Restorative Exam 3 (with dental casting alloys cards)
Captek
"Captek" sintered alloy infiltrated with gold. Palladium-based alloy layer is adapted to the die and then it is Heated: causing particles bond by sintering creating capillary channels. Then the Gold-rich layer (97% gold) is adapted and Heated again so the gold infiltrates into structure by capillary action. Then Porcelain is then applied
Other techniques for fabricating metal prostheses
-Captek -electroforming -CAD/CAM -3D printing
RPD alloys physical properties: color temp density any shrinkage? thermal conductivity high or low?
-Color: lustrous silvery white Typical fusion temperatures: 1400 to 1454 oC Density (lighter weight than gold counterparts): 8-9 gm/cm3 Linear casting shrinkage: 2.05 to 2.33% (vs 1.4 to 1.7% for gold alloys) --> more shrinkage than gold! Thermal conductivity: high
For PFMs, what materials are in those
-Feldspar -glass -opacifiers -pigments -flourescent agents
Metal-ceramic bonding mechanisms? 4 factors
-mechanical interlocking -compressive forces -van der waal's forces -chemical bonding between oxide layer and porcelain
What are the different porcelain layers?
-opaque porcelain -body/gingival/dental porcelain -enamel/incisal porcelain -shoulder porcelain (applied to buccal margin for esthetics with no metal collar underneath.) -porcelain glaze which can be natural glaze aka autoglaze or overglaze aka add on glaze. WIth natural glaaze there is no additional layer on top of teh porcelain. you are just firing crn in oven at low temp and suprfcicial layer will fuse together and give shiney surface and its more durable and wont wear on function but the add on glaze, is added and then crn is fired and over the years it can wear and its shiney and doesnt look natural
metals composition can be
-pure metals -intermetallic compounds -alloys
types of titanium alloys and what are they used for and why difficult
2 Types: Commercially pure (CP) Ti or the titanium alloy which is Ti/6Al/4V. Used as dental implants! o The titanium alloys used for § Excellent biocompatibility § Good mechanical properties § These make for difficult casting.. · Low density (4.5gm/cm3) · High casting temperatures (~1700 oC) · Oxidizes, reacts with casting investments and HARD to fabricate itanium alloys so instead use Machinable (CAD/CAM) to fabricate titanium alloys
Which has more corrosion resistance, base metal alloys or gold
BMA have reasonable corrosion resistance but get more corrosion than gold
Bond strength of porcelain to BMAs
Bond strength of porcelain to these metal-ceramic restoration alloys are comparable to those on noble metal alloys
What is the importance of compressive forces in metal-ceramic bonding?
CTE of the metal is slightly higher than that of porcelain (1x10^-6 / degrees C), which will cause the porcelain to "draw" towards the metal copings when the restoration cools after firing thus placing the porcelain in a state of compression
· How does the stiffness aka elastic modulus of alloys affect the deflection?
Changing from a high gold alloy to a high modulus alloy (e.g. Ni-Cr). If all dimensions are kept the same, the deformation would be 50% less
*****Ingredients in base metal alloys to promote bond to porcelain in PFM?
Chromium (Cr) Boron (B) Silicon (Si) *Form the oxide layer on the metal. Form an oxide layer (adherent to metal) to which the porcelain will bond covalently
Cobalt chromium has high...? Nickel chromium has high...?
Cobalt chromium has high yield strength and high tensile strength and high modulus and excellent for rigid portions of RPD like a lingual bar Nickel chromium has high yield strength and tensile strength and modulus too..more so than cobalt chromium
First ceramic material used in dentistry?
Denture teeth can be made of whiteware ceramics was first ceramics used in dentistry. Now we use acrylic resin teeth in dentures and not really use porcelain teeth anymore and now we use acrylic resin teeth in dentures. Maybe if you have to use opposing teeth that are porcelain then use opposing porcelain teeth but these click when patient talk and tooth and no chemical bond between porcelain teeth and denture base.
What is the chemical microstructure of dental porcelain? What happens and what is formed?
