Gold Alloys Flashcards
What are noble metals?
- metals that are stable in elemental form
- inert to environment
- e.g. Au, Pt
What are precious metals?
Noble metals + Ag
General classification of alloys w noble metals.
- High noble alloys
- 40wt% Au, 60wt% other noble - Noble alloys
- 25wt% noble, no limit for gold content - Predominantly base metal alloys
- 75wt% base metal, 25wt% noble
Effect of gold on alloys.
- 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
Effects of platinum on alloy.
- hardens alloy
- increases melting temp of alloy
- whitens alloy
- principal active constituent of white gold used in dentistry
Effect of palladium on alloy
- in sufficient conc, acts as an effective hardener
- increases tarnish and corrosion resistance
- increases solidification temp of alloy
- whitens gold alloy
Effect of copper on alloy
- 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
Effect of zinc on alloy
- acts as an oxide scavenger
- improves castability
- reduces melting temp of alloy
- hardens alloy in combination w palladium
Effect of indium on alloys
- oxide scavenger
- promotes uniform grain size n casting fluidity
Types of gold alloys:
- Type I (Soft)
- Type II (Medium)
- Type III (Hard)
- Type VI (Extra hard)
Properties of Type I (soft) gold alloy
- 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
Properties of Type II (medium) gold alloy
- alloys w gold, silver, cu, platinum/palladium,zn
- 76% gold content
- not emenable to age hardening
- usually used for class III, V & onlay
Properties of Type III (hard) gold alloy
- 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
Properties of Type IV (extra hard) gold alloy
- 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
General characteristics going from type I to type IV:
Decrease in:
- gold content
- ductility
- melting range
Increase in: (mechanical properties)
- proportional limit
- elastic modulus
- strength n hardness
Casting shrinkage is the..
- 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
What is the average casting shrinkage value for gold alloys:
- 1.25 ± 0.1%
How does high melting range relate to thermal contraction.
High melting range = larger range of values from solidification to room temp => increase in thermal contraction
List the alternatives to dental casting gold alloys:
- Low gold alloys (replace Au w Pd, Cu, Ag)
- Silver palladium precious alloys (Ag-Pd)
- Non-precious alloys (Ni + Cr + Be)
Corrosion of gold alloys:
- 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
