DMT Flashcards
1
Q
What is the range of dental materials ?
A
polymers
ceramics
metals and alloys
inorganic salts
2
Q
What are the properties of polymers ?
A
soft and flexible
can be deformed
3
Q
What is a metal ?
A
an element
4
Q
What is an alloy ?
A
mixture of metals
5
Q
What are inorganic salts used for ?
A
impressions and root fillings
6
Q
What is the success of treatment dependent on ?
A
selection of material
use of material
quality of material
7
Q
What is the success of treatment dependent on ?
A
selection of material
use of material
quality of material
8
Q
What guarantees safety of material ?
A
C e mark
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
10
Q
How is testing of materials carried out ?
A
products should have been evaluated and quality assurace programme set ot
11
Q
What are standard specficiations ?
A
ISO and BSI Standards
dont cover all aspects though
12
Q
What are laboratory evaluations ?
A
test sustaianability
ensure methods are given that correlate with clinical experience
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
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
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
16
Q
What is the ideal material for mixing and manipulation ?
A
should be easy to proportion
not drip or stick to instruments
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
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
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
20
Q
What is rheology ?
A
study of flow and deformation of materials
refers to viscosity
21
Q
What does high viscosity mean ?
A
more gloopy
22
Q
What is viscosity related to ?
A
pressure and rate
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
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
25
What does it mean if viscosity increases with shear rate ?
dilatant
| k>1
26
What does it mean if a material is thixotropic ?
dont flow until sufficinet pressure applied
27
Which type of viscosity is easier to mix ?
lower
28
Why is high viscosity good sometimes ?
stops spillages when transferring
29
When is viscosity important to consider ?
viscosity needed- to adapt to cavities- low
| impression material- low - flow and take detail
30
Why are some materials hard to mix ?
high initial viscosity
31
What can be done to counteract an increase in viscoity during setting ?
solvents- reduce viscosity
| retarders- increase working time and reduce the increase in viscosity
32
What is the nature of temperature during setting ?
setting is an exothermic reaction
33
What happens during an exothermic reaction ?
energy released
max temperature reached
setting reaction slows down
34
What is the time from mixing to reaching max temperature ?
setting time
35
How are thermal and dimensional changes linked ?
increase in temperature can lead to porosity - lower strength and integrity of material
36
How is porosity caused ?
materials vapourise at high temperature
leave gaps
filled with air bubbles
37
Are enamel and dentine conductors or insulators ?
insulators - protect pulp from temperature change
38
What type of material should filling materials be ?
filling materials should be good insulators
| when setting exo reaction can damage pulp
39
When does expansion happen ?
crystal growth in the setting of amalgum and gypsum products
damage restorations
inaccuracy in fitting of crowns
40
When does contraction happen ?
polymerisation
| shrinking can lead to gaps and increase risk of secondary caries and marginal staining
41
What influences the oral environment ?
pH and temperature
42
What can cause pH fluctuations in the mouth ?
plaque
acidic rinks
alkaline mediction
43
What happens if the pH falls below the critical pH ?
saliva becomes unsaturated and enamel dissolves
44
High stress in the form of a large force lead to ?
fracture
45
What do low stresses over time lead to ?
fatigue
46
What can abrasive factors do ?
abrasive food and toothpaste can lead to softening and reduced integrity of enamel
47
What does biocompatibility of a material refer to ?
whether its toxic
irritant
allergies
48
What are the chemical properties of set materials ?
solubility
corrosion
leaching
49
What does accuracy of a material refer to ?
impression materials
| how good it can replicate oral anatomy
50
What does durability refer to ?
how long it can survive in the mouth before needing to be replaced
51
What does conservation mean ?
minimally invasive
preserve natural dentition
of all teeth- opposing teeethe tc
52
How can restorations change colour ?
over time naturally
| diet and abrasion factors
53
What must we consider with material solubility ?
materials can be water soluble, alcohol soluble and saliva soluble
to be durable- must have low solubility
54
What is leaching ?
loss of one components into a liquid likemsaliva
55
What is positive leaching ?
loss of one component into another- GIC fluoride leaches into saliva and can from fluorapatite
56
What is negative leaching ?
plasticisers from denture liners leach out and are uncomfortable to wear
57
Which materials does corrosion occur in ?
metals and alloys
58
What is a galvanic cells and how is it formed ?
when 2 metals comes into contact in an electrolyte
| flow of electrons
59
What is an elecetrolyte ?
ionic solution
60
What can act as an electrolyte ?
saliva- can corrode amalgam fillings
61
What are the consequences of corrosion of amalgam fillings ?
