The metallurgy of Ni-Cr alloys for fixed prosthodontics

Evaluation and comparison of shear bond strength of porcelain to a beryllium-free alloy of nickel-chromium, nickel and beryllium free alloy of cobalt-chromium, and titanium: An in vitro study

Aims:

The aim of this study was to evaluate and compare the shear bond strength of porcelain to the alloys of nickel-chromium (Ni-Cr), cobalt-chromium (Co-Cr), and titanium.

Materials and Methods:

A total of 40 samples (25 mm × 3 mm × 0.5 mm) were fabricated using smooth casting wax and cast using Ni-Cr, Co-Cr, and titanium alloys followed by porcelain buildup. The samples were divided into four groups with each group containing 10 samples (Group A1–10: sandblasted Ni-Cr alloy, Group B1–10: sandblasted Co-Cr alloy, Group C1–10: nonsandblasted titanium alloy, and Group D1–10: sandblasted titanium alloy). Shear bond strength was measured using a Universal Testing Machine.

Statistical Analysis Used:

ANOVA test and Tukey's honestly significance difference post hoc test for multiple comparisons.

Results:

The mean shear bond strength values for these groups were 22.8960, 27.4400, 13.2560, and 25.3440 MPa, respectively, with sandblasted Co-Cr alloy having the highest and nonsandblasted titanium alloy having the lowest value.

Conclusion:

It could be concluded that newer nickel and beryllium free Co-Cr alloys and titanium alloys with improved strength to weight ratio could prove to be good alternatives to the conventional nickel-based alloys when biocompatibility was a concern.

Evaluation of the Effect of recasting Nickel-chromium Base Metal Alloy on the Metal-ceramic Bond Strength: An in vitro Study

AIM

The aim of this study was to evaluate the effect of recasting base metal alloy on the metal-ceramic bond strength.

MATERIALS AND METHODS

A total of 60 test samples were prepared from new and recast nickel-chromium alloy and divided into six groups. In group A0, test samples were prepared from 100% new alloy. Groups A1, A2, A3, A4, and A5 were prepared from 50% new alloy and 50% casting remnants (sprue and buttons) of the previous group by weight. All these samples were coated with ceramic (IPS d.SIGN and Ivoclar-Vivadent), and samples were then subjected to three-point bending test to evaluate metal-ceramic bond strength.

RESULTS

The values were analyzed using one-way analysis of variance and Tukey's post hoc test. The mean bond strength of group A0 was higher than that of groups A1, A2, A3, A4, and A5.

CONCLUSION

Metal-ceramic bond strength decreased significantly with multiple recastings.

CLINICAL SIGNIFICANCE

Bond strength between metal and ceramic is a crucial factor for the clinical performance of metal-ceramic restorations. Recasting of alloys may affect the metal oxide layer composition and thickness of metal-ceramic interface, and thereby the metal-ceramic bond.

Effect of casting atmosphere on the marginal deficiency and misfit of Ni-Cr alloys with and without beryllium

STATEMENT OF PROBLEM

The marginal adaptation of prosthetic crowns is still a significant clinical problem.

PURPOSE

The purpose of this in vitro study was to evaluate the marginal deficiency and misfit of Ni-Cr alloys with and without beryllium under different casting conditions.

MATERIAL AND METHODS

Four casting conditions were selected: flame-torch, induction/argon, induction/vacuum, and induction/air; and 2 alloys were used, Ni-Cr-Be and Ni-Cr. For each group, 10 metal specimens were prepared. Silicone indirect impressions and analysis of the degree of rounding were used to evaluate the marginal deficiencies of metal copings, and a standardized device for the setting pressure associated with optical microscopy was used to analyze the marginal misfit. Results were evaluated with 2-way ANOVA (α=.05), followed by the Tukey honest significant difference post hoc test, and the Pearson correlation test (α=.05).

RESULTS

Alloy (P<.001) and casting technique (P<.001) were shown to affect marginal deficiencies. The Ni-Cr cast using the torch technique showed the highest marginal deficiency, and the Ni-Cr-Be cast in a controlled argon atmosphere showed the lowest (P<.001). Alloy (P=.472) and casting techniques (P=.206) did not affect the marginal misfit, but significant differences were found in the interaction (P=.001); the lowest misfit was achieved using the Ni-Cr-Be, and the highest misfit occurred with the molten Ni-Cr, using the cast torch technique. No correlation was found between deficiency and marginal misfit (r=.04, P=.69).

