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BDS stage 2 > DMT > Flashcards

DMT Flashcards

1
Q

What is the range of dental materials ?

A

polymers
ceramics
metals and alloys
inorganic salts

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2
Q

What are the properties of polymers ?

A

soft and flexible

can be deformed

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3
Q

What is a metal ?

A

an element

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4
Q

What is an alloy ?

A

mixture of metals

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5
Q

What are inorganic salts used for ?

A

impressions and root fillings

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6
Q

What is the success of treatment dependent on ?

A

selection of material
use of material
quality of material

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7
Q

What is the success of treatment dependent on ?

A

selection of material
use of material
quality of material

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8
Q

What guarantees safety of material ?

A

C e mark

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9
Q

What are the 2 factors that determine use of a material ?

A

does it meet requirements
is it available
choose by seeing if it meets

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10
Q

How is testing of materials carried out ?

A

products should have been evaluated and quality assurace programme set ot

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11
Q

What are standard specficiations ?

A

ISO and BSI Standards

dont cover all aspects though

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12
Q

What are laboratory evaluations ?

A

test sustaianability

ensure methods are given that correlate with clinical experience

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13
Q

What are clinical trials used for ?

A

RCTs and verdict of practitioners after use- some materials produce good lab results but lack clinical use

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14
Q

What are the stages to selecting and using materials ?

A 
diagnosis 
what are the requirements of the materials
look at available materials and compare 
selection 
review
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15
Q

What is important to consider during storage of materials ?

A

shelf life- do not use if exceeds shelf life

temperature sensitive and light sensitive

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16
Q

What is the ideal material for mixing and manipulation ?

A

should be easy to proportion

not drip or stick to instruments

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17
Q

What are the methods of mixing ?

A

hand mixing- cheap and technique sensitive
mechanical mixing- capsule but less technique sensitive
no mixing- need to store well to prevent mixing - lowest technique sensitivity

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18
Q

What is the working time ?

A

time between start of mixing and material can no longer be effectively ised
measured at RT
eg time between impression material being mixed and when it can no longer be set

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19
Q

What is the setting time ?

A

Time between start of mix and achieving the minimum properties for fucntion
not 100% reaction over
measured at appropriate temperature

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20
Q

What is rheology ?

A

study of flow and deformation of materials

refers to viscosity

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21
Q

What does high viscosity mean ?

A

more gloopy

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22
Q

What is viscosity related to ?

A

pressure and rate

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23
Q

What does it mean if a fluid exhibits newtonian behaviour ?

A

flow rate does not change with pressure
k=1
viscosity does not increase with shear rate

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24
Q

What does it mean if viscosity decreases with shear rate ?

A

pseudoplastic
k<1
ketchup- more you shake the more watery it gets

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25
Q

What does it mean if viscosity increases with shear rate ?

A

dilatant

k>1

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26
Q

What does it mean if a material is thixotropic ?

A

dont flow until sufficinet pressure applied

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27
Q

Which type of viscosity is easier to mix ?

A

lower

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28
Q

Why is high viscosity good sometimes ?

A

stops spillages when transferring

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29
Q

When is viscosity important to consider ?

A

viscosity needed- to adapt to cavities- low

impression material- low - flow and take detail

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30
Q

Why are some materials hard to mix ?

A

high initial viscosity

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31
Q

What can be done to counteract an increase in viscoity during setting ?

A

solvents- reduce viscosity

retarders- increase working time and reduce the increase in viscosity

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32
Q

What is the nature of temperature during setting ?

A

setting is an exothermic reaction

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33
Q

What happens during an exothermic reaction ?

A

energy released
max temperature reached
setting reaction slows down

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34
Q

What is the time from mixing to reaching max temperature ?

A

setting time

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35
Q

How are thermal and dimensional changes linked ?

A

increase in temperature can lead to porosity - lower strength and integrity of material

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36
Q

How is porosity caused ?

A

materials vapourise at high temperature
leave gaps
filled with air bubbles

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37
Q

Are enamel and dentine conductors or insulators ?

A

insulators - protect pulp from temperature change

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38
Q

What type of material should filling materials be ?

A

filling materials should be good insulators

when setting exo reaction can damage pulp

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39
Q

When does expansion happen ?

A

crystal growth in the setting of amalgum and gypsum products
damage restorations
inaccuracy in fitting of crowns

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40
Q

When does contraction happen ?

A

polymerisation

shrinking can lead to gaps and increase risk of secondary caries and marginal staining

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41
Q

What influences the oral environment ?

A

pH and temperature

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42
Q

What can cause pH fluctuations in the mouth ?

A

plaque
acidic rinks
alkaline mediction

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43
Q

What happens if the pH falls below the critical pH ?

A

saliva becomes unsaturated and enamel dissolves

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44
Q

High stress in the form of a large force lead to ?

A

fracture

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45
Q

What do low stresses over time lead to ?

A

fatigue

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46
Q

What can abrasive factors do ?

A

abrasive food and toothpaste can lead to softening and reduced integrity of enamel

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47
Q

What does biocompatibility of a material refer to ?

A

whether its toxic
irritant
allergies

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48
Q

What are the chemical properties of set materials ?

A

solubility
corrosion
leaching

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49
Q

What does accuracy of a material refer to ?

A

impression materials

how good it can replicate oral anatomy

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50
Q

What does durability refer to ?

A

how long it can survive in the mouth before needing to be replaced

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51
Q

What does conservation mean ?

A

minimally invasive
preserve natural dentition
of all teeth- opposing teeethe tc

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52
Q

How can restorations change colour ?

A

over time naturally

diet and abrasion factors

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53
Q

What must we consider with material solubility ?

A

materials can be water soluble, alcohol soluble and saliva soluble
to be durable- must have low solubility

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54
Q

What is leaching ?

A

loss of one components into a liquid likemsaliva

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55
Q

What is positive leaching ?

A

loss of one component into another- GIC fluoride leaches into saliva and can from fluorapatite

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56
Q

What is negative leaching ?

