Porcelain adherence to high-palladium alloys

Influence of a bonding agent on the bond strength between a dental Co-Cr alloy and nine different veneering porcelains

Adequate bonding between dental veneering porcelains and non-precious metal alloys is a main factor for the long-term functionality of porcelain fused to metal restorations. Although a huge number of veneering porcelains are on the market, only few studies have reported about the role of bonding agents for the bond strength at their respective interface to cobalt-chromium (Co-Cr). The aim of this study was to compare the influence of a metal-ceramic bonding agent for Co-Cr alloys on the bond strength of metal-ceramic systems. The bond strength test was done according to ISO 9693 with additional detection of the first acoustic crack initiated signal while testing. The bonding agent had only minor effects on the bond strength of the different Co-Cr/ceramic systems. Only three of the nine studied systems showed statistically significant differences (p<0.05) upon applying the bonding agent. Scanning electron microscopy (SEM) showed cracks predominantly caused by adhesive failure. Based on this study, Co-Cr alloys veneered with porcelains with and without a bonding agent exceeded the minimum bond strength of 25 MPa required according to ISO 9693. However, if bond strength values based on acoustic signals were calculated, values below the threshold of 25 MPa could be observed. Such findings are important for failures caused by the occurrence of early cracks.

HRTEM observation of bonding interface between Ce-TZP/Al2O3 nanocomposite and porcelain

The surface of a ceria-stabilized tetragonal zirconia polycrystal (Ce-TZP/Al2O3) nanocomposite was sandblasted by alumina particles and veneered with feldspathic porcelain via a conventional condensation method. The part of each specimen containing the interface layer was sliced to ultrathin sections with an argon ion slicer, and these sliced sections were observed using high-resolution transmission electron microscopy (HRTEM). For both interfaces, Ce-TZP/porcelain and Al2O3/porcelain, no transition layers due to abrupt changes in atomic distributions were observed. Besides, the porcelain layers of both interfaces consisted of homogeneous amorphous phases. These results suggested that both Ce-TZP and Al2O3 could be directly bonded to porcelain by Van der Waals forces arising from the close contact between them.

The effect of zinc levels in a gold-based alloy on porcelain-metal bonding

OBJECTIVES

The purpose of this study was to ascertain whether the amount of Zn in gold alloys contributes to porcelain-metal bonding.

METHODS

Experiments were carried out using a commercial Pd-free gold alloy with a nominal composition of 88.7 wt% Au, 9.49 wt% Pt, 1.5 wt% Zn, 0.1 wt% Mn, 0.2 wt% Rh, and 0.1 wt% Ir, which contains Zn and no other elements (In, Sn, Fe) known to affect porcelain-metal bond strength. To establish the effect of oxidation of the metal surface, porcelain was applied both to preoxidized and to non-preoxidized metal specimens. The bond strength was evaluated by means of the ISO 9693: 1999 crack initiation test. A conventional gold alloy was used as a control. The elemental distributions at the porcelain/alloy interfaces were analyzed in cross section by electron probe microanalysis. Additionally, after the bond strength test, cross-sections of the interfaces of the debonded specimens were microscopically analyzed to characterize the fracture mode.

RESULTS

The Pd-free gold alloy joints showed significantly higher bond strength values than joints made with conventional gold alloy. Preoxidation treatment significantly increases the bond strength, in the preoxidized joints Zn was highly localized at the interface and diffused into the porcelain up to about 10 μm from the interface, and the joint failed by cohesive fracture in the porcelain. In contrast, the non-preoxidized joint showed mainly adhesive fracture at the porcelain/alloy interface.

SIGNIFICANCE

The presence of Zn in gold alloys plays a part in establishing chemical bonding thus improving the bond strength between porcelain and alloy.

Bond strength of three dental porcelains to Ni-Cr and Co-Cr-Ti alloys

Ceramometal bond strength has played an important role for the replacement of gold alloys by nickel-chromium alloys in dentistry. This study evaluated the metal/porcelain bond strength of three ceramic systems (Vita VMK 88, Williams and Duceram) associated with three nickel-chromium alloys (Durabond, Lite Cast B and Resistal P) and one experimental cobalt-chromium-titanium alloy. Thirty cast cylinder specimens (15 mm in height; 6 mm in diameter) were obtained for each alloy, in away that 10 specimens of each alloy were tested with each porcelain. Bond strength was measured with an Emic screw-driven mechanical testing machine by applying parallel shear forces to the specimens until fracture. Kruskal-Wallis and Mann-Whitney U tests were used for statistical analysis of the alloy/ceramic combinations (p<0.05). Resistal P/Duceram had significantly higher bond strength (44.38±9.12 MPa) (p<0.05) than the other combinations, except for Co-Cr-Ti alloy/Vita VMK 88 (38.41±12.64 MPa). The association of the experimental Co-Cr-Ti alloy with Williams porcelain had significantly higher bond strength (28.20±3.86 MPa) than the combination of other alloys with the same porcelain (p<0.05). Based of these results and within the limitations of an in vitro study, it may be concluded that the bond strength of the three ceramic systems to the Ni-Cr and Co-Cr-Ti alloys varied significantly, indicating that metal/ceramic compatibility was very important to the bond strength.