CONCLUSIONS

The interactions demonstrated that the alloy containing beryllium that was cast in an argon atmosphere led to reduced marginal deficiency. Improved marginal adaptation can be achieved for the same alloy by using the torch technique.

Effect of PFM Firing Cycles on the Mechanical Properties, Phase Composition, and Microstructure of Nickel-Chromium Alloy

PURPOSE

The purpose of this study was to compare the mechanical properties of beryllium-free nickel-chromium (Ni-Cr) dental casting alloy before and after each porcelain firing cycle (once fired, twice fired, and thrice fired) and to relate these properties to the microstructural changes and changes in X-ray diffraction patterns of Ni-Cr alloy that occur after each porcelain firing cycle.

MATERIAL AND METHODS

Forty tensile bar specimens and 20 disc-shaped specimens of Ni-Cr alloy were prepared. These specimens were divided into four groups. The first group was not heat treated and tested in the as-cast condition, thus serving as control group. The second, third, and fourth groups were fired once, twice, and thrice, respectively. Tensile bar specimens were loaded to failure in tension using a universal testing machine. Values of ultimate tensile strength, 0.1% yield strength, and percentage elongations were determined. Microstructural study and hardness testing were done using an optical microscope and digital Vickers hardness tester, respectively, on disc-shaped specimens. Disc-shaped specimens were again used to obtain the X-ray diffraction patterns by using diffractometer Bruker D8 focus. Statistical comparisons of the mechanical properties and hardness of the alloy were made with ANOVA. Intergroup comparisons of the data in the as-cast and fired specimens were analyzed by applying Tukey's HSD multiple comparison tests.

RESULTS

Before porcelain firing, the alloy exhibited higher ultimate tensile strength (548 MPa), 0.1% yield strength (327 MPa), hardness (192 HV), and lower elongation values (18%). After each firing cycle, there was a significant (p < 0.001) decrease in ultimate strength (464 MPa for three times fired specimens), 0.1% yield strength (284 MPa for three times fired group), and hardness (164 HV for three times fired group) and significant (p < 0.001) increase in elongation value (28% for three times fired group) of Ni-Cr alloy. The microstructure of the control group specimen exhibited heterogeneous microstructure, and after each firing, microstructure of the alloy was gradually homogenized by formation of grain boundaries at the interdendritic interfaces. X-ray diffraction pattern shows that the alloy exhibited four strong diffraction peaks within the range of 2θ = 40° to 100°. After a third firing, intensity of these planes increased.

CONCLUSIONS

Results of this study confirmed that nickel-based alloys become weaker after each firing process. After firing treatment, the microstructure of alloys showed decreased dendritic structure (i.e., homogenization, which was responsible for decrease in strength and an increase in ductility of the alloy); however, this decreased strength and hardness of Ni-Cr alloy after heat treatment was still superior to those of the most noble metal alloys used in dentistry. X-ray diffraction study showed that firing process led to relieving of stresses, which ultimately resulted in stability in the crystal structure of alloy.

Evaluation of Surface Roughness and Tensile Strength of Base Metal Alloys Used for Crown and Bridge on Recasting (Recycling)

BACKGROUND

Dental casting alloys play a prominent role in the restoration of the partial dentition. Casting alloys have to survive long term in the mouth and also have the combination of structure, molecules, wear resistance and biologic compatibility. According to ADA system casting alloys were divided into three groups (wt%); high noble, Noble and predominantly base metal alloys.

AIM

To evaluate the mechanical properties such as tensile strength and surface roughness of the new and recast base metal (nickel-chromium) alloys.

MATERIALS AND METHODS

Recasting of the base metal alloys derived from sprue and button, to make it reusable has been done. A total of 200 test specimens were fabricated using specially fabricated jig of metal and divided into two groups- 100 specimens of new alloy and 100 specimens of recast alloys, which were tested for tensile strength on universal testing machine and surface roughness on surface roughness tester.

RESULTS

Tensile strength of new alloy showed no statistically significant difference (p-value>0.05) from recast alloy whereas new alloy had statistically significant surface roughness (Maximum and Average surface roughness) difference (p-value<0.01) as compared to recast alloy.

CONCLUSION

Within the limitations of the study it is concluded that the tensile strength will not be affected by recasting of nickel-chromium alloy whereas surface roughness increases markedly.

