Dental Material Science Flashcards
what does success of dental materials depend on
Selection of material- which material is most suitable
Use of material (instructions)- has it been mixed/ set correctly
Quality of material- has it been manufactured well. Must follow instructions. To must look for the ‘CE’
how can dental materials be tested
Clinical trials- may not answer everything, expensive
Laboratory evaluation- may be better, cheaper, no patients, compare a new product with a product already used that works well.
ISO and BSI standards
what are the various stages that the properties of dental materials is important at and why
After manufacture, during storage (shelf life)
- Shelf life – long shelf life allows large quantities to be bought
- Storage conditions – temperature and light exposure
- Dispensing mixing/manipulation
- During setting
- after setting – the patient e.g. young/old, diet, work
- after ageing – once patient has left the clinic. E.g. Durability
what forms can the dental materials be in
powder/liquid, pastes
what is shelf life
- How long the materials can be stored and still be used as the manufacturer intends
- Long shelf life means large orders can be placed – reduced costs
- Careful stock control
what is shelf life affected by
Temperature:
- Products that contain volatile components may require refrigeration. This may affect performance.
- liquids may be unstable at low temperature, e.g. components may crystallise- changes the chemical structure
Light:
- Some products may be unstable in visible light – need storage in cupboard or require special packaging e.g. foil packaging or amber glass bottle. Must inspect the packaging to see if it has been damaged as it may not behave as you expect. DO NOT use product if this is the case.
If the shelf life is exceeded, you must not use the product.
what are the methods of mixing
- Hand mixing
- Mechanical mixing
- No mixing
what are the features of hand mixing
Powder/liquid, paste/paste, paste/liquid
Mixed using a spatula on a pad or a mixing bowl
Cheap equipment e.g. spatulas and bowls
Technique sensitive:
- Must learn how to mix them correctly within a given time
- Quality of materials and procedure depends on skill level
- unpredictable results, quality depends on skill
- reduction in technique sensitivity (below)
what are the features of mechanical mixing
Capsules and cartridges
Consistent mixing in capsule
Mixed using special equipment
More expensive – due to need of equipment e.g. rotating mixture
Less technique sensitive
what are the features of No mixing
Single paste
Mixed by manufacture
Need careful storage to prevent premature setting
Needs special equipment to start setting – more expensive
Lowest technique sensitivity
what are the Properties important during setting
Working time (WT)-How long have you got to mix the components
Setting time (ST)- How long until you can proceed to the next procedure
what is the working time (WT)
- Measured from start of mix
- Till material can no longer be effectively used
e. g. filling must be in cavity by this stage
e. g. impression material should be seated in mouth at this stage
Measured at room temperature
what is the setting time (ST)
- From start of mix
- Till material achieves minimum properties for function
- e.g. filling can be polished. Impression can be removed
- Does not imply 100% completion of reaction
- Sufficient elasticity for impression to be removed from patients mouth
Measured at appropriate temperature
- e.g. at mouth temperature for materials which set in mouth
- Set in the mouth - 32 and 37 degrees- difference between an open and closed mouth
- Set chair site- setting time at room temp
what are the methods to measure WT and ST
- Rheology
- Thermal effects
- Dimensional changes
what is rheology
The study of deformation and flow of materials
Measure how viscosity changes
How fluid a material is
- Fluid does not mean liquid – fluid means IT FLOWS
what are thermal effects
- Exothermic reactions
* Temperature rise
what are Dimensional changes
Expansion or contraction
outline rheology (viscosity) in terms of extruding from a syringe
Low viscosity – plunger moves easily – e.g. water- FLOWS EASILY
High viscosity – plunger harder to move – e.g. treacle
-FLOWS SLOWER
what is viscosity related to (2 things)
pressure and speed (rate)
in terms of K what is the Newtonian (linear) behaviour and for what compound would this be true for
K=1
It doesn’t matter how fast we push plunger the viscosity will always be the same
water
in terms of K what is the pseudoplasticity behaviour and for what compound would this be true for
K < 1
viscosity reduces as shear rate increases (push the plunger faster rate increases as viscosity decreases)
e. g. ketchup
- Shake the ketchup it will start to flow
- As we increase the shear rate the viscosity goes down
in terms of K what is the dilatant behaviour and for what compound would this be true for
K>1
viscosity increases as shear rate increases (as you mix something it gets harder to mix
e. g bullet proof vests
- Some Endontontic materials are dilatant- as you place them down the canal they get more and more viscous
what is thioxtropy
no flow until sufficient pressure is applied
E.g. Nail varnish- until you brush sufficiently hard it wont move
What are the implications of viscosity for practice?
- Ease of manipulating
* Ability to flow and adapt
how is ease of manipulating implicated in practice
Ease of mixing
- Low viscosity is better- easier to get the components to mix
- May affect how you mix materials (e.g. may not be able to mechanically mix)
Ease of transfer (e.g. to impression tray)
- High viscosity is better (stops spills) e.g. impressions- move material into patients mouth it won’t spill.
- E.g cup of tea- if its full its likely to spill.
what is the importance of the ability of dental materials to flow and adapt
e. g. detail in impression
e. g. adaptation of fillings to cavity- low enough viscosity to to take the detail but high enough to transfer it.
(to lower the viscosity the easier it is to flow)
why are many dental materials difficult to mix
A - a high initial viscosity
B – a rapid increase in viscosity during setting
How are manufacturers helping dental professionals with mixing dental materials ?
solvents – reduce viscosity.
Retarders – delay setting- delays the increase of visocity over a given period of time so there is time to mix things
how can Temperature change during setting affect the materials
- Many materials set through an exothermic reaction
- Once it gets to a maximum temperature from mixing, that is the setting time
- Useful for determining rate of set
- The setting reaction slows down after the max. temperature
Can influence structure / properties
- High temperature rise can cause porosity – leads to weak strength
- Trapped air bubbles make something weak e.g. filling/denture
what clinical problems can temperature change cause
Pulp is sensitive to temperature change (rise of 5°C can damage it)- dentine protects the pulp but if there is a deep cavity, we wont have as much dentine left- can damage the pulp by putting the restorative (filling) material in.
what are dimensional changes that can take place during setting
- Expansion
* Contraction
what is expansion and what clinical issues can this cause
Reactions involving crystal growth, e.g. amalgam, gypsum
- If things expand when made in the lab it may not fit
- Meet each other- expansion
- Potential damage to tooth (restorations)
- Inaccuracies in devices fitting (crowns, orthodontics)
what is Contraction and what clinical issues can this cause
Reactions involving polymerisation
- Greyish brown marks- tooth coloured restoration has shrunk during setting-
- May lead to marginal staining, secondary caries
- Inaccurate impressions- shrink
Casting of alloys
- Large temperature decrease (heating then cool- hot things expand, cold things contract)
- Inaccuracies in devices fitting (crowns)
what are the features of the oral environment that can affect the properties of dental materials
- Temperature
- pH variations- restoring function
- Mechanical stress
- Abrasive factors
- Bacteria
how does temperature affect materials
Cold drinks snd Hot food/drinks can cause Thermal cycling (5°C to 60°C) -natural material in the patients mouth cope well. If we are restoring teeth we should consider how those materials react to temperature changes.
how do pH variations affect restoring function
Plaque (~pH4) oCaused by fermentation oBelow critical pH-l loss of enamel and dentine oEnamel and dentine don’t cope well oDental materials struggle at low pHs
Acidic drinks (pH 1-3) oCarbonated drinks
Alkaline medication (~pH 12)
Toothpaste with chalk (~pH 12)
we require our materials to be stable in these pH ranges
how does mechanical stress affect teeth and dental materials (what are the types of mechanical stress)
High stress leads to fracture
e.g. on Incisal edges
Low stress over repeated over time (low repeated cycling)
- Fatigue- not necessarily high stress in one bite
Sudden, rapidly applied stress
- Impact failures
- Enamel and dentine good at absorbing energy but they do fail
- Dentures susceptible to breaking. Shatter- breaking into lots of different pieces.