Effects of silicon coating on bond strength of two different titanium ceramic to titanium

OBJECTIVES

This study investigated the effect of silicon coating (SiO2) by magnetron sputtering on bond strength of two different titanium ceramics to titanium.

METHODS

Sixty cast titanium specimens were prepared following the protocol ISO 9693. Titanium specimens were divided into two test and control groups with 15 specimens in each. Test groups were silicon coated by the magnetron sputtering technique. Two titanium ceramics (Triceram and Duceratin) were applied on both test (coated) and control (uncoated) metal specimens. The titanium-ceramic specimens were subjected to a three point flexural test. The groups were compared for their bond strength. SEM and SEM/EDS analyses were performed on the delaminated titanium surfaces to ascertain bond failure.

RESULTS

The mean bond strength of Ti-Duceratin, Ti-Triceram, Si-coated Ti-Duceratin and Si-coated Ti-Triceram were 17.22+/-2.43, 23.31+/-3.18, 23.21+/-3.81 and 24.91+/-3.70 MPa, respectively. While the improvement in bond strength was 30% for Duceratin, it was statistically insignificant for Triceram. An adhesive mode of failure was observed in the Duceratin control group. In the silicoated Duceratin specimen, the bonded ceramic boundaries were wider but less than in the silicoated Triceram specimen. In the coated Triceram specimen, the ceramic retained areas were frequent and the failure mode was generally cohesive.

SIGNIFICANCE

Silicon coating was significantly effective in both preventing titanium oxide layer formation and in improving bond strength for Duceratin. However, it was of less value for Triceram.

Shear bond strengths of a ceramic system to alternative metal alloys

STATEMENT OF PROBLEM

The success of metal-ceramic restorations is influenced by the compatibility between base metal alloys and porcelains. Although porcelain manufacturers recommend their own metal systems as the most compatible for fabricating metal-ceramic prostheses, a number of alloys have been used. Purpose This study evaluated the shear bond strength between a porcelain system and 4 alternative alloys.

MATERIAL AND METHODS

Two Ni-Cr alloys: 4 ALL and Wiron 99, and 2 Co-Cr alloys: IPS d.SIGN 20 and Argeloy NP were selected for this study. The porcelain (IPS d.Sign porcelain system) portion of the cylindrical metal-ceramic specimens was 4 mm thick and 4 mm high; the metal portion was machined to 4x4 mm, with a base that was 5 mm thick and 1 mm high. Forty-four specimens were prepared (n=11). Ten specimens from each group were subjected to a shear load on a universal testing machine using a 1 mm/min crosshead speed. One specimen from each group was observed with a scanning electron microscope. Stress at failure (MPa) was determined. The data were analyzed with a 1-way analysis of variance (alpha=.05).

RESULTS

The groups, all including IPS d.Sign porcelain, presented the following mean bond strengths (+/-SD) in MPa: 4 ALL, 54.0+/-20.0; Wiron, 63.0+/-13.5; IPS d.SIGN 20, 71.7+/-19.2; Argeloy NP, 55.2+/-13.5. No significant differences were found among the shear bond strength values for the metal-ceramic specimens tested.

CONCLUSION

None of the base metal alloys studied demonstrated superior bond strength to the porcelain tested.

