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BDS2 CLINICAL Dental Materials > Partial Denture Alloys > Flashcards

Partial Denture Alloys Flashcards

1
Q

What are the ideal properties of a partial denture alloy

A 
▪ Rigid (Young's Modulus)
	▪ Strong (ultimate tensile strength and elastic limit)
	▪ Hard
	▪ Ductile 
	▪ Precise casting (shrinkage) 
	▪ Melting point (investment material)
	process
Density
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2
Q

Why is melting point important for partial denture alloys

A

Partial denture alloys are produced using the casting technique - one of the challenges with this method is that the melting point will dictate the type of investment material that will be used in that casting process

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3
Q

What are the partial denture alloys

A

ADA type IV gold
White gold (Ag-Pd)
Co-Cr
Titanium

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4
Q

What properties do we want a denture base to have

A

a HIGH YOUNG’S MODULUS to withstand forces and maintain its shape in use and a HIGH ELASTIC LIMIT to avoid plastic deformation

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5
Q

What properties do we want the denture clasp to have

A

a LOW YOUNG’S MODULUS to allow flexure over the tooth and can disengage and a HIGH ELASTIC LIMIT to maintain its elasticity over a wide range of movement

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6
Q

To compromise the properties we want for a denture base and clasp what do we do

A

we use THICK SECTIONS for a rigid base and THIN SECTIONS for a flexible clasp

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7
Q

What are the ADA specifications of gold alloys

A

Type I: simple alloys
Type II: larger (2-3 surface) inlays
Type III: crown and bridge alloys
Type IV: partial dentures

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8
Q

What is the composition of type 4 gold

A

Gold (60-70%)

Silver (4-20%)

Copper (11-16%)

Zinc (1-2%)

Palladium (0-5%)

Platinum (0-4%)

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9
Q

What does the phase diagram for Au-Cu show

A

the liquidus and solidus are CONTINUOUS meaning that we can have all sorts of combinations for gold and copper

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10
Q

What are the effect of copper as an alloying element in a gold alloy

A

Solid solution in all proportions
Solution hardening
Order hardening - if 40-80% Gold and correct heat treatment
Reduced melting point
No coring - solidus is close to the liquidus
Imparts red colour (if sufficient quantity)
Reduces density
Base metal so copper is more likely to make it corrode

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11
Q

What does the phase diagram for Au/Ag show

A

This phase diagram shows that any combination of gold and silver can coexist in the same grain structure

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12
Q

What are the effect of using silver as an alloying element in a gold alloy

A

Solid solution in all proportions
Solution hardening
Precipitation hardening with COPPER and heat treatment and this can be used to improve the properties of the materials
Downside is that it can allow tarnishing
Molten silver absorbs gas (CO2) - porosities during casting
It whitens the alloy

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13
Q

What does molten silver absorbing gas result in

A

it creates porosities during the casting process

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14
Q

Why is silver whitening the alloy an advantage

A

compensates for the presence of copper

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15
Q

What is the phase diagram for Ag-Cu

A

shows that it has partial solid solubility

has solubility limit lines

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16
Q

Because the Ag-Cu is partially soluble solid solution what can it suffer from

A

precipitation hardening

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17
Q

What is the phase diagram for Au-Pt

A

straight forward

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18
Q

What is the effect of using platinum as an alloying element in a gold alloy

A

Solid solution with Gold
Solution hardening
Fine grain structure
Coring can occur (wide liquidus-solidus gap)

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19
Q

What is the effect of using palladium as an alloying element in a gold alloy

A

Similar to Pt but less expensive
Less coring than Pt
Coarser grains than Pt
Absorbs gases when molten - porous casting

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20
Q

What is the function of zinc in type 4 gold

A

scavenger

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21
Q

What is the function of nickel in type 4 gold

A

increase hardness and strength

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22
Q

What is the function of indium in type 4 gold

A

fine grain structure

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23
Q

Describe the heat treatment for type 4 gold

A

1) Quench after casting (fine grains)
2) Homogenising anneal (700 degrees Celsius for ten minutes)
3) If cold worked then stress relief annealing is required
4) Heat harden - (order and precipitation) at 450 degrees and then cool slowly through 15-30 minutes to 200 degrees then quench it

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24
Q

What are the properties of type 4 gold (heat treated) more suitable for

A

Its properties more suitable for clasps

You need thickness for the base

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25
Q

What are the uses for cobalt chromium

A

Wires
Surgical implants
Cast partial dentures (connectors)

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26
Q

In thick sections what are the properties of COCr

A

high EL

high YM

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27
Q

In thin sections what are the properties of CoCr

A

High EL, low YM

28
Q

What is the composition of CoCr

A

Cobalt (35-65%)

Chromium (25-30%)

Nickel (0-30%)

Molybdenum (5-6%)

Carbon (0.2–0.4%)

