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Metallurgical, surface, and corrosion analysis of Ni–Cr dental casting alloys before and after porcelain firing
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 378–385
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Metallurgical, surface, and corrosion analysis of Ni–Cr dental casting alloys before and after porcelain firing Hsin-Yi Lin a,∗ , Bonnie Bowers b , John T. Wolan c , Zhuo Cai d , Joel D. Bumgardner e a
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec 3, Zhongxiao E Rd., Taipei 106, Taiwan b Department of Biological Engineering, Mississippi State University, Mississippi State, MS, USA c Department of Chemical Engineering, University of South Florida, Tampa, FL, USA d Department of Biomaterials Science, Baylor College of Dentistry, Dallas, TX, USA e Department of Biomedical Engineering, University of Memphis, Memphis, TN, USA
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Article history:
Objectives. A porcelain veneer is often fired on nickel–chromium casting alloys used in dental
Received 29 January 2007
restorations for aesthetic purposes. The porcelain-fused-to-metal (PFM) process brings the
Accepted 5 June 2007
temperature to over 950 ◦ C and may change the alloy’s corrosion properties. In this study, the metallurgical, surface, and corrosion properties of two Ni–Cr alloys were examined, before and after PFM firing.
Keywords:
Methods. Two types of alloy were tested—a high Cr, Mo alloy without Be and a low Cr, Mo
Biomaterials
alloy with Be. Before the PFM firing, specimens from both alloys were examined for their
Ni–Cr dental casting alloys
microstructures, hardness, electrochemical corrosion properties, surface composition, and
Corrosion
metal ion release. After the PFM firing, the same specimens were again examined for the
Biocompatibility
same properties.
Surface analysis
Results. Neither of the alloys showed any differences in their electrochemical corrosion prop-
Metal ion release
erties after the PFM firing. However, both alloys exhibited new phases in their microstructure
X-ray diffraction
and significant changes in hardness after firing. In addition, there was a slight increase in
Metallography
CrOx on the surface of the Be-free alloy and increased Mo–Ni was observed on the surface
Hardness
of both alloys via X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). This might be one of the reasons why both alloys had increased Ni and Mo ion release after firing. Significance. The PFM firing process changed the alloys’ hardness, microstructure, and surface composition. No significant changes in the alloys’ corrosion behavior were observed, however, the significant increase in metal ion release over a month may need to be further investigated for its clinical effects. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nickel–chromium casting alloys used in dental crown and bridge applications may, for aesthetic purposes, have a porcelain veneer fired onto the restoration. This type of metal–ceramic restoration is referred to as porcelain-fused-
∗
to-metal (PFM). While most nickel–chromium restorations perform well clinically, corrosion products and components of these alloys are known to have the potential to cause hypersensitivity and other tissue reactions [1,2]. Hence, it is critical that corrosion and metal ion release from the alloys be kept at a minimum. The PFM firing process is conducted in a vac-
Corresponding author. Tel.: +886 2 27712171; fax: +886 2 27317117. E-mail address: [email protected] (H.-Y. Lin). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.06.010
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uum by means of three to four stages from 950 to 1010 ◦ C. Often, only the facial aspect of the restoration is veneered leaving the lingual and occlusal aspect exposed, as well as the sub-gingival margins. To keep the release of alloy corrosion products from exposed surfaces to host tissues minimal, it is important to evaluate the effects of PFM firing processes on alloy corrosion behavior. Corrosion properties of nickel–chromium alloys depend on their bulk composition [3,4], microstructure and development of protective surface oxides [5,6]. Alloys with 16–27% Cr, 6–17% Mo, and no Be display homogeneous protective surface oxides, low corrosion rates, large passivation ranges, and resistance to pitting/crevice corrosion in electrochemical corrosion tests [3,6–8]. Be containing alloys exhibit higher corrosion rates, smaller passivation ranges, and increased susceptibility to pitting/crevice corrosion as compared to non-Be containing alloys [4,7,8], which results from a Ni–Be eutectic formed in the microstructure of Ni–Cr–Be alloys that leads to the development of non-homogeneous Cr surface oxides which are less corrosion resistant [7]. High-temperature heat treatments and simulated PFM firing processes have also been shown to affect alloy microstructures, surface oxides, corrosion, and physical properties [6,9–16]. Ni–Cr alloys subjected to heat treatments at temperatures used in PFM processes have exhibited coarsening of the dendritic microstructure, and decreases in alloy hardness and strength [13–16]. Roach et al. [6] found that, after the PFM firing procedure, alloys with 14–22% Cr and 9–17% Mo and no Be had a decrease in Cr and Mo content in the surface oxide and showed an increase in corrosion, though not significant in every case. However, PFM firing did not significantly affect surface oxide composition or corrosion rates of alloys containing lower levels of Cr and Mo and or Be. While Roach et al. noted changes in surface oxide composition with corrosion, the test corrosion electrolyte contained an unusually high concentration of chlorides, which limits its clinical relevance. In a related study, the anodic corrosion behavior of three commercial non-Be low Mo containing Ni–Cr alloys were not affected by heat treatments similar to those experienced during PFM firing [17]. However, this study did not evaluate changes in microstructure or other properties, nor did their heat treatment (950 ◦ C for 20 min) simulate clinical conditions of multiple heat treatments under vacuum conditions [6]. Understanding the corrosion behavior of Ni–Cr dental alloys in the oral environment is vital to their biocompatibility. Alloy properties may be affected by many factors including PFM firing processes which may, in turn, alter alloy surface oxides and corrosion properties. The purpose of this study was to evaluate the effects of a clinical PFM firing process on the microstructure, hardness, surface oxide composition, and corrosion behavior of two commercial Ni–Cr alloys. This information will advance the understanding of clinical performance and the health benefit–risks of these alloys.
2.