A

plasticisers from denture liners leach out and are uncomfortable to wear

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57
Q

Which materials does corrosion occur in ?

A

metals and alloys

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58
Q

What is a galvanic cells and how is it formed ?

A

when 2 metals comes into contact in an electrolyte

flow of electrons

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59
Q

What is an elecetrolyte ?

A

ionic solution

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60
Q

What can act as an electrolyte ?

A

saliva- can corrode amalgam fillings

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61
Q

What are the consequences of corrosion of amalgam fillings ?

A 
loss of metal ions 
metallic taste
weakening of restoration 
release of electrons to pulp causes pain 
release of mercury
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62
Q

The amount of expansion and contraction is ….

A

equal and opposite

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63
Q

How can we measure thermal expansion and contraction ?

A

coefficient of thermal expansion

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64
Q

What is the meaning of thermal conductivity ?

A

rate of heat flow per unit temperature gradient

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65
Q

What needs to be cosnidered about amalgam being a conductor ?

A

a conductor- when using in a filling use an insulating liner to stop the feeling of pain

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66
Q

What materials are filling insulating linings made from ?

A

zinc phosphate

zinc oxide

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67
Q

Composites amde from acrylic resin are ?

A

insulators- dont need liner

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68
Q

What is the disadvantage about thermal conductivity ?

A

only measures static temperature

oral temperature changes over time

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69
Q

What is thermal diffusivity ?

A

indicates the way in which a material will respond to transient thermal stimuli
how quickly will the material respond to temperature changes
heat required to change the temperature of body before it can be conducted

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70
Q

What does a low diffusivity mean ?

A

takes a long time to reach that temperature

low diffusivity and conductivity is desirable in materials

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71
Q

What is a high diffusivity ?

A

temperature reached afte rhte stimulus

needed for dentures to replace palate sensation

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72
Q

How are conductivity and diffusivity mesured ?

A

using a thermcouple in test material

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73
Q

What is a force ?

A

outside agency acting on body to change momentum

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74
Q

How do we calculate force ?

A

weight x acceleration

weight= mg

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75
Q

What is stress ?

A

internal forces set up in a body to oppose externally applied force
like the internal forces set up to oppose biiting forces

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76
Q

What is the stress resisting a tensile force called ?

A

tensile stress

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77
Q

How can the magnitude of the stress be calculated ?

A

applied force/ dimensions of area applied to

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78
Q

What are the simple stresses ?

A

tensile
compressive
shear

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79
Q

What are the complex stress types ?

A

flexural
torsional
diametral

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80
Q

What is a flexural stress ?

A

in a denture - mix of compression and tensile

not supported in middle

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81
Q

What is strength ?

A

the maximum stress that can be withstood by a material before breakage

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82
Q

What is strain ?

A

applying an external force can lead toa change in dimensions

like tensile stress can lead to extension

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83
Q

How can strain be calculated ?

A

change in length/original lengtj

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84
Q

What is elastic deformation ?

A

material returns to original dimensions when force is removed

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85
Q

What is plastic deformation ?

A

material is permanently damaged after force is removed

useful for fillings

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86
Q

What is viscoelastic deformation ?

A

force is removed
returns partially to original dimensions
takes a long time to return to original dimesions

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87
Q

What is used as a model for elastic deformation ?

A

spring

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88
Q

What is used as a model for plastic deformation ?

A

dashpot

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89
Q

What are the 2 models for viscoelastic deformation ?

A

maxwell

voigt

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90
Q

What is the maxwell model ?

A

dashpot and spring
load deforms
load removed and the dashpot is permnenantly deformed
cant return to original dimensions

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91
Q

What is the voigt model ?

A

application of a load

removal of the load and slow return to original state

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92
Q

What is stiffness ?

A

a measure of how resistant something is to deformation

higher stiffness-more rigid and harder to deform

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93
Q

What is the modulus of elasticity ?

A

on stress/strain graph

gradient of straight line

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94
Q

What does the modulus of elasticity tell us ?

A

how difficult it is to deform

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95
Q

What is the ultimate tensile strenght ?

A

pulling at this strength leads to deformation

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96
Q

What is the yield stress ?

A

point at which something stops elastic deformation and is now deformed plastically
required to permenantly deform soemthing
change in gradient

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97
Q

What is used instead of yiels stress ?

A

proof stress

easier to measure

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98
Q

What is malleability ?

A

how much something can be squahed to failure

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99
Q

What is resilience ?

A

how much energy something can take before it permenanatly deforms
area under the black line

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100
Q

What is toughness ?

A

how much energy something can take before it fails

area under whole gra[h

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101
Q

What is ductile behaviour ?

A

can be deformed by large amounts
leads to thinning called necking
deformed elastically and then plastically

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102
Q

What is brittle behaviour ?

A

can only be deformed by small amount
deformed elastically
smallest amount of stress can fail them

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103
Q

Are materials ductile or brittle ?

A

all materials are ductile or brittle depending on temperature and shear rate

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104
Q

How can toughness be measured ?

A

total area under the stress/strain graph

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105
Q

For something to break what must be initiated ?

A

a crack must be initiated and then propagated across the material

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106
Q

What type of test is used for toughness ?

A

an impact test

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107
Q

What happens in the toughness test ?

A

potential energy in pendulum converted to kinetic energy to fracture the specimen

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108
Q

What does the impact test compare ?

A

compares the energy required to break notched and unnotched specimens
energy to break unnotched»
notched specimen is notch sensitive

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109
Q

What does notch sensitive mean ?

A

cracks and scratches break the material easier

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110
Q

What is fatigue ?

A

materials can fail due to repeated cycles of stress

they fail at stress lower than strength

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111
Q

Where is fatigue common ?

A

in dentures- flexural stress down the midline mean they fracture

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112
Q

What is fatigue life ?

A

the maximum cyclic stress a material can take until failure

number of cycles

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113
Q

What is fatigue limit ?

