Dental casting alloy.ppt

“What we will be tomorrow is because of what we are
today, and what we are today is because of what we were
yesterday”.

INTRODUCTION
In dentistry, metals represent one of the three major classes
of materials used for the reconstruction of damaged or missing
oral tissues in the form casting metal alloys.

Pure metals
 apt to be soft
 many tends to corrode rapidly
 high cost their use is quite limited in dentistry.
To optimize properties, most metals commonly used in dentistry
are mixtures of two or more metallic elements or one or more
metal and/or non metals (THE ALLOY).

History is the best teacher and a brief description of
the evolution makes you understand the rationale for
the development of the wide variety of alloy
formulations.
HISTORY

HISTORY OF METALS IN DENTISTRY
Dentistry as a specialty is believed
to have begun about 3000 BC.
Gold bands and wires were used by the
Phoenicians after 2500 BC.
Modern dentistry began in 1728
when Fauchard published different
treatment modalities describing many types
of dental restorations, including a method
for the construction of artificial dentures
made from ivory.

Gold shell crowns were described by Mouton in 1746 but they
were not patented until in 1873 by Beers.
In 1885 Logan patented porcelain fused to platinum post
replacing the unsatisfactory wooden post previously used to
build up intra- radicular areas of teeth.
In 1907 a detached post crown was introduced which was more
easily adjustable.

Year Event
1907 Introduction of Lost-Wax Technique
(TAGGART’s presentation – new York
odontological group on fabrication of cast inlay
restoration developed in 1905)
1933 Replacement of Co-Cr for Gold in RPD’s
Advantages
Reduced cost
Lighter weight
Greater stiffness
1950 Development of Resin Veneers for Gold Alloys
-To improve esthetics

1959 Introduction of the Porcelain Fused-to-Metal Technique
(Added platinum, palladium in gold alloys to reduce
coefficient of thermal expansion)
1968 Palladium-Based Alloys as Alternatives to Gold Alloy.
1971 Nickel-Based Alloys as Alternatives to Gold Alloys
1971 – THE GOLD STANDARD
The United States abandoned the gold standard in 1971.
Gold then became a commodity freely traded on the open
markets.
Increasing price of gold, new dental alloys were introduced
through the following changes:
Gold was replaced with palladium.
Base metal alloys with nickel as the major element.

1980s Introduction of All-Ceramic Technologies
(Aesthetics as main concern)
1999 Gold Alloys as Alternatives to Palladium-Based Alloys
(1993-1999 more demand & less availability as they
used in palladium containing converter to reduce n2 &
co emission in automobile industry)
COST
pd $125$1000 per troy oz(31.1g)
au $300 per troy oz(31.1g)

KEY TERMS
Grain – A microscopic single crystal, in the microstructure of a
metallic material.
Applied- alloy strength
- workability
- corrosion susceptibility.
Metal – An element whose atomic structure readily loses electrons
to form positively charged ions, which exhibits
metallic bonding capacity, good light reflectance from a
polished surface and high electrical and thermal
conductivity.

Noble metal – which are highly resistant to oxidation and
dissolution in inorganic acids. Gold and platinum
group metals (Platinum, palladium, rhodium,
ruthenium, iridium and osmium).
Base metal – A metal that readily oxidizes or dissolves to release
ions.
Noble metal – which are highly resistant to oxidation and
dissolution in inorganic acids. Gold and platinum
group metals (Platinum, palladium, rhodium,
ruthenium, iridium and osmium).
Base metal – A metal that readily oxidizes or dissolves to release
ions.

PHYSICAL PROPERTIES AND
EFFECTS OF DENTAL CASTING
ALLOYS

NOBLE METALS
The noble metals have been the basis of inlays, crowns and
bridges because of their resistance to corrosion in the oral cavity.
Gold, platinum, palladium, rhodium, ruthenium, iridium,
osmium, and silver are the eight noble metals. However, in the
oral cavity, silver is more reactive and not considered as a noble
metal.

GOLD
 Pure gold is a soft and
ductile. Yellow “Gold” hue.
 Density of 19.3 gms/cm3.
Melting point of 1063oC.
Boiling point of 2970 oC.
 CTE of 14.2×10-6/°C.
 Good luster and high polish.
 Good chemical stability.
 Does not tarnish and
corrode.
Dissolves in aqua regia, pot cyanide, br, cl

Gold content:
Traditionally the gold content of dental casting alloys have been
referred to in terms of:
• Karat
• Fineness
Karat:
Parts of pure gold in 24 parts of alloys.
For Eg: a) 24 Karat gold is pure gold
b) 22 Karat gold is 22 parts of pure gold
Fineness:
parts per thousand of pure gold.
For Eg: Pure gold is 1000 fine.
Thus, if ¾ of the gold alloy is pure gold, it is said to be 750 fine.

SILVER
The “Whitest” of all metals.
Helps to counteract the
reddish colour of copper.
To a slight extent it increases
strength and hardness.
In large amounts however, it
reduces tarnish resistance.
lowest density 10.4gms/cm3.
melting point of 961oC.
boiling point of 2216 oC.
 CTE19.710-6/oC.
(comparatively high)

PLATINUM
Increases the strength and
corrosion resistance.
Increases the melting point
and has a whitening effect
on the alloy.
Helps to reduce the grain
size.
Highest density of 21.45
gms/cm3 ,
 Highest melting point of
1769oC,
Boiling point of 4530 oC and
the lowest CTE 8.910-6/oC

PALLADIUM
It is similar to platinum in its
effect. It hardens as well as
whitens the alloy.
Raises the fusion temperature
and provides tarnish resistance.
Less expensive than platinum.
Density of 12.02gms/cm3.
Higher melting point of1552oC.
Boiling point of 3980 oC and
lower CTE which is 11.810-/oC,
when compared to gold.

