Partial Denture Alloys Flashcards
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)
Why is electric induction or oxyacetylene the preferred method to melt the CoCr alloy?
To avoid carbon pickup
What can overheating the CoCr alloy result in?
Coarse grains
What can result from cooling the CoCr alloy too fast or too slow?
Can result in carbides which makes the alloy brittle
What methods are used for surface finishing of CoCr?
- Sandblasting
- Electroplate
- Abrasive wheel
- Polishing buff
What is the hardness of CoCr?
370 hardness units
- much harder than gold
What is an advantage and a disadvantage of CoCr compared to type 4 gold?
Advantage: Wear in the mouth better
Disadvantage: finishing/polishing is time consuming
What is precision casting needed for CoCr?
- Elongation 4%
- Work hardens rapidly
- Adjustment is difficult
therefore precision casting is needed
What are used of titanium in dentistry?
- Potentially use as a cast material
- Other uses:
- implants
- crown and bridge (cast)
- Maxillo-Facial skull plates
- partial dentures (cast)
What are advantages of using titanium in dentistry?
- Good biocompatibility
- Good corrosion resistance (passive oxide layer)
- Parts joined by laser welding
What are disadvantages of using titanium?
- Electric arc melting is required
- As titanium absorbs gases:
- specialised investment and casting equipment needed
(Basically needs more high tech equipment to work with)
What is the ductility of titanium?
15% - this is 5% higher than gold (the next highest ductility)
List the ductility’s of:
- Gold
- Silver/platinum
- CoCr
- Titanium
- Steel
From highest to lowest
- Titanium 15%
- Gold 10%
- Silver/platinum 9%
- CoCr 4%
- Steel 1%
List the ultimate tensile strength of :
- Gold
- silver/platinum
- CoCr
- Titanium
- Steel
From highest to lowest
In MPa:
- Steel 1900
- Gold approx 700
- Titanium approx 700
- Silver/platinum approx 450-475
How does the density of CoCr compare to Type 4 gold?
CoCr = 8g/cm3
- just over half that of gold
How does the rigidity (YM (GPa)) compare to type 4 gold?
CoCr = 250 GPa
This is over twice that of gold
What is the shrinkage of CoCr and how does this compare to type 4 gold?
- CoCr = approx 2.25%
This is greater than gold (1.25%)
What factors need to be considered when using CoCr rather than gold?
- More difficult to produce defect free casting than gold
- Cannot use gypsum-bonded investment
- More difficult to polish (as it is harder) than gold
- but retains polish better - Work hardens rapidly, so needs precision casting
Summarise the comparisons between CoCr and type 4 gold in terms of:
- Shrinkage (during casting)
- Density
- Rigidity (YM)
- Fracture strength (UTS)
- Ductility
- Hardness
CoCr:
- Shrinks more
- Just over Half the density
- Double the rigidity
- Similar fracture strength
- Half as ductile
- Much harder
