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


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.
Vermilyea
S G
SG
Section of Primary Care, College of Dentistry, Ohio State University, Columbus 43210-1241, USA.
Cai
Z
Z
Brantley
W A
WA
Mitchell
J C
JC
D000704:Analysis of Variance; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003300:Copper / Q000737:chemistry; D003722:Dental Alloys / Q000737:chemistry*; D004548:Elasticity; D005708:Gallium / Q000737:chemistry; D006244:Hardness; D006358:Hot Temperature; D007204:Indium / Q000737:chemistry; D008422:Materials Testing; D010165:Palladium / Q000737:chemistry*; D013718:Tensile Strength
D003722:Dental Alloys; D007204:Indium; D010165:Palladium; D003300:Copper; D005708:Gallium