Partial Denture Alloys Flashcards
(67 cards)
What are the ideal properties of a partial denture alloy
▪ Rigid (Young's Modulus) ▪ Strong (ultimate tensile strength and elastic limit) ▪ Hard ▪ Ductile ▪ Precise casting (shrinkage) ▪ Melting point (investment material) process Density
Why is melting point important for partial denture alloys
Partial denture alloys are produced using the casting technique - one of the challenges with this method is that the melting point will dictate the type of investment material that will be used in that casting process
What are the partial denture alloys
ADA type IV gold
White gold (Ag-Pd)
Co-Cr
Titanium
What properties do we want a denture base to have
a HIGH YOUNG’S MODULUS to withstand forces and maintain its shape in use and a HIGH ELASTIC LIMIT to avoid plastic deformation
What properties do we want the denture clasp to have
a LOW YOUNG’S MODULUS to allow flexure over the tooth and can disengage and a HIGH ELASTIC LIMIT to maintain its elasticity over a wide range of movement
To compromise the properties we want for a denture base and clasp what do we do
we use THICK SECTIONS for a rigid base and THIN SECTIONS for a flexible clasp
What are the ADA specifications of gold alloys
Type I: simple alloys
Type II: larger (2-3 surface) inlays
Type III: crown and bridge alloys
Type IV: partial dentures
What is the composition of type 4 gold
Gold (60-70%)
Silver (4-20%)
Copper (11-16%)
Zinc (1-2%)
Palladium (0-5%)
Platinum (0-4%)
What does the phase diagram for Au-Cu show
the liquidus and solidus are CONTINUOUS meaning that we can have all sorts of combinations for gold and copper
What are the effect of copper as an alloying element in a gold alloy
Solid solution in all proportions
Solution hardening
Order hardening - if 40-80% Gold and correct heat treatment
Reduced melting point
No coring - solidus is close to the liquidus
Imparts red colour (if sufficient quantity)
Reduces density
Base metal so copper is more likely to make it corrode
What does the phase diagram for Au/Ag show
This phase diagram shows that any combination of gold and silver can coexist in the same grain structure
What are the effect of using silver as an alloying element in a gold alloy
Solid solution in all proportions
Solution hardening
Precipitation hardening with COPPER and heat treatment and this can be used to improve the properties of the materials
Downside is that it can allow tarnishing
Molten silver absorbs gas (CO2) - porosities during casting
It whitens the alloy
What does molten silver absorbing gas result in
it creates porosities during the casting process
Why is silver whitening the alloy an advantage
compensates for the presence of copper
What is the phase diagram for Ag-Cu
shows that it has partial solid solubility
has solubility limit lines
Because the Ag-Cu is partially soluble solid solution what can it suffer from
precipitation hardening
What is the phase diagram for Au-Pt
straight forward
What is the effect of using platinum as an alloying element in a gold alloy
Solid solution with Gold
Solution hardening
Fine grain structure
Coring can occur (wide liquidus-solidus gap)
What is the effect of using palladium as an alloying element in a gold alloy
Similar to Pt but less expensive
Less coring than Pt
Coarser grains than Pt
Absorbs gases when molten - porous casting
What is the function of zinc in type 4 gold
scavenger
What is the function of nickel in type 4 gold
increase hardness and strength
What is the function of indium in type 4 gold
fine grain structure
Describe the heat treatment for type 4 gold
1) Quench after casting (fine grains)
2) Homogenising anneal (700 degrees Celsius for ten minutes)
3) If cold worked then stress relief annealing is required
4) Heat harden - (order and precipitation) at 450 degrees and then cool slowly through 15-30 minutes to 200 degrees then quench it
What are the properties of type 4 gold (heat treated) more suitable for
Its properties more suitable for clasps
You need thickness for the base
