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GOLD CASTING ALLOYSGOLD CASTING ALLOYS

Stephen C. BayneStephen C. Bayne

University of MichiganUniversity of MichiganSchool of DentistrySchool of DentistryAnn Arbor, MichiganAnn Arbor, Michigansbayne@umich.edusbayne@umich.edu

INDIRECT RESTORATIONSINDIRECT RESTORATIONSOverview of ErrorsOverview of Errors

ERRORS:ERRORS:• ImpressionsImpressions• Models/CastsModels/Casts• WaxingWaxing• InvestingInvesting• CastingCasting• Finishing/PolishingFinishing/Polishing• CementingCementing

0.1 to 0.2%------------+1.5 to 1.7%-1.5 to 1.7%------------

Calculation of ideal permissible error:2 x 25 m / 10,000 m = 0.5%

*Image source: Steve Bayne, University of Michigan, 2008**Image source: Undetermined

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*

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0.0. Fabrication requirements vs final properties:Fabrication requirements vs final properties:

1.1. Physical Properties:Physical Properties:a.a. Reasonably low MP (flow)Reasonably low MP (flow)

b.b. Moderately high density (castability) Moderately high density (castability)

c.c. Low coefficient of thermal expansion (Low coefficient of thermal expansion ())

2.2. Chemical Properties:Chemical Properties:a.a. Chemical corrosion (tarnish) resistanceChemical corrosion (tarnish) resistanceb.b. Electrochemical corrosion resistanceElectrochemical corrosion resistancec.c. Solubility (solderability)Solubility (solderability)

GENERAL REQUIREMENTSGENERAL REQUIREMENTS

PlatinumPlatinum = 12.45 gms/cm= 12.45 gms/cm33 CobaltCobalt = 8.90= 8.90GoldGold = 18.88= 18.88 NickelNickel = 8.90= 8.90PalladiumPalladium = 12.02= 12.02 IronIron = 7.87= 7.87SilverSilver = 10.50= 10.50 ChromiumChromium = 7.17= 7.17CopperCopper = 8.96= 8.96

ToothTooth = 9-11 ppm/= 9-11 ppm/CC Amalgam Amalgam = 25= 25PFM alloysPFM alloys = 14= 14 Composite Composite = 35-45= 35-45Gold alloysGold alloys = 18= 18

3.3. Mechanical Properties:Mechanical Properties:a.a. High E (stiffness)High E (stiffness)b.b. Moderately high YS and H (resistance to plastic Moderately high YS and H (resistance to plastic

deformation)deformation)c.c. Hardenable by heat treatment (retention of polish)Hardenable by heat treatment (retention of polish)

4.4. Biological Properties:Biological Properties:a.a. Biocompatible: no toxic soluble phasesBiocompatible: no toxic soluble phasesb.b. Non-reactive in the oral environmentNon-reactive in the oral environment

GENERAL REQUIREMENTSGENERAL REQUIREMENTS(continued)(continued)

ST

RE

SS

ST

RE

SS

STRAINSTRAIN

x

Yield Strength (or Hardness)Yield Strength (or Hardness)

Modulus (Stiffness, Slope of Line)Modulus (Stiffness, Slope of Line)

x

Graph source: Steve Bayne, University of Michigan, 2008

REVIEW OF CORROSIONREVIEW OF CORROSION1.1. Types: Types: Chemical CorrosionChemical Corrosion, , Electrochemical CorrosionElectrochemical Corrosion

2.2. Requirements for Electrochemical Corrosion: Requirements for Electrochemical Corrosion: Anode, Cathode, Circuit, ElectrolyteAnode, Cathode, Circuit, Electrolyte

3.3. Electrochemical Corrosion Categories:Electrochemical Corrosion Categories:a.a. Galvanic CorrosionGalvanic Corrosion (macro-galvanic) (macro-galvanic)b.b. Local Galvanic CorrosionLocal Galvanic Corrosion (structure-selective corrosion) (structure-selective corrosion)c.c. Concentration Cell Corrosion (Concentration Cell Corrosion (crevice corrosioncrevice corrosion))d.d. Stress CorrosionStress Corrosion

