DMS - bonding/GI/Am/Comp/imp/alloys Flashcards
1
Q
Constituents of enamel
A
- Hydroxyapatite (95% weight / 90% volume)
- Water (4% weight / 5-10% volume)
- Organic matrix (1% weight / 1-2% volume)
proteins, amelogenins, enamelins, peptides and amino acids
2
Q
Constituentss of dentine
A
- Hydroxyapatite - (70% weight / 50% volume)
- Water (10% weight / 20% volume)
- organic matrix - (20% weight / 30% volume )
3
Q
content of dentinal tubules
A
- odontoblast process
- Unmyelinated nerve terminals (sensory)
- dendritic cells (immune system)
- dentinal fluid/EC fluid
4
Q
which is more minaeralised: surface or deep enamel
A
SURFACE ENAMEL MORE MINERALISED,
hardness < from cusp tip /incisal edge to cervical region
5
Q
Bonding to enamel
A
acid etch technique - application of acid , this orughens the surface enamel causing characteristic etched pattern.
MOISTURE CONTAMINATION RUINS THIS
- rough surface allows micromechanical interlocking of resin filling materials.
- etching INCREASES surface energy of enamel surface (remove surface contaminants) and INCREASES the WETTABILITY of the enamel.
6
Q
How can enamel be etched - acid?
A
30-50 organic/inorganic acid (aqueous solution of phosphroric acid-37%)
7
Q
constitients of GI cement
A
-acid - tartaric/polyacrylic
-base -Silica, SiO2(Silicone dioxide) 30% -40%
Alumina, Al2O3(Aluminium dioxide) 15% -30%
Calcium Fluoride, CaF215% -35%
Aluminium Fluoride 2% -10%
Aluminium phosphate 4% -20%
Sodium fluoride 4% -10%
acid base reaction - glass and acid =salt and water
8
Q
GI cement - setting reaction stages
A
- dissolution - glass surface is attacked by H ions causing the release of ions (Ca/F/Al/Na), leaves silica gel around unreacted glass
- Gelation - initial set. Bivalent Ca crosslinks with polyacid by chelation to form Calcium polyacrylate
Takes a few minutes, will appear hard but will need to be protected (vaseline) - Hardening - final set. Trivalent aluminium crosslinks with polyacid by chelation.
Takes a long time - 30min-weeks
9
Q
why should GI be protected following gelation
A
-aluminium ions escape
-excessive drying - water lost
-saliva contamination - water absorption
=weak material that will break up easily
10
Q
how does GI bond to tooth
A
chelation with Ca on tooth, reprecipitation of complex of calcium phosphate from apatite, H bonding/metallic ion bridging
11
Q
what factors are needed to achieve a good bond
A
Clean surface
Conditioned surface
Conditioned, not etched.
Little or no tissue is removed.
Best conditioner appears to be polyacrylic acid
Purpose is to produce clean smooth surface
12
Q
- advantages of GI
- Disadvantages
A
Adv - stable chemical bond to enamel and dentine
low microleakage and fluoride release, good thermal properties, no contraction on setting
DIS - Brittle
- Poor wear resistance
- Moisture susceptible when first placed
- poor aesthetics
- Poor handling characteristics
- Susceptible to acid attack and drying out over time
- Possible problems bonding to composite
- Etching damages surface
13
Q
components of RMGIC
A
powder - fluoro alumino silcate glass, barium glass (radiopaque), vacuum dried polyacrylic acid,
potassioum persulphate (allows redox reaction in dark)
ascorbic acid, pigments
liquid -HEMA, polyacrylic acid and pendant methacylate groups, tartaric acid, water and photoinitiators
14
Q
how does RMGIC set - Dual-curing
A
Dual curing
Initially on mixing the acid base reaction begins in the same way as conventional GIC.
