Dental Casting Alloys Prepared by Dental Materials Department

Dental Casting Alloys Prepared by: Dental Materials Department Yenepoya Dental College Yenepoya University, Mangalore.

Applications All metal restoration Removable Partial Denture (RPD) Metal-ceramic or Porcelain-fused to metal restoration

Objectives • Understand the alloy classifications • Know the roles of each element in dental casting alloys • Know the requirements of porcelain-fused to metal (or metal-ceramic) alloys • Understand the relation between the TCOE of PFM alloys and that of ceramics • Recognize the importance of some properties of the alloys 3

History • 1907 : Lost wax technique by Taggart • 1932 - 1948 : Standardization of dental casting alloys • 1950 s -1960 s : Development of porcelainfused-to-metal (PFM) alloys – Found that adding Pd and Pt to gold (Au) would lower coefficient of thermal expansion sufficiently to ensure physical compatibility between the porcelain veneer and the metal substructure. 4

History • 1970 s : Placement of gold on the free market – Increased prices stimulates the search for alternative low gold and base metal alloys. 5

Terminology • Noble metals – Elements with good metallic surfaces that retain their luster in clean dry air – Indicate the relative inertness of the element in relation to the standard EMF series – Resist oxidation, tarnish and corrosion during heating casting and soldering • Platinum group (6 metals) – Platinum, Iridium, Osmium (atomic wt 190, density 22 g/cc) – Palladium, Rhodium, Ruthenium (atomic wt 100, density 11 -12 g/cc) • Gold (atomic wt 196, density 19. 3 g/cc) • (Silver? ) 6

Terminology • Precious metals – Indicates how expensive a metal is based on supply and demand. – **The descriptors precious and semiprecious should be avoided because they are imprecise terms. 7

Terminology • Gold content of a dental alloy – Karat, Carat (K) • Parts of pure gold per 24, e. g. 18 K, 24 K – Fineness • Parts of pure gold per 1, 000 – e. g. a 650 fine alloy has a gold content of 65% • Primarily used for gold solders • Pennyweight (dwt. ) – 1 dwt = 1. 555 gm = 0. 05 oz 8

Classification ADA Specification #5 ADA’s Classification Principal Elements Descriptive Classification 9

ANSI/ADA Specification #5 • Referred to Gold-based alloys – Alloys can have any composition as long as they pass the tests for toxicity, tarnish, yield strength, and percent elongation. Strength Type %Au & Pt VHN Restoration I (soft) 83 50 -90 Inlay II (medium) 78 90 -120 Inlay/onlay III (hard) 78 120 -150 Onlay/Crown& Bridge IV (extra-hard) 75 150 -250 Crown&Bridge /RPD 10

ADA’s Classification (1984( 1. High noble (HN( 2. Noble (N( 3. )Predominantly) Base metal (PB( Alloy Type Total Noble Metal Content High noble metal Contains > 40 wt% Au and > 60% of the noble metal elements Noble metal Contains > 25 wt% of the noble meal elements (Au, Pd, Pt) Base metal Contains < 25 wt% of the noble metal elements *No discrimination among alloys within a given category* 11

Principal Elements • When an alloy is identified according to the elements it contains, the components are listed in declining order of composition, with the largest constituent first followed by the second largest constituent. – e. g. Au-Ag-Pt (Au ~ 78%, Ag ~ 12%, Pt ~10%) • Exception: Certain elements that significantly affect physical properties or that represent potential biocompatibility concerns are often designated (regardless of their small amounts). – e. g. Au-Cu-Ag-Pd (Au ~40%, Cu ~7. 5%, Ag ~47%, Pd~4%) 13

Descriptive Classification • Normal-fusing alloys – – Medium-gold Low-gold Silver-palladium Silver-indium • High-fusing alloys (mostly for PFM) – Gold-platinum-palladium – Gold-palladium-silver – Gold-palladium – High-palladium – Palladium-silver – Base-metal • Cr/Co; Cr/Ni 14

