Porcelain Fused Alloys

Question 1

What are the mechanical properties of porcelain?

Answer 1

Hard - withstands abrasion well
Strong - high compressive strength but low tensile strength (leads to fracture at low stress)
Rigid - large stress required to cause strain
Brittle - will fracture at low strain

Question 2

What are the mechanical properties of alloys?

Answer 2

Hard
Strong
Rigid
Ductile

Question 3

How do porcelain and alloys interact in a restoration?

Answer 3

Bonding of metal oxide to porcelain helps eliminate defects and cracks on porcelain surface
Alloys act as a support and limit the strain that porcelain experiences

Question 4

Describe the thermal expansion of porcelain and alloys in restorations

Answer 4

Must have matching thermal expansion coefficients
So when heated they will expand at same rate and contract at same rate when being cooled
Avoids thermal stresses and ensures good bond with metal oxide

Question 5

Which different alloys can be bonded to porcelain?

Answer 5

High gold alloy
Low gold alloy
Silver palladium
Nickel chromium
Cobalt chromium - different to partial denture framework

Question 6

What are the required properties of porcelain fused alloys?

Answer 6

Must form good bond to porcelain
Must have similar thermal expansion coefficient
Must avoid discolouration of porcelain - tooth-like aesthetics
High bond strength
Hard
High elastic modulus to prevent fracture
Melting recrystallisation temperature of alloy must be higher than fusion temperature of porcelain - otherwise creep may occur

Question 7

What is creep?

Answer 7

A gradual increase in permanent strain experienced under prolonged application of stress

Question 8

What is the composition of high gold alloys?

Answer 8

Au - 80%
Pt/Pd - 14%
Ag - 1%
Small amounts of Indium and Tin

Question 9

What are the disadvantages of high gold alloys?

Answer 9

Melting point is too low
Young’s modulus is too low - not sufficiently rigid

Question 10

What is the composition of low gold alloys?

Answer 10

Au - 50%
Pd - 30%
Ag - 10%
Indium and tin - 10%

Question 11

What is the purpose of Pd and Pt in porcelain fused alloys?

Answer 11

Helps to match the thermal expansion and increase melting point which helps minimise creep

Question 12

What is the purpose of Indium and Tin in porcelain fused alloys?

Answer 12

Enables metal oxide layer to form enabling the bonding to porcelain

Question 13

What is the composition of silver palladium alloys?

Answer 13

Pd - 60%
Ag - 30%
Indium and Tin - 10%

Question 14

What are the properties of Silver Palladium alloys?

Answer 14

High melting point
Care needed in casting - challenge for technicians

Question 15

What is the composition of Nickel Chromium alloys?

Answer 15

Ni - 70-80%
Cr - 10-25%

Question 16

What are the properties of Nickel Chromium alloys?

Answer 16

High melting point, high Young’s modulus - good
High casting shrinkage - challenge to use
Low bond strength - bad

Question 17

What are the properties of Cobalt Chromium alloys?

Answer 17

High melting point
High casting shrinkage - bad
Low bond strength
High Young’s modulus - good
High tensile strength - good
High hardness - good

Question 18

Which porcelain fused alloys are used in GDH?

Answer 18

CoCr most used
Some gold and palladium
Not NiCr

Question 19

What type of bond is formed between the alloy and porcelain?

Answer 19

Mechanical
Stressed skin
Chemical
Or a combination of these

Question 20

Describe a porcelain - metal mechanical bond

Answer 20

Surface irregularities allows them to interlock

Question 21

Describe a porcelain - metal stressed skin effect bond

Answer 21

Slight differences in thermal contraction coefficients of porcelain and alloy lead to compressive forces which aid bonding
After furnace stage, alloy contracts slightly more on cooling - this generates compressive forces on the porcelain, gripping it

Question 22

Describe a porcelain - metal chemical bond

Answer 22

May be electron sharing in oxides
During firing porcelain flows and oxides in the metal oxide coating migrate

Question 23

What is the ideal mode of failure in a porcelain fused restoration?

Answer 23

Failure within the porcelain

Question 24

In what undesirable ways may a porcelain fused restoration fail?

Answer 24

Porcelain detaching from the oxide layer
Oxide layer delaminating from the alloy
Oxide layer itself fracturing