Dental casting alloys /certified fixed orthodontic courses by Indian dental academy


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Dental casting alloys /certified fixed orthodontic courses by Indian dental academy

  1. 1. DENTAL CASTING ALLOYS INDIAN DENTAL ACADEMY Leader in Continuing Dental Education
  2. 2. INTRODUCTION In dentistry, metals represent one of the threemajor classes of materials used for the reconstructionof damaged or missing oral tissues. Although metalsare readily distinguished from ceramics and polymers.
  3. 3. The wide varieties of complex dental alloycompositions consist of the following:Dental amalgams containing the major elementsmercury, silver, tin, and copper.Noble metal alloys in which the major elementsare some combination of gold, palladium, silverand important secondary elements includingcopper, platinum, tin, indium and gallium.Base metal alloys with a major element of nickel,cobalt, iron or titanium and many secondaryelements that are found in the alloy compositions.
  4. 4. HISTORY OF METALS IN DENTISTRY Dentistry as a specialty is believedto have begun about 3000 BC. Gold bandsand wires were used by the Phoeniciansafter 2500 BC. Modern dentistry began in 1728when Fauchard published differenttreatment modalities describing many typesof dental restorations, including a methodfor the construction of artificial denturesmade from ivory. Gold shell crowns weredescribed by Mouton in 1746 but they werenot patented until in 1873 by Beers. In1885 Logan patented porcelain fused toplatinum post replacing the unsatisfactorywooden post previously used to build upintra-radicular areas of teeth. In 1907 adetached post crown was introduced which www.indiandentalacademy.comwas more easily adjustable.
  5. 5. Year Event1907 Introduction of Lost-Wax Technique1933 Replacement of Co-Cr for Gold in Removable Partial Dentures1950 Development of Resin Veneers for Gold Alloys1959 Introduction of the Porcelain Fused-to-Metal Technique1968 Palladium-Based Alloys as Alternatives to Gold Alloy1971 Nickel-Based Alloys as Alternatives to Gold Alloys1980s Introduction of All-Ceramic Technologies1999 Gold Alloys as Alternatives to Palladium-Based Alloys
  6. 6. 1971 – THE GOLD STANDARD The United States abandoned the goldstandard in 1971. Gold then became a commodityfreely traded on the open markets. As a result, theprice of gold increased steadily over the next nineyears. In response to the increasing price of gold,new dental alloys were introduced through thefollowing changes: In some alloys, gold was replaced withpalladium. In other alloys, palladium eliminated goldentirely. Base metal alloys with nickel as themajor element eliminated the exclusive need fornoble metals.
  7. 7. KEY TERMSGrain–A microscopic single crystal in the microstructureof a metallic material.Metal – An element whose atomic structure readilyloses electrons to form positively charged ions, andwhich exhibits metallic bonding (through a spatialextension of valence electrons), opacity, good lightreflectance from a polished surface and high electricaland thermal conductivity.Noble metal – which are highly resistant to oxidationand dissolution in inorganic acids. Gold and platinumgroup metals (Platinum, palladium, rhodium, ruthenium,iridium and osmium).Base metal – A metal that readily oxidizes or dissolvesto release ions.
  8. 8. Alloy – A crystalline substance with metallic properties that is composed of two or more chemical elements, at least one of which is metal.Solid solution (metallic) – A solid crystalline phase containing two or more elements, at least one of which is a metal, that are intimately combined at the atomic level.Liquidus temperature – Temperature at which an alloy begins to freeze on cooling or at which the metal is completely molten on heating.Solidus temperature – Temperature at which an alloy becomes solid on cooling or at which the metal begins to melt on heating.
  10. 10. Dimitri Ivanovich Mendeleyev (1834-1934)
  11. 11. Of the 115 elements currently listed in mostrecent versions of the periodic tables of theelements, about 81 can be classified as metals.(Additional elements that have been created withnuclear reactors have short half-lives.) It is ofscientific interest that the metallic elements can begrouped according to density, ductility, melting pointand nobility. This indicates that the properties ofmetals are closely related to their valence electronconfiguration. The groupings of pure metal elementscan be seen in the periodic chart of the elements.Several metals of importance for dental alloys aretransition elements, in which the outermost electronsubshells are occupied before the interior subshellsare completely filled.
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  18. 18. INTERATOMIC PRIMARY BONDS The forces that hold atoms togetherare called cohesive forces. Theseinteratomic bonds may be classified asprimary or secondary. The strength of thesebonds and their ability to reform afterbreakage determine the physical propertiesof a material. Primary atomic bonds may beof three different types. 1. Ionic 2. Covalent 3. Metallic
  19. 19. 1. IONIC BOND FORMATION Characterized by electrontransfer from one element (positive)to another (negative).
  20. 20. 2. COVALENT BOND FORMATION Characterized by electronsharing and very precise bondorientations.
  21. 21. 3. METALLIC BOND FORMATION Since the outer-shell valence electrons can beremoved easily from atoms in metals, the nuclei containingthe balance of the bound electrons form positively chargedionic cores. The unbound or free valence electrons form a“cloud” or “gas”, resulting in electrostatic attraction betweenthe free electron cloud and the positively charged ioniccores. Closed-shell repulsion from the outer electrons of theionic cores balances this attractive force at the equilibriuminteratomic spacing for the metal.
  22. 22. The free electrons act as conductors of boththermal energy and electricity. They transfer energyby moving readily from areas of higher energy tothose of lower energy, under the influence of eithera thermal gradient or an electrical field (potentialgradient). Metallic bonding is also responsible forthe luster or mirror-reflecting property, of polishedmetals and their typical capability of undergoingsignificant permanent deformation (associated withthe properties of ductility and malleability) atsufficiently high mechanical stresses. Thesecharacteristics are not found in ceramic andpolymeric materials in which the atomic bondingoccurs through a combination of the covalent andionic
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  24. 24. INTERATOMIC SECONDARY BONDS In contrast with primary bonds, secondary bonds donot share electrons. Instead, charge variations amongmolecules or atomic groups induce polar forces that attractthe molecules. VAN DER WAALS FORCES Fluctuating dipole that binds inert gas moleculestogether. The arrows show how the fields may fluctuate sothat the charges become momentarily positive and negative.
  25. 25. PHYSICAL PROPERTIESStressWhen a force is applied to a material there is aresistance in the material to the external force.The force is distributed over an area and theratio of the force to the areais called stress. STRESS= F/AStrainThe change in length or deformation per unitlength when a material is subjected to a stressis defined as strain.
  26. 26. STRESS vs STRAIN CURVE If one plots stress vs. strain on a graph, a stress-strain curve will result. The properties of various dentalmaterials, such as alloys, can be compared by analysisof their respective stress-strain curves.P = Elastic modulus or Proportional LimitY-X curve = Yield StrengthX = Ultimate Strength
  27. 27. STRENGTHIt is the maximal stress required to fracture a structure. Types of Strength: - Compressive - Tensile - Shearing
  28. 28. TOUGHNESSIt is defined as the energy required to fracture a material. It is a property of the material which describes how difficult the material would be to break.DUCTILITYIt is the ability of a material to withstand permanent deformation under a tensile load without rupture. A metal may be drawn readily into a wire and is said to be ductile. Ductility is dependent on tensile strength.MALLEABILITYIt is the ability of the material to withstand rupture under compression, as in hammering or rolling into a sheet. It is not dependent on strength as is ductility.
