Dental Casting Alloys34 / orthodontic courses by Indian dental academy

DEPARTMENT OF PROSTHODONTICS

Seminar on

Dental casting alloys

Dr nikhil .j Post Graduate Student

Under the Guidance of'Dr. N.P. Patil

Prof & Head of Department

S.D.M. COLLEGE OF DENTAL SCIENCES & HOSPITAL,DHARWAD, KARNATAKA, INDIA.

S.D.M. COLLEGE OF DENTAL SCIENCES ANDHOSPITAL, DHARWAD, KARNATAKA, INDIA

CERTIFICATE

This is to certify that the Seminar on "DENTAL

CASTING ALLOYS" was presented by Dr. Nikhil

under my guidance, during the period 2003-2006.

Date Dr. Narendra P. Patil,M.D.S.Place: Dharwad Prof & Head

INTRODUCTION because pure metals are apt to be soft and many tends to corrode rapidly and also because high cost their use is quite limited in dentistry. To optimize properties, most metals commonly used in dentistry are mixtures of two or more metallic elements or one or more metal and/or non metals (THE ALLOY).

CASTING is one of the most widely used methods for fabrication of metal restorations out of the mouth. A pattern of lost tooth structure or the dental prosthesis to be reproduced in metal is constructed in wax. The wax is surrounded by an investment. After the investment has hardened the wax is removed and the molten metal is forced into the mold space.

My seminar deals with these dental casting alloys.

“What we will be tomorrow is because of what we are today, and what we are today is because of what we were yesterday”.

Somebody haws rightly said that History is the best teacher and a brief description of the evolution of the currently marketed alloys is appropriate to understand the rationale for the development of the wide variety of alloy formulations.

TABLE 19-1 ANUSAVICE PG NO 5651. The technological changes of dental prostheses.2. Metallurgic advancements.3. Price changes of noble metals since 1968

Taggart’s presentation to the New York Odontological Group in 1907 on the fabrication of cast inlay restorations often has been acknowledged as the first reported application of lost wax technique in dentistry. The inlay technique described by Taggart was an instant success and it soon lead to the casting of complex restorations such as inlays, onlays, crowns, fixed partial dentures and removable partial denture frameworks. Because pure gold did not have physical properties required for those dental restorations existing jewelry alloys were quickly adopted. These gold alloys were further strengthened with copper, silver or platinum. In 1932 the dental materials Group at the National Bureau of Standards surveyed the alloys being used and roughly classified them as a Type I, II, III and IV. In the following years several patents were issued for alloys containing Palladium, as a substitute for Platinum. By 1948 the composition of dental noble metal alloys for lost metal restorations had become rather diverse, wirth these formulations, the tarnishing tendency of the original alloys apparently had disappeared. It is now known that in gold alloys Palladium is added to counteract the tarnishing potential of silver. The base metal removable partial denture alloys were introduced in the 1930s. Since that time, both Ni-Cr and Co-Cr formulati0ons have become increasingly popular compared with conventional type IV gold alloys, which previously were the predominant metals used for such prostheses. The obvious advantages of base metal alloys are their lighter weight, increased mechanical properties and their reduced cost.

Likewise by 1978, the price of gold was climbing so rapidly attention focused on the noble, metal alloys. To reduce the the precious metal content and yet retain the advantages of noble metals for dental use. DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS:

All casting alloys must first be biocompatible and then exhibit sufficient physical and mechanical properties to ensure adeqwuate function and structural durability overlong periods of time.

1. BIOCOMPASTIBILITY: Their material must tolerate oral fluids and must not release any harmful products into the oral environment.

2. CORROSION RESISTANCE: Corrosion is defined as a chemical or a electrochemical process in which a solid usually a metal, is attacked by an environmental agent, resulting in partial or complete dissolution. Metals are generally more susceptible to such attacks because of electrochemical reactions. Corrosion resistance is derived either by the component being too noble to react in the oral environment(E.g.: gold, palladium) or by its ability to form an adherent passivating surface film which inhibits any subsurface reactions( E.g.: Co-Cr, Ni-Cr and Co-Cr alloys and Ti alloys in CP Ti and in Ti-6Al-4V alloys).

