty of Dental Sciences (FDS), Shree Guru Gobind Singh Tricentenary University, Gurugram, Haryana, India

Corresponding Author: Ripul Pahwa, Department of Prosthodontics, Inderprastha Dental College & Hospital, Ghaziabad, Uttar Pradesh, India, Phone: +91 9990820891, e-mail: ishita.dureja2@gmail.com

Received on: 23 June 2023; Accepted on: 04 September 2023; Published on: 29 September 2023

ABSTRACT

Purpose: An in vitro study to evaluate and compare the bond strength of veneered porcelain as affected by the addition of recast nickel-chromium alloy in varying percentages using the universal testing machine (UTM) and to analyze the oxide layer on the metal specimens using the scanning electron microscope (SEM) and energy-dispersive X-ray (EDX).

Materials and methods: Forty disk specimens of dimension 8 × 2 mm were fabricated from Bellabond Plus alloy (Bego, Germany) using recast metal in different (n = 10 samples/group) (group I: 100% fresh metal; group IIA: 75% fresh metal and 25% recast metal; group IIB: 50% fresh metal and 50% recast metal; and group IIC: 25% fresh metal and 75% recast metal). Application of commercially available porcelain (Ceramco 3) was done up to a thickness of 2 mm. Samples were then placed under SEM for EDX examination to evaluate ionic changes that occurred at the metal–ceramic interface. Bond strength of each sample was evaluated under UTM. The statistical analysis was done using one-way analysis of variance (ANOVA) test and post hoc test.

Results: The one-way ANOVA showed that for the oxide layer content, a significant difference was seen in the values of chromium, nickel, and oxygen (p < 0.001). As the content of recast metal in sample increases, there is a significant decrease in the bond strength value, thus making the metal−ceramic bond weaker (p < 0.001). On performing the Tukey honestly significant difference (HSD) test, values were significant for nickel, chromium, and oxygen in the oxide layer content among different groups of new and recasted metal alloys (p < 0.05) and were not significant for silica, aluminum, and sulfur (p > 0.05).

Conclusion: As the content of recasted metal increases, there is a decrease in the bond strength and increase in oxygen content in oxide layer. An adequate bond strength of 40 MPa was not achieved if the recasted metal is >50%. Thus, the data revealed that we could use up to 50% of recasted metal for prosthesis fabrication; beyond this limit, the bond strength is not adequate for dental use.

How to cite this article: Pahwa R, Dureja I, Pahwa A, et al. Evaluation of the Effect of Varying Percentage of Recast Metal–Ceramic Alloy on Bond-strength and Oxide-layer Composition using SEM and EDX: An In Vitro Study. Int J Prosthodont Restor Dent 2023;13(3):154–162.

Source of support: Nil

Conflict of interest: None

Keywords: Base metal, Bond strength, Ceramic, Energy-dispersive X-ray, Scanning electron microscope.

INTRODUCTION

The earth may soon experience a natural imbalance and catastrophe due to the quick depletion of its resources. The materials used in daily life have been recycled and reused by a variety of scientists working in a variety of sectors. For the preparation of crowns, bridges, and partial dentures, cast gold alloys, and base metal alloys are widely employed in dentistry.1,2 The majority of the time, the metallic dental restorations or appliances are made using the casting technique, which results in 50–60% of the alloy being wasted as buttons and sprues.3 Gold alloys’ noble composition has made it possible to repeatedly recast the material without losing any of its required properties.4,5 However, a lacuna exists in literature regarding recasting of base metal alloys. The manufacturers of base metal alloys almost always advise using the alloy just once. But, recycling or recasting the alloy repeatedly, with or without adding fresh alloy, will be extremely advantageous from an economic and environmental standpoint.5

Since recasting lowers mechanical qualities like tensile strength, yield strength, and elastic modulus,6,7 the impact of base metal alloy recasting is still debatable. Recasting basemetal alloys did not modify their physical characteristics, according to studies, which also indicated that no appreciable changes in the alloy’s composition were discovered after six more castings.8 Even though studies have shown that alloy can be reused, the effect on its bond strength with ceramic is not well-documented. It will be of definite scientific advantage if the properties of the recast alloys are studied in detail and directions given to prosthodontists and laboratory technique.9,10

