alloys for cast partial dentures (3).pptx

8/10/2019 Alloys for cast partial dentures (3).pptx

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Alloys for cast partial dentures


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Definition

An alloy is defined as a solid mixture of metal

with one or more metals or with non metal.


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Alloy Type by Function

i. Hardness increases from Type I to Type IV.

ii. Type I and Type II alloys are otherwise calledinlay alloys.

iii. Types III and IV are also called as crown and

bridge alloys


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Type I (Soft Tissue)

i. Used for small inlays, class III and class V

cavities, which are not subjected to high

stress.

ii. They are easily burnished on dies using

indirect technique.

iii. These being highly malleable and ductile

have self-sealing margins like gold foil fillings.


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Type II (Medium)

i. These are indicated for restorations, inlayssubjected to moderate stress like full crowns,abutments, pontics and occasionally smallsaddles.

ii. These alloys are malleable and ductile but toa lesser degree than Type I alloys because of theirslightly higher hardness value and yield strength.

iii. They are less resistant to tarnish andcorrosion than Type I. They are not self-sealinglike Type I gold alloys.


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Type III (Hard type)

i. These are indicated for restorations, which

are subjected to high stress like abutments,

pontics, full crowns, short span bridges and

denture bases.

ii. They can be age hardened.

iii. Harder and stronger than Type I and II, but

less malleable and ductile than Type II and I.


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Type IV (Extrahard type)

i. These alloys are indicated for long spanbridges, denture bases, bars and clasps of

removable partial dentures, for inlays in patientswith heavy masticatory forces, precisionattachment and three quarter crowns.

ii. These are the strongest and hardest of all

gold alloys and lack ductility and malleability. iii. These are less resistant to tarnish and

corrosion than Type I, II and III alloys


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According to Application

There are three types of alloys

i. Crown and bridge alloys ii. Metal ceramic alloys

iii. Removable partial denture alloys


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Crown and bridge alloys:

Noble and base metal alloys have been used in thefabrication of full metal and partial veneer crowns andfixed partial dentures

i. Noble metal alloys

Gold based: Type III and IV gold alloys, low gold alloys.

Non-gold based: Silver-palladium alloys.

ii. Base metal alloys Nickel based

Cobalt based


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Metal Ceramic alloys

i. In metal ceramic alloys a strong bond is formed at the porcelain- metal interface. Referredto as porcelain fused to metal or ceramo-metal alloys.

ii. High noble alloys

Gold -platinum-palladium alloys

Gold-Platinum-silver alloys

Gold -palladium alloys

iii. Noble metal alloys for porcelain bonding High palladium

Palladium silver alloys

Palladium copper alloys

Palladium cobalt alloys

Palladium gallium silver alloys

iv. Base metal alloys for porcelain bonding

Nickel-chromium Cobalt -chromium

Titanium and its alloys


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Removable partial denture alloys

Although type IV gold alloys are occasionally employed, mostpartial denture frameworks are constructed using base metal alloys.

i. Gold alloy: (only Type IV is used).

ii. Base metal alloys

Cobalt-chromium

Nickel-chromium

Cobalt chromium-nickel

Titanium and its alloy

According to ADA Specification In 1984, ADA proposed a simple classification for Dental casting

alloys


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Classification of dental casting alloys (ADA specification)

Alloy type

Total noble metal content

1.

High noble

Contains greater than or equal to 40wt percent Au and 60 wt percent of the noble metal elements(Au+Ir+Os+Pt+Ph+Ru)

2.

Noble metal

Contains greater than or equal to 25 wt percent of the noble metal alloys used for inlay, crown andbridge, metal ceramic alloys, by virtue of their tarnish and corrosion resistance.

3.

Predominantly base metal

Contains less than 25wt percent of the noble metal elements. Base metals are Ni, Cr, Co, Ti,V, Mo.They are used in preference to noble metal alloys because of their low cost, increased mechanicalproperties, lighter weight and oxide formation (required for bonding to porcelain)


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Common Requirements of the Alloys

1.Stiffness

Important in long span bridges and partialdenture frameworks.Palladium based alloys

have higher stiffness than gold alloys.

2.Resilience

Amount of energy absorbed by a structure

when it is stressed to its proportional limit.


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Crown and bridge alloys

It should have suitable mechanical propertieslike high yield strength as these restorations

are subjected to greater masticatory stress.Ifthe alloy is insufficiently ductile, it mayfracture during the burnishingprocess.Hardness of the alloy is an indicationof the difficulty of grinding and finishing of thealloy.


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Metal ceramic alloys

The main function of metal ceramic alloys is toreinforce porcelain, thus increasing its resistanceto fracture.The alloy must form surface oxides tobond to porcelain.The coefficient of thermalexpansion should be compatible with that ofporcelain'Its melting temperature should behigher than the porcelain firing temperature. It

should be able to resist creep(or) sag at thesetemperatures.It should not stain or discolorporcelain


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Removable partial denture alloys

The material for the connectors of a partial denture should havethe following mechanical properties.

-A high modulus of elasticity, so that it is rigid in thin sections.

-A high proportional limit, so that permanent deformation is

unlikely to occur. For clasp construction, the materials should have a high

proportional limit.

But a lower modulus of elasticity, so that the clasp is flexibleenough to be withdrawn over undercuts, without either the tooth

or clasp is flexible enough to be withdrawn over undercuts, withouteither the tooth or clasps being overstressed.