SEM/EDS evaluation of porcelain adherence to gold-coated cast titanium

The adhesion between titanium and dental porcelain is related to the diffusion of oxygen to the reaction layer formed on cast-titanium surfaces during porcelain firing. The diffusion of oxygen could be suppressed by coating the titanium surface with a thin gold layer. This study characterized the effects of gold coating on titanium-ceramic adhesion. ASTM grade II CP titanium was cast into a MgO-based investment (Selevest CB, Selec). The specimen surfaces were air abraded with 110-microm Al(2)O(3) particles. Gold coating was applied on titanium surfaces by three methods: gold-paste (Deck Gold NF, Degussa-Ney) coating and firing at 800 degrees C for three times, single gold-paste coating and firing followed by sputter coating (40 mA, 500 s), and sputter coating (40 mA, 1000 s). Surfaces only air abraded with Al(2)O(3) particles were used as controls. An ultra-low-fusing dental porcelain (Vita Titankeramik, Vident) was fused on titanium surfaces. Specimen surfaces were characterized by SEM/EDS and XRD. The titanium-ceramic adhesion was evaluated by a biaxial flexure test (N = 8), and area fraction of adherent porcelain (AFAP) was determined by EDS. Numerical results were statistically analyzed by one-way ANOVA and the Student-Newman-Keuls test at alpha = 0.05. SEM fractography showed a substantial amount of porcelains remaining on the gold-sputter-coated titanium surfaces. A new Au(2)Ti phase was found on gold-coated titanium surface after the firing. Significantly higher (p <.05) AFAP values were determined for the gold-sputter-coated specimens compared to the others. No significant differences were found among the other groups and the control. Results suggested that gold coatings used in this study are not effective barriers to completely protect titanium from oxidation during the porcelain firing, and porcelain adherence to cast titanium can be improved by gold-sputter coating used in the present study.

Comparison of the load at failure of soldered and nonsoldered porcelain-fused-to-metal crowns

STATEMENT OF PROBLEM

The application of solder material to metal frameworks could affect the porcelain-to-metal failure load and possibly compromise the longevity of porcelain-fused-to-metal crowns.

PURPOSE

This in vitro study assessed the influence of solder on the porcelain failure load between metal and porcelain, using crown-shaped specimens.

MATERIAL AND METHODS

Forty standardized crown patterns were fabricated on a metal die and cast with noble alloy (Protocol). The specimens were divided into test (soldered, n=20) and control (nonsoldered, n=20) groups. A 2-mm-diameter perforation was produced on the buccal surface of the test specimens, 1.5 mm apical to the junction between occlusal and buccal surfaces. The perforation was repaired with high-fusing white ceramic solder material. All castings were finished with aluminum oxide stones and airborne-particle abraded with 50-microm aluminum oxide at 75 psi of pressure. Two layers each of opaque and dentin porcelain were applied on all specimens. All crowns were subjected to a vertical compressive load with a universal testing machine at a crosshead speed of 0.5 mm/min until fracture. The loading apparatus of the device contacted the buccal cusp only. In both control and test groups, 10 crowns were loaded with a stainless steel pin (phase I) whereas the remaining 10 in each group were loaded with a carbon steel pin (phase II). Means of loads at failure were recorded in each group for each pin type and compared (soldered vs nonsoldered) by use of a t test (alpha=.05).

RESULTS

Mean failure load for nonsoldered crowns was 349 +/- 63 kgf in phase I and 228 +/- 49 kgf in phase II. Soldered crowns showed a significantly lower failure load (265 +/- 82 kgf [phase I; P<.02] and 154 +/- 108 kgf [phase II; P<.04]) and a greater standard deviation.

CONCLUSION

Soldered metal-porcelain crown specimens demonstrated a significantly lower load at failure. Within the limits of this study, it was concluded that solder material might negatively affect porcelain-metal crowns.

Effects of interfacial variables on ceramic adherence to cast and machined commercially pure titanium

STATEMENT OF THE PROBLEM

Titanium-ceramic bonding is less optimal than conventional metal-ceramic bonding, due to excessive oxidation of titanium during porcelain firing.

PURPOSE

This in vitro study evaluated the effects of porcelain firing atmosphere and gold sputter coating on titanium surfaces on porcelain bonding to machined and as-cast titanium substrates. Material and methods Eight groups of ASTM grade 2 commercially pure (CP) titanium specimens (13 mm x13 mm x1 mm) were prepared (n=10). A conventional Au-Pd-In metal-ceramic alloy (Orion) and an ultra low-fusing porcelain (Finesse) served as the control (n=10). Forty machined titanium specimens were prepared from 1.00-mm thick titanium sheets with a diamond band saw. Forty titanium specimens were produced in a centrifugal dental titanium casting machine. All titanium specimens were airborne particle abraded with 110-microm alumina particles, whereas the control specimens were airborne particle abraded with 50-microm alumina particles. Forty titanium specimens (20 specimens each of as-cast and machined titanium) were randomly selected for gold sputter coating before ceramic firing. An ultra low-fusing porcelain (Vita Titankeramik) was fused on the central 6-mm diameter circular area on each titanium specimen. Porcelain firing environments for the titanium specimens consisted of vacuum and a reduced argon atmosphere. Porcelain was debonded by a biaxial flexure, constant strain test at a cross-head speed of 0.25 mm/min. Specimens were analyzed by standardized SEM/EDS analysis 3 times throughout the study to determine the silicon atomic percentage (Si at %): (1) after airborne particle abrasion, before porcelain application; (2) after the application of the first layer of porcelain; and (3) after the fracture of porcelain from the metal substrate. The titanium-ceramic adhesion was characterized by determining the area fraction of adherent porcelain (AFAP). Results were analyzed by analysis of variance and the Student-Newman-Keuls test (alpha=.05).

