Gold Alloys Flashcards
Composition of high noble alloy
More than 40wt% gold + 60wt% other noble metals
Composition of noble alloys
> 25wt% noble metals, no limit for gold content
Composition of predominantly base metal alloy
> 75wt% base metal, less than 25wt% noble metals
What does gold do
- increase tarnish and corrosion resistance if greater than 75wt%
- give high ductility to alloy
- harden alloy on heat treatment
- anti microbial
- yellow colour
What does copper do
- increase strength and hardness when >4%
- lower tarnish and corrosion resistance because more copper means less gold
- reduce melting temperature
- increase ductility
- reduce difference in temperature between upper and lower melting range
- reddish colour
What does palladium do
- harden alloy (if sufficient concentration)
- increase tarnish and corrosion resistance
- increase solidification temp
- whitens gold alloy
What does platinum do
- harden
- whitens gold alloy
- increase melting temperature
What does zinc do
- oxide scavenger
- improve castability
- reduce alloy melting temp
- harden alloy
What does indium do
- oxide scavenger
- promote uniform grain size and casting fluidity
What does an oxide scavenger do
React with O2 during melting and solidification of alloy to form zinc oxide instead of other detrimental oxides
Percentage of gold in type 1 gold alloys
Soft gold alloy, 90 to 94 % gold
What class cavities can type 1 alloys be used for
Class 3 and class 5 cavities (not subjected to high stress)
What is type 2 gold alloy
Harder than type 1, 78% gold
Not amenable to age hardening
What is type 3 gold alloy
Hard, higher percentage (4%) of platinum and palladium
Can be age hardened
Crowns, bridges, abutments
What is type 4 gold alloy
Extra hard, lowest gold content (70%)
Can be readily age hardened
Very hard, lowest tarnish resistance
Lower melting range due to increased zinc and copper
Used for partial and complete denture bases
What us the average dental gold alloy casting shrinkage
1.25 +/- 0.1%
What is galvanic corrosion
In contact with restoration of dissimilar metal, generate electrical current in ionic solution between 2 different metals
What is low gold alloy
Gold replaced mainly be silver, copper, indium. Some gold replaced by palladium. Noble metal content lower than 60wt%.
Lower density
Lower tarnish and corrosion resistance
What is silver-palladium precious alloy
60-70% silver, 25% palladium
White
Tarnish and corrosion resistance clinically satisfactory
Should not be pickled
What does non precious alloy mainly contain
Nickel and chromium
Some contain beryllium to improve castability
Properties of non precious alloys
High mechanical properties
Low cost
Nickel may give rise to allergic reactions
Heat treatment softening of gold alloy
Heat to 700ºC for 10min then quench
Decreases tensile strength and hardness, increases ductility
Heat treatment hardening of alloy
Heat to 424ºC for 2 min cool to 250ºC for 30min then quench
Increase tensile strength and hardness, decrease ductility
