Alloys for cast metal restorations

Question 1

compressive strength

Answer 1

ability to withstand loads aimed at reducing size

Question 2

elastic modulus

Answer 2

measure of rigidity , stress required for material to change shape

Question 3

what cant be deduced from a stress strain curve

Answer 3

hardness

Question 4

discuss porcelain alone as a cast restoration

Answer 4

pros - good aesthetics, hard, rigid
cons - brittle , useless when biting

Question 5

discuss metal alloys alone as a cast restoration

Answer 5

pros - hard, strong, rigid , ductile
cons - awful aesthetics

Question 6

how do porcelain fused alloys bond together

Answer 6

when porcelain and alloy substructure heated in a furnace, alloy produces a metal oxide which bonds to the ceramic fusing the two together.

Question 7

effect of metal oxide produced from heating porcelain and alloy substructure on restoration

Answer 7

fuses alloy and porcelain together
helps prevents defects and micro cracks forming on the porcelain
allows the underlying allow to act as support for the porcelain and limit the strain porcelain experiences

Question 8

why should copper not be present in alloys being used for cast metal restorations

Answer 8

copper can discolour porcelain

Question 9

name 3 potential alloys for cast metal restorations

Answer 9

high gold
low gold
cobalt chrome
silver palladium

Question 10

discuss low gold as an alloy for cast metal restorations

Answer 10

similar expansion co efficient to porcelain
good melting point
mechanical properties sufficient

Question 11

what is cobalt chromes downfall as an alloy for cast metal restorations

Answer 11

shrinks during casting

Question 12

what are the 3 bonding mechanisms proposed of how the oxide layer fuses porcelain and alloy in cast metal restorations

Answer 12

mechanical
stressed skin effect
chemical

Question 13

mechanical bonding theory of metal oxide bonding

Answer 13

irregularities on surface allow interlocking

Question 14

stressed skin effect theory of metal oxide bonding

Answer 14

relies on slight differences between thermal expansion co efficients
alloy contracts slightly more during cooling generating compressive forces upon porcelain (gripping it)

Question 15

chemical theory of metal oxide bonding

Answer 15

‘electron sharing’
during firing (very high temps) porcelain flows and oxides from the metal oxide migrate into porcelain itself

Question 16

give 4 modes of failure for a cast metal restoration

Answer 16

oxide layer fractures
oxide layer debonds from alloy
oxide layer debonds from porcelain
porcelain layer fractures