Dental Casting Alloys / orthodontic courses by Indian dental academy

FORMAT

i) Introduction

ii) Historical Perspective on Dental Casting Alloys.

iii) Metallic Element used in dentistry.

iv) Desirable Properties of casting Alloys.

v) Classification of Dental casting Alloys.

vi) Alloys for all Metal and Resin Veneer Restoration

a. Gold Alloys.

b. Silver Palladium Alloy

c. Aluminium Bronze Alloy.

vii) Heat Treatment of high noble and Nobel metal alloy.

viii) High Noble Alloy for Metal Ceramic Restoration

a. Gold – Platinum- Palladium alloy

b. Gold – Palladium Silver alloy

c. Gold – Palladium alloy.

ix) Noble Alloys for Metalic Ceramic Restoration

a. Palladium Based Alloy

i) Palladium silver alloy

ii) Palladium Copper alloy

iii) Palladium Cobalt alloy

iv) Palladium – Gallium – Silver and Palladium – Gallium – Silver

– Gold alloy.

x) Base Metal Alloys for Cast Metal and Ca Metal Ceramic

Restoration.

a. Classification

b. Handling Hazard and Patients Hazard

c. Cobalt – chromium alloys.

d. Nickel – Chromium Alloys.

e. Aluminum Bronze.

x) Metals for Partial Denture Alloys.

xi) Titanium

xii) Conclusion.

Dental Casting Alloys

Metals and alloys have many uses in dentistry. Steel alloys are

commonly used for the construction of instruments and of wires for

orthodontics. Gold alloys and alloys containing chromium are used for

making crowns, inlays and denture bases whilst dental amalgam, an alloy

containing mercury, is the most widely used dental filling material.

With the exception of Hg, metals are generally hard and lustrous at

ambient temperatures, and have crystalline structures in which the atoms

are closely packed together. Metals are opaque and are good conductors

of both heat and electricity.

The shaping of metals and alloys for dental use can be

accomplished by one of three methods, namely, casting, cold working or

amalgamation. Casting involves heating the material until it becomes

molten. When it can be forced into an investment mould which has been

prepared from wax pattern.

Cold working involves mechanical shaping of the metal at relatively low

temperatures, taking advantage of the high values of ductility and

Malleability possessed by many metals. Some alloys can be mixed with

mercury to form a plastic mass which gradually hardens by a chemical

reaction followed by crystallization. The material is shaped by packing it

into a tooth cavity whilst still in the plastic state.

HISTORICAL PERSPECTIVE ON DENTAL CASTING

ALLOYS

The 20th century generated substantially new changes to dental prosthetic

materials. The major factors that are driving new developments are:

i)Economy The new material performs the same function as the old

material but at a lower cost.

ii)Performance The new material performs better than the old product

in some desirable way, such as ease of processing, improved handlinig

characteristics, or increased fracture resistance.

iii) Aesthetics The new material Provides a more aesthetic result, such

as increased translucency.

1905 – The Lost – Wax Process

i) Taggart’s presentation to the New York Odontological group in

1907 on the fabrication of cast inlay restorations developed in

1905 often has been acknowledged as the first reported

application of the lost – wax technique in dentistry;. It was an

instant success.

ii) It soon led to the casting of inlays, onlays, crowns, FPDs, and

frame works for RPD.

iii) Jewelry alloys were quickly adopted. These gold alloys were

further strengthened with additions of copper, silver, or

platinum. Gold alloys were used because of their

biocompatibility and ease of use.

1932 – Classification of Gold – Based Casting Alloys:

i) In 1932, the dental materials group at the National Bureau of

standards surveyed the alloys being used and classified them as

Type I (Soft , VHN between 50 and 90)

Type II (Medium VHN between 90 and 120)

Type III (Hard VHN between 120 and 150)

Type IV (Extra hard, VHN 150)

ANSI/ADA Specification no.5

ISO standard 15592.

ii) During this period, the results of some tarnish tests suggest that

alloys with a gold content lower than 65% to 75% tarnished too

readily for dental use.

iii) It is now known that, in gold alloys, palladium counteracts the

tarnish potential of silver, allowing alloys with a lower gold

content to be used successfully.

1933 – Cobalt – chromium Partial Denture Alloys

i) Base metal removable partial denture alloys were introduced in

the 19305. Since that time, both nickel – chromium and cobalt –

chromium formulations have become increasingly popular

compared with conventional Type IV gold alloys.

ii) The advantages of the base metal alloys are their lighter weight,

greater stiffness, other beneficial mechanical properties, and

reduced costs.

iii) For these reasons, nickel – and cobalt – based alloys have

largely replaced noble metal alloys for removable partial

denture.

iv) Likewise, by 1978 the price of gold was increasing so rapidly

that attention was focused on the noble metal alloys.

1959 – Porcelain – Fused – to – Metal Process

i) 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.

ii) 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.

iii) The first commercially successful alloy contained gold,

platinum, and palladium.

1971 – The Gold Standard

i) The United States abandoned the gold standard in 1971.

ii) Prices of gold increased, in response to that, new dental alloys

were introduced through the following charges.

a. In some alloys, gold was replaced with palladium.

b. In other alloys, palladium eliminated gold entirely.

c. Base metal alloys with nickel as the major element

eliminated the exclusive need for noble metals.

1976 – The Medical and Dental Devices Act

i) Dental alloys for prosthetics were classifies as passive implants.

ii) All materials on the market before 1976 were automatically

grandfathered as acceptable for market distribution.

Manufacturers were required to have a quality system in place, but no

product standards were established.

1996 – The European Medical Devices Directive

i) The European Union established that any imports of dental

devices required a CE mark.

ii) Information and data on the development process were also

required. Again, no specific product standards were established.

1998 – The Clean Air Acts

i) To meet the requirements of reduced nitrogen and carbon

monoxide emissions, automakers use palladium – containing

catalytic converters.

ii) The demand for palladium soared sevenfold from 1993 to 1999.

iii) Supply could not meet the demand, and the price of palladium

increased to new record highs.

iv) At the same time the price of gold was trading during the

decade.

The result was an increased demand for gold – based dental alloys.

Desirable Properties of Dental Casting alloys

All casting alloys must first be biocompatible and then exhibit

sufficient physical and mechanical properties to ensure adequate function

and structural durability over long periods of time.

The only nearly pure metal cast for dental applications is

commercially pure titanium (often written as CPTi).

From a stand point of patient safety and to minimize the risk for

medico-legal situations, it is highly important to understand the following

clinically important requirements and properties of dental casting alloys.

Biocompatibility The material must tolerate oral fluids and not release

any harmful products into the oral environment.

Corrosion Resistance Corrosion is the physical dissolution of a

material in an environment. Corrosion resistance is derived from the

material components being either too noble to react in the oral

environment (e.g., gold and palladium) or by the ability of one or more of

the metallic elements to form an adherent passivating surface film, which

inhibits any subsurface reaction (e.g., chromium in Ni – Cr and Co – Cr

alloys and titanium in commercially pure titanium [CPTi] and in Ti – 6Al

– 4V alloy).

Tarnish Resistance Tarnish is a thin film of a surface deposit or an

interaction layer that is adherent to the metal surface. These films are

generally found on gold alloys with relatively high silver content or on

silver alloys.

Allergic Components in casting Alloys

A restorative material should not cause adverse health

consequences to a patient.

The patient’s “right – to – know” extends to having some knowledge

of what is being placed into their bodies. Laws in some states are

explicit in this respect. It is wise for the dentist to maintain a record of

the material used for each restoration or prostheses, as well as an

understanding of any known allergies stated by the patient.

