Precious metal alloys

precious metal alloys and there uses in dentistry.

1. Precious metal alloys Presented by Dr. ARUNIMA UPENDRAN 1 st year MDS 1

2. introduction Although popular press dental journals have occasionally promoted "metal-free” dentistry as desirable, the metals remain the only clinically proven materials for many long-term dental applications 2

3. Definitions –gpt9 3

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5. history As early as the seventh century B.C Etruscan dental prostheses made by passing thin strips of gold round teeth on each side of a space from which a tooth or teeth had been lost and riveting the strip so as to hold the substitute teeth in place 5

6. The discoveries were dated back to 550 B.C . A canine tooth like object made of two piece of calcite having hardness similar to natural teeth showing wear on the chewing surface & secured with gold wires wrapped around the neck of adjacent teeth 6

7. The first printed book on dentistry, entitled ' Artzney Buchlein ' ('The Little Pharmacopoeia'), was published by Michael Blum in Leipzig in 1530 . Under this title or as ' Zene Artznei ' (' Dental Medicine') “ Scrape and clean the hole and the area of decay with a fine small chisel or a little knife or a file, or with another suitable instrument, and then to preserve the other part of the tooth, fill the cavity with gold leaves.” 7

8. Maggilio in 1809 , a dentist at the university of Nancy , France, author of the book called “THE ART OF THE DENTIST”. The first reference to modern style implants. He has described the implant & placement. He made the tooth root shaped implant with 18 carat gold with three prongs at the end to hold it in place in the bone . The implant was placed in the freshly extracted socket site retained with the prongs. After the tissues healed the crown was attached with the help of post placed into the hole of root section of the implant. He placed the single stage gold implant 8

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10. In 1886 Harris treated a Chinese patient in Grass valley , California . He placed the tooth root shaped platinum post with lead coating, lasted for 27 yrs Reported in Dental Comos . In 1888, Charles Henry Land who fused porcelain on thin platinum caps for use as crowns. This technique is still used in making jacket crowns.  In 1890, a Massachusetts minister had his lower jaw resected & was restored with an extensive system of gold crowns soldered & joined to hinged device attached to the remaining dentition 10

11. Bonwill in 1895 reported on the implantation of one or two tubes of gold or Iridium as a support for individual teeth or crown. In 1898 R. E Payne at the National Dental Association meeting gave the first clinical demonstration by placing the silver capsule in the extracted tooth socket.  In 1896 B. F. Philbrook , attempted to make soft, fusible metal inlays by a lost wax process, he fitted several white metal inlays and one gold inlay. 11

12. In 1897 George B. Martin demonstrated gold dummy or artificial teeth, called ` pontics ', for use on fixed bridges; these were soldered to gold crowns on the abutment teeth. In 1900, J. G.Schottler used a method to restore the biting edges of front teeth by placing a platinum wire in the root canal, building the required shape on the tooth with wax. Invested and casted it in gold. In 1906 John A. Lenz obtained a patent for devising a method for lost wax casting a gold chewing surface onto a gold band made to fit around a tooth. 12

13. At a meeting of the New York Odontological Society on January 15, 1907, William H.Taggart of Chicago read a lecture entitled `A New and Accurate Method of Casting Gold Inlays' in which he described a lost wax technique which can truly be said to have revolutionized restorative and prosthetic dentistry 13

14. In 1907, Dr. Solbrig , in Paris. introduced his casting pliers which achieved enormous popularity for the rapid production of small inlays. In 1913 Dr. Edward J. Greenfield , fabricated the hollow cylindrical basket root of 20 gauge iridio platinum soldered with 24 carat gold. In 1948, metallurgists experimenting with various alloys were able to decrease the gold content while maintaining their resistance to tarnish. This breakthrough was due to palladium. It counteracted the tarnish potential of silver. 14

15. 1950 -Developments of resin veneers for gold alloys. 1959- Introduction of porcelain fused to metal technique. In the late 1950s , there was the successful Veneering of a metal substructure with dental porcelain. Until that time, dental porcelain had a markedly lower coefficient of thermal expansion than did gold alloys. This thermal mismatch often led to impossible to attain a bond between the two structural components. 15

16. It was found that adding both platinum and palladium to gold lowered the coefficient of thermal expansion/contraction of the alloy sufficiently to ensure physical compatibility between the porcelain Veneer and the metal substructure. In 1968-Palladium based alloys as alternative to gold alloys. In 1971-nickel based alloys as alternative to gold alloys. 1971 – The Gold Standard 16

17. The United States abandoned the gold standard in 1971. Prices of gold increased, in response to that, new dental alloys were introduced through the following charges. In some alloys, gold was replaced with palladium. In other alloys, palladium eliminated gold entirely. Base metal alloys with nickel as the major element eliminated the exclusive need for noble metals . 17

18. Palladium-silver alloy type was introduced to the US market in 1974 as the first gold free noble alloy available for metal ceramic restorations. The first alloy of the gold palladium type (group V according to ADA)Olympia was introduced in 1977 by JF JELENKO AND CO. 18

19. 1980’s –Introduction of All ceramic technologies. Using a mesh screen pattern as a castability monitor, WHITLOCK ET AL in 1985 measured percent castability values of fourteen metal –ceramic alloys. 1999 -Gold alloys as alternatives to palladium based alloys 19

20. Thus the history of dental casting alloys has been influenced by 3 major factors: 1.The Technologic changes of dental prosthesis. 2.Metallurgic Advancements 3.Price changes of Noble metals since 1968. 20

