Base metal alloys

DENTAL CASTING ALLOYS
INTRODUCTION
For casting dental restorations and for the fabrication of
wire and other structures, it is necessary to combine various
metals to produce alloys with adequate properties, for dental
applications. These alloys are produced largely from gold
combined with other noble metals and certain base metals. The
alloys are compounded to produce properties most acceptable for
their intended dental applications, such as simple inlays, bridges,
removable cast restorations, solders, (or) wrought wire forms.
The noble metals are those elements with a good metallic
surface that retain their surfaces in dry air.
The six metals of the platinum group they are, platinum,
iridium, radium, osmium and ruthemium; along with gold they are
called noble metals.
HISTORICAL PERSPECTIVE ON DENTAL CASTING
ALLOYS
The history of dental casting alloys has been influenced by
3 major factors:
1

1. The technologic changes of dental prosthesis.
2. Metallurgic advancements; and
3. Price changes of the noble metals since 1968.
Taggart’s presentation to the New York odontological
group in 1907 on the fabrication of cast inlay restorations often
has been acknowledged as the first reported application of the lost
wax technique in dentistry. The inlay technique described by
Taggart was an instant success. It soon led to the casting of
complex inlays such as onlays, crowns, FPD, and RPD frame
works. Because pure gold did not have the physical properties
required of these dental restorations, existing jewellery alloys
were quickly adopted. These gold alloys were further strengthened
with copper, silver (or) palladium.
In 1932, the dental materials group at the National Bureau
of Standards surveyed the alloys being used and roughly classified
them as Type 1, 2, 3 and 4.
In the following years, several patents were issued for
alloys containing palladium as a substitute for platinum.
2

By 1948, the composition of Dental Noble metal alloys for
cast metal restorations had become rather diverse, with these
formulations, the tarnishing tendency of the original alloys
apparently had disappeared. It is now known that in gold alloys,
palladium is added to counter act the tarnish potential of silver.
The base metal removable partial denture alloys were
introduced in the 1930s. Since that time, both Nickel-chromium
and cobalt-chromium formulations have become increasingly
popular compared with conventional Type IV gold alloys, which
previously were the predominant metals used for such prosthesis.
The obvious advantages of base metal alloys are their lighter
weight, increased mechanical properties and reduced costs.
Likewise, by 1978, the price of gold was climbing so
rapidly the attention focused on the noble metal alloys. To reduce
the precious metal content yet retain the advantages of the noble
metals for dental use.
PROPERTIES OF NOBLE METAL ALLOYS
Since these metals have a wide range of properties and are
widely used in dentistry, it is worth while to describe some of
their properties.
3

a. GOLD
1. Pure gold is a soft, malleable, ductile metal that does
not oxidize under atmospheric conditions and is
attacked by only a few of the most powerful
oxidizing agents.
2. It has a rich yellow colour with a strong metallic
luster.
3. Although it is the most ductile and malleable of all
metals, it ranks much lower in strength.
The pure metal fuses at 106°C, which is only 20° below the
melting point of copper (1083°C).
4. 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 will cause gold to be
extremely brittle.
Mercury in small quantities also has a harmful effect on its
properties.
5. Gold is nearly as soft as lead, with the result that in
dental alloys, coins, and articles of jewellery it must
4

be alloyed with copper, silver, platinum and other
metals to develop the necessary hardness, durability
and elasticity.
6. The specific gravity of pure gold is between 19.30
and 19.33, making it one of the heavy metal.
7. Air (or) water at any temperature does not affect (or)
tarnish gold.
8. Gold is not soluble in sulfuric, nitric (or)
hydrochloric acids.
PALLADIUM
1. Palladium is not used in the pure state in dentistry, but it is
used in many dental alloys, combined with either gold (or)
silver.
It is cheaper than platinum and since it imparts many of the
properties of platinum to dental alloys, it is often used as a
replacement for platinum.
2. Palladium is a white metal some what darker than platinum.
5

