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Graduate student, Graduate Prosthodontics, Fujian Key Laboratory of Oral Diseases & Fujian Provincial Engineering Research Center of Oral Biomaterial & Stomatological Key Lab of Fujian College and University, School and Hospital of Stomatology, Fujian Medical University, Fuzhou, Fujian, PR China
Graduate student, Graduate Prosthodontics, Fujian Key Laboratory of Oral Diseases & Fujian Provincial Engineering Research Center of Oral Biomaterial & Stomatological Key Lab of Fujian College and University, School and Hospital of Stomatology, Fujian Medical University, Fuzhou, Fujian, PR China
Graduate student, Graduate Prosthodontics, Fujian Key Laboratory of Oral Diseases & Fujian Provincial Engineering Research Center of Oral Biomaterial & Stomatological Key Lab of Fujian College and University, School and Hospital of Stomatology, Fujian Medical University, Fuzhou, Fujian, PR China
Researcher, Institute of Stomatology & Research Center of Dental Esthetics and Biomechanics, School and Hospital of Stomatology, Fujian Medical University, Fuzhou, Fujian, PR China
Corresponding author: Dr Hui Cheng, School and Hospital of Stomatology, Fujian Medical University, 246 Yangqiao Road, Fuzhou, Fujian 350002, Box 350001, Fuzhou 86-18960883888, PR CHINA
Professor, Institute of Stomatology & Research Center of Dental Esthetics and Biomechanics, School and Hospital of Stomatology, Fujian Medical University, Fuzhou, Fujian, PR China
The complex oral environment leads to the corrosion of dental alloy materials and the release of metal ions that may have a negative impact on health. Digital manufacturing is increasingly being used in dentistry, but whether digitally manufactured prostheses have better resistance to corrosion than traditional cast prostheses is unclear.
Purpose
The purpose of this in vitro study was to determine the surface properties and corrosion resistance of dental cobalt-chromium (Co-Cr) alloys fabricated by lost-wax casting (CAST), selective laser melting (SLM), and computer numerical control milling (CNC).
Material and methods
The surface characteristics of the specimens were analyzed via scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS), metallurgical observation, and X-ray diffraction (XRD). For corrosion resistance, the specimens were immersed in artificial saliva at a pH 2.3 and 6.8 for 1, 4, and 7 weeks. Then, inductively coupled plasma-mass spectrometry (ICP-MS) was used to detect the main metal ion. Electrochemical impedance spectroscopy (EIS) was conducted based on a 3-electrode system to assess the electrochemical corrosion resistance. An ANOVA test was used to evaluate statistically significant differences among the groups (α=.05).
Results
The SLM and CNC specimens showed more homogenous microstructures, less ion release at different times and pH, and more charge transfer resistance than CAST specimens.
Conclusions
Compared with casting, SLM-printing and CNC-milling have advantages in terms of surface properties and corrosion resistance.
Clinical Implications
Biocompatibility is closely related to the corrosion of dental alloys. Manufacturing defects can be minimized through digital SLM-printing and CNC-milling, both of which provide better corrosion resistance than lost-wax casting.
Although there is an increasing demand for ceramics, metal-ceramic restorations are still widely used as single crowns or fixed partial dentures, especially in developing countries,
Traditionally, cobalt-chromium (Co-Cr) frameworks have been fabricated by the lost-wax casting technique (CAST), comprised of wax pattern buildup, investment, wax elimination, and casting.
Recently, computer-aided design and computer-aided manufacturing (CAD-CAM) technology has been introduced to fabricate dental restorations with both additive, including selective laser melting (SLM), and subtractive, including computer numerical control milling (CNC), manufacturing processes.
Moreover, SLM produces metal substrates almost without porosity by melting alloy powder layer by layer, and the machinable metal disk is also manufactured under highly standardized industrial conditions.
Selective laser melting technique of Co-Cr dental alloys: a review of structure and properties and comparative analysis with other available techniques.
