alloys in prosthodontics
TRANSCRIPT
ALLOYS IN PROSTHODONTICS
PRESENTED BY-DR.KELLY NORTON POST GRADUATE STUDENTDEPT. OF PROSTHODONTICS
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2INTRODUCTION
What is an alloy?
A mixture of two or more metals or metalloids that are mutually soluble in the molten state; distinguished as binary, ternary, quaternary, etc., depending on the number of metals within the mixture
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Year Event1907 Introduction of Lost-Wax Technique1933 Replacement of Co-Cr for Gold in Removable Partial Dentures1950 Development of Resin Veneers for Gold Alloys1959 Introduction of the Porcelain Fused-to-Metal Technique1968 Palladium-Based Alloys as Alternatives to Gold Alloy1971 Nickel-Based Alloys as Alternatives to Gold Alloys1980s Introduction of All-Ceramic Technologies1999 Gold Alloys as Alternatives to Palladium-Based Alloys
HISTORY
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4METALS
Metals can be classified as 1. Noble metals which have high resistance to oxidation, corrosion
and dissolution in organic acids Eg. Gold, Platinum, Palladium, Iridium, Osmium, Ruthenium, Silver,
Rhodium 2. Base Metals undergo oxidation and corrosion easily
Eg. Iron, Nickel, Tin, Zinc,Chromium, Aluminium, Titanium etc
3. Metalloids: Few elements carbon, boron, silicon,
sometimes behave like metals and some times nonmetals.
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SOLIDIFICATIONAND
CRYSTALLIZATION OF METALS
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SOLIDIFICATION OF METALS
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solidification temperature melting point or fusion temperature
SUPERCOOLING
• During the supercooling process, crystallization of the pure metal begins.
• Once the crystals begin to form, the release of the latent heat of fusion causes the temperature to rise to Tf, where it remains until crystallization is completed at point C.
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Nucleation can occur by two processes. The first, called homogeneous nucleation, is enhanced by rapid cooling so the nuclei are supercooled.
The more nuclei that are formed by rapid cooling, the smaller the grain size Another means of decreasing the grain size (grain refining) is by adding to
the melt a foreign solid particle or surface to which the atoms are attracted, such as a very fine high-melting metal or oxide powder. This process of seeding the nuclei is called heterogeneous nucleation.
All modern noble metal alloys are fine grained. Smaller the grain size of the metal, the higher yield stress, better ductility, and improved ultimate strength
A large grain size reduces the strength and increases the brittleness of the metal.
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8CRYSTALLISATION OF METALS
When a molten alloy cools to the solid state, crystals form around tiny nuclei (clusters of atoms).
As the temperature drops, these crystals grow until the crystal boundaries meet each other in the solid state.
At this point, each crystal is called a grain and the boundaries between crystals are grain boundaries.
Characteristically, a pure metal crystallizes from nuclei in a pattern that often resembles the branches of a tree, yielding elongated crystals that are called Dendrites.
Predominantly base metal (PB) alloys for dental prostheses typically solidify with a dendritic microstructure, most high noble (HN) and noble (N) metal casting alloys solidify with an equiaxed polycrystalline microstructure (grain).
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9NEWTON’S LAW OF COOLING
According to this law, the quantity of heat lost per second from a hot body i.e. rate of cooling is directly proportional to the means excess of its temperature above the cooler surrounding
Temperature against time graph, i.e the cooling curve is exponential, indicating , infinite time is required for the cooling of the hot body to reach the external temperature, if not disturbed
COOLING PATTERN OF A LIQUID METAL DURING SOLIDIFICATION
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The upper temperature for the liquid-solid alloy range is called the liquidus temperature, and the lower temperature limit is called the solidus temperature.
When a liquid alloy melt is being cooled or heated, the liquidus temperature is the temperature at which solid crystals start to nucleate or dissolve into liquid respectively.
The solidus temperature is the temperature at which the last liquid solidifies on cooling or the first liquid is formed on heating.107
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11COOLING CURVE
For a binary solid solution alloy of two metals, A and B, in which the melting point of metal A is greater than that of metal B, the first material to crystallize, at just below temperature T1, will be rich in the higher melting point metal A close to the nucleus , whilst the last material to crystallize, at a temperature just above T2, is rich in the lower melting point metal B close to the grain boundaries . A - close to the nucleus
B - close to the boundary
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12To increase hardness
and strength.
To increase fluidity of liquid metal
To make casting or working on the metal
easy.
To increase resistance to tarnish and
corrosion. To lower or increase
the melting point
To change the microscopic structure
of the metal.
To change the color of the metal.
To provide special electrical and
magnetic properties.
WHY IS ALLOYING DONE??
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13SOLID SOLUTION
In the molten state metals usually show mutual solubility, one within another. When the molten mixture is cooled to below the melting point the component metals may remain soluble in each other forming a solid solution.
CONDITIONS FAVORING SOLID-SOLUBILITY Atom size - if the atom sizes of the mixing metal are same, it will produce solid
solution type alloy. Valency - metals of the same valency will produce solid-solution alloy. Space-lattice type - if same, preferably if face centered will favour solid
solubility. Chemical affinity - must be less to produce solid-solution alloy.
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14TYPES OF SOLID SOLUTION
There are two principal atomic arrangements for binary solid solutions.
One of these is the substitutional solid solution in which the atoms of the solute metal occupy the positions in the crystal structure that are normally occupied by the solvent atoms in the pure metal.
Can be Disordered : when the substitution is random in the crystal lattices
• E.g Pd-Ag alloy in which Pd is the solvent metal, Ag atoms replace the Pd
atoms randomly in the crystal structure. Can be Ordered: when new ordered phases are formed by diffusion
of atoms which precipitate as superlattice. Eg. Cu in Au
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Another type of solid solution is the interstitial solid solution. In this case, the solute atoms are present in random positions (interstices) between the atoms in the crystal structure of the solvent metal. Commercially pure titanium (CPTi), which is important for implants and restorative dentistry, consists of high-purity (99 wt% or higher) titanium, with oxygen, carbon, nitrogen, and hydrogen atoms dissolved interstitially.
