DMS - Porcelain Fused Alloys

Question 1

what are the advantages of using porcelain fused to an alloys? (1)

Answer 1

Alloys are ductile and limits the strain the that the porcelain is subjected to.
porcelain is not ductile/ brittle.

Question 2

what is the advantage of having the metal oxides bonded to the porcelain? (1)

Answer 2

helps eliminate the defects/cracks on the porcelain surface.

Question 3

what alloys can you use for a porcelain fused alloy? (5)

Answer 3

High gold alloy
Low gold alloy 
Silver palladium 
Nickel chromium 
Cobalt chromium

Question 4

what properties must the alloys have? (5)

Answer 4

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

Question 5

what should the thermal expansion coefficient of alloys be compared to the porcelain and why?

Answer 5

Alloys value should just be a little greater (0.5ppm) so that during the cooling process the alloy is slightly compressing the porcelain

Question 6

why should the melting/recrystallisation temperature of the alloy be greater than that of the porcelain?

Answer 6

to prevent creep

Question 7

what alloy is the best to use in porcelain fused alloys (1) and why (5) ?

Answer 7

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

Question 8

describe the mechanical bonding mechanisms in porcelain fused alloys.

Answer 8

mechanically interlock - Due to surface irregularities on the (alloy’s) metal oxide layer and the porcelain

Question 9

describe the stressed skin bonding mechanism in porcelain fused alloys.

Answer 9

Alloy contracts slightly more than the porcelain on cooling and this generates compressive forces on the porcelain from the alloy gripping it

Question 10

describe the chemical bonding mechanism in porcelain fused alloys.

Answer 10

Electron Sharing that occurs during the firing stage

Question 11

describe the modes of failure in porcelain fused alloys (4)

Answer 11

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.