1. DMS Flashcards
Stress calculation
Strain calculation
YM relates to material
Material failure mechanisms (8)
Blacks cavity classifications (6)
Stress (Pa/Nm2) = force/unit area
Strain = change in length/original length
Rigidity
Fracture, hardness, abrasion, abrasion resistance, fatigue, creep, deformation, de-bond, impact
Class I - pit and fissure carues Class II - posterior approximal caries Class III - anterior approximal caries Class IV - approximal caries involving incisal edge Class V - affecting cervical surface Class VI - affecting cusp tips
What type of bonding is enamel/restorative material
How is this bonding obtained
How does the acid-etch process work
Effect of increased surface energy
Mechanical
Acid-etch
Micromechanical interlocking of resin-filled material and increase surface energy
Better gettability and adaptation of resin to surface
Why is dentine bonding difficult (2)
Definition of smear layer
What is done to the smear layer (2)
DBA requirements (2)
Type of bonding in dentine/DBA (2)
Contact angle of a hydrophobic surface
Dentine has low surface energy and is hydrophilic
Adherent layer of organic debris that remains on the dentine surface after restoration preparation (0.5-5um)
Can be removed (acid-etch) or infiltrated and incorporated (DBA)
Low viscosity, adhesion to substrate
Chemical (electrostatic dipole interaction - strength depends on contact angle) and mechanical (interlocking)
<90
Definition of critical surface energy
Liquid/solid surface energy relationship
Wet dentine/composite surface energy relationship
Function of DBA in regards to surface energy
How can adhesion occur
Surface tension of a liquid that will just spread on the surface of a solid
A liquid must have a lower surface energy that the surface it is attaching to, to flow and stick
Dentine < composite
A DBA will increase the surface energy of dentine, allowing composite (liquid) to flow and stick
Through molecular entanglement
Types of DBAs (2)
Function of dentine conditioner
Definition of dentine primer
Function of dentine primer
Examples of dentine primer (3)
Components of dentine adhesive
Function of adhesive
Components of total etch
Toth-etch problems (3)
Action of self-etching primers
Advantage of self-etching primer
Disadvantage of self-etching primer
Why are MDP and 4-META better than HEMA
Bis-GMA, NPG-GMA
Remove smear layer, open tubules
Coupling agent - bifunctional molecule
To bond to dentine (hydrophilic head) and resin (hydrophobic head - methacrylate)
HEMA, 4-META, MDP
Mix of resins with filler particles (increase strength) and camphorquinone (photo-activator catalyst, initiates resin polymerisation when activated by blue light - 430-490nm)
Penetrates primed dentine and forms micro mechanical bond with tubules (molecular entanglement)
Conditioner, primer and adhesive
Over-etching –> collapse of exposed dentine fibres –> no resin penetration; too dry –> dentine surface collapses; too wet –> dilution –> reduces bond strength
Infiltrate and incorporate themselves into the smear layer
Less technique sensitive
Weaken bond integrity
Less acidic and absorb less water –> increased bond durability
What does bonding involve and how does this work (2)
What is the AD concept
How does it work (2)
Mineral exchange - minerals removed from dental hard tissues and replaced by resin, which mechanically interlocks the porosities (molecular entanglement)
Interaction of resins with hydroxyapatite based tissue
All acid monomers bond to calcium in HA. Monomers with a lower pKa do not form a stable bond but continue to dissolve HA.