Images source: Steve Bayne, University of Michigan, 2008

CLASSIFICATION OF ALLOYSCLASSIFICATION OF ALLOYS1.1. Full Gold Crown and Bridge Alloys (based on precious metals)Full Gold Crown and Bridge Alloys (based on precious metals)

a.a. ADA Classification System (see phase diagrams)ADA Classification System (see phase diagrams)(1)(1) Type IType I 83% Au+ 83% Au+ (Non-heat hardenable)(Non-heat hardenable) -- inlay-- inlay(2)(2) Type IIType II 78% Au+ 78% Au+ (Non-heat hardenable)(Non-heat hardenable) -- inlay, onlay, …-- inlay, onlay, …(3)(3) Type IIIType III 78% Au+ 78% Au+ (Heat hardenable)(Heat hardenable) -- onlay, crown-- onlay, crown(4)(4) Type IVType IV 75% Au+ 75% Au+ (Heat hardenable)(Heat hardenable) -- crown, bridge-- crown, bridge

b.b. Effects of Alloys Components:Effects of Alloys Components:(1)(1) GoldGold (Au) (Au) (2)(2) CopperCopper (Cu) (Cu) (3)(3) SilverSilver (Ag) (Ag) (4)(4) Palladium Palladium (Pd) (Pd) (5)(5) Platinum Platinum (Pt) (Pt) (6)(6) Zinc Zinc (Zn) (Zn)

Corrosion resistanceCorrosion resistanceHardnessHardnessCounteract orange color of copperCounteract orange color of copperIncrease MP and hardnessIncrease MP and hardnessIncrease MPIncrease MPPrevent oxidation during melting (OPrevent oxidation during melting (O22 getter) getter)

TERMINOLOGYTERMINOLOGY

1.1. Precious MetalPrecious Metal = containing metals of high economic value such as = containing metals of high economic value such as gold, platinum, palladium, silver, (rhodium), (iridium), (rhuthenium), gold, platinum, palladium, silver, (rhodium), (iridium), (rhuthenium), and (osmium).and (osmium).

2.2. Noble MetalNoble Metal = a precious metal that is resistant to tarnish. This = a precious metal that is resistant to tarnish. This excludes “silver” by definition.excludes “silver” by definition.

3.3. Low Gold AlloysLow Gold Alloys = Alloys containing <75% gold (less than 50 a/o gold) = Alloys containing <75% gold (less than 50 a/o gold) which means that gold atoms represent less than every other atom.which means that gold atoms represent less than every other atom.

4.4. Gold-substitute AlloysGold-substitute Alloys = precious metal alloys not containing gold. = precious metal alloys not containing gold.

5.5. Base-Metal AlloysBase-Metal Alloys = alloys not containing precious metals to impart = alloys not containing precious metals to impart their corrosion resistance.their corrosion resistance.

ATOMIC PERCENT GOLDATOMIC PERCENT GOLDCUCU AUAU

100

600

700

800

900

1000

500

400

300

200

10 70 80 90 10030 40 50 60200

1100

WEIGHT PERCENT GOLDWEIGHT PERCENT GOLD3010 70 80 90 10040 50 6020 75 85 95

LIQUIDUSLIQUIDUS

SOLIDUSSOLIDUS

DENTALDENTALALLOYSALLOYS

TE

MP

ER

AT

UR

E (

T

EM

PE

RA

TU

RE

( C

)C

)

