alloys for cast metal Rxs

Question 1

why porcelain and metal are used together based on properties

Answer 1

porcelain
 - good aesthetics
 - but forms micro cracks - prone to mechanical failure
alloys
 - good mechanical properties

Question 2

elastic modulus/rigidity

Answer 2

stress/strain ratio

stress required to cause change in shape

Question 3

compressive strength

Answer 3

stress to cause fracture

Question 4

brittleness/ductility

Answer 4

dimensional change experienced before fracture

Question 5

hardness

Answer 5

resistance of surface to indentation or abrasion

Question 6

what doesn’t a stress-strain curve tell you?

Answer 6

hardness

Question 7

what does a stress-strain curve tell you?

Answer 7

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

Question 8

how to assess ductility on stress strain graph

Answer 8

look at change in shape between PL and FS

Question 9

on a stress strain graph what can’t you predict the relationship beyond?

Answer 9

PL

Question 10

porcelain properties

Answer 10

hard
rigid - need large stress to cause strain
strong - high compressive strength
brittle - low fracture toughness (max strain 0.1% before fracturing)
low tensile strength
tendency to form surface defects (micro cracks)
- fractures at low stress

Question 11

alloy properties

Answer 11

hard
strong
rigid
ductile

Question 12

porcelain-metal restoration

Answer 12

fused together via a metal oxide layer
bonding helps eliminate defects/cracks on porcelain surface
alloy acts as support and limits the strain porcelain experiences
- alloy small % strain
- porcelain - large % strain - fracture
- alloy and porcelain - small % strain experienced by “porcelain+alloy”

Question 13

required properties

Answer 13

good bond to porcelain i.e. good wetting
TEC similar to porcelain
avoid discolouration of porcelain
mechanical
melting, recrystallisation temp of alloy must be higher than fusion temp of porcelain

Question 14

required properties - good bond to porcelain

Answer 14

bonds with metallic oxides on surface

NiCr alloys more difficult

Question 15

required properties - TEC similar to porcelain

Answer 15

14ppm/degrees - ideally difference of 0.5 in alloys favour

avoid stresses during fusing

all alloys ok

Question 16

required properties - avoid discolouration of porcelain

Answer 16

Ag in AgPd - green

Cu not used in high gold alloy

Question 17

required properties - mechanical

Answer 17

bond strength
- 3 alloys adequate (not NiCr)
hardness
- all adequate (though early NiCr alloys too hard)
EM
- high (rigid) to support porcelain and prevent fracture (NiCr best)

Question 18

required properties - melting, recrystallisation temp of alloy

Answer 18

must be higher than fusion temp of porcelain - otherwise creep

Question 19

need porcelains and alloys with matching TECs

Answer 19

high gold alloy
low gold alloy
silver palladium (AgPd)
NiCr
CoCr

Question 20

creep

Answer 20

gradual increase in permanent strain experienced under prolonged application of stress (

Question 21

high gold alloys

Answer 21

Au 80%
Pt/Pd 14% - match TEC and increase mp
Ag 1%
indium, tin - forms oxides - bonding
no Cu - otherwise green hue
melting range may be too low
YM too low

Question 22

low gold alloys

Answer 22

Au 50%
Pd 30%
Ag 10%
indium, tin 10%
increased melting temp
slightly better mechanical properties

Question 23

Ag-Pd alloys

Answer 23

Pd 60%
Ag 30%
In, Sn 10%
high mp
care needed in casting

Question 24

NiCr alloys

Answer 24

Ni 70-80%
Cr 10-25% (oxide bond)
high mp
high YM
high casting shrinkage
lowish bond strength

Question 25

which alloys are usually used?

Answer 25

CoCr
some gold and palladium
NOT NiCr

Question 26

CoCr alloys

Answer 26

low density
high mp
casting shrinkage
lowish bond strength
high YM
high tensile strength
high hardness

Question 27

failure mode of porcelain-metal bond

Answer 27

porcelain
- only acceptable failure should be that porcelain splits - weakest point

porcelain-MO
MO-alloy
MO
- no - indicates process by which it was made isn’t as good as it should be

Question 28

high gold properties

Answer 28

castable
biocompatible
corrosion resistant
bad creep
YM too low

Question 29

low gold properties

Answer 29

castable
better EM
corrosion resistant

Question 30

AgPd properties

Answer 30

good EM

not easy casting

Question 31

NiCr properties

Answer 31

not castable
not biocompatible
good creep
good EM

Question 32

porcelain-metal bond proposed mechanisms

Answer 32

mechanical
- surface irregularities, probably least important
stressed skin effect
- slight differences in thermal contraction coefficients lead to compressive forces which aid bonding
chemical
- may be electron sharing in oxides
- during firing porcelain flows and oxides in the metal-oxide coating migrate
(Van der Waals forces - now disregarded)

Question 33

bonding to porcelain

Answer 33

can get feldspathic glass to bond to an alloy surface by adding leucite crystals - added to match TEC
if proper bond - internal cracks eliminated - metal barrier to propagation of cracks - high fracture toughness
need v clean surface
migration of metal oxides into the ceramic

Question 34

quality of bond

Answer 34

amount of micro mechanical bonding
thermal coefficient compatibility
chemical interaction between the MOs and the ceramic