Function of cavity liners (2)
Features of a cavity base
Thickness of a cavity liner
Function of a cavity liner (4)
Ideal liner properties (4)
Ideal thermal properties (3)
Definition of thermal conductivity
Definition of thermal expansion coefficient
Definition of thermal diffusivity
Ideal mechanical properties
Types of liners (8)
Prevent gaps and act as a protective layer
Thick, for metal restorations
<0.5mm
Protects pulp from chemical and thermal stimuli, bacteria and endotoxins
Easy to use, radiopaque, cariostatic, biocompatible
Low conductivity, expansion coefficient and diffusivity should be similar to/lower than dentine
How well heat energy is transferred through material
Change in length per unit length for a 1C rise
Similar to conductivity, different measurement
High compressive strength, YM similar to dentine
CaOH, ZnO-based (ZnPO4, Zn polycarboxylate, ZOE, RMZOE, EBAZOE), GI and RMGI
CaOH
Components of base (4)
Components of catalyst (4)
Setting reaction
Action (2)
Advantages (2)
Disadvantages (2)
CaOH, ZnO filler, Zn stearate filler, N-ethyl toluene sulphonamide plasticiser
Butylene glycol disalicylate reactive element, titanium dioxide filler, calcium tungsten filler
Chelation between zinc oxide and butylene glycol disalicylate
Bactericidal to cariogenic bacteria, irritation (leads to reparative tertiary dentine formation)
Quick setting time, radiopaque
Low compressive strength, soluble
ZnPO4
Type of reaction (2)
Powder components
Liquid components
Initial setting reaction
Final setting reaction
Function of AlO
Problems (5)
Acid/base - powder/liquid
Magnesium dioxide (white, increases compressive strength), other oxides (Al, silica - improve physical properties and alter shade)
Aqueous phosphoric acid, oxides (buffer solution - AlO2), ZnO slows reaction (better working time)
ZnO + 2H3PO4 –> Zn(H2PO4)2 + H2O
ZnO + Zn(H2PO4)2 + 2H2O –> Zn3(H2PO4).4H2O
Prevents crystallisation –> glassy matrix (insoluble but porous with water)
Low initial pH, exothermic setting, not adhesive/cariostatic, 24hr set, brittle, opaque
Zn polycarboxylate
Difference between this and ZnPO4
Action
Problems (3)
Polyacrylic acid, not phosphate
Bonds to tooth like GIC
Difficult to mix/manipulate, soluble in oral environment at low pH, lower YM/compressive strength than ZnPO4
ZOE
Uses (4)
Basic reaction
Setting reaction (3)
Properties (5)
Deep linings/bases, temporary restorations, root canal sealer, periodontal dressings
ZOE = ZnO + eugenol –> salt + water
Chelation reaction of ZnO and eugenol –> Zn eugenolate matrix, which bonds unreacted ZnO particles
Adequate working time, relatively rapid setting time, low thermal conductivity, low strength, high solubility
RMZOE
What does adding resin do
Advantages of RMZOE (2)
Strengthen backbone to set material - increases compressive strength (>40MPa)
Increased compressive strength, decreases solubility
EBA ZOE
Components of powder (2) Components of liquid (2) Setting reaction Feature of adding EBA Advantages of EBA ZOE (2)
ZnO, quartz/alumina - reinforcing hydrogenated rosin
Eugenol, EBA - reactive
Similar set to ZOE
EBA encourages crystalline structure –> greater strength
Less soluble and stronger
Advantages of GI liners (5)
Feature of RMGIC
Advantages of RMGIC (4)
Disadvantage
Release F, easy to use, thermal conductivity/diffusivity < dentine, high compressive strength, radiopaque
Only material to predictably seal dentinal tubules
Reduced microleakage, prevents post-op sensitivity, release F, cytotoxic (benzoyl iodine and bromide released during polymerisation)
Complete cure required or unreacted HEMA may damage pulp
Composition of composite resin (5)
Composition of filler particles (3)
What are resins made from (2)
Function of resins
Function of camphorquinone
Function of silane coupling agent
Filler particles, camphorquinone, resin, low weight dimethacrylates, silane coupling agents
Glass of different sizes - microfine silica, quartz and borosilicate glass
Bis-GMA and urethane dimethacrylaes
Bifunctional molecules (C=C facilitates crosslinking) that can undergo free radical additional polymerisation
Photo-activator catalyst that initiates resin polymerisation when activated by blue light (430-490nm)
Coupling agents used to preferentially bond glass and resin
Uses of composite (4)
Advantages of adding filler particles to resin (6)
Advantages of light-cured composite (4)
Disadvantages of light-cured composite (4)
Safety considerations when light-curing (3)
Where aesthetics are important, labial veneers, indirect restorations, class III, IV and V restorations
Improves mechanical properties, improves aesthetics, improves abrasion resistance, improves radiopacity, reduces thermal expansion, lowers polymerisation shrinkage
