d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 263–270
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Surface characteristics and corrosion properties of selective laser melted Co–Cr dental alloy after porcelain firing X-Z. Xin a,1 , J. Chen a,1 , N. Xiang a , Y. Gong b,∗ , B. Wei a,∗ a
Stomatology Special Consultation Clinic, Department of Prosthodontics, Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai Key Laboratory of Stomatology, Shanghai 200011, China b Department of orthodontics, Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai Key Laboratory of Stomatology, Shanghai, 200011, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. We examined the surface characteristics and corrosion properties of selective laser
Received 15 September 2012
melted (SLM) cobalt–chromium (Co–Cr) dental alloys before and after porcelain-fused-to-
Received in revised form
metal (PFM) firing.
4 August 2013
Methods. Samples were manufactured utilizing SLM techniques and control specimens were
Accepted 27 November 2013
fabricated using traditional casting methods. The microstructure and surface composition were examined using metallographic microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Corrosion properties were evaluated using electrochemi-
Keywords:
cal impedance spectroscopy. Student’s t-test was used to evaluate differences in numerical
Co–Cr alloy
results of electrochemical corrosion tests between SLM and cast specimens before or after
Selective laser melting (SLM)
PFM firing. The results of electrochemical corrosion tests of the SLM and cast samples before
Corrosion
and after firing were analyzed using one-way ANOVA.
Firing
Results. Although PFM firing altered the microstructure of the SLM specimens, they still exhibited a compact and homogeneous structure, and XPS analysis indicated that there were no significant differences in the surface composition of the specimens after firing. In artificial saliva at pH 5, the Rp value of the SLM specimens was 6.21 M cm−2 before firing and 2.84 M cm−2 after firing, suggesting there was no significant difference in electrochemical corrosion properties (P > 0.05). In artificial saliva at pH 2.5, the Rp value of the SLM group was 4.80 M cm−2 before firing and 2.88 M cm−2 after firing, again indicating no significant difference in electrochemical corrosion properties (P > 0.05). At pH 2.5, there was a significant difference in corrosion behavior between the cast and SLM groups, with the Rp value of the cast group being 0.78 M cm−2 vs. 2.88 M cm−2 for the SLM group. Significance. The improved post-firing corrosion resistance of SLM specimens provides further support for their use in prosthodontic applications, as the oral environment may become temporarily acidic following meals. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +86 021 23271699x5276; fax: +86 021 63133174. E-mail addresses: [email protected], [email protected] (B. Wei). 1 Both authors have contributed equally to this work. 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.11.013
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1.
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Introduction
Selective laser melting (SLM) is a widely used production process in which three-dimensional objects are created by selectively melting regions of a powder layer using a laser heat source [1–3]. The melted areas are specified using a CAD file, and the solid object is progressively formed by melting layers on top of one another. Compared with traditional casting techniques, SLM reduces the probability of operator error and minimizes casting defects, providing greater production precision [4,5]. SLM products exhibit higher density and improved corrosion and surface properties [6]. Because of these advantages, the technique has recently been applied to denture manufacture and has attracted the attention of dentists. Traditional casting methods have been used to manufacture Co–Cr restorations for many years [7,8], but SLM has only recently been employed to fabricate Co–Cr products [3,6]. For the sake of clinical safety, it is important to investigate the properties of Co–Cr products fabricated using SLM techniques. Dental alloys used in fixed prostheses typically have a porcelain veneer fired onto the base metal for esthetic reasons. This type of metal–ceramic restoration is known as a porcelain-fused-to-metal (PFM) restoration. The facial surfaces of PFM restorations are veneered while the lingual and occlusal surfaces and sub-gingival margins are exposed. The porcelain firing process requires high-temperature treatment consisting of four operations between 950 and 1010 ◦ C. The heating process inevitably changes the microstructure of the dental alloys, which may in turn affect their corrosion behavior. Materials for dental applications have unique requirements including non-toxicity, biocompatibility, and mechanical strength [9]. The biocompatibility of a material is influenced by the corrosion behavior, as corrosion may induce the release of toxic ions. It is therefore important to study the influence of the porcelain firing process on the corrosion behavior of the final product. Relatively few researchers have examined the effect of the PFM firing process on the surface characteristics and corrosion properties of alloys such as Ni–Cr [10]. Other studies have evaluated the corrosion properties of Co–Cr alloys in artificial saliva [11,12], and Qiu et al. [13] described the effects of the PFM firing process on the surface and corrosion properties of these alloys. However, the influence of the PFM firing process on the surface and corrosion properties of Co–Cr components fabricated using SLM has not been assessed. The pH within the oral cavity can become substantially lower after meals. The resulting ion exchange may affect the surface properties of the alloy and potentially modify the corrosion behavior [14,15]. We evaluated the effects of a simulated PFM firing process on the surface characteristics and corrosion properties of Co–Cr samples fabricated using SLM. The properties were compared to those of cast control specimens in artificial saliva at pH 2.5 and 5.
2.
Materials and methods
2.1.
