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3. BDS2 CDS Revision > 1. DMS > Flashcards

1. DMS Flashcards

1
Q

Stress calculation
Strain calculation
YM relates to material
Material failure mechanisms (8)

Blacks cavity classifications (6)

A

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
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2
Q

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

A

Mechanical
Acid-etch
Micromechanical interlocking of resin-filled material and increase surface energy
Better gettability and adaptation of resin to surface

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3
Q

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

A

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

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4
Q

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

A

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

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5
Q

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

A

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

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6
Q

What does bonding involve and how does this work (2)

What is the AD concept
How does it work (2)

A

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.

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7
Q

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)

A

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

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8
Q

CaOH

Components of base (4)
Components of catalyst (4)

Setting reaction

Action (2)

Advantages (2)
Disadvantages (2)

A

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

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9
Q

ZnPO4

Type of reaction (2)
Powder components
Liquid components

Initial setting reaction
Final setting reaction

Function of AlO

Problems (5)

A

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

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10
Q

Zn polycarboxylate

Difference between this and ZnPO4

Action
Problems (3)

A

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

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11
Q

ZOE

Uses (4)
Basic reaction
Setting reaction (3)

Properties (5)

A

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

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12
Q

RMZOE

What does adding resin do
Advantages of RMZOE (2)

A

Strengthen backbone to set material - increases compressive strength (>40MPa)

Increased compressive strength, decreases solubility

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13
Q

EBA ZOE

Components of powder (2)
Components of liquid (2)
Setting reaction
Feature of adding EBA
Advantages of EBA ZOE (2)
A

ZnO, quartz/alumina - reinforcing hydrogenated rosin

Eugenol, EBA - reactive

Similar set to ZOE
EBA encourages crystalline structure –> greater strength
Less soluble and stronger

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14
Q

Advantages of GI liners (5)

Feature of RMGIC

Advantages of RMGIC (4)

Disadvantage

A

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

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15
Q

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

A

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

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16
Q

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)

A

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

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17
Q

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

A

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

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18
Q

Composite thermal properties (2)

Thermal expansion coefficient relationship between tooth and restorative materials

Other advantages of composite

Disadvantages of composite (2)

A

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

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19
Q

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

A

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

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20
Q

Features of gamma phase
Feature of gamma 1 phase
Feature of gamma 2 phase

Tensile strengths (4)

Benefit of removal gamma 2

A

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

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21
Q

Traditional material setting dimensional changes (2)
Modern material setting dimensional changes (2)

Why is amalgam now zinc free (2)

A

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

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22
Q

Factors decreasing amalgam strength (4)

Definition of creep
What does creep affect

Thermal properties of amalgam (2)

A

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

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23
Q

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)

A

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

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24
Q

Other names for copper enriched amalgam
Types (2)

Advantages over traditional amalgam (4)

Dispersion modified setting reaction (2)
Single composition setting reaction

A

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

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25
Q 
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
A

Amalgam
Amalgam
Amalgam
Amalgam

Amalgam > dentine > enamel
Dentine > amalgam > enamel
Enamel > amalgam > dentine
Enamel > amalgam > dentine

26
Q

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)

A

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

27
Q

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

A

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

28
Q

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
A

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

29
Q

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)

A

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

30
Q

Why were cermets developed
How do they do this
Do they actually do this

A

To overcome GI brittleness
Adding silver to glass, increasing toughness/wear resistance
No evidence

31
Q

RMGICs

Composition of powder (5)
Composition of liquid (4)

Dual-curing reaction (2)
Tri-Curing reaction (3)

Properties vs GIC (3)

Disadvantages (4)

A

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

32
Q

Impression material function

Classifications of impression materials (4) and descriptions (2)

Ideal elastic behaviour (3)
Actual elastic behaviour (3)

A

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

33
Q

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

A

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)

34
Q

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

A

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

35
Q

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

A

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

36
Q

Ideal properties of dentures (5)

PMMA properties (10)

Definition of elastic limit

Ideal mechanical properties (3)

A

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

37
Q

Acrylic resin bonding mechanism

Stages (4) and descriptions

A

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)

38
Q

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

A

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

39
Q

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)

A

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

40
Q

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
A

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

41
Q

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)

A

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

42
Q

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

A

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

43
Q

What type of grains are better
What properties do they show (4)

Fast cooling factors (4)

A

Small, fine grains
High EL, increased UTS, increased hardness, decreased ductility

Small bulk, heat metal/alloy just above melting temperature, mould, quench

44
Q

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

A

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

45
Q

Definition of cold work
What can cold work cause
What does this lead to (2)

A

Work being done on metal/alloy (bending, rolling, swaging) at low temperatures
SLIP
A stronger, harder material and internal stresses

46
Q

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)

A

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

47
Q

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)

A

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)

48
Q

Types of solid solutions (2) and structures (3)

A

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

49
Q

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
A

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

50
Q

What is used to reverse coring
Describe this process

Considerations for this process

A

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

51
Q

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)

A

Distorted grain structure

Impedes dislocation movement and improves mechanical properties

52
Q

Describe dislocation movement in a metal lattice (2)

Describe dislocation movement in a solid solution (2)

Why are alloys inherently stronger (2)

A

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

53
Q

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)

A

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

54
Q

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

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

55
Q

Ideal properties of partial denture alloys (6)

Materials in ADA type IV gold (6)
Materials in CoCr (7)

A

Rigid, strong, hard, ductile, precise casting (no shrinkage), low density

Gold, zinc, copper, silver, palladium, platinum
Cobalt, chromium, nickel, molybdenum, others (carbon, zinc, aluminium)

56
Q

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

A

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

57
Q

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

A

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

58
Q

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

A

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

59
Q

Uses of titanium (4)
Titanium prostheses preparation involves (2)

Advantages of titanium (2)
Disadvantage of titanium

A

Implants, partial dentures, crowns and bridges, maxillofacial implants

Electric arc melting, specialised investment/casting

Biocompatible, good corrosion resistance
Absorbs gases (when molten)
60
Q

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)
A

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

61
Q

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

A

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)

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