Alloys for Cast Metal Restorations Flashcards
what are cast metal restorations
crown and bridges and porcelain fused to metal
how can you describe a porcelain fused to metal crown
cast metal coping onto which is fired a ceramic veneer
porcelain has good aesthetics but what may be the downside
microcracks tend to form at the fitting surface which means its prone to mechanical failure
what forces is porcelain unable to withstand
biting forces
what is meant by compressive strength
stress to cause fracture
what is elastic modulus definition
the stress required to change shape
what is the definition of brittleness (ductility)
dimensional change experienced before fracture
what is hardness definition
resistance of surface to indentation or abrasion
what three properties can be ascertained by the stress/ strain curve
strength (compressive and tensile)
brittleness
elastic modulus
what cannot be ascertained from a stress/ strain curve
hardness
what are the properties of porcelain
ductile
hard
strong
rigid
brittle
what are the properties of an alloy
hard
strong
rigid
ductile
what is the definition of rigid
large stress required to cause strain
what is the definition of hard
surface withstands abrasion/ indentation well
what is the definition of strong
high compressive strength but low tensile strength which means a tendency to form surface defects
what is the definition of brittle
low fracture toughness
what is the definition of brittle
low fracture toughness
what is the definition of brittle
low fracture toughness
what bonds the alloy structure to the porcelain
metal oxide
what does the bonding of a metal oxide to porcelain help to eliminate
defects/ cracks on the porcelain surface
what does the alloy act as in a PFM
acts as support and limits the strain that porcelain experiences
what are the 5 required properties of alloys used in PFM
forms good bond with porcelain
thermal expansion coefficient similar to porcelain
avoid discolouring of porcelain
mechanical strength
melting recrystallisation temp of alloy
why should the thermal expansion coefficient of the metal be similar to that of porcelain
it avoids setting up stresses during fusing of porcelain to alloy
what metals need to be avoided to reduce chances of discolouration of porcelain
Ag in AgPd can produce green discolouration
Copper is not used in high gold alloy
what are the five alloys that are suitable for PFM
high gold alloys
low gold alloy
silver palladium (AgPd)
Nickle chromium (NiCr)
cobalt chromium
what alloy is more difficult to achieve good bonding to porcelain with
Nickel chromium
what must be the melting/ recrystallisation temperature of the alloy be in relation to porcelain
must be higher than the fusion temperature of porcelain otherwise creep may occur
what is the definition of creep
gradual increase in strain experienced under prolonged application of stress
when does creep occur
when the material temperature is more than half its MPt
what metals are included in high gold alloys
gold (80%)
platinum and palladium
silver
why is there no copper in high gold alloys
so no green hue is imparted to the porcelain
what are two disadvantages of high gold alloys
melting range may be too low
young’s modulus too low
what are the metals included in low gold alloys
gold (50%)
palladium
silver
what are two advantages of using low gold alloys over high gold alloys
increased melting temperature
better mechanical properties
what metals are incorporated into silver-palladium alloys
palladium
silver
tin
what percentage of nickel and chromium are in nickel chromium alloys
nickel 70%
chromium 30%
give one advantage and one disadvantage of nickel chromium alloys
high melting point and young - advantage
low-ish bond strength and high casting shrinkage - disadvantage
what alloys are the best for casting
high gold and low gold
what alloys have excellent properties for creep
low gold
silver palladium
nickel chromium
what alloys have the best elastic modulus
nickel chromium
cobalt chromium
what alloys have the best bonding properties
high gold
low gold
silver palladium
what alloy has the best biocompatibility
high gold
what alloy has the worst biocompatibility
nickel chromium
where is it best to have failure to occur in PFM
the porcelain
what is the stressed skin effect of porcelain-metal bonds
slight differences in thermal contraction coefficiencies leads to compressive forces that aid bonding