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Guidelines for selection of alloys

1. The composition of alloys used in the laboratoryshould be known.

2. The alloy to which the patient has known allergy

should be specified in the laboratory prescription. 3. Selection of the alloy should be based on long term

usage and documented clinical results.

4. Alloys with minimal tarnish and corrosion should

be chosen. 5. Single phase alloys should be chosen in

comparison to multiphase alloys.


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Recent development has been the use of asintered metal composite as a metallicsubstructure for metal-ceramic restorations.

These composites consist of a sintered highnoble alloy . The result is a compositebetween two gold alloys that is of low costand fired into a special refractory die. The

porcelain does not bond to an oxide layer inthese systems but presumably bondsmechanically to a micro-rough gold surface.


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Noble metal alloys

1. Noble metals according to the periodic table areplatinum, palladium, rhodium, ruthenium, gold,iridium, osmium and silver.

2. Common property to all these metals is their nobility,i.e. resistance to tarnish and corrosion.

3. This is attributed to their inertness or inability to formsurface oxides. they occupy a higher position in theelectromotive series.

4. In dentistry, silver is not noble as it undergoes tarnishin the oral fluids.

5. The most commonly used noble metals in dentistryinclude gold, palladium and platinum.


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Units of Gold

Karat: It refers to the parts of pure gold in 24parts of an alloy, e.g. 22 karat means 22 parts ofpure gold and 2 parts of other metals.

Fineness: it refers to the number of parts per1000 parts of gold, e./g. 750- fineness means 75percent of pure gold had 25 percent of othermetals.

High noble alloy contains40wt percent Au and60wt percent of the noble metal elements(Au+Ir+Os+Pt+Rh+Pd+Ru)


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Uses

i. High noble for all metal and resin veneeredrestorations: the alloys contain gold, silver,

copper and platinum. ii. High noble alloys for metal ceramic

restorations; Metal -ceramic restoration is one inwhich the core is made of an ally to which the

porcelain is veneered, the advantage of thisrestoration is it combines the strength of themetal with esthetics of porcelain.


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Requirements of alloys for metal ceramic bonding.

i. Its melting temperature should be higher than the porcelain-firingtemperature. It should be able to resist creep or sag at thesetemperatures.

ii. Its coefficient of thermal expansion should be compatible with that ofporcelain, the alloy and porcelain when subjected to different

temperatures should expand and contract at the same rate. iii. They should be able to bond with porcelain,. that is form an oxide layer

that bonds chemically with porcelain. that is form an oxide layer that bondschemically with porcelain.

iv. It should be rigid when mechanical forces are applied. Lack of stiffnesscauses the porcelain to fracture.

v.It should not discolor porcelain. this is called greening effect, in which the

corrosion products cause discoloration at the metal procelain junction. Minimised by: Adding less than 1 percent of oxide forming elements such as iron,

indium, and tin to high gold alloys to improve bond strength. This additionprovides an oxide film on the surface of the substructure to achieve a chemicalbond.


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High noble alloys used are the following

Gold based metal ceramic alloys

PFM alloys containing 740wt/gold at least

60wt/of noble metals. Gold platinum-palladium alloys

Au-88% , with varying amounts of Pd, Pt andsmall amounts of base metal.These alloys areyellow in color.

Properties; Susceptible to sag deformation.


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Gold palladium silver alloys

Au- 39.77%

Od - 3%

Ag-22%

Gold - palladium alloys Au-44.55%

Pd- 35-45%

Lack of silver results in decrease in coefficient of thermal

expansion. Absence of discoloration of porcelain. So alloysof this type must be used with porcelains having lowcoefficient of thermal expansion to avoid tensile stress inporcelain


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Noble alloys

It contains 25 percent wt. of noble metal

elements.The noble alloys have moderately high

solidus temperature.Reducing the gold contentlowers the density, which makes the casting less

reliable. Less dense alloys have lower kinetic

energy during casting which in turn reducespenetration of molten metal into the mold. As a

result these alloys are more difficult to cast


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Noble alloys for metal and resin veneer restoration

Silver palladium alloys: these alloys are white and predominantly silver incomposition.

Ag- 70-72

Pd - 25%

Twenty five percent of palladium provides nobility and promotes corrosion and

tarnish resistance. Palladium based alloys: These are alloys that contain 54 to 88 wt. palladium. They

are economical. workability and scrap value.Palladium has a strong whiteningeffect, which means that most of these alloys will have a silvery appearance.

Pd- 53-63%

Ag- 28-40%

Tin or indium-increases alloy hardness and promotes oxide formation for bondingto porcelain.

Advantages:Ease of castability and workability, adequate porcelain bonding.

Disadvantages ;Discoloration of porcelain during firing is termed as greening.


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Greening: The silver vapor escapes from the surface of these alloys during firing of porcelain,diffuses as ionic silver into the porcelain, and is reduced to form colloidal metallic silver into thesurface layer of porcelain.

Minimizing greening effect

Reducing silver content.

Gold metal conditioners or ceramic coating agents.

Palladium copper alloy

it contains 74 to 80 percent palladium and 9 to 15 percent copper.These alloys are multi-phased.

Copper (i) imparts reddish color and hardens the alloy through solid solution hardening andformation of phases (ii) reduces melting point.

Advantages

-increased hardness, yield strength and elongation values.

-easier to handle.

Disadvantages

-susceptible to creep deformation because of low melting range. -dark brown or black oxide formation during oxidation and porcelain firing cycles.

-copper ions have been implicated in producing lichenoid reactions


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alloys for cast partial dentures (3).pptx

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