A

the value of cycle stress needed to cause a fracture

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114
Q

What is hardness ?

A

how likely a material will be scratched

resistance to indentation under load

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115
Q

What is the significance of scratches ?

A

they can be colonised by bacteria

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116
Q

How can hardness be measured ?

A

VHN

KHN

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117
Q

What is the significance of porcelain being harder than enamel ?

A

porcelain being harder means it can damage the opposing tooth if part of a crown

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118
Q

What are synthetic polymers ?

A

based on petrochemicals

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119
Q

What are polymers ?

A

made of regularly repeating units called monomers
covalently bonded
as monomers join the molecular weight increases

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120
Q

What are cross linked and linear polymers ?

A

cross linked- links between polymer chains

linear- no cross links

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121
Q

What is a homopolymer ?

A

contains only 1 type of monomer

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122
Q

What is a copolymer ?

A

2 or more monomers

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123
Q

What are the different ways monomers in copolymers can join ?

A

randomly
regularly
block

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124
Q

What is addition polymerisation ?

A

addition of 2 molecules to form a third bigger molecule

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125
Q

What is condensation polymerisation ?

A

tow molecules join to form a larger molecule and a by product

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126
Q

What happens in addition polymerisation ?

A

reaction of a reactive species with a monomer to form a larger reactive species
chain lengthening and formation

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127
Q

What are the reactive species used in addition polymerisation ?

A

ion
free radial
with a spare electron

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128
Q

What are the 4 steps of addition polymerisation ?

A

activation
initiation
propagation
termination

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129
Q

How are reactive species formed ?

A

from initiators- molecules with weak bonds where heat and light is applied

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130
Q

What are initiators ?

A

molecules with weak bonds that can be broken down to make reactive species via heat and light

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131
Q

What are vinyl polymers ?

A

have C-C double bonds

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132
Q

Minor changes in R groups of monomers can lead to ?

A

different polymers

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133
Q

What is activation ?

A

decomposition of benzyl peroxide by thermal or light energy

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134
Q

What is initiation ?

A

Polymerisation is initiated when the monomer reacts witth the radical

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135
Q

How can benzoyl peroxide act as an initiator ?

A

O-O bond broken down to give 2 radicals

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136
Q

What is propagation ?

A

newly formed radical reacts with further monomer to produce another radical
increases chain length and average molecular weight reached

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137
Q

What is termination ?

A

reactions produce dead polymer which arent capable of further reaction

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138
Q

What happens to viscosity as polymers are made ?

A

increases- making it harder to join

leaves residual monomer

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139
Q

What can termiante polymerisation ?

A

impurities like oxygen

residual monomers - premature

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140
Q

What does cross linking do ?

A

joins chains together to form a network

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141
Q

What happens to properties with cross linking ?

A

polymer is harder to deform
stiffness increases
improve stability in liquids

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142
Q

What are the properties of linear polymers ?

A

flexible

lower stiffness

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143
Q

What does cross linking require ?

A

difunctional monomers

have more than 1 C-C double bond

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144
Q

Give an example of condensation polymerisation ?

A

esterification reaction between alcohol and acid produces water and estre

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145
Q

How many reactive groups are needed for AP, CP and CS ?

A

1
2
3

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146
Q

Which materials set by condensation polymerisation ?

A

silicone impression material
ethanol evaporated quickly
changes dimensions

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147
Q

What are the changes that occur during polymrisation ?

A

monomers usually fluids

polymerisation increases viscosity

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148
Q

What are thermal changes that occur in polymerisation ?

A

it is an exo reaction
temperature rise is proportional to volume
continual energy releases- runaway

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149
Q

What is the consequence of the temperature getting too high in polymerisation ?

A

monomer vapourises
air bubbles form
weakness leads to porosity

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150
Q

What are the dimensional changes that occur in polymerisation ?

A

polymers are denser than monomers

polymerisation lead to shrinkage

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151
Q

What is the consequence of shrinkage ?

A

gaps forming

staining and secondary caries

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152
Q

What is contraction porosity ?

A

conraction leads to weaknesses

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153
Q

How can we counter act with contraction ?

A

overfilling

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154
Q

What are the methods used to reduce polymer shrinkage ?

A

BisGMA- bigger than MMA- bigger monomers take up more space leading to less shrinkage

Fillers- fillers dont polymerise-

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155
Q

What are the 2 structures of polymers ?

A

amorphous

crystalline

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156
Q

What are amorphous polymers ?

A

randomly arranged

short range order

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157
Q

What are crystalline polymers ?

A

regularly arranged

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158
Q

What is a short range order ?

A

regular and predictable arrangement of monomers

monomers are sitting next to same type

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159
Q

What is a long range order ?

A

atoms show periodicity over large distances

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160
Q

Can crystalline polymers melt ?

A 
yes
they have a melting temperature 
melting leads to a loose chain arrangement 
loss od long range roder 
cooling can recover long range order
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161
Q

Do amorphous polymers melt ?

A

they burn at high temperatures

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162
Q

Which temperatures do crystalline polymers have ?

A

Tg

Tm

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163
Q

Which temperatures do amorphous polymers have ?

A

Tg

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164
Q

What are thermoplastic polymers ?

A

soften on heating
harden on cooling
crystalline

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165
Q

What are thermosetting polymers ?

A

dont soften on heating
harden on setting
cross linked

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166
Q

Can thermoplastic or thermosetting polymers be recycled ?

A

thermoplastic only

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167
Q

What is the Tg ?

A

temperature which they change from glassy to rubbery

easier to defrom

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168
Q

What happens at the Tg ?

A

10 x reduction in modulus of elasticity

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169
Q

What are polymers like in the mouth if their Tg is above mouth temperature ?

A

stay rigid in the mouth

good for dentures and fillings

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170
Q

What are polymers like with a Tg below mouth temoerature in the mouth ?

A

rubbery in the mouth

good for impression materials

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171
Q

How does molecular weight affect Tg ?