Eutectic Solid Solution refers to different solid solutions of limited solubilities, precipitate as alternate layers
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PHASE DIAGRAM FOR ALLOYS
A phase is a state of matter that is distinct in some way from the matter around it. Phase diagrams are maps of the phases that occur when metals are mixed together . The x axis -------- composition of element The y axis -------- temperature of the alloy system. shows the composition and types of phases at a given temperature and at equilibrium. Every phase diagrams divides an alloy system into at least three areas :the liquid phase, the
liquid –solid phase and solid phase. If a series of cooling curves for alloys of different composition within a given alloy system are
available a phase diagram can be constructed from which many important predictions regarding
coring and other structural variations can be made.
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17SILVER PALLADIUM SYSTEM
Liquidus temp
Solidus temp
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CORING
For solid solution alloys a cored structure exists in which the first material to crystallize is rich in the metal with the higher melting point (A), whilst the last material to solidify is rich in the other metal (B)
An indication of the degree of coring is given by the separation of the solidus and liquidus lines on the phase diagram.
With slow cooling the crystallization process is accompanied by diffusion and a random distribution of atoms results, with no coring.
Rapid cooling quickly denies the alloy the energy and mobility required for diffusion of atoms to occur and the cored structure is ‘locked in’ at low temperatures.
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This involves heating the alloy to a temperature just below the solidus temperature for a few minutes to allow diffusion of atoms and the establishment of an homogeneous structure and then normally quenched in order to prevent grain growth from occurring.
HOMOGENIZATION 19
A, Copper-silver alloy (1%) as cast. B, The same castalloy after homogenization heat treatment
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EUTECTIC ALLOYS The eutectic alloy is one in which the
components exhibit complete solubility in the liquid state but limited solid solubility
The term eutectic means lowest melting point.
In silver copper system ---- M.P. silver is around 960.5°C and that of
copper is 1083° C. But that of the eutectic composition is
779.4° C. Eutectic –alternative layer of alpha (silver
rich)and beta(copper rich) phases.
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It can be written as :
LIQUID α SOLID SOLUTION + ß SOLID SOLUTION
INVARIANT TRANSFORMATION- OCCURS AT SINGLE TEMPERATURE AND COMPOSITION
SILVER-COPPER SYSTEM:
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PROPERTIES OF EUTECTIC ALLOYS
• These in contrast to other alloys do not have a solidification range ;
instead they have a solidification point.
• Hard and Brittle, because the presence of alternate alpha and beta
phases inhibits slip.
• The silver rich alpha solid solution or copper rich beta solid solution are
hard and have higher strength. They are ductile and malleable.
• They have a low melting point and therefore are important as solders.
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Peritectic is a phase where there is
limited solid solubility.
They are not of much use in dentistry
except for silver tin system.
Eg: Silver-tin
Silver –platinum
Palladium-ruthenium
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PERITECTIC ALLOYS
Liquid + solid solution solid solution
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23GOLD COPPER SYSTEM
These are disordered substitutional alloys below solidus and meet each other at 911 C when the gold is about 80 wt %.
When the AuCu phase is slowly cooled below 375 C or 410 C, the attraction between gold and copper atoms cause intermetallic alloy phases.
AuCu3 phase: If amount gold = 40-65 wt% then, solid state reaction takes place by ordering the copper atoms int the middle of the faces and gold atoms at the corners of the F.C.C. unit cell.
AuCu phase: when the gold is more 65- 85 wt % the solid state reaction takes place by forming intermetallic alloy Au-Cu equilibrium phase with alternate layers of gold and copper
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INTERMETALLIC COMPOUNDSInter metallic compounds are those when the metals are soluble in the liquid state but unite and form a chemical compound on solidifying.
Eg ; Ag3 – Sn,
Sn7 – Hg8
They are called inter metallic compounds because the alloy is formed by a chemical reaction between a metal and metal.
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25TYPES HEAT TREATMENT
Precipitation hardening or order hardening.
Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice
Technique: The alloy is heat soaked at temperature between 200 C
and 450 C for 15-30 minutes and then rapidly cooled by quenching
Strength Hardness Proportional Limit
Ductility
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Solid solutions are generally harder, stronger and have higher values of elastic limit than the pure metals from which they are derived. The hardening effect, known as solution hardening, is thought to be due to the fact that atoms of different atomic radii within the same lattice form a mechanical resistance to the movement of dislocations along slip planes.
Age Hardening : After solution heat treatment, the alloy is once again heated to bring about further precipitation. This also causes hardening of the alloy and is known as age hardening because the alloy will maintain its quality for many years. Ideally, before age hardening an alloy, it should first be subjected to a softening heat treatment
1) To relieve all strain and 2) starting the age hardening treatment when the alloy is in a disordered solid solution - allows better control of the hardening process
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In a summary, Solid solution strengthening involves formation of a single-phase solid solution via quenching. Precipitation heat treating involves the addition of impurity particles to increase a material's strength
A heat treatment is sometimes used to eliminate the cored structure. Such a heat treatment is termed a homogenization heat treatment. Homogenization heat treatment INCREASES DUCTILITY AND CORROSION RESISTANCE
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Clinical significance of heat treatment Type I and II gold alloys usually do not harden or they harden to a
lesser degree than do the types III and IV gold alloys. The type III and IV gold alloys that can be hardened or strengthened
from quenching, can also be softened by heat treatments.
