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BDS2: Dental Material Science > Partial Denture Alloys > Flashcards

Partial Denture Alloys Flashcards

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1
Q

List 7 ideal properties of PDAs

A
  1. Rigid
  2. Strong
  3. Hard
  4. Ductile
  5. Precise casting (shrinkage)
  6. Melting point ( investment material)
  7. Density
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2
Q

List 4 types of PDA

A
  1. ADA type IV gold
  2. Co-Cr
  3. White gold [Ag-Pd]
  4. Titanium
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3
Q

Why should the Young’s modulus for the base of a denture be high?

A

To maintain shape in use

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4
Q

Why should the elastic limit of a denture base be high?

A

To avoid plastic deformation

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5
Q

Why should a clasp have a low YM and a high EL?

A
  1. Lower YM - to allow flexure over tooth

2. High EL - maintain elasticity over wide range of movement

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6
Q

What is pure gold used for?

A

Class 3 and class 4 cavities

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7
Q

What uses to types 1 to 4 gold alloys have?

A

Type 1: simple alloys

Type 2: larger (2-3 surface) inlays

Type 3: crown and bridge alloys

Type 4: partial dentures

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8
Q

What is the composition of type 4 gold alloy?

A
  1. Gold (Au) - 65%
  2. Silver (Ag) - 14%
  3. Copper (Cu) - 14%
  4. Zinc (Zn) - 2%
  5. Palladium (Pd) - 3%
  6. Platinum (Pt) - 2%
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9
Q

What part of the phase diagram indicates what level of coring will occur?

A

The distance between solidus and liquidus lines

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10
Q

What effect does copper have in gold alloys?

A
  1. Solid solution in all proportions
  2. Solution hardening
  3. Order hardening - if 40-80% gold and correct heat treatment
  4. Reduced melting point
  5. Little or no coring
  6. Imparts red colour (if sufficient quantity)
  7. Reduces density
  8. Base metal - can corrode if too much
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11
Q

What are the effects of silver in gold alloys?

A
  1. Solid solution in all proportions
  2. Solution hardening
  3. Precipitation hardening with copper and heat treatment
  4. Can allow tarnishing
  5. Molten silver absorbs gas (e.g. CO2)
  6. Whitens alloy - compensates for copper
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12
Q

What are the effects of adding platinum to a gold alloy?

A
  1. Solid solution with gold
  2. Solution hardening
  3. Fine grain structure
  4. Coring can occur
    - wide liquidus-solidus gap
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13
Q

What are the effects of adding Palladium to gold alloys?

A
  1. Less coring than Pt
  2. Coarser grains than Pt
  3. Absorbs gases when molten - porous casting
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14
Q

What are the effects of zinc, nickel and indium in gold alloys?

A
  1. Zinc: scavenger
  2. Nickel: increase hardness and strength (wrought alloys)
  3. Indium: fine grains structure
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15
Q

State the process of events involved during heat treatment of gold alloys
(This process makes type 4 gold alloy more suitable for clasp)

A
  1. Quench after casting (fine grains)
  2. Homogenising anneal (700 degrees C, 10 mins)
  3. If cold worked - stress relief anneal
  4. Heat harden - (order and precipitation)
    - 450 degrees C cool slowly (15-30 mins) to 200 then quench
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16
Q

List uses of CoCr

A
  1. Wires
  2. Surgical implants
  3. Cast partial dentures
  4. Connectors
    - high EL + YM = thick section
    - high EL + low YM = thin section
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17
Q

List the composition of different metals in CoCr

A
  1. Cobalt - 54%
  2. Chromium - 25%
  3. Nickel - 15%
  4. Mo - 5%
  5. C - 0.4%
18
Q

How does cobalt effect the CoCr alloy?

A
  1. Forms solid solution with Cr
  2. Increased strength, hardness, rigidity
  3. Coring possible
19
Q

How does chromium effect CoCr alloys?

A
  1. Forms solid solution with Co
  2. Increased strength, hardness, rigidity
  3. Coring possible
  4. Forms passive layer - corrosion resistance
20
Q

How does nickel effect CoCr alloys?