3. Its specific gravity is 11.4 (or) about half that of platinum
and a little more than half that of gold.
4. It is a malleable and ductile metal with a melting point of
1555°C, which is the lowest of the platinum group of
metals.
5. The metals has the quality of absorbing (or) occluding large
quantities of hydrogen gar when heated. This can be an
undesirable quality when alloys containing palladium are
heated with an improperly adjusted gas air torch.
IRIDIUM, RUTHENIUM, AND RHODIUM
1. Small amounts of iridium are some times present in dental
alloys, either as impurities combined with platinum (or) as
additions to modify the properties.
2. As little as 0.005% (50 ppm) is effective in refining the
grain size of cast gold alloys.
Ruthenium produces a similar effect.
3. Iridium is a hard metal that is quite brittle, white with a
high sp. Gravity of 22.42 and an exceptionally high
melting point estimated to be about 2440'C.
6

SILVER
1. Silver is malleable and ductile, white, the best-known
conductor of heat and electricity, and stronger and harder
than gold but softer than copper.
2. It melts at 960.5'C, which is below the melting point of both
gold and copper.
3. It is unaltered in clean, dry air at any temperature but
combines with sulfur, chlorine and phosphorous, (or)
vapors containing these elements (or) their compounds.
Foods containing sulfur compounds cause severe tarnish on
silver.
4. Pure silver is seldom employed in dental restorations
because of the black sulfide formation on the metal in the
mouth, although it is used extensively for small additions to
many gold alloys.
Addition of small amounts of palladium to silver containing
alloys prevents the rapid corrosion of such alloys in the oral
environment.
7

KARAT AND FINENESS OF GOLD
For many years the gold content of gold alloys has been
described on the basis of karat, (or) in terms of fineness, rather
than by weight %.
The term karat refers to the parts of pure gold in 24 parts of
an alloy.
For example, 24-karat gold is pure gold, where as 22-karat
gold is an alloy containing 22 parts pure gold and 2 parts of other
metals.
Fineness describes gold alloys by the number of parts per
1000 of gold. For example, pure gold has a fineness of 1000, and
650 fine alloy has a gold content of 65%. Thus the fineness rating
is to timer the gold % in an alloy.
Fineness is considered as more practical term than karat.
The terms karat and fineness are rarely used to describe the
gold content of current alloys.
8

