Application of metals in Application of metals in prosthodontics with reference to prosthodontics with reference to

solidification range of alloysolidification range of alloy

IntroductionIntroduction

Metals and alloys play a prominent and Metals and alloys play a prominent and important role in dentistry. These materials are important role in dentistry. These materials are used in almost all aspects of dental practice, used in almost all aspects of dental practice, including the dental laboratory, direct and including the dental laboratory, direct and indirect dental restorations, and instruments indirect dental restorations, and instruments used to prepare and manipulate teeth.used to prepare and manipulate teeth.

METALMETAL

Metals Hand book (1992) defines the metal as an Metals Hand book (1992) defines the metal as an opaque lustrous chemical substance that is good opaque lustrous chemical substance that is good conductor of heat and electricity and when polished conductor of heat and electricity and when polished good reflector of light. A clean metallic surface good reflector of light. A clean metallic surface exhibits a luster that is difficult to duplicate in other exhibits a luster that is difficult to duplicate in other types of solid materialtypes of solid material

GPT 8 (2005) defines “METAL” as any strong GPT 8 (2005) defines “METAL” as any strong relatively ductile substance that provides relatively ductile substance that provides electropositive ions to a corrosive environment and electropositive ions to a corrosive environment and that can be polished to a high lustre. Characterized by that can be polished to a high lustre. Characterized by metallic atomic bonding metallic atomic bonding

Occurrence and extractionOccurrence and extraction

Metals can occur either as pure elements or in compound with Metals can occur either as pure elements or in compound with other elements in ores.Examples,Gold occur as the pure other elements in ores.Examples,Gold occur as the pure metal,Silver occurs as the pure metal or silver sulphide or silver metal,Silver occurs as the pure metal or silver sulphide or silver chloride.chloride.

An ore contains the compound of metal, together with unwanted An ore contains the compound of metal, together with unwanted earthy material.Ores often go through the process such as earthy material.Ores often go through the process such as grinding,grading according to size and quality and concentartion grinding,grading according to size and quality and concentartion before conversion to a metal. The various methods of before conversion to a metal. The various methods of producing of metals are producing of metals are

Thermal methods Thermal methods Hydro-metallurgical methodsHydro-metallurgical methods, , Thermo-electrolyticThermo-electrolytic

ALLOYALLOY The use of pure metals is quite limited in dentistry. To The use of pure metals is quite limited in dentistry. To

optimize properties, most metals commonly used in optimize properties, most metals commonly used in engineering and dental applications are mixtures of two or engineering and dental applications are mixtures of two or more metallic elements or in some cases one or more metals more metallic elements or in some cases one or more metals and/or nonmetals. They are generally prepared by fusion of the and/or nonmetals. They are generally prepared by fusion of the elements above their melting points. A solid material formed elements above their melting points. A solid material formed by combining a metal with one or more other metals or by combining a metal with one or more other metals or nonmetals is called an alloy. nonmetals is called an alloy.

An alloy is a mixture of two or more metals or metalloids An alloy is a mixture of two or more metals or metalloids that are mutually soluble in the molten state; distinguishable as that are mutually soluble in the molten state; distinguishable as binary, ternary,quaternary, etc., depending on the number of binary, ternary,quaternary, etc., depending on the number of metals with the mixture. Alloying elements are added to alter metals with the mixture. Alloying elements are added to alter the hardness, strength, and toughness of a metallic element, the hardness, strength, and toughness of a metallic element, thus obtaining properties found in pure metal (GPT-7).thus obtaining properties found in pure metal (GPT-7).

HISTORY OF METALS IN DENTISTRYHISTORY OF METALS IN DENTISTRY

Dentistry as a specialty is believed to have begun about 3000 Dentistry as a specialty is believed to have begun about 3000 BC. Gold bands and wires were used by the Phoenicians after 2500 BC. Gold bands and wires were used by the Phoenicians after 2500 BC.BC.

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

Year Year EventEvent

1907 Introduction of Lost-Wax Technique.1907 Introduction of Lost-Wax Technique. 1933 Replacement of Co-Cr for Gold in Removable1933 Replacement of Co-Cr for Gold in Removable Partial Dentures.Partial Dentures. 1950 Development of Resin Veneers for Gold Alloys 1950 Development of Resin Veneers for Gold Alloys 1959 Introduction of the Porcelain Fused-to metal1959 Introduction of the Porcelain Fused-to metal Technique.Technique. 1968 Palladium-Based Alloys as Alternatives to gold1968 Palladium-Based Alloys as Alternatives to gold Alloy.Alloy. 1971 Nickel-Based Alloys as Alternatives to Gold 1971 Nickel-Based Alloys as Alternatives to Gold

Alloys.Alloys. 1980 Introduction of All-Ceramic Technologies. 1980 Introduction of All-Ceramic Technologies. 1999 Gold Alloys as Alternatives to Palladium-Base1999 Gold Alloys as Alternatives to Palladium-Base Alloys.Alloys.

Classification Of Metals and AlloysClassification Of Metals and Alloys

According to periodic chart of elements out of 103 According to periodic chart of elements out of 103 elements approximately 81 are metals elements approximately 81 are metals arsenic,silicon,boron and others are metalloids arsenic,silicon,boron and others are metalloids (located between metals and non metals on the (located between metals and non metals on the periodic table)all the remaining elements are non periodic table)all the remaining elements are non metals. It is of scientific interest that the metallic metals. It is of scientific interest that the metallic elements can be grouped according to density, elements can be grouped according to density, ductility, melting point and nobility.Several metals of ductility, melting point and nobility.Several metals of importance for dental alloys are transition elements, importance for dental alloys are transition elements, in which the outermost electron subshells are in which the outermost electron subshells are occupied before the interior subshells are completely occupied before the interior subshells are completely filled.filled.

Metals can be broadly classified according to Metals can be broadly classified according to composition ascomposition as

NOBLE METALSNOBLE METALS • The term noble identifies elements in terms of their The term noble identifies elements in terms of their

chemical stability i.e., they resist oxidation and are chemical stability i.e., they resist oxidation and are impervious to acids.impervious to acids.

• Gold, Platinum, Palladium, Rhodium, Ruthenium, Gold, Platinum, Palladium, Rhodium, Ruthenium, Iridium, Osmium, and Silver are the eight noble Iridium, Osmium, and Silver are the eight noble metals. metals.

• In the oral cavity In the oral cavity Silver is more reactiveSilver is more reactive and and therefore is not considered as a noble metal. therefore is not considered as a noble metal.

PRECIOUS METALSPRECIOUS METALS The term “precious” merely indicates whether The term “precious” merely indicates whether

a metal has intrinsic value, the noble metals a metal has intrinsic value, the noble metals (all eight) are also precious metals (all eight) are also precious metals

All noble metals are precious but all precious All noble metals are precious but all precious metals are not noble.metals are not noble.

Silver is usually the major ingredient in most Silver is usually the major ingredient in most alloys considered as precious.alloys considered as precious.

BASE METALSBASE METALS These are Ignoble elements. These remain These are Ignoble elements. These remain

invaluable components of dental casting alloys invaluable components of dental casting alloys because of their influence on physical because of their influence on physical properties, control of the amount and type of properties, control of the amount and type of oxidation, or for their strengthening effects.oxidation, or for their strengthening effects.

e.g., Chromium, Cobalt, Nickel, Iron, e.g., Chromium, Cobalt, Nickel, Iron, titanium,berillieum,Copper etc.titanium,berillieum,Copper etc.

Another Classification Of MetalsAnother Classification Of Metals Light Metal – e.g., Al.Light Metal – e.g., Al. Heavy Metal – e.g., Fe.Heavy Metal – e.g., Fe. High Melting Metal – e.g., Co, Cr.High Melting Metal – e.g., Co, Cr. Low Melting Metal – e.g., Sn.Low Melting Metal – e.g., Sn. High Ductile and Malleable metal – e.g., Au.High Ductile and Malleable metal – e.g., Au.

1. ALLOY TYPES BY FUNCTIONS1. ALLOY TYPES BY FUNCTIONS:: In 1927, the Bureau of Standard established gold casting alloys, type I In 1927, the Bureau of Standard established gold casting alloys, type I

to type IV according to dental function with hardness increasing from to type IV according to dental function with hardness increasing from type I to type IV. type I to type IV.

Type I (Soft):Type I (Soft): It is used for fabrication of small inlays, class III and class V It is used for fabrication of small inlays, class III and class V

restorations which are not subjected to great stress . These alloys restorations which are not subjected to great stress . These alloys are easily burnishable.are easily burnishable.

Type -II (Medium):Type -II (Medium): These are used for fabrication of inlays subjected to moderate These are used for fabrication of inlays subjected to moderate

stress, thick 3/4 crowns, abutments, pontics, full crowns and soft stress, thick 3/4 crowns, abutments, pontics, full crowns and soft saddles.saddles.

Type -III (Hard):Type -III (Hard): It is used for fabrication of inlays subjected to high stress, thin 3/4 It is used for fabrication of inlays subjected to high stress, thin 3/4

crowns, thin cast backing abutments, pontics, full crowns, denture crowns, thin cast backing abutments, pontics, full crowns, denture bases and short span FPDs . bases and short span FPDs .

Type-IV (Extra hard):Type-IV (Extra hard): It is used for fabrication of inlays subjected to high stress, denture It is used for fabrication of inlays subjected to high stress, denture

bases, bars and clasps, partial denture frameworks and long span bases, bars and clasps, partial denture frameworks and long span FPDs. These alloys can be age hardened by heat treatment.FPDs. These alloys can be age hardened by heat treatment.

Later, in 1960, metal ceramic alloys were introduced Later, in 1960, metal ceramic alloys were introduced and removable partial denture alloys were added in and removable partial denture alloys were added in this classification. this classification.

