Alloys for Cast Metal Restorations

Question 1

what is a porcelain fused metal crown? + the benefits and cons

Answer 1

Porcelain surface and a metal (alloy) substructure

Porcelain:
- Good aesthetics
- Microcracks can form on the fitting surface prone to mechanical failure

Alloys:
- good mechanical properties

Question 2

what make a dental material have good mechanical properties?

Answer 2

• compressive stress (stress to cause FRACTURE)
• Elastic modulus (how rigid it is, before permanently changing shape)
• Brittleness/ductility
• Hardness (resistance to indentation/abrasion)

Question 3

what properties does a stress-strain curve contain?

Answer 3

• strength (compressive/tensile)
• brittleness/ductility
• elastic modulus (rigidity)

Question 4

what are the mechanical properties of porcelain?

Answer 4

• brittle
• hard
• strong
• rigid

Question 5

what are the mechanical properties of alloys?

Answer 5

• hard
• strong
• ductile
• rigid

Question 6

why are porcelain-metal restorations 3 layers. (porcelain - metal oxide - alloy)?

Answer 6

Bonding of metal oxide to porcelain helps eliminate any defects on the porcelains surface

the alloy acts as a support & limits strain that porcelain experiences

Question 7

what are the required properties of a metal alloy to be fused with porcelain?

Answer 7

1) good bond to porcelain
2) thermal expansion coefficient (must be similar to porcelain)
3) avoid discoloring the porcelain (some alloys can cause discolouration)
4) mechanical (hard, rigid, good bond strength)
5) melting, recrystallisation temperature of alloy [MUST be higher than temp of porcelain, if not = creep]

Question 8

what is creep?

Answer 8

slow deformation

gradual increase in STRAIN (permanent) experienced under prolonged application of STRESS