Alloys for Cast Metal Restorations

Question 1

Porcelain has good aesthetic so why can’t this be used on its own as a restorative material?

Answer 1

During the production of porcelain, microcracks will inevitably form within porcelain or on its surfaces. This means that porcelain on is own isnt able to withstand the large biting forces it will be subjected to

Question 2

What is compressive strength?

Answer 2

the stress required to fracture a material – making it no longer fit for purpose.

Question 3

What is elastic modulus/young’s modulus a measure of?

Answer 3

The rigidity (stress strain ratio) which is the stress required to change the shape of a material

Question 4

What is is the brittleness/ductility of a material related to?

Answer 4

How much dimensional change the material undergoes before fracturing

Question 5

What is the hardness of a material?

Answer 5

Measure how readily a material’s surface will resist indentation or abrasion

Question 6

What property cant be ascertained from a material’s stress-strain curve? (out of strength, brittleness/ductility, rigidity, hardness)

Answer 6

Hardness

Question 7

Which material is more rigid?

Answer 7

Material A

The initial gradient of each curve indicates the materials elastic modulus. The greater the gradient = more rigid

Question 8

What is the tensile strength of each material?

Answer 8

A = approx. 170MPa

B = approx 125MPa

Note: the vertcal axis indicates the tensile strength being applied

Question 9

Is each material ductile or brittle? Explain.

Answer 9

Material A = brittle

Material B = ductile

Looking at the same stress-strain curves again, we can see that material A is BRITTLE.

Between the proportional limit stress – the yellow dot – and the FRACTURE stress (the red dot), we can see that material A undergoes just 0.5% strain or change in length.

Contrast that with material B that stretches from just under 3% strain at the proportional limit stress to 6% when it fractures. Material B is therefore DUCTILE.

Question 10

How is a brittle/ductile material defined in terms of stretching?

Answer 10

A BRITTLE material will change shape a fraction of a per cent of its length, and then break (Ceramics fall into this category) e.g. a material stretches approx 0.5% before fracturing

WHEREAS a DUCTILE material will stretch several per cent of its length, then fracture. (Alloys tend to be ductile)

Question 11

Descibe the mechanical properties of porcealin. What mechanical property would be different ideally?

Answer 11

Reasonably HARD and STRONG

QUITE RIGID

HOWEVER, it’s BRITTLE, not DUCTILE (would prefer it to be ductile)

Question 12

Describe the mechanical properties of alloys. (compare to porcelain)

Answer 12

Alloys are much stronger, harder, more rigid and are ductile

Therefore, alloys can withstand greater degrees of permanent STRAIN when subjected to very large stresses e.g. biting

Question 13

Summarise the characteristics of porcelain.

Answer 13

• Rigid - large stress required to cause strain
• Hard - surface withstands abrasion/indention well
• Srong - High compressive strength but LOW TENSILE STRENGTH (tendency to form surface defected which lead to fracture at low stress)
• Brittle/low fracture toughness (maximum strain before fracture = 0.1% which is really small)

Question 14

What is the simple structure of porcelain-metal restorations?

Answer 14

Porcelain - metal oxide - alloy

Question 15

What does the bonding of metal oxide to porcelain help with?

Answer 15

Helps eliminate defects/cracks on the porcelain surface

Question 16

How is the metal oxide layer formed in porcelain bonded to alloys?

Answer 16

The porcelain and alloy substructure is subject to high temps in a furnace which produces an oxide layer on the alloy which in turn bonds to the ceramic

Question 17

The alloy supports the porcelain and limits what?

Answer 17

Limits the strain that porcelain experiences (provides mechanical support to porcelain)

Question 18

Explain how the alloy limits the strain that the porcelain experiences.

Answer 18

The alloy is more rigid than the porcelain. When it is subjected to a large stress the alloy changes shape a little and then returns to its original dimensions.

The same stress on porcelain would fracture it.

When the same stress is applied to the porcelain fused to the alloy, the strain experienced is less than porcelain alone as the alloy limits the stain.

Question 19

The porcelain and alloy needs to have matching whats in order to fuse to each other? Why?

Answer 19

Similar/matching thermal expansion coefficients

Dont want thermal stresses caused when heating and cooling if not matching

Question 20

What are some alloys that have been developed to bond to porcelain?

Answer 20

• High gold
• Low Gold
• Silver palladium
• Nickel chromium
• Cobalt chromium (Note that this is a different type of Cobalt chromium alloy from that used as a partial denture framework.)

Question 21

What are the 5 requires properties of an alloy for bonding to porcelain?

Answer 21

• Form a good bond to porcelain (good weting)
• Similar thermal expnasion coefficient to porcelain
• Avoid discolouration of porcelain (dont want to compromise aesthetics of porcelain)
• Good mechanical properties
• Melting (recrystalisation) temperature of the alloy MUST BE HIGHER than the fusion temperature of porcelain

Question 22

Describe the required ‘good mechanical properties’ required of the alloy.

Answer 22

Good bond strength - THREE alloys adequate (not Ni-Cr)

Hardness (all alloys adequate)

Elastic modulus - want a high value (rigid) to support porcelain and prevent fracture

Question 23

What can occur is the melting temp of the alloy is not higher than the fusion temp of porcelain?

Answer 23

Creep may occur

Question 24

What is creep?

Answer 24

When a gradual increase in strain (permanent deformation) experienced under prolonged application of stress

Question 25

When is creep more likely to occur in an alloy?

Answer 25

Whenever the alloy reaches half its melting temp

Question 26

The strengths and weaknesses of each of the FIVE alloys is illustrated in this TABLE

No one alloy excels according to all the criteria,

But LOW GOLD proves to be satisfactory in each criteria

NOTE that

Nickel chromium is the only one where there are biocompatibility concerns – due to allergic responses attributed to nickel

if it weren’t for its tendency to undergo CREEP, HIGH GOLD would be a strong contender

for THREE alloys – AgPd, NiCr and CoCr – the casting process is difficult.

Question 27

What are the 3 mechanisms put forward for the bonding between the alloys and porcelain?

Answer 27

• Mechanical
• Stressed skin
• Chemical

Question 28

Describe the mechanical mechanism of bonding.

Answer 28

The mechanical mechanism is said to be due to irregularities on the surface of the alloy’s metal oxide layer and porcelain which allows them to interlock.

Question 29

Describe the stressed skin effect as a mechanism of bonding.

Answer 29

The stressed skin effect depends on the slight differences in the thermal expansion/ and contraction of the porcelain and the alloy.

During the production process, after the furnace stage, the alloy contracts slightly more on cooling, and this generates compressive forces on the porcelain – in essence gripping it.

Question 30

Describe the chemical mechanism for bonding.

Answer 30

Oxides in the metal oxide migrating with oxides within the porcelain itself. (electron sharing in oxides)

This occurs during the firing stage, where high temperatures are reached.

This mechanism has been described as ELECTRON SHARING

Question 31

What are the possible modes of failure for porcelain-metal bonds?

Answer 31

The oxide layer itself fracturing

The oxide layer delaminating from the alloy

Porcelain detaching from the oxide layer

Note: Ideally any failure would occur within the porcelain