Partial Denture Alloys Flashcards
(42 cards)
List 7 ideal properties of PDAs
- Rigid
- Strong
- Hard
- Ductile
- Precise casting (shrinkage)
- Melting point ( investment material)
- Density
List 4 types of PDA
- ADA type IV gold
- Co-Cr
- White gold [Ag-Pd]
- Titanium
Why should the Young’s modulus for the base of a denture be high?
To maintain shape in use
Why should the elastic limit of a denture base be high?
To avoid plastic deformation
Why should a clasp have a low YM and a high EL?
- Lower YM - to allow flexure over tooth
2. High EL - maintain elasticity over wide range of movement
What is pure gold used for?
Class 3 and class 4 cavities
What uses to types 1 to 4 gold alloys have?
Type 1: simple alloys
Type 2: larger (2-3 surface) inlays
Type 3: crown and bridge alloys
Type 4: partial dentures
What is the composition of type 4 gold alloy?
- Gold (Au) - 65%
- Silver (Ag) - 14%
- Copper (Cu) - 14%
- Zinc (Zn) - 2%
- Palladium (Pd) - 3%
- Platinum (Pt) - 2%
What part of the phase diagram indicates what level of coring will occur?
The distance between solidus and liquidus lines
What effect does copper have in gold alloys?
- Solid solution in all proportions
- Solution hardening
- Order hardening - if 40-80% gold and correct heat treatment
- Reduced melting point
- Little or no coring
- Imparts red colour (if sufficient quantity)
- Reduces density
- Base metal - can corrode if too much
What are the effects of silver in gold alloys?
- Solid solution in all proportions
- Solution hardening
- Precipitation hardening with copper and heat treatment
- Can allow tarnishing
- Molten silver absorbs gas (e.g. CO2)
- Whitens alloy - compensates for copper
What are the effects of adding platinum to a gold alloy?
- Solid solution with gold
- Solution hardening
- Fine grain structure
- Coring can occur
- wide liquidus-solidus gap
What are the effects of adding Palladium to gold alloys?
- Less coring than Pt
- Coarser grains than Pt
- Absorbs gases when molten - porous casting
What are the effects of zinc, nickel and indium in gold alloys?
- Zinc: scavenger
- Nickel: increase hardness and strength (wrought alloys)
- Indium: fine grains structure
State the process of events involved during heat treatment of gold alloys
(This process makes type 4 gold alloy more suitable for clasp)
- Quench after casting (fine grains)
- Homogenising anneal (700 degrees C, 10 mins)
- If cold worked - stress relief anneal
- Heat harden - (order and precipitation)
- 450 degrees C cool slowly (15-30 mins) to 200 then quench
List uses of CoCr
- Wires
- Surgical implants
- Cast partial dentures
- Connectors
- high EL + YM = thick section
- high EL + low YM = thin section
List the composition of different metals in CoCr
- Cobalt - 54%
- Chromium - 25%
- Nickel - 15%
- Mo - 5%
- C - 0.4%
How does cobalt effect the CoCr alloy?
- Forms solid solution with Cr
- Increased strength, hardness, rigidity
- Coring possible
How does chromium effect CoCr alloys?
- Forms solid solution with Co
- Increased strength, hardness, rigidity
- Coring possible
- Forms passive layer - corrosion resistance
How does nickel effect CoCr alloys?
- Replaces some Co
- Improves ductility
- Slight reduction in strength
- Sensitivity:
- 6% of females
- 2% of males
Why must we be careful not to add too much carbon to a CoCr alloy?
Makes the alloy too hard and brittle
How does Mo and tungsten effect CoCr alloys?
- Mo - reduced grain size Therefore increasing strength
2. Tungsten - increases strength
How does Al (aluminium) effect CoCr alloys?
Increases proportional limit stress (PL)
Why are silica or phosphate bonded investment materials used as opposed to gypsum?
They must be able to cope with high temperatures (1200-1400 Degrees C)