CLASSIFICATION OF DENTAL CASTING ALLOYS (OR)
DENTAL GOLD ALLOYS
According to ADA specification No.5 these casting alloys
are described simply as:
Type I
Type II
Type III and
Type IV
Type I (Soft): These alloys are limited to use in inlays that are
subject only to slight stress during mastication.
This would include inlays for the gingival and
interproximal areas of a 91 tooth and for certain occlusal inlays of
such design (or) location that they are not subjected to severe
stress applications.
Alloys of this type often are useful for inlays prepared by
the direct technique, which requires the finishing operation to be
completed on the tooth with relatively simple hand instruments.
Type II (Medium): These medium alloys can be used for all types
of cast inlays and onlays.
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Type III (Hard): These alloys are most acceptable for crowns,
thin 3/4th
crowns, and anterior and posterior bridge abutments,
which should not be cast from the softer and weaker Type I and
Type II alloys.
Type IV (Extra hard): These alloys are designed to have
sufficient strength and adequate properties for cast removable
partial dentures with clasps, precision cast fixed bridges and ¾
crowns, which are not subjected to hard working (or) burnishing
operations.
Composition:
The composition of the gold casting alloys that meet the
requirements of ADA Sp. No. 5 are given in the Table below:
TYPE GOLD PLATINUM PALLADIUM
I 81-83% - 0.2-4.5%
II 76-78% - 1-3%
III 73-77% - 2-4%
IV 71-74% 0-1% 2-5%
It is apparent from the Table that there is some reduction in
gold content when a comparison is made between Type I and IV
alloys.
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An increase in copper content is observed as the gold
content is decreased.
An increase in the zinc content also occurs in Type IV
alloys.
Platinum is rarely added to Type 1 gold alloys, but a small
amount of palladium is always added to all 4 types.
PROPERTIES OF GOLD CASTING ALLOYS
Type
Vickers hardness
number (Kg/mm2
)
Softened
Yield strength,
(0.1% Offset) MPa
Elongation
Minimum Maximum
Softened
minimum
Hardened
minimum
Softened
minimum
Hardened
minimum
(%)
I 50 90 None None 18 None
II 90 120 140 None 12 None
III 120 150 200 None 12 None
IV 150 None 340 500 10 2
From the above table it may be seen that as the hardness
increases from Type I to Type IV, the yield strength and tensile
strength values are also increased, and the elongation generally
decreased.
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Since the yield strength represents in general the resistance
to permanent deformation under stress, it can be seen that alloys
with increased hardness values offer an increased resistance to
permanent bending (or) deformation.
Soft alloys have a higher degree of elongation and a
relatively greater quality of ductility than the alloys of higher
hardness values.
FUSION TEMPERATURES OF DENTAL GOLD CASTING
ALLOYS
METAL (or) ALLOY
TYPICAL FUSION
TEMPERATURE (°C)
Type I 1005-1070
Type II 900-970
Type III 875-1000
Type IV 875-1000
From the above table it is observed that the fusion
temperature of the 4 types of alloys decreases from Type I to Type
IV.
The fusion temperatures are important factors in choosing
the type of investment to be used.
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Alloys having fusion temperatures higher than about
1100°C should not be cast into calcium sulfate bonded
investment.
Numerous classification systems have been proposed to
categorize the wide variety of commercial gold-based and
palladium based alloys.
In 1984 the ADA proposed a simple classification of dental
casting alloys.
ALLOY TYPE
TOTAL NOBLE METAL
CONTENT
High noble metal
Contains ≥ 40 wt% AV and ≥
60% wt of the noble metal
elements
(Au+Ir+OS+Pt+Rh+Ru).
Noble metal
Contains ≥ 25% of the noble
metal elements.
Predominantly base metal
Contains < 25 wt% of noble
metal elements.
Many manufacturers have adopted this classification to
simplify the communication between dentists and dental
laboratory technologists.
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Some insurance companies use it as well to determine the
cost of crown and bridge treatment.
CLASSIFICATION OF ALLOYS FOR ALL METAL
RESTORATIONS METAL, CERAMIC RESTORATIONS
AND FRAME WORKS FOR REMOVABLE PARTIAL
DENTURES
Alloy Type All-Metal
Metal-
Ceramic
Removable
Partial
Dentures
High noble
Au-Ag-Cu-Pd
metal ceramic
alloys
Au-Pt-Pd
Au-Pd-Ag (5-
12 wt% Ag)
Au-Pd-Ag
(>wt 12% Ag)
Au-Pd (no Ag)
Au-Ag-Cu-Pd
Noble
Ag-Pd-Au-Cu
Ag-Pd
Metal ceramic
alloys
Pd-Au (no Ag)
Pd-Au-Ag
Pd-Ag
Pd-Cu
Pd-Co
Pd-Ga-Ag
Ag-Pd-Au-Cu
Ag-Pd
In this we have all the 4 types of alloys, described earlier
with both high noble and noble metal alloys.
Heat treatment of high noble and noble metal alloys:
Gold alloys can be significantly hardened if the alloy
contains a sufficient amount of copper. Types I and E alloys
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usually do not harden, (or) harden to a lesser degree than do the
types HI and IV alloys. The actual mechanism of hardening is
probably the result of several different solid - solid
transformations.
Alloys that can be hardened can of course, also be softened.
In metallurgical terminology the softening heat treatment is
referred to as solution heat treatment. The hardening heat
treatment is termed age hardening.
Softening heat treatment:
The casting is placed in an electric furnace for 10 minutes
at a temperature of 700°C (129°F) and then it is quenched in
water.
The tensile strength, hardness and proportional limit are
reduced by such a treatment but the ductility is increased.
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.
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Hardening heat treatment:
The age hardening of the dental alloys can be accomplished
'in several ways. One of the most practical hardening treatments
is by "Soaking" (or) aging the casting at a specific temperature for
a definite time, usually 15 to 30 minutes, before it is water
quenched. The aging temperature depends on the alloy
composition but is generally between 200°C and 450°C.
Ideally, before the alloy is given an age-hardening
treatment, it should be subjected to a softening heat treatment to
relieve all strain hardening, if it is present and to start the
hardening treatment with the alloy as a disordered solid solution.
The hardening heat treatment is indicated for metallic
partial dentures, bridges, and other similar structures.
Casting Shrinkage:
Most metals and alloys, including gold and noble metal
alloys, shrink when they change from the liquid to the solid state.
The values for the casting shrinkage differ for the various
alloys presumably because of differences in their composition. It
has been shown, for example, that platinum, palladium and copper
16

