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
Effects of alloying element - molybdenum
Reduces grain size, hence increases strength
Effects of alloying element - tungsten
Increases strength
Effects of alloying element - aluminium
Increases plastic limit
Effects of alloying element - zinc
Scavenger
Zinc oxidises preferentially to avoid unfavourable oxidation/reduction reactions of other metals
CoCr techniques (3)
Investment material
Melting
Casting
Features of CoCr investment material
High temperature – 1200-1400C – silica or phosphate bonded
Features of CoCr melting
Electric induction preferred; oxyacetylene – avoid carbon pickup
Features of CoCr casting (3)
Centrifugal force required
Avoid overheating (coarse grains)
Cooling too fast/slow carbides (brittle)
Types of CoCr finishing (4)
Sandblast
Electroplate
Abrasive wheel
Polishing buff
Difference between CoCr and gold hardness (3)
CoCr harder
CoCr has better wear
CoCr finishing and polishing can be time-consuming
Features of CoCr ductilty
Cobalt chromium shows low ductility as elongation (4%) of the material can occur
Definition of elongation (3)
Elongation is the amount of strain it can experience before failure in tensile testing
The lower the amount of elongation a material shows, the less ductile it is
A ductile material (most metals and polymers) will record a high elongation
Why is precision casting utilised in CoCr casting
Due to low ductility, any CoCr work hardens rapidly making adjustment difficult
Uses of titanium (4)
Implants
Partial dentures (cast)
Crowns and bridges (cast)
Maxillofacial craniofacial/skull implants
Features of titanium (2)
Good biocompatibility
Good corrosion resistance (passive oxide layer)
Titanium parts are joined by laser welding
Titanium prosthesis preparation involves (2)
Electric arc melting
Specialised investment and casting (because titanium absorbs gases)
Disadvantages of CoCr (4)
More difficult to produce defect-free casting as opposed with gold
Cannot use gypsum-bonded investment
More difficult to polish (harder) than gold, but retains polish better
Work hardens rapidly, so precision casting is required
Relationship between elongation and ductility
The greater the amount of elongation (%), the greater ductility the material has
Relationship between UTS and the materials ability to resist tension
The greater the ultimate tensile strength (UTS), the greater the materials ability to resist tension (being pulled apart)
Definition of UTS (2)
The capacity of a material or structure to withstand loads tending to elongate
Definition of compressive strength
The capacity of a material or structure to withstand loads tending to reduce size
Difference between tensile strength and compressive strength
Tensile strength resists tension (being pulled apart), whereas compressive strength resists compression (being pushed together)
UTS on a stress-strain curve (3)
The UTS is usually found by performing a tensile test and recording the engineering stress versus strain
The highest point of the stress–strain curve is the UTS. It is an intensive property
Tensile strength is defined as a stress, which is measured as force per unit area
Relationship between density and how appropriate material is for use
The lower the density (g/cm2) of a material, the more appropriate for use as a partial denture alloy (not as heavy, increased flexibility)
Relationship between hardness and indentation resistance
The greater the hardness of a material, the greater the resistance to abrasion/indentation
Definition of hardness
A measure of the resistance to localised plastic deformation induced by either mechanical indentation or abrasion Some materials (metals) are harder than others (plastics) The hardness of a material defines the relative resistance that its surface imposes against the penetration of a harder body
Relationship between shrinkage and how appropriate material is for use
The lower the shrinkage of a material, the more suitable it is for use (less shrinkage leads to less stress, strain and shape alteration)
Relationship between rigidity and how appropriate material is for use
The greater the rigidity of a material, the better it is (less chance of breaking/fracturing/deforming)
Definition of YM (3)
Young’s modulus, also known as the elastic modulus, is a measure of the stiffness of a solid material
A mechanical property of linear elastic solid materials
Defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material
Features of straight stress-strain curve (3)
A solid material will deform when a load is applied to it
If it returns to its original shape after the load is removed, this is called elastic deformation
In the range where the ratio between load and deformation remains constant, the stress–strain curve is linear
Relationship of YM between gold, CoCr, titanium and stainless steel (4)
Stainless steel > titanium > CoCr > gold
Relationship of shrinkage between gold, CoCr, titanium and stainless steel (2)
Gold > CoCr
Relationship of melting range between gold, CoCr, titanium and stainless steel (3)
Titanium > CoCr > gold
Relationship of density between gold, CoCr, titanium and stainless steel (4)
Ti > stainless steel = CoCr > gold
Relationship of proportional limit between gold, CoCr, titanium and stainless steel (4)
Stainless steel > CoCr > titanium > gold
Relationship of UTS between gold, CoCr, titanium and stainless steel (4)
Stainless steel > titanium > CoCr > gold
Relationship of elongation between gold, CoCr, titanium and stainless steel (4)
Gold > titanium = CoCr > stainless steel
Relationship of hardness between gold, CoCr, titanium and stainless steel (4)
CoCr > stainless steel > titanium > gold