On light activation a free radical methacrylate reaction occurs resulting in a resin matrix being formed
Quickly light activation is complete (20s)
Acid Base reaction continues within the resin matrix for several hours
15
Q
how does RMGIC set - tricuring
A
Tri curing
Initially on mixing the acid base reaction begins in the same way as conventional GIC.
The REDOX reaction begins
On light activation a free radical methacrylate reaction occurs resulting in a resin matrix being formed
Quickly light activation is complete (20s)
The REDOX reaction continues for about 5 minutes after initial mixing
Acid Base reaction continues within the resin matrix for several hours
Final hardening of the acid/base phase with aluminium polyacrylateformation can take days
16
Q
properties of RMGIC
A
Good bond to enamel and dentine Superior to conventional GIC ?? Difficult to know what is being measured Definitely better initially Better physical properties Lower solubility Fluoride release Better translucency and aesthetics Better handling DIS - Polymerisation Contraction Exothermic setting reaction both polymerisation and dark cure Swelling due to uptake of water HEMA is extremely hydrophilic Monomer leaching HEMA is toxic to the pulp it must be polymerisedcompletely Reduced strength if not light cured
17
Q
properties for impression materials - elastomers
A
flow /viscosity • surface detail (reproduction) • contact angle / wettability • elastic recovery (%) • stiffness (flexibility) • tear strength • mixing time (min) • working time (min)
18
Q
- surface detail properties of elastomers
- accuracy
- removal from undercuts
- dimentsional stability
A
- Quality of surface interaction between
material & tooth/soft tissue surfaces
viscocity/surface wetting/contact angle - replication of surface detail/viscoelastic behaviour
- Flow under pressure (”shark fin” test)
- Tear/tensile strength
- Rigidity - Setting shrinkage
- Thermal expansion/contraction
- Storage
19
Q
explain the chemistry of elastomers
A
Elastomers - formed by polymerisation with cross-linking of polymer chains • Cross-linking:- – generates ELASTIC properties – causes FLUID SOLID transition • Polymerisation MAY produce BYPRODUCTS (H2O, H2, alcohol) which affect DIMENSIONAL STABILITY and cast compatibility
20
Q
conventional silicone - constituents
base paste
catalyst paste
A
Base paste
– silicone prepolymer with terminal hydroxyl groups
– filler
• Catalyst paste (or liquid)
– crosslinking agents*
– activator - organo-tin compound
*alkoxy orthosilicate or organohydrogen siloxane
21
Q
addition cured silicone - constituents
base paste
catalyst paste
A
Base paste - • polydimethylsiloxane - some methyl (CH3) groups replaced by hydrogen • filler - variations change viscosity • Catalyst paste - • polydimethylsiloxane - some methyl groups replaced by vinyl (CH2 =CH) • filler - variations change viscosity • platinum catalyst eg chloroplatinic acid
22
Q
polyether constituents
base paste
catalyst paste
A
Base paste • imine terminated prepolymer - cross linking • inert filler - viscosity, strength • Catalyst paste • ester derivative of aromatic sulphonic acid - initiates polymerisation • inert oils - form paste • inert fillers - form paste
23
Q
requirements of a DBA
A
Ability to flow Potential for intimate contact with dentine surface Low viscosity Adhesion to substrate Mechanical Chemical Van der Waals Combination of the above
24
Q
how does van der waal forces cause bonding
A
Van der Waals Adhesion
Based on electrostatic or dipole interaction between
bonding agent and substrate
Strength of interaction depends on CONTACT ANGLE,
which is a good indication of WETTABILITY of a solid
by a specific liquid. A contact angle of <90o means the
solid surface is hydrophilic
Best adhesion/bonding is achieved when Van der
Waals forces are optimised
25
what is critical suface energy
A liquid must have a lower surface energy than the
surface it is being placed on for it to flow onto it and
stick.