During the processing/baking of feldspathic porcelain, the Feldspar (K2O x Al2O3 x 6SiO2) recrystallizes to Leucite (K2O x Al2O3 x 4SiO2) and is responsible for making the CTE of porcelain match the metal. there are also some porosities and some tine-oxides. If there are porosities then weakens porcelains and it becomes opaque. On repeated bakes by the dental tech, the leucite can transform to sandine, which has the same composition but a higher thermal expansion than leucite. This can cause crazing or internal cracking of the porcelain.
What is involved in mechanical interlocking in metal-ceramic bonding?
Finishing the metal surface with special stones and air particle abrasion/sand blasting (50 micron Al2O3) Air abrasion (sandblasting) -->Enhances the wettability of the metal substrate by porcelain. It also Provides mechanical interlocking. And it Increases surface area for chemical bonding
Explain the processing of PFM restorations
First step is to oxidize the metals. The metal coping goes in oven for oxidation cycle and metal will oxidize and have oxide layer on surface and is important responsible for chemical bond with porcelain specifically the opaque which is the 1st layer of porcelain. Then bake it then apply gingival ceramic aka body porcelain aka dental porcelain. It has more translucency and contributes to color shade of porcelain then apply the incisor porcelain aka enamel porcelain and its more translucent and give you a translucency for incisal edge of the enamel then last step is stain and glaze the ceramic. § Number of baking cycles · Excluding the oxidizing of the metal, when you apply the opaque you have to bake it so that is the first baking cycle. Then you apply the gingival porcelain and incisal on top and then bake it for 2nd cycle. Then 3rd apply the stain and glaze and bake. So minimum 3 baking cycles. More cycles will happen if porcelain doesn't have enough proximal contact. Edie can add more porcelain and then fire it. If you have long span FPD and you keep adjusting and firing, then that is not good and affect porcelain properties in a bad way so don't over bake the porcelain!
Are BMAs harder or stronger than gold
Harder adn stronger than gold alloys but finishing and polishing is more difficult than with gold alloys
Importance of the chromium oxide layer on the base metal alloys
Important for passivation so it prevents tarnish and corrosion and that's why the base metal alloys can be used in the oral environment. If base metal alloy gets scratched, formation of new chromium oxide layer will form
Again why is coefficient of thermal expansion important in PFMs?
In order to put the porcelain into compression: the coefficient of thermal expansion of the porcelain must be slightly less than the CTE of the metal!
Coefficient of thermal expansion of the porcelain and metal
In order to put the porcelain into the compression: coefficient of thermal expansion of the porcelain must be slightly less than the CTE of the metal --> CTE porcelain < CTE metal. When cooled the metal will shrink and porcelain will shrink but metal will shrink more than porcelain cause CTE of metal is more than the porcelain. But in reality, there is chemical bond between metal and porcelain so when cooling, the metal is shrinking and porcelain is shrinking and metal shrinking more than porcelain and forcing porcelain atoms to shrink more than they want to and as a result the compressive stress builds up inside the porcelain! The metal shrinks more than porcelain and porcelain usually shrinks less but because they are bonded the metal is forcing the porcelain to shrink even more creating compressive stress inside the porcelain which is good. Porcelain is brittle and weak under tension and strong under compression! This applies also to amalgam and composite. So if you induce tensile stress on porcelain, it will break but if you induce compressive stresses on the porcelain, it will not break. So porcelain withstands compression and not tension. Lab techs use a certain brand of metal that is compatible with porcelain but if switched to other brand then a failure may result. So how to control that? You have to evaluate what is good and bad. When you establish relationship with lab, know the metal and porcelain they use and if they are compatible or when writing the prescription, then specify the brand of the metal and porcelain to cover your butt. SO write on your prescription the type of metal and porcelain to use.
So we talked about oxidizing the metal and now the lab technician will apply the porcelain......What is sintering?
Initial stage is particles touching and now it gets heated in oven. Then necking stage occurs where particles are fusing together aka coalescing. Has some vacancy inside which is air and bad because it can create porosity. So try to minimize porosity as much as possible. Then densification stage where there is minimum porosity content and particles getting tinier and tinier. Then its fully densified which is no porosity at all this happens in material like high strength ceramics like alumina or zirconia ceramics. This full densified is done by the manufacturer though. In feldspathic porcelain, there are always some porosity inside and that's why it is weaker.