how do abrasive factors affect dental materials
Abrasive food- e.g. seeds
Abrasive toothpaste- removes plaque and stains by being abrasive. Restorative/ denture materials not as hard - scratches which bacteria can enter
Solvents causing softening- alcohols are solvents. Softening means they can be scratched easier. Mouthwashes are also solvents
how do bacteria affect dental materials
- Breakdown of resins
- Oral bacteria can break down the fillings
what are properties of the set enamel
- Biocompatibility- toxicity, irritancy, allergies
- Appearance - aesthetics
- Thermal properties - expansion/contraction & heat transfer
- Chemical properties - solubility, corrosion, leaching
- Mechanical properties - strength, toughness, stiffness, hardness etc
- Adhesion - bonding of filling to tooth
why is safety of materials important
Important for all materials
Patient should not be harmed by treatment- allergies e.g. nickel. Consider medical history
Don’t forget dental staff
why is accuracy of materials important
- Important for impressions and models
- Very good reproduction of the oral anatomy
- Cant get accurate adhesive
why is durability of materials important
- Important for restorations, prosthodontic devices
- Things places in patients mouth
- Things that are a permanent solution (lasts around 5 years)
why is conservation of materials important
- Very important for restorations
- Conservative dentistry -move away from ‘drill and fill” and conserve as much of natural hard tissue as possible – minimal intervention
why is prevention of materials important
- Longevity of ‘restored tooth’ more important than the material
- Make sure that every treatment doesn’t cause damage to the tooth or other teeth
why is aesthetics of materials important
- Important for ‘visible’ restorations
- Invisible restorations
- Less aesthetic may last longer
how are dental materials developed towards to satisfying appearance and aesthetics
Colour, shade, translucency
•Not available for all materials (e.g. amalgam, gold)
•Tooth-coloured materials come in many shades
•Use of shade guide to match with natural tooth
•Build up different shades to match the tooth
• However Can (and will) change over time- diet/abrasion etc. should advise patients.
Surface roughness, gloss
•Altered by scratching, wear, erosion, stains
•Affected by polishing
what are the chemical properties of materials
- Solubility-
- Leaching
- Corrosion
what is the effect of solubility on materials
Dissolution in a solvent
(CaOH2) – pulp capping- water soluble so cover needs to be put on so it maintains it function
Durability requires low solubility
Dietary factors by the patient can affect this
what are the positive effects of leaching
Stable in an aqueous environment
fluoride leaching– GIC naturally contains fluoride so when you put in an aqueous solution such as saliva the fluoride leaches come out
oFluoride is antibacterial
oit might allow fluoride appetite to forms- less susceptible to erosion and caries. Better fluoride releasing dental materials = better materials
what are the negative effects of leaching
Aligners contain plasticisers. (plasticisers make things softer). When plasticiser leaches out the denture gets harder and harder and the denture will have to be removed
how does corrosion effect dental materials
if we have 2 metals and they come into contact then we can get a galvanic cell formed- flow of electrons
they must be in an electrolyte
-Saliva is a very good electrolyte
2 different metal touching above the electrolyte
-Part of amalgam filling covered in saliva- this is still sufficient for corrosion to occur
how is corrosion graded
in terms of electronegativity – which is likely to Form anode and cathode
-More difference in electronegativity the more likely they are to form a galvanic cell.
what are the consequences of erosion of dental materials (amalgam)
Metallic taste tells us we have a change in chemistry
We start Weakening the material – restoration fails
Amalgam composed o different alloys- if there’s saliva its possible to get an electrochemical cell to develop and get corrosion
Mercury produced from amalgam- not toxic to patients or practitioners
what are the thermal properties of materials
•Materials expand and contract as temperature changes (Hot it expands, Cold it contracts)
•For restorations this can lead to marginal gaps forming
- Possibly leading to staining and secondary caries
what is the amount of expansion and contractions is related to
the coefficient of thermal expansion (Units °C -1 )- governs expansion and contraction
how are the Selected values for Coefficient of Thermal Expansion clincally relavent
Dental materials Expansion & Contraction different to tooth
When things cool down- amalgam shrinks twice as fast as tooth and composite 4x as much so may have gap forming. (staining)
As we heat up and cool down it starts to pump liquid around. May allow bacteria in - secondary caries
what can changes in oral temp cause
Cause pain – hypersensitivity (Cold) , burns (hot)
Damage the pulp- shine a bright curing light/hot drink may get temp rise above 5 degrees
what is thermal conductivity and what are enamel, dentine, amalgam and composite
How well materials transfer heat is termed
- Enamel and dentine are insulators (low conductivity)
- amalgam -conductor
- composite- conductor
what are the clinical implications of amalgam being a conductor
- If deep cavity is produced an insulating liner may be needed
- Amalgam restoration may feel pain when they drink a hot or cold drink.
what are the clinical implications of Composites (made from acrylic resin) being insulators
Are liners (between restoration and pulp) still needed? – lack of clinical evidence on which is the best
Gold- 200x better
what is thermal diffusivity
How quickly material reacts to a sudden temperature change
-i.e. quickly raises to normal after cold drink-
(Conductivity only really covers static conditions- (mouth stays at a constant temp) – not what happens therefore diffusivity is needed.
- Low diffusivity doesn’t react
- High diffusivity does react
what is the equation related to thermal diffusivity (D)
. D (m2 s-1) = l / r Cp
where r is density
Cp is heat capacity
- heat capacity - heat required to raise 1g of material by 1°C
- Units J g-1 °C-1
- Measured using a thermocouple in a defined volume of material
- Low diffusivity doesn’t react
- High diffusivity does react
in most cases what type of diffusivity is required and why
low diffusivity is desired
-i.e. a restoration does not transfer the heat from a hot drink to the pulp
why would dentures ideally have high diffusivity
to prevent scalding
If the denture has a low diffusivity they don’t recognise that the liquid is too hot as they lose some sensation and burn mouth
what are the mechanical properties of dental materials related to
Force
Stress
how does force affect materials and what is its eq
Results from an outside agency acting upon a body to change its momentum
Force (N) = load (kg) x acceleration (ms-2)
(Weight x acceleration due to gravity)
•Static load acts under gravity (e.g. 1 kg acting under gravity gives force of 9.8N)
how does stress affect materials and what is its eq
•internal forces are set up inside a body to oppose an externally applied force
- e.g. internal forces need to develop to oppose biting on restoration
Magnitude of stress is function of applied force and dimensions of the object to which force is applied
what are the types of stress
what are most stresses in the mouth
simple – tensile, compressive, shear
complex- flexural (tension and compression), torsional (twisting) & diametral
In the mouth most stresses are complex
i.e. they combine simple stresses
what stress to dentures undergo
e. g. dentures undergo flexural stress (mixture tension and compression)
- top is compression
- under is tension
what is stress
Internal forces are set up inside a body to oppose an externally applied force
Magnitude of stress is function of applied force and dimensions of the object to which force is applied
what are the types of stress
- simple – tensile (pulling) , compressive (squashing) , shear (pushing out of line) - stress applied in one direction
- complex- flexural (dentures) , torsional (twisting) , diametral - stress applied in different directions
outline the calculation of simple stress with units
- Stress = Force/Area
- Units of stress Nm-2 = Pa
- (MPa = 1000000 Pa)
- (GPa = 1000000000 Pa)
what is strength
- The maximum stress which can be withstood before breaking
- E.g. if you bite and apply so much stress that you break the filling, the stress would be greater than the strength of the filling.
what is strain
When stress is applied the material will change dimensions
The amount of change that occurs due to an applied stress
how can strain be calculated
Ratio of new length / original length
no units
May be expressed as a %.
what are the different types of deformation
elastic deformation
plastic deformation
what is elastic deforamation
Material returns to original dimension
when is elastic deformation clinically applicable
If we have made a denture or placed a filling
Because we don’t want the filing/denture to change dimensions everytime the patient chews as it will fail very quickly
what is plastic deformation
Material is permanently changed
when is plastic deformation clinically applicable
When placing filling material into the cavity and pushing it into the cavity you want the deformation to be plastic- filling material deformed permanently so it fits into the cavity.
what is viscoelastic deformation
Material slowly return to original dimensions OR material only partially returns to original dimensions
explain elastic deformation in terms of a spring
- When load applied deforms quickly
- If load held it stays deformed
- When load taken off, quickly returns to the original dimensions
- Pulling spring down- very quickly itll reach the length being pulled too. Stays deformed while we apply the load and returns to normal length when removed.