RESULTS

Statistical analysis showed a significant difference in the AFAP values among all the groups. AFAP value of the control group was significantly higher (135.35 +/- 23.68) than those of the experimental groups (P<.001). For the machined titanium, AFAP value of gold sputter-coated/argon group (91.38 +/- 7.93) was significantly higher than the rest of the groups (P<.001). For the as-cast titanium fired in vacuum, significantly lower AFAP values (P<.001) were found in the gold sputter-coated group (50.2 +/- 11.26 vs 66.15 +/- 10.41). AFAP values between the argon groups with or without the gold coating were not significantly different (P=.303); however, both argon groups (93.83 +/- 4.65 and 98.09 +/- 6.35) showed significantly higher AFAP values compared with the vacuum groups (P<.001).

CONCLUSION

Firing porcelain in a reduced argon atmosphere significantly improved titanium-ceramic bonding for machined and as-cast titanium. The sputter-coated gold layer on titanium provided improved titanium-ceramic bonding only when combined with firing porcelain in reduced argon atmosphere. When porcelain was fired in vacuum in the presence of the gold layer, the titanium-ceramic bonding was weakened in as-cast titanium and was not affected in machined titanium. Conventional noble metal-ceramic bonding was superior to titanium-ceramic bonding regardless of the interfacial variables examined in this study.

Transmission electron microscopic studies of deformed high-palladium dental alloys

Objective. The purpose of this study was to employ transmission electron microscopy (TEM) to investigate the microstructures of Pd-Cu-Ga and Pd-Ga dental alloys that had been permanently deformed, in order to obtain information about the deformation behavior of individual phases and changes in microstructure brought about by that deformation.Methods. Heavily deformed regions taken from fractured tensile test bars of the two alloys in the as-cast condition were prepared for TEM analysis, using mechanical grinding and polishing, ion milling, and plasma cleaning. The specimens were examined in the TEM using bright-field and dark-field diffraction contrast imaging. Selected-area and convergent-beam electron diffraction patterns were employed to analyze the structures of the phases, and standardless energy-dispersive X-ray spectrometry was used to determine their mean compositions.Results. For both alloys, tweed structures underwent permanent deformation by twinning, whereas dislocation movement occurred in the face-centered cubic (fcc) palladium solid solution matrix. A body-centered cubic (bcc) phase, previously unreported in our TEM studies and containing a high density of dislocations, was identified in the Pd-Cu-Ga alloy, while fine-scale, stress-induced precipitates were found in some regions of the fcc matrix in the Pd-Ga alloy.Significance. The present results have provided novel information about the mechanical deformation behavior of high-palladium alloys. The stress-induced precipitation in the Pd-Ga dental alloy studied may be a critical component of strengthening mechanisms.

Alloys for prosthodontic restorations

The numbers and types of alloys for prosthodontic restorations have increased dramatically over the past 25 years, making selection of an alloy for a given clinical situation difficult. Factors such as cost, the need for better strength, and worries about alloy corrosion have pressured the alloy market to change significantly. A number of properties--including yield strength, hardness, elastic modulus, microstructural phases, grain size, corrosion performance, coefficient of thermal expansion, oxide color, and melting range--are relevant to the proper selection of an alloy for a given clinical problem. In this article, a brief historical look at prosthodontic alloys and the nomenclature for alloys is followed by a discussion of the most important physical properties of alloys for clinical practice. A summary of the types of alloys available today and their classification is then presented. Finally, speculations about future trends for alloys are made, and simple guidelines are suggested to help dentists choose appropriate alloys for their practices. This review excludes implant alloys, dental amalgam, and alloys for orthodontic and endodontic applications.

Inductively coupled plasma-mass spectroscopy measurements of elemental release from 2 high-palladium dental casting alloys into a corrosion testing medium

STATEMENT OF PROBLEM

The biocompatibility of high-palladium alloy restorations has been of some concern due to the release of palladium into the oral environment and sensitivity reactions in patients.

PURPOSE

This study measured the in vitro elemental release from a Pd-Cu-Ga alloy and a Pd-Ga alloy into a corrosion testing medium.