Materials and methods
Two commercial Ni–Cr dental casting alloys for PFM applications were selected to represent high Cr and Mo containing
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alloys (Litecast, Ivoclar Vivadent Inc., Amherst, NY) and low Cr and Mo containing alloys with Be additions (NPXIII, Nobilium, Albany, NY). Manufacturers reported Litecast alloy contains (wt%) 68.5% Ni, 15.5% Cr, 14.0% Mo, and 1.0% Al and NPXIII alloy contains 76.5% Ni, 14.0% Cr, 4.5% Mo, 2.5% Al, 1.6% Be, 0.5% Co, and 0.4% Ti. The alloys were flame-cast using a 50/50 oxygen–propane gas mixture into cylinders (0.15 cm diameter × 1 cm length) and discs (1 cm diameter × 0.3 cm thickness) by a private dental laboratory (Oral Tech, Pearl, MS). The cast specimens were wet-polished using silicon carbide abrasive sandpaper up to 1500 grit to simulate clinical procedures [6], then ultrasonically cleaned for 5 min each in acetone, ethanol, and de-ionized water to remove surface contaminants. The microstructure, hardness, surface composition, and corrosion properties of the alloys were evaluated. Subsequently, the alloy specimens were re-polished to 1500 grit, and then subjected to a PFM firing cycle in a dental porcelain furnace at the School of Dentistry, University of Mississippi, Jackson, MS [6]. 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 fired at 970 ◦ C under vacuum and air cooled, and finally glaze fired at 980 ◦ C and air cooled. The specimens were re-polished as above, and their microstructure, hardness, surface composition, and corrosion properties were reevaluated.
2.1.
Microstructural evaluation
Two discs of each were prepared metallographically. Here, the alloy specimens were mounted in Bakelite (manufacturer, location), wet-polished to 1 m alumina on metallographic polishing tables, cleaned with acetone and distilled water and then air dried. Specimens were etched according to ASTM E407-70. Due to the resistance of the alloys to etching and difficulties with the etching process, the same etchant was not used for both alloys. For the non-Be containing Litecast alloy, the specimens were etched using 10 ml HNO3 and 40 ml HCl for 20 s. For the Be containing NPXIII alloy, the specimens were electrolytically etched at 6 V in 10% (w/v) oxalic acid in distilled water solution for 1 min and 40 s. The microstructure of the alloys was observed using an Olympus PME3 optical microscope and photomicrographs were taken at 100× and 500×. The average dendrite arm spacing in pixels was calculated using a Leco Image Analysis System. Values were determined from 5 randomly selected images of each alloy specimen and 30 measurements were made for each image. Differences in dendrite arm spacing due to the PFM process for each alloy were determined using Student’s t-test at a 95% level of significance.
2.2.
Hardness testing
The hardness of the alloy specimens used in the metallographic analyses was measured using a Clark Rockwell Hardness Tester, Model CTT (Leco Co., St. Joseph, MI). Since the discs were only 3 mm thick, all tests were conducted under superficial mode using a minor load of 10 kgf and a major load of 30 kgf. A 1/16 in. ball-penetrator and 30 T scale was used for Litecast and a diamond penetrator and 30 N scale were used for NPXIII. Rockwell hardness values were then converted to
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Vickers hardness values for comparison. Differences in the hardness of the PEM treated alloys were also determined using Student’s t-test at 95% level of significance.
2.3.
Surface analysis
X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the surface and near-surface of the alloy before and after PFM firing. Two specimens and two representative spots on each surface were examined. Specimens were polished and cleaned before and after PFM as described previously. XPS was performed (Model 1600 surface analysis system, Physical Electronics, Eden Praire, MN) utilizing a Mg K␣ electrode at 12 kV and 200 W at a 30◦ takeoff angle. The analysis area was approximately 800 m in diameter. Survey and high-resolution spectra were obtained using pass energies of 49.5 and 23.5 eV, respectively. Reference binding energies of each element were obtained from the National Institute of Standards and Technology (NIST) XPS Online Database. All spectral features were referenced to the binding energy of adventitious carbon (284.6 eV). X-ray diffraction (XRD) was used to obtain bulk information on metallurgy and phase composition of the before and after treatment specimens. XRD patterns were obtained at room temperature over a 2 range of 20–90◦ using a diffractometer (Miniflex CN2005, Rigaku Americas Co., Tokyo, Japan) equipped with a Ni filter and calibrated using a silicon standard (640b Silicon powder XRD spacing, Standard Reference material, National Bureau of Standards, Gaithersburg, MD, USA). The diffractometer was operated with a Cu K␣ electrode at 0.02◦ step−1 and a 6 s/step photon counting time. Features on the XRD pattern were identified using Jade 3.1 computer software (MDI, Livermore, CA, USA) and indexed to the International Center for Diffraction Data (ICDD, Swathmore, PA) Powder Diffraction Files (Data Sets 1-48, 1998).
2.4. Electrochemical impedance spectroscopy (EIS) corrosion test and metal ion release Corrosion tests were performed using an EG&G Model 6310 electrochemical impedance analyzer via a test cell specified in ASTM G5-94 with a high-purity platinum wire across the salt bridge, a standard calomel reference electron (SCE) and graphite auxiliary electrodes. Corrosion tests were performed on triplicate cylindrical specimens of each alloy in an artificial saliva solution (1.5 g/l KCl, 1.5 g/l NaHCO3 , 0.5 g/l NaH2 PO4 –H2 O, 0.5 g/l KSCN, 0.9 g/l lactic acid, pH 4.8) [6] at room temperature and pressure. Each specimen was allowed to reach open circuit potential (Ecorr ) for 1 h. The Ecorr was recorded and then a 10 mV amplitude sine wave potential was applied through a frequency range of 100 kHz to 500 Hz. Consecutive multi-sine and single-sine combination tests using the EG&G impedance software (EIS Software Model 398) were implemented to expedite the determination of polarization resistance (Rp ) and double layer capacitance (Cdl ) of the alloys. Rp is used as an indicator of corrosion resistance and is inversely proportional to corrosion rate (Icorr = B/Rp ; B = ba bc /(ba + bc ), where ba is the anodic Tafel slope and bc is the cathodic Tafel slope). Cdl is the surface oxide capacitance and is an indicator of the insulating or protective property of the
alloy surface. Bode impedance (frequency versus impedance), Bode phase (frequency versus phase shift), and complex plane capacitance (real versus imaginary) plots were used to determine Rp and Cdl values. Dissolution tests were performed using two cylinders and two discs of the alloys immersed in artificial saliva solution at 3:1 volume of solution to surface area of specimens to evaluate the types and relative amounts of metal ions released from the alloys. Test alloy specimens were incubated in closed sterile 50 ml centrifuge tubes at 37 ◦ C and solutions were changed every 2–3 days for 30 days. Solutions were analyzed for Ni, Cr, Mo, and Be ion release by inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Optima 4300DV, MA) using matrix matched standards.