explain plastic deformation in terms of a dashpot
- When load applied deforms quickly
- If load held it stays deformed
- When load taken off, stays deformed
what are the models of viscoelasticity
•Viscoelasticity is a combination of elastic and plastic deformation
Models involve both springs and dashpots
- Maxwell model – describes when only some deformation returns (partial)
- Voigt model – all deformation returns but slowly
explain viscoelasticity in terns of a spring connected to a dashpot
maxwell model -only some defromation returns
Spring connected to dashpot- starts deform, slow increase. When we let go the spring will close, however dashpot not closed and wont return to original dimensions. the deformation that happened after we wont be able to cover it. Left with permanent deformation, it will never return to original dimensions.
how do Liners on dentures to make more comfortable behave
they are viscoelastic (maxwell model)
called ‘Temporary’ liners as they need to be replaced- wont fit as well over time
how do impression materials behave
viscoelastic
if you place impression in patients mouth, let it set and try and remove it. removing it might do some deformation and
elastic deformation recovered but we also recover the plastic deformation until we’re back in the original dimension.
when does more deformation occur
More deformation and the longer time period we do it over
either the more permanent deformation we get or the longer it takes us to return to the original dimension
Technique- not much deformation in impression tray then it wont take much time to return to original.
what is stiffness
the measure of resistance to deformation
- It does not matter whether this is elastic, plastic or viscoelastic deformation
- The higher the stiffness the harder it is to deform something
- How easy is it to change its dimensions
- Stifness and strength work together in dental materials
what factors contribute to strength and stiffness of a material. relate this to dentures
strength- how easily it BREAKS
stiffness- how easily it DEFORMS
Denture in patients mouth and the patient bites with such force that the denture will break the denture will fail. If they bite and it deforms then it may fail too- the flexing may cause discomfort and patient may stop wearing denture
what is ultimate tensile
point where apply sufficient strength it breaks
how is stiffness shown on the graph
• Stiffness is the gradient of the curve where it starts to bend
- Lower stiffness - shallow
- Higher stiffness - steeper
how can stiffness me measured
Modulus of elasticity
Young’s modulus
what is Modulus of elasticity
and Young’s modulus
Modulus of elasticity - the rate of change of unit stress with respect to unit strain
Young’s modulus- measure of the ability of a material to withstand changes in length when under lengthwise tension or compression.
what is yield stress
Stress required to permanently deform material- precise definition.
what is Proof stress
Easier to measure than yield stress as its more expensive to do yield. Draw a line parallel to that line to 0.1% across on the x axis – stress required to do 0.1% permanent deformation
the offset point of yield stress which is not easily defined on a graph
what is - Proportional limit
Easier to measure than yield stress- must be similar to yield stress
what is - Ductility (elongation)
How much can something be pulled until it breaks- measure the value. More you can pull the more ductile
what is - Malleability
How much can something be compressed till it breaks
what is - Resilience
– How much energy can something take before it deforms- measure area under straight line on graph
what is Toughness
How much energy can something take before it breaks- all the area under the curve.
how do ductile materials behave
Ductile materials can be deformed large amounts
• The are often deformed elastically and, then, plastically
• They may show “necking” – thin areas prior to breaking. Happens too fast in dentistry to be used in mouth
• Pull- weaknesses, stress drops and fails
how do brittle materials behave
- The can only be deformed elastically
- Less than 1% plastic deformation will break then
- The smallest amount of deformation breaks them
- Depends on temperature and how fast we’re deforming them.
most materials can be ductile and brittle, what does the behaviour type depend on
- Temperature
* Strain rate (how fast it is being deformed).
what is Toughness
How much energy something takes before it breaks
• Useful to understand what happens when things are deformed very quickly:
• E.g. dropped or patient trips
why is toughness used over stress
• Energy is more useful than stress, so toughness used.
how can toughness be measured
- To measure toughness an impact test is used.
- Compare energy required to break pre-cracked (notched) and un-notched (no cracks) specimens.
- If energy to break un-notched specimens is»_space; than notched material is notch sensitive
what is meant when a material is notch sensitive
any cracks and scratches can make material break easier. If energy to break an unnotched specimen than that to break a notched
what is Fatigue
- Materials fail due to:
- Repeated cycles of stress
- Often fail at stresses much lower than strength
- may fail at the biting stress due to fatigue
when is fatigue common in the mouth
- E.g. Dentures
- They flex repeatedly during chewing and talking
- Fracture down the midline
what is fatigue life and what are the 2 ways materials can act
cycles (or time/age) survived at a value of stress
A-Initially a higher strength, the more cycles we apply the lower the strength becomes till it breaks at 1/5 of its ultimate tensile strength. Traditionally chosen for a denture but now we thing B might be better
B- starts off with a lower strength but doesn’t decrease over time. Going to survive longer and better. ‘Fatigue life”. Material B will never get to 60
what is fatigue limit
stress below which material survives indefinitely
what is Hardness and how is it measured
• How likely a material is to be scratched?
pressing something hard into the surface of something (usually a diamond)
Polishing discs contain hard particles to polish- things in mouth allow scratches
why are dentures likely to be scratched
Acrylic resin has a VHN of 20 which is v low so can be scratched by harder material
what is Resistance to indentation under load related to
- A factor in determining durability
* Related to scratching, wear
what are some uses of polymers
- Impressions
- Dental composites
- Denture bases
- Artificial teeth
What are the important characteristics of polymers?
Made up of many regular repeating units – termed mers • Based on C, O, N, H • Covalently bonded • Monomer – one mer • Polymer – many mers
The mers join to form long chains
• Formed by covalent bonds mainly
• As the mers join the chain the molecular weight increases- want a high molecular weight (lots of the monomers to join onto chains)
what is the difference between a linear polymer and crosslinked(network) polymer
No links – called a linear polymer
With links (cross links) – termed a network or cross-linked polymer o Changes of properties
what is the difference between a homopolymer and copolymer
Only one monomer makes up polymer – homopolymer
Two or more monomers – copolymers
what are the 3 ways Monomers may join, explain them
Random (Homopolymer)- Most likely- join together randomly, no distinct structure
Regular- monomers alternate in every chain (condensation polymers form in this way)
Block- blocks of M1, blocks of M2 etc.
what are the two methods of polymerisation common in dentistry
Addition polymerisation
- two molecules join to form a bigger molecule
Condensation polymerisation
- two molecules join to form a bigger molecule and a bi-product (often water but H202, O2, CO, CO2, ethanol etc can also be produced)
(toxic bi-product should be considered if the materials is to set in the patients mouth)
what are the distinct reaction stages that take place in addition polymerisation
- Activation
- Initiation
- Propagation
- Termination
what must free radicals have and give an example of an initiator used in dentures
- A molecule with a weak bond (initiator)
- A means to break (activate) this bond
- Energy used to break bond – heat, light, etc.
An example initiator – benzyl peroxide (BPO)
•Commonly used initiator for dentures
•The O-O bond is a weak bond
•As temperature increases this breaks up more (atoms vibrate more and move further away)
what are the Types of monomers
•Vinyl monomers – have a carbon-carbon double bond C=C
can change the R1 and R2 groups to form -Very similar structures -the minor changes lead very different polymers
what are the different polymers that can be made from vinyl monomers
PMMA – Plexiglass, denture bases
- Hard
- Add pigments
- Transparent
PE – drinking bottles, hip replacements
-Difficult to break by hand
PS – heat-proof cups, packaging
- Not transparent
PVC – clothing, food packaging
-Fake leather
outline the activation stage of free radical polymerisation
- Heat activation Apply heat- add reactants to container and heat up
- Light activated materials- light used to set/cure material
- Chemically activated polymers
- Forms radicals
outline the initiation stage of free radical polymerisation
- The free radicals first react with the monomer
- W/o monomers would randomly join
- Add initiator which starts the reaction
- Initiator must be stable
outline the propagation stage of free radical polymerisation
- Each time a monomer meets a radical the C-C double bond opens
- This extends Chain length- chains have different lengths
- Average molecular weight – average of how many monomers have joined on onto polymer chain
Outline Termination of free radical polymerisation. is there a 100% conversion rate from monomers to polymers? why?