  29. 29. COEFFICIENT OF THERMAL EXPANSION (Linear Coefficient Of Expansion ) Change in length per unit of original length of a materialwhen its temperature is raised 1 ° K
  30. 30. HARDNESSIn mineralogy the hardness is described on the basis of the material to resist scratching. In metallurgy and in most other fields, the amounts of the resistance of indentation is taken as the measure of hardness for the respective material).Brinell hardness number ( BHN )Rockwell hardness number ( RHN )Vickers hardness test (VHN )Knoop hardness test ( KHN )
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  32. 32. TARNISH AND CORROSION High-noble alloys used indentistry are so stable chemically thatthey do not undergo significantcorrosion in the oral environment; themajor components of these alloys aregold, palladium and platinum. (Iridium,osmium, rhodium and ruthenium arealso classified as noble metals.) Silveris not considered noble by dentalstandards, since it will react with air,water and sulfur to form silver sulfide, adark discoloration product. Gold resists chemical attack verywell. Thus it was natural that this mostnoble metal was employed early inmodern dental history for theconstruction of dental appliances.
  33. 33. TARNISH is observable as a surface discoloration on a metal, oras a slight loss or alteration of the surface finish or luster. In theoral environment, tarnish often occurs from the formation of hardand soft deposits on the surface of the restoration. Calculus is theprincipal hard deposit, and its color varies from light yellow tobrown. The soft deposits are plaques and films composed mainlyof microorganisms and mucin. Stain or discoloration arises frompigment-producing bacteria, drugs containing such chemicals asiron or mercury and adsorbed food debris.CORROSION is not merely a surface deposit. It is a process inwhich deterioration of a metal is caused by reaction with itsenvironment. Frequently, the rate of corrosion attack may actuallyincrease over time, especially with surfaces subjected to stress,with intergranular impurities in the metal or with corrosion productsthat do not completely cover the metal surface. Sulfur is probably the most significant factor causing surfacetarnish on casting alloys that contain silver, although chloride hasalso been identified as a contributor.
  34. 34. 1. Chemical or Dry CorrosionIn this the metal reacts to form oxides, sulphides in theabsence of electrolytes 2. Electrochemical or Wet Corrosion
  35. 35. a. Galvanic corrosion b. Heterogeneous Surface CompositionDifference in potential c. Stress Corrosion Occurs due to fatigue or cyclic loading
  36. 36. d. Concentration Cell Corrosion or Crevice Corrosion• Pitting type (Oxygen concentration cell) • Cervical type (Electrolyte concentration cell)
  37. 37. PROTECTION AGAINST CORROSIONi. Passivationii. Increase noble metal contentiii. Polishing restorationsiv. Avoid dissimilar metal restorations Certain metals readily form strong adherent oxide film ontheir surface, which protects them from corrosion. Such a metalis said to be passive. Chromium, titanium and aluminium areexamples of such metals. Since this film is passive to oxidativechemical attack, their formation is called passivation. Chromium provides this corrosion resistance by forming avery thin, adherent surface oxide that prevents the diffusion ofoxygen or other corroding species to the underlying bulk metal.Ifmore than 12% Cr is added to iron or cobalt, we get stainlesssteel or cobalt chromium alloys, which are lightly corrosionresistant and therefore suitable for dental use.
  38. 38.  Noble metals resist corrosionbecause their electromotive force ispositive with regard to any of thecommon reduction reactions found inthe oral environment. In order tocorrode a noble metal under suchconditions, an external current (overpotential) is required. At least half the atoms should benoble metals (gold, platinum, andpalladium) to ensure against corrosion.Palladium has been found to beeffective in reducing the susceptibilityto sulfide tarnishing for alloyscontaining silver.
  40. 40. SOLIDIFICATION OF METALS The temperature decreases steadily from point A topoint B . An increase in temperature then occurs from pointB to point B, at which time the temperature remainsconstant until the time indicated at point C is reached.Subsequently, the temperature of the metal decreasessteadily to room temperature.
  41. 41. The temperature Tf, as indicated by the straight or“plateau” portion of the curve at BC, is the freezing point, orsolidification temperature of the pure metal. This is also themelting point, or fusion temperature. During melting, thetemperature remains constant. During freezing orsolidification, heat is released as the metal changes fromthe higher-energy liquid state to the lower-energy solidstate. The initial cooling of the liquidmetal from Tf to point B is termedsuper cooling. During the supercooling process, crystallizationbegins for the pure metal. Once thecrystals begin to form, release of thelatent heat of fusion causes thetemperature to rise to Tf where itremains until crystallization iscompleted point C.
  42. 42. CRYSTALLIZATION OF METALS Characteristically, apure metal crystallizes fromnuclei in a pattern that oftenresembles the branches of atree, yielding elongatedcrystals that are calledDendrites. In threedimensions, their generalappearance is similar to thatof the two dimensional frostcrystals that form on awindow pane in the winters.
  43. 43. Extensions or elevated areas(termed protuberances) formspontaneously on theadvancing front of thesolidifying metal and grow intoregions of negative temperaturegradient. Secondary andtertiary protuberances result ina three dimensional dendriticstructure.
  44. 44. Although dental base metal casting alloys typicallysolidify with a dendritic micro-structure, most nobel metalcasting alloys solidify with an Equiaxed polycrystallinemicrostructure. The microstructural features in thisfigure are called grains, and the term Equiaxed meansthat the three dimensions of each grain are similar, incontrast to the elongated morphology of the dendrites.
  45. 45. All modern noble metal alloys are finegrained. Smaller the grain size of the metal, themore ductile and stronger it is. It also produces amore homogenous casting and improves thetarnish resistance. A large grain size reduces thestrength and increases the brittleness of themetal. Factors controlling the grain size are therate of cooling, shape of the mold, andcompositon of the alloy.
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  48. 48. NOBLE METALS The noble metals have been the basis of inlays,crowns and bridges because of their resistance to corrosionin the oral cavity. Gold, platinum, palladium, rhodium, ruthenium,iridium, osmium, and silver are the eight noble metals.However, in the oral cavity, silver is more reactive andtherefore is not considered as a noble metal. Of the eight noble metals, four are of majorimportance in dental casting alloys, i.e., gold, platinum,palladium and silver. All four have a face-centered cubiccrystal structure and all are white coloured except for gold.
  49. 49. GOLD Pure gold is a soft andductile metal with a yellow“Gold” hue. It has a densityof 19.3 gms/cm3 , meltingpoint of 1063oC, boiling pointof 2970 oC and CTE of14.2×10-6/°C. Gold has agood luster and takes up ahigh polish. It has goodchemical stability and doesnot tarnish and corrode.
  50. 50. Gold content: Traditionally the gold content of dental casting alloys have been referred to in terms of:• Karat• FinenessKarat: It is the parts of pure gold in 24 parts of alloys. For Eg: a) 24 Karat gold is pure gold b) 22 Karat gold is 22 parts of pure gold and remaining 2 parts of other metal. The term Karat is rarely used to describe gold content in current alloys.Fineness: Fineness of a gold alloy is the parts per thousand of pure gold. Pure gold is 1000 fine. Thus, if ¾ of the gold alloy is pure gold, it is said to be 750 fine.
  51. 51. Physical and mechanical properties of cast pure gold, gold alloys, and condensed gold foil Material Density Hardness (VHN/BHN) Tensile Elongation (g/cm3) (kg/mm2) Strength (%) (MPa)Cast 24k gold 19.3 28(VHN) 105 30Cast 22k gold -- 60(VHN) 240 22Coin gold -- 85 (BHN) 395 30Typical Au-based 15.6 135/195(VHN) 425/525 30/12casting alloy (70 wt% Au)*Condensed gold 19.1 60 (VHN) 250 12.8foil* Values are for softened / hardened condition.
