Partial Denture Alloys Flashcards
What are the ideal properties for partial denture alloys? (7 points)
- Rigid (YM)
- Strong (UTS, EL)
- Hard
- Ductile
- Precise casting (shrinkage)
- Melting point ( will indictae what type of investment material you will use in the process)
- Density
What are 4 examples of partial denture alloys?
- ADA type IV gold
- Which gold (Ag-Pd)
- Co-Cr
- Titanium
What do we want the base of a RPD to be like?
- High YM - to maintain shape in use
- High EL - to avoid plastic deformation
What do we want the clasp of an RPD to be like?
- Lower YM - to allow flexure over tooth
- High EL - maintain elasticity over wide range of movement
What is pure gold used for in dentistry?
Class III and Class IV cavities
What are type I gold alloys used for?
- Simple alloys
What are type II gold alloys used for?
Larger (2-3 surface) inlays
What are type III gold alloys used for?
Crown and bridge alloys
What are type IV gold alloys used for?
Partial dentures
What is the composition of type IV gold? (6 points)
- Au
- Ag
- Cu
- Zn
- Pd
- Pt
What % of Au is present in type IV gold?
65%
What % of Ag is present in type IV gold?
14%
What % of
Cu is present in type IV gold?
14%
What % of Zn is present in type IV gold?
2%
What % of Pd is present in type IV gold?
3%
What % of Pt is present in type IV gold?
2%
What effect doe the presence of copper have on type IV alloy? (8 points)
- Solid solution in all proportions
- Solution hardening
- Order hardening - if 40%-80% gold and correct hea t treatment
- Reduced melting point
- No coring - solidus close to liquidus
- Imparts red colour (if sufficient quantity)
- Reduces density
- Base metal - can corrode if too much
What effect does the presence of silver have on type IV alloy? (6 points)
- Solid solution in all proportions
- Solution hardening
- Precipitation hardening with COPPER & heat treatment
- Can allow tarnishing
- Molten silver absorbs gas - creates porosity
- Whitens alloy - compensates for copper
Can the presence of silver in type IV gold alloy cause coring?
- Yes
What effect does the presence of platinum have on type IV alloy? (4 points)
- Solid solution with gold
- Solution hardening
- Fine grain structure (better mechanical properties)
- Coring can occur (wide liqui dous - solidus gap)
What effect does the presence of palladium have on type IV alloy? (4 points)
- Similar to Pt but less expensive
- LESS CORING than Pt
- COARSER GRAINS than Pt
- ABSORBS GASSES when molten - porous casting
What effect does the presence of zinc have on type IV alloy?
Scavenger
What effect does the presence of nickel have on type IV alloy?
- Increases hardness & strength (wrought alloys)
What effect does the presence of indium have on type IV alloy?
- Fine grain structure
What is the process of heat treatment of Type IV gold alloy? (4 points)
1) Quench after casting (fine grains)
2) Homogenising anneal (700 degrees, 10 mins) as some degree of coring has been created previously
3) If cold worked - stress relief anneal
4) Heat harden - (order and precipitation) -> 450 degrees cool slowly (15-30 mins) to 200 degrees then quench
(all of these processes try to improve the properties of the partial denture)
What are CoCr alloys used for? (3 points)
- Wires
- Surgical implants
- Cast partial dentures
What is involved in the composition of CoCr alloy? (5 points)
- Co
- Cr
- Ni
- Mo
- C
What is the % composition of Co in a CoCr alloy?
54%
What is the % composition of Cr in a CoCr alloy?
25%
What is the % composition of Ni in a CoCr alloy?
15%
What is the % composition of Mo in a CoCr alloy?
5%
What is the % composition of C in a CoCr alloy?
0.4%
What effects does the presence of cobalt in a CoCr alloy have? (3 points)
- Forms solid solution with Cr
- Increased strength, hardness, rigidity
- CORING possible
What effects does the presence of cobalt in a CoCr alloy have? (4 points)
- Forms solid solution with Co
- Increased strength, hardness, rigidity
- Coring possible
- Forms passive layer - corrosive resistance
What effects does the presence of nickel in a CoCr alloy have? (4 points)
- Replaces some Co
- Improves ductility
- Slight reduction in strength
- Sensitivity: 6% females, 2% males
What effects does the presence of carbon in a CoCr alloy have?
- Undesirable, carbide grain boundaries hard and brittle
- Ideally wouldn’t have any carbon at all but during the casting process some of it is inevitable
What effects does the presence of Mo in a CoCr alloy have? (2 points)
Reduces grain size - hence increases strength
What effects does the presence of W (Tungsten) in a CoCr alloy have? (3 points)
- Increases strength
- Al - increases PL
- Others: scavengers
What is the temperature of investment material and give 2 examples of these in making a CoCr alloy?
- High temp: 1200-1400 degrees
- Hence - silica or phosphate bonded
How is a CoCr alloy melted?
- Electric induction preferred
- Oxyacetylene - avo id carbon pickup
How should casting of a CoCr alloy be carried out? (3 points)
- Centrifugal force required
- Avoid overheating - coarse grains
- Cooling too fast or slow - carbides - brittle
What 4 things can be used to finish/polish CoCr?
- Sandblast
- Electroplate
- Abrasive wheel
- Polishing buff
Which is harder gold or CoCr?
CoCR
- It wears in the mouth better
What is the elongation/ductility of CoCr like? (4 points)
- Low ductility (4%)
- Work hardens rapidly
- Adjustment difficult
- Precision casting
What are 4 uses if titanium in dentistry?
- Implants
- Partial dentures (cast)
- Crown & bridge (cast)
- MAxillo-facial skull plates
What is one of the main key advantages of using titanium?
Good biocompatibility
Does titanium have good or bad corrosion resistance?
Good (passive oxide layer)
What are parts of titanium joined by?
Laser welding
What is the best RPD alloy in terms of elongation?
- Titanium - but it is V expensive
Which RPD alloy has the best UTS?
Stainless steel
What is the best RPD alloy in term of density?
Titanium
What is the best RPD alloy in relation to rigidity (YM)?
- Ag-Pd
- Closely followed by Au (hard) and titanium
What is the best RPD alloy in relation to hardness?
- CoCr is very hard
- Gold is not as difficult to polish but will wear down more easily
What is the best RPD alloy in terms of shrinkage?
Gold (hard)
Look at property comparison table in lecture
:)