A

higher the molecular weight the greater the Tg

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172
Q

How does the amount of residual monomer affect molecular weight ?

A

lower the amount of residua monomer the greater the Tg

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173
Q

What type of polymers do C-C bonds produce ?

A

vinyl

rigid

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174
Q

What type of polymers do Si-O bonds make ?

A

flexible polymers

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175
Q

How do co polymers affect Tg ?

A

the Tg will depend on the specific combination of monomers

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176
Q

What are pendant groups ?

A

hang off the polymer chain

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177
Q

What do bigger pendant groups produce ?

A

produce more rubbery polymers

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178
Q

What do pendant groups do ?

A

push chains further apart

make it easier to remove chains

179
Q

How do pendant groups affect Tg ?

A

pendant groups reduce Tg

180
Q

How does the amount of initiator affect the Tg ?

A

more initiator produces more chains
these have a low molecular weight
reduce Tg
easy to deform

181
Q

What are activators ?

A 
tertiary amine 
unstable at RT 
eg- DMPT 
donate electrons allowing weak bonds to form 
control chain growth 
too much reduces chain length- low Tg
182
Q

How does cross linking affect Tg ?

A

the more cross links the greater the Tg

183
Q

What are plasticisers ?

A

they are added to reduce Tg

make more flexible

184
Q

What can act as a plasticiser ?

A

residual monomer

185
Q

What are fillers ?

A

change properties of polymer

186
Q

What are the methods of fabrication of polymers ?

A

injection moulding- high temperature and pressure
compression moulding- mould
dough moulding- powder and liquid
paste - pastes placed in mould

187
Q

What are some of the uses of metals and alloys ?

A

denture frameworks
implants
crowns/bridges/inlays

188
Q

What is a direct use of metals and alloys ?

A

fillings- mixed and then placed into cavity

189
Q

What are indirect uses of metals and alloys ?

A

made by the technician- crowns

190
Q

What are the characteristics of metals ?

A

dense
crystalline structure
conductors of heat and electricity
opaque

191
Q

Why is it important metals are strong ?

A

difficult to break

instruments need to be strong to cut into teeth

192
Q

Why is it important metals are stiff ?

A

theyre not easily deformed

193
Q

Why is it important metals are tough ?

A

need a lot of energy to shatter them

194
Q

Why is it important metals are strrong, tough and stiff ?

A

withstand the high forces of masticaton and biting

195
Q

Why is it important metals are strong ?

A

they are easily scratched

196
Q

Why is it important metals are lustrous ?

A

dont easily loos polish

197
Q

What is the problem with metals and alloys being conductors of heat ?

A

they can conduct heat to the pulp

important to put insulating liner in

198
Q

What is the crystalline structure of metals and alloys ?

A

they form crystals
atoms are defined
long and short range order

199
Q

What is the colour of metals and alloys ?

A

opaque

dont add pigments to the m

200
Q

What can corrosion do to metals and alloys ?

A

weaken the structure
change colour
form oxides that flake off
eg. Copper oxide originally orange copper oxide is green and flakes

201
Q

What is a metal ?

A

an element

202
Q

What is an alloy ?

A

combinations of 2 or more metals

can also be a combination of metals and non metals

203
Q

What is amalgam a combination of ?

A

Hg, Ag, An, Cu and Zn

204
Q

What is stainless steel a combination of ?

A

Fe, Cr, Ni and C

205
Q

What are the 3 ways of shaping metals ?

A

casting
working
amalgamation

206
Q

What is casting ?

A

metal is melted and molten metal is poured into mould

207
Q

What is working ?

A

metal is soft and can be hammered into shape
use the ductility and malleability of metals
simple shapes originally now complex shapes can be made by CAD and CAM

208
Q

What is amalgamation ?

A

mixed with mercury
form a plastic mass which gradually hardens by cooling
material flows because it mixes with mercury

209
Q

What is the temperature at which metals turn into liquid ?

A

Tm

210
Q

What is the state of metals above Tm ?

A

liquid

211
Q

What is the state of metals below the Tm ?

A

solid

212
Q

Is melting reversible ?

A

yes

213
Q

What happens to atoms when metals cool ?

A

the atoms get into an increasingly confined space and start bumping into each other
solidifcation occurs

214
Q

What are the distinct phases of solidification ?

A

decrease temperature
atoms vibrate less and move closer together
groups of 4 atoms begin to form and join together
crystal growth starts and small areas of solid to form
plateau- temperature remains constant in crystallisation in metals
cooling down continues

215
Q

Why does temperature remain constant during crystallisation ?

A

energy loss from cystallsiation is equal to energy loss from decreasig temperature

216
Q

Where does solidifcation occur ?

A

metals solidify onto solid surfaces

like moulds

217
Q

What are seeds and nuclei ?

A

these usually have a Tm above the solidifying Tm
they are solid
they are seeds/nuclei that act as places for initiation to start

218
Q

What are seeds/nuclei generally ?

A

impurities

219
Q

What do crystals grow as ?

A

dendrites

3D structures

220
Q

What are dendrites ?

A

tree branches

atoms join onto branches

221
Q

Why are metals known as equiaxed ?

A

equal in all directions from central nuclei

222
Q

What are crystals also known as ?

A

grains

223
Q

What are the boundaries between crystals ?

A

grain boundaries

224
Q

What happens within the grains ?

A

the atoms stack into planes
atomic planes
3D lattices

225
Q

What are the 3 different cystals structures ?

A

simple cubic
face centred cubic
body centred cubic

226
Q

What is a simple cubic structure ?

A

metals and alloys are stacked
atoms are hard to stack in planes because they dont have ionic bonds
metals and alloys dont usually simple cubic structure

227
Q

What is a body centred cubic ?

A

atoms stack into gaps between adjacent atoms
cant stack a lot of atoms
they are far apart

228
Q

What is face centred cubic ?

