Flashcards in 8 - Dental Ceramics 1 Deck (52):
dental porcelain: what is the fusing temperature for high fusing ceramics?
dental porcelain: what is the fusing temperature for low fusing ceramics?
decorative porcelain: what is its distribution of kaolin, silica, feldspar and glasses?
high fusing dental porcelain: what is its distribution of kaolin, silica, feldspar and glasses?
low fusing dental porcelain: what is its distribution of kaolin, silica, feldspar and glasses?
kaolin - what type of material?
feldspar: what type of material?
it is a mixture of potassium and sodium aluminosilicates
role of feldspar? how does it achieve this role?
- it acts as a binder, uniting components in a solid mass
- it melts and flows on firing
other additives in ceramics:
- what is opalescence?
- what is added to achieve opalescence?
- what other additive is used to simulate tooth fluorescence but is no longer used? why is this so?
- opalescence is a light scattering effect
- small amounts of metallic oxides
- uranium. it gives unnatural appearance under UV light and may create a potential health hazard
porcelain powder - how is it built up?
it is mixed with water to produce a plastic mass of material to be moulded/carved before firing
porcelain powder - what is added to improve working properties? (facilitate binding?)
a binder e.g. starch or sugar
compaction of porcelain:
- how does compaction help?
- how can it be done?
- how is moisture removed?
- what else is beneficial about a well compacted crown?
- reduces the size of the spaces between particles, thereby reducing firing shrinkage
- 1) light vibration of moulded crown to help settle the powder particles
2) powder can be "patted" with a spatula to be compacted
- moisture is removed by blotting
- shows regular contraction over entire surface, therefore maintaining the original form on a slightly reduced scale
what is the next stage following compaction?
firing: what does a porcelain furnace consist of? what does a pyrometer do?
- electrically heated muffler, containing a pyrometer that indicates the temperature in that part of the muffler where the porcelain is placed
what is the effect of firing under vacuum?
it reduces porosity in the finished material from 4.6% to about 0.5%
firing: what happens when wet structure is placed directly into hot furnace? what should be done instead?
- when wet structure is placed into hot furnace directly, it will rapidly steam rapidly and explode or crumble
- wet structure should be dried in a warm atmosphere before being placed in a hot furnace
what happens to the sugar/starch binder when the temperature of the furnace elevates?
how will this affect the structure?
how is this issue resolved?
- the sugar/starch binder ignites
- this causes the structure to blacken
- the door of the furnace is left slightly ajar during this stage to allow products of combustion to escape. door is then shut to firing completion
how should porcelain work be cooled? why and how is this important?
how should porcelain be re-fired if needed?
- porcelain work should be cooled slowly. this is because porcelain is brittle and a poor conductor of heat. rapid cooling of porcelain would result in cracking and loss of strength
- re-firing would require incremental buildup of heat.
porcelain properties? x6
- good aesthetics
- prone to crack propagation
- relatively resistant to chemical attack
- good thermal insulator
the brittleness of dental ceramics is compounded by what?
brittleness of dental ceramics is compounded by their tendency to undergo static fatigue
what is known as static fatigue? how does it occur?
- it is a time dependent decrease in strength even in the absence of applied load
- it occurs through the alkaline hydrolysis of Si-O groups within the porcelain structure
static fatigue: what results in alkalinity within the dental ceramic material?
this forms which component of porcelain?
- alkalinity results from the solubilization of Na2O and K2O, which:
- forms part of the feldspathic component of porcelain
crack propagation in dental ceramics:
how does it propagate?
what stops crack propagation?
- cracks propagate from within, and continue outwards
- compressive forces
what are the methods of generating compressive forces to limit crack propagation in dental ceramics?
- ion strengthening: potassium ions occupy a greater volume and thereby create compressive stresses
- thermal strengthening: inner layers of material that sets later will set up a compressive stress on the outer layer when it solidifies
methods to limit crack propagation? x4
1) aluminium core: core of pure alumina on which porcelain crown is constructed. alumina is hard, opaque, less susceptible to crack propagation
2) adding powdered alumina to porcelain -> achieve significant strengthening. alumina particles act as "crack stoppers" preventing the propagation of a crack.
3) sintered alumina core
4) metal core - PFM
besides the good mechanical properties of alumina, how else does it help to improve the properties of porcelain?
- good compatibility with porcelain. close matching values of
1) coefficient of thermal expansion
2) modulus of elasticity
sintered alumina core - what does adding zirconium oxide help to achieve?
a sintered alumina core containing significant quantities of zirconium oxide will help to achieve further strengthening and a higher flexural strength
porcelain fused to metal (PFM):
- what substructure?
- restorations generally consist of?
- feldspathic porcelains used for PFM normally contain significant amounts of what material?
- alloy substructure
- alloy substructure bonded with porcelain veneers
addition of leucite has what effect on PFM?
- increases coefficient of thermal expansion of the porcelain (to a value closer to that of metal) -> reduces thermal stress during cooling
- leucite also helps to strengthen ceramic
alloys used for PFM - desirable properties? x4
1) withstand porcelain firing - does not melt
2) sufficiently rigid to support the brittle porcelain
3) bond to porcelain; prevent detaching
4) similar coefficient of thermal expansion to porcelain
list the 4 alloys currently available for porcelain bonding
1) high gold alloys
2) low gold content alloys
3) silver-palladium alloys
4) nickel-chromium alloys
high gold alloys for PFM:
- why is copper absent?