Metal ceramic alloys (hard and extra hard):Metal ceramic alloys (hard and extra hard): It is suitable for veneering with dental porcelain, It is suitable for veneering with dental porcelain,

copings, thin walled crowns, short span FPDs and copings, thin walled crowns, short span FPDs and long span FPDs. These alloy vary greatly in long span FPDs. These alloy vary greatly in composition and may be gold, palladium, nickel or composition and may be gold, palladium, nickel or cobalt based. cobalt based.

Removable partial denture alloysRemovable partial denture alloys : : It is used for removable partial denture frameworks. It is used for removable partial denture frameworks.

Now a days, light weight, strong and less expensive Now a days, light weight, strong and less expensive nickel or cobalt based have replaced type IV alloys .nickel or cobalt based have replaced type IV alloys .

2. ALLOY TYPES BY DESCRIPTION:2. ALLOY TYPES BY DESCRIPTION:

A)A)CROWN AND BRIDGE ALLOYSCROWN AND BRIDGE ALLOYS This category of alloys include both noble and base metal This category of alloys include both noble and base metal

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

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

low gold alloyslow gold alloys ii)ii) Non-gold based alloy-Silver -palladium alloyNon-gold based alloy-Silver -palladium alloy 2.2. Base metal alloys:Base metal alloys: i)Nickel-based alloysi)Nickel-based alloys ii)ii) Cobalt based alloysCobalt based alloys 3.3. Other alloys:Other alloys: i)Copper-zinc with Indium and nickeli)Copper-zinc with Indium and nickel ii)Silver-indium with palladiumii)Silver-indium with palladium

B) METAL CERAMIC ALLOYB) METAL CERAMIC ALLOYNoble metal alloys for porcelain bondingNoble metal alloys for porcelain bonding::• Gold-platinum -palladium alloyGold-platinum -palladium alloy• Gold-palladium-silver alloyGold-palladium-silver alloy• Gold-palladium alloyGold-palladium alloy• Palladium silver alloyPalladium silver alloy• High palladium alloyHigh palladium alloyBase metal alloys for porcelain bondingBase metal alloys for porcelain bonding:: Nickel -chromium alloyNickel -chromium alloy Cobalt-chromium alloyCobalt-chromium alloy

J. Robert Kelly and Thomas C.Rose in 1983 discussed J. Robert Kelly and Thomas C.Rose in 1983 discussed

the various physical properties, biocompatibility, the various physical properties, biocompatibility,

porcelain bonding and corrosion resistance of various porcelain bonding and corrosion resistance of various

non precious alloys.non precious alloys.

They concluded that though the manipulation of non They concluded that though the manipulation of non

precious alloys is technique sensitive and exacting, precious alloys is technique sensitive and exacting,

their better physical properties and clinical their better physical properties and clinical

performance merited considerationperformance merited consideration ..

JPD 1983,VOL49,N03

c)c) REMOVABLE PARTIAL DENTURE ALLOYREMOVABLE PARTIAL DENTURE ALLOY

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

1. Cobalt-chromium alloy1. Cobalt-chromium alloy

2. Nickel-chromium alloy2. Nickel-chromium alloy

3.3. Cobalt-chromium-nickel alloy Cobalt-chromium-nickel alloy

4.4. Silver-palladium alloy Silver-palladium alloy

5.5. Aluminum -bronze alloy Aluminum -bronze alloy

Kamal Asgar, (1970) evalvated the properties of Kamal Asgar, (1970) evalvated the properties of new alloy (vitallium, Ticonium) for partial new alloy (vitallium, Ticonium) for partial denture the benefits of these alloys are denture the benefits of these alloys are

partial denture clasps could last longer, partial denture clasps could last longer, reduced hardness facilitates ajustments partial reduced hardness facilitates ajustments partial

dentures, dentures, wear on natural teeth is reduced. wear on natural teeth is reduced. JPD 1970 Vol- 23 No. 1.

3.ALLOY TYPE BY NOBILITY3.ALLOY TYPE BY NOBILITY Alloy Classification of the American Dental Association Alloy Classification of the American Dental Association

(1984) classified according to nobility as High noble, noble, (1984) classified according to nobility as High noble, noble, and predominantly base metaland predominantly base metal

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

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

5.ALLOY TYPE BY PRINCIPAL THREE ELEMENTS5.ALLOY TYPE BY PRINCIPAL THREE ELEMENTS:: Such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, 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 Ti-Al-V and Fe-Ni-Cr.(If two metals are present, a binary alloy is formed; if three or four metals are binary alloy is formed; if three or four metals are present, ternary and quaternary alloys, respectively, present, ternary and quaternary alloys, respectively, are produced and so on.) are produced and so on.)

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

General characteristics and properties of metalsGeneral characteristics and properties of metals

A metal is an element that ionizes positively in A metal is an element that ionizes positively in solution.solution.

Metals have certain typical and characteristic Metals have certain typical and characteristic properties that tend to identify and distinguish them properties that tend to identify and distinguish them from the nonmetallic elements, such as luster, opacity, from the nonmetallic elements, such as luster, opacity, density, thermal and electrical conductivity. density, thermal and electrical conductivity.

Extreme ductility and malleability are often desirable Extreme ductility and malleability are often desirable

in metals used in dentistry and these are found to in metals used in dentistry and these are found to predominate in pure metals rather than in alloys.predominate in pure metals rather than in alloys.

Metals have high fracture toughness which is Metals have high fracture toughness which is described by the property hardness of metal or described by the property hardness of metal or alloysalloys

HARDNESSHARDNESSIn mineralogy the hardness is described on the basis of In mineralogy the hardness is described on the basis of

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

Brinell hardness number ( BHN )Brinell hardness number ( BHN ) Rockwell hardness number ( RHN )Rockwell hardness number ( RHN ) Vickers hardness test (VHN )Vickers hardness test (VHN ) Knoop hardness test ( KHN ) Knoop hardness test ( KHN ) JPD 2002, Vol 87, No 4, Pg 351 – 363.JPD 2002, Vol 87, No 4, Pg 351 – 363.

Solid metals are stronger and denser than other non Solid metals are stronger and denser than other non metallic elementsmetallic elements

Some metals require alloying elements to resist Some metals require alloying elements to resist tarnish and corrosiontarnish and corrosion

passivationpassivation

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

Microscopic Structure Of MetalsMicroscopic Structure Of Metals

Most metals have crystalline structure in solid Most metals have crystalline structure in solid state which are held together by metallic state which are held together by metallic bonds.bonds.

METALLIC BOND FORMATION

•Metal is a crystalline material that consists of

positively charged ions in an ordered, closely

packed arrangement and bonded with a cloud of

electrons. This type of bond, called a metallic

bond, is responsible for many of the properties of

metals-electrical and thermal conductivity,

metallic luster, and (usually) high strength

Solidification Of Pure MetalSolidification Of Pure Metal

Pure metal has a melting point-known as Fusion Temperature, Pure metal has a melting point-known as Fusion Temperature, and has specific heat. and has specific heat.

To melt a crystalline substance (metal) some what more heat To melt a crystalline substance (metal) some what more heat energy is required to convert it from solid to liquid. This extra energy is required to convert it from solid to liquid. This extra heat is stored away within the atoms in the form of latent heat heat is stored away within the atoms in the form of latent heat of fusion.of fusion.

When the solid metal changes into liquid, its crystalline When the solid metal changes into liquid, its crystalline pattern disappears, and the atoms are randomly distributed in pattern disappears, and the atoms are randomly distributed in the mass of liquid and they have more energy and are the mass of liquid and they have more energy and are therefore move about freely. In the reverse process of therefore move about freely. In the reverse process of changing into solid, temperature of the melt goes gradually changing into solid, temperature of the melt goes gradually (cooling); atoms make an attempt to reform the crystalline (cooling); atoms make an attempt to reform the crystalline arrangementarrangement

Mechanism of CrystallizationMechanism of Crystallization Metals usually have crystalline structures in the solid Metals usually have crystalline structures in the solid

state.When a molten metal or alloy is cooled, the solidification state.When a molten metal or alloy is cooled, the solidification process is one of crystallization and is initiated at specific sites process is one of crystallization and is initiated at specific sites called called nuclei.nuclei.

The nuclei are formed from impurities within the molten mass The nuclei are formed from impurities within the molten mass of metal. The initial nuclei are small in size and few in number of metal. The initial nuclei are small in size and few in number known as EMBRYO, which do not stabilize in the melt and known as EMBRYO, which do not stabilize in the melt and soon disappearsoon disappear

As the temperature of the metal gradually goes down, a stable As the temperature of the metal gradually goes down, a stable NUCLEUS is formed.Such nucleus formations are called NUCLEUS is formed.Such nucleus formations are called DENDRITESDENDRITES,,

Crystals grow as dendrites, which can be described as three-Crystals grow as dendrites, which can be described as three-dimensional, branched network structures emanating from the central dimensional, branched network structures emanating from the central nucleusnucleus

Crystal growth continue until all the material has solidified and all the Crystal growth continue until all the material has solidified and all the dendrite crystals are in contact.dendrite crystals are in contact.

The metal is therefore made of thousands of tiny crystals, The metal is therefore made of thousands of tiny crystals, such a metal is called polycrystalline and each crystal in such a metal is called polycrystalline and each crystal in the structure is called a the structure is called a GRAIN.GRAIN.

The area between two grains in contact is the The area between two grains in contact is the grain grain boundaryboundary

After crystallization, the grains, have approximately After crystallization, the grains, have approximately the same dimensions in each direction, measured the same dimensions in each direction, measured from the central nucleus. from the central nucleus.

They are not perfectly spherical or cubic however, They are not perfectly spherical or cubic however, nor do they conform to any other geometric shape. nor do they conform to any other geometric shape. They are said to have an They are said to have an equiaxed grain structureequiaxed grain structure..