IRIDIUM, RUTHENIUM
They decrease the grain size.
They are added in very small
quantities (about 100 to 150 ppm).
IRIDIUM
high melting point of 2454°C ,
boiling point of 5300 °C ,
density of 22.5gm/cm3
CTE 6.810-6/oC.
RUTHENIUM
melting point of 1966°C ,
boiling point of 4500 °C ,
density of 12.44 gm/cm3
CTE 8.310-6/oC

BASE METALS
These are non-noble metals.
Properties
Influences on physical properties,
control of the amount
type of oxidation, or their strengthening effect.
Although they are frequently referred as non precious, the
preferred term is base metal.
Examples of base metals are chromium, cobalt, nickel, iron,
copper, manganese etc.

COBALT
hardness,
strength
rigidity to the alloy.
high melting point of 1495°C
boiling point of 2900 °C
density of 8.85 gm/cm3
CTE 13.810-6/oC

NICKEL
Decreases
Strength, hardness, modulus
of elasticity and fusion temp.
Increases ductility.
Melting point of 1453°C ,
Boiling point of 2730 °C ,
Density of 8.9 gm/cm3
CTE 13.310-6/oC
Nickel, which is the most
common metal to cause
Contact Dermatitis.

CHROMIUM
Passivating effect ensures
corrosion resistance.
Higher proportion greater
tarnish and corrosion resistance.
Reduces the melting point.
30% chromium is the upper
limit to get maximum
mechanical properties.
CTE 6.210-6/ oC

COPPER
It is the principal hardener.
Reduces the melting point and
density of gold.
Reddish colour.
In greater amounts it reduces
resistance to tarnish and corrosion
of the gold alloy.
Therefore, the maximum content
should NOT exceed 16%.
Density of 8.96 gm/cm³
CTE 16.5 10-6/°C .

ZINC
Scavenger for oxygen.
Without zinc the silver in
the alloy causes absorption
of oxygen during melting.
Later during solidification,
the oxygen is rejected
producing gas porosities in
the casting.
Density of 7.133gm/cm3
CTE 39.710-6/oC

MOLYBDENUM OR
TUNGSTEN
They are effective
hardeners.
Molybdenum is preferred as
it reduces ductility to a lesser
extent than tungsten.
Molybdenum refines grain
structure.
CTE 4.9 10-6/oC

IRON,BERYLLIUM
They help to harden the metal ceramic gold - palladium
alloys, iron being the most effective. In addition, beryllium reduces
fusion temperature and refines grain structure . IRON has melting
point of 1527°C , boiling point of 3000 °C , density of 7.87
gm/cm3 and CTE 12.3 10-6/oC .

GALLIUM
It is added to
compensate for the
decreased coefficient of
thermal expansion that
results when the alloy is
made silver free.
The elimination of silver
reduces the tendency for
green stain at the margin
of the metal-porcelain
interface.

MANGANESE AND SILICON
Primarily oxide scavengers to prevent oxidation of other
elements during melting. They are hardeners. MANGANESE
has melting point of 650°C , boiling point of 1107 °C , density
of 1.74 gm/cm3 and CTE 25.2 10-6/oC , where as SILICON
has melting point of 1410°C , boiling point of 2480 °C , density
of 2.33 gm/cm3 and CTE 7.3 10-6/oC .

CARBON:
Small amounts may have a
pronounced effect on strength,
hardness and ductility.
Carbon forms carbides
important factor in strengthening
the alloy.
However when in excess it
increases brittleness.
CTE 6 10-6/oC .

BORON
It is a deoxidizer and
hardener, but
reduces ductility.

ALLOYS
They are generally prepared by fusion of the elements
above their melting points.
For example,
A certain amount of chromium is added to iron, carbon,
and other elements to form stainless steel, an alloy that
is highly resistant to corrosion.
Also to nickel or cobalt alloys, which comprise two of the
major groups of base metal alloys used in dentistry.

At least four factors determine the extent of solid solubility of
metals; atom size, valence, chemical affinity and crystal
structure.
ATOM SIZE:
If the sizes of two metallic atoms differ by less than
approximately 15% (first noted by Hume-Rothery), they
possess a favorable size factor for solid solubility.

VALENCE:
Metals of the same valence and size are more
likely to form extensive solid solutions than are metals
of different valences.
CHEMICAL AFFINITY:
When two metals exhibit a high degree of
chemical affinity, they tend to form an intermetallic
compound upon solidification rather than a solid
solution.

CRYSTAL STRUCTURE:
Only metals with the same type of crystal structure can
form a complete series of solid solutions.
Few crystal structure
body-centered cubic (bcc)

CLASSIFICATION OF DENTAL
CASTING ALLOYS

1. ALLOY TYPES BY FUNCTIONS:
In 1927, the Bureau of Standard established gold casting
alloys, type I to type IV according to dental function with
hardness increasing from type I to type IV.
Type I (Soft):
It is used for fabrication of small inlays, class III and class V
restorations which are not subjected to great stress . These
alloys are easily burnishable.
Type -II (Medium):
These are used for fabrication of inlays subjected to moderate
stress, thick 3/4 crowns, abutments, pontics, full crowns and
soft saddles.
Type I and II are usually referred to as inlay gold.

Type -III (Hard):
It is used for fabrication of inlays subjected to high stress, thin
3/4 crowns, thin cast backing abutments, pontics, full crowns,
denture bases and short span FPDs . Type III alloys can be
age hardened.
Type-IV (Extra hard):
It is used for fabrication of inlays subjected to high stress,
denture bases, bars and clasps, partial denture frameworks
and long span FPDs. These alloys can be age hardened by
heat treatment.
Type III and Type IV gold alloys are generally called "Crown
and Bridge Alloys", although type IV alloy is used for high
stress applications such as RPD framework.

