Partial Denture Alloys Flashcards
What are the ideal properties of a partial denture alloy
Rigid (YM) Strong (UTS, EL) Hard Ductile Precise casting (shrinkage)
What are the different types of PDA
Type IV gold
CoCr
White gold (AgPd)
Titanium
What causes conflict in ideal rigidity of PDAs
The base must be rigid so large stresses produce little strain
The clasp should be removed easily with little stress required - should be flexible
How is a PDA designed for ideal rigidity
Using a thick section of alloy for the base and a thin section for the clasp
What are the dental uses of pure gold
Class III and IV cavities
What are the dental applications of the different types of gold
Type I - simple alloys
Type II - larger inlays
Type III - crown and bridge alloys
Type IV - partial dentures
What is the composition of type IV gold
Au - 65% Ag - 14% Cu - 14% Zn - 2% Pd - 3% Pt - 2%
Describe the phase diagram of AuCu
The liquidus and solidus are separated a little, so there will be a little coring when quenched
There are 2 regions below the solidus so two ordered solutions will form
What effect does adding Cu to type IV gold have on the phase diagram
Solid solutions of all proportions are possible
Benefits from solution hardening as it will impede dislocations
Benefits from order hardening for certain proportions
Has a reduced melting point
Little to no coring as the solidus is close to the liquidus
What other effects does adding Cu to type IV gold have
Imparts red colour - to be avoided
Reduces density
As it is a base metal, can cause corrosion if too much is added
What effects does adding silver to type IV gold have on the phase diagram
A solid solution will exist in all proportions
Benefits from solution hardening
After annealing there is precipitation hardening due to Ag and Cu being partially soluble
What other effects does adding silver to type IV gold have
Can allow tarnishing - undesirable
Molten silver absorbs CO2 so the alloy may become porous and so less robust
Whitens alloy - may compensate for copper
What effects does adding platinum have on type IV gold
Will form a solid solution and there will be solution hardening
Introduces a fine grain structure
Will be coring after quenching
What effects does adding palladium have on type IV gold
Similar to platinum but less expensive and less coring
Coarser grains than platinum
Absorbs gases when molten which may make the alloy porous and so less robust
What effects do other elements have on type IV gold
Zinc - scavenger, removes impurities
Nickel - increases strength and hardness
Indium - fine grain structure
What is done to eliminate coring in type IV gold and why
Homogenous annealing - raises alloy’s temperature for about 10 minutes
Removes the concentration gradient but doesn’t affect grain size or mechanical properties
What other processes are carried out on type IV gold to enhance strength and hardness
Cold working
Stress relief annealing
What are the dental uses of CoCr
Wires
Surgical implants
Cast partial dentures
Connectors
What is the composition of CoCr
Co - 54% Cr - 25% Ni - 15% Mo - 5% C - 0.4%
What are the effects of cobalt in CoCr
Forms a solid solution with Cr
Increased strength, hardness and rigidity
Coring occurs
What are the effects of chromium in CoCr
Forms a solid solution with Co
Increased strength, hardness and rigidity
Coring occurs
Forms a passive layer on the alloy surface enhancing corrosion resistance
What are the effects of nickel in CoCr
Replaces some Co as it improves ductility and causes a reduction in strength
Possibility of experiencing an allergic reaction
What effect does carbon have on CoCr
Avoid taking too much in as carbides will form at the grain boundaries making the alloy too hard and brittle
What effect do other elements have on CoCr
Mo - reduces grain size, increasing strength
W - increases strength
Al- increases proportional limit
Other elements can act as scavengers