The atoms with in each grain are arranged in regular The atoms with in each grain are arranged in regular three dimensional latticethree dimensional lattice

A space lattice can be defined as any arrangement of A space lattice can be defined as any arrangement of atoms in space such that every atom is situated atoms in space such that every atom is situated similarly to every other atom. It is also called a similarly to every other atom. It is also called a crystal. There are 14 possible lattice types or forms, crystal. There are 14 possible lattice types or forms, but many of the metals used in dentistry belong to the but many of the metals used in dentistry belong to the cubic system arrangement.cubic system arrangement.

o simple cubic,simple cubic,o body centered cubic, body centered cubic, o face centered cubic, face centered cubic, o Close packed hexagonalClose packed hexagonal

The arrangement adopted by any one crystal depends on The arrangement adopted by any one crystal depends on specific factors such as atomic radius and charge specific factors such as atomic radius and charge distributions on the atoms. Although there is a tendency distributions on the atoms. Although there is a tendency towards a perfect crystal structure, occasional defects occurstowards a perfect crystal structure, occasional defects occurs

a)imperfection in a crystal structureb)under the influence of sufficient

force atoms may move to establish a more perfect arrangement

Such defects are normally referred to as Such defects are normally referred to as dislocations dislocations and their and their occurrence has an effect on the ductility of the metal occurrence has an effect on the ductility of the metal

When the material is placed under a sufficiently high stress the When the material is placed under a sufficiently high stress the dislocation is able to move through the lattice until it reaches a dislocation is able to move through the lattice until it reaches a grain boundary. The plane along which the dislocation moves is grain boundary. The plane along which the dislocation moves is called a called a slip planeslip plane and the stress required to initiate movement is and the stress required to initiate movement is the elastic limit.the elastic limit.

Grain boundaries form a natural barrier to the Grain boundaries form a natural barrier to the movement of dislocations. movement of dislocations.

The concentration of grain boundaries increases The concentration of grain boundaries increases as the grain size decreases.as the grain size decreases.

Metals with finer grain structure are generally Metals with finer grain structure are generally harder and have higher values of elastic limit harder and have higher values of elastic limit than those with coarser grain structure.than those with coarser grain structure.

Hence it can be seen that material properties can be Hence it can be seen that material properties can be controlled to some extent by controlling the grain controlled to some extent by controlling the grain size. size.

A fine grain structure can be achieved by rapid A fine grain structure can be achieved by rapid cooling of the molten metal or alloy following cooling of the molten metal or alloy following casting. This process, often referred to as casting. This process, often referred to as quenchingquenching, , ensures that many nuclei of crystallization are ensures that many nuclei of crystallization are formed, resulting in a large number of relatively formed, resulting in a large number of relatively small grains. small grains.

Small grains have been found to improve the Small grains have been found to improve the elongation and tensile strength of cast gold alloys.elongation and tensile strength of cast gold alloys.

Slow cooling causes relatively few nuclei to be Slow cooling causes relatively few nuclei to be formed which results in a larger grain sizeformed which results in a larger grain size

JPD 2002, Vol 87, No 4, Pg 351 – 363.JPD 2002, Vol 87, No 4, Pg 351 – 363.

Grain size is determined by number of variablesGrain size is determined by number of variables.. cooling rate of the solidifying alloy.cooling rate of the solidifying alloy. Presence of special grain refining elements in Presence of special grain refining elements in

the alloy composition. the alloy composition. Grain size of 30 micrometer or less is desirable Grain size of 30 micrometer or less is desirable

in dental alloys in dental alloys JPD 2002, Vol 87, No 4, Pg 351 – 363.JPD 2002, Vol 87, No 4, Pg 351 – 363.

A)RAPID COOLINGA)RAPID COOLING

B)SLOW COOLINGB)SLOW COOLING

HARDENING OF METALHARDENING OF METAL

COLD WORKING COLD WORKING when the stress is greater than the elastic limit and at when the stress is greater than the elastic limit and at

relatively low temperatures the cold working not only relatively low temperatures the cold working not only produces a change in microstructure, with produces a change in microstructure, with dislocations becoming concentrated at grain dislocations becoming concentrated at grain boundaries, but also a change in grain shape.boundaries, but also a change in grain shape.

The grains are no longer equiaxed but take up a more The grains are no longer equiaxed but take up a more fibrous structurefibrous structure

the properties of material are altered, becoming the properties of material are altered, becoming harder and stronger and the ductility and malleability harder and stronger and the ductility and malleability is decreasedis decreased

Cold working is sometimes referred to as work hardening due to the effect on mechanical properties.

When mechanical work is carried out on a metal or alloy at a more elevated temperature it is possible for the object to change shape without any alteration in grain shape or mechanical properties.

examples of cold working in dentistry are formation of wires, bending of wires or clasps during the construction and alteration of appliances.

The temperature below which work hardening is The temperature below which work hardening is possible is termed the possible is termed the recrystallization temperature.recrystallization temperature.

If the material is maintained above the If the material is maintained above the recrystallization temperature for sufficient time, recrystallization temperature for sufficient time, diffusion of atoms across grain boundaries may diffusion of atoms across grain boundaries may occur, leading to occur, leading to grain growthgrain growth. .

It is clear that grain growth should be avoided if the It is clear that grain growth should be avoided if the

properties are not to be adversely affected. properties are not to be adversely affected.

AnnealingAnnealing

It is process of heating a metal to reverse the It is process of heating a metal to reverse the effects associated with cold working such as effects associated with cold working such as strain hardening, low ductility and distorted strain hardening, low ductility and distorted grains.grains.

In general it has 3 stages.In general it has 3 stages.1) Recovery1) Recovery2) Recrystallization2) Recrystallization3) Grain growth.3) Grain growth.

Cold working may cause the formation of Cold working may cause the formation of internal stresses within a metal object. If these internal stresses within a metal object. If these stresses are gradually relieved they may cause stresses are gradually relieved they may cause distortion which could lead to loss of fit of, for distortion which could lead to loss of fit of, for example, an orthodontic appliance. example, an orthodontic appliance.

For certain metals and alloys the internal For certain metals and alloys the internal stresses can be wholly or partly eliminated by stresses can be wholly or partly eliminated by using a low temperature heat treatment using a low temperature heat treatment referred to as referred to as stress relief annealingstress relief annealing. .

Structure and phase diagrams for alloysStructure and phase diagrams for alloys

When two metals are completely miscible in When two metals are completely miscible in liquid state, they are capable of forming any liquid state, they are capable of forming any alloy. When such a combination is cooled, one alloy. When such a combination is cooled, one of the three possibilities may take place of the three possibilities may take place

a) Solid solutiona) Solid solution b) Intermetallic compoundb) Intermetallic compound c) Eutectic formationc) Eutectic formation

SOLID SOLUTIONS OR ISOMORPHOUS SOLID SOLUTIONS OR ISOMORPHOUS STATE OR SINGLE PHASESTATE OR SINGLE PHASE

The two metals are said to be mutally soluble in the solid The two metals are said to be mutally soluble in the solid state ,and the alloys are called state ,and the alloys are called solid solutionsolid solution..

It consists of a solute and a solvent. It consists of a solute and a solvent.

Solvent is that metal whose space lattice persists and solute is the Solvent is that metal whose space lattice persists and solute is the other metal. other metal.

E.g. E.g. Au – AgAu – Ag

Au – CuAu – Cu

Au – PtAu – Pt

Au – PdAu – Pd

Ag – PdAg – Pd

The solid solution may take one of three forms. The solid solution may take one of three forms. It may be a It may be a random solid solutionrandom solid solution in which the in which the component metal atoms occupy random sites component metal atoms occupy random sites in a common crystal latticein a common crystal lattice

Another possibility is the formation of an Another possibility is the formation of an ordered solid solutionordered solid solution in which component in which component metal atoms occupy specific sites within a metal atoms occupy specific sites within a common crystal lattice. common crystal lattice.

The third type of solid solution is the The third type of solid solution is the interstitial solid solutioninterstitial solid solution in which, for binary alloys, the primary lattice sites are in which, for binary alloys, the primary lattice sites are occupied by one metal atom and the atoms of the second occupied by one metal atom and the atoms of the second component do not occupy lattice sites but lie within the component do not occupy lattice sites but lie within the interstices of the lattice. This is normally found where the interstices of the lattice. This is normally found where the atomic radius of one component is much smaller than that atomic radius of one component is much smaller than that of the other.of the other.

Conditions Favoring Solid-SolubilityConditions Favoring Solid-Solubility

Atom size - Atom size - if the atom sizes of the mixing metal are same, it if the atom sizes of the mixing metal are same, it will produce solid solution type of alloy.will produce solid solution type of alloy.

Valency - Valency - metals of the same valency will produce solid-metals of the same valency will produce solid-solution alloy.solution alloy.

Crystal structure- Crystal structure- Only metals with the same type of crystal Only metals with the same type of crystal lattice can form a series of solid solutionslattice can form a series of solid solutions..

Chemical affinity - Chemical affinity - must be less to produce solid-solution must be less to produce solid-solution alloy.alloy.

PROPERTIES OF A SOLID SOLUTION PROPERTIES OF A SOLID SOLUTION ALLOYALLOY

The solid solution possesses: The solid solution possesses: Increased hardnessIncreased hardness Increased strengthIncreased strength Increased proportional limitIncreased proportional limit Decreased ductilityDecreased ductility Decreased resistance to corrosion due to Decreased resistance to corrosion due to

coringcoring Melting range rather than a pointMelting range rather than a point

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

EQUILIBRIUMEQUILIBRIUM PHASE DIAGRAM FOR ALLOYSPHASE DIAGRAM FOR ALLOYS

Liquidus temperatureaSolidus temperatureLiquidusSolidus

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

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

PHASE DIAGRAM OF A SOLID SOLUTION PHASE DIAGRAM OF A SOLID SOLUTION

ALLOYALLOY 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 state Point 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.

CORINGCORING In the coring process the last liquid to solidify is metal In the coring process the last liquid to solidify is metal

with lower solidus temperature and solidifies between with lower solidus temperature and solidifies between the dendrites. the dendrites.