3. TARNISH RESISTANCE “process by which a metal surface is dulled or discolored when a reaction with sulfide, oxide, chloride or chemical causes a the film to form. These films are generally found on gold alloys with relatively high silver content or on silver alloys.

4. ALLERGENIC COMPONENTS INC ASTING ALLOYS: A restorative material should not cause adverse health consequences to a patient. Beryllium potentially toxic under uncontrolled conditions.

5. ESTHETICS : Considerable controversy exists over the optimal balance among the properties of esthetics, fit, abrasive potential, clinical survivability, and cost of cast metal prosthesis compared with direct filling restorations, ceramic based prosthesis(all ceramic and metal ceramic) and resin veneer prosthesis.

6. THERMAL PROPERTIES: For metal ceramic restorations the alloys or metals must have closely, matching thermal Expansion to be compatible with as given porcelain, and they must tolerate high processing temperature.

7. MELTING RANGE: Must be low enough to g\form smooth surfaces with the mold wall of the casting investment.

8. COMPENSATION FOR SOLIDIFICATION: Compensation for casting shrinkage must be achieved either through computer generated oversized dies or through controlled mold expansion. The fit of the cemented prosthesis must be tailored to accommodate the alloys of bonding adhesive and the luting cement.

9. STRENGTH REQUIREMWNTS: Alloys for bridgework require higher strength than alloys for single crowns. Copings for metal ceramic restorations require sufficient elastic modulus (stiffness) to prevent elastic deflection from functional forces.

10. CASTABILITY: The molten metal must be able to wet the investment mold material very well (decreased contact angle) and flow into the most intricate

regions of the mold without any appreciable interaction with the investment and without forming porosity within the subsur5face ands surface regions.

11. FINISHING OF CAST METALS: Hardness, ductility (percentage elongation) and ultimate tensile strength are important properties in this regard.

12. PORCELAIN BONDING: A substrate metal must be able to form a thin, adherent oxide, preferably one that is light in color and also it must have thermal expansion/contraction coefficients that are closely matched to that of porcelain.

13. ECONOMIC CONSIDERATIONS: The cost of metals used in restorative dentistry is a function of the metal density and the cost per unit mass.

CLASSIFICATION OF DENTAL CASTING ALLOYS“Even the proverbial needle in the haystack can be found if there is a system and method to the search”

In 1984, the ADA proposed a simple classification for dental casting alloys.BOX NO 19-1 ANUSAVICE PG 570.Cast dental alloys can be classified according to the following five categories:

1. USE (All metal inlays, crowns and bridges, metal and ceramic prostheses, posts and cores, removable partial dentures, implants).2. MAJOR ELEMENTS :( Gold based, palladium based, silver based, nickel based, cobalt based and titanium based)3. NOBILITY: High noble, noble, predominantly base metal.4. PRINCIPAL THREE ELEMENTS: E.g.: Au-Pd-Ag Pd-Ag- Sn

Ni-Cr-BeCo-Cr-Mo etc

4. DOMINANT PHASE SYSTEM: Single phase, Eutectic, Peritectic, and Intermetallic

Mechanical properties (ANSI/ ADA Specifications, 1997)- Table 19-2 pg 571 anusavice. MICROSTRUCTURE OF ALLOYS

METAL: An element whose atomic structure rapidly loses electron to form positively charged ions, and which exhibits metallic bonding, opacity good light reflectance from a poloished surface, and high electrical ands thermal conductivity.Of the 103 elemnts listed oinm the periodic table 80 are classed as metals and they exhibit the following properties:

1. Metallic luster2. Metallic ring.3. Harder, stronger and denser than other elements4. Solids at room temperature( Exception, mercury and gallium which are liquids at

room temperature and hydrogen which is a gas at room temperature).5. Good conductors of heat and electricity.6. Opaque

7. Ductile and malleable8. Electropositive

ALLOY: A crystalline substance with metallic properties that is composed of two or more chemical elements, at least one of which is a metal.ALLOY SYSTEM is an aggregate of two or more metals in all combinations. In order to specify a particular alloy it is necessary to list the metals or elements \present in the alloy and the amount of each element present. Two methods are available.:1. The weight percentageof each element.2. The atomic fraction or percentage.The properties of an alloy relate more to the atomic percentage.