Materials and sample preparation
The specimens were manufactured from a commercially available Co–Cr alloy containing 63.9 wt% Co, 24.7 wt% Cr, 5.4 wt% W, 5.0 wt% Mo, and trace quantities of Si (Wirobond C+, Bego Dental, Bremen, Germany). The 16 cylindrical specimens (10 mm in diameter and 3 mm thick) were fabricated using an SLM system (BEGO MEDIFACTURING-SYSTEM, BEGO Medical, Germany) equipped with a Yb (Ytterbium) fiber laser with a wavelength between 1060 and 1100 nm and a maximum power of 200 W. The laser translation speed utilized in the study was under 7000 mm/s. The beam diameter was approximately 0.1 mm. Another 16 control specimens of the same shape and size were produced using a flame-casting method employing an oxygen–propane (50/50, v/v) gas mixture [16]. The specimens were polished with a series of silicon carbide (SiC) papers (400, 800, and 1200 grit) on a grinding and polishing machine (Beta, Buehler, Lake Bluff, IL, USA). The polished samples were ultrasonically cleaned in ethanol and deionized water. In order to imitate the porcelain veneering procedure, half of the samples were selected at random and subjected to the firing cycle used in fabricating PFM restorations. Firing was performed under vacuum in a dental porcelain furnace (Multimat C, Dentsply Int., York, PA, USA). Briefly, the specimens were degassed at 1010 ◦ C under vacuum for 5 min, opaque fired at 980 ◦ C under vacuum, and cooled in air. Body firing was performed under vacuum at 970 ◦ C and was followed by a second air cooling step. The final procedure consisted of glaze firing at 980 ◦ C under vacuum followed by air cooling. After firing, the samples were re-polished and cleaned according to the procedure described above [13].
2.2.
Microstructural observation
One specimen of each group was selected before and after firing and etched for 30 s using hydrochloric acid/hydrogen peroxide (80:20, v/v) at room temperature [16]. The microstructure of the specimen was examined using a metallographic microscope (Carl Zeiss Image AIM, Germany). Micrographs were obtained at 100×. An additional sample from each group was selected for Xray diffraction (XRD) analysis and was processed as described above. The X-ray diffraction pattern was recorded between 15 and 100◦ 2 using Cu K␣ radiation (Rigaku Ultima IV, Japan).
2.3.
Surface analysis
X-ray photoelectron spectroscopy (XPS) (Axis Ultra DLD surface analysis system, Kratos Analytical, Hadano, Japan) was utilized to identify the elemental constituents and chemical composition of the sample surfaces. One specimen of each group was examined before and after firing. The analysis was carried out using a monochromatic Al K␣ electrode at 15 kV
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265
Fig. 1 – Metallographic photographs of cast specimen before (A) and after (B) firing and SLM specimen before (C) and after (D) firing (magnification 100×).
and 150 W and a 45◦ take-off angle. Survey and high-resolution spectra were acquired separately at pass energies of 160 and 40 eV. Elemental reference binding energies were obtained from the National Institute of Standards and Technology XPS online database. All spectral features were referenced to the binding energy of adventitious carbon (C) (284.8 eV). Surface oxide film thicknesses were determined by alternating XPS analysis and argon ion sputtering at an acceleration voltage of 4 keV until a pure metal surface was obtained [13,16].
Before testing, the samples were allowed to reach open circuit potential (Ecorr ) for 30 min. A 10 mV amplitude sine wave potential was applied over a frequency range of 100 kHz to 10 mHz. Electrochemical impedance spectroscopy (EIS) tests were performed using the dedicated PowerSine software. An appropriate equivalent circuit was constructed using the ZsimpWin software and the acquired data was analyzed using Nyquist plots, Bode impedance (|Z|) plots, and Bode phase diagrams.
2.5. 2.4.
Statistical analysis
Electrochemical corrosion testing
Before testing, five each of fired and unfired SLM and cast specimens were carefully mounted in self-curing epoxy resin. The specimen surfaces were exposed, polished, and ultrasonically cleaned as previously described. Corrosion tests were performed using an electrochemical potentiostat (PARSTAT 2273, Princeton Applied Research, Oak Ridge, TN, USA) and a test cell containing the embedded sample as the working electrode, a high-purity platinum wire as the counter electrode, and a Ag/AgCl reference electrode. Corrosion tests were carried out in quintuplicate in Fusayama artificial saliva solution. This solution (normal pH ∼5) contains 0.4 g/L NaCl, 0.4 g/L KCl, 0.795 g/L CaCl2 ·2H2 O, 0.690 g/L NaH2 PO4 ·H2 O, 0.005 g/L Na2 S·9H2 O, and 1.0 g/L urea. Since the pH of the oral cavity can fall below 5.0 following a meal [13], more aggressively acidic conditions in the oral cavity were also simulated by decreasing the to pH 2.5 using lactic acid. Corrosion tests were carried out at 37 ± 0.5 ◦ C. Each specimen was immersed in the artificial saliva solution for 24 h.
The electrochemical corrosion test results were analyzed using the SAS 8.0 software package (SAS, NC, USA). Student’s t-test was used to evaluate differences in numerical results between the SLM and cast specimens before and after PFM firing. Electrochemical corrosion tests of the SLM samples before and after firing or cast samples before and after firing were analyzed using one-way ANOVA. The probability level for statistical significance was set at P < 0.05.
3.
Results
3.1.