Aesthetics Considerable controversy exists over the optimal balance

among the properties of aesthetics, fit, abrasive potential, clinical

survivability, and cost of cast metal prostheses compared with direct-

filling restorations, ceramic- based prostheses (all-ceramic and metal –

ceramic), and resin-veneered prostheses.

Thermal PropertiesFor metal – ceramic restorations, the alloys or

metals must have closely matching thermal expansion to be compatible

with a porcelain, and they must tolerate high processing temperatures.

Melting RangeThe melting range of the alloys and metals for cast

appliances must be low enough to form smooth surfaces with the mold

wall of the casting investment.

Compensation for Solidification: To achieve accurately fitting cast

inlays, on lays , crowns and more complex frameworks or prostheses,

compensation for casting shrinkage from the solid us temperature to room

temp must be achieved either through Computer – generated oversized

dies or through controlled mold expansion. In addition, the fit of a

cemented prosthesis must be tailored to accommodate the layers of

bonding adhesive (if used) and the luting cement.

Strength Requirements:

i) For the full cast alloys the strength requirements increase as the

number of tooth surfaces being replaced increases.

ii) Likewise, alloys for bridge work require higher strength than

alloys for single crowns.

iii) Copings for metal – ceramic pros these are finished in thin

sections and require a sufficient elastic modulus (stiffness) to

prevent excessive elastic deflection from functional

forces ,especially when used for long – span frameworks.

iv) The elastic moduli of many base metal alloys are considerably

greater than gold – based alloys.

Values for the elastic modulus of dental alloys are as follows:

Co - Cr125 to 220Gpa

Ni - Cr145 to 190Gpa

CPTi117 Gpa

Pd-based alloys 110-135 Gpa

Au-based alloys75 to 119 Gpa

Fabrication of cast Prosthese and Frame works

i) The use of cobalt – chromium alloys rather than gold alloys for

partial denture applications may require different casting

investment products and casting equipment in order to produce

high – quality restorations consistently.

ii) Selection of a suitable casting investment is a major problem

when a dentist decides to use titanium for all metal prosthese or

as a metal- ceramic restorative material.

Castability To achieve accurate details in a cast frame work or

prosthesis, the molten metal must be able to wet the investment mold

material very well and flow into the most intricate regions of the mold

without any appreciable interaction with the investment and without

forming porosity with in the surface or subsurface regions.

i) The castability of some base metals is extremely challenging in

this regard, because these alloys tend to readily form oxides or

interact chemically with the mold wall during the casting

process.

Finishing of Cast Metal Cutting, grinding, of some metals is quite

demanding, and extra time is required to produce a satisfactory surface

finish.

i) Hardness, ductility (percent elongation), and ultimate strength

are important properties in this regard.

ii) The hardness of the alloy is a good primary indicator of cutting

and grinding difficulty, and this property varies widely among

the current casting metals. For example, Co – Cr and Ni – Cr

alloys are quite hard compared with other metals.

List of Vickers hardness numbers:

Co - Cr450 to 650

Ni - Cr330 to 400

Ti – 6 Al –4 V 320

Tooth enamel 300 to 400

Type IV Au alloy 250

Pd – based alloys235 to 400

CPTi210 (bulk)

Ag - Pd143 to 154

Dentin 60

Type I Au alloy 55

Porcelain BondingTo achieve a sound chemical bond to ceramic

veneering materials, a substrate metal must be able to form a thin,

adherent oxide, preferably one that is light in color so that it does not

interfere with the aesthetic potential of the ceramic.

i) The metal must have a thermal expansion/contraction

coefficient that is closely matched to that of the porcelain. Stresses

that develop in the ceramic adjacent to the metal/ceramic interface can

enhance the fracture resistance of a metal – ceramic prosthesis or they

can increase the susceptibility to crack fo;rmation. (if tensile in nature)

Economic Considerations The cost of metals used for single – unit

prostheses or as frame works for fixed or removable partial dentures is a

function of the metal density and the cost per unit mass. For example,

compared with a palladium alloy having a density of 11g/cm3 , a gold

alloy with a density of 18g/cm3 will cost 164% (18/11x100) more for the

same volume and unit cost of metal.

Laboratory Costs The metal cost is a major concern for the dental

Laboratory owner who must guarantee prices of prosthetic work for a

certain period of time. Because of the fluctuating prices of noble metals

over the past two decades, the cost of fabricating prostheses made from

noble elements must be adjusted periodically to reflect these changes.

FUNCTIONS OF EACH INGREDIENT METAL IN

CASTING ALLOYGold

i) Yellow in colour

ii) Ductility

iii) Resistance to tarnish and corrosion.

Silver

i) Hardness and strength

ii) Whitens the alloy thus over comes the reddening effect of

copper. But tarnishes the alloy.

iii) Forms solid solution with gold and partial solubility with

copper.

Copper

i) Hardness and strength

ii) Reddish color but lowers tarnish resistance.

iii) Lowers fusion temperature.

iv) Forms solid solution with gold

v) Reduces the density of the alloy.

Palladium

i) Increases resistance to tarnish and corrosion.

ii) Whitens the alloy

iii) Cheap

iv) Absorbs gases formed during casting, and thus reduces porosity.

v) Increases hardness.

Zinc

i) Acts as a scavenger and removes the oxides.

Makes the alloy more castable

CLASSIFICATION OF DENTAL CASTING ALLOYS

IMPORTANCE

i) The dental casting alloy classification is useful for estimating

the relative cost of alloys, because the cost is dependent on the

noble metal content as well as on the alloy density.

ii) It is also useful for identification of the billing code that is used

for insurance reimbursement.

iii) It simplify the communication between dentists and dental

laboratory technologists.

Dental casting alloys are classified according to:

(According to Anusavice)

I) According to American Dental Association (1984)

II) According to ANSI/ADA specification No.5 (1997)

III) According to mechanical property Requirements

proposed

In ISO Draft international standard 1562 for Casting Gold

Alloys (2002)

IV) Classification of casting metals for Full – metal and

Metal – ceramic Prostheses and Partial Dentures

Classification according to Anusavice

I) According to American Dental Association (1984)

Alloy Type Total Nobel Metal content

High Noble (HN) Must contain 40 wt% Au

And 60 wt% of noble metal

elements (Au, Pt, Pd, Rh,Ru,Ir, Os)

Noble (N) Must contain 25wt% of noble

metal elements (Au, Pt, Pd, Rh, Ru,

Ir, Os)

Predominantly Base Metal (PB) Contain 25 wt% of noble metal

elements.

II) According to ANSI/ADA Specification No.5 (1997)

Mechanical Property Requirements

Yield strength (0.2% offset) Elongation

Annealed Hardened Annealed Hardened

Alloy

type

Max.

(Mpa)

Mini.

