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BDS3 Dental Materials Science > Alloys for cast metal restorations > Flashcards

Alloys for cast metal restorations Flashcards

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1
Q

What are the two alloys used for cast metal restorations?

A
  • Crown and bridge alloys
  • Porcelain fused-to-metal alloys
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2
Q

What is a PFM crown?

A
  • Porcelain surface
  • Metal alloy substructure
  • Porcelain fused metal crown
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3
Q

Why must porcelain be bonded to a metal alloy?

A
  • Porcelain has good aesthetics
  • But microcracks tend to form at fitting surface due to large biting forces
  • Makes it prone to mechanical failure
  • Alloys withstand large stresses readily as have good mechanical properties
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4
Q

What is compressive strength?

A
  • Stress to cause fracture
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5
Q

What is elastic modulus (rigidity)?

A
  • Stress/strain ratio
  • Stress required to cause change in shape
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6
Q

What is brittleness/ductility?

A
  • Dimensional change experienced before fracture
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7
Q

What is hardness?

A
  • Resistance of surface to indentation or abrasion
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8
Q

What can be ascertained from a stress -strain curve and what can’t?

A
  • Strength (compressive/tensile)
  • Brittleness/ductility
  • Elastic modulus (rigidity)
  • Hardness is not
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9
Q

What are some key features of a stress strain curve?

A
  • Initial gradient = Elastic modulus (steeper gradient means more rigid)
  • Red dots at end of curve = fracture stress
  • If small gap between fracture stress and proportional limit = brittle
  • If large gap between FS and PL = ductile
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10
Q

What materials tend to be brittle and what tends to be ductile?

A

Brittle = Ceramics
Ductile = Alloys

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11
Q

What are the properties of procelain?

A
  • Hard so surface withstands abrasion/indentation well
  • Quite rigid so large stress required to cause strain
  • Strong so high compressive strength
  • Low tensile strength so tendency to form surface defects leads to fracture at low stress
  • Quite brittle so low fracture toughness
  • Not ductile
  • Maximum strain is approx 0.1% before fracturing
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12
Q

What are the properties of porcelain-fused alloys?

A
  • Alloys much stronger
  • Much harder and more rigid
  • More ductile
  • Withstand greater degrees of permanent strain when subjected to large stresses like biting
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13
Q

What is the structure of porcelain-metal restorations?

A
  • Metal oxide bonded to both the porcelain and the alloy
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14
Q

What is the purpose of the metal oxide?

A
  • Helps eliminate defects/cracks on porcelain surface
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15
Q

What is the purpose of the alloy?

A
  • Alloy supports and limits strain that porcelain experiences
  • More rigid so change shape very little and return to original dimensions
  • Helps it not reach level for brittle failure
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16
Q

To avoid developing defects or micro-cracks what must the porcelain-fused-ally undergo?

A
  • Both porcelain and alloy should have similar thermal expansion coefficients
  • Due to needing to be fired in furnace then cooling
  • If didn’t have similar then defects would occur
  • So they expand at same rate when heated and contract at same rate when cooled
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17
Q

What are some different alloys that have been developed for bonding to porcelain?

A
  • High gold alloy
  • Low gold alloy
  • Silver palladium (AgPd)
  • Nickel chromium (NiCr)
  • Cobalt chromium
18
Q

Why must the alloy form a good bond to porcelain?

A
  • AKA good wetting
  • Means goo surface contact
  • Porcelain forms bond with metallic oxides on surface
  • To ensure the restoration does not fall apart in patients mouth and swallow
19
Q

Why must the thermal expansion coefficient of alloy be similar to porcelain?

A
  • Porcelain is 14ppm per degree C
  • To avoid setting up stresses during fusing of porcelain to alloy
  • So defects and micro-cracks don’t form on cooling stage
20
Q

What must a property of the alloy be to ascertain good aesthetics?

A
  • Needs to avoid discolouration of porcelain as porcelain has good aesthetics
  • Ag in AgPd can produce green discolouration
  • Copper not used in high gold alloy for this reason
21
Q

What mechanical properties are desirable of an alloy?

A
  • High bond strength
  • Good hardness
  • High elastic modulus (rigid) to support porcelain and prevent fracture
22
Q

What must the melting, recrystallisation temp of alloy be compared to porcelain?

A
  • Must be higher than fusion temp of porcelain
  • Or creep may occur
23
Q

What is creep?

A
  • Gradual increase in strain (permanent) experienced under prolonged application of stress (<EL)
  • Occurs when material temp is more than half its melting point
24
Q

What are the constituents of High gold alloys?

A
  • Gold (Au) = 80%
  • Platinum/Palladium (Pt/Pd) = 14%
  • Silver (Ag) = 1%
  • Small amount Indium, Tin
  • No Copper (green hue)
25
Q

Why is Pt/Pd used in high gold alloys?

A
  • Matches thermal expansion of porcelain
  • Increases its melting point which Helps minimise potential for creep
26
Q

Why is Indium and Tin used in high gold alloys?

A
  • Enable metal oxide layer to form
  • Enables bonding to porcelain
27
Q

What are the disadvantages of high gold alloys?

A
  • Melting range too low
  • Young’s modulus too low (not rigid)
28
Q

What are the constituents for low gold alloy?

A

Au = 50%
Pd = 30%
Ag = 10%
Indium, Tin = 10%

29
Q

Why is low gold better than high gold?

A
  • Increased melting temperature
  • Slightly better mechanical properties
30
Q

What are the constituents of silver palladium alloys?

A

Pd = 60%
Ag = 30%
Indium, Tin = 10%

31
Q

A negative and positive about silver palladium alloys?

A
  • High melting point
  • Care needed in casting and is challenge for technicians
32
Q

What are the constituents of Nickel-chromium alloys?

A

Ni = 70-80%
Cr = 10-25% (creates oxide bond)

33
Q

What are the advantages and disadvantages for Nickel-chromium alloy?

A
  • High melting point
  • High young modulus (rigid)

But - has high casting shrinkage so challenging to use
- Low-ish bond strength to porcelain

34
Q

What are the advantages and disadvantages to Cobalt-chromium alloys?

A
  • High melting point (1300C-1400C)
  • High young modulus (220 GPa)
  • High tensile strength (850MPa)
  • High hardness (360-430 VHN)

But
- Low ish bond strength (220GPa)
- Casting shrinkage 2.3%

35
Q

Why are there biocompatibility concerns for nickel chromium?

A
  • Allergy to nickel
36
Q

What alloys have high casting shrinkage?

A
  • AgPd
  • NiCr
  • Cocr
37
Q

What is the most used alloy in GDH labs?

A
  • CoCr
38
Q

What are the 3 mechanisms for bond between oxide layer, alloy and procelain?

A
  • Mechanical
  • Stressed skin
  • Chemical
39
Q

What is the mechanical bond between oxide layer, alloy and porcelain?

A
  • Due to surface irregularities on alloys metal oxide layer and porcelain
  • Allows them to interlock
  • Least important of the 3
40
Q

What is the stressed skin effect?

A
  • Depends on slight differences in thermal contraction coefficients
  • After furnace stage, alloy contracts slightly on cooling
  • Generates compressive forces on porcelain
  • Aids bonding
41
Q

What is the chemical bond?

A
  • Electron sharing in metal oxide coating alloy and the porcelain
  • Occurs during firing of porcelain where high temps reached
  • Described as electron sharing
42
Q

What are the modes of failure in porcelain fusion?

A
  • Oxide layer fracturing
  • Oxide layer delaminating from alloy
  • Porcelain detaching from oxide layer (most ideal situation)

BDS3 Dental Materials Science (10 decks)

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