Amalgamation Process/ Basic Setting Reaction

Low-copper alloysHigh-copper alloys

admixed alloys single composition alloys

Endanus Harijanto

BASIC COMPOSITION

• A silver-mercury matrix containing filler particles of silver-tin

• Filler (bricks)– Ag3Sn called gamma

• can be in various shapes– irregular (lathe-cut), spherical,

or a combination

• Matrix– Ag2Hg3 called gamma 1

• cement – Sn8Hg called gamma 2

• voids

Low-copper Alloys (1)

• When a powder (Ag3Sn, ) is triturated, the Ag and Sn in the outer portion of the particles dissolve into Hg (mercury).

• Hg also diffuses into the alloy particles.– Solubility for Ag = 0.035 wt%, for Sn = 0.6 wt%

Low-copper Alloys (2)

• When the solubility is exceeded, crystals of two binary metallic compounds precipitate into the mercury.– Ag2Hg3 compound (1) precipitates first.– Sn7-8Hg compound (2) precipitates later.

As the remaining mercury dissolves the alloy particles, 1 and 2 crystals grow.

trituration condensable, carvable

Low-copper Alloys (3)

• As the mercury disappears, the amalgam hardens. Particles become covered with newly formed crystals, mostly 1.

Unconsumed particles (smaller after being partly dissolved) are surrounded and bound together by solid 1 and 2 phases.

trituration condensable, carvable harden, no longer workable

Low-copper Alloys (4)

• A typical low-copper amalgam is a composite in which the unconsumed particles are embedded in 1 and 2 phases.

P = and Ag-SnE = (Cu3Sn)

G1 = (Ag2Hg3)G2 = (Sn7-8Hg)

Low-copper Alloys (5)

• In summary,Alloy particles (+ ) + Hg 1 + 2 + unconsumed alloy particles

(+ )

• Physical properties. -phase strongest, 2 phase weakest

2. Hardness: > 1 >>> 2

3. 2 poor corrosion resistance

High-Cu: Admixed Alloys (1)

• Spherical silver-copper (Ag-Cu) eutectic alloy particles are added to lathe-cut low-copper amalgam alloy particles (Ag-Sn or +).

• The final powder is composed of two kinds of particles. “admixed”

• Ag-Cu particles act as strong fillers, strengthening the amalgam matrix.

High-Cu: Admixed Alloys (2)

• Silver-Copper eutectic alloy– 71.9 wt% Ag, 28.1 wt% Cu

– alloy, eutectic,n any combination of metals the melting point of which is lower than that of any of the individual metals of which it consists. An alloy in which the components are mutually soluble in the solid state. A eutectic alloy has a nonhomogeneous grain structure and is therefore likely to be brittle and subject to tarnishing and corrosion.

Tempe

rature

Eutecticcomposition

High-Cu: Admixed Alloys (3)

• Ag dissolves into the Hg from the Ag-Cu alloy particles.

• Both Ag and Sn dissolve into Hg from the Ag-Sn alloy particles. (same as in low-Cu alloy)

• Sn in solution diffuses to the surface of Ag-Cu alloy particles and reacts with the Cu to form the phase (Cu6Sn5) (therefore, the Sn7-8Hg or 2 is eliminated)

High-Cu: Admixed Alloys (4)

• A layer of forms around unconsumed Ag-Cu particles.

1 phase is the matrix.

• The final structure composes of the phase, Ag-Cu particles, particles, 1 matrix, and reaction layers.

High-Cu: Admixed Alloys (5)

• In summary:Alloy particles (+ ) + Ag-Cu eutectic + Hg 1 + + unconsumed alloy

of both types of particles

2 has been eliminated in this reaction, being replaced by .

• The effectiveness in eliminating 2 depends on % of copper-containing particles.

(net copper concentration of > 12% in alloy powder)

High-Cu: Single-composition Alloys (1)

• Each particle has the same chemical composition.

• Major components: Ag-Cu-Sn

High-Cu: Single-composition Alloys (2)

• Phases found in each single-composition alloy particle are (Ag-Sn), (Ag3Sn), and (Cu3Sn).

crystals are found as meshes of rod crystals at the surfaces of alloy particles (P), as well as dispersed in the matrix.

1

P

High-Cu: Single-composition Alloys (3)

• In summary:Alloy particles (Ag-Sn-Cu) + Hg 1 + + unconsumed alloy particles

• Little or none of 2 phase can form.