all are effective in reducing the casting shrinkage of the alloy. It
is of interest that the value for the casting shrinkage of pure gold
closely approaches that of its maximal linear thermal contraction.
Alloy Casting shrinkage (%)
Type I, Gold base 1.56%
Type II, Gold base 1.37%
Type III, Gold base 1.42%
Ni-Cr-Mo 2.3%
Silver Palladium alloys:
These alloys are white and predominantly silver 'm
composition but have substantial amounts of palladium (at least
25/o) that provide nobility and promote the silver tarnish
resistance. They may (or) may not contain copper and a small
amount of gold.
The copper (Cu) free Ag-Pd alloys may contain 70% to 72%
silver and 25% palladium and may have physical properties of a
Type III gold alloy.
Other silver-based might contain roughly 60% silver 25%
palladium and as much as 15% or more of copper 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.
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Because of the increasing interest in aesthetics by dental
patients, a decreased use of all metal restorations has occurred
during the past decade. The use of metal ceramic restorations in
posterior sites has increased relative to the use of all metal crowns
and onlays.
HIGH NOBLE ALLOYS FOR METAL CERAMIC
RESTORATIONS
The original metal ceramic alloys contained 88% gold and
were much to soft for stress bearing restorations such as FPD.
Because there was no evidence of a chemical bond between these
alloys and dental porcelain, mechanical retention and undercuts
were used to prevent detachment of the ceramic veneer.
Using the stress bond test, it was found that the bond
strength of the porcelain to this type of alloy was less than the
cohesive strength of the porcelain itself. This mean that if the
failure occurred in the metal-ceramic restoration, it would most
probably arise at the porcelain metal interface.
By adding less than 1% of oxide forming elements such as
iron, idium, and tin to this high gold content alloy, the porcelain
18