What are ceramics
Inorganic, non-metallic man-made solid objects formed by baking naturally occurring minerals at high temperatures. They are beautiful, brittle and they break --> think a vase. Theyare brittle and break becuase they don't have high yield strength and no ductility.
*****Ingredients in high noble or noble metal alloys to promote bond to porcelain in PFM?
Iron (Fe) Indium (In) Tin (Sn) *These are responsible for forming the oxide layer on the metal and that will make a covalent chemical bond with the 1st layer called opaque porcelain
What is the feldspar in porcelain PFM crn made of
It is a crytalline material. · KnO, Al2O3, 6SiO2 aka: "Orthoclase" --> potassium when heated it expands and responsible for increasing the thermal expansion of teh porcelain Aluminum silicate and orthoclase
What is the first layer of the porcelain that is applied? WHat is the function?
Opaque porcelain: Tin oxide or Ti oxide § Function: · Mask the color of underlying metal (0.2mm thick) · The basis for the total color tone · Bonds to metal oxide layer
What is the role of chemical bonding in metal-ceramic bonding?
Plays the major role in metal-ceramic bonding mechanisms. Attributed to the formation of an oxide layer on the metal surface by firing in an oxidizing atmosphere (degassing). When fired in air, trace elements in the gold alloy such as tin, indium, gallium or iron migrate to the surface, form oxides layer and subsequently bond to similar oxides in the opaque layer of the porcelain § Base metal alloys readily form chromium oxide layer that bond to the porcelain without addition of any trace elements § In Ni-Cr base metal alloys (most commonly used for FPD is using base metal alloys), elements are added to control oxide layer thickness, which is important in preventing premature bond failure through the metal oxide or at the metal-metal oxide surface
How to fix little failure or chip in porcelain
Porcelain failure repair (for small chip of porcelain).Repair of porcelain requires etching with hydrofluoric acid gel (can't be applied to metal). Then application of silane coupling agent then dentin bonding agent § Application of composite resin and light-curing
Precautions with BMAs
Possible hypersensitivity reactions (Ni, 10-15% of population) and beryllium effects (potential hazard to laboratory personnel who might inhale beryllium dust) beryillum limits oxide formation and improves castability and thats why its added
sag resistance
Sag resistance is to prevent the deformation of metal during oxidation cycle and during firing of porcelain
electroforming
The die is duplicated with gypsum. THen Conductive silver layer is applied to the surface of the die. THen The die is placed in a plating solution and A thick layer of gold or other metal is deposited
Is there shrinkage of the porcelain during sintering?
There is 20-25% shrinkage because the particles are coalescing together? Know the percentage of shrinkage in porcelain during sintering (baking in oven). This is important cause lab technician has to over build the porcelain by 20-25% in order to compensate for the shrinkage to get the desired crown dimension in the end
What is the purpose of porcelain condensation (blotting)?
To remove the excess water so as to decrease firing shrinkage o To adapt the paste in the required form o To decrease the porosity (also can decrease porosity with vacuum in oven)
o Role of metal-ceramic alloy substructure metal framework...typical thickness and why?
