cobalt chromium

Question 1

What is the function of chromium do in cobalt chromium alloy

Answer 1

• Inert layer of chromium oxide to prevent tarnish

- Solid solution hardening

Question 2

What are the disadvantages of chromium in cobalt chromium alloy

Answer 2

• Difficult to cast when alloy contains more than 30% chromium
• Hard to solder due to presence of oxide layer on metal surface

Question 3

How does cobalt affect mechanical properties of cobalt chromium alloy

Answer 3

Increase strength, hardness, modulus of elasticity

Question 4

What happens as nickel replaces cobalt in cobalt chromium alloy

Answer 4

Less strength, less hardness, lower modulus of elasticity

Lower fusion temperature

Increase in ductility

Question 5

Function of manganese and silicon in cobalt chromium alloy

Answer 5

Oxide scavengers to prevent oxidation of other elements during melting

Solid solution hardeners

But tend to cause brittleness

Question 6

Function of boron in cobalt chromium alloy

Answer 6

Deoxidiser, oxide scavenger to prevent oxidation of other elements during melting

Hardener

Reduce ductility. Increase hardness

Question 7

Function of beryllium in cobalt chromium alloy

Answer 7

Hardener, grain structure refiner, reduce fusion temperature

Question 8

What does carbon do in cobalt chromium alloy

Answer 8

Formation of carbides, which can precipitate at grain boundaries

Question 9

How is carbide structure seen in cobalt chromium alloy

Answer 9

Large grains with solid solution matrix showing cored structure with islands of visible interdendritic carbide

Question 10

Two types of carbide precipitation

Answer 10

Discontinuous and continuous

Question 11

Effect of carbide on cobalt chromium alloy

Answer 11

Lower ductility. Greater strength. Brittle hence little to no clasp adjustment without fracture [discontinuous carbide precipitation provides greater ductility]

Carbide structures may act as core to increase energy required for fracture but cracks occasionally pass core and result in greater percentage elongation

Question 12

How to control carbide formation in cobalt chromium alloy to have less decrease in ductility

Answer 12

High fusion temperature and faster cooling of metal more likely to result in discontinuous precipitation

Higher mould temperature will increase carbide spacing, increasing ductility

Question 13

How does modulus of elasticity of CoCr compare to gold

Answer 13

CoCr modulus of elasticity twice that of gold. Rigid and hard to trim. Less undercut can be used for retentive arm of clasp

Question 14

Why does cobalt chromium alloy undergo a lot of shrinkage when cooling

Answer 14

2.2-2.5% because high melting temperature of 1250-1500ºC

Question 15

What are the two types of cobalt chromium alloy

Answer 15

High fusing and low fusing alloy (fusion temperature above and below 1300ºC)

Question 16

What are the benefits of low fusing alloy

Answer 16

Can be cast into gypsum bonded investment. can be cast against other high fusing precious metal/wrought alloy metals as will not melt other components when molten metal goes into mould space

Question 17

How to improve ductility of cobalt chromium alloy

Answer 17

Less molybdenum
Less carbon content since carbides make it brittle
Add 5% tantalum
increase melting temperature (discontinuous carbide, increased carbide spacing; but this will result in rougher surface of casting hence better to just accept that cast clasps have low ductility)

Question 18

What is yield strength

Answer 18

Amount of stress required to cause permanent deformation

Question 19

How much yield strength should dental alloys have to be used as clasps

Answer 19

4200kg/cm2

Question 20

How to compensate for casting shrinkage of cobalt chromium alloy

Answer 20

Expansion of investment

Question 21

Factors affecting casting shrinkage

Answer 21

Surface area to volume ratio. Increased surface area to volume ratio = less casting shrinkage

Strength of investment eg phosphate bonded investment relatively strong