One of the primary requisites 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 to ceramic application. This oxide layer is analyzed under scanning electron microscope (SEM) using energy-dispersive X-ray (EDX), sometimes called energy dispersive X-ray analysis (EDXA).11 This is a method of analysis used to characterize a sample’s chemical composition or elemental composition. It is dependent on the interaction between an X-ray excitation source and a sample.11,12 Its ability to characterize things is largely a result of the core idea that every element has a special atomic structure that allows for a special collection of peaks on its X-ray emission spectrum.13

James et al.6 studied the bond strength of porcelain when different percentages of recycled silver-palladium alloy and came to the conclusion that each casting should have at least 50% new alloy. Agrawal et al.7 evaluated the shear bond strength between a porcelain system and four alloy system [two nickel-chromium (Ni-Cr) alloys and two cobalt-chromium (Co-Cr) alloys] and reported bond strength values ranging between 54 and 71.7 MPa. However, the impact of recasting on the bond strength of porcelain to both the Ni-Cr and Co-Cr alloys was not compared in any of these tests, and which test for bond strength most accurately forecasts the bond strength of the metal−ceramic contact is another question that remains unanswered in the literature.

Therefore, the purpose of this in vitro study is to evaluate and compare the bond strength of veneered porcelain as affected by addition of recast nickel-chromium alloy in varying percentages using universal testing machine (UTM) and to analyze the oxide layer on the metal specimens using SEM and EDX. The null hypothesis of the study is that there is no effect of recasting on bond strength of alloy.

MATERIALS AND METHODS

An in vitro study was conducted at the Department of Prosthodontics, Shree Guru Gobind Singh Tricentenary University, Gurugram. The duration of the study was 3 months, in which 40 disk specimens of commercially available Ni-Cr alloys (Bellabond plus, Bego, Bremen, Germany) were fabricated in this study.

Study Design

Based on the concentration of recast metal, the samples were categorized into four groups: 100% fresh metal for group I, 75% fresh and 25% recasted alloy (by weight) for group IIA, 50% fresh and 50% recasted alloy (by weight) for group IIB, and 25% fresh and 75% recasted alloy (by weight) for group IIC. The research was divided into two parts. Part I consisted of the SEM (EVO 40, ZIESS, Baden, Germany) and EDX (Bruker EDX system, Massachusetts, United States of America) for the evaluation of the changes in chemical composition at the metal–ceramic interface in different groups. Part II was a three-point bending test for the bond strength between the porcelain and base metal alloys using UTM.

Sampling Criteria

Wax patterns of size 8 mm diameter and 2 mm thickness were fabricated in shape of a disk using blue inlay wax (Bego, Bremen, Germany). These wax patterns were then divided into four groups of 10 samples each. Sprue of 3.0 mm diameter and 4.00 mm length was attached at the center of the prepared pattern, and each group was invested separately with phosphate-bonded investment (Deguvest Impact, DeguDent GmbH, Hanau, Germany) using a vacuum mixer and invested using a ring liner. A two-stage programmed burnout process was followed; that is, the investment was allowed to bench set for 1 hour and then was placed in a burnout furnace (Unident, Delhi, India). The casting was carried out in the induction casting machine (Ducatron Quattro, France).

Grouping of Samples

Metal casting using the induction casting machine was done using 100% fresh Ni-Cr alloy for group I, 75% fresh and 25% recasted alloy (by weight) for group IIA, 50% fresh and 50% recasted alloy (by weight) for group IIB, and 25% fresh and 75% recasted alloy (by weight) for group IIC. Thus, a total of 40 cast specimens were prepared (Fig 1). The standardized specimens were air-abraded with 120 µm alumina particles using sandblasting unit and ultrasonically cleaned and divided into groups for ceramic application.2

Fig. 1: Specimens of dimension 8 × 2 mm were fabricated with metal in different proportions

