Porcelain Metal Alloys Flashcards Preview

BDS3 Dental Materials Science > Porcelain Metal Alloys > Flashcards

Flashcards in Porcelain Metal Alloys Deck (25)
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1
Q

what is compressive strength

A

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

2
Q

what is elastic modulus (young’s)

A

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

3
Q

what is brittleness

A

how much dimensional change a material can undergo before it fractures

4
Q

what is ductility

A

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

5
Q

what is hardness

A

resistance to surface indentation / abrasion

6
Q

what characteristics can be obtained from a stress : strain curve

A

elastic modulus, compressive strength, brittleness/ductility

7
Q

what characteristics cannot be obtained from a stress : strain curve

A

hardness

8
Q

3 good qualities of porcelain

A

rigid, hard , strong

9
Q

biggest issue with porcelain

A

very brittle and not ductile

10
Q

how are porcelain metal alloys made and why

A

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

11
Q

what happens after porcelain metal alloys are heated

A

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

12
Q

5 required alloy properties to be bonded to porcelain

A
  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
13
Q

5 alloys used for bonding to porcelain

A

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

14
Q

pros of high gold alloy

A

matches TEC, increases mpt

15
Q

cons of high gold alloy

A

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

16
Q

pros of low gold alloy

A

higher mpt and better mechanical properties compared to high gold

17
Q

pros of silver palladium

A

high mpt

18
Q

cons of silver palladium

A

casting is a challenge

19
Q

pros of nickel chromium

A

hight mpt, high young’s modulus

20
Q

cons of nickel chromium

A

high casting shrinkage, low bond strength to porcelain

21
Q

pros of cobalt chromium

A

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

22
Q

cons of cobalt chromium

A

significant casting shrinkage 2.3%, low bond strength 50MPa

23
Q

which of the alloys has biocompatibility issues

A

NiCr has due to allergic responses attributed to nickel

24
Q

3 bonding mechanisms of porcelain to oxide layer and alloy

A
  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
25
Q

3 main modes of failure of porcelain metal bond

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