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module 6 - dental materials > Ceramics > Flashcards

Ceramics Flashcards

1
Q

What are 5 components to ceramic?

A
  • Quartz (silica).
  • Feldspar.
  • Clay (kaolin - very small amount).
  • Glass
  • Metallic colourants
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2
Q

What are the fusing temperatures for the 2 main types of traditional dental porcelain?

A
  • High fusing: 1300-1400.
  • Low fusing: 850-1100.
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3
Q

What do traditional dental porcelain and decorative porcelain have in common?

A

Feldspar Binder.

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4
Q

When does Feldspar binder form a glass? What happens if this is heated repeatedly?

A
  • If K over 11% and melted and cooled quickly it will form GLASS.
  • Glass on repeat heat gains new crystalline component, LEUCITE.
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5
Q

What is the coefficient of thermal expansion of NORMAL traditional dental porcelain vs LEUCITE CONTAINING traditional dental porcelain?

A
  • Normal: 8
  • Leucite Containing: 20
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6
Q

What is the ideal expansion/ contraction of dental porcelain relative to the underlying dental alloy? Why?

A
  • Aim to have slightly HIGHER expansion/ contraction than the underlying alloy to put porcelain in slight TANGENTIAL COMPRESSION.
  • This helps with CRACK STOPPING (prevents crack propagation in the ceramic).
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6
Q

What is kaolin better known as? Chemical compound name?

A
  • Better known as CLAY.
  • Hydrated aluminosilicate.
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6
Q

Why do manufacturers add leucite to dental porcelain?

A
  • Allows for customization of ceramics EXPANSION/CONTRACTION to match range of dental alloys.
  • Has a dispersion strengthening effect upon the ceramic (if at 35-5- mass %).
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7
Q

What is Felspar composed of (2)?

A
  • Mixture of aluminosilicates (potassium and sodium).
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8
Q

2 properties of kaolin at room temperature?

A
  1. Forms COLLOIDAL suspension in water.
  2. By SURFACE TENSION EFFECTS provides COHERENCE and PLASTICITY to porcelain powder, permitting MANIPULATION.
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9
Q

What happens when kaolin is heated to 450 C?

A
  • Unstable metakaolinite forms. and water is let off.
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10
Q

What happens to kaolin when heated to 1000C?

A
  • Metakaolinite decomposes to form 3Al2O3 2SiO2.
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11
Q

2 things that happen when ceramic is left at a high temperature for an EXCESS of time?

A
  • Lowers the slumping time.
  • Promotes pyroplastic flow.
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12
Q

What is the term for scattering of light and colouration? What is used to achieve this in dental ceramics?

A
  • OPALESCENCE.
  • Use particles of metal oxides.
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13
Q

Uranium use in dental ceramics?

A
  • NEVER now due to HEALTH HAZARD.
  • Poor appearance under UV light.
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14
Q

Steps to a traditional porcelain buildup?

A

1. PORCELAIN POWDER mixed with WATER to form a paste.
- (some products also have sugar/ starch to facilitate binding).
2. Built up to desired shape on FOIL or CORE.
3. Compaction
- achieved via patting/ vibration.
- Brings particles together and brings moisture to surface.
- Blotting removes moisture. - Compaction also reduces firing shrinkage of the ceramic.
4. Firing (porcelain furnace, electrically heated muffler, vacuum).
5. GLAZE using low fusing transparent glass.

15
Q

2 effects of compaction on ceramic?

A
  • Brings particles of ceramic closer together and brings moisture to surface.
  • Reduces firing shrinkage of the ceramic.
16
Q

What happens to dental ceramic at 1150C? On cooling?

A
  • Feldspar melts and decomposes (flows and consolidates powder particles).
  • On cooling a glassy silicate matrix results.
17
Q

What happens during the firing stage (7 things)?

A
  • Ceramic to be fired enters porcelain furnace.
  • Built up crown introduced INCREMENTALLY (explode, crumble).
  • Enters an electrically heated muffler (surrounds the porcelain).
  • PYROMETER indicates the muffle temperature (as it must be tightly controlled).
  • VACUUM reduces porosity from 4.6% to 0.5% - do not wish inclusions of 0 strength.
  • Starch/ sugar binder IGNITES (must be VENTED).
  • After firing, cooling must occur slowly to AVOID CRACKING.
18
Q

What is pyroplastic flow?