MATERIAL AND METHODS

Both alloys were cast into 12-mm-diameter x 1-mm-thick disks, subjected to heat treatment that simulated porcelain firing cycles, polished to a 0.05-mm surface finish, and ultrasonically cleaned in ethanol. Two specimens of each alloy were immersed 3 times (at 7, 70, and 700 hours) in an aqueous lactic acid/NaCl solution used for in vitro corrosion testing and maintained at 37 degrees C. The specimens were removed after each immersion time, and the elemental compositions of the solutions were analyzed with inductively coupled plasma-mass spectroscopy (ICP-MS). Elemental concentrations for the 2 alloys at each immersion time were compared with Student t test (alpha=.05).

RESULTS

No significant differences in palladium release were found for the 7- and 70-hour solutions, but significant differences were found for the 700-hour solutions. Mean concentrations of palladium and gallium in the 700-hour solutions, expressed as mass per unit area of alloy surface, were 97 (Pd) and 46 (Ga) microg/cm(2) for the Pd-Cu-Ga alloy and 5 (Pd) and 18 (Ga) microg/cm(2) for the Pd-Ga alloy.

CONCLUSION

Relative proportions of the elements in the solutions were consistent with the release of palladium and breakdown of microstructural phases found in the alloys. The results suggest that there may be a lower risk of adverse biological reactions with the Pd-Ga alloy than with the Pd-Cu-Ga alloy tested.

Potentiodynamic polarization study of the in vitro corrosion behavior of 3 high-palladium alloys and a gold-palladium alloy in 5 media

STATEMENT OF PROBLEM

Corrosion of cast alloy restorations may lead to their failure or adversely affect their biocompatibility. Although some documentation of the corrosion behavior of the high-palladium dental alloys exists, questions remain about their corrosion resistance and mechanisms.

PURPOSE

This study compared the in vitro corrosion characteristics of 3 high-palladium alloys and 1 gold-palladium alloy in simulated body fluid and oral environments.

MATERIAL AND METHODS

Two Pd-Cu-Ga alloys and 1 Pd-Ga alloy were selected; an Au-Pd alloy served as the control. The corrosion behavior for the as-cast and simulated porcelain-firing (heat-treated) conditions of each alloy (N = 5) was evaluated in 0.9% NaCl, 0.09% NaCl, and Fusayama solutions. Heat-treated specimens of each alloy (N = 5) were also tested in N(2)-deaerated 0.09% NaCl and Fusayama solutions (pH 4). After immersion in the electrolyte for 24 hours, the open-circuit potential (OCP) was measured, and linear polarization was performed from -20 mV to +20 mV (vs. OCP) at a scanning rate of 0.125 mV/s. Cyclic polarization was performed from -300 mV to +1000 mV and back to -300 mV (vs. OCP) at a scanning rate of 1 mV/s. Data were evaluated with analysis of variance and the Ryan-Einot-Gabriel-Welsch multiple-range test (alpha=.05).

RESULTS

The OCP of each alloy varied with the condition (as-cast or heat-treated) and electrolyte used. Corrosion resistance was similar for the 4 alloys tested. For cyclic polarization, all alloys showed active-passive or spontaneous passive behavior in nearly all electrolytes. During some reverse scans, the 3 high-palladium alloys displayed 3 or 5 anodic peaks. No positive hysteresis was observed for any of the alloy/electrolyte combinations evaluated.

CONCLUSION

The corrosion resistances of the 3 high-palladium alloys in simulated body fluid and oral environments were comparable to that of the gold-palladium alloy. The similar corrosion resistance for the 3 high-palladium alloys was attributed to their high noble metal content and theorized stable structure at the submicron level. Selective corrosion of different phases and elements, surface enrichment of palladium, and adsorption of species are possible corrosion mechanisms. The cyclic polarization results suggest that none of the 4 alloys would be prone to pitting or crevice corrosion under in vivo conditions, but crevice conditions should nonetheless be avoided for these alloys in the oral environment.

Porcelain adherence to dental cast CP titanium: effects of surface modifications

OBJECTIVES

A reaction layer forms on cast titanium surfaces due to the reaction of the molten titanium with the investment material. Such a layer prevents strong adhesion between titanium and porcelain. This study characterized the effects of surface modifications on cast titanium surfaces and titanium-ceramic adhesion.

