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DENTAL CASTING ALLOYS

INDIAN DENTAL ACADEMYLeader in Continuing Dental Education


INTRODUCTION In dentistry, metals represent one of the three

major classes of materials used for the reconstruction of damaged or missing oral tissues. Although metals are readily distinguished from ceramics and polymers.


The wide varieties of complex dental alloy compositions consist of the following:Dental amalgams containing the major elements mercury, silver, tin, and copper.Noble metal alloys in which the major elements are some combination of gold, palladium, silver and important secondary elements including copper, platinum, tin, indium and gallium.Base metal alloys with a major element of nickel, cobalt, iron or titanium and many secondary elements that are found in the alloy compositions.


HISTORY OF METALS IN DENTISTRYDentistry as a specialty is believed

to have begun about 3000 BC. Gold bands and wires were used by the Phoenicians after 2500 BC.

Modern dentistry began in 1728 when Fauchard published different treatment modalities describing many types of dental restorations, including a method for the construction of artificial dentures made from ivory. Gold shell crowns were described by Mouton in 1746 but they were not patented until in 1873 by Beers. In 1885 Logan patented porcelain fused to platinum post replacing the unsatisfactory wooden post previously used to build up intra-radicular areas of teeth. In 1907 a detached post crown was introduced which was more easily adjustable.


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


1971 – THE GOLD STANDARD The United States abandoned the gold

standard in 1971. Gold then became a commodity freely traded on the open markets. As a result, the price of gold increased steadily over the next nine years. In response to the increasing price of gold, new dental alloys were introduced through the following changes:

In some alloys, gold was replaced with palladium.

In other alloys, palladium eliminated gold entirely.

Base metal alloys with nickel as the major element eliminated the exclusive need for noble metals.


KEY TERMSGrain–A microscopic single crystal in the microstructure of a metallic material.Metal – An element whose atomic structure readily loses electrons to form positively charged ions, and which exhibits metallic bonding (through a spatial extension of valence electrons), opacity, good light reflectance from a polished surface and high electrical and thermal conductivity.Noble metal – which are highly resistant to oxidation and dissolution in inorganic acids. Gold and platinum group metals (Platinum, palladium, rhodium, ruthenium, iridium and osmium).Base metal – A metal that readily oxidizes or dissolves to release ions.


Alloy – A crystalline substance with metallic properties that is composed of two or more chemical elements, at least one of which is metal.

Solid solution (metallic) – A solid crystalline phase containing two or more elements, at least one of which is a metal, that are intimately combined at the atomic level.

Liquidus temperature – Temperature at which an alloy begins to freeze on cooling or at which the metal is completely molten on heating.

Solidus temperature – Temperature at which an alloy becomes solid on cooling or at which the metal begins to melt on heating.


PERIODIC TABLE


Dimitri Ivanovich Mendeleyev(1834-1934)


Of the 115 elements currently listed in most recent versions of the periodic tables of the elements, about 81 can be classified as metals. (Additional elements that have been created with nuclear reactors have short half-lives.) It is of scientific interest that the metallic elements can be grouped according to density, ductility, melting point and nobility. This indicates that the properties of metals are closely related to their valence electron configuration. The groupings of pure metal elements can be seen in the periodic chart of the elements. Several metals of importance for dental alloys are transition elements, in which the outermost electron subshells are occupied before the interior subshells are completely filled.








INTERATOMIC PRIMARY BONDS The forces that hold atoms together

are called cohesive forces. These interatomic bonds may be classified as primary or secondary. The strength of these bonds and their ability to reform after breakage determine the physical properties of a material. Primary atomic bonds may be of three different types.

1. Ionic2. Covalent3. Metallic


1. IONIC BOND FORMATION Characterized by electron transfer from one element (positive) to another (negative).


2. COVALENT BOND FORMATION

Characterized by electron sharing and very precise bond orientations.


3. METALLIC BOND FORMATION Since the outer-shell valence electrons can be removed easily from atoms in metals, the nuclei containing the balance of the bound electrons form positively charged ionic cores. The unbound or free valence electrons form a “cloud” or “gas”, resulting in electrostatic attraction between the free electron cloud and the positively charged ionic cores. Closed-shell repulsion from the outer electrons of the ionic cores balances this attractive force at the equilibrium interatomic spacing for the metal.


The free electrons act as conductors of both thermal energy and electricity. They transfer energy by moving readily from areas of higher energy to those of lower energy, under the influence of either a thermal gradient or an electrical field (potential gradient). Metallic bonding is also responsible for the luster or mirror-reflecting property, of polished metals and their typical capability of undergoing significant permanent deformation (associated with the properties of ductility and malleability) at sufficiently high mechanical stresses. These characteristics are not found in ceramic and polymeric materials in which the atomic bonding occurs through a combination of the covalent and ionic modes.



INTERATOMIC SECONDARY BONDS In contrast with primary bonds, secondary bonds do not share electrons. Instead, charge variations among molecules or atomic groups induce polar forces that attract the molecules.

VAN DER WAALS FORCES Fluctuating dipole that binds inert gas molecules together. The arrows show how the fields may fluctuate so that the charges become momentarily positive and negative.


PHYSICAL PROPERTIES Stress When a force is applied to a material there is a resistance in the material to the external force. The force is distributed over an area and the ratio of the force to the areais called stress. STRESS= F/A

StrainThe change in length or deformation per unit length when a material is subjected to a stress is defined as strain.


STRESS vs STRAIN CURVEIf one plots stress vs. strain on a graph, a stress-

strain curve will result. The properties of various dental materials, such as alloys, can be compared by analysis of their respective stress-strain curves.

P = Elastic modulus or Proportional Limit

Y-X curve = Yield Strength

X = Ultimate Strength


STRENGTH It is the maximal stress required to fracture a structure. Types of Strength: - Compressive - Tensile - Shearing


TOUGHNESS It is defined as the energy required to fracture a

material. It is a property of the material which describes how difficult the material would be to break.

DUCTILITYIt is the ability of a material to withstand permanent

deformation under a tensile load without rupture. A metal may be drawn readily into a wire and is said to be ductile. Ductility is dependent on tensile strength.

MALLEABILITYIt is the ability of the material to withstand rupture

under compression, as in hammering or rolling into a sheet. It is not dependent on strength as is ductility.


COEFFICIENT OF THERMAL EXPANSION (Linear Coefficient Of Expansion )

Change in length per unit of original length of a material when its temperature is raised 1 ° K


HARDNESSIn mineralogy the hardness is described on the

basis of the material to resist scratching. In metallurgy and in most other fields, the amounts of the resistance of indentation is taken as the measure of hardness for the respective material).

Brinell hardness number ( BHN )Rockwell hardness number ( RHN )Vickers hardness test (VHN )Knoop hardness test ( KHN )



TARNISH AND CORROSION High-noble alloys used in dentistry are so stable chemically that they do not undergo significant corrosion in the oral environment; the major components of these alloys are gold, palladium and platinum. (Iridium, osmium, rhodium and ruthenium are also classified as noble metals.) Silver is not considered noble by dental standards, since it will react with air, water and sulfur to form silver sulfide, a dark discoloration product. Gold resists chemical attack very well. Thus it was natural that this most noble metal was employed early in modern dental history for the construction of dental appliances.