```
loss of metal ions
metallic taste
weakening of restoration
release of electrons to pulp causes pain
release of mercury
```
62
The amount of expansion and contraction is ....
equal and opposite
63
How can we measure thermal expansion and contraction ?
coefficient of thermal expansion
64
What is the meaning of thermal conductivity ?
rate of heat flow per unit temperature gradient
65
What needs to be cosnidered about amalgam being a conductor ?
a conductor- when using in a filling use an insulating liner to stop the feeling of pain
66
What materials are filling insulating linings made from ?
zinc phosphate
| zinc oxide
67
Composites amde from acrylic resin are ?
insulators- dont need liner
68
What is the disadvantage about thermal conductivity ?
only measures static temperature
| oral temperature changes over time
69
What is thermal diffusivity ?
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
70
What does a low diffusivity mean ?
takes a long time to reach that temperature
| low diffusivity and conductivity is desirable in materials
71
What is a high diffusivity ?
temperature reached afte rhte stimulus
| needed for dentures to replace palate sensation
72
How are conductivity and diffusivity mesured ?
using a thermcouple in test material
73
What is a force ?
outside agency acting on body to change momentum
74
How do we calculate force ?
weight x acceleration
| weight= mg
75
What is stress ?
internal forces set up in a body to oppose externally applied force
like the internal forces set up to oppose biiting forces
76
What is the stress resisting a tensile force called ?
tensile stress
77
How can the magnitude of the stress be calculated ?
applied force/ dimensions of area applied to
78
What are the simple stresses ?
tensile
compressive
shear
79
What are the complex stress types ?
flexural
torsional
diametral
80
What is a flexural stress ?
in a denture - mix of compression and tensile
| not supported in middle
81
What is strength ?
the maximum stress that can be withstood by a material before breakage
82
What is strain ?
applying an external force can lead toa change in dimensions
| like tensile stress can lead to extension
83
How can strain be calculated ?
change in length/original lengtj
84
What is elastic deformation ?
material returns to original dimensions when force is removed
85
What is plastic deformation ?
material is permanently damaged after force is removed
| useful for fillings
86
What is viscoelastic deformation ?
force is removed
returns partially to original dimensions
takes a long time to return to original dimesions
87
What is used as a model for elastic deformation ?
spring
88
What is used as a model for plastic deformation ?
dashpot
89
What are the 2 models for viscoelastic deformation ?
maxwell
| voigt
90
What is the maxwell model ?
dashpot and spring
load deforms
load removed and the dashpot is permnenantly deformed
cant return to original dimensions
91
What is the voigt model ?
application of a load
| removal of the load and slow return to original state
92
What is stiffness ?
a measure of how resistant something is to deformation
| higher stiffness-more rigid and harder to deform
93
What is the modulus of elasticity ?
on stress/strain graph
| gradient of straight line
94
What does the modulus of elasticity tell us ?
how difficult it is to deform
95
What is the ultimate tensile strenght ?
pulling at this strength leads to deformation
96
What is the yield stress ?
point at which something stops elastic deformation and is now deformed plastically
required to permenantly deform soemthing
change in gradient
97
What is used instead of yiels stress ?
proof stress
| easier to measure
98
What is malleability ?
how much something can be squahed to failure
99
What is resilience ?
how much energy something can take before it permenanatly deforms
area under the black line
100
What is toughness ?
how much energy something can take before it fails
| area under whole gra[h
101
What is ductile behaviour ?
can be deformed by large amounts
leads to thinning called necking
deformed elastically and then plastically
102
What is brittle behaviour ?
can only be deformed by small amount
deformed elastically
smallest amount of stress can fail them
103
Are materials ductile or brittle ?
all materials are ductile or brittle depending on temperature and shear rate
104
How can toughness be measured ?
total area under the stress/strain graph
105
For something to break what must be initiated ?
a crack must be initiated and then propagated across the material
106
What type of test is used for toughness ?
an impact test
107
What happens in the toughness test ?
potential energy in pendulum converted to kinetic energy to fracture the specimen
108
What does the impact test compare ?
compares the energy required to break notched and unnotched specimens
energy to break unnotched>>
notched specimen is notch sensitive
109
What does notch sensitive mean ?
cracks and scratches break the material easier
110
What is fatigue ?
materials can fail due to repeated cycles of stress
| they fail at stress lower than strength
111
Where is fatigue common ?
in dentures- flexural stress down the midline mean they fracture
112
What is fatigue life ?
the maximum cyclic stress a material can take until failure
| number of cycles
113
What is fatigue limit ?
the value of cycle stress needed to cause a fracture
114
What is hardness ?
how likely a material will be scratched
| resistance to indentation under load
115
What is the significance of scratches ?
they can be colonised by bacteria
116
How can hardness be measured ?