-Doesn’t mean 100% of monomers joins to become polymer (never 100% conversion rate)
- 2 chains that are radicals will cancel each other out- residual monomer that hasn’t reacted
- Change from Liquid to viscous so its hard for monomers to meet chains to react
what are common reasons for termination
Impurities are common reasons for termination
- Oxygen can terminate polymerisation
- Need to stop oxygen from getting to it as It will prematurely terminate it
why is it easier to deform linear polymers
they have a lower STIFFNESS compared to those that are cross linked
how does cross linking change the properties of polymers
Makes it harder to deform - increase stiffness- must move all chains for it to deform
Makes it more ductile – can be deformed more- (the polyisoferen is cross linked by sulfur elastic deformity)
Improves stability in liquids- linear polymers in the solution water can push strands apart, when cross linked they’re unlikely to be separated
what does cross linking require
Requires difunctional (or higher) monomers
More than one C-C double bond
E.g. Ethylene Glycol Dimethacrylate (EGDMA) – added to dentures
-If we add this to monomers we can get 2 chains- improve cross linked
how does BisGMA form cross links by difunctional monomers and what is its clinical use
- Methylmethapolate groups at each end
- Monomers naturally difunctional
- Naturally cross link
used in making dental composites
what occurs in condensation polymerisation
the reaction f an acid with an alcohol to produce an ester and water
by product given off
what must monomers have to complete polymerisation to occur
must have 2 OR MORE reactive groups capable of condensation reaction
how many reactive groups are needed on monomers to form cross linking and how does that differ from addition polymers
3 OR MORE
addition only needed one active group and there was still polymerisation and 2 for cross linking
what are examples of condensation polymers
- Polyester – used for fabrics
- Polyamide (e.g. Nylon) – clothing, fabrics, denture bases
- Bakelite – furniture, jewellery
- Polydimethylsiloxane (silicone rubber)- impressions, bath sealant
why are addition silicones more desirable than condensation one e.g. Polydimethylsiloxane (silicone rubber) in impressions
in polymerisations Ethanol is the by product (evaporates quickly) , so the condensation silicones need the model to pour very quickly otherwise the impression will change direction.
Addition silicones are more desirable as they are more dimensionally stable
what are Changes that occur during condensation polymerisation:
• Change in nature
• Monomers are often liquids or gases
• Polymers are normally viscous liquids, solids, glasses or rubbers- polymerisation increases viscosity
• Effect of this change in nature
• Exothermic reaction- release of energy
o Increase in temperature
• Dimensional change
outline how Thermal changes during polymerisation occur
Monomers joining chains leads to a release of energy- Exothermic reaction
Temperature rise is proportional to volume of the material you have
what is the significance of Temperature rise being proportional to volume of the material you have in industry
• Danger on an industrial scale – explosions. When there is very high volume temperature rise can lead to explosions.
what are the problems in dentistry of Temperature rise being proportional to volume of the material you have
Porosity – High volume e.g. denture base. Increase in temp can cause weakness, potentially leading to early failure. temp may be so high that we vaporise the monomer forming pores.
- If porosity is formed in denture (at thickest bit) this can lead to early failure of denture.
Damage to the pulp – if filling close to the pulp (set in patients mouth)- sufficient temperature rise can damage the pulp.
what are the Dimensional changes during polymerisation:
Polymers more dense than monomers
So, monomer occupies more volume than polymer
- (assume equal weights)
- Polymerisation ‘normally’ causes shrinkage
what are the Problems in dentistry of Polymerisation causing shrinkage
Fillings - Marginal gap formation – staining, secondary caries
- Gaps formed due to shrinkage during polymerisation causing gap between tooth and restoration
Denture – contraction porosity – weakness, early failure
- Should overfill to avoid this
what are the two Methods to reduce polymer shrinkage
- Different monomers
* Add filler that do not polymerise
how does Different monomers reduce polymer shrinkage in fillings
- E.g. BisGMA (monomer) is much bigger than MMA
- Bigger monomers take up more volume when joined up– less shrinkage
- Reactive ends are further away
- MMA not used in fillings, rather BisGMA
how does adding a filler that does not polymerise reduce polymer shrinkage in fillings
PMMA beads added to MMA in dentures
•PMMA already polymerised, takes up volume
•Reduced the amount of shrinkage
outline the Structure of Polymers
amorphous structure
crystalline polymers
what is meant by an amorphous structure
chains have a short range order- arrangement of monomers
Chains do not have long range order – chains are randomly arranged
what are crystalline polymers and example, why are they not common dentistry
Some polymers, can have long range order e.g. nylon
sometimes require increased temperatures
what happens when you heat crystalline polymers
can melt, so have a melting temperature (Tm)
- Lose regular chain arrangements
- Cool down and it can be recovered
- Heating and cooling means we can change its shape
what happens when you heat amorphous polymers
do not melt, at high temperature they burn
- When we heat the chains move apart but then they char
- When we cool down we still have the char mess
- Can’t change structure by heating up
Polymers form one of two structures, what are they and what is the effect on recycling
- Polymers form one of two structures
- Thermoplastic (crystalline polymers mainly, small number of amorphous) - soften on heating, harden on cooling
- Thermosetting - harden on setting, cannot be softened on heating
- Crosslinked polymers are thermosets, most dental polymers are crosslinked.
- Only thermoplastics can be recycled therefore recycling problem
what is the Glass transition temperature (Tg)
what is meant by glassy and rubbery
The temperature at which polymers change from glassy to rubbery is the Tg
- At low temperature they are stiff (or glassy) shatter easily- hard to deform
- At high temperature they are flexible (or rubbery) – easy to deform
• There is a 10x reduction in modulus when we go from glass to rubber
Ex. As we approach 80 degrees it gets x10 easier to deform the polymers as we have gone from glass to rubbery
what is the effect of polymers with a Tg below and above the mouth temperature
• Polymers with a Tg below mouth temperature will be rubbery
• Polymers with a Tg above mouth temperature will be glassy hard and rigid
•
what does Tg allow for in clinics
Tg allows you to work out what application a polymer will be useful for
- Glassy/ rigid at room temp - Denture, filling
- Rubber at mouth temp- impression material, a rubber dam
what are the Factors Affecting Properties:
- Degree of polymerisation
* Molecular structure of the monomer/monomers
how does degree of polymerisation affect polymerisation
Higher average molecular weight – higher Tg
- Tg is related to a theoretical value Tgo (it can polymerasie 100%)
- Take away K and molecular weight- higher the molecular wright the higher the denominator- closer we are to the theoretical Tg- closer it is to behaving how we expect.
The lower the amount of residual monomer – higher Tg
how does the nature of repeating units affect the degree of polymerisation
- C=C bonds are rigid - bonds open up. Anything that has C=C bonds is generally rigid
- Si-O bonds are flexible- polymers generally more flexible
how do pendant groups affect the degree of polymerisation
- Hang off the polymer chain
* Bigger pendant groups lead to more rubbery polymers
how doCo-polymers affect the degree of polymerisation
- Tg, and other properties, is a combination from the monomers
- When we go from glass to rubber we make the polymer easier to deform
- Boil and bite materials have Tg of around 40 degress
- If you have a denture (Tg 60 degrees) it is possible that when a patient cleans the denture it may change dimensions- should let water cool down. polyMM used in dentures
why do monomers with larger pendant groups have a lower Tg
• When we deform a polymers we are trying to move chains past each other
• The closer the chains are the harder it is to move them
• The bigger the pendant group the further apart our chains will be
• When we look at the Tg if these polymers- the one with the smallest pendant will have a higher Tg
- Generally the are all rigid- but the pendants effect them
- Polybutly (table)- easy to deform in the mouth
what is the effect of the amount of initiator added on the properties of the polymer
- More initiator leads to greater number of chains that form
- More chains lower average molecular weight- lots of small chains
- The lower the molecular weight the lower the molecular transition temperature
what is the effect of Chemical activators on the properties of the polymer
- Typically a tertiary amine – for instance N,N-Dimethyl-p-toluidine (DMPT)- have an unstable structure at room temperature. Can donate electrons causing weak bonds to break down.
- Concentration of activator has similar effect to initiator
what is the effect of Cross link concentration on the properties of the polymer
- Cross links make polymer hard to deform (increase Tg).
