Precious metal alloys

Vinay Pavankumar .K

1ST Year P.G

Department of Prosthodontics

AECS Maaruti Dental College

Precious metal alloys

Classification

Composition

Properties

History

Alloy

Treatment of noble and high

noble alloys

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.

Two or more metals that are mutually soluble in each other in the molten state.

Noble metal Those metal elements that resist oxidation,

tarnish, and corrosion during heating, casting, or soldering and when used intraorally; examples include gold and platinum.

Good metallic surface that retain their surface in dry air.

Eight noble metals

Metallurgists consider silver a noble metal, but it is not considered a noble metal in dentistry

Precious metal alloyPrecious 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

HISTORICAL PERSPECTIVE ON PRECIOUS METAL & ALLOYS IN DENTISTRY

C. Spence Bate at the 1883,annual meeting of the British Dental Association, in Plymouth, entitled `A Review of the Scientific Progress of Dental Surgery from 1771 to 1883, contains the passage:

“We must congratulate ourselves on the great improvement that has taken place in the power of retaining diseased teeth and restoring to usefulness such as would, a few years ago, have been considered hopelessly irrecoverable. The extent of this process of repair can best be understood by saying that, independent of amalgam and cement stoppings, 20 000 ounces [620 kg] of fine gold is annually used in filling teeth ...”

As early as the seventh centry 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 rivetting the strip so as to hold the substitute teeth in place.

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

The first printed book on dentistry, entitled 'ArtzneyBuchlein' ('The Little Pharmacopaeia'), 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.”

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.

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

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.

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.

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

In 1907, a 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 iridioplatinum soldered with 24 carat gold.

The recent development of gold abutment retainaing screws and cylinder.

Gold

Soft, rich yellow color and a strong metallic lusterMost malleable and ductile0.2% lead – brittleSoluble in aqua regiaAlloyed with copper, silver, platinum – increases hardness ,

durability and elasticity

Platinum

Bluish white metalHardness similar to copperHigher melting point ( 1772°C) than porcelainCoefficient of thermal expansion close to porcelainLighten the color of yellow gold based alloysCommon constituent in precision prosthetic attachments

Silver (Ag)

Malleable, ductile; white metal.Stronger and harder than gold, softer than copper. Absorbs oxygen in molten state-difficult to castForms series of solid solutions with palladium and

gold . Neutralizes reddish color of alloys containing

copper

Palladium (Pd)

White metal darker than platinumDensity little more than half that of Pt and AuAbsorbs hydrogen gas when heatedNot used in pure state in dentistry Whitens yellow gold based alloys.

Iridium(Ir ) Ruthenium(Ru ), Rhodium(Rh) & Osmium(Os)

Grain refinersImproves mechanical properties and uniformity of

properties within alloy Extremely high melting point of Ir - 2410°C and Ru -

2310°C – serve as nucleating centersOsmium(Os) has a very high melting point, and is very

expensive, hence not used in dentistry.

Physical properties of noble and precious metals

Mechanical properties of noble and precious metals

History of Dental casting alloy

1907 : Lost wax process technique -W H Taggart.

1932 : Classification of gold based casting alloys.

1933 : Introduction of Co-Cr and Ni-Cr alloys.

1959 : Porcelain-fused to metal prothesis.

1971 : End of Bretton Woods system.

1976 : The medical and dental devices act.

1996 : European medical devices directive.

1998 : The clean air act.

Classification of dental casting alloys:

According to their use: All metal inlaysCrowns and bridgesMetal ceramic prothesisPost and coresRemovable partial dentures.Implants

According to major elements :•Gold based •Palladium based•Silver based•Nickel based•Cobalt based •Titanium based

According Dominant Phase system:• Single • Eutectic • Peritectic • Intermetallic

Alloy Classification by Noble Metal Content

Alloy type Total noble metal content

High noble (HN) Contain ≥40 wt% Au and ≥60 wt% of noble metal elements (Au, Pt, Pd, Rh , Ru, Ir, Os)

Noble (N) Contain ≥25 wt% of noble metal elements

Predominantly Base metal (PB)

Contain <25wt% of noble metal elements

In 2003, the Council for Scientific Affairs revised the classification to include titanium in a separate category because of its extensive usage and similar properties with noble metals

class Required noble

content (%)

Required gold

content (%)

Required titanium content

(%)

High noble alloys ≥60 ≥40

Titanium and titanium alloys

≥85

Noble alloys ≥25

Predominantly base metals

≥25

Givan A A, Precious Metals in Dentistry, Dent Clin N Am vol 51 2007;591-601

Classification of Mettallic Materials for Dental Application ISO 22674 (2006)

Type Yield Strength (Mpa)

Elongation (%)

Examples of Application

0 -- -- Single fixed tooth fixed restorations

1 80 18 Single fixed tooth fixed restorations veneered or non veneered

2 180 10 For Single fixed tooth fixed restorations

3 270 5 For multiple unit fixed restorations

4 360 2 For appliances with this cross sections that are subject to very high forces

5 500 2 For thin removable partial dentures, parts with thin cross sections

According to yield strength and percentage elongation (proposed in ISO draft international standard 1562 for casting

gold alloys 2002)

Alloy type Hardness Yield strength (MPa)

Percent elongation

%

Type 1: Low strength Soft 80 18

Type 2:Medium strength

Medium 180 10

Type 3: high strength Hard 270 5

Type 4: Extra-high strength

Extra hard

360 3

Classification of casting metals for full metal, Metal Ceramic and Partial Dentures

Metal type

All metal Metal Ceramic Partial Denture

High noble

Au-Ag-PdAu-Pd-Cu-Ag

Au-Pt-PdAu-Pd-Ag (5-12 wt% Ag)Au-Pd-Ag (>12 wt% Ag)

