DMS - Porcelain Fused Alloys Flashcards
what are the advantages of using porcelain fused to an alloys? (1)
Alloys are ductile and limits the strain the that the porcelain is subjected to.
porcelain is not ductile/ brittle.
what is the advantage of having the metal oxides bonded to the porcelain? (1)
helps eliminate the defects/cracks on the porcelain surface.
what alloys can you use for a porcelain fused alloy? (5)
High gold alloy Low gold alloy Silver palladium Nickel chromium Cobalt chromium
what properties must the alloys have? (5)
Form a good bond to porcelain i.e. good wetting/surface contact
Thermal expansion coefficient must be similar porcelain/ higher by 0.5ppm
Avoid discolouration of the porcelain
Elastic modulus - want a high value i.e. the more rigid the alloy the lower the amount of strain the porcelain will be subjected too and prevent fracture.
Melting/recrystallisation higher than the porcelain
what should the thermal expansion coefficient of alloys be compared to the porcelain and why?
Alloys value should just be a little greater (0.5ppm) so that during the cooling process the alloy is slightly compressing the porcelain
why should the melting/recrystallisation temperature of the alloy be greater than that of the porcelain?
to prevent creep
what alloy is the best to use in porcelain fused alloys (1) and why (5) ?
Low gold
Castable
low creep = melting temp higher than porcelain
has a high elastic modulus = limits strain on porcelain
Good bond to the porcelain
Biocompatible
describe the mechanical bonding mechanisms in porcelain fused alloys.
mechanically interlock - Due to surface irregularities on the (alloy’s) metal oxide layer and the porcelain
describe the stressed skin bonding mechanism in porcelain fused alloys.
Alloy contracts slightly more than the porcelain on cooling and this generates compressive forces on the porcelain from the alloy gripping it
describe the chemical bonding mechanism in porcelain fused alloys.
Electron Sharing that occurs during the firing stage
describe the modes of failure in porcelain fused alloys (4)
Metal oxide layer fracture
Alloy detaching from the metal oxide and porcelain
Porcelain detaching from the metal oxide and the alloy
Porcelain fracture - preferred mode of failure since porcelain is the weakest.