- Too much cross linker makes polymer brittle. Can shatter if dropped
what is the effect of Plasticisers on the properties of the polymer
- Added to reduce the Tg
- Insertion of material
- Liners added to dentures have plastiscers to lower Tg
- Act like a lubricant between the chains
- Residual monomer acts as a plasticiser- if we don’t get high degrees of polymerisation we are making things that are more flexible
what is the effect of fillers on the properties of the polymer
- Fibres and particles added to make composites
- Concentration of filler changes properties
In industry polymers are usually made using what
- Injection moulding
- Compression moulding
- Both require high temperature and pressure (inappropriate in mouth)
what are the dental appliactions of
• Dough moulding
• Paste moulding
Dough moulding
• Mix a power and liquid to form a dough and place in mould and it sets
• E.g. acrylic dentures
Paste moulding
• Mix two pastes to place into mould
• E.g. composite fillings- more modern use one paste
Features of WT:
- Always plastic
- Goes from low viscosity to high viscosity
features of Setting time
- Always elastic
- Measured by manufacturer at oral temperature
what are the uses of metals and alloys
- Denture Frameworks
- Implants- part screwed into the jaw
- Crowns/Bridges/Inlays- occlusal surface replicate the opposing teeth. May eb Indiret (made by technician and dentist tooth)
- Direct Filling- mixed and placed into cavity by dental professional in the clinic.
- Wires- aesthetic and function
- Instruments
what are the characteristics of metals and alloys
- Hard and Lustrous
- Dense and Crystalline
- Conduct Heat and Electricity
- Opaque
what are the useful properties of metals and alloys
They tend to be strong, stiff and tough
They are hard and lustrous
They are conductors
why is it important metals and alloys are strong, stiff and tough
- Difficult to break (strong)
- Difficult to deform (stiff)
- Lots of energy before they shatter (tough)
- Biting and chewing are high stress activities
why is it important metals and alloys are hard and ludicrous
- Hard means they do not scratch easily (harder it is, the harder it is to scratch)
- Lustrous means they retain their polish (retaining shine) – requires scratching. Polishing is scratching with fine particles. If something starts off being hard, its hard to scratch it (bacteria colonise in scratches)
why is it important metals and alloys are conductors
• Both thermal and electrical
• Note, this may not be an advantage (i.e. heat conduction to the pulp)
Need to protect the pulp e.g. aligner into the cavity before amalgam filling.
What are metals and what are alloys and what are dental examples
- Metals are elements
- Alloys are combinations of elements
- Two or more metals –
- Dental example: amalgam – Hg, Ag, Sn, Cu, Zn
- or
- Metals and non-metals
- Dental example (orthodontic brackets) – stainless steel – Fe, Cr, Ni, C
what is the importance of metals and alloys being crystalline
- Atoms form in well-defined ways
- The atoms have both short- and long-range order
- Polymers have short range order, monomer form and the chains form anywhere so there no order -no long range order
- Remember polymers were amorphous
- no long range order
what is the importance of metals and alloys being opaque
- This may affect potential aesthetics
- Pigments cannot be added to change appearance
- Painting is not used in dentistry- the mouth is so aggressive they would chip off and it would be unsatisfactory
what is the importance of metals and alloys being corrode
- Corrosion may weaken the materials- in the mouth they may bot be as strong for as long as we like
- Corrosion may change the appearance- oxides can flake off easily
what are the three methods commonly used for Metals and alloys to be shaped into complex shapes:
• Casting – FIRST STEP melting and pouring into a mould. Liquid cool down and form shape. Complex shapes can be made. Always have casting
• Working – bending, pulling hammering to shape e.g. scalpol shape- punching out hole
• Amalgamation – mix with mercury (don’t worry about one for now)
-Only dentistry still uses amalgamation
-Casting is always the first step
-Capsules containing greyish material (flakes) mixed with pure mercury- mercury makes the alloys flow, it can be put into cavities and help shape it
why is working cheaper
Working does need such high temperatures- don’t need to melt things however • Traditionally only simple shapes possible
how can complex shapes be made using CAD/CAM
- Manufacturer cats a disc and sends it to lab which feeds it into computer which will make that shape.
what is the state of the metals and below the melting temp (Tm)
- Above Tm they are liquid
- Below Tm the are solid
- The process is reversible
Solidification occurs in distinct phases, what are they
As temperature decreases atoms get closer together
Eventually groups of 4 atoms start to join together
more and more atoms join on the four atoms – crystal growth
what is the latent heat of fusion and what is this like for metals
the amount of heat energy released or absorbed when a solid changing to liquid
• For metals the temperature remains constant during crystal growth
where else will metals solidify
- Metals will also solidify onto solid surfaces
* So will start to solidify against the walls of reaction vessels first.
what are seeds or nuclei
Small amounts of metals with high Tms often added
help to control the properties of the casting. Of hugh melting pt mental and these would act as places for solidification to take place.
These metals still might be solid above the melting temperature
what occurs in Crystal Growth:
The atoms join in structures called dendrites
• Like tree branches
• The atoms join from the liquid onto the branches
• Cause more atoms to solidify until all the liquid is used up and the metal becomes solid
• As the branches grow the liquid becomes used up
• Eventually all of the metal is solid
how do crystals grow
- The grow randomly but are roughly equal in all directions- when we heat the metal a nd cool it down we have an equiaxed structure
- some bigger than other but roughly equal
- Termed equiaxed – equal in all axes (directions)
what are the crystals called
grains
• Where the grains meet are termed grain boundaries
• Grain boundaries are important for mechanical properties
what are atomic planes
- Within the grains the atoms form into layers
* Termed atomic planes-
what are the 2 competing factors when metal planes stack on top of eachother
- Atoms want to join up to be solid
* Different planes need to stack in a stable way
what are Common unit cells in metals called
- BCC – dental example: chromium, (dental wires, fillings)
- FCC – dental example: aluminium, copper, gold, nickel
- HCP – dental example: titanium (implants)
- Some metals can change unit cell as they cool down/heat up
- Certain metals wont mix as they want to be BCC and FCC
what point Defects (appear at locations) can form in the grains
Two types:- VERY COMMON
• Impurities – atoms of different elements (purple dot)
• Vacancies – gaps caused during solidification
what defects can involve whole plains
- Termed line defects or dislocations (also very common)
- Planes of atoms joining up and you have an odd number of planes joining up
- Can be either too many or two few planes
- Dislocations are important for metal properties
outline the casting process, how is cooling down accelerated
During casting liquid metal is poured into a mold- the metal will be very hot
The mold will be at a lower temperature than the liquid metal
• This starts the solidification process
- Cooling down could be quickened by placing the mold in a liquid (water, oil)-
- Advantagoues for mechanical properties- This process is called quenching (routine in dentistry)
what are molds in the casting process made from
Molds are made from insulators – so will take a long time to cool down – slow process
what is the effect of temperature on metal structure:
the speed of cooling down effects how many grains form
what is the effect of cooling down slowly
we get fewer groups of 4 atoms joining and get fewer grains
what is quenching and what is the effect of it
rapid cooling of metal to adjust the mechanical properties of its original state
this produced more grains per unit volume
what is the effect of having more grains per unit volume
the grains will be smaller
there will be more grain boundaries per unit volume (these boundaries are areas where atoms aren’t well joined up- disorganised)
what is the relationship between grain size and mechanical properties determined by
yield strength
what is yield strength
the stress required for plastic deformation
Important property – governs how metal/alloy can be used
what is the effect of more grain boundaries on yield strength
more grain boundaries the higher the yield strength -areas of disorder is harder for atoms to move
what do we want the yield strength to be like in dentistry
want it to be higher than strength in daily life otherwise material will return to its original dimensions
how is yield strength related to grain size - what is the dental application of this
the Hall-Petch equation
Smaller grain size higher yield strength so harder for plastic deformation (want crown to not change dimensions- we quench so we can get small grain sizes)
what is the Hall-Petch equation
theoretical strength + (constant/ √ grain diameter)
what are Dental examples for temperature changes of metals and alloys during use
joining by soldering, heat treatments to strengthen, fusing porcelain for crowns (Need to heat metal to apply porcelain)
what does An increase in temperature in metals leads to
- Increased inter-atomic distance, atomic vibration and diffusion rate
- Don’t line up with each other
- Atoms can start to “jump” over grain boundaries leading to grains growing
what is the effect of atoms starting to ‘jump’ over grain boundaries when heated
- reduces yield stress – can lead to device failing in service.