(Mpa)

Minimum

(Mpa)

Minimum

(%)

Minimum

(%)

Type I 80 180 ------- 18 -----

Type II 180 240 12

Type III 240 12

Type IV 300 450 10 3

(III) According to mechanical property requirements

proposed in ISO Draft International standard 1562 for

casting Gold alloys (2002)

Alloy Type

Minimum yield strength (0.2%) or

proof stress of nonproportional elongation (Mpa)

Minimum elongation after fracture (%)

Type 1 80 18Type 2 180 10Type 3 270 5Type 4 360 3

(IV) Classification of Casting Metals for full- metal and

Metal – ceramic Prostheses and Partial Dentures

Metal Type All- Metal

Prostheses

Metal –

Ceramic

Prostheses

Partial denture

frameworks

High Noble

(HN)

Au-Ag-Pd

Au-Pd-Cu-Ag

HN Metal-

Ceramic alloys

Pure Au (99.7

wt%)

Au-Pt-Pd

Au-Pd-Ag

(5-12 wt% Ag)

Au-Pd-Ag

(12wt% Ag)

Au-Ag-Cu-Pd

Au-Pd

Noble(N) Ag-Pd-Au-Cu

Ag-Pd

Pd-Au

Pd-Au-Ag

Noble Metal –

Cesamic alloys

Pd-Ag

Pd-Cu-Ga

Pd-Ga-Ag

Predominantly

(Base Metal (PB)

CPTi CPTi CPTi

Ti – Al – V Ti – Al – V Ti – Al – V

Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo-Be

Ni-Cr-Mo Ni-Cr-Mo Ni-Cr-Mo

Co-Cr-Mo Co-Cr-Mo Co-Cr-Mo

Co-Cr-W Co-Cr-W Co-Cr-W

Cu-Al

METALLIC ELEMENTS USED IN DENTAL ALLOYS

For dental restorations, it is necessary to combine various elements

to produce alloys with adequate properties for dental applications because

none of the elements themselves have properties that are suitable. These

alloys may be used for dental restorations as cast alloys, or may be

manipulated into wire. The metallic elements that make up dental alloys

can be divided into two major groups, the noble metals and the base

metals.

BASE METAL ALLOYS

INTRODUCTION

Base metal alloys contain no gold, silver, platinum or palladium.

The two most commonly used base metal alloys in dentistry are the

nickel – chromium (Ni/Cr) alloys which are commonly used for crown

and bridge casting, including porcelain fused to metal (PFM) restorations,

and the cobalt- chromium (Co/Cr) alloys which are commonly used for

partial denture frame work castings.

i) These alloys contain less than 25% of nobel metals

ii) They are tarnish and corrosion resistant due to the presence of

chromium (passivation)

iii) These alloys are presently widely used for their superior

mechanical properties and low cost.

Base metals are invaluable components of dental casting alloys

because of their low cost and their influence on weight , strength,

stiffness, and oxide formation (which is required for bonding to

porcelain)

iv) Compared with noble metals are still frequently referred to as

non precious or no noble, the preferred designation is

predominantly base metal. One reason for this designation is

that some base metal alloys in the past have contained a minor

amount of palladium, but because the properties of these alloys

were controlled primarily by the base metals present, they

should not have been classified as noble alloys of these alloys

were controlled primarily by the base metals present, they

should not have been classified as noble alloys.

Noble metals are not currently included in most of the base metal

alloys in use.

The percentage of base metal use in dentistry decreased between

1981 and 1995. Although the increased acceptance of these alloys

during this period was greatly influenced by the rapidly fluctuating

international cost of gold and other noble metals, the subsequent

decline in the cost of noble metals has had a small effect on reversing

this trend.

The Ni – Cr – Be alloys have retained their popularity despite the

potential toxicity of beryllium and the allergenic potential of nickel.

There are several reasons for the use of nickel – chromium alloys

in dentistry:

i) Nickel is combined with chromium to form a highly corrosion

resistant alloy.

ii) Ni – Cr alloys became popular in the early 1980s as low cost

metals ($2 to $3 per conventional avoirdupois ounce) when the

price of gold rose to more than $ 500 per troy ounce. Because

metal – ceramic restorations made with Ni – Cr – Be alloys

have exhibited high success rates from the mid – 1980s to the

present, many dentists have continued to use these alloys.

iii) Alloys such as Ticonium 100 have been used in removable

partial denture frameworks for many years with few reports of

allergic reactions. However, it is believed that palatal

epithelium may be more resistant to allergic reactions (contact

dermatitis ) than gingival secular epitheliums .

iv) The Ni – Cr and Ni – Cr – Be alloys are relatively inexpensive

compared with high noble or noble alloys. The price of nickel –

base alloys is stable, unlike the price of palladium based alloys.

v) Although beryllium is a toxic metal, dentists and patients

should not be affected because the main risk occurs primarily;

in the vapor form, which is a concern for technicians who melt

and cast large quantities of Ni – Cr – Be alloys without

adequate ventilation or fume hoods in the melting area.

vi) Nickel alloys have excellent mechanical properties, such as high

elastic modulus (stiffness), high hardness, and a reasonably high

elongation (ductility).

The majority of nickel – chromium alloys for crowns and FPD

prostheses contain 61 wt% to 81 wt% nickel, 11 wt% to 27 wt%

chromium and 2wt% to 4wt% molybdenum.

i) These alloys may also contain one or more of the following

elements:

aluminum, beryllium, boron, carbon, cobalt, copper,; cerium, gallium,

iron, manganese, niobium, silicon, tin, titanium, and zirconium.

The cobalt – chromium alloys typically contain 53 wt% to 67 wt%

cobalt, 25 wt% to molybdenum, which could affect the metal ceramic

bond strength.

Classification of Base Metal Alloys

i) Nickel – cobalt – Chromium alloys

i) Cobalt – Chromium:

Co – Cr – Mo

Co – Cr - W

ii) Nickel – Chromium:

Ni –Cr – Mo – Be.

Ni – Cr – Mo.

iii) Cobalt – Chromium – nickel

ii) Titanium alloys:

Pure Ti.

Ti – Al - V

iii) Others:

Aluminum bronze.

Nickel – Cobalt – Chromium Alloys

Composition: Percentage of alloying elements.

i) Nickel – Chromium:

Ni up to 80%

Cr – 13 – 22%

Be – up to 2%

ii) Cobalt – chromium:

Co – 55 – 68%

Cr – up to 25 – 27%

Cobalt – chromium: (vitallium)

Co – 60%

Cr – 25 – 30%

Nickel – chromium:

Ni – 67%

Cr –26%

Cobalt – chromium – nickel:

Co – 54%

Cr – 26%

Ni – 14%

Advantages And Disadvantages Of Base Metal Alloys

ADVANTAGES DISADVANTAGES

Cheaper and harder than gold

alloys

Density is low

High yield strength

High melting range and high

modulus of elasticity

Casting shrinkage is more.

Exceptional strength at high

temperature.

Oxidize readily.

Superior sag resistance –

means less deformation

than gold alloys.

Not resistant to tarnish

and corrosion.

APPLICATION OF BASE METAL ALLOYS

i) Inlays and onlays.

ii) Cast post

iii) Orthodontic appliances.

iv) For metal ceramic restorations

Base metal alloys generally have higher hardness and elastic modulus

values are more sag resistant at elevated temp.

v) For making cast removable partial dentures. It has the

following disadvantages when used metal ceramic alloys.

a) They are more difficult to cast and presolder than Au – Pd or

Pd – Ag alloys. More technique sensitive than noble metal

alloys.

b) Ni – based or Co – based alloys have a potential for

porcelain debonding due to separation of a poorly adherent

oxide layers from the metal substrate.

c) Small differences in composition may produce wide

variations in metal ceramic bond strength.

Composition

COBALT – CHROMIUM ALLOYS

The chemical composition of these alloys specified in the ISO

standard for Dental Base Metal Casting Alloys is as follows:

Cobalt Main constituent

Chromium No less than 25%

Molybdenum No less than 4%

Cobalt + nickel +chromium No less than 85%

A typical material would contain 35 – 65% cobalt, 25 – 35%

chromium, 0-30% nickel, a little molybdenum and trace quantities of

other elements such as beryllium, silicon and carbon.

i) Cobalt and Nickel are hard, strong metals the main purpose of the

chromium is to further harden the alloy by solution hardening and

also to impart corrosion resistance by the passivating effect.

