High-Copper Amalgam: Types, Composition, and Properties

High-Copper Amalgam: Types, Composition, and Properties

Classify dental amalgams and describe high-copper amalgams.
Answer:

High-copper alloys contain more than 6% copper. They are preferred because of their improved mechanical properties, resistance to corrosion, and better marginal integrity because the weakest γ₂ phase is eliminated in high-copper amalgam.

Types of  High-copper amalgam.

1. Admixed alloy powder:
2. Single composition alloy powder.

1. Admixed Alloy Powder:

• They are made by mixing one part silver-copper eutectic alloy (high-copper spherical particles) with two parts silver tin alloy (low-copper lathe-cut particles).
• Amalgam made from admixed powder is stronger than amalgam made from lathe-cut low-copper powder, because of the silver-copper particles which act as filers in the amalgam matrix, hence strengthening the amalgam.
• In its setting reaction, the weakest g2 phase is eliminated by η phase

Composition:

Silver    –   69%

Tin        –   17%

Copper –   13%

Zinc      –      1%

Setting Reaction (Amalgamation):

When the alloy is mixed with mercury, mercury begins to dissolve the outer portion of the particles. Silver from the silver-copper eutectic alloy particles and both silver and tin from the silver-tin alloy particles enter the mercury. The tin dissolved in the mercury reacts with the copper of the silver-copper particles and forms the Cu₆Sn₅(η or Eta).

The η crystals form around the unreacted silver-copper particle. At the same time, γ₁ phase is also formed. As in the low-copper alloys γ₁ surrounds everything forming the matrix. γ₂ is also formed at the same time but is later replaced by η. Thus in admixed alloy the undesirable γ₂ phase is greatly reduced.

Ag₃Sn  +  Ag-Cu  +  Hg Ag₂ Hg₃  +  Cu₆Sn₅  +  Ag₃Sn unreacted  +  Ag-Cu unreacted
(b+g)     (eutectic)    (γ₁) (η)  (b+g)    (eutectic)

Microstructure:

The microstructure of set high-copper admixed amalgam consists of core particles of unreacted γ phase and unreacted Ag–Cu eutectic which is surrounded by the halo of η phase. The core particles are embedded in a matrix of γ₁. Here the γ₂ phase is eliminated and replaced by a stronger η phase.

2. Single Composition Alloy

Unlike admixed alloy powders, each particle of the alloy powder has the same composition. Therefore, they are called single composition or uncompositional alloys.

Composition

Silver  –   40 – 60%

Tin      –   20 – 30%

Copper –  13 – 30%

Zinc      –     0 – 4%

Indium or Palladium – Small amounts

Setting Reaction (Amalgamation):

• Though each particle has the same composition, silver, tin, and copper present exist in various phases within the particle. Thus each particle contains Ag₃Sn (γ), AgSn (β), and Cu₃Sn (ε). When triturated, silver and tin from the particle dissolve in mercury forming the γ₁ (Ag₂ Hg) crystal matrix that binds together the partially dissolved alloy particles.
• At this stage, very little copper dissolves. Later, a layer of η (Cu₆Sn₅ ) crystals is formed at the surface of alloy particles. Some η (Cu₆Sn₅) crystals also form in the matrix.

Ag-Sn-Cu  +  Hg  → Cu₆Sn₅  +  Ag₂ Hg₃ +  Ag-Sn-Cu
(γ + β + ε)                  (γ₁)         (unreacted)

Microstructure:

• The difference between the elimination of γ₂ phase in an admixed and uni compositional alloy is that in the admixed type the γ₂ forms around the silver, and tin particles and is eliminated around the silver-copper particles.
• In the uni-compositional type, the particles at the beginning of the reaction function like silver tin particles of the admixed type and later the same particles function like the silver-copper particles of the admixed type, eliminating γ₂ phase.

Set amalgam consists of core particles of unreacted γ, β, and ε phases which are surrounded by a mesh of rod-shaped η phase. The core particles and η phase are embedded in a matrix of γ₁.