Typical thickness of 0.3-0.5 mm Provides high modulus substrate (120-200 GPa) for prevention of porcelain bending (tension) under loading. Metal framewok supporting porcelain has to be rigid to avoid bending and avoid risk of porcelain fracture if framework bends
RPD alloys chemical properties
Undergo Electrochemical corrosion: but good passivation by chromium oxide
Baking porcelain on metal...what happens ***What are the important temps/coefficient of thermal expansion
When oven heats, the temp goes from room temp to 1200 degree F then turn vacuum on so to reduce the porosities in the porcelain. If you forget the vacuum on the there will be more porosities then usual. Then temp increased more to 1700 degree F and particle fuse and sintering and melting and starting to look like molten glass and porosities getting smaller and smaller and now turn the vacuum off so the air pressure in oven to press on the structure to make the porosities even less..than continue heating to 1800 degree F and then take out of oven and cool down to room temp. So when heated the porcelain looks like viscous honey. When heating a material it will expand and when cool it it will shrink. How much does it expand or shrink depends on coefficient of thermal expansion (CTE). The metal is solid and does not melt down in porcelain oven but the porcelain is liquid and metal cool it shrinks and the porcelain is liquid and flows all over the metal. Then at 1200 F this is called the glass transition temperature and below this temp the liquid glass becomes solid. Think about the transition temp as melting point however it is not a point but rather a range close to 1200 F and above 1200 is viscous liquid glass and below is 1200 F is sold porcelain. The glass is solid and metal is solid and both cooling down and both are shrinking together. You may assume that CTE for both glass and metal is same but this is not true. The metal has CTE that is slightly higher than porcelain! Know the number on the pic!! CTE of metal is 14.0 x 10^-6 degrees C and CTE for porcelain is 13.5 x10^-6 degrees C
Compare the PFM structure to the all ceramic crown
When we have PFM, the metal coping is inside then the porcelain is placed on top. For all ceramic crown, the inside is called a ceramic core and not a ceramic coping! It is covered by feldspathic porcelain.
What is blotting of the porcelain as it is added?
blotting with napkin to absorb excess water from inside to bring particles condesed and closer together
Metals and metalloys form what type of structure
crystalline
What is the 3rd layer of porcelain applied?
enamel/incisal porcelain § Translucent to imparts the natural translucency to the final restoration
What is the second layer of porcelain appplied? Function?
gingival/body § Fired onto the opaque usually together with the enamel porcelain § Contains coloring oxides to match the proper color as it forms the main bulk of the porcelain § Body and enamel porcelain shrink 20-25% and thats why porcelain has to be overbuilt by 20-25%
***Role of metals in high noble alloys osmium, ruthenium, iridium
grain refiners and act as grain nucleation sites (Fine grains = better casting, higher strength + elongation) fine grains lead to better casting and higher strength
Are BMAs more or less ductile than gold alloys
less ductile
DO BMAs have lower or higher density than gold alloys? Lower or higher melting temps than gold alloys?
lower density and higher melting point than gold alloys and BMAs require a special casting technique
· Best Sequence for Trying in Crowns
o 1) Check the proximal contacts first. Make sure that the crown is not prevented from seating on the tooth due to tight contacts - check with floss, interproximal articulating paper, etc o 2) If contacts are OK and crown still does not seat, check interior surface with "fit checker" and selectively grind high spots o 3) Next, check the margins with an explorer. Make sure that all margins are closed. If you have an open margin on the tooth, but not on the die, make a new impression o 4) After you are sure that the crown fits, check the occlusion last o 5) If you can't get the casting to fit within 15 minutes, make a new impression. You will ultimately save a lot of time
What is a metal alloy .... binary system vs ternary system
o A blend (mixture) of two or more metals: § Binary system · Brass: zinc + copper § Ternary system · Dental gold alloy: gold, copper, & silver Implant alloy: titanium, vanadium, & aluminum
Indications for using base metals alloys
o All-metal crowns o Metal-ceramic crowns o Long span fixed partial dentures (FPDs) o Resin-bonded (Maryland) FPDs (with wings like Dr. Hyun made and now only one wing is better she said according to research) o Dental implants
What is important about metal-ceramic alloys
o Alloy and Porcelain must be matched!*** Meaning Alloy melting temperature must be 300 to 500°F (165 to 280°C) higher than the firing temperature of porcelain. If not then you risk having melting of the metal and sag. Coefficient of Thermal Expansion (CTE) of the alloy must be near that of porcelain. Both alloy oxide and porcelain opaque layer must be designed for bonding
Base metals vs noble metals and corrosion
o Base metals - corrode easily o Noble metals - least oxidation potential meaning resistance to tarnish and corrosion in oral environment
Burnishing of gold alloys...what leads to good burnishability?