Chromium exposed at surface of the alloy rapidly becomes oxidized

to form a thin, passive, surface layer of chromic oxide which

prevents further attack on the bulk of the alloy.

ii) The minor elements are generally added to improve casting and

handling characteristics and modify mechanical properties. E.g.

silicon imparts good casting properties to a nickel – containing alloy

and increases its ductility.

Molybdenum and beryllium are added to refine the grain structure and

improve the behavior of base metal alloys during casting.

iii) Carbon affects the hardness, strength and ductility of the alloys and

the exact concentration of carbon is one of the major factors

controlling alloy properties.

iv) The presence of too much carbon results in a brittle alloy with very

low ductility and an increased danger of fracture.

v) During crystallization the carbides become precipitated in the

interdendritic regions which form the grain boundaries. The grains

are generally much larger than those produced on casting gold alloy.

If this occurs the alloy becomes extremely hard and brittle as the

carbide phase acts as a barrier to slip. A discontinuous carbide phase

is preferable since it allows some slip and reduces brittleness.

vi) Whether a continuous or discontinuous carbide phase is formed

depends on the amount of carbon present and on the casting

technique. High melting temp during

vii) In general, cobalt – chromium alloys are resistant to pitting and

crevice corrosion, even with in the body. By contrast, relatively little

is known about their susceptibility to stress corrosion cracking or

corrosion fatigue.

viii) Co – Cr alloys may undergo fretting corrosion quite readily. The

process of fretting is a mechanical one and involves rubbing in the

form of a prolonged series of cyclic micro – movements. The result

is localized damage to one or both surfaces.

ix) In fretting corrosion, the process continually exposes new surfaces,

and these undergo oxidation. The fretting debris that becomes

trapped between the surface damage and exposure of new metal, and

the whole process leads to loss of metal from the assembly.

BIOCOMPATIBILITY OF COBALT – CHROMIUM ALLOYS

USE OF CHROME – COBALT – BASED ALLOY

i) As a denture base to complete denture, as a denture base to

partial denture.

ii) As a part of implant denture.

iii) For making surgical screws and plates.

iv) In orthopedic surgery.

v) For making dental wires.

Casting favour discontinuous carbide phases but there is a limit to

which this can be used to any advantage since the use of very high casting

temperatures can cause interactions between the alloy and the mould.

NICKEL – CHROMIUM ALLOYS

The chemical composition of these alloys specified in the ISO Standard

for Dental Base Metal Casting Alloys (Part 2) is as follows.

Nickel Main constituent

Chromium No less than 20%

Molybdenum No less than 4%

Beryllium No more than 2%

Nickel + Cobalt + Chromium No less than 85%

As for the Co/Cr alloys the concentrations of minor ingredients can

Have a profound effect on properties. The concentration of carbon and

the nature of the grain boundaries are major factors in controlling the

properties of these alloys.

MANIPULATION OF BASE METAL CASTING ALLOYS

i) The fusion temperatures of the Ni/Cr and Co/Cr alloys vary with

composition but are generally in the range 1200 – 15000c. This is

considerably higher than for the casting gold alloys which rarely have

fusion

Temperatures above 9500c.

ii) Melting of gold alloys can readily be achieved using a gas – air

mixture.

iii) For base metal alloys, however, either acetylene – oxygen flue

or an electrical induction furnance is required. The latter method is

to be favored since it is carried out under more controlled conditions.

iv) When using oxyacetylene flames the ration of oxygen to

acetylene must be carefully controlled. Too much oxygen may cause

oxidation of the alloy whilst an excess of acetylene produces an

increase in the metal carbide content leading to embrittlement.

v) Investment moulds for base metal alloys must be capable of

maintaining their integrity at the high casting temperatures used.

Silica bonded and phosphate bonded materials are favored with the

latter product being most widely used.

vi) Gypsum – bonded investments decompose above 12000c to

form sulphur dioxide which may be absorbed by the casting, causing

embrittlement. This effect can be reduced by the incorporation of

oxalate in the investment; however the problem is generally avoided

by choosing an investment which is more stable at elevated

temperatures.

vii) The density values of base metal alloys are approximately half

those of the casting gold alloys. For this reason the thrust developed

during casting may be somewhat lower, with the possibility that the

casting may not adequately fill the mould. Casting machines used for

base metal alloys must therefore be capable or producing extra thrust

which overcomes this deficiency.

viii) The problem may be aggravated if the investment is not

sufficiently porous to allow escape of trapped air and other gases.

Careful use of vents and sprues of adequate size is normally

sufficient to overcome such problems.

ix) The greatest expense involved in producing a Co/Cr dental

casting is in the time required for trimming and polishing.

x) In the ‘as cast’ state, the alloy surface is normally quite rough,

partially due to the coarse nature of some investment powders. Finer

investments can be used to give a smoother surface requiring less

finishing.

xi) One common technique involves painting the wax pattern with

fine investment –this then forms the inner surface of the investment

mould. The bulk of the mould is then formed from the coarser grade

material.

xii) Base metal alloys, and particularly the Co/Cr type are very hard

and consequently difficult to polish. After casting, it is usual to

sandblast the metal to remove any surface roughness or adherent

investment material as well as the green layer of oxide which coats

the surface after casting. Electrolytic polishing may then be carried

out. This procedure is essentially the opposite of electroplating.

xiii) If a rough metal surface is connected as the anode in a bath of

strongly acidic electrolyte, a current passing between it and the

cathode will cause the anode to ionize and lose a surface film of

metal. With a suitable electrolyte and the correct current density, the

first products of electrolysis will collect in the hollows of the rough

metal surface and so prevent further attack in these areas. The

prominences of the metal surface will continue to be dissolved and

in this way the contours of the surface are smoothed. Final polishing

can be carried out using a high – speed polishing buff.

xiv) The process of electro polishing is not generally used for Ni/Cr

alloy castings. These products are normally used for crown and

bridge work and it is essential to maintain the accuracy of fit,

particularly at the margins of crowns. This accuracy may be lost

during polishing procedures and care is required to avoid such

problems.

COMPARATIVE PROPERTIES OF Ni/Cr AND TYPE 3

CASTING GOLD ALLOYS FOR SMALL CAST

RESTORATION

PROPERTY Ni/Cr Type 3

Gold alloy

Comments

Density (gcum-

3)8 15 More difficult to produce defect –

free castings for Ni/Cr alloys.

Fusion

temperatureAs high

as

13500c

Normally

lower than

10000c

Ni/Cr alloys require electrical

induction furnance or oxyacetylene

equipment.

Casting

shrinkage (1%)2.0 1.4 Mostly compensated for by correct

choice of investment.

Tensile strength

(Mpa)600 540 Both adequate for the applications

being considered.

Proportional

limit (Mpa)500 290 Both high enough to prevent

distortions for applications being

considered; note that values are

lower than for partial denture alloys

Table 8-1)

Modulus of

elasticity (Gpa)220 85 Higher modulus of Ni/Cr is an

advantage for larger restorations,

e.g. bridges and for porcelain –

bounded restorations.

Hardness

(Vickers)300 150 Ni/Cr more difficult to polish during

service.