Nevertheless, whether the fabrication method affects corrosion resistance of dental Co-Cr alloys is unclear. The changeable oral environment, including temperature, pH, and pressure, has been reported to accelerate corrosion and the release of metal ions, which may induce deoxyribonucleic acid (DNA) damage and cell apoptosis.
Therefore, the purpose of this in vitro study was to investigate the surface and corrosion properties of Co-Cr dental alloys fabricated by different methods (CAST, SLM, and CNC). The null hypothesis was that the SLM, CNC, and CAST specimens would have similar microstructures and corrosion resistance.
Material and methods
The casting Co-Cr alloy ingots (Biodur soft; DFS Diamon GmbH) tested had a composition of Co 61.0 wt%, Cr 24.0 wt%, W 8.0 wt%, Mo 2.5 wt%, and other ≤1.0 wt%. The Co-Cr alloy powder (remanium star CL; Dentaurum GmbH & Co KG) used for SLM consisted of Co 60.5 wt%, Cr 28.0 wt%, W 9.0 wt%, Si 1.5 wt%, and other ≤1.0 wt%. The machinable Co-Cr alloy disk (KERA-Disc; Eisenbacher Dentalwaren GmbH) had a composition of Co 61.65 wt%, Cr 27.75 wt%, W 8.45 wt%, and other <1.0 wt%. All the specimens were fabricated according to the manufacturers' instructions, and cylindrical specimens (Ø10.0 ±0.01×2.0 ±0.01 mm) were prepared. The brand names, manufacturers, elemental composition of the alloys, and fabrication conditions are presented in Tables 1 and 2. Any apparent surface defects were removed, and the specimens were polished with a series of SiC papers (400, 600, 800, and 1200 grits), ultrasonically cleaned in acetone, ethanol, and deionized water for 15 minutes, dried with oil-free compressed air, steam-sterilized at 121 °C for at least 30 minutes, and put in a drying oven at 65 °C for 24 hours.
Table 1Fabrication methods, brand names, manufacturers, and elemental compositions of Co-Cr alloys
Fabrication Methods
Brand Names
Manufacturers
Element Composition (wt%)
Co
Cr
W
Mn
Fe
Nb
Si
Mo
CAST
Biodur Soft
DFS Diamon GmbH
61.0
24.0
8.0
1.0
1.0
1.0
1.0
2.5
SLM
remanium star CL
Dentaurum GmbH & Co KG
60.5
28.0
9.0
1.0
1.0
1.0
1.5
NA
CNC
Kera Disc
Eisenbacher Dentalwaren GmbH
61.65
27.75
8.45
0.25
0.20
NA
NA
NA
CAST, lost-wax casting; CNC, computer numerical control milling; NA, not available; SLM, selective laser melting.
For microstructural characterization and chemical composition analysis, the Co-Cr alloys were examined by using SEM-EDS (Quanta 250; FEI). The specimens were etched with hydrochloric acid/hydrogen peroxide (80:20 v/v) for 30 seconds to observe the grain structure.
Representative SEM images were made, and the EDS spectra were obtained by spot scanning. Qualitative and semiquantitative analyses of the spectra were conducted by using a software program (ESPRIT 2, Bruker Nano Analytics; Bruker Nano GmbH).
The specimens were polished with a metallographic polishing disk covered with a piece of polishing velour cloth at a speed of 900 rpm before the metallographic analysis (XJZ-6A; Shanghai clicking optical equipment Co Ltd). For the observation of the phase structure, the chromium oxide polishing solution was continuously added to the surface until the grinding mark almost disappeared; the specimens were then etched for 15 seconds with hydrochloric acid/nitric acid (3:1 v/v) before the test.
International Organization for Standardization ISO 4499-1: Hard metals-metallographic determination of microstructure. Part 1: Photomicrographs and description.
Phase identification was performed by using XRD equipment (Xpert pro MPD; Philips) and Cu-Kα1 radiation operated at an accelerating voltage of 40 kV, a beam current of 30 mA, a 2θ-angle scanning range from 30 to 95 degrees, and a scanning speed of 0.5 degree/S.