A

stacking leads to more gaps forming

fits a lot more atoms

229
Q

What type of structure does chromium form ?

A

body centred cubic

230
Q

What does aluminium, copper, gold and nickel form ?

A

face centred cubic

231
Q

What is the significance of different metals having different structures ?

A

hard to mix together

232
Q

What type of structure is titanium ?

A

hexagonal closed packed

233
Q

What is the smallest metal in a unit called ?

A

unit cell

234
Q

How can metals change unit cell ?

A

by heating and cooling down

235
Q

What leads to defects in grains ?

A

cooling down metals

its a random and rapid process

236
Q

What are point defects ?

A

defects in specific point in plane

impurities and vacancies

237
Q

What are impurities ?

A

atoms of a different element in the plane
might be from the reaction vessel
different sized atom

238
Q

What are vacancies ?

A

gap in plane formed during solidification

239
Q

What are defects called that effect whole planes ?

A

dislocations or line defects

odd number of planes

240
Q

How does solidifcation start in casting ?

A

the mould is at a lower temperature than the metal

241
Q

Which material type are moulds made from ?

A

insulators

242
Q

How can we make the cooling down process faster ?

A

placing the mould into liquid

243
Q

What is the name of the process for when a mould is placed into liquid ?

A

quenching

244
Q

What does the speed of cooling down effect ?

A

how many grains are formed

245
Q

What happens as the cooling speed increases ?

A

groups of atoms join up

246
Q

What does quenching produce ?

A

more grains per volume
smaller grains
more grain boundaries

247
Q

What is the yield strenght ?

A

the point at which deformation becomes plastic

248
Q

What is the ideal yield stress ?

A

ideally you want yield stress to be higher than biting force

249
Q

What does the Hall-petch equation relate ?

A

yield strength is inversely related to the square root of the grain size

250
Q

What is the relationship between grain diameter size and yield strength ?

A

the smaller the diameter of the grain the greater the yield strength - harder to plastically deform

251
Q

What does quenching do to grain diameter ?

A

reduces grain diameter

252
Q

Describe the process of recrystallisation ?

A

Increasing temperature of metals leads to an increase in their atomic distance
atomic vibration increases and the diffusion rate increases
atoms eventually jump over grain boundaries and join other planes leading to an increase in grain growth

253
Q

What does an increase in grain growth mean ?

A

there are less grains per unit volume
bigger grains
less grain boundaries

254
Q

What does a reduction in grain boundaries lead to ?

A

reduction in yield strenght

255
Q

Is the RcT lower than Tm usually ?

A

yes

like 30-70% of the Tm

256
Q

Which state does recrystallisation happen in ?

A

solid

below the Tm

257
Q

What is diffusion rate like ?

A

low diffusion rate below the RcT

high diffusion rate above the RcT

258
Q

What type of alloy is amalgam ?

A

quinary

259
Q

What type of alloy is stainless steel ?

A

quarternary

260
Q

What is an alloy system ?

A

all possible combinations of metal proportions within the alloy

261
Q

Are alloys or metals more expensive ?

A

metals

262
Q

Are metals or alloys harder /?

A

alloys

263
Q

Are metals or alloy stronger ?

A

alloys

264
Q

Do alloys have a Tm ?

A

no they have a melting range

265
Q

What is the melting range of an alloy ?

A

the highest Tm and the lowest Tm of the components

266
Q

What happens to atoms in liquid alloys ?

A

the atoms cam move around each other and can be combined

267
Q

What are the 4 different types of solids that alloys can form ?

A

solid solutions
insoluble metals
partial solubility
intermetallic compounds

268
Q

What are solid solutions ?

A

the atoms of different components are happy to form planes

269
Q

What is an example of a solid solution ?

A

gold and silver

270
Q

What is an insoluble metal ?

A

the atoms of one of the metals wont from planes with the other
the solutions can segregate out

271
Q

What is an example of insoluble metals ?

A

lead-tin

272
Q

What is partial solubility ?

A

the atoms are happy to form planes up to certain concentrations

273
Q

What is an example of partial solubility ?

A

copper and silver

274
Q

What is an intermetallic compound ?

A

the atoms form ionic compounds instead of metallic compounds

275
Q

What are the properties of intermetallic compounds ?

A

hard and brittle

276
Q

What are the factors determining if a solid solution will form ?

A

relative size of atoms
relative electronegativity
relative valency
crystal structure

277
Q

How does the relative size of the atoms effect solid solution formation ?

A

there must be a small difference in the size of atoms to reduce the amount of distortion - less than 15%

can also have a very large difference between atom size - more than 59% - this allows smaller atoms to form in the gaps- this forms interstitial solid solutions

278
Q

How does the relative electronegativity effect solid solution formation ?

A

large differences in electronegativity mean the alloy is more likely to form ionic bonds and intermetallic structures

279
Q

How does the relative valency effect solid solution formation ?

A

valecny is the availability of electrons for reaction
equal valnecy needed for full solubility
bigger the difference in valency thelow valency metal will dissolve in high valency metal

280
Q

How does the crystal sturcture effect solid solution form ?

A

metals must have the same crystal structure for planes to form.

281
Q

What is shown on a phase diagram ?

A

possible combinations of metals as alloys
liquidous line- above is liquid
solidous- below is solid
S+L

282
Q

What does a phase diagram show ?

A

depending on the proportion of A/B in the alloy- solidifiying starts and finishes differently

283
Q

What is coring ?

A

cooling down produces grains with different compositions which make up the alloy
grains are cored

284
Q

What is the effect of coring ?

A

affects corrosion resistance

different metals in proximity to each other in electrolyte can be corroded

285
Q

What can we do to remove the effect of coring ?

A

homogenisation

286
Q

What happens in homogenisation ?

A

the alloy is heated just below the RcT to prevent recrystallisation
atoms move and can form homoegenous structure again

287
Q

How can phase diagrams predict the amount of coring ?

A

bigger the S+L spacing - More coring

288
Q

Why do we quench ?