2.5.
Statistical analysis
Data from dendrite arm spacing, hardness, corrosion resistance and capacitance, metal ion release, was statistically analyzed using two-way analysis of variance (ANOVA). Student’s t-test was used to compare differences in numerical results before and after the PFM firing. Significance is declared when p < 0.05.
3.
Results
3.1.
Microstructure
The microstructure of the test alloys before and after PFM firing are shown in Fig. 1(a and b). Both alloys exhibited a solid solution matrix in a typical dendritic arrangement. NPXIII alloys (Fig. 1(b)) contained script metal precipitates in the interdendritic eutectic structure. After PFM firing, a new second phase was observed in the microstructure of Litecast (Fig. 1(a)), the Be-free alloy; additional interdendritic precipitates were apparent in the eutectic structure of the Be containing NPXIII alloy. Since different etching procedures were used for each alloy, direct comparison of dendrite arm spacing between alloy microstructures is not possible. Table 1 shows the average number of five dendrite arm spacing measurements from Litecast and NPXII alloys before and after PFM firing. The average numbers of dendrite arm spacing for either alloy after PFM firing, although slightly increased, did not increase significantly.
3.2.
Hardness
The Vickers hardness (Hv) for each alloy before and after PFM firing is shown in Table 1. The hardness of LC was significantly lower than that of NPXIII in both test conditions. The hardness of LC increased after PFM firing (p < 0.01) while the hardness of NPXIII decreased (p < 0.01) after firing.
3.3.
Surface analysis (XPS)
The XPS survey spectra of Litecast and NPXIII alloys are shown in Fig. 2(a and b), respectively. After PFM firing, carbon (C 1s), oxygen (O 1s), and molybdenum (Mo 3d) peak intensities increased in the near-surface region of the Litecast alloy
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 378–385
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Fig. 1 – (a) Metallography pictures of Be-free Litecast alloy before (L) and after (R) the PFM firing process. Magnification = 500×. A new interdendric second phase appeared after the alloy was fired. (b) Metallography pictures of Be containing NPXIII alloy before (L) and after (R) the PFM firing process. Magnification = 500×. Additional precipitations appeared in the eutectic structure after firing.
(Fig. 2(a)). The NPXIII alloys exhibited an increase in C 1s, O 1s, Mo 3d, and Ni 2p peak intensities after PFM firing (Fig. 2(b)). High-resolution analysis (results not shown) of the binding energies indicated that molybdenum and nickel were in the form of MoO3 and NiO2 , respectively, carbon existed primarily as carbonyl compounds (C O), and oxygen was present in hydroxyl compounds (O–H) as well as the metal Mo and Ni oxides.
3.4.
Surface analysis (XRD)
Representative X-ray diffraction patterns for the cast alloys before and after PFM firing are shown in Fig. 3(a and b). Consistent results were obtained from the two specimens. For the Be containing NPXIII, large prominent peaks for orthorhombic MoNi and cubic BeNi phases and smaller peaks for Cr3 Ni2 phase were identified on the as-polished specimens (Fig. 3(a),
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Table 1 – List of changes in dendritic arm spacing, hardness, corrosion resistance, and surface oxide double layer capacitance of Be-free Litecast alloy and Be containing NPXIII alloys before and after the PFM firing Alloys
Condition
Dendrite arm spacing (pixel) (n = 5)
Litecast (Be-free)
Before PFM After PFM
8.10 (1.64) 10.69 (2.01)
NPXIII (Be-addition)
Before PFM After PFM
18.61 (3.09) 20.23 (1.81)
Vickers hardness (Hv) (n = 6)
Corrosion resistance, Rp () (n = 4)
Surface oxide double layer capacitance, Cdl (F) (n = 4)
138.5 (4.4)a 149.3 (6.3)b
60.3 (20.7) 54.8 (28.5)
24.9 (1.6) 29.9 (5.0)
354.5 (8.1)c 281.2 (11.5)d
7.4 (7.1) 7.7 (2.8)
25.0 (1.4) 32.5 (7.8)
For both alloys, there was no significant increase in dendritic arm spacings (pixel) in their microstructures after the PFM firing. The hardness (Hv) of Litecast increased and that of NPXIII decreased after the PFM firing. Both changes were significant (p < 0.01). The corrosion resistance (Rp , ) of neither alloy changed after the PFM firing. The double layer capacitance (Cdl , F) of neither alloy changed after the PFM firing. All values are expressed in the format: average (standard deviation); n: number of specimens. Values with “a and b” are statistically different; values with “c and d” are statistically different.
Fig. 2 – (a) X-ray photoelectron spectroscopy survey spectra of Be-free Litecast dental alloy before and after PFM firing. Peak intensities of Mo 3d, C 1s, and O 1s increased after firing. (b) X-ray photoelectron spectroscopy survey spectra of Be containing NPXIII dental alloy before and after PFM firing. Peak intensities of Mo 3d, Ni 2p, C 1s, and O 1s increased after firing.
Fig. 3 – (a) XRD diffractogram of Be containing NPXIII alloy before (top) and after (bottom) PFM firing. (b) XRD diffractogram of Be-free Litecast alloy before (top) and after (bottom) PFM firing.
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˚ from the as-polished top). A low angle peak (d = 3.432 A) specimens was not identified. On the PFM fired specimens, additional BeO and Cr2 O3 phases were identified (Fig. 3(a), bottom). For the beryllium-free Litecast alloy, large prominent peaks for MoNi and Cr3 Ni2 phases were identified on as-polished specimens (Fig. 3(b), top). For re-polished PFM fired specimens, prominent peaks for Cr2 O3 phases were identified along with MoNi phases (Fig. 3(b), bottom).
3.5. Corrosion resistance (Rp ) and surface double layer capacitance (Cdl ) Fig. 4 shows typical impedance plots for both alloys before and after PFM firing. Results from the EIS corrosion tests indicated that the Litecast alloy (non-Be containing alloy) exhibited approximately ten times higher corrosion resistance (Rp ) than the Be containing alloy, NPXIII, both before and after the PFM treatment (Table 1). The surface oxide double layer capacitance (Cdl ) of both alloys was similar for both the as-polished and after the PFM firing and polished (Table 1). Corrosion resistance (Rp ) and metal oxide double
Fig. 4 – Typical impedance plots from EIS test for Litecast and NPXIII.
layer capacitance (Cdl ) of both alloys did not change significantly after PFM firing. Though not significant, the capacitance values of both alloys seem to increase slightly after the firing process.