VHN
| KHN
117
What is the significance of porcelain being harder than enamel ?
porcelain being harder means it can damage the opposing tooth if part of a crown
118
What are synthetic polymers ?
based on petrochemicals
119
What are polymers ?
made of regularly repeating units called monomers
covalently bonded
as monomers join the molecular weight increases
120
What are cross linked and linear polymers ?
cross linked- links between polymer chains
| linear- no cross links
121
What is a homopolymer ?
contains only 1 type of monomer
122
What is a copolymer ?
2 or more monomers
123
What are the different ways monomers in copolymers can join ?
randomly
regularly
block
124
What is addition polymerisation ?
addition of 2 molecules to form a third bigger molecule
125
What is condensation polymerisation ?
tow molecules join to form a larger molecule and a by product
126
What happens in addition polymerisation ?
reaction of a reactive species with a monomer to form a larger reactive species
chain lengthening and formation
127
What are the reactive species used in addition polymerisation ?
ion
free radial
with a spare electron
128
What are the 4 steps of addition polymerisation ?
activation
initiation
propagation
termination
129
How are reactive species formed ?
from initiators- molecules with weak bonds where heat and light is applied
130
What are initiators ?
molecules with weak bonds that can be broken down to make reactive species via heat and light
131
What are vinyl polymers ?
have C-C double bonds
132
Minor changes in R groups of monomers can lead to ?
different polymers
133
What is activation ?
decomposition of benzyl peroxide by thermal or light energy
134
What is initiation ?
Polymerisation is initiated when the monomer reacts witth the radical
135
How can benzoyl peroxide act as an initiator ?
O-O bond broken down to give 2 radicals
136
What is propagation ?
newly formed radical reacts with further monomer to produce another radical
increases chain length and average molecular weight reached
137
What is termination ?
reactions produce dead polymer which arent capable of further reaction
138
What happens to viscosity as polymers are made ?
increases- making it harder to join
| leaves residual monomer
139
What can termiante polymerisation ?
impurities like oxygen
| residual monomers - premature
140
What does cross linking do ?
joins chains together to form a network
141
What happens to properties with cross linking ?
polymer is harder to deform
stiffness increases
improve stability in liquids
142
What are the properties of linear polymers ?
flexible
| lower stiffness
143
What does cross linking require ?
difunctional monomers
| have more than 1 C-C double bond
144
Give an example of condensation polymerisation ?
esterification reaction between alcohol and acid produces water and estre
145
How many reactive groups are needed for AP, CP and CS ?
1
2
3
146
Which materials set by condensation polymerisation ?
silicone impression material
ethanol evaporated quickly
changes dimensions
147
What are the changes that occur during polymrisation ?
monomers usually fluids
| polymerisation increases viscosity
148
What are thermal changes that occur in polymerisation ?
it is an exo reaction
temperature rise is proportional to volume
continual energy releases- runaway
149
What is the consequence of the temperature getting too high in polymerisation ?
monomer vapourises
air bubbles form
weakness leads to porosity
150
What are the dimensional changes that occur in polymerisation ?
polymers are denser than monomers
| polymerisation lead to shrinkage
151
What is the consequence of shrinkage ?
gaps forming
| staining and secondary caries
152
What is contraction porosity ?
conraction leads to weaknesses
153
How can we counter act with contraction ?
overfilling
154
What are the methods used to reduce polymer shrinkage ?
BisGMA- bigger than MMA- bigger monomers take up more space leading to less shrinkage
Fillers- fillers dont polymerise-
155
What are the 2 structures of polymers ?
amorphous
| crystalline
156
What are amorphous polymers ?
randomly arranged
| short range order
157
What are crystalline polymers ?
regularly arranged
158
What is a short range order ?
regular and predictable arrangement of monomers
| monomers are sitting next to same type
159
What is a long range order ?
atoms show periodicity over large distances
160
Can crystalline polymers melt ?
```
yes
they have a melting temperature
melting leads to a loose chain arrangement
loss od long range roder
cooling can recover long range order
```
161
Do amorphous polymers melt ?
they burn at high temperatures
162
Which temperatures do crystalline polymers have ?
Tg
| Tm
163
Which temperatures do amorphous polymers have ?
Tg
164
What are thermoplastic polymers ?
soften on heating
harden on cooling
crystalline
165
What are thermosetting polymers ?
dont soften on heating
harden on setting
cross linked
166
Can thermoplastic or thermosetting polymers be recycled ?
thermoplastic only
167
What is the Tg ?
temperature which they change from glassy to rubbery
| easier to defrom
168
What happens at the Tg ?
