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Module 6 - Dental Materials > Precious and Low Gold Alloys > Flashcards

Precious and Low Gold Alloys Flashcards

1
Q

3 Types of gold used in dentistry?

A
  1. Pure gold.
  2. Traditional casting gold alloys.
  3. Alloys with noble metal content of at least 25% but less than 75% (low gold casting alloys, silver palladium alloys).
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2
Q

Alloys with noble metal content of at least 25% but less than 75%?

A
  • Low gold casting alloys.
  • Silver palladium alloys.
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3
Q

What is the cohesive gold foil technique?

A
  • Cavity lined with an oxyphosphate lining material.
  • Tiny pieces of gold placed into a cavity and pressure applied with SPRING LOADED MALLET.
  • Cavity is incrementally filled up - takes MANY HOURS.
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4
Q

What technique is used for cohesive gold foil technique?

A

COLD WELDING.
- Pressure applied.
- Metallic bonds form at point of contact.
- Prior to placement heat to 250μm - drives off grease.
- Foil is 1μm.

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5
Q

What did patients complain about with cohesive gold foil technique?

A

Soreness of PDL due to pressure being applied with spring loaded mallet.

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6
Q

2 applications for cohesive gold foil technique?

A
  • Occlusal.
  • Buccal pit of molar.
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7
Q

5 disadvantages of pure gold in dentistry?

A
  • Time consuming
  • No cement lute.
  • Ok in protected cavities (class I or buccal pit).
  • Rigidity and elasticity insufficient in high stress situations.
  • Overworking at placement can cause work hardening.
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8
Q

4 types of traditional casting gold alloys?

A
  • Type 1: Low strength - casting subject to SLIGHT stresses (ex. inlays).
  • Type 2: Medium strength - castings subject to MODERATE stresses (ex. inlays/ onlays).
  • Type 3: High strength - high stresses (ex. onlays, thin cast backing, pontics, full crowns).
  • Type 4: Extra high strength - castings THIN IN CROSS SECTION (ex. saddles, bars, clasps, crowns, bridges and partial denture frameowrks).
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9
Q

What are two ways in which gold content is expressed?

A
  • Carat: parts by weight of gold in 24 parts of alloy.
  • Fineness: parts by weight of gold in 1000 parts of alloy.
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10
Q

What does an alloy with 75% gold mean in carat and fineness?

A
  • 18 carat.
  • 750 fine.
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11
Q

6 elements present in gold casting alloys?

A

Gold, silver, copper, platinum, palladium, zinc.

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12
Q

What happens to the percentage of gold content when going from Type 1 to Type 4 traditional casting gold alloys?

A
  • At strength increases, gold content DECREASES.
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13
Q

What happens to the percentage of silver content when going from Type 1 to Type 4 traditional casting gold alloys?

A

Stays about the same (some fluctuation).

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14
Q

What happens to the percentage of copper content when going from Type 1 to Type 4 traditional casting gold alloys?

A

As strength increases, copper content INCREASES.

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15
Q

What happens to the percentage of platinum/ palladium content when going from Type 1 to Type 4 traditional casting gold alloys?

A

As strength increases, platinum/palladium content INCREASES.

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16
Q

What happens to the percentage of zinc content when going from Type 1 to Type 4 traditional casting gold alloys?

A

Stays the same (1).

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17
Q

What are the changes in properties when we move from traditional casting gold alloy 1 to 4.

A

Increase in:
- Hardness.
- Proportional limit.
- Strength.

Decrease in:
- Ductility.
- Corrosion Resistance.

18
Q

Why does hardness increase as gold content decreases?

A

Due to SOLUTION HARDENING - formation of solid solutions with gold.

19
Q

What is the effect of silver in gold alloy (2)?

A
  • Slight strengthening effect.
  • Counteracts reddish copper tint.
20
Q

What is the effect of copper in gold alloys (3)?

A
  • Increases strength.
  • Lowers MP.
  • If content higher than 16% alloy will TARNISH.
21
Q

What is the effect of platinum/palladium in gold alloys (2)?

A
  • Increase strength.
  • Increase MP.
22
Q

What is the effect of zinc in gold alloys (1)?

A
  • Acts as a SCAVENGER (forms complex with OXYGEN).
23
Q

What happens if zinc is used up in the gold alloy? How can this be prevented?

A
  • Zinc acts as a SCAVENGER.
  • When used up its function is taken over by COPPER. This impairs the physical properties of the copper.
  • Prevent by always having fresh alloy in the button.
24
Q

What types of alloys can be heat treated?

A
  • Types 3 and 4
25
Q

What happens when type 3 and 4 alloys are heat treated? How does this occur?

A

further HARDENING by PRECIPITATION HARDENING of Ag/Cu and Au/Cu systems.

26
Q

What happens when type 1 and 2 alloys are heat treated? How does this occur?

A
  • CANNOT be heat treated.
  • Due to lack of silver and copper required for the necessary atomic diffusions.
27
Q

How are heat treatments achieved? What influences the structure?

A
  • Melting alloy and cooling it –> formation of a NEW CRYSTAL STRUCTURE.
  • LENGTH OF TIME alloy is allowed to cool to room temperature influences the structure.
28
Q

What happens when an alloy is allowed to cool SLOWLY to room temperature?

A
  • Premature precipitation and order hardening.
29
Q

How can premature precipitation and order hardening be avoided?

A
  • Casting cooled rapidly from excess of 600 (red color).
  • Once red color lost through cooling, do a COLD WATER PLUNGE.
30
Q

How do we know the casting has reached above 600 celsius?

A
  • RED color of sprue region of casting.
31
Q

What are the two effects of doing a cold plunge once the red color has been lost?

A
  • Disintegrates investment (helps remove casting).
  • Results in FINE GRAIN structure of casting.
32
Q

What is coring?

A

Zones of concentration of a metal (of constituents) which can result in CORROSION.

33
Q

What is the effect of eliminating coring?

A

Improves the CORROSION RESISTANCE of the metal.

34
Q

What may happen after cold water cooling in alloys containing Pt/Pd? How can this be avoided?

A
  • risks CORING.
    Avoid by HOMOGENISATION HEAT TREATMENT:
  • Heating to **700* for 10 minutes.
  • Quenching (rapid cooling).
35
Q

What is the process and aim of homogenisation heat treatment?

A
  • Heat to 700C for 10 minutes.
  • Quenching.
  • Aim is to ELIMINATE CORING (in alloys containing Pt/Pd).
36
Q

What are the main contents (2) and color of low gold content alloys?

A
  • Normally AU 45%-50% but could be down to 10%.
  • High PALLADIUM.
  • WHITE in color.
37
Q

What are the main contents (2) of silver palladium alloys?

A
  • Ag and Pd.
  • Little/ no Au.
38
Q

How are low gold alloys classified?

A
  • As for conventional gold casting alloys, classified into 4 TYPES.
39
Q

3 advantages of low gold casting alloys?

A
  1. Can utilize same casting equipment as gold.
  2. Low cost (relative to conventional gold casting alloys).
  3. Good clinical performance.
40
Q

Why is the fact that silver palladium alloys have a lower density compared to gold alloys?

A

Lower density means the alloys requires casting equipment that exerts a GREATER FORCE than for a conventional gold casting alloy.

41
Q

4 disadvantages of silver-palladium alloys?

A
  1. Lower density compared to gold alloys (affects CASTABILITY).
  2. May dissolve oxygen - POROSITY.
  3. Lower ductility than conventional gold alloys.
  4. Care must be taking in their use.

Module 6 - Dental Materials (3 decks)

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