TARNISH is observable as a surface discoloration on a metal, or as a slight loss or alteration of the surface finish or luster. In the oral environment, tarnish often occurs from the formation of hard and soft deposits on the surface of the restoration. Calculus is the principal hard deposit, and its color varies from light yellow to brown. The soft deposits are plaques and films composed mainly of microorganisms and mucin. Stain or discoloration arises from pigment-producing bacteria, drugs containing such chemicals as iron or mercury and adsorbed food debris.

CORROSION is not merely a surface deposit. It is a process in which deterioration of a metal is caused by reaction with its environment. Frequently, the rate of corrosion attack may actually increase over time, especially with surfaces subjected to stress, with intergranular impurities in the metal or with corrosion products that do not completely cover the metal surface. Sulfur is probably the most significant factor causing surface tarnish on casting alloys that contain silver, although chloride has also been identified as a contributor.


1. Chemical or Dry Corrosion

2. Electrochemical or Wet Corrosion

In this the metal reacts to form oxides, sulphides in the absence of electrolytes


a. Galvanic corrosion b. Heterogeneous Surface Composition

c. Stress Corrosion

Difference in potential

Occurs due to fatigue or cyclic loading


d. Concentration Cell Corrosion or Crevice Corrosion

• Pitting type (Oxygen concentration cell)

• Cervical type (Electrolyte concentration cell)


PROTECTION AGAINST CORROSIONi. Passivationii. Increase noble metal contentiii. Polishing restorationsiv. Avoid dissimilar metal restorations

Certain metals readily form strong adherent oxide film on their surface, which protects them from corrosion. Such a metal is said to be passive. Chromium, titanium and aluminium are examples of such metals. Since this film is passive to oxidative chemical attack, their formation is called passivation.

Chromium provides this corrosion resistance by forming a very thin, adherent surface oxide that prevents the diffusion of oxygen or other corroding species to the underlying bulk metal.If more than 12% Cr is added to iron or cobalt, we get stainless steel or cobalt chromium alloys, which are lightly corrosion resistant and therefore suitable for dental use.


Noble metals resist corrosion because their electromotive force is positive with regard to any of the common reduction reactions found in the oral environment. In order to corrode a noble metal under such conditions, an external current (over potential) is required.

At least half the atoms should be noble metals (gold, platinum, and palladium) to ensure against corrosion. Palladium has been found to be effective in reducing the susceptibility to sulfide tarnishing for alloys containing silver.


SOLIDIFICATIONAND

CRYSTALLIZATION OF METALS


SOLIDIFICATION OF METALS

The temperature decreases steadily from point A to point B' . An increase in temperature then occurs from point B' to point B, at which time the temperature remains constant until the time indicated at point C is reached. Subsequently, the temperature of the metal decreases steadily to room temperature.


The temperature Tf, as indicated by the straight or “plateau” portion of the curve at BC, is the freezing point, or solidification temperature of the pure metal. This is also the melting point, or fusion temperature. During melting, the temperature remains constant. During freezing or solidification, heat is released as the metal changes from the higher-energy liquid state to the lower-energy solid state.

The initial cooling of the liquid metal from Tf to point B' is termed super cooling. During the super cooling process, crystallization begins for the pure metal. Once the crystals begin to form, release of the latent heat of fusion causes the temperature to rise to Tf where it remains until crystallization is completed at point C.


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.Dendrites. In three dimensions, their general appearance is similar to that of the two dimensional frost crystals that form on a window pane in the winters.

CRYSTALLIZATION OF METALS


Extensions or elevated areas (termed protuberances) form spontaneously on the advancing front of the solidifying metal and grow into regions of negative temperature gradient. Secondary and tertiary protuberances result in a three dimensional dendritic structure.


Although dental base metal casting alloys typically solidify with a dendritic micro-structure, most nobel metal casting alloys solidify with an Equiaxed polycrystalline Equiaxed polycrystalline microstructure.microstructure. The microstructural features in this figure are called grains, and the term Equiaxed means that the three dimensions of each grain are similar, in contrast to the elongated morphology of the dendrites.


All modern noble metal alloys are fine grained. Smaller the grain size of the metal, the more ductile and stronger it is. It also produces a more homogenous casting and improves the tarnish resistance. A large grain size reduces the strength and increases the brittleness of the metal. Factors controlling the grain size are the rate of cooling, shape of the mold, and compositon of the alloy.


PHYSICAL PROPERTIES AND EFFECT OF

NOBLE METALS AND BASE METALS ON DENTAL

CASTING ALLOYS



NOBLE METALSThe noble metals have been the 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 are the eight noble metals. However, in the oral cavity, silver is more reactive and therefore is not considered as a noble metal.

Of the eight noble metals, four are of major importance in dental casting alloys, i.e., gold, platinum, palladium and silver. All four have a face-centered cubic crystal structure and all are white coloured except for gold.


GOLD

Pure gold is a soft and ductile metal with a yellow “Gold” hue. It has a density of 19.3 gms/cm3 , melting point of 1063oC, boiling point of 2970 oC and CTE of 14.2×10-6/°C. Gold has a good luster and takes up a high polish. It has good chemical stability and does not tarnish and corrode.


Gold content: Traditionally the gold content of dental casting alloys have been

referred to in terms of:• Karat• FinenessKarat: It is the parts of pure gold in 24 parts of alloys. For Eg: a) 24 Karat gold is pure gold b) 22 Karat gold is 22 parts of pure gold and

remaining 2 parts of other metal. The term Karat is rarely used to describe gold content in current alloys.Fineness: Fineness of a gold alloy is the parts per thousand of pure gold.

Pure gold is 1000 fine. Thus, if ¾ of the gold alloy is pure gold, it is said to be 750 fine.


Material Density(g/cm3)

Hardness (VHN/BHN)(kg/mm2)

TensileStrength

(MPa)

Elongation(%)

Cast 24k gold 19.3 28(VHN) 105 30

Cast 22k gold -- 60(VHN) 240 22

Coin gold -- 85 (BHN) 395 30

Typical Au-basedcasting alloy

(70 wt% Au)*

15.6 135/195(VHN) 425/525 30/12

Condensed goldfoil

19.1 60 (VHN) 250 12.8

* Values are for softened / hardened condition.

Physical and mechanical properties of cast pure gold, gold alloys, and condensed gold foil


SILVER It is sometimes described as the “Whitest” of all metals. It whitens the alloy, thus helping to counteract the reddish colour of copper. To a slight extent it increases strength and hardness. In large amounts however, it reduces tarnish resistance. It has the lowest density 10.4gms/cm3 and melting point of 961oC, boiling point of 2216 oC among the four precious metals used in dental casting alloys. Its CTE is 19.710-6/oC , which is comparatively high.


PLATINUM

It increases the strength and corrosion resistance. It also increases the melting point and has a whitening effect on the alloy. It helps to reduce the grain size.It has the highest density of 21.45 gms/cm3 , highest melting point of 1769oC, boiling point of 4530 oC and the lowest CTE 8.910-6/oC among the four precious metals used in dental casting alloys.