Thus under rapid freezing conditions, the alloy has a Thus under rapid freezing conditions, the alloy has a cored structurecored structure. .

The The corecore consists of the dendrites composed of consists of the dendrites composed of compositions with higher solidus temperature, and the compositions with higher solidus temperature, and the matrix matrix is the portion of the micro-structure between the is the portion of the micro-structure between the dendrites that contains compositions with lower solidus dendrites that contains compositions with lower solidus temperatures. temperatures.

Since coring may markedly reduce the corrosion Since coring may markedly reduce the corrosion resistance of some alloys, a heat treatment is some resistance of some alloys, a heat treatment is some times used to eliminate the cored structure. Such a heat times used to eliminate the cored structure. Such a heat treatment is termed a treatment is termed a homogenization heat treatmenthomogenization heat treatment

For homogenization heat treatment, the cast alloy is held For homogenization heat treatment, the cast alloy is held at a temperature near its solidus to achieve the maximum at a temperature near its solidus to achieve the maximum

amount of diffusion without melting. (This process amount of diffusion without melting. (This process required 6 hr. for the alloy). Little or no grain growth required 6 hr. for the alloy). Little or no grain growth

occurs when a casting receives this type of heat treatment occurs when a casting receives this type of heat treatment eg. Annealing eg. Annealing done mainly for wrought alloys . The done mainly for wrought alloys . The

ductility of an alloy usually increases after ductility of an alloy usually increases after homogenization heat treatment .homogenization heat treatment .

EUTECTIC ALLOYSEUTECTIC ALLOYS

Many binary alloy systems do not exhibit complete Many binary alloy systems do not exhibit complete solubility in both the liquid and the solid states. solubility in both the liquid and the solid states.

The eutectic system is an example of an alloy for The eutectic system is an example of an alloy for which the component metals have limited solid which the component metals have limited solid solubility. solubility.

Two metals, A and B, which are completely insoluble Two metals, A and B, which are completely insoluble in each other in the solid state, provide the simplest in each other in the solid state, provide the simplest illustration of a eutectic alloy. illustration of a eutectic alloy.

The term eutectic means The term eutectic means lowest melting point.lowest melting point.

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.

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

PROPERTIES OF EUTECTIC ALLOYSPROPERTIES OF EUTECTIC ALLOYS

Since there is a heterogeneous composition, Since there is a heterogeneous composition, they are susceptible to electrolytic corrosion.they are susceptible to electrolytic corrosion.

They are brittle, because the present of They are brittle, because the present of insoluble phases inhibits slip.insoluble phases inhibits slip.

They have a low melting point and therefore They have a low melting point and therefore are important as solders.are important as solders.

PERITECTIC ALLOYSPERITECTIC ALLOYS

• Peritectic is a phase where there is limited Peritectic is a phase where there is limited solid solubility. solid solubility.

• They are not of much use in dentistry except They are not of much use in dentistry except for silver tin system. for silver tin system.

• This type of reaction occurs when there is a This type of reaction occurs when there is a big differences in the melting points of the big differences in the melting points of the components. components.

Liquid + solid solution solid solution

Inter metallic CompoundsInter metallic Compounds

Inter metallic compounds are those when the metals Inter metallic compounds are those when the metals are soluble in the liquid state but unite and form a are soluble in the liquid state but unite and form a chemical compound on solidifying.chemical compound on solidifying.

E.g. E.g. Ag3 – Sn,Ag3 – Sn, Sn7 – Hg8Sn7 – Hg8

They are called inter metallic compounds because the They are called inter metallic compounds because the alloy is formed by a chemical reaction between a alloy is formed by a chemical reaction between a metal and metal. metal and metal.

Application of metals and alloys in prosthodonticsApplication of metals and alloys in prosthodontics

NOBLE METALSNOBLE METALS The noble metals have been the basis of inlays, The noble metals have been the basis of inlays,

crowns and bridges because of their resistance to crowns and bridges because of their resistance to corrosion in the oral cavity.corrosion in the oral cavity.

Gold, platinum, palladium, rhodium, ruthenium, Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver are the eight noble metalsiridium, osmium, and silver are the eight noble metals

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

Solidification temperature of metals in detistrySolidification temperature of metals in detistry

GOLDGOLD Pure gold is a soft and ductile metal with a yellow Pure gold is a soft and ductile metal with a yellow

“Gold” hue. “Gold” hue. It has a density of 19.3 gms/cm3 , melting point of It has a density of 19.3 gms/cm3 , melting point of

1063oC, boiling point of 2970 oC and CTE of 1063oC, boiling point of 2970 oC and CTE of 14.2×10-6/°C.14.2×10-6/°C.

Gold has a good luster and takes up a high polish. It Gold has a good luster and takes up a high polish. It has good chemical stability and provides a high level has good chemical stability and provides a high level of corrosion and tarnish resistance , increases an of corrosion and tarnish resistance , increases an alloy's melting range slightly, improves workability, alloy's melting range slightly, improves workability, burnish ability, and raises the density .However, gold burnish ability, and raises the density .However, gold imparts a very pleasing yellow color to an alloy (if imparts a very pleasing yellow color to an alloy (if present in sufficient quantity)present in sufficient quantity)

UsesUses Majority of alloys for prosthodontic applications Majority of alloys for prosthodontic applications

were gold based in the pre 1975 erawere gold based in the pre 1975 era Gold alloys are used for Gold alloys are used for

inlays ,onlays,crowns ,bridges and partial inlays ,onlays,crowns ,bridges and partial denture frameworkdenture framework

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

MaterialMaterial DensityDensity(g/cm(g/cm33))

Hardness (VHN/BHN)Hardness (VHN/BHN)(kg/mm(kg/mm22))

TensileTensileStrength Strength

(MPa)(MPa)

ElongationElongation(%)(%)

Cast 24k goldCast 24k gold 19.319.3 28(VHN)28(VHN) 105105 3030

Cast 22k goldCast 22k gold ---- 60(VHN)60(VHN) 240240 2222

Coin goldCoin gold ---- 85 (BHN)85 (BHN) 395395 3030

Typical Au-basedTypical Au-basedcasting alloy casting alloy

(70 wt% Au)*(70 wt% Au)*

15.615.6 135/195(VHN)135/195(VHN) 425/525425/525 30/1230/12

Condensed goldCondensed goldfoilfoil

19.119.1 60 (VHN)60 (VHN) 250250 12.812.8

JPD 2002, Vol 87, No 4, Pg 351 – 363.JPD 2002, Vol 87, No 4, Pg 351 – 363.

SILVERSILVER It is sometimes described as the “Whitest” of all It is sometimes described as the “Whitest” of all

metals. It whitens the alloy, thus helping to counteract metals. It whitens the alloy, thus helping to counteract the reddish colour of copper. To a slight extent it the reddish colour of copper. To a slight extent it increases strength and hardness. It has lowest density increases strength and hardness. It has lowest density 10.4gms/cm3 and melting point of 961oC, boiling 10.4gms/cm3 and melting point of 961oC, boiling point of 2216 oC among the four precious metals point of 2216 oC among the four precious metals used in dental casting alloys. Its CTE is used in dental casting alloys. Its CTE is 19.719.710-6/oC , which is comparatively high10-6/oC , which is comparatively high

AdvantagesAdvantages lowers the melting range, improves fluidity, and helps lowers the melting range, improves fluidity, and helps

to control the coefficient of thermal expansion in to control the coefficient of thermal expansion in gold- and palladium-based alloysgold- and palladium-based alloys

disadvantagesdisadvantages Silver-containing porcelain alloys have been known to induce Silver-containing porcelain alloys have been known to induce

discoloration (yellow, brown, or green) with some porcelains. discoloration (yellow, brown, or green) with some porcelains. Silver possesses a high affinity for oxygen absorption, which Silver possesses a high affinity for oxygen absorption, which can lead to casting porosity can lead to casting porosity

However, small amounts of zinc or indium added to gold However, small amounts of zinc or indium added to gold and silver-based alloys help to control silver's absorption of and silver-based alloys help to control silver's absorption of oxygen. oxygen.

Silver will also corrode and tarnish in the presence of Silver will also corrode and tarnish in the presence of sulfur. Although silver is a precious element, it is not sulfur. Although silver is a precious element, it is not universally regarded as noble in the oral cavityuniversally regarded as noble in the oral cavity

PLATINUMPLATINUM

. Platinum increases the strength, melting range, . Platinum increases the strength, melting range, and hardness of gold-based alloys while and hardness of gold-based alloys while improving their corrosion, tarnish, and sag improving their corrosion, tarnish, and sag resistance.It whitens an alloy and increases the resistance.It whitens an alloy and increases the density of non gold-based metals because of density of non gold-based metals because of its high density.its high density.

PALLADIUMPALLADIUM It is less expensive than platinum, thus reducing cost It is less expensive than platinum, thus reducing cost

of alloy. increase the strength, hardness (with of alloy. increase the strength, hardness (with copper), corrosion and tarnish resistance of gold-copper), corrosion and tarnish resistance of gold-based alloys.based alloys.

will elevate an alloy's melting range and improve its will elevate an alloy's melting range and improve its sag resistance. sag resistance.

has a very strong whitening effecthas a very strong whitening effect possesses a high affinity for hydrogen, oxygen, and possesses a high affinity for hydrogen, oxygen, and

carbon.carbon. lowers the density of the gold-based alloys slightly lowers the density of the gold-based alloys slightly

but has little similar effect on silver-based metals. but has little similar effect on silver-based metals.

IRIDIUM, RUTHENIUMIRIDIUM, RUTHENIUM They help to decrease the grain size. They are added in very They help to decrease the grain size. They are added in very

small quantities . small quantities . IRIDIUM has a high melting point of 2410°C .IRIDIUM has a high melting point of 2410°C . RUTHENIUM has a melting point of 2310°C.these elements RUTHENIUM has a melting point of 2310°C.these elements

do not melt during the casting of the alloy and serve as do not melt during the casting of the alloy and serve as nucleating centers for the melt as it cools,resulting in a fine nucleating centers for the melt as it cools,resulting in a fine grained alloy. grained alloy.