Low Gold AlloysLow Gold Alloys High Gold AlloysHigh Gold Alloys

RandomRandomSolidSolid

SolutionSolution

OrderedOrderedSolidSolid

SolutionSolutionGraph source: Steve Bayne, University of Michigan, 2008

AGAG CUCU

AUAU

+

5

10

15

20

25

30

35

40

50

45

AU-AG-CUAU-AG-CUTenaryTenary

Phase DiagramPhase DiagramType IType IType IIType IIType IIIType III

Type IVIType IVI

T

Graph source: Steve Bayne, University of Michigan, 2008

COMMERCIAL EXAMPLESCOMMERCIAL EXAMPLES

Graph source: Steve Bayne, University of Michigan, 2008

PROCESSING CYCLESPROCESSING CYCLES

inve

stm

ent

all

oy

TIME

Alloy LCTE = 16-18 ppm/C

CASTCASTGOLDGOLDALLOYSALLOYS

Tm 890C

Trt

Graph source: Steve Bayne, University of Michigan, 2008

A.A. DistortionDistortion::

1.1. Margins:Margins: Probability highest in thinner portions of pattern. Probability highest in thinner portions of pattern.a.a. Wax Deformation:Wax Deformation: improper removal or handling of pattern. improper removal or handling of pattern.b.b. Premature Quenching:Premature Quenching: wait until button loses red color. wait until button loses red color.c.c. Investment Expansion/Contraction:Investment Expansion/Contraction:

CASTING PROBLEMSCASTING PROBLEMSfor Gold Alloysfor Gold Alloys

ab

c

Image source: Steve Bayne, University of Michigan, 2008

B.B. Surface IrregularitiesSurface Irregularities::

1.1. Fine Surface Roughness:Fine Surface Roughness: Inherent particle size of investment Inherent particle size of investmenta.a. High W/P ratio increases surface roughness.High W/P ratio increases surface roughness.b.b. Low W/P ratio decreases investment adaptation or flow.Low W/P ratio decreases investment adaptation or flow.c.c. Prolonged burnout encourages investment decomposition.Prolonged burnout encourages investment decomposition.d.d. Overheating alloy encourages investment decomposition.Overheating alloy encourages investment decomposition.e.e. Overheating alloy encourages reaction with investment.Overheating alloy encourages reaction with investment.

2.2. Surface Defects:Surface Defects:a.a. Nodules: air bubbles trapped on the pattern during investing.Nodules: air bubbles trapped on the pattern during investing.

(Use surfactant; paint pattern; vacuum invest; vibrate)(Use surfactant; paint pattern; vacuum invest; vibrate)b.b. Ridges or Veins: poor wetting causing water films on pattern. Ridges or Veins: poor wetting causing water films on pattern.

(Use surfactant; vacuum invest; vibrate investment carefully).(Use surfactant; vacuum invest; vibrate investment carefully).3.3. Gross Surface Defects:Gross Surface Defects:

a.a. Fins: cracked investment (from overheating)Fins: cracked investment (from overheating)

INVESTMENTALLOY

NodulesNodules

Ridges or veinsRidges or veins

FinsFins

SurfaceSurfaceroughnessroughness

CASTING PROBLEMSCASTING PROBLEMSfor Gold Alloysfor Gold Alloys

Image source: Steve Bayne, University of Michigan, 2008

C.C. Incomplete CastingsIncomplete Castings::

1.1. Internal Porosity:Internal Porosity: due to improper solidification. due to improper solidification.a.a. Improper Spruing: Diameter too small or too long.Improper Spruing: Diameter too small or too long.b.b. Low Temperature: Investment or metal too cold.Low Temperature: Investment or metal too cold.c.c. Included Gases: Contaminated gold or oxidized old gold.Included Gases: Contaminated gold or oxidized old gold.d.d. Occluded Gases: Improper burnout of pattern.Occluded Gases: Improper burnout of pattern.

2.2. Incomplete External Shape:Incomplete External Shape:a.a. Insufficient casting pressure.Insufficient casting pressure.b.b. Excessive back pressure from investment.Excessive back pressure from investment.c.c. Suck back into sprue.Suck back into sprue.

InternalInternalporosityporosity

IncompleteIncompletemarginmargin

GasGasbubblebubble

CASTING PROBLEMSCASTING PROBLEMSfor Gold Alloysfor Gold Alloys

Image source: Steve Bayne, University of Michigan, 2008