METHODS

ASTM grade II CP titanium was cast into an MgO-based mold. Castings were devested by sandblasting with alumina particles, and subjected to surface modification by immersion in one of the following solutions: (1) 35% HNO3-5% HF at room temperature for 1min; (2) 50% NaOH-10% CuSO4 x 5H2O at 105 degreesC for 10min; (3) the NaOH-CuSO4 solution followed by the HNO3-HF solution; (4) 50% NaOH-10% NaSO4 at 105 degreesC for 10 min; (5) the NaOH-NaSO4 solution followed by the HNO3-HF solution; and (6) 50% NaOH solution at 105 degreesC for 10min. Surfaces only sandblasted with alumina were used as controls. Specimen surfaces were characterized by XRD and SEM/EDS, and hardness-depth profiles were determined. All specimens were sandblasted with 110 microm alumina particles before porcelain firing. An ultra-low-fusing porcelain (Vita Titankeramik) was fused on the titanium surfaces. The titanium-ceramic adhesion was characterized by a biaxial flexure test, and area fraction of adherent porcelain (AFAP) was determined by X-ray spectroscopy.

RESULTS

EDS analyses revealed a substantial amount (13-17 wt%) of Al on the control, and specimens modified with Methods 2, 4, and 6. XRD revealed residual stress in the titanium surfaces and corundum on the control, and Methods 2, 4, and 6 specimens. A new Ti(Cu, Al)2 phase was identified on the titanium surfaces modified by immersion in 50% NaOH-10% CuSO4 x 5H2O aqueous solution. Reduced residual stress was observed on Method 1, 3, and 5 specimens. No corundum peaks were detected on these specimens. Compared to the control, significantly lower (P < 0.05) hardness values were found for Methods 1-3 and Method 5 specimens at 20 microm below the surfaces and for Method 1-5 specimens at 50 microm below the surfaces. Significantly higher (P < 0.05) AFAP values were found for surfaces modified with Methods 2-6 compared to the control and Method 1, and no significant differences were found among Methods 1-6, and between control and Method 1.

SIGNIFICANCE

Based on the results from the present study, porcelain adherence to cast CP titanium can be improved by the caustic baths used in the study.

Effect of soldering on metal-porcelain bond strength in repaired porcelain-fused-to-metal castings

STATEMENT OF PROBLEM

Chemical bonding plays a major role in the adherence between metal and porcelain. The formation of an oxide layer on solder material has not been described in the literature. It is unknown whether the application of solder negatively affects the bond strength between porcelain and metal.

PURPOSE

This in vitro study assessed the effect of solder on the bond strength between metal and porcelain.

MATERIAL AND METHODS

Forty 20 x 6 x 0.5 mm patterns were divided into test (20) and control (20) groups. Test samples were perforated and repaired with solder, and 2 layers of opaque and dentin porcelain subsequently were applied on all samples. The samples were subjected to a 3-point flexural test on a screw-driven mechanical testing machine at a crosshead speed of 0.5 mm/min. Failure type (adhesive vs cohesive) was quantified by digitizing photographs of test and control samples. Three samples in each group also were examined with an SEM coupled with an x-ray energy-dispersive spectroscopy apparatus (SEM/EDS). Means and standard deviations of loads at failure, sample thickness, and surface area covered with porcelain were calculated, and data were analyzed with Student t test (P < or = 0.05).

RESULTS

The mean fracture load for test samples was significantly greater than for control samples (P = 0.0038). Test samples also were significantly thicker (mean thickness difference 0.14 mm) (P = 0.0001). When the data were controlled for thickness by using a multiple linear regression analysis, no significant difference was found (P = 0.68). Test samples had a greater surface area covered with opaque porcelain (P = 0.0006) as determined by visual inspection.

CONCLUSION

In this study, soldered and nonsoldered samples did not show any significant difference in porcelain-to-metal bond strength. Visual analysis revealed a significant difference in the amount of porcelain remaining on the fracture surface of the test and control samples; a complete quantitative elemental analysis with SEM/EDS is in progress.

In vitro corrosion resistance of high-palladium dental casting alloys

OBJECTIVE

The objective of this study was to characterize the in vitro corrosion behavior of five high-palladium dental alloys in two media using a potentiodynamic polarization technique.

METHODS

Potentiodynamic cyclic polarization between -1000 and +1000 mV (SCE), 12 h open-circuit potential measurement, and linear polarization were performed on cast specimens of three Pd-Cu-Ga alloys and two Pd-Ga alloys in deaerated 0.09% NaCl solution and Fusayama artificial saliva at 37 degrees C. Zero-current potential, corrosion current density, open-circuit potential, and polarization resistance were determined. The Kruskal-Wallis test and the Mann-Whitney U-test were used to analyze the numeric findings.

