Porcelain Flashcards
What is a ceramic?
Inorganic, non metallic materials
What are the 3 types of ceramics?
Crystalline, amorphous (glass) and mixed (glass-ceramics)
Name a specific type of ceramic material
Porcelain
What are the clinical uses of porcelain in dentistry?
Crowns, onlays, veneers and bridges
What are the main components of porcelain?
Kaolinite (or kaolin)
Quartz (crystallised silica)
Feldspar
What is kaolinite?
China clay (hydrated aluminosilicate)
What is quartz?
The second most abundant element in the earths crust
What is feldspar?
A mixture of potassium and sodium aluminosilicates
What is the composition of dental porcelain?
It contains little pr no kaolin, it is mainly silica and feldspar (i.e. feldspathic glass with crystalline inclusion of silica)
What is the composition of dental porcelain?
(Pic)
N.b. high fusing means high melting temp
Which other components may be included in porcelain?
- Borax (sodium borate) - added to feldspar to reduce the melting temperature
- Metal oxides - pigments (e.g. iron = brown, copper = green and cobalt = blue) & opalescence (e.g. zinc oxide & titanium oxide)
What are the physical properties of porcelain?
- Excellent aesthetics
- Low thermal conductivity
What determines opacity/transparency of porcelain/ceramic/composite material?
- Particle/crystal/grain size
- Index of refraction
What is the positive implication of porcelains low thermal conductivity?
Does not conduct heat to the pulp
What is the negative implication of porcelains low thermal conductivity?
Thermal stresses can lead to cracks or fatigue (heat accumulates in some areas)
Why are the aesthetics provided by porcelain so amazing?
= wide range of shades and levels of translucency - to mimic different natural tissues
What are the chemical and biological properties of porcelain?
- Pretty stable (inert)
- biocompatible
- polished surfaces are rougher than glazed surfaces = accumulates more plaque
What are the mechanical properties of porcelain?
- Brittle
- Hard
- Weak (tensile)
- Strong (compression)
- Low fracture toughness
What are the common failure modes for brittle materials?
- catastrophic failure (suddenly breaks)
2. static fatigue (slowly degrades over time, even in absence of applied load -> can propogate)
How does catastrophic failure occur?
Crack propagation (usually along inner surface of crowns)
What reduces the strength of a porcelain (leading to catastrophic failure)?
- Surface roughness (micro cracks -> stress concentration)
- Internal voids
- Porosity
Why does static fatigue happen?
SiO2 bonds slowly hydrolysed = generates OH- = elevates pH -> dissolved Na2O & K2O (feldspathic components) = accelerated by dynamic mechanical loading
How do we improve the mechanical properties of porcelain?
- Alter processing -> fabrication condition optimisation
- produce stronger porcelains -> composition and microstructure optimisation
- reinforce porcelain by providing a solid supportive core -> structure optimisation i.e. pfm
How do we improve processing?
- Use fine grained powder = gives more uniform microstructure and surface
- Fire at vacuum or under pressure = reduces porosity
- Cool slowly after removing from the furnace = reduces thermal stresses which cause cracks
- Smoothing or further fusing = reduces surface roughness and imperfections
- Careful tooth preparation = reduces possibility for stresses at inner surface of crown
How do we produce stronger/tougher porcelains?
Need compressive stress to compensate for the stresses created on surface irregularities under force
- > ion exchange strengthening
- > thermal strengthening
- > particulate strengthening/toughening
What is ion exchange strengthening?
= creates compressive forces all over surface = soak the fired porcelain in a molten potassium salt
What is thermal strengthening?
Can be applied by careful application of heat (during initial firing or later heat tampering)…
Molten porcelain = solidifies from outside inwards = liquid centre pulls inwards on outer edge = compressive forces
What is particulate strengthening/toughening?
Alumina particles act as a “crack stopper” = improves strength and fracture toughness
= either plate like mica or needle like crystals such as lithium dislocate
= (large S.A. on interface = absorbs lots of energy along the interface)
Which material can we use to make a strong/tough ceramic core?
- Alumina
- Alumina-reinforced feldspathic porcelain
- Glass-infiltrated alumina
- Spinel (magnesium aluminate)
- Zirconia
What are the strengths of porcelain?
Aesthetics, High strength (compression), chemically inert, high hardness, high elastic modulus
What are the weaknesses of porcelain?
Brittle (low fracture toughness), fatigue (any defect can cause fatigue) & tooth wear (due to roughness and high hardness)
What is the general preparation of porcelain?
Powder mixed with water and binder (sugar or starch = improves working properties)-> moulded, carved, compacted and dried -> fired (scintering = melts together and fuses) and cooled -> glazed or polished
What is the name of the powder porcelain is supplied in?
A frit
How are the components of porcelain mixed?
- Ground
- Mixed
- Fused
- Rapidly cooled
- Ground again
= more even mix
How does feldspar fuse the other components together?
It melts at the lowest temperature
Why is the porcelain compacted onto platinum foil mould?
The foil is not affected by the firing temperature = porcelain shrinks inwards towards fold = assures good crown fir on tooth
How do some companies overcome setting shrinkage of porcelain?
Produce partially sintered moulds i.e. for CADCAM
How do we achieve wanted compaction?
Light vibration/patting
What does compaction achieve?
- settles particles = reduces shrinkage
- gives uniform contraction over whole surface
- brings excess water to surface
What do modern manufacturers do to make dense ‘green’ porcelain ceramics?
Cold isostatic pressing (CIP)
Which method can be used t make dense ceramics directly?
Hot isostatic pressing (HIP)
What is the drying and firing process?
- Placed into warm atmosphere to dry ( 1000 degrees C (sometimes under vacuum)
What happens if you put wet porcelain straight into the oven?
It will bloat
How do we glaze?
Coat the completed restoration with low-fusing transparent glass
What are the benefits of glazing?
- improves appearance = greater translucency
- gives impervious, smooth coating which protects against chemical attack
- lower hardness = reduces wear
What is CAD/CAM?
A computer that produces ceramics or composites in a lab or chair side
For ceramics can use fully sintered (zirconia and just put glaze on top to mach colour and strength -> tools wear a lot) or partially sintered (not filly dense, must then scienter again = shrinkage but pretty similar and predictable shrinkage = 10%)
What are the 3 main categories of bonding of porcelains?
- cemented using luting agents
- fused to metal then cemented to tooth
- resin bonded ceramics have recently been developed
What are the benefits of porcelain fused to metal crowns?
Combines properties of cast dental alloys and porcelain = good mechanical properties and aesthetics
- materials must be compatible to form a good bona
- alloys typically used = high-gold, low-gold, silver-palladium and nickel chromium
What are the costs, health and safety of porcelains?
Materials: relatively expensive for patient £3-400
Environment: materials plentiful
Time: several visits required, less so with CAD/CAM
Longevity: adv. crown lasts 10 years, may be longer
Health and safety: uses fine powders = needs to use extractor