PHASE DIAGRAMS(CONSTITUTION DIAGRAM).A graph of the phase field limits as a function of temperature and composition. Phase diagrams usually represent equilibrium conditions.USES: They show the phases that are present in an alloy system for different compositions and temperatures.DIAGRAM:Fig 6-3 pg 126 and fig 6-4 pg 127 anusavice.

CLASSIFICATION OF ALLOYS BASED ON THEIR MISCIBILITY:1. SOLID SOLUTION SUBSTITUTIONAL AND INTERSTITIAL TYPE.2. CORING3. EUTECTIC ALLOYS4. PERITEC SYSTEM5. INTERMETALLIC COMPOUNDS

PROPERTIES OF DIFFERENT ALLOYING ELEMENTS:

GOLD:1. Pure gold is soft, malleable, ductile metal that does not oxidize under atmospheric conditions and is attacked by only a few of the most powerful oxidizing agents.2. It has a rich yellow color with a strong metallic luster.3. Although it is the most malleable and ductile metal, it ranks much lower in strength.4. The pure metal fuses at 1060 degree Celsius 5. Small amounts of impurities have a pronounced effect on mechanical properties of gold and its alloys6. The presence of les than 2% lead will cause the metal to become extremely brittle.7. Mercury in small quantities also has a harmful effect on its properties.8. Gold is nearly as soft as lead with the result that in dental alloys it must be alloyed with copper, silver, platinum and other metals to develop the necessary hardness, durability And elasticity9. The specific gravity of pure gold is between19.30 and 19.33 making it one of the heavy metals10. Air or water at any temperature doesn’t tarnish gold.11. It is not soluble in sulfuric, nitric or hydrochloric acids.

13.BHN of 2514. boiling point of 2970 degree Celsius 15.Linear coefficient of thermal expansion 0.142The puritiy of gold is expressed in karat or finenessKarat refers to parts of pure gold in 24 parts of gold alloyFineness refers to parts of pure gold in 1000 parts of gold alloy.

PALLADIUM:1. Palladium is not used in pure state in dentistry but it is used in many dental alloys combined with either gold or silver.2. It is cheaper than platinum and since it imparts many of the properties of platinum to dental alloys it is often used as a replacement for platinum.3. Platinum is a white metal some what darker than platinum.4. Its specific gravity is 11.4 i.e., about half that of platinum and a little more than half of gold.5. It is a malleable and ductile metal with a melting point of1555 degree Celsius which is the lowest of the platinum group of metals.6. It hardens the alloy, imparts it whiter color and compensates the reddening effect of copper. Increase the melting point of the alloy and renders silver tarnish resistant.

SILVER:1. Silver is malleable, ductile, white in color and best known for its conduction of

heat and electricity. It is stronger and harder than gold but softer than copper.2. Melting point of 960.5 degree Celsius 3. It combines with sulfur, chlorine and phosphorus or their vapors 4. Pure silver is seldom employed in dental restorations because of the black sulfide

formation on themetal in the mouth although it is used as small additions to many gold alloys.

5. Addition of palladium to silver containing alloys prevents the rapid corrosiojn of such alloys in the oral environment.

6.Silver increases the hardness slightly,whitens the alloy to over come the reddening effect of copper. Molten silver can dissolve oxygen and cause porosity in the casting and silver can encourage corrosion.

COPPER1.Hardens the alloy. 2.Reduces the melting point of alloy.3. Reduces the density of the alloy4. Excessive copper renders the alloy more susceptible to tarnish and corrosion and reddens the alloy.

ZINC:1. It is an oxide scavenger during melting of the alloy for casting procedure.

2. in the absence of zinc silver absorbs oxygen at high temperature from the atmosphere. This oxygen is rejected during solidification tending to produce porosity in the casting.

IRIDIUM, RUTHENIUM AND RHODIUM:Iridium is a hard metal that is quite brittle white with a high specific gravity of 22.42 and an exceptionally high melting point of 2440 degree Celsius.1.As little as 0.005% of Iridium is effective in refining the grain size of cast gold alloys.2. Ruthenium produces a similar effect.