PALLADIUM

It is similar to platinum in its effect. It hardens as well as whitens the alloy. It also raises the fusion temperature and provides tarnish resistance. It is less expensive than platinum, thus reducing cost of alloy. It has a density of 12.02gms/cm3. Palladium has a higher melting point of 1552oC, boiling point of 3980 oC and lower CTE which is 11.810-6/oC, when compared to gold.


IRIDIUM, RUTHENIUMThey help to decrease the grain size. They are added in very small quantities (about 100 to 150 ppm). 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


BASE METALS

These are non-noble metals. They are invaluable components of dental casting alloys because of their influences on physical properties, control of the amount and type of oxidation, or their strengthening effect. Such metals are reactive with their environment, and are referred to as ‘base metals’. Some of the base metals can be used to protect an alloy from corrosion (passivation). Although they are frequently referred as non precious, the preferred term is base metal. Examples of base metals are chromium, cobalt, nickel, iron, copper, manganese etc.


COBALT

Imparts hardness, strength and rigidity to the alloy . It has a high melting point of 1495°C , boiling point of 2900 °C , density of 8.85 gm/cm3 and CTE 13.810-6/oC


NICKEL Cobalt and nickel are interchangeable.It decreases strength, hardness, modulus of elasticity and fusion temperature. It increased ductility. Bio-incompatibility due to nickel, which is the most common metal to cause Contact Dermatitis. It has a melting point of 1453°C , boiling point of 2730 °C , density of 8.9 gm/cm3 and CTE 13.310-

6/oC


CHROMIUM Its passivating effect ensures corrosion resistance. The chromium content is directly proportional to tarnish and corrosion resistance. It reduces the melting point. Along with other elements, it also acts in solid solution hardening. Thirty percent chromium is considered the upper limit for attaining maximum mechanical properties. It has melting point of 1875°C , boiling point of 2665 °C , density of 7.19 gm/cm3 and CTE 6.210-6/ oC


COPPERIt is the principal hardener. It reduces the melting point and density of gold. If present in sufficient quantity, it 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 .


It acts as a scavenger for oxygen. Without zinc the silver in the alloy causes absorption of oxygen during melting. Later during solidification, the oxygen is rejected producing gas porosities in the casting. It has a melting point of 420°C , boiling point of 906 °C , density of 7.133gm/cm3 and CTE 39.710-6/oC

ZINC


MOLYBDENUM OR TUNGSTEN

They are effective hardeners. Molybdenum is preferred as it reduces ductility to a lesser extent than tungsten. Molybdenum refines grain structure. It has melting point of 2610°C , boiling point of 5560 °C , density of 10.22 gm/cm3 and CTE 4.9 10-6/oC


IRON,BERYLLIUM They help to harden the metal ceramic gold - palladium alloys, iron being the most effective. In addition, beryllium reduces fusion temperature and refines grain structure . IRON has melting point of 1527°C , boiling point of 3000 °C , density of 7.87 gm/cm3 and CTE 12.3 10-6/oC .


GALLIUM

It is added to compensate for the decreased coefficient of thermal expansion that results when the alloy is made silver free. The elimination of silver reduces the tendency for green stain at the margin of the metal-porcelain interface.


MANGANESE AND SILICON Primarily oxide scavengers to prevent oxidation of other elements during melting. They are also hardeners. MANGANESE has melting point of 650°C , boiling point of 1107 °C , density of 1.74 gm/cm3 and CTE 25.2 10-

6/oC , where as SILICON has melting point of 1410°C , boiling point of 2480 °C , density of 2.33 gm/cm3 and CTE 7.3 10-6/oC .


CARBON:Carbon content is most

critical. 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. However when in excess it increases brittleness. Thus, control of carbon content in the alloy is important. It has melting point of 3700°C , boiling point of 4830 °C , density of 2.22 gm/cm3 and CTE 6 10-6/oC .


BORON

It is a deoxidizer and hardener, but reduces ductility.


ALLOYS


ALLOYS

The use of pure metals is quite limited in dentistry. To optimize properties, most metals commonly used in engineering and dental applications are mixtures of two or more metallic elements or in some cases one or more metals and/or nonmetals. They are generally prepared by fusion of the elements above their melting points. A solid material formed by combining a metal with one or more other metals or nonmetals is called an alloy. For example, a small amount of carbon is added to iron to form steel. A certain amount of chromium is added to iron, carbon, and other elements to form stainless steel, an alloy that is highly resistant to corrosion. Chromium is also used to impart corrosion resistance to nickel or cobalt alloys, which comprise two of the major groups of base metal alloys used in dentistry.


At least four factors determine the extent of solid solubility of metals; atom size, valence, chemical affinity and chemical structure.

ATOM SIZE: If the sizes of two metallic atoms differ by less than approximately 15% (first noted by Hume-Rothery), they possess a favorable size factor for solid solubility.

VALENCE: Metals of the same valence and size are more likely to form extensive solid solutions than are metals of different valences.

CHEMICAL AFFINITY: When two metals exhibit a high degree of chemical affinity, they tend to form an intermetallic compound upon solidification rather than a solid solution.


CRYSTAL STRUCTURE: Only metals with the same type of crystal structure can form a complete series of solid solutions. The simplest alloy is a solid solution, in which atoms of two metals are located in the same crystal structure such as body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed (hcp).



EQUILIBRIUM PHASE DIAGRAM FOR

ALLOYS


Equilibrium phase diagram are of central importance to the metallurgy of alloys, since the phases that are present in an alloy system for different compositions and temperatures. Eg Single phase [isomorphous], eutectic, peritectic and intermetallic.

Phase diagrams are useful for understanding the structure of dental alloys and can provide microstructural predictions when some cast dental alloys are subjected to heat treatment.

This concept equilibrium phase diagram was introduced by using the table salt-water system


Liquidus temperatureSolidus temperatureLiquidusSolidus

EQUILIBRIUM PHASE DIAGRAM FOR ALLOYS


Liquidus temperature – Temperature at which an alloy begins to freeze on cooling or at which the metal is completely molten on heating.

Solidus temperature – Temperature at which an alloy becomes solid on cooling or at which the metal begins to melt on heating.


This equilibrium phase diagram is for palladium 65% and silver 35%. When an alloy composition is undergoing equilibrium solidification, the percentage of the liquid and solid phases present at a given temp. can be calculated by the lever rule.

Dashed line PO perpendicular to composition line is drawn.

A point on line PO corresponds temp. 1500°C, the alloy is clearly in liquid statePoint R - Temp is approx. 1400°C and first solid is formed (crystal), but the composition is different from 65% Palladium and 35% Silver. To determine the composition of first solid extend the tie line to point M. This when projected to the base line gives the composition of first solid which is 77% of Palladium. Point S - The alloy is midway through its freezing range, and the composition of solid and liquid may be determined by drawing tie line Y-W. These points when projected to the base line gives liquid composition of 57% Palladium at point Y and solid composition of 71%Palladium at point W.Point T – As the temp. reaches point T (solid phase) the concentration is 65% Palladium.


