Porcelain Flashcards
What is in kaolin?
Al2o3. 2sio2. 2H2O
Hydrated aluminosilicate
What are the components of porcelain?
Kaolinite
Quartz
Feldspar
What is quartz made from?
SiO2
What is the most common element in the earth?
Silica
What is feldspar?
Mixture of potassium and sodium aluminosilicates
K20 Al2O3 6SiO2
N20
What are dental porcelains mainly composed of?
mainly Felspar some quart hardly any qaolin
What is the differnce in compostition of high fusing and low fusin porcelain?
percetntage: Kaolin, Silica, Feldpar, Glasses
High fusing: 4, 15, 80, 0
Low fusing:, 25, 60, 15
Metal oxides less than 10%
Wen are high fuse porcelains used?
they are used mainly for PJC
they fuse at around 2400 degress
When are low fuse porcelains used?
these are used to provide the glaze
What are the other componenents added to porcelain?
Borax: sodium borate
Metal oxides
What is the purpse of adding sodium borate, and to what component is it added to?
this is added to feldspar
added to reduce melting temp
What are the two types of metal oxides added for?
pigments and opalscence
Which metal oxides are added for pigmentation?
iron: brown
copper: green
coablt: blue
Which metal oxides are added for opalscence *transparity?
Zno
Titanium oxide
What are the physical properties of porcelain?
Low thermal conductivity: good since does conduct heat to pulp, bad since thermal stresses can lead to fracture
Excellent aesthetcs
wide range of shades so can mimic dentine and enamel
What determines the opacity or trasparency of porcelains composte and ceramic materials?
partcle size and index of refraction of the differnt components
WHat was the issue with having kaolin in porcelain?
it was initally added to increase the abilty to mould and shape the porcelain however, since it was opaque it made it difficlt to achieve a high transluceny of porcelain
What are the chemical and biological properties of porcelain?
bicompatoble
stable
need to ensure that porcelain is glazed since only polishing makes the surface quite rough and promotes plaque accumulation
t/f porcelain is harder than enamel?
t
vickers hardness is 450 vs 350 for enamel
What are the mechanical properties of procelain?
brittle
lacks fracture toughness
fails via catostrophic failure and static fatigue
What are is static fatigue?
decrease in strength over time even in the abscence of load but is accelrated by dynamic loading
SiO bonds slowly hydrolysed and this genreats a rise in OH which elevates pH. this then dissolves oout the Na2O and the K2O (feldpsathic components)
WHat is catostrphoc failure?
this is when the porcelain actually chips away and due to its brittle nature and is rapid.
How is the stength of the porcelain reduced?
surface roughness
internal voids
porosity
How can the mechaical properties of porcelains be improved?
- Alter processing: optimising the fabrication condition
- produce stronger procelains: optimising the compostions and microstructure
- Reinforce porcelain by providing a solid core : structure optimisation
What are the 5 processing imprvments?
Careful tooth prep: redices stresses inside the crown
Use finer grain powder: gives a more unifrom microstructure and surface
Fire at vacuum or under pressure: reduces porosity
Cool slowly from furncae: reudces thermal stresses which can cause cracks
Smooth or further fusing: reduces surface roughness and imprefections
WHat are the three ways of priducing stronger porcelains?
most common method of failure is via crack propagation from the surface irregularties so need a compressive strenght to combat the froces created at surface imperfections
- Ion exchange: xreates compressive forces all over surface by soacking the fired porcelain in molten potassium salt
- thermal stengthening: careful applicaton of heat during inital firing or later heat tempering.
- adding strengthening particles:alumina added will act as crack stoppers. plate like mica or needle like crystals eg lithium disiclicate are added
What are the names of the various stengthening cores that can be added?
alumina alumina reinforces feldspathic porcelain glass infiltrated alumina spinel zirconia
WHat are the advnatages of porcelain?
aestehtics
chemically inert
Wgat are the disadvntages of porcelain?
brittle (low fracture toughness)
High elastic modulus
high vicker hardness so wear to enamel
How do you prepare porcelains?