10 x reduction in modulus of elasticity
169
What are polymers like in the mouth if their Tg is above mouth temperature ?
stay rigid in the mouth
| good for dentures and fillings
170
What are polymers like with a Tg below mouth temoerature in the mouth ?
rubbery in the mouth
| good for impression materials
171
How does molecular weight affect Tg ?
higher the molecular weight the greater the Tg
172
How does the amount of residual monomer affect molecular weight ?
lower the amount of residua monomer the greater the Tg
173
What type of polymers do C-C bonds produce ?
vinyl
| rigid
174
What type of polymers do Si-O bonds make ?
flexible polymers
175
How do co polymers affect Tg ?
the Tg will depend on the specific combination of monomers
176
What are pendant groups ?
hang off the polymer chain
177
What do bigger pendant groups produce ?
produce more rubbery polymers
178
What do pendant groups do ?
push chains further apart
| make it easier to remove chains
179
How do pendant groups affect Tg ?
pendant groups reduce Tg
180
How does the amount of initiator affect the Tg ?
more initiator produces more chains
these have a low molecular weight
reduce Tg
easy to deform
181
What are activators ?
```
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
How does cross linking affect Tg ?
the more cross links the greater the Tg
183
What are plasticisers ?
they are added to reduce Tg
| make more flexible
184
What can act as a plasticiser ?
residual monomer
185
What are fillers ?
change properties of polymer
186
What are the methods of fabrication of polymers ?
injection moulding- high temperature and pressure
compression moulding- mould
dough moulding- powder and liquid
paste - pastes placed in mould
187
What are some of the uses of metals and alloys ?
denture frameworks
implants
crowns/bridges/inlays
188
What is a direct use of metals and alloys ?
fillings- mixed and then placed into cavity
189
What are indirect uses of metals and alloys ?
made by the technician- crowns
190
What are the characteristics of metals ?
dense
crystalline structure
conductors of heat and electricity
opaque
191
Why is it important metals are strong ?
difficult to break
| instruments need to be strong to cut into teeth
192
Why is it important metals are stiff ?
theyre not easily deformed
193
Why is it important metals are tough ?
need a lot of energy to shatter them
194
Why is it important metals are strrong, tough and stiff ?
withstand the high forces of masticaton and biting
195
Why is it important metals are strong ?
they are easily scratched
196
Why is it important metals are lustrous ?
dont easily loos polish
197
What is the problem with metals and alloys being conductors of heat ?
they can conduct heat to the pulp
| important to put insulating liner in
198
What is the crystalline structure of metals and alloys ?
they form crystals
atoms are defined
long and short range order
199
What is the colour of metals and alloys ?
opaque
| dont add pigments to the m
200
What can corrosion do to metals and alloys ?
weaken the structure
change colour
form oxides that flake off
eg. Copper oxide originally orange copper oxide is green and flakes
201
What is a metal ?
an element
202
What is an alloy ?
combinations of 2 or more metals
| can also be a combination of metals and non metals
203
What is amalgam a combination of ?
Hg, Ag, An, Cu and Zn
204
What is stainless steel a combination of ?
Fe, Cr, Ni and C
205
What are the 3 ways of shaping metals ?
casting
working
amalgamation
206
What is casting ?
metal is melted and molten metal is poured into mould
207
What is working ?
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
What is amalgamation ?
mixed with mercury
form a plastic mass which gradually hardens by cooling
material flows because it mixes with mercury
209
What is the temperature at which metals turn into liquid ?
Tm
210
What is the state of metals above Tm ?
liquid
211
What is the state of metals below the Tm ?
solid
212
Is melting reversible ?
yes
213
What happens to atoms when metals cool ?
the atoms get into an increasingly confined space and start bumping into each other
solidifcation occurs
214
What are the distinct phases of solidification ?
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
Why does temperature remain constant during crystallisation ?
energy loss from cystallsiation is equal to energy loss from decreasig temperature
216
Where does solidifcation occur ?
metals solidify onto solid surfaces
| like moulds
217
What are seeds and nuclei ?
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
What are seeds/nuclei generally ?
impurities
219
What do crystals grow as ?
dendrites
| 3D structures
220
What are dendrites ?
tree branches
| atoms join onto branches
221
Why are metals known as equiaxed ?
equal in all directions from central nuclei
222
What are crystals also known as ?
grains
223
What are the boundaries between crystals ?
grain boundaries
224
What happens within the grains ?
the atoms stack into planes
atomic planes
3D lattices
225
What are the 3 different cystals structures ?
simple cubic
face centred cubic
body centred cubic
226
What is a simple cubic structure ?