29
Q

What are the effects of cobalt on the alloy

A

Forms solid solution with chromium
Increased strength, hardness, rigidity
CORING possible

30
Q

What are the effect of chromium on the CoCr alloy

A

Forms solid solution with cobalt
Increased strength, hardness, rigidity
Coring possible
Forms passive oxide layer, corrosion resistance

31
Q

What are the effect of nickel on the alloy (CoCr)

A

Replaces some Co
Improves ductility
Slight reduction in strength
Causes sensitivity in patients

32
Q

What is the effect of carbon on the alloy (CoCr)

A

undesirable, forms carbide grain boundaries and can make it hard and brittle

33
Q

What is the effect of molybdenum on the alloy (CoCr)

A

reduces grain size and so increases strength

34
Q

What is the effect of tungsten on the alloy (CoCr)

A

increases strength

35
Q

What is the effect of aluminum on the alloy (CoCr)

A

increases plastic limit

36
Q

What is the effect of other things in the CoCr alloy

A

scavengers

37
Q

Describe the technique for cobalt chromium

A

high temperature
melting
casting

38
Q

What is the high temperature stage for cobalt chromium

A

1200-1400 degrees Celsius hence silica or phosphate bonded

39
Q

What is the melting stage for cobalt chromium

A

electric induction preferred. Oxyacetylene allows it to avoid carbon pickup

40
Q

What is the casting stage for cobalt chromium

A

centrifugal force required. Avoid overheating or will result in coarse grains. Cooling too fast or slow will result in carbides causing it to be brittle

41
Q

What happens in the finishing of cobalt chromium

A

Sandblast
Electroplate
Abrasive wheel
Polishing buff

42
Q

What are the properties of cobalt chrome in terms of hardness

A

Much harder than gold
Wear in mouth better
Finishing/polishing time consuming

43
Q

What is elongation value for cobalt chromium

A

4%

44
Q

What are the properties of cobalt chromium

A

○ Low ductility
○ Work hardens rapidly
○ Adjustment difficult
Precision casting requires

45
Q

What are the uses for titanium

A

Implants
Partial dentures (cast)
Crown and bridges (cast)
Maxillo-facial skull plates

46
Q

What are the properties of titanium

A

good biocompatibility
good corrosion resistance (passive oxide layer)
parts joined by laser welded
titanium absorbs gases

47
Q

Why is titanium having good biocompatibility advantageous

A

if you use a maxillofacial skull plate then it Is a more demanding situation than that for a partial denture framework so titanium is used for that purpose

48
Q

Why is titanium parts being welded together an advantage

A

alloys individual parts to be laser welded together so that means that casting process is not as challenging

49
Q

What is electric arc melting used for

A

melt titanium

50
Q

What is the effect of titanium

A

absorbs gases it requires specialised investment and casting

51
Q

Why is titanium absorbing gas a disadvantage

A

it requires specialised investment and casting

52
Q

What is the order of elongation % from highest to lowest

A 
titanium 
Au (hard)
Ag-Od
CoCr
S. steel
53
Q

What is the order of ultimate tensile strength (MPa) highest to lowest

A 
s. steel
titanium 
Au (hard)
CoCr
Ag-Pd
54
Q

What is the order of density highest to lowest

A

Gold
Ag-PD
CoCr/s. steel
titanium

55
Q

What is the order of rigidity highest to lowest (Gpa)

A 
cobalt chromium
s. steel 
titanium
au (hard0
ag-pd
56
Q

What is the order of hardness highest to lowest

A 
cobalt chromium 
s. steel
au (hard)
titanium 
ag-pd
57
Q

What is the order of shrinkage highest to lowest

A

cobalt chromium
ag pd
au

COBALT CHROMIUM NEEDS DIFF INVESTMENT MATERIAL

58
Q

Which alloy is harder to produce a defect free casting - gold or CoCr

A

Cocr

59
Q

What investment can you not use for cobalt chrome

A

gypsum bonded

60
Q

Is cocr harder to polish or gold

A

cocr but retains polish better

61
Q

Since CoCr hardens rapidly what type of casting does it require

A

so need precision casting

Casting process more challenging to avoid defects

62
Q

What has higher shrinkage, gold or cobalt chrome

A

cobalt chrome
2.3% for CoCr
1% difference

63
Q

Which has higher density gold or CoCr

A

gold - almost double CoCr

CoCr is 8

64
Q

What has higher young modulus, CoCr or gold

A

CoCr
1.5 x higher than gold
250 MPa for CoCr

65
Q

What is the UTS (fracture strength) higher for, gold or CoCr?

A

Gold
100Mpa higher than Cobalt chromium
690 for cobalt chromium

66
Q

What has a higher elongation, gold or CoCr

A

gold
more than double
4% for CoCr

67
Q

What has a greater hardness, Cobalt chromium or gold

A

cobalt chromium
around 150 BHN difference
370 BHN for CoCr

BDS2 CLINICAL Dental Materials (15 decks)

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