  52. 52. SILVER It is sometimes describedas the “Whitest” of all metals. Itwhitens the alloy, thus helpingto counteract the reddish colourof copper. To a slight extent itincreases strength andhardness. In large amountshowever, it reduces tarnishresistance. It has the lowestdensity 10.4gms/cm3 andmelting point of 961oC, boilingpoint of 2216 oC among the fourprecious metals used in dentalcasting alloys. Its CTE is19.7×10-6/oC , which iscomparatively
  53. 53. PLATINUM It increases the strength andcorrosion resistance. It alsoincreases the melting pointand has a whitening effect onthe alloy. It helps to reducethe grain size.It has thehighest density of 21.45gms/cm3 , highest meltingpoint of 1769oC, boiling pointof 4530 oC and the lowestCTE 8.9×10-6/oC among thefour precious metals used indental casting alloys.
  54. 54. PALLADIUM It is similar to platinum in itseffect. It hardens as well aswhitens the alloy. It also raisesthe fusion temperature andprovides tarnish resistance. Itis less expensive thanplatinum, thus reducing cost ofalloy. It has a density of12.02gms/cm3. Palladium hasa higher melting point of1552oC, boiling point of 3980o C and lower CTE which is11.8×10-6/oC, when comparedto gold.
  55. 55. IRIDIUM, RUTHENIUMThey help to decrease the grain size. They are added invery small quantities (about 100 to 150 ppm). IRIDIUMhas a high melting point of 2454°C , boiling point of 5300°C , density of 22.5gm/cm3 and CTE 6.8×10-6/oC.RUTHENIUM has a melting point of 1966°C , boilingpoint of 4500 °C , density of 12.44 gm/cm 3 and CTE8.3×10-6/oC
  56. 56. BASE METALS These are non-noble metals. They are invaluablecomponents of dental casting alloys because of theirinfluences on physical properties, control of the amount andtype of oxidation, or their strengthening effect. Such metalsare reactive with their environment, and are referred to as‘base metals’. Some of the base metals can be used toprotect an alloy from corrosion (passivation). Although theyare frequently referred as non precious, the preferred term isbase metal. Examples of base metals are chromium, cobalt,nickel, iron, copper, manganese etc.
  57. 57. COBALT Imparts hardness,strength and rigidity tothe alloy . It has a highmelting point of1495°C , boiling pointof 2900 °C , density of8.85 gm/cm3 and CTE 13.8×10-6/oC
  58. 58. NICKEL Cobalt and nickel areinterchangeable.It decreasesstrength, hardness, modulusof elasticity and fusiontemperature. It increasedductility. Bio-incompatibilitydue to nickel, which is themost common metal tocause Contact Dermatitis. Ithas a melting point of1453°C , boiling point of2730 °C , density of 8.9gm/cm3 and CTE 13.3×10- /C6 o
  59. 59. CHROMIUM Its passivating effectensures corrosion resistance.The chromium content isdirectly proportional to tarnishand corrosion resistance. Itreduces the melting point. Alongwith other elements, it also actsin solid solution hardening.Thirty percent chromium isconsidered the upper limit forattaining maximum mechanicalproperties. It has melting pointof 1875°C , boiling point of2665 °C , density of 7.19 www.indiandentalacademy.comgm/cm3 and CTE 6.2×10-6/ oC
  60. 60. COPPERIt is the principal hardener. Itreduces the melting point anddensity of gold. If present insufficient quantity, it gives thealloy a reddish colour. It alsohelps to age harden gold alloys.In greater amounts it reducesresistance to tarnish andcorrosion of the gold alloy.Therefore, the maximum contentshould NOT exceed 16%. It hasmelting point of 1083°C , boilingpoint of 2595 °C , density of8.96 gm/cm³ and CTE 16.5×10-6/°C .
  61. 61. ZINC It acts as a scavenger foroxygen. Without zinc thesilver in the alloy causesabsorption of oxygenduring melting. Laterduring solidification, theoxygen is rejectedproducing gas porositiesin the casting. It has amelting point of 420°C ,boiling point of 906 °C ,density of 7.133gm/cm3and CTE 39.7×10-6/oC
  62. 62. MOLYBDENUM OR TUNGSTEN They are effectivehardeners. Molybdenum ispreferred as it reducesductility to a lesser extentthan tungsten.Molybdenum refines grainstructure. It has meltingpoint of 2610°C , boilingpoint of 5560 °C , densityof 10.22 gm/cm3 and CTE4.9 ×10-6/oC
  63. 63. IRON,BERYLLIUM They help to harden the metal ceramic gold - palladiumalloys, iron being the most effective. In addition, berylliumreduces fusion temperature and refines grain structure . IRONhas melting point of 1527°C , boiling point of 3000 °C , densityof 7.87 gm/cm3 and CTE 12.3 ×10-6/oC .
  64. 64. GALLIUMIt is added tocompensate for thedecreased coefficientof thermal expansionthat results when thealloy is made silverfree. The elimination ofsilver reduces thetendency for greenstain at the margin ofthe metal-porcelaininterface.
  65. 65. MANGANESE AND SILICON Primarily oxide scavengers to prevent oxidation ofother elements during melting. They are also hardeners.MANGANESE has melting point of 650°C , boiling pointof 1107 °C , density of 1.74 gm/cm 3 and CTE 25.2 ×10- / C , where as SILICON has melting point of 1410°C ,6 oboiling point of 2480 °C , density of 2.33 gm/cm 3 and CTE 7.3 ×10-6/oC .
  66. 66. CARBON: Carbon content is mostcritical. Small amounts mayhave a pronounced effect onstrength, hardness andductility. Carbon formscarbides with any of themetallic constituents which isan important factor instrengthening the alloy.However when in excess itincreases brittleness. Thus,control of carbon content in thealloy is important. It hasmelting point of 3700°C ,boiling point of 4830 °C ,density of 2.22 gm/cm3 and www.indiandentalacademy.comCTE 6 ×10-6/oC .
  67. 67. BORONIt is a deoxidizerand hardener, butreduces ductility.
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  69. 69. ALLOYS The use of pure metals is quite limited in dentistry. Tooptimize properties, most metals commonly used in engineeringand dental applications are mixtures of two or more metallicelements or in some cases one or more metals and/ornonmetals. They are generally prepared by fusion of theelements above their melting points. A solid material formed bycombining a metal with one or more other metals or nonmetalsis called an alloy. For example, a small amount of carbon is added to iron toform steel. A certain amount of chromium is added to iron,carbon, and other elements to form stainless steel, an alloy thatis highly resistant to corrosion. Chromium is also used to impartcorrosion resistance to nickel or cobalt alloys, which comprisetwo of the major groups of base metal alloys used in dentistry.
  70. 70. At least four factors determine the extent of solid solubility ofmetals; atom size, valence, chemical affinity and chemicalstructure.ATOM SIZE: If the sizes of two metallic atoms differ by less thanapproximately 15% (first noted by Hume-Rothery), theypossess a favorable size factor for solid solubility.VALENCE: Metals of the same valence and size are more likely toform extensive solid solutions than are metals of differentvalences.CHEMICAL AFFINITY: When two metals exhibit a high degree of chemicalaffinity, they tend to form an intermetallic compound upon www.indiandentalacademy.comsolidification rather than a solid solution.
  71. 71. CRYSTAL STRUCTURE: Only metals with the same type of crystal structure canform a complete series of solid solutions. The simplest alloy is a solid solution, in which atoms oftwo metals are located in the same crystal structure such asbody-centered cubic (bcc), face-centered cubic (fcc) andhexagonal close-packed (hcp).