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
What is a body centred cubic ?
atoms stack into gaps between adjacent atoms
cant stack a lot of atoms
they are far apart
228
What is face centred cubic ?
stacking leads to more gaps forming
| fits a lot more atoms
229
What type of structure does chromium form ?
body centred cubic
230
What does aluminium, copper, gold and nickel form ?
face centred cubic
231
What is the significance of different metals having different structures ?
hard to mix together
232
What type of structure is titanium ?
hexagonal closed packed
233
What is the smallest metal in a unit called ?
unit cell
234
How can metals change unit cell ?
by heating and cooling down
235
What leads to defects in grains ?
cooling down metals
| its a random and rapid process
236
What are point defects ?
defects in specific point in plane
| impurities and vacancies
237
What are impurities ?
atoms of a different element in the plane
might be from the reaction vessel
different sized atom
238
What are vacancies ?
gap in plane formed during solidification
239
What are defects called that effect whole planes ?
dislocations or line defects
| odd number of planes
240
How does solidifcation start in casting ?
the mould is at a lower temperature than the metal
241
Which material type are moulds made from ?
insulators
242
How can we make the cooling down process faster ?
placing the mould into liquid
243
What is the name of the process for when a mould is placed into liquid ?
quenching
244
What does the speed of cooling down effect ?
how many grains are formed
245
What happens as the cooling speed increases ?
groups of atoms join up
246
What does quenching produce ?
more grains per volume
smaller grains
more grain boundaries
247
What is the yield strenght ?
the point at which deformation becomes plastic
248
What is the ideal yield stress ?
ideally you want yield stress to be higher than biting force
249
What does the Hall-petch equation relate ?
yield strength is inversely related to the square root of the grain size
250
What is the relationship between grain diameter size and yield strength ?
the smaller the diameter of the grain the greater the yield strength - harder to plastically deform
251
What does quenching do to grain diameter ?
reduces grain diameter
252
Describe the process of recrystallisation ?
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
What does an increase in grain growth mean ?
there are less grains per unit volume
bigger grains
less grain boundaries
254
What does a reduction in grain boundaries lead to ?
reduction in yield strenght
255
Is the RcT lower than Tm usually ?
yes
| like 30-70% of the Tm
256
Which state does recrystallisation happen in ?
solid
| below the Tm
257
What is diffusion rate like ?
low diffusion rate below the RcT
| high diffusion rate above the RcT
258
What type of alloy is amalgam ?
quinary
259
What type of alloy is stainless steel ?
quarternary
260
What is an alloy system ?
all possible combinations of metal proportions within the alloy
261
Are alloys or metals more expensive ?
metals
262
Are metals or alloys harder /?
alloys
263
Are metals or alloy stronger ?
alloys
264
Do alloys have a Tm ?
no they have a melting range
265
What is the melting range of an alloy ?
the highest Tm and the lowest Tm of the components
266
What happens to atoms in liquid alloys ?
the atoms cam move around each other and can be combined
267
What are the 4 different types of solids that alloys can form ?
solid solutions
insoluble metals
partial solubility
intermetallic compounds
268
What are solid solutions ?
the atoms of different components are happy to form planes
269
What is an example of a solid solution ?
gold and silver
270
What is an insoluble metal ?
the atoms of one of the metals wont from planes with the other
the solutions can segregate out
271
What is an example of insoluble metals ?
lead-tin
272
What is partial solubility ?
the atoms are happy to form planes up to certain concentrations
273
What is an example of partial solubility ?
copper and silver
274
What is an intermetallic compound ?
the atoms form ionic compounds instead of metallic compounds
275
What are the properties of intermetallic compounds ?
hard and brittle
276
What are the factors determining if a solid solution will form ?
relative size of atoms
relative electronegativity
relative valency
crystal structure
277
How does the relative size of the atoms effect solid solution formation ?
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
How does the relative electronegativity effect solid solution formation ?
large differences in electronegativity mean the alloy is more likely to form ionic bonds and intermetallic structures
279
How does the relative valency effect solid solution formation ?
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
How does the crystal sturcture effect solid solution form ?
metals must have the same crystal structure for planes to form.
281
What is shown on a phase diagram ?
possible combinations of metals as alloys
liquidous line- above is liquid
solidous- below is solid
S+L
282
What does a phase diagram show ?
depending on the proportion of A/B in the alloy- solidifiying starts and finishes differently
283
What is coring ?
cooling down produces grains with different compositions which make up the alloy
grains are cored
284
What is the effect of coring ?
affects corrosion resistance
| different metals in proximity to each other in electrolyte can be corroded
285
What can we do to remove the effect of coring ?
homogenisation
286
What happens in homogenisation ?
the alloy is heated just below the RcT to prevent recrystallisation
atoms move and can form homoegenous structure again
287
How can phase diagrams predict the amount of coring ?
bigger the S+L spacing - More coring
288
Why do we quench ?
rapid cooling to get small grains
289
What does the hall-petch equation tell us ?
smaller gain size higher yield stress- reduces the chances of permenant deformation
290
Why do we carry out recrystallisation ?
heating atoms so they diffuse more and jump over grain boundaries
reduces yield stress
above the RcT
291
What is the side effect of quenching ?
coring- grains are layered with different compositions of allys- reduces corrosion resistance
292
How do we get rid of coring ?
via homogenisation
| diffusion within grains
293
Are insoluble metals common ?
no theyre rare
294
Describe the phase diagram for an insoluble metals ?