- Easier for grains to join up- grains growing. Likely to get bigger grains per unit volume so we have less boundaries per unit volume
- If yield stress reduces the crown may deform when patients bite down
what is the Recrystallisation temperature (RcT) in relation to the melting point , what state is the metal
- Lower than melting point (Tm)
- Between 0.3 (metal) – 0.7 (alloy)
- Within solid state
during recrystallisation the diffusion rate within a grain is
- Low below RcT (atoms jumping around is low)
- High above RcT (can jump over grain boundaries)
Atoms on cup of water- atoms can jump out of surface and form steam- similar to atoms at grain boundaries
what is dental example of a 2 or more metals
amalgam – Hg, Ag, Sn, Cu, Zn
what is an example of metals and non-metals
stainless steel – Fe, Cr, Ni, C (quaternary)
what can alloys be described as
binary, ternary, etc.
what does an alloy system describe
describes all of the possible combinations
How do alloys compare to pure metals:
They can be cheaper
• Assuming expensive metal replaced by cheaper
• E.g. gold alloys cheaper than pure gold
Mechanical Properties
• Harder - solution hardening
• Stronger
Wider melting range
• Reflecting different melting points of pure metals
when metals are liquids they are normally soluble in each other , what does this mean
the atoms are happy to move around each other – can be combined
When metals solidify what do the the atoms want to do
join into planes – form grains
• How the different metal atoms form grains governs what the solid alloy is like
what are the four groups of alloys that can form
Solid Solutions
Insoluble metals
Partial Solubility
Intermetallic Compounds
what occurs in Solid Solutions, what is the dental example
: the atoms are happy to form into planes with each other. Mix well
Dental example – gold and silver (used in gold alloys for crowns, bridges, etc.)
what occurs in insoluble metals , what is the dental example
- Insoluble metals: the atoms of one metal do not want to form into planes with atoms of the other metal. When we cool they don’t want to be together at all.
- No dental example, but lead-tin used in solder is an example
what occurs in partial solubility , what is the dental example
the atoms are happy to form into planes up to certain concentrations.
• Dental example – copper and silver (used in gold alloys for crowns, bridges, etc.)
what occurs in intermetallic compounds , what is the dental example
(rare) the atoms can form ionic compounds
• Dental example – silver tin (one of the main components of dental amalgam)
• Due to the ionic bonds these tend to be very hard but brittle materials
What are the 4 key factors which govern whether a solid solution will be formed?
- The relative sizes of the atoms
- The relative electronegativity
- Relative valency of the metals
- Crystal structures
how does The relative sizes of the atoms governs whether a solid solution will be form
The atoms must be either be very similar in size (small difference in atom size)
• Difference in atomic radii < 15 %
• When they sit together we don’t get a lot of distortion
Or they must be very different in size
• Difference in atomic radii > 59% - these are termed interstitial solid solution
how does the relative electronegativity governs whether a solid solution will be form
Big difference may lead to ionic bonding – form intermetallics
how does the Relative valency of the metals governs whether a solid solution will be form
Full solubility requires the same valency.
If valency differences exist:
• metal with lower valency more likely to dissolve in metal with higher valency
• may not be symmetrical
how does Crystal structures governs whether a solid solution will be form
• The metals must have the same crystal structure for planes to form
Alloys have a melting / crystallisation range, what does this reflect
- Reflects Tms of the metals
* Melting behaviour depends on concentrations of metals
what is a cooling curve and why are phase diagrams used instead
A cooling curve is a line graph that represents the change of phase of matter, typically from a gas to a solid or a liquid to a solid.
• BUT this is very time consuming
Phase diagram summarise all the cooling curves in one (bottom) – not graph
what do Phase diagrams show
all the compositions at the bottom
the full temperature range on the sides
Shows the temperature above which everything is liquid- liquidus line
Shows the temperature below which everything is solid- solidus line (solidus temperature)
Shows that solidifying starts and finishes differently per alloy
Shows that there is a region of solid and liquid existing together (Like ice in water, solid and liquid exist together)
what are tie lines and what do they show
- A tie line is an isothermal (constant temperature) line connecting the compositions of the two phases in a two phase field. It is used to find the compositions of the phases in the two phase field.
- The ends of the tie lines show the compositions of the two phases that exist in equilibrium with each other at this temperature
what is Coring
when we cool down too quickly (e.g. by quenching) it can lead to different structures in grain
what does coring effect
corrosion resistance (c.f. properties lectures)
• Many different metals all closely joined together
• Poor corrosion resistance is not good for in mouth use
how do we remove coring
homogenisation
how do we carry out homogenisation
- Heat the alloy so that the atoms can move around
- If heat to temperature below Rct – atoms won’t “jump” grain boundaries
- As long as we don’t go above the recrystallisation temperature and the grains don’t go over the grain boundary this is okay.
how does a phase diagram allow us to predict the amount of coring
- A indicates gross coring (top)
- B indicates moderate coring (bottom)
- The bigger the solid and liquid region the more coring will occur.
what do phase diagrams allow us to see
allow us to see the melting range for all alloys in that system
Can find the liquidous line and solidous line and then can work out what temperature it needs to be heated and cooled to
what do tie lines allow us to see
allow us to see what the composition of things are as they cool down. If we are quenching we may encounter coring. This is where we may have corrosion the mouth due to different composition metals sat in different amounts. Homogenisation removes this (must be done below the RcT).
how does the phase diagram for a insoluble metal and solid solution vary
Phase diagram is similar to that for solid solutions
Liquidus line, solidus line, region of solid and liquid
However insoluble phase diagrams have a eutectic point which forms at a specific composition (eutectic composition)
and Has a melting POINT temperature – not a melting range
• All other compositions have a melting range
outline the Solidification of Insoluble Metals:
Case 1: alloy containing 90%A and 10%B
(need to look at phase diagram)
Case 1: alloy containing 90%A and 10%B
- The tie line crosses the temperature line for metal A first
- So pure metal A solidifies first.
- Tie lines in the solid + liquid field always cross the metal A line
- So, until the solidus line is reached only metal A solidifies
- Below the solidus line everything must solidify
- The remaining metal A and B must solidify
- BUT metal A and B are insoluble, so regions of pure A and B form
- Below the solidus line the alloy is extremely cored
- Homogenisation will not work – the metals are insoluble. They aren’t used within the mouth
outline the Solidification of Insoluble Metals:
Case 2: alloy containing 20%A and 80%B
- The tie line crosses the temperature line for metal B first
- So pure metal B solidifies first.
- Tie lines in the solid + liquid field always cross the metal B line
- So, until the solidus line is reached only metal B solidifies
- Below the solidus line everything must solidify
- The remaining metal A and B must solidify
- BUT metal A and B are insoluble, so regions of pure A and B form
- Below the solidus line the alloy is extremely cored
- Homogenisation will not work – the metals are insoluble
Even If we melt and cool slowly we sill still get coring- these alloys have bad corrosion resistance. Insoluble metals will not be used for metals that go into the mouth for this reason
outline the Solidification of Insoluble Metals:
Case 3: alloy containing 45%A and 55%B
- The liquidus lines and solidus line meet at the same point
- The eutectic point
- Above this temperature the atoms resist solidifying
- At this temperature there is not enough energy – they must solidify
- BUT metal A and B are insoluble, so regions of pure A and B form
- Below the solidus line the alloy is extremely cored
- Homogenisation will not work – the metals are insoluble
- Having a single melting point is useful
- Solders – used to join metals and alloys have eutectics
what are Partially Soluble Alloys and the dental example
- The majority of alloys are partially soluble
- There is a limit to how much one metal can dissolve in another
- When they dissolve – a solid solution is formed
- Above the solubility limit – they are insoluble metals.