21. definitions 21 GPT 9

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23. Of the 118 elements currently listed in the periodic table, about 88 (74.6%) can be classified as metals. Groups 3 to 12 - Transition metals. Groups 14 to 16 – Non metals Dental alloys are transition elements (typically 21 to 80, although groups 89 to 112 are also included) 23

24. METALLURGY The science and art of the extraction of metals from their ores together with the refinement of these metals and their adaption to various uses 24 CHEMICAL PHYSICAL MECHANICAL TYPES

25. CHEMICAL METALLURGY Production and refinement of metals. “ Process ” metallurgy - processing of ores for the production of metals. 25

26. PHYSICAL METALLURGY Physical metallurgy is newer science and deals with the possible alteration in structure as well as the characteristic physical properties of metals. In some respects physical metallurgy and metallography are closely related. Investigates the effects of composition, casting processes, deformation, and heat treatment on the physical and mechanical properties of metals 26

27. MECHANICAL METALLURGY It includes various processes in the fabrication of a structure such as the casting , rolling or drawing operations . In restorative materials, physical metallurgy combined with metallography and the mechanical phase of metallurgy are of greatest importance. 27

28. General characteristics of metals A metal - ionizes positively in solution. Typical and characteristic properties - lustre , opacity, density, thermal and electrical conductivity. Extreme ductility and malleability - often desirable in metals used in dentistry - predominate in pure metals rather than in alloys. 28

29. STRUCTURE Crystalline structures in the solid state. SPACE LATTICE / CRYSTAL - any arrangement of atoms in space such that every atom is situated similarly to every other atom. Types - Length of each of three unit cell edges (called the axes) and the angles between the edges. 29

30. 14 possible lattice types or forms Simple cubic space lattice - Single cells of cubic space lattice Simple cubic Face-centered cubic - Cu Body-centered cubic - Fe 30

31. Other simple lattice types of dental interest. Rhombohedral Orthorhombic Monoclinic Triclinic Tetragonal Simple hexagonal Close packed hexagonal – Ti, Zn, Zr Rhombic . 31

32. DEFORMATION OF METALS Slip planes 32

33. The plane along which an edge dislocation moves is known as a slip plane The crystallographic direction in which the atomic planes have been displaced is termed the slip direction Combination of a slip plane and a slip direction is termed a slip system . 33

34. CRYSTAL IMPERFECTIONS Point defects Line defect/ Dislocations Slip plane Slip direction Slip system 34

35. ALLOY Alloy —a mixture of two or more metals or metalloids that are mutually soluble in the molten state; distinguished as binary, ternary, quaternary, etc., depending on the number of metals within the mixture; alloying elements are added to alter the hardness, strength, and toughness of a metallic element, thus obtaining properties not found in a pure metal; alloys may also be classified on the basis of their behavior when solidified. Alloy system —All possible alloyed combinations of two or more elements at least one of which is a metal. 35

36. ALLOY - GROUPS Dental amalgams – Hg, Ag, Sn , Cu High noble (HN) alloys At least 40 wt% Au and 60 wt% of noble metals. Noble (N) metal alloys Palladium (Pd) - main noble metal content - 25 weight percent Also contain Au, Ag, Cu, Ga , In, Pt, Sn . Predominantly base (PB) metal alloy Less than 25 wt% of noble metals, Ni-Cr; Co-Cr; Fe-C-Cr; CP-Ti; Ti-Al-V 36

37. SOLIDIFICATION AND MICROSTRUCTURE OF CAST DENTAL ALLOYS Microstructure —Structural features of a metal, including grains, grain boundaries, phases, and defects such as porosity, revealed by microscopic imaging of the chemically or electrolytic ally etched surface of a flat, polished specimen 37

38. During solidification - liquid changes in to solid - cooling Energy of liquid is < solid above the melting point. Liquid is stable above the melting point Below the melting point, the energy of liquid >solid. Solid becomes more stable At the melting point, liquid gets converted in to solid during cooling. This transformation of liquid into solid below melting point is known as solidification 38

39. Thermodynamically, both liquid and solid equal energy at melting point - equally stable at melting point - no solidification or melting at the melting point 3.Under-cooling - essential for solidification 4.Solidification occurs by two process : nucleation and growth. 39

40. SOLIDIFICATION OF METALS During the super cooling process, crystallization of the pure metal begins. Once the crystals begin to form, the release of the latent heat of fusion causes the temperature to rise to T f , where it remains until crystallization is completed at point C 40

41. NUCLEATION Solidification begins with the formation of embryos in the molten metal—(small clusters of atoms that form nuclei of crystallization) At temperatures > Tf - embryos form spontaneously in the molten metal - unstable, since the liquid state has a lower free energy than the solid state. 41

42. Nucleation and Growth Transformation EMBRYO Tiny particle of solid that forms from the liquid as atoms cluster together. Unstable - either grow into a stable nucleus or re-dissolve. NUCLEUS Large enough to be stable, nucleation has occurred and growth of the solid can begin. 42

43. I step - creation of tiny, stable, nuclei in the liquid metal Cooling the liquid below its equilibrium freezing temperature, or under cooling, provides the driving force for solidification Once a cluster reaches a critical size, it becomes a stable nucleus and continues to grow The mold walls and any solid particles present in the liquid make nucleation easier 43 Cluster of atoms Embryo Nuclei Crystals Grains r < Ro r > Ro

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45. Heterogeneous nucleation —Formation of solid nuclei on the mold walls or on particles within a solidifying molten metal. Homogeneous nucleation —Formation of nuclei that occur at random locations within a supercooled molten metal in a clean, inert container No external interface 45