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  74. 74. Equilibrium phase diagram are of centralimportance to the metallurgy of alloys, since thephases that are present in an alloy system fordifferent compositions and temperatures. Eg Singlephase [isomorphous], eutectic, peritectic andintermetallic. Phase diagrams are useful for understandingthe structure of dental alloys and can providemicrostructural predictions when some cast dentalalloys are subjected to heat treatment. This concept equilibrium phase diagram wasintroduced by using the table salt-water system
  75. 75. EQUILIBRIUM PHASE DIAGRAM FOR ALLOYS Liquidus temperature Solidus temperature Liquidus Solidus
  76. 76. Liquidus temperature – Temperature atwhich an alloy begins to freeze on coolingor at which the metal is completely moltenon heating. Solidus temperature – Temperature atwhich an alloy becomes solid on coolingor at which the metal begins to melt onheating.
  77. 77. This equilibrium phase diagram is for palladium 65% and silver 35%. When analloy composition is undergoing equilibrium solidification, the percentage ofthe liquid and solid phases present at a given temp. can be calculated by thelever rule.Dashed line PO perpendicular to composition line is drawn.A point on line PO corresponds temp. 1500°C, the alloy is clearly in liquid statePoint R - Temp is approx. 1400°C and firstsolid is formed (crystal), but the compositionis different from 65% Palladium and 35%Silver. To determine the composition of firstsolid extend the tie line to point M. This whenprojected to the base line gives thecomposition of first solid which is 77% ofPalladium.Point S - The alloy is midway through itsfreezing range, and the composition of solidand liquid may be determined by drawing tieline Y-W. These points when projected to thebase line gives liquid composition of 57%Palladium at point Y and solid composition of71%Palladium at point W.Point T – As the temp. reaches point T (solid www.indiandentalacademy.comphase) the concentration is 65% Palladium.
  78. 78. CORING In the coring process the last liquid to solidifyis metal with lower solidus temperature andsolidifies between the dendrites. Thus under rapidfreezing conditions, the alloy has a corde structure.The core consists of the dendrites composed ofcompositions with higher solidus temperature, andthe matrix is the portion of the micro-structurebetween the dendrites that contains compositionswith lower solidus temperatures.
  79. 79. HOMOGENIZATION For homogenization heat treatment, the cast alloy isheld at a temperature near its solidus to achieve themaximum amount of diffusion without melting. (This processrequired 6 hr. for the alloy). Little or no grain growth occurswhen a casting receives this type of heat treatment eg.Annealing done mainly for wrought alloys . The ductility ofan alloy usually increases after homogenization heattreatment . Gold alloys are heat treated by softening (solutionheat treat) or hardening (age hardening heat treat)
  80. 80. EUTECTIC ALLOYSMany binary alloy systems do not exhibit complete solubility inboth the liquid and the solid states. The eutectic system is anexample of an alloy for which the component metals have limitedsolid solubility. Two metals, A and B, which are completelyinsoluble in each other in the solid state, provide the simplestillustration of a eutectic alloy.In this case, some grains arecomposed solely of metal Aand the remaining grains arecomposed of metal B. Thesalt and water moleculesintermingle randomly insolution, the result uponfreezing is a mixture of saltcrystals and ice crystals thatform independently of eachother.
  81. 81. 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 (α) containing a small amount of copper atoms.• A copper-rich substitutional solid solution phase (β) containing a small amount of silver atoms. The α and β phases are sometimes referred to as terminal solid solutions because of their locations at the left and right sides of the phase diagram. Boundary ABEGD is the solidus and AED is the liquidus. The major portion of diagram below 780°C is composed of a two phase region.
  82. 82. 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 α solid solution + β solid solution
  83. 83. PERITECTIC ALLOYLiquid + β solid solution α solid solution
  85. 85. 1. ALLOY TYPES BY FUNCTIONS:In 1927, the Bureau of Standard established gold casting alloys, typeI to type IV according to dental function with hardness increasing fromtype I to type IV.Type I (Soft):It is used for fabrication of small inlays, class III and class Vrestorations which are not subjected to great stress . These alloysare easily burnishable.Type -II (Medium):These are used for fabrication of inlays subjected to moderate stress,thick 3/4 crowns, abutments, pontics, full crowns and soft saddles.Type I and II are usually referred to as inlay gold.Type -III (Hard):It is used for fabrication of inlays subjected to high stress, thin 3/4crowns, thin cast backing abutments, pontics, full crowns, denturebases and short span FPDs . Type III alloys can be age hardened.Type-IV (Extra hard):It is used for fabrication of inlays subjected to high stress, denturebases, bars and clasps, partial denture frameworks and long span www.indiandentalacademy.comFPDs. These alloys can be age hardened by heat treatment.
  86. 86. Type III and Type IV gold alloys are generally called "Crownand Bridge Alloys", although type IV alloy is used for highstress applications such as RPD framework.Later, in 1960, metal ceramic alloys were introduced andremovable partial denture alloys were added in thisclassification.Metal ceramic alloys (hard and extra hard):It is suitable for veneering with dental porcelain, copings, thinwalled crowns, short span FPDs and long span FPDs. Thesealloy vary greatly in composition and may be gold, palladium,nickel or cobalt based.Removable partial denture alloys :It is used for removable partial denture frameworks. Now adays, light weight, strong and less expensive nickel or cobaltbased have replaced type IV alloys .
  87. 87. 2. ALLOY TYPES BY DESCRIPTION:By description, these alloys are classified intoA) CROWN AND BRIDGE ALLOYSThis category of alloys include both noble and base metal alloys that have been or potentially could be used in the fabrication of full metal or partial veneers.1. Noble metal alloys: i) Gold based alloy - type III and type IV gold alloys , low gold alloys ii) Non-gold based alloy-Silver -palladium alloy2. Base metal alloys: i) Nickel-based alloys ii) Cobalt based alloys3. Other alloys: i) Copper-zinc with Indium and nickel ii) Silver-indium with palladium
  88. 88. B) METAL CERAMIC ALLOY1. Noble metal alloys for porcelain bonding:i) Gold-platinum -palladium alloyii) Gold-palladium-silver alloyiii) Gold-palladium alloyiv) Palladium silver alloyv) High palladium alloy2. Base metal alloys for porcelain bonding:i) Nickel -chromium alloyii) Cobalt-chromium alloy
  89. 89. C) REMOVABLE PARTIAL DENTURE ALLOY Although type-IV noble metal alloy may be used, majority of removable partial framework are made from base metal alloys:1. Cobalt-chromium alloy2. Nickel-chromium alloy3. Cobalt-chromium-nickel alloy4. Silver-palladium alloy5. Aluminum -bronze alloy
  90. 90. 3.ALLOY TYPE BY NOBILITY High noble, noble, and predominantly base metal. Alloy Classification of the American Dental Association (1984) ALLOY TYPE TOTAL NOBLE METAL CONTENT Contains > 40 wt% Au and > 60 wt High noble metal % of the noble metal elements (Au + Ir + Os + Pd + Pt + Rh + Ru) Noble metal Contains > 25 wt % of the noble metal elementsPredominantly base metal Contains < 25 wt % of the noble metal elements
  91. 91. Classification of alloys for All-Metal restorations, metal ceramic restorations, and frameworks for removable partial dentures.Alloy type All-metal Metal-ceramic Removable partial denturesHigh noble Au-Ag-Cu-Pd Au-Pt-Pd Au-Ag-Cu-Pd Metal ceramic alloys Au-Pd-Ag (5-12wt% Ag) Au-Pd-Ag (>12wt%Ag) Au-Pd (no Ag)Noble Ag-Pd-Au-Cu Pd-Au (no Ag) Ag-Pd-Au-Cu Ag-Pd Pd-Au-Ag Ag-Pd Metal-ceramic alloys Pd-Ag Pd-Cu Pd-Co Pd-Ga-AgBase Metal Pure Ti Pure Ti Pure Ti Ti-Al-V Ti-Al-V Ti-Al-V Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo Ni-Cr-Mo Ni-Cr-Mo Co-Cr-Mo Co-Cr-Mo Co-Cr-Mo Co-Cr-W Co-Cr-W Co-Cr-W Al bronze
  92. 92. 4. ALLOY TYPE BY MAJOR ELEMENTS: Gold-based, palladium-based, silver-based, nickel-based, cobalt-based and titanium-based .5. ALLOY TYPE BY PRINCIPAL THREE ELEMENTS: Such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V and Fe-Ni-Cr. (If two metals are present, a binary alloy is formed; if three or four metals are present, ternary and quaternary alloys, respectively, are produced and so on.)6. ALLOY TYPE BY DOMINANT PHASE SYSTEM: Single phase [isomorphous], eutectic, peritectic and intermetallic.
