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Dental materials > noble metal alloys > Flashcards

noble metal alloys Flashcards

1
Q

What are noble metals?

A
  • are those with good and retained metallic luster in dry air
  • their resistance to tarnish and corrosion during heating, casting, soldering or use in the mouth is very good
  • gold and platinum group (platinum, palladium, iridium, rhodium, osmium and ruthenium)
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2
Q

What are precious metals?

A

noble metals together with silver

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3
Q

What is casing?

A

process of pouring a molten alloy into a mold to form restorations

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4
Q

What are the functional requirements of dental casting alloys?

A
  • fatigue strength and fatigue limit should be high to resist cyclic loading
  • yield strength should be high to resist permanent deformation
  • tarnish and corrosion resistance should be high
  • stiffness (elastic modulus) should be high to resist elastic deformation of long span bridges and denture framework
  • resilience should be high to absorb mechanical energy without permanent deformation
  • compatibility to porcelain
  • sag resistance
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5
Q

What is sag resistance?

A

resistance of the alloy to flow or melt during porcelain firing or soldering (important property)

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6
Q

What should the dental casting alloy be compatible with porcelain in?

A
  • coefficient of thermal expansion
  • modulus of elasticity
  • the oxides formed should help bonding and all of its components must not discolor the porcelain
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7
Q

What are the working requirements of dental casting alloys?

A
  • ease of casting i.e. easy melting and flow to fill the mold easily
  • ease of soldering i.e. easy wetting by the solder alloy
  • ease of burnishing i.e. easy adaptation of its margins to tooth
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8
Q

What are the properties of gold?

A
  • soft, malleable, ductile metal with high nobility that resist tarnish and corrosion
  • rich yellow color with strong metallic luster
  • has a low strength and hardness
  • melting temperature is 1063 degrees and the specific gravity is 19
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9
Q

What are the properties of platinum?

A
  • noble metal which resists tarnish and corrosion
  • blue white in color and tends to lighten the color of yellow gold
  • increases the hardness and strength of gold
  • ductile
  • melting temperature is 1755 degrees and specific gravity of 21 which increase the weight of gold alloys
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10
Q

What are the properties of palladium?

A
  • cheaper than platinum and it is often used as replacement for platinum
  • white metal
  • specific gravity is 11
  • ductile and malleable noble metal - - the melting point is 1555 degrees
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11
Q

Why do gold alloys used with porcelain should contain higher amounts of platinum & palladium?

A

to increase its sag resistance, so it does not flow during porcelain firing

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12
Q

What effect does 5% palladium have?

A

whitening the color of gold alloy and it decreases the greening of silver and the reddening

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13
Q

What are the properties of silver?

A
  • ductile and malleable metal
  • stronger and harder than gold
  • white in color and so it is important in developing the white color of gold alloys particularly when combined with platinum and palladium
  • its melting temperature is 960 degrees and the specific gravity is 10.4
  • also improves the mechanical properties of gold alloys
  • pure silver occludes oxygen in the molten state which is evolved during solidification as a result, porous and rough casting may develop
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14
Q

What is th eproperty of iridium?

A

effective in refining the grain size of gold alloys

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15
Q

What are some base metals added to gold?

A
  • copper
  • zinc
  • tin
  • indium
  • iron
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16
Q

What arethe properties of copper?

A
  • ductile and malleable metal with characteristic red color
  • increases strength and hardness of gold alloys and is an important factor in heat treatment
  • its melting temperature is 1083 degrees
  • specific gravity is 8
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17
Q

What are the properties of zinc?

A
  • blue white in color with melting temperature of 419 degrees
  • it is present in gold alloys to act as a scavenger (deoxidizing agent) during melting and casting of the alloys
  • improves the fluidity and castability of the alloy
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18
Q

What re the properties of tin?

A

lustrous white metal with melting temperature of 232 degrees
- it produce oxides to help bonding with porcelain

19
Q

What are the properties of indium?

A
  • white metal with melting temperature of 156 degrees
  • the same as tin it produces white oxides in alloys for porcelain which help bonding with porcelain
20
Q

What is the property of iron?

A

added with porcelain alloys to produce precipitation hardening (Pt3Fe)

21
Q

What is the classification of gold alloys according to gold content?

A
  • carat (k)
  • fineness (F)
22
Q

What is carat?

A
  • the number of parts of pure gold in 24 parts of alloy
  • for example, 24 K is pure gold, 21 K means 21 parts pure gold and 3 parts other metals, and 18 K means 18 parts pure gold and 6 parts other metals
23
Q

What is fineness?

A
  • the number of parts of pure gold in 1000 parts of the alloys
  • for example, 1000 F is pure gold, 750 F means 750 parts pure gold and 250 parts other metals
24
Q

Is carat used in dentistry?