46. Crystals grow as dendrites, which can be described as three-dimensional, branched network structures emanating from the central nucleus 46

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48. Volume free energy ΔGV – free energy difference between the liquid and solid Surface energy ΔGs – the energy needed to create a surface for the spherical particles Total free energy Change, ΔGT = ΔGV + ΔGs At low temperatures atoms form small cluster or groups. Embryos formed may either form into stable nuclei or may re-dissolve in the liquid. Beyond the critical radius of the nuclei it will remain stable and growth occurs 48

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52. LIQUID-TO-SOLID TRANSFORMATION OF CAST METALS 52

53. Dendrite formation occurs during solidification of alloys because of constitutional supercooling Dendritic microstructures are not desirable in cast dental alloys because interdendritic regions serve as sites for crack formation and propagation. Microcracks , called “hot tears,” form at elevated temperatures in thin cast areas of these alloys . 53

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55. Crystal growth continue until all the material has solidified and all the dendritic crystals are in contact Grain —A single crystal in the microstructure of a metal Grain boundary —The interface between adjacent grains in a polycrystalline metal Dendritic microstructure —A cast alloy structure of highly elongated crystals with a branched morphology Equiaxed grain microstructure —A cast alloy microstructure with crystal (grain) dimensions that are similar along all crystal axes 55

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60. GRAIN SIZE AND GRAIN BOUNDARY Material placed under sufficiently high stress - dislocation is able to move through the lattice until it reaches a grain boundary The plane along which the dislocation moves is called a slip plane Stress required to initiate movement is the elastic limit 60

61. Grain boundaries - Natural barrier to the movement of dislocations. Concentration of grain boundaries increases as the grain size decreases. Metals with finer grain structure - Harder Higher values of elastic limit 61

62. Fine grain structure achieved by rapid cooling of the molten metal or alloy following casting. Referred as quenching - many nuclei of crystallization are formed - large number of relatively small grains 62

63. Fine grain sizes - noble metal alloys Rapid solidification conditions – time is inadequate for the growth of large crystals. Compositional uniformity and corrosion resistance - superior for a fine grain size Less opportunity for microsegregation . Controls yield strength – inversely proportional 63

64. Some metals and alloys are said to have a refined grain structure. Achieved by seeding the molten metal with an additive metal which forms nuclei crystallization 64

65. STRUCTURE AND PROPERTIES OF ALLOYS 65

66. A lloy Mixture of two or more metals. A lloy system Refers to all possible compositions of an alloy. Eg . silver-copper system refers to all alloys with compositions ranging between 100% silver and 100% copper. Metals usually show mutual solubility, one within another. When the molten mixture is cooled to below the melting point:- The component metals may remain soluble in each other forming a solid solution . 66

67. Solid solution When in a solid, the atoms of solute are present in the lattice of the solvent, it is known as solid solution. It is considered a solution rather than a compound Crystal structure of the solvent remains unchanged by addition of the solutes. 67 Two types. Substitutional solid solution Interstitial solid solution

68. Substitutional solid solution When the atoms of solute substitute for the atoms of the solvent in its lattice, the solution is known as Substitutional solid solution. The solute may incorporate into the solvent crystal lattice substitutionally by replacing a solvent particle in the lattice. Substitutional element replaces host atoms in its lattice 68

69. Substitutional solid solutions can be of two types Ordered solid solution Disordered solid solution 69

70. Ordered solid solution Atoms of the solute occupy certain preferred sites in the lattice of the solvent, Occur only at certain fixed ratios of the solute and solvent atoms. In Cu – Au system, Cu atoms occupying the face-centered sites and Au atoms occupying the corner sites of the FCC unit cell. 70

71. Disordered solid solutions Atoms of the solute are present randomly in the lattice of the solute Most of the solid solutions are disordered solid solutions 71

72. Interstitial solid solution Atoms of the solute occupy the interstitial spaces in the lattice of the solvent If the size of the solute is less than 40% that of solvent, interstitial solid solution may be formed. The solute may incorporate into the solvent crystal lattice interstitially , by fitting into the space between solvent particles. Only H, Li, Na and B form interstitial solid solution. 72

73. IMPORTANCE OF SOLID SOLUTIONS ??? Solid solutions are generally harder, stronger and have higher values of elastic limit than the pure metals from which they are derived. This explains why pure metals are rarely used. The hardening effect, known as solution hardening , -atoms of different atomic radii within the same lattice form a mechanical resistance to the movement of dislocations along slip planes 73

74. CONDITIONS FOR SOLID SOLUBILITY Atom size difference-diameters of the solute atoms Valence- SS forms - same valence; chemical affinity Type of crystal structure - same type of crystal structure Potential for solvent atoms to become ordered 74

75. PHYSICAL PROPERTIES OF SOLID SOLUTIONS Solid solution strengthening Greater concentrations of the solute atoms Increasingly dissimilar sizes of the solvent and solute atoms. For binary alloys - maximum hardness at concentrations of approx. 50% for each metal 75

76. Cooling curve of a solid solution 76

77. EQUILIBRIUM-PHASE DIAGRAMS Equilibrium- or constitution-phase diagrams Identify the phases present in an alloy system for different compositions and temperatures . 77

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79. Phase diagram For Solid Solution 79

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81. All possible combinations of 2 metals - completely soluble at all compositions in both the solid and liquid states. Liquid, liquid-plus solid, and solid regions separated by the liquidus and solidus curves 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 81