CORING

In the coring process the last liquid to solidify is metal with lower solidus temperature and solidifies between the dendrites. Thus under rapid freezing conditions, the alloy has a corde structure. The core consists of the dendrites composed of compositions with higher solidus temperature, and the matrix is the portion of the micro-structure between the dendrites that contains compositions with lower solidus temperatures.


For homogenization heat treatment, the cast alloy is held at a temperature near its solidus to achieve the maximum amount of diffusion without melting. (This process required 6 hr. for the alloy). Little or no grain growth occurs when a casting receives this type of heat treatment eg. Annealing done mainly for wrought alloys . The ductility of an alloy usually increases after homogenization heat treatment . Gold alloys are heat treated by softening (solution heat treat) or hardening (age hardening heat treat)

HOMOGENIZATION


Many binary alloy systems do not exhibit complete solubility in both the liquid and the solid states. The eutectic system is an example of an alloy for which the component metals have limited solid solubility. Two metals, A and B, which are completely insoluble in each other in the solid state, provide the simplest illustration of a eutectic alloy.

EUTECTIC ALLOYS

In this case, some grains are composed solely of metal A and the remaining grains are composed of metal B. The salt and water molecules intermingle randomly in solution, the result upon freezing is a mixture of salt crystals and ice crystals that form independently of each other.


SILVER-COPPER SYSTEM: The phase diagram for this system is presented in where 3 phases are

found:• A liquid phase (L)• A silver-rich substitutional solid solution phase () containing a small

amount of copper atoms.• A copper-rich substitutional solid solution phase () containing a small

amount of silver atoms. The and phases are sometimes referred to as terminal solid solutions because of their locations at the left and right sides of the phase diagram.

Boundary ABEGD is the solidus and AED is the liquidus. The major portion of diagram below 780°C is composed of a two phase region.


Liquidus and solidus meet at point E. This composition (72% silver and 28% copper) is known as the eutectic composition or simply the eutectic. The following characteristics of this special composition should be noted.

The temperature at which the eutectic composition melts (779°C or 1435°F) is lower than the fusion temperature of silver or copper (eutectic literally means “lowest melting”).

There is no solidification range for composition E. The eutectic reaction is sometimes written schematically as follows.Liquid solid solution + solid solution


PERITECTIC ALLOY

Liquid + solid solution solid solution


CLASSIFICATION OF DENTAL CASTING ALLOYS


1. ALLOY TYPES BY FUNCTIONS:In 1927, the Bureau of Standard established gold casting alloys, type I to type IV according to dental function with hardness increasing from type I to type IV. Type I (Soft):It is used for fabrication of small inlays, class III and class V restorations which are not subjected to great stress . These alloys are easily burnishable.Type -II (Medium): These are used for fabrication of inlays subjected to moderate stress, thick 3/4 crowns, abutments, pontics, full crowns and soft saddles.Type I and II are usually referred to as inlay gold.Type -III (Hard):It is used for fabrication of inlays subjected to high stress, thin 3/4 crowns, thin cast backing abutments, pontics, full crowns, denture bases and short span FPDs . Type III alloys can be age hardened.Type-IV (Extra hard):It is used for fabrication of inlays subjected to high stress, denture bases, bars and clasps, partial denture frameworks and long span FPDs. These alloys can be age hardened by heat treatment.


Type III and Type IV gold alloys are generally called "Crown and Bridge Alloys", although type IV alloy is used for high stress applications such as RPD framework. Later, in 1960, metal ceramic alloys were introduced and removable partial denture alloys were added in this classification.

Metal ceramic alloys (hard and extra hard):It is suitable for veneering with dental porcelain, copings, thin walled crowns, short span FPDs and long span FPDs. These alloy vary greatly in composition and may be gold, palladium, nickel or cobalt based.

Removable partial denture alloys :It is used for removable partial denture frameworks. Now a days, light weight, strong and less expensive nickel or cobalt based have replaced type IV alloys .


2. ALLOY TYPES BY DESCRIPTION:By description, these alloys are classified into

A) CROWN AND BRIDGE ALLOYSThis category of alloys include both noble and base metal

alloys that have been or potentially could be used in the fabrication of full metal or partial veneers.

1. Noble metal alloys: i) Gold based alloy - type III and type IV gold alloys ,

low gold alloys ii) Non-gold based alloy-Silver -palladium alloy2. Base metal alloys:

i) Nickel-based alloys ii) Cobalt based alloys3. Other alloys:

i) Copper-zinc with Indium and nickelii) Silver-indium with palladium


B) METAL CERAMIC ALLOY

1. Noble metal alloys for porcelain bonding:i) Gold-platinum -palladium alloyii) Gold-palladium-silver alloyiii) Gold-palladium alloyiv) Palladium silver alloyv) High palladium alloy

2. Base metal alloys for porcelain bonding:i) Nickel -chromium alloyii) Cobalt-chromium alloy


C) REMOVABLE PARTIAL DENTURE ALLOY

Although type-IV noble metal alloy may be used, majority of removable partial framework are made from base metal alloys:

1. Cobalt-chromium alloy2. Nickel-chromium alloy3. Cobalt-chromium-nickel alloy4. Silver-palladium alloy5. Aluminum -bronze alloy


High noble, noble, and predominantly base metal.

ALLOY TYPE TOTAL NOBLE METAL CONTENT

High noble metal Contains > 40 wt% Au and > 60 wt% of the noble metal elements (Au

+ Ir + Os + Pd + Pt + Rh + Ru)

Noble metal Contains > 25 wt % of the noble metal elements

Predominantly base metal Contains < 25 wt % of the noble metal elements

Alloy Classification of the American Dental Association (1984)

3.ALLOY TYPE BY NOBILITY


Alloy type All-metal Metal-ceramic Removable partial dentures

High noble Au-Ag-Cu-Pd Au-Pt-Pd Au-Ag-Cu-Pd

Metal ceramic alloys Au-Pd-Ag (5-12wt% Ag)

Au-Pd-Ag (>12wt%Ag)

Au-Pd (no Ag)

Noble Ag-Pd-Au-Cu Pd-Au (no Ag) Ag-Pd-Au-Cu

Ag-Pd Pd-Au-Ag Ag-Pd

Metal-ceramic alloys Pd-Ag

Pd-Cu

Pd-Co

Pd-Ga-Ag

Base Metal Pure Ti Pure Ti Pure Ti

Ti-Al-V Ti-Al-V Ti-Al-V

Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo-Be

Ni-Cr-Mo Ni-Cr-Mo Ni-Cr-Mo

Co-Cr-Mo Co-Cr-Mo Co-Cr-Mo

Co-Cr-W Co-Cr-W Co-Cr-W

Al bronze

Classification of alloys for All-Metal restorations, metal ceramic restorations, and frameworks for removable partial dentures.


4. ALLOY TYPE BY MAJOR ELEMENTS: Gold-based, palladium-based, silver-based, nickel-based, cobalt-based and titanium-based .

5. 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.)

