Porcelain Metal Alloys
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what is compressive strength
stress required to fracture a material making it no longer fit for purpose
what is elastic modulus (young’s)
this is a measure of rigidity which is the stress : strain ratio
what is brittleness
how much dimensional change a material can undergo before it fractures
what is ductility
the ability of a material to be drawn or plastically deformed without fracture
what is hardness
resistance to surface indentation / abrasion
what characteristics can be obtained from a stress : strain curve
elastic modulus, compressive strength, brittleness/ductility
what characteristics cannot be obtained from a stress : strain curve
hardness
3 good qualities of porcelain
rigid, hard , strong
biggest issue with porcelain
very brittle and not ductile
how are porcelain metal alloys made and why
cast to a desired shape beforehand then subjected to high temp to produce oxide layer that turns bond to ceramic this helps to eliminate defects on porcelain surface
what happens after porcelain metal alloys are heated
both materials must be cooled without developing thermal stresses as metal oxide layer could develop defects so both porcelain and alloy must have similar thermal expansion coefficients
5 required alloy properties to be bonded to porcelain
• good wetting - to form good bond • TEC should be similar - 14ppm/degreesC • avoid discolouration of porcelain • mechanical - bond strength, hardness, elastic modulus should all be high • melting temp - higher than porcelain to prevent creep
5 alloys used for bonding to porcelain
high gold alloy, low gold alloy, silver palladium AgPd, nickel chromium, cobalt chromium (note different to CoCr for RPD)
pros of high gold alloy
matches TEC, increases mpt
cons of high gold alloy
melting range is too low and young’s modulus is too low
pros of low gold alloy
higher mpt and better mechanical properties compared to high gold
pros of silver palladium
high mpt
cons of silver palladium
casting is a challenge
pros of nickel chromium
hight mpt, high young’s modulus
cons of nickel chromium
high casting shrinkage, low bond strength to porcelain
pros of cobalt chromium
high mpt 1300-1400, high young’s modulus 220GPa, high tensile strength 850MPa, high hardness 360-430 VHN
cons of cobalt chromium
significant casting shrinkage 2.3%, low bond strength 50MPa
which of the alloys has biocompatibility issues
NiCr has due to allergic responses attributed to nickel
3 bonding mechanisms of porcelain to oxide layer and alloy
• mechanical - surface irregularities allows them to interlock • stressed skin effect - slight differences in TEC leads to compressive forces to aid bonding • chemical - electron sharing in oxides