Six specimens were selected from each group with a random number table to determine the surface roughness before and after polishing treatment by profilometry (DSF 600; Kosaka Laboratory Ltd), and the data were retrieved from 4 quadrants of 1 side of the specimen. The average roughness (Ra) was obtained over the measuring length of 4.0 mm by sampling a length of 0.8 mm at a scanning speed of 0.1 mm per second.
International Organization for Standardization ISO 13565-1: Geometrical product specifications (GPS)-Surface texture: Profile method; surfaces having stratified functional properties. Part 1: Filtering and general measurement conditions.
Static immersion was performed to measure relative numbers of main metal ions in the extract. The specimens (n=6) were immersed in a neutral solution and then acidified by using the Fusayama artificial saliva solution. The Fusayama artificial saliva consisted of NaCl (400 mg L-1), KCl (400 mg L-1), CaCl2·2H2O (795 mg L-1), NaH2PO4 (690 mg L-1), KSCN (300 mg L-1), NaS·9H2O (5 mg L-1), and urea (1000 mg L-1), and the acidified saliva was prepared by adding lactic acid to adjust the pH from 6.8 to 2.3,
Each polished specimen was incubated in a closed sterile centrifuge tube containing 2.199 mL of artificial saliva solution (1 mL solution per 1 cm2 of sample surface area) at 37.0 °C for 1, 4, and 7 weeks,
and 2 blank control groups were set with the same volume of artificial saliva at different pH levels. The quantitative and qualitative analyses on metal ions were conducted by using ICP-MS (7900; Agilent), and the results were converted to units of μg·cm-2 of alloy surface area.
In the electrochemical corrosion test, one side of the specimens (n=6) was selected as a test surface with an exposure area of 0.714 cm2, and the other sides were embedded with epoxy resin (Quikfirm; Truer). The EIS was performed by using a potentiostat (Autolab PGSTAT302; Metrohm Ltd) controlled by a software program (Nova, v2.1; Metrohm Ltd). The 3-electrode test system containing the Co-Cr alloy specimens operated as the working electrode, saturated calomel electrode (SCE) was used as the reference electrode, and a Pt plate acted as the counter electrode. Each specimen was evaluated in the electrolyte of the neutral Fusayama artificial saliva solution at 37.0 °C. The frequencies for EIS ranged from 10-2 Hz to 105 Hz with an alternating current sinusoidal potential of ±10 mV, following the open-circuit potential (EOCP). The EIS was tested until the fluctuation range of potential was kept less than 2 mV for 30 minutes. The Nyquist plots and Bode plots were analyzed, and a Randles circuit was modeled and fitted to the EIS data.
Furthermore, the impedance parameters, including the solution resistance (Rs), charge transfer resistance (Rct), constant phase elements (Yo-CPE), diffusion coefficient (n), and fitting efficiency, evaluated with the chi-square analysis, were obtained with a software program (ZSimpWin, v3.2; Bruno Yeum).
Statistical analyses were performed by using a statistical software program (IBM SPSS Statistics, v21.0; IBM Corp). The Shapiro-Wilk and Levene tests were applied to assess the assumptions of data normality and the homogeneity of variance, respectively. One-way ANOVA and the Fisher LSD multiple comparisons post hoc analyses of the roughness and impedance parameters were conducted. Statistical analyses of the main ion release were performed with a 3 (fabrication methods) by 3 (immersion time) by 2 (pH level), 3-way analysis of variance to evaluate interaction effects between these independent variables (α=.05 for all tests).
