1. Introduction
The use of dental prostheses with metal frames dates to ancient civilizations including the Egyptians, Greeks, and Romans who manufactured dental reconstructions out of gold, silver, and other metals. Simple metal wires or plates were joined to false teeth in these early prostheses, which were simple and straightforward. But the widespread use of metal frameworks in dentistry did not start until the 18th century [
1]. French dentist Pierre Fauchard developed a technique in 1728 for attaching prosthetic teeth to neighboring healthy teeth using gold wire. This method, referred to as the “bridge,” served as the model for contemporary dental bridges, which frequently incorporate metal frameworks to support the false teeth.
Beginning in the twentieth century, dental laboratories started to create metal alloys expressly for dental use, such as gold alloys. However, because gold is an expensive metal, other substitute alloys with lower costs and densities have also been created for tooth restoration (for instance, nickel and cobalt–chromium alloys). With the aid of these alloys, dental restorations that could withstand the strains of chewing and biting were made to be more robust and long-lasting. Today, metal frameworks from Ni-Cr (nickel–chromium) and Co-Cr (cobalt–chromium) dental alloys are commonly used in dentistry to fabricate dental prostheses, including crowns, bridges, and partial dentures.
The human body contains extremely little amounts of nickel; yet, elevated concentrations have the potential to be dangerous [
2]. The Nickel Directive, which the European Union introduced in 1994, substantially strengthened the position against the inclusion of nickel in materials. The defined threshold is 0.5 μg/cm
2/week, which applies to consumer goods containing nickel that come into prolonged and direct contact with the skin. The maximum allowable amount of nickel released from articles placed into pierced regions of the body is 0.2 μg/cm
2/week [
3]. Nickel is now thought to be a harmful element. It has been estimated that approximately 1 in 10 individuals have a nickel allergy [
4]. The objective of this legislation was to set a maximum allowable level of nickel emission in order to prevent the development of primary nickel sensitization. However, given the vague symptoms of this allergy, it is plausible that the official number is underestimated and that there are more people who are allergic. Being exposed to nickel has been related to several systemic illnesses [
5]. But the nickel–chromium alloys have the advantages of very good ceramic adhesion and easy processing; the predisposition to corrosion is highly dependent of chromium content [
6] and the biocompatibility can be increased using new digital techniques of fabrication [
7]. According to its composition and nature, the oral mucosa diffuses nickel ions more quickly than the skin does, and there is very little chance of sensitization inside the mouth [
8]. As of this moment, Ni accumulations in the body are not documented.
Co-Cr alloys, on the other hand, are highly resistant to wear and corrosion [
9], and are used primarily for anterior (front) teeth restorations where esthetics are important. Co-Cr alloys also have a high strength-to-weight ratio, making them ideal for partial dentures where the prosthesis needs to be lightweight, but their shear bond strength of porcelain is dependent on multiple firings [
10], post treatment [
11], technique of fabrication [
12,
13,
14,
15,
16], and surface properties [
17].
Both Ni-Cr and Co-Cr alloys can be cast or milled, allowing for the fabrication of precise dental restorations that closely match the shape and color of natural teeth. They are also compatible with a variety of dental ceramics, allowing for the creation of highly esthetic restorations that mimic the appearance of natural teeth. Overall, Ni-Cr and Co-Cr alloys are widely used in dentistry due to their biocompatibility, strength, durability, and esthetic qualities but there are few studies far available referring to the comparison among materials in terms of their properties [
18].
The specific aim of this study was to compare two commercial dental alloys, one Co-Cr and other Ni-Cr, analyzing their microstructure, corrosion behavior, elastic modulus, and microhardness of ingot and cast crown in order to help clinicians to predict the appropriate behavior of them in oral cavity.
Author Contributions
Conceptualization, J.C.M.-R. and A.P.; methodology, A.F.; software, S.B.-G.; validation, J.C.M.-R.; formal analysis, A.F.; investigation, A.F. and A.P.; data curation, A.F.; writing—original draft preparation, A.P.; writing—review and editing, J.C.M.-R.; visualization, S.B.-G. All authors have read and agreed to the published version of the manuscript.
Funding
This work was supported by the infrastructure of Cabildo de Gran Canaria, projects number CABINFR2019-07 and CABINFR2019-08.
Data Availability Statement
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
Conflicts of Interest
The authors declare there are no conflicts of interest relevant to this study.
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Figure 1.
Graphic illustration of the material preparation.
Figure 2.
Optical microstructure after etching: for Ni-Cr sample (a,b) and for Co-Cr sample (c,d).
Figure 3.
(a) Microstructure; (b) EDS on selected area; (c) elemental mapping of Ni-Cr sample.
Figure 4.
(a) Microstructure; (b) EDS on selected area; (c) elemental mapping of Co-Cr sample.
Figure 5.
XRD spectra for Ni-Cr and Co-Cr dental alloys.
Figure 6.
Corrosion potential curves for Ni-Cr and Co-Cr samples after 24 h immersion.
Figure 7.