A low surface energy liquid will spread on a higher
surface energy substrate because this leads to a
lower surface energy of the material as a whole.
dentine has a low surface energy so a DBA increases this
26
what is the smear layer
The smear layer is an adherent layer of organic debris that remains
on the dentine surface after the preparation of the dentine during
the restoration of a tooth.
It is 0.5 – 5 microns in thickness.
It is variably attached to the dentine surface.
It is generally contaminated with bacteria.
Originally it was thought of as a protective barrier reducing
permeability of the dentine and protecting the pulp.
Now it is considered to interfere with adhesion
27
components of total etch bonding agent
Dentine conditioner: An acid, usually 35% phosphoric.
Primer: Really the adhesive part of the agent with a
hydrophilic/hydrophobic molecule
Adhesive: A resin which penetrates into the surface of
the dentine attaching to the primers hydrophobic
surface
28
what is a dentine conditioner
Generally Phosphoric acid, can be EDTA or Nitric
acid in some older systems.
Removes smear layer
Opens dentinal tubules by removing smear plugs
Decalcifies the uppermost layer of the dentine
The etchant is washed off with water.
The collagen network in this top 10um of the dentine
is exposed and subsequently penetrated by the next
two components
29
what is a dentine primer
Primer
The primer is really the adhesive element in the
process. A coupling agent.
It has a bifunctional molecue with a hydrophilic end to
bond to the hydrophilic dentine surface and a
hyrdophobic, methacrylate end to bond to the resin.
The molecule must also have a spacer group to make
it long enough to be flexible when bonding. Lack of
flexibility reduces bonding sites and bond strength.
This molecule or group of molecules is dissolved in a
suitable solvent. Ethanol, acetone or water
30
name an adhesive and how it works
It is predominantly hydrophobic.
It may contain some filler particles to make it stronger.
It will contain Camphorquinone to allow it to light
cure
Penetrates the primed dentine which now has a
hydrophobic surface.
Forms a micromechanical bond within the tubules and exposed dentinal collagen fibres.
Forms the HYBRID LAYER of collagen plus resin.
31
what are some isssues with total etch
Over etching à to collapse of the collagen fibres so no
resin can penetrate
Over etching à too deep an etch and the primer
cannot penetrate to the full depth of the etch.
Moisture dependent:
Too dry and the dentine surface collapses
Too wet and the primer is diluted à reduced strength
32
what are some issues with self etching primers
Self etching primers
These materials work in a different way from all the
previous materials.
They do not attempt to remove the smear layer. They
infiltrate it and incorporated themselves into it.
They are not washed off.
This removes the problem of how dry to make the
dentine.
Not as technique sensitive but bond doesn’t seem to
be as strong.
33
components of composite resin
- filler particles - quartz
- resin - Bis-GMA
- camphorquinone -produce free radicals to break c=c bonds
- low weight dimethacrylates - viscocity and reactivity
- silane coupling agent - ensure good bond
34
types of composite to be used in areas of mouth
- anterior
- posterior
- universal
-anterior - microfilled, or submicron
hybrid
– posterior - heavily filled
– universal - submicron hybrid
35
classification by handling characteristics
```
condensable - “amalgam
feeling” - greater porosity
– syringeable - good
adaptation, less porosities,
easy to apply
– flowable - lower filler
content, more shrinkage,
difficult to apply, place for
them -with fibre ribbons
```
36
effect of adding more filler particles
improved mechanical properties
– strength, hardness, rigidity etc
• improved aesthetics
• increased abrasion resistance
• lower thermal expansion (still not perfect)