BASE METALSBASE METALS

These are Ignoble elements. These remain These are Ignoble elements. These remain invaluable components of dental casting alloys invaluable components of dental casting alloys because of their influence on physical because of their influence on physical properties, control of the amount and type of properties, control of the amount and type of oxidation, or for their strengthening effects.oxidation, or for their strengthening effects.

e.g., Chromium, Cobalt, Nickel, Iron, Copper e.g., Chromium, Cobalt, Nickel, Iron, Copper etc.etc.

COPPER:COPPER: It reduces the resistance to tarnish and corrosion of Gold alloy if It reduces the resistance to tarnish and corrosion of Gold alloy if

present more than 16%. present more than 16%. Copper serves as a hardening and strengthening agent, can lower the Copper serves as a hardening and strengthening agent, can lower the

melting range of an alloy, and interacts with platinum, palladium, melting range of an alloy, and interacts with platinum, palladium, silver, and gold to provide a heat-treating capability in gold-, silver-, silver, and gold to provide a heat-treating capability in gold-, silver-, and palladium-based alloys.and palladium-based alloys.

Copper helps to form an oxide for porcelain bonding, lowers the Copper helps to form an oxide for porcelain bonding, lowers the density slightly, and can enhance passivity in the high palladium-density slightly, and can enhance passivity in the high palladium-copper alloys.copper alloys.

ZINC:ZINC: lowers the melting range of an alloy lowers the melting range of an alloy acts as a deoxidizer acts as a deoxidizer Zinc improves the castability of an alloy and contributes to Zinc improves the castability of an alloy and contributes to

hardness when combined with palladium.hardness when combined with palladium. Scavenger for OxygenScavenger for Oxygen Without Zinc, the Silver in the alloy causes absorption of Without Zinc, the Silver in the alloy causes absorption of

Oxygen during melting. Later during solidification, the Oxygen during melting. Later during solidification, the Oxygen is rejected producing gas porosities in the casting.Oxygen is rejected producing gas porosities in the casting.

MOLYBDENUM MOLYBDENUM They are effective hardener They are effective hardener Molybdenum refines grain structure. Molybdenum refines grain structure.

Molybdenum improves corrosion resistanceMolybdenum improves corrosion resistance influences oxide productioninfluences oxide production is helpful in adjusting the coefficient of is helpful in adjusting the coefficient of

thermal expansion of nickel-based alloys.thermal expansion of nickel-based alloys.

CHROMIUMCHROMIUM

Its passivating effect ensures corrosion resistance. The Its passivating effect ensures corrosion resistance. The chromium content is directly proportional to tarnish chromium content is directly proportional to tarnish and corrosion resistance. and corrosion resistance.

It reduces the melting point. Along with other elements, It reduces the melting point. Along with other elements, it also acts in solid solution hardening. it also acts in solid solution hardening.

Thirty percent chromium is considered the upper limit Thirty percent chromium is considered the upper limit for attaining maximum mechanical properties. for attaining maximum mechanical properties.

BerylliumBeryllium beryllium lowers the melting range of nickel-based beryllium lowers the melting range of nickel-based

alloys, improves castability, improves polishability, is alloys, improves castability, improves polishability, is a hardener, and helps to control oxide formation. a hardener, and helps to control oxide formation.

Beryllium may decrease the ability of nickel and Beryllium may decrease the ability of nickel and cobalt to passivate.cobalt to passivate.

JPD 1983 ,Vol49 ,No3

IRONIRON They help to harden the metal ceramic gold - They help to harden the metal ceramic gold -

palladium alloys, iron being the most effective.palladium alloys, iron being the most effective. IRON has melting point of 1527°C , boiling IRON has melting point of 1527°C , boiling

point of 3000 °C , density of 7.87 gm/cm3 .point of 3000 °C , density of 7.87 gm/cm3 .

GALLIUMGALLIUM

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

COBALTCOBALT Imparts hardness, strength and rigidity to the alloy . It Imparts hardness, strength and rigidity to the alloy . It

has a high melting point of 1495°C , boiling point of has a high melting point of 1495°C , boiling point of 2900 °C , density of 8.85 gm/cm3 Cobalt is an 2900 °C , density of 8.85 gm/cm3 Cobalt is an alternative to the nickel-based alloys, but the cobalt-alternative to the nickel-based alloys, but the cobalt-based metals are more difficult to process.based metals are more difficult to process.

Cobalt is included in some high-palladium alloys to Cobalt is included in some high-palladium alloys to increase the alloy's coefficient of thermal expansion increase the alloy's coefficient of thermal expansion and to act as a strengthener and to act as a strengthener

NICKELNICKELIt decreases strength, hardness, modulus of elasticity and fusion It decreases strength, hardness, modulus of elasticity and fusion

temperature. It increased ductility. temperature. It increased ductility.

Nickel has been selected as a base for porcelain alloys because its Nickel has been selected as a base for porcelain alloys because its coefficient of thermal expansion approximates that of gold and coefficient of thermal expansion approximates that of gold and

it provides resistance to corrosion.it provides resistance to corrosion.

Bio-incompatibility due to nickel, which is the most common Bio-incompatibility due to nickel, which is the most common metal to cause Contact Dermatitis. metal to cause Contact Dermatitis.

Unfortunately, nickel is a sensitizer and a known carcinogen.Unfortunately, nickel is a sensitizer and a known carcinogen.

Estimates of nickel sensitivity among women in the United States Estimates of nickel sensitivity among women in the United States range from 9% to 31.9% and from 0.8% to 20.7% among men .range from 9% to 31.9% and from 0.8% to 20.7% among men .

JPD 1991 VOl.66 Pg 213-221

MANGANESE MANGANESE Primarily oxide scavengers to prevent Primarily oxide scavengers to prevent

oxidation of other elements during melting and oxidation of other elements during melting and a hardening agent in nickel- and cobalt-based a hardening agent in nickel- and cobalt-based alloys.alloys.

CARBON:CARBON: Carbon content is most critical. Small amounts may have Carbon content is most critical. Small amounts may have

a pronounced effect on strength, hardness and ductility. a pronounced effect on strength, hardness and ductility. Carbon forms carbides with any of the metallic Carbon forms carbides with any of the metallic constituents which is an important factor in strengthening constituents which is an important factor in strengthening the alloy. However when in excess it increases brittleness. the alloy. However when in excess it increases brittleness.

Thus, control of carbon content in the alloy is important.Thus, control of carbon content in the alloy is important.

BORONBORON It is a deoxidizer and hardener, but reduces ductility. It is a deoxidizer and hardener, but reduces ductility. For nickel-based alloys, it is a hardening agent and an For nickel-based alloys, it is a hardening agent and an

element that reduces the surface tension of the molten element that reduces the surface tension of the molten alloy to improve castability.alloy to improve castability.

The nickel-chromium beryllium-free alloys that The nickel-chromium beryllium-free alloys that contain boron will pool on melting, as opposed to the contain boron will pool on melting, as opposed to the Ni-Cr-Be alloys that do not pool. Ni-Cr-Be alloys that do not pool.

Boron reduces ductility and increases hardness.Boron reduces ductility and increases hardness.

TITANIUMTITANIUM Titanium is called “material of choice” in dentistry. Titanium is called “material of choice” in dentistry.

This is attributed to the oxide formation property This is attributed to the oxide formation property which forms basis for corrosion resistance and which forms basis for corrosion resistance and biocompatibility of this material. The term 'biocompatibility of this material. The term 'titanium'titanium' is used for all types of pure and alloyed titanium.is used for all types of pure and alloyed titanium.

Like aluminum and beryllium, titanium is added to Like aluminum and beryllium, titanium is added to lower the melting range and improve castability.lower the melting range and improve castability.

Tita nium also acts as a hardener and influences oxide Tita nium also acts as a hardener and influences oxide formation at high temperatures.formation at high temperatures.

Dental casting alloysDental casting alloys

UsesUses 1) Fabrication of inlay, onlays1) Fabrication of inlay, onlays 2) Fabrication of crowns, conventional all 2) Fabrication of crowns, conventional all

metal bridges, metal-ceramic bridges, resin metal bridges, metal-ceramic bridges, resin bonded bridges.bonded bridges.

3) Endodontic posts.3) Endodontic posts. 4) Removable partial denture frameworks. 4) Removable partial denture frameworks.

