Dental Materials · MU Sofia

20. Noble dental alloys - characteristics

Covers the general makeup of noble dental alloys, their melting range, density, hardness and elasticity in a summary table, and their main strengths and weaknesses. It then sorts them by strength into four types and by composition into gold, white gold, white noble and palladium groups, with approximate percentages for each, and notes which suit metal ceramics and which need care with gases or porcelain.

Inside: 4 sections · 2 labelled pictures · 1 table · 12 flashcards · 2 min

Topic 20 of 52 in the MU Sofia syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 4 sections, about 323 words (2 min of reading), with 12 flashcards.

In this note

  1. Introduction
  2. General properties
  3. Types by strength
  4. Groups by composition

Introduction

Noble dental alloys contain mostly noble metals: gold, platinum, palladium, iridium, rhodium, ruthenium and osmium, with silver usually counted too, though it oxidizes. Noble metals resist oxidation and do not dissolve in most acids. The amount is given in carats or as fineness: parts per 1000 by weight.

General properties

  • Strengths: very high corrosion resistance and biological tolerance, excellent castability, easy soldering, and good ductility. Hardening can improve mechanical properties by about 20%.
  • Weaknesses: high cost, and high thermal and electrical conductivity.

Groups by composition

Gold-silver-copper alloys

These are made at fineness 916 (22 carat), 780 and 530. Silver and copper make the gold harder, and copper lowers the melting point and adds a reddish shade. They are heterogeneous and can tarnish, and they are unsuitable for metal ceramics. Heating at about 800 °C for 30 minutes improves homogeneity. Abroad they have not been produced or certified by the ADA since 1984.

Gold-platinum alloys

These contain 70 to 90% gold and 3 to 20% platinum, with iridium and ruthenium as grain refiners. They are type IV alloys with the highest density (19.5 g/cm³), the highest mechanical properties of noble alloys, a homogeneous structure and excellent biological tolerance. They turn white above 5 to 8% platinum. High cost and weight limit their use.

White gold alloys

  • Gold-palladium: about 50% gold and 40% palladium, suitable for metal ceramics.
  • Gold-silver: about 50% gold and 40% silver. Silver can turn porcelain green, so they are not recommended for metal ceramics.

White noble alloys

  • Silver-palladium: about 70% silver and 25% palladium; not for metal ceramics.
  • Palladium-silver: about 60% palladium and 30% silver; the cheapest alloys for metal ceramics.

These are the cheapest and lightest noble alloys. Palladium and silver absorb gases, so avoid open flames and graphite.

Palladium alloys

Offered since 1987 for metal ceramics, these contain about 80% palladium with copper, gold or cobalt. They are white, relatively cheap, melt above 1200 °C and are the hardest noble alloys (Vickers hardness 425).

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