82. INTERPRETATION OF THE PHASE DIAGRAM – Determines - Composition of alloy Weight percentage of alloy 82

83. PO - 65% Pd and 35% Ag PO intersects liquidus curve at point R - first solid forms Temp 1400 ᵒC - Composition of the first solid formed Tie line RM - R on the liquidus curve to M on the solidus curve = 77% Pd Temp - 1370 °C Compositions of the solid and liquid determined YW -liquid 57% Pd, and solid 71% Pd Temp – 1340 ᵒC last portion of liquid that solidifies - 52% Pd (point N). The solid phase contains 65% Pd (point T). 83

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86. Percentages of two phases in equilibrium at a given temperature – LEVER RULE Length of the tie line segment opposite a given boundary curve ÷ Total length of the tie line connecting the two boundary curves is the percentage of the phase. 86 FULCRUM

87. CORING AND HOMOGENIZATION HEAT TREATMENT Coring process Under rapid solidification conditions, the alloy has a cored structure. The core consists of dendrites composed of compositions that developed at higher solidus temperatures Matrix is the portion of the microstructure between the dendrites that contains compositions that developed at lower solidus temperatures 87

88. 88 Indication of the degree of coring - Separation of the solidus and liquidus lines on the phase diagram. The potential for coring is greater when there is wide separation of solidus and liquidus lines

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90. Homogenization heat treatment Promotes atomic diffusion - eliminate as-cast compositional difference Produce equiaxed grains Reduce microsegregation as can be confirmed by the appearance of distinct grain boundaries in the microstructure 90

91. Homogenizing the cast structure Heated near its solidus temperature to promote the most rapid diffusion without melting – about 6 hrs Little grain growth occurs because the grain boundaries are immobilized by secondary or impurity elements and phases 91

92. Other adv. of homogenisation - Reduces tarnish and corrosion Increases ductility Decreases brittleness 92

93. Wrought alloys are heat treated by Annealing - Ductility increases Gold alloys are heat treated by softening (solution heat treatment ) or hardening (age hardening heat treatment ) 93

94. EUTECTIC ALLOYS The term eutectic greek word ‘ euctectos ’- easily fused Correspond to a composition with the lowest melting point in an alloy system. Used in joining metal components - brazing or soldering The eutectic alloy is one in which the components exhibit complete solubility in the liquid state but limited solid solubility Eg ; Ag – Cu system Au – Ir system Reaction during the cooling process : Liquid α solid solution + β solid solution 94

95. BINARY EUTECTIC SYSTEM - THE SILVER-COPPER SYSTEM 95

96. 3 phases Liquid phase (L) α Phase - A silver-rich substitutional solid solution phase + small percentage of copper atoms; β Phase – A copper-rich substitutional solid solution phase + small percentage of silver atoms. α and β phases - Terminal solid solutions . Solidus curve - ABEGD. Liquidus curve - AED Below 780 °C - two-phase α-plus-β region - a mixture of the silver-rich and copper-rich phases in the alloy microstructures. 96

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98. Liquidus and solidus phases meet at composition E (Temp - 779 °C, corresponding to line BEG) This composition, of 72% Ag and 28% Cu by weight, which corresponds to point E, is known as the eutectic compos ition, 98

99. Characteristics of this special composition Temp at which the eutectic composition melts – 779 ᵒC- Lower than fusion temp of either metal No solidification range for composition E 99

100. Solidus slope - AB & DG AB - copper content of the silver-rich α phase - 0% to nearly 9%. DG - silver content of the copper-rich β phase - 0% to 8%. Solvus lines – CB & FG CB - solid solubility of copper in the α phase increases - 1% at 300 °C to nearly 9% at B. FG - solid solubility of silver in the β phase increases - extremely small value at 300 °C to a maximum of approx. 8% at point G 100

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102. Phase diagram used to determine - Composition – Application of tie line Weight percentage – Application of lever rule 102

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104. PROPERTIES OF HYPOEUTECTIC AND HYPEREUTECTIC ALLOYS Alloys with a composition less than that of the eutectic are called hypoeutectic alloys Those with a composition greater than the eutectic are known as hypereutectic alloys 104

105. Hypoeutectic alloys – Primary crystals - α solid solution Hypereutectic alloys – Primary crystals - β solid solution. Hypoeutectic or hypereutectic alloys containing eutectic constituent in their microstructures (compositions between B and G) - brittle Alloys with microstructures lacking this constituent (compositions to the left of B or to the right of G) are ductile and tarnish resistant Alternating lamellae of the α and β phase inhibit the movement of dislocation increases strength and hardness, decreases ductility (or increases brittleness). The tarnish resistance of these alloys without the eutectic is superior 105

106. PROPERTIES OF EUTECTIC ALLOYS Since there is a heterogeneous composition, they are susceptible to electrolytic corrosion. They are brittle, because the present of insoluble phases inhibits slip. They have a low melting point and therefore are important as solders. 106

107. PERITECTIC ALLOYS Another example of the limited solid solubility of two metals is the peritectic transformation Eg ; Ag – Sn system Ag – Pt system Pd – Ru system Reaction during the cooling process : Liquid + β solid solution α solid solution 107

108. Peritectic is a phase where there is limited solid solubility. Not of much use in dentistry except for silver tin system. Reaction occurs when there is a big differences in the melting points of the components. 108

