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BDS2 Dental Materials Sciences > Partial Denture Alloys > Flashcards

Partial Denture Alloys Flashcards

1
Q

what are 5 good mechanical properties of alloys to remember

A

Stress

Rigidity

elastic limit

hardness

ductility

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2
Q

4 ways an alloy can be hardened

A

work hardening

solution hardening

order hardening

precipitation hardening

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3
Q

2 types of alloy annealing

A

homogenisation annealing

stress relief annealing

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4
Q

7 ideal properties of partial denture alloys

A

rigid (YM)

strong (UTS, EL)

hard

ductile

precise casting (shrinkage)

melting point (investment material)

density

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5
Q

4 types of partial denture alloys

A

ADA Type IV Gold

White Gold (Ag-Pd)

Co-Cr

Titanium

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6
Q

what technique is used in partial denture manufacture

A

one piece casting

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7
Q

YM and EL of base for one piece casting

A

high YM - to maintain shape in use
- withstand large stresses and not change shape greatly

high EL - to avoid plastic deformation

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8
Q

YM and EL of clasp for one piece casting

A

lower YM - to allow flexure over tooth
- flex easily and disengage readily

high EL - maintain elasticity over wide range of movement (i.e. strain)

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9
Q

why do both the clasp and base need a high EL

A

avoid any permanent/plastic deformation even if apply large stress to material

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10
Q

what is the compromise that is needed in one-piece manufacture to benefit both the base and clasp

A

thick section - rigid base;

thin section - flexible clasp

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11
Q

2 ways to describe Pure Gold

A

Carat : 24 - pure gold

Fineness : 1000 fine - 100% Au

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12
Q

example use of pure gold

A

class III and IV cavities in some situations

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13
Q

uses of different types of gold

A

Type I : simple alloys

Type II : larger (2-3 surface) inlays

Type III : Crown & Bridge alloys

Type IV : Partial Dentures

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14
Q

uses of Type IV gold

A

partial dentures

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15
Q

what metals are in type IV gold

A

Au gold

Zn zinc

Cu cooper

Ag silver

Pd palladium

Pt platinum

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16
Q

type IV gold % composition of gold

A

60-70% (65)

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17
Q

type IV gold % composition zinc

A

1-2% (1)

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18
Q

type IV gold % composition cooper

A

11-16% (14)

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19
Q

type IV gold % composition of silver

A

4-20% (14)

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20
Q

type IV gold % composition palladium

A

0-5% (3)

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21
Q

type IV gold % composition platinium

A

0-4% (2)

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22
Q

what are the liquidous and solidus line like for Au-Cu phase diagram

A

Continuous

  • Have all sorts of combination of gold and cooper
  • All the way through
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23
Q

8 effects of adding cooper to gold alloy

A

solid solution in all proportions

solution hardening

order hardening

reduced melting point

no coring

imparts red colour
- due to cooper if sufficient quantity

reduces density
- more cooper leads to lower density

base metal - can corrode if too much

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24
Q

how can you tell if there will be little to no coring

A

solidus relatively close to liquidus

so little to none coring on quenching

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25
Q

what is order hardening

A

if 40-80% Gold and correct heat treatment

  • Once taken alloy quenched to room temp and then heat treat
  • Undergoes some form of heat treatment

forms an ordered solid solution (Row of Au row of Cu)
- Little peak in phase diagram

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26
Q

6 effects of adding silver to gold alloy

A

solid solution in all proportions

solution hardening

precipitation hardening with COPPER & heat treatment
- silver and cooper can benefit from precipitation hardening, utilise to improve mechanical properties

can allow tarnishing

molten silver absorbs gas e.g. CO2
- can lead to porosity in casting process

whitens alloy – compensates for copper

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27
Q

describe the phase diagram for Ag-Au

A

Simple

Close together

Any value an coexist in same grain structure

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28
Q

describe the phase diagram for Ag-Cu (in type IV Gold)

