Alloys for Cast Metal Restorations Flashcards
(46 cards)
Why is porcelain prone to mechanical failures
microcracks tend to form at the fitting surface making it prone to mechanical failure and make it unable to withstand the large biting forces
What is compressive strength defined as
the strength required to fracture a material making it no longer fit for purpose
What is elastic modulus defined as
a measure of its rigidity and it is the stress/strain ratio that tells us for a given stress how much strain (how much of a change in shape) will the material undergo
What is brittleness/ductility be defined as
is the dimensional change experienced before fracture and this is where porcelain and alloys are very different
What is hardness defined as
how readily a material surface resists indentation or abrasion
What mechanical properties can be retrieved from a stress strain curve
fracture strength
elastic modulus
brittleness
ductility
What are the characteristics of porcelain
○ Rigid - large stresses required to cause strain
○ Hard - surface withstands abrasion/indentation well
○ Strong - high compressive strength BUT low tensile strength
They have a tendency to form surface defects
Brittle/low fracture toughness
Describe where the defects are in porcelain
these defects are in the crystals.
It may take time for the crystals to grow into a substantial defect) which can lead to fracture at a low stress
How brittle is porcelain?
(maximum strain is 0.1% before fracturing, very brittle)
What are the mechanical properties of an alloy
- Alloys are much stronger, harder, rigid and ductile
* They can withstand a greater degree of permanent strain when subjected to large stresses when biting for example
What is the structure of porcelain-metal restorations
porcelain-metal oxide-alloy
Describe how the porcelain metal restorations are made
- The alloy substructure has been cast to the desired shape beforehand, a process undertaken by the technician
- The next stage is to subject the porcelain and the alloy substructure to very high temperatures (hundreds of degrees Celsius in a furnace) and this process produces a metal oxide layer on the allow which goes onto the ceramic
What does the bonding of metal oxide to porcelain help in
to eliminate defects/cracks on the porcelain surface
How is the alloy of use to porcelain
• Alloys act as a support & limits the strain that porcelain experiences, it does this because the applied stress has to cause a change in dimensions of the porcelain AND the alloy and the alloy is more rigid.
Why is it important that the porcelains and alloys have matching thermal expansion cfefficeints
○ When forming a porcelain fused metal alloy, they have to be fired in the furnace with the temperature of both materials raised by hundreds of degrees
○ The materials must be cooled without developing any thermal stresses that would result in either the material or the metal oxide layer to develop defects or microcracks
○ To avoid this outcome, both the porcelain and alloy should have similar expansion coefficients so they will expand and contract the same rate when heated and cooled
○ This avoids thermal stresses in either materials or in contact surfaces
What are the metal alloy options
- High gold alloy
- Low gold alloy
- Silver palladium (AgPd)
- Nickel chromium (NiCr)
- Cobalt chromium (not the same as what is used in dentures)
What are the property requirements of a metal alloy
forms good bond to porcelain thermal expansion coefficient must be similar to that of porcelain avoid discoloration of porcelain mechanical bond strength hardness elastic modulus melting/recrystallization temperature of the alloy
Why is a good bond required
○ Aka good wetting
○ Porcelain forms bond with the metallic oxides on the surface
○ Ni-Cr alloys are more difficult to achieve good bonding
Why must thermal expansion be similar to porcelain
○ Due to the firing process involving high temperatures the porcelain and the alloy must have similar thermal expansion coefficients - for porcelain this is 14ppm/C
○ Ideally there should be a difference of 0.5ppm.C in the alloy’s favour (so the alloy’s is greater) so that the alloy is slightly compressing the porcelain in the cooling stage
○ This is to avoid setting up stresses during the fusion of porcelain on to the alloy
○ This is generally okay for all the alloys
How is discoloration of porcelain dealt with
○ The key advantage of porcelain is its aesthetics and tooth like appearance and if the underlying alloy disturbs that then that is undesirable and makes it unfit for purpose
○ Ag in AgPd can produce green discolouration
○ Copper is not used in high gold alloys for this reason
Why is bond strength important
○ Needs to bond well to metal oxide layer during firing stage
○ 3 alloys are adequate
Not nickel chromium
Why is hardness important
○ All alloys are adequate
○ Though early nickel chromium ones were too hard
Why is elastic modulus important
○ Want a high value (i.e rigid) to support porcelain and prevent fracture as the more rigid the alloy the lower the amount of strain porcelain is subjected to
○ Nickel chromium is best
What is the melting/recrystallization temperature of the alloy
○ Must be higher than fusion temperature of porcelain otherwise creep may occur
○ Creep is the gradual increase in permanent strain experienced under prolonged application of STRESS (