Later, in 1960, metal ceramic alloys were introduced and
removable partial denture alloys were added in this
classification.
Metal ceramic alloys (hard and extra hard):
It is suitable for veneering with dental porcelain, copings, thin
walled crowns, short span FPDs and long span FPDs. These
alloy vary greatly in composition and may be gold, palladium,
nickel or cobalt based.
Removable partial denture alloys :
It is used for removable partial denture frameworks. Now a days,
light weight, strong and less expensive nickel or cobalt based
have replaced type IV alloys .

2. ALLOY TYPES BY DESCRIPTION:
A) CROWN AND BRIDGE ALLOYS
used in the fabrication of full metal or partial veneers.
1. Noble metal alloys:
i) Gold based alloy - type III and type IV gold alloys ,
low gold alloys
ii) Non-gold based alloy-Silver -palladium alloy
2. Base metal alloys:
i) Nickel-based alloys
ii) Cobalt based alloys
3. Other alloys:
i) Copper-zinc with Indium and nickel
ii) Silver-indium with palladium

B) METAL CERAMIC ALLOY
1. Noble metal alloys for porcelain bonding:
i) Gold-platinum -palladium alloy
ii) Gold-palladium-silver alloy
iii) Gold-palladium alloy
iv) Palladium silver alloy
v) High palladium alloy
2. Base metal alloys for porcelain bonding:
i) Nickel -chromium alloy
ii) Cobalt-chromium alloy

C) REMOVABLE PARTIAL DENTURE ALLOY
Although type-IV noble metal alloy may be used, majority of
removable partial framework are made from base metal alloys:
1. Cobalt-chromium alloy
2. Nickel-chromium alloy
3. Cobalt-chromium-nickel alloy
4. Silver-palladium alloy
5. Aluminum -bronze alloy

ALLOY TYPE TOTAL NOBLE METAL
CONTENT
High noble metal
Contains > 40 wt% Au and > 60
wt% of the noble metal elements
(Au + Ir + Os + Pd + Pt + Rh + Ru)
Noble metal Contains > 25 wt % of the noble
metal elements
Predominantly base metal Contains < 25 wt % of the noble
metal elements
Alloy Classification of the American Dental
Association (1984)
3.ALLOY TYPE BY NOBILITY

Alloy type All-metal Metal-ceramic
High noble Au-Ag-Cu-Pd Au-Pt-Pd
Metal ceramic alloys Au-Pd-Ag (5-12wt% Ag)
Au-Pd-Ag (>12wt%Ag)
Au-Pd (no Ag)
Noble Ag-Pd-Au-Cu Pd-Au (no Ag)
Ag-Pd Pd-Au-Ag
Metal-ceramic alloys Pd-Ag
Pd-Cu
Pd-Co
Pd-Ga-Ag
Base Metal Pure Ti Pure Ti
Ti-Al-V Ti-Al-V
Ni-Cr-Mo-Be Ni-Cr-Mo-Be
Ni-Cr-Mo Ni-Cr-Mo
Co-Cr-Mo Co-Cr-Mo
Co-Cr-W Co-Cr-W
Al bronze

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

DESIRABLE PROPERTIES OF DENTAL CASTING
ALLOYS
Biocompatibility
Ease of melting
Ease of casting
Ease of brazing (soldering)
Ease of polishing
Little solidification shrinkage
Minimal reactivity with the mold material
Good wear resistance
High strength
Excellent corrosion resistance
Porcelain Bonding

GOLD CASTING ALLOYS:
ADA specification No. 5 classify dental gold casting
alloys as:
1. High Gold Alloys Type I
Type II
Type III
Type IV
2. Low Gold Alloys
3. White Gold Alloys
Inlay Gold Alloy
Crown & Bridge Alloy

HIGH GOLD ALLOY:
These alloys contain 70% by weight or more of gold
palladium and platinum. ADA specification No.5 divides this into
four depending upon mechanical properties.
Type I (Soft):-
They are weak, soft and highly ductile, useful only in
areas of low occlusal stress designed for simple inlays such as
used in class I, III & V cavities.
These alloys have a high ductility so they can be
burnished easily. Such a characteristic is important since these
alloys are designed to be used in conjunction with a direct wax
pattern technique. Since such a technique occasionally results
in margins that are less than ideal . At present, these are used
very rarely.

PROPERTIES
1. Hardness VHN (50 – 90)
2. Tensile Strength Quite Low
276 MPa or 40,000 PSi
3. Yield Strength 180 MPa or 26,000 PSi
4. Linear Casting Shrinkage 1.56% (according to
Anusavice)
5. Elongation or ductility 46% - William O Brien
18% - Anusavice
COMPOSITION
Au Ag Cu Pt Pd Zn&Ga
83% 10% 6% - 0.5% balance

Type II (Medium):-
These are used for conventional inlay or onlay
restorations subject to moderate stress, thick three quarter
crowns, pontics and full crowns. These are harder and have
good strength.
Ductility is almost same as that of type I alloy however,
yield strength is higher. Since burnishability is a function of
ductility and yield strength, greater effort is required to deform
the alloy. They are less yellow in color due to less gold.