Evaluation of effect of galvanic corrosion between nickel-chromium metal and titanium on ion release and cell toxicity

PURPOSE

The purpose of this study was to evaluate cell toxicity due to ion release caused by galvanic corrosion as a result of contact between base metal and titanium.

MATERIALS AND METHODS

It was hypothesized that Nickel (Ni)-Chromium (Cr) alloys with different compositions possess different corrosion resistances when contacted with titanium abutment, and therefore in this study, specimens (10×10×1.5 mm) were fabricated using commercial pure titanium and 3 different types of Ni-Cr alloys (T3, Tilite, Bella bond plus) commonly used for metal ceramic restorations. The specimens were divided into 6 groups according to the composition of Ni-Cr alloy and contact with titanium. The experimental groups were in direct contact with titanium and the control groups were not. After the samples were immersed in the culture medium - Dulbecco's modified Eagle's medium[DMEM] for 48 hours, the released metal ions were detected using inductively coupled plasma mass spectrometer (ICP-MS) and analyzed by the Kruskal-Wallis and Mann-Whitney test (P<.05). Mouse L-929 fibroblast cells were used for cell toxicity evaluation. The cell toxicity of specimens was measured by the 3-{4,5-dimethylthiazol-2yl}-2,5-diphenyltetrazolium bromide (MTT) test. Results of MTT assay were statistically analyzed by the two-way ANOVA test (P<.05). Post-hoc multiple comparisons were conducted using Tukey's tests.

RESULTS

The amount of metal ions released by galvanic corrosion due to contact between the base metal alloy and titanium was increased in all of the specimens. In the cytotoxicity test, the two-way ANOVA showed a significant effect of the alloy type and galvanic corrosion for cytotoxicity (P<.001). The relative cell growth rate (RGR) was decreased further on the groups in contact with titanium (P<.05).

CONCLUSION

The release of metal ions was increased by galvanic corrosion due to contact between base metal and titanium, and it can cause adverse effects on the tissue around the implant by inducing cytotoxicity.

Characterization of Ni-Cr alloys using different casting techniques and molds

This study differentiated the mechanical properties of nickel-chromium (Ni-Cr) alloys under various casting techniques (different casting molds and casting atmospheres). These techniques were sampled by a sand mold using a centrifugal machine in ambient air (group I) and electromagnetic induction in an automatic argon castimatic casting machine (group II). The specimen casting used a graphite mold by a castimatic casting machine (group III). The characteristics of the Ni-Cr alloys, yield and ultimate tensile strength, bending modulus, microhardness, diffraction phase, grindability, ability to spring back, as well as ground microstructure and pattern under different casting conditions were evaluated. The group III specimens exhibited the highest values in terms of strength, modulus, hardness, and grindability at a grind rate of 500 rpm. Moreover, group III alloys exhibited smaller grain sizes, higher ability to spring back, and greater ductility than those casted by sand investment (groups I and II). The main factor, "casting mold," significantly influenced all mechanical properties. The graphite mold casting of the Ni-Cr dental alloys in a controlled atmosphere argon casting system provided an excellent combination of high mechanical properties and good ability to spring back, and preserved the ductile properties for application in Ni-Cr porcelain-fused system. The results can offer recommendations to assist a prosthetic technician in selecting the appropriate casting techniques to obtain the desired alloy properties.

Nickel alloys in the oral environment

The use of nickel casting alloys for long-term restorations in dentistry has long been controversial. A 'tug-of-war' between economic, engineering and biological considerations is central to this controversy; nickel-casting alloys have low costs and favorable physical properties, but are corrosion-prone in the oral environment. Clinicians and researchers have questioned the safety of nickel-containing dental alloys because several nickel compounds are known to cause adverse biological effects in vivo and in vitro in contexts outside of dentistry. The debate revolves around the extent to which corrosion products from oral restorations cause intraoral or systemic biological problems. Current evidence suggests that nickel alloys may be used successfully and safely in dentistry if clinical risks are taken into account. However, these alloys may cause significant clinical problems, primarily allergenic and inflammatory, if the risks are ignored.

Does the casting mode influence microstructure, fracture and properties of different metal ceramic alloys?