Au-Pd

Au-Ag-Cu-Pd

Noble Ag-Pd-Au-CuAg-Pd

Pd-AuPd-Au-Ag

Pd-AgPd-Cu-GaPd-Ga-Ag

Desirable properties of casting alloys

 

BiocompatibilityTarnish and Corrosion resistance Compatible thermal propertiesCastabilityAesthetics Economical

Functional Mechanical Properties of Casting alloys

Elastic modulus

Yield strength

Ductility

Hardness

Fatigue Resistance

Selected properties of major types of high-noble alloys

Alloy type

Solidus-liquidus(C)

Color Phasestructure

Elasticmodulus(staticGPa)

Vicker’shardness(kg/mm2)

Yieldstrength(tension,0.2%, Mpa)

Au-Pt (Zn) 1060–1140

Yellow Multiple 65–96 165–210 360–580

Au-Pd (Ag)

1160–1260

White Single 105 280 385

Au-Cu-Ag 905–960 White Single 100 210 450

Mechanical properties of noble and precious metal casting alloys

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.

 

Softening Heat Treatment Increases ductility reduces tensile strength ,proportional limit and hardness

Method:Casting placed in electric furnace10 minutes,700°Cquenched in water resulting disordered

solid solution Indicated-alloys that are to be ground, shaped or otherwise

cold worked either in or out of mouth.

Hardening of Noble metals Increases strength, proportional limit, and hardness, but

decreases ductilityIf positioning of two elements become ordered-ordered

solutionCopper present in gold alloy helps in this process.

Method:Soaking/ageing casting-15 to 30 minutes before water

quenching 200°C to 450°CIdeally, before age hardening it should first be subjected

to softening heat treatment

Alloys for metal ceramic prosthesis

Classification of Noble PFM alloys Au Based Pd Based

Au-Pt-Pd (21 K) Pd-Ag

Au- Pd (13 K) Pd-Cu

Au-Pd-Ag (13 K) Pd-Co

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

Physical and Chemical properties1. Noble metal content2. Hardness3. Yield strength4. Elongation5. Fusion temperature6. Porcelain-Metal Compatibility7. Color stability8. Biocompatibility

Typical properties of alloys for PFM restorations

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. 

Gold-platinum alloyDeveloped 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)

Gold-palladium alloyUsed 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).

Gold-copper-silver-palladium alloyHave 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%.

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. 

Gold–copper-silver-palladium alloyMore 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%

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

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

 

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)

Greening Discoloration of porcelainDue to silver ionsMechanism not clear

Cervical regionMetal - ceramic interface

Prevented -silver free alloys metal coating agents

Precious metals & dental implantsGOLD 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

Alloy Melting range

Coefficient of thermal linear expansion

Plastic sleeveBurnout

temperature

Au 60%, Pt 24%, Pd 15%, Ir 1%

1400–1460°C/ 2552-2660°F

25–500°C 12.3 (10–6K-1)25–600°C 12.7 (10–6K-1

700°C/1292°F

Review of literatureThe 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

• This study evaluated the influence of a composite interlayer (at the metal–ceramic interface) on the shear bond strength of a metal–ceramic composite when compared with a conventional porcelain fused to metal (PFM).

The shear bond strength results for all composites bonded to metal and to ceramic substrates were significantly higher (>150 MPa) than those registered in the upper range of conventional porcelain fused to metal (PFM) techniques ( 80 MPa). The use of a ∼composite interlayer proved to enhance metal/ceramic adhesion in 160%.

B. Henriques et al. Optimization of bond strength between gold alloy and porcelain through a composite interlayer obtained by powder metallurgy .Materials Science and Engineering A 528 (2011) 1415–1420

The aim of this study were to investigate the fatigue limits of two Pd–Ag alloys (Ivoclar Vivadent) with differing mechanical properties and varying proportions of secondary alloying elements, examine the effect of casting porosity on fatigue behavior,and determine the effect of casting size on microstructures and Vickers hardness

Tension test bars, heat-treated to simulate dental porcelain application, were subjected to cyclic loading at 10 Hz, with R-ratio of −1 for amplitudes of compressive and tensile stress. Two replicate specimens were tested at each stress amplitude. Fracture surfaces were examined with a scanning electron microscope (SEM). Sectioned fatigue specimens and additional cast specimens simulating copings for a maxillary central incisor restoration were also examined with the SEM, and Vickers hardness was measured using 1 kg load. Casting porosity was evaluated in sectioned fatigue fracture specimens, using an image analysis program.

•Baba.D.Li.N etal, Study of Pd–Ag dental alloys: examination of effect of casting porosity on fatigue behavior and microstructural analysis Journal of Materials Science: Materials in Medicine October 2010, Volume 21, Issue 10, pp 2723-2731

relatively low ratios of fatigue limit to 0.2% yield strength are similar to those found previously for high-palladium dental alloys, and are attributedto their complex microstructures and casting porosity.

Complex fatigue fracture surfaces with striations were observed forboth alloys. Substantial further decrease in the number of cycles for fatigue failure only occurred when the pore size and volume percentage became excessive.

While the heat-treated alloys had equiaxed grains with precipitates, the microstructural homogenization resulting from simulated porcelain firing differed considerably for the coping and fatigue test specimens; the latter specimens had significantly higher values of Vickers hardness.

•Baba.D.Li.N etal, Study of Pd–Ag dental alloys: examination of effect of casting porosity on fatigue behavior and microstructural analysis Journal of Materials Science: Materials in Medicine October 2010, Volume 21, Issue 10, pp 2723-2731

CONCLUSION

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