A

rapid cooling to get small grains

289
Q

What does the hall-petch equation tell us ?

A

smaller gain size higher yield stress- reduces the chances of permenant deformation

290
Q

Why do we carry out recrystallisation ?

A

heating atoms so they diffuse more and jump over grain boundaries
reduces yield stress
above the RcT

291
Q

What is the side effect of quenching ?

A

coring- grains are layered with different compositions of allys- reduces corrosion resistance

292
Q

How do we get rid of coring ?

A

via homogenisation

diffusion within grains

293
Q

Are insoluble metals common ?

A

no theyre rare

294
Q

Describe the phase diagram for an insoluble metals ?

A

the liquidus line reaches the solidus line at eutectic point
2 regions of pure solid which is metal and liquid
tie lines cross the solidus first telling us that the first solid to form is usuually pure metal A
you can find out the composition of the liquid from the composition on the bottom
the solidus line is constant unlike in solid solution phase diagrams

295
Q

What is the eutectic point ?

A

where the liquidus and solidus lines meet

296
Q

What are eutectuc alloys ?

A

formed at a specific composition

have a melting point and not a melting range like conventional alloys

297
Q

What happens above the eutectic temperature ?

A

the atoms will do everything they can to say liquid and resist solidification

298
Q

What happens at the eutectic temperature ?

A

not enough energy to support liquid state
turn solid
layers form of pure metal A and B as they are insoluble

299
Q

Can we use homogenisation for eutectic alloys and insoluble metals ?

A

no - theyre insoluble

300
Q

Why are eutectic alloys useful ?

A

they can be used as dental solder

they can easily go above and below the Tm as it is a single point and not a range

301
Q

Why are insoluble metals not used in the mouth ?

A

they cant be homogenised- dont use in the mouth

302
Q

What is the Tl and how can we find it ?

A

Tl is the liquid temperature and is found by drawing a line from the composition of the alloy to the liquidus line where it first hits.

303
Q

What can we work out from the Tl ?

A

from the Tl we can work out the composition of the first solid that begins to form by drawing tie lines across to the solidus line and down

304
Q

What does the Ts tell us and what can we find out from it ?

A

the solidus temperature

we can work out the composition of the last solid that forms by drawing across time lines

305
Q

What is TsL and what can we find out from it ?

A

a temperature intermediate between Ts and Tl
solid and liquid both present
work out the composition of the solid and the remainining liquid by hitting the solidus and liquidus lines in a solid solution phase diagram

306
Q

What do phase diagrams tell us about coring >?

A

support the idea that the first solid to form is high in composition of the higher melting point metal
the last solid that forms is highest in composition of the lowermelting point metals- this is corning

307
Q

What temperature do we carry out homogensiation at ?

A

below RcT

308
Q

Where are the solidus and liquidus lines in a eutectic insoluble metal phase diagram ?

A

liquidus line- the bent line on top

solidus line- the surrounding lines including the sides

309
Q

The majority of alloys are which type ?

A

partially soluble

310
Q

What forms do partially soluble metals take above and within the solubiltiy limit ?

A

within- act like solid solutions

above- act as insoluble metals

311
Q

What is solubulity related to ?

A

temperature and composition

312
Q

What does the phase diagram for partially soluble metals look like ?

A

alpha and beta regions
eutectic point
solvus lines

313
Q

What is an example of a partially soluble alloy system ?

A

silver and copper

314
Q

What are alpha and beta in partially soluble phase diagrams ?

A

alpha- 9% copper and 91% silver

beta- 8% silver and 92% copper

315
Q

What do solvus lines show ?

A

decreasing solubility of copper/silver at lower temperatures

316
Q

What happens in the solidification of insoluble metals ?

A

pure metal A and B form and the remaining liquid can be worked out in the composition

317
Q

What happens in the solidification of partially soluble metals ?

A

alpha and beta are formed instead of pure metal like in insoluble metals
alpha and beta are solid solutions really

318
Q

What happens when we quench copper ans silver alloys that are alpha and beta ?

A

there is too much copper and silver
they want to come out in equilibrium
over time they will diffuse out the grains and form pure copper or silver at the grain boundaries

319
Q

What is age hardening ?

A

alloy will become harder over time as the copper and silver diffuse out and gather at the grain boundaries

320
Q

What is precipitation hardening ?

A

we can increase the hardness and yield stress of partially soluble alloys by heating below the RcT to speed up the diffsuion of copper an d silver to grain boundaries

321
Q

What state does homogenisation happen in ?

A

solid

322
Q

What is order hardening ?

A

increase hardness and yield stress by causing atoms to form ordered solid solutions - go from a random structure to an ordered lattice structure
improve mechanical properties
cool down slowly

323
Q

What type of alloy do gold and copper form ?

A

solid solutions a they have similar atomic radii

324
Q

What is the problem with cooling down gold and copper alloys quickly ?

A

the atoms form into random planes in solid solutions

makes them weak/easily deformed and not suitable for partial dentures

325
Q

How can we make gold and copper alloys suitable for partial dentures ?

A

cool down slowly
form ordered lattices instead of random lattices - intermetallic structures which are harder
ie. order hardening

326
Q

At what temperature can we order harden gold and copper alloys ?

A

450 degrees

below RcT

327
Q

How is the gold and copper phase diagram different ?

A

solidus and liquidus lines meet at M

intermetallic fields

328
Q

How can we describe brittle behaviour ?

A

if we want to plastically deform something a little bit
we need to break bonds and ensure they reform to stop breakage
need to break 5 bonds and get 4 to reform- unlikley the bonds will reform so the material will be brittle - cant plastically deform a lot - easily fails

329
Q

How can we describe ductile behaviour ?

A

the bonds have a high chance of reforming
it can be plastically deformed a lot before failure
less force is also needed to palstically deform.

330
Q

How can yield stress be applied to dislocations ?

