Porcelain Fused to Metal Alloys Flashcards
(20 cards)
Why is porcelain fused to metal alloys?
PORCELAIN - good aesthetics
but microcracks tend to form at the fitting surface, making it prone to mechanical failure.
ALLOYS - good mechanical properties
What is compressive strength, elastic modulus (rididity), brittleness/ductility, hardness?
- Compressive strength
– stress to cause fracture - Elastic modulus (rigidity)
– stress/strain ratio
(ie stress required to cause change in shape) - Brittleness / ductility
– dimensional change experienced before fracture - Hardness
– resistance of surface to indentation or abrasion
What is the difference between brittle and ductile?
Brittle materials fracture easily and without warning, while ductile materials can be stretched or bent without breaking.
What are the mechanical properties of porcelain?
brittle
hard
strong
rigid
high compressive strength
low tensile strength
What are the mechanical properties of an alloy?
hard
strong
rigid
ductile
What is a rigid material?
large stress required to cause strain
How does bonding of metal oxide help porcelain?
Bonding of metal oxide to porcelain helps eliminate defects/cracks on porcelain surface.
* Alloy acts as a support & limits the strain that porcelain experiences.
What are the alloys that porcelain can be fused to?
High gold alloy
Low gold alloy
Silver palladium (AgPd)
Nickel chromium (NiCr)
Cobalt chromium
What are the required properties for porcelain to fuse to metal alloys?
1) Form good bond to porcelain. i.e. good wetting.
2) Thermal expansion coefficient - must be similar to that of porcelain to avoid setting up stresses during fusing of porcelain on to alloy
3) Avoid discolouration of porcelain
4) Mechanical properties must be cooperative
5) Melting, recrystallisation temperature of alloy must be higher
What must the mechanical properties be do allow porcelain to fuse to metal?
- Bond Strength : (not Ni-Cr)
- Hardness: all alloys adequate
(though early Ni-Cr alloys too hard) - Elastic modulus: want high value (ie rigid) to support porcelain and prevent fracture (Ni-Cr best)
What must the melting, recrystallisation temperature of alloy be?
MUST be higher than fusion temperature of porcelain - otherwise CREEP may occur.
What is creep?
when does it occur?
gradual increase in STRAIN (permanent) experienced under prolonged application of STRESS (< EL)
occurs when material temperature is more than about half its MPt
What are the advantages and disadvantages for high gold alloys?
- advantages
match thermal expansion
corrosive resistance - disadvantages
melting range may be too low
Young’s Modulus - too low
What are the advantages of low gold alloys?
- increased melting temperature
- slightly better mechanical properties
What are the advantages/disadvantages of silver palladium alloys?
- high melting point
- care needed in casting
What are the advantages/disadvantages of nickel-chromium alloys?
advantages
- high melting point
- high Young’s Modulus
disadvantages
- high casting shrinkage
- low bond strength due to thick oxide layer
What are the advantages/disadvantages of cobalt-chromium alloys?
disadvantages
high casting shrinkage
low-ish bond strength
advantages
high Young’s Modulus
high tensile strength
high hardness
How does porcelain bond to metal mechanically?
surface irregularities
How does porcelain bond to metal via the stressed skin effect?
slight differences in thermal contraction coefficients
lead to compressive forces which aid bonding
Imagine wrapping cling film tightly over a bowl — if the film is stretched just a little more than the bowl, it hugs the surface tightly.
That’s what the stressed skin effect does — it creates a natural hug between the porcelain and the metal, strengthening the bond.
How does porcelain bond to metal chemically?
- may be electron sharing in oxides
- during firing porcelain flows and oxides in the metal- oxide coating migrate
