Partial Denture Alloys Flashcards
what are 5 good mechanical properties of alloys to remember
Stress
Rigidity
elastic limit
hardness
ductility
4 ways an alloy can be hardened
work hardening
solution hardening
order hardening
precipitation hardening
2 types of alloy annealing
homogenisation annealing
stress relief annealing
7 ideal properties of partial denture alloys
rigid (YM)
strong (UTS, EL)
hard
ductile
precise casting (shrinkage)
melting point (investment material)
density
4 types of partial denture alloys
ADA Type IV Gold
White Gold (Ag-Pd)
Co-Cr
Titanium
what technique is used in partial denture manufacture
one piece casting
YM and EL of base for one piece casting
high YM - to maintain shape in use
- withstand large stresses and not change shape greatly
high EL - to avoid plastic deformation
YM and EL of clasp for one piece casting
lower YM - to allow flexure over tooth
- flex easily and disengage readily
high EL - maintain elasticity over wide range of movement (i.e. strain)
why do both the clasp and base need a high EL
avoid any permanent/plastic deformation even if apply large stress to material
what is the compromise that is needed in one-piece manufacture to benefit both the base and clasp
thick section - rigid base;
thin section - flexible clasp
2 ways to describe Pure Gold
Carat : 24 - pure gold
Fineness : 1000 fine - 100% Au
example use of pure gold
class III and IV cavities in some situations
uses of different types of gold
Type I : simple alloys
Type II : larger (2-3 surface) inlays
Type III : Crown & Bridge alloys
Type IV : Partial Dentures
uses of Type IV gold
partial dentures
what metals are in type IV gold
Au gold
Zn zinc
Cu cooper
Ag silver
Pd palladium
Pt platinum
type IV gold % composition of gold
60-70% (65)
type IV gold % composition zinc
1-2% (1)
type IV gold % composition cooper
11-16% (14)
type IV gold % composition of silver
4-20% (14)
type IV gold % composition palladium
0-5% (3)
type IV gold % composition platinium
0-4% (2)
what are the liquidous and solidus line like for Au-Cu phase diagram
Continuous
- Have all sorts of combination of gold and cooper
- All the way through
8 effects of adding cooper to gold alloy
solid solution in all proportions
solution hardening
order hardening
reduced melting point
no coring
imparts red colour
- due to cooper if sufficient quantity
reduces density
- more cooper leads to lower density
base metal - can corrode if too much
how can you tell if there will be little to no coring
solidus relatively close to liquidus
so little to none coring on quenching
what is order hardening
if 40-80% Gold and correct heat treatment
- Once taken alloy quenched to room temp and then heat treat
- Undergoes some form of heat treatment
forms an ordered solid solution (Row of Au row of Cu)
- Little peak in phase diagram
6 effects of adding silver to gold alloy
solid solution in all proportions
solution hardening
precipitation hardening with COPPER & heat treatment
- silver and cooper can benefit from precipitation hardening, utilise to improve mechanical properties
can allow tarnishing
molten silver absorbs gas e.g. CO2
- can lead to porosity in casting process
whitens alloy – compensates for copper
describe the phase diagram for Ag-Au
Simple
Close together
Any value an coexist in same grain structure
describe the phase diagram for Ag-Cu (in type IV Gold)
have solubility limit lines
- Partially soluble solid solution
End up with potential precipitation hardening
describe the phase diagram of AgPt
Simple
Reasonable degree of separation
- Coring can occur on Rapid cooling
- Grains have concentration gradient
Have to further process
what feature does a partially soluble solid have on phase diagram
have solubility limit lines
4 effects of adding platinum to gold alloy
solid solution with Gold
solution hardening
fine grain structure
- greater mechanical properties
coring can occur
- wide Liquidus - Solidus gap
4 effects of adding pallidium to gold alloy
similar to Pt
less expensive
less coring than Pt
coarser grains than Pt
absorbs gases when molten
porous casting – weaken end RPD
contribution of zinc to alloy
scavenger
contribution of nickel to alloy
increase hardness and strength (wrought alloys)
contribution of indium to alloys
fine grain structure
why would you use heat treatment on partial denture alloys
All to improve properties of RPD and minimise negatives
4 types of heat treatment on gold alloys
Quench after casting
fine grains – good mechanical properties
Homogenising anneal (700C, 10 mins)
If cold worked - stress relief anneal
- Homogenous grain composition
- Raise temperature for some time but not to melting point, cool
Cold working to manipulate – if doesn’t fit the pt exactly
Heat harden - (order & precipitation)
- 450C cool slowly (15 - 30 mins) to 200C then quench
- —After cold work, raise temperature then quench
role of stress relief annealing of type IV gold
Homogenous grain composition
- Raise temperature for some time but not to melting point, cool
Cold working to manipulate – if doesn’t fit the pt exactly