metal bond strength was improved by a factor of 3. Iron also
increase the proportional limit and strength of the alloy.
This 1% addition of base metals to the gold, palladium and
platinum alloy was all that was necessary to produce a slight
oxide filum on the surface of the substructure to achieve a
porcelain-metal bond strength level that surpassed the cohesive
strength of porcelain itself.
The high noble alloys for metal ceramic restorations are:
A. Gold-Platinum - Palladium Alloys:
These alloys have a gold content ranging up to 88% with
varying amounts of palladium, platinum and small amounts of
base metals.
Some of these alloys are yellow in colour. Alloys of this
type are susceptible to sag deformation and FPD's should be
restricted to 3-unit spans, anterior cantilevers (or) crowns.
B. Gold-Palladium-Silver Alloys:
These gold based alloys contain between 39% and 77% upto
35% palladium and silver levels as high as 22%.
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The silver increases thermal contraction coefficient but it
also has a tendency to discolor some porcelains.
C. Gold-Palladium Alloys:
A gold content ranging from 44% to 55% and a palladium
level of 35% to 45% is present in these metal ceramic alloys,
which have remained popular despite their relatively high cost.
The lack of silver results in a decreased thermal contraction
coefficient and the freedom from silver discolouration of
porcelain.
Alloys of this type must be used with porcelains that have
low coefficients on to avoid the development of axial and
circumferential of thermal contraction to avoid the development
of axial and circumferential tensile stresses in Porcelain during
the cooling part of the porcelain firing cycle.
NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS
A. Palladium Based Alloys
1. Palladium - Silver alloys.
2. Palladium Copper alloys:
3. Palladium - Cobalt alloys:
4. Palladium - Gallium - Silver and Palladium - Gallium –
Silver – Gold alloys:
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a. Palladium based alloys
Noble palladium based alloys offer a compromise between
the high noble gold alloys and the predominantly base metal
alloys. The price per ounce of a palladium alloy is generally one
half to one third that of a gold alloys. The density of a palladium
based alloy is midway between that of base metal and of high
noble alloys.
2) Palladium - Silver Alloys:
Pd-Ag alloys were introduced widely in the late 1970s. This
alloy type was introduced to the U.S. market in 1974 as the first
gold free noble metal available for metal ceramic restorations.
Pd-Ag alloys enjoyed wide spread popularity for a few
years after they were introduced, but their popularity has declined
some what in recent years because of their tendency to discolor
porcelain during firing. One theory that has been proposed for
this greenish yellow discoloration, popularly termed "Selling" is
that the silver vapor escapes from the surface of these alloys
during firing of the porcelain, diffuses as ionic silver into the
porcelain and is reduced to form colloidal metallic silver in the
surface layer of porcelain.
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The compositions of Pd-Ag alloys fall within a narrow
range: 53% to 61% palladium and 28% to 40% silver.
Tin (or) Indium or both are usually added to increase alloy
hardness and to promote oxide formation for adequate bonding of
porcelain. In some of these alloys, the formation of an internal
oxide rather than an external oxide has been reported.
Other palladium alloys contain 75% to 90% palladium and
no silver and were developed to eliminate the greening problem
some of the high palladium alloys develop a layer of dark oxide
on their surface during cooling from the degassing cycle, and this
oxide layer has proven difficult to mask by the opaque porcelain.
Other high palladium alloys such as the Pd-Ga-Ag-Au type
seem not to be plagued by this problem.
The replacement of gold by palladium raises the melting
range but lower the contraction coefficient of an alloy. Increases
the silver content tends to lower the melting range and raises the
contraction coefficient.
Because of their high silver contents compared with the
gold based alloys, the silver discoloration effect is most severe for
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these alloys. Gold metal conditioners or ceramic coating agents
may minimize this effect.
The low specific gravity of these alloys (10.7-11.1)
combined with their low intrinsic cost makes these alloys
attractive as economical alternatives to the gold based alloys.
3) PALLADIUM-COPPER ALLOYS:
This alloy is comparable in cost to the Pd-Ag alloys.
Because of their low melting range of approximately
1170°C to 1190°C, these alloys are expected to be susceptible to
creep deformation (Sag) at elevated firing temperatures. Thus, one
should exercise caution in using these alloys for long-span FPDs
with relatively small connectors.
As is true for some Pd-Ag alloys, several of these products
contain 2% gold.
These alloys contain between 74-80% palladium and 9-15%
copper.
Porcelain discolouration due to copper is possible but does
not appear to be a major problem.
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One should be aware of the potential effect on aesthetics of
the dark brown (or) black oxide formed during oxidation and
subsequent porcelain firing cycles. Care should be taken, to mask
this oxide completely with opaque porcelain and to eliminate the
unaesthetic dark band that develops at metals porcelain junctions.
The Pd-Cu alloy have yield strengths upto 1145MPa.
Elongation values of 5% to 11% and hardness values as
high as some base metal alloys.
Thus, these alloys would appear to have a poor potential for
burnishing except when the marginal areas are relatively thin.
Although thermal incompatibility is not considered to be a
major concern, distortion of ultra thin metal copings (0.1mm) has
been occasionally reported.
4) PALLADIUM-COBALT ALLOYS:
This alloy group is comparable in cost to the Pd-Ag and Pd-
Cu alloys. They are often advertised as gold free, nickel-free,
beryllium-free, and silver-free alloys. The reference to nickel and
beryllium indicates that these alloys, as is true with the other
noble metals, are generally considered biocompatible.
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Like many noble metals, these alloys have a fine grain size
to minimize hot tearing during the solidification process.
This Pd-Co group is the most sag resistant of all noble
metal alloys.
The noble metal content (based on palladium) ranges from
78% to 88%.
The cobalt content ranges between 4 and 10wt% over
commercial alloy contains 8% gallium.
An example of typical properties of a Pd-Co alloy is as follows:
Hardness 250DPH
Yield strength 586MPa
Elongation 20% and
Modulus of elasticity 85.2Gpa
Although these alloys are silver-free, discolouration of
porcelain can still result because of the presence of cobalt. Any
way this is not considered a significant problem. Failure of the
technician to completely mask out the dark metal oxide color with
opaque porcelain is a more common cause of unacceptable
aesthetic results.
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5) PALLADIUM-GALLIUM-SILVER
AND PALLADIUM-GALLIUM-SILVER-GOLD ALLOYS
These alloys are the most recent of the noble metals. This
group of alloys was introduced because they tend to have a
slightly lighter coloured oxide than Pd-Cu (or) Pd-Co alloys and