- So the phase diagram is a combination of solid solution and insoluble metals
- Dental example: Silver and Copper
- Both want to form FCC crystals- atoms stack up happily
- Difference in atomic radii is > 15% (but not by much)- just big enough that after a while we struggle to get them to sit together
- So there is a solubility limit
what is • Solubility related to
• Related to temperature and concentration – bigger the temperature difference the more complex
outline what happens to partially soluble alloys: Silver-Copper (on phase diagram)
- At low concentrations of Cu a solid solution forms
- a – 9%Cu, 91%Ag
- At low concentrations to Ag a solid solution forms
- b – 8%Ag, 92%Cu
- The lines ABF and DCG are called solvus lines
- Between 9%Cu and 92%Cu the solubility limit has been reached
- There is a eutectic point – 28% Cu, 72% Ag, solidifies at 780°C
- This is used as dental solder and in amalgam
- Solidification is similar to insoluble metals but a and b form
- Note: a and b are solid solutions not pure metals
outline Solidification of Silver-Copper Alloys:
- Take an alloy made from 60% Cu, 40% Ag
- At liquidus line draw tie line towards the Cu line
- Crosses the DCG solvus line – b solidifies first
- Similarly as temperature decreases b solidifies
- At solidus line a and b solidify in alternate regions
what happens if we quench the silver-copper
• There is too much Cu in a and too much Ag in b – but they are trapped
- e.g. Sponge has expanded too much, the atoms want to jump out
Over time Cu and Ag will diffuse out of the grains and form pure Cu and Ag at boundaries, forming regions of pure copper and pure silver - what does this lead to
• Age hardening – alloy will become harder but takes a long time
what happens if we heat the Silver-Copper Alloys to below RcT
we can speed up the diffusion (some silver jumping out and some Cu jumping out)
Precipitation hardening – alloy becomes harder quickly
• We don’t need all Ag and Cu to jump out
Heat Treatments used after casting when the alloy is solid to improve properties
Homogenisation
• Done to remove coring
• Done when solid
Precipitation Hardening
• Can increase hardness and yield strength of partially soluble alloys
Order hardening
• Can increase hardness and yield strength by causing metal atoms to form ordered solid solutions
• Dental example – gold and copper alloys
outline Solidification of Gold-Copper Alloys
- Gold and copper have very similar atomic radii
- Form solid solutions
- If cool down quickly the atoms form up into planes randomly
- M is a narrow region where the atoms are very similar and want to sit together so melt spontaneously
- Gold and copper can form intermetallics if cooled down slowly
- Intermetallics are hard and brittle
- Slow cooling would produce big grains – very weak
what heat treatment is used in the Solidification of Gold-Copper Alloys:
order hardening used
- Heat to above 450°C- below RcT temp (close however)
- Allow atoms to diffuse (slowly cool)
- Form some ordered alloy (alternate layer of copper/gold).
- Cool down
- Certain amounts of intermetallic formed
when do Intermetallics form
only form is we cool slowly BUT we don’t get small grains so need a compromise (order hardening)
what are the characteristics of metals and alloys
- Dense and Crystalline
- Conduct Heat and Electricity
- Opaque
- Hard (difficult to scratch) and Lustrous
- High strength and modulus (difficult to deform, low modulus is flexible)
- Ductile- work to bend metals/alloys. Plastic deformation can be done without breaking
what are three ways that give an indication of ductility
- Yield strength
- Hall-Petch equation (sy = sO + ky/√d)- wont appear in exam. Small grains give high yield strength.
- Dislocations- can work to a benefit
Why are metals and alloys ductile? why do they not break immediately
- When stress is applied the alloy will change shape
- Up to the yield stress if the stress is removed the alloys returns to its shape (Elastic deformation)
- If stress above the yield stress is applied the alloy changes shape
- Plastic deformation
- Porcelain cannot plastically deform as it will break
onset of necking first then failure
outline brittle behaviour for a perfect crystal i.e. no dislocations
- We need apply enough force (F) to break all 5 bonds
- If we don’t break the alloy we need the 4 bonds to reform
- We need to break lots of bonds and get them to reform to get a small amount of deformation.
outline brittle behaviour for a real alloy
- Break millions of bonds
- Get the bonds to reform
- This would take a very large force
- The chances of the bonds reforming is low
- It is more likely the alloy will break – brittle behaviour
- In ceramic we can apply a lot of stress but as we do a little bit of plastic deformation we cause millions of binds to break, and they stay broken.
outline ductile behaviour for a crystal with a dislocation
- We need to apply enough force to break one bond
- All the surrounding bonds remain the same
- High chance that will not break when we move the dislocations
outline ductile behaviour for a real alloy
- Break many of bonds – but far fewer than for perfect crystals
- Get the bonds to reform – all the remaining bonds are sill there so high chance
- This would a lower force than without crystals (1/5th in our example)
- The chances of the bonds reforming is higher
- If we permanently move by 1 atom that is plastic deformation
- Within metals and alloys we have millions of dislocations so they yield stress is much lower than the strength of the material
how do dislocations appear when an alloy solidifies
there will be many dislocations in each grain
- Randomly placed, some near the centre, some near the grain boundaries- extra planes appear
- When we apply stress above the yiels stress we get more deformation so more dislocations form
- Dislocations travel in specific directions – slip planes (different striped lines in different directions)
- When different grain meet at grain boundaries there is a lot of disorder
- As dislocations reach the grain boundary they become trapped- they cant change direction
how does the Hall-Petch equation (sy = sO + ky/√d) relate to dislocations
- The smaller the grains, the more boundaries, more places to trap dislocations
- Dislocations cant move anymore so more stress needs to be applied for deformation- yield stress increases
- Yield stress dependant on quenching (grain size) and dislocations
why do alloys break? mention what occurs up to the yield stress and above the yield stress
there is a limit of ductility
Up to yield stress
• Not enough stress applied to move dislocations
• elastic deformation
Above yield stress
• Dislocations can move and more dislocations form
• Dislocations start to coalesce at grain boundaries from centre as we dorm things- get trapped
• Stress required to move dislocations increases – more barriers
• Coalesced dislocations start to form pores
• Necking occurs and then failure (not seen in dentistry as things break too fast)
• Alloys fail- dislocations have moved to the grain boundaries, pores form and it breaks
what is The effect of repeated deformation – working on alloys
Repeated deformation above the yield stress causes:
• Causes dislocations to move
• Causes dislocations to form
• Causes dislocations to coalesce at grain boundaries
• Cause the yield stress to increase and the ductility to decrease
• Modulus stays the same- how stiff it is
Every cycle makes it harder to do the next cycle as dislocation migrate to boundaries and get trapped. Eventually forms pores and breaks (Used up ductility)
Can increase yield stress of wires in this way- orthodontic wires (as long as they are below their yield stress when applied they will move teeth as will want to pull straight- can deform them elastically and want to come back)
What happens to the grains during work?
As dislocations flow to grain boundaries
• The grains start to deform
• The turn fibrous- start to line up in a preferential direction
• Work hardening is going from equiaxed to fibrous
- Ductility decreases
- Grain size decreases – due to moving of dislocations
what is the effect of temperature on recrystallisation
Recrystallisation temperature (RcT)
• If temperature increased to allow diffusion over grain boundaries above RcT
• Filling in pores
• Atoms can start to “heal” dislocations at boundaries
• New grains can start to form
• If more heat applied grain growth can occur- more energy, atoms jump about and form. Small grains can start to join up, get bigger grains
this is because as temperature increases:
• Increased inter-atomic distance
• Increased atomic vibration
• Increase in diffusion rate
what is cold work
heating below RcT
• Grains go from equiaxed to fibrous
• Yield strength and hardness increases – work hardening
• Limit to ductility – may lead to failure if we deform too much
what is hot work
heating Above RcT
- Grains remain equiaxed- don’t get fibrous structure
- Dislocations flow but can also be recovered (as atoms move around – diffusion)
- Do not get work hardening but have full ductility
outline how in dentistry hot working and cold working can be combined
- Start off with hot work – no ductility problems (full range of ductility can be used) – change in dimension but stay with equiaxed grains
- Final cycle is cold work – get the good mechanical properties
what does dislocation enable, what happens if there is no dislocations
enable plastic deformation
• No dislocations – brittle material (intermetallics form by ionic bonds)
what occurs if there is more deformation
- More plastic deformation
- More dislocations formed
what occurs if there is a Change in grain shape
- Dislocations flow towards grain boundaries
- Change from equiaxed to fibrous
what are the Mechanical Properties that Change in Work Hardening
- Harder, higher PL (proportional limit)
- Special characteristics of e.g. wires (work hardening for orthodontic wires)
- Any Excess deformation may cause fractures
what are wrought alloys
Alloys that can be used to make devices using work by either cold and/or hot working , as casting is expensive due the high temps and expensive equipment
outline dental devices that can be made by working
- Orthodontic wires and brackets (simpler to cast then pull into wires over casting into wire shapes)
- Dental instruments – drills, rasps, scalpels
- Implants
- CAD/CAM crowns and bridges
- Many things are made by work- even cutting is work!
