alloys for cast metal restorations

Question 1

give 3 examples of a cast metal restoration

Answer 1

crown alloy
bridge alloy
porcelain-fused metal alloy

Question 2

why is porcelain supported by a metal/alloy substructure in cast restorations?

Answer 2

it is not able to withstand the loads and biting forces it will be subjected to on its own

Question 3

what forms on/in porcelain during its production?

Answer 3

microcracks

Question 4

define comprehensive strength

Answer 4

the stress required to fracture a material, making it no longer fit for purpose

Question 5

define elastic modulus

Answer 5

a measure of the rigidity (the stress strain ratio)
i.e. the stress required to cause a change in shape

Question 6

what is brittleness/ductility a measure of?

Answer 6

how much dimensional change a material can undergo before it fractures

Question 7

define ‘hardness’

Answer 7

resistance of surface to indentation or abrasion

Question 8

which 3 qualities of a material can be determined from its stress-strain curve?

Answer 8

strength
rigidity
brittleness/ductility

Question 9

what does the initial gradient on a stress-strain curve indicate?

Answer 9

the material’s elastic modulus

Question 10

what does the end point of the curve on a stress-strain curve represent?

Answer 10

the fracture stress of the material

Question 11

describe the qualities of porcelain

Answer 11

rigid
hard
strong- high compressive strength but low tensile strength
brittle

Question 12

what is the purpose of subjecting the porcelain and alloy substructure to a very high temperature in a furnace?

Answer 12

to produce an oxide layer on the alloy which will bond to the ceramic
the metal oxide layer also helps prevent defects or micro-cracks forming on the porcelain surface

Question 13

what is the benefits of porcelain and the alloy having similar thermal expansion coefficients?

Answer 13

they expand at the same rate when being heated and contract at same rate when being cooled
this avoids thermal stresses within either material or at their contact surfaces
ensures a good bond with the metal-oxide layer sandwiched between them

Question 14

name 5 alloys which have been developed for bonding to porcelain

Answer 14

high gold
low gold
silver palladium
nickel chromium
cobalt chromium

Question 15

name 5 required properties in an alloy to bond to porcelain

Answer 15

• good wetting
• similar thermal expansion coefficient to porcelain
• avoid discolouration of porcelain
• good mechanical properties
• higher melting temperature than porcelain

Question 16

what part of the alloy does porcelain bond to?

Answer 16

the metal oxide layer

Question 17

why must the thermal expansion coefficient of the alloy be similar to porcelain in order to bond them?

Answer 17

to avoid setting up stresses during fusing

Question 18

what is the thermal expansion coefficient of porcelain?

Answer 18

14ppm per degree Celcius

Question 19

where should the thermal expansion coefficient of the alloy be in comparison to porcelain and why?

Answer 19

0.5ppm per degree Celcius greater so that during the cooling stage, the alloy is slightly compressing the porcelain

Question 20

why is a high elastic modulus desirable when bonding an alloy to porcelain?

Answer 20

the more rigid the alloy, the lower the amount of strain that the porcelain will be subjected to

Question 21

why must the melting temperature of the alloy be greater than that of porcelain during bonding?

Answer 21

to stop creep occurring

Question 22

define creep

Answer 22

the gradual increase in strain experienced under prolonged application of stress

Question 23

when does creep occur?

Answer 23

when the material temperature is more than about half its melting temperature

Question 24

what alloy is most desirable for bonding to porcelain?

Answer 24

low gold

Question 25

why does the alloy nickel chromium have biocompatibility concerns?

Answer 25

nickel allergy

Question 26

what are the constitutes of high gold?

Answer 26

80% gold
14% platiunum and/or palladium
1% silver
small amount of indium and/or tin

Question 27

what are the disadvantages of using high gold as an alloy to bond to porcelain?

Answer 27

melting point too low
not sufficiently rigid

Question 28

what are the constitutes of low gold?

Answer 28

50% gold
30% palladium
10% silver
10% indium and tin

Question 29

what are the constitutes of silver palladium?

Answer 29

30% silver
60% palladium
10% indium and tin

Question 30

what are the constitutes of nickel chromium?

Answer 30

70-80% nickel
10-25% chromium

Question 31

what are 2 disadvantages of using nickel chromium as an alloy to bond to porcelain?

Answer 31

exhibits a lot of shrinkage during the casting process
has a low bond strength to porcelain

Question 32

what is the melting point of cobalt chromium?

Answer 32

1300-1400C

Question 33

what are the mechanical properties of cobalt chromium?

Answer 33

low bond strength
high elastic modulus
high tensile strength
high hardness

Question 34

what are the 3 ways in which metal bonds?

Answer 34

mechanical
chemical
stressed skin effect

Question 35

what causes mechanical metal bonding with porcelain?

Answer 35

irregularities on the surface of the alloy’s metal oxide layer and porcelain which allows them to interlock

Question 36

how does chemical bonding occur between an alloy and porcelain?

Answer 36

oxides in the metal oxide coating on the alloy migrating with oxides within the porcelain (called electron sharing)

Question 37

explain the stressed skin effect

Answer 37

depends on the slight differences in the thermal expansion and contraction of the porcelain and the alloy
during the production process (after furnace stage) the alloy contracts more and cooling
this generates compressive forces on the porcelain- aids bonding

Question 38

name 3 possible modes of failure during the bonding of an alloy and porcelain

Answer 38

1. the oxide layer itself fracturing
2. the oxide layer delaminating from the alloy
3. pocelain detaching from the oxide layer