The aim of the present study was to evaluate the tensile strength, elongation, microhardness, microstructure and fracture pattern of various metal ceramic alloys cast under different casting conditions. Two Ni-Cr alloys, Co-Cr and Pd-Ag were used. The casting conditions were as follows: electromagnetic induction under argon atmosphere, vacuum, using blowtorch without atmosphere control. For each condition, 16 specimens, each measuring 25 mm long and 2.5 mm in diameter, were obtained. Ultimate tensile strength (UTS) and elongation (EL) tests were performed using a Kratos machine. Vickers Microhardness (VM), fracture mode and microstructure were analyzed by SEM. UTS, EL and VM data were statistically analyzed using ANOVA. For UTS, alloy composition had a direct influence on casting condition of alloys (Wiron 99 and Remanium CD), with higher values shown when cast with Flame/Air (p < 0.05). The factors 'alloy" and 'casting condition" influenced the EL and VM results, generally presenting opposite results, i.e., alloy with high elongation value had lower hardness (Wiron 99), and casting condition with the lowest EL values had the highest VM values (blowtorch). Both factors had significant influence on the properties evaluated, and prosthetic laboratories should select the appropriate casting method for each alloy composition to obtain the desired property.

Influence of alloy microstructure on the microshear bond strength of basic alloys to a resin luting cement

The aim of this study was to evaluate the influence of microstructure and composition of basic alloys on their microshear bond strength (µSBS) to resin luting cement. The alloys used were: Supreme Cast-V (SC), Tilite Star (TS), Wiron 99 (W9), VeraBond II (VBII), VeraBond (VB), Remanium (RM) and IPS d.SIGN 30 (IPS). Five wax patterns (13mm in diameter and 4mm height) were invested, and cast in a centrifugal casting machine for each basic alloy. The specimens were embedded in resin, polished with a SiC paper and sandblasted. After cleaning the metal surfaces, six tygon tubes (0.5 mm height and 0.75 mm in diameter) were placed on each alloy surface, the resin cement (Panavia F) was inserted, and the excess was removed before light-curing. After storage (24 h/37°C), the specimens were subjected to µSBS testing (0.5 mm/min). The data were subjected to a one-way repeated measures analysis of variance and Turkey's test (α=0.05). After polishing, their microstructures were revealed with specific conditioners. The highest µSBS (mean/standard deviation in MPa) were observed in the alloys with dendritic structure, eutectic formation or precipitation: VB (30.6/1.7), TS (29.8/0.9), SC (30.6/1.7), with the exception of IPS (31.1/0.9) which showed high µSBS but no eutectic formation. The W9 (28.1/1.5), VBII (25.9/2.0) and RM (25.9/0.9) showed the lowest µSBS and no eutectic formation. It seems that alloys with eutectic formation provide the highest µSBS values when bonded to a light-cured resin luting cement.

Effect of recasting of nickel: chromium alloy on its castability

Castability plays an important role in selection of an alloy for cast dental restorations. This study was conducted to assess the effect of recasting of nickel-chromium alloy on its castability. Different percentage combinations of new and once casted alloy were used to produce a total of twentyfive cast samples using modified Whitlock's method and castings obtained from new alloy were used as control group. Castability value was obtained by using Whitlock's formula. Results were analyzed using student 't' test. There is no statistical difference between the castability value of the new alloy and the recasted alloy (confidence level 95%). Within the limitations of the study it is concluded that the castability value will not be affected by recasting the nickel-chromium alloy. Complete castings of any metal restoration are mandatory and to know the completeness of castings of any alloy, castability test is of prime importance.

Effect of recasting of nickel-chromium alloy on its porosity

STATEMENT OF PROBLEM

As per the review of literature very few studies have been carried on recasting of dental casting alloy and in particlular its effect on occurrence of porosities.

PURPOSE OF STUDY

This study was designed to find out occurrence of porosities in new alloy and recasted alloy using a scanning electron microscope.

MATERIALS AND METHODS

Different percentage combinations of new and once casted alloy were used to produce twenty five samples. Castings obtained from new alloy were used as control group. All the samples were scanned under scanning electron mocroscope and photographs were taken from three specific sites for comparison.

RESULTS

There is no significant difference in occurrence of porosities in casting obtained by using new alloy and recasted alloy.

CONCLUSION

With in the limitations of the study it is conducted that the prorosities will not be affected by recasting of neckel-chromium alloy.

CLINICAL IMPLICATION

Porosities in dental casting alloy can alter physical and mechanical properties of the mental which inturn may lead to failure of crown and bridge, and also cast partial denture prosthesis.