• lower polymerisation shrinkage (still a problem)
• less heat of polymerisation (BUT not negligible)
• some radiopaque
37
how do self cure and light cure occur
```
self curing:
benzoyl peroxide + aromatic tertiary amine
free radicals (break resin C=C bonds)
• light curing:
camphorquinone + blue light (430-490 nm)
```
38
advantage of light cure acrylic
```
extended working time
ie on-demand set
• less finishing
• immediate finishing
• less waste
• higher filler levels
(not mixing two pastes)
• less porosity
(not mixing two paste
```
39
what is depth of cure
```
the depth at which the composite
resin polymerises sufficiently,
such that its hardness is about
half that of the cured surface
2mm
```
40
comparison of:
conventional
microfine
hybrid
Conventional:
– strong but problems with finishing and staining due to soft resins and hard particles
• Microfine:
– smaller particles - smoother surface better aesthetics
for longer period
– but inferior mechanical properties (Elastic limit &
Young’s Modulus
hybrid - improved filler loading and coupling agents have
led to improvement in mechanical properties
41
factors affecting wear of composite
```
filler material
• particle size distribution
• filler loading
• resin formulation
• coupling agent
```
42
properties of composite
-abrasion resistance
-Thermal Conductivity
• low – which is good
-Thermal Expansion coefficient
- high - which is poor
(NB for composite: 28 ppm/ 0C
enamel: 11.4 ppm/ 0C
dentine: 8.3 ppm/ 0C)
Thermal Conductivity
• low – which is good
-bond to tooth
-aesthetic
-radiopaque
-handling/viscocity
-smooth finish
-NOT anticariogenic
-low setting shrinkage
-biocompatible
43
amalgam powder contents
| liquid content
Silver, Tin- intermetallic compound Ag3Sn
- g phase, reacts with Hg liquid to form amalgam
• Copper- increases strength & hardness
• Zn - scavenger during production - preferentially
oxidises & slag formed / removed - some zinc free
• Hg in powder - (few materials)
– “pre-amalgamated” alloys - react faster
-liquid - mercury
44
name 2 types of amalgam particles
```
lathe cut
– coarse, medium, fine
– formed by filing ingots
• spherical, spheroidal
– range of particle sizes
– formed by spraying molten
metal into inert atmosphere
```
45
amalgam reaction
Ag3Sn + Hg --> Ag3Sn + Ag2Hg3 + Sn7Hg9
| gamma gamma gamma 1 gamma 2
46
properties of gamma/gamma 1/gamma 2
```
g good strength &
corrosion resistance
• g1 good corrosion
resistance
• g2 weak and poor corrosion
resistance
```
47
describe the issues with zinc
interaction of zinc with saliva / blood -
Zn + H2O ZnO + H2
• bubbles of H2 formed within amalgam
– pressure build up causes expansion
– downward pressure cause pulpal pain
– upward - restoration sitting proud of surface
• Hence zinc-free materials
48
name some properties of amalgam
```
Strength
– generally consider compressive but others
important
– early (1hr) - Traditional materials, poor(ish)
– late (> 24hrs) - OK
– see also comparison later
• Abrasion Resistance - high, suitable for posterior teeth/too high for deciduous
-3x thermal expansion of tooth
-high thermal conductivity
-no bond to tooth
Aesthetics - poor
• Radiopaque - yes
• Anticariogenic - no
• Smooth surface - yes, if polished well,
may deteriorate over time
• Setting shrinkage - modern materials
tend to have net overall shrinkage
```
49
factors decreasing strength of amalgam
```
undermixing
• too high Hg content after condensation
• too low condensation pressure
• slow rate of packing
– increments do not bond
• corrosion
```
50
what is creep
When a material is repeatedly stressed
for long periods at low stress levels ie
stress below elastic limit, it may flow,
resulting in permanent deformation
51
ADV of spherical particles
```
less Hg required
• higher tensile strength
• higher early compressive strength
• less sensitive to condensation
• easier to carve
```
52
name 2 types of copper enriched amalgam
dispersion modified
| single composition