Properties:
1. Hardness VHN (90-120)
2. Tensile Strength 345 MPa
3. Yield Strength 300 MPa
4. Linear Casting Shrinkage 1.37%
5. Elongation 40.5% -
William O Brien
10% -
Anusavice
Composition:
Au Ag Cu Pt Pd
Zn&Ga
77% 14% 7% - 1%
balance

Type III (Hard):
Inlays subject to high stress and for crown and bridge in
contrast to type I and type II, this type can be age hardened.
The type III alloy, burnishing is less important than strength.
Properties:
1. Hardness (VHN) 120 – 150
5. Elongation or ductility 39.4% - William O Brien
5% - Anusavice
Composition:
Au Ag Cu Pt Pd Zn & Ga
75% 11% 9% - 3.5% balance

Type IV (Extra Hard):
These are used in areas of very high stress, crowns and
long span bridges. It has lowest gold content of all four type (Less
than 70%) but has the highest percentage of silver, copper,
platinum and Palladium. It is most responsive to heat treatment
and yield strength but lowers ductility.
Properties:
1. Hardness VHN (150-200)
5. Elongation or ductility 17% - William O Brien
3% - Anusavice
Composition:
Au Ag Cu Pt Pd Zn&Ga
56% 25% 14% - 4% balance

Type Hardness Proportional
limit
Strength Ductility Corrosion
resistance
I
II INCREASES DECREASES
III
IV

Composition Range (weight percent) of traditional type I to IV alloys and
four metal -ceramic alloys
Alloy type
Main elements Au Cu Ag Pd Sn, In, Fe, Zn,
Ga
I High noble (Au base) 83 6 10 0.5 Balance
II High noble (Au base) 77 7 14 1 Balance
III High noble (Au base) 75 9 11 3.5 Balance
III Noble (Au base) 46 8 39 6 Balance
III Noble (Ag base) 70 25 Balance
IV High noble (Au base) 56 14 25 4 Balance
IV Noble (Ag base) 15 14 45 25 Balance
Metal-
ceramic
High noble (Au base) 52 38 Balance
Metal-
ceramic
Noble (Pd base) 30 60 Balance
Metal-
ceramic
High noble (Au base) 88 1 7 (+4Pt) Balance
Metal-
ceramic
Noble (Pd base) 0-6 0-15 0-
10
74-88 Balance

HEAT TREATMENT OF GOLD ALLOYS:
Heat treatment of alloys is done in order to alter its
mechanical properties.
Gold alloys can be heat treated if it contains sufficient
amount of copper. Only type III and type IV gold alloys can be
heat-treated.
There are two types of heat treatment.
1. Softening Heat Treatment (Solution heat treatment)
2. Hardening Heat Treatment (Age hardening)

1. SOFTENING HEAT TEMPERATURE
Softening heat treatment increases ductility, but reduces
tensile strength, proportional limit, and hardness.
Indications:
It is indicated for appliances that are to be grounded,
shaped, or otherwise cold worked in or outside the mouth.
Method:
The casting is placed in an electric furnace for 10 minutes
at a temperature of 700oC and then it is quenched in water.
During this period, all intermediate phases are presumably
changed to a disordered solid solution, and the rapid quenching
prevents ordering from occurring during cooling.
Each alloy has its optimum temperature. The
manufacturer should specify the most favorable temperature and
time.

2. HARDENING HEAT TREATMENT
Hardening heat treatment increases strength, proportional
limit, and hardness, but decreases ductility. It is the copper present
in gold alloys, which helps in the age hardening process.
Indications:
It is indicated for metallic partial dentures, saddles, bridges
and other similar structures. It is not employed for smaller
structures such as inlays.
Method:
It is done by “soaking” or ageing the casting at a specific
temperature for a definite time, usually 15 to 30 minutes. It is then
water quenched. The aging temperature depends on the alloy
composition but is generally between 200°C and 450°C. During
this period, the intermediate phases are changed to an ordered
solid solution.

The proper time and temperature for age hardening an
alloy are specified by the manufacturer.
Ideally, before age hardening an alloy, it should first be
subjected to a softening heat treatment to relieve all strain
hardening and to start the age hardening treatment when the
alloy is in a disordered solid solution. This allows better control
of the hardening process.

METAL CERAMIC ALLOYS
The main function of metal-ceramic alloys is to reinforce
porcelain, thus increasing its resistance to fracture.
Requirements:
They should be able to bond with porcelain.
Its coefficient of thermal expansion should be compatible with
that of porcelain.
Its melting temperature should be higher than the porcelain
firing temperature. It should be able to resist creep or sag at
these temperatures.
It should not stain or discolor porcelain.

The alloys used for metal-ceramic purposes are grouped
under two categories:
i) Noble metal alloys
ii) Base metal alloys.
In case of noble metal alloys for porcelain bonding ,
addition of 1% base metals (iron, indium, tin etc.) increases
porcelain-metal bond strength, which is due to formation of
an oxide film on its surface. It also increases strength and
proportional limit.

Modulus of elasticity:
The base metal alloys have a modulus of elasticity
approximately twice that of gold alloys. Thus it is suited for long
span bridges. Similarly, thinner castings are possible.
Hardness:
The hardness of base metal alloys ranges from 175 to 360
VHN. Thus, they are generally harder than noble metal alloys.
Thus, cutting, grinding and polishing requires high speed and
other equipment.
Ductility:
It ranges from 10 to 28% for base metal alloys. Noble metal
alloys have an elongation of 25 to 40%.
PROPERTIES

Density:
The density of base metal alloys are less, which is
approximately 8.0 gms/cm3 as compared to 18.39 gms/cm3
for noble metal alloys.
Sag Resistance:
Base metal alloys resist creep better than gold alloy
when heated to high temperatures during firing.
Bond Strength: Varies according to composition.
Technique Sensitivity: Base metals are more technique
sensitive than high noble metal-ceramic alloys.

Shillingburg HT, Hobo S and Fisher DW (1977)
Studied Preparation design and margin distortion in
porcelain-fused-to-metal restorations.
The results of this study suggested that thermal
incompatibility stresses were likely to cause margin distortion
in metal ceramic crowns.
However, subsequent studies support other potential
mechanisms, including the effect of excessive sand blasting
time and/or pressure.

The Gold-Platinum-Palladium (Au-Pt-Pd) System:
This is one of the oldest metal ceramic alloy system. But
these alloys are not used widely today because they are very
expensive.
Composition:
Gold – 75% to 88%
Palladium – Upto 11%
Platinum – Upto 8%
Silver – 5%
Trace elements like Indium, Iron and Tin for porcelain bonding.