Ductility

(%elongation)3-30 20 Relatively large values suggest that

burnishing is possible; however,

large proportional limit values

suggest wish forces would be

required.

METALS AND ALLOYS FOR IMPLANTS

Implants offer an alternative method of treatment for the replacement of

missing teeth which can be used instead of dentures or fixed bridges.

Biocompatibility and stability are often seen as closely related in

that some materials are known to encourage bone growth which produces

a very intimate interface between bone and in plant which helps to

stabilize the latter. Function primarily depends upon the rigidity of the

implant structure. This in turn is related to the dimensions and the

modulus of elasticity of the material from which the implant is

manufactured.

Dental implants are normally classified according to the way in

which they are stabilized. The three most common types are:

- Subperiosteal

- Blade – vent end osseous

- Osseo integrated.

Subperiosteal implants consist of an open framework of cast alloy

which rests on top of the bony ridge but beneath the mucosa.

Cost cobalt – chromium alloys are most commonly used for these

applications. The very high modulus of elasticity of these materials

combined with reasonable cast ability is the main factors affecting this

choice. Attempts have been made to improve the biocompatibility of the

alloys by using hydroxyapatite coatings.

- Blade – vent implants are normally constructed from titanium

which has excellent biocompatibility.

BIOLOGICAL HAZARDS AND PRECAUTIONS: RISKS

FOR DENTAL LABORATORY TECHNICIANS

Laboratory technicians may be exposed occasionally or routinely

to excessively high concentrations of beryllium and nickel dust and

beryllium vapor. Although the beryllium concentration in dental alloys

rarely exceeds 2% by weight, the amount of beryllium vapor released into

the breathing space during the melting of Ni-Cr- beryllium alloys may be

significant over an extended period of time.

i) Actually, the potential hazards of beryllium should be based on

its atomic concentration in an alloy.

ii) One can demonstrate that an alloy which contains 80% Ni,

11.4% Cr, 5% Mo, 1.8% Fe, and 1.8% Be on a weight basis contains

73.3% Ni, 11.8% Cr, 2.6% Mo, 1.6% Fe, and 10.7% Be on an

atomic basis. Thus toxicity considerations for beryllium should be

based on the atomic concentration rather than the weight percentage.

iii) The Vapor pressure of pure beryllium is app 0.1 torr (mmHg) at

an assumed casting temp of 1370 o C. Comparable vapor pressures

for chromium, nickel, and Molybdenum are 5x10-3 torr, 8x10-4 torr,

and 3x10-11 torr, respectively.

iv) The risk for beryllium Vapor exposure is greatest for dental

technicians during alloy melting, especially in the absence of an

adequate exhaust and filtration system.

v) The Occupational Health and Safety Administration (OSHA)

specifies that exposure to beryllium dust in an should be limited to

particulate beryllium concentration of 2g/m3 of air (both respirable

and non respirable particles) determined from an 8-h time-weighted

average the allowable maximum concentration is 5g/m3 (not to be

exceeded for a 15-min period). For a minimum duration of 30 min, a

maximum ceiling concentration of 25g/m3 is allowed. The National

Institute for Occupational Safety and Health (NIOSH) recommends a

limit of 0.5 g/m3 based on a 130 – min sample.

vi) Moffa et al (1973) reported that high levels of beryllium were

accumulating during finishing and polishing when a local exhaust

system was not used. When an exhaust system was used, the

concentration of beryllium in the breathing zone was reduced to

levels considered safe by the authors. Workers exposed to

moderately high conc. of beryllium dusts over a short period of time,

or prolonged exposure to low conc., may experience signs and

symptoms representing acute disease states.

vii) Physiological responses vary from contact dermatitis to severe

chemical pneumonitis, which can be fatal. The chronic disease state

is characterized by symptoms persisting for more than 1 year, with

the onset of symptoms separated by a period of years from coughing,

chest pain, and general weakness to pulmonary dysfunction.

Prevention

i) Well –ventilated work areas.

ii) Protection against inhalation of dust particles during trimming

with masks.

Nickel

Nickel is common in the general population. The source can also

be due to other contacts like utensils and artificial jewelry. The most

common manifestation is contact dermatitis. A patient with a base metal

alloy bridge can show erythematous inflammation in the area of contact.

Other manifestations due to inhalation include pulmonary irritation,

pneumoconiosis, lung carcinomas, leading to decrease in lung function

and death.

Prevention Patch test to confirm allergy. Use of alternative metals like

palladium or titanium alloy.

Titanium alloys Their major advantages are biocompatibility to oral

tissues, significant strength and ductility.

Composition:

i) Titanium alloy

ii) Chromium – 5 – 15%

iii) Nickel – 5 – 15%

iv) Molybdenum – 3%

v) Silicon, manganese, iron and carbon- small quantities.

Advantages

i) High modulus of elasticity

ii) Easy cast ability.

iii) Excellent bio compatibility

iv) Has high tarnish and corrosion resistance and does not form

corrosion products.

v) Oxidizes upon contact with air or oral fluids.

vi) Low thermal conductivity.

vii) Capability of bonding t resin and porcelain.

Disadvantages Special equipment is required.

Aluminium – Bronzei) Aluminium Bronze 7-11 wt%

ii) Copper 71-88 wt%

iii) Nickel 2-4 wt%

iv) Iron 1-4 wt%

These alloys are still in experimental stage. No particular clinical

trial has been done. Poor resistance to tarnish is a major drawback.

NOBLE METALS

- Elements with a good metallic surface that retain their surface in

dry; air. They react easily with sulfur to form sulfides, but their

resistance to oxidation, tarnish, and corrosion during heating,

casting, soldering, and use in the mouth is very good.

The noble metals are gold, platinum, palladium, iridium, sodium,

osmium, and ruthenium.

- The noble metals, together with silver, are some times called

precious metals. Some metallurgists consider silver a noble metal

in dentistry because it corrodes considerably in the oral cavity.

Thus the terms noble and precious are not synonymous in dentistry.

GOLD (Au) Pure gold is a soft, malleable ductile metal that has a rich

yellow color with a strong metallic luster.

i) It ranks much lower in strength.

ii) Small amounts of impurities have a pronounced effect on the

mechanical properties of gold and its alloys. The presence of less

than 0.2% lead causes gold to be extremely brittle.

iii) Air or water at any temp doesn’t affect or tarnish gold.

iv) Gold is not soluble in sulfuric, nitric or hydrochloric acids.

However, it readily dissolves in combinations of nitric and HCl

(aqua rugia, 18 Vol% nitric acid and 82 vol% Hclacids ) to form the

trichloride of gold (Aucl3).It is also dissolved by a few other

chemicals, such as potassium cyanide and solutious of bromine or

chlorine.

v) Gold must be alloyed with Cu, Ag, Pt and other metals to

develop the hardness, durability, and elasticity necessary in dental

alloys, coins, and jewelry.

PLATINUM (Pt) Platinum is a bluish – white metal, and is toughs

ductile, malleable, and can be produced as foil or fine – drawn wire.

i) Platinum has hardness similar to copper.

ii) Pure pt has numerous applications in dentistry because of its

high fusing point and resistance to oral conditions and elevated

temp.

iii) Pt has been used for pins and posts in crown and bridge

restorations and alloys may be cast or soldered to the posts

without damage.

iv) Adds greatly to the hardness and elastic qualities of gold.

v) Tends to lighten the color of yellow gold based alloys.