Microstructure
Fig. 1A and B illustrates the microstructure of the cast specimens before and after firing. At 100× magnification, the unfired cast specimens (Fig. 1A) displayed a typical inhomogeneous dendritic solidification microstructure consisting of dendrites (light areas), interdendritic regions (dark areas), and a third portion manifesting as dark lines in the light
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Fig. 2 – XPS survey spectra before and after PFM firing for cast (A) and SLM (B) specimens.
areas. Based on XRD analysis, the dendrites and interdendritic regions were a Co–Cr solid solution and the third portion was Co3 Mo(W). The XRD analysis also revealed that the dendrites and interdendritic regions were fcc and hcp allotropes of the element cobalt [8]. Based on this analysis, the cast specimens were not homogeneous. After firing, the cast specimens (Fig. 1B) retained a dendritic solidification microstructure with
more pronounced dendrites. However, the third portion was not evident, suggesting that the cast specimens remained inhomogeneous after firing. Fig. 1C and D depicts the microstructure of the SLM specimens before and after firing. The SLM specimens before firing (Fig. 1C) exhibited a homogeneous and compact fingerlike structure which consisted of Co–Cr solid solution and a
Fig. 3 – Representative high-resolution XPS spectra of O 1s, Cr 2p, Co 2p, and Mo 3d peaks for cast and SLM specimens.
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267
Fig. 4 – Nyquist plots for SLM and traditional cast samples in artificial saliva solution at pH 5 (A) and 2.5 (B).
Co3 Mo(W) phase. However, the Co3 Mo(W) regions were too small to appear in the metallographic photographs. After firing (Fig. 1D), the finger-like structure became indistinct but the samples remained homogeneous and compact.
3.2.
Surface analysis
XPS survey spectra of cast and SLM specimens are presented in Fig. 2A and B. The elements Co, Cr, Mo, O, and C were present on the surfaces of both groups. Adventitious C peaks were present due to contamination during laboratory handling. Fig. 3 contains high-resolution binding energy analyses. The O 1s peak displayed a wide asymmetric shape suggesting the presence of both metal oxides and hydroxides. The complex peaks for both Cr 2p groups suggest that Cr is predominantly present as Cr2 O3 , while the dissymmetric Co 2p peaks indicate the presence of CoO and metallic Co. The complex shape of the Mo 3d peak suggests the presence of metallic Mo as well as MoO3 . There was little change in the intensities of the Co 2p, Cr 2p, and Mo3d peaks in the cast (Fig. 2A) or SLM (Fig. 2B) specimens following firing. The oxide layer on the cast specimens was approximately 3.3 nm thick before firing and 3.6 nm thick after firing. The oxide layer on the SLM specimens was approximately 3.6 nm thick before firing and 5.4 nm after firing.
3.3.
Corrosion properties
Fig. 4A and B depicts typical EIS results for cast and SLM samples before and after PFM firing in the form of a Nyquist plot at two pH values (5 and 2.5). The measurements for all samples described a single approximately semicircular curve. At pH 5 (Fig. 4A), the semicircle diameters were similar for both cast and SLM samples and before and after firing. At pH 2.5 (Fig. 4B), the semicircle diameters for the SLM samples before and after firing were larger than for the cast samples. The spectra were analyzed in terms of an equivalent circuit model (Fig. 5). In this model, Rs represents the electrolyte resistance, Rp is the corrosion resistance of the surface oxide layer (which is inversely proportional to the corrosion rate), and Q represents the constant phase elements (CPE) of the interbarrier layer. The CPE includes Y0 and n and describes the shift from ideal capacitive
behavior resulting from surface roughness acquired during preparation with SiC abrasive paper. The value of n varied from 0.84 to 0.94, suggesting that dissolution of the samples was under mixed control. Tables 1 and 2 list the corresponding Rp , Y0-CPE, n, and 2 values. The 2 values were within 10−3 , indicating excellent agreement between the experimental data and the model. Based on one-way ANOVA analysis, there were no significant differences in Rp between the cast or SLM specimens before or after PFM firing at either pH value (P > 0.05) (Table 1). However, according to Student’s t-test results, the Rp value of the post-firing SLM specimens was significantly higher than the Rp value of the post-firing cast specimens at pH 2.5. Fig. 6A–D is representative Bode |Z| and phase diagram and equivalent circuit model curves at pH 5 for the SLM and cast groups before and after firing. The maximum phase values were approximately 80◦ between 10 and 0.1 Hz. Fig. 7A–D is typical Bode |Z| and phase diagram and equivalent circuit plots at pH 2.5 for the cast and SLM groups before and after firing. The maximum phase angle for the SLM specimens was approximately 85◦ , dropping to approximately 75◦ at lower frequencies. In contrast, the cast samples before and after firing exhibited a maximum phase value of approximately 80◦ , dropping to 70◦ at lower frequencies.
4.
Discussion
Selective laser melting produces structures by locally melting a metal powder using a laser scanned across the part surface under computer control [17,18]. Because this technique has only recently been applied to prosthodontics, there has
Fig. 5 – Equivalent circuit used for fitting experimental data.
