Explains why noble alloys suit porcelain fused to metal restorations, from the need for small additions of easily oxidised metals that form the bonding oxide layer. Covers the main noble alloy types, including which are suitable and which are not, with a table of approximate contents. Also compares noble and base alloys in a summary table, and notes handling precautions for palladium and silver alloys.
Topic 35 of 52 in the MU Sofia syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 7 sections, about 213 words (2 min of reading), with 13 flashcards.
In this note
Introduction
Which noble alloys can be used
What makes them suitable
Gold-platinum alloys
Gold-palladium alloys
Palladium-silver and palladium alloys
Noble against base alloys for metal ceramics
Introduction
Noble alloys for porcelain fused to metal combine high biological tolerance with a chemical bond to porcelain. Pure noble metals do not oxidize, so small amounts of easily oxidized metals, such as tin, indium, zirconium, niobium and iron, must be added. These form the thin oxide layer that the porcelain bonds to.
What makes them suitable
All alloys used for metal ceramics are type IV, the extra-high-strength type, so that the frame does not bend under chewing.
Gold-platinum alloys
These have a melting range of 1050 to 1150 °C, a homogeneous structure, high corrosion resistance and excellent castability. For metal ceramics, they also contain copper, indium, tin or zirconium, which form a strong oxide layer.
Gold-palladium alloys
About 50% gold and 40% palladium, with alloying elements such as platinum, indium, zinc, tin, iridium and gallium. These lower the melting temperature, improve castability, refine the grain and form the bonding oxide layer. They are the cheapest gold alloys, with a density of about 11 to 13 g/cm³.
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