Alloys for Cast Metal Restorations Flashcards
(31 cards)
Porcelain has good aesthetic so why can’t this be used on its own as a restorative material?
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
What is compressive strength?
the stress required to fracture a material – making it no longer fit for purpose.
What is elastic modulus/young’s modulus a measure of?
The rigidity (stress strain ratio) which is the stress required to change the shape of a material
What is is the brittleness/ductility of a material related to?
How much dimensional change the material undergoes before fracturing
What is the hardness of a material?
Measure how readily a material’s surface will resist indentation or abrasion
What property cant be ascertained from a material’s stress-strain curve? (out of strength, brittleness/ductility, rigidity, hardness)
Hardness
Which material is more rigid?
Material A
The initial gradient of each curve indicates the materials elastic modulus. The greater the gradient = more rigid
What is the tensile strength of each material?
A = approx. 170MPa
B = approx 125MPa
Note: the vertcal axis indicates the tensile strength being applied
Is each material ductile or brittle? Explain.
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.
How is a brittle/ductile material defined in terms of stretching?
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)
Descibe the mechanical properties of porcealin. What mechanical property would be different ideally?
Reasonably HARD and STRONG
QUITE RIGID
HOWEVER, it’s BRITTLE, not DUCTILE (would prefer it to be ductile)
Describe the mechanical properties of alloys. (compare to porcelain)
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
Summarise the characteristics of porcelain.
- 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)
What is the simple structure of porcelain-metal restorations?
Porcelain - metal oxide - alloy
What does the bonding of metal oxide to porcelain help with?
Helps eliminate defects/cracks on the porcelain surface
How is the metal oxide layer formed in porcelain bonded to alloys?
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
The alloy supports the porcelain and limits what?
Limits the strain that porcelain experiences (provides mechanical support to porcelain)
Explain how the alloy limits the strain that the porcelain experiences.
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.
The porcelain and alloy needs to have matching whats in order to fuse to each other? Why?
Similar/matching thermal expansion coefficients
Dont want thermal stresses caused when heating and cooling if not matching
What are some alloys that have been developed to bond to porcelain?
- 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.)
What are the 5 requires properties of an alloy for bonding to porcelain?
- 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
Describe the required ‘good mechanical properties’ required of the alloy.
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
What can occur is the melting temp of the alloy is not higher than the fusion temp of porcelain?
Creep may occur
What is creep?
When a gradual increase in strain (permanent deformation) experienced under prolonged application of stress