GALLIUM:Used mainly in silver free alloys to compensate for the the decreased thermal expansion seen in silver free alloys. (Silver is avoided in metal ceramics as it has as greening effect)

IRON,TIN:Increases the hardness. Also provides an oxide coat which improves bonding of porcelain to alloy.

HEAT TREATMENT OF HIGH NOBLE AND NOBLE METAL ALLOYS:

HOMOGENIZATION:The cast alloy is held at a temperature near its solidus to achieve the maximum amount of diffusion without melting (up to a period of 6 hours in some instances)This treatment allows atomic diffusion to occur which eliminates as-cast compositional nonuniformity. This treatment results in:

1. Increase in tarnish and corrosion resistance.2. Increase in the ductility of the alloy.

SOLUTION HEAT TREATMENT:It involves heating the casting to a temperature below the solidus(usually 700 degree Celsius) , holding for a short period of time(typically 10 min) so that the alloy returns to random substitutional solid solution , and then quenching to retain this atomic arrangement at room temperature. The tensile strength, hardness and proportional limit are reduced by such a treatment but the ductility is increased. This treatment is indicated for structures that are to be ground, shaped or otherwise cold worked, either in or out of the mouth.

HARDENING HEAT TREATMENT:The age hardening of the dental alloys can be accomplished in several ways. One of the most practical hardening treatments is by SOAKING or AGEING the casting at a specific temperature for a definite time,usually 15 to 30 minutes, before it is water quenched. The ageing temperature depends on the alloy composition but is generally between 200 and 450 degree Celsius. This treatment is indicated for metallic partial dentures, bridges and other similar structures.

CASTING SHRINKAGE:All metals and alloys of practical dental interest shrink when they change from liquid to solid state. This occurred in three stages:

1. The thermal contraction of the liquid metal between the temperature to which it is heated and the liquidus.

2. The contraction of the metal inherent in its change from the liquid to the solid state.

3. The thermal contraction of the solid metal that occurs on further cooling to room temperature.

TABLE FOR CASTING SHRINKAGE:Pg 577, ANUSAVICE. This casting shrinkage must be compensated for by adequate casting technique( selection of proper investment material which compensates the shrinkage and yet will be able to withstand the fusion temperature of the alloy, for example)

SILVER PALLADIUM ALLOYS:These alloys are white and predominantly silver in composition but have substantial amounts of palladium(at least 25%)that provides nobility and promote tarnish resistance. They may or may not have copper and a small amount of gold. ADVANTAGES:

1. Adequate strength properties.2. Acceptable castability3. Low cost

LIMITATION:1. Great potential for tarnish and corrosion.

HIGH NOBLE AND NOBLE ALLOYS FOR METAL-CERAMIC PROSTHESISThe chief objections to the use of dental porcelain as a restorative material are its low strength under tensile and shear conditions. A method by which this disadvantage can be minimized is to bond the porcelain directly to a cast alloy substructure made to fit the prepared tooth. In spite of vastly different chemical compositions all such alloys must share at least three common features:

1. They must have the potential to bond to dental porcelain.2. They posses coefficients of thermal contraction compatible with those of dental

porcelains.3. Their solidus temperature is sufficiently high to permit application of low fusing

porcelains.4. the coefficients ofd thermal expansion tend to be reciprocal to melting point of the

alloys.5. high sag resistance.

SAG DEFORMATION Fig 19.1

GOLD PALLADIUM SILVER ALLOYS (LOW SILVER CONTENT)ADVANTAGES:

1. Economical

2. Excellent resistance to tarnish and corrosion3. Relative freedom from technique sensitivity

DISADVANTAGE:The potential for porcelain discoloration when silver vapor is released.

GOLD- PALLADIUM-SILVER ALLOYS (HIGH SILVER CONTENT)Gold alloys that contain 12% Ag or more account for approximately 20% of the current alloy market.ADVANTAGES:

1. Lower cost2. Favorable physical properties

DISADVANTAGE:Potential for porcelain discoloration.