Advantages Disadvantages
1. Excellent castability 1. High cost
2. Excellent porcelain bonding 2. Poor sag resistance
so not suited for
3. Easy to adjust and finish long span fixed partial dentures
4. High nobility level 3. Low hardness (Greater
wear)
5. Excellent corrosion and tarnish 4. High density (fewer casting
resistance. ounce)
6. Biocompatible
7. Some are yellow in color
8. Not “Technique Sensitive”
9. Burnishable

Gold-Palladium-Silver (Au-Pd-Ag) System:
These alloys were developed in an attempt to overcome the
major limitations in the gold-platinum-palladium system (mainly
poor sag resistance, low hardness & high cost)
Two variations on the basic combination of gold, palladium
and silver were created and are identified as either high-silver or
low-silver group.
Composition (High Silver Group):
Gold – 39% to 53%
Silver – 12% to 22%
Palladium – 25% to 35%
trace amount of oxidizable elements are added for porcelain
bonding.

1. Less expensive than Au-Pt-Pd alloys 1. High silver content
creates potential
2. Improved rigidity and sag resistance. for porcelain
discoloration.
3. High malleability. 2. High Cost.
3. High coefficient of
thermal expansion.
4. Less Tarnish and
corrosion resistant.

Composition (Low Silver Group):
Gold – 52% to 77%
Silver- 5% to 12%
Trace amounts of oxidizable elements for porcelain bonding.
1. Less expensive than the Au-Pt-Pd alloys 1. Silver creates
potential for porcelain discoloration
(but less than high silver group)
2. Improved sag resistance 2. High cost.
3. High noble metal content 3. High coefficient
of thermal expansion.
4. Tarnish and corrosive resistant

Gold-Palladium (Au-Pd) System:
This particular system was developed in an attempt to
overcome the major limitations in the Au-Pt-Pd system and Au-
Pd-Ag system. Mainly-
-Porcelain discoloration.
-Too high coefficient of thermal expansion & contraction.
Composition:
Gold – 44% to 55%
Gallium – 5%
Indium & Tin – 8% to 12%
Indium, Gallium and Tin are the oxidizable elements
responsible for porcelain bonding.

1. Excellent castability 1. Not thermally
compatible with high
expansion dental porcelain.
2. Good bond strength 2. High cost
3. Corrosion and tarnish resistance
4. Improved hardness
5. Improved strength ( sag resistance)
6. Lower density

Palladium-Silver (Pd-Ag) System
This was the first gold free system to be introduced in the
United States (1974) that still contained a noble metal (palladium).
It was offered as an economical alternative to the more expensive
gold-platinum-silver and gold-palladium-silver (gold based) alloy
systems.
Composition: (available in two compo.)
1. Palladium – 55% to 60% Silver – 25% to 30%
Indium and Tin
2. Palladium – 50% to 55% Silver – 35% to 40%
Tin (Little or no Indium)
Trace elements of other oxidizable base elements are also
present.

1. Low Cost 1. Discoloration (yellow, brown or green) may
occur with some dental porcelains.
2. Low density 2. Some castibility problems reported (with
induction casting)
3. Good castibility (when torch 3. Pd and Ag prone to absorb gases.
casting) 4. Require regular purging of the porcelain
4. Good porcelain bonding, furnace.
5. Burnishability 5. May form internal oxides (yet porcelain
6. Low hardness bonding does not appear to be a problem)
7. Excellent sag resistance 6. Should not be cast in a carbon crucible.
8. Moderate nobility level 7. Non-carbon phosphate bonded investments
9. Good tarnish and corrosion recommended.
resistance. 8. High coefficient of thermal expansion.
10. Suitable for long-span fixed
partial dentures.

Berzins DW, Sarkar NK et al (2000)
Did an in-vitro electrochemical evaluation of high palladium alloys
in relation to palladium allergy.
The high incidence of allergic reaction was associated with Pd-
Cu based alloys. The “Pd-skin” of these alloys when in contact
with saliva release some Pd++ ions (an allergen) which can
trigger the cascade of biological reaction involved in allergy and
hypersensitivity. It is a time dependent process.
In Pd alloys containing Ag, formation of Ag-Cl film on the alloy
surface is supposed to prevent Pd in coming in contact with
oral fluids, having a masking effect and thus avoiding allergy.

DISCOLORATION OF PORCELAIN BY SILVER:
The colloidal dispersion of silver atoms entering the body and incisal
porcelain or the glazed surface from vapour transport or surface
diffusion may cause color changes including green, yellow-green,
yellow-orange, orange and brown hues. This phenomenon is termed
GREENING.
Porcelains with higher sodium content are believed to exhibit more
intense discoloration because of more rapid silver diffusion in
sodium containing glass.
The intensity of discoloration increases for higher silver content alloys,
is more in the cervical region, lighter shades, multiple firing
procedures and certain brands of porcelain and also in silver free
alloys due to vaporization of silver from the walls of contaminated
furnaces.

PREVENTION OF DISCOLORATION:
Use of ultra low fusing porcelain or non greening
porcelain.
A pure gold film can be fired on a metal substrate to
reduce the surface silver concentration.
A ceramic conditioner can be fired as a barrier between
the alloy and the porcelain.
Use of a graphite block routinely to maintain a reducing
atmosphere.

HIGH PALLADIUM SYSTEM
Several types of high palladium systems were originally
introduced (Tuccillo, 1987).
More popular composition groups contained cobalt and
copper.
Composition (PALLADIUM-COBALT ALLOY):
Palladium – 78% to 88% Cobalt – 4% to 10%
(Some high palladium-cobalt alloys may contain 2% gold)
Trace amounts of oxidizable elements (such as gallium and
indium) are added for porcelain bonding.