Results
The microstructures of the Co-Cr alloys are shown in Figure 1. In the CAST group, the corrosion pits were distributed evenly, and the different diameter and depth of the pits represented the degrees of corrosion (Fig. 1A, 1B). The SLM specimens exhibited fine grains and clear grain boundaries. Furthermore, large high-contrast particles were distributed in the interior of the grain boundary with local intergranular corrosion (Fig. 1C, 1D). Only a few polishing scratch marks and sparse corrosion pits were found in the CNC group, and it also exhibited uniform grains and sharp boundaries (Fig. 1E, 1F). Figure 1G–I and Table 3 illustrate the elemental composition at different scanning spots with EDS analysis. The greatest number of element types were found on the surface of the CAST group; the SLM specimen contained a higher level of Cr inside the grains than in the grain boundaries; and the distribution of the chemical constitution of the CNC specimen was more uniform than with the other 2 groups. Moreover, the CAST specimen displayed a typical inhomogeneous dendritic microstructure that consisted of light and dark regions (Fig. 2A, 2B). In the SLM group, homogeneous and compact finger-like structures were covered with many scattered second-phase particles (Fig. 2C, 2D). Only a small number of island-shaped intermetallic compounds were found in the CNC group (Fig. 2E, 2F).
Figure 1Representative scanning electron micrographs of Co-Cr alloys fabricated by different methods. A, B, CAST; C, D, SLM; E, F, CNC. Original magnification A, C, E, ×1000. Original magnification B, D, F, ×5000. Red arrows, corrosion pits; yellow arrow, intergranular corrosion; blue arrow, grinding mark. Pink, yellow, and blue spots, EDS analysis. CAST, lost-wax casting; CNC, computer numerical control milling; EDS, energy-dispersive spectroscopy; SLM, selective laser melting.
Figure 2Metallographic images of Co-Cr alloys. A, B, CAST; C, D, SLM; E, F, CNC. Original magnification A, C, E, ×100. Original magnification B, D, F, ×400. CAST, lost-wax casting; CNC, computer numerical control milling; SLM, selective laser melting.
The XRD analysis indicated that in addition to the face-centered cubic (FCC) α-Co phase (ICDD card no.: 15-0806), the hexagonal close-packed (HCP) ε-Co phase (ICDD card no.: 05-0727) was identified in all specimens (Fig. 3). The Ra values are presented in Figure 4A-C and Table 4, and the SLM group had a rougher surface than the CAST and CNC groups before the specimens were polished (P<.001). Figure 4D-F shows the grainy surface of the CAST specimens, incompletely melted surface of the SLM specimens, and regular fine texture on the CNC specimens. However, no significant difference in roughness (P>.05) or morphology was found among groups after polishing.
Figure 3X-ray diffraction spectra of Co-Cr alloys. Red line, CAST; green line, SLM; blue line, CNC. Orange square, α-Co FCC phase; pink hexagon, ε-Co HCP phase. CAST, lost-wax casting; CNC, computer numerical control milling; SLM, selective laser melting.
Figure 4Surface roughness of Co-Cr alloys before (red, green, blue line) and after polishing (black line). A, CAST; B, SLM; C, CNC; Morphology of Co-Cr alloys before (lower-left section) and after polishing (upper-right section). Original magnification D, E, F, ×40. D, CAST; E, SLM; F, CNC. CAST, lost-wax casting; CNC, computer numerical control milling; SLM, selective laser melting.
Figure 5 exhibits the main metal ions (Co, Cr, Mn, W) released from the Co-Cr alloy specimens after immersion in neutral and acidic artificial saliva for 1, 4, and 7 weeks. Mo ion was detected only in the CAST group, and a trace amount of Fe ion was found in the blank control groups. An increase in Co, Cr, and Mn ion release was found with increased immersion time and decreased pH value, and the highest ion concentrations were observed for the CAST specimens. However, Table 5 indicates the fabrication methods had a noticeable effect on Co and Cr ion release (P<.001) and that the interactions between fabrication methods and immersion time and pH level were statistically significant (P<.005).