Extended working time, less/immediate finishing, less porosity, less waste
Premature polymerisation from dental light, optimistic depth of cure values, recommended setting time too short, polymerisation shrinkage
Exothermic reaction, divergent light beam, ocular damage
What does surface roughness affect (3)
Types of composite (3)
Which is best and why
Composite tooth wear process
What does good material/tooth bonding lead to (2)
Typical composite bond strength
Appearance, plaque retention, sensation to the tongue
Hybrid, microfine, conventional
Hybrid - compromise - improved filler loading/coupling agents –> increased mechanical properties
Resin removed –> exposed filler particle - if enough resin removed, filler particle dislodged –> repeated
Reduced microleakage, counteract polymerisation contraction shrinkage
40MPa
Composite thermal properties (2)
Thermal expansion coefficient relationship between tooth and restorative materials
Other advantages of composite
Disadvantages of composite (2)
Thermal conductivity - low
Thermal expansion coefficient - high (should be equal to tooth to reduce microleakage)
Dentine < enamel = GIC < amalgam < composite
Radiopaque (allows detection of secondary caries), relatively biocompatible (if monomer fully polymerised)
Not anticariogenic, low polymerisation shrinkage
What is amalgam
Function of silver and tin in powder
Function of copper in powder
Function of zinc in powder
Function of mercury in powder
Composition of liquid
Types of amalgam particles (2)
Setting reaction of conventional amalgam
Alloy formed from mercury (liquid) and powder (other metals - tin, copper, mercury)
Intermetallic compound - gamma-phase reacts with mercury
Increases strength/hardness
Scavenger - oxidises preferentially
Faster reaction
Triple distilled reactive mercury
Lathe cut (coarse, fine, medium - formed by filling ingots), spherical (ranges of sizes, formed by spraying molten metal into inert atmosphere)
Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn7Hg9
Features of gamma phase
Feature of gamma 1 phase
Feature of gamma 2 phase
Tensile strengths (4)
Benefit of removal gamma 2
Good strength and corrosion resistance
Good corrosion resistance
Weak strength, poor corrosion resistance (voids) - most electronegative and weakens material at margins
Gamma > amalgam > gamma 1 > gamma 2
Stronger amalgam
Traditional material setting dimensional changes (2)
Modern material setting dimensional changes (2)
Why is amalgam now zinc free (2)
Initial contraction, expansion (gamma 1 crystallisation)
Small contraction, solid solution of mercury
Zinc reacts with blood/saliva, forming H gas –> expansion (pressure), pain (pressure) and protrusion of filling
Factors decreasing amalgam strength (4)
Definition of creep
What does creep affect
Thermal properties of amalgam (2)
Under-mixing, slow packing, too high mercury content after condensation, corrosion
Repeated low level stresses for long periods of time, eventually leading to permanent deformation
Marginal integrity
Thermal conductivity high (liners), thermal expansion coefficient 3x tooth tissue
How does amalgam stay in place
Other advantages of amalgam (5)
Other disadvantages of amalgam (8)
Amalgam indication for use
Amalgam contraindications for use (2)
Mechanical retention of cavity
Strong, user friendly, durable, good long term clinical performance, cheap
Corrosion, leakage, poor aesthetics, not anticariogenic, contain mercury, no bond, amalgam tattoo, lichenoid reactions (type IV hypersensitivity)
Posterior moderate/large cavity
Limited tooth tissue remaining (retention cannot be created), excessive tooth tissue removal required
Other names for copper enriched amalgam
Types (2)
Advantages over traditional amalgam (4)
Dispersion modified setting reaction (2)
Single composition setting reaction
Non-gamma 2
Dispersion modified, single composition
Higher early strength, less creep, higher corrosion resistance, increased marginal durability
Gamma + Hg –> gamma + gamma 1 + gamma 2
Gamma 2 + AgCu –> Cu6Sn5 + gamma 1
AgSnCu + Hg –> AgSnCu + gamma 1 + Cu6Sn5
Amalgam vs hybrid composite - which is higher: Compressive strength Tensile strength Elastic modulus Hardness
Relationship between amalgam, dentine and enamel for: Compressive strength Tensile strength Elastic modulus Hardness
Amalgam
Amalgam
Amalgam
Amalgam
Amalgam > dentine > enamel
Dentine > amalgam > enamel
Enamel > amalgam > dentine
Enamel > amalgam > dentine
Features of self-retentive box preparation (4)
Features of proximo-occlusal preparation (3)
Function of pins
What does finishing involve (2)
Effect of moisture contamination on amalgam restoration/cavity preparation (4)
Functions of matrices (4)
Functions of wedges (4)