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Table 1 – Corrosion parameters for SLM and cast groups before and after firing. Measurements were performed in pH 5 artificial saliva solution. Group
Condition
Impedance parameters (n = 5), mean (SD) SLM Before firing CAST
Rp (M cm−2 )
Yo-CPE (F cm2 )
n
2
After firing
6.21(3.91) 2.84(1.30)
30.17(0.72)a 25.52(1.10)
0.89(0.02) 0.88(0.04)
10−3 10−3
Before firing After firing
3.74(1.86) 2.90(1.74)
22.21(0.24)b 23.87(0.45)
0.91(0.02) 0.89(0.02)
10−3 10−3
Values marked a or b are statistically different.
Table 2 – Corrosion parameters for SLM and cast groups before and after firing. Measurements were obtained in pH 2.5 artificial saliva solution. Group
Condition
Impedance parameters (n = 5), mean (SD) SLM Before firing CAST
Rp (M cm−2 )
Yo-CPE (F cm2 )
n
2
After firing
4.80(1.55)a 2.88(1.29)c
19.79(0.09)e 24.79(0.65)
0.94(0.005) 0.89(0.03)
10−3 10−3
Before firing After firing
1.65(0.90)b 0.78(0.39)d
28.61(0.45)f 34.55(0.69)
0.90(0.02) 0.84(0.04)
10−3 10−3
Values marked a–f are statistically different.
been little investigation of the effect of the high porcelain firing temperature on the corrosion and surface properties of SLM Co–Cr alloys. The high temperature may cause alterations to the microstructure of the alloy and possibly affect the corrosion behavior. We examined the surface and corrosion
properties of SLM and traditional cast Co–Cr alloys in artificial saliva solution at two pH values before and after a simulated firing procedure. The corrosion resistance of Co–Cr alloys is mainly due to the existence of a passive oxide layer [19–21] spontaneously
Fig. 6 – Typical Bode |Z| and phase diagrams containing points calculated using the equivalent circuit for SLM samples before (A) and after (B) firing and cast samples before (C) and after (D) firing at pH 5.
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Fig. 7 – Typical Bode |Z| and phase diagrams with points calculated using the equivalent circuit for SLM samples before (A) and after (B) firing and cast samples before (C) and after (D) firing at pH 2.5.
formed when the alloy is exposed to atmospheric oxygen after polishing [22]. These oxide layers protect against corrosion by acting as a barrier to electron flow (resistor) between the alloy and the surrounding electrolyte [23]. Previous investigators determined that the composition of the oxide layers on the surface of Co–Cr alloy is primarily (approximately 90%) Cr2 O3 and Cr(OH)3 [22,24,25]. Although cobalt is the predominant constituent of the alloy, its concentration in the oxide films is only 5% [13]. Hence, changes in the concentration of Co and Cr may change the composition of the passive oxide layer on the alloy surface and affect the corrosion resistance. In this study, XPS measurements indicated little change in the concentration of Co and Cr in SLM specimens before and after firing, and it was not surprising that there was no significant difference in Rp value for SLM specimens before and after firing at either pH. Electrical impedance spectroscopy was used to study the pre- and post-firing corrosion properties of the cast and SLM specimens in artificial saliva solution at pH 5 and 2.5. This method is a superior means of describing the corrosion characteristics of metal specimens with surface oxide layers because it is non-destructive in the open circuit mode [13]. In the Nyquist plots, the impedance curves were approximately semicircular for all of the specimens. The diameter of the semicircle was similar before and after firing for SLM samples tested at pH 5. In the Bode phase diagrams, the
maximum phase angle for both fired and unfired SLM samples was approximately 80◦ between 10 and 0.1 Hz at pH 5. These results suggest that the corrosion behavior of the SLM samples was unchanged during the firing process. This conclusion was also supported by the Rp values. At pH 2.5, the semicircle diameters in the Nyquist plots of the SLM samples were similar before and after firing and were slightly larger than the semicircle diameter of the post-firing cast specimens. In the Bode phase diagrams, the maximum phase angle of the SLM samples was 85◦ , dropping to 75◦ at lower frequencies. In contrast, the cast samples after firing exhibited a maximum phase value of approximately 80◦ , dropping to 70◦ at lower frequencies. A higher phase angle at lower frequencies suggests better passivation, and a larger semicircle diameter indicates better corrosion properties of the alloy [10,13]. Both of these results suggest that the SLM samples possessed better corrosion resistance at pH 2.5. The improved corrosion resistance may be due to a thicker oxide layer on the surface of the SLM specimens or may be due to microstructural differences. After firing, the SLM specimens exhibited a compact homogenous microstructure, while the microstructure of the cast specimens exhibited a less compact heterogeneous dendritic microstructure. Corrosion of Co–Cr alloys releases Co and Cr, which have been reported to have cytotoxic, genotoxic, and metal sensitizing effects [13,16]. Further investigations should examine the
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pre- and post-firing ion release from alloys used in selective laser melting.
5.
Conclusions
1. SLM specimens exhibited similar corrosion behavior before and after firing at both pH 5 and 2.5. 2. Following firing, SLM samples displayed significantly better corrosion resistance than traditional cast specimens at pH 2.5. 3. Within the limitations of this study, the corrosion properties of SLM Co–Cr specimens meet the needs of clinical applications and may exceed those of traditional cast specimens.