Fig. 5 – Total metal ion release of (a) Ni, (b) Cr, (c) Mo, and (d) Be over a month for Litecast (LC) and NPXIII dental alloys before and after firing. In general, the Ni, Cr, Mo ion release of both alloys increased after PFM firing process. Be release in NPX III increased after firing. Asterisk (*) indicates statistical significance between two sets of data.
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3.6. Corrosion (total metal ion release measured by ICP-AES) Fig. 5 shows the cumulative metal ion release from Litecast and NPXIII before and after PFM firing over 30 days in artificial saliva. Generally speaking, total metal ion release of Ni, Cr, and Mo from both alloys increased after PFM firing. Some larger error bars may be due to the limited number of specimens (four from each alloy) and the low metal ion concentration. For Litecast alloy, ion release of Ni, Cr, and Mo increased significantly after the PFM firing. The only metal ion that increased significantly in NPXIII was Mo.
4.
Discussion
Bauers et al. [18] reported that the chemical composition in the interdendritic eutectic structure of a Ni–Cr–Be alloy was Ni–Be. The XRD data in the current study showed that there were prominent Be–Ni and Mo–Ni peaks on the PFM fired specimens. Hence the additional precipitation appeared in the eutectic structure in NPXIII after PFM firing (Fig. 1(b)) could be Ni–Be and/or Mo–Ni. The formation of BeO on the oxidized alloy surface was reported in an earlier study [1] on a similar Ni–Cr–Be alloy. This result from the XRD test showing an increase in Mo–Ni and Be–Ni after PFM is consistent with the XPS (Fig. 2(a and b)) result that shows an increase in Mo and Ni peak after the PFM firing and results from the dissolution (Fig. 5) tests that show increases in Mo, Ni and Be release from the alloy specimens. For the Be-free Litecast alloy, the XRD result showed MoNi and Cr3 Ni2 existed before PFM. After the PFM firing, MoNi and small Cr2 O3 peaks appeared on the surface. This might explain why Ni and Mo ion release increased after the PFM firing. This was consistent with the finding in XPS that Mo 3d and Ni peak intensities increased after the PFM firing. The second phase that appeared on Litecast after PFM could be CrMo. This might contribute to the significant increase in Cr and Mo ion release from Litecast after PFM firing. The surface examination from XPS showed no significant difference before and after PFM, except for the increase in Mo and Ni peak intensities (Fig. 2(a and b)). These peaks appeared not to come from the formation of protective surface metal oxides such as MoOx but from the formation of Mo–Ni intermetallic precipitates based on XRD results (Fig. 3). As a result, very little difference in the alloys’ corrosion resistance (Rp ) (Table 1) and double layer capacitance (Cdl ) (Table 1) were observed. However, there were definite changes in the bulk composition for both alloys (Fig. 1) resulting in significant changes in hardness (Table 1), total metal ion release (Fig. 5), and a new precipitate observed in the micrographs. Though not statistically significant, the slight increase in Cdl (Table 1) of both alloys after PFM firing indicated slightly lower corrosion resistance. This lower corrosion resistance is consistent with the increased levels of released metal ions from the specimen after the PFM firing procedure. Morris [16] found a statistically significant reduction in strength after porcelain firing in a Ni–Cr–Be alloy and a high-strength Ni–Cr alloy without Be. Winkler et al. [14] and
Marinello et al. [15] demonstrated a decrease in hardness of Ni–Cr alloys as a result of heat treatment. In the current experiment, similar results were found for both the Be containing alloy and NPXIII. The Vickers hardness value of NPXIII was also very close to other literature values [18]. However, the current test showed Litecast alloy had a Vickers hardness value of around 140, which was about one-third of what Bauer et al. found for non-Be containing Ni–Cr alloys (around 420). Also, the Litecast alloy used in this study had an increase in hardness after PFM firing. This result was the opposite of the effect of PFM firing on a high-strength Ni–Cr alloy described by Morris, which had a decrease in hardness after PFM. The difference in the hardness results for the non-Be containing alloy used in this study and those of Bauer et al. [18] and Morris [16] may be due in part to the alloys’ bulk composition. The Litecast alloy used in this study had a 15% Mo content without Ti and Nb additions, whereas the Ni–Cr alloys used by Bauer et al. had about 5% of Mo and 4% of Ti or Nb. Among all the metal ions evaluated, the increase in Ni ion release was the greatest after PFM firing, especially for the Be containing alloy (∼11 ppm) (Fig. 5(a)). This was consistent with the results from Roach et al. [6] who found Ni–Cr alloys’ corrosion rates increased after PFM firing. Bumgardner and Lucas [4] and Bumgardner et al. [19] also found Be containing Ni–Cr-based alloys released more Ni ions than Be-free Ni–Cr alloys. They reported increased Ni ion concentration caused decreased cell proliferation and cellular energy metabolism without changing cellular morphology and viability. Ni ions, known to cause hypersensitivity, have been reported to indirectly cause lipopolysaccharide (LPS) induced human monocytes to release pro-inflammatory cytokines [20,21]. The Ni ion concentrations in the current study (maximum 200 M) fell in the range of the reported dose (0–90 M) [22] that caused LPS-activated monocytes to release more cytokine than the non-activated ones. Further investigation of the in vivo impact of Ni-ion release from Be containing alloys is required, especially at inflammatory sites.
5.
Conclusions
PFM firing changed the alloys’ bulk composition and thus their hardness values. New surface oxides were formed on the alloy surfaces after the PFM firing. The changes on the surface was not significant enough to change the alloys’ corrosion resistance measured by potentiostat, however, there were significant increases in metal ion release detected by ICPAES in vitro after a month. The in vivo effects of Ni ion release from Ni–Cr alloys after PFM firing requires further investigation.
Acknowledgements This work is supported by a Faculty Research Grant from the Office of Research at Mississippi State University and by the Mississippi Agricultural and Forestry Experiment Station, Starkville, MS.