Solidification range of some of alloys commonly usedSolidification range of some of alloys commonly used

13001215base metalCo-Cr

13101250base metalNi-Cr (Cr>20 wt %)

13901330base metal Ni-Cr (Cr<20 wt %)

12701160base metal Ni-Cr-Be (Cr<20 wt %)

1270990NobleAg-Pd

10451185NoblePd-Ag

12301145NoblePd-Cu

1270880NobleAu-Cu-Ag-Pd

960905High nobleAu-Cu-Ag-Pd

12601160High nobleAu-Pd

11401060High Noble Au-Pt

Liquidus temperature (°C)

Solidus temperature (°C)

ADA classificationAlloy type

DESIRABLE PROPERTIES OF DENTAL DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYSCASTING ALLOYS

BiocompatibilityBiocompatibility Ease of meltingEase of melting Ease of castingEase of casting Ease of brazing (soldering)Ease of brazing (soldering) Ease of polishingEase of polishing Little solidification shrinkageLittle solidification shrinkage Minimal reactivity with the mold materialMinimal reactivity with the mold material Good wear resistanceGood wear resistance High strengthHigh strength Excellent corrosion resistanceExcellent corrosion resistance Porcelain Bonding Porcelain Bonding

Balance0-150-1074-880-6Noble (Ag-based)

Metal Ceramic

Balance-1788High Noble

(Au-based)Metal

Ceramic

Balance-3060-Noble (Ag-based)

Metal Ceramic

Balance--3852High Noble

(Au-based)Metal

Ceramic

Balance14452515Noble (Ag-based)

IV

Balance1425456High Noble

(Au-based)IV

Balance-7025-Noble (Ag-based)

III

Balance839646High Noble

(Ag-based)

III

Balance9113.575High Noble

(Au-based)III

Balance714177High Noble

(Au-based)II

Balance 6100.583High Noble

(Au-based)I

Ga, In, and ZnCuAgPdAu

Elemental composition (wt%)ClassificationAlloy type

Typical compositions of Casting Alloys for Full-Metal, Resin-Veneered and Metal- Ceramic Prostheses

Gold casting alloysGold casting alloys

ADA specification No. 5 classify dental gold casting alloys as:ADA specification No. 5 classify dental gold casting alloys as: Type1 :low strength – for castings subject to very slight Type1 :low strength – for castings subject to very slight

stress,eg;inlaysstress,eg;inlays Type2 :medium strength –for casting subject to moderate Type2 :medium strength –for casting subject to moderate

stress ,eg ;inlays and onlaysstress ,eg ;inlays and onlays Type3: high strength –for casting subject to high stress, Type3: high strength –for casting subject to high stress,

eg;onlays, thin cast backings,pontics,full crowneg;onlays, thin cast backings,pontics,full crown Type 4: extra high strength –for casting subject to very high Type 4: extra high strength –for casting subject to very high

stress and thin in cross stress and thin in cross section,eg;saddles,bars,clasps,crowns,bridges,and partial section,eg;saddles,bars,clasps,crowns,bridges,and partial denture frame work. denture frame work.

JPD 2002, Vol 87, No 4, Pg 351 – 363.JPD 2002, Vol 87, No 4, Pg 351 – 363.

HEAT TREATMENT OF GOLD ALLOYS:HEAT TREATMENT OF GOLD ALLOYS: Heat treatment of alloys is done in order to Heat treatment of alloys is done in order to

alter its mechanical properties.alter its mechanical properties. Gold alloys can be heat treated if it contains Gold alloys can be heat treated if it contains

sufficient amount of copper. Only type III and sufficient amount of copper. Only type III and type IV gold alloys can be heat-treattype IV gold alloys can be heat-treat

There are two types of heat treatment.There are two types of heat treatment. Softening Heat Treatment (Solution heat Softening Heat Treatment (Solution heat

treatment)treatment) Hardening Heat Treatment (Age hardening) Hardening Heat Treatment (Age hardening) JPD 1964, Vol 14, No 5, Pg 955 – 960JPD 1964, Vol 14, No 5, Pg 955 – 960

1. SOFTENING HEAT TEMPERATURE1. SOFTENING HEAT TEMPERATURE

Softening heat treatment increased Softening heat treatment increased ductility, but reduces tensile strength, ductility, but reduces tensile strength, proportional limit, and hardness.proportional limit, and hardness.

Indications:Indications: It is indicated for appliances that are to be It is indicated for appliances that are to be

grounded, shaped, or otherwise cold worked in grounded, shaped, or otherwise cold worked in or outside the mouth.or outside the mouth.

Method:Method: The casting is placed in an electric furnace The casting is placed in an electric furnace

for 10 minutes at a temperature of 700oand for 10 minutes at a temperature of 700oand then it is quenched in water. During this then it is quenched in water. During this period, all intermediate phases are presumably period, all intermediate phases are presumably changed to a disordered solid solution, and the changed to a disordered solid solution, and the rapid quenching prevents ordering from rapid quenching prevents ordering from occurring during cooling.occurring during cooling.

2. HARDENING HEAT TREATMENT2. HARDENING HEAT TREATMENT Hardening heat treatment increases strength, proportional limit, and hardness, Hardening heat treatment increases strength, proportional limit, and hardness,

but decreases ductility. It is the copper present in gold alloys, which helps but decreases ductility. It is the copper present in gold alloys, which helps in the age hardening process.in the age hardening process.

Indications:Indications: It is indicated for metallic partial dentures, saddles, bridges and other It is indicated for metallic partial dentures, saddles, bridges and other

similar structures. It is not employed for smaller structures such as inlays.similar structures. It is not employed for smaller structures such as inlays.Method:Method: It is done by “soaking” or ageing the casting at a specific temperature for a 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 definite time, usually 15 to 30 minutes. It is then water quenched. The aging temperature depends on the alloy composition but is generally aging temperature depends on the alloy composition but is generally between 200°C and 450°C. During this period, the intermediate phases are 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 changed to an ordered solid solution. The proper time and temperature for age hardening an alloy are specified by the manufacturer.age hardening an alloy are specified by the manufacturer.Ideally, before age hardening an alloy, it should first be subjected to a 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 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 hardening treatment when the alloy is in a disordered solid solution. This allows better control of the hardening process.allows better control of the hardening process.

LOW GOLD CONTENT ALLOYSLOW GOLD CONTENT ALLOYS : : SILVER - PALLADIUM ALLOYSSILVER - PALLADIUM ALLOYS

• Contains predominantly silver Contains predominantly silver • Palladium upto 25%Palladium upto 25%• May or may not contain gold and copperMay or may not contain gold and copper• Traces of zinc and indiumTraces of zinc and indium

High casting temp High casting temp Low densityLow density Good tarnish and corrosion resistanceGood tarnish and corrosion resistance Properties similar to type III and type IV gold.Properties similar to type III and type IV gold.

ALUMINIUM BRONZE ALLOYSALUMINIUM BRONZE ALLOYS Cu 81-88 wt% Cu 81-88 wt% Al 7-11 wt%Al 7-11 wt% Ni 2-4 wt%Ni 2-4 wt% Fe 1-4 wt%Fe 1-4 wt%

Increasing the gold levels of concentration Increasing the gold levels of concentration with in an alloy decreased the degree of tarnish with in an alloy decreased the degree of tarnish

Increasing the palladium levels of Increasing the palladium levels of concentration in an alloy decreased degree of concentration in an alloy decreased degree of tarnishtarnish

Increased concentration of silver to copper will Increased concentration of silver to copper will increase degree of tarnish.increase degree of tarnish.

JPD1982,VOL48,NO3

In noble metal alloys

METAL CERAMIC ALLOYSMETAL CERAMIC ALLOYSThe main function of metal-ceramic alloys is to reinforce The main function of metal-ceramic alloys is to reinforce

porcelain, thus increasing its resistance to fracture.porcelain, thus increasing its resistance to fracture.Requirements:Requirements: Fusion temperature should be 100°C greater than the fusion Fusion temperature should be 100°C greater than the fusion

temperature of porcelain.temperature of porcelain. Contact angle between the ceramic and metal should be less Contact angle between the ceramic and metal should be less

than 60°than 60° Should form oxides on the surface for bonding to porcelain. -Should form oxides on the surface for bonding to porcelain. -

Tin, Indium and Iron are added.Tin, Indium and Iron are added. should have compatible co-efficient of thermal expansion(0.5 should have compatible co-efficient of thermal expansion(0.5

x 10-6/°C). x 10-6/°C). Adequate stiffness and strengthAdequate stiffness and strength High sag resistance High sag resistance Lab workability and Casting Accuracy – in order to provide Lab workability and Casting Accuracy – in order to provide

clinically acceptable castingsclinically acceptable castings

The alloys used for metal-ceramic purposes are grouped under The alloys used for metal-ceramic purposes are grouped under two categories:two categories:

i) Noble metal alloys i) Noble metal alloys ii) Base metal alloys.ii) Base metal alloys. In case of noble metal alloys for porcelain bonding , In case of noble metal alloys for porcelain bonding ,

addition of 1% base metals (iron, indium, tin etc.) increases addition of 1% base metals (iron, indium, tin etc.) increases porcelain-metal bond strength, which is due to formation of an porcelain-metal bond strength, which is due to formation of an oxide film on its surface. It also increases strength and oxide film on its surface. It also increases strength and proportional limit. proportional limit.

Base metal alloys generally cast better than noble alloysBase metal alloys generally cast better than noble alloys

JPD 1996 Vol : 75,No. 4 ,367-374

The Gold-Platinum-Palladium (Au-Pt-Pd) System:The Gold-Platinum-Palladium (Au-Pt-Pd) System:This is one This is one of the oldest metal ceramic alloy system. But these alloys are of the oldest metal ceramic alloy system. But these alloys are not used widely today because they are very expensive.not used widely today because they are very expensive.

Composition:Composition:Gold – 75% to 88%,Palladium – Upto 11%,Platinum – Upto Gold – 75% to 88%,Palladium – Upto 11%,Platinum – Upto

8%,Silver – 5%8%,Silver – 5%Trace elements like Indium, Iron and Tin for porcelain bonding.Trace elements like Indium, Iron and Tin for porcelain bonding. Gold-Palladium-Silver (Au-Pd-Ag) System:Gold-Palladium-Silver (Au-Pd-Ag) System:These alloys were developed in an attempt to overcome the major These alloys were developed in an attempt to overcome the major

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

Composition (High Silver Group):Composition (High Silver Group):Gold – 39% to 53% ,Silver – 12% to 22%,Palladium – 25% to Gold – 39% to 53% ,Silver – 12% to 22%,Palladium – 25% to

35% trace amount of oxidizable elements are added for 35% trace amount of oxidizable elements are added for porcelain bonding.porcelain bonding.

Composition (Low Silver Group):Composition (Low Silver Group):Gold – 52% to 77%,Silver- 5% to 12%,Palladium – 10% to 33%Gold – 52% to 77%,Silver- 5% to 12%,Palladium – 10% to 33%Trace amounts of oxidizable elements for porcelain bonding. Trace amounts of oxidizable elements for porcelain bonding.