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
What is the eutectic point ?
where the liquidus and solidus lines meet
296
What are eutectuc alloys ?
formed at a specific composition
| have a melting point and not a melting range like conventional alloys
297
What happens above the eutectic temperature ?
the atoms will do everything they can to say liquid and resist solidification
298
What happens at the eutectic temperature ?
not enough energy to support liquid state
turn solid
layers form of pure metal A and B as they are insoluble
299
Can we use homogenisation for eutectic alloys and insoluble metals ?
no - theyre insoluble
300
Why are eutectic alloys useful ?
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
Why are insoluble metals not used in the mouth ?
they cant be homogenised- dont use in the mouth
302
What is the Tl and how can we find it ?
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
What can we work out from the Tl ?
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
What does the Ts tell us and what can we find out from it ?
the solidus temperature
| we can work out the composition of the last solid that forms by drawing across time lines
305
What is TsL and what can we find out from it ?
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
What do phase diagrams tell us about coring >?
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
What temperature do we carry out homogensiation at ?
below RcT
308
Where are the solidus and liquidus lines in a eutectic insoluble metal phase diagram ?
liquidus line- the bent line on top
| solidus line- the surrounding lines including the sides
309
The majority of alloys are which type ?
partially soluble
310
What forms do partially soluble metals take above and within the solubiltiy limit ?
within- act like solid solutions
| above- act as insoluble metals
311
What is solubulity related to ?
temperature and composition
312
What does the phase diagram for partially soluble metals look like ?
alpha and beta regions
eutectic point
solvus lines
313
What is an example of a partially soluble alloy system ?
silver and copper
314
What are alpha and beta in partially soluble phase diagrams ?
alpha- 9% copper and 91% silver
| beta- 8% silver and 92% copper
315
What do solvus lines show ?
decreasing solubility of copper/silver at lower temperatures
316
What happens in the solidification of insoluble metals ?
pure metal A and B form and the remaining liquid can be worked out in the composition
317
What happens in the solidification of partially soluble metals ?
alpha and beta are formed instead of pure metal like in insoluble metals
alpha and beta are solid solutions really
318
What happens when we quench copper ans silver alloys that are alpha and beta ?
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
What is age hardening ?
alloy will become harder over time as the copper and silver diffuse out and gather at the grain boundaries
320
What is precipitation hardening ?
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
What state does homogenisation happen in ?
solid
322
What is order hardening ?
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
What type of alloy do gold and copper form ?
solid solutions a they have similar atomic radii
324
What is the problem with cooling down gold and copper alloys quickly ?
the atoms form into random planes in solid solutions
| makes them weak/easily deformed and not suitable for partial dentures
325
How can we make gold and copper alloys suitable for partial dentures ?
cool down slowly
form ordered lattices instead of random lattices - intermetallic structures which are harder
ie. order hardening
326
At what temperature can we order harden gold and copper alloys ?
450 degrees
| below RcT
327
How is the gold and copper phase diagram different ?
solidus and liquidus lines meet at M
| intermetallic fields
328
How can we describe brittle behaviour ?
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
How can we describe ductile behaviour ?
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
How can yield stress be applied to dislocations ?
yield stress is the stress applied to make dislocations move
331
What happens to alloys with dislocations when stress is applied higher than the yiedl stress ?
more dislocations form
| dislocations move along slip planes towards grain boundaries where they are trapped
332
What do dislocations travel long ?
slip planes
333
How do grain size and grain boundaries link to the hall petch equation ?
smaller grains
more grain boundaries
more places to trap dislocations
more stress needed to plastically deform
334
What is the effect of dislocations on yield stress ?
the higher the dislocations the higher the yield stress
335
What is yield stress dependent on ?
quenching and trapped dislocations
336
Below the yield stress can we plastically deform ?
no
| we have to be above the yield stress to plastically deform
337
What happens to dislocations when stress is applied higher than the yield stress ?
dislocations move
| coalesce of dislocations at grain boundaries
338
What do coalesced grain boundaries do ?
form pores
necking starts
eventual failure
339
When do pores form and what do they do ?
above the yield stress
| reduce ductiltiy
340
How can we form alloys into shapes ?
repeated deformation aboe the yield stress
341
What happens with each application of stress above the yield stress ?