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journal homepage: www.intl.elsevierhealth.com/journals/dema
Metallurgical, surface, and corrosion analysis of Ni–Cr dental casting alloys before and after porcelain firing Hsin-Yi Lin a,∗ , Bonnie Bowers b , John T. Wolan c , Zhuo Cai d , Joel D. Bumgardner e a
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec 3, Zhongxiao E Rd., Taipei 106, Taiwan b Department of Biological Engineering, Mississippi State University, Mississippi State, MS, USA c Department of Chemical Engineering, University of South Florida, Tampa, FL, USA d Department of Biomaterials Science, Baylor College of Dentistry, Dallas, TX, USA e Department of Biomedical Engineering, University of Memphis, Memphis, TN, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. A porcelain veneer is often fired on nickel–chromium casting alloys used in dental
Received 29 January 2007
restorations for aesthetic purposes. The porcelain-fused-to-metal (PFM) process brings the
Accepted 5 June 2007
temperature to over 950 ◦ C and may change the alloy’s corrosion properties. In this study, the metallurgical, surface, and corrosion properties of two Ni–Cr alloys were examined, before and after PFM firing.
Keywords:
Methods. Two types of alloy were tested—a high Cr, Mo alloy without Be and a low Cr, Mo
Biomaterials
alloy with Be. Before the PFM firing, specimens from both alloys were examined for their
Ni–Cr dental casting alloys
microstructures, hardness, electrochemical corrosion properties, surface composition, and
Corrosion
metal ion release. After the PFM firing, the same specimens were again examined for the
Biocompatibility
same properties.
Surface analysis
Results. Neither of the alloys showed any differences in their electrochemical corrosion prop-
Metal ion release
erties after the PFM firing. However, both alloys exhibited new phases in their microstructure
X-ray diffraction
and significant changes in hardness after firing. In addition, there was a slight increase in
Metallography
CrOx on the surface of the Be-free alloy and increased Mo–Ni was observed on the surface
Hardness
of both alloys via X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). This might be one of the reasons why both alloys had increased Ni and Mo ion release after firing. Significance. The PFM firing process changed the alloys’ hardness, microstructure, and surface composition. No significant changes in the alloys’ corrosion behavior were observed, however, the significant increase in metal ion release over a month may need to be further investigated for its clinical effects. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nickel–chromium casting alloys used in dental crown and bridge applications may, for aesthetic purposes, have a porcelain veneer fired onto the restoration. This type of metal–ceramic restoration is referred to as porcelain-fused-
∗
to-metal (PFM). While most nickel–chromium restorations perform well clinically, corrosion products and components of these alloys are known to have the potential to cause hypersensitivity and other tissue reactions [1,2]. Hence, it is critical that corrosion and metal ion release from the alloys be kept at a minimum. The PFM firing process is conducted in a vac-
Corresponding author. Tel.: +886 2 27712171; fax: +886 2 27317117. E-mail address: [email protected] (H.-Y. Lin). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.06.010
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uum by means of three to four stages from 950 to 1010 ◦ C. Often, only the facial aspect of the restoration is veneered leaving the lingual and occlusal aspect exposed, as well as the sub-gingival margins. To keep the release of alloy corrosion products from exposed surfaces to host tissues minimal, it is important to evaluate the effects of PFM firing processes on alloy corrosion behavior. Corrosion properties of nickel–chromium alloys depend on their bulk composition [3,4], microstructure and development of protective surface oxides [5,6]. Alloys with 16–27% Cr, 6–17% Mo, and no Be display homogeneous protective surface oxides, low corrosion rates, large passivation ranges, and resistance to pitting/crevice corrosion in electrochemical corrosion tests [3,6–8]. Be containing alloys exhibit higher corrosion rates, smaller passivation ranges, and increased susceptibility to pitting/crevice corrosion as compared to non-Be containing alloys [4,7,8], which results from a Ni–Be eutectic formed in the microstructure of Ni–Cr–Be alloys that leads to the development of non-homogeneous Cr surface oxides which are less corrosion resistant [7]. High-temperature heat treatments and simulated PFM firing processes have also been shown to affect alloy microstructures, surface oxides, corrosion, and physical properties [6,9–16]. Ni–Cr alloys subjected to heat treatments at temperatures used in PFM processes have exhibited coarsening of the dendritic microstructure, and decreases in alloy hardness and strength [13–16]. Roach et al. [6] found that, after the PFM firing procedure, alloys with 14–22% Cr and 9–17% Mo and no Be had a decrease in Cr and Mo content in the surface oxide and showed an increase in corrosion, though not significant in every case. However, PFM firing did not significantly affect surface oxide composition or corrosion rates of alloys containing lower levels of Cr and Mo and or Be. While Roach et al. noted changes in surface oxide composition with corrosion, the test corrosion electrolyte contained an unusually high concentration of chlorides, which limits its clinical relevance. In a related study, the anodic corrosion behavior of three commercial non-Be low Mo containing Ni–Cr alloys were not affected by heat treatments similar to those experienced during PFM firing [17]. However, this study did not evaluate changes in microstructure or other properties, nor did their heat treatment (950 ◦ C for 20 min) simulate clinical conditions of multiple heat treatments under vacuum conditions [6]. Understanding the corrosion behavior of Ni–Cr dental alloys in the oral environment is vital to their biocompatibility. Alloy properties may be affected by many factors including PFM firing processes which may, in turn, alter alloy surface oxides and corrosion properties. The purpose of this study was to evaluate the effects of a clinical PFM firing process on the microstructure, hardness, surface oxide composition, and corrosion behavior of two commercial Ni–Cr alloys. This information will advance the understanding of clinical performance and the health benefit–risks of these alloys.
2.