6. ALLOY TYPE BY DOMINANT PHASE SYSTEM: Single phase [isomorphous], eutectic, peritectic and intermetallic.


DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS

Biocompatibility Ease of melting Ease of casting Ease of brazing (soldering) Ease of polishing Little solidification shrinkage Minimal reactivity with the mold material Good wear resistance High strength Excellent corrosion resistance Porcelain Bonding


To achieve a sound chemical bond to ceramic veneering materials, a substrate metal must be able to form a thin, adherent oxide, preferably one that is light in color so that it does not interfere with the aesthetic potential of the ceramic. The metal must have a thermal expansion/contraction coefficient that is closely matched to that of the porcelain.


GOLD CASTING ALLOYS


Composition Range (weight percent) of traditional type I to IV alloys and four metal -ceramic alloys

Alloy typeMain elements Au Cu Ag Pd Sn, In, Fe, Zn, Ga

I High noble (Au base) 83 6 10 0.5 Balance

II High noble (Au base) 77 7 14 1 Balance

III High noble (Au base) 75 9 11 3.5 Balance

III Noble (Au base) 46 8 39 6 Balance

III Noble (Ag base) 70 25 Balance

IV High noble (Au base) 56 14 25 4 Balance

IV Noble (Ag base) 15 14 45 25 Balance

Metal-ceramic High noble (Au base) 52 38 Balance

Metal-ceramic Noble (Pd base) 30 60 Balance

Metal-ceramic High noble (Au base) 88 1 7 (+4Pt) Balance

Metal-ceramic Noble (Pd base) 0-6 0-15 0-10

74-88 Balance


GOLD CASTING ALLOYS:

ADA specification No. 5 classify dental gold casting alloys as:

1. High Gold Alloys Type I Type II

Type III

Type IV

2. Low Gold Alloys

3. White Gold Alloys

Inlay Gold Alloy

Crown & Bridge Alloy


HIGH GOLD ALLOY: These alloys contain 70% by weight or more of gold palladium and platinum. ADA specification No.5 divides this into four depending upon mechanical properties.

Type I (Soft):-

They are weak, soft and highly ductile, useful only in areas of low occlusal stress designed for simple inlays such as used in class I, III & V cavities. These alloys have a high ductility so they can be burnished easily. Such a characteristic is important since these alloys are designed to be used in conjunction with a direct wax pattern technique. Since such a technique occasionally results in margins that are less than ideal it is necessary to use a metal that can be burnished. At present, these are used very rarely.


PROPERTIES

1. Hardness VHN (50 – 90)2. Tensile Strength Quite Low 276 MPa or 40,000 PSi3. Yield Strength 180 MPa or 26,000 PSi4. Linear Casting Shrinkage 1.56% (according to

Anusavice)5. Elongation or ductility 46% - William O Brien

18% - Anusavice

COMPOSITION

Au Ag Cu Pt Pd Zn&Ga 83% 10% 6% - 0.5% balance


Type II (Medium):-

These are used for conventional inlay or onlay restorations subject to moderate stress, thick three quarter crowns, pontics and full crowns. These are harder and have good strength. Ductility is almost same as that of type I alloy however, yield strength is higher. Since burnishability is a function of ductility and yield strength, greater effort is required to deform the alloy. They are less yellow in color due to less gold.Properties:1. Hardness VHN (90-120)2. Tensile Strength 345 MPa3. Yield Strength 300 MPa4. Linear Casting Shrinkage 1.37%5. Elongation 40.5% - William O Brien 10% - AnusaviceComposition: Au Ag Cu Pt Pd Zn&Ga 77% 14% 7% - 1% balance


Type III (Hard):

Inlays subject to high stress and for crown and bridge in contrast to type I and type II, this type can be age hardened. The type III alloy, burnishing is less important than strength.

Properties:1. Hardness(VHN) 120 – 1502. Tensile Strength 360 MPa3. Yield Strength 331 MPa4. Linear Casting Shrinkage 1.42%5. Elongation or ductility 39.4% - William O Brien 5% - Anusavice

Composition: Au Ag Cu Pt Pd Zn&Ga 75% 11% 9% - 3.5% balance


Type IV (Extra Hard):

These are used in areas of very high stress, crowns and long span bridges. It has lowest gold content of all four type (Less than 70%) but has the highest percentage of silver, copper, platinium and Palladium. It is most responsive to heat treatment and yield strength but lowers ductility.

Properties:1. Hardness VHN (150-200)2. Tensile Strength 462 MPa3. Yield Strength 703 MPa4. Linear Casting Shrinkage 2.30%5. Elongation or ductility 17% - William O Brien 3% - Anusavice

Composition: Au Ag Cu Pt Pd Zn&Ga 56% 25% 14% - 4% balance


Type Hardness Proportional limit

Strength Ductility Corrosion resistance

I

II INCREASES DECREASES

III

IV


HEAT TREATMENT OF GOLD ALLOYS:

Heat treatment of alloys is done in order to alter its mechanical properties.

Gold alloys can be heat treated if it contains sufficient amount of copper. Only 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)


1. SOFTENING HEAT TEMPERATURE

Softening heat treatment increased ductility, but reduces tensile strength, proportional limit, and hardness.Indications: It is indicated for appliances that are to be grounded, shaped, or otherwise cold worked in or outside the mouth.Method: The casting is placed in an electric furnace for 10 minutes at a temperature of 700oC and then it is quenched in water. During this period, all intermediate phases are presumably changed to a disordered solid solution, and the rapid quenching prevents ordering from occurring during cooling. Each alloy has its optimum temperature. The manufacturer should specify the most favorable temperature and time.


2. HARDENING HEAT TREATMENT

Hardening heat treatment increases strength, proportional limit, and hardness, but decreases ductility. It is the copper present in gold alloys, which helps in the age hardening process.Indications: It is indicated for metallic partial dentures, saddles, bridges and other similar structures. It is not employed for smaller structures such as inlays.Method: It is done by “soaking” or ageing the casting at a specific temperature for a definite time, usually 15 to 30 minutes. It is then water quenched. The aging temperature depends on the alloy composition but is generally between 200°C and 450°C. During this period, the intermediate phases are changed to an ordered solid solution.


The proper time and temperature for age hardening an alloy are specified by the manufacturer.

Ideally, before age hardening an alloy, it should first be subjected to a softening heat treatment to relieve all strain hardening and to start the age hardening treatment when the alloy is in a disordered solid solution. This allows better control of the hardening process.


METAL CERAMIC ALLOYS



METAL CERAMIC ALLOYS4,8,11,15,16,27,31,32,41&43 The main function of metal-ceramic alloys is to reinforce porcelain, thus increasing its resistance to fracture.Requirements:1.They should be able to bond with porcelain.2.Its coefficient of thermal expansion should be compatible with that of porcelain.3.Its melting temperature should be higher than the porcelain firing temperature. It should be able to resist creep or sag at these temperatures.4.It should not stain or discolor porcelain.

The alloys used for metal-ceramic purposes are grouped under two categories: i) Noble metal alloys ii) Base metal alloys. In case of noble metal alloys for porcelain bonding , addition of 1% base metals (iron, indium, tin etc.) increases porcelain-metal bond strength, which is due to formation of an oxide film on its surface. It also increases strength and proportional limit.