GOLD PALLADIUM ALLOYS:The first alloy of this type was introduced in 1977 by J.F.Jelenko and Co. This alloy was designed to overcome the porcelain discoloration effect (because it is silver free) and also to provide an alloy with a lower thermal contraction coefficient than that of either Au-Pd-Ag or Pd-Ag alloys. Their gold content varies from 45 to 52% and palladium content varies between 37-45%. A slight thermal contraction mismatch is recommended to develop compressive hoop and axial stresses in porcelain which are protective in nature. However, significantly higher mismatches may lead to porcelain cracking or metal-ceramic bond failure because of the development of tensile stresses which exceed the tensile strength of porcelain or the strength of the metal ceramic bond. Another cause is the development of radial tensile stresses that exceed the tensile strength of porcelain. ADVANTAGES: These alloys are considered nearly ideal because:

1. Contain no silver2. Their surface oxide layer is virtually indiscernible3. Their sag resistance is better than that of Au-Pt-Pd alloys4. Their castability, corrosion resistance and adherence to porcelain are excellent5. Cost effective

PROPERTIES:1. HV of about 2002. Yield strength of 570 MPa3. Elongation of 20%

PALLADIUM GOLD ALLOYS:Relatively few alloys are available in the market due to the price volatility of palladium and their laboratory and clinical performance has not been adequately documented.

PALLADIUM-GOLD-SILVER ALLOYS:These have a potential for porcelain discoloration. Gold content is from 5-32% and silver content from 6.5-14%.

These alloys have a range of thermal contraction coefficients that increase with an increase in silver content.

PALLADIUM-SILVER ALLOYS:This alloy type was introduced in the market in 1974 as the first gold free noble alloy available for metal ceramic restorationsThe Pd content is 53-61% and 28-40% Ag in addition to tin and/or indium.ADVANTAGES:

1. The low specific gravity and their low cost make them attractive economic alternatives to gold based alloys.

2. Adequate physical properties 3. Alloys of this type are easy to polish and burnish4. Adherence to porcelain is acceptable although a predominantly mechanical type

of bonding has been suggested for this alloy.DISADVANTAGES:

1. Silver discoloration effect is most severe for these alloys. Gold metal conditioners or ceramic coating agents may minimize this effect. In addition many of today’s porcelains are formulated to minimize this problem.

PALLADIUM-COPPER-GALLIUM ALLOYS:First introduced in 1983, these alloys were very popular in 1990s. However the price volatility of palladium required dentists to use other alloys. A brown or black oxide layer is formed during oxidation and subsequent porcelain firing cycles. Because of all these factors these alloys have not been well accepted in the dental practice.

PALLADIUM-GALLIUM-SILVER ALLOYS: These alloys have a slightly lighter oxide layer than Pd-Cu alloys and they are thermally compatible with lower expansion porcelains. In addition they have a coparatively low hardness which allows the alloy to be adjusted in the dental lab or the chair side.

DISCOLORATION OF PORCELAIN BY SILVER:1. The colloidal dispersion of silver atoms entering the body and incisal porcelain or

the glazed surface from vapour transport or surface diffusion may cause color changes including green, yellow-green, yellow-orange, orange and brown hues. This phenomenon is termed GREENING.

2. Porcelains with higher sodium content are believed to exhibit more intense discoloration because of mmore rapid silver diffusion in sodium containing glass.

3. The intensity of discoloration increases for higher silver content alloys, is more in the cervical region, lighter shades, multiple firing procedures and certain brands of porcelain and also in silver free alloys due to vaporization of silver from the walls of contaminated furnaces.

PREVENTION OF DISCOLORATION:1. Use of ultra low fusing porcelain or non greening porcelain

2. A pure gold film can be fired on a metal substrate to reduce the surface silver concentration.

3. A ceramic conditioner can be fired as a barrier between the alloy and the porcelain.

4. Use of a graphite block routinely to maintain a reducing atmosphere.

THERMAL COMPATIBILITY AND INCOMPATIBILITY OF METAL-CERAMIC SYSTEMSThermal compatibility refers to the ability of a metal and its veneering porcelain to contract at similar rates during cooling from the ceramic sintering temperature (>871 degree Celsius for low fusing porcelains and <871 degree Celsius for ultra low fusing porcelains). FIG 19-4 DESCRIBE IN DETAIL Stresses develop because of the difference in the thermal coefficients between metal and porcelain as a prosthesis is cooled below the glass transition temperature of porcelain. For today’s porcelain this temperature lies within the range of 500 and 650 degree Celsius. It is also possible to develop failure level stresses in near the metal porcelain junction when the contraction coefficient of the porcelain is much lower than that of the metal. Incompatibility failures may result with compatible systems when atypical cooling rates, excessive porcelain metal thickness ratios, when improper framework or coping geometries are used, or when the number of cycles exceeds the number recommended by the manufacturer. Leucite(K2O.Al2O3.4SWiO2) is the principal high expansion microstructural component of dentalporcelains and its presence may cause large increases in the contraction coefficients of porcelain when more than five firing cycles are necessary.