  93. 93. DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS Biocompatibility Ease of melting Ease of casting Ease of brazing (soldering) Ease of polishing Little solidification shrinkage Minimal reactivity with the mold material Good wear resistance High strength Excellent corrosion resistance Porcelain Bonding
  94. 94. To achieve a sound chemical bond toceramic veneering materials, a substratemetal must be able to form a thin, adherentoxide, preferably one that is light in color sothat it does not interfere with the aestheticpotential of the ceramic. The metal must havea thermal expansion/contraction coefficientthat is closely matched to that of theporcelain.
  96. 96. Composition Range (weight percent) of traditional type I to IV alloys and four metal -ceramic alloys Main elements Au Cu Ag Pd Sn, In, Fe, Zn, Ga Alloy type I High noble (Au base) 83 6 10 0.5 Balance II High noble (Au base) 77 7 14 1 Balance III High noble (Au base) 75 9 11 3.5 Balance III Noble (Au base) 46 8 39 6 Balance III Noble (Ag base) 70 25 Balance IV High noble (Au base) 56 14 25 4 Balance IV Noble (Ag base) 15 14 45 25 BalanceMetal-ceramic High noble (Au base) 52 38 BalanceMetal-ceramic Noble (Pd base) 30 60 BalanceMetal-ceramic High noble (Au base) 88 1 7 (+4Pt) BalanceMetal-ceramic Noble (Pd base) 0-6 0-15 0- 74-88 Balance 10
  97. 97. GOLD CASTING ALLOYS: ADA specification No. 5 classify dental gold casting alloys as:1. High Gold Alloys Type I Inlay Gold Alloy Type II Type III Crown & Bridge Alloy Type IV2. Low Gold Alloys3. White Gold Alloys
  98. 98. HIGH GOLD ALLOY: These alloys contain 70% by weight or more of goldpalladium and platinum. ADA specification No.5 divides thisinto four depending upon mechanical properties. Type I (Soft):- They are weak, soft and highly ductile, useful only inareas of low occlusal stress designed for simple inlays suchas used in class I, III & V cavities. These alloys have a high ductility so they can beburnished easily. Such a characteristic is important sincethese alloys are designed to be used in conjunction with adirect wax pattern technique. Since such a techniqueoccasionally results in margins that are less than ideal it isnecessary to use a metal that can be burnished. At present,these are used very rarely.
  99. 99. PROPERTIES1. Hardness VHN (50 – 90)2. Tensile Strength Quite Low 276 MPa or 40,000 PSi3. Yield Strength 180 MPa or 26,000 PSi4. Linear Casting Shrinkage 1.56% (according to Anusavice)5. Elongation or ductility 46% - William O Brien 18% - AnusaviceCOMPOSITION Au Ag Cu Pt Pd Zn&Ga 83% 10% 6% - 0.5% balance
  100. 100. Type II (Medium):- These are used for conventional inlay or onlay restorationssubject to moderate stress, thick three quarter crowns, pontics andfull crowns. These are harder and have good strength. Ductility is almost same as that of type I alloy however,yield strength is higher. Since burnishability is a function of ductilityand yield strength, greater effort is required to deform the alloy.They are less yellow in color due to less gold.Properties:1. Hardness VHN (90-120)2. Tensile Strength 345 MPa3. Yield Strength 300 MPa4. Linear Casting Shrinkage 1.37%5. Elongation 40.5% - William O Brien 10% - AnusaviceComposition: Au Ag Cu Pt Pd Zn&Ga 77% 14% 7% - 1% balance
  101. 101. Type III (Hard): Inlays subject to high stress and for crown and bridge in contrast to type I and type II, this type can be age hardened. The type III alloy, burnishing is less important than strength.Properties:1. Hardness(VHN) 120 – 1502. Tensile Strength 360 MPa3. Yield Strength 331 MPa4. Linear Casting Shrinkage 1.42%5. Elongation or ductility 39.4% - William O Brien 5% - AnusaviceComposition: Au Ag Cu Pt Pd Zn&Ga 75% 11% 9% - 3.5% balance
  102. 102. Type IV (Extra Hard): These are used in areas of very high stress, crowns andlong span bridges. It has lowest gold content of all four type (Lessthan 70%) but has the highest percentage of silver, copper,platinium and Palladium. It is most responsive to heat treatmentand yield strength but lowers ductility.Properties:1. Hardness VHN (150-200)2. Tensile Strength 462 MPa3. Yield Strength 703 MPa4. Linear Casting Shrinkage 2.30%5. Elongation or ductility 17% - William O Brien 3% - AnusaviceComposition: Au Ag Cu Pt Pd Zn&Ga 56% 25% 14% - 4% balance
  103. 103. Type Hardness Proportional Strength Ductility Corrosion limit resistance I II INCREASES DECREASES III IV
  104. 104. HEAT TREATMENT OF GOLD ALLOYS: Heat treatment of alloys is done in order to alter its mechanical properties. Gold alloys can be heat treated if it contains sufficient amount of copper. Only type III and type IV gold alloys can be heat-treated. There are two types of heat treatment.1. Softening Heat Treatment (Solution heat treatment)2. Hardening Heat Treatment (Age hardening)
  105. 105. 1. SOFTENING HEAT TEMPERATURE Softening heat treatment increased ductility, butreduces tensile strength, proportional limit, and hardness.Indications: It is indicated for appliances that are to be grounded,shaped, or otherwise cold worked in or outside the mouth.Method: The casting is placed in an electric furnace for 10minutes at a temperature of 700oC and then it is quenched inwater. During this period, all intermediate phases arepresumably changed to a disordered solid solution, and therapid quenching prevents ordering from occurring duringcooling. Each alloy has its optimum temperature. Themanufacturer should specify the most favorable temperatureand time.
  106. 106. 2. HARDENING HEAT TREATMENT Hardening heat treatment increases strength,proportional limit, and hardness, but decreases ductility. It isthe copper present in gold alloys, which helps in the agehardening process.Indications: It is indicated for metallic partial dentures, saddles,bridges and other similar structures. It is not employed forsmaller structures such as inlays.Method: It is done by “soaking” or ageing the casting at aspecific temperature for a definite time, usually 15 to 30minutes. It is then water quenched. The aging temperaturedepends on the alloy composition but is generally between200°C and 450°C. During this period, the intermediatephases are changed to an ordered solid solution.
  107. 107. The proper time and temperature for agehardening an alloy are specified by themanufacturer. Ideally, before age hardening an alloy, itshould first be subjected to a softening heattreatment to relieve all strain hardening and to startthe age hardening treatment when the alloy is in adisordered solid solution. This allows better controlof the hardening process.
  109. 109.