RESULTS

The Pd-Cu-Ga and Pd-Ga alloys in both the as-cast and heat-treated conditions showed spontaneous passive behavior under electrochemical conditions similar to those in the oral environment. The Pd-Cu-Ga alloys Liberty and Spartan Plus exhibited increased activity in the Fusayama artificial saliva after the porcelain-firing heat treatment, which may have arisen from internal oxidation of casting defects. Heat-treated Freedom Plus had an unstable oxide film to form on the surface during anodic polarization and may have a limited tendency for pitting corrosion in the Fusayama artificial saliva. The two heat-treated Pd-Ga alloys remained passive in the test media. As a group, the high-palladium alloys exhibited satisfactory corrosion resistance.

SIGNIFICANCE

Corrosion of high-palladium alloys in the oral environment involves a release of ions that is related to their biocompatibility. Knowledge of the in vitro corrosion behavior of these alloys may lead to better understanding of any biologically adverse effects in vivo.

Effects of dental laboratory processing variables and in vitro testing medium on the porcelain adherence of high-palladium casting alloys

PURPOSE

This study assessed the effects of dental laboratory processing variables and testing medium on porcelain adherence of representative high-palladium alloys.

MATERIAL AND METHODS

The processing variables were recasting two and three times, porcelain stripping with hydrofluoric acid and rebuilding, and changing the recommended oxidation procedures. The testing medium was air for these four groups and the nontreatment group. In the last group, the specimens were stored and tested in artificial saliva. The metal ceramic specimens were fractured with biaxial flexure in constant strain. The area fraction of adherent porcelain (%) was calculated with a standardized spectrometric technique. A gold-palladium alloy served as the control.

RESULTS

Recasting without adding new alloy negatively affected some high-palladium alloys. Porcelain stripping did not cause a decrease in adherence of the tested alloys. The alternate oxidation treatment reduced significantly the porcelain adherence of the Au-Pd and one Pd-Cu-Ga alloy. Storage and testing in artificial saliva did not significantly affect the porcelain adherence of the alloys. The Au-Pd alloy exhibited the highest porcelain adherence.

Porcelain adherence vs force to failure for palladium-gallium alloys: a critique of metal-ceramic bond testing

OBJECTIVES

The purpose of this study was to characterize the metal-ceramic bond of four commercial Pd-Ga alloys by two separate tests: porcelain adherence and bond failure force. A Pd-Ag alloy was the control. A secondary goal was to investigate a possible correlation between the percent porcelain retained and bond failure forces.

METHODS

For adherence testing, five cast plates of each alloy were air abraded and oxidized. The porcelain was applied on a circular area in the center of each plate and fractured using constant-strain flexure. The area fraction of adherent porcelain (%) was calculated via a standardized spectrometric technique. Bond failure forces were measured in three-point bending using bar specimens. The modulus of elasticity of the five alloys was also measured. No attempt was made to calculate bond strength since the residual thermal stresses at the interfacial region were unknown. Data were compared via ANOVA and the Tukey multiple range test (p < 0.05).

RESULTS

All failures occurred at the porcelain termination sites with no tensile fracture of the porcelain. Two Pd-Ga alloys exhibited porcelain adherence comparable with previously studied Pd-Cu-Ga alloys. There was no correlation between the porcelain adherence and the force to failure (r2 = 0.0159). Additionally, no statistically significant differences were found among the moduli of elasticity of the five alloys.

SIGNIFICANCE

The new Pd-Ga alloys have been commercially introduced with little information on the quality of the bond with porcelain. The use of two different means for assessing the metal-ceramic bond provides information on the relationship of data using different experimental techniques.

Metallurgical structure and microhardness of four new palladium-based alloys

PURPOSE

This investigation compared the Vickers hardness and microstructures of four recently marketed, palladium-based alloys for metal-ceramic restorations.

MATERIALS AND METHODS

Wax patterns simulating copings for maxillary central incisors were invested in a fine-grained, carbon-free, phosphate-bonded investment. Following burnout, the palladium alloys were fused with a gas-oxygen torch, centrifugally cast, and bench-cooled. Representative castings were embedded in transparent metallographic resin and sectioned to yield two mirror-image specimens. The specimens were evaluated in either the as-cast condition or following heat treatment simulating the firing cycles for Vita VMK porcelain. Vickers hardness measurements (n = 50) were made using a 1-kg load, and photomicrographs of polished and etched specimens were obtained with a scanning electron microscope.