Less tissue removed, sound tooth tissue retained, more challenging, further pit and fissure caries treatment may be required
Very retentive, treats pit and fissure caries, destroys tooth tissue
Increase retention in large, non-retentive cavities
Removing all caries, smooth lines, angles and margins
Reduces strength and increases creep, porosity and corrosion
Recreate cavity walls, allows creation of proximal form, allows adequate condensation, confines amalgam to cavity
Produces matrix adaptation (temporary tooth separation), prevents excess material gingivally, aids proximal wall contour, prevents movement of matrix band
Definition of microleakage
What can microleakage cause (4)
Action of condensation
Function of condensation
Effect of inadequate condensation
Definition of corrosion
Effect of corrosion
How is creep reduced
Passage of fluid and bacteria in micro gaps between restoration and tooth
Pulpal irritation, infection, discolouration, secondary caries
Expel excess mercury
Eliminates voids
Inferior mechanical properties
Detrimental change in amalgam character due to reactions in the mouth (associated with gamma 2 phase)
Marginal ridge breakdown
Copper incorporation
Uses for GIC (2)
Composition of GIC acid
Composition of GIC base
Advantage of adding strontium and lithium salts
Silica/alumina and translucency relationship
Setting reaction
Dissolution description (3) Gelation description (3) Maturation description
Liners, temporary/permanent restorations
Tartaric acid, polyacrylic acid (both control setting characteristics)
Glass powder - silica, alumina, CaF, AlF, AlPO4, NaF
Increase radiopacity
More silica, more translucent
MO.SiO2 + H2A –> MA + SiO2 + H2O
Acid added to solution, H ions interact and attack glass surface. Glass ions (Ca, Al, Na, F) leach out, leaving silica gel around unreacted glass
Initial set due to Ca ion crosslinking with polyacrylic acid (chelation with COO- groups) –> Ca polyacrylate (Ca ions bivalent - not ideally, can chelate with two COO- groups in same molecule)
Trivalent Al ensures good crosslinking, increasing strength
GIC bonding mechanisms (2)
After gelation, why must GIC be protected from moisture contamination/dessication (6)
How is conventional GIC protected following placement (3)
Mechanical properties vs composite (5)
Main advantage of GIC
Other advantages (3)
Disadvantages (3)
Chelation between COO- in cement and Ca2+ on tooth surface, H bonding/metallic ion bridging to collagen
Al diffusion out –> excessive drying –> water loss –> saliva absorption –> contamination –> material failure
Varnishes, resins (DBAs/EBAs, unfilled Bis-GMAs), greases/gels (vaseline)
Poor tensile strength, lower compressive strength, poorer wear resistance, lower hardness, higher solubility
F release - act as F reservoir (absorb/recharge F from toothpaste)
Stable chemical bond to tooth, low microleakage, good thermal properties
Brittle, poor wear resistance, poor aesthetics
Why were cermets developed
How do they do this
Do they actually do this
To overcome GI brittleness
Adding silver to glass, increasing toughness/wear resistance
No evidence
RMGICs
Composition of powder (5)
Composition of liquid (4)
Dual-curing reaction (2)
Tri-Curing reaction (3)
Properties vs GIC (3)
Disadvantages (4)
Fluoro-alumio-silicate glass, barium glass, vacuum dried polyacrylic acid, K persulphate (redox catalyst), ascorbic acid
HEMA, polyacrylic acid with pendant methacrylate groups, tartaric acid, photo-initiators
Acid/base (as GIC) on mixing and occurs for several hours; light-activation - free radical methacrylation occurs
Acid/base (as GIC) on mixing and occurs for several hours; light-activation - free radical methacrylation occurs; redox reaction initiated 5 mins after mixing (final hardening with aluminium polyacrylate can take days)
Better physical/mechanical properties, lower solubility, better aesthetics
Polymerisation contraction, exothermic setting reaction, swelling due to water uptake, monomer leaching
Impression material function
Classifications of impression materials (4) and descriptions (2)
Ideal elastic behaviour (3)
Actual elastic behaviour (3)
Produce an accurate replica of surface and shape of hard and soft oral tissues
Mucostatic (fluid materials, slightly displace soft tissues) and mucocompressive (record impression of mucosa under load - displace soft tissue) or elastic and non-elastic
When removed, material reaches maximum amount of strain almost instantly. This is held during removal. When fully removed, material instantly returns to original strain and pre-removal shape
When removed, strain increases gradually to just below the maximum amount. This is held during removal. When fully removed, the material quickly returns to just above its original strain, resulting in a permanent deformation