Application of Ceramic Veneering Material

A standardized metal jig was fabricated with dimension of 5 × 5 cm, with a central groove of 8 mm diameter and depth of 2 mm. A metal plate of size 5 × 5 cm and thickness of 2 mm was fabricated over the jig (Fig. 2). This plate provided a standardized thickness for ceramic veneering. All the samples underwent oxidation heat treatment under vacuum at 980°C for 1 minute before the application of ceramic. Porcelain (Ceramco 3, Dentsply prosthetic, York, United States of America) was applied to the casted metal disk in a conventional manner according to manufacturer’s instructions. A thin layer of wash opaque paste was applied to the surface of metal disk and was vacuum fired at 970°C for 8 minutes. The application of the second layer of paste opaque was done in the uniform thickness and fired at 970°C for 8 minutes. Body porcelain was built up to 2 mm (including the thickness of the opaque porcelain) using custom-made measuring jig on the opaque porcelain. The body porcelain was condensed by vibration and was fired at 960°C for 12 minutes, using a vacuum press system (Ceramic Furnace, Ney CeramfireS, Dentsply, York, United States of America).2

Fig. 2: Metallic jig for standardized thickness of ceramic veneering

Testing of Samples for Oxide Layer Using SEM for EDX

After the completion of the metal surface preparation and porcelain application, the samples were placed on carbon platform for observation under SEM (Fig. 3) for EDX (EVO 40, ZEISS, Baden, Germany) examination. The specimens were sputtered with carbon in a low vacuum chamber for 5–15 minutes, and then the samples were observed for the oxide layer under SEM at a magnification of 100× (Fig. 4). Then the samples were evaluated for the chemical composition at metal–ceramic interface. In the EDX analysis of the samples, the task is divided into three major parts. First is the excitation of the sample by an electron beam. Next, the emitted X-rays are collected, sorted, and counted, and finally, the energetic emissions of an electron-excited sample get translated into analyzable data. Six elements for each alloy were selected to compare the chemical composition at the interface zone, that is, oxygen (O), aluminum (Al), silicon (Si), chromium (Cr), sulfur (S), and nickel (Ni). The results of composition were obtained on a graph (Fig. 5), and the values obtained for different specimen was compared to evaluate the changes in their composition.

Fig. 3: Sample seen under 50× magnification

Fig. 4: Oxide layer seen under 100× magnification

Fig. 5: EDX graph showing elemental composition of oxide layer at metal–ceramic interface

Testing of Samples for Ceramic Bond Strength Using UTM

The flexural bond strength of each sample was tested using a three-point bending device and was measured with a UTM (WDWS Electronic UTM, Banbros Engineering Pvt. Ltd., Ghaziabad, India) (Fig. 6). The samples were subjected to a compressive load on the metal–ceramic junction. A 5 kN load cell with hitting blade of thickness 1 mm and with a crosshead speed of 1 mm/minute was applied at the metal–ceramic junction. The load was applied until a drop in the graph was obtained. The bond 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 the bond strength.

Fig. 6: Samples placed in UTM for testing bond strength

Then, the bond strength was calculated using the formula:

Bond strength = applied load/cross-sectional area

Cross-sectional area of disk = πd2/4

π = 3.14

d = diameter of metal disk

Statistical Analysis

The values of the flexural bond strength of the four groups were compared by one-way analysis of variance (ANOVA) test (p < 0.001), and post hoc test (p < 0.05) was used to do the multiple comparisons within groups. Then, the oxide layer contents were also compared using one-way ANOVA, and post hoc test was used to do multiple comparisons within groups. In order to determine the correlation between oxide layer and bond strength, Pearson’s correlation (p < 0.01) test was done to compare oxygen content and the bond strength using NCSS statistical software 2022 (NCSS, LLC. Kaysville, Utah, United States of America, ncss.com/software/ncss).

RESULTS

Bond Strength

Table 1 depicts the descriptive values for bond strength, and Table 2 depicts the one-way ANOVA test for analysis of load (kN) and bond strength (MPa) of samples between groups and within groups. The mean square between groups for load is 4.411. F-value for load between groups was 42.062. The mean square between groups for bond strength was 1745.175, and the F-value was 42.07. These values showed that the p-value (<0.001) is significant for both bond strength and load. Table 3 depicts the multiple comparisons within groups of samples for load and bond strength using the Tukey honestly significant difference (HSD) test. The values were significant for both load and bond strength (p < 0.05).

