Porcelain Metal Alloys

Question 1

what is compressive strength

Answer 1

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

Question 2

what is elastic modulus (young’s)

Answer 2

this is a measure of rigidity which is the stress : strain ratio

Question 3

what is brittleness

Answer 3

how much dimensional change a material can undergo before it fractures

Question 4

what is ductility

Answer 4

the ability of a material to be drawn or plastically deformed without fracture

Question 5

what is hardness

Answer 5

resistance to surface indentation / abrasion

Question 6

what characteristics can be obtained from a stress : strain curve

Answer 6

elastic modulus, compressive strength, brittleness/ductility

Question 7

what characteristics cannot be obtained from a stress : strain curve

Answer 7

hardness

Question 8

3 good qualities of porcelain

Answer 8

rigid, hard , strong

Question 9

biggest issue with porcelain

Answer 9

very brittle and not ductile

Question 10

how are porcelain metal alloys made and why

Answer 10

cast to a desired shape beforehand then subjected to high temp to produce oxide layer that turns bond to ceramic
this helps to eliminate defects on porcelain surface

Question 11

what happens after porcelain metal alloys are heated

Answer 11

both materials must be cooled without developing thermal stresses as metal oxide layer could develop defects so both porcelain and alloy must have similar thermal expansion coefficients

Question 12

5 required alloy properties to be bonded to porcelain

Answer 12

1. good wetting - to form good bond
2. TEC should be similar - 14ppm/degreesC
3. avoid discolouration of porcelain
4. mechanical - bond strength, hardness, elastic modulus should all be high
5. melting temp - higher than porcelain to prevent creep

Question 13

5 alloys used for bonding to porcelain

Answer 13

high gold alloy, low gold alloy, silver palladium AgPd, nickel chromium, cobalt chromium (note different to CoCr for RPD)

Question 14

pros of high gold alloy

Answer 14

matches TEC, increases mpt

Question 15

cons of high gold alloy

Answer 15

melting range is too low and young’s modulus is too low

Question 16

pros of low gold alloy

Answer 16

higher mpt and better mechanical properties compared to high gold

Question 17

pros of silver palladium

Answer 17

high mpt

Question 18

cons of silver palladium

Answer 18

casting is a challenge

Question 19

pros of nickel chromium

Answer 19

hight mpt, high young’s modulus

Question 20

cons of nickel chromium

Answer 20

high casting shrinkage, low bond strength to porcelain

Question 21

pros of cobalt chromium

Answer 21

high mpt 1300-1400, high young’s modulus 220GPa, high tensile strength 850MPa, high hardness 360-430 VHN

Question 22

cons of cobalt chromium

Answer 22

significant casting shrinkage 2.3%, low bond strength 50MPa

Question 23

which of the alloys has biocompatibility issues

Answer 23

NiCr has due to allergic responses attributed to nickel

Question 24

3 bonding mechanisms of porcelain to oxide layer and alloy

Answer 24

1. mechanical - surface irregularities allows them to interlock
2. stressed skin effect - slight differences in TEC leads to compressive forces to aid bonding
3. chemical - electron sharing in oxides

Question 25

3 main modes of failure of porcelain metal bond

Answer 25

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