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Y2 DM > Gold Alloys > Flashcards

Gold Alloys Flashcards

1
Q

What are noble metals?

A
  • metals that are stable in elemental form
  • inert to environment
  • e.g. Au, Pt
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2
Q

What are precious metals?

A

Noble metals + Ag

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3
Q

General classification of alloys w noble metals.

A
  1. High noble alloys
    - 40wt% Au, 60wt% other noble
  2. Noble alloys
    - 25wt% noble, no limit for gold content
  3. Predominantly base metal alloys
    - 75wt% base metal, 25wt% noble
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4
Q

Effect of gold on alloys.

A
  • imparts tarnish resistance to alloy (must be at least 75wt% to impart this)
  • gives ductility to alloy
  • tgt w copper, hardens alloy on heat treatment
  • contributes colour
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5
Q

Effects of platinum on alloy.

A
  • hardens alloy
  • increases melting temp of alloy
  • whitens alloy
  • principal active constituent of white gold used in dentistry
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6
Q

Effect of palladium on alloy

A
  • in sufficient conc, acts as an effective hardener
  • increases tarnish and corrosion resistance
  • increases solidification temp of alloy
  • whitens gold alloy
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7
Q

Effect of copper on alloy

A
  • increases strength & hardness when more than 4% of copper present
  • lowers tarnish resistance of alloy
  • reduces melting temp of alloy
  • reduces diff in temp between upper n lower limits of melting range
  • increases ductility
  • imparts reddish colour
  • cheap
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8
Q

Effect of zinc on alloy

A
  • acts as an oxide scavenger
  • improves castability
  • reduces melting temp of alloy
  • hardens alloy in combination w palladium
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9
Q

Effect of indium on alloys

A
  • oxide scavenger
  • promotes uniform grain size n casting fluidity
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10
Q

Types of gold alloys:

A
  1. Type I (Soft)
  2. Type II (Medium)
  3. Type III (Hard)
  4. Type VI (Extra hard)
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11
Q

Properties of Type I (soft) gold alloy

A
  • alloys of gold, silver & copper (Au, Ag, Cu)
  • 87% gold content
  • alloy cannot be hardened (cuz too much gold)
  • usually used for class III, V (no stress) & inlay
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12
Q

Properties of Type II (medium) gold alloy

A
  • alloys w gold, silver, cu, platinum/palladium,zn
  • 76% gold content
  • not emenable to age hardening
  • usually used for class III, V & onlay
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13
Q

Properties of Type III (hard) gold alloy

A
  • higher % of platinum(pt) & palladium(pd)
  • 70% gold content
  • can be age hardened (Au:Cu ratio allows heat treatment)
  • usually used for crowns or bridge abutments
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14
Q

Properties of Type IV (extra hard) gold alloy

A
  • Ag & Cu increased to lower melting range
  • lowest: 65% gold content
  • readily age hardened (Au:Cu ratio allows heat treatment)
  • lowest tarnish resistance due to high Cu
  • usually used for cast partial n complete denture base
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15
Q

General characteristics going from type I to type IV:

A

Decrease in:
- gold content
- ductility
- melting range

Increase in: (mechanical properties)
- proportional limit
- elastic modulus
- strength n hardness

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16
Q

Casting shrinkage is the..

A
  • allows shrink when alloy changes from liquid to solid state

L cooling: thermal contraction of metal between temperature to which it is heated to liquidus temp - not significant
L to S: contraction during change from liquid to solid state
S cooling: thermal contraction of solid metal from solidification to room temp

17
Q

What is the average casting shrinkage value for gold alloys:

A
  • 1.25 ± 0.1%
18
Q

How does high melting range relate to thermal contraction.

A

High melting range = larger range of values from solidification to room temp => increase in thermal contraction

19
Q

List the alternatives to dental casting gold alloys:

A
  1. Low gold alloys (replace Au w Pd, Cu, Ag)
  2. Silver palladium precious alloys (Ag-Pd)
  3. Non-precious alloys (Ni + Cr + Be)
20
Q

Corrosion of gold alloys:

A
  • occurs when restoration is in contact w a restoration of a DISSIMILAR metal, such as amalgam or CoCr alloy
  • gold alloys can also tarnish in the mouth if there is surface inhomogeneity due to coring inhomogeneity due to coring and inclusions
    Thus, a narrower range btw liquidus and solidus temp wanted=> decreases coring

Y2 DM (14 decks)

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