53
dispersion modified copper enriched amalgam
| benefits
originally Ag-Cu spheres + conventional lathe cut alloy
(now some single composition dispersed alloys - spheres & lathe
cut particle same composition)
• originally thought spherical particles would act as
strengthening agent, but
• increased copper content gave beneficial modifications
to setting reaction
54
setting reaction of dispersion modified
- g + Hg g + g1 + g2
| - g2 + Ag-Cu Cu6Sn5 + g1
55
benefits of copper enriched
Higher early strength
• Less creep
• Higher corrosion resistance
• Increased durability of margins
56
setting reaction of single composition
Ag-Sn-Cu + Hg Ag-Sn-Cu + g1 + Cu6Sn5
57
what is a stress concentration point in an endo file
Abrupt changes in the geometric shape of a file that leads to a higher stress at that point
58
what is strain
the change in dimension over the original (deformation)
59
what is the elastic limit
A set value representing the maximal strain that when applied to a file, allows the file to return to original dimensions
60
what is plastic deformation
Permanent bond displacement occurring when elastic limit exceeded
61
what is plastic limit
the point at which a plastic deformed fine breaks
62
what is cyclic fatigue
Freely rotating in a curvature
•Generation of tension/compression cycles
•Cyclic fatigue
•Failure
Tension
63
what is work hardening
Strengthening of a metal by plastic deformation
•Crystal structure dislocation
•Dislocations interact and create obstructions in crystal lattice
•Resistance to dislocation formation develops
•Observed work hardening
64
sodium hypochlorite - which ions predominate at different pH - neutral/acid and alkaline
NaOCl ionises in water into NA+ and the hypochlorite ion, OCl-
•Establishes equilibrium with hypochlorous acid (HOCl)
•Acid/Neutral HOCl predominates
•pH 9 and above OCl- predominates
•HOCl is responsible for antibacterial activity
65
factors important for NaOCl function
```
Concentration
•Volume
•Contact
•Mechanical agitation
•Exchange
```
66
how to remove smear layer
17% EDTA
•10% Citric Acid
•MTAD (Mixture of a Tetracycline isomer, an Acid, and a Detergent
•Sonic and Ultrasonic irrigation
67
what is in GP
20% Gutta-percha
•65% Zinc Oxide
•10% Radiopacifiers
•5% Plasticizers
68
constituents of green stick
carnauba/stearic acid/wax/talc
69
2 types of imp material
MUCOSTATIC:
(eg zinc oxide eugenol, low viscosity alginates)
- fluid materials that displace the soft tissues slightly
- ie give an impression of the undisplaced mucosa.
MUCOCOMPRESSIVE
eg impression compound, high viscosity alginates/elastomers)
- viscous materials that record an impression of the mucosa under load
ie give impression of displaced soft tissue.
70
components of alginate
```
Component % Function
Salt of alginic acid - 12% reacts with Ca ions
(eg Na alginate)
Calcium sulphate - 12% provides Ca ions
Trisodium phosphate - 2% delays gel formation
Filler - 70% cohesion, strength
modifiers, - small improve surface,
flavourings - taste,
chemical indicators - pH colour change
```
71
ideal partial denture alloy properties
```
rigid (YM)
strong (UTS, EL)
hard
ductile
precise casting (shrinkage)
melting point (investment material)
density
```
72
4 examples of partial denture alloys
ADA Type IV Gold
* White Gold (Ag-Pd)
* Co-Cr
* Titanium
73
type IV gold composition
```
Au (60-70%) 65%
Zn (1-2%) 2%
Cu (11-16%) 14%
Ag (4-20%) 14%
Pd (0-5%) 3%
Pt (0-4%) 2%
```
74
effect of copper as alloying element with gold
1) solid solution in all proportions
2) solution hardening
3) order hardening - if 40-80% Gold
and correct heat treatment
4) reduced melting point
5) no coring - solidus close to liquidus
6) imparts red colour (if sufficient quantity)
7) reduces density
75
effect of silver as alloying element with gold
1) solid solution in all proportions
2) solution hardening
3) precipitation hardening with COPPER
& heat treatment
4) can allow tarnishing
