Alloys for Cast Metal Restorations Flashcards
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 (
What is the composition of high gold alloys
80% Au 14% Pu/Pd 1% Ag Indium tin NO COPPER
What is the function of Pu/Pd in high gold alloys
matches thermal expansion and increases the melting point which helps minimise potential for creep
What is the function of indium/tin in high gold alloys
form the oxides required for bonding
What is the reason for no copper in high gold alloys
otherwise a green hue would be imparted to porcelain
What are the disadvantages of high gold alloys
○ Melting range may be too low
○ Young’s modulus may be too low
What is the composition of Low gold alloys
○ Au (50%)
○ Pd (30%) (double that of high gold)
○ Ag (10%)
○ Indium, Tin (10%)
What is the advantages of low gold alloys
○ Increased melting temperature
○ Slightly better mechanical properties
What is the composition of silver palladium alloys
○ Pd (60%)
○ Ag (30%)
In + Sn (10%)
What are the advantages of silver palladium alloys
○ High melting point
○ High young’s modulus
What are the disadvantages of the silver palladium alloys
○ Care needed in casting
What i the composition of nickel chromium alloys
○ Nickel (70-80%)
○ Chromium (10-25%) (oxide bond)
What are the advantages of nickel chromium alloys
○ High melting point
○ High young’s modulus
What are the disadvantages of nickel chromium
○ High casting shrinkage
○ Low-ish bond strength
What are the advantages of cobalt chromium alloys
○ High melting point (1300-1400 degrees celcius)
○ High Young’s modulus (220 Gpa)
○ High tensile strength (850MPa)
○ High hardness (360-430 VHN)
What are the disadvantages of cobalt chromium alloys
○ Casting shrinkage making it difficult to use
○ Low-ish bond strength (50MPa)
Why is biocompatibility an issue for nickel chromium
due to allergic response attributed to nickel
What are the 4 mechanisms outlined for the bond in textbooks
mechanical
chemical
stressed skin
Van der Waals but now DISREGRADED
Describe the mechanical bond
§ Said to be due to irregularities in the surface of the alloy metal oxide layer and porcelain which allows them to mechanically interlock
§ Probably the least important
Describe the chemical bond
§ May be electron sharing
§ During the firing porcelain flows and oxides in the metal oxide coating migrate with oxides in the procelain iteslf
Describe the stressed skin bond
§ Slight differences in thermal contraction coefficients leading to compressive forces which aid bonding
§ During the production process, after the furnace stage, the alloy contracts slightly more on cooling and this generates the compressive forces essentially gripping it
What are the different failure modes
○ Oxide layer itself fracturing
○ Oxide layer detaching from alloy
○ Porcelain detaching from oxide layer
Porcelain itself fracturing
Which failure mode is preferred
○ Porcelain itself fracturing - this is the preferred mode of failure as it is the weakest link
