Alloys for Cast Metal Restorations Flashcards
Why are porcelain and metal alloys combined to create crowns?
- Porcelain has excellent aesthetics and the alloy acts to support the porcelain
What are the disadvantages of porcelain in porcelain fused alloy restorations?
- microcracks can form in porcelain and on its surface during production
- prone to mechanical failure
- cannot withstand loads from occlusal forces
What are the advantages of alloys in porcelain fused alloy restorations?
- good mechanical properties
- can withstand large forces
What are the properties of porcelain?
- very brittle
- low fracture toughness
- maximum strain ~0.1% before fracture
- reasonably hard
- surface withstands abrasion and indentation well
- reasonably strong
- high compressive strength
- low tensile strength
- surface defects can form
- can fracture at low stress
- reasonably rigid
- large stress required to cause strain
What are the properties of alloys?
- very hard
- very strong
- very rigid
- reasonably ductile
How are porcelain-metal restorations bonded?
metal oxide layer
- alloy is cast to desired shape to create substructure covered in porcelain
- subjected to high temperatures in furnace producing oxide layer which bonds to ceramic
- metal oxide layer prevents defects/microcracks on porcelain surface
What is the main function of the alloy in porcelain fused restoration?
- provides mechanical support to the porcelain to limit the strain it is subjected to therefore reducing the risk of brittle failure
- overall strain experienced when stress is applied is less than that required to fracture porcelain
What must be similar in the alloy and porcelain?
thermal expansion coefficients
- restoration must be fired in a furnace then cooled
- to avoid thermal stresses causing defects and microcracks expansion and contraction of the materials should occur at the same rate
- ensures good bond with metal oxide layer
What alloys can be fused to porcelain?
- high gold alloy
- low gold alloy
- silver palladium (AgPd)
- nickel chromium (NiCr)
- cobalt chromium (CoCr)
What are the required properties of alloys being bonded to porcelain?
- good bond with porcelain
- good wetting/surface contact
- porcelain bonds to metal oxide produced on surface
- thermal expansion coefficient
- must be similar to porcelain (14ppm/oC)
- ideally 0.5ppm/oC more for alloy
- alloy then able to compress porcelain
- unwanted stresses not set up
- does not discolour porcelain
- porcelain has very good aesthetic
- underlying alloy cannot interfere with this
- Ag in AgPd can produce green discolouration
- Cu is not used in high gold alloy
- desired mechanical properties
- bond strength (adequate in all alloys)
- hardness (adequate in all alloys)
- elastic modulus (must be rigid, less strain
experienced by porcelain)
- melting and recrystallisation temperatures
- must be higher than fusion temperature of porcelain
- if lower creep can occur
What is the composition of high gold alloys?
Au - 80%
Pt/Pd - 14%
Ag - 1%
In/Sn
- indium and tin form oxides which are vital for bonding
- no copper as porcelain can take up green hue
- platinum and palladium help match the thermal expansion of the alloy with porcelain to minimise creep. they also increase the melting point
What are the advantages and disadvantages of high gold alloys?
Advantages
- easy casting process
- reasonable bonding to porcelain
- good biocompatibility
Disadvantages
- melting range is low so may result in creep
- Young’s modulus is too low, material is not sufficiently rigid
What is the composition of low gold alloys?
Au - 50%
Pd - 30%
Ag - 10%
In/Sn - 10%
What are the advantages and disadvantages of low gold alloys
Advantages
- increased melting temperature, less creep experienced
- relatively easy casting process
- good rigidity
- reasonable bonding
- reasonable biocompatibility
Although no alloy excels in every characteristic, low gold alloys are the only type to satisfy each criteria
What is the composition of silver-palladium alloys?
Pd - 60%
Ag - 30%
In/Sn - 10%