Gold-Palladium (Au-Pd) System:Gold-Palladium (Au-Pd) System: This particular system was developed in an attempt to overcome the This particular system was developed in an attempt to overcome the

major limitations in the Au-Pt-Pd system and Au-Pd-Ag system. major limitations in the Au-Pt-Pd system and Au-Pd-Ag system. Mainly-Mainly-

Porcelain discoloration.Porcelain discoloration.Too high coefficient of thermal expansion & contraction.Too high coefficient of thermal expansion & contraction.Composition:Composition:Gold – 44% to 55%,Gallium – 5% ,Palladium – 35% to 45%,Indium & Gold – 44% to 55%,Gallium – 5% ,Palladium – 35% to 45%,Indium &

Tin – 8% to 12%Tin – 8% to 12%Indium, Gallium and Tin are the oxidizable elements responsible for Indium, Gallium and Tin are the oxidizable elements responsible for

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

Composition:Composition: (available in two compo.)(available in two compo.)1. Palladium – 55% to 60% Silver – 25% to 30%1. Palladium – 55% to 60% Silver – 25% to 30% Indium and TinIndium and Tin2. Palladium – 50% to 55% Silver – 35% to 40%2. Palladium – 50% to 55% Silver – 35% to 40% Tin (Little or no Indium)Tin (Little or no Indium) Trace elements of other oxidizable base elements are also present. Trace elements of other oxidizable base elements are also present.

HIGH PALLADIUM SYSTEMHIGH PALLADIUM SYSTEMSeveral types of high palladium systems were originally Several types of high palladium systems were originally

introduced introduced More popular composition groups contained cobalt and copper.More popular composition groups contained cobalt and copper.Composition (PALLADIUM-COBALT ALLOY):Composition (PALLADIUM-COBALT ALLOY):Palladium – 78% to 88% Cobalt – 4% to 10%Palladium – 78% to 88% Cobalt – 4% to 10%(Some high palladium-cobalt alloys may contain 2% gold)(Some high palladium-cobalt alloys may contain 2% gold)Trace amounts of oxidizable elements (such as gallium and Trace amounts of oxidizable elements (such as gallium and

indium) are added for porcelain bonding.indium) are added for porcelain bonding.COMPOSITION (PALLADIUM-COPPER ALLOYS)COMPOSITION (PALLADIUM-COPPER ALLOYS)Palladium – 70% to 80% Copper – 9% to 15%Palladium – 70% to 80% Copper – 9% to 15%Gold – 1% to 2% Platinum – 1Gold – 1% to 2% Platinum – 1 Some, but not all, high palladium-copper alloys contain small Some, but not all, high palladium-copper alloys contain small

quantities ( 1% to 2%) of gold and/or platinum. Trace amounts quantities ( 1% to 2%) of gold and/or platinum. Trace amounts of the oxidizable elements gallium, indium and tin are added of the oxidizable elements gallium, indium and tin are added for porcelain bonding. for porcelain bonding.

JOP 2009, Vol 18, Pg 188 – 184.JOP 2009, Vol 18, Pg 188 – 184.

BASE METAL ALLOYSBASE METAL ALLOYSLow costLow cost

Low densityLow densityGreater stiffnessGreater stiffnessHigher hardnessHigher hardness

High resistance to tarnish and corrosion.High resistance to tarnish and corrosion.

Properties of base metal alloy ca be altered by heat Properties of base metal alloy ca be altered by heat treatment to enable its use for a wide variety of fixed treatment to enable its use for a wide variety of fixed

dental restoration.dental restoration.

JPD 1984 Vol 52 No.6 Pg 821-827JPD 1984 Vol 52 No.6 Pg 821-827

The melting temperature of base metal alloys The melting temperature of base metal alloys is 1150-1500 c as compared to cast gold alloys is 1150-1500 c as compared to cast gold alloys

Addition of 1%-2% beryllium lowers the Addition of 1%-2% beryllium lowers the melting temperature of ni-cr alloys about 100 cmelting temperature of ni-cr alloys about 100 c

Melting temperature is important for selection Melting temperature is important for selection casting equipment,investment and selection of casting equipment,investment and selection of casting techique.casting techique.

HighHighFairExcellentBond to

porcelain

ExtremelyModeratelyModerately

HighMinimal

Technique sensitivity

GoodExcellentExcellentPoor-excellentSag resistance

103207145-22090Elastic

Modulus (GPa)

4.58.77.514Density (g/cm3)

ExcellentFairExcellentExcellentBiocompatibilit

y

CPTiNi-Cr-BeCo-CrHigh noble

alloyProperty

COMPARISON OF PROPERTIES OF THE VARIOUS TYPES OF BASE METAL ALLOYS

According to the ADA the following combinations are According to the ADA the following combinations are available :available :

Cobalt – chromium Cobalt – chromium Nickel – chromiumNickel – chromium Nickel – chromium – beryllium Nickel – chromium – beryllium Nickel – cobalt – chromiumNickel – cobalt – chromium Titanium – aluminium – vanadiumTitanium – aluminium – vanadiumAccording to functionAccording to functionCROWN AND BRIDGE ALLOYCROWN AND BRIDGE ALLOY NICKEL basedNICKEL based COBALT basedCOBALT basedMETAL CERAMIC ALLOYSMETAL CERAMIC ALLOYS:: NICKEL- CHROMIUMNICKEL- CHROMIUM TITANIUMTITANIUMREMOVABLE PARTIAL DENTURE ALLOYSREMOVABLE PARTIAL DENTURE ALLOYS:: COBALT- CHROMIUMCOBALT- CHROMIUM NICKEL- CHROMIUMNICKEL- CHROMIUM COBALT- CHROMIUM- NICKELCOBALT- CHROMIUM- NICKEL

Nickel –chromium-beryllium alloysNickel –chromium-beryllium alloysCompositionComposition Nickel - 62 - 82%Nickel - 62 - 82%,, Chromium -11 - 20% Chromium -11 - 20%,, Beryllium - upto 2.0% Beryllium - upto 2.0%AdvantagesAdvantages Low costLow cost Low densityLow density High sag resistanceHigh sag resistance Can produce thin castingsCan produce thin castings Poor thermal conductorPoor thermal conductordisadvantagesdisadvantages Cannot use with Nickel sensitive patientsCannot use with Nickel sensitive patients Beryllium exposure may be potentially harmful to technicians and patientsBeryllium exposure may be potentially harmful to technicians and patients Proper melting and casting is a learned skillBond failure more common in Proper melting and casting is a learned skillBond failure more common in

the Oxide layerthe Oxide layer High hardness (may wear opposing teeth)High hardness (may wear opposing teeth) Difficult to solderDifficult to solder Difficult to cut through cemented castingsDifficult to cut through cemented castings

Strength and rigidity of nickel chromium alloys suggested potential usefulness in

fixed prosthodontic procedures.

JPD 1978 Vol- 40 No. 6. Pg 637-641

Nickel - Chromium - Beryllium - Free Alloys Nickel - Chromium - Beryllium - Free Alloys compositioncomposition

Nickel - 62 - 77%Nickel - 62 - 77%Chromium Chromium - - 1111 - - 22% 22%Boron, Iron, Molybdenum, Niobium and Tantalum (small Boron, Iron, Molybdenum, Niobium and Tantalum (small

amount)amount) Don't contain Beryllium Don't contain Beryllium Low costLow cost Low density so difficult to cast and polishLow density so difficult to cast and polish Cannot used in patients who are Nickel sensitiveCannot used in patients who are Nickel sensitive Produce more oxides than Nickel - Chromium - Beryllium Produce more oxides than Nickel - Chromium - Beryllium

alloysalloys

Porcelain bonded test revealed that Nickel Porcelain bonded test revealed that Nickel Chromium Beryllium alloys produce Chromium Beryllium alloys produce significantly better porcelain metal bond ,than significantly better porcelain metal bond ,than Nickel Chromium aloys without beryllium.Nickel Chromium aloys without beryllium.

Palladium Copper alloys bond better with Palladium Copper alloys bond better with porcelain than Palladium Cobalt alloys .porcelain than Palladium Cobalt alloys .

JPD 1996 Vol : 75,No. 4 ,367-374

Cobalt-chromium alloysCobalt-chromium alloys composition:composition: cobalt - 53-68%cobalt - 53-68% chromium - 25-34%chromium - 25-34%Trace elements include molybdenum, rhuthenium. Trace elements include molybdenum, rhuthenium. AdvantagesAdvantages:: Do not contain nickelDo not contain nickel Do not contain berylliumDo not contain beryllium poor thermal conductorspoor thermal conductors low density low costlow density low costDisadvantagesDisadvantages More difficult to process than Nickel - Base alloysMore difficult to process than Nickel - Base alloys High hardness (may wear opposing dentition) High hardness (may wear opposing dentition) Oxidize more than both Nickel based alloys Oxidize more than both Nickel based alloys

Titanium alloysTitanium alloys

Uses:Uses: Commercially pure titanium is used for dental implants, surface coatings, Commercially pure titanium is used for dental implants, surface coatings,

crowns, partial dentures, complete dentures and orthodontic wires crowns, partial dentures, complete dentures and orthodontic wires According to the American Society for Testing and Materials (ASTM), According to the American Society for Testing and Materials (ASTM),

there are five unalloyed grades of CP Ti (Grades 1-4, and Grade 7), there are five unalloyed grades of CP Ti (Grades 1-4, and Grade 7), based on the concentration of based on the concentration of

oxygen (0.18 wt% to 0.40 wt%) andoxygen (0.18 wt% to 0.40 wt%) and iron (0.2 wt% to 0.5 wt%). iron (0.2 wt% to 0.5 wt%). Other impurities include nitrogen (0.03 wt% to 0.05 wt%), Other impurities include nitrogen (0.03 wt% to 0.05 wt%), carbon (0.1 m%), and hydrogen (0.015 wt%).carbon (0.1 m%), and hydrogen (0.015 wt%).

Grade 1 CP Ti is the purest and softest form Grade 1 CP Ti is the purest and softest form It has a moderately high tensile strengthIt has a moderately high tensile strength moderately high stiffnessmoderately high stiffness low densitylow density low thermal expansion coefficient.low thermal expansion coefficient.