A

yield stress is the stress applied to make dislocations move

331
Q

What happens to alloys with dislocations when stress is applied higher than the yiedl stress ?

A

more dislocations form

dislocations move along slip planes towards grain boundaries where they are trapped

332
Q

What do dislocations travel long ?

A

slip planes

333
Q

How do grain size and grain boundaries link to the hall petch equation ?

A

smaller grains
more grain boundaries
more places to trap dislocations
more stress needed to plastically deform

334
Q

What is the effect of dislocations on yield stress ?

A

the higher the dislocations the higher the yield stress

335
Q

What is yield stress dependent on ?

A

quenching and trapped dislocations

336
Q

Below the yield stress can we plastically deform ?

A

no

we have to be above the yield stress to plastically deform

337
Q

What happens to dislocations when stress is applied higher than the yield stress ?

A

dislocations move

coalesce of dislocations at grain boundaries

338
Q

What do coalesced grain boundaries do ?

A

form pores
necking starts
eventual failure

339
Q

When do pores form and what do they do ?

A

above the yield stress

reduce ductiltiy

340
Q

How can we form alloys into shapes ?

A

repeated deformation aboe the yield stress

341
Q

What happens with each application of stress above the yield stress ?

A 
permenantly deform 
dislocations move 
dislocations form 
dislocations coalesce at the grain boundaries 
as a result yield stress will increase
342
Q

What happens when yield stress increases ?

A

ductility decreases

becomes harder to deform plastically

343
Q

What is work hardening ?

A

by working materials we can increase their yield stress- useful property
eg. orthodontic wires

344
Q

What does ductility mean ?

A

undergo significant amounts of permenant deformation before failure

345
Q

What happens to grains as dislocations move ?

A

grains deform
turn from equiaxed structure to fibrous structure
line up

346
Q

What happens when an allo goes from equiaxed to fibrous ?

A

hardness and strenght increase

ductiltiy decreases

347
Q

What happens as temperature increases in metals/slloys ?

A

interatomic vibration increases
interatomic distance increases
increased diffusion rate

348
Q

What happens if we heat above the RcT ?

A

atoms can diffuse across the grain boundaries

heal dislocations and fill in missing planes

349
Q

What happens if we heat the metal/alloy that has been worked to increase hardness and strength ?

A

loose the dislocations
reduced hardness and strength
ductility increases

350
Q

What is cold working ?

A

grains go from equixaed to fibrous
yield stress/hardness increase- work hardening
below RcT
limit to ductility

351
Q

What is hot working ?

A 
above the RcT
grains remain equaixed 
dislocations flow but the dislocations can be recovered as the atoms move into place 
no work hardening 
full ductility
352
Q

How can we use a combination of hot and cold working ?

A

start off with hard work, no ductility problems

final cycle is cold work- get work hardening for the properties we want. ductility decreased

353
Q

What do dislocations allow in metals/alloys ?

A

plastic deformation

bc they have metallic bonds

354
Q

What happens to materials with no dislocations ?

A

they are brittle

eg. intermetallc- no metllic bonds

355
Q

What does excess work hardening lead to ?

A

decreases ductility

increases chance of fracture

356
Q

What are wrought alloys ?

A

alloys that can be used to make devices through work

cold/hot working

357
Q

Which processes can be used to shape metals/alloys in dentistry ?

A

forging
milling
drawing and rolling

358
Q

What is forging ?

A

shaping by heating and hammering

above/below RcT

359
Q

What is milling ?

A

cutting shapes using rotating tools

360
Q

What is drawing and rolling ?

A

shaping by being pulled through die/dies
above or below the RcT
permenant deofrmation

361
Q

What happens to grain structure in cold working ?

A

equiaxed to fibrous

362
Q

What can happen if you heat cold worked alloys above the RcT ?

A

overheating can destroy crystal structure

increased hardness and strength

363
Q

What is the disadvantage of cold working ?

A

limit of ductility reached

sensitive to fracture

364
Q

What type of work does milling and forging require ?

A

little work

365
Q

What type of work does drawing and rolling require ?

A

high work

366
Q

What happens to yield stress and ductility with cold worked substances ?

A

yield stress increases

ductility decreases

367
Q

What is ductility ?

A

how much something can be plastically deformed before failure

368
Q

What happens to grains and dislocations in cold work ?

A

dislocations flow

grains gather a lot of internal stress

369
Q

What happens to atoms that have formed dislocations in cold worked materials ?

A

atoms try to get back into equilibrium and relax- this can change appearance

370
Q

What type of treatment can be done to remove distortions in worked alloys ?

A

stress relief annealing

371
Q

What happens in stress relief annealing ?

A

atoms move within grain and get back to original structure

372
Q

What are the phases of stress relief annealing ?

A

recovery
recrystallisation
grain growth

373
Q

What happens in recovery phase of stress relief annealing ?

A

atoms move back to equilibrium

stress and ductility dont change much

374
Q

What happens in the recrystallisation phase of stress relief annealing ?

A

atoms can diffuse across grain boundaries
grain size increases and yield stress decrease
ductility increases

375
Q

What happens if stress relief annealing is not done properly ?

A

enter recrystallisation phase
grain growth
ruin properties and structure

376
Q

What are the methods of joining metals and alloys ?

A

loops
soldering
welding

377
Q

What happens when making loops ?

A

bend parts around each other
require ductility
difficult if high yield stress- cant plastically deform

378
Q

What is soldering ?

A

using a liquid alloy to join together

usually a eutectic alloy - has a TM and not a melting range- solder is an intermediate

379
Q

What is welding ?

A

use an electric current to locally heat components

high localised temperature rise

380
Q

What can happen in welding ?

A

weld decay

ionic solids for at high temperatures

381
Q

What are the properties of ionic solids ?

A

hard but brittle

382
Q

What is the problem with soldering and welding ?

A

can lead to overheating
above the RcT
recystallisation and grain growth
weaker joint produced and device failure

383
Q

Why are alloys harder and stronger than metals ?

