Alloys Flashcards
When is a material considered ductile?
Stretches ~3.5% before fracturing
What is the fracture stress?
The load applied for the material to fracture
What is the proportional limit?
The highest stress at which stress is directly proportional to strain
What is the fracture stress of porcelain?
0.1%
What are the properties of porcelain?
Rigid, hard, high comprehensive strength
BUT
Low tensile strength, tendency to fracture, brittle/low fracture toughness
What is the advantage of porcelain-metal materials?
Good aesthetics of the porcelain combined with mechanical strength of the alloy.
Alloy acts as a support & limits the strain porcelain experiences.
What are examples of porcelain fused to metal alloys?
High gold alloy
Low gold alloy
Silver palladium (AgPd)
Nickel chromium
What are the required properties of porcelain fused to metal alloys?
Forms good bond with porcelain
Thermal expansion coefficient similar to porcelain
Avoid discolouration of porcelain
Good mechanical properties
Melting, recrystallisation temp of alloy higher than fusion temp of porcelain
What problem can occur if the melting, recrystallisation temp. Of the alloy is not higher than the fusion temp of porcelain?
Creep may occur
What is creep?
Gradual increase in strain (permanent) experienced under prolonged application of stress.
When is a material considered brittle?
Stretches