Acknowledgements This study was supported by Shanghai Leading Academic Discipline Project (Project Number: T0202, S30206) and Science and Technology committee of Shanghai (08DZ2271100, 12441903001 and 13140902701). And thanks to School of Materials Science and Engineering, Shanghai Jiao Tong University.
references
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Surface characteristics and corrosion properties of selective laser melted Co–Cr dental alloy after porcelain firing X-Z. Xin a,1 , J. Chen a,1 , N. Xiang a , Y. Gong b,∗ , B. Wei a,∗ a
Stomatology Special Consultation Clinic, Department of Prosthodontics, Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai Key Laboratory of Stomatology, Shanghai 200011, China b Department of orthodontics, Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai Key Laboratory of Stomatology, Shanghai, 200011, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. We examined the surface characteristics and corrosion properties of selective laser
Received 15 September 2012
melted (SLM) cobalt–chromium (Co–Cr) dental alloys before and after porcelain-fused-to-
Received in revised form
metal (PFM) firing.
4 August 2013
Methods. Samples were manufactured utilizing SLM techniques and control specimens were
Accepted 27 November 2013
fabricated using traditional casting methods. The microstructure and surface composition were examined using metallographic microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Corrosion properties were evaluated using electrochemi-
Keywords:
cal impedance spectroscopy. Student’s t-test was used to evaluate differences in numerical
Co–Cr alloy
results of electrochemical corrosion tests between SLM and cast specimens before or after
Selective laser melting (SLM)
PFM firing. The results of electrochemical corrosion tests of the SLM and cast samples before
Corrosion
and after firing were analyzed using one-way ANOVA.
Firing
Results. Although PFM firing altered the microstructure of the SLM specimens, they still exhibited a compact and homogeneous structure, and XPS analysis indicated that there were no significant differences in the surface composition of the specimens after firing. In artificial saliva at pH 5, the Rp value of the SLM specimens was 6.21 M cm−2 before firing and 2.84 M cm−2 after firing, suggesting there was no significant difference in electrochemical corrosion properties (P > 0.05). In artificial saliva at pH 2.5, the Rp value of the SLM group was 4.80 M cm−2 before firing and 2.88 M cm−2 after firing, again indicating no significant difference in electrochemical corrosion properties (P > 0.05). At pH 2.5, there was a significant difference in corrosion behavior between the cast and SLM groups, with the Rp value of the cast group being 0.78 M cm−2 vs. 2.88 M cm−2 for the SLM group. Significance. The improved post-firing corrosion resistance of SLM specimens provides further support for their use in prosthodontic applications, as the oral environment may become temporarily acidic following meals. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +86 021 23271699x5276; fax: +86 021 63133174. E-mail addresses: [email protected], [email protected] (B. Wei). 1 Both authors have contributed equally to this work. 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.11.013
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Introduction
Selective laser melting (SLM) is a widely used production process in which three-dimensional objects are created by selectively melting regions of a powder layer using a laser heat source [1–3]. The melted areas are specified using a CAD file, and the solid object is progressively formed by melting layers on top of one another. Compared with traditional casting techniques, SLM reduces the probability of operator error and minimizes casting defects, providing greater production precision [4,5]. SLM products exhibit higher density and improved corrosion and surface properties [6]. Because of these advantages, the technique has recently been applied to denture manufacture and has attracted the attention of dentists. Traditional casting methods have been used to manufacture Co–Cr restorations for many years [7,8], but SLM has only recently been employed to fabricate Co–Cr products [3,6]. For the sake of clinical safety, it is important to investigate the properties of Co–Cr products fabricated using SLM techniques. Dental alloys used in fixed prostheses typically have a porcelain veneer fired onto the base metal for esthetic reasons. This type of metal–ceramic restoration is known as a porcelain-fused-to-metal (PFM) restoration. The facial surfaces of PFM restorations are veneered while the lingual and occlusal surfaces and sub-gingival margins are exposed. The porcelain firing process requires high-temperature treatment consisting of four operations between 950 and 1010 ◦ C. The heating process inevitably changes the microstructure of the dental alloys, which may in turn affect their corrosion behavior. Materials for dental applications have unique requirements including non-toxicity, biocompatibility, and mechanical strength [9]. The biocompatibility of a material is influenced by the corrosion behavior, as corrosion may induce the release of toxic ions. It is therefore important to study the influence of the porcelain firing process on the corrosion behavior of the final product. Relatively few researchers have examined the effect of the PFM firing process on the surface characteristics and corrosion properties of alloys such as Ni–Cr [10]. Other studies have evaluated the corrosion properties of Co–Cr alloys in artificial saliva [11,12], and Qiu et al. [13] described the effects of the PFM firing process on the surface and corrosion properties of these alloys. However, the influence of the PFM firing process on the surface and corrosion properties of Co–Cr components fabricated using SLM has not been assessed. The pH within the oral cavity can become substantially lower after meals. The resulting ion exchange may affect the surface properties of the alloy and potentially modify the corrosion behavior [14,15]. We evaluated the effects of a simulated PFM firing process on the surface characteristics and corrosion properties of Co–Cr samples fabricated using SLM. The properties were compared to those of cast control specimens in artificial saliva at pH 2.5 and 5.
2.
Materials and methods
2.1.