Modulus of elasticity: The base metal alloys have a modulus of elasticity approximately twice that of gold alloys. Thus it is suited for long span bridges. Similarly, thinner castings are possible.Hardness: The hardness of base metal alloys ranges from 175 to 360 VHN. Thus, they are generally harder than noble metal alloys. Thus, cutting, grinding and polishing requires high speed and other equipment.Ductility: It ranges from 10 to 28% for base metal alloys. Noble metal alloys have an elongation of 25 to 40%.Density: The density of base metal alloys are less, which is approximately 8.0 gms/cm3 as compared to 18.39 gms/cm3 for noble metal alloys.Sag Resistance: Base metal alloys resist creep better than gold alloy when heated to high temperatures during firing.Bond Strength: Varies according to composition.Technique Sensitivity: Base metals are more technique sensitive than high noble metal-ceramic alloys.

PROPERTIES


The 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.

Composition:Gold – 75% to 88%Palladium – Upto 11%Platinum – Upto 8%Silver – 5%Trace elements like Indium, Iron and Tin for porcelain bonding. Advantages Disadvantages1. Excellent castability 1. High cost2. Excellent porcelain bonding 2. Poor sag resistance so not suited for 3. Easy to adjust and finish long span fixed partial dentures.4. High nobility level 3. Low hardness (Greater wear)5. Excellent corrosion and tarnish 4. High density (fewer casting per resistance. ounce)6. Biocompatible7. Some are yellow in color8. Not “Technique Sensitive”9. Burnishable


Gold-Palladium-Silver (Au-Pd-Ag) System:

These alloys were developed in an attempt to overcome the major limitations in the gold-platinum-palladium system (mainly poor sag resistance, low hardness & high cost) Two variations on the basic combination of gold, palladium and silver were created and are identified as either high-silver or low-silver group.

Composition (High Silver Group):

Gold – 39% to 53%Silver – 12% to 22%Palladium – 25% to 35%trace amount of oxidizable elements are added for porcelain bonding.

Advantages Disadvantages1. Less expensive than Au-Pt-Pd alloys 1. High silver content creates potential2. Improved rigidity and sag resistance. for porcelain discoloration.3. High malleability. 2. High Cost. 3. High coefficient of thermal expansion. 4. Less Tarnish and corrosion resistant.


Composition (Low Silver Group):

Gold – 52% to 77%Silver- 5% to 12%Palladium – 10% to 33%Trace amounts of oxidizable elements for porcelain bonding.

Advantages Disadvantages1. Less expensive than the Au-Pt-Pd alloys 1. Silver creates potential for porcelain discoloration (but less than high silver group)2. Improved sag resistance 2. High cost.3. High noble metal content 3. High coefficient of thermal expansion.4. Tarnish and corrosive resistant


Gold-Palladium (Au-Pd) System:

This particular system was developed in an attempt to overcome the major limitations in the Au-Pt-Pd system and Au-Pd-Ag system. Mainly--Porcelain discoloration.-Too high coefficient of thermal expansion & contraction.

Composition:

Gold – 44% to 55%Gallium – 5%Palladium – 35% to 45%Indium & Tin – 8% to 12%Indium, Gallium and Tin are the oxidizable elements responsible for porcelain bonding.

Advantages Disadvantages1. Excellent castability 1. Not thermally compatible with high expansion dental porcelain.2. Good bond strength 2. High cost3. Corrosion and tarnish resistance4. Improved hardness5. Improved strength ( sag resistance)6. Lower density


Palladium-Silver (Pd-Ag) System8

This was the first gold free system to be introduced in the United States (1974) that still contained a noble metal (palladium). It was offered as an economical alternative to the more expensive gold-platinum-silver and gold-palladium-silver (gold based) alloy systems.

Composition: (available in two compo.)1. Palladium – 55% to 60% Silver – 25% to 30% Indium and Tin2. Palladium – 50% to 55% Silver – 35% to 40% Tin (Little or no Indium) Trace elements of other oxidizable base elements are also present.



Advantages Disadvantages

1. Low Cost 1. Discoloration (yellow, brown or green) may occur with some dental porcelains.2. Low density 2. Some castibility problems reported (with induction casting)3. Good castibility (when torch 3. Pd and Ag prone to absorb gases. casting) 4. Require regular purging of the porcelain 4. Good porcelain bonding, furnace.5. Burnishability 5. May form internal oxides (yet porcelain 6. Low hardness bonding does not appear to be a problem)7. Excellent sag resistance 6. Should not be cast in a carbon crucible.8. Moderate nobility level 7. Non-carbon phosphate bonded investments 9. Good tarnish and corrosion recommended. resistance. 8. High coefficient of thermal expansion.10. Suitable for long-span fixed partial dentures.


HIGH PALLADIUM SYSTEM8,11,31,32,41&43

Several types of high palladium systems were originally introduced (Tuccillo, 1987).More popular composition groups contained cobalt and copper.

Composition (PALLADIUM-COBALT ALLOY):

Palladium – 78% to 88% Cobalt – 4% to 10%(Some high palladium-cobalt alloys may contain 2% gold)Trace amounts of oxidizable elements (such as gallium and indium) are added for porcelain bonding.

Advantages Disadvantages1. Low cost 1. More compatible with higher expansion 2. Reportedly good sag resistance porcelains.3. Low density means more casting 2. Are more prone to over-heating than per ounce then gold based alloys. high Pd-Cu.4.They Melt and cast easily 3. Produces a thick, dark oxide5. Good polishability (Supposed 4. Colored oxide layer may cause bluing of the to be similar to Au-Pd alloys) porcelain.6. Reportedly easier to presolder 5. Prone to gas absorption than Pd-Cu alloys. 6. Little information on long-term clinical success.


COMPOSITION (PALLADIUM-COPPER ALLOYS)Palladium – 70% to 80% Copper – 9% to 15%Gold – 1% to 2% Platinum – 1% Some, but not all, high palladium-copper alloys contain small quantities ( 1% to 2%) of gold and/or platinum. Trace amounts of the oxidizable elements gallium, indium and tin are added for porcelain bonding.Advantages Disadvantages1. Good castability 1. Produces dark, thick oxides2. Lower cost (than gold based alloys) 2. May discolor (gray) some dental 3. Low density means more castings porcelains. Per ounce 3. Must visually evaluate oxide color to 4. Tarnish and corrosion resistance determine if proper adherent oxide was 5. Compatible with many dental formed. Porcelains. 4. Should not be cast in carbon crucibles6. Some are available in one unit ingots. (electric casting machines) 5. Prone to gaseous absorption. 6. Subject to thermal creep. 7. May not be suitable for long span fixed partial denture prosthesis. 8. Little information on long term clinical success. 9. Difficult to polish 10. Resoldering is a problem


BASE METAL ALLOYS


BASE METAL ALLOYS1,3,4,7,9,10,15,16,18,20,23&34

-Nickel based-Cobalt basedAlloys in both systems contain chromium as the second largest constituent. A classification of base metal casting alloys

Base metal Casting alloy

RemovablePartial denture

Co-Cr

Co-Cr-Ni

Ni-Cr

Co-Cr-Mo

Co-Cr-MoSurgical Implant

Ni-Cr

Co-Cr (Class-III)Fixed Partial denture

Be. Cont.(Class-II)

No Be. (Class-I)


Nickel-chromium (Ni-Cr) System1,7

These metal-ceramic alloy offer such economy that they are also used for complete crown and all metal fixed partial denture prosthesis (Bertolotti, 1983). The major constituents are nickel and chromium, with a wide array of minor alloying elements.

