Dental Material Science
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Porcelain has great aesthetics, but why is it not used on its own for restorations
Because on its own it isn’t able to withstand all of the loads that it is subjected to ( e.g. biting) so it isn’t inevitable that it will produce micro cracks and eventually fail
What is compressive strength
The stress needed to cause fracture
What is elastic/ Young’s modulus
Measure of ridigty The stress strain ratio - the stress required to cause a change in shape Measure of objects resistance to be deformed elastically ( not permanetly)
What is hardness
The resistance of the surface to indentation or abrasion
What three properties can we ascertain from a stress strain curve and which one can we not?
Strength , brittleness/ductility, elastic modulus ( rigitdty) - CAN Hardness cannot be seen on a stress strain curve
On a stress strain curve what does the initial gradient represent
The elastic modulus Steeper gradient would mean the material is more rigid
What is a brittle material ?
A brittle material will change shape a FRACTION a percentage of its length and then break Small distance between proportional limit and fracture stress on stress strain curve
What is a ductile material?
A ductile material will stretch SEVERAL percent of its length and then fracture Much greater distance on stress train curve between proportional limit and fracture stress on diagram
What is the proportional limit of a material
This is the maximum stress that a dental material sustains without any deviation
Name 3 good characteristics of porcelain
It is rigid - large stress required to cause a strain It is hard - surface with stands indentations and abrasion well It is strong - it has a high compressive strength
What are the downfalls of porcelain
Low tensile strength - so has a tendency to form surface defects, this leads to fracture at low stresses It is a brittle material which means it has only a small strain( stretch ability) before it fractures
How is the metal oxide layer produced between the porcelain and the alloy in porcelain fused to metal alloys
The porcelain and alloy are subjected to very high temperatures in a furnace which produces the metal oxide layer.
What is the benefit of the metal oxide layer
The bonding of the metal oxide to the porcelain helps eliminate defect and cracks on the porcelains surface
What is the purpose of the alloy in porcelain metal restorations
The alloy with its own metal oxide layer provides MECHANICAL SUPPORT Being more rigid, when subjected to large stresses the alloy will change shape very little and return to its original dimensions. Acts as a support and limits the strain that the porcelain experiences
How is the porcelain alloy restoration made and what must be considered during this
It has to be fired in a furnace - raising temp of both materials by hundreds of degrees and then cooling them without developing thermal stresses which could cause the material or the metal oxide layer to form micro cracks To avoid this the porcelain and the alloy should have similar thermal expansion coefficients.
What is the thermal expansion coefficient of porcelain
14ppm degrees Celsius
Why do we want the alloys thermal expansion to be similar to porcelains but ever so slightly greater?
This is so that during the cooling stage the alloy is slightly compressing the porcelain
Why would we not use silver palladium under the porcelain
Porcelains key feature is the aesthetics, the silver palladium can cause discolouration so don’t use that
Why do we want the melting temp of the alloy higher than the porcelain
Because this could lead to creep
What is creep
Gradual increase n STRAIN( permanent ) experienced under prolonged appplicaton of stress below the elastic limit ‘’Subjected to low level stresses over a prolonged period of time”
At what temp is creep most likely to occur in an alloy
When the alloy reaches half of its melting point
Why is high gold not used in porcelain alloys
Melting range too low and Young’s modulus too low
What is the main issue with nickel chromium alloys
During the casting process they exhibit a lot of shrinkage making it challenging to use It’s bond strength to porcelain is also low
Why is there a biocompatibility issue in nickel chromium alloys
Because of the allergic response to nicekl