o Burnishability is a measure of the yield strength and high elongation (ductility) of a metal o Low yield strength and high elongation = excellent burnishability
CAD/CAM
o CAD/CAM for Subtractive (milling) or 3D printing § Electronic or digital records of the prepared teeth stored in a computer. Image data can be retrieved immediately to mill or grind a prosthesis from a solid metal block. Prosthesis can be fabricated within minutes. Optical scanning procedure (digital impression) eliminates the need for a physical impression. CP Ti, Ti alloys, & Co-Cr alloys (base metal alloys) can be milled using this CAD/CAM technology
2 main types of base metal alloys used in dentistry
o Chromium containing alloys with nickel and cobalt § 2 main types used in dentistry are nickel chromium and cobalt chromium
· Properties of high noble/noble metal alloys
o Hardness, strength, & stiffness (easy finishing & polishing) o High ductility & malleability o Higher density & lower melting temperature than base metal alloys thus Easy to manipulate in the laboratory § High corrosion resistance o Biocompatible
·**********ADA Classification (1984) for Casting Alloys
o High noble -->noble metal content has to be greater than or equal to 60% gold, platinum, palladium and greater than or equal to 40% gold o Noble metal -->noble metal content has to be greater than or equal to 25% gold, platinum, palladium (does not have to contain gold) o Predominantly base metal -->has to be noble metal content less than 25% gold, platinum, palladium · By mixing and matching certain metals in the alloy you are achieving certain set of physical and mechanical properties to use the metal alloy
**********high noble metals vs. noble metals vs. base metal alloys
o High noble metals § Most expensive § Easy to manipulate in the lab § Highest corrosion resistance o Noble metal alloys § Expensive § Strengths and hardness are equal to or greater than higher-noble alloys § High corrosion resistance o Base metal alloys § Very high strengths, hardness and rigidity § Most difficult to manipulate in the lab § Low cost § Least corrosion resistance
gold content measures
o Karat: A measure of the gold content of an alloy, with 24 karat being 100% gold (e.g. 18 karat gold is 75% gold mixed with 25% of other metals like Cu or Ag) o Fineness: The percentage of gold content in an alloy multiplied by a factor of 10 (e.g. 75% gold is 750 fine)
What are the requirements of porcelain for a PFM? Temps? CTE?
o Low fusing temperature, must be 300-500 degree F (165-280oC) below the melting range of the metal coping so metal doesnt melt or sag o High viscosity to resist slumping in order to maintain shape during firing o Resist devitrification = crystallization (when porcelain is fired too many time it may devitrify, thus becomes milky and difficult to glaze) o CTE should be slightly lower than that of the metal (1x10-6 / degree C)
Noble metal vs precious metal
o Noble metal § A metal that is resistant to oxidation; includes gold, platinum, palladium (less expensive than gold), and the other platinum group metals o Precious Metal § A metal that is relatively high in cost; includes gold, platinum, palladium, and silver
How can dental casting alloys be classified
o Noble metal content § High noble § Noble § Base o Cost (not used, she said don't use) § Precious § Semi-precious § Non-precious o Physical properties § Type I § Type II § Type III § Type IV
· Misfits that can occur during casting process
o Preparation errors - Dentist's Responsibility § Undercuts o Impression inaccuracies - Dentist's Responsibility § Permanent deformation § Dimensional stability o Die inaccuracies - Dentist / Lab's Responsibility § Under sized die (epoxy) § Altered dies § Improper trimming o Molding errors - Lab's Responsibility § Distortion of the wax § Improper expansion of investment o Distortion of casting - Lab / Dentist's Responsibility
Uses for base metal Alloys?
o RPD, FPD, surgical implant § RPD --> cobalt-chromium, cobalt-chromium nickel, nickel chromium, titanium (most common use) § FPD --> nickel chromium (with or without beryllium), cobalt chromium, titanium § Surgical implant --> cobalt-chromium molybdenum, nickel-chromium cobalt
· *******American National Standards Institute/American Dental Association Specification no. 5 for Dental Casting Gold Alloy
o Type I § Soft and indicated for inlays o Type II § Medium, indicated inlays o Type III § Hard, indicated for inlays/onlays, crowns fixed partial dentures o Type IV § Extra hard, indicated for cast posts and cores, long span brdiges, partial denture clasps
Properties of base metal alloys
o Wide range of uses and compositions o Removable partial dentures (RPDs), fixed prosthodontics (FPDs), maxillofacial prosthetics, and orthopaedic implant applications o Low corrosion because of chromium oxide film o Higher stiffness and hardness than noble metal alloys
Careful oxidation of metal-ceramic alloys is critical for...?