A

have solubility limit lines
- Partially soluble solid solution

End up with potential precipitation hardening

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29
Q

describe the phase diagram of AgPt

A

Simple

Reasonable degree of separation
- Coring can occur on Rapid cooling
- Grains have concentration gradient
Have to further process

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30
Q

what feature does a partially soluble solid have on phase diagram

A

have solubility limit lines

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31
Q

4 effects of adding platinum to gold alloy

A

solid solution with Gold

solution hardening

fine grain structure
- greater mechanical properties

coring can occur
- wide Liquidus - Solidus gap

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32
Q

4 effects of adding pallidium to gold alloy

A

similar to Pt

less expensive

less coring than Pt

coarser grains than Pt

absorbs gases when molten
porous casting – weaken end RPD

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33
Q

contribution of zinc to alloy

A

scavenger

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34
Q

contribution of nickel to alloy

A

increase hardness and strength (wrought alloys)

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35
Q

contribution of indium to alloys

A

fine grain structure

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36
Q

why would you use heat treatment on partial denture alloys

A

All to improve properties of RPD and minimise negatives

37
Q

4 types of heat treatment on gold alloys

A

Quench after casting
fine grains – good mechanical properties

Homogenising anneal (700C, 10 mins)

If cold worked - stress relief anneal
- Homogenous grain composition
- Raise temperature for some time but not to melting point, cool
Cold working to manipulate – if doesn’t fit the pt exactly

Heat harden - (order & precipitation)

  • 450C cool slowly (15 - 30 mins) to 200C then quench
  • —After cold work, raise temperature then quench
38
Q

role of stress relief annealing of type IV gold

A

Homogenous grain composition
- Raise temperature for some time but not to melting point, cool
Cold working to manipulate – if doesn’t fit the pt exactly

39
Q

role of heat hardening type IV gold

A

order and precipitation

450C cool slowly (15 - 30 mins) to 200C then quench
—-After cold work, raise temperature then quench

40
Q

advantage and disadvantage for partial denture of heat treated type IV gold alloy

A

Properties more suitable for clasp

Need thickness for base (expense)

41
Q

3 uses for CoCr

A

Wires

Surgical Implants

Cast Partial Dentures
Connectors :
- High EL, High YM - thick section
- High EL, Low YM - thin section

42
Q

metals in CoCr

A

cobalt

chromium

nickle

molybdenum

carbon

43
Q

% composition in CoCr of cobalt

A

35-65% (54)

44
Q

% composition in CoCr of chromium

A

25-30% (25)

45
Q

% composition in CoCr of nickle

A

0-30% (15)

46
Q

% composition in CoCr of molybdenum

A

5-6% (5)

47
Q

% composition in CoCr of carbon

A

0.2-0.4%

48
Q

3 effects of cobalt in CoCr

A

Forms solid solution with Cr

Increase strength, hardness, rigidity

Coring possible

49
Q

solid solution alloy arrangement

A

2 metals coexist in normal lattice arrangement

- Across whole spectrum of compositions

50
Q

4 effects of chromium in solid CoCr alloy

A

Forms solid solution with Co

Increase strength, hardness, rigidity

Coring possible

Forms passive layer – corrosion resistance
- Passive oxide layer to resist corrosion – KEY

51
Q

4 effects of nickel in CoCr alloy

A

Replaces some Co

Improves ductility

COMPROMISE
Slight reduction in strength
Sensitivity
- 6% females; 2% males

52
Q

2 effects of carbon in CoCr alloy

A

Undesirable
- Ideally not have any, but inevitable to make in casting

Carbide grain boundaries hard & brittle

53
Q

role of molybdenum in CoCr

A

Reduces grain size

- increase strength

54
Q

role of tungsten (W) in CoCr alloy

A

Al: increases PL

Other: scavengers

55
Q

what is the point of having a range of metals of varying compositions in an alloy

