DMS - Porcelain Fused Alloys Flashcards

1
Q

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

A

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

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2
Q

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

A

helps eliminate the defects/cracks on the porcelain surface.

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3
Q

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

A
High gold alloy
Low gold alloy 
Silver palladium 
Nickel chromium 
Cobalt chromium
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4
Q

what properties must the alloys have? (5)

A

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

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5
Q

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

A

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

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6
Q

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

A

to prevent creep

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7
Q

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

A

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

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8
Q

describe the mechanical bonding mechanisms in porcelain fused alloys.

A

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

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9
Q

describe the stressed skin bonding mechanism in porcelain fused alloys.

A

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

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10
Q

describe the chemical bonding mechanism in porcelain fused alloys.

A

Electron Sharing that occurs during the firing stage

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11
Q

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

A

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.

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