Table 1: Descriptive values for load and bond strength
Load and bond strength
N Mean Standard deviation Standard error 95% confidence interval for mean Minimum Maximum
Lower bound Upper bound
Load (kN) Group I 10 3.34 0.40 0.13 3.06 3.63 3 4.28
Group IIA 10 2.89 0.07 0.02 2.84 2.95 2.76 2.99
Group IIB 10 2.51 0.13 0.04 2.41 2.60 2.31 2.68
Group IIC 10 1.78 0.49 0.15 1.43 2.12 1 2.3
Total 40 2.63 0.66 0.10 2.42 2.84 1 4.28
Bond strength (MPa) Group I 10 66.49 7.99 2.53 60.77 72.20 59.71 85.1
Group IIA 10 57.58 1.48 0.47 56.51 58.64 54.87 59.39
Group IIB 10 49.84 2.54 0.80 48.02 51.66 45.94 53.34
Group IIC 10 35.32 9.67 3.06 28.40 42.23 19.83 45.68
Total 40 52.30 13.14 2.08 48.10 56.50 19.83 85.1
Table 2: One-way ANOVA test for analysis of bond strength
ANOVA
Sum of Squares Degree of freedom (df) Mean square F p-value
Load (kN) Between groups 13.234 3 4.411 42.062 <0.001*
Within groups 3.776 36 0.105
Total 17.01 39
Bond strength (MPa) Between groups 5235.526 3 1745.175 42.07 <0.001*
Within groups 1493.361 36 41.482
Total 6728.887 39

*Indicates p < 0.001 as statistically significant

Table 3: Tukey HSD (post hoc test) for multiple comparison of bond strength
Multiple comparisons
Tukey HSD
Dependent variable (I) Group (J) Group Mean difference (I − J) Standard error Significance 95% confidence interval
Lower bound Upper bound
Load (kN) Group I Group IIA 0.44800* 0.14483 0.019 0.0579 0.8381
Group IIB 0.83690* 0.14483 <0.05 0.4468 1.227
Group IIC 1.56710* 0.14483 <0.05 1.177 1.9572
Group IIA Group I −0.44800* 0.14483 0.019 −0.8381 −0.0579
Group IIB 0.3889 0.14483 0.051 −0.0012 0.779
Group IIC 1.11910* 0.14483 <0.05 0.729 1.5092
Group IIB Group I −0.83690* 0.14483 <0.05 −1.227 −0.4468
Group IIA −0.3889 0.14483 0.051 −0.779 0.0012
Group IIC 0.73020* 0.14483 <0.05 0.3401 1.1203
Group IIC Group I −1.56710* 0.14483 <0.05 −1.9572 −1.177
Group IIA −1.11910* 0.14483 <0.05 −1.5092 −0.729
Group IIB −0.73020* 0.14483 <0.05 −1.1203 −0.3401
Bond strength (MPa) Group I Group IIA 8.91000* 2.88036 0.019 1.1525 16.6675
Group IIB 16.64600* 2.88036 <0.05 8.8885 24.4035
Group IIC 31.16900* 2.88036 <0.05 23.4115 38.9265
Group IIA Group I −8.91000* 2.88036 0.019 −16.6675 −1.1525
Group IIB 7.736 2.88036 0.051 −0.0215 15.4935
Group IIC 22.25900* 2.88036 <0.05 14.5015 30.0165
Group IIB Group I −16.64600* 2.88036 <0.05 −24.4035 −8.8885
Group IIA −7.736 2.88036 0.051 −15.4935 0.0215
Group IIC 14.52300* 2.88036 <0.05 6.7655 22.2805
Group IIC Group I −31.16900* 2.88036 <0.05 −38.9265 −23.4115
Group IIA −22.25900* 2.88036 <0.05 −30.0165 −14.5015
Group IIB −14.52300* 2.88036 <0.05 −22.2805 −6.7655

*The mean difference is significant at the p <0.05 level

Oxide Layer Content

Table 4 depicts the descriptive values for oxide layer content, and Table 5 depicts the one-way ANOVA test for analysis of oxide layer content (weight%) of samples between groups and within groups. The mean square between groups for silica, chromium, nickel, sulfur, aluminum, and oxygen was 66.96, 117.43, 2699.12, 0.26, 11.28, and 3469.87, respectively. F-value for silica, chromium, nickel, sulfur, aluminum, and oxygen between groups was 2.64, 32.96, 25.87, 3.17, 3.71, and 31.86, respectively. These values showed that the p-value (<0.001) is significant for chromium, nickel, and oxygen, and the values were not significant for silica, sulfur, and aluminum. Table 6 depicts the multiple comparisons within groups of samples for oxide layer content using the Tukey HSD test. The values were significant (p < 0.05) for nickel, chromium, and oxygen and were not significant for silica, aluminum, and sulfur.