Materials and methods
Two commercial Ni–Cr dental casting alloys for PFM applications were selected to represent high Cr and Mo containing
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alloys (Litecast, Ivoclar Vivadent Inc., Amherst, NY) and low Cr and Mo containing alloys with Be additions (NPXIII, Nobilium, Albany, NY). Manufacturers reported Litecast alloy contains (wt%) 68.5% Ni, 15.5% Cr, 14.0% Mo, and 1.0% Al and NPXIII alloy contains 76.5% Ni, 14.0% Cr, 4.5% Mo, 2.5% Al, 1.6% Be, 0.5% Co, and 0.4% Ti. The alloys were flame-cast using a 50/50 oxygen–propane gas mixture into cylinders (0.15 cm diameter × 1 cm length) and discs (1 cm diameter × 0.3 cm thickness) by a private dental laboratory (Oral Tech, Pearl, MS). The cast specimens were wet-polished using silicon carbide abrasive sandpaper up to 1500 grit to simulate clinical procedures [6], then ultrasonically cleaned for 5 min each in acetone, ethanol, and de-ionized water to remove surface contaminants. The microstructure, hardness, surface composition, and corrosion properties of the alloys were evaluated. Subsequently, the alloy specimens were re-polished to 1500 grit, and then subjected to a PFM firing cycle in a dental porcelain furnace at the School of Dentistry, University of Mississippi, Jackson, MS [6]. 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 fired at 970 ◦ C under vacuum and air cooled, and finally glaze fired at 980 ◦ C and air cooled. The specimens were re-polished as above, and their microstructure, hardness, surface composition, and corrosion properties were reevaluated.
2.1.
Microstructural evaluation
Two discs of each were prepared metallographically. Here, the alloy specimens were mounted in Bakelite (manufacturer, location), wet-polished to 1 m alumina on metallographic polishing tables, cleaned with acetone and distilled water and then air dried. Specimens were etched according to ASTM E407-70. Due to the resistance of the alloys to etching and difficulties with the etching process, the same etchant was not used for both alloys. For the non-Be containing Litecast alloy, the specimens were etched using 10 ml HNO3 and 40 ml HCl for 20 s. For the Be containing NPXIII alloy, the specimens were electrolytically etched at 6 V in 10% (w/v) oxalic acid in distilled water solution for 1 min and 40 s. The microstructure of the alloys was observed using an Olympus PME3 optical microscope and photomicrographs were taken at 100× and 500×. The average dendrite arm spacing in pixels was calculated using a Leco Image Analysis System. Values were determined from 5 randomly selected images of each alloy specimen and 30 measurements were made for each image. Differences in dendrite arm spacing due to the PFM process for each alloy were determined using Student’s t-test at a 95% level of significance.
2.2.
Hardness testing
The hardness of the alloy specimens used in the metallographic analyses was measured using a Clark Rockwell Hardness Tester, Model CTT (Leco Co., St. Joseph, MI). Since the discs were only 3 mm thick, all tests were conducted under superficial mode using a minor load of 10 kgf and a major load of 30 kgf. A 1/16 in. ball-penetrator and 30 T scale was used for Litecast and a diamond penetrator and 30 N scale were used for NPXIII. Rockwell hardness values were then converted to
380
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Vickers hardness values for comparison. Differences in the hardness of the PEM treated alloys were also determined using Student’s t-test at 95% level of significance.
2.3.
Surface analysis
X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the surface and near-surface of the alloy before and after PFM firing. Two specimens and two representative spots on each surface were examined. Specimens were polished and cleaned before and after PFM as described previously. XPS was performed (Model 1600 surface analysis system, Physical Electronics, Eden Praire, MN) utilizing a Mg K␣ electrode at 12 kV and 200 W at a 30◦ takeoff angle. The analysis area was approximately 800 m in diameter. Survey and high-resolution spectra were obtained using pass energies of 49.5 and 23.5 eV, respectively. Reference binding energies of each element were obtained from the National Institute of Standards and Technology (NIST) XPS Online Database. All spectral features were referenced to the binding energy of adventitious carbon (284.6 eV). X-ray diffraction (XRD) was used to obtain bulk information on metallurgy and phase composition of the before and after treatment specimens. XRD patterns were obtained at room temperature over a 2 range of 20–90◦ using a diffractometer (Miniflex CN2005, Rigaku Americas Co., Tokyo, Japan) equipped with a Ni filter and calibrated using a silicon standard (640b Silicon powder XRD spacing, Standard Reference material, National Bureau of Standards, Gaithersburg, MD, USA). The diffractometer was operated with a Cu K␣ electrode at 0.02◦ step−1 and a 6 s/step photon counting time. Features on the XRD pattern were identified using Jade 3.1 computer software (MDI, Livermore, CA, USA) and indexed to the International Center for Diffraction Data (ICDD, Swathmore, PA) Powder Diffraction Files (Data Sets 1-48, 1998).
2.4. Electrochemical impedance spectroscopy (EIS) corrosion test and metal ion release Corrosion tests were performed using an EG&G Model 6310 electrochemical impedance analyzer via a test cell specified in ASTM G5-94 with a high-purity platinum wire across the salt bridge, a standard calomel reference electron (SCE) and graphite auxiliary electrodes. Corrosion tests were performed on triplicate cylindrical specimens of each alloy in an artificial saliva solution (1.5 g/l KCl, 1.5 g/l NaHCO3 , 0.5 g/l NaH2 PO4 –H2 O, 0.5 g/l KSCN, 0.9 g/l lactic acid, pH 4.8) [6] at room temperature and pressure. Each specimen was allowed to reach open circuit potential (Ecorr ) for 1 h. The Ecorr was recorded and then a 10 mV amplitude sine wave potential was applied through a frequency range of 100 kHz to 500 Hz. Consecutive multi-sine and single-sine combination tests using the EG&G impedance software (EIS Software Model 398) were implemented to expedite the determination of polarization resistance (Rp ) and double layer capacitance (Cdl ) of the alloys. Rp is used as an indicator of corrosion resistance and is inversely proportional to corrosion rate (Icorr = B/Rp ; B = ba bc /(ba + bc ), where ba is the anodic Tafel slope and bc is the cathodic Tafel slope). Cdl is the surface oxide capacitance and is an indicator of the insulating or protective property of the
alloy surface. Bode impedance (frequency versus impedance), Bode phase (frequency versus phase shift), and complex plane capacitance (real versus imaginary) plots were used to determine Rp and Cdl values. Dissolution tests were performed using two cylinders and two discs of the alloys immersed in artificial saliva solution at 3:1 volume of solution to surface area of specimens to evaluate the types and relative amounts of metal ions released from the alloys. Test alloy specimens were incubated in closed sterile 50 ml centrifuge tubes at 37 ◦ C and solutions were changed every 2–3 days for 30 days. Solutions were analyzed for Ni, Cr, Mo, and Be ion release by inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Optima 4300DV, MA) using matrix matched standards.
