Alloys For Cast Metal Restorations

Question 1

What is added to the porcelain in a crown?

Answer 1

Metal alloy (alloys withstand large forces readily).

Question 2

What is the other name for the elastic modulus?

Answer 2

Rigidity.

Question 3

What is a materials compressive strength?

Answer 3

The amount of stress to cause fracture.

Question 4

What is the elastic modulus of a material?

Answer 4

Stress required to cause a change in shape.

Question 5

What is the definition of the hardness of a material?

Answer 5

Resistance of surface to indentation or abrasion.

Question 6

What is the ductility of a material?

Answer 6

The dimensional change experienced before fracture.

Question 7

What 3 properties of a material can be determined by a stress-strain curve?

Answer 7

Strength
Ductility
Elastic Modulus.

Question 8

There is a good example of a stress-strain curve on these slides.

Answer 8

.

Question 9

What feature on a stress-strain diagram lets you know that one material is more brittle than the other?

Answer 9

When the proportional limit and the fracture stress are closer together means that the material is brittle (less ductile).

Question 10

What are the characteristics of porcelain?

Answer 10

It is rigid, hard, strong but it has a low tensile strength.

Question 11

What happens as result of porcelain having a low tensile strength?

Answer 11

This makes porcelain have a tendency to form surface defects which leads to fractures at low stresses. Defects are in the crystals – may take a little time to grow into a substantial defect which triggers fracture line.

Question 12

What substance is bonded to porcelain to help eliminate defects/cracks on the porcelain surface?

Answer 12

Metal oxide.

Question 13

Why are alloys added to porcelain?

Answer 13

To support and limit he strain that porcelain experiences. It does this because the applied stress has to cause a change in the dimensions of the porcelain AND the alloy, and the alloy is more rigid than porcelain.
Let’s say an applied stress would cause a 1% change in porcelain but only 0.1% in the alloy.
Then once the alloy is bonded and is supporting the porcelain, the applied stress is only capable of moving the alloy 0.1% - for the porcelain to move, so must the alloy.

*With the TWO materials bonded together, the stress applied causes a small strain to be experienced, small enough for the porcelain to withstand it and remain intact!!!

Question 14

What are different types of porcelain fused metal alloys?

Answer 14

High gold
Low gold
Silver palladium
Nickel chromium
Cobalt chromium.

Question 15

What is creep?

Answer 15

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

Question 16

What are the required properties of porcelain fused metal alloys?

Answer 16

1. forms a good bond to porcelain (good wetting)
2. the thermal expansion coefficients must be similar
3. avoid discolouration of the porcelain
4. has to have good bond strength, hardness and a high elastic modulus
5. the melting/recrystallisation temperature of the alloy must be higher than fusion temp of porcelain otherwise creep may occur.

Question 17

Why must the thermal expansion coefficient of porcelain must be similar to the metal oxide?

Answer 17

To avoid setting up stresses during fusing of porcelain on alloy. The alloys melting temp should be no more than 0.5ppm/degrees centigrade so the alloy is slightly compressing porcelain on the cooling stage.

Question 18

Why is copper not used in high gold alloys?

Answer 18

Can produce a green discolouration.

Question 19

Why do we want a high elastic modulus?

Answer 19

Elastic modulus- more rigid the alloy- lower amount of stain the porcelain will be subjected to.

Question 20

What are examples of metals in a high gold alloys?

Answer 20

Pt-pd- 14%- helps match up thermal expansion of porcelain. Increases melting point- potential for alloy to exhibit creep diminishes
Ag 1%
Au 80%
Indium and tin form oxides for bonding.

Question 21

What are the disadvantages of using high gold alloys with porcelain?

Answer 21

Melting range may be too low

Youngs modulus is also too low (not sufficiently rigid).

Question 22

Why are low gold alloys better to use than high gold?

Answer 22

Increased melting temperature and slightly better mechanical properties.

Question 23

What materials are in a low gold alloy?

Answer 23

Au 50%
Pd 30%
Ag 10%
Indium+tin 10%.

Question 24

What is the composition of silver-palladium alloys?

Answer 24

Pd 60%
Ag 30%
In+Sn 10%.

Question 25

What are the characteristics of a silver-palladium alloy?

Answer 25

High melting point

Care needed in casting.

Question 26

What is the composition of nickel-chormium alloys?

Answer 26

Ni 70-80%

Cr 10-25%.

Question 27

What are the characteristics of a nickel-chromium alloy?

Answer 27

High melting point (high young’s modulus)
High casting shrinkage (low bond strength)
Challenging to use.

Question 28

What are the characteristics of cobalt-chromium alloys?

Answer 28

High melting point (1300-1400 degrees celcius)
Casting shrinkage (2.3%)
Low bond strength (50MPa)
High YM (220 GPa)
High tensile strength (850MPa)
High hardness (360-430 VHN).

Question 29

What effects the mechanical bond to porcelain?

Answer 29

Surface irregularities.

Question 30

What is the ‘stressed skin effect’?

Answer 30

During production after furnace stage- alloy contacts slightly more on cooling- generates compressive forces on porcelain due to the alloy gripping it. Meaning compressive forces are generated to aid bonding.

Question 31

What is the preferred mode of failure for a porcelain metal alloy?

Answer 31

When just the porcelain breaks off.

Question 32

What effects the mechanical bond to porcelain?

Answer 32

There may be electron sharing in oxides. During the firing process the porcelain flows and oxides in the metal oxide coating migrate.