Types of elastic impression materials (2) and examples (2)
Examples of non-elastic impression materials (2)
What influences accuracy of impression material (5)
Properties that affect accuracy accuracy (6)
Ideal properties of an impression material (6)
Definition of colloid
Hydrocolloids (alginate - irreversible), elastomers (polyethers, silicones)
Impression compound, impression paste
Flow, setting changes, removal, storage, decontamination
Viscosity, setting mechanism, thermal expansion coefficient, hydrophobic/hydrophilic, elasticity, tear strength
Non-toxic, non-irritant, acceptable taste/smell, short setting time, simple, convenient working/setting times, can be decontaminated
2-phase system - fine particles of one phase dispersed in another phase - hydrocolloid (in water)
Alginate
Composition (7)
What does setting involve
Role of trisodium phosphate and setting mechanism (2)
Setting reaction
Requirements for correct alginate manipulation (3)
Properties
Definition of synersis
Definition of imbibition
Alginic acid, CaSO4, trisodium PO4, fillers, modifiers, flavourings, chemical indicators
Long crosslinking fibrils entangling undissolved particles
Trisodium phosphate preferentially reacts with Ca in CaSO4 to delay the set; sodium alginate then reacts with Ca ions
2Na3PO4 + 3CaSO4 –> Ca3(PO4)2 + 3Na2SO4
Correct powder/liquid ratio, water 18-24C, perforated tray/adhesive
Non-toxic, non-irritant, relatively easy to use, adequate setting time, acceptable taste/smell
Water release
Water uptake
Types of elastomeric impression materials used clinically (2)
Why are condensation silicones not used
Important material properties (8)
Normal setting time
Normal working time
Normal elastic recovery
Normal tear strength
Polyethers, addition silicones
Give off water during curing, affecting dimensions
Viscosity/flow, surface detail (ISO - 50-75um), surface wetting, elastic recovery, stiffness, tear strength, mixing time, working time
5-6mins
2-4mins
98-99.5%
1.8-9MPa
Ideal properties of dentures (5)
PMMA properties (10)
Definition of elastic limit
Ideal mechanical properties (3)
Replace function of normal teeth, fits well in mouth, dimensionally accurate, high softening temperature, unaffected by oral fluids
Non-toxic, non-irritant, low thermal conductivity, low mechanical properties, good colour, low density, high softening temperature, dimensionally accurate, stable in use
Limit at which material will return to its original shape if distorted
High YM, PL and EL
Acrylic resin bonding mechanism
Stages (4) and descriptions
Free radical addition polymerisation of a methacrylate monomer (chemical union of two molecules to form a larger molecule without the elimination of a smaller molecule)
Activation (of initiator - benzoyl peroxide - giving 2 free radicals), initiation (free radicals break monomer C=C and transfer of free radicals), propagation (growing polymer chain), termination (of polymerisation)
Composition of heat-cured PMMA powder (6)
Composition of heat-cured PMMA liquid (3)
Advantages of having to mix PMMA (2)
Requirement of heat curing
What causes internal stresses
How is it relieved
Initiator, PMMA, pre-polymerised beads, plasticiser, pigments, co-polymers
Methacrylate monomer, co-polymers, initiator
Dough-like material reduces heat of reaction and minimises polymerisation shrinkage
Efficient polymerisation (to give high polymer molecular weight)
During cooling, the mould and acrylic have different thermal expansion coefficients
Slow cooling
Disadvantages of internal stresses (2)
Effects of under-curing (2)
Effects of fast-curing
Effects of too much monomer
Effect of too little monomer
What does porosity affect (3)
How does gaseous porosity occur
What is contraction porosity and how does it occur (3)
Decreased strength, warping
Reduced mechanical properties, free monomer
Gaseous porosity
Contraction porosity
Granularity
Strength, appearance, saliva absorption (absorbs saliva)
When curing temperature exceeds 100C, the monomer boils and small bubbles are formed
Voids due to polymerisation shrinkage - too much monomer, insufficient excess material, insufficient clamp pressure
Gypsum
Uses
Definition of study cast
Purpose of creating a study cast
Types of materials (3) and their structures (3)
Setting reaction Setting mechanism Function of impurities Setting process (2) How are voids produced on completion of setting
Study casts
Positive replica (created from impression) of patients dentition
Record position/shape of teeth, aid manufacture of prostheses
Plaster (B-hemihydrate - larger, porous irregular crystals), stone (a-hemihydrate - non-porous regular crystals), improved stone/densite (compact smoother crystals)
(CaSO4)2.H2O + 3H2O –> 2CaSO4 + 2H2O
