Porcelain Fused Alloys Flashcards
What are the mechanical properties of porcelain?
Hard - withstands abrasion well
Strong - high compressive strength but low tensile strength (leads to fracture at low stress)
Rigid - large stress required to cause strain
Brittle - will fracture at low strain
What are the mechanical properties of alloys?
Hard
Strong
Rigid
Ductile
How do porcelain and alloys interact in a restoration?
Bonding of metal oxide to porcelain helps eliminate defects and cracks on porcelain surface
Alloys act as a support and limit the strain that porcelain experiences
Describe the thermal expansion of porcelain and alloys in restorations
Must have matching thermal expansion coefficients
So when heated they will expand at same rate and contract at same rate when being cooled
Avoids thermal stresses and ensures good bond with metal oxide
Which different alloys can be bonded to porcelain?
High gold alloy
Low gold alloy
Silver palladium
Nickel chromium
Cobalt chromium - different to partial denture framework
What are the required properties of porcelain fused alloys?
Must form good bond to porcelain
Must have similar thermal expansion coefficient
Must avoid discolouration of porcelain - tooth-like aesthetics
High bond strength
Hard
High elastic modulus to prevent fracture
Melting recrystallisation temperature of alloy must be higher than fusion temperature of porcelain - otherwise creep may occur
What is creep?
A gradual increase in permanent strain experienced under prolonged application of stress
What is the composition of high gold alloys?
Au - 80%
Pt/Pd - 14%
Ag - 1%
Small amounts of Indium and Tin
What are the disadvantages of high gold alloys?
Melting point is too low
Young’s modulus is too low - not sufficiently rigid
What is the composition of low gold alloys?
Au - 50%
Pd - 30%
Ag - 10%
Indium and tin - 10%
What is the purpose of Pd and Pt in porcelain fused alloys?
Helps to match the thermal expansion and increase melting point which helps minimise creep
What is the purpose of Indium and Tin in porcelain fused alloys?
Enables metal oxide layer to form enabling the bonding to porcelain
What is the composition of silver palladium alloys?
Pd - 60%
Ag - 30%
Indium and Tin - 10%
What are the properties of Silver Palladium alloys?
High melting point
Care needed in casting - challenge for technicians
What is the composition of Nickel Chromium alloys?
Ni - 70-80%
Cr - 10-25%
What are the properties of Nickel Chromium alloys?
High melting point, high Young’s modulus - good
High casting shrinkage - challenge to use
Low bond strength - bad
What are the properties of Cobalt Chromium alloys?
High melting point
High casting shrinkage - bad
Low bond strength
High Young’s modulus - good
High tensile strength - good
High hardness - good
Which porcelain fused alloys are used in GDH?
CoCr most used
Some gold and palladium
Not NiCr
What type of bond is formed between the alloy and porcelain?
Mechanical
Stressed skin
Chemical
Or a combination of these
Describe a porcelain - metal mechanical bond
Surface irregularities allows them to interlock
Describe a porcelain - metal stressed skin effect bond
Slight differences in thermal contraction coefficients of porcelain and alloy lead to compressive forces which aid bonding
After furnace stage, alloy contracts slightly more on cooling - this generates compressive forces on the porcelain, gripping it
Describe a porcelain - metal chemical bond
May be electron sharing in oxides
During firing porcelain flows and oxides in the metal oxide coating migrate
What is the ideal mode of failure in a porcelain fused restoration?
Failure within the porcelain
In what undesirable ways may a porcelain fused restoration fail?
Porcelain detaching from the oxide layer
Oxide layer delaminating from the alloy
Oxide layer itself fracturing