109. Phase diagram of peritectic alloy α phase - silver-rich β phase - platinum-rich Two -phase ( α+β ) region results from : Limited solid solubility of Ag in Pt at 700 °C - < 12% (point F) Limited solid solubility of Pt in Ag at 700 °C- 56 % (point G) 109

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111. Peritectic transformation occurs at point P Liquid composition at B and the platinum-rich β phase (composition at point D) transform into the silver-rich α phase (composition at point P) 111

112. Extensive diffusion is required in these phases for transformation, Peritectic alloys are susceptible to coring during rapid cooling. Cored structure has inferior corrosion resistance More brittle than the homogeneous solid solution phase. 112

113. COLD WORKING AND ANNEALING 113

114. Cold working When the stress is greater than the elastic limit at relatively low temperatures – Ductile and malleable Produces a change in microstructure , with dislocations becoming concentrated at grain boundaries, but also a change in grain shape . The grains are no longer equiaxed - more fibrous . 114

115. Cold working is sometimes referred to as work hardening due to the effect on mechanical properties. When mechanical work is carried out - at a more elevated temperature - object change shape without any alteration in grain shape or mechanical properties. 115

116. The temperature below which work hardening is possible is termed the recrystallization temperature. If the material is maintained above the recrystallization temperature for sufficient time, diffusion of atoms across grain boundaries may occur, leading to grain growth . Grain growth should be avoided if the properties are not to be adversely affected. 116

117. Annealing Process of heating a metal to reverse the effects associated with cold working such as strain hardening, low ductility and distorted grains. In general it has 3 stages. 1) R ecovery 2) R ecrystallization 3) G rain growth. 117

118. Recovery : Stage at which the cold work properties begin to disappear before any significant visible changes are observed under the microscope. Recrystallization : Occurs after the recovery stage. The old grains disappear completely Replaced by a new set of strain free grains . Grain growth : The crystallized structure has a certain average grain size , depending on the number of nuclei . The more severe the cold working, the greater the number of such nuclei. Grain size for completely recrystallized material can range from rather fine to fairly coarse . 118

119. Annealing 119

120. Cold working may cause the formation of internal stresses within a metal object. If these stresses are gradually relieved they may cause distortion which could lead to loss of fit of, for example, an orthodontic appliance. For certain metals and alloys the internal stresses can be wholly or partly eliminated by using a low temperature heat treatment referred to as stress relief annealing . 120

121. This heat treatment is carried out well below the recrystallization temperature and has no deleterious effect on mechanical properties since the original grain structure is maintained. 121

122. Heat treatment Heat treatment of metals in the solid state is called SOLID STATE REACTIONS. Results in diffusion of atoms of the alloy by heating a solid metal to a certain temperature and for certain period of time. This will result in the changes in the microscopic structure and physical properties. 122

123. Purpose of heat treatment Shaping and working on the appliance in the laboratory is made easy when the alloy is soft. - First stage called softening heat treatment. To harden the alloy for oral use, so that it will withstand oral stresses. The alloy is again heated and this time it is called hardening heat treatment . 123

124. Types Of Heat treatment Softening Heat treatment/ Solution Heat treatment Hardening Heat treatment/ Age Hardening 124

125. Softening / SOLUTION Heat treatment Also known as ANNEALING. This is done for structures which are cold worked. Eg ; Gold (High noble & noble metal alloys) Phase change – disordered solid solution Technique - alloy is placed in an electric furnace at a temperature of 700°C for 10 minutes and then rapidly cooled (quenched). Result of this is reduction in strength, hardness and pro­portional limit but increase in ductility. In other words the metal becomes soft. This is also known as HOMOGENIZATION TREATMENT. Indication- Structures to be ground, shaped or cold worked 125

126. Age Hardening/ hardening Heat treatment This is done for cast removable partial dentures, saddles, bridges, but not for Inlays. Technique - The appliance (alloy) is heat soaked at a temperature between 200-450°C for 15-30 minutes and then rapidly cooled by quenching. The result of this is increase in strength (yield strength), hardness and proportional limit but reduction in ductility(percent elongation). Also known as ORDER HARDENING or PRECIPITATION HARDENING . Phase change – ordered solid solution 126

127. Age Hardening After solution heat treatment, the alloy is once again heated to bring about further precipitation and this time it shows in the metallography as a fine dispersed phase. This also causes hardening of the alloy and is known as age hardening because the alloy will maintain its quality for many years. 127

128. CLASSIFICATION OF ALLOYS 128

129. In order of increasing melting temperature, they include gold, palladium, platinum, rhodium, ruthenium, iridium, and osmium. Only gold, palladium, and platinum, which have the lowest melting temperatures of the seven noble metals, are currently of major importance in dental casting alloys. 129

130. ALLOY CLASSIFICATION BY MECHANICAL PROPERTIES 130

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132. ALLOY TYPE BY MAJOR ELEMENTS: Gold-based, palladium-based, silver-based, nickel-based, cobalt-based and titanium-based . ALLOY TYPE BY PRINCIPAL THREE ELEMENTS: Such as Au-Pd-Ag, Pd-Ag- Sn , Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V and Fe-Ni-Cr. (If two metals are present, a binary alloy is formed; if three or four metals are present, ternary and quaternary alloys, respectively, are produced and so on.) ALLOY TYPE BY DOMINANT PHASE SYSTEM: Single phase [ isomorphous ], eutectic, peritectic and intermetallic 132

133. ALLOY CLASSIFICATION BY DENTAL APPLICATIONS 133

134. Published in the March 2003 Journal of the American Dental Association 134

135. Metallic Elements Used in Dental Alloys NOBLE METALS Noble Metal are corrosion and oxidation resistant because of inertness and chemical resistance. Basis of inlays, crowns and bridges because of their resistance to corrosion in the oral cavity. Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver 135