  110. 110. METAL CERAMIC ALLOYS4,8,11,15,16,27,31,32,41&43 The main function of metal-ceramic alloys is to reinforceporcelain, thus increasing its resistance to fracture.Requirements:1.They should be able to bond with porcelain.2.Its coefficient of thermal expansion should be compatible with that ofporcelain.3.Its melting temperature should be higher than the porcelain firingtemperature. It should be able to resist creep or sag at thesetemperatures.4.It should not stain or discolor porcelain.The alloys used for metal-ceramic purposes are grouped under twocategories: i) Noble metal alloys ii) Base metal alloys. In case of noble metal alloys for porcelain bonding , addition of1% base metals (iron, indium, tin etc.) increases porcelain-metal bondstrength, which is due to formation of an oxide film on its surface. It www.indiandentalacademy.comalso increases strength and proportional limit.
  111. 111. PROPERTIESModulus of elasticity: The base metal alloys have a modulus of elasticity approximately twicethat of gold alloys. Thus it is suited for long span bridges. Similarly, thinnercastings are possible.Hardness: The hardness of base metal alloys ranges from 175 to 360 VHN. Thus,they are generally harder than noble metal alloys. Thus, cutting, grinding andpolishing requires high speed and other equipment.Ductility: It ranges from 10 to 28% for base metal alloys. Noble metal alloys havean elongation of 25 to 40%.Density: The density of base metal alloys are less, which is approximately 8.0gms/cm3 as compared to 18.39 gms/cm3 for noble metal alloys.Sag Resistance: Base metal alloys resist creep better than gold alloy when heated to hightemperatures during firing.Bond Strength: Varies according to composition.Technique Sensitivity: Base metals are more technique sensitive than highnoble metal-ceramic alloys.
  112. 112. The Gold-Platinum-Palladium (Au-Pt-Pd) System: This is one of the oldest metal ceramic alloy system. But these alloys are notused widely today because they are very expensive.Composition:Gold – 75% to 88%Palladium – Upto 11%Platinum – Upto 8%Silver – 5%Trace elements like Indium, Iron and Tin for porcelain bonding.Advantages Disadvantages1. Excellent castability 1. High cost2. Excellent porcelain bonding 2. Poor sag resistance so not suited for3. Easy to adjust and finish long span fixed partial dentures.4. High nobility level 3. Low hardness (Greater wear)5. Excellent corrosion and tarnish 4. High density (fewer casting per resistance. ounce)6. Biocompatible7. Some are yellow in color8. Not “Technique Sensitive”9. Burnishable
  113. 113. Gold-Palladium-Silver (Au-Pd-Ag) System: These alloys were developed in an attempt to overcome the major limitations inthe gold-platinum-palladium system (mainly poor sag resistance, low hardness & highcost) Two variations on the basic combination of gold, palladium and silver werecreated and are identified as either high-silver or low-silver group.Composition (High Silver Group):Gold – 39% to 53%Silver – 12% to 22%Palladium – 25% to 35%trace amount of oxidizable elements are added for porcelain bonding.Advantages Disadvantages1. Less expensive than Au-Pt-Pd alloys 1. High silver content creates potential2. Improved rigidity and sag resistance. for porcelain discoloration.3. High malleability. 2. High Cost. 3. High coefficient of thermal expansion. 4. Less Tarnish and corrosion resistant.
  114. 114. Composition (Low Silver Group):Gold – 52% to 77%Silver- 5% to 12%Palladium – 10% to 33%Trace amounts of oxidizable elements for porcelain bonding.Advantages Disadvantages1. Less expensive than the Au-Pt-Pd alloys 1. Silver creates potential for porcelain discoloration (but less than high silver group)2. Improved sag resistance 2. High cost.3. High noble metal content 3. High coefficient of thermal expansion.4. Tarnish and corrosive resistant
  115. 115. Gold-Palladium (Au-Pd) System: This particular system was developed in an attempt to overcome the majorlimitations in the Au-Pt-Pd system and Au-Pd-Ag system. Mainly--Porcelain discoloration.-Too high coefficient of thermal expansion & contraction.Composition:Gold – 44% to 55%Gallium – 5%Palladium – 35% to 45%Indium & Tin – 8% to 12%Indium, Gallium and Tin are the oxidizable elements responsible for porcelainbonding.Advantages Disadvantages1. Excellent castability 1. Not thermally compatible with high expansion dental porcelain.2. Good bond strength 2. High cost3. Corrosion and tarnish resistance4. Improved hardness5. Improved ( sag resistance)6. Lower density
  116. 116. Palladium-Silver (Pd-Ag) System8 This was the first gold free system to be introduced in theUnited States (1974) that still contained a noble metal(palladium). It was offered as an economical alternative to themore expensive gold-platinum-silver and gold-palladium-silver(gold based) alloy systems.Composition: (available in two compo.)1. Palladium – 55% to 60% Silver – 25% to 30% Indium and Tin2. Palladium – 50% to 55% Silver – 35% to 40% Tin (Little or no Indium) Trace elements of other oxidizable base elements are alsopresent.
  117. 117.
  118. 118. Advantages Disadvantages1. Low Cost 1. Discoloration (yellow, brown or green) may occur with some dental porcelains.2. Low density 2. Some castibility problems reported (with induction casting)3. Good castibility (when torch 3. Pd and Ag prone to absorb gases. casting) 4. Require regular purging of the porcelain4. Good porcelain bonding, furnace.5. Burnishability 5. May form internal oxides (yet porcelain6. Low hardness bonding does not appear to be a problem)7. Excellent sag resistance 6. Should not be cast in a carbon crucible.8. Moderate nobility level 7. Non-carbon phosphate bonded investments9. Good tarnish and corrosion recommended. resistance. 8. High coefficient of thermal expansion.10. Suitable for long-span fixed partial dentures.
  119. 119. HIGH PALLADIUM SYSTEM8,11,31,32,41&43Several types of high palladium systems were originally introduced (Tuccillo, 1987).More popular composition groups contained cobalt and copper.Composition (PALLADIUM-COBALT ALLOY):Palladium – 78% to 88% Cobalt – 4% to 10%(Some high palladium-cobalt alloys may contain 2% gold)Trace amounts of oxidizable elements (such as gallium and indium) are added forporcelain bonding.Advantages Disadvantages1. Low cost 1. More compatible with higher expansion2. Reportedly good sag resistance porcelains.3. Low density means more casting 2. Are more prone to over-heating than per ounce then gold based alloys. high Pd-Cu.4.They Melt and cast easily 3. Produces a thick, dark oxide5. Good polishability (Supposed 4. Colored oxide layer may cause bluing of the to be similar to Au-Pd alloys) porcelain.6. Reportedly easier to presolder 5. Prone to gas absorption than Pd-Cu alloys. 6. Little information on long-term clinical success.
  120. 120. COMPOSITION (PALLADIUM-COPPER ALLOYS)Palladium – 70% to 80% Copper – 9% to 15%Gold – 1% to 2% Platinum – 1% Some, but not all, high palladium-copper alloys contain small quantities ( 1% to 2%)of gold and/or platinum. Trace amounts of the oxidizable elements gallium, indium andtin are added for porcelain bonding.Advantages Disadvantages1. Good castability 1. Produces dark, thick oxides2. Lower cost (than gold based alloys) 2. May discolor (gray) some dental3. Low density means more castings porcelains. Per ounce 3. Must visually evaluate oxide color to4. Tarnish and corrosion resistance determine if proper adherent oxide was5. Compatible with many dental formed. Porcelains. 4. Should not be cast in carbon crucibles6. Some are available in one unit ingots. (electric casting machines) 5. Prone to gaseous absorption. 6. Subject to thermal creep. 7. May not be suitable for long span fixed partial denture prosthesis. 8. Little information on long term clinical success. 9. Difficult to polish 10. Resoldering is a problem
  122. 122. BASE METAL ALLOYS1,3,4,7,9,10,15,16,18,20,23&34 -Nickel based -Cobalt based Alloys in both systems contain chromium as the second largest constituent. A classification of base metal casting alloys Co-Cr Removable Partial denture Co-Cr-Ni Ni-Cr Co-Cr-MoBase metal SurgicalCasting alloy Implant Co-Cr-Mo No Be. (Class-I) Ni-Cr Fixed Be. Cont.(Class-II) Partial denture Co-Cr (Class-III)
  123. 123. Nickel-chromium (Ni-Cr) System1,7 These metal-ceramic alloy offer such economy that they arealso used for complete crown and all metal fixed partial dentureprosthesis (Bertolotti, 1983). The major constituents are nickel and chromium, with awide array of minor alloying elements. The system contains two major groups: -Beryllium free (class 1) -Beryllium (class 2) Of the two, Ni-Cr-Beryllium alloy are generally regarded aspossessing superior properties and have been more popular(Tuccillo and Cascone,1984).
