Partial denture alloys

Question 1

what are the ideal properties of partial denture alloys?

Answer 1

• rigid (high YM)
• strong (high UTS, EL)
• hard
• ductile
• precise casting (shrinkagae)
• melting point (investment material)
• density

Question 2

what are some partial denture alloys?

Answer 2

• ADA type IV gold
• CoCr
• white gold (AuPt)
• titanium

Question 3

what properties would a partial denture connector need?

Answer 3

• high YM (rigid) - to maintain shape in use
• high EL - to avoid plastic deformation
• thick material

Question 4

what properties would a partial denture clasp need?

Answer 4

• low YM (ductile) - to allow flexture over the tooth
• high EL - to maintain elasticity over a wide range of movement i.e. strain
• thin material

Question 5

describe the phase diagram of type IV gold alloy

Answer 5

• liquidus and solidus are close together so there will be little coring when quenched
• the area under the graph shows the two possible ordered solid solutions that can form after annealing (order hardening)

Question 6

what is the composition of type IV gold?

Answer 6

Question 7

what is the effect of copper in type IV gold?

Answer 7

1. solid solutions of all proportions possible - both metals are completely soluble in one another
2. benefits from solution hardening - forms common lattice to impede dislocations (increased EL, UTS, hardness)
3. benefits from order hardening - as long as 40-80% Au then ordered solution is created when quenched
4. reduced melting point
5. little/no coring - solidus close to liquidus
6. imparts red colour - if sufficient quantity added
7. reduces density
8. can cause corrosion if too much added

Question 8

what is the effect of silver in type IV gold

Answer 8

1. solid solutions of all proportions - both completely soluble in one another
2. benefits from solution hardening
3. benefits from precipitation hardening with copper - after annealing as Ag and Cu are partially soluble
4. can allow tarnishing
5. molten silver absorbs gas - care when casting or alloy may become porous and less robust
6. whitens the alloy - compensates for the redness

α-grains (high Ag%) β-grains (high Cu%)

Question 9

what is the effect of platinum in type IV gold?

Answer 9

1. solid solution of all proportions - both completely soluble in each other
2. benefits from solution hardening
3. fine grain structure
4. coring can occur - big difference between liquidus and solidus

Question 10

what is the effect of palladium on type IV gold?

Answer 10

1. similar to Pt but less expensive
2. less coring than Pt
3. coarser, large grains than with Pt
4. absorbs gases when molten - care when casting as alloy may become porous

Question 11

what is the effect of zinc, nickel and indium in type IV gold?

Answer 11

• zinc: acts as scavenger, oxidising during the process
• nickel: increases hardness and strength
• indium: encourages smaller/finer grains

Question 12

what are the heat treatments applied to type IV gold alloys?

Answer 12

1. quenching after casting - causes coring of AuPt and AuCu (fine grains produced)
2. homogenous anneal - eliminates coring by removing the concentration gradient (700degrees, 10 mins)
3. cold working - pushes dislocations to grain boundaries but induces residual stress within the alloy
4. stress-relieving annealing - alleviates residual stress
5. order hardening between Au and Cu
6. precipitation hardening between Ag and Cu
7. heated to 450 degrees then cooled slowly to 200 degrees then quenched
   * all increases alloy’s strength and hardness so is more robust (EL, UTS, hardness)

Question 13

what are the uses of cobalt chrome in dentistry?

Answer 13

• wires
• surgical implants
• casat partial dentures

Question 14

what is the composition of CoCr?

Answer 14

Question 15

what is the effect of cobalt in CoCr?

Answer 15

1. forms solid solution with Cr - both co-exist in same lattice network
2. increases strength, hardness and rigidity
3. allows coring

Question 16

what is the effect of chromium in CoCr?

Answer 16

1. forms solid solution with Co - both co-exist in same lattice network
2. increases strength, hardness and rigidity
3. allows coring
4. forms a passive layer on the alloy on the alloy surface - increases corrosion resistance

Question 16

what is the effect of nickel in CoCr?

Answer 16

1. replaces some Co
2. improves ductility
3. slight reduction in strength
4. patients may have allergic reaction

Question 17

how does carbon, molybdenum, tungsten and aluminium effect CoCr?

Answer 17

• carbon: small quantities - too much can make the alloy extremely brittle
• molybdenum: reduces grain size, increases strength
• tungsten: increases strength
• aluminium: increases PL - Polymer Laminate

Question 18

what are the techniques used with CoCr when making partial dentures?

Answer 18

• investment material used when casting CoCr has to cope with** high temperatures: 1200-1400 degrees**
• melting the alloy can be done by electric conduction or oxyacetylene
• casting requires centrifugal force - avoid overheating (coarse grains) and cooling too fast or slow may induce carbides and make framework brittle

Question 19

what are some finishing methods for CoCr?

Answer 19

• sandblast
• electroplate
• abrasive wheel
• polishing buff

Question 20

what are the properties of CoCr?

Answer 20

• hard - resists wear in mouth
• low ductility
• work hardens rapidly

Question 21

what are the uses of titanium in dentistry?

Answer 21

• implants
• crown and bridge (cast)
• maxillo-facial skull plates
• partial dentures (cast)

Question 22

what are the advantages of titanium?

Answer 22

• good biocompatability
• good corrosion resistance (passive oxide layer)
• parts can be joined by laser welding

Question 23

what are the disadvantages of titanium?

Answer 23

• required electric arc melting
• absorbs gases so needs specialised investment and casting equipment

Question 24

what component of partial dentures is titanium best for?

Answer 24

clasps

Question 25

what partial denture alloy has the best ductility?

Answer 25

**titanium** offers best ductility - can be stretched, altered in shape without fracturing to ensure the appliance is a good fit

Question 26

what partial denture alloy has the best UTS?

Answer 26

**stainless steel** has highest UTS which is indicative of fracture strength - Au slightly higher than CoCr

Question 27

what partial denture alloy has the lowest density?

Answer 27

**titanium** has the lowest density so offers the best patient comfort - CoCr is second best

Question 28

what partial denture alloy has the best rigidity?

Answer 28

**CoCr** has the highest rigidity so there will be less flexing when large stresses are applied to it

Question 29

what partial denture alloy has the best hardness?

Answer 29

**CoCr** has the greatest surface hardness so can resist wear in the teeth

Question 30

what partial denture alloy has the best shrinkage?

Answer 30

**Au and Ag-Pd** has the best shrinkage, making them easier to process - all alloys have some shrinkage but should be as little as possible