Materials and sample preparation
The specimens were manufactured from a commercially available Co–Cr alloy containing 63.9 wt% Co, 24.7 wt% Cr, 5.4 wt% W, 5.0 wt% Mo, and trace quantities of Si (Wirobond C+, Bego Dental, Bremen, Germany). The 16 cylindrical specimens (10 mm in diameter and 3 mm thick) were fabricated using an SLM system (BEGO MEDIFACTURING-SYSTEM, BEGO Medical, Germany) equipped with a Yb (Ytterbium) fiber laser with a wavelength between 1060 and 1100 nm and a maximum power of 200 W. The laser translation speed utilized in the study was under 7000 mm/s. The beam diameter was approximately 0.1 mm. Another 16 control specimens of the same shape and size were produced using a flame-casting method employing an oxygen–propane (50/50, v/v) gas mixture [16]. The specimens were polished with a series of silicon carbide (SiC) papers (400, 800, and 1200 grit) on a grinding and polishing machine (Beta, Buehler, Lake Bluff, IL, USA). The polished samples were ultrasonically cleaned in ethanol and deionized water. In order to imitate the porcelain veneering procedure, half of the samples were selected at random and subjected to the firing cycle used in fabricating PFM restorations. Firing was performed under vacuum in a dental porcelain furnace (Multimat C, Dentsply Int., York, PA, USA). Briefly, the specimens were degassed at 1010 ◦ C under vacuum for 5 min, opaque fired at 980 ◦ C under vacuum, and cooled in air. Body firing was performed under vacuum at 970 ◦ C and was followed by a second air cooling step. The final procedure consisted of glaze firing at 980 ◦ C under vacuum followed by air cooling. After firing, the samples were re-polished and cleaned according to the procedure described above [13].
2.2.
Microstructural observation
One specimen of each group was selected before and after firing and etched for 30 s using hydrochloric acid/hydrogen peroxide (80:20, v/v) at room temperature [16]. The microstructure of the specimen was examined using a metallographic microscope (Carl Zeiss Image AIM, Germany). Micrographs were obtained at 100×. An additional sample from each group was selected for Xray diffraction (XRD) analysis and was processed as described above. The X-ray diffraction pattern was recorded between 15 and 100◦ 2 using Cu K␣ radiation (Rigaku Ultima IV, Japan).
2.3.
Surface analysis
X-ray photoelectron spectroscopy (XPS) (Axis Ultra DLD surface analysis system, Kratos Analytical, Hadano, Japan) was utilized to identify the elemental constituents and chemical composition of the sample surfaces. One specimen of each group was examined before and after firing. The analysis was carried out using a monochromatic Al K␣ electrode at 15 kV
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Fig. 1 – Metallographic photographs of cast specimen before (A) and after (B) firing and SLM specimen before (C) and after (D) firing (magnification 100×).
and 150 W and a 45◦ take-off angle. Survey and high-resolution spectra were acquired separately at pass energies of 160 and 40 eV. Elemental reference binding energies were obtained from the National Institute of Standards and Technology XPS online database. All spectral features were referenced to the binding energy of adventitious carbon (C) (284.8 eV). Surface oxide film thicknesses were determined by alternating XPS analysis and argon ion sputtering at an acceleration voltage of 4 keV until a pure metal surface was obtained [13,16].
Before testing, the samples were allowed to reach open circuit potential (Ecorr ) for 30 min. A 10 mV amplitude sine wave potential was applied over a frequency range of 100 kHz to 10 mHz. Electrochemical impedance spectroscopy (EIS) tests were performed using the dedicated PowerSine software. An appropriate equivalent circuit was constructed using the ZsimpWin software and the acquired data was analyzed using Nyquist plots, Bode impedance (|Z|) plots, and Bode phase diagrams.
2.5. 2.4.
Statistical analysis
Electrochemical corrosion testing
Before testing, five each of fired and unfired SLM and cast specimens were carefully mounted in self-curing epoxy resin. The specimen surfaces were exposed, polished, and ultrasonically cleaned as previously described. Corrosion tests were performed using an electrochemical potentiostat (PARSTAT 2273, Princeton Applied Research, Oak Ridge, TN, USA) and a test cell containing the embedded sample as the working electrode, a high-purity platinum wire as the counter electrode, and a Ag/AgCl reference electrode. Corrosion tests were carried out in quintuplicate in Fusayama artificial saliva solution. This solution (normal pH ∼5) contains 0.4 g/L NaCl, 0.4 g/L KCl, 0.795 g/L CaCl2 ·2H2 O, 0.690 g/L NaH2 PO4 ·H2 O, 0.005 g/L Na2 S·9H2 O, and 1.0 g/L urea. Since the pH of the oral cavity can fall below 5.0 following a meal [13], more aggressively acidic conditions in the oral cavity were also simulated by decreasing the to pH 2.5 using lactic acid. Corrosion tests were carried out at 37 ± 0.5 ◦ C. Each specimen was immersed in the artificial saliva solution for 24 h.
The electrochemical corrosion test results were analyzed using the SAS 8.0 software package (SAS, NC, USA). Student’s t-test was used to evaluate differences in numerical results between the SLM and cast specimens before and after PFM firing. Electrochemical corrosion tests of the SLM samples before and after firing or cast samples before and after firing were analyzed using one-way ANOVA. The probability level for statistical significance was set at P < 0.05.
3.
Results
3.1.
