Study of Pd-Ag dental alloys: examination of effect of casting porosity on fatigue behavior and microstructural analysis

Electrochemical impedance spectroscopy study of corrosion characteristics of palladium-silver dental alloys

Electrochemical impedance spectroscopy (EIS) has been used to obtain insight into corrosion processes for three Pd-Ag alloys, and compare their behavior with an Au-Pd alloy. Five specimens of each alloy received clinically-appropriate simulated porcelain-firing heat treatment. EIS testing was performed at ambient temperature, using 0.09% NaCl, 0.9% NaCl and Fusayama solutions. EIS data are presented as Bode plots. At the open-circuit potential (OCP), the data fit a modified Randles equivalent electrical circuit with a constant phase element (CPE), and the charge-transfer resistance (R_CT ) and the two CPE parameters (CPE-T and CPE-P) were determined. The area-normalized capacitance of the double layer (C_dl ) was also calculated. The EIS data at two relevant elevated potentials in the passive range were also found to fit well a modified Randles equivalent circuit with different values for the charge transfer resistance and CPE parameters. At the OCP no significant effect on R_CT was found for the alloys and electrolytes, and both alloy and electrolyte significantly affected CPE-P. In vitro corrosion was controlled by charge transfer and charge accumulation processes, and the behavior differed at the elevated potentials compared to the OCP. Significant effects were found for alloy, electrolyte, and alloy/electrolyte interaction on C_dl at the OCP. The EIS parameters at elevated potentials indicate that the Pd-Ag alloys should have satisfactory clinical corrosion resistance. The EIS analyses yielded information about in vitro corrosion of these alloys that cannot be obtained from potentiodynamic polarization testing.

Effect of cooling rate on hardness and microstructure of Pd-Ag-In-Sn-Ga alloy during porcelain firing simulation

This study examined the effect of the cooling rate on the hardness and its effect on the microstructure during porcelain firing simulation of a Pd-Ag-In-Sn-Ga metal-ceramic alloy. In practice, after each firing step for porcelain bonding, the prosthesis is cooled to room temperature before proceeding to the next firing step. The cooling step is known to allow the hardness of the metal substructure to increase. The aim of the study was to determine whether controlling the cooling rate after each porcelain-firing step increases the hardness of the Pd-Ag-based metal-ceramic alloy. The results showed that the hardness of specimens cooled at a higher cooling rate increased after each firing step compared to specimens cooled at a lower cooling rate (p < 0.05). During cooling after the firing simulation the InPd₃-based phase of tetragonal structure precipitated from the Pd-Ag-rich matrix of the face-centered cubic structure. Hardening by cooling at a higher cooling rate after firing was the result of the coherency strains that formed at the interface of the Pd-Ag-rich matrix and the metastable phase based on the InPd₃ phase. . The reduced hardness obtained in the specimen cooled at a lower cooling rate after firing resulted from the loss of coherency strains as the fine metastable phases based on the InPd₃ phase were transformed into the coarser stable phase with decreased (c/a) of 0.88. This finding revealed that controlling the cooling rate during porcelain firing simulation improves the hardness of the Pd-Ag-In-Sn-Ga metal-ceramic alloy without an additional heat treatment of the alloy.

Discrepancy in alloy composition of imported and non-imported porcelain-fused-to-metal (PFM) crowns produced by Norwegian dental laboratories

Purpose: Even though the use of full ceramic crowns have become a well-established practice in dental clinics compare to the last decade, the use of imported casted porcelain-fused-to-metal (PFMs) crowns is still prevalent. The use of imported PFMs is often economically driven; however, when dentists order PFMs, they do not have capabilities to examine its true alloy content. Therefore, we raise the questions whether cheaper imported PFMs have more discrepancies in alloy content compared to domestically produced PFMs? Materials and Methods: This study included 62 porcelain-fused-to-metal crowns: 41 produced in Norway and 21 imported. Their alloy-composition was determined non-destructively by EDX and SEM. Results and Conclusions: Imported PFMs demonstrated larger deviations compared with non-imported PFMs. Significant deviation was found in key metallic elements in the different alloys (W, In, Pd, Ag). The detected deviations in key element such as Wolfram and Indium could influence the PFMs service time. These finding may be of international concern.