Figure 5Co, Cr, Mn, and W ions were released from Co-Cr alloys in artificial saliva at 1, 4, and 7 weeks. A, artificial saliva at pH 6.8; red line, CAST; green line, SLM; blue line, CNC; B, acidified saliva at pH 2.3; pink line, CAST; yellow line, SLM; cyan line, CNC. CAST, lost-wax casting; CNC, computer numerical control milling; SLM, selective laser melting.
Figure 6A shows a schematic view of the electrochemical 3-electrode system. The electrochemical measurements for all specimens described a single arc, the larger radius of which indicated improved corrosion resistance (Fig. 6B).
Table 6 lists the corresponding corrosion parameters, and the χ2 values were less than 10-3, suggesting an excellent fitting characteristic of the model.
No statistical difference was found in the Rs and n values among the groups (P>.05). The greatest Rct was obtained from the SLM group, followed by the CNC group (P<.005) and CAST group (P<.001) (Fig. 6D). Additionally, Figure 6E shows the magnitude Bode plots: The low-frequency impedance modulus |Z| has been reported to be positively associated with the corrosion resistance, and the SLM and CNC groups had larger |Z| than the CAST group.
Moreover, the larger phase angles (which are proportional to the consistency and protective effect of the surface oxide film) were observed for the SLM and CNC groups in the low-frequency region (10-2-100 Hz) compared with the CAST group (Fig. 6F).
Figure 6Electrochemical corrosion behaviors of Co-Cr alloys. A, Schematic view of the electrochemical three-electrode system; B, Nyquist plots of specimens recorded in neutral artificial saliva; C, Randles circuit model based on EIS measurements; D, mean charge transfer resistance (Rct) of CAST, SLM, and CNC, ∗P<.05, ∗∗P<.005, ∗∗∗P<.001; E, magnitude Bode plots; F, Bode phase diagrams. CAST, lost-wax casting; CNC, computer numerical control milling; SLM, selective laser melting.
The null hypothesis was rejected, as the surface properties and corrosion resistance of the Co-Cr dental alloy were dependent on the fabrication method. Alloy corrosion not only reduces physicochemical performance but also adversely affects biocompatibility.
In the present study, the CAST group exhibited the widely distributed corrosion pits, visible after etching, while regular grains and clear grain boundaries were observed in the SLM and CNC groups (Fig. 1). Moreover, the CAST specimen demonstrated a typical dendritic structure for an alloy with a low cooling and solidification rate (Fig. 2), and the bright and dark regions represented the difference in the solute distribution between the dendrite and interdendritic regions, indicating the dendritic segregation during the casting process.
However, it could be avoided with the SLM, as the alloy powders are entirely melted and rapidly solidified and the machinable disks for CNC are also fabricated under the strict thermomechanical process. The digital manufacturing minimizes defects and produces a more homogeneous microstructure that may contribute to improved corrosion resistance.
Additionally, there is a phase transformation from the high-temperature α-Co (FCC) phase to the low-temperature ε-Co (HCP) phase in Co-Cr alloy, and the XRD analysis suggested the peak strength of 2θ=44 degrees (α-Co phase) in the SLM and CNC groups was stronger than that in the CAST group (Fig. 3). A higher proportion of FCC phase in SLM and CNC specimens exposed to a higher cooling rate, cold rolling, and annealing treatments, and eventually the overall properties of alloy were remarkably improved by grain refinement.
In terms of chemical composition, although the presence of Cr-rich carbide is associated with the formation and thickness of the surface oxide film of Co alloy,
the EDS determined no apparent difference in Cr content among the groups (Table 3). The addition of Mo improves the corrosion performance by refining the grain and optimizing the wettability between carbides and binder
A link between ion release and some oral symptoms, such as metallic taste, gray mucositis, and grey-bluish discoloration of the gingiva, has been reported.
