Gold alloys and semi-precious alloys Flashcards
Define noble
Metals that do not oxidise easily
Examples of noble metals
Gold, platinum and palladium
Define precious
Related to how valuable the metal is
Examples of precious metals
Gold, platinum and silver
Why is gold good for restoring teeth?
Ductile - easily shape to fill cavity
Soft - easily distorted
Stable/un-reactive
Coefficient of thermal expansion similar to tooth
Advantages of gold foil
Ductile so conform to fit cavity
Disadvantages of gold foil
Expensive
Never fully dense = marginal staining
Poor mechanical properties so limited to small cavities
Advantages of gold alloys
Cheaper
Harder
Alloys are less ductile than pure gold, what does this mean for restorations?
Can’t be shaped to fit cavity, must now be done by casting
How are dental gold alloys classified?
Type I = soft
Type II = medium
Type III = hard
Type IV = extra hard
Through what process do gold alloys improve mechanical strength and hardness?
Solution hardening
What is quenching?
Mold placed in cool liquid and liquid metal poured into this = increased rate of cooling
Effect of quenching?
Improve mechanical properties
smaller grains = higher yield strength
Consequence of quenching
Cooling down quickly leads to coring
Effect of coring?
Effects corrosion resistance
How is coring removed?
Homogenization
How are dental alloys cast? What is the effect of this?
Quenching
Small grains = increase yield strength but also coring = homogenisation
How can properties of gold alloys be improved?
Solution hardening
Precipitation hardening
Order hardening
Work hardening
What is order hardening?
Controlled Cu and Au content - heat treatment
What type of gold alloy may be used for small occlusal restorations?
Pure gold or Type I alloys (inlays)
What type of gold alloy may be used for medium stress bearing applications like large inlays?
Type II
What type of gold alloy may be used for high stress bearing applications like full crowns, denture components
Type III and IV
Hardening
Advantages of medium/low gold content materials?
Cheap
Whitish appearance
Similar properties to type III and IV gold alloys = strong, rigid
Disadvantages of medium/low gold content?
Lower density than high gold content = less accurate
Properties of silver/palladium
Lower density than gold alloys = less accurate
Oxygen dissolves in molten alloy = porous
Adequate corrosion resistance
Similar to type III gold alloys