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30CLASSIFICATION OF ALLOYS
ALLOY CLASSIFICATION BY NOBLE METAL CONTENT
In order of increasing melting temperature, they include gold, palladium, platinum,rhodium, ruthenium, iridium, and osmium. Only gold, palladium, and platinum, which have the lowest melting temperaturesof the seven noble metals, are currently of major importance in dental casting alloys.
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31ALLOY CLASSIFICATION BY MECHANICAL PROPERTIES
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ALLOY TYPE BY MAJOR ELEMENTS: Gold-based, palladium-based, silver-based, nickel-based, cobalt-based and titanium-based .
ALLOY TYPE BY PRINCIPAL THREE ELEMENTS: Such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V and Fe-Ni-Cr.
(If two metals are present, a binary alloy is formed; if three or four metals are present, ternary and quaternary alloys, respectively, are produced and so on.)
ALLOY TYPE BY DOMINANT PHASE SYSTEM: Single phase [isomorphous], eutectic, peritectic and intermetallic.
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33ALLOY CLASSIFICATION BY DENTAL APPLICATIONS
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Published in the March 2003 Journal of the American Dental Association.
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Metallic Elements Used in Dental Alloys
NOBLE METALS
• Noble Metal are corrosion and oxidation resistant because of inertness and chemical resistance.
• Basis of inlays, crowns and bridges because of their resistance to corrosion in the oral cavity.
• Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver
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GOLD
Pure gold --soft, malleable, ductile, rich yellow color, strong metallic
luster.
Lowest in strength and surface hardness.
Highest ductility, malleability and high density
High level of corrosion and tarnish resistance
High melting point, low C.O.T.E value and very good conductivity
Improves workability, burnish ability, raises the density .
Alloyed with copper, silver, platinum, and other metals to develop the
hardness, durability, and elasticity 36
• Density 19.3 g/cm3
• Melting point 1063oc
• Boiling point of 2970oc
• KHN 25
• CTE of 14.2×10-6/°c.
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Gold content: Traditionally the gold content of dental casting alloys have been referred to in terms
of:Carat: The term carat refers only to the gold content of the alloy; a carat represents a 1⁄24
part of the whole. Thus 24 carat indicates pure gold. The carat of an alloy is designated by a small letter k, for example, 18k or 22k gold.
Fineness: Fineness also refers only to the gold content, and represents the number of parts of
gold in each 1000 parts of alloy. Thus 24k gold is the same as 100% gold or 1000 fineness (i.e., 1000 fine) or an 18k gold would be designated as 750 fine.
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Silver
Lowers the melting range
Low corrosion resistance In gold-based alloys, silver is effective in neutralizing the reddish
color of copper. Silver also hardens the gold-based alloys via a solid-solution
hardening mechanism. Increases CTE in gold- and palladium-based alloys Foods containing sulfur compounds cause severe tarnish on silver,
and for this reason silver is not considered a noble metal in dentistry.
Pure silver is not used in dental restorations because of the black sulfide that forms on the metal in the mouth.
density 10.4gms/cm3
melting point 961oC
boiling point 2216 oC CTE 19.710-6/oC ,
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Platinum
High density, ductile and malleable
increases the strength and corrosion resistance.
increases the melting point
whitening effect on the alloy.
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• High density 21.45 g/cm3
• High melting point 1769oC
• Boiling point of 4530 oC
• Low CTE 8.910-6/oC
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Palladium
hardens + whitens the alloy.
increases the melting point.
Improves--tarnish resistance.
Lowers the C.O.T.E value
Absorbs or occluding large quantities of hydrogen gases when heated with an improperly adjusted gas torch.
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• density 12.02gms/cm3
• melting point 1552oC
• boiling point 3980 oC
• lower CTE 11.810-6/oC when compared to gold.
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Iridium and Ruthenium
grain refiners for gold- and palladium-based alloys
Reduces grain size. Improve the mechanical properties & tarnish
resistance. IRIDIUM has a high melting point of 2454°C ,
boiling point of 5300 °C , density of 22.5gm/cm3 and CTE 6.810-6/oC.
RUTHENIUM has a melting point of 1966°C , boiling point of 4500 °C , density of 12.44 gm/cm3 and CTE 8.310-6/oC 41107
BASE METALS
Cobalt
• INCREASES hardness, strength and elastic modulus.
• high melting point of 1495°C
• boiling point of 2900 °C
• density of 8.85 gm/cm3 and
• CTE 13.810-6/oC
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Nickel
Chosen base for porcelain alloys because its COTE
approximates that of gold
provides resistance to corrosion.
sensitizer and a known carcinogen.----contact dermatitis
melting point of 1453°C
boiling point of 2730 °C
density of 8.9 gm/cm3
CTE 13.310-6/oC
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Chromium
passivating effect Chromium content is directly
proportional to tarnish and corrosion resistance.
solid solution hardening. It has melting point of 1875°C boiling point of 2665 °C density of 7.19 gm/cm3
CTE 6.210-6/ oC45107
Copper
principal hardener. reduces the melting point and density of gold. gives the alloy a reddish colour. It also helps to age harden gold alloys. In greater amounts it reduces resistance to tarnish
and corrosion of the gold alloy. Therefore, the
maximum content should NOT exceed 16%. It has melting point of 1083°C , boiling point of 2595
°C , density of 8.96 gm/cm³ and CTE 16.5 10-6/°C
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scavenger for oxygen. Makes the alloy brittle. Later during solidification, the oxygen is rejected
producing gas porosities in the casting because of low density.
melting point of 420°C boiling point of 906 °C density of 7.133gm/ cm3 CTE 39.710-6/oC
ZINC47
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MOLYBDENUM
• effective hardener
• Molybdenum is preferred as it reduces ductility to a lesser extent than tungsten.
• refines grain structure.
