Partial Denture Alloys Flashcards
What are the ideal properties of a partial denture alloy
▪ Rigid (Young's Modulus) ▪ Strong (ultimate tensile strength and elastic limit) ▪ Hard ▪ Ductile ▪ Precise casting (shrinkage) ▪ Melting point (investment material) process Density
Why is melting point important for partial denture alloys
Partial denture alloys are produced using the casting technique - one of the challenges with this method is that the melting point will dictate the type of investment material that will be used in that casting process
What are the partial denture alloys
ADA type IV gold
White gold (Ag-Pd)
Co-Cr
Titanium
What properties do we want a denture base to have
a HIGH YOUNG’S MODULUS to withstand forces and maintain its shape in use and a HIGH ELASTIC LIMIT to avoid plastic deformation
What properties do we want the denture clasp to have
a LOW YOUNG’S MODULUS to allow flexure over the tooth and can disengage and a HIGH ELASTIC LIMIT to maintain its elasticity over a wide range of movement
To compromise the properties we want for a denture base and clasp what do we do
we use THICK SECTIONS for a rigid base and THIN SECTIONS for a flexible clasp
What are the ADA specifications of gold alloys
Type I: simple alloys
Type II: larger (2-3 surface) inlays
Type III: crown and bridge alloys
Type IV: partial dentures
What is the composition of type 4 gold
Gold (60-70%)
Silver (4-20%)
Copper (11-16%)
Zinc (1-2%)
Palladium (0-5%)
Platinum (0-4%)
What does the phase diagram for Au-Cu show
the liquidus and solidus are CONTINUOUS meaning that we can have all sorts of combinations for gold and copper
What are the effect of copper as an alloying element in a gold alloy
Solid solution in all proportions
Solution hardening
Order hardening - if 40-80% Gold and correct heat treatment
Reduced melting point
No coring - solidus is close to the liquidus
Imparts red colour (if sufficient quantity)
Reduces density
Base metal so copper is more likely to make it corrode
What does the phase diagram for Au/Ag show
This phase diagram shows that any combination of gold and silver can coexist in the same grain structure
What are the effect of using silver as an alloying element in a gold alloy
Solid solution in all proportions
Solution hardening
Precipitation hardening with COPPER and heat treatment and this can be used to improve the properties of the materials
Downside is that it can allow tarnishing
Molten silver absorbs gas (CO2) - porosities during casting
It whitens the alloy
What does molten silver absorbing gas result in
it creates porosities during the casting process
Why is silver whitening the alloy an advantage
compensates for the presence of copper
What is the phase diagram for Ag-Cu
shows that it has partial solid solubility
has solubility limit lines
Because the Ag-Cu is partially soluble solid solution what can it suffer from
precipitation hardening
What is the phase diagram for Au-Pt
straight forward
What is the effect of using platinum as an alloying element in a gold alloy
Solid solution with Gold
Solution hardening
Fine grain structure
Coring can occur (wide liquidus-solidus gap)
What is the effect of using palladium as an alloying element in a gold alloy
Similar to Pt but less expensive
Less coring than Pt
Coarser grains than Pt
Absorbs gases when molten - porous casting
What is the function of zinc in type 4 gold
scavenger
What is the function of nickel in type 4 gold
increase hardness and strength
What is the function of indium in type 4 gold
fine grain structure
Describe the heat treatment for type 4 gold
1) Quench after casting (fine grains)
2) Homogenising anneal (700 degrees Celsius for ten minutes)
3) If cold worked then stress relief annealing is required
4) Heat harden - (order and precipitation) at 450 degrees and then cool slowly through 15-30 minutes to 200 degrees then quench it
What are the properties of type 4 gold (heat treated) more suitable for
Its properties more suitable for clasps
You need thickness for the base
What are the uses for cobalt chromium
Wires
Surgical implants
Cast partial dentures (connectors)
In thick sections what are the properties of COCr
high EL
high YM
In thin sections what are the properties of CoCr
High EL, low YM
What is the composition of CoCr
Cobalt (35-65%)
Chromium (25-30%)
