Dental Material Science Flashcards
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
How does the mechanical mechanism of porcelain and alloy work
There is said to be irregularities on the surface of thr alloys metal oxide layer and porcelain which allows them to interlock
What is the stressed skin effect
Slight differences in the thermal expansion and contraction of hte porcelian and the alloy.
During the furnace stage - the alloy contracts slightly more on cooling and this generates compressive forces on the porcelain essentially gripping it
How does the chemical mechanism of porcelains alloys work
Oxides in the metal oxide coating on the allot migrating with the oxides within the porcelain itself - occurs in the firing stage when high temps are reached.
Described as ELECTRON SHARING
Ideally if there a was going to be a failure in the porcelain metal bond where would you want it to occur
Within the porcelain
By what process are wrought alloys manipulated and shaped?
Via cold working
Iron is allotropic - what does this mean
This means that it undergoes two solid state phase changes with temperature
What are the two solid crystalline forms of iron
Temp >1400 = BBC ( body centred cubic lattice)
900 <temp < 1400= FCC ( face centered cubic)
Temp< 900 - BCC
What is an alloy
Two metals that form a common lattice structure
If in the alloy the two metals in the common lattice are soluble in one another, what kind of solution would this form?
Solid solution
If two atoms are markedly different in size what kind of solid solution do. They form
Interstitial solid solution
What is the gamma phase of iron called and what kind of solid solution is it
Austenite and it is an intestinal solid solution with a FCC structure
Fast cooling of austenite iron leads to what kind of iron
Martensite
- distorted lattice
- hard
-brittle
What percentage of chromium must steel coat win in order for it to be stainless
> 12% chromium
How does stainless steel have its corrosion resistance property
Due to the chromium oxide layer
Name 2 functions of nickel in SS
Improves fracture strength and corrosion resistance
What are three dental uses of Austenitic steel?
- Dental equipment and instruments to be sterilised
- Wires e.g. ortho
- Sheet forms for denture base
Why can stainless steel not be continuously manipulated when trying to form desired shape
It work hardens rapidly
What is cold working
Cold working is the work done on a metal/alloy at LOW temperatures ( e.g bending, rolling)
This causes SLIP - dislocations collect at the grain boundaries, therefore the alloy has fewer defects within the lattice structure, making it stronger and harder.
What is the springiness of a material ( specific to wire used in ortho)
Ability of a material to undergo large defections ( to form an arc) without permanent deformation ( i.e. returning to its natural shape)
Why do we want a high ductility in stainless steel wires
So that it can undergo bending without fracture
What is soldering of stainlesssteel
This can be used to Join SS together
What happens in weld decay of stainless steel
When welding steel there is a risk that weld decay occurs.
This occurs when the temp i raised to 500-900 degrees and this can push the Cr and C atoms to the grain boundaries, allowing CrC to precipitate there.
This causes the SS to become brittle, limiting the amount of manipulation available and it also makes it more likely to corrode
As with many alloys which have undergone various process (e.g. cold working), what kind of process is needed to ensure the configuration of the metal atoms
Stress. Relief annealing
Name 4 advantages of using stainless steel as a denture base?
Fracture resistant
Corrosion resistant
High thermal conductivity
Thin
Name some disadvantages of using stainless steel as a denture base
- Possible dimensional inaccuracy
- Elastic recovery of steel can lead to inaccuracy
- Difficult to ensure uniform thickness
Name some ideal properties for an acrylic resin denture base
Dimensionally stable
Unaffected by oral fluids
Low density
High softening temperature
Non toxic
Easy and inexpensive to manufacture
What is transverse/flexural strength
Flexural strength is the ability of the material to withstand bending forces applied perpendicular to its longitudinal axis.
What is free radical addition polymerisation
Chemical union of two molecules either the same or different to form a larger molecule without the Elimination of a smaller molecule
Involves molecules with C-C double bonds
What are the stages of acrylic polymerisation
Activation - of initiator to provide free radicals
Initiation - free radicals break the C-C double bonds in the monomer and transfer the free radical
Propagation - growing polymer chain
Termination - of polymerisation
What is the initiator in acrylic polymerisation?
Benxzoyl peroxide
What is the function of the plasticiser in the heat cured acrylic powder
Allows quicker dissolving in monomer liquid
In the heat cure acrylic liquid there is methacrylate monomer what is its function
Dissolves PMMA particles
What is the inhibitor in the heat cure acrylic liquid and what does it do
Hydroquinone
Prolongs the shelf life - reacts with any free radicals produced by heat, UV light
What is the function of the co-polymers in the heat cured acrylic liquid?
Improve mechanical properties
What is the difference with thermal expansion when using acrylic teeth in denture base compared to porcelain?
The thermal expansion is ok if the acrylic tooth is used however if porcelain is used it tends to be slighly higher.
The porcelain teeth expand less than the acrylic base causing a mismatch
What is the softening temperature of an acrylic denture and what must we remind the patient of
75 degrees
They will be ok with injested hot fluids, however the denture will not cope with boiling water when washing the denture so must tell the patient this.
During its usage how does the head cured PMMA make up for the 0.4% contraction that happens during the heating stage?
Through water absorption
What activates the benzoyl perioxide in heat cured acrylic?
Heat
Benzoyl peroxide is the initiator in self cure PMMA ( Aswell as heat cured), what is the activator of the initiator in this case
Tertiary amine
Why might self cured PMMA dentures be more accurate fitting than heat cured?
Because the polymerisation stage requires no heating stage - so there will be little thermal contraction experience
Why do self cure PMMA have poorer mechanical properties to heat used PMMA
Chemical activation of the polymerisation is less efficient, hence it has a lower molecular weight and so poorer mechanical properties
Given the polymerisation of self-crushed acrylic is less efficient. One consequence is there is more unreacted monomer, what are some problems with this;
It can act as a plasticiser, softening the denture base
Can make the denture more vulnerable to failure
Also means there is more likelihood of the monomer leaking out and causing irritation
What % of açylric resin dentures fracture within the first 3 years
10%