Microstructure
Fig. 1A and B illustrates the microstructure of the cast specimens before and after firing. At 100× magnification, the unfired cast specimens (Fig. 1A) displayed a typical inhomogeneous dendritic solidification microstructure consisting of dendrites (light areas), interdendritic regions (dark areas), and a third portion manifesting as dark lines in the light
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Fig. 2 – XPS survey spectra before and after PFM firing for cast (A) and SLM (B) specimens.
areas. Based on XRD analysis, the dendrites and interdendritic regions were a Co–Cr solid solution and the third portion was Co3 Mo(W). The XRD analysis also revealed that the dendrites and interdendritic regions were fcc and hcp allotropes of the element cobalt [8]. Based on this analysis, the cast specimens were not homogeneous. After firing, the cast specimens (Fig. 1B) retained a dendritic solidification microstructure with
more pronounced dendrites. However, the third portion was not evident, suggesting that the cast specimens remained inhomogeneous after firing. Fig. 1C and D depicts the microstructure of the SLM specimens before and after firing. The SLM specimens before firing (Fig. 1C) exhibited a homogeneous and compact fingerlike structure which consisted of Co–Cr solid solution and a
Fig. 3 – Representative high-resolution XPS spectra of O 1s, Cr 2p, Co 2p, and Mo 3d peaks for cast and SLM specimens.
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Fig. 4 – Nyquist plots for SLM and traditional cast samples in artificial saliva solution at pH 5 (A) and 2.5 (B).
Co3 Mo(W) phase. However, the Co3 Mo(W) regions were too small to appear in the metallographic photographs. After firing (Fig. 1D), the finger-like structure became indistinct but the samples remained homogeneous and compact.
3.2.
Surface analysis
XPS survey spectra of cast and SLM specimens are presented in Fig. 2A and B. The elements Co, Cr, Mo, O, and C were present on the surfaces of both groups. Adventitious C peaks were present due to contamination during laboratory handling. Fig. 3 contains high-resolution binding energy analyses. The O 1s peak displayed a wide asymmetric shape suggesting the presence of both metal oxides and hydroxides. The complex peaks for both Cr 2p groups suggest that Cr is predominantly present as Cr2 O3 , while the dissymmetric Co 2p peaks indicate the presence of CoO and metallic Co. The complex shape of the Mo 3d peak suggests the presence of metallic Mo as well as MoO3 . There was little change in the intensities of the Co 2p, Cr 2p, and Mo3d peaks in the cast (Fig. 2A) or SLM (Fig. 2B) specimens following firing. The oxide layer on the cast specimens was approximately 3.3 nm thick before firing and 3.6 nm thick after firing. The oxide layer on the SLM specimens was approximately 3.6 nm thick before firing and 5.4 nm after firing.
3.3.
Corrosion properties
Fig. 4A and B depicts typical EIS results for cast and SLM samples before and after PFM firing in the form of a Nyquist plot at two pH values (5 and 2.5). The measurements for all samples described a single approximately semicircular curve. At pH 5 (Fig. 4A), the semicircle diameters were similar for both cast and SLM samples and before and after firing. At pH 2.5 (Fig. 4B), the semicircle diameters for the SLM samples before and after firing were larger than for the cast samples. The spectra were analyzed in terms of an equivalent circuit model (Fig. 5). In this model, Rs represents the electrolyte resistance, Rp is the corrosion resistance of the surface oxide layer (which is inversely proportional to the corrosion rate), and Q represents the constant phase elements (CPE) of the interbarrier layer. The CPE includes Y0 and n and describes the shift from ideal capacitive
behavior resulting from surface roughness acquired during preparation with SiC abrasive paper. The value of n varied from 0.84 to 0.94, suggesting that dissolution of the samples was under mixed control. Tables 1 and 2 list the corresponding Rp , Y0-CPE, n, and 2 values. The 2 values were within 10−3 , indicating excellent agreement between the experimental data and the model. Based on one-way ANOVA analysis, there were no significant differences in Rp between the cast or SLM specimens before or after PFM firing at either pH value (P > 0.05) (Table 1). However, according to Student’s t-test results, the Rp value of the post-firing SLM specimens was significantly higher than the Rp value of the post-firing cast specimens at pH 2.5. Fig. 6A–D is representative Bode |Z| and phase diagram and equivalent circuit model curves at pH 5 for the SLM and cast groups before and after firing. The maximum phase values were approximately 80◦ between 10 and 0.1 Hz. Fig. 7A–D is typical Bode |Z| and phase diagram and equivalent circuit plots at pH 2.5 for the cast and SLM groups before and after firing. The maximum phase angle for the SLM specimens was approximately 85◦ , dropping to approximately 75◦ at lower frequencies. In contrast, the cast samples before and after firing exhibited a maximum phase value of approximately 80◦ , dropping to 70◦ at lower frequencies.
4.
Discussion
Selective laser melting produces structures by locally melting a metal powder using a laser scanned across the part surface under computer control [17,18]. Because this technique has only recently been applied to prosthodontics, there has
Fig. 5 – Equivalent circuit used for fitting experimental data.
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Table 1 – Corrosion parameters for SLM and cast groups before and after firing. Measurements were performed in pH 5 artificial saliva solution. Group
Condition
Impedance parameters (n = 5), mean (SD) SLM Before firing CAST
Rp (M cm−2 )
Yo-CPE (F cm2 )
n
2
After firing
6.21(3.91) 2.84(1.30)
30.17(0.72)a 25.52(1.10)
0.89(0.02) 0.88(0.04)
10−3 10−3
Before firing After firing
3.74(1.86) 2.90(1.74)
22.21(0.24)b 23.87(0.45)
0.91(0.02) 0.89(0.02)
10−3 10−3
Values marked a or b are statistically different.