Hemihydrate dissolves, dihydrate forms. Dihydrate crystals precipitate on impurities as crystals
Impurities act as nuclei for crystallisation
Initial set - dehydrate crystals come into contact, expansion starts, weak solid formed. Final set - strength continues to develop
On completion of setting, water evaporates and voids are produced
Properties of gypsum
What affects gypsum set (4) and what effect does this have in setting time and expansion
Advantages of gypsum (2)
Disadvantages of gypsum (3)
Medium/high compressive strength, low hardness,
Increased spatulation - breaks down growing crystals, decreasing setting time, increasing expansion
Increased powder - more nuclei of crystallisation per unit volume, faster set/greater expansion
Temperature - as temperature increases, rate of diffusion of ions increases and solubility of hemihydrate decreases
Chemical additives - KSO4 produces sygenite - crystallises rapidly, decreasing setting time; borax forms Ca borate, delaying setting process
Convenient setting time, dimensionally accurate and stable
Low tensile strength, poor abrasion resistance, very brittle
Definition of metal
Definition of alloy
Definition of ductility
What affects mechanical properties (2)
Process of crystal growth (2)
Definition of grain boundary
Types of grain structures (3) with definitions (3)
Methods of crystal growth (2)
Which method of crystal growth is better and why
Function of nucleating agents
Aggregate of atoms in a crystalline structure
Combination of metal atoms in a crystalline structure
Amount of plastic deformation prior to fracture
Crystalline structure, grain size/imperfections
Atoms act as nuclei of crystallisation. Crystals grow to form dendrites and grow until they impinge on other crystals
Regions where grains make contact
Equi-axed grains (crystal growth of equal dimension in each direction), radial (molten metal cooled quickly in cylindrical mould), fibrous (wire pulled through die)
Fast cooling/quenching, slow cooling
Fast cooling - has more nuclei so produces more grains, produces small fine grains
Impurities/additives that act as foci for enhanced crystal growth
What type of grains are better
What properties do they show (4)
Fast cooling factors (4)
Small, fine grains
High EL, increased UTS, increased hardness, decreased ductility
Small bulk, heat metal/alloy just above melting temperature, mould, quench
Definition of dislocation
How do defects move along a plane and what is this due to (2)
Where do dislocations accumulate and why
How is dislocation movement impeded and what effect does impeding dislocation have
Imperfections/defects in crystal lattice –> alteration of lattice shape and structure. Weak points
SLIP - due to propagation of dislocation. Involves rupture of only a few bonds at a time
Grain boundaries because they cannot move from one grain to another
Grain boundaries, alloys, cold working - increases EL, UTS and hardness and decreases ductility and impact resistance
Definition of cold work
What can cold work cause
What does this lead to (2)
Work being done on metal/alloy (bending, rolling, swaging) at low temperatures
SLIP
A stronger, harder material and internal stresses
What can residual stress cause
How is residual stress relieved and how is this done
What eliminates internal stresses caused by cold work and how
What can occur when heated and what effect does this have on a cold worked material
Describe this process
What does this lead to (4)
How can the correct/desired shape be obtained
How is the recrystallisation temperature lowered
What is the effect of excessive temperature rises (2)
Lattice instability leading to distortion over time - undesirable
Through annealing - heating metal/alloy so that greater thermal vibrations alloy migration/re-arrangment of atoms
Stress relief annealing - allows atoms to rearrange within grains
Recrystallisation - spoils benefits of cold working
Occurs when heated, leading to smaller, equi-axed grains
Lower EL, UTS and hardness and increased ductility
Repeating recrystallisation and cold work
Greater amount of cold work
Large grains replace small grains –> poorer mechanical properties
Advantages of alloys over metals (2)
Uses of alloys (4)
Definition of phase
Definition of solution
Definition of solid solution
What do metals form upon cooling (2)
Improved properties, lower melting point
Steel, amalgam, gold alloy, NiCr
Physically distinct homogenous structure
Homogenous mixture at an atomic scale
Common lattice structure containing two metals that are soluble in one another
Intermetallic compound (insoluble), solid solution (soluble)
Types of solid solutions (2) and structures (3)
Substitutional - atoms of one metal replace the other metal in lattice/grain. Can be random or ordered (regular lattice).
