Alloys for Cast Metal Restorations Flashcards
What is compressive strength?
How much stress a material can take before it fractures
What is the elastic modulus of a material?
The stress required to cause a change in shape (stress/strain ratio)
What are the characteristics of porcelain?
- rigid
- hard (withstands abrasion/indentation well)
- strong (high compressive strength)
- low tensile strength (formation of surface defects)
- brittle/low fracture toughness
Why are metal oxides bonded to porcelain in porcelain-metal restorations?
- metal oxide helps eliminate defects/cracks on porcelain surface
Why are alloys bonded to metal oxide/porcelain in porcelain-metal restorations?
alloy acts as a support & limits the strain that porcelain experiences
What are the required properties of metal alloys used in porcelain fused-to-metal alloys?
- forms a good bond to porcelain (good wetting)
- thermal expansion coefficient must be similar to that of porcelain
- won’t discolour porcelain
- high hardness
- high elastic modulus (rigid) to support porcelain and prevent fracture
What must the melting & recrystallisation temperature of alloy in a porcelain fused-to-metal alloy be?
HIGHER that the fusion temperature of porcelain (otherwise CREEP may occur)
Are high gold alloys suitable for porcelain-fused-to-metal alloys?
- melting range may be too low
- youngs modulus too low
Are low gold alloys suitable for porcelain-fused-to-metal alloys?
- increased melting temperature
- slightly better mechanical properties compared to high gold alloys
Are silver-palladium alloys suitable for porcelain-fused-to-metal alloys?
- high melting point
- care needs to be taken in casting
Are nickel-chromium alloys suitable for porcelain-fused-to-metal alloys?
- high melting point
- high Youngs Modulus
- high casting shrinkage
- low-ish bond strength
Are cobalt-chromium alloys suitable for porcelain-fused-to-metal alloys?
- high melting point
- low-ish bond strength
- high Young’s Modulus
- high tensile strength
- high hardness
