Porcelain fused alloys

Question 1

Benefit of combining porcelain and metal alloy for crowns?

Answer 1

Combines good aesthetics of porcelain and the good mechanical properties of alloy.
Without the alloy - micro cracks form at fitting surface = mechanical failure.
Bonding of metal oxide to porcelain helps eliminate defects on porcelain surface. Acts as a support and limits the strain that porcelain experiences.

Question 2

Compressive strength

Answer 2

Compressive stress resulting in fracture.

Question 3

Elastic modulus

Answer 3

Measurement of materials elasticity/rigidity.
High elastic modulus = rigid.
Steeper slope on stress/strain curve indicates material is more rigid.

Question 4

Brittleness/ductility

Answer 4

Dimensional change experienced before fracture e.g. glass is brittle, plastic is ductile.
Distance from PL (proportional limit) to FS (fracture stress) = brittleness. Short distance = brittle.

Question 5

Hardness

Answer 5

Resistance of surface to indentation.

Question 6

What information can be provided from stress-strain curve?

Answer 6

Rigidity
Ductility
Strength
NOT surface hardness.

Question 7

Mechanical properties of porcelain

Answer 7

Hard
Strong
Rigid
BRITTLE

Question 8

Mechanical properties of alloy

Answer 8

Hard
Strong
Rigid
Ductile

Question 9

Characteristics of porcelain

Answer 9

Rigid - large stresses required to cause strain.
Hard - surface withstands indentation well.
Strong - high compressive strength.
BUT
Low tensile strength - tendency to form surface defects leading to fracture at a low stress.
Brittle

Question 10

Required properties of metal alloys (porcelain fused alloy)

Answer 10

Good bond strength i.e. good wetting.
Similar thermal expansion coefficient.
Avoid discolouration of porcelain (reason why copper is not used in high gold alloy).
Good mechanical properties.
Melting, recrystallisation temperature of alloy must be higher than fusion temperature of porcelain.

Question 11

Why is it important that the melting, recrystallisation temperature of alloy is higher than fusion temperature of porcelain?

Answer 11

Otherwise creep may occur.

Question 12

Creep

Answer 12

Gradual increase in strain (permanent) experienced under prolonged application of stress.

Question 13

Constituents of high gold alloys

Answer 13

80% gold
14% platinum
1% silver
Indium and tin forms oxides for bonding.
No copper - otherwise green discolouration of porcelain.

Question 14

Constituents of low gold alloys

Answer 14

50% gold
30% platinum
10% silver
10% indium and tin - increased melting temperature, slightly better mechanical properties.

Question 15

Constituents of silver-palladium alloys

Answer 15

60% platinum
30% silver
10% indium and tin

Question 16

constituents of nickel-chromium alloys

Answer 16

70-80% nickel
10-25% chromium

Question 17

properties of nickel-chromium alloys

Answer 17

high melting point
high Youngs modulus
high casting shrinkage
low-ish bond strength

Question 18

properties of CoCr alloys

Answer 18

High melting point
Low-ish bond strength
High Youngs modulus
High tensile strength
High hardness

Question 19

Failure modes of porcelain fused alloys

Answer 19

fracture of metal oxide
fracture between alloy and metal oxide
fracture between metal oxide and porcelain
fracture of porcelain

Question 20

Which alloy does NOT provide adequate bond strength to porcelain?

Answer 20

Ni-Cr

Question 21

Which alloy has highest elastic modulus i.e. most rigid?

Answer 21

Ni-Cr

Question 22

Problems with high gold alloys?

Answer 22

Melting range may be too low.
Youngs modulus is too low - i.e. not rigid enough.

Question 23

Bonding mechanisms of porcelain fused alloys

Answer 23

mechanical
stressed skin effect
chemical