Oral Keratinocyte Responses to Nickel-based Dental Casting Alloys In Vitro

Adverse reactions of oral mucosa to nickel-based dental casting alloys are probably due to corrosion metal ion release. We exposed H400 oral keratinocytes to two Ni-based dental alloys (Matchmate and Dsign10) as well as NiCl 2 (1—40 μg/mL Ni2+). Alloy derived Ni2+ media concentrations were determined. Direct culture on both alloys resulted in inhibited growth with a greater effect observed for Dsign10 (higher ion release). Indirect exposure of cells to conditioned media from Dsign10 negatively affected cell numbers (~64% of control by 6 days) and morphology while Matchmate-derived media did not. Exposure to increasing NiCl2 negatively affected cell growth and morphology, and the Granulocyte-macrophage colony-stimulating factor (GM-CSF) transcript was significantly up-regulated in cells following direct and indirect exposure to Dsign10. NiCl2 exposure up-regulated all cytokine transcripts at 1 day. At day 6, IL-1β and IL-8 transcripts were suppressed while GM-CSF and IL-11 increased with Ni2+ dose. Accumulation of Ni2+ ions from alloys in oral tissues may affect keratinocyte viability and chronic inflammation.

Base metal alloys used for dental restorations and implants

One of the primary reasons for the development of base metal alloys for dental applications has been the escalating cost of gold throughout the 20th century. In addition to providing lower cost alternatives, these nonprecious alloys were also found to provide better mechanical properties and aesthetics for some oral applications. Additionally, certain base metal alloy systems are preferred because of their superior mechanical properties, lower density, and in some cases, their capability to osseo-integrate. The base metal alloy systems most commonly used in dentistry today include stainless steels, nickel-chromium, cobalt-chromium, titanium, and nickel-titanium alloys. Combined, these alloy systems provide a wide range of available properties to choose the correct material for both temporary and long-term restoration and implant applications.

Corrosion of nickel-based dental casting alloys

OBJECTIVES

To study the microstructure, corrosion behaviour and cell culture response of two nickel-based dental casting alloys before and after a heat treatment to simulate porcelain firing.

METHODS

The microstructure was studied using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Corrosion behaviour was evaluated by electrochemical measurements in artificial saliva at different values of pH in the presence of a crevice. 3T3 mouse fibroblasts were exposed indirectly to alloy specimens and the number of viable cells counted after 3 and 6 days compared to a control culture.

RESULTS

Small changes in microstructure were observed after heat treatment but had a negligible effect on the corrosion properties in the conditions tested. The alloy with a lower bulk level of Cr (12.6 wt.%) showed lower corrosion resistance, indicated by an increased passive current density and this stability was greatly reduced at pH 2.5, where crevice corrosion was observed. Selective dissolution occurred at regions within the microstructure containing lower levels of Cr and Mo. Furthermore, the proliferation of 3T3 mouse fibroblasts was reduced (p<0.05) when exposed indirectly to this alloy. The alloy containing a higher level of Cr (25 wt.%) showed superior corrosion resistance, which was associated with a more uniform distribution of Cr in the alloy microstructure.

SIGNIFICANCE

The presence of crevices combined with an inhomogeneous distribution of Cr in the microstructure can lead to accelerated corrosion of Ni-based alloys with lower Cr contents. This effect can be avoided by increasing the Cr content of the alloy.

Microhardness of Ni-Cr alloys under different casting conditions

This study evaluated the microhardness of Ni-Cr alloys used in fixed prosthodontics after casting under different conditions. The casting conditions were: (1-flame/air torch) flame made of a gas/oxygen mixture and centrifugal casting machine in a non-controlled casting environment; (2-induction/argon) electromagnetic induction in an environment controlled with argon; (3-induction/vacuum) electromagnetic induction in a vacuum environment; (4-induction/air) electromagnetic induction in a non-controlled casting environment. The 3 alloys used were Ni-Cr-Mo-Ti, Ni-Cr-Mo-Be, and Ni-Cr-Mo-Nb. Four castings with 5 cylindrical, 15 mm-long specimens (diameter: 1.6 mm) in each casting ring were prepared. After casting, the specimens were embedded in resin and polished for Vickers microhardness (VH) measurements in a Shimadzu HMV-2 (1,000 g for 10 s). A total of 5 indentations were done for each ring, one in each specimen. The data was subjected to two-way ANOVA and Tukey's multiple comparison tests (alpha = 0.05). The VH values of Ni-Cr-Mo-Ti (422 ± 7.8) were statistically higher (p < 0.05) than those of Ni-Cr-Mo-Nb (415 ± 7.6). The lowest VH values were found for Ni-Cr-Mo-Be (359 ± 10.7). The VH values obtained in the conditions induction/argon and induction/vacuum were similar (p > 0.05) and lower than the values obtained in the conditions induction/air and flame/air torch (p < 0.05). The VH values in the conditions induction/air and flame/air were similar (p > 0.05). The microhardness of the alloys is influenced by their composition and casting method. The hardness of the Ni-Cr alloys was higher when they were cast with the induction/air and flame/air torch methods.