RESULTS

The measured values of microhardness for the as-cast alloys were in excellent agreement with values reported by the manufacturer. The hardness in the as-cast condition was significantly greater for the Pd-Cu-Ga-In alloy, compared with the other three alloys, which did not contain copper. For the three high-palladium (> or = 75 wt%) alloys, there were small (4%-8%) decreases in hardness following heat treatment, whereas a larger decrease (13%) in hardness occurred for the Pd-Ag-In-Sn alloy after heat treatment. The porcelain firing cycles caused microstructural homogenization for all four alloys, and the relatively thick near-surface oxidation region in the Pd-Cu-Ga-In and Pd-Ag-In-Sn alloys was not observed in the two heat-treated Pd-Ga-Ag-In-Au alloys.

CONCLUSIONS

The multiphasic microstructures of these alloys may have some significance for the in vitro and clinical corrosion behavior and the metal-ceramic bond strength. The hardness for the three high-palladium alloys may be controlled by submicroscopic precipitates that remain unaltered by heat treatment. The significantly greater hardness for the Pd-Cu-Ga-In alloy may cause greater difficulty for finishing castings in the dental laboratory compared with the other three alloys studied. The strengthening mechanism for the Pd-Ag-In-Sn alloy has significant temperature dependence, which might be exploited to achieve optimum mechanical properties.

X-ray diffraction studies of oxidized high-palladium alloys

OBJECTIVE

The purpose of this study was to use x-ray diffraction (XRD) to obtain new information about the oxide layers on four representative oxidized high-palladium alloys.

METHODS

Cast specimens of two Pd-Cu-Ga alloys and two Pd-Ga alloys, with both polished and etched surfaces and air-abraded surfaces, were subjected to oxidation procedures recommended by the manufacturers. The specimens were analyzed by x-ray diffraction using CuK alpha radiation, and the peaks were compared to appropriate Joint Committee on Powder Diffraction Standards (JCPDS).

RESULTS

The surface preparation procedure had a profound effect on the phases present in the oxide layers. For the specimens that had been polished and etched, CuGa2O4 and beta-Ga2O3 were detected on the 79Pd-10Cu-9Ga-2Au alloy, whereas SnO2 and CuGa2O4 were detected on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy. The oxide layers on both Pd-Cu-Ga alloys contained Cu2O1 and the oxide layer on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy may contain beta-Ga2O3. The principal phase in the oxide layers on both Pd-Ga alloys that had been polished and etched was ln2O3, which exhibited extreme preferred orientation. No other phase was detected in the oxide layer on the 85Pd-10Ga-2Au-1Ag-1 ln alloy, whereas beta-Ga2O3 was found in the oxide layer on the 75Pd-6Ga-6Au-6Ag-6.5ln alloy. For the air-abraded specimens, beta-Ga2O2 was not present in the oxide layers on the Pd-Cu-Ga alloys, and beta-Ga2O3 was the major phase in the oxide layers on the Pd-Ga alloys. Palladium oxide(s) in varying amounts were detected for both surface preparations of the Pd-Cu-Ga alloys and for the air-abraded Pd-Ga alloys. Except for the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy, the oxide layer caused minimal change in the lattice parameter of the palladium solid solution compared to that for the as-cast alloy.

SIGNIFICANCE

Knowledge of the phases found in the oxide layers on these high-palladium alloys is of fundamental importance for interpreting differences in the adherence of dental porcelain to the metal substrates under static and dynamic conditions, and may provide guidance in the development of new high-palladium alloys with improved metal-ceramic bonding.

X-ray diffraction studies of as-cast high-palladium alloys

OBJECTIVES

The purpose of this study was to use x-ray diffraction (XRD) to investigate four representative high-palladium alloys in the as-cast condition and obtain new information about the metallurgical phases.

METHODS

Two Pd-Cu-Ga alloys and two Pd-Ga alloys were cast into plate-shaped specimens (20 mm x 1.5 mm) which were bench-cooled. Polished and etched specimens were analyzed using two different x-ray diffractometers and CuKa radiation.

RESULTS

All four alloys exhibited strong XRD peaks for the face-centered cubic (fcc) palladium solid solution matrix, and variations in the lattice parameter were consistent with alloy compositions. Comparison of the relative peak intensities for the alloys and the pure palladium powder standard indicated that the as-cast microstructures had preferred crystallographic orientation. Because the two Pd-Cu-Ga alloys contained appreciable amounts of near-surface lamellar interdendritic or eutectic constituents, it was possible to determine previously unreported XRD peaks for the Pd2Ga phase in these alloys. Low-intensity peaks in the Pd-Ga alloys were attributed to small amounts of secondary phases observed in the microstructures.

SIGNIFICANCE

Knowledge of the metallurgical structures of these high-palladium alloys can be important for interpretation of microstructural observations, mechanical properties, corrosion behavior and clinical performance.