Restoration Type Alloy Type High Noble > 40 wt% Au and > 60% of the noble metal elements Noble > 25 wt% of the noble metal elements Base Metal < 25 wt% of the noble metal elements All-Metal Restorations Au-Ag-Cu-Pd Ag-Pd-Au-Cu Ag-Pd Metal-Ceramic and All-Metal Restorations RPD Au-Pt-Pd Au-Ag-Cu-Pd Au-Pd-Ag (5 -12 wt% Ag) Au-Pd-Ag (>12 wt% Ag) Au-Pd (no Ag) Pd-Au-Ag Pd-Cu Pd-Co Pd-Ga-Ag Ag-Pd-Au-Cu Ag-Pd Pure Ti Ti-Al-V Ni-Cr-Mo-Be Ni-Cr-Mo Co-Cr-Mo Co-Cr-W

Fundamental Properties of Noble Metals • Gold (Au) • Platinum (Pt) • Palladium (Pd) • Silver (Ag) • Minor alloying elements

Gold (Au( • Soft, (most) malleable and ductile • Relatively low strength • Tarnish resistant in air and water at any temp. • Attacked by only a few of the most powerful oxidizing agents • Insoluble in sulfuric, nitric, or hydrochloric acids • Soluble in a combination of nitric and sulfuric acids (aqua-regia( • Small amounts of impurities (ie. lead, mercury, base metals) have a pronounced and usually detrimental effect on its properties. Fusion temp = 1063°C Density = 19. 3 g/cm 3 Thermal coef. of exp. = 14. 2 x 10 -6/°C MOE = 80 GPa 17

Platinum (Pt( • Tough, malleable and ductile • Very high cost (usually replaced by Pd in most modern alloys( • High corrosion resistance • Higher melting temp than porcelain Fusion temp = 1755°C >Au Density = 21. 37 g/cm 3 >Au Thermal coef. of exp. = 8. 9 x 10 -6/°C <Au MOE = 147 GPa >Au 18

Palladium (Pd( • Not used in the pure state dentistry • Has replaced Pt in dental casting alloys • Decreased cost v. s. Pt • Helps prevent corrosion of silver in the oral environment • Absorbs H 2 gas when heated improperly Fusion temp = 1555°C Density = 11. 4 g/cm 3 Thermal coef. of exp. = 11. 1 x 10 -6/°C MOE = 112 GPa 19

Silver (Ag( • “Noble”? • Malleable and ductile • Best known conductor of heat and electricity • Harder than gold • Unaltered in clean dry air, however, combines with sulfur, chlorine and phosphorus resulting in severe tarnish in the oral environment • Occludes large quantities of O 2 in molten state • O 2 gas will evolve during solidification resulting in pits and porosities. Fusion temp = 960. 5°C Density = 10. 4 g/cm 3 Thermal coef. of exp. = 19. 7 x 10 -6/°C MOE = 120 GPa 20

Minor Alloying Elements • Iridium (Ir) - grain refining • Ruthenium (Ru) - grain refining 21

• Grain refining – The addition of as little as 50 ppm (0. 005%) of Ir and Ru results in a 100 x increase in the no. of grains per unit volume. – Increases the alloy’s tensile strength and %elongation by >30% – Increases tarnish resistance, slightly increases yield strength – Does not appreciable affect hardness 22

Alloys for All-Metal Restoration • High-noble and Noble Metal Alloys –Au-Ag-Cu-Pd –Ag-Pd –Metal Ceramic Alloys • Base Metal Alloys

Au-Ag-Cu-Pd Alloys • Primarily ternary alloys of Au, Ag and Cu, with minor amounts of Pt, Pd and Zn. • Approx. >90% of the total alloy content is Au, Ag and Pd 24

Au-Ag-Cu-Pd: Composition • Gold (Au) – Tarnish and corrosion resistance • Tarnish is an inverse function of gold content. – Contributes burnishability, ductility, and ability to heat harden the alloy • Silver (Ag) – Helps control the color of the alloy, neutralizing the red color imparted by Cu – Promotes ductility • Au/Cu alloys (75% Au) break apart at grain boundaries during heat treatment if no Ag is present. 25

