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Dental Materials > Partial Denture Alloys > Flashcards

Partial Denture Alloys Flashcards

1
Q

What are the ideal properties of a partial denture alloy

A 
Rigid (YM)
Strong (UTS, EL)
Hard
Ductile
Precise casting (shrinkage)
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2
Q

What are the different types of PDA

A

Type IV gold
CoCr
White gold (AgPd)
Titanium

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3
Q

What causes conflict in ideal rigidity of PDAs

A

The base must be rigid so large stresses produce little strain
The clasp should be removed easily with little stress required - should be flexible

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4
Q

How is a PDA designed for ideal rigidity

A

Using a thick section of alloy for the base and a thin section for the clasp

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5
Q

What are the dental uses of pure gold

A

Class III and IV cavities

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6
Q

What are the dental applications of the different types of gold

A

Type I - simple alloys
Type II - larger inlays
Type III - crown and bridge alloys
Type IV - partial dentures

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7
Q

What is the composition of type IV gold

A 
Au - 65%
Ag - 14%
Cu - 14%
Zn - 2%
Pd - 3%
Pt - 2%
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8
Q

Describe the phase diagram of AuCu

A

The liquidus and solidus are separated a little, so there will be a little coring when quenched
There are 2 regions below the solidus so two ordered solutions will form

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9
Q

What effect does adding Cu to type IV gold have on the phase diagram

A

Solid solutions of all proportions are possible
Benefits from solution hardening as it will impede dislocations
Benefits from order hardening for certain proportions
Has a reduced melting point
Little to no coring as the solidus is close to the liquidus

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10
Q

What other effects does adding Cu to type IV gold have

A

Imparts red colour - to be avoided
Reduces density
As it is a base metal, can cause corrosion if too much is added

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11
Q

What effects does adding silver to type IV gold have on the phase diagram

A

A solid solution will exist in all proportions
Benefits from solution hardening
After annealing there is precipitation hardening due to Ag and Cu being partially soluble

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12
Q

What other effects does adding silver to type IV gold have

A

Can allow tarnishing - undesirable
Molten silver absorbs CO2 so the alloy may become porous and so less robust
Whitens alloy - may compensate for copper

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13
Q

What effects does adding platinum have on type IV gold

A

Will form a solid solution and there will be solution hardening
Introduces a fine grain structure
Will be coring after quenching

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14
Q

What effects does adding palladium have on type IV gold

A

Similar to platinum but less expensive and less coring
Coarser grains than platinum
Absorbs gases when molten which may make the alloy porous and so less robust

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15
Q

What effects do other elements have on type IV gold

A

Zinc - scavenger, removes impurities
Nickel - increases strength and hardness
Indium - fine grain structure

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16
Q

What is done to eliminate coring in type IV gold and why

A

Homogenous annealing - raises alloy’s temperature for about 10 minutes
Removes the concentration gradient but doesn’t affect grain size or mechanical properties

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17
Q

What other processes are carried out on type IV gold to enhance strength and hardness

A

Cold working

Stress relief annealing

18
Q

What are the dental uses of CoCr

A

Wires
Surgical implants
Cast partial dentures
Connectors

19
Q

What is the composition of CoCr

A 
Co - 54%
Cr - 25%
Ni - 15%
Mo - 5%
C - 0.4%
20
Q

What are the effects of cobalt in CoCr

A

Forms a solid solution with Cr
Increased strength, hardness and rigidity
Coring occurs

21
Q

What are the effects of chromium in CoCr

A

Forms a solid solution with Co
Increased strength, hardness and rigidity
Coring occurs
Forms a passive layer on the alloy surface enhancing corrosion resistance

22
Q

What are the effects of nickel in CoCr

A

Replaces some Co as it improves ductility and causes a reduction in strength
Possibility of experiencing an allergic reaction

23
Q

What effect does carbon have on CoCr

A

Avoid taking too much in as carbides will form at the grain boundaries making the alloy too hard and brittle

24
Q

What effect do other elements have on CoCr

A

Mo - reduces grain size, increasing strength
W - increases strength
Al- increases proportional limit
Other elements can act as scavengers

25
Q

How can alloys be melted

A

Electric induction - preferred

Oxyacetylene - risks excess carbon being taken up

26
Q

Which investment materials can and can’t be used with CoCr

A

Silica or phosphate bonding

Not gypsum

27
Q

Why does casting of CoCr require a centrifugal force

A

Important to avoid overheating as coarse grains will form

Cooling too fast or slow may introduce carbides making the framework brittle

28
Q

By what methods may surface finishing of CoCr be carried out

A

Sandblast
Electroplate
Abrasive wheel
Polishing buff

29
Q

Describe the hardness of CoCr

A

370 hardness units
Much harder than gold
Will resist surface wear in mouth better
Requires a significant amount of time to finish and polish before use

30
Q

Describe the ductility of CoCr

A

Elongation - 4% so low ductility
Therefore it work hardens rapidly
Makes adjustment of the finished appliance difficult so you need precision casting

31
Q

What are the dental uses of pure titanium

A

Implants
Crown and bridge (cast)
Maxilla-facial skull plates
Partial dentures (cast)

32
Q

What are the advantages of titanium

A

Good biocompatibility
Good corrosion resistance
Parts can be joined by laser welding so complex structures can be dealt with

33
Q

What are the disadvantages of titanium

A

It requires electric arc melting

As it absorbs gases it needs special investment and casting equiptment

34
Q

Compare the ductility of different PDAs

A

High ductility is advantageous
Type IV gold has an elongation of 10% before fracture compared to CoCr 4% so type IV gold is better here
Titanium has the greatest ductility

35
Q

Compare the UTS of type IV gold and CoCr

A

A little higher for type IV gold

36
Q

Compare the density of type IV gold and CoCr

A

CoCr has half the density of type IV gold and is therefore better

37
Q

Compare the rigidity (Young’s Modulus) of type IV gold and CoCr

A

CoCr has more than twice the stiffness of type IV gold so there will be much less flexing when large stresses are applied

38
Q

Compare the surface hardness of type IV gold and CoCr

A

Surface hardness of CoCr is much greater than type IV gold

39
Q

Compare the shrinkage of type IV gold and CoCr

A

CoCr shrinks around 2.3%, almost double that of type IV gold so processing CoCr appliances is more difficult for labs

40
Q

What is important to note when dealing with CoCr

A

More difficult to produce defect free casting than gold
Cannot use gypsum-bonded investment
More difficult to polish as it is harder
Work hardens rapidly, so need precision casting

Dental Materials (19 decks)

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