PHYSICAL PROPERTIES OF HIGH NOBLE AND NOBLE ALLOYSTable 19-7 pg 594 anusavice

BASE METAL ALLOYS FOR CAST METAL AND METAL CERAMIC PROSTHESIS

The no of dental laboratories using base metal alloys steadily increased through 70s and 80s. Although the increased acceptance of these alloys during this period was greatly influenced by the price fluctuation of the noble metals, the trend continued through 90s even when the prices of noble metals had come down. The Ni-Cr-Be alloys have retained their popularity despite the potential toxicity of beryllium and the allergenic potential of nickel.

There are several reasons for the use of nickel chromium and/or cobalt chromium alloys in dentistry

1. Nickel is combined with Chromium to form a highly corrosion resistant alloy2. Cost effectiveness3. Alloys such as Ticonium 100 have been used in re4movable partial denture

frameworks for many years with few reports of allergic reactions.4. Although Beryllium is a toxic metal, dentists and patients should not be affected

because the main risk occurs mainly in the vapor form which is a concern for the technician.

5. Nickel alloys have excellent mechanical properties such as high elastic modulus, high hardness, high sag resistance and a reasonably high elongation (ductility)

6. Lower density On the other hand it is also important the realize the limitations of these alloys, particularly Vis-a –Vis metal ceramic restorations:

1. These alloys are more difficult to cast and presolder 2. The ability to obtain acceptable fitting castings may require special procedures to

adequately compensate for the higher solidification shrinkage3. Potential for porcelain delamination as a result of separation of poorly adherent

oxide layer from the metal substrate. 4. Finishing and polishing require special procedures and is not easy either in the lab

or at chairside.5. Removal of defective restorations may take time.6. Repair of crowns with fractured porcelain veneers which may be simply

performed on noble metal substrates using pin-retained facings or metal ceramic onlays, is more difficult to accomplish in base metal frameworks.

COMPOSITION AND PROPERTIES OF BASE METAL ALLOYS: TABLE 19-8,19-9 Pg 599 Anusavice.

BIOLOGICAL HAZARDS AND PRECAUTIONS

Lab technicians may be exposed occasionally or routinely to excessively high concentrations of beryllium and nickel dust and beryllium vapour. Although the amount

of beryllium rarely exceeds 2% by weight, the atomic concentration of beryllium is around 10.7%. The risk for beryllium vapour exposure is grteatest to dental technicians during alloy melting , especially in the absencedof an adequate exhaust and filtration system. The Occupational Health and Saftey Administration (OSHA) specifies that the exposure to beryllium dust in air should be limited to a particulate beryllium concentration of 2micrograms/m3 of air ( both respirable and nonrespirable particles) determined from an 8 hr time weighted average. The allowable maximum concentration is 5microgram/m3(not to be exceeded for a 15 min period). The National Institute for Occupation Safety and Health (NIOSH) recommends a limit of 0.5 micrograms /m3 based on a 130 min sample. Moffa et al reported that when a local exhaust system was used the cocn of beryllium was reduced to safe levels.

Physiologic responses to beryllium vary from contact dermatitis to severe chemical pneumonitis which can be fatal. Symptoms may range from coughing, chest pain and general weakness to pulmonary dysfunction.