```
permenantly deform
dislocations move
dislocations form
dislocations coalesce at the grain boundaries
as a result yield stress will increase
```
342
What happens when yield stress increases ?
ductility decreases
| becomes harder to deform plastically
343
What is work hardening ?
by working materials we can increase their yield stress- useful property
eg. orthodontic wires
344
What does ductility mean ?
undergo significant amounts of permenant deformation before failure
345
What happens to grains as dislocations move ?
grains deform
turn from equiaxed structure to fibrous structure
line up
346
What happens when an allo goes from equiaxed to fibrous ?
hardness and strenght increase
| ductiltiy decreases
347
What happens as temperature increases in metals/slloys ?
interatomic vibration increases
interatomic distance increases
increased diffusion rate
348
What happens if we heat above the RcT ?
atoms can diffuse across the grain boundaries
| heal dislocations and fill in missing planes
349
What happens if we heat the metal/alloy that has been worked to increase hardness and strength ?
loose the dislocations
reduced hardness and strength
ductility increases
350
What is cold working ?
grains go from equixaed to fibrous
yield stress/hardness increase- work hardening
below RcT
limit to ductility
351
What is hot working ?
```
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
How can we use a combination of hot and cold working ?
start off with hard work, no ductility problems
| final cycle is cold work- get work hardening for the properties we want. ductility decreased
353
What do dislocations allow in metals/alloys ?
plastic deformation
| bc they have metallic bonds
354
What happens to materials with no dislocations ?
they are brittle
| eg. intermetallc- no metllic bonds
355
What does excess work hardening lead to ?
decreases ductility
| increases chance of fracture
356
What are wrought alloys ?
alloys that can be used to make devices through work
| cold/hot working
357
Which processes can be used to shape metals/alloys in dentistry ?
forging
milling
drawing and rolling
358
What is forging ?
shaping by heating and hammering
| above/below RcT
359
What is milling ?
cutting shapes using rotating tools
360
What is drawing and rolling ?
shaping by being pulled through die/dies
above or below the RcT
permenant deofrmation
361
What happens to grain structure in cold working ?
equiaxed to fibrous
362
What can happen if you heat cold worked alloys above the RcT ?
overheating can destroy crystal structure
| increased hardness and strength
363
What is the disadvantage of cold working ?
limit of ductility reached
| sensitive to fracture
364
What type of work does milling and forging require ?
little work
365
What type of work does drawing and rolling require ?
high work
366
What happens to yield stress and ductility with cold worked substances ?
yield stress increases
| ductility decreases
367
What is ductility ?
how much something can be plastically deformed before failure
368
What happens to grains and dislocations in cold work ?
dislocations flow
| grains gather a lot of internal stress
369
What happens to atoms that have formed dislocations in cold worked materials ?
atoms try to get back into equilibrium and relax- this can change appearance
370
What type of treatment can be done to remove distortions in worked alloys ?
stress relief annealing
371
What happens in stress relief annealing ?
atoms move within grain and get back to original structure
372
What are the phases of stress relief annealing ?
recovery
recrystallisation
grain growth
373
What happens in recovery phase of stress relief annealing ?
atoms move back to equilibrium
| stress and ductility dont change much
374
What happens in the recrystallisation phase of stress relief annealing ?
atoms can diffuse across grain boundaries
grain size increases and yield stress decrease
ductility increases
375
What happens if stress relief annealing is not done properly ?
enter recrystallisation phase
grain growth
ruin properties and structure
376
What are the methods of joining metals and alloys ?
loops
soldering
welding
377
What happens when making loops ?
bend parts around each other
require ductility
difficult if high yield stress- cant plastically deform
378
What is soldering ?
using a liquid alloy to join together
| usually a eutectic alloy - has a TM and not a melting range- solder is an intermediate
379
What is welding ?
use an electric current to locally heat components
| high localised temperature rise
380
What can happen in welding ?
weld decay
| ionic solids for at high temperatures
381
What are the properties of ionic solids ?
hard but brittle
382
What is the problem with soldering and welding ?
can lead to overheating
above the RcT
recystallisation and grain growth
weaker joint produced and device failure
383
Why are alloys harder and stronger than metals ?
alloys have different sized atoms
planes are deformed
harder for dislocations to move
increases strength and hardness
384
What is it called when hardness/strength increase because dislocations cant move due to plane deformations ?
solution strengthening
385
What happens to yield stress when we turn a pure metal into an alloy ?
adding nickel- differetn sized atoms
| harder for dislocations to move
386
What happens to ductility when we turn a metal into an alloy ?
ductility decreases
| dislocations get trapped
387
What type of ductilty do wrought alloys have ?
have a high ductility initiallly
388
What is steel ?
an alloy if iron and carbon
389
What is the structure of steel ?
carbon sits in interstitial areas between iron atoms
390
Is there a carbon solubility with steel ?
yes - 2%
| the carbon cannot fit between the iron atoms anymore
391
What does steel first form when it solidfies ?
austenite
392
What is the structure of austenite ?