• melting point of 2610°C • boiling point of 5560 °C • density of 10.22 gm/cm3 • CTE 4.9 10-6/oC
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Iron
• Fe---helps to harden the metal ceramic gold - palladium alloys
• melting point 1527°C • boiling point 3000 °C • density 7.87 gm/cm3 • CTE 12.3 10-6/oC .
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Beryllium
Be---reduces fusion temperature and refines grain structure . improves castability & polishability is a hardener controls oxide formation The etching of nickel-chromium-beryllium alloys removes a Ni-
Be phase to create the micro retention so important to the etched
metal resin-bonded retainer.
Potential Health risk - Berylliosis 50107
Tin
hardening agent
lower the melting range of an alloy.
assists in oxide production for porcelain bonding in gold and palladium-based alloys.
Tin is one of the key trace elements for oxidation of the palladium-silver alloys.
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Aluminium
Lowers the melting range of nickel-based alloys.
Act as a hardening agent and influences oxide formation.
With the cobalt - chromium alloys used for metal ceramic restorations.
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Gallium
Added to silver-free porcelain alloys to compensate for
the decreased COTE created by the removal of silver.
The oxides of gallium are important to bonding of
ceramic to metal.
It has a very low melting point of 29.8 C and density of
only 5.91g/cm3.
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Indium
oxide-scavenging agent (to protect molten alloy).
High COTE value ( 33ppm/°C) and very low melting temperature (156°C)
Enhance tarnish resistance as it is not tarnished by air or water
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CARBON:
• Small amounts may have a pronounced effect on strength, hardness and ductility.
• Carbon forms carbides with any of the metallic constituents which is an important factor in strengthening the alloy.
• when in excess it increases brittleness
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melting point of 3700°c boiling point of 4830 °C
density of 2.22 gm/cm3
CTE 6 10-6/oC . 107
BORON
It is a deoxidizer and hardener, but reduces ductility.
In Nickel-based alloys it is a hardening agent and an element that reduces the surface tension of the molten alloy to improve castability
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57PROPERTIES OF ELEMENTS:
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58DESIRABLE PROPERTIES OF DENTALCASTING ALLOYS
They must not tarnish and corrode in the mouth. They must be biocompatible (nontoxic and nonallergic). Alloys for bridgework require higher strength than alloys for single crowns. Alloys for metal-
ceramic prostheses are finished in thin sections and require sufficient stiffness to prevent excessive elastic deflection from functional forces, especially when they are used for long-span frameworks.
They must be easy to melt, cast, cut and grind (easy to fabricate). The melting range of the casting alloys must be low enough to form smooth surfaces with the
mold wall of the casting investment.
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For metal ceramic prostheses, the alloys must have closely matching thermal expansion coefficients to be compatible with given porcelains, and they must tolerate high processing temperatures without deforming via a creep process.
They must flow well and duplicate fine details during casting. They must have minimal shrinkage on cooling after casting. They must be easy to solder. To achieve a sound chemical bond to ceramic veneering materials, the alloy must
be able to form a thin adherent oxide, preferably one that is light in color so that it does not interfere with the esthetic potential of the ceramic.
ALLOYS FOR ALL-METAL PROSTHESES- HIGH NOBLE AND NOBLE ALLOYS
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Gold-Based Alloys These alloys are generally yellow in color. Type 1 gold alloys are soft and designed for
inlays supported by teeth and not subjected to significant mastication forces.
Type 2 alloys are widely used for inlays because of their superior mechanical properties, but they have less ductility than type 1 alloys.
Type 3 alloys are used for constructing crowns and onlays for high-stress areas. Increasing the Pt or Pd content raises the melting temperature, which is beneficial when components are to be joined by soldering (or brazing).
Type 4 gold alloys are used in high-stress areas such as bridges and partial denture frameworks.
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HEAT TREATMENT OF GOLD ALLOYS:
Heat treatment of alloys is done in order to alter its mechanical properties.
Type III and type IV gold alloys can be heat-treated.
There are two types of heat treatment.
1. Softening Heat Treatment (Solution heat treatment)
2. Hardening Heat Treatment (Age hardening)
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SILVER – PALLADIUM ALLOYS:
• Offered as an economical alternative to the more expensive gold-platinum-silver and
gold-palladium-silver (gold based) alloy systems.• Palladium – cheaper
tarnish resistance
Ag – Pd (non copper) : Ag 70 – 72 %
Pd 25 %
Ag – Pd – Cu : Ag 60%
Pd 25 %
Cu 15% The major limitation of Ag-Pd alloys in general and in the Ag-Pd-Cu alloys in particular is
their greater potential for tarnish and corrosion. Silver, copper, and/or gold can be added to increase the ductility and improve the castability
of the alloy for dental applications
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NICKEL - CHROMIUM AND COBALT - CHROMIUM ALLOYS:Also known as base metal alloys , extensively used
The Ni-Cr alloys can be divided into those with and without beryllium, which improves castability and promotes the formation of a stable metal oxide for porcelain bonding.
Advantages :low cost
strong and hard
Disadvantage : difficult to work (cutting , grinding , polishing)
TITANIUM AND TITANIUM ALLOYS :
can be used for metal and metal ceramic restorations as well as partial dentures .
ALLOYS FOR METAL-CERAMIC PROSTHESES
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67COMMON FEATURES OF PFM ALLOYS
(1) they have the potential to bond to dental porcelain: The addition of a small quantity of base metal to noble and high noble alloys promotes oxide
formation on the surface, which promotes chemical bonding between the alloy and the porcelain (2) they exhibit coefficients of thermal contraction compatible with those of dental porcelain: The thermal contraction differential between metal alloys and dental porcelains may, under
certain conditions, contribute to high levels of stress in porcelain, which can induce cracking of porcelain or delayed fracture.
(3) their solidus temperature is sufficiently high to resist softening during the sintering of porcelain:
When an alloy is heated close to its solidus temperature, it may become susceptible to flow under its own mass (creep). All metal-ceramic alloys should have a solidus temperature that is significantly higher than the sintering temperature of the porcelain so as to minimize creep deformation.