Au-Ag-Cu-Pd: Composition • Platinum (Pt) – Very expensive ingredient – Contributes strength – Whitens the alloy – Increases the fusion temperature • Palladium (Pd) – Like Pt but more effective and less expensive than Pt Alloying metal of choice v. s. Pt 26

Au-Ag-Cu-Pd: Composition • Copper (Cu) *** – Principle hardener in gold alloys – Conc. >12% of Au amount alloy can be heat treated – Conc. >18% decrease the melting temp of the alloy 27

Au-Ag-Cu-Pd: Composition • Copper (Cu) *** – When alloyed with Ag, Cu increases the alloy’s hardness and decreases melting temp. – Cu imparts a reddish color to the metal and contributes most to the corrosion of gold alloys. – Ag/Cu ratio is important to tarnish resistance (but not as important as the Ag/Pd ratio). – Cu is not found in PFM alloys due to its tendency to discolor the porcelain. 28

Au-Ag-Cu-Pd: Composition • Zinc (Zn) – O 2 scavenger – 1 -2% helps to counteract the absorption of O 2 by silver. – Increases the castability, decreases porosities, and increases the hardness and brittleness of the alloy • Indium (In), Tin (Sn), Iron (Fe) – Hardens the alloy – (Provides oxides for ceramic bonding in PFM alloys) 29

Au-Ag-Cu-Pd: Composition • Iridium (Ir), Ruthenium (Ru), Rhenium (Rh) – Grain refining • Gallium (Ga) – Added to high Pd alloys or non-silver Au/Pd metal ceramic alloys to compensate for a decrease in the TCOE caused by the elimination of the Ag. – (Also provides oxides for ceramic bonding) 30

Au-Ag-Cu-Pd: Composition Alloy Type Main Elements Cu/ Au Au Cu Ag Pd Sn, In, Fe, Zn, Ga I High noble (Au base) 7% 83 6 10 0. 5 Balance II High noble (Au base) 9% 77 7 14 1 Balance III High noble (Au base) Noble (Au base) 12% 17% 75 46 9 8 11 39 3. 5 6 Balance IV High noble (Au base) 25% 56 14 25 4 Balance Copper: – Conc. >12% of Au amount alloy can be heat treated – Conc. >18% decrease the melting temp of the alloy • Types I and II gold can’t be heat treated and have a higher melting temp v. s. Types III and IV. 31

Heat Treatment Cu/Au system is the basis for heat treatment – Cu: Au ratio > 12: 88 the alloy is heat treatable. 100%Au 32

Heat Treatment • Above 424°C solid solution – Quenching from above 424°C will result in a softer, more ductile alloy with decreased strength solid solution 424°C 33

Heat Treatment • Below 424°C ordered crystal lattice – Alloy has increased strength, hardness and decreased ductility. – The amount of transformation is time and temperature dependent and the process is reversible. 424°C Ordered crystal lattice 34

Softening Heat Treatment (Solution Heat Treatment) • Heat alloy to 700°C for 10 min. then quench. – Decreased tensile strength, proportional limit and hardness – Increases ductility and %elongation – MOE not significantly altered. 700°C 424°C • Indicated prior to adjusting, burnishing and polishing 35

Hardening Heat Treatment (Age Hardening) • Heat alloy to 450°C for 2 min. , cool slowly to 250°C over 30 mins then quench. Or Heat to 350°C for 10 – 15 min. and quench – Increases strength, proportional limit and hardness – Decreases ductility and %elongation • Indicated for RPD frameworks and long span FPD’s 424°C 36

Silver-Palladium Alloys (Ag-Pd) • Ag: Pd ratio approx 3: 1 (60 -70% Ag, 25% Pd) to render silver tarnish resistant in the oral cavity. • Both Ag and Pd absorb gases during heating, casting is very technique sensitive. • ≠ Pd-Ag alloys (for PFM restorations) 37