A

no

25
Q

Is fineness used in dentistry?

A

yes, in describing gold solder alloys

26
Q

What is the classification of gold alloys according ADA, hardness and uses?

A
  • type 1 (soft)
  • type 2 (medium)
  • type 3 (hard)
  • type 4 (extra hard)
27
Q

What types have highest and lowest gold composition?

A
  • type 1 (highest gold composition)
  • type 2
  • type 3
  • type 4 (lowest gold composition)
28
Q

What is the use and properties of type 1 (soft)?

A
  • use: inlays subjected to very slight stress
  • heat treatment: do not respond to hardening heat treatment
  • ductility: 30% (highest)
  • hardness: 70 (lowest)
  • Y.S. in MPA: 100 (lowest)
  • M.P.: 1000 degrees
29
Q

What is the use and properties of type 2 (medium)?

A
  • use: inlays subjected to moderate stress
  • heat treatment: do not respond to hardening heat treatment
  • ductility: 20%
  • hardness: 85
  • Y.S. in MPA: 150
  • M.P.: 950 degrees
30
Q

What is the use and properties of type 3 (hard)?

A
  • use: onlays subjected to high stress and as crowns and bridges
  • heat treatment: respond to hardening heat treatment
  • ductility: 20% (soft) 10% (hard)
  • hardness: 100 (soft) 115 (hard)
  • Y.S. in MPA: 200 (soft) 300 (hard)
  • M.P.: 930 degrees (soft) 1000 degrees (hard)
31
Q

What is the use and properties of type 4 (extra hard)?

A
  • use: onlays subjected to very high stress and as denture frameworks
  • heat treatment: respond to hardening heat treatment
  • ductility: 20% (soft) 5% (hard)
  • hardness: 130 (soft) 225 (hard)
  • Y.S. in MPA: 300 (soft) 500 (hard)
  • M.P.: 900 degrees (soft) 950 degrees (hard)
32
Q

What is the modulus of elasticity in noble metal alloys types?

A
  • ranges between 75,000 and 150,000 MPa, depending on the composition of the alloy
  • not altered by heat treatment
33
Q

What is the classification of gold alloys according to alloy description?

A
  • white gold away
  • economic gold alloy
34
Q

What are white gold alloys?

A
  • gold alloys which are white in color because palladium % is more than 5%, also silver and platinum are present
35
Q

What are the advantages of white gold alloys?

A
  • more aesthetic
  • less expensive
36
Q

What are economic gold alloys?

A
  • gold alloys that contain gold in the range of 42% - 55% which is below than that required by the ADA specification (65% Au in type IV gold alloys)
  • these alloys are comparable to type III and IV gold alloys in properties and uses
37
Q

What is found in economic gold alloys?

A
  • 1% palladium is needed for every 3% silver to stop the high tarnish effect of silver
  • if the silver–copper ratio were not carefully balanced, the alloy would also tarnish even with the correct palladium and silver content
  • this is due to the precipitation of a silver-rich phase in the microstructure
38
Q

What is heat treatment of gold alloys?

A
  • conventional cast gold alloys that contain sufficient copper (above 12%) may be easily heat treated to produce a softer or a harder alloy
  • they are softened by heating gold alloys at 700 degrees for 15 minutes and then cooling rapidly to room temperature in water
  • a hardening heat treatment takes place when gold alloys are heated at 700 degrees for 15 minutes and then cooled slowly
  • hardening heat treatments produces an increase in strength but also a reduction in ductility
  • in practice, dental gold castings are heat treated by either quenching or slow cooling the casting while it is still in the mold
  • to produce a softer casting, the mold is quenched in water when the metal appears dull red
  • however, a harder and stronger but less ductile casting is produced by allowing the casting to cool to room temperature in the mold
39
Q

What is the result of softening heat treatment?

A
  • decrease strength
  • decrease hardness
  • increase ductility
40
Q

What is the result of hardening heat treatment?

A
  • increase strength
  • increase hardness
  • decrease ductility
41
Q

What is the explanation of softening heat treatment?

A
  • heating causes atomic movement
  • quenching stop them in a discolored state
42
Q

What is the hardening heat treatment?

A
  • heating causes atomic movement
  • slow cooling gives atoms the chance to arrange themselves ideally in an ordered state
43
Q

What is recasting of gold alloys?

A
  • gold alloys can be recast for 2 or 3 time without a change in composition
  • the only elements likely to get lost are the more volatile base metals such as zinc, indium and iron
  • this loss can be treated by the addition of at least equal amounts of the new alloy to the scrap (old) when recast
  • the scrap should be cleaned and foreign materials removed before recasting

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