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CHEMICAL BONDING
The addition of 1% passivating base metals to gold palladium and platinum alloys was sufficient to produce a slight oxide film on surface of sub structure to achieve porcelain metal bond strength that surpassed the cohesive strength of porcelain.
By electro-depostion of Gold on an article followed by Tin which forms a tin oxide layer that helps to chemically bond to cermaics
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69MECHANICAL INTERLOCKING
The ceramic penetrating into a roughened metal surface can mechanically attach to the metal, improving adhesion.
Roughness provides increased surface area for adhesion and more room for chemical bond to form.
Sandblasting is often used to roughen the surface of the metal coping to improve the bonding of the ceramic.
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70THERMAL COMPATIBILITY
When the co efficient of thermal expansion of metal and porcelain are incompatible the tensile stress that develop during cooling are sufficient to cause immediate cracking of porcelain or delayed cracking after cooling at room temperature.
Porcelains have coefficient of thermal expansion between 13.0 and 14.0 X 10-6 and metal between 13.5 and 14.5 X 10-6.
The difference of 0.5 X10-6 in thermal expansion between metal and porcelain causes the metal to contract slightly more than does the ceramic during cooling after firing the porcelain
The C.O.T.E of alloys is decreased by adding platinum or palladium.
HIGH NOBLE AND NOBLE ALLOYS
Most alloys contain palladium, whose high melting point improves sag resistance during firing, and whose thermal contractioncoefficient is lower than that of silver, gold, and platinum, which is helpful in developing lightweight metal-ceramic alloys that are compatible with currently used dental ceramics.
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72The Gold-Platinum-Palladium (Au-Pt-Pd) System:
This is one of the oldest metal ceramic alloy system. But these alloys are not used widely today because they are very expensive.
These alloys have adequate elastic modulus, strength, hardness, and elongation but are low in sag resistance.
Therefore, they should be limited to crowns and three-unit FDPs.
Their use has decreased over time, since more economical alloys have been developed with significantly better mechanical properties and sag resistance.
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73Gold-Palladium-Silver Alloys
These alloys are economical alternatives to the Au-Pt-Pd or Au-Pd-Pt alloys.
Excellent tarnish and corrosion resistance Increased melting ranges. Improved resistance to creep deformation (sag) at elevated
temperatures. A study revealed that Ag–Pd–Au alloy has a lower
potential to abrade opposing enamel than do indirect resin composite, disilicate glass ceramic, and tooth enamel.
Journal of Prosthodontic Research. 2015;59(3):210-212. Japanese Dental Science Review. 2011;47(1):82-87.
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74Gold - Palladium Alloys
Lower thermal contraction coefficient than that of either the Au-Pd-Ag or Pd-Ag alloys.
The esthetic quality of metal-ceramic prostheses made with Au-Pd alloys is comparable to that obtained with Au-Pt-Pd alloys.
The sag resistance of these alloys is better than that of Au-Pt-Pd alloys.
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75PALLADIUM-SILVER ALLOYS
Alternative to gold and base metal alloys Disadvantage-
high silver content causes greening (green yellow discoloration of metal ceramic alloy)
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PALLADIUM BASED ALLOYS
PALLADIUM-COBALT ALLOYSCompositionPalladium 78 to 88 wt%Cobalt 4 to 10 wt%Gallium up to 9 wt% (in some brands)Base metal around 1 wt%
Esthetics – cobalt causes insignificant discoloration Sag resistance-good
PALLADIUM-COPPER ALLOYS CompositionPalladium 74 to 80 wt%Copper 5 to 10 wt%Gallium 4 to 9 wt%Gold 1 to 2 wt% (in some brands)Base metals around 1 wt%
Esthetics –Copper -causes slight discoloration, darker brown black oxide layer
PALLADIUM-GALLIUM ALLOYSCompositionPalladium 75 wt%Gallium 6 wt%Silver 5 to 8 wt%Gold 6 wt%(when present)Base metals around 1 wt%Esthetics – oxide layer is lighter than Pd Cu and Pd Co alloys
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BASE METAL ALLOYS FOR METAL-CERAMIC RESTORATIONS
Base metal alloys used for metal ceramic are:o Nickel - chromium alloys o Titanium and titanium alloyso Cobalt -chromium alloys
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78GENERAL PROPERTIES OF NICKEL BASED ALLOYS
Cost: cheapest
Colour: white
Melting range: 1155 to 1304° c high
Density: 7.8 to 8.4 gm/cm3
Hardness and workability: 175 to 360 VHN, much harder than high noble metal ceramic alloys, hardness makes them very difficult to cut grind and polish, more chair time, high hardness results in rapid wear of carbide and diamond burs
Yield strength: 310 to 828 Mpa, stronger
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Modulus of elasticity: 150 to 218 Gpa,
base metal alloys are twice as stiff as gold ceramic, gold alloys are require a minimum thickness of atleast 0.3 to 0.5mm, whereas base metal copings can be reduced to 0.3mm
Percent elongation: 10 to 28%,ductility of alloy , not easily burnishable
porcelain bonding: adequate oxide layer, essential for successful porcelain bonding
Sag resistance: more stable at porcelain firing, higher sag resistance
Esthetics: a dark oxide layer seen at porcelain metal
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Tarnish and corrosion resistance: highly resistant, this is due to the property known as passivation.
passivation is the resistant oxide layer on surface of chrome containing alloys
Soldering: base metal alloys much more difficult to solder than gold alloys
Casting shrinkage: higher casting shrinkage than gold alloys, greater mould expansion is to compensate for inadequate compensation for casting shrinkage
Etching: the alloy's surface can etched electrochemically to create micromechanical retention for resin-bonded FPD’s (Maryland Bridges).
