Partial denture alloys Flashcards
What are soem partial denture alloys
ADA Type IV Gold
Co-Cr
White Gold (Ag-Pd)
Titanium
What properties do the base of the denture and the clasps need to have
Base : high rigidity - to maintain shape in use
high strength - to avoid plastic deformation
Clasp : lower rigidity - to allow flexure over tooth
high strength - maintain elasticity over wide range of movement (ie strain)
How many types of gold alloys are used and when
Type I : simple alloys
Type II : larger (2-3 surface) inlays
Type III : Crown & Bridge alloys
Type IV : Partial Dentures
What is the composition of type 4 gold
Au - 65%
Ag - 14%
Cu - 14%
Zn - 2%
Pd - 3%
Pt - 2%
What are the properties of gold alloyed with Copper
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) little/no coring - solidus close to liquidus
6) imparts red colour (if sufficient quantity)
7) reduces density
8) base metal - can corrode if too much
What effect does Gold alloyed with silver have
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 - compensates for copper
What changes does platinum bring to gold as an alloy
1) solid solution with Gold
2) solution hardening
3) fine grain structure
4) coring can occur (wide Liquidus - Solidus gap)
Compare palladium to platinum as an alloy
similar to Pt - less expensive
- less coring than Pt
- coarser grains than Pt
- absorbs gases when molten - porous casting
What heat treatment is carried out on type 4 gold alloys
1) Quench after casting (fine grains)
2) Homogenising anneal (700oC, 10 mins)
3) If cold worked - stress relief anneal
4) Heat harden - (order & precipitation)
–> 450oC cool slowly (15 - 30 mins) to 200oC then quench
What is the composition of Co Cr
Co - 54%
Cr - 25%
Ni - 15%
Mo - 5%
C - 0.4%
What state is the solution formed by Co and Cr
Solid
What is the benefit of nickel
Improves ductility while slightly reduces strength
What are the effects of different metals on Co-Cr alloy
C:- avoid including too much - as carbides will form at the grain boundaries making the alloy too hard & brittle
Mo:- reduces grain size,
hence increases strength
W:- increases strength
Al:- increases PL
others:- scavengers
What are the techniques used to shape Co-Cr
Investment Material :
High temp. 1200-1400oC
Hence - silica or phosphate bonded
Melting :
Electric Induction preferred
Oxyacetylene - avoid carbon pickup
Casting :
Centrifugal force required
Avoid overheating - coarse grains
Cooling too fast or slow - carbides - brittle
What techniques are used to finish Co-Cr
sandblast
electroplate
abrasive wheel
polishing buff
properties of Co-Cr
Harder than gold, time consuming to finish/polish
Low ductility - adjustment difficult
work hardens rapidly
When is titanim used in dentistry
Implants
Crown & Bridge (Cast)
Maxillo-Facial Skull Plates
Partial Dentures (Cast)
Benefits of titanium
Good biocompatibility
Good corrosion resistance (passive oxide layer)
Parts joined by laser welding
What alloy has greatedt ductility/elongation
Titanium
Of gold, silver, Co-Cr, titanium and steel which has the greatest strength
Stainless steel
Which alloy is most dense - gold, silver, Co-Cr, titanium or steel
Gold
What metal alloy has the greatest rigitidy
Co - Cr, followed by stainless steel
What alloy is hardest - gold, silver, Co-Cr, titanium and steel
Co-Cr, then steel
What alloys have shrinkage
Co-Cr - most shrinkage
then gold and silver
What are the disadvantages of CoCr
-more difficult to produce defect free casting than GOLD
-cannot use gypsum-bonded investment
-more difficult to polish (ie harder) than GOLD but retains polish better
-work hardens rapidly, so need precision casting
How is an alloy made stronger and harder based on the solution formed
Solid solution - Solution hardening
Ordered solid solution - Order hardening
Partially soluble alloy - Precipitation hardening
Compare properties of gold and Co-Cr
Gold is more dense, greater fracture strength, more ductile
Co-Cr shrinks more, is more rigid, and harder
What are the ideal properties of a partiaal denture alloy
rigid (YM)
strong (UTS, EL)
hard
ductile
precise casting (shrinkage)
melting point (investment material)
density