Ag-Pd: Composition Alloy Type Main Elements III Noble (Ag base) IV Noble (Ag base) Cu/ Au Au 15 Cu 14 Ag Pd Sn, In, Fe, Zn, Ga 70 25 Balance 45 25 Balance 38

Alloys for PFM or Metal Ceramic Restoration No Copper! Au-Pt-Pd Au-Pd-Ag Au-Pd Pd-Ag High Pd

Firing 40

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some important Requirements • Must have the potential to bond to dental porcelain – need oxide-forming elements (small amount of base metals( • Posses coefficient of thermal contraction compatible with those of dental porcelains • Sufficiently high solidus temp (fusing temp) to permit the application of low-fusing porcelains – >100°C than the firing temp of the ceramic 42

Ceramic-Metal Bond • Typically, TCOE of porcelain = 13. 0 to 14. 0 x 10 -6/°C and the metals = 13. 5 to 14. 5 x 10 -6/°C. • The difference of 0. 5 x 10 -6/°C causes the metal to contract slightly more than does the ceramic during cooling after firing the porcelain. • This condition puts the ceramic under slight residual compression, which makes it less sensitive to applied tensile forces. 43

Gold-Platinum-Palladium Alloys (Au-Pt-Pd) • Composition – Au (84 -86%); Pt (4 -10%); Pd (5 -7%); Ag (0 -5%); Fe, In, Sn (2 -3%( – (high noble) • Advantages – – – Excellent bonding to porcelain Reproduces fine margins and occlusal detail Easily finished and polished Corrosion resistant and non-toxic Adequate yield strength and MOE (most cases( 45

• Disadvantages – low sag and creep resistance – not strong enough for long span FPDs – High cost 46

Gold-Palladium-Silver Alloys (Au-Pd-Ag) • Composition – Au (45 -52%); Pd (26 -31%); Ag (6 -16%); In, Sn (5 -7%( – (high noble) • Advantages – – – Higher melting range Better sag and creep resistance Higher yield strength and MOE for long span FPDs Good castability Easily finished and polished Non-toxic and lower cost v. s. Au-Pt-Pd alloys 47

• Disadvantages – Ag may cause greening of porcelain. – White color may show through tissues as gray and may not be as acceptable as gold collars. – High Pd content may increase the risk of H 2 gas absorption during casting, and bonding of porcelain may be affected by oxidizing procedures. 48

Gold-Palladium Alloys (Au-Pd) • Composition – Au (45 -52%); Pd (38 -45%); In (8. 5%); Ga (1. 5%( – (high noble) • Advantages – same as for Au-Pd-Ag alloys with the addition of potentially better porcelain color due to lack of Ag • Disadvantages – same as for Au-Pd-Ag alloys with the exception of porcelain greening 49

Palladium-Silver Alloys (Pd-Ag) • Composition – Pd (53 -88%); Ag (30 -37%); In (4 -7%); Sn (4 -7%( – (noble) • Advantages – High yield strength and MOE – Better sag and creep resistance – Non-toxic and low cost • Disadvantages – Castability < gold alloys – High Ag porcelain greening, ↓bonding – High Pd ↑gas absorption and poor color 50

High Palladium Alloys • Composition – Pd (74 -88%); Cu (10 -15%); Ga (9%); Au (0 -2%); Co (4 -5%); In (0 -5%( – (noble) • Advantages – – High yield strength and sag and creep resistance Non-toxic, low cost Castability = gold alloys (easy( Excellent porcelain color 51

• Disadvantages – Porcelain bond strength may be variable. – High Pd content ↑H 2 gas absoption, poor solderability – Can’t be used with carbon investments or crucibles • Carbon or Silicon contamination will cause brittle castings which may crack or tear at grain boundaries under stress. 52

Palladium in PFM Alloys • • Hardens the alloy Whitens the alloy Increases the alloy’s casting temp. Increases the alloy’s MOE Renders silver tarnish resistant Decreases the alloy’s density Decreases the alloy’s thermal coef. of exp. 53

Minor Elements in PFM Alloys • In, Sn, Fe, Ga - provide metallic oxides for porcelain bonding, and harden the alloy. • Ga - increases thermal coef. of exp. to compensate for decreased or absence of Ag. 54