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Biological considerations: nickel may produce allergic, potential carcinogen, beryllium potentially toxic, inhalation of beryllium containing dust or fumes known as berylliosis, flu-like symptoms and granulomas of the lungs.
Precautions: wear mask, well-ventilated, good exhaust
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82
NICKEL-CHROMIUM (NI-CR) ALLOYS
used for complete crown and all metal fixed partial denture prosthesis major constituents - nickel -61 to 71 %
chromium – 11 to 27 % The system contains two major groups:
-Beryllium free (class 1)
-Beryllium (class 2)
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83NICKEL CHROMIUM ALLOYS
Composition
Basic elements
Nickel : 61 To 81 Wt%
Chromium : 11 To 27 Wt%
Molybdenum : 2 To 9 Wt%The minor additions include Beryllium : 0.5 To 2.0 Wt% Aluminum : 0.2 To 4.2 Wt% Iron : 0.1 To 0.5 Wt% Silicon : 0.2 To 2.8 Wt% Copper : 0.1 To 1.6 Wt% Manganese : 0.1 To 3.0 Wt% Cobalt : 0.4 To 0.5 Wt% Tin : 1.25 wt%
ADVANTAGES• Low cost• Low Density permits more castings• High sag resistance• Poor thermal conductor• Can be etched
DISADVANTAGES• Cannot be used with nickel-sensitive
patients• Beryllium exposure may be
potentially harmful • Bond failure more common in oxide
layer• High hardness, may wear opposing
tooth
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84
NICKEL-CHROMIUM BERYLLIUM FREE ALLOYS
Composition:
Nickel – 62% to 77%
Chromium – 11% to 22%
Boron , iron, molybdenum, Niobium or columbium and tantalum (trace elements).
Advantages1. Do not contain beryllium2. Low cost .3. Low density means more
casting alloys
Disadvantages1. Cannot use with Nickel sensitive patients.2. Cannot be etched.(Cr doesn’t dissolve in acids) 3. May not cast as well as Ni-Cr-Be per ounce 4. Produces more oxide than Ni-Cr-Be
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Comparative properties of Ni / Cr alloys and type III casting gold alloys for small cast restorations 85
Property (Units) Ni/Cr Type III gold alloy
Comments
Density (g/cm3) 8 15 More difficult to produce defect free casting for Ni/Cr alloys.
Fusion temperature As high as 1350°C
Normally lower than 1000°C
Ni/Cr alloys require electrical induction furnace or oxyacetylene equipment.
Casting shrinkage (%) 2 1.4 Mostly compensated for by correct choice of investment
Tensile strength (MPa) 600 540 Both adequate for the applications being considered.
Proportional limit (MPa)
230 290 Both high enough to prevent distortion for applications being considered; not that values are lower than for partial denture alloys
Modulus of elasticity (GPa)
220 85 Higher modulus of Ni/Cr is an advantage for large restoration e.g. bridges and for porcelain bonded restoration.
Hardness (VHN) 300 150 Ni/Cr more difficult to polish but retains polish during service
Ductility(% elongation)
upto 30% 20 (as cast)10 (hardened)
Relatively large values suggest that burnishing is possible; however, large proportional limit value suggests higher forces would be require.
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86
TITANIUM AND TITANIUM ALLOYS : Titanium derives its corrosion protection from a thin passivating oxide film
(approximately 10 nm thick), which forms spontaneously with surrounding oxygen
Titanium has a high melting point (1668 °C) and high rate of oxidation above 900 °C.
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87Commercially Pure Titanium
CP Ti is often selected for its excellent corrosion resistance, especially in applications for which high strength is not required.
CP Ti has a hexagonal closepacked (HCP) crystal lattice, which is denoted as the alpha (α) phase. On heating, an allotropic phase transformation occurs. At 883° C, a body-centered cubic (BCC)
phase, which is denoted as the beta (β) phase, forms. A component with predominantly β phase is stronger but more brittle than a component with α-phase
microstructure
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88
PROPERTIES OF COMMERCIALLY PURE TITANIUM
Phases: in metallic form at ambient temperature- has a hexagonal, close-packed crystal lattice, transforms into a body-centered cubic format 883 °c- phase is susceptible to oxidation.
Color: white
Density: light weight metal 4.5 gm/cm3 compared nickel chrome 8gm/cm3 and gold alloys 15 gm/cm3
Modulus of elasticity: half as rigid as base metal alloys
Melting point: quite high 1668° c
Yield strength: 460 to 600 MPa
Tensile strength: 560 to 680 Mpa
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89
Coefficient of thermal expansion: CTE(8.4-10)too low to be compatible with porcelain. for this reason special low fusing porcelains have been developed to get around this problems
Biocompatibility: nontoxic excellent biocompatibility
Tarnish and corrosion: • Ability to self-passivate, oxidizes in air to form a tenacious and stable oxide
layer, oxide layer protects the metal from further oxidation.• Oxide layer allows for bonding of fused porcelains, adhesive polymers, • In case of endosseous implants, plasma-sprayed or surface-nucleated apatite
coatings.
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90
Titanium Alloys
By incorporating α and/or β microstructural stabilizers, four possible types of titanium alloys can be produced: α, near-α, α-β, and β.
Alpha-phase stabilizers, such as aluminum, carbon, nitrogen, and gallium, cause the transformation from α to β phase to occur at a higher temperature on heating.
Beta-phase stabilizers, such as molybdenum, cobalt, nickel, niobium, copper, palladium, tantalum, and vanadium, cause the transformation from β to α phase to occur at lower temperatures on cooling.
In general, alpha-titanium is weldable, but difficult to form or work with at room temperature.
Beta titanium, however, is malleable at room temperature and is thus used in orthodontics.