1. Low cost 1. More compatible with
higher expansion
2. Reportedly good sag resistance 2.porcelains.
3. Low density means more casting Are more prone to over-
heating than per ounce than
gold based alloys or high
Pd-Cu.
4.They Melt and cast easily 3. Produces a thick, dark
oxide
5. Good polishability (Supposed 4. Colored oxide layer may to
be similar to Au-Pd alloys) cause bluing of the porcelain.
6. Reportedly easier to presolder 5. Prone to gas absorption
than Pd-Cu alloys. 6. Little information on long-
term clinical success.

COMPOSITION (PALLADIUM-COPPER ALLOYS)
Palladium – 70% to 80% Copper – 9% to 15%
Gold – 1% to 2% Platinum – 1%
Some, but not all, high palladium-copper alloys
contain small quantities ( 1% to 2%) of gold and/or
platinum. Trace amounts of the oxidizable elements
gallium, indium and tin are added for porcelain bonding.

1. Good castability 1. Produces dark, thick oxides
2. Lower cost (than gold based alloys) 2. May discolor (gray) some dental
3. Low density means more castings porcelains.
Per ounce 3. Must visually evaluate oxide color to
4. Tarnish and corrosion resistance determine if proper adherent oxide was
5. Compatible with many dental formed.
Porcelains. 4. Should not be cast in carbon crucibles
6. Some are available in one unit ingots. (electric casting machines)
5. Prone to gaseous absorption.
6. Subject to thermal creep.
7. May not be suitable for long span fixed
partial denture prosthesis.
8. Little information on long term clinical
success.
9. Difficult to polish
10. Resoldering is a problem

Tufekci E, Mitchell JC et al (2002)
Did a study on spectroscopy measurements of elemental
release from high palladium dental casting alloys into a
corrosion testing medium.
A highly sensitive analytical technique shows that the
release of individual elements over a one month period,
suggesting that there may be low risk of biological reaction
with the Pd-Ga alloys than with the Pd-Cu-Ga alloys tested.

Carr A.B., Cai Z., Brantley W.A.(1993) did a study on new
high palladium casting alloys (generation 1&2).
For the 5 high-palladium alloys studied,
the following conclusions were drawn:
1. An increase in the investment burn out temperature from
1400°F to 1500 °F had little effect on microstructure and
hardness, but grain or dendrites size was found to vary
substantially.
2. Hot tears were more prevalent in the alloys when the higher
burnout temperature was used.
3. Heat treatment simulating porcelain firing cycles for these
alloys generally caused decrease in hardness.

Reisbick NH and Brantley WA (1995)
conducted a study on mechanical properties and micro
structural variations for recasting low gold alloys.
They concluded that significant decrease in yield strength and
percentage elongation were observed for recasting these
alloys but not in tensile strength when the Type III gold alloys
were recasted upto 3 times.
Scanning electron microscope examination revealed that the
number of casting defects (principally porosity) increased with
the number of times the alloy was remelted.

Nickel-chromium (Ni-Cr) System
These alloys offer such economy that they are also used
for complete crown and all metal fixed partial denture prosthesis
(Bertolotti, 1983).
The system contains two major groups:
-Beryllium free (class 1)
-Beryllium (class 2)
Of the two, Ni-Cr-Beryllium alloy are generally regarded
as possessing superior properties and have been more popular
(Tuccillo and Cascone,1984).

NICKEL-CHROMIUM BERYLLIUM FREE ALLOYS
Composition:
Nickel – 62% to 77% Chromium – 11% to 22%
Boron , iron, molybdenum, Niobium or columbium and tantalum (trace
elements).
1. Do not contain beryllium 1. Cannot use with Nickel sensitive
patients.
2. Low cost 2. Cannot be etched. (Cr doesn’t
dissolve in acid)
3. Low density 3. May not cast as well as Ni-Cr-Be
alloys
4. Produces more oxide than Ni-Cr-
Be alloys

NICKEL-CHROMIUM-BERYLLIUM ALLOY
Composition:
Nickel – 62% to 82% Chromium – 11% to 20%
Beryllium – 2.0%
Numerous minor alloying elements include aluminum,
carbon, gallium, iron, manganese, molybdenum, silicon,
titanium and /or vanadium are present.

1. Low cost 1. Cannot use with nickel sensitive
patients
2. Low density, permits more 2. Beryllium exposure may be
casting per ounce. potentially harmful to technicians and
patients.
3. High sag resistance 3. Proper melting and casting is a
learned skill.
4. Can produce thin casting 4. bond failure more common in the
oxide layer.
5. Poor thermal conductor 5. High hardness (May wear opposing
teeth)
6. Can be etched to increase 6. Difficult to solder
retention 7. Ingots do not pool
8. Difficult to cut through cemented
castings

DISADVANTAGES OF NICKEL-CHROMIUM ALLOYS:
Nickel may produce allergic reactions in some
individuals (contact dermatitis). It is also a potential
carcinogen.
Beryllium which is present in many base metal alloys is
a potentially toxic substance. (Moffa JP)
Inhalation of beryllium containing dust or fumes is the main
route of exposure. It causes a condition know as ‘berylliosis’.
It is characterized by flu-like symptoms and granulomas of
the lungs.

Comparative properties of Ni / Cr alloys and type III casting gold
alloys for small cast restorations
Property (Units) Ni/Cr Type III gold
alloy
Comments
Density (g/cm3) 8 15 More difficult to produce defect free casting for
Ni/Cr alloys.
Fusion temperature As high as
1350°C
Normally lower
than 1000°C
Ni/Cr alloys require electrical induction furnace or
oxyacetylene equipment.
Casting shrinkage (%) 2 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)
230 290 Both high enough to prevent distortion for
applications being considered; not that values are
lower than for partial denture alloys
Modulus of elasticity
(GPa)
220 85 Higher modulus of Ni/Cr is an advantage for large
restoration e.g. bridges and for porcelain bonded
restoration.
Hardness (VHN) 300 150 Ni/Cr more difficult to polish but retains polish
during service
Ductility
(% elongation)
upto 30% 20 (as cast)
10 (hardened)
Relatively large values suggest that burnishing is
possible; however, large proportional limit value
suggests higher forces would be require.

