Thermal expansion of dental casting alloys and phosphate-bonded investments.
For a dental investment to render accurate castings, it is essential 1) that the setting expansion is low enough not to cause any distortion of the wax pattern, and 2) that the thermal expansion is high enough to compensate fully for the solid thermal contraction of the casting alloy used1-8). However, all phosphate-bonded investments known today when used according to the manufacturers' instructions, are characterized by a relatively high setting expansion, and by a thermal expansion insufficient to compensate adequately for the solid thermal contraction of conventional casting alloys. Recent studies9) have shown that the setting expansion of representative European brands of phosphate-bonded investments may be reduced to an acceptable low value by modifying the mixing liquid. It was the purpose of the present work to study the thermal expansion of thus modified products, and for comparison also the thermal expansion of several dental casting alloys.
Corrosion properties of soldered joints. Part I: Electrochemical action of dental solder and dental nickel-chromium alloy.
The corrosion properties of contact between six Ni-Cr alloys and two dental solders were evaluated. Ni-Cr alloys with low Cr content coupled with silver solder induced continuous galvanic current, whereas Ni-Cr alloys with high Cr and some Mo content coupled with silver or gold solder induced galvanic current for a short time. Six Ni-Cr alloys were divided into two groups according to the dissolution of Ni, one with low resistance and the other with high resistance. The initial equilibrium was induced between high resistance Ni-Cr alloys and both solders by less current density, which implies a smaller degree of corrosive current than that in low-resistance alloys. Corrosion was observed in the silver solder coupled with high-resistance Ni-Cr alloy, but was not found in the gold solder coupled with both alloys. In soldering Ni-Cr alloy, it is better to use a gold solder and a high-resistance Ni-Cr alloy for corrosion resistance.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.