PALLADIUM (Pd)

i) White metal some what darker than Pt.

ii) Its density is a little more than half that of Pt and gold.

iii) It has a quality of absorbing or occluding large quantities of

hydrogen gas when heated. This can be an undesirable quality

when alloys combining Pd are heated with an improperly

adjusted gas – air torch

iv) Palladium can be combined with gold, silver, Cu, Co, Sn, In or

Ga for dental alloys.

Iridium (Ir), Ruthenium (Ru), and Rhodium (Rh)

i) Iridium and Ruthenium are used in small amounts in dental alloys as

grain refiners to keep the grain size small.

A small grain size is desirable because in improves the mechanical

properties and uniformity of properties with in an alloy. As little as

0.005% of Ir is effective in reducing the grain effect.

ii) Ru has a similar effect. The grain refining properties of these elements

occurs largely because of their extremely high melting points.

iii) Ir melts at 24100C and Ru at 23100C. Thus these elements don’t melt

during the casting of the alloy and serve as nucleating centers for the

melt as it cools, resulting in a fine – grained alloy.

iv) Rh also has a high melting point (199660C) and has been used in

alloys with Pt to form wire for thermocouples. These thermocouples

help measure the temp in porcelain furnaces used to make dental

restorations.

Osmium (Os) Because of its tremendous expense and extremely high

welting point Os is not used in dental casting alloys.

i) a deoxidizing agent.

ii) Because of its low density, the resulting ZuO large behind the

denser molten mass during casting, and is therefore excluded from

the casting.

iii) If too much Zinc is present, it will markedly increase the

brittleness of the alloy.

Indium (In)

i) In is a soft, gray- white metal with a low melting point of

156.60c.

ii) It is not tarnished by air or water. It is used in some gold- based

alloys as a replacement for Zn, and is a common minor

component of some noble ceramic dental alloys.

iii) Recently, Indium has been used in greater amounts (up to 30%

by wt) to impart a yellow color Pd – Ag alloys.

BINARY COMBINATIONS OF METALS

Although most noble casting alloys have three or more elements, the

properties of certain binary alloys are imp because these binary

combinations constitute the majority of the mass of many – noble alloys.

An understanding of the physical and manipulative properties of these

binary – combinations constitute the majority of the mass of many noble

alloys. Among the noble alloys, six binary combinations of elements are

important:

i) Au – Cu, Pd – Cu, Au – Ag, Pd – Ag, Au – Pd, and Au – Pt

Phase diagrams are powerful tools for understanding the physical

and manipulative properties of binary alloys.

ALLOY COMPOSITION AND TEMPERATURE

i) In each phase diagram, the horizontal axis represents the

composition of the binary alloy.

ii) For example, in fig A, the horizontal axis represents a series of

binary alloys of gold and copper ranging in composition from 0% gold

(or 100% copper) to 100% gold.

iii) The composition can be given In atomic percent (at %) or

weight percent (wt%)

iv) Weight percent compositions give the relative mass of each

element in the alloy, where as atomic percentages give the relative

numbers of atomies in the alloys. It is a simple calculation to convert

weight percentages to atomic percentages, or vice versa.

v) Note that for the binary alloys shown in fig, the atomic percent

composition is shown along the bottom of the phase diagram whereas

the weight percent composition is shown along the top.

vi) The atomic and weight percent compositions of the binary

alloys can differ considerably.

vii) For example, for the Au – Cu system, an alloy that is 50% gold

by weight is only 25% gold by atoms.

viii) For other systems, like the Au –Pt system ‘F’, there is little

difference between atomic and weight percentages.

The difference between atomic and weight percentages depends on the

differences in the atomic masses of the elements involved.

ix) The bigger the difference in atomic mass, the bigger the

difference between the atomic and weight percentages in the binary

phase diagram.

x) Because it more convenient to use masses in the manufacture of

alloys, the most common method to report composition is by weight

percentages. However, the physical and biological percentages.

However, the physical and biological properties of alloys relate best

to atomic percentages. It is therefore important to keep the difference

between atomic and weight percent in mind when selecting and

using noble dental casting alloys. Alloys that appear high in gold by

weight percentage may in reality contain for fewer gold atoms than

might be thought.

xi) Other aspects of the phase diagrams that deserve attention are

the liquid us and solid us lines. The y axes show temperature.

xii) If the temp is above the liquid us line (marked L), the alloy will

solid us line (marked S), the alloy will be solid. If the temp lies

between the liquid us and solid us lines, the alloy will be partially

molten.

xiii) Note that the distance between the liquidus and solidus lines

varies among systems in Fig. For example the temp difference

between these lines is small for the Ag – Au system, much larger for

the Au – Pt system (‘F’) and varies considerably with composition

for the Au – Cu system. (‘A’)

xiv) If the liquidus – solidus line is broad, the alloy will remain at

least partially molten for a longer period after it is cast.

xv) The temp. of the liquid us line is also imp, and varies

considerably among alloys and with composition. For example the

liquidus line of the Au- Ag system ranges from 9620 to 10640C (‘C’),

but the liquidus line of the Au – Pd system ranges from 10640 to

15540C [‘E’]. It is often desirable to have an alloy with a liquidus

line at lower temperatures; the method of heating is easier, fewer

side reactions occur, and shrinkage is generally less of a problem.

PHASE STRUCTURE OF NOBLE ALLOYS:

i) The area below the solidus lines in fig is also imp to the

behavior of the alloy.

ii) If this area contains no boundaries, then the binary system is a

series of solid solutions. This means that the two elements are

completely soluble in one another at all temp and compositions.

iii) The Ag –Pd system (‘D’) and Pd – Au system (‘F’) are

examples of solid solution systems.

iv) If the area below the solidus line contains dashed lines, then an

ordered solution is present with in the dashed lines. An ordered

solution occurs when the two elements in the alloy assume specific

and regular positions in the crystal lattice of the alloy. This situation

differs from a solid solution where the positions of the elements in

the crystal lattice are random.

Examples of systems containing ordered solutions are the Au

– Cu system (‘A’).The Pd – Cu system ‘B’ and Au – Ag

system ‘C’.

v) Note that the ordered solutions occur over a limited range of

compositions because the ratios between the elements must be

correct to support the regular positions in the crystal lattices.

vi) If the area below the solidus line contains a solid line, it

indicates the existence of a second phase. A second- phase is an area

with a composition distinctly different from the first phase.

vii) In the Au – Pt system (‘F’) a second phase forms between 20

and 90 at% platinum. If the temp. is below the phase boundary live

with in these compositions, two phases exist in the alloy. The

presence of a second phase is imp because it significantly changes

the corrosion properties of an alloy.

HARDENING OF NOBLE ALLOYS

i) The use of pure cast gold is not practical for dental restorations

because cast gold lacks sufficient strength and hardness.

ii) Solid – solution and ordered solution hardening are two

common ways of strengthening noble dental alloys sufficiently for

use in the mouth.

iii) By mixing two elements in the crystal lattice randomly (forming

a solid solution), the force needed to distort the lattice may be

significantly increased.

For example, adding just 10% by weight of copper to gold, the

tensile strength increases from 105 to 395 Mpa and the Brinell hardness

increases from 28 to 85.

The 90:10 Au – Cu mixture is the composition used in U.S. gold coins.

iv) If the positions of the two elements become ordered (forming an

ordered solution), the properties of the alloy are improved further.