136. Precious metal alloy Precious metal: a metal containing primarily elements of the platinum group, gold, and silver. Precious metal alloy: an alloy predominantly composed of elements considered precious, i.e., gold, the six metals of the platinum group (platinum, osmium, iridium, palladium, ruthenium, and rhodium), and silver Term precious stems from the trading of these metals on the commodities market 136

137. DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS 137

138. Biocompatibility Tarnish and Corrosion resistance Compatible thermal properties Castability Aesthetics Economical 138

139. Biocompatibility Tolerate oral fluids Do not release harmful products 139

140. Corrosion resistance Physical dissolution of material - corrosion Too noble to react Metallic elements form adherent passivating surface film eg :- Cr in Ni – Cr and Ti 140

141. Tarnish Resistance Thin film of surface deposit/ interaction layer on metal surface - tarnish Allergenic Components in Casting Alloys Morally and legally to minimize risk Aesthetics optimal balance among properties 141

142. Compatible thermal properties Compensation for solidification Compensation for casting shrinkage from solidus temperature to room temperature achieved either through- computer- generated oversized dies controlled mold expansion Alloys - closely matching thermal expansion coefficients to be compatible with porcelains 142

143. Castability Alloy should flow freely into the most intricate regions of the investment mold Measured by percent completion of a cast mesh screen pattern or other castability patterns 143

144. Strength requirements Alloys for bridgework require higher strength than alloys for single crowns. Alloys for metal-ceramic prostheses are finished in thin sections and require sufficient stiffness 144

145. Porcelain bonding Sound chemical bond to ceramic veneering materials, - thin adherent oxide, preferably one that is light in color Aesthetic 145

146. Economic considerations Cost of fabricating prostheses must be adjusted periodically to reflect the fluctuating prices of casting metals - high noble and noble metal alloys. 146

147. Alloys used for metal ceramic restoration can be used for all metal prosthesis But Alloys for all metal restorations should not be used for metal ceramic restoration Reasons :- 1) May not form thin, stable oxide layers to promote atomic bonding to porcelain. 2) Melting range too low to resist sag deformation or melting at porcelain firing temperatures. 3) Thermal contraction co- efficients not close to porcelain. 147

148. GOLD Soft, rich yellow color and a strong metallic luster Most malleable and ductile 0.2% lead – brittle Soluble in aqua regia ( combination of nitric and hydrochloric acid ) Alloyed with copper, silver, platinum – increases hardness , durability and elasticity 148

149. Lowest in strength and surface hardness High level of corrosion and tarnish resistance High melting point, low C.O.T.E value and very good conductivity Improves workability, burnish ability, raises the density Calcium improves its mechanical properties Cohesive , welded at room temp. 149

150. Gold content: Traditionally the gold content of dental casting alloys have been referred to in terms of: Carat: The term carat refers only to the gold content of the alloy; a carat represents a 1⁄24 part of the whole. Thus 24 carat indicates pure gold. The carat of an alloy is designated by a small letter k , for example, 18k or 22k gold. Fineness: Fineness also refers only to the gold content, and represents the number of parts of gold in each 1000 parts of alloy. Thus 24k gold is the same as 100% gold or 1000 fineness (i.e., 1000 fine) or an 18k gold would be designated as 750 fine. 150

151. Platinum Bluish white metal Hardness similar to copper Higher melting point ( 1772°C) than porcelain Coefficient of thermal expansion close to porcelain Lighten the color of yellow gold based alloys Common constituent in precision prosthetic attachments High density 21.45 g/cm 3 High melting point 1772 o C Boiling point of 4530 o C Low CTE 8.9  10 -6 / o C 151

152. Silver Malleable, ductile; white metal. Stronger and harder than gold, softer than copper. Absorbs oxygen in molten state-difficult to cast Forms series of solid solutions with palladium and gold density 10.4gms/cm 3 melting point 961 o C boiling point 2216 o C CTE 19.7  10 -6 / o C 152

153. Lowers the melting range Low corrosion resistance In gold-based alloys, silver is effective in neutralizing the reddish color of copper. Silver also hardens the gold-based alloys via a solid-solution hardening mechanism. Increases CTE in gold- and palladium-based alloys Foods containing sulfur compounds cause severe tarnish on silver, and for this reason silver is not considered a noble metal in dentistry. Pure silver is not used in dental restorations because of the black sulfide that forms on the metal in the mouth. 153

154. Palladium (Pd) White metal darker than platinum Density little more than half that of Pt and Au Absorbs hydrogen gas when heated Not used in pure state in dentistry Whitens yellow gold based alloys . density 12.02gms/cm 3 melting point 1552 o C boiling point 3980 o C lower CTE 11.8  10 -6 / o C when compared to gold. 154

155. Iridium (Ir ) Ruthenium (Ru ), Rhodium (Rh) & Osmium (Os) Grain refiners Improves mechanical properties and uniformity of properties within alloy Extremely high melting point of Ir - 2410°C and Ru - 2310°C – serve as nucleating centers Osmium (Os) has a very high melting point, and is very expensive, hence not used in dentistry. 155

156. Physical properties of noble and precious metals 156

157. Mechanical properties of noble and precious metals 157

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159. 159

160. Binary composition of metals Constitute majority of mass of many noble alloys Useful in understanding the behavior of more complex alloys Six important combinations Au –Cu Pd - Ag Pd –Cu Au - Pd Au – Ag Au - Pt 160

