20171126 Published by Wiley-Blackwell written by J. Anthony von Fraunhofer『Dental Materials at a Glance』p.35より抜粋

Published by Wiley-Blackwell
written by J. Anthony von Fraunhofer
『Dental Materials at a Glance』p.35より抜粋
ISBN-10: 1118459962
ISBN-13: 978-1118459966


15.1 Gold and noble metals
1 Pure gold: Now of limited use, gold formerly was used as a direct filling material because it welds to itself under pressure. Gold is embrittled by Pb, Bi, and Hg and typically is alloyed with Cu, Ag, and Pt to improve its mechanical properties.


2 Noble metals: The noble metals are elements with high electropositive potential, i.e. resist electrochemical corrosion. They are dense, have a high melting point（MP）, and resist oxidation at temperature.


３ Platinum: Pt foil is used as a matrix for fused porcelain（PFM）restorations because of oxidation resistance, high MP, and expansion coefficient similar to porcelain that prevent metal buckling or porcelain fracturing during temperature changes. Pt addition of ≤ to gold improve its hardness and elasticity but lighten its color.


4 Palladium: Pd has lower cost and is less dense than Pt and has the lowest MP of noble metals; it is used as alloying element for Au and Ag but whitens color of Au. White golds contain large amounts of Pd and Ag. Pd occludes hydrogen when heated, requiring care during melting.


5 Iridium, ruthenium, and rhodium: These are added in small amounts（ca. 50ppm）as grain refiners for casting alloys.


15.2 Precious metals
Precious metals have high cost but are not necessarily noble（e.g., silver）. Silver is a precious but non-noble metal that is ductile, malleable, highly conductive, and harder than Au but has lower MP. It is susceptible to corrosion（improved by Pd addition）, especially in S and Cl media. Pure Ag occludes air and O2 on melting, reduced by 5-10% additions of Cu. Ag readily alloys with Au and reduces red color of Au-Cu alloys.


15.3 Gold alloys
15.3.1 Carat and fineness of gold
Gold content is traditionally designated by the carat（1k=1/24 of the gold content ）or fineness; these terms now are rarely used in dentistry.


15.3.2 Gold copper alloys
Au and Cu form a continuous series of solid solutions, stable above 424℃ but transforming into the ordered phase AuCu and AuCu3 below 424℃. The face-centered cubic AuCu3 phase structure is not found in dental alloys. AuCu has face-centeredtetragonal structure, the unit cell being a cube with Au atoms at the center of the side faces. The high-temperature cubic lattice transforms to a tetragonal lattice at lower temperatures, hardening the alloy by inducing localized strains that inhibit dislocation movement.

Specification of gold content of yellow alloys is 62-92.5 wt.% or 34-78 wt.%. AuCu contains 75% gold by weight but only 50% of the number of atoms; also, increasing Cu content decreases MP. Slow cooling to RT allows transformation to ordered tetragonal AuCu with attendant strength increase, which is the basis of gold alloy heat treatment. At minimum 75% gold content is required for corrosion resistance and for good castability.