Alloys for cast metal restorations Flashcards
What are the two alloys used for cast metal restorations?
- Crown and bridge alloys
- Porcelain fused-to-metal alloys
What is a PFM crown?
- Porcelain surface
- Metal alloy substructure
- Porcelain fused metal crown
Why must porcelain be bonded to a metal alloy?
- 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
What is compressive strength?
- Stress to cause fracture
What is elastic modulus (rigidity)?
- Stress/strain ratio
- Stress required to cause change in shape
What is brittleness/ductility?
- Dimensional change experienced before fracture
What is hardness?
- Resistance of surface to indentation or abrasion
What can be ascertained from a stress -strain curve and what can’t?
- Strength (compressive/tensile)
- Brittleness/ductility
- Elastic modulus (rigidity)
- Hardness is not
What are some key features of a stress strain curve?
- 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
What materials tend to be brittle and what tends to be ductile?
Brittle = Ceramics
Ductile = Alloys
What are the properties of procelain?
- 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
What are the properties of porcelain-fused alloys?
- Alloys much stronger
- Much harder and more rigid
- More ductile
- Withstand greater degrees of permanent strain when subjected to large stresses like biting
What is the structure of porcelain-metal restorations?
- Metal oxide bonded to both the porcelain and the alloy
What is the purpose of the metal oxide?
- Helps eliminate defects/cracks on porcelain surface
What is the purpose of the alloy?
- 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
To avoid developing defects or micro-cracks what must the porcelain-fused-ally undergo?
- 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
What are some different alloys that have been developed for bonding to porcelain?
- High gold alloy
- Low gold alloy
- Silver palladium (AgPd)
- Nickel chromium (NiCr)
- Cobalt chromium
Why must the alloy form a good bond to porcelain?
- 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
Why must the thermal expansion coefficient of alloy be similar to porcelain?
- 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
What must a property of the alloy be to ascertain good aesthetics?
- 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
What mechanical properties are desirable of an alloy?
- High bond strength
- Good hardness
- High elastic modulus (rigid) to support porcelain and prevent fracture
What must the melting, recrystallisation temp of alloy be compared to porcelain?
- Must be higher than fusion temp of porcelain
- Or creep may occur
What is creep?
- Gradual increase in strain (permanent) experienced under prolonged application of stress (<EL)
- Occurs when material temp is more than half its melting point
What are the constituents of High gold alloys?
- Gold (Au) = 80%
- Platinum/Palladium (Pt/Pd) = 14%
- Silver (Ag) = 1%
- Small amount Indium, Tin
- No Copper (green hue)
Why is Pt/Pd used in high gold alloys?
- Matches thermal expansion of porcelain
- Increases its melting point which Helps minimise potential for creep
Why is Indium and Tin used in high gold alloys?
- Enable metal oxide layer to form
- Enables bonding to porcelain
What are the disadvantages of high gold alloys?
- Melting range too low
- Young’s modulus too low (not rigid)
What are the constituents for low gold alloy?
Au = 50%
Pd = 30%
Ag = 10%
Indium, Tin = 10%
Why is low gold better than high gold?
- Increased melting temperature
- Slightly better mechanical properties
What are the constituents of silver palladium alloys?
Pd = 60%
Ag = 30%
Indium, Tin = 10%
A negative and positive about silver palladium alloys?
- High melting point
- Care needed in casting and is challenge for technicians
What are the constituents of Nickel-chromium alloys?
Ni = 70-80%
Cr = 10-25% (creates oxide bond)
What are the advantages and disadvantages for Nickel-chromium alloy?
- High melting point
- High young modulus (rigid)
But - has high casting shrinkage so challenging to use
- Low-ish bond strength to porcelain
What are the advantages and disadvantages to Cobalt-chromium alloys?
- 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%
Why are there biocompatibility concerns for nickel chromium?
- Allergy to nickel
What alloys have high casting shrinkage?
- AgPd
- NiCr
- Cocr
What is the most used alloy in GDH labs?
- CoCr
What are the 3 mechanisms for bond between oxide layer, alloy and procelain?
- Mechanical
- Stressed skin
- Chemical
What is the mechanical bond between oxide layer, alloy and porcelain?
- Due to surface irregularities on alloys metal oxide layer and porcelain
- Allows them to interlock
- Least important of the 3
What is the stressed skin effect?
- Depends on slight differences in thermal contraction coefficients
- After furnace stage, alloy contracts slightly on cooling
- Generates compressive forces on porcelain
- Aids bonding
What is the chemical bond?
- Electron sharing in metal oxide coating alloy and the porcelain
- Occurs during firing of porcelain where high temps reached
- Described as electron sharing
What are the modes of failure in porcelain fusion?
- Oxide layer fracturing
- Oxide layer delaminating from alloy
- Porcelain detaching from oxide layer (most ideal situation)