ALLERGY POTENTIAL OF NICKEL:

Of greater concern to dental patient is intra oral exposure to nickel, especially for patient with a known allergy to this element. Inhalation, ingestion and dermal contact on nickel or nickel containing alloys are common because nickel is found in environment sources such as air , soil and food as well as synthetic objects such as coins, kitchen utensils and jewelry.Nickel allergy is determined by patch test using 5% Nickel sulfate. The effects of nickel exposure to humans have included dermatitis, casncer of nasal sinus and larynx, irritation and perforation of nasal septum, loss of smell, asthmatic lung disease, pulmonary pneumoconiosis, lung dysfunction and death.OSHA standard: 8 hr time weighted average concentration limit of 1000 microgram/m3 of nickel and nickel compounds.

PARTIAL DENTURE ALLOYS AND GUIDELINES FOR SELECTION EFFECT OF EACH ALLOY CONSTITUENT:CHROMIUM:Chromium content is responsible for the tarnish resistance and stainless properties of these alloys. When the chromium of an alloy is over 30% the alloy is more difficult to cast. It also forms a brittle phase known as the zigma phase. Therefore dental alloys of these types should not contain more than 28-29% chromium.

COBALT AND NICKEL:They are some2what interchangeable to a certain extent. Cobalt increases the elastic modulus, strength and harness of the alloy more than nickel does. Nickel may increase ductility.

CARBON CONTENT;

The hardness of cobalt based alloys is increased by the increased content of carbon. A change in the carbon content in the order of 0.2 % in these alloys changes their properties to c\such an extent that the alloy would no longer be usable in dentistry.

MOLYBDENUM:The presence of 3-6% molybdenum contributes to the strength of the alloy.

ALUMINIUM:Al in Ni containing alloys forms a compound of Nickel and Aluminium (Ni3-Al).This compound increases the ultimate tenmsile and yield strength.

BERYLLIUM:1 % of this element to Nickel based alloys reduces the fusion range of the alloy by about 100 degree Celsius. It also aids in solid solution hardening. It improves the casting characteristics which possibly aid in porcelasin bonding.

SILICON AND MANGANESE:These are added to increase the castability of these alloys. They are present primarily on oxide to prevent oxidation of other elements during melting. The presence of nitrogen which cannot be controlled unless the castings are made in a controlled atmosphere as in vacuum or argon also contributes to the brittle qualities of these cast alloys. When the nitrogen content of the final alloy ois more thamn 0.1 % the ncasatings loose some of their ductility since the minor ingredients of carbon, nitroigen and ygen effectively increase the properties of the final formulated and designerd in such aweay as rto maximize the rigidity of the prosthesis. The obvious approach would be to increase the thickness of metal substructure since doubling the tyhi9ckness increases the rigidity in bendingby a factor of 8. however, the maximum thickness of the overall restorastion is limited externally by occlusion and proper anatomical contour internally by the desire to retain as mucxh tooth structure as possible(esthtics requir44es a minimal thickness of overlaying porvcelain that results in severe ;limits asstothew maximum thickneres s ofdthe metal)

TITANIUM FOR CASTING APPLICATIONS:Titanium was first isolated thern named 200 years ago but the metal we know is less than 40 years old. It is thew fourth most abundant metalin the easrth’s crust. It is a reactive metal and hence difficult ti extract pure titanium. Dr. Wilhelm kroll invented useful metallurgical processes for the commercial production of titanium metal and hence heis called the father of titanium industry.PROPERTIES OF TITANIUMThe physical and mechanical properties of titanium and its alloys vary greatly with the addition of traces of other elements such as oxygen, iron and nitrogen. Commercially pure Ti is available in four grades (Grade I to Grade IV) based on the incorporation of small amounts of oxygen, nitrogen, hydrogen, iron and carbon during purification procedures. The most commonly used and important Ti alloy is Ti-6Al-4V alloy because of its desirable proportion and predictable producibility.

PROPERTIES OF TITANIUM AND ITS ALLOY (Ti-6Al-4V)Table 2 –Wang/ Fenton

1. Ti is the most biocompatible metal used for dental prostheses.2. High melting point of 1668 degree Celsius.3. It is highly resistant to tarnish and corrosion due to the formation of a coat of

titanium oxide on the surface.4. But as the oxidation rate of Ti increases rapidly above a temperature of 850

degree Celsius, it is desirable to use ultra low fusing porcelains for Ti-ceramic prostheses.

5. A special casting machine with arc melting capability and argon atmosphere is used along with a compatible investment are used to ensure acceptable castability.