Table 4: Descriptive values of oxide layer content
N Mean Descriptives 95% confidence interval for mean Minimum Maximum
Standard deviation Standard error Lower bound Upper bound
Silica Group I 10 16.63 3.10 1.39 12.78 20.48 14.18 21.23
Group IIA 10 21.39 4.37 1.95 15.97 26.81 15.2 27.05
Group IIB 10 24.24 4.08 1.83 19.18 29.31 18.16 28.65
Group IIC 10 16.99 7.47 3.34 7.71 26.27 10.17 25.58
Total 40 19.81 5.65 1.26 17.17 22.46 10.17 28.65
Chromium Group I 10 11.19 0.78 0.35 10.22 12.15 9.99 12.1
Group IIA 10 6.16 3.51 1.57 1.80 10.51 2.52 10.37
Group IIB 10 1.89 1.12 0.50 0.51 3.28 0.62 2.93
Group IIC 10 0.39 0.31 0.14 0.01 0.78 0.06 0.78
Total 40 4.91 4.64 1.04 2.73 7.08 0.06 12.1
Nickel Group I 10 51.07 3.74 1.67 46.43 55.72 44.55 54.02
Group IIA 10 25.73 19.71 8.82 1.25 50.21 4.89 47.9
Group IIB 10 4.13 3.83 1.71 -0.62 8.88 0.6 9.53
Group IIC 10 0.76 0.17 0.08 0.55 0.98 0.59 0.98
Total 40 20.43 22.67 5.07 9.82 31.04 0.59 54.02
Sulphur Group I 10 0.56 0.30 0.13 0.19 0.93 0.15 0.98
Group IIA 10 0.42 0.43 0.19 −0.11 0.95 0.06 1.14
Group IIB 10 0.42 0.24 0.11 0.12 0.71 0.07 0.63
Group IIC 10 0.03 0.04 0.02 −0.02 0.08 0 0.09
Total 40 0.36 0.33 0.07 0.20 0.51 0 1.14
Aluminum Group I 10 3.95 0.95 0.43 2.76 5.13 3.12 5.41
Group IIA 10 5.60 2.16 0.97 2.92 8.29 3.2 8.59
Group IIB 10 7.39 1.25 0.56 5.84 8.95 5.62 8.85
Group IIC 10 4.60 2.24 1.00 1.82 7.38 2.4 7.09
Total 40 5.39 2.08 0.47 4.41 6.36 2.4 8.85
Oxygen Group I 10 16.60 3.02 1.35 12.85 20.34 11.42 19.06
Group IIA 10 40.70 17.88 8.00 18.50 62.90 21.56 59.8
Group IIB 10 61.91 1.95 0.87 59.49 64.33 59.9 64.26
Group IIC 10 77.22 10.15 4.54 64.62 89.82 66.03 86.55
Total 40 49.11 25.29 5.65 37.27 60.94 11.42 86.55
Table 6: Post hoc test for multiple comparison of oxide layer content
Multiple comparisons
Tukey HSD
Dependent variable (I) Group1 (J) Group1 Mean difference (I − J) Standard error Significant 95% confidence interval
Lower bound Upper bound
Silica Group I Group IIA −4.756 3.18124 0.463 −13.8576 4.3456
Group IIB −7.612 3.18124 0.119 −16.7136 1.4896
Group IIC −0.362 3.18124 0.999 −9.4636 8.7396
Group IIA Group I 4.756 3.18124 0.463 −4.3456 13.8576
Group IIB −2.856 3.18124 0.806 −11.9576 6.2456
Group IIC 4.394 3.18124 0.528 −4.7076 13.4956
Group IIB Group I 7.612 3.18124 0.119 −1.4896 16.7136
Group IIA 2.856 3.18124 0.806 −6.2456 11.9576
Group IIC 7.25 3.18124 0.145 −1.8516 16.3516
Group IIC Group I 0.362 3.18124 0.999 −8.7396 9.4636
Group IIA −4.394 3.18124 0.528 −13.4956 4.7076
Group IIB −7.25 3.18124 0.145 −16.3516 1.8516
Chromium Group I Group IIA 5.03000* 1.19379 0.003 1.6145 8.4455
Group IIB 9.29400* 1.19379 <0.05 5.8785 12.7095
Group IIC 10.79400* 1.19379 <0.05 7.3785 14.2095
Group IIA Group I −5.03000* 1.19379 0.003 −8.4455 −1.6145
Group IIB 4.26400* 1.19379 0.012 0.8485 7.6795