Effect of ice quenching after oxidation with or without vacuum on the hardness of Pd-Ag-Au-In alloy during porcelain firing simulation

This study examined the effect of ice-quenching after oxidation treatment with or without vacuum, on the change in the hardness of a Pd-Ag-Au-In metal-ceramic alloy during porcelain-firing simulation. The aim of the present study is to determine whether ice quenching after oxidation renders a Pd-Ag metal-ceramic alloy soft enough for easy processing without the need for an additional solution treatment and whether it affects the final hardness of the Pd-Ag metal-ceramic alloy after porcelain firing simulation. The oxidation treatment resulted in the as-cast specimen becoming homogenized, which was progressed further by vacuuming. The bench-cooling to room temperature after the oxidation resulted in precipitation. From the next firing step, the precipitation occurred during the firing process as well as subsequent bench-cooling; this was owing to the presence of a phase boundary wherein the single phase is separated into two phases between the temperatures of the oxidation treatment (1010 °C) and post-oxidation treatment (~960 °C). During the firing process until main bake, the specimen that was ice-quenched after oxidation maintained a higher hardness than the bench-cooled specimen, particularly for the oxidation treated specimen with vacuum (p < 0.05). The mechanism of ice-quenching after oxidation with vacuum to induce further hardening during the firing process involved more active precipitation; this was attributed to the further progress of homogenization during the oxidation. The precipitation reaction in the present study corresponded to the Pd-In binary phase diagram, illustrating that as the firing proceeded, the Pd₃(In,Sn,Ga) phase was precipitated from the Pd-Ag-Au-rich matrix. The findings revealed that ice quenching after oxidation makes the alloy soft enough for easy processing without an additional heat treatment and does not affect the final hardness of the Pd-Ag metal-ceramic alloy after porcelain firing simulation.

Effect of ice-quenching after oxidation treatment on hardening of a Pd-Cu-Ga-Zn alloy for bonding porcelain

This study examined the effect of ice-quenching after oxidation treatment on hardness change of a Pd-Cu-Ga-Zn metal-ceramic alloy during porcelain firing simulation. Although not statistically significant, the alloy was softened slightly during porcelain firing simulation with conventional slow cooling rate. On the other hand, the hardness increased significantly by ice-quenching instead of the slow cooling after oxidation (p<0.001). The gap in the final hardness depending on ice-quenching occurred in the matrix and plate-like precipitates but not in the particle-like structure without plate-like precipitates (p<0.05). The mechanism of ice-quenching after oxidation to prevent softening and induce hardening during porcelain firing simulation involved the more active precipitation and retardation of microstructural coarsening. In conclusion, for practical work on Pd-Cu-Ga-Zn alloys, the oxidation treatment followed by ice-quenching instead of slow cooling is recommended for the simultaneous oxidation and hardening effects on the alloy.

Quantitative description of short-range order and its influence on the electronic structure in Ag-Pd alloys

We investigate the effect of short-range order (SRO) on the electronic structure in alloys from the theoretical point of view using density of states (DOS) data. In particular, the interaction between the atoms at different lattice sites is affected by chemical disorder, which in turn is reflected in the fine structure of the DOS and, hence, in the outcome of spectroscopic measurements. We aim at quantifying the degree of potential SRO with a proper parameter. The theoretical modeling is done with the Korringa-Kohn-Rostoker Green's function method. Therein, the extended multi-sublattice non-local coherent potential approximation is used to include SRO. As a model system, we use the binary solid solution Ag c Pd1-c at three representative concentrations c  =  0.25, 0.5 and 0.75. The degree of SRO is varied from local ordering to local segregation through an intermediate completely uncorrelated state. We observe some pronounced features, which change over the whole energy range of the valence bands as a function of SRO in the alloy. These spectral variations should be traceable in modern photoemission experiments.

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.