porcelain bonding! § Careful calibration of oven § Metal preparation and cleaning § Removal of abrasive particles and contamination
What is the 4th layer of porcelain applied?
shoulder porcelain: § Used to build up the cervical area of a metal- ceramic crown to produce an esthetic butt-joint margin § More opaque than body or incisal porcelains Has a higher sintering temperature than the adjacent body porcelain so as to retain the sharp edge at the margin during subsequent sintering processes otherwise you will have and open margin
Stiffness and elastic modulus of BMAs compared to gold alloys?
stiffness (elastic modulus) of BMAs is 2x that of gold alloys
Advantages of metals offer
strength and ductility
What is whiteware ceramics used for?
the whiteware ceramics are used for porcelain denture teeth! When talking about porcelain in PFM, that is different. That is composed of feldspar and not clay or silica but has glass.
adjusting RPD clasps and fatigue
when adjusting rpd claps and rpd is made of metal alloy so all the properties like yield strength, elongation, modulus, they are all important but fatigue resistance and that is because when patient talks the clasp in and out, its cyclic loading so its fatigue and if you keep adjusting the clasp it fatigues and will break at point below its fracture strength because its fatigued. Like bending paper clip back and forth over and over it will fatigue and break
o Three Dimensional Printing (Additive Manufacturing Technology)
§ A three-dimensional object is created by depositing successive layers of a material. [SLS (selective laser sintering) / SLM (selective laser melting)] § Laser (or a high-energy electron beam) traces the outline of each part being printed fusing metal powder to the layer below it § The process is repeated until the printing process is completed § The metal powder can be titanium, Co-Cr, or a noble metal § Marginal and internal fit is as good as or superior to the traditional casting method
Casting alloys used by dental labs for RSDM cases
§ High Noble Metal (Dentrex DTX 63) · used for "yellow gold" crowns, inlays, and onlays · Has 63%Au, 23%Ag, 8%Cu, 3%Pd · (it is considered high noble because has more than 60% of combine gold and palladium recall) Noble Metal (Argen Noblecrown NF) · used for PFM crowns, "white" noble metal cast crowns, inlays and onlays · HAS 43% Co, 25% Pd, 20%Cr, 12% Mo Noble metal because more than 25% palladium ALSO noble metal don't have to include gold like high noble metal does
properties of metal-ceramic alloys
§ High strength § Defined coefficient of thermal expansion § Castable in thin cross section § Show Controlled oxide formation
What is the importance of Van der Waals forces in metal-ceramic bonding?
§ Minor forces § Molecular attraction § Significant in the initiation of the most important mechanism, the chemical bond
What are the porcelain glaze types?
§ Natural glaze = autoglaze § Overglaze = add on glaze
How many baking cycles needed to process the porcelain for PFM
§ Number of baking cycles · Excluding the oxidizing of the metal, when you apply the opaque you have to bake it so that is the first baking cycle. Then you apply the gingival porcelain and incisal on top and then bake it for 2nd cycle. Then 3rd apply the stain and glaze and bake. So minimum 3 baking cycles. More cycles will happen if porcelain doesn't have enough proximal contact. Eddie can add more porcelain and then fire it. If you have long span FPD and you keep adjusting and firing, then that is not good and affect porcelain properties in a bad way so don't over bake the porcelain!
· ***Role of metals in high noble alloys copper
§ hardening heat treatment (if more than 15% by weight), dark red color
· ***Role of metals in high noble alloys platinum
§ hardening heat treatment, increase casting temperature
· ***Role of metals in high noble alloys silver
§ low cost diluent, color correction for copper
· ***Role of metals in high noble alloys palladium
§ lower cost, increase casting temperature
· ***Role of metals in high noble alloys Gold:
§ soft, malleable, corrosion resistance
What is the glass in the porcelain for PFMs made of
· (amorphous material) contains SiO2, Al2O3, Na2O, CaO, Li2O · Has silicon dioxide aka silica, aluminum oxide, sodium oxide, calcium oxide and lithium oxide · It is mainly alumina silicate!
