Alloys for cast metal restorations

Question 1

What are the characteristics of porcelain

Answer 1

• Rigid = less stress required to cause strain
  -Hard = surface withstands abrasion
• High Compressive strength
• Low tensile strength

Question 2

Why is porcelain prone to mechanical failure?

Answer 2

it tends to form micro cracks at the fitting surface

Question 3

What is meant by the term ‘elastic modulus’?

Answer 3

the stress required to cause a change in shape

Question 4

what is meant by ‘compressive strength’?

Answer 4

the stress required on an object to cause fracture

Question 5

define the term ‘hardness’ in reference to dental materials

Answer 5

the resistance of the surface of an object to indentation or abrasion

Question 6

What are the benefits of porcelain metal restorations?

Answer 6

• the bonding of a metal oxide to porcelain helps eliminate porcelain surface defects
• the alloy acts as a support, limiting the strain experienced by the porcelain

Question 7

Give expamples of alloys that can be used for pfm alloys

Answer 7

• high gold alloy
• low gold alloy
• silver palladium
• nickel chromium
• cobalt chromium

Question 8

list the required properties of an alloy that is bonded to porcelain

Answer 8

• forms good bond to porcelain
• thermal expansion coefficient similar to that of porcelain
• avoids discolouration of porcelain e.g. Ag in AgPd can produce green discolouration
• high bond strength
• high hardness
• high elastic modulus
• higher melting, recrystallisation temperature than the fusion temperature to ensure creep does not occur

Question 9

porcelain fused to metal alloys - required properties

Answer 9

• form good bond to porcelain
• thermal expansion coefficient similar to porcelain to avoid setting stresses during fusion
• avoid discolouration of porcelain
• high elastic modulus
• high bond strength
• high hardness value
• melting and recrystallisation temperature of alloy must be higher than fusion temperature to prevent creep

Question 10

What is creep?

Answer 10

gradual increase in strain experienced under prolonged application of stress

Question 11

Why isn’t copper used in high gold alloys in dentistry?

Answer 11

can cause green hue

Question 12

High gold alloy constituents

Answer 12

Au 80%
Pt/Pd 14%
Ag 1%
indium and tin - forms oxides

Question 13

Low gold alloys - how do they differ from high gold alloys?

Answer 13

increased melting temperature
slightly better mechanical properties