A

Multiple ingredient – best mix for greater properties

56
Q

investment material for CoCr

A

High temp. 1200-1400C

Hence - silica or phosphate bonded
- NOT GYPSUM

57
Q

2 melting techniques for CoCr

A

Electric Induction (preferred)

Oxyacetylene - avoid carbon pickup (too many problems to be used now)

58
Q

what is the preferred melting technique for CoCr

A

electric induction

59
Q

technique for casting CoCr

A

Centrifugal force required
- Avoid overheating
—-Risk coarse grains
Cooling too fast or slow - carbides - brittle

60
Q

techniques for finishing CoCr (4)

A

sandblast

electroplate

abrasive wheel

polishing buff

61
Q

why is CoCr hard to polish

A

very hard material

  • but then equally hard to roughen up in use
  • – so maintains surface longer
62
Q

hardness of CoCr

A

BHN 370

63
Q

why is the high hardness of CoCr good (2)

A

Much harder than Gold

Wear in mouth better
- Experience less wear

64
Q

downside of high hardness of CoCr

A

Finishing/polishing time consuming

65
Q

elongation/ductility value for CoCr

A

4%

low

66
Q

what is elongation/ductility

A

Ability of material to be stretched, change shape

67
Q

effect of CoCr having low elongation value

A

Work hardens rapidly

Adjustment difficult, thus precision casting

  • Only small amount can be done to fit pt
  • Casting needs to be really precise so 4% is adequate
68
Q

uses of pure titanium (4)

A

Implants

Partial Dentures (Cast)

Crown & Bridge (Cast)

Maxillo-Facial Skull Plates

69
Q

3 advantages of titanium

A

Good biocompatibility

Good corrosion resistance (passive oxide layer )

Individual parts joined by laser welding
- Casting process less of a challenge

70
Q

why is good biocompatibility key to pure titanium use

A

Can be used in more demanding situations than RPD e.g. maxilla-facial skull plates

71
Q

% of titanium in pure titanium

A

99.5%

72
Q

how is titanium melted

A

Electric Arc Melting

Specialised investment and casting – not in GDH

Can buy in from commercial outlets

73
Q

why is electric arc melting used for titianium

A

As titanium absorbs gases

74
Q

what alloy has the highest elongation/ductility value

A

Titanium easy engage and disengage in pt dentition

Expensive
- generally between Type IV or CoCr

75
Q

UTS is

A

fracture strength (MPa)

76
Q

compare UTS of type IV gold, CoCr and titanium

A

Not identical

- But not tremendous difference in fracture strength

77
Q

why is density (g/cm3) important for partial denture alloys

A

Pt comfort

78
Q

compare densities of type IV Au and CoCr

A

Pt comfort

Au is the most dense

CoCr is half
- favoured for pt as lighter

79
Q

compare the rigidity (young’s/elastic modulus (GPa)) of CoCr, type IV Au and Titanium

A

CoCr 250
Au 100
Titanium 110

CoCr is least likely to undergo dimension change due to stress

80
Q

compare the hardness between CoCr and Au

and explain this effect on finsihing

A

CoCr 370
Au 220

Au not as hard to polish but wears down easier

81
Q

what does shrinkage of partial denture alloys impact on

A

investment material used

large shrinkage is a challenge

82
Q

which is more difficult to produce a defect free casting CoCr or gold

A

CoCr

83
Q

can you use conventional gypsum bonded investment material with CoCr

A

no

84
Q

is polishing CoCr easy

A

no as its surface is harder than gold

- but due to this retains polish longer

85
Q

why is precise casting needed for CoCr

A 
work hardens (cold hardens) rapidly 
- casting process is harder to avoid defects
86
Q

Au Vs CoCr rigidity

A

Au less than half rigidity of CoCr

87
Q

Au Vs CoCr fracture strength UTS

A

Fracture strength is similar

88
Q

Au Vs CoCr ductility

A

Ductility Au exceeds CoCr

89
Q

AU VS CoCr hardness

A

CoCr hardness is significantly more than type IV Au

BDS2 Dental Materials Sciences (13 decks)

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