COBALT CHROMIUM ALLOYS
Cobalt chromium alloys have been available since
the 1920’s. They possess high strength. Their excellent
corrosion resistance especially at high temperatures makes
them useful for a number of applications.
These alloys are also known as ‘satellite’ because
they maintained their shiny, star-like appearance under
different conditions.
They have bright lustrous, hard, strong and non-
tarnishing qualities.

APPLICATIONS:
1. Denture base
2. Cast removable partial denture framework.
3. Surgical implants.
COMPOSITION:
Cobalt - 55 to 65%
Chromium - 23 to 30%
Nickel - 0 to 20%
Molybdenum - 0 to 7%
Iron - 0 to 5%
Carbon - upto 0.4%
Tungsten, Manganese, Silicon and Platinum in traces.
According to A.D.A specification No. 14 a minimum of 85% by
weight of chromium, cobalt, and nickel is required. Thus the gold
base corrosion resistant alloys are excluded.

1. Physical Properties:
Density: The density is half that of gold alloys, so they are lighter
in weight.
8 to 9 gms/cm3.
Fusion temperature: The casting temperature of this alloy is
considerably higher than that of gold alloys. 1250oC to 1480oC.
PROPERTIES
The Cobalt-Chromium alloys have replaced Type IV
gold alloys because of their lower cost and adequate
mechanical properties. Chromium is added for tarnish
resistance since chromium oxide forms an adherent and
resistant surface layer.

A.D.A. specification No. 14 divides it into two types,
based on fusion temperature (which is defined as the
liquidus temperature)
Type-I (High fusing) – liquidus temperature greater than
1300oC
Type-II (Low fusing) – liquidus temperature lower than
1300oC

2. Mechanical Properties:
Yield strength: It is higher than that of gold alloys. 710Mpa
(103,000psi).
Elongation: Their ductility is lower than that of gold alloys.
Depending on the composition, rate of cooling, and the fusion
and mold temperature employed, it ranges from 1 to 12%.
Modulus of elasticity: They are twice as stiff as gold alloys
22.5103Mpa. Thus, casting can be made more thinner.

Hardness: These alloys are 50% harder than gold alloys 432
VHN.
Thus, cutting, grinding and finishing is difficult.
3. Tarnish and corrosion resistance: Formation of a layer
of chromium oxide on the surface of these alloys.
Solutions of hypochlorite and other compounds that
are present in some denture-cleaning agents will cause
corrosion in such base metal alloys.
Even the oxygenating denture cleansers will stain such
alloys. Therefore, these solutions should not be used for
cleaning cobalt-chromium base alloys.

4. Casting Shrinkage: The casting shrinkage is much greater
than that of gold alloys (2.3%), so limited use in crown & bridge.
The high shrinkage is due to their high fusion temperature.
5. Porosity: As in gold alloys, porosity is due to shrinkage and
release of dissolved gases which is not true in case of Co-Cr
alloys.
Porosity is affected by the composition of the alloys and its
manipulations.

Comparative properties of Co / Cr alloys and type IV casting gold alloys
for partial denture
Property (Units) Co/Cr Type IV gold
alloy
Comments
Density (g/cm3) 8-9 15 More difficult to produce defect free
casting for Co/Cr alloys but denture
frameworks are lighter
Fusion
temperature
as high
as
1500°C
Normally
lower than
1000°C
Co/Cr alloys require electrical induction
furnace or oxyacetylene equipment.
Can not use gypsum bonded
investments for Co/Cr alloys
Casting shrinkage
(%)
2.3 1.4 Mostly compensated for by correct
choice of investment
Tensile strength
(MPa)
850 750 Both acceptable
Proportional limit
(MPa)
710 500 Both acceptable; can resist stresses
without deformation
Modulus of
elasticity (GPa)
225 100 Co/Cr more rigid for equivalent
thickness; advantage for connectors;
disadvantage for clasps
Hardness (Vickers) 432 250 Co/Cr more difficult to polish but retains
polish during service
Ductility (%
elongation)
2 15 (as cast)
8 (hardened)
Co/Cr clasps may fractured if
adjustments are attempted.

Property Ni-Cr without
Be
Ni-Cr with Be Co-Cr
Strength
(MPa)
255-550 480-830 415-550
Ultimate
tensile
strength
(MPa)
550-900 760-1380 550-900
% elongation 5-35 3-25 1-12
Modulus of
elasticity
(MPa)
13.8-20.7 x 104 17.2-20.7 x 104 17.2-22.5x104
Vickers
hardness
175-350 300-350 300-500
Casting
temperature
(°C)
1430-1570 1370-1480 1430-1590

Titanium is called “material of
choice” in dentistry.
oxide formation property which
forms basis for corrosion
resistance & biocompatibility of
this material.
The term 'titanium' is used for all
types of pure and alloyed
titanium.
TITANIUM AND TITANIUM ALLOYS

Properties of titanium:
-Resistance to electrochemical degradation
-Good biological response
-Relatively light weight
-Low density (4.5 g/cm3)
-Low modulus (100 GPa)
-High strength (yield strength = 170-480
MPa; ultimate strength = 240-550 MPa)
-Passivity
-Low coefficient of thermal expansion (8.5 x
10–6/°C)
-Melting & boiling point of 1668°C & 3260°C

Uses:
Commercially pure titanium is used for dental implants,
surface coatings, crowns, partial dentures, complete
dentures and orthodontic wires
Commercially Pure Titanium (CP Ti):
It is available in four grades (according to American Society
for Testing and Materials ASTM) which vary according to the
oxygen (0.18-0.40 wt.%),
iron (0.20-0.50 wt%) and other impurities.
It has got an alpha phase structure at room temperature and
converts to beta phase structure at 883°C which is stronger
but brittle.