Group IIC 5.76400* 1.19379 0.001 2.3485 9.1795
Group IIB Group I −9.29400* 1.19379 <0.05 −12.7095 −5.8785
Group IIA −4.26400* 1.19379 0.012 −7.6795 −0.8485
Group IIC 1.5 1.19379 0.602 −1.9155 4.9155
Group IIC Group I −10.79400* 1.19379 <0.05 -14.2095 −7.3785
Group IIA −5.76400* 1.19379 0.001 −9.1795 −2.3485
Group IIB −1.5 1.19379 0.602 −4.9155 1.9155
Nickel Group I Group IIA 25.34200* 6.46006 0.006 6.8596 43.8244
Group IIB 46.94000* 6.46006 <0.05 28.4576 65.4224
Group IIC 50.31000* 6.46006 <0.05 31.8276 68.7924
Group IIA Group I −25.34200* 6.46006 0.006 −43.8244 −6.8596
Group IIB 21.59800* 6.46006 0.019 3.1156 40.0804
Group IIC 24.96800* 6.46006 0.007 6.4856 43.4504
Group IIB Group I −46.94000* 6.46006 <0.05 −65.4224 −28.4576
Group IIA −21.59800* 6.46006 0.019 −40.0804 −3.1156
Group IIC 3.37 6.46006 0.953 −15.1124 21.8524
Group IIC Group I −50.31000* 6.46006 <0.05 −68.7924 −31.8276
Group IIA −24.96800* 6.46006 0.007 −43.4504 −6.4856
Group IIB −3.37 6.46006 0.953 −21.8524 15.1124
Sulphur Group I Group IIA 0.146 0.18165 0.852 −0.3737 0.6657
Group IIB 0.146 0.18165 0.852 −0.3737 0.6657
Group IIC 0.53400* 0.18165 0.043 0.0143 1.0537
Group IIA Group I −0.146 0.18165 0.852 −0.6657 0.3737
Group IIB 0 0.18165 1 −0.5197 0.5197
Group IIC 0.388 0.18165 0.184 −0.1317 0.9077
Group IIB Group I −0.146 0.18165 0.852 −0.6657 0.3737
Group IIA 0 0.18165 1 −0.5197 0.5197
Group IIC 0.388 0.18165 0.184 −0.1317 0.9077
Group IIC Group I −0.53400* 0.18165 0.043 −1.0537 −0.0143
Group IIA −0.388 0.18165 0.184 −0.9077 0.1317
Group IIB −0.388 0.18165 0.184 −0.9077 0.1317
Aluminum Group I Group IIA −1.658 1.10219 0.458 −4.8114 1.4954
Group IIB −3.44800* 1.10219 0.03 −6.6014 −0.2946
Group IIC −0.654 1.10219 0.933 −3.8074 2.4994
Group IIA Group I 1.658 1.10219 0.458 −1.4954 4.8114
Group IIB −1.79 1.10219 0.394 −4.9434 1.3634
Group IIC 1.004 1.10219 0.799 −2.1494 4.1574
Group IIB Group I 3.44800* 1.10219 0.03 0.2946 6.6014
Group IIA 1.79 1.10219 0.394 −1.3634 4.9434
Group IIC 2.794 1.10219 0.092 −0.3594 5.9474
Group IIC Group I 0.654 1.10219 0.933 −2.4994 3.8074
Group IIA −1.004 1.10219 0.799 −4.1574 2.1494
Group IIB −2.794 1.10219 0.092 −5.9474 0.3594
Oxygen Group I Group IIA −24.10200* 6.59968 0.01 −42.9838 −5.2202
Group IIB −45.31800* 6.59968 <0.05 −64.1998 −26.4362
Group IIC −60.62200* 6.59968 <0.05 −79.5038 −41.7402
Group IIA Group I 24.10200* 6.59968 0.01 5.2202 42.9838
Group IIB −21.21600* 6.59968 0.025 −40.0978 −2.3342
Group IIC −36.52000* 6.59968 <0.05 −55.4018 −17.6382
Group IIB Group I 45.31800* 6.59968 <0.05 26.4362 64.1998
Group IIA 21.21600* 6.59968 0.025 2.3342 40.0978
Group IIC −15.304 6.59968 0.135 −34.1858 3.5778
Group IIC Group I 60.62200* 6.59968 <0.05 41.7402 79.5038
Group IIA 36.52000* 6.59968 <0.05 17.6382 55.4018
Group IIB 15.304 6.59968 0.135 −3.5778 34.1858