Castability and resistance of ceramometal bonding in Ni-Cr and Ni-Cr-Be alloys

STATEMENT OF PROBLEM

Because of its pathogenic potential, the importance of the use of beryllium with Ni-Cr alloys must be determined.

PURPOSE

This study compared fundamental properties for the clinical use of Ni-Cr alloys, determining the advantage of the addition of beryllium, despite the involved risks.

MATERIAL AND METHODS

Two Be-free commercial alloys (Vera Bond II and Wiron 99) and 1 Be-free experimental alloy (E3) with Nb and/or Mo in their formulations and 1 experimental Ni-Cr alloy (E4) with 1.1% Be were submitted to ceramometal bonding resistance, castability, and hardness tests.

RESULTS

Analysis of variance showed significant differences (P=.05) between Vera Bond II and Wiron 99 (Be-free) and the E4 alloy for the castability test. Vera Bond II, Wiron 99, and E4 presented higher castability values than E3. There were no statistically significant differences for the ceramometal bonding resistance test. The Kruskal-Wallis test showed significant differences (P=.01) among the alloys with Rockwell 30 T values for the hardness test.

CONCLUSION

Within the limitations of this study, the presence of Be in Ni-Cr alloys was not necessary to guarantee the castability and the ceramometal bond resistance of the alloys tested.

Pilot study of the relationship between the hardness and abrasion resistance of two base metal alloys used for metal-ceramic restorations

STATEMENT OF PROBLEM

Base metal alloys present high values of strength and hardness, which have been associated with the greater abrasion resistance and polishing of metal-ceramic restorations. However, surface hardness has been shown to be a poor indicator of abrasion resistance.

PURPOSE

The study aimed to compare the hardness and abrasion resistance of Ni-Cr alloys and determine whether there is a correlation between these 2 properties.

MATERIAL AND METHODS

Two Ni-Cr alloys for metal-ceramic restorations with different hardness values were subjected to the following procedures: (1) initial measurement of Vickers hardness, (2) a series of abrasion cycles, (3) measurement of mass loss after each cycle, and (4) Vickers hardness measurements after each cycle.

RESULTS

For each alloy, linear regression revealed a negative correlation between hardness and reduction in mass. The higher hardness of alloy A was associated with higher mass loss during abrasion when compared with alloy B.

CONCLUSION

There was no significant correlation between hardness and mass loss for either alloy.

Use of x-ray photoelectron spectroscopy and cyclic polarization to evaluate the corrosion behavior of six nickel-chromium alloys before and after porcelain-fused-to-metal firing

STATEMENT OF PROBLEM

Nickel-chromium casting alloys rely on a surface oxide layer for corrosion resistance to the oral environment. Porcelain-fused-to-metal (PFM) firing procedures may alter the surface oxides and corrosion properties of these alloys. Changes in alloy corrosion behavior affect metal ion release and therefore local and/or systemic tissue responses.

PURPOSE

The aim of this study was to evaluate changes in alloy surface oxides and electrochemical corrosion properties after PFM firing.

MATERIAL AND METHODS

The electrochemical corrosion behavior of 6 commercial nickel-chromium alloys was evaluated in the as-cast/polished and PFM fired/repolished states. Surface chemistries of the alloys were analyzed by x-ray photoelectron spectroscopy.

RESULTS

Results indicated an increase in corrosion rates after PFM firing and repolishing for alloys containing 14% to 22% Cr and 9% to 17% Mo. This increase in corrosion rates was attributed to a decrease, caused by the PFM and repolishing process, in the Cr and Mo levels in the surface oxides of these alloys. The PFM firing and repolishing process did not alter the corrosion behavior of the alloys containing lower levels of Cr and Mo and/or Be additions in their bulk composition. These alloys exhibited low levels of Cr and Mo surface oxides in both test conditions. Si particles became embedded in the surfaces of the fired alloys during repolishing and may have contributed to the changes in surface oxides and the corrosion behavior of some alloys.