The (α + β) alloys are strong and formable but difficult to weld
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91
The most widely used titanium alloy in dentistry is Ti-6Al-4V (Ti-10.2Al- 3.6V in atomic percent), which is an α-alloy.
Greater strength than that of CP Ti. However Vanadium is highly toxic both in the elemental state and in
oxide forms, and aluminum has been reported to cause potential neurological disorders.
Vanadium can be replaced with the same atomic percentage of niobium yields Ti-6Al-7Nb (Ti-10.5Al-3.6Nb in atomic percent) which acts as a β stabiliser.
Niobium has not been associated with any known toxic or adverse reactions in the body
Microstructure of equiaxed Ti-6Al-4V (×200). Equiaxed microstructures are characterized by small, rounded α-grains, with aspect ratios near unity
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92
ADVANTAGES of titanium High strength Light weight Bioinert Low tarnish and corrosion because of ability to passivate Can be laser welded Limited thermal conductivity
DISADVANTAGES Highly technique sensitive Require expensive machines for casting and machining Low fusing porcelains required to prevent beta phase transformation
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93
REMOVABLE DENTURE ALLOYS
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95
ADDITIONAL REQIREMENTS FOR PARTIAL DENTURE ALLOYS Light in weight, lighter weight aids in retention in the mouth High stiffness, making the casting more thinner, especially in the palate
region, more comfortable to the patient, stiffness prevents bending under occlusal forces
Have good fatigue resistance for clasps,- clasps have to flex when inserted or removed from the mouth, if do not have good fatigue resistance break repeated insertion and removal
Should be economical, cost should be low Not react to denture cleansers
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96
TYPES alloys used for removable dentures Cobalt chromium alloys Nickel chromium alloys Aluminum and its alloys Type 4 noble alloys Titanium
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97
COBALT-CHROMIUM ALLOYS
Posses high strength, excellent corrosion resistance
COMPOSITION:
Cobalt : 35 to 65%
Chromium : 23 to 30%
Nickel : 0 to 20%
Molybdenum: 0 to 7%
Iron : 0 to 5%
Carbon : up to 0.4%
Tungsten, manganese, silicon and platinum in traces
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98
PROPERTIES OF COBALT CHROMIUM
Cost Lower and good mechanical properties
Density: half that of gold alloys, lighter in weight (8 to 9 gm/cm3)
Fusion temperature:
Type-1(high fusing)-liquidus temperature greater than 1300° c
Type-2(low fusing)-liquidus temperature not greater than 1300 °c
Yield strength : higher than the gold alloys
Elongation: ductility is lower
Modulus of elasticity: twice as stiff as gold alloys, casting made thinner, the weight of the RPD
Hardness: harder than gold alloys,cutting,grinding,finishing are difficult.
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99
Tarnish and corrosion resistance:
Chromium oxide prevents tarnish and corrosion - passivating effect
Caution: hypochlorite denture cleaning cause corrosion ,should not be used to clean chromium based alloys
Casting shrinkage: shrinkage is much greater due to high fusion temperature
Porosity: is due to shrinkage of alloy and release of dissolved gases
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100
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101RECENT ALLOYS
Palladium- Copper- Gallium Palladium- 75%Copper- 10%Gallium- 5-10%
Palladium –Gallium- SilverPalladium- 80-85%Gallium- 5-10%Silver- 0.5-8%
Cobalt-Chromium-Neobium-Molybdenum-Zirconia- an increased corrosion resistance and overall biocompatibilityCo 60%Cr 26.5%Mo 4.5%Nb 6.0%Zr 0.8%
• These alloys have superior mechanical properties.
• Gallium and Copper causes brownish discoloration of ceramic
Si 1.0 %C 0.4 %N 0.2 %Mn 0.8 %
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102Comparison of titanium and cobalt-chromium removable partial denture clasps.
This study assessed the characteristics of cast clasps made of titanium and titanium alloys to determine whether these materials are suitable alternatives for removable partial denture applications.
Removable partial denture clasps at two undercut depths were fabricated from commercially pure titanium, titanium alloy (Ti-6A1-4V), and cobalt-chromium.
Results showed that for the 0.75 mm undercut specimens, there was less loss of retention for clasps made from pure titanium and titanium alloy than for cobalt-chromium clasps.
Porosity was more apparent in the pure titanium and titanium alloy clasps than in those made from cobalt-chromium.
The Journal of Prosthetic Dentistry. 1997;78(2):187-193.
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103Microstructure, elemental composition, hardness and crystal structure study of the interface between a noble implant component and cast noble alloys. The Journal of Prosthetic Dentistry. 2011;106(3):170-178.
Casting a high-gold alloy to a wrought prefabricated noble implant-component increases the cost of an implant. Selecting a less expensive noble alloy would decrease implant treatment costs.
The purpose of this study was to investigate the interfacial regions of a representative noble implant component and cast noble dental alloys and to evaluate the effects of porcelain firing cycles on the interface.
Six representative alloys gold-platinum-palladium (Aquarius XH), gold-platinum (Brite Gold XH), gold-palladium (IPS d.SIGN 91), palladium-silver (IPS d.SIGN 59), and palladium-silver-gold (Capricorn 15) systems and ANSI/ADA Type IV (non-ceramic) gold alloy were cast to gold implant abutments (ComOcta).
Less expensive reduced-gold and palladium alloy alternatives provided comparable results to high-gold alloys for joint quality. Consequently, such noble metal alternatives to high gold alloys for conventional partial fixed dental prostheses might provide clinically acceptable implant superstructures.
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104The Release of Elements from the Base Metal Alloys in a Protein Containing Biologic Environments and Artificial Saliva – An Invitro Study JCDR. 2016;
This study aims to determine whether the solution in which an alloy is submerged and the exposure time have any effect on the amount of release of elements from the Ni-Cr and Co-Cr alloys.