Heat Treatment • PFM alloys can be heat tx however clinical condition is dependant on ceramic application. 55

Base Metal Alloys • Ni-Cr, Co-Cr • Pure Ti, Ti alloy

Co-Cr and Ni-Cr alloys Co-Cr Ni-Cr 57

Composition • Chromium (11 -20%( – responsible for tarnish and corrosion resistance due to its passivity “ passivation” – if >30% difficult to cast and brittle • Cobalt or Nickel (65 -78%( – Co and Ni are pretty much interchangeable. – Ni alloys have decreased strength, hardness, MOE, fusion temps and increased ductility and %elongation v. s. Co alloys. 58

Composition • Minor alloying elements control the majority of the physical properties – Carbon (0. 1 -0. 5%) • increases strength, hardness, and brittleness. • increased by 0. 2% alloy too hard and brittle for dental use • decreased by 0. 2% decreases yield strength and UTS to unacceptable levels. – Molybdenum (3 -6%( • increases strength, hardness, and %elongation 59

Composition – Aluminum (4 -5%) • forms a Ni 3 Al in Ni. Cr alloys which contributes to precipitation hardening resulting in increased tensile and yield strength. – Beryllium (0. 5 -2%) • decreases the fusion temp by approx 100°C • increases fluidity during casting • allows for electrolytic etching (with resin bond prosthesis( 60

Composition – Manganese (5%) and Silicone (0. 5%( • increases fluidity and castability of the molten alloy • +Boron deoxidizers (essential in Ni containing alloys( – Iron and Copper • increase hardness 61

Heat Treatment • Most desirable properties are in the as cast condition. (= no need for heat tx( 62

Titanium and Titanium Alloys • Forms a very stable oxide layer • Commercially pure titanium (cp Ti) is used for dental implants, surface coatings, and crowns, partial and complete dentures, and orthodontic wires. • Ti-6 Al-4 V is the most widely used. 63

Cast Titanium • Problems – High melting point (~ 1700°C) – Chemical reactivity • Reacts with gaseous elements easily, esp. at high temp (>600°C) • Need a well-controlled vacuum in processing • The technology required makes casting Ti so expensive. 64

Considerations on Properties

Melting Range • The solidus-liquidus range should be narrow to avoid having the alloy in a molten state for extended times during casting. To decrease oxides and contamination • Liquidus temp determines the burnout temp, type of investment, and type of heat-source. – Burnout temp liquidus temp – 500°C – Burnout temp >700°C, cannot use gypsum-bonded investment • Liquidus temp: Base-metal 1400°-1500°C vs. cast gold Type I-IV 800° 1050°C – Liquidus temp < 1100°C gas-air torch, >1100°C gas-oxygen torch or electrical induction • Solidus temp is important to soldering and formation of ordered phases. – Limit heating to 50°C below the solidus temp. 66

Density • Alloys with high densities will generally accelerate into the mold during casting faster and tend to form complete castings more easily. – Base-metal 7 -8 g/cc vs. High Noble 13 -18 g/cc • Alloys with lower density lighter 67

Yield Strength • Can be increased with treatment and changing the compositions 68

Hardness • Is a good indicator of the ability of an alloy to resist local permanent deformation under occlusal load • Gives some indication of the difficulty in polishing the alloy • Most noble casting alloys < enamel (343 Kg/mm 2) and < base-metal alloys 69

Elongation/Fatigue • Important property for RPD alloys • For crown and bridge applications, a low value of elongation for an alloy is not a big concern. – However, the elongation will indicate if the alloy can be burnished. 70

Biocompatibility • Noble alloys related to elemental release from the alloys (i. e. , from the corrosion process). • Base-metal alloys – Be from contact dermatitis to severe chemical pheumonitis – Ni sensitivity • 5 -10 times higher for females • 5%-8% of females 71

End of Dental Casting Alloys

Noble Casting Alloys 73

Properties of Elements in Dental Casting Alloys 74

Inlay, onlay 76