CONCLUSION

The effects of PFM firing and repolishing on Ni-Cr dental casting alloy surface oxides and corrosion properties appear to be alloy dependent.

Effect of beryllium on the castability and resistance of ceramometal bonds in nickel-chromium alloys

STATEMENT OF PROBLEM

Castability and ceramometal bond resistance play an important role in accepting nickel-chromium alloys as a substitute for gold alloys in dentistry.

PURPOSE

This study was developed to verify the effect of beryllium on these factors in several compositions of nickel-based alloys by submitting them to castability and ceramometal bonding resistance tests.

MATERIAL AND METHODS

Three experimental compositions of Ni-Cr alloys with different amounts of beryllium were used. One beryllium-free alloy was used as the control.

RESULTS

Analysis of variance and Tukey's test showed significant differences (alpha = .001) for the castability test results and significant differences (alpha = .05) for ceramometal bond resistance between alloys.

CONCLUSIONS

Although the amounts of chromium, manganese, and niobium were maintained, the variations in the amounts of beryllium allowed the estimation that Be-containing alloys presented better castability than Be-free alloys. The 0.9% Be-containing alloy demonstrated higher resistance of the ceramometal bond than the Be-free alloy.

Tensile bond strengths of three chemical and one electrolytic etching systems for a base metal alloy

This study compared the tensile bond strength of Rexillium III disks etched by three chemical etching systems and the electrochemical etching technique. Cast Rexillium III disks were etched by the following etching systems: electrochemical, Assure-Etch, Met-Etch gel, and Etch-It gel. Thermocycled samples were loaded to failure in tension on an Instron testing machine using a crosshead speed of 5 mm/minute. Samples etched electrochemically yielded significantly greater bond strengths than those etched chemically (p less than 0.05). The rank of the groups was: Electrochemical greater than Assure-Etch = Met-Etch greater than Etch-It. Scanning electron micrograph evaluations of the etching patterns supported the bond strength determinations.

Veterans Administration Cooperative Studies Project No. 147. Part IX: A comparison of the mechanical properties of several alternative metal ceramic alloys cast in clinical and research laboratories

Comparison of mechanical properties of five metal ceramic alloys cast in clinical and research laboratories resulted in significant differences. Tested alloys included one Au-Pd (control), one Pd-Ag, and three Ni-Cr. Mechanical properties included strength, elongation, and microhardness. Tensile bars were torch cast simultaneously with restorations in the clinical laboratory and induction cast by using a preformed design in the research laboratory. Mechanical properties of most alloys showed larger variations when cast in the clinical laboratory. In the clinical laboratory the Au-Pd alloy showed significantly higher values for all properties. One Ni-Cr alloy was not significantly affected by environment for all properties. The remaining alloys were significantly affected for some properties by environment. The different laboratories had no greater or lesser effect on the base metals as a group than on the noble metals. Variation in handling, spruing, casting, and specimen size may account for some of the differences between laboratories.

Influence of heat treatment on the surface texture of an etched cast nickel-chromium base alloy: an evaluation by profilometric records

Profilometric measurements were performed to test the electrochemically etched surface roughness of a Ni-Cr-Be alloy in the as-cast state and after porcelain firing simulation. A Student's t test at 5% risk leads to the conclusion that a heat-treated surface is significantly rougher than an as-cast surface after electrolytic etching. The exact cause of the increased roughness and how it improves metal-to-resin bonding must be evaluated by further research.

Long-term corrosion studies in vitro of gold, cobalt-chromium, and nickel-chromium alloys in contact

Specimens of various types of dental casting alloys were stored in phosphate-buffered 0.9% NaCl solution for 35 weeks. Every 7 weeks the solutions were changed and analyzed with regard to elements released from the alloys. The release of Cu from type III gold alloy increased in contact with gold alloys for metallo-ceramic use in a 1:2 area relation. However, when the area relation was reversed, no difference in the amounts of elements released was observed. Crevice corrosion was initiated with one of the Co-Cr alloys in contact with type III gold alloy. The crevice corrosion increased the amounts of Co and Cr released into the solutions. The release of Ni and Cr from the Ni-Cr alloys was prominent. These alloys were very susceptible to crevice corrosion. With one of the Ni-Cr alloys the release of elements increased in contact with type III gold alloy.