A total of 126 specimens were made from the Ni-Cr alloy and 42 specimens were made from Co-Cr alloy. Dissolution experiments were carried out in Bovine Serum Albumin (BSA) and artificial saliva for a period of seven weeks and atomic absorption spectrophotometer was used for elemental analysis.
Results: The release of elements from the Ni-Cr alloy showed the predominant release of Cr. Conclusion: The protein containing solution showed maximum release of elements from Ni-Cr
and Co-Cr alloys. The elements that released from the alloys never reached their threshold for toxic effects. Hence these alloys can be safely used in fabrication of metal restorations without any ill effects
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105Evaluation of effect of galvanic corrosion between nickel-chromium metal and titanium on ion release and cell toxicity. The Journal of Advanced Prosthodontics. 2015;7(2):172. .
The purpose of this study was to evaluate the metal ion release caused by electrochemical corrosion due to contact between metals and to assess the cell toxicity effect.
A prosthesis was made of a base metal on the titanium abutment using three types of Ni-Cr alloys with different components and compositions.
The amount of metal ions released was increased by galvanic corrosion in all of the groups in which Ni-Cr alloys were in contact with titanium.
Cytotoxicity was significantly increased in all of the groups in which Ni-Cr alloys were in contact with titanium as compared to that in the group in which Ni-Cr alloys were not in contact with titanium.
A large amount of ions were released and high cytotoxicity was observed in the Ni-Be alloy with a relatively low corrosion resistance.
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106Effect of PFM Firing Cycles on the Mechanical Properties, Phase
Composition, and Microstructure of Nickel-Chromium Alloy. Journal of Prosthodontics. 2015;24(8):634-641.
The purpose of this study was to compare the mechanical properties of beryllium-free nickel-chromium (Ni-Cr) dental casting alloy before and after each porcelain firing cycle (once fired, twice fired, and thrice fired) and to relate these properties to the microstructural changes and changes in X-ray diffraction patterns of Ni-Cr alloy that occur after each porcelain firing cycle.
Results showed that After each firing cycle, there was a significant (p < 0.001) decrease in ultimate strength , 0.1% yield strength, and hardness and significant (p < 0.001) increase in elongation value of Ni-Cr alloy. The microstructure of the control group specimen exhibited heterogeneous microstructure, and after each firing, microstructure of the alloy was gradually homogenized by formation of grain boundaries at the interdendritic interfaces.
Results of this study confirmed that nickel-based alloys become weaker after each firing process. After firing treatment, the microstructure of alloys showed decreased strength and hardness of Ni-Cr alloy.
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107
EACH ALLOY SYSTEM HAS ITS OWN PROS AND CONS AND IS DEVELOPED FOR A SPECIFIC APPLICATION IN DENTISTRY OVERCOMING THE DRAWBACKS OF ITS PREDECESSORS
A SOUND KNOWLEDGE OF THE PROPERTIES AND HANDLIING OF THE CASTING METALS AND ALLOYS IS ESSENTIAL TO ENSURE PROPER APPLICATION IN CLINICAL PRACTICE AND DIAGNOSIS OF FAILURES IN CASE THEY OCCUR.
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108REFERENCES
1. Phillips' Science Of Dental Materials, 12th Edition Anusavice & Shen & Rawls
2. Craig’s Restorative Dental Materials / Edited By Ronald L. Sakaguchi, John M. Powers. -- 13th Ed.
3. Dental Materials And Their Selection - 3rd Ed. (2002) By William J. O'brien
4. Applied Dental Materials –Mccabbes And Walls- 9th Ed.
5. Science of Dental Materials- Clinical Application – V Shama Bhat & B.T Nandish
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109CROSS REFERENCES
1. Taira Y, Nakashima J, Sawase T, Sakihara M. Wear of tooth enamel against silver–palladium–gold alloy and two other restorative materials in vitro. Journal of Prosthodontic Research. 2015;59(3):210-212.
2. Bridgemana J, Marker V, Hummel S, Benson B, Pace L. Comparison of titanium and cobalt-chromium removable partial denture clasps. The Journal of Prosthetic Dentistry. 1997;78(2):187-193.
3. Jorge J, Barão V, Delben J, Faverani L, Queiroz T, Assunção W. Titanium in Dentistry: Historical Development, State of the Art and Future Perspectives. The Journal of Indian Prosthodontic Society. 2012;13(2):71-77.
4. Ucar Y, Brantley W, Johnston W, Iijima M, Han D, Dasgupta T. Microstructure, elemental composition, hardness and crystal structure study of the interface between a noble implant component and cast noble alloys. The Journal of Prosthetic Dentistry. 2011;106(3):170-178.
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5. McGinley E, Moran G, Fleming G. Biocompatibility effects of indirect exposure of base-metal dental casting alloys to a human-derived three-dimensional oral mucosal model. Journal of Dentistry. 2013;41(11):1091-1100.
6. Pangi A. The Release of Elements from the Base Metal Alloys in a Protein Containing Biologic Environments and Artificial Saliva – An Invitro Study. JCDR. 2016;.
7. Andrei M, Galateanu B, Hudita A, Costache M, Osiceanu P, Calderon Moreno J et al. Electrochemical comparison and biological performance of a new CoCrNbMoZr alloy with commercial CoCrMo alloy. Materials Science and Engineering: C. 2016;59:346-355
8. Lee J, Song K, Ahn S, Choi J, Seo J, Park J. Evaluation of effect of galvanic corrosion between nickel-chromium metal and titanium on ion release and cell toxicity. The Journal of Advanced Prosthodontics. 2015;7(2):172. .
9. Anwar M, Tripathi A, Kar S, Sekhar K. Effect of PFM Firing Cycles on the Mechanical Properties, Phase Composition, and Microstructure of Nickel-Chromium Alloy. Journal of Prosthodontics. 2015;24(8):634-641.
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