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BDS3 DMS > Alloys for Cast Metal Restorations > Flashcards

Alloys for Cast Metal Restorations Flashcards

1
Q

what are cast metal restorations

A

crown and bridges and porcelain fused to metal

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2
Q

how can you describe a porcelain fused to metal crown

A

cast metal coping onto which is fired a ceramic veneer

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3
Q

porcelain has good aesthetics but what may be the downside

A

microcracks tend to form at the fitting surface which means its prone to mechanical failure

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4
Q

what forces is porcelain unable to withstand

A

biting forces

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5
Q

what is meant by compressive strength

A

stress to cause fracture

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6
Q

what is elastic modulus definition

A

the stress required to change shape

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7
Q

what is the definition of brittleness (ductility)

A

dimensional change experienced before fracture

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8
Q

what is hardness definition

A

resistance of surface to indentation or abrasion

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9
Q

what three properties can be ascertained by the stress/ strain curve

A

strength (compressive and tensile)
brittleness
elastic modulus

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10
Q

what cannot be ascertained from a stress/ strain curve

A

hardness

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11
Q

what are the properties of porcelain

A

ductile
hard
strong
rigid
brittle

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12
Q

what are the properties of an alloy

A

hard
strong
rigid
ductile

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13
Q

what is the definition of rigid

A

large stress required to cause strain

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14
Q

what is the definition of hard

A

surface withstands abrasion/ indentation well

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15
Q

what is the definition of strong

A

high compressive strength but low tensile strength which means a tendency to form surface defects

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16
Q

what is the definition of brittle

A

low fracture toughness

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17
Q

what is the definition of brittle

A

low fracture toughness

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18
Q

what is the definition of brittle

A

low fracture toughness

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19
Q

what bonds the alloy structure to the porcelain

A

metal oxide

20
Q

what does the bonding of a metal oxide to porcelain help to eliminate

A

defects/ cracks on the porcelain surface

21
Q

what does the alloy act as in a PFM

A

acts as support and limits the strain that porcelain experiences

22
Q

what are the 5 required properties of alloys used in PFM

A

forms good bond with porcelain
thermal expansion coefficient similar to porcelain
avoid discolouring of porcelain
mechanical strength
melting recrystallisation temp of alloy

23
Q

why should the thermal expansion coefficient of the metal be similar to that of porcelain

A

it avoids setting up stresses during fusing of porcelain to alloy

24
Q

what metals need to be avoided to reduce chances of discolouration of porcelain

A

Ag in AgPd can produce green discolouration
Copper is not used in high gold alloy

25
Q

what are the five alloys that are suitable for PFM

A

high gold alloys
low gold alloy
silver palladium (AgPd)
Nickle chromium (NiCr)
cobalt chromium

26
Q

what alloy is more difficult to achieve good bonding to porcelain with

A

Nickel chromium

27
Q

what must be the melting/ recrystallisation temperature of the alloy be in relation to porcelain

A

must be higher than the fusion temperature of porcelain otherwise creep may occur

28
Q

what is the definition of creep

A

gradual increase in strain experienced under prolonged application of stress

29
Q

when does creep occur

A

when the material temperature is more than half its MPt

30
Q

what metals are included in high gold alloys

A

gold (80%)
platinum and palladium
silver

31
Q

why is there no copper in high gold alloys

A

so no green hue is imparted to the porcelain

32
Q

what are two disadvantages of high gold alloys

A

melting range may be too low
young’s modulus too low

33
Q

what are the metals included in low gold alloys

A

gold (50%)
palladium
silver

34
Q

what are two advantages of using low gold alloys over high gold alloys

A

increased melting temperature
better mechanical properties

35
Q

what metals are incorporated into silver-palladium alloys

A

palladium
silver
tin

36
Q

what percentage of nickel and chromium are in nickel chromium alloys

A

nickel 70%
chromium 30%

37
Q

give one advantage and one disadvantage of nickel chromium alloys

A

high melting point and young - advantage
low-ish bond strength and high casting shrinkage - disadvantage

38
Q

what alloys are the best for casting

A

high gold and low gold

39
Q

what alloys have excellent properties for creep

A

low gold
silver palladium
nickel chromium

40
Q

what alloys have the best elastic modulus

A

nickel chromium
cobalt chromium

41
Q

what alloys have the best bonding properties

A

high gold
low gold
silver palladium

42
Q

what alloy has the best biocompatibility

A

high gold

43
Q

what alloy has the worst biocompatibility

A

nickel chromium

44
Q

where is it best to have failure to occur in PFM

A

the porcelain

45
Q

what is the stressed skin effect of porcelain-metal bonds

A

slight differences in thermal contraction coefficiencies leads to compressive forces that aid bonding

BDS3 DMS (6 decks)

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