Powder mixed with water and a binder
moulded and carved and the compacted and dried
fired and cooled
glazed or poloshed
WHat is the name of the porcelain powder?
a frit
How is rhe frit made?
feldspar
kaolin
silica
ground up, mixed and fused *fedpar acts as the flux. rapidly cooled and then ground again
What is the binder?
this is made from sugar and starch so improves the working time of the porcelain
What do we place the porcelain on when firing?
placed on to foil mould and the porcelain shrinls towards this and assures a god fit of the crown to the tooth
what is the purpose of compating?
light vibrations and patting whichsettles the partciles and reduces shirnkage. this gives a uniform contraction over the whjole surface and brings any excess water to the surface.
What do modern mnufactures use to make green dense ceramics?
cold isotatic pressing
How are drying and sintering carried out>
plced in warm atmosphere less than 100 degrees
since if wet procelain placed straight into hot oven it will bloat. once dried at lower temperature, it is then fired at a high temperature by placing in a firnace of more than 1000 degrees sometimes under vaccuum
How is glazing performed?
Low fusing transparent glass is used to coat the porcelain and improves the appearance provding greater translucency
this proves a smooth impervous finsih protecting agaisnt chemica attack and lowering hardness to reduce wear
Which component to porcelain has the lowest melting point?
Feldspar
Acts as a flux
T/Fporcelaim is strong in compression?
T
T/f porcelain is week I tension?
T
This is why cannot be used in thin section
What is the name for the feldpathic components?
Soda Na2Oand potash K2O
T/F porcelain is a thermal insulator?
T
What is a glaze?
Glass put on the surface
What is the purpose of the porcelain glaze?
Smooths the surface
Protects against chemical attack
What temp are core porcelains fired at?
850-1050 for 5 mins
What temp are the enamel and dentine porcelains fired at?
750-900 for 5 mins
By how much does porcelain shrink after firing?
40%
What are the options for sintered porcelains?
Aluminous porcelain: this is strong and used for cores but not great aesthetics since more opaque
Feldpathic reinforces with zirconia or leucite
How do you prepare for a PJC?
Incisal reduction by 1/4 or 1/3
Labial reduction in two planes
5% taper
Even Palatal reduction
1mm shoulder
What are resin bonded crowns?
Made in feldpathic porcelains
Minimally intervention
Thin section
Strength derived from underlying bond to tooth
What are the ad and dis of PJVC?
AD: good aesthetics and easy to manufacture
Dis: abrasive, brittle, irreversible ,damaging to tooth
What is good about PFM?
Strength of metal restoration and Appearance of porcelain
When are PFM indicated?
Heavy masticatory load
Limited Palatal space
Incorpated rest seat
What are the dis of PFM?
Destructive
Difficult to remove
Cost
Technqie sensitve
How does metal bond to porcelain?
Early theory: van der waals, setting
Now: mechanical interlocking, chemical bond to oxide layer, compressive force
What are the causes of failure for PFM?
Insufficient metal support
Unequal thickness of porcelain
Faulty design
Fault occlusion
What should you avoid with PFM?
Unsupported porcelain
Interfaced of porcelain and metsl under heavy occlusal load
What is the function of a Pontic?
Appearance
Stabilise occlusion
Masticatory
Phonetics
What are the options for a bridge replacing anterior tooth?
Conventional fixed fixed
Cantilever bridge
Spring cantilever
RRB
When preparing the labial preparation phases?
2 stages
First cut is incisal third
Second cut: gingival third
What type of preps are there for PFM?
Winged prep : more resistance and more conservative
Wingless Prep : easier to prepare, less chance of leaving undercut
What is the depth of the PFM shoulder?
1.3mm
How deep is the Palatal chamfer?
0.7mm
What are the stages of bridge prep for anterior teeth?
Incisal reduction by 2mm Paired mesial and sitar prep Paired axial prep Cingulam reduction of 1mm Shoulder Finnish