For a typical gold – based casting alloy, the formation of an ordered

solution may increase yield strength by 50%, tensile strength by 25%

and hardness by at least 10%. It is important to note that the

elongation of an alloy is reduced by formation of the ordered

solution. For the typical gold – based alloy, the percentage

elongation will decrease from 30% to about 12%.

v) The formation of ordered solutions has been commonly used to

strengthen cast dental restorations, particularly in gold – based

alloys. As shown in fig ‘A’, the Au- Cu system supports ordered

solutions between about 20 and 70 at %gold. However, the

manipulation of the alloy during casting will determine if the ordered

solution will form.

vi) If Au – Cu containing about 50 at % gold is heated to the

molten state and then cooled slowly, the mass will solidify at about

8800C as solid solutions. As the mass cools slowly to 4240 C, the

ordered solutions will then form and will remain present at room

temp.

vii) However, if the mass is cooled rapidly to room temp. after the

initial solidification, the ordered solution will not form because there

is insufficient time for the mass reorganize. Thus the alloy will be

trapped in a non – equilibrium state of a solid solutions and will be

softer, weaker, and have greater elongation.

viii) By heating an alloy in either condition above 4240C, the state of

the alloy can be selected by picking the cooling rate.

ix) Rapid cooling will preserve the solid solution and the soft

condition, whereas slow cooling will allow the formation of the

ordered solution and the hardened condition.

FORMULATION OF NOBLE ALLOYS

The desired qualities of noble dental casting alloys determine the

selection of elements that will be used to formulate the alloys.

The ideal noble casting alloy should have

i) A low melting range and narrow solidus – liquidus temperature

range.

ii) Adequate strength, hardness, and elongation

iii) A low tendency to corrode in the oral environment

iv) Low cost among other properties.

- Solid – solution systems are desirable for the formulation of alloys

because for the formulation of alloys because they are generally

easier to manufacture and manipulate, have a lower tendency to

corrode than multiple- phase systems, and provide increased

strength through solid – solution or ordered – solution hardening.

CARAT AND FINENESS OF GOLD – BASED ALLOYS

For many years the gold content of gold containing alloys has been

described on the basis of the carat, or in terms of fineness, rather than by

wt%. 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 R, for example, 18k

or 22k gold.

The use of term carat to designate the gold content of dental alloy

is less common now. It is not unusal to find the weight percentage of gold

listed or to have the alloy described in terms of finer ness. 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). The fineness represents a precise measure

of the gold content of the alloy and is often the preferred measurement.

- An 18k gold would be designated as 750 fine, or, when the decimal

system is used, it would be 0.750 fine; this indicates that 750/1000

of the total is gold.

- The fineness system is somewhat less relevant to day because of

the introduction of alloys that are not gold- based. It is imp to

emphasize that the terms carat and fineness refer only to gold

content, not noble –metal content.

ALLOYS FOR ALL – METAL AND RESIN – VENEERED

RESTORATIONS

In 1927, The National Bureau of standards established gold casting alloy

types I through IV according to dental function, with hardness increasing

from type I to type IV.

a) Gold Alloys

b) Silver Palladium alloy

c) Aluminium Bronze alloy.

HEAT TREATMENT OF HIGH NOBLE AND NOBLE

METAL ALLOYS

i) Gold alloys can be significantly hardened if the alloy contains a

sufficient amount of copper. Types I and II alloys usually don’t

harden, or they harden to a lesser degree than do the types III and IV

gold alloys.

ii) The actual mechanism of hardening is probably the result of

several different solid state transformations.

iii) In metallurgical engineering terminology the softening heat

treatment is – referred to as a solution heat treatment. The hardening

heat treatment is termed age hardening.

SOFTENING HEAT TREATMENT OF GOLD CASTING

ALLOYS

i) The casting is placed in an electric furnace for 10 min at a temp

of 7000c (12920F) and then it is quenched in h20. During this period,

all intermediate phases are presumably changed to a disordered

solid, solution, and the rapid quenching prevents ordering from

occurring during cooling.

ii) The tensile strength, proportional limit, and hardness are

reduced by such a treatment and the ductility is in creased.

iii) The softening heat treatment is indicated for structures that are

to be ground, shaped, or otherwise cold worked, either in or out of

the mouth. Although 7000c is an adequate average softening temp,

each alloy has its optimum temp, and the manufacturer should

specify the most favorable temp. and time.

HARDENING HEAT TREATMENT OF GOLD CASTING

ALLOYS

It can be accomplished in several ways.

i) One of the most practical hardening treatments is by soaking or

aging the casting at a specific temp, for a definite time, usually 15 to

30 minutes, before it is water – quenched.

ii) The aging temp. depends on the alloy composition but is

generally between 2000C (3920F) and 4500C (8420F).

iii) Ideally, before the alloy is given an age – hardening treatment,

it should be subjected to a softening heat treatment to relieve all

strain hardening and to start the hardening treatment with the alloy

as a disordered solid solution.

iv) This treatment is indicated for metallic partial dentures, saddles,

FPDs, and other similar sites. For small sites, such as inlays, a

hardening treatment is not usually employed. Age hardening

substantially increases the yield strength.

v) The hardness values for noble metal alloys correlate quite well

with their yield strengths.

vi) Age hardening reduces the percent elongation in some cases.

Alloys with low elongation are relatively brittle materials and

fracture readily if loaded beyond the proportional limit or yield

strength.

B) SILVER – PALLADIUM ALLOYS

i) Silver – Pd alloys are white and predominantly silver in

composition but have substantial amounts of Pd (at least 25%) that

provide nobility and promote tarnish resistance. They may or may

not contain copper and a small amount of gold.

ii) The Cu-free Ag – Pd alloys may contain 70% to 72% silver and

25% Pd and may have physical properties similar to those for a type

III gold alloy.

iii) Other Ag – based alloys might contain roughly 60% Ag, 25%

Pd, and as much as 15% or more Cu and may have properties more

like a Type IV gold alloy.

Despite early reports of poor castability, the Ag – Pd alloys can

produce acceptable castings.

iv) The use of metal – ceramic restorations in posterior sites has

increased relative to the use of all metal crowns and onlays.

The compositions of representative high noble and noble alloys

(including Ag- Pd alloys) for all meal restorations (Type 1 to Type IV).

C) ALUMINUM BRONZE ALLOY

Bronze is traditionally defined as a copper – rich, copper – tin (Cu- Sn)

alloy with or without other elements such as Zn and phosphorus, there

exist essentially two – component (binary), three component(ternary),and

four component (quaternary) bronze alloys that contain aluminum bronze

(Cu – Al), silicon bronze copper – silicon, and beryllium bronze (Cu –

Be).

i) The Al- bronze family of alloys may contain between 81 wt%

and 88 wt% Cu, 7 wt% to; 11 wt% Al, 2 wt% to 4 wt% Ni, and 1 wt

to 4 wt% iron.

ii) There is a potential for copper alloys to react with sulfur to form

copper sulfide which may tarnish the surface of this alloy in the

same manner that Ag sulfide darkens the surface of gold – base or

Ag – base alloys that contain a significant Ag content.