they are thermally compatible with lower expansion porcelains.
The oxide that is required for bonding to porcelain is
relatively dark, but it is somewhat lighter than those of the Pd-Cu
and Pd-Co alloys. The silver content is relatively low (508 wt%)
and is usually inadequate to cause porcelain “greening”.
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TECHNICAL DATA CHART FROM DEGUSSA
HIGH GOLD CONTAINING CROWN AND BRIDGE ALLOYS
Alloy Type Colour Au
Vicker’s
hardness
Degulor A 1, soft
Deep
yellow
87.5 55
Degulor B 2, medium Yellow 75.7 95
Degulor C 3, hard Yellow 74.0 145
Degulor
Mo
4, extra
hard
Yellow 65.5 195
SILVER PALLADIUM CROWN AND BRIDGE ALLOYS
Alloy Type Colour Au Pd
Vickers
hardness
Palliag
MJ
4, extra
hard
White 55.0 20.9 150
HIGH GOLD CONTAINING ALLOYS FOR CERAMICS
Vicker’s
hardness
Degulent G Extra hard Yellow 86 150
Biobond III Extra hard
Bright
yellow
82.6 160
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PALLADIUM-BASE ALLOYS FOR CERAMICS
Vicker’s
hardness
Bond-on 4 Extra hard White 79.7 260
These Pd-Ga-Ag alloys generally tend to have a relatively
low thermal contraction coefficient and would be expected to be
more compatible with lower expansion porcelains such as vita
porcelains.
BASE METAL ALLOYS FOR DENTAL CASTINGS
The pressures of economics, as well as a search for
improved mechanical properties, have led to the development of
base metal alloys for the construction of dental prosthesis devices.
Composition:
The principal elements present in cast base metals for
partial dentures are chromium, cobalt and nickel, which together
make up approximately 90% of the most alloys used for dental
restorations. Representation compositions for typical dental
casting alloys are listed in the table.
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Composition of cast base metal alloys used in dentistry:
Ingredients Alloys (% of weight)
Vitallium Toconium
Jelenko
LG
Nobilium
Chromium
Cobalt
Nickel
Molybdenum
Aluminium
Iron
Carbon
Beryllium
Silicon
Manganese
Gallium
30.0
Balance
-
5.0
-
1.0
0.5
-
0.6
0.5
-
17.0
-
Balance
5.0
5.0
0.5
0.1
1.0
0.5
5.0
-
27.0
Balance
13.0
4.0
-
1.0
0.2
-
0.6
0.7
-
30.0
Balance
-
5.0
-
-
0.35
-
0.35
-
0.05
On close examination of the table, one can observe the
following points:
1. Chromium is the only major metal that exists in all alloys of
the type. Cobalt is present in all alloys except Ticonium,
whereas nickel is absent in vitallium and nobelium.
2. The total wt of chromium, cobalt and nickel in these alloys
is over 90% yet, their effect on the physical properties of
these alloys are controlled by the presence of minor
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alloying elements such as carbon, molybdenum, beryllium,
tungsten and aluminium.
Effect of each alloy constituents
Chromium: Chromium content is responsible for the tarnish
resistance and stainless properties of these alloys. When the
chromium content of an alloy is over 30% of the alloy is more
difficult to cast. It also forms a brittle phase, known as the zigma
phase. Therefore dental alloys of these types should not contain
more than 28% or 29% chromium.
Cobalt and nickel: are somewhat interchangeable to a certain
percentage cobalt increases the elastic modulus, strength and
hardness of the alloy more than nickel does. Nickel may increase
ductility.
Carbon content: The hardness of cobalt base alloys is increased
by the increased content of carbon. A change in the carbon
content in this order of 0.2% in these alloys changes the properties
to such an extent that the alloy would no longer be usable in
dentistry.
Molybdenum: The presence of 3% to 6% molybdenum contributes
to the strength of the alloy.
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Aluminium: Aluminium in nickel-containing alloys forms a
compound of nickel and aluminium (Ni3-Al). This compound
increases the ultimate tensile and yield strength.
Berylium: Addition of 1% beryllium to nickel-base alloy reduces
the fusion range of the alloy by about 100°C. It also aids in solid
solution hardening. It improves the casting characteristic are
possibly participate in porcelain bonding.
Silicon and Manganese: are added to increase and castability of
these alloys. They are present primarily on oxide to prevent
oxidation of other element during melting. The presence of
nitrogen which cannot be controlled unless the castings are made
in a controlled atmosphere as in vacuum or argon, also contributes
to the brittle qualities of these cast alloys. When the nitrogen
content of the final alloy is more than 0.1%, the castings lose
some of their ductility. Since the minor ingredients of carbon,
nitrogen and oxygen effectively influence the properties of the
final formulated and designed in such a way as to maximize the
rigidity of the prosthesis.
The obvious approach would be to increase the thickness of
the metal substructure, since doubling the thickness increases the
rigidity in bending by a factor of 8. However, the maximum
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thickness of the overall restoration is limited externally by
occlusion and proper anatomical contour internally by the desire
to retain as much tooth structure as possible. (Esthetics requires a
minimal thickness of overlying porcelain that results in severe
limits as to the maximum thickness of the metal).
An examination of the mechanical properties of base metal
alloys and a gold alloy shows that in general the base metal alloys
have a modulus of elasticity approximately twice that of
previously used gold alloys. Since elastic modulus is a measure of
the stiffness of rigidity of materials, this property would enhance
the application of base metal alloys for long-span bridges where
flexure, is a major cause of failure. Given an equal thickness of
precious metal alloy and base metal alloy, the base metal alloy
bridge would flex only half as much as the precious alloy material
under the same occlusal forces. In a similar manner, the higher
modulus of elasticity may be utilized to permit thinner castings.
The Vicker’s hardness of base metal alloys may range from
approximately 175 to 360DPH. Although certain of the base metal
alloys may approach the hardness of noble metal alloy
(approximately 160DPH), the majority of these alloys are
considerably harder. Clinically, it is improbable that significant
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occlusal wear of the alloy will occur. Therefore, particular
attention must be directed toward perfecting occlusal
equilibration. The removal of defective clinical units is also more
difficult than with noble metal alloys, since the high hardness
results in rapid wear of carbide burs and diamond points.
The durability, as measured by the percentage elongation,
of base metal alloys ranges between approximately to 10 and 28
percent. Noble metal alloys have an elongation of approximately 5
to 10 percent.
The density of base metal alloys is approximately
8.0gm/cm3
, as compared with 18.39gm/cm3
for comparable noble
metal alloys. Since casting alloys are purchased on a weight basis,
a lower density is indirectly reflected to the purchaser, who
receives more than twice the volume of material for each unit
weight acquired. Also, the intrinsic value of the component
elements in base metal alloys is significantly les than that of
comparable noble metal alloys. Thus, on the basis of their lower
density and the low intrinsic value of the component metals, the
cost differential between base metal and noble metal alloys can be
substantial.
33

