Porcelain Fused Alloy Flashcards
Porcelain fused metal crown
With only porcelain
- good aesthetics
- but microcracks tend to form at fitting surface, making it prone to mechanical failure
alloy- has good mechanical properties
Mechanical properties
Compressive strength
- stress to cause fracture
Elastic modulus/ Young’s modulus
- rigidity
- stress/ strain ratio
- stress required to cause change in shape
Brittleness/ ductile
- dimensional change experienced before fracture
Hardness
- resistance of surface to indentation/ abrasion
What does stress/ strain curve represent?
- strength (compressive/ tensile)
- brittleness/ ductile
- elastic modulus (rigidity)
** cannot test for hardness as it is the feature of material surface
Stress- strain curve
- material A is more rigid and stronger than B
- material A is more brittle and less ductile than B
Mechanical properties of porcelain
- hard, strong, rigid
- but brittle
Mechanical properties of Alloy
- hard
- strong
- rigid
- ductile, withstand when bending
Summary of Characteristics of porcelain
- rigid - large stress required to cause strain
- hard - surface withstand abrasion/ indentation well
- strong - high compressive strength, but low tensile strength
- tendency to form surface defects, leading to fracture at low stress
- brittle (max strain is 0.1%)
Porcelain- metal restorations
- bonding of metal oxide to porcelain helps eliminate defect/ cracks on porcelain surface
- alloy- acts as support and limits strain that porcelain experiences
- metal oxide forms when alloy and porcelain are in high temp furnace
Thermal expansion coefficient
- both porcelain and alloy need to have similar thermal expansion
- avoid thermal stresses on contact surface, hence good bond
Range of alloys
- high gold alloy
- low gold alloy
- silver palladium
- nickel chromium
- cobalt chromium: different from RPD CoCr
Required properties needed for porcelain fused alloy
- Form good bond to porcelain
- good wetting
- porcelain forms bond with metallic oxides on surface
-NiCr more diff to achieve good bonding
- Thermal expansion coefficient
- must be similar to porcelain
- (14ppm/dc)
- avoid setting up stresses during fusing of porcelain to alloy
- alloy can be slightly higher so it can compress porcelain - Avoid discolouration of porcelain
- AgPd can produce green discolouration - Mechanical (bond strength, hardness, elastic modulus)
- BS - all adequate , except NiCr
- hardness - all similar, NiCr may be too hard
- EM- want a high value to prevent fracture (NiCr best) - Melting/ recrystallisation temp of alloy
- must be higher than fusion temp to prevent creep
What is creep?
- gradual increase in strain experienced under prolonged application of stress
- happens when material temp is more than about half its MP
Why not high gold alloys
- melting point maybe too low
- not rigid
Why low gold alloys
- higher MP
- better mechanical properties
AgPd alloys
- high MP
- need casting and may be a challenge
NiCr alloy
- high MP
- rigid
- exhibit a lot of shrinkage during casting
- bonding to porcelain is quite low
CoCr alloys
Properties of 5 alloys
Stressed skin effect
- alloy contracts more during cooling state
- leads to compressive forces which aid bonding
- slight differences in thermal contraction
Chemical reaction
- during firing state
- electron sharing
- oxides in metal oxide coating migrate