The system contains two major groups: -Beryllium free (class 1)

-Beryllium (class 2)

Of the two, Ni-Cr-Beryllium alloy are generally regarded as possessing superior properties and have been more popular (Tuccillo and Cascone,1984).


NICKEL-CHROMIUM BERYLLIUM FREE ALLOYS9,10,23

Composition:

Nickel – 62% to 77% Chromium – 11% to 22%Boron , iron, molybdenum, Niobium or columbium and tantalum (trace elements).

Advantages Disadvantages1. Do not contain beryllium 1. Cannot use with Nickel sensitive patients.2. Low cost 2. Cannot be etched. (Cr doesn’t dissolve in acid)3. Low density means more casting 3. May not cast as well as Ni-Cr-Be alloys per ounce 4. Produces more oxide than Ni-Cr-Be alloys.


NICKEL-CHROMIUM-BERYLLIUM ALLOY9,10,23

Composition:

Nickel – 62% to 82% Chromium – 11% to 20%Beryllium – 2.0% Numerous minor alloying elements include aluminum, carbon, gallium, iron, manganese, molybdenum, silicon, titanium and /or vanadium are present.

Advantages Disadvantages1. Low cost 1. Cannot use with nickel sensitive patients2. Low density, permits more 2. Beryllium exposure may be potentially casting per ounce. harmful to technicians and patients.3. High sag resistance 3. Proper melting and casting is a learned skill.4. Can produce thin casting 4. bond failure more common in the oxide layer.5. Poor thermal conductor 5. High hardness (May wear opposing teeth)6. Can be etched to increase 6. Difficult to solder retention 7. Ingots do not pool 8. Difficult to cut through cemented castings


DISADVANTAGES OF NICKEL-CHROMIUM ALLOYS:

Nickel may produce allergic reactions in some individuals (contact dermatitis). It is also a potential carcinogen. Beryllium which is present in many base metal alloys is a potentially toxic substance.21,23 Inhalation of beryllium containing dust or fumes is the main route of exposure. It causes a condition know as ‘berylliosis’. It is characterized by flu-like symptoms and granulomas of the lungs. Adequate precautions must be taken while working with base metal alloys. Fumes from melting and dust from grinding beryllium-containing alloys should be avoided. The work area should be well ventilated.


Comparative properties of Ni / Cr alloys and type III casting gold alloys for small cast restorations

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.


COBALT CHROMIUM ALLOYS4,6,15,16,22&25

Cobalt chromium alloys have been available since the 1920’s. They possess high strength. Their excellent corrosion resistance especially at high temperatures makes them useful for a number of applications. These alloys are also known as ‘satellite’ because they maintained their shiny, star-like appearance under different conditions. They have bright lustrous, hard, strong and non-tarnishing qualities.APPLICATIONS:1. Denture base2. Cast removable partial denture framework.3. Surgical implants.4. Car spark plugs and turbine blades.COMPOSITION:Cobalt - 55 to 65%Chromium - 23 to 30%Nickel - 0 to 20%Molybdenum - 0 to 7%Iron - 0 to 5%Carbon - upto 0.4%Tungsten, Manganese, Silicon and Platinum in traces. According to A.D.A specification No. 14 a minimum of 85% by weight of chromium, cobalt, and nickel is required. Thus the gold base corrosion resistant alloys are excluded.


1.Physical Properties:Density: The density is half that of gold alloys, so they are lighter in

weight. 8 to 9 gms/cm3.Fusion temperature: The casting temperature of this alloy is

considerably higher than that of gold alloys. 1250oC to 1480oC.

A.D.A. specification No. 14 divides it into two types, based on fusion temperature (which is defined as the liquidus temperature) Type-I (High fusing) – liquidus temperature greater than

1300oC Type-II (Low fusing) – liquidus temperature lower than 1300oC

PROPERTIESThe Cobalt-Chromium alloys have replaced Type IV

gold alloys because of their lower cost and adequate mechanical properties. Chromium is added for tarnish resistance since chromium oxide forms an adherent and resistant surface layer.


2. Mechanical Properties:

Yield strength: It is higher than that of gold alloys. 710Mpa (103,000psi).

Elongation: Their ductility is lower than that of gold alloys. Depending on the composition, rate of cooling, and the fusion and mold temperature employed, it ranges from 1 to 12%.

These alloys work harden very easily, so care must be taken while adjusting the clasp arms of the partial denture.

Modulus of elasticity: They are twice as stiff as gold alloys 22.5103Mpa. Thus, casting can be made more thinner, thus decreasing the weight of the R.P.D. Adjustment of clasp is not easy.

Hardness: These alloys are 50% harder than gold alloys 432 VHN. Thus, cutting, grinding and finishing is difficult.


3. Tarnish and corrosion resistance: Formation of a layer of chromium oxide on the surface of these alloys prevents tarnish and corrosion in the oral cavity.

Solutions of hypochlorite and other compounds that are present in some denture-cleaning agents will cause corrosion in such base metal alloys. Even the oxygenating denture cleansers will stain such alloys. Therefore, these solutions should not be used for cleaning cobalt-chromium base alloys.

4. Casting Shrinkage: The casting shrinkage is much greater than that of gold alloys (2.3%), so limited use in crown & bridge. The high shrinkage is due to their high fusion temperature.

5. Porosity: As in gold alloys, porosity is due to shrinkage and release of dissolved gases which is not true in case of Co-Cr alloys. Porosity is affected by the composition of the alloys and its manipulations.


Comparative properties of Co / Cr alloys and type IV casting gold alloys for partial denture

Property (Units) Co/Cr Type IV gold alloy

Comments

Density (g/cm3) 8-9 15 More difficult to produce defect free casting for Co/Cr alloys but denture frameworks are lighter

Fusion temperature as high as

1500°C

Normally lower than 1000°C

Co/Cr alloys require electrical induction furnace or oxyacetylene equipment.Can not use gypsum bonded investments for Co/Cr alloys

Casting shrinkage (%)

2.3 1.4 Mostly compensated for by correct choice of investment

Tensile strength (MPa)

850 750 Both acceptable

Proportional limit (MPa)

710 500 Both acceptable; can resist stresses without deformation

Modulus of elasticity (GPa)

225 100 Co/Cr more rigid for equivalent thickness; advantage for connectors; disadvantage for clasps

Hardness (Vickers) 432 250 Co/Cr more difficult to polish but retains polish during service

Ductility (% elongation)

2 15 (as cast)8 (hardened)

Co/Cr clasps may fractured if adjustments are attempted.