161. Alloy composition and temperature Differences between liquidus and solidus line small for Ag – Au system larger for Au - Pt system Desirable to have narrow liquidus – solidus range – Potential for coring less 161

162. With slow cooling the crystallization - diffusion and a random distribution of atoms - with no coring. Rapid cooling - denies the alloy the energy and mobility required for diffusion - cored structure is ‘locked in’ Reducing the cooling rate - self-defeating – Results in alloy with large grain size - inferior mechanical properties 162

163. Heat treatment - to eliminate the cored structure - homogenization heat treatment . Heating the alloy to a temperature just below the solidus temperature for a few minutes - allow diffusion of atoms The alloy is then normally quenched - prevent grain growth from occurring. Eg : Gold-silver system. – Pt or Pd are present - homogenization heat treatment Heating to 700ºC for 10 minutes, then quenching 163

164. Hardening of noble alloys Hardening heat treatments are not beneficial for the types 1 and 2 alloys - insufficient quantities of copper and silver. Solid solution & ordered solution hardening Precipitation Grain refiners such as Ir , Rh , and Ru 164

165. Solid solutions - stronger and harder than either component pure metal. Presence of atoms of unequal size - difficult for atomic planes to slide by each other. Ordered solutions - Further strengthen a solid - pattern of dissimilar sizes throughout the alloy's crystal structure 165

166. Au-Cu system ,heated to molten state-cooled slowly –mass solidify at 880 C - Solid solution Cools slowly to 424 o C –ordered solution 166

167. Precipitation hardening - By heating some cast alloys carefully, a second phase - appear in the body of the alloy. Blocks the movement of dislocations - increasing strength and hardness. The effectiveness greater - if precipitate is still part of the normal crystal lattice. - coherent precipitation. Overheating may reduce alloy properties - second phase grow outside of the original lattice structure. 167

168. 168

169. Grain refiners - Ir , Rh , and Ru Fine grained - grain sizes below 70 pm in diameter. 0.005% or 50 ppm of iridium and ruthenium Tensile strength and elongation are improved significantly (30%) Hardness and yield strength - less effect 169

170. Cold working an alloy will significantly strengthen it. But - less ductile 170

171. Treatment of noble and high noble alloys Type lll and type lV gold alloys can be hardened and softened. Softening heat treatment/homogenizing-Solution heat treatment. Hardening heat treatment-Age hardening.   171

172. Softening Heat Treatment Increases ductility Reduces tensile strength ,proportional limit and hardness Method: Casting placed in electric furnace 10 minutes,700°C  quenched in water resulting disordered solid solution Indicated-alloys that are to be ground, shaped or otherwise cold worked either in or out of mouth. 172

173. Hardening of Noble metals Increases strength, proportional limit, and hardness, but decreases ductility If positioning of two elements become ordered-ordered solution Copper present in gold alloy helps in this process. Method: Soaking/ageing casting-15 to 30 minutes before water quenching 200°C to 450°C Ideally, before age hardening it should first be subjected to softening heat treatment 173

174. The hardening heat treatment is indicated for metallic partial dentures, saddles, FDPs - Rigidity of the prosthesis is needed. For small structures, such as inlays, a hardening treatment is not usually required. Age hardening reduces the ductility of gold alloy 174

175. Functional Mechanical Properties of Casting alloys Elastic modulus Yield strength Ductility Hardness Fatigue Resistance 175

176. Alloys for metal ceramic prosthesis Classification of Noble PFM alloys 176 Au Based Pd Based Au-Pt-Pd (21 K) Pd-Ag Au- Pd (13 K ) Pd-Cu Au-Pd-Ag (13 K) Pd-Co

177. 177 Noble alloys Gold-copper-silver-palladium Palladium-copper-gallium Palladium-silver and silver-palladium High noble alloys Gold-Platinum alloy Gold-Palladium alloy Gold-copper-silver-palladium alloys

178. Physical and Chemical properties 1. Noble metal content 2. Hardness 3. Yield strength 4. Elongation 5. Fusion temperature 6. Porcelain-Metal Compatibility 7. Color stability 8. Biocompatibility 178

179. Typical properties of alloys for PFM restorations 179

180. High noble alloys Minimum of 60% noble metals (any combination of gold, palladium and silver) with a minimum of 40% by weight of gold.   Tin, indium and/or iron  oxide layer formation  chemical bond for the porcelain 180

181. Gold-platinum alloy Developed alternative to palladium alloys For full cast as well as metal-ceramic restorations.  More prone to sagging, they should be limited to short span bridges.  A typical composition is Gold 85%;  Platinum12%; Zinc 1%;  Silver (in few brands 181 Large two-phase region

182. Gold-palladium alloy Used for full cast /metal-ceramic restorations. Palladium - high melting temperature - impart a white or gray color - improves sag resistance These alloys usually contain indium, tin or gallium to promote an oxide layer. A typical composition Gold 52%; Palladium 38%;  Indium 8.5%; Silver (in some brands). 182

183. Gold-copper-silver-palladium alloy Have low melting temperature Not used for metal-ceramic applications.  Greening of porcelain – due to silver Copper tends to cause sagging during porcelain processing.  A typical composition is Gold 72%, Copper 10%;  Silver 14%;  Palladium 3%. 183

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185. Noble alloys Contain at least 25% by weight of noble metal (gold, palladium or silver) Have relatively high-strength, durability, hardness, ductility.  They may be yellow or white in color .  185