- how does tin and indium promote bonding to porcelain?
- why must copings be produced?
1) copper will give a greenish hue to the porcelain veneer
2) they become oxidised at the surface, and the oxidised layer forms a chemical bond with the porcelain during firing
3) to prevent flexing, which would result in porcelain fracture. (high gold alloys have bad modulus of elasticity)
what does the presence of platinum/palladium achieve?
- raises the melting temperature of the alloy, therefore reducing the risk of softening and creep during firing
low gold alloys - composition of:
palladium? + function?
silver & indium? + function?
how do its properties differ from high gold alloys?
- gold 50%
- palladium 30%. helps increase MP and lower coefficient of thermal expansion
- silver 10% & indium 10% - for porcelain bonding
- similar mechanical properties
- slightly greater modulus of elasticity (good for porcelain bonding)
- high melting range (better creep resistance during porcelain firing)
- what advantages compared to high gold alloys?
- presence of Ag (silver) may lead to?
- how does it compare with high-gold materials in cost?
- higher MP
- higher modulus of elasticity
- may cause green hue in some ceramics
- silver-palladium brings considerable cost saving
what is the significance of higher modulus of elasticity or higher MP?
- greater modulus of elasticity (good for porcelain bonding)
- high melting range - better creep resistance during porcelain firing
- high modulus of elasticity
- higher melting temperature
- high casting shrinkage: may affect accuracy of fit if investment does not compensate
- poor castability: pron to voids in castings
- poorer bond strength compared with other alloys
- poor biocompatability: nickel risks contact dermatitis
alternatives to PFM alloys - capillary technology:
how does it work?
1) wax strip with high Pd content adapted onto cast
2) cast is fired: burns off wax and sinters the metal
3) second wax strip with pure gold applied to sintered layer
4) fired again: molten gold infiltrates capillary to form metal substructure
5) layer of veneer porcelain baked onto surface
* bonding between metal and porcelain achieved through mechanical attachment
alternatives to PFM alloys - CAD-CAM:
what does it stand for?
how does it work?
- computer aided design-computer aided manufacture
1) optical impression taken (prepared tooth coated with optically reflective powder)
2) digital model produced
3) computer controlled milling of final restoration & coping
*porcelain build up on coping thereafter required
alternatives to PFM alloys - injection moulded ceramics: how does it work?
1) wax pattern formed on an epoxy resin die
2) variation of lost wax technique used to maintain correct shape and size of ceramic attachment
3) ceramic mix injected under pressure at 180deg into prepared mould
4) fired in special furnace -> formation of spinel (mixed metal oxide)
5) veneer porcelain then baked onto surface of coping
alternatives to PFM alloys - injection moulded ceramics:
what does the ceramic mix contain?
magnesium, aluminium oxides, glass, kaolin, calcium stearate, wax
alternatives to PFM alloys - injection moulded ceramics: what happens when pressed ceramics during firing?
magnesium oxide reacts with alumina to form mixed metal oxide (spinel)
alternatives to PFM alloys - injection moulded ceramics: how is firing shrinkage compensated for?
- the spinel is less dense than original mixture -> it gives a resultant expansion therefore compensating for firing shrinkage
alternatives to PFM alloys - pressed ceramics:
name 2 examples of molten ceramics that will be pressed into the pre-formed mould?
IPS Empress (feldspathic type glass)
IPS Empress 2 (lithium disilicate ceramic)
alternatives to PFM alloys - pressed ceramics:
- IPS Empress injected into pre-formed mould
- after molten glass forced into mould under pressure, it is cooled under conditions allowing leucite crystals to form
how does IPS Empress 2 differ from IPS Empress?
it is a lithium disilicate glass that has a higher flexural strength similar to sintered alumina core
alternatives to PFM alloys - cast glass:
- process is similar to which technique?
- how are the crowns formed? at what temperature?
- describe the casting process
- what does heat treatment of the crown result in?
- how is color matching achieved?
- lost wax technique (for alloy casting)
- crowns are formed from the wax patterns which are invested on phosphate bonded investment. wax burn out and heat soak of the investment is carried out at 950deg
- molten ceramic cast centrifugally into mould at 1350deg.
- heat treatment causes partial crystallization, forming mica-like crystals
- apply series of tinted porcelains to surface and refiring
alternatives to PFM alloys - cast glass:
what do mica-like crystals contain?
describe the dual effect the formation of mica-like crystals have on the crown?
- K, Mg, Si oxides with significant amounts of Fl
- reduced translucency of previous clear material
- increased strength
alternatives to PFM alloys - polycrystalline ceramics:
- Y-TZP: yttrium tetragonal zirconia polycrystals - a blend of what chemicals?
- what is "transformation toughening" and how is it initiated
- how do internal stresses change the crystal structure?
- a blend of zirconium oxide and yttrium oxide, resulting in a multiphase material
- internal stresses causes change in crystal structure, leading to "transformation toughening", which is the increase in volume that sets up compressive stresses
- increased volume -> sets up compressive stresses around crack tips, therefore reducing the likelihood of crack propagation
repair sometimes possible - helps to avoid what?
- avoids need for restoration removal and remake