A

alloys have different sized atoms
planes are deformed
harder for dislocations to move
increases strength and hardness

384
Q

What is it called when hardness/strength increase because dislocations cant move due to plane deformations ?

A

solution strengthening

385
Q

What happens to yield stress when we turn a pure metal into an alloy ?

A

adding nickel- differetn sized atoms

harder for dislocations to move

386
Q

What happens to ductility when we turn a metal into an alloy ?

A

ductility decreases

dislocations get trapped

387
Q

What type of ductilty do wrought alloys have ?

A

have a high ductility initiallly

388
Q

What is steel ?

A

an alloy if iron and carbon

389
Q

What is the structure of steel ?

A

carbon sits in interstitial areas between iron atoms

390
Q

Is there a carbon solubility with steel ?

A

yes - 2%

the carbon cannot fit between the iron atoms anymore

391
Q

What does steel first form when it solidfies ?

A

austenite

392
Q

What is the structure of austenite ?

A

FCC

393
Q

What happens to austenite above carbon conc of 2%?

A

it becomes stable

394
Q

What happens as austenite cools ?

A

solubiltiy of carbon decreases

carbon is chucked out

395
Q

What is Tc ?

A

critical temperature

396
Q

What happens as Tc ?

A

critical temperature

iron changes from FCC to BCC

397
Q

What is the critical temperture ?

A

723 degrees

398
Q

What type of steel forms at low carbon cocnentrations ?

A

ferrite forms
carbon is chuckes out
BCC structure

399
Q

What is the structure of ferrite ?

A

BCC

400
Q

What is the carbon content of ferrite ?

A

0.02% carbon

401
Q

What happens below 723 degrees ?

A

no austenite left

form ferrite and cementite

402
Q

What do ferrite and cementite form ?

A

pearlite

403
Q

What happen as the carbon concentration increases ?

A

you get cementitie forming

404
Q

What is cementite ?

A

ionic

Fe3C

405
Q

What happens below 0.76% Carbon ?

A

austenite and ferrite form

406
Q

What happens above 0.76% carbon ?

A

austenite and cementite

407
Q

What is point X on the steel phase diagram ?

A

eutectoid

start to form pearlite

408
Q

What is a eutectoid ?

A

a solid eutectic
solid to solid
no change from liquid to solid like a eutectic

409
Q

What are the different phases of steel ?

A

austenite
ferrite
cementite
pearlite

410
Q

What are the properties of austenite ?

A

stable above useful temperatures for dental applications- not used

411
Q

What are the properties of ferrite ?

A

solid solution of Fe and C , medium strength

412
Q

What are the properties of cementite ?

A

Fe3C
ionic
hard but brittle

413
Q

What are the properties of pearlite ?

A

combined ferrite and cementite

properties depend on the carbon concentration

414
Q

What happens if we increase the carbon content of pearlite ?

A

the yield stress increases
ductility decreases
hardness increases

415
Q

How is martensite formed ?

A

by rapid cooling

416
Q

How does martensite form ?

A

quenching to get small grains
carbon gets trapped
can no longer leave to form BCC

417
Q

What is the structure of martensite ?

A

body centred tetragonal

418
Q

What are the properties of martensite ?

A

hard and brittle

too brittle for use

419
Q

How can the amount of martensite that forms be controlled ?

A

use tempering

420
Q

What happens in tempering ?

A

heat steel to below 723 degrees
no austenite
pearlite forms from martensite

421
Q

If you temper for a higher temperature and longer time what happens ?

A

more pearlite forms

422
Q

What happens to form martensite from austenite ?

A

austenite is quenched

423
Q

What does tempering allow ?

A

control properties of steel

- any property combo possible

424
Q

Describe the steel triangle ?

A

pearlite heated form austenite
austenite quenched makes martensite
martensite tempered to make pearlite

425
Q

What are the properties of quenched materials compared to slowly cooled ?

A

quenched are harder

426
Q

Why is steel not useful in dentistry ?

A

iron readily corrodes and rusts

iron oxide binds weakly to iron and flakes easily

427
Q

What is different between steel and stainless steel ?

A

stainles steel has iron, nickel and chromium added

428
Q

Why is chromium added to stainless steel ?

A

chromium corrodies faste than iron but chromic oxide binds stronger to chromium - harder to flake off
chromic oxide also retains that chromium colour

429
Q

What happens if chromic oxide does flake off ?

A

more chromic oxide will form

430
Q

What is passivation ?

A

forming a stable oxide layer

passive oxide layer

431
Q

What are the types of steel ?

A

austenitic

martensitic

432
Q

What is the composition of austenitic stainless steel ?

A

18% Cr and 8% Ni added to iron and carbon

433
Q

What are the properties of martensitic stainless steel ?

A

chromium passivation

Cr and Ni form solid solutions with steel

434
Q

What is solution hardening ?

A

when solid solutions form giing better mechanical properties

435
Q

What happens with austenitic stainless steel ?

A

no austenite to pearlite transformation

no ferrite made as iron atoms dont want to go into BCC sturcture

436
Q

What is the consequence of a lack of austenitic to pearlite transition ?

A

cant form martensite
cant temper to control properties
limited use

437
Q

What is martensitic stainless steel ?

A

12/0 steel

12% Cr and 0% Ni

438
Q

What are the properties of martensitic stainless steel ?

A

chomium forms passive layer in passiviation

Chromium and nickel form solid solutions with steel- solution hardening

439
Q

Does the austenite to pearlite transition occur in martensitic steel ?

A

yes
martensite can be formed via quenching
tempering of martensite cna happen

440
Q

What are properties of high carbon 12/0 steel ?

A

more cementite

harder

441
Q

What are the properties of low carbon 18/8 steel ?

A

more ferrite

softer

442
Q

What are uses of 18/8 stainless steel ?

A

orthodontics

443
Q

What are the uses of 12/0 stainless steel ?

A

drills

burs

BDS stage 2 (28 decks)

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