alloys for cast metal Rxs Flashcards
why porcelain and metal are used together based on properties
porcelain - good aesthetics - but forms micro cracks - prone to mechanical failure alloys - good mechanical properties
elastic modulus/rigidity
stress/strain ratio
stress required to cause change in shape
compressive strength
stress to cause fracture
brittleness/ductility
dimensional change experienced before fracture
hardness
resistance of surface to indentation or abrasion
what doesn’t a stress-strain curve tell you?
hardness
what does a stress-strain curve tell you?
strength (compressive/tensile)
brittleness/ductility
elastic modulus (ductility)
how to assess ductility on stress strain graph
look at change in shape between PL and FS
on a stress strain graph what can’t you predict the relationship beyond?
PL
porcelain properties
hard
rigid - need large stress to cause strain
strong - high compressive strength
brittle - low fracture toughness (max strain 0.1% before fracturing)
low tensile strength
tendency to form surface defects (micro cracks)
- fractures at low stress
alloy properties
hard
strong
rigid
ductile
porcelain-metal restoration
fused together via a metal oxide layer
bonding helps eliminate defects/cracks on porcelain surface
alloy acts as support and limits the strain porcelain experiences
- alloy small % strain
- porcelain - large % strain - fracture
- alloy and porcelain - small % strain experienced by “porcelain+alloy”
required properties
good bond to porcelain i.e. good wetting TEC similar to porcelain avoid discolouration of porcelain mechanical melting, recrystallisation temp of alloy must be higher than fusion temp of porcelain
required properties - good bond to porcelain
bonds with metallic oxides on surface
NiCr alloys more difficult
required properties - TEC similar to porcelain
14ppm/degrees - ideally difference of 0.5 in alloys favour
avoid stresses during fusing
all alloys ok
required properties - avoid discolouration of porcelain
Ag in AgPd - green
Cu not used in high gold alloy
required properties - mechanical
bond strength
- 3 alloys adequate (not NiCr)
hardness
- all adequate (though early NiCr alloys too hard)
EM
- high (rigid) to support porcelain and prevent fracture (NiCr best)
required properties - melting, recrystallisation temp of alloy
must be higher than fusion temp of porcelain - otherwise creep
need porcelains and alloys with matching TECs
high gold alloy low gold alloy silver palladium (AgPd) NiCr CoCr
creep
gradual increase in permanent strain experienced under prolonged application of stress (
high gold alloys
Au 80% Pt/Pd 14% - match TEC and increase mp Ag 1% indium, tin - forms oxides - bonding no Cu - otherwise green hue melting range may be too low YM too low
low gold alloys
Au 50% Pd 30% Ag 10% indium, tin 10% increased melting temp slightly better mechanical properties
Ag-Pd alloys
Pd 60% Ag 30% In, Sn 10% high mp care needed in casting
NiCr alloys
Ni 70-80% Cr 10-25% (oxide bond) high mp high YM high casting shrinkage lowish bond strength