RPD alloys clinical properties
· Adjustments: difficult due to high hardness and Elastic modulus · Ni sensitivity allergy · Wear: can contribute to excessive wear of teeth or restorations · Hygiene: clean with soap and water or very dilute solutions avoiding chlorine (passive film is attacked by chlorine). Use soft bristle brush. Avoid abrasive toothpastes
What is clay made of?
· Al2O3, 2SiO2, H2O · It is also aluminum silicate but it has a diff crystalline structure then feldspar and has water inside.
Biological properties of dental casting alloys
· Biocompatible: no toxic soluble phases · Non-reactive in the oral environment
Other properties for dental asting alloys (inclduing sag)
· Casting Temperature · **Sag Resistance à o In pic there are 2 teeth prepared to support a fixed partial denture. Then there is a cast metal framework and the metal frame work before going in patients mouth has to undergo oxidation cycle where some distortion may happen and therefore sag and open margin. Then when porcelain is added, then more sag may happen so need sag resistance and achieved by firing temp being much less than melting temp of metal alloy · Bond (to porcelain and cements) · Etching Characteristics
chemical properties of dental casting alloys
· Chemical corrosion (tarnish) resistance · Electrochemical corrosion resistance · Solubility - insoluble!
RPD Alloys most used alloys and commercial names
· Co-Cr (Vitallium) the bold are the 2 commonly used types and know the commercial name too! · Ni-Cr (Ticonium)
RPD alloys mechanical properties: -elastic modulus -Hardness -% elongation
· Elastic modulus = 200-240 GPa (about 2X that of comparable cast gold alloy) · Hardness (30% harder than Type IV golds) · % Elongation: 1-2% (Cromium alloys are quite brittle) = · ****(challenged question) Fatigue is much more important for clasps than connectors --> major connector on RPD needs to be stiff and for clasp, if you adjust claps a lot of when patient takes the RPD in and out several times a day that leads to fatigue in the clasps and eventually breaking
mechanical properties of dental casting alloys
· High rigidity (stiffness) = Modulus of elasticity · High yield strength · High hardness · Ductility · Burnishability
Physical properties of casting dental alloys
· Reasonably low melting point (flow) · Moderately high density (castability) o Gold, platinum palladium have high density and so when an alloy has high density it is easier to cast and combined with a low melting point is easier to cast. Cobalt, nickel and iron have low density and are harder to cast · Low coefficient of thermal expansion o Tooth has 9-11ppm/oC degree of thermal expansion and PFM are close as are gold alloys but amalgam and composite are not
What is Feldspar made of?
· RnO, Al2O3, 6SiO2 (R = Na, K, Ca) · Mostly aluminum silicate. The R in the chemical formula is an alkalate--> Either sodium, potassium or calcium
What is quartz made of?
· SiO2 --> silica dioxide
What are the fluorescent agents in PFMs? What is their role?
· cerium and dysprosium oxides · these needed because natural teeth fluorescent under UV light and bone does too. So the hydroxyapatite is responsible for this fluorescence. The emit a light blue light. Under black light your teeth will florescence but not the composite so teeth like like they have holes under blacklight... when choosing shade of porcelain go to window and choose it under the sunlight
What are the raw materials for whiteware ceramics?
· feldspar, quartz and clay · the clay is white in color
WHat are the pigments in porcelain for PFMs? What do they do?
· metal and rare earth oxides · responsible for the porcelain shades
What are the opacifiers in the porcelain for PFMs? Why are they needed in PFMs?
· tin oxide, titanium oxide · Why these needed in PFM? These are also used in house paint which is primer. Opacifier is needed to hide the grey of the metal in PFM. This is the 1st layer applied on the metal and also creates the chemical bond aka oxide layer on the metal