When porcelain is first fired to a metal substructure, the
alloy is subjected to considerable temperature variations and
stresses induced by the shrinkage of the overlying porcelain. Sag
resistance is the property that has been used to describe the ability
of an alloy to resist the permanent deformation of creep induced
by thermal stresses. It is particularly important in long-span
bridges where the porcelain firing temperature may cause the
unsupported structure to deform permanently. Under controlled
conditions, it has been found that a base metal alloy will deform
less than 0.001 inch, while a noble metal alloy will deform 0.009
inch. It is likely that the higher fusion temperature common to
base metal alloys is a factor that contributes to the superior sag
resistance properties of these alloys.
The question of metal ceramic compatibility is basic to the
selection of an alloy system for this type of restoration. Two
requirements are implicit. The metal must not interact with the
ceramic in such a way to discolour the porcelain at the interface or
marginal regions. Moreover, the metal ceramic system must form
a stable bond at the interface that can withstand normal stresses in
the mouth.
34

Alloys for complete metal restorations are cast into calcium
surface bonded investment molars then the alloys have been
melted with gas-air blow porhes. The cast base metal alloys
cannot be melted with the conventional blowtorch uses for gold
alloys, and so it has been necessary to develop special electric
melting facilities or less commonly to melt the alloys which or
oxygen-oxetylene torch.
Electrical somers of melting are often used to advantage,
such as carbon areas, argon arcs, high frequency induction, or
silicon-confide resistance furnaces. In some insurances,
sophisticated electronic equipment is used to control the
temperature, casting time, and similar variables in order to
regulate the gain formation and confide precipitation. Less
commonly oxygen-nityfine torch is used to melt the alloys. The
confurizing section of the oxygen-acetylene flame caudd carbon to
the alloy. The extra carbon changes not only the microstructure
but also the mechanical properties. (In general, hardness and yield
strength increases whereas ductility decreases).
Therefore, when melting the alloy with an oxygen-acetylen
torch, the proportion of the two gases, the length of the flame and
the distance of the torch tip from the alloy should be standardized.
35