HIGH NOBLE AND NOBLE ALLOYS FOR METAL

CERAMIC RESTORATION (PROSTHESES)

a) Gold – Platinum – Palladium alloy

b) Gold – Palladium silver alloy

c) Gold- Palladium alloy.

i) The chief objection to the use of dental porcelain as a

restorative material is its low strength under tensile and shears stress

conditions.

ii) Although porcelain can resist compressive stresses with

reasonable success, the substructure design should not include

shapes in which significant tensile stresses are produced during

loading. A method by which the disadvantage can be minimized is to

bond the porcelain directly to cast alloy substructure made to fit the

prepared tooth.

iii) Adding less than 1% of oxide forming element such as iron,

indium, and tin to this high gold content alloy the porcelain metal

bond strength was improved. Iron increases proportional limit and

strength of alloy.

iv) The 1% addition of base metal to gold, Pd and Pt alloys was all

that was necessary to produce a slight oxide film on surface of

substructure to achieve porcelain metal bond strength.

v) Inspite of vastly different chemical compositions, all the alloys

described in the following according to their principal chemical

elements share at least three common features:

a) They have the potential to bond to dental porcelain

b) They possess coefficients of thermal contraction compatible

with those of dental porcelains.

c) Their solidus temp is sufficiently high to permit the

application of low fusing porcelains.

vi) The coefficients of thermal expansion (CTE) tend to have a

reciprocal relationship with the melting points of alloys (because of

an inverse dependence on the relative strength of interatomic

bonding), as well as the melting range of alloys; that is, the higher

the melting temp of a metal, the lower its CTE. Metal ceramic alloys

of a metal, the lower its CTE. Metal ceramic alloys are also often

referred to as porcelain fused to metal (PFM) or ceramo metal alloys.

GOLD – PALLADIUM – SILVER ALLOYS (Low Silver Content)

i) Au – Pd – Ag alloys, which contain 5% to 11.99% Ag are

economical alternatives to the Au – Pt – Pd or Au – Pd – Pt alloys.

These are resistant to tarnish and corrosion.

ii) The principal disadvantage of this alloy group is the potential

for porcelain discoloration when Ag vapor is released and deposited

on the porcelain surface.

GOLD – PALLADIUM – SILVER ALLOYS (High silver content)

i) Gold alloys that contain 12% Ag or more account for

approximately; 20% of the current alloy market. These include Au –

Pd – Ag, Pd – Au – Ag and Pd – Ag alloys.

ii) The Au – Pd alloys with high silver contents (12% to 22%)

have been popular alternatives to the higher gold content alloys for

many years despite their potential for porcelain discoloration.

iii) These alloys are white – colored and are used primarily for their

lower cost and comparable physical properties.

iv) The commonly used alloys in this gp contain between 39%

and53% Au and 25% to; 35% Pd.

v) The potential for porcelain discoloration is greatest with alloy

which has the highest silver contents.

vi) The factors that intensify the porcelain color changes because of

the release of Ag were identified previously. In general, it is

advisable to avoid these types of alloys when using lighter shades

and ceramic products that are sensitive to silver discoloration.

GOLD – PALLADIUM ALLOYS

This alloy was designed to overcome the porcelain discoloration effect

(because it is Ag – free) and also to provide an alloy with a lower thermal

contraction coefficient than that of either the Au – Pd – Ag or Pd – Ag

alloys.

i) The contents are gold ranging from 44% - 55% and a Pd level

35 –45%

ii) Alloys of this type must be used with porcelains that have low

coefficient of thermal contraction to avoid the development of axial

and circumferential tensile stresses in porcelain during the cooling

part of porcelain firing cycle.

iii) The yield strength, modulus of elasticity;, tensile strength and

hardness of Au – Pd – Ag and Au – Pd alloys are greater and density

lower, then those are Au – Pt –Pd alloys, which implies that

combination; will be more resistant to masticatory force and stiffer

then restoration made of Au – Pt – Pd alloys. Lower densities also

mean prosthesis will be lighter in weight.

PALLADIUM – GOLD ALLOYS

Its popularity has been diminished by the recent price volatility of Pd.

These are free of Ag. Therefore they don’t contribute to porcelain

discoloration. Physical properties are similar to those of the Au – Pd

alloys. Thermal compatibility with commercial porcelain products has not

yet been reported in the dental literature.

PALLADIUM – GOLD – SILVER ALLOYS

i) The Pd – Au – Ag alloy group is similar to the Au – Pd – Ag

types of alloys in their potential for porcelain discoloration. These

alloys have gold contents ranging from 5% to 32% and Ag contents

varying between 6.5% and 14%

ii) One would expect the potential for porcelain discoloration to be

greater for the higher Ag – content alloys in this group.

iii) These alloys have a range of thermal contraction coefficients

that increase with an increase in Ag content.

PALLADIUM – SILVER ALLOYS

i) It was introduced to the U.S. market in 1974 as the first gold –

free noble alloy available for metal – ceramic restorations.

ii) The compositions of Pd – Ag alloys fall with in a narrow range

of 53% to 61% Pd and 28% to 40% Ag.

iii) Tin and /or indium are usually added t increase alloy hardness

and to promote oxide formation and adequate bonding to porcelain.

iv) A proper balance is needed to maintain a reasonably low casting

temp and a compatible coefficient of thermal contraction.

v) Because of their increase Ag content compared with that of gold

based alloys, the Ag discoloration effect is most severe for these

alloys. Gold metal conditioners or ceramic coating agents may

minimize this effect.

vi) The low specific gravity of these alloys (10.7 to 11.1),

combined with their low intrinsic cost, make them attractive as

economical alternatives to the gold – based alloys.

vii) Adherence to porcelain is considered to be acceptable for most

of the Pd – Ag alloys.

viii) Instead of the formation of the desired external oxide, Pd – Ag

nodules may develop on the surface, which enhance retention of

porcelain by mechanical rather than chemical bonding.

PALLADIUM-COPPER-GALLIUM ALLOYS:

No clinical reports of adverse events have been reported for

Pd-Cu-Ga alloys.

The clinician should be aware of the potential effect on

aesthetics of the dark brown or black oxide formed during oxidation

and subsequent porcelain-firing cycles.

PALLADIUM-GALLIUM-SILVER ALLOYS:

They tend to have a slightly lighter colored oxide than the

Pd-Cu alloys and they are thermally compatible with lower expansion

porcelains.

The silver consent is generally relatively low (5 wt% to 8 wt

% in most cases) and is usually inadequate to cause significant

porcelain greening.

Pd-Ga-Ag alloys generally have relatively low thermal

contraction coefficients are expected to be more compatible with

lower expansion porcelains.

METALS FOR PARTIAL DENTURE ALLOYS:

The majority of removable partial denture frameworks are

made from alloys based primarily on nickel, cobalt, or titanium as the

principal metal component.

Ni is a malleable, ductile, silver-colored transition element

with atomic numbers and a melting point of about 14500C.

CO is a silver-colored transition element with atomic

number 27, having a melting point of about 15000C and little ductility

at room temperature.

All CO-based and Ni-based alloys container to prevent

corrosion and tarnish. The passivation mechanism of the alloy occurs

through a thin surface layer of chromium oxide (Cr2O3). Most CO-Cr

alloys contain MO (CO-Cr-MO), and some may contain Ni (CO-Cr-

Ni). Some Ni-Cr alloys contain beryllium (Be), which lowers the

melting point to improve castability.

Frameworks may also be made from CPTi and Ti-6Al-4V.

The most biocompatible metal for frameworks is CPTi.

Porcelain with Na contents are believed to exhibit a more intense

discoloration because of more rapid silver diffusion in Na-containing

glass.

CONCLUSION

So its important to have the proper knowledge of metals and the alloys for the proper use in dentistry.

REFERENCES:

1) Dental material properties and manipulation CRAIG

2) Notes on dental materialsE.C.COMBE

3) Text book of dental materialsSHARMILLA HUSSAIN

3) Essential of dental materialsSH SORATUR

4) Applied dental materialsJOHN F.MCCABE

5) The chemistry of medical and dental materialsJ.W.NICHOLSON

6) Dental materialsANUSAVICE