JOP 2009, Vol 18, Pg 188 – 184.JOP 2009, Vol 18, Pg 188 – 184.

The elastic modulus of CP Ti is comparable to that of The elastic modulus of CP Ti is comparable to that of tooth enamel and noble alloys, but it is lower than tooth enamel and noble alloys, but it is lower than that of other base metal alloysthat of other base metal alloys

Casting of titanium alloys is difficult due to a high Casting of titanium alloys is difficult due to a high casting temperature – 2000° c casting temperature – 2000° c

Rapid oxidation and reactions with investmentsRapid oxidation and reactions with investments Melting is done in specially designed furnaces with Melting is done in specially designed furnaces with

an argon atmosphere an argon atmosphere Ti-6Al-4v has been used for PFM restorationsTi-6Al-4v has been used for PFM restorations Used with low expansions porcelainsUsed with low expansions porcelains

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

WROUGHT BASE METAL AND GOLD WROUGHT BASE METAL AND GOLD ALLOYSALLOYS

When a casting is plastically deformed in any manner, it When a casting is plastically deformed in any manner, it is called wrought metal.is called wrought metal.

Wrought base metal alloys are used in dentistry, mainly Wrought base metal alloys are used in dentistry, mainly as wires for orthodontics and as clasp arms for as wires for orthodontics and as clasp arms for removable partial dentures. removable partial dentures.

The alloys include:The alloys include: Stainless steel : iron-chromium-nickel alloy Stainless steel : iron-chromium-nickel alloy Co-Cr-NiCo-Cr-Ni Ni-TiNi-Ti ββ- Titanium alloys.- Titanium alloys.

The titanium nickel cast clasp may be suitable The titanium nickel cast clasp may be suitable in removable prosthodontic constructions in removable prosthodontic constructions because of its significantly less permanent because of its significantly less permanent deformation during service. deformation during service.

Int J Prothodont 1997; 10:547– 552.Int J Prothodont 1997; 10:547– 552.

CARBON STEELS:

Steels are iron based alloys that usually contain Steels are iron based alloys that usually contain less than 1.2% carbonless than 1.2% carbon. .

The different classes of steels are based on The different classes of steels are based on three possible lattice arrangements of iron.three possible lattice arrangements of iron.

STAINLESS STEELSTAINLESS STEEL

When 12-30% Cr is added to steel, the alloy is called as When 12-30% Cr is added to steel, the alloy is called as Stainless steelStainless steel

Ferritic stainless steel:Ferritic stainless steel: Often designated as American Iron and Steel Often designated as American Iron and Steel

institute (AISI) series 400 stainless steels.institute (AISI) series 400 stainless steels. Good corrosion resistance.Good corrosion resistance. Is not hardenable by heat treatment.Is not hardenable by heat treatment. Limited application in dentistry.Limited application in dentistry.

Martensitic stainless steel:Martensitic stainless steel: Share the AISI 400 designation.Share the AISI 400 designation. Have high strength and hardness, so used for surgical Have high strength and hardness, so used for surgical

and cutting instruments.and cutting instruments. Poor corrosion resistance.Poor corrosion resistance.

Austenitic stainless steel:Austenitic stainless steel:

•Most corrosion resistant of all.Most corrosion resistant of all.•AISI 302 is basic type, containing 18% or 8% Ni AISI 302 is basic type, containing 18% or 8% Ni and 0.15% carbon.and 0.15% carbon.•Type 304 has 0.08% of carbon.Type 304 has 0.08% of carbon.•Both are designated as 18-8 stainless steelBoth are designated as 18-8 stainless steel•Type 316L (0.03% carbon) is ordinarily employed Type 316L (0.03% carbon) is ordinarily employed for implants.for implants.

DENTAL IMPLANT MATERIALSDENTAL IMPLANT MATERIALS

Most commonly, metals and alloys are used. Initially Most commonly, metals and alloys are used. Initially surgical grade stainless steel and Co-Cr alloys were surgical grade stainless steel and Co-Cr alloys were used because of their acceptable physical properties used because of their acceptable physical properties and relatively good corrosion resistance and and relatively good corrosion resistance and biocompatibility.biocompatibility.

Commercially pure Ti (CPTi) has become one of the Commercially pure Ti (CPTi) has become one of the material of choice because of its predictable material of choice because of its predictable interaction with the biological environment.interaction with the biological environment.

OTHER METALS:OTHER METALS:Gold, Palladium, Tantalum, Platinum and alloys of Gold, Palladium, Tantalum, Platinum and alloys of

these metals. these metals.

The properties of two surgical casting alloys The properties of two surgical casting alloys (vitallium and Ticonium) were determined. It (vitallium and Ticonium) were determined. It was found that vitallium was Co-Cr based was found that vitallium was Co-Cr based alloy. Ticonium Ni, Cr, and Co. Both had alloy. Ticonium Ni, Cr, and Co. Both had cored microstructures. Vitalluium was superior cored microstructures. Vitalluium was superior in strength and ticonium had significantly in strength and ticonium had significantly lower hardness and greater ductility. lower hardness and greater ductility.

JPD 1972 ,Vol- 28 ,No. 1.

CONCLUSIONCONCLUSION

Great variety of alloys currently available can lead to Great variety of alloys currently available can lead to uncertainty in choosing an optimal alloy for a given patient uncertainty in choosing an optimal alloy for a given patient and situation. Avoid selecting an alloy based on its color and situation. Avoid selecting an alloy based on its color unless all other factors are equal. Know the complete unless all other factors are equal. Know the complete composition of alloys, and avoid elements that are allergic to composition of alloys, and avoid elements that are allergic to the patient. Use alloy that have been tested for elemental the patient. Use alloy that have been tested for elemental release and corrosion and have the lowest possible release of release and corrosion and have the lowest possible release of elements.Focus on long term clinical performance.Finally it is elements.Focus on long term clinical performance.Finally it is important for the dentist to remember and take up the important for the dentist to remember and take up the responsibility of being responsible for the safety and efficacy responsibility of being responsible for the safety and efficacy of any restoration. of any restoration.

JPD 2002, Vol 87, No 4, Pg 351 – 363.JPD 2002, Vol 87, No 4, Pg 351 – 363.

REFERENCESREFERENCES Science of dental materials- Anusavice, 11th Edn.Science of dental materials- Anusavice, 11th Edn. Restorative dental materials - Craig ,12th Edn.Restorative dental materials - Craig ,12th Edn. Dental biomaterials- E.C. CoombeDental biomaterials- E.C. Coombe.. Applied dental Materials - John F. Mc Cabe,8th Edn.Applied dental Materials - John F. Mc Cabe,8th Edn. Dental materials, Properties & Manipulation -Robert G. Craig et.al, 5th Edn. Dental materials, Properties & Manipulation -Robert G. Craig et.al, 5th Edn. Materials in Dentistry Principles and Application 2nd edition Jack L Ferracane. Materials in Dentistry Principles and Application 2nd edition Jack L Ferracane. John C.Wataha , Alloys for prosthodontic restoration JPD, vol87,no4,April 2oo2,John C.Wataha , Alloys for prosthodontic restoration JPD, vol87,no4,April 2oo2, Timothy k. jones :dental implications of nickel hypesesitive,JPD,vol56,no4,1986Timothy k. jones :dental implications of nickel hypesesitive,JPD,vol56,no4,1986 Howard W.Roberts:metal ceramic alloys in dentistry,Journal of prosthodontics Howard W.Roberts:metal ceramic alloys in dentistry,Journal of prosthodontics

18,200918,2009 Brien R. Lang: Tarnish and corrosion of noble metal alloys, JPD, 48. 245, 1982.Brien R. Lang: Tarnish and corrosion of noble metal alloys, JPD, 48. 245, 1982. Eugene F. Huget, .Characterization of two ceramic – base – metal alloys, JPD, 40, Eugene F. Huget, .Characterization of two ceramic – base – metal alloys, JPD, 40,

637, 1978.637, 1978.

Randolph P. O’ Cornnor, Castability, opaque masking,n and porcelain Randolph P. O’ Cornnor, Castability, opaque masking,n and porcelain bonding of 17 porcelain-fused-to-metal alloys. JPD, 75, 367, 1996.bonding of 17 porcelain-fused-to-metal alloys. JPD, 75, 367, 1996.

Thomas D. Taylor, Clcerative lesions of the palate associated with Thomas D. Taylor, Clcerative lesions of the palate associated with removable partial denture castings. JPD 66, 213, 1991.removable partial denture castings. JPD 66, 213, 1991.

Sheldon Winkler, Changes in mechanical properties and microstructure Sheldon Winkler, Changes in mechanical properties and microstructure following heat treatment of a nickel – chromium base alloy. JPD, 52, following heat treatment of a nickel – chromium base alloy. JPD, 52, 821,1984.821,1984.

Harold F. Morris, Mechanical properties of metal ceramic alloys, JPD, 61, Harold F. Morris, Mechanical properties of metal ceramic alloys, JPD, 61, 160, 1989. 160, 1989.

Simon Civjan, Properties of surgical casting alloys. JPD, 28, 77, 1972.Simon Civjan, Properties of surgical casting alloys. JPD, 28, 77, 1972. John C. Wataha, Biocompatibility of dental casting alloys: a review. JPD, John C. Wataha, Biocompatibility of dental casting alloys: a review. JPD,

83, 223, 2000.83, 223, 2000. J. Robert Kelly, Nonprecious alloys for use in fixed alloys. JPD, 49, 363, J. Robert Kelly, Nonprecious alloys for use in fixed alloys. JPD, 49, 363,

1983.1983. Masato Kotake, Fatigue resistance of Titanium – Nickel Alloy Cast Clasps. Masato Kotake, Fatigue resistance of Titanium – Nickel Alloy Cast Clasps.

JPD, Int/ Prosthodont 1997; 10;547-552.JPD, Int/ Prosthodont 1997; 10;547-552.