Alloys for Metal Resotrations Revision Questions

Question 1

Procelain Characterstics

Answer 1

rigid - large stress required to cause strain
hard - surface withstands abrasion
strong - high compressive strength, low tensile strength
form surface defects - leads to fracture at low stress
brittle/low fracture toughness - 0.1% before fracturing

Question 2

Examples of porcelain metal restorations

Answer 2

high and low gold alloy
silver palladium
nickel palladium
cobalt chromium

Question 3

Mechanical properties of porcelain metal restiorations

Answer 3

good hardness
good bond strength
want high elastic modulus
melting/recrystalisation temp - alloys higher than procelain so creep does not occur

Question 4

Characterstics of alloy bond to porcelain

Answer 4

form good bond to porcelain (good wetting)
thermal expansion coeffiecent similar
avoid discolouration of porcelain - no copper in high gold alloy

Question 5

Modes of failure in porcelain fused resotrations

Answer 5

oxide layer fracturing
oxide layer delaminating from alloy
porcelain detaching from oxide layer

Question 6

Bond between oxide layer, alloy and porcelain

Answer 6

Mechanical - due to irregularities on surface of alloys and porcelain causing interlocking
Stressed skin effect - during production process after furnance stage the alloy contracts slightly more on cooling and generates compressive forces on porcelain
Chemical - oxides in the metal oxides coating on the alloy migrating with oxides on the porcelain itself. occurs during firing stage where high temps reached (electron sharing)

Question 7

High gold alloys

Answer 7

80% gold, 1% silver, 14% platinium
small amount of indium/tin
platinium/palladium - helps match thermal expansion and increases melting point

disadvatnages - mp too low, young’s modulus too low (not rigid)

Question 8

Low gold alloys

Answer 8

50% gold, 30% palladium, 10% silver, 10% indium/tin
increased melting temp
better mechanical propterties than high gold

Question 9

silver palladium

Answer 9

30% silver, 60% palladium
high melting temp
casting a challenge

Question 10

Nickel chromium

Answer 10

70-80% nickel, 10-25% chromium
high melting point and young’s modulus

disadvatanges - high casting shrinkage, low bond strength to porcelain

Question 11

Cobalt Chromium

Answer 11

advatanges - high EM, hard material, high tensile strength, high MP

Disadvantges - low bond strength, significant shrinkage during casting - challenging