Flashcards
Dental Materials Exam Prep
Cards in this deck (47)
- 1What is the chemical composition of Porcelaininorganic, non-metallic material.
Covalent or ionic bonding.
Contains either silica, zironcia, alumina - 2What is the Atomic Structures of Porcelain?Amorphous or Crystalline
- 3Describe amorphousTends to be glass.
No defined atomic structure - 4Define crystallineDefined atomic structure
- 5What are the 3 processing classifications of porcelain?1. Powder/Liquid (glass based) with or without crystalline fillers
2. Machinable/Pressable (glass based) with or without crystalline fillers
3. CAD/CAM or slurry (crystalline based) all crystalline-alumina or zirconia based - 6Describe glassProduct of fusion which has cooled to a rigid condition without crystallising.
An amorphous solid - 7Describe how Powder/Liquid systems are processed1. blending of raw materials
2. melted in a crucible (pyrochemical reaction)
3. fluxes partially combine with silica
4. red hot glass rapidly cooled
5. molten mass turn to small particles= Porcelain powder (frit) - 8Name and describe the materials in feldspathic porcelains. (without crystalline fillers)Structure- vitreous
Silica (quartz)
Kaolin (clay), chemical name- aluminosilicate. High melting point
Feldspar (glass),, either silica combined with sodium/calcium or potassium. The main constituent or powder/liquid porcelains - 9Name and describe the materials in Aluminous/Leucitic porcelains (with crystalline fillers)Alumina (kaolin) fillers, helps crack propagation
- 10Why are fillers added to powder/liquid porcelains?Feldspathic porcelainlow in strength, addition of free crystalline phase particles as fillers adds strength
- 11Name the properties of AluminaHigh modulus of elasticity
high compressive strength
high tensile strength
high flexural strength
(crystalline structure) - 12Describe crack propagationMicrocracks form from tensile stresses within the porcelain.
Surface flaws, porosity and thermal stressing can cause this.
Crack will travel into the porcelain until it hits a free alumina particle. - 13*Remember* Aluminas grain size effects strength but also opacity. Finer the grain size greater the strength, higher the opacity-
- 14Describe the procedure in which Powder/Liquid porcelains are used1. porcelain powder mixed with fluid to create slurry.
2. adapted to shape and fired
3. particles combine by the process (sintering)
(sintering- edges of the particles become rounded and stick together)
4. fired in a partial vacuum to reduce risk of porosity (increases strength) - 15Name the different porcelain powders and what they're used for1.Core porcelain-base or core of PJC. Good mechanical properties and provides strength for final structure.
2. Opaque porcelain-used in metal-bonding porcelains. Bonds veneer to metal and masks the colour
3. Dentine porcelain-more translucent. Governs the shape and shade. Largest bulk of porcelain
4. Enamel porcelain-translucent, forms incisal/occlusal portion of crown
5. Stain and Glaze-air-fired (no vacuum) Produces high glaze. Glaze is clear. Stain is highly pigmented - 16List the build-up techniques for Powder/Liquid porcelain1. slurry applied over the core
2. condensed/compacted to required shape (this reduces the gaps between particles removing excess fluid, increasing density)
3. shrinkage of 15-40% possible
Modern feldspathic/alumina porcelains little to no shrinkage occures and blotting with tissue is substantial. (should not be vibrated) - 17List the methods of condensationvibration
blotting with tissue
after compressing whipping with a brush
sprinkling dry powder - 18*Particle Size*If powder particle size is the same throughout, then 45%of the mass will consist of voids.
Blends create a better degree of condensation. - 19Explain the technique for firing powder/liquid porcelainelectrically heated furnace
1. initial rate of heating slow, prevents liquid turning to steam too quickly which will lead to it collapsing of exploding
2.slow and uniform heating allows interior ad exterior surfaces to heat at the same rate (prevents thermal shock)
3. door initially left open to allow vapour to escape (not on modern furnaces) - 20Name and describe the stages porcelain goes through during firingLow Bisque, surface of porcelain very porous. Grains starting to soften at contact points. Body weak and friable.
Medium Bisque, surface slightly porous. Flow of grain boundaries will have trapped air left in the porcelain.
High Bisque, surface completely sealed. Shrinkage complete
Glazing, pyroplastic flow on surface produces high shine.
Cooling, carried out slowly otherwise contraction can lead to stress, forming superficial cracks - 21List the requirements for Powder/Liquid porcelains1. suspension plastic enough to allow it to be shaped
2. high green strength (unfired strength)
3. low shrinkage on drying
4. fire shrinkage low
5. no warpage or cracking during firing
6. free of airbubbles - 22Describe machine/pressable porcelainsSimilar to powder/liquid porcelains as they are glass-based, with or without fillers.
Main advantage is their homogeneity (difficult to achieve when building by hand)
This increases strength and aesthetics.
Manufactured metal-free - 23Describe the process of Machinable porcelains1. (in labs) CAD/CAM creates digital impression using touch or laser scanners.