2.5.
Statistical analysis
Data from dendrite arm spacing, hardness, corrosion resistance and capacitance, metal ion release, was statistically analyzed using two-way analysis of variance (ANOVA). Student’s t-test was used to compare differences in numerical results before and after the PFM firing. Significance is declared when p < 0.05.
3.
Results
3.1.
Microstructure
The microstructure of the test alloys before and after PFM firing are shown in Fig. 1(a and b). Both alloys exhibited a solid solution matrix in a typical dendritic arrangement. NPXIII alloys (Fig. 1(b)) contained script metal precipitates in the interdendritic eutectic structure. After PFM firing, a new second phase was observed in the microstructure of Litecast (Fig. 1(a)), the Be-free alloy; additional interdendritic precipitates were apparent in the eutectic structure of the Be containing NPXIII alloy. Since different etching procedures were used for each alloy, direct comparison of dendrite arm spacing between alloy microstructures is not possible. Table 1 shows the average number of five dendrite arm spacing measurements from Litecast and NPXII alloys before and after PFM firing. The average numbers of dendrite arm spacing for either alloy after PFM firing, although slightly increased, did not increase significantly.
3.2.
Hardness
The Vickers hardness (Hv) for each alloy before and after PFM firing is shown in Table 1. The hardness of LC was significantly lower than that of NPXIII in both test conditions. The hardness of LC increased after PFM firing (p < 0.01) while the hardness of NPXIII decreased (p < 0.01) after firing.
3.3.
Surface analysis (XPS)
The XPS survey spectra of Litecast and NPXIII alloys are shown in Fig. 2(a and b), respectively. After PFM firing, carbon (C 1s), oxygen (O 1s), and molybdenum (Mo 3d) peak intensities increased in the near-surface region of the Litecast alloy
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 378–385
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Fig. 1 – (a) Metallography pictures of Be-free Litecast alloy before (L) and after (R) the PFM firing process. Magnification = 500×. A new interdendric second phase appeared after the alloy was fired. (b) Metallography pictures of Be containing NPXIII alloy before (L) and after (R) the PFM firing process. Magnification = 500×. Additional precipitations appeared in the eutectic structure after firing.
(Fig. 2(a)). The NPXIII alloys exhibited an increase in C 1s, O 1s, Mo 3d, and Ni 2p peak intensities after PFM firing (Fig. 2(b)). High-resolution analysis (results not shown) of the binding energies indicated that molybdenum and nickel were in the form of MoO3 and NiO2 , respectively, carbon existed primarily as carbonyl compounds (C O), and oxygen was present in hydroxyl compounds (O–H) as well as the metal Mo and Ni oxides.
3.4.
Surface analysis (XRD)
Representative X-ray diffraction patterns for the cast alloys before and after PFM firing are shown in Fig. 3(a and b). Consistent results were obtained from the two specimens. For the Be containing NPXIII, large prominent peaks for orthorhombic MoNi and cubic BeNi phases and smaller peaks for Cr3 Ni2 phase were identified on the as-polished specimens (Fig. 3(a),
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Table 1 – List of changes in dendritic arm spacing, hardness, corrosion resistance, and surface oxide double layer capacitance of Be-free Litecast alloy and Be containing NPXIII alloys before and after the PFM firing Alloys
Condition
Dendrite arm spacing (pixel) (n = 5)
Litecast (Be-free)
Before PFM After PFM
8.10 (1.64) 10.69 (2.01)
NPXIII (Be-addition)
Before PFM After PFM
18.61 (3.09) 20.23 (1.81)
Vickers hardness (Hv) (n = 6)
Corrosion resistance, Rp () (n = 4)
Surface oxide double layer capacitance, Cdl (F) (n = 4)
138.5 (4.4)a 149.3 (6.3)b
60.3 (20.7) 54.8 (28.5)
24.9 (1.6) 29.9 (5.0)
354.5 (8.1)c 281.2 (11.5)d
7.4 (7.1) 7.7 (2.8)
25.0 (1.4) 32.5 (7.8)
For both alloys, there was no significant increase in dendritic arm spacings (pixel) in their microstructures after the PFM firing. The hardness (Hv) of Litecast increased and that of NPXIII decreased after the PFM firing. Both changes were significant (p < 0.01). The corrosion resistance (Rp , ) of neither alloy changed after the PFM firing. The double layer capacitance (Cdl , F) of neither alloy changed after the PFM firing. All values are expressed in the format: average (standard deviation); n: number of specimens. Values with “a and b” are statistically different; values with “c and d” are statistically different.
Fig. 2 – (a) X-ray photoelectron spectroscopy survey spectra of Be-free Litecast dental alloy before and after PFM firing. Peak intensities of Mo 3d, C 1s, and O 1s increased after firing. (b) X-ray photoelectron spectroscopy survey spectra of Be containing NPXIII dental alloy before and after PFM firing. Peak intensities of Mo 3d, Ni 2p, C 1s, and O 1s increased after firing.
Fig. 3 – (a) XRD diffractogram of Be containing NPXIII alloy before (top) and after (bottom) PFM firing. (b) XRD diffractogram of Be-free Litecast alloy before (top) and after (bottom) PFM firing.
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˚ from the as-polished top). A low angle peak (d = 3.432 A) specimens was not identified. On the PFM fired specimens, additional BeO and Cr2 O3 phases were identified (Fig. 3(a), bottom). For the beryllium-free Litecast alloy, large prominent peaks for MoNi and Cr3 Ni2 phases were identified on as-polished specimens (Fig. 3(b), top). For re-polished PFM fired specimens, prominent peaks for Cr2 O3 phases were identified along with MoNi phases (Fig. 3(b), bottom).
3.5. Corrosion resistance (Rp ) and surface double layer capacitance (Cdl ) Fig. 4 shows typical impedance plots for both alloys before and after PFM firing. Results from the EIS corrosion tests indicated that the Litecast alloy (non-Be containing alloy) exhibited approximately ten times higher corrosion resistance (Rp ) than the Be containing alloy, NPXIII, both before and after the PFM treatment (Table 1). The surface oxide double layer capacitance (Cdl ) of both alloys was similar for both the as-polished and after the PFM firing and polished (Table 1). Corrosion resistance (Rp ) and metal oxide double
Fig. 4 – Typical impedance plots from EIS test for Litecast and NPXIII.
layer capacitance (Cdl ) of both alloys did not change significantly after PFM firing. Though not significant, the capacitance values of both alloys seem to increase slightly after the firing process.