186. Gold–copper-silver-palladium alloy More copper and silver Have a fairly low melting temperature More prone to sagging during application of porcelain. Used mostly for full cast restorations rather than PFM applications.  A typical formula is: gold 45%  Copper 15% Silver 25% Palladium 5% 186

187. Palladium-copper-gallium alloys Introduced in 1983 Very rigid  excellent full cast or PFM restorations. Contain copper  prone to sagging during porcelain firing.  Gallium  reduces the melting temperature   A typical composition is Palladium 79% Copper 7%; Gallium 6% Hardness is comparable to base metal alloy but are not burnishable 187 2 superlattice transformations

188. Palladium-silver and silver-palladium alloys Higher palladium alloys - PFM frameworks.  Higher silver alloys - susceptible to corrosion - greening of porcelain High resistance to sagging very rigid - good for long spans 188

189. More castable (more fluid in the molten state) Easier to solder and easier to work with than the base metal alloys. Typical composition for Palladium- silver alloy: Palladium 61%;  silver 24%;  Tin (in some)  Silver-palladium alloy: Silver 66%;  Palladium 23%; gold (in some formulation) 189

190. Discoloration of Porcelain by Silver Greening : colloidal dispersion of silver atoms entering porcelain - from vapor transport or surface diffusion - green , yellow – green, yellow – orange ,orange and brown hues (ii) More near cervical region – marginal metal , localized silver concentration 190

191. (iii) Certain porcelain resistant to silver discoloration - silver ionization by porcelains with high oxygen potential (iv) Greening when porcelains fired on silver – free alloys- vaporization of silver from walls of contaminated furnaces 191

192. (v) Reduce porcelain discoloration by metal coating agents :- (a) gold film fired on metal substrate (b) ceramic conditioner 192

193. Color of cast metal alloys 193 Compositions of casting alloys determine their color. Palladium content >10 wt%, - white Copper -reddish color Silver lightens either the red or yellow color of alloys.

194. Biological hazards and precautions : Dental Laboratory Technicians Exposed to high concentrations of beryllium and nickel dust and beryllium vapor. Beryllium concentration in dental alloys rarely exceed 2% by weight. On melting of Ni – Cr – Be alloys the beryllium vapor remain over an extend period of time in absence of exhaust and filtration system. 194

195. The Occupational health and Safety Administration (OSHA) limit beryllium concentration to 2 µg/m 3 Symptoms include contact dermatitis, coughing , chest pain, general weakness to pulmonary dysfunction. 195

196. Potential patient hazard Intraoral exposure to nickel – Nickel allergy Inhalation , ingestion and dermal contact of nickel and nickel alloys Incidence – 5 to 10 times higher for females 196

197. National institute for occupational safety and health (NIOSH ) recommend standard limit to employee exposure to inorganic nickel to workplace to 15mg/m 3 To minimize exposure , a high speed evacuation system used. Patient informed of potential allergic effects. Medical history of the patient taken to rule out nickel allergy. 197

198. Precious metals & dental implants GOLD UCLA-TYPE ABUTMENTS • 64% gold, 22% palladium • Melting range 2400˚F-2500˚F (1320˚C-1370˚C) •  Gold alloy abutment screw retention increases the preloading force there by  assuring precision fit to implant Ceraone & Mirus cone abutments SEMI-BURNOUT CYLINDER Non-oxidizing, high precious gold platinum alloy with a plastic wax-up sleeve Cylinder base 198

199. Review of literature 199

200. The study evaluated the cervical and internal fit of complete metal crowns that were cast and recast using palladium-silver alloy and 3 different marginal configurations used were straight shoulder, 20-degree bevel shoulder, and 45-degree chamfer. Results showed - The new alloy provided significantly better adaptation than the recast alloy for both marginal and internal discrepancy measurements. Marginal designs did not shown any statistical differences when the new metal was used Lopes,S.Consani et al ,Influence of recasting palladium-silver alloy on the fit of crowns with different marginal configurations J Prosthet dent,2005;94,5:430-434 200

201. The study was to evaluate the bond strength of 4 recently introduced noble alloys by using 2 techniques for porcelain application For both conventional layering and press-on-metal techniques, all 4 noble alloys had a mean metal-to-ceramic bond strength that substantially exceeded the 25 MPa minimum in the ISO Standard 9693. The results for Aries support the manufacturer’s recommendation not to use the press-on-metal technique for alloys that contain more than 10% silver . 201 Mofida R. Khmaj et al ;Comparison of the metal-to-ceramic bond strengths of four noble alloys with press-on-metal and conventional porcelain layering techniques (J Prosthet Dent 2014;112:1194-1200)

202. The purpose of this in vitro study was to determine whether heat treatment affects the metal ceramic bond strength of 2 Pd-Ag alloys containing different trace elements. Conclusions : Heating under reduced atmospheric pressure effectively improved the bond strength of the ceramic-to-Pd-Ag alloys. 202 Jie -yin Li et al ;Effect of heating palladium-silver alloys on ceramic bond Strength (J Prosthet Dent 2015;114:715-724)

203. conclusion The diversity of alloys available to the dental practitioner has never been more extensive. We now have the opportunity to select the alloys based on the individual patient’s specific biological, functional, and economic requirements. There is no one alloy suitable for all applications, because in metallurgy there is a constant trade off in properties as changes in formulations are made. To make optimal uses of the choices available, and for ethical and medico legal considerations, it is incumbent upon the practitioner to be aware of the identity and composition of the alloys prescribed 203