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REPOSITORY - Dental Materials Science > Alloys for Cast Metal Restorations > Flashcards

Alloys for Cast Metal Restorations Flashcards

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 = CeramicsDuctile = 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 (
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)

REPOSITORY - Dental Materials Science (19 decks)

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