Porcelains Flashcards
Porcelains high or low strength
Low strength, but can be improved with bonding
What is the minimal thickness required for porcelain to be reliable restoration
0.7mm
What are traditional feldspathics used for
Artificial teeth for dentures, full porcelain crown, metal ceramic crown, inlays, veneers
Do traditional feldspathics have high or low melting point
High
What is the thermal and electrical conductivity of dental ceramics
Low
Is strength of feldspathics high or low
Compressive strength high, very stiff
Low tensile strength
Why does traditional feldspathic have good biocompatibility
No conversion or leeching of monomer, very inert
Is porcelain ductile
No. Very stiff
Function of leucite in traditional feldspathics (3)
Leucite is a glass ceramic. It is a filler to reduce polymerisation shrinkage. It strengthens and toughens the ceramic. It increases the coefficient of thermal expansion.
Describe process of producing traditional feldspathics
Melt the feldspar. Alkali unite with aluminium and silica to form sodium or potassium silicate. Glassy phase and free crystalline silica phase formed. Ground blocks into porcelain powders. Filter to separate glassy from leucite particles. Firing
Describe porcelain at low bisque stage of firing
Very porous. Minimal shrinkage. Fired body is weak and brittle. Grains of porcelain have started to soften and lense at certain points
Describe porcelain at medium bisque of firing
Surface porous but increased flow of glass grains. Any entrapped furnace atmosphere that has not escaped grain boundaries may become trapped in porcelain. Definite shrinkage has taken place
Describe porcelain at high bisque of firing
Surface of porcelain completely sealed. Much smoother. Porcelain is strong.
What happens if too much leucite crystal is artificially added to the porcelain
Too opaque
What type of structure is porcelain
Vitreous (formed by vitrification). meaning it is not well structured, this random organisation with increased space between atoms allows light to pass through
How to adjust colour of porcelain
Add concentrated colour frit. Add high temperature resistant pigments such as metallic oxides
Why are opacifying agents needed in porcelain
Porcelain is too translucent
What is fluorescence
Absorb radiation of wavelength and re emit as radiation of longer wave length
How to make porcelain fluoresce
Small percentage of sodium diuranate added into the frit
Why is it important that porcelain crown is well adjusted
If there is space between ceramic and tooth, ceramic is unsupported. Not ductile at all, any deformity will cause fracture
Why do traditional feldspathics have low tensile strength
Surface defects eg minute cracks, porosities, unevenness
Leucite vs lithium disilicate as porcelain filler
Leucite take shorter time to break as crack path less tortuous
Compare effects of porcelain surface defects
Cracks are very detrimental to mechanical properties especially surface cracks
Bubbles are not much a concern due to rounded surface
Inclusions mean porcelain not homogenous any more
How do surface defects cause brittle fracture in traditional feldspathics
Surface defects cause a concentration of stresses. Under tensile stresses, concentration of stresses exceed strength of ceramic body. Not ductile unable to relieve stress
Polycrystalline solids vs alumina ceramics
Polycrystalline solids are twice as strong and tough
Crack propagation triggers tetragonal to monoclinic transformation with accompanying 4,5% increase in volume to stop further propagation