5) molten silver absorbs gas (e.g. CO2)
6) whitens alloy - compensation for
copper
76
effect of platinum as alloying element with gold
1) solid solution with Gold
2) solution hardening
3) fine grain structure
4) coring can occur
(wide Liquidus - Solidus gap
77
composition of CoCr
```
Co (35-65%) 54%
Cr (25-30%) 25%
Ni (0-30%) 15%
Mo (5-6%) 5%
C (0.2–0.4%) 0.4%
```
78
CoCr properties as partial denture alloy
```
much harder than Gold
* wear in mouth better
* finishing/polishing time
consuming
Elongation : 4%
* low ductility
* work hardens rapidly
* adjustment difficult
* precision casting
```
79
Definition:
Elastic limit
ductility
EL - maximum stress without plastic
deformation
Ductility - amount of plastic deformation
prior to fracture (ie measure of
the extent that a material can be
shaped/manipulated = (y-x)%
80
In alloys, how do crystals grows
-Atoms at these sites act
as nuclei of crystallisation
-Crystals grow to form dendrites (3D branched
lattice network)
-Crystals (or GRAINS) grow until they impinge
on other crystals
-Region where grains make contact is called
GRAIN BOUNDARY
81
what are equi-axed grains
if crystal growth of equal dimension in
| each direction – EQUI-AXED grains
82
- what is quenching and what effect does it have on grains
| - what effect does slow cooling have on grains
```
fast cooling (QUENCHING):-
-more nuclei
-small fine grains
slow cooling :-
-few nuclei
-large coarse grains
```
83
what is a grain
Grains:
- each grain is a single crystal (lattice)
with atoms orientated in given directions
(Dendrites)
84
what is grain boundary
Grain boundary:
- change in orientation of the crystal
planes (impurites concentrate here)
85
what are dislocations
| -what is slip
Dislocations are imperfections/defects in
the crystal lattice
-SLIP is due to Propagation of Dislocations
and involves rupture of only a few bonds at
a time
86
what factors can impede the movement of dislocations
```
INCREASES
elastic limit
UTS
hardness
DECREASES
ductility
impact resistance
```
87
what is cold working
| -effect
working done at LOW TEMPERATURE
(ie below recrystallisation temperature)
-causes SLIP – so dislocations collect at
grain boundaries
```
modifies grain structure:
higher
-Elastic Limit
-UTS
-hardness
-
lower
-ductility
-impact strength
-lower corrosion resistance
```
88
what is annealing
```
heating metal (or alloy) so that
greater thermal vibrations allows
migration of atoms
(ie re-arrangement of atoms)
-residual stress eliminated
```
89
definition
- phase
- solution
PHASE - physically distinct homogeneous
structure (can have more than one component)
SOLUTION - homogeneous mixture at an
atomic scale
90
on crystallisation, what 3 forms can alloys be in
ON CRYSTALLISATION, metals may :-
– a) be insoluble, no common lattice - 2 phases
– b) form intermetallic compound with specific
chemical formulation (eg Ag3Sn)
OR
– c) be SOLUBLE and form a SOLID SOLUTION,
ie form common lattice… 3 types of solid solution.
91
what is a sustitutional solid solution
- random
- ordered
```
Substitutional
atoms of one metal replace the other metal in the
crystal lattice/grain.
- A. RANDOM:- metal atoms similar in:-
SIZE, VALENCY, CRYSTAL STRUCTURE (eg fcc)
eg AuAg, AuCu
– B. ORDERED:- metal atoms in regular
lattice arrangement, conditions as above
```
92
what is interstitial solid solution
2. Interstitial
– atoms markedly different in size
– smaller atoms located in spaces in lattice/grain
structure of larger atom (eg Fe-C)
93
on a phase diagram, what does the liquidus and solidus represent
LIQUIDUS
line representing the temperatures which different
alloy compositions begin to crystallise
SOLIDUS
line representing the temperatures which different
alloy compositions have completely crystallised
94
what does rapid cooling of a molten alloy cause
RAPID COOLING of MOLTEN ALLOY
prevents atoms diffusing through lattice
-causes CORING
as composition varies throughout grain.