Fig. 5 – Total metal ion release of (a) Ni, (b) Cr, (c) Mo, and (d) Be over a month for Litecast (LC) and NPXIII dental alloys before and after firing. In general, the Ni, Cr, Mo ion release of both alloys increased after PFM firing process. Be release in NPX III increased after firing. Asterisk (*) indicates statistical significance between two sets of data.
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3.6. Corrosion (total metal ion release measured by ICP-AES) Fig. 5 shows the cumulative metal ion release from Litecast and NPXIII before and after PFM firing over 30 days in artificial saliva. Generally speaking, total metal ion release of Ni, Cr, and Mo from both alloys increased after PFM firing. Some larger error bars may be due to the limited number of specimens (four from each alloy) and the low metal ion concentration. For Litecast alloy, ion release of Ni, Cr, and Mo increased significantly after the PFM firing. The only metal ion that increased significantly in NPXIII was Mo.
4.
Discussion
Bauers et al. [18] reported that the chemical composition in the interdendritic eutectic structure of a Ni–Cr–Be alloy was Ni–Be. The XRD data in the current study showed that there were prominent Be–Ni and Mo–Ni peaks on the PFM fired specimens. Hence the additional precipitation appeared in the eutectic structure in NPXIII after PFM firing (Fig. 1(b)) could be Ni–Be and/or Mo–Ni. The formation of BeO on the oxidized alloy surface was reported in an earlier study [1] on a similar Ni–Cr–Be alloy. This result from the XRD test showing an increase in Mo–Ni and Be–Ni after PFM is consistent with the XPS (Fig. 2(a and b)) result that shows an increase in Mo and Ni peak after the PFM firing and results from the dissolution (Fig. 5) tests that show increases in Mo, Ni and Be release from the alloy specimens. For the Be-free Litecast alloy, the XRD result showed MoNi and Cr3 Ni2 existed before PFM. After the PFM firing, MoNi and small Cr2 O3 peaks appeared on the surface. This might explain why Ni and Mo ion release increased after the PFM firing. This was consistent with the finding in XPS that Mo 3d and Ni peak intensities increased after the PFM firing. The second phase that appeared on Litecast after PFM could be CrMo. This might contribute to the significant increase in Cr and Mo ion release from Litecast after PFM firing. The surface examination from XPS showed no significant difference before and after PFM, except for the increase in Mo and Ni peak intensities (Fig. 2(a and b)). These peaks appeared not to come from the formation of protective surface metal oxides such as MoOx but from the formation of Mo–Ni intermetallic precipitates based on XRD results (Fig. 3). As a result, very little difference in the alloys’ corrosion resistance (Rp ) (Table 1) and double layer capacitance (Cdl ) (Table 1) were observed. However, there were definite changes in the bulk composition for both alloys (Fig. 1) resulting in significant changes in hardness (Table 1), total metal ion release (Fig. 5), and a new precipitate observed in the micrographs. Though not statistically significant, the slight increase in Cdl (Table 1) of both alloys after PFM firing indicated slightly lower corrosion resistance. This lower corrosion resistance is consistent with the increased levels of released metal ions from the specimen after the PFM firing procedure. Morris [16] found a statistically significant reduction in strength after porcelain firing in a Ni–Cr–Be alloy and a high-strength Ni–Cr alloy without Be. Winkler et al. [14] and
Marinello et al. [15] demonstrated a decrease in hardness of Ni–Cr alloys as a result of heat treatment. In the current experiment, similar results were found for both the Be containing alloy and NPXIII. The Vickers hardness value of NPXIII was also very close to other literature values [18]. However, the current test showed Litecast alloy had a Vickers hardness value of around 140, which was about one-third of what Bauer et al. found for non-Be containing Ni–Cr alloys (around 420). Also, the Litecast alloy used in this study had an increase in hardness after PFM firing. This result was the opposite of the effect of PFM firing on a high-strength Ni–Cr alloy described by Morris, which had a decrease in hardness after PFM. The difference in the hardness results for the non-Be containing alloy used in this study and those of Bauer et al. [18] and Morris [16] may be due in part to the alloys’ bulk composition. The Litecast alloy used in this study had a 15% Mo content without Ti and Nb additions, whereas the Ni–Cr alloys used by Bauer et al. had about 5% of Mo and 4% of Ti or Nb. Among all the metal ions evaluated, the increase in Ni ion release was the greatest after PFM firing, especially for the Be containing alloy (∼11 ppm) (Fig. 5(a)). This was consistent with the results from Roach et al. [6] who found Ni–Cr alloys’ corrosion rates increased after PFM firing. Bumgardner and Lucas [4] and Bumgardner et al. [19] also found Be containing Ni–Cr-based alloys released more Ni ions than Be-free Ni–Cr alloys. They reported increased Ni ion concentration caused decreased cell proliferation and cellular energy metabolism without changing cellular morphology and viability. Ni ions, known to cause hypersensitivity, have been reported to indirectly cause lipopolysaccharide (LPS) induced human monocytes to release pro-inflammatory cytokines [20,21]. The Ni ion concentrations in the current study (maximum 200 M) fell in the range of the reported dose (0–90 M) [22] that caused LPS-activated monocytes to release more cytokine than the non-activated ones. Further investigation of the in vivo impact of Ni-ion release from Be containing alloys is required, especially at inflammatory sites.
5.
Conclusions
PFM firing changed the alloys’ bulk composition and thus their hardness values. New surface oxides were formed on the alloy surfaces after the PFM firing. The changes on the surface was not significant enough to change the alloys’ corrosion resistance measured by potentiostat, however, there were significant increases in metal ion release detected by ICPAES in vitro after a month. The in vivo effects of Ni ion release from Ni–Cr alloys after PFM firing requires further investigation.
Acknowledgements This work is supported by a Faculty Research Grant from the Office of Research at Mississippi State University and by the Mississippi Agricultural and Forestry Experiment Station, Starkville, MS.
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