Interstitial - atoms markedly different in size; smaller atoms are located in spaces in lattice/grain structure of larger atoms
What do cooling curves show
Definition of liquidus
Definition of solidus
Advantage of slow cooling
Disadvantage of slow cooling
Advantages of fast cooling (3)
Disadvantage of fast cooling
Coring conditions (2) Effect of coring
Crystallisation of metal (one temp) or alloy (temp range - TL to TS)
TL - temperature at which alloy begins to crystallise at different compositions
TS - temperature at which alloy has completely crystallised
Ensures grain composition is homogenous by allowing metal atoms to diffuse through lattice
Results in larger grains
Generates small grains, impeding dislocation movement and improving mechanical properties
Results in coring as composition varies through the grain because atoms have not diffused through lattice
Rapid cooling of liquid state, liquidus and solidus must be separate
May reduce corrosion resistance of solid alloy
What is used to reverse coring
Describe this process
Considerations for this process
Homogenising annealing
Reheat alloy after coring to allow atoms to diffuse through lattice causing grain composition to become homogenous
Keep below recrystallisation temperature otherwise grain structure will be altered
Allots forming a(n ordered) solid solution and consisting of metals of different atomic sizes have what sort of grain structure
What effect does this have (2)
Distorted grain structure
Impedes dislocation movement and improves mechanical properties
Describe dislocation movement in a metal lattice (2)
Describe dislocation movement in a solid solution (2)
Why are alloys inherently stronger (2)
Defect rolls over atoms in lattice plane. Little energy/force is required for defect to move along slip plane
Defect falls into larger space existing between large and small atoms. More energy/force is required for defect to overcome different-sized atoms and move along to the grain boundary
Greater stress is required to move dislocation in a solid solution (alloys are solid solution), causing a greater fracture resistance than metals
Definition of eutectic alloys
What do they show
Composition on a phase diagram (2)
Features of eutectic alloys (3)
Description of non-eutectic composition of eutectic alloys (3)
Contain metals that are soluble as liquids but insoluble as solids
Complete insolubility between the metals composing the alloy
Liquidus and solidus coincide, where grains of individual metals are formed simultaneously
Hard, brittle, poor corrosion resistance
Excess metal crystallises, liquid reaches eutectic composition, both metals crystallise
Features of a partially soluble alloy (2)
Definition of solubility limit line
What is formed when partially soluble alloys cool rapidly
Upon annealing, supersaturated alloys will undergo
Why do alloys have better mechanical properties than metals (3)
How is a cored structure (from rapid cooling) removed
a-phase (mostly a-rich), B-phase (mostly B-rich)
Dashed line on phase diagram - indicates the range of compositions of metals that are not possible
Grains of a and b (not a 50/50 grain composition)
Precipitation hardening
Due to solution hardening, order hardening and precipitation hardening
Annealing
Ideal properties of partial denture alloys (6)
Materials in ADA type IV gold (6)
Materials in CoCr (7)
Rigid, strong, hard, ductile, precise casting (no shrinkage), low density
Gold, zinc, copper, silver, palladium, platinum
Cobalt, chromium, nickel, molybdenum, others (carbon, zinc, aluminium)
Effects of adding copper to gold (8)
Effects of adding silver to gold (6)
Effects of adding platinum to gold (4)
Effects of adding palladium to gold (4)
Effect of adding zinc to gold
Effects of adding nickel to gold (2)
Effect of adding indium to gold
Solid solution in all proportions, solution hardening, order hardening, reduced melting point, no coring, red colour, reduced density, increased corrosion