The qualitative and quantitative analyses of the released elements are regarded as the most effective measure of the corrosion behavior and biological effect of metallic biomaterial.
reported the toxicity rank order (Cr6+>Co2+>Cr3+) by assessing corrosion products produced by the Co-Cr alloy. In the present work, factors contributing to corrosion were investigated, with an increase in ion release found with increased immersion time and decreased pH value. The most notable growth tendency and the highest concentrations of Co, Cr, and Mn ions were observed in the CAST group, while the ion release of the CNC group barely grew over the 7-week test period. A low pH level has been reported to cause the electrochemical equilibrium to shift toward metal dissolution and increase ion release.
The release of Co and Cr ions interacted with both the fabrication method and pH level (P<.005). Fewer metal ions have been previously reported to be released from the CNC and SLM Co-Cr specimens after immersion in artificial saliva at pH 2.3 and 7.1 for 15 and 30 days.
Overall, given the ion release and its sensitivity to time and pH, it was concluded that the SLM and CNC Co-Cr alloy presented a more long-lasting resistance to corrosion than the CAST Co-Cr alloy.
EIS has been extensively used to study the electrode surface reactions, with the electrochemical reaction mechanisms being analyzed by recording the variation relations between impedance and frequency after creating small perturbations to the reaction system.
reported the improved corrosion resistance of SLM Co-Cr alloy with EIS for the microstructural homogeneity compared with that of CAST Co-Cr alloy. A positive correlation has been reported between the radius of the capacitive reactance arc in the Nyquist diagram and the Rct value of the investigated electrode.
Figure 6B shows that all specimens had similar arc shapes, suggesting that the corrosion mechanism was similar, while the SLM and CNC groups exhibited a larger arc than the CAST group. With the method of fitting, the Rct values in descending order were SLM (250.06 ±37.40 kΩ∙cm2), CNC (128.70 ±32.26 kΩ∙cm2), and CAST (54.70 ±14.60 kΩ∙cm2). A larger Rct value represents a more difficult loss of electrons and release of metal ions.
The magnitude Bode plots analysis indicated that the resistance values of SLM and CNC groups were significantly larger than that of the CAST group in the low-frequency region (10-2 to 100 Hz) and that the values of the middle- and high-frequency regions were close to zero for all specimens, suggesting a solution impedance that lacked the time-related feature (Fig. 6E).
Therefore, the SLM and CNC Co-Cr alloy showed better electrochemical corrosion resistance than the CAST Co-Cr alloy through a short-term EIS measurement.
Although the metal ion concentrations of all specimens were much lower than 200 μg·cm-2 over a 1-week immersion according to the International Organization for Standardization (ISO) standard,
it remains unclear whether metal ion released from restorations has an impact on the patient; however, the electrochemical reaction would extend over 5 to 10 years in the oral cavity.
and the static and dynamic effect of the saliva, temperature, pH, and stress on restorations should be further investigated. Limitations of this in vitro study included that the Co-Cr alloys tested did not have identical chemical compositions. For toxicity assessments, the effect of metal ions of different valences should also be taken into consideration.
Conclusions
Based on the findings of this in vitro study, the following conclusions were drawn:
1.
The surface properties and corrosion resistance of Co-Cr dental alloys were affected by the fabrication method.
2.
SLM-printing and CNC-milling exhibit a more homogenous microstructure and better corrosion resistance than the casting process.
The authors thank Huajian Liu for the ICP-MS testing and Shanfeng Huang and Keke Feng from the School of Materials Science and Engineering, Fuzhou University, for help with EIS measurement.
References
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Kakar K.
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Which mechanical and physical testing methods are relevant for predicting the clinical performance of ceramic-based dental prostheses?.
Selective laser melting technique of Co-Cr dental alloys: a review of structure and properties and comparative analysis with other available techniques.
ISO 13565-1: Geometrical product specifications (GPS)-Surface texture: Profile method; surfaces having stratified functional properties. Part 1: Filtering and general measurement conditions.
Supported by National Natural Science Foundation of China (grant no. 81870794), Science and Technology Innovation Joint Project of Fujian Province (grant no. 2018Y9103), and Medical Innovation Project, Health and Family Planning Commission of Fujian Province (grant no. 2018-CXB-12).
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