Bode impedance. Bode phase and Nyquist diagrams of Ni-Cr (a,c,e) and Co-Cr (b,d,f) samples at ±0.400 V.
Figure 8.
Equivalent circuit R(QR) to adjust impedance data from −0.400 V to 0.400 V.
Table 1.
Composition (in wt%) of the alloys under study.
Sample | Ni | Co | Cr | W | Si | Al | Mo | Fe |
---|
Ni-Cr | 61.4 | - | 25.7 | - | 1.5 | <1.0 | 11 | - |
Co-Cr | - | 59.5 | 31.5 | 3.0 | 2.0 | - | 5 | <1.0 |
Table 2.
EDS quantification results of the samples studied.
Elements | Ni-Cr | Co-Cr |
---|
wt.% | at.% | wt.% | at.% |
---|
AlK | 2.72 | 5.67 | - | - |
SiK | 3.50 | 7.00 | - | - |
MoL | 1.06 | 0.62 | - | - |
CrK | 21.11 | 22.83 | 30.44 | 34.53 |
NiK | 64.36 | 61.66 | - | - |
W L | 7.25 | 2.22 | 3.40 | 1.09 |
NbL | - | - | 2.48 | 1.57 |
MnK | - | - | 1.33 | 1.43 |
CoK | - | - | 60.89 | 60.95 |
HgL | - | - | 1.47 | 0.43 |
Table 3.
Corrosion potential results: initial, after 3 h, and after 24 h for the two samples submerged in artificial saliva and kinetic parameters of corrosion process.
Alloy | Ecorr, V vs. SCE | icorr | Rp | Βa | Βc | ipass | Ebd |
---|
Initial | After 3 h | After 24 h | µA/cm2 | KΩ/cm2 | mV/DIV | mV/DIV | µA/cm2 | mV |
---|
Ni-Cr | −0.359 | −0.452 | −0.390 | 0.20 | 112 | 157 | 88 | 3.82 | 620 |
Co-Cr | −0.139 | −0.121 | −0.094 | 0.24 | 98 | 164 | 84 | 4.14 | 600 |
Table 4.
Results obtained in the Bode diagrams of the samples studied.
Potential (V) | Alloys | Max. Impedance (Ω) | Max. Phase Angle (°) |
---|
−0.400 | Ni-Cr | 7.24 × 103 | 53 |
Co-Cr | 5.76 × 104 | 66 |
−0.200 | Ni-Cr | 1.18 × 104 | 55 |
Co-Cr | 5.41 × 104 | 66 |
0.000 | Ni-Cr | 2.36 × 104 | 60 |
Co-Cr | 6.82 × 104 | 68 |
0.200 | Ni-Cr | 3.48 × 104 | 63 |
Co-Cr | 7.78 × 104 | 69 |
0.400 | Ni-Cr | 3.38 × 104 | 62 |
Co-Cr | 6.03 × 104 | 67 |
Table 5.
Equivalent circuit R(QR) of the studied samples when applying potentials from −0.400 V to 0.400 V.
Potential (V) | Samples | Parameters |
---|
Rsol (Ω·cm2) | Y01 (S·secn/cm2) | n1 | R1 (Ω·cm2) |
---|
−0.400 | Ni-Cr | 17.33 | 1.30 × 10−4 | 0.66 | 4.97·103 |
Co-Cr | 21.00 | 2.78 × 10−5 | 0.75 | 6.75·104 |
−0.200 | Ni-Cr | 17.31 | 1.22 × 10−4 | 0.66 | 4.97·104 |
Co-Cr | 21.00 | 2.88 × 10−5 | 0.75 | 5.83·104 |
0.000 | Ni-Cr | 17.33 | 1.05 × 10−4 | 0.68 | 7.55·104 |
Co-Cr | 20.91 | 2.47 × 10−5 | 0.76 | 8.92·104 |
0.200 | Ni-Cr | 17.50 | 7.62 × 10−5 | 0.71 | 2.75·105 |
Co-Cr | 20.90 | 2.15 × 10−5 | 0.77 | 1.02·105 |
0.400 | Ni-Cr | 17.51 | 7.18 × 10−5 | 0.71 | 7.46·104 |
Co-Cr | 20.97 | 2.68 × 10−5 | 0.76 | 6.48·104 |
Table 6.
Values of microhardness, elasticity modulus, and tensile strength for the tested samples of Ni-Cr and Co-Cr (IN—ingot; CC—cast crown).
Mean Parameter ± Standard Deviation | Ni-Cr | Co-Cr |
---|
IN | CC | IN | CC |
---|
Vickers microhardness | 231.6 ± 11.2 | 182.2 ± 11.5 | 425.4 ± 16.2 | 326.6 ± 14.2 |
Young´s modulus (GPa) | 201.5 ± 16.2 | 128.9 ± 22.9 | 282.2 ± 15.8 | 110.7 ± 12.9 |
Tensile strength (MPa) | 720.3 ± 26.5 | 386.6 ± 12.4 | 1310.2 ± 28.8 | 623.3 ± 12.2 |
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