Summary of base metal alloy propertiesProperty Ni-Cr without

BeNi-Cr with Be Co-Cr

Strength (MPa) 255-550 480-830 415-550

Ultimate tensile

strength (MPa)

550-900 760-1380 550-900

% elongation 5-35 3-25 1-12

Modulus of elasticity

(MPa)

13.8-20.7 x 104 17.2-20.7 x 104 17.2-22.5x104

Vickers hardness

175-350 300-350 300-500

Casting temperature

(°C)

1430-1570 1370-1480 1430-1590


TITANIUM AND TITANIUM ALLOYS4,13,19,45,46&48

Titanium is called “material of choice” in dentistry. This is attributed to the oxide formation property which forms basis for corrosion resistance and biocompatibility of this material. The term 'titanium' is used for all types of pure and alloyed titanium. Properties of titanium:

-Resistance to electrochemical degradation-Begins biological response-Relatively light weight-Low density (4.5 g/cm3)-Low modulus (100 GPa)-High strength (yield strength = 170-480 MPa; ultimate strength = 240-550 MPa)-Passivity-Low coefficient of thermal expansion (8.5 x 10–6/°C)-Melting & boiling point of 1668°C & 3260°C

Uses:Commercially pure titanium is used for dental implants, surface coatings, crowns, partial dentures, complete dentures and orthodontic wires


Commercially Pure Titanium (CP Ti):It is available in four grades (according to American Society for Testing and Materials ASTM) which vary according to the oxygen (0.18-0.40 wt.%), iron (0.20-0.50 wt%) and other impurities. It has got an alpha phase structure at room temperature and converts to beta phase structure at 883°C which is stronger but brittle.


TITANIUM ALLOYS

Alloying elements are added to stabilize alpha or the beta phase by changing beta transformation temperature e.g. in Ti-6Al-4V48, Aluminum is an alpha stabilizer whereas Vanadium as well as copper and palladium are beta stabilizer. Alpha titanium is weld able but difficult to work with at room temperature. Beta titanium is malleable at room temperature and is used in orthodontics, but is difficult to weld.

Pure titanium is used to cast crowns, partial denture, and complete denture.


CAST TITANIUM:Cast titanium has been used for more than 50 years, and it has been recently that precision casting can be obtained from it. The two most important factors in casting titanium based materials are its high melting point (1668°C) and chemical reactivity. Because of the high melting point, special melting procedures, cooling cycles, mold materials, and casting equipments are required to prevent metal contamination, because it readily reacts with hydrogen, oxygen and nitrogen at temperatures greater than 600°C. So casting is done in a vacuum or inert gas atmosphere. The investment materials such as phosphate bonded silica and phosphate investment material with added trace metal are used. It has been shown that magnesium based investment cause internal porosity in casting.


Because of its low density, it is difficult to cast in centrifugal casting machine. So advanced casting machine combining centrifugal, vacuum, pressure and gravity casting with electric arc melting technology have been developed.

Difficulties in casting Titanium :

-High melting point-High reactivity-Low casting efficiency-Inadequate expansion of investment-Casting porosity-Difficulty in finishing-Difficulty in welding-Requires expensive equipments


REVIEW OF LITERATURE

Moffa JP, Guckes AD, Okawa MT and Lilly GE (1973)23 did an evaluation of nonprecious alloys for use with porcelain veneers and provided quantitative information about the levels of beryllium produced during the finishing and polishing of cast base metal dental alloys with there harmful effects.

Shillingburg HT, Hobo S and Fisher DW (1977)39 Studied Preparation design and margin distortion in porcelain-fused-to-metal restorations. The results of this study suggested that thermal incompatibility stresses were likely to cause margin distortion in metal ceramic crowns. However, subsequent studies support other potential mechanisms, including the effect of excessive sand blasting time and/or pressure.

Baran GR (1983)7 did an extensive study on metallurgy of sixteen commercially available Ni-Cr alloys for fixed prosthodontics and compared their alloy compositions, mechanical properties (yield strength, tensile strength, %elongation and hardness number), microstructures and clinically relevant considerations for the use of these alloys.


Carr A.B., Cai Z., Brantley W.A.(1993)11 did a study on new high palladium casting alloys (generation 1&2). For the five high-palladium alloys studied, the following conclusions were drawn:1. An increase in the investment burn out temperature from 1400°F to 1500 °F had little effect on microstructure and hardness, but grain or dendritic size was found to vary substantially.2. Hot tears were more prevalent in the alloys when the higher burnout temperature was used.3. Heat treatment simulating porcelain firing cycles for these alloys generally caused decrease in hardness.

Reisbick NH and Brantley WA (1995)36 conducted a study on mechanical properties and microstructural variations for recasting low gold alloys. They concluded that significant decrease in yield strength and percentage elongation were observed for recasting these alloys but not in tensile strength when the Type III gold alloys were recasted upto 3 times. Scanning electron microscope examination revealed that the number of casting defects (principally porosity) increased with the number of times the alloy was remelted.


Berzins DW, Sarkar NK et al (2000)8 did an in-vitro electrochemical evaluation of high palladium alloys in relation to palladium allergy. The high incidence of allergic reaction was associated with Pd-Cu based alloys. The “Pd-skin” of these alloys when in contact with saliva release some Pd++ ions (an allergen) which can trigger the cascade of biological reaction involved in allergy and hypersensitivity. It is a time dependent process.In Pd alloys containing Ag, formation of Ag-Cl film on the alloy surface is supposed to prevent Pd in coming in contact with oral fluids, having a masking effect and thus avoiding allergy.

Tufekci E, Mitchell JC et al (2002)43 did a study on spectroscopy measurements of elemental release from high palladium dental casting alloys into a corrosion testing medium. A highly sensitive analytical technique shows that the release of individual elements over a one month period, suggesting that there may be low risk of biological reaction with the Pd-Ga alloys than with the Pd-Cu-Ga alloys tested.


Ahmad SAH, Omar MB, Homa D. (2003)1 did an investigation of the cytotoxic effects of commercially available dental casting alloys and concluded the following:

1.The high noble alloy Bioherador N was significantly less cytotoxic than all the base metal alloys tested in this study (Ni-Cr, Co-Cr, Cu-based)2. The Ni-Cr alloy CB Soft was significantly more cytotoxic than all the Ni-Cr and Co-Cr alloys tested. This could be related to the content of Cu, low content of Cr and absence of Mo in its composition.3. Cu based alloys Thermobond showed a more severe cytotoxic reaction than all the other alloys.

O’Brien WJ (2004)29 Biomaterial Properties Database, University of Michigan:

http://www.lib.umich.edu/dentlib/Dental tables/. This database provides an electronic reference to the following properties of dental materials; strength between restorative materials and tooth structures, BHN, coefficient of thermal friction, coefficient of thermal expansion (linear), colours of dental shade guide, contact angles, creep, density, dynamic modulus, elastic modulus, heat of fusion, KHN, melting temperatures and ranges, %elongation, permanent deformities, proportional limit, shear strength, tear energy, tear strength, ultimate compressive strength, VHN and yield strength.

http://www.lib.umich.edu/dentlib/Dental%20tables/




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