Solid solution in all proportions, solution hardening, precipitation hardening (with Cu), tarnishing, absorbs gas when molten, whitens alloys
Solid solution (with gold), solution hardening, fine grain structure, coring
Similar to platinum but cheaper, less coring, coarser grains, absorbs gases when molten (porous casting)
Scavenger - oxidises preferentially
Increases hardness and wrought strength
Fine grain structure
Uses of CoCr (3)
Effects of adding cobalt to CoCr (5)
Effects of adding chromium to CoCr (6)
Effects of adding nickel to CoCr (3)
Effects of adding carbon to CoCr (4)
Effects of adding molybdenum to CoCr (2)
Effect of adding aluminium to CoCr
Effect of adding zinc to CoCr
Wires, surgical implants, cast partial dentures (connectors)
Forms solid solution with Cr, increased hardness, rigidity and strength, coring possible
Forms solid solution with Co, increased hardness, rigidity and strength, coring possible, forms passive layer (improves corrosion resistance)
Replaces some Co - improves ductility, slight reduction in strength, nickel allergy/sensitivity
Undesirable, carbide grain boundaries, hard, brittle
Reduces grain size, increases strength
Increases PL
Scavenger - oxidises preferentially to avoid unfavourable redox reactions of other metals
CoCr techniques (3) and descriptions (3)
CoCr finishing involves (4)
Is CoCr softer or harder than gold and what are the effects (2) of this
Definition of elongation
Relationship between elongation, ductility and CoCr
CoCr work hardens rapidly, so what is used to make adjustments
Investment material (1200-1400C - silica/PO4 bounded), melting (electric induction preferred - avoids C pickup), casting (centrifugal forces required; overheating –> coarse grains; cooling too fast/slow –> brittle carbides)
Sandblast, electroplate, abrasive wheel, polishing buff
Harder - improved wear, but time-consuming finishing or polishing
Amount of strain a material can experience before failing in tensile testing - lower elongation, less ductile
CoCr has low elongation and therefore low ductility
Precision casting
Uses of titanium (4)
Titanium prostheses preparation involves (2)
Advantages of titanium (2)
Disadvantage of titanium
Implants, partial dentures, crowns and bridges, maxillofacial implants
Electric arc melting, specialised investment/casting
Biocompatible, good corrosion resistance Absorbs gases (when molten)
Relationship between stainless steel, CoCr, gold and titanium in regards to:
YM/elastic modulus (4) Shrinkage (2) Melting range (3) Density (4) Proportional limit (4) UTS (4) Elongation (4) Hardness (4)
SS >Ti > CoCr > gold
Gold > CoCr
Ti > CoCr > gold
Ti > SS = CoCr > gold
SS > CoCr > Ti > gold
SS > Ti > CoCr > gold
Au = Ti > CoCr > SS
CoCr > SS > Ti > gold
Setting reactions and reaction processes
Amalgam - traditional
Amalgam - copper-enriched (dispersion modified and single composition types)
GIC
Alginate
Role of trisodium phosphate
Gypsum
Factors affecting reaction (4)
Acrylic resin polymerisation stages
Role of camphorquinone in composite resins
Amalgam - traditional -
Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn7Hg9
Amalgam - copper-enriched - dispersion modified -
G + Hg –> G + G1 + G2
G2 + AgCu –> Cu6Sn5 + G1
Amalgam - copper-enriched - single composition -
AgSnCu + Hg –> AgSnCu + G1 + Cu6Sn5
GIC - MO.SiO2 + H2A –> MA + SiO2 + H2O
Dissolution, gelation, maturation
Alginate - 2Na3PO4 + 3CaSO4 –> Ca3(PO4)2 + 3Na2SO4
Trisodium phosphate preferentially reacts with Ca in CaSO4 to delay the set
Gypsum -
(CaSO4)2.H2O + 3H2O –> 2CaSO4 + 2H2O
Increased spatulation, increased powder, temperature, chemical additives
Fee radical addition polymerisation of a methacrylate monomer
Stages - activation , initiation, propagation, termination
Camphorquinone in composite resins - photo-activator catalyst that initiates resin polymerisation when activated by blue light (430-490nm)
