Ceramics Flashcards
What are 5 components to ceramic?
- Quartz (silica).
- Feldspar.
- Clay (kaolin - very small amount).
- Glass
- Metallic colourants
What are the fusing temperatures for the 2 main types of traditional dental porcelain?
- High fusing: 1300-1400.
- Low fusing: 850-1100.
What do traditional dental porcelain and decorative porcelain have in common?
Feldspar Binder.
When does Feldspar binder form a glass? What happens if this is heated repeatedly?
- If K over 11% and melted and cooled quickly it will form GLASS.
- Glass on repeat heat gains new crystalline component, LEUCITE.
What is the coefficient of thermal expansion of NORMAL traditional dental porcelain vs LEUCITE CONTAINING traditional dental porcelain?
- Normal: 8
- Leucite Containing: 20
What is the ideal expansion/ contraction of dental porcelain relative to the underlying dental alloy? Why?
- 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).
What is kaolin better known as? Chemical compound name?
- Better known as CLAY.
- Hydrated aluminosilicate.
Why do manufacturers add leucite to dental porcelain?
- 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 %).
What is Felspar composed of (2)?
- Mixture of aluminosilicates (potassium and sodium).
2 properties of kaolin at room temperature?
- Forms COLLOIDAL suspension in water.
- By SURFACE TENSION EFFECTS provides COHERENCE and PLASTICITY to porcelain powder, permitting MANIPULATION.
What happens when kaolin is heated to 450 C?
- Unstable metakaolinite forms. and water is let off.
What happens to kaolin when heated to 1000C?
- Metakaolinite decomposes to form 3Al2O3 2SiO2.
2 things that happen when ceramic is left at a high temperature for an EXCESS of time?
- Lowers the slumping time.
- Promotes pyroplastic flow.
What is the term for scattering of light and colouration? What is used to achieve this in dental ceramics?
- OPALESCENCE.
- Use particles of metal oxides.
Uranium use in dental ceramics?
- NEVER now due to HEALTH HAZARD.
- Poor appearance under UV light.
Steps to a traditional porcelain buildup?
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.
2 effects of compaction on ceramic?
- Brings particles of ceramic closer together and brings moisture to surface.
- Reduces firing shrinkage of the ceramic.
What happens to dental ceramic at 1150C? On cooling?
- Feldspar melts and decomposes (flows and consolidates powder particles).
- On cooling a glassy silicate matrix results.
What happens during the firing stage (7 things)?
- 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.
What is pyroplastic flow?
- 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.
What happens if the crown is suddenly introduced into the furnace for firing?
- If not incrementally introduced can lead to TOO RAPID STEAM/ CRUMBLE/ EXPLODE.
What is the importance of starch/ sugar binder during firing?
- 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.
How is ceramic cooled after firing? Why?
- CONTROLLED cooling to AVOID CRACKING.
Are additions possible after firing?
- YES, however must then be RE-FIRED.
- Heat must build up incrementally again to avoid thermal stresses.
What is used for ceramic glazing?
Low fusing transparent glass.
In what direction do cracks propagate in porcelain? How can this propagation be stopped?
- Cracks propagate from WITHIN OUTWARDS.
- COMPRESSIVE FORCES stop crack propagation.
- things that can be used to limit crack propagation in porcelain?
- Compressive forces.
- Aluminium core (pure).
- Additions of alumina powder to porcelain (similar coefficients of thermal expansion, opaque).
- Sintered alumina core (w or w/o zirconium oxide for further strengthening).
- Metal core (porcelain fused metal).
What are the 2 components of a Porcelain Fused to Metal (PFM) crown?
- Alloy substructure.
- Bonded porcelain veneer.
What is a characteristic of porcelain made to become fused to a metal crown? How is this achieved? Why is this beneficial?
- Porcelain has INCREASED COEFFICIENT OF THERMAL EXPANSION via addition of LEUCITE.
- Metal and porcelain thus have better matched THERMAL PROPERTIES –> REDUCES STRESS CONCENTRATIONS
4 desirable properties for the alloy used for PFM crowns?
- Not melt when firing porcelain.
- Rigid to support thin porcelain veneer.
- Bond to porcelain.
- Similar coefficient of thermal expansion to porcelain.
What are the alloys available for PFM crown?
- High gold alloy.
- Low gold alloy
- Silver palladium.
- Nickel chromium
High gold as a PFM alloy? (3).
- 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).
4 metals included in low gold alloy?
- 50% Au.
- 30% Pd.
- 10% Ag.
- 10% Indium.
Name the 4 metals and their functions found in LOW GOLD ALLOYS.
- Au and Pd –> increase melting temperature, decrease coefficient of thermal expansion.
- Silver (Ag) and Indium –> bonding.
3 advantages and 1 disadvantage of silver palladium as a PFM alloy?
- Advantages: Cost saving, higher modulus value and melting range (compared to high gold).
- Disadvantages: Ag may lead to greening.
2 advantages and 3 disadvantages of nickel-chromium as a PFM alloy?
- 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).
What causes nickel-chromium to have poor biocompatibility?
- Be: animal carcinogen.
- Ni: risks contact dermatitis.