*The mean difference is significant at the p <0.05 level

Table 7 depicts the comparisons of the values of oxygen concentration (wt%) in oxide layer with the bond strength (MPa), showing Pearson’s correlation and value of significant difference. The comparison showed that as the content of oxygen is increasing, the bond strength is decreasing as the value is −0.770, which is a negative value, the p-value (0), which is significant in this test of comparison as it is <0.001.

Table 7: Comparison between the bond strength and amount of oxygen within all groups
Correlations
Oxygen Bond strength (MPa)
Oxygen Pearson correlation 1 −0.770**
p-value <0.001
N 20 20
Bond strength (MPa) Pearson correlation −0.770** 1
p-value 0
N 20 20

**Correlation is significant at the 0.01 level (two-tailed)

DISCUSSION

There have been ongoing efforts to use base metal alloys more efficiently and sparingly due to the exponential growth in the demand for them in restorative dentistry and the corresponding rise in their price. The purpose of this study was to evaluate the effect of varying percentages of recasted porcelain-fused-to-metal alloy on bond strength using UTM and to analyze the oxide layer comparison using SEM and EDX.

There is evident proof that noble metals can be recasted without much change in their physical and mechanical properties, but the same is with base metal is not clear in the literature. Tripuraneni and Namburi14 analyzed the effect of various castings on the bond strength of a single chosen base metal alloy with a dental ceramic; the findings from two distinct tests employed in this study for bond load evaluation agreed that a decrease in bond was seen as the number of recasting increases, whereas the results of studies by James et al.6 found that recasting the base-metal alloys had negligible effect on their physical properties. So, this study was undertaken to evaluate the effect of recasting on oxide layer content and bond strength and to what percentage by weight the recasted metal should be used for recasting. The effect of recasted alloy on the elemental composition of the oxide layer with addition of 25% once-recasted alloy (group IIA), 50% once-recasted alloy (group IIB), and 75% once-recasted alloy (group IIC) was evaluated at the metal–ceramic junction using SEM with EDX, and it was compared with 100% new alloy (group I). Various elements observed at the metal–ceramic interface were O, Al, Si, Cr, S, and Ni.

In the present study, a significant difference was seen in the values of chromium, nickel, and oxygen. The values obtained in this study depict that as the content of recasted alloy in samples increases, the content of oxygen also increases simultaneously, making the oxide layer thicker.15 It was observed that the mean oxygen concentration value was the highest and most significant in the group with 25% fresh metal, followed by the group with 50% fresh metal, followed by group with 75% fresh metal, and the least values were found in the group with 100% fresh metal. The content of oxygen was increasing as the content of recast metal increased, maybe because the metal gets oxidized on every casting. The data obtained from the study was comparable with the other published values using the same testing method.16,17

The results of the present study showed that an increase in recasted metal content in sample causes a significant decrease in the bond strength value, thus making the metal–ceramic bond weaker. Nandish et al.18 stated that a decreased bond strength can lead to bond failure and, hence, porcelain fractures within the prosthesis.

According to Kul et al.,4 reduced modulus of elasticity is one potential factor in the weakening of the metal−ceramic connection. Ceramic fractures at the weakest point, which is the metal−ceramic link, because it is delicate and cannot withstand the alloy’s deformation. The modulus of elasticity, however, largely stays constant as long as the amount of strain or change in interatomic distance is <1%. The modulus of elasticity is increased or decreased by excessive compression or tension.