A
  • As temperature increases, the binders are burnt off and the ceramic particles begin to fuse at the points of contact making a POROUS MASS.
  • The voids in the porous material start to disappear as the molten glass flows between the particles, bringing them closer –> PYROPLASTIC FLOW.
19
Q

What happens if the crown is suddenly introduced into the furnace for firing?

A
  • If not incrementally introduced can lead to TOO RAPID STEAM/ CRUMBLE/ EXPLODE.
20
Q

What is the importance of starch/ sugar binder during firing?

A
  • They IGNITE.
  • Thus combustion products must be VENTED through an open door.
  • If not vented, the porcelain will have a BLACK TINGE.
  • Door closed by computer program after ventilation completion.
21
Q

How is ceramic cooled after firing? Why?

A
  • CONTROLLED cooling to AVOID CRACKING.
22
Q

Are additions possible after firing?

A
  • YES, however must then be RE-FIRED.
  • Heat must build up incrementally again to avoid thermal stresses.
23
Q

What is used for ceramic glazing?

A

Low fusing transparent glass.

24
Q

In what direction do cracks propagate in porcelain? How can this propagation be stopped?

A
  • Cracks propagate from WITHIN OUTWARDS.
  • COMPRESSIVE FORCES stop crack propagation.
25
Q
  1. things that can be used to limit crack propagation in porcelain?
A
  1. Compressive forces.
  2. Aluminium core (pure).
  3. Additions of alumina powder to porcelain (similar coefficients of thermal expansion, opaque).
  4. Sintered alumina core (w or w/o zirconium oxide for further strengthening).
  5. Metal core (porcelain fused metal).
26
Q

What are the 2 components of a Porcelain Fused to Metal (PFM) crown?

A
  • Alloy substructure.
  • Bonded porcelain veneer.
27
Q

What is a characteristic of porcelain made to become fused to a metal crown? How is this achieved? Why is this beneficial?

A
  • Porcelain has INCREASED COEFFICIENT OF THERMAL EXPANSION via addition of LEUCITE.
  • Metal and porcelain thus have better matched THERMAL PROPERTIES –> REDUCES STRESS CONCENTRATIONS
28
Q

4 desirable properties for the alloy used for PFM crowns?

A
  1. Not melt when firing porcelain.
  2. Rigid to support thin porcelain veneer.
  3. Bond to porcelain.
  4. Similar coefficient of thermal expansion to porcelain.
29
Q

What are the alloys available for PFM crown?

A
  1. High gold alloy.
  2. Low gold alloy
  3. Silver palladium.
  4. Nickel chromium
30
Q

High gold as a PFM alloy? (3).

A
  • No Copper as risks staining porcelain GREEN.
  • Tin and Indium become oxidized and chemically bond porcelain to the gold alloy.
  • Need thick coping to prevent flexure (prevent cracking of the veneer/porcelain).
31
Q

4 metals included in low gold alloy?

A
  • 50% Au.
  • 30% Pd.
  • 10% Ag.
  • 10% Indium.
32
Q

Name the 4 metals and their functions found in LOW GOLD ALLOYS.

A
  • Au and Pd –> increase melting temperature, decrease coefficient of thermal expansion.
  • Silver (Ag) and Indium –> bonding.
33
Q

3 advantages and 1 disadvantage of silver palladium as a PFM alloy?

A
  • Advantages: Cost saving, higher modulus value and melting range (compared to high gold).
  • Disadvantages: Ag may lead to greening.
34
Q

2 advantages and 3 disadvantages of nickel-chromium as a PFM alloy?

A
  • Advantages: High modulus, high melting temperature.
  • Disadvantages: Prone to high casting shrinkage with voids, poorer bond strength to porcelain and other alloys, poor biocompatibility (Be, Ni).
35
Q

What causes nickel-chromium to have poor biocompatibility?

A
  • Be: animal carcinogen.
  • Ni: risks contact dermatitis.
36
Q
A

module 6 - dental materials (3 decks)

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