what are the Wrought alloys in dentistry and what are their used
steel- stainless steel
• Orthodontic wires, orthodontic brackets
• Instruments
• Implants
Titanium and titanium alloys
• Implants
• Orthodontic wires
Others (historical) Gold alloys (historical) • Orthodontic wires
Base metal alloys (Co/Cr and Ni/Cr)
• Fixed prosthodontics, removable prosthodontics
• Implants
What processes are used in dentistry to make wrought alloys
Forging
• Shaping by heating and hammering (as we increase temp its easier to deform)
• Can be above or below RcT
Milling
• Cut to shape with a rotating tool
Drawing and Rolling
• How wires are made
• Shaping by being pulled through a die or dies
• Can be above or below RcT
outline how Cold working produces desirable properties
Change in crystal structure
- Dislocation to flow down slip planes
- grain change from being Equiaxed to fibrous (increase in yield strength
Improved properties
o Hardness, strength
o “springy”
o Difficult to do plastic deformation with them but we can deform a lot and they’ll return to original
why do we need to be careful when doing cold working
Work hardening
o Uses up ductility
o Danger of fracture- don’t have much range left to deform
o Must apply a lot of strength to deform e.g. orthodontic wires/dentures may break if large forced applied for more desirable fit
Sensitive to temperature change
o Overheating may destroy crystal structure
what does cold working depend on
Depends on shaping method- rule of thumb
o Milling and forging – little work
o Drawing and rolling – high work
what is stress relief annealing and what does it do
it is a heat treatment used to prevent distortion of grains that occurs when cold working stores stress in the grains. the atoms can relax cause distortion
why is stress relief annealing carried out below the RcT
it is carried out below the RcT to relieve the stress without changing crystal size or shape
this means that atoms can diffuse within the grains but NOT over grain boundaries, and they can move closer to eqm (moving atoms means they are not in eqm)
Do not re-crystallise - will lose hardness, springiness
in what phase does stress relief annealing occur (graph) and what are the other phases
occurs in the recovery phase- the atoms move back to their eqm position
other phases:
recrystallisation
grain growth
as a result of higher temperature faster things occur however careful not to go over boundaries and yield strength will decrease
Stress relief anneal may not done properly
what are the methods available to join metals and alloys
loops
soldering
welding
outline what occurs in loops
bend the parts around each other
• Difficult - there is high yield stress as permanent deformation is needed
• Requires ductility – have to be able to bend the wires without breaking
• Alternative methods are better as this is difficult
outline what occurs in soldering (why is eutectic alloy better to use)
using a liquid alloy to join together
• Use a eutectic alloy – melting point rather than melting range gives control
• Need to solidify liquid alloy
• E.g. silver solder (Ag/Cu eutectic)- eutectic alloy has a melting point not a melting range! So is easier to control
- Most common dental solder
outline what occurs in welding
– use an electric current to locally heat the components
• High localised temperature rise when electric current is applied
• Weld decay may occur – ionic solids can form at high temperature (hard but brittle)
• Do not need to remember equation in image
outline why high temperatures may cause problems Joining Metals and Alloys
- If we cant join things together it may lead to localised temperature increase
- Could lead to recrystallisation and grain growth
- The join could be much weaker than expected – device failure possible
- Special technique to avoid localised recrystallisation at the join
Why are alloys harder and stronger than metals? what is this termed
• Around the different sized atoms the planes are deformed
- This deformation makes it harder for dislocations to move
- This increases the strength and hardness of the alloy
This is termed SOLUTION HARDENING
In alloys the effects are related to concentration of other atoms, outline how
• Yield strength increases
• Ductility decreases
- More obstacles- easier for dislocations to get trapped (similar to grain boundaries)
- Wrought alloys (to make thru work) start of with good ductility
• As we added nickel the yield strength increases
what is steel composed of
• An alloy of iron and carbon (Fe & C)
carbon has a solubility limit, outline why this is important in the steel alloy
there is a point where we can’t add more carbon atoms into the gap as its too destructive (2% limit then not useful)
when steel FIRST forms a solid, what does it form and what structure is this form and when is it unstable (1/5)
austenite - FCC
at high Carbon concentrations (above 2%) the austenite is unstable
as the temp decreases, the solubility of C decreases
what occurs At the critical temperature (723°C) (2/5)
iron wants to change form FCC to BCC (atoms slide over each other and rearrange new form)
In the BCC structure the atoms are closer together so not much carbon can eneter (lower concentration of Carbon =0.02% C max)
Iron and carbon also want to form an ionic solid , name this solid (3/5)
cementite (Fe3C)
what happens as the temperature decreases in the steel phase diagram , and what effects this (4/5)
the austenite starts to become ferrite and cementite
o The concentration of C effects this
o Below 0.76% C – ferrite form, above 0.76%C – cementite forms
At 0.76%C there is a eutectoid point, what is this
eutectoid is a eutectic in the solid state
what occurs below 723°C in steel
a composite material forms Pearlite =Ferrite (BCC) + Cementite (Fe3C)
there are number of phases that form with steel that have different properties- what are they and their properties
Austenite
• Stable above useful temperatures for dental applications
Ferrite
• Solid solution of Fe and C
• Medium strength
Cementite
• Ionic solid: Fe3C
• Hard but brittle
Pearlite
• Composite of ferrite and cementite
• Properties depend on concentrations of Fe and C
what is The effect of C content on mechanical properties
- Pearlite
- Composite of ferrite and cementite
- Properties depend on concentrations of Fe and C
- As the C content increases
- Hardness increases
- Yield strength increases
- Ductility decreases
- Higher yield strength lower ductility
what is Martensite and when it is formed
- If steel is cooled very quickly (quenched) the carbon gets trapped – (we want small grains)
- Steel can no longer change to ferrite (BCC)
- Martensite forms – body centred tetragonal
- (water is frozen so extend sponge)
Martensite is very hard and brittle
what can be done to control steel properties thus control the amount of martensite we get
tempering- heat the steal below 723 degrees so we get ferrite + cementite (pearlite)
Higher temperature and longer time means more pearlite forms
Quenching will increase the formation of martensite from austenite- this is much harder(the material) and if we temper we can control this
why are stainless steels used instead of just steel
- Iron readily corrodes (rusts)
- Iron oxide bonds weakly to iron – easy to flake off
- So, steel (Fe and C, only) is not useful in dentistry
what is added to stainless steel that makes it better for use in dentistry
- Stainless steel is used instead
- Chromium and nickel added
- Chromium corrodes faster than iron
- Chromic oxide binds to chromium strongly – hard to flake off
- Forming this STABLE oxide layer is called passivation
- This means stainless steels can be used in the mouth (aluminium does too but not used in dentistry)
what are the 2 types of stainless steal
Austenitic stainless steel – 18/8 stainless steel
Martensitic stainless steel – 12/0 stainless steel
outline the composition of Austenitic stainless steel and when is it used
- 18% Cr and 8% Ni added to steel (Fe-C)
- Chromium forms a passive layer
- Cr and Ni form solid solutions with steel
- Get solution hardening occurring- improvement in mechincal properties
Austenitic stainless steel used for orthodontics
no austenite to pearlite transition occurs in stainless steel, why is this
- This is because by adding more bigger atoms C solubility limit not met
- This means no martensite can form
- Steel atoms don’t want to change for FCC to BCC (there is enough energy for them to stay in austenitic form)
outline the compositon of Martensitic stainless steel and what is it used for
- 12% Cr and 0% Ni added to steel (Fe-C)
- Beware there can still be trace amounts of Ni (allergies)
- Chromium still forms a passive layer
- Cr and Ni form solid solutions with steel
- Get solution hardening occurring
Martensitic stainless steel used drills, burs, scalpels
- Drills/burs -high carbon
- Low forceps - low carbon
outline why we do get a Austenite to pearlite transition to occur
- Solubility limit of C can be reached
- Martensite can now form
- Quenching – tempering possible
- Martensite will form