Bandela and Kanaparthi17 stated that apart from the variation in elemental composition, thermal expansion coefficient of the dental alloy utilized also varies after numerous castings. To relate the oxygen content and the bond strength value, a two-tailed Pearson’s correlation test was done. In group I (p = 0.313), the value of oxygen was taken as “1,” and the bond strength was calculated as 0.572, which depicts that as oxygen content increases, the bond strength also increases. In case of group IIA (p = 0.629), the value of oxygen was taken as “1,” and the bond strength was calculated as −0.269, which depicts that as oxygen increases, the bond strength decreases. In case of group IIB (p = 0.533), the value of oxygen was taken as “1,” and the bond strength was calculated as −0.376, which depicts that as oxygen increases, the bond strength decreases. In case of group IIC (p = 0.59), the value of oxygen was taken as “1,” and the bond strength was calculated as −0.328, which depicts that as oxygen increases, the bond strength decreases. Thus, we note that the bond strength of group I increased with increase in elemental oxygen, whereas in all the other three groups, the result was the opposite; that is, increase in elemental oxygen decreased the bond strength.

According to Ramirez et al.,15 the properties of the surface oxide layer of base metal alloy can affect the bond strength between metal and ceramic as the main components of surface oxide on Ni-Cr alloy under high temperatures are NiO and Cr2O3 which is the most ample type of oxide. But as the percentage of recast metal increases, due to oxidation of alloy, excessive Cr2O3 is formed, which induces an internal stress through change in the coefficient of thermal expansion, thus decreasing the bond strength between metal and ceramic. Recasting results in appearance of the new rains with size that may differ from the original one. When the melting temperature and casting conditions are maintained uniformly, the new grain size depends mainly on the concentration of the grain formation nuclei.5

On evaluating the results of the present study, it was observed that there is a significant difference between the bond strength and oxide layer content. The bond strength decreased when the quantity of recasted metal increased. Bond strength for group I > group IIA > group IIB > group IIC and oxygen content for group IIC > group IIB > group IIA > group I. This showed that as the oxygen content in oxide layer increases, bond strength decreases.

The apparatus used in the present study, namely SEM and EDX, were inadequate to quantify the thickness of the oxide layer at the interface. Yoo et al.19 stated that the role of minimum thickness of the oxide layer required for the optimum porcelain-metal bond strength could not be established. There is a further need to study the various metal alloy types, namely high noble, noble, and base metal alloys, on similar grounds for understanding the influence of alloy constituents on bond strength.

Thus, the present study highlights the different oxide layer content and bond strength in samples. The results and the aforementioned observations based on statistical analysis led to the following conclusions—as the content of recasted metal increases, there is a decrease in bond strength and increase in oxygen content in oxide layer. An adequate bond strength of 40 MPa was not achieved if the recasted metal was >50%. Thus, the data revealed that we can use up to 50% of recasted metal for prosthesis fabrication; beyond this limit, the bond strength is not adequate for dental use.

CONCLUSION

Within the limitations of the study, it can be concluded that as the content of recast metal in sample increases, there is a significant decrease in the bond strength value, thus making the metal−ceramic bond weaker. There was a significant change in the content of nickel, chromium, and oxygen in the oxide layer content among different groups of new and recasted alloys. The content of nickel and chromium decreased with increase in content of recast metal, while the content of oxygen increased with the increase in recast metal content. There was a significant decrease in bond strength value as the oxygen content increased. Also, to some extent, an increase in oxygen content increases the bond strength.

Table 5: One-way ANOVA for analysis of oxide layer content
ANOVA
Sum of squares df Mean square F p-value
Silica Between groups 200.899 3 66.966 2.647 0.084
Within groups 404.811 16 25.301
Total 605.711 19
Chromium Between groups 352.306 3 117.435 32.961 <0.001*
Within groups 57.006 16 3.563
Total 409.312 19
Nickel Between groups 8097.385 3 2699.128 25.871 <0.001*
Within groups 1669.297 16 104.331
Total 9766.682 19
Sulphur Between groups 0.786 3 0.262 3.176 0.053
Within groups 1.32 16 0.082
Total 2.106 19
Aluminum Between groups 33.855 3 11.285 3.716 0.033
Within groups 48.593 16 3.037
Total 82.448 19
Oxygen Between groups 10409.62 3 3469.873 31.866 <0.001*
Within groups 1742.232 16 108.89
Total 12151.852 19

*Indicates p < 0.001 as statistically significant

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