2. takes images of models and converts them to digital impression
3. technician digitally creates restoration
1. (in clinics) optical scanner inserted into patients mouth for a digital impression
2. dentist builds restoration digitally - 24Describe the process of milling1. restoration is sent to a milling machine
2. milled out of solid blocks of porcelain
3. porcelain soft enough to be trimmed, yet hard enough to reproduce fine details without chipping
4. manual work still required (glazing/staining) - 25Describe the process for pressable porcelain(Lost wax process)
1. restoration created in wax
2. sprued and invested
3. heated in furnace, leaving a void
4. porcelain comes in ingot form, placed into sprue hole, whilst still hot
5. plunger is placed into sprue hole, pressing the molten porcelain into the void
shape and size precise so not to cause damage to furance - 26Name the materials used as fillers for machinable/pressable porcelainsleucite or lithium disilicate
Lithium disilicatemore common and twice the strength.
Leucite porosity- 9%
LD porosity- 1% The crystals are much smaller, forming a denser crystalline matrix
During pressing, crystalline alignment results in the ability to withstand crack propagation in any direction - 27Describe CAD/CAM or slurry systems (crystalline based with or without glass fillers)Highly aesthetic, high in strength, cost effective and very versatile.
Composition:
Glass-infiltrated alumina, solid sintered alumina and versions of zirconia - 28CAD/CAM or Slurry systems uses(core/substructures)
Sintered alumina and glass infiltrated alumina used for 3-unit bridges in anterior and posterior
Zirconia used for up to 14-unit full mouth bridges.
(Standard porcelain is then applied by technician for both) - 29Describe CAD/CAM or slurry porcelains strength factorsIts strength comes from the layered porcelain or bond between the layers and core.
Strength limited to ,100MPa due to feldspathic porcelain
Core strength due to crystals being sintered directly together with no intervening matrix - 30Describe Glass-infiltrated Alumina(also known as slip-casts/slurry)
Created with crystalline infrastructure, then fired with a tanthanum-based glass, making two interpenetrating networks.
Crystalline phases- alumina/spinel or zirconia-alumina
Alumina slipcasts - 5% porosity
Porosity is detrimental, greater porosity increased opacity - 31Describe Sintered AluminaLarge volume of shrinkage (12-20%)
Compensated by CAM produced die, larger than desired core. - 32Describe ZirconiaDivided into 3 types-
1. Pure zirconia
2. Fully stabilised zirconia
3. Partially stabilised zirconia
Strength of all zirconias depends on factors during manufacturing process.
Final grain size determined by sintering phase (firing duration and temperature) - 33What is zirconia used for?Restoration substructures
Implant abutments
Implant fixtures - 34List the requirements for Porcelain Bonding Alloys1. good corrosion resistance
2. high melting range (no melting/deformation during fusion of porcelain)
3. close match of thermal properties
4. high MoE
5. high yield strength (avoid stress on porcelain)
6. alloy shouldn't discolour porcelain - 35* Porcelain is fired onto surface of cast metal structure creating a strong bond *-
- 36List the basic alloys used in porcelain-metal bonding1. High gold bonding alloys, 80-90% gold and platinum/palladium to harden
2. Gold palladium, 50% gold, 30% palladium, 10% tin/indium and silver
3. Palladium silver, 60% palladium, 30% silver, 10% tin/indium
4. High palladium, (atleast) 80% palladium, 10% tin/indium and gallium
5. Nickel chrome, 70-80% nickel, 10-25% chromium and other components - 37List the failures between metal and porcelain1. Metal-porcelain, interfacial fracture, leaving clean surface on metal. Cause- no oxide prior to firing
2. Metal oxide-porcelain, fractures at the metal oxide surface. Oxide firmly attached to metal (common)
3. Metal-metal oxide, metal oxide breaks from metal substrate. Cause- Overproduction of chromium/nickel oxides
4. Metal oxide-metal oxide, fracture through oxide from overproduction of oxide (sandwich effect)
5. Cohesive within metal- joint area in bridge breaks
6. Cohesive within porcelain, tensile failure in porcelain - 38What are stainless steel alloys used for?Used for clasps
bars
orthodontic appliances
swaged plates
(austentic stainless steel-oral use only) - 39What is the composition of stainless steel?Nickname: 18/8 s/s
73% iron
18% chromium
8% nickel
0.12% carbon/tungsten - 40What is Cobalt Chromium alloys used for?(stellite alloys-solid solution, coarse grained)
partial denture framework - 41What is the composition of Cobalt Chromium?33-60% cobalt
25-30% chromium
0-30% nickel
5-6% molybdenum
0-5% tungsten
manganese/silicon/iron
0.2-0.4% carbon - 42What problem can be encountered when casting co/cr?co/cr has half the density value of gold, meaning thrust must be great to pull down the solution, or it'll stay in the crucible.
After casting it is left to cool to prevent warpage and brittleness.
A large grain size is better than brittleness - 43Describe the properties of denture frameworkConnectors- rigid, high modulus of elasticity. Shouldn't be deformed by mechanical stress.
Clasps- high proportional limit (prevents deformation) low MoE for flexibility into the undercuts - 44Talk about Orthodontic wireStainless steel cannot be heat treated because if it is quenched it will soften the wire but if it is left it will be too brittle.
Wire can be stabilised by adding titanium and a heat treatment of 450oC for 10min - 45Talk about S/S platesCan be annealed during swaging.
restricted to a minimum to avoid precipitation of chromium carbides - 46What is the fusion temperature for Stainless steel1200-1500oC
Because it is such a high temp. an oxy-acetylene flame ot induction electrical system is required - 47—
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- Dental Technology Flashcards on Dental Materials Exam Prep, created by Alice Dolman on 08/05/2018.
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