  124. 124. NICKEL-CHROMIUM BERYLLIUM FREE ALLOYS9,10,23Composition:Nickel – 62% to 77% Chromium – 11% to 22%Boron , iron, molybdenum, Niobium or columbium and tantalum (trace elements).Advantages Disadvantages1. Do not contain beryllium 1. Cannot use with Nickel sensitive patients.2. Low cost 2. Cannot be etched. (Cr doesn’t dissolve in acid)3. Low density means more casting 3. May not cast as well as Ni-Cr-Be alloys per ounce 4. Produces more oxide than Ni-Cr-Be alloys.
  125. 125. NICKEL-CHROMIUM-BERYLLIUM ALLOY9,10,23Composition:Nickel – 62% to 82% Chromium – 11% to 20%Beryllium – 2.0% Numerous minor alloying elements include aluminum, carbon, gallium, iron,manganese, molybdenum, silicon, titanium and /or vanadium are present.Advantages Disadvantages1. Low cost 1. Cannot use with nickel sensitive patients2. Low density, permits more 2. Beryllium exposure may be potentially casting per ounce. harmful to technicians and patients.3. High sag resistance 3. Proper melting and casting is a learned skill.4. Can produce thin casting 4. bond failure more common in the oxide layer.5. Poor thermal conductor 5. High hardness (May wear opposing teeth)6. Can be etched to increase 6. Difficult to solder retention 7. Ingots do not pool 8. Difficult to cut through cemented castings
  126. 126. DISADVANTAGES OF NICKEL-CHROMIUM ALLOYS: Nickel may produce allergic reactions in someindividuals (contact dermatitis). It is also a potentialcarcinogen. Beryllium which is present in many base metalalloys is a potentially toxic substance.21,23 Inhalation ofberyllium containing dust or fumes is the main route ofexposure. It causes a condition know as ‘berylliosis’. Itis characterized by flu-like symptoms and granulomasof the lungs. Adequate precautions must be taken whileworking with base metal alloys. Fumes from meltingand dust from grinding beryllium-containing alloysshould be avoided. The work area should be wellventilated.
  127. 127. Comparative properties of Ni / Cr alloys and type III casting gold alloys for small cast restorations Property (Units) Ni/Cr Type III gold Comments alloyDensity (g/cm3) 8 15 More difficult to produce defect free casting for Ni/Cr alloys.Fusion temperature As high as Normally lower Ni/Cr alloys require electrical induction 1350°C than 1000°C furnace or oxyacetylene equipment.Casting shrinkage (%) 2 1.4 Mostly compensated for by correct choice of investmentTensile strength (MPa) 600 540 Both adequate for the applications being considered.Proportional limit 230 290 Both high enough to prevent distortion for(MPa) applications being considered; not that values are lower than for partial denture alloysModulus of elasticity 220 85 Higher modulus of Ni/Cr is an advantage for(GPa) large restoration e.g. bridges and for porcelain bonded restoration.Hardness (VHN) 300 150 Ni/Cr more difficult to polish but retains polish during serviceDuctility upto 30% 20 (as cast) Relatively large values suggest that burnishing(% elongation) 10 (hardened) is possible; however, large proportional limit www.indiandentalacademy.comsuggests higher forces would be require. value
  128. 128. COBALT CHROMIUM ALLOYS4,6,15,16,22&25 Cobalt chromium alloys have been available since the 1920’s. They possesshigh strength. Their excellent corrosion resistance especially at high temperaturesmakes them useful for a number of applications. These alloys are also known as ‘satellite’ because they maintained their shiny,star-like appearance under different conditions. They have bright lustrous, hard, strong and non-tarnishing qualities.APPLICATIONS:1. Denture base2. Cast removable partial denture framework.3. Surgical implants.4. Car spark plugs and turbine blades.COMPOSITION:Cobalt - 55 to 65%Chromium - 23 to 30%Nickel - 0 to 20%Molybdenum - 0 to 7%Iron - 0 to 5%Carbon - upto 0.4%Tungsten, Manganese, Silicon and Platinum in traces. According to A.D.A specification No. 14 a minimum of 85% by weight ofchromium, cobalt, and nickel is required. Thus the gold base corrosion resistant www.indiandentalacademy.comalloys are excluded.
  129. 129. PROPERTIES The Cobalt-Chromium alloys have replaced Type IV gold alloys because of their lower cost and adequate mechanical properties. Chromium is added for tarnish resistance since chromium oxide forms an adherent and resistant surface layer.1.Physical Properties:Density: The density is half that of gold alloys, so they are lighter in weight. 8 to 9 gms/cm3.Fusion temperature: The casting temperature of this alloy is considerably higher than that of gold alloys. 1250oC to 1480oC. A.D.A. specification No. 14 divides it into two types, based on fusion temperature (which is defined as the liquidus temperature) Type-I (High fusing) – liquidus temperature greater than 1300oC Type-II (Low fusing) – liquidus temperature lower than 1300oC
  130. 130. 2. Mechanical Properties:Yield strength: It is higher than that of gold alloys. 710Mpa (103,000psi).Elongation: Their ductility is lower than that of gold alloys. Depending on the composition, rate of cooling, and the fusion and mold temperature employed, it ranges from 1 to 12%. These alloys work harden very easily, so care must be taken while adjusting the clasp arms of the partial denture.Modulus of elasticity: They are twice as stiff as gold alloys 22.5×103Mpa. Thus, casting can be made more thinner, thus decreasing the weight of the R.P.D. Adjustment of clasp is not easy.Hardness: These alloys are 50% harder than gold alloys 432 VHN.Thus, cutting, grinding and finishing is difficult.
  131. 131. 3. Tarnish and corrosion resistance: Formation of alayer of chromium oxide on the surface of these alloys preventstarnish and corrosion in the oral cavity. Solutions of hypochlorite and other compounds that arepresent in some denture-cleaning agents will cause corrosion insuch base metal alloys. Even the oxygenating denture cleanserswill stain such alloys. Therefore, these solutions should not beused for cleaning cobalt-chromium base alloys.4. Casting Shrinkage: The casting shrinkage is muchgreater than that of gold alloys (2.3%), so limited use in crown &bridge. The high shrinkage is due to their high fusion temperature.5. Porosity: As in gold alloys, porosity is due to shrinkage andrelease of dissolved gases which is not true in case of Co-Cralloys. Porosity is affected by the composition of the alloys and itsmanipulations.