95
what does coring leave an alloy susceptible to
CORING:
may reduce corrosion resistance of the
solid form of alloy
96
what is homogenous annealing
| -why is it completed below recrystallisation temperature
HOMOGENISING ANNEAL
-once solid cored alloy formed
REHEAT to allow atoms to diffuse and so cause
grain composition to become homogeneous
NOTE: keep below recrystallisation temperature,
otherwise grains altered
97
why are alloy inherintly more # resistant that metal
More energy/force is needed for the defect to
overcome the different-sized atoms, and move along
lattice to the grain boundary.
-Hence, it requires greater stress to move dislocations
in a solid solution – making alloys inherently more
fracture resistant (ie stronger) than metals
98
why do alloys forming an ordered solid solution have better properties, what are they
```
Alloys forming an ORDERED SOLID
SOLUTION (atoms distributed at specific
lattice sites) have a distorted grain
structure (eg Au-Cu)
which IMPEDES dislocation movement and
so improves mechanical properties (EL,
UTS, hardness)
```
99
Iron is allotropic, what does this mean
| -what temperature do these occur
Allotropic - undergoes TWO solid state
phase changes with temperature.
(1) Temp. > 1400 C
BCC lattice structure; low Carbon solubility (0.05%)
(2) 900 < Temp. < 1400 C
FCC lattice; higher Carbon solubility (2%)
(3) Temp < 900 C: as (1)
100
definition of
- austentite
- ferrite
- cementite
- pearlite
```
AUSTENITE: interstitial solid solution, FCC;
exists at high temp (ie >720 0C)
FERRITE: very dilute solid solution;
exists at low temp
CEMENTITE: Fe3C ; exists at low temp
PEARLITE: Eutectoid mixture of
Ferrite and Cementite
```
101
how is martensite produced
quenching of austenite (fast cooling)
102
what does slow cooling of austenite produce
pearlite
103
composition of stainless steel
| -how much chromium (%) must be in the alloy to be classed as stainless
iron/nickel/chromium/carbon
| 13% chromium
104
purpose of chromium in stainless steel
```
lowers Austenite to Martensite temperature
: lowers Austenite to Martensite rate
: decreases % carbon at which Eutectoid
formed
: corrosion resistance
```
105
purpose of nickel in SS
lowers Austenite to Martensite transition
temperature
- improves UTS
- improves corrosion resistance
106
uses of austenitic SS
1. dental instrument (not cutting edge)
2. wires - ortho
3. denture bases
107
composition of SS for ortho wire
18% Chromium
8% Nickel
0.1% Carbon
74% Iron
108
requirement for ortho wires (properties)
high springiness ( EL / YM) (IE undergo large deflections without permanent
deformation)
• stiffness (YM) - depends on required force for tooth movement
• high ductility - bending without fracture
• easily joined without impairing properties - soldered, welded
• corrosion resistant
Springiness ( EL / YM)-Ability of a material to undergo large deflections (to form arc) without
permanent deformation (ie it returns to its
original shape)
109
how does weld decay occur and at what temperature
| -effect
```
Occurs between 500 - 900 0C
Chromium carbides precipitate at
grain boundaries
-alloy becomes brittle
• less chromium in central region of
solid solution
• more susceptible to corrosion
```
110
how to minimise weld decay
```
Low carbon content steels - expensive
2. Stabilised stainless steel
- contain small quantities of
titanium or niobium
- forms carbides preferentially
- not at grain boundaries
```
111
how are enamel crystalites deposited in relation to ameloblast membrane
Crystallite orientation is determined during enamel formation
•
Crystallites are deposited at right angles to ameloblast membrane
