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

Abstract

The goals of this study were to investigate the fatigue limits of two Pd-Ag alloys (Ivoclar Vivadent) with differing mechanical properties and varying proportions of secondary alloying elements, examine the effect of casting porosity on fatigue behavior, and determine the effect of casting size on microstructures and Vickers hardness. The alloys selected were: IPS d.SIGN 59 (59.2Pd-27.9Ag-8.2Sn-2.7In-1.3Zn); and IS 64 (59.9Pd-26.0Ag-7.0Sn-2.8Au-1.8 Ga-1.5In-1.0Pt). Tension test bars, heat-treated to simulate dental porcelain application, were subjected to cyclic loading at 10 Hz, with R-ratio of -1 for amplitudes of compressive and tensile stress. Two replicate specimens were tested at each stress amplitude. Fracture surfaces were examined with a scanning electron microscope (SEM). Sectioned fatigue specimens and additional cast specimens simulating copings for a maxillary central incisor restoration were also examined with the SEM, and Vickers hardness was measured using 1 kg load. Casting porosity was evaluated in sectioned fatigue fracture specimens, using an image analysis program. The fatigue limit (2 × 10(6) loading cycles) of IS 64 was approximately 0.20 of its 0.2% yield strength, while the fatigue limit of d.SIGN 59 was approximately 0.25 of its 0.2% yield strength. These relatively low ratios of fatigue limit to 0.2% yield strength are similar to those found previously for high-palladium dental alloys, and are attributed to their complex microstructures and casting porosity. Complex fatigue fracture surfaces with striations were observed for both alloys. Substantial further decrease in the number of cycles for fatigue failure only occurred when the pore size and volume percentage became excessive. While the heat-treated alloys had equiaxed grains with precipitates, the microstructural homogenization resulting from simulated porcelain firing differed considerably for the coping and fatigue test specimens; the latter specimens had significantly higher values of Vickers hardness.

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

“…Alloys that contain Pd are known for their capacity to absorb gases from the environment during the casting process, particularly when heated with an inadequately set blowtorch. 9 Perhaps this capacity of Pd alloy to absorb gases did not have such a pronounced effect on the mechanical properties in this study. An advantage of controlling the casting temperature is that it prevents the materials from being overheated, and consequently, precludes the creation of porosities by volatilization of components with lower fusion temperatures.…”

“…It is known that the casting process may increase the porosity of noble alloys, particularly those containing palladium (Pd) in their composition. 9 Another source of alloy contamination during the casting process is Declaration.of.Interests:.The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript.…”

Hardening effect of pre- and post-firing heat treatment for a firing-simulated Au-Pd-In metal-ceramic alloy

“…By the subsequent simulated firing schedule of the main bake, correction and glaze, the hardness decreased gradually and finally became similar to that of the as-cast state (226.9 HV). From the above, the porcelain firing full cycle did not result in hardening of the as-cast specimen [11], unlike the case of the Au-Pt and Au-Pd-Ag metal-ceramic alloys [9,10,12]. After the pre-firing heat treatment after casting, the hardness decreased to 207 HV [10,13].…”

Change in hardness of an as-cast and softening heat-treated low-gold-content alloy for bonding porcelain by simulated porcelain firing and its mechanism

“…Then during cooling to room temperature, the alloys can be hardened to some extent by atomic ordering or precipitation of the alloy components [1,2]. Therefore, depending on the composition, the alloy becomes much softer after several porcelain-firing cycles [3][4][5].…”