  132. 132. Comparative properties of Co / Cr alloys and type IV casting gold alloys for partial denture Property (Units) Co/Cr Type IV gold Comments alloyDensity (g/cm3) 8-9 15 More difficult to produce defect free casting for Co/Cr alloys but denture frameworks are lighterFusion temperature as high Normally lower Co/Cr alloys require electrical induction as than 1000°C furnace or oxyacetylene equipment. 1500°C Can not use gypsum bonded investments for Co/Cr alloysCasting shrinkage 2.3 1.4 Mostly compensated for by correct(%) choice of investmentTensile strength 850 750 Both acceptable(MPa)Proportional limit 710 500 Both acceptable; can resist stresses(MPa) without deformationModulus of 225 100 Co/Cr more rigid for equivalent thickness;elasticity (GPa) advantage for connectors; disadvantage for claspsHardness (Vickers) 432 250 Co/Cr more difficult to polish but retains polish during serviceDuctility (% 2 15 (as cast) Co/Cr clasps may fractured ifelongation) 8 (hardened) adjustments are attempted.
  133. 133. Summary of base metal alloy properties Property Ni-Cr without Ni-Cr with Be Co-Cr BeStrength (MPa) 255-550 480-830 415-550 Ultimate 550-900 760-1380 550-900 tensilestrength (MPa)% elongation 5-35 3-25 1-12 Modulus of 13.8-20.7 x 104 17.2-20.7 x 104 17.2-22.5x104 elasticity (MPa) Vickers 175-350 300-350 300-500 hardness Casting 1430-1570 1370-1480 1430-1590 temperature (°C)
  134. 134. TITANIUM AND TITANIUM ALLOYS4,13,19,45,46&48Titanium is called “material of choice” in dentistry. This is attributed to the oxideformation property which forms basis for corrosion resistance and biocompatibility ofthis material. The term titanium is used for all types of pure and alloyed titanium.Properties of titanium:-Resistance to electrochemical degradation-Begins biological response-Relatively light weight-Low density (4.5 g/cm3)-Low modulus (100 GPa)-High strength (yield strength = 170-480 MPa; ultimatestrength = 240-550 MPa)-Passivity-Low coefficient of thermal expansion (8.5 x 10–6/°C)-Melting & boiling point of 1668°C & 3260°CUses:Commercially pure titanium is used for dental implants, surface coatings, crowns, www.indiandentalacademy.compartial dentures, complete dentures and orthodontic wires
  135. 135. Commercially Pure Titanium (CP Ti):It is available in four grades (according to American Society for Testing andMaterials ASTM) which vary according to the oxygen (0.18-0.40 wt.%), iron (0.20-0.50 wt%) and other impurities. It has got an alpha phase structure at roomtemperature and converts to beta phase structure at 883°C which is stronger butbrittle.
  136. 136. TITANIUM ALLOYS Alloying elements are added to stabilize alpha or the betaphase by changing beta transformation temperature e.g. inTi-6Al-4V48, Aluminum is an alpha stabilizer whereas Vanadiumas well as copper and palladium are beta stabilizer. Alphatitanium is weld able but difficult to work with at roomtemperature. Beta titanium is malleable at room temperature andis used in orthodontics, but is difficult to weld. Pure titanium is used to cast crowns, partial denture, andcomplete denture.
  137. 137. CAST TITANIUM:Cast titanium has been used for more than 50 years, and ithas been recently that precision casting can be obtained fromit. The two most important factors in casting titanium basedmaterials are its high melting point (1668°C) and chemicalreactivity. Because of the high melting point, special meltingprocedures, cooling cycles, mold materials, and castingequipments are required to prevent metal contamination,because it readily reacts with hydrogen, oxygen and nitrogenat temperatures greater than 600°C. So casting is done in avacuum or inert gas atmosphere. The investment materialssuch as phosphate bonded silica and phosphate investmentmaterial with added trace metal are used. It has been shownthat magnesium based investment cause internal porosity incasting.
  138. 138. Because of its low density, it is difficult to cast in centrifugalcasting machine. So advanced casting machine combiningcentrifugal, vacuum, pressure and gravity casting with electricarc 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
  139. 139. REVIEW OF LITERATUREMoffa JP, Guckes AD, Okawa MT and Lilly GE (1973)23 did an evaluation ofnonprecious alloys for use with porcelain veneers and provided quantitativeinformation about the levels of beryllium produced during the finishing and polishingof cast base metal dental alloys with there harmful effects.Shillingburg HT, Hobo S and Fisher DW (1977)39 Studied Preparation design andmargin distortion in porcelain-fused-to-metal restorations. The results of this study suggested that thermal incompatibility stresses werelikely to cause margin distortion in metal ceramic crowns. However, subsequentstudies support other potential mechanisms, including the effect of excessive sandblasting time and/or pressure.Baran GR (1983)7 did an extensive study on metallurgy of sixteen commerciallyavailable Ni-Cr alloys for fixed prosthodontics and compared their alloycompositions, mechanical properties (yield strength, tensile strength, %elongationand hardness number), microstructures and clinically relevant considerations forthe use of these alloys.
  140. 140. Carr A.B., Cai Z., Brantley W.A.(1993)11 did a study on new highpalladium casting alloys (generation 1&2). For the five high-palladiumalloys studied, the following conclusions were drawn:1. An increase in the investment burn out temperature from 1400°F to1500 °F had little effect on microstructure and hardness, but grain ordendritic size was found to vary substantially.2. Hot tears were more prevalent in the alloys when the higher burnouttemperature was used.3. Heat treatment simulating porcelain firing cycles for these alloysgenerally caused decrease in hardness.Reisbick NH and Brantley WA (1995)36 conducted a study onmechanical properties and microstructural variations for recasting lowgold alloys. They concluded that significant decrease in yield strengthand percentage elongation were observed for recasting these alloys butnot in tensile strength when the Type III gold alloys were recasted upto 3times. Scanning electron microscope examination revealed that thenumber of casting defects (principally porosity) increased with thenumber of times the alloy was remelted.
  141. 141. Berzins DW, Sarkar NK et al (2000)8 did an in-vitro electrochemical evaluation of high palladium alloys in relation to palladium allergy. The high incidence of allergic reaction was associated with Pd-Cu based alloys. The “Pd-skin” of these alloys when in contact with saliva release some Pd++ ions (an allergen) which can trigger the cascade of biological reaction involved in allergy and hypersensitivity. It is a time dependent process. In Pd alloys containing Ag, formation of Ag-Cl film on the alloy surface is supposed to prevent Pd in coming in contact with oral fluids, having a masking effect and thus avoiding allergy.Tufekci E, Mitchell JC et al (2002)43 did a study on spectroscopymeasurements of elemental release from high palladium dental castingalloys into a corrosion testing medium. A highly sensitive analyticaltechnique shows that the release of individual elements over a onemonth period, suggesting that there may be low risk of biologicalreaction with the Pd-Ga alloys than with the Pd-Cu-Ga alloys tested.
  142. 142. Ahmad SAH, Omar MB, Homa D. (2003)1 did an investigation of the cytotoxic effects of commercially available dental casting alloys and concluded the following: 1.The high noble alloy Bioherador N was significantly less cytotoxic than all the base metal alloys tested in this study (Ni-Cr, Co-Cr, Cu-based) 2. The Ni-Cr alloy CB Soft was significantly more cytotoxic than all the Ni-Cr and Co-Cr alloys tested. This could be related to the content of Cu, low content of Cr and absence of Mo in its composition. 3. Cu based alloys Thermobond showed a more severe cytotoxic reaction than all the other alloys.O’Brien WJ (2004)29 Biomaterial Properties Database, University of Michigan: tables/.This database provides an electronic reference to the following properties ofdental materials; strength between restorative materials and tooth structures,BHN, coefficient of thermal friction, coefficient of thermal expansion (linear),colours of dental shade guide, contact angles, creep, density, dynamicmodulus, elastic modulus, heat of fusion, KHN, melting temperatures andranges, %elongation, permanent deformities, proportional limit, shearstrength, tear energy, tear strength, ultimate compressive strength, VHN andyield strength.
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  148. 148. THANK