Table 2 – Corrosion parameters for SLM and cast groups before and after firing. Measurements were obtained in pH 2.5 artificial saliva solution. Group
Condition
Impedance parameters (n = 5), mean (SD) SLM Before firing CAST
Rp (M cm−2 )
Yo-CPE (F cm2 )
n
2
After firing
4.80(1.55)a 2.88(1.29)c
19.79(0.09)e 24.79(0.65)
0.94(0.005) 0.89(0.03)
10−3 10−3
Before firing After firing
1.65(0.90)b 0.78(0.39)d
28.61(0.45)f 34.55(0.69)
0.90(0.02) 0.84(0.04)
10−3 10−3
Values marked a–f are statistically different.
been little investigation of the effect of the high porcelain firing temperature on the corrosion and surface properties of SLM Co–Cr alloys. The high temperature may cause alterations to the microstructure of the alloy and possibly affect the corrosion behavior. We examined the surface and corrosion
properties of SLM and traditional cast Co–Cr alloys in artificial saliva solution at two pH values before and after a simulated firing procedure. The corrosion resistance of Co–Cr alloys is mainly due to the existence of a passive oxide layer [19–21] spontaneously
Fig. 6 – Typical Bode |Z| and phase diagrams containing points calculated using the equivalent circuit for SLM samples before (A) and after (B) firing and cast samples before (C) and after (D) firing at pH 5.
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Fig. 7 – Typical Bode |Z| and phase diagrams with points calculated using the equivalent circuit for SLM samples before (A) and after (B) firing and cast samples before (C) and after (D) firing at pH 2.5.
formed when the alloy is exposed to atmospheric oxygen after polishing [22]. These oxide layers protect against corrosion by acting as a barrier to electron flow (resistor) between the alloy and the surrounding electrolyte [23]. Previous investigators determined that the composition of the oxide layers on the surface of Co–Cr alloy is primarily (approximately 90%) Cr2 O3 and Cr(OH)3 [22,24,25]. Although cobalt is the predominant constituent of the alloy, its concentration in the oxide films is only 5% [13]. Hence, changes in the concentration of Co and Cr may change the composition of the passive oxide layer on the alloy surface and affect the corrosion resistance. In this study, XPS measurements indicated little change in the concentration of Co and Cr in SLM specimens before and after firing, and it was not surprising that there was no significant difference in Rp value for SLM specimens before and after firing at either pH. Electrical impedance spectroscopy was used to study the pre- and post-firing corrosion properties of the cast and SLM specimens in artificial saliva solution at pH 5 and 2.5. This method is a superior means of describing the corrosion characteristics of metal specimens with surface oxide layers because it is non-destructive in the open circuit mode [13]. In the Nyquist plots, the impedance curves were approximately semicircular for all of the specimens. The diameter of the semicircle was similar before and after firing for SLM samples tested at pH 5. In the Bode phase diagrams, the
maximum phase angle for both fired and unfired SLM samples was approximately 80◦ between 10 and 0.1 Hz at pH 5. These results suggest that the corrosion behavior of the SLM samples was unchanged during the firing process. This conclusion was also supported by the Rp values. At pH 2.5, the semicircle diameters in the Nyquist plots of the SLM samples were similar before and after firing and were slightly larger than the semicircle diameter of the post-firing cast specimens. In the Bode phase diagrams, the maximum phase angle of the SLM samples was 85◦ , dropping to 75◦ at lower frequencies. In contrast, the cast samples after firing exhibited a maximum phase value of approximately 80◦ , dropping to 70◦ at lower frequencies. A higher phase angle at lower frequencies suggests better passivation, and a larger semicircle diameter indicates better corrosion properties of the alloy [10,13]. Both of these results suggest that the SLM samples possessed better corrosion resistance at pH 2.5. The improved corrosion resistance may be due to a thicker oxide layer on the surface of the SLM specimens or may be due to microstructural differences. After firing, the SLM specimens exhibited a compact homogenous microstructure, while the microstructure of the cast specimens exhibited a less compact heterogeneous dendritic microstructure. Corrosion of Co–Cr alloys releases Co and Cr, which have been reported to have cytotoxic, genotoxic, and metal sensitizing effects [13,16]. Further investigations should examine the
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pre- and post-firing ion release from alloys used in selective laser melting.
5.
Conclusions
1. SLM specimens exhibited similar corrosion behavior before and after firing at both pH 5 and 2.5. 2. Following firing, SLM samples displayed significantly better corrosion resistance than traditional cast specimens at pH 2.5. 3. Within the limitations of this study, the corrosion properties of SLM Co–Cr specimens meet the needs of clinical applications and may exceed those of traditional cast specimens.
Acknowledgements This study was supported by Shanghai Leading Academic Discipline Project (Project Number: T0202, S30206) and Science and Technology committee of Shanghai (08DZ2271100, 12441903001 and 13140902701). And thanks to School of Materials Science and Engineering, Shanghai Jiao Tong University.
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