6. The most widely used Ti alloy used in dentistry is Ti-6Al-4V which is a alpha-beta alloy. Although it is stronger than CP Ti, it is not as attractive from a biocompatibility point of view due to slow release of Al and V atoms in vivo.

ALTERNATIVES TO CAST METAL TECHNOLOGYTo avoid the challenges and cost of associated with metal casting process, four technologies are available;

SINTERING OF BURNISHED FOIL:The Captek system consists of three pairs of materials:1. CaptekP layer which is adapted first to the die and fired at a temperature of 1075

degree Celsius2. Captek G which is applied over the Captek P coping and the former is drawen by

capillary action into the network structure of the Captek P coping vacated by the adhesive binder.

3. Captek Repair paste and Capfil which are used to add material to Captek structures.

The main ADVANTAGE of Captek structures is the very low thichness of metal tjhat can be achievedehich ensures minimal tooth preparation and hence improved esthetics.

CAD-CAM PROCESSINGA CAD-CAM System electronically or digitally records surface co-ordinates of the prepared tooth and stores these retrived data in the memory of a computer. The image data can then be retrieved immediately to mill or grind a metal, ceramic or composite prosthesis by computer control from a solid block of the chosen material. Within minutes the prosthesis can be fabricated and placed in a prepared tooth and bonded or cemented in the mouth of a patient. The optical scanning procedure eliminates the need for an impression. An advantage of ceramics is that homogeneous, high quality materials with minimal porosity and other typical defects are designed for CAD-CAM applications.

COPY MILLINGThis process is based on the principle of tracing the surface of a pattern that is then replicated from a blank of ceramic, composite, or metal that is ground, cut or milled by a

rotating wheel whose motionis controlled by a link through the tracing device. Eg : The Celay : Mikrona Technologies, Spreintenbach, Switzerland)

ELECTROFORMINGA master cast of the prepared tooth is prepared and coated with a special die spacer to facilitate separation of the duplicating material. After applying aconductive silver layer to the duplicated surface (Gypsum product) , the die is connected to a plating head and connected to a power source and then placed in a plating solution. After a sufficentlyu thick layer of gold or other material is deposited, the gypsum is removed and the coping is sandblasted. Subsequent ceramic layers are condensed and sintered in a conventional way.

REVIEW OF LITERATUREP.J Brockhurst and R.W.S Canon in 1981 examined the requirements of alloys for metal-ceramic crowns and bridgework and discussed the functional requirements and manipulative behavior of as well as cost of alternatives to high gold alloys. They concluded that base metal alloys functioned satisfactorily as compared to high noble alloys provided proper dental lab procedures were employed. Nickel and beryllium did not appear to be health hazards for them.

J. Robert Kelly and Thomas C.Rose in 1983 discussed the various physical properties, biocompatibility, porcelain bonding and corrosion resistance of various non precious alloys and concluded that though the manipulation of non precious alloys is technique sensitive and exacting, their better physical properties and clinical performance merited consideration. They were of the opinion that beryllium was not a health hazard provided proper exhaust and ventilation was used in the dental lab and that the allergenic potential of nickel needed further research.

Russel R. Wang and Aaron Fenton in 1996 reviewed the literature on Titanium for prosthodontic applications. They described the development and properties of titanium for the purpose of evaluating the present status and future trends in its use.

M. Bagby, S.J Marshall and G.W Marshall in 1990 reviewed the literature on metal ceramic bio compatibility. They discuseed the various tests to predict thermomechanical compatibility and also for measuring compatibility at the metal ceramic interface.

Selcuk Oruc and Ybrahim Tulunoglu in 2000 evaluated the marginal and inner fit of metal –cramic restorations and frameworks made with a Nickel-Chromium alloy (Remanium CS) and a commercially pure Titanium (Rematitan). They concluded that the fit of base metal alloy metal ceramic crowns was better than the commercially pure Titanium metal ceramic crowns. However both the artificial crowns were clinically acceptable.

John C. Wataha in 2002 discussed the properties that are relevant tot eh proper selection of an alloy for a given clinical problem. He summarized the various alloys available till

date and their classification and also provided simple guidelines to help dentists choose appropriate alloys for their practices.