A
  1. Replaces some Co
  2. Improves ductility
  3. Slight reduction in strength
  4. Sensitivity:
    - 6% of females
    - 2% of males
21
Q

Why must we be careful not to add too much carbon to a CoCr alloy?

A

Makes the alloy too hard and brittle

22
Q

How does Mo and tungsten effect CoCr alloys?

A
  1. Mo - reduced grain size Therefore increasing strength

2. Tungsten - increases strength

23
Q

How does Al (aluminium) effect CoCr alloys?

A

Increases proportional limit stress (PL)

24
Q

Why are silica or phosphate bonded investment materials used as opposed to gypsum?

A

They must be able to cope with high temperatures (1200-1400 Degrees C)

25
Q

Why is electric induction or oxyacetylene the preferred method to melt the CoCr alloy?

A

To avoid carbon pickup

26
Q

What can overheating the CoCr alloy result in?

A

Coarse grains

27
Q

What can result from cooling the CoCr alloy too fast or too slow?

A

Can result in carbides which makes the alloy brittle

28
Q

What methods are used for surface finishing of CoCr?

A
  1. Sandblasting
  2. Electroplate
  3. Abrasive wheel
  4. Polishing buff
29
Q

What is the hardness of CoCr?

A

370 hardness units

- much harder than gold

30
Q

What is an advantage and a disadvantage of CoCr compared to type 4 gold?

A

Advantage: Wear in the mouth better

Disadvantage: finishing/polishing is time consuming

31
Q

What is precision casting needed for CoCr?

A
  1. Elongation 4%
  2. Work hardens rapidly
  3. Adjustment is difficult

therefore precision casting is needed

32
Q

What are used of titanium in dentistry?

A
  1. Potentially use as a cast material
  2. Other uses:
    - implants
    - crown and bridge (cast)
    - Maxillo-Facial skull plates
    - partial dentures (cast)
33
Q

What are advantages of using titanium in dentistry?

A
  1. Good biocompatibility
  2. Good corrosion resistance (passive oxide layer)
  3. Parts joined by laser welding
34
Q

What are disadvantages of using titanium?

A
  1. Electric arc melting is required
  2. As titanium absorbs gases:
    - specialised investment and casting equipment needed

(Basically needs more high tech equipment to work with)

35
Q

What is the ductility of titanium?

A

15% - this is 5% higher than gold (the next highest ductility)

36
Q

List the ductility’s of:

  1. Gold
  2. Silver/platinum
  3. CoCr
  4. Titanium
  5. Steel

From highest to lowest

A
  1. Titanium 15%
  2. Gold 10%
  3. Silver/platinum 9%
  4. CoCr 4%
  5. Steel 1%
37
Q

List the ultimate tensile strength of :

  1. Gold
  2. silver/platinum
  3. CoCr
  4. Titanium
  5. Steel

From highest to lowest

A

In MPa:

  1. Steel 1900
  2. Gold approx 700
  3. Titanium approx 700
  4. Silver/platinum approx 450-475
38
Q

How does the density of CoCr compare to Type 4 gold?

A

CoCr = 8g/cm3

  • just over half that of gold
39
Q

How does the rigidity (YM (GPa)) compare to type 4 gold?

A

CoCr = 250 GPa

This is over twice that of gold

40
Q

What is the shrinkage of CoCr and how does this compare to type 4 gold?

A
  1. CoCr = approx 2.25%

This is greater than gold (1.25%)

41
Q

What factors need to be considered when using CoCr rather than gold?

A
  1. More difficult to produce defect free casting than gold
  2. Cannot use gypsum-bonded investment
  3. More difficult to polish (as it is harder) than gold
    - but retains polish better
  4. Work hardens rapidly, so needs precision casting
42
Q

Summarise the comparisons between CoCr and type 4 gold in terms of:

  1. Shrinkage (during casting)
  2. Density
  3. Rigidity (YM)
  4. Fracture strength (UTS)
  5. Ductility
  6. Hardness
A

CoCr:

  1. Shrinks more
  2. Just over Half the density
  3. Double the rigidity
  4. Similar fracture strength
  5. Half as ductile
  6. Much harder

BDS2: Dental Material Science (8 decks)

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