15. Partial Denture Alloys Flashcards
Ideal properties of partial denture alloys (7)
Rigidity (Young’s Modulus) Strong (high elastic limit and ultimate tensile strength) Hard Ductile Precise casting (no shrinkage) Melting point (investment material) Low density
Materials used in partial denture alloys (3)
ADA type IV gold Cobalt chromium (Co-Cr) Titanium
Components of one-piece casting (2)
Base
Clasp
Ideal mechanical properties of base in one-piece casting (2)
High YM to maintain shape in use
High elastic limit to avoid plastic deformation
Ideal mechanical properties of clasp in one-piece casting (2)
Low YM to allow flexure over bulbous tooth (removal)
High elastic limit to maintain elasticity over wide range of movement (strain)
Actual mechanical properties compromise of base and clasp in one-piece casting (2)
Thick section - rigid base
Thin section - flexible clasp
ADA gold specifications and uses (4)
Type I – simple alloys
Type II – larger (2-3 surface) inlays
Type III – crown and bridge alloys
Type IV – partial dentures
Composition of Type IV gold (6)
60-70% (65%) gold 1-2% (2%) zinc 11-16% (14%) copper 4-20% (14%) silver 0-5% (3%) palladium 0-4% (2%) platinum
Effects of alloying element - copper (8)
Solid solution in all proportions Solution hardening Order hardening (if 40-80% gold and correct heat treatment) Reduced melting point No coring (solidus close to liquidus) Imparts red colour Reduces density Base metal – can corrode if too much
Effects of alloying element - silver (6)
Solid solution in all proportions
Solution hardening
Precipitation hardening with copper and heat treatment
Can allow tarnishing
Molten silver absorbs gas (CO2)
Whitens alloy – compensates for copper discolouration
Effect of alloying element – platinum (4)
Solid solution with gold
Solution hardening
Fine grain structure
Coring can occur (wide liquidus – solidus gap)
Effect of alloying element – palladium (4)
Similar to platinum (less expensive)
Less coring than platinum
Coarser grains than platinum
Absorbs gases when molten (porous casting)
Effect of alloying element – zinc
Scavenger
Effect of alloying element – nickel
Increases hardness and wrought strength (wrought alloys)
Effect of alloying element – indium
Fine grain structure
Heat treatment for type IV gold alloys (4)
Quench after casting (fine grains)
Homogenising annealing (700C, 10 minutes)
If cold worked –> stress relief annealing
Heat harden (order and precipitation) - 450C, cool slowing (15-30 minutes) to 200C then quench
Reasons for heat treatment of type IV gold alloys
Result in properties more suitable for the clasps
Uses of CoCr
Wires
Surgical implants
Cast partial dentures - connectors
Mechanical properties of CoCr thick section connectors (2)
High EL
High YM
Mechanical properties of CoCr thin section connectors (2)
High EL
Low YM
Composition of CoCr (5)
35-65% (54%) cobalt 25-30% (25%) chromium 0-30% (15%) nickel 5-6% (5%) molybdenum 0.2-0.4% (0.4%) carbon
Effect of alloying element - cobalt (3)
Forms solid solution with chromium
Increased strength, hardness and rigidity
Coring possible
Effect of alloying element - chromium (4)
Forms solid solution with cobalt
Increased strength, hardness and rigidity
Coring possible
Forms passive layer (corrosion resistance)
Effects of alloying element - nickel (4)
Replaces some cobalt
Improves ductility
Slight reduction in strength
Nickel allergy/sensitivity (6% of females, 2% of males)
Effects of alloying element - carbon
Undesirable, carbide grain boundaries, hard and brittle