TITANIUM ALLOYS
Alloying elements are added to stabilize alpha or the
beta phase by changing beta transformation temperature
e.g. in Ti-6Al-4V.
Aluminum is an alpha stabilizer whereas Vanadium as
well as copper and palladium are beta stabilizer.
Alpha titanium is weld able but difficult to work with at
room temperature.
Beta titanium is malleable at room temperature and is
used in orthodontics, but is difficult to weld.

CAST TITANIUM:
Cast titanium has been used for more than 50 years,
most important factors high melting point (1668°C) and
chemical reactivity.
Because of which, special melting procedures, cooling
cycles, mold materials, and casting equipments are required
to prevent metal contamination, because it readily reacts with
hydrogen, oxygen and nitrogen at temperatures greater than
600°C.
So casting is done in a vacuum or inert gas atmosphere.
It has been shown that magnesium based investment cause
internal porosity in casting.

Because of its low density, it is difficult to cast in centrifugal
casting machine. So advanced casting machine combining
centrifugal, vacuum, pressure and gravity casting with electric
arc melting technology have been developed.
Difficulties in casting Titanium :
-High melting point
-High reactivity
-Low casting efficiency
-Inadequate expansion of investment
-Casting porosity
-Difficulty in finishing
-Difficulty in welding
-Requires expensive equipments

Ahmad SAH, Omar MB, Homa D. (2003)
Did an investigation of the cytotoxic effects of commercially
available dental casting alloys and concluded the following:
1.The high noble alloy Bioherador N was significantly less
cytotoxic than all the base metal alloys tested in this study (Ni-
Cr, Co-Cr, Cu-based)
2. The Ni-Cr alloy CB Soft was significantly more cytotoxicity than
all the Ni-Cr and Co-Cr alloys tested. This could be related to
the content of Cu, low content of Cr and absence of Mo in its
composition.
3. Cu based alloys Thermobond showed a more severe cytotoxic
reaction than all the other alloys.

BIOLOGICAL HAZARDS AND PRECAUTIONS
Although the amount of beryllium rarely exceeds 2% by weight,
the atomic concentration of beryllium is around 10.7%.
The risk for beryllium vapour exposure is grteatest to dental
technicians during alloy melting , especially in the absencedof
an adequate exhaust and filtration system.
The Occupational Health and Saftey Administration (OSHA)
specifies that the exposure to beryllium dust in air should be
limited to a particulate beryllium concentration of
2micrograms/m3 of air ( both respirable and nonrespirable
particles) determined from an 8 hr time weighted average.

The allowable maximum concentration is 5microgram/m3(not
to be exceeded for a 15 min period).
The National Institute for Occupation Safety and Health
(NIOSH) recommends a limit of 0.5 micrograms /m3 based
on a 130 min sample.
Moffa et al reported that when a local exhaust system was
used the conc. of beryllium was reduced to safe levels.

ALLERGY POTENTIAL OF NICKEL:
Nickel allergy is determined by patch test using 5% Nickel
sulfate.
The effects of nickel exposure to humans have included
dermatitis, cancer of nasal sinus and larynx, irritation and
perforation of nasal septum, loss of smell, asthmatic lung
disease, pulmonary pneumoconiosis, lung dysfunction and
death.
OSHA standard: 8 hr time weighted average concentration
limit of 1000 microgram/m3 of nickel and nickel compounds.

ALTERNATIVES TO CAST METAL TECHNOLOGY
To avoid the challenges and cost of associated with metal casting
process, four technologies are available;
SINTERING OF BURNISHED FOIL:
 The Captek system consists of three pairs of materials:
 CaptekP layer which is adapted first to the die and fired at a
temperature of 1075 degree Celsius
 Captek G which is applied over the Captek P coping and the former is
drawn by capillary action into the network structure of the Captek
P coping vacated by the adhesive binder.
 Captek Repair paste and Capfil which are used to add material to
Captek structures.
ADVANTAGE of Captek structures is the very low thickness of metal
that can be achieved which ensures minimal tooth preparation and
hence improved esthetics.

CAD-CAM PROCESSING
A CAD-CAM System electronically or digitally records surface
co-ordinates of the prepared tooth and stores these retrieved
data in the memory of a computer.
The image data can then be retrieved immediately to mill or
grind a metal, ceramic or composite prosthesis by computer
control from a solid block of the chosen material. Within
minutes the prosthesis can be fabricated and placed in a
prepared tooth
The optical scanning procedure eliminates the need for an
impression. An advantage of ceramics is that homogeneous,
high quality materials with minimal porosity and other typical
defects are designed for CAD-CAM applications.

COPY MILLING
This process is based on the principle of tracing the surface of
a pattern that is then replicated from a blank of ceramic,
composite, or metal that is ground, cut or milled by a rotating
wheel whose motions controlled by a link through the tracing
device. (similar to key duplication)
Eg : The Celay : Mikrona Technologies, Spreintenbach,
Switzerland)

ELECTROFORMING
A master cast of the prepared tooth is prepared and coated with
a special die spacer to facilitate separation of the duplicating
material.
After applying conductive silver layer to the duplicated surface
(Gypsum product) , the die is connected to a plating head and
connected to a power source and then placed in a plating
solution.
After a sufficiently thick layer of gold or other material is
deposited, the gypsum is removed and the coping is
sandblasted.
Subsequent ceramic layers are condensed and sintered in a
conventional way.

CONCLUSION
Though they are many dental casting alloys we use
in routine procedures, one should all ways keep in
mind the application of basic properties in these
alloys for the best success.

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