role of heat hardening type IV gold
order and precipitation
450C cool slowly (15 - 30 mins) to 200C then quench
—-After cold work, raise temperature then quench
advantage and disadvantage for partial denture of heat treated type IV gold alloy
Properties more suitable for clasp
Need thickness for base (expense)
3 uses for CoCr
Wires
Surgical Implants
Cast Partial Dentures
Connectors :
- High EL, High YM - thick section
- High EL, Low YM - thin section
metals in CoCr
cobalt
chromium
nickle
molybdenum
carbon
% composition in CoCr of cobalt
35-65% (54)
% composition in CoCr of chromium
25-30% (25)
% composition in CoCr of nickle
0-30% (15)
% composition in CoCr of molybdenum
5-6% (5)
% composition in CoCr of carbon
0.2-0.4%
3 effects of cobalt in CoCr
Forms solid solution with Cr
Increase strength, hardness, rigidity
Coring possible
solid solution alloy arrangement
2 metals coexist in normal lattice arrangement
- Across whole spectrum of compositions
4 effects of chromium in solid CoCr alloy
Forms solid solution with Co
Increase strength, hardness, rigidity
Coring possible
Forms passive layer – corrosion resistance
- Passive oxide layer to resist corrosion – KEY
4 effects of nickel in CoCr alloy
Replaces some Co
Improves ductility
COMPROMISE
Slight reduction in strength
Sensitivity
- 6% females; 2% males
2 effects of carbon in CoCr alloy
Undesirable
- Ideally not have any, but inevitable to make in casting
Carbide grain boundaries hard & brittle
role of molybdenum in CoCr
Reduces grain size
- increase strength
role of tungsten (W) in CoCr alloy
Al: increases PL
Other: scavengers
what is the point of having a range of metals of varying compositions in an alloy
Multiple ingredient – best mix for greater properties
investment material for CoCr
High temp. 1200-1400C
Hence - silica or phosphate bonded
- NOT GYPSUM
2 melting techniques for CoCr
Electric Induction (preferred)
Oxyacetylene - avoid carbon pickup (too many problems to be used now)
what is the preferred melting technique for CoCr
electric induction
technique for casting CoCr
Centrifugal force required
- Avoid overheating
—-Risk coarse grains
Cooling too fast or slow - carbides - brittle
techniques for finishing CoCr (4)
sandblast
electroplate
abrasive wheel
polishing buff
why is CoCr hard to polish
very hard material
- but then equally hard to roughen up in use
- – so maintains surface longer
hardness of CoCr
BHN 370
why is the high hardness of CoCr good (2)
Much harder than Gold
Wear in mouth better
- Experience less wear
downside of high hardness of CoCr
Finishing/polishing time consuming
elongation/ductility value for CoCr
4%
low
what is elongation/ductility
Ability of material to be stretched, change shape
effect of CoCr having low elongation value
Work hardens rapidly
Adjustment difficult, thus precision casting
- Only small amount can be done to fit pt
- Casting needs to be really precise so 4% is adequate
uses of pure titanium (4)
Implants
Partial Dentures (Cast)
Crown & Bridge (Cast)
Maxillo-Facial Skull Plates
3 advantages of titanium
Good biocompatibility
Good corrosion resistance (passive oxide layer )
Individual parts joined by laser welding
- Casting process less of a challenge
why is good biocompatibility key to pure titanium use
Can be used in more demanding situations than RPD e.g. maxilla-facial skull plates
% of titanium in pure titanium
99.5%
how is titanium melted
Electric Arc Melting
Specialised investment and casting – not in GDH
Can buy in from commercial outlets
why is electric arc melting used for titianium
As titanium absorbs gases
what alloy has the highest elongation/ductility value
Titanium easy engage and disengage in pt dentition
Expensive
- generally between Type IV or CoCr
UTS is
fracture strength (MPa)
compare UTS of type IV gold, CoCr and titanium
Not identical
- But not tremendous difference in fracture strength
why is density (g/cm3) important for partial denture alloys
Pt comfort
compare densities of type IV Au and CoCr
Pt comfort
Au is the most dense
CoCr is half
- favoured for pt as lighter
compare the rigidity (young’s/elastic modulus (GPa)) of CoCr, type IV Au and Titanium
CoCr 250
Au 100
Titanium 110
CoCr is least likely to undergo dimension change due to stress
compare the hardness between CoCr and Au
and explain this effect on finsihing
CoCr 370
Au 220
Au not as hard to polish but wears down easier
what does shrinkage of partial denture alloys impact on
investment material used
large shrinkage is a challenge
which is more difficult to produce a defect free casting CoCr or gold
CoCr
can you use conventional gypsum bonded investment material with CoCr
no
is polishing CoCr easy
no as its surface is harder than gold
- but due to this retains polish longer
why is precise casting needed for CoCr
work hardens (cold hardens) rapidly - casting process is harder to avoid defects
Au Vs CoCr rigidity
Au less than half rigidity of CoCr
Au Vs CoCr fracture strength UTS
Fracture strength is similar
Au Vs CoCr ductility
Ductility Au exceeds CoCr
AU VS CoCr hardness
CoCr hardness is significantly more than type IV Au