Nickel (0-30%)
Molybdenum (5-6%)
Carbon (0.2–0.4%)
What are the effects of cobalt on the alloy
Forms solid solution with chromium
Increased strength, hardness, rigidity
CORING possible
What are the effect of chromium on the CoCr alloy
Forms solid solution with cobalt
Increased strength, hardness, rigidity
Coring possible
Forms passive oxide layer, corrosion resistance
What are the effect of nickel on the alloy (CoCr)
Replaces some Co
Improves ductility
Slight reduction in strength
Causes sensitivity in patients
What is the effect of carbon on the alloy (CoCr)
undesirable, forms carbide grain boundaries and can make it hard and brittle
What is the effect of molybdenum on the alloy (CoCr)
reduces grain size and so increases strength
What is the effect of tungsten on the alloy (CoCr)
increases strength
What is the effect of aluminum on the alloy (CoCr)
increases plastic limit
What is the effect of other things in the CoCr alloy
scavengers
Describe the technique for cobalt chromium
high temperature
melting
casting
What is the high temperature stage for cobalt chromium
1200-1400 degrees Celsius hence silica or phosphate bonded
What is the melting stage for cobalt chromium
electric induction preferred. Oxyacetylene allows it to avoid carbon pickup
What is the casting stage for cobalt chromium
centrifugal force required. Avoid overheating or will result in coarse grains. Cooling too fast or slow will result in carbides causing it to be brittle
What happens in the finishing of cobalt chromium
Sandblast
Electroplate
Abrasive wheel
Polishing buff
What are the properties of cobalt chrome in terms of hardness
Much harder than gold
Wear in mouth better
Finishing/polishing time consuming
What is elongation value for cobalt chromium
4%
What are the properties of cobalt chromium
○ Low ductility
○ Work hardens rapidly
○ Adjustment difficult
Precision casting requires
What are the uses for titanium
Implants
Partial dentures (cast)
Crown and bridges (cast)
Maxillo-facial skull plates
What are the properties of titanium
good biocompatibility
good corrosion resistance (passive oxide layer)
parts joined by laser welded
titanium absorbs gases
Why is titanium having good biocompatibility advantageous
if you use a maxillofacial skull plate then it Is a more demanding situation than that for a partial denture framework so titanium is used for that purpose
Why is titanium parts being welded together an advantage
alloys individual parts to be laser welded together so that means that casting process is not as challenging
What is electric arc melting used for
melt titanium
What is the effect of titanium
absorbs gases it requires specialised investment and casting
Why is titanium absorbing gas a disadvantage
it requires specialised investment and casting
What is the order of elongation % from highest to lowest
titanium Au (hard) Ag-Od CoCr S. steel
What is the order of ultimate tensile strength (MPa) highest to lowest
s. steel titanium Au (hard) CoCr Ag-Pd
What is the order of density highest to lowest
Gold
Ag-PD
CoCr/s. steel
titanium
What is the order of rigidity highest to lowest (Gpa)
cobalt chromium s. steel titanium au (hard0 ag-pd
What is the order of hardness highest to lowest
cobalt chromium s. steel au (hard) titanium ag-pd
What is the order of shrinkage highest to lowest
cobalt chromium
ag pd
au
COBALT CHROMIUM NEEDS DIFF INVESTMENT MATERIAL
Which alloy is harder to produce a defect free casting - gold or CoCr
Cocr
What investment can you not use for cobalt chrome
gypsum bonded
Is cocr harder to polish or gold
cocr but retains polish better
Since CoCr hardens rapidly what type of casting does it require
so need precision casting
Casting process more challenging to avoid defects
What has higher shrinkage, gold or cobalt chrome
cobalt chrome
2.3% for CoCr
1% difference
Which has higher density gold or CoCr
gold - almost double CoCr
CoCr is 8
What has higher young modulus, CoCr or gold
CoCr
1.5 x higher than gold
250 MPa for CoCr
What is the UTS (fracture strength) higher for, gold or CoCr?
Gold
100Mpa higher than Cobalt chromium
690 for cobalt chromium
What has a higher elongation, gold or CoCr
gold
more than double
4% for CoCr
What has a greater hardness, Cobalt chromium or gold
cobalt chromium
around 150 BHN difference
370 BHN for CoCr