FCC
393
What happens to austenite above carbon conc of 2%?
it becomes stable
394
What happens as austenite cools ?
solubiltiy of carbon decreases
| carbon is chucked out
395
What is Tc ?
critical temperature
396
What happens as Tc ?
critical temperature
| iron changes from FCC to BCC
397
What is the critical temperture ?
723 degrees
398
What type of steel forms at low carbon cocnentrations ?
ferrite forms
carbon is chuckes out
BCC structure
399
What is the structure of ferrite ?
BCC
400
What is the carbon content of ferrite ?
0.02% carbon
401
What happens below 723 degrees ?
no austenite left
| form ferrite and cementite
402
What do ferrite and cementite form ?
pearlite
403
What happen as the carbon concentration increases ?
you get cementitie forming
404
What is cementite ?
ionic
| Fe3C
405
What happens below 0.76% Carbon ?
austenite and ferrite form
406
What happens above 0.76% carbon ?
austenite and cementite
407
What is point X on the steel phase diagram ?
eutectoid
| start to form pearlite
408
What is a eutectoid ?
a solid eutectic
solid to solid
no change from liquid to solid like a eutectic
409
What are the different phases of steel ?
austenite
ferrite
cementite
pearlite
410
What are the properties of austenite ?
stable above useful temperatures for dental applications- not used
411
What are the properties of ferrite ?
solid solution of Fe and C , medium strength
412
What are the properties of cementite ?
Fe3C
ionic
hard but brittle
413
What are the properties of pearlite ?
combined ferrite and cementite
| properties depend on the carbon concentration
414
What happens if we increase the carbon content of pearlite ?
the yield stress increases
ductility decreases
hardness increases
415
How is martensite formed ?
by rapid cooling
416
How does martensite form ?
quenching to get small grains
carbon gets trapped
can no longer leave to form BCC
417
What is the structure of martensite ?
body centred tetragonal
418
What are the properties of martensite ?
hard and brittle
| too brittle for use
419
How can the amount of martensite that forms be controlled ?
use tempering
420
What happens in tempering ?
heat steel to below 723 degrees
no austenite
pearlite forms from martensite
421
If you temper for a higher temperature and longer time what happens ?
more pearlite forms
422
What happens to form martensite from austenite ?
austenite is quenched
423
What does tempering allow ?
control properties of steel
| - any property combo possible
424
Describe the steel triangle ?
pearlite heated form austenite
austenite quenched makes martensite
martensite tempered to make pearlite
425
What are the properties of quenched materials compared to slowly cooled ?
quenched are harder
426
Why is steel not useful in dentistry ?
iron readily corrodes and rusts
| iron oxide binds weakly to iron and flakes easily
427
What is different between steel and stainless steel ?
stainles steel has iron, nickel and chromium added
428
Why is chromium added to stainless steel ?
chromium corrodies faste than iron but chromic oxide binds stronger to chromium - harder to flake off
chromic oxide also retains that chromium colour
429
What happens if chromic oxide does flake off ?
more chromic oxide will form
430
What is passivation ?
forming a stable oxide layer
| passive oxide layer
431
What are the types of steel ?
austenitic
| martensitic
432
What is the composition of austenitic stainless steel ?
18% Cr and 8% Ni added to iron and carbon
433
What are the properties of martensitic stainless steel ?
chromium passivation
| Cr and Ni form solid solutions with steel
434
What is solution hardening ?
when solid solutions form giing better mechanical properties
435
What happens with austenitic stainless steel ?
no austenite to pearlite transformation
| no ferrite made as iron atoms dont want to go into BCC sturcture
436
What is the consequence of a lack of austenitic to pearlite transition ?
cant form martensite
cant temper to control properties
limited use
437
What is martensitic stainless steel ?
12/0 steel
| 12% Cr and 0% Ni
438
What are the properties of martensitic stainless steel ?
chomium forms passive layer in passiviation
| Chromium and nickel form solid solutions with steel- solution hardening
439
Does the austenite to pearlite transition occur in martensitic steel ?
yes
martensite can be formed via quenching
tempering of martensite cna happen
440
What are properties of high carbon 12/0 steel ?
more cementite
| harder
441
What are the properties of low carbon 18/8 steel ?
more ferrite
| softer
442
What are uses of 18/8 stainless steel ?
orthodontics
443
What are the uses of 12/0 stainless steel ?
drills
| burs