Carbon crucibles and carbon-containing investments should be
avoided.
Casting shrinkage compensation
Because of the high fusion temperature, the casting
shrinkage of the base metal alloys is greater than that of the fold
casting alloys. It is in the order of 2.3%, which requires that the
mold be expanded more than when the dental gold alloys are cast.
(Approximately 2% for Ni-Cr alloys). Thermal expansion
represents the principal method of mold expansion for
compensation of the alloy shrinkage. The use of special phosphate
of silicote-bonded investments permit adequate thermal expansion
of the molds when they are probably located and one can produce
castings that display the proper fit and adequate compensation
have yield strengths of at least 450MPa (60,000 lb/inch2
) to
withstand permanent deformation when used as partial denture
clasps.
Tensile strength studies have indicated that the ultimate
tensile strength of the cast base metal alloys is less influenced by
variations in test conditions than some other properties such as
elongation are.
36

Elongation: The percentage elongation of the an alloy is
important as an indication of the relative brittleness or ductility
that the restoration will exhibit. (There are many occasions
therefore when it can be considered to be an important property
for comparison of alloys for removable partial denture
appliances). The combined effect of elongation and ultimate
tensile strength is an indication of toughness of any material.
Partial denture claps cast of alloys with a high elongation and
tensile strength do not fracture in service as often as those with
low elongation, because of their toughness.
The percentage elongation is one of the properties that is
critical to test accurately and to control properly during test
preparatio. A very small amount of microporosity that may exist
in the test specimen will alter the elongation considerably,
whereas its effect on yield strength, elastic modulus and tensile
strength is rather limited. One can therefore assume that practical
castings may exhibit similar variations in elongation from one.
Casting to another. To some degree this is borne out in
practice, with some castings from the same product showing a
greater tendency toward brittleness than others. This observation
37

indicates that the control of the melting and casting variables is of
extreme importance if reproducible results are to be obtained.
Although nickel and cobalt are interchangeable in cobalt-
nickel-chromium alloys, in general increasing the nickel content
with a corresponding reduction in cobalt will increase the ductility
and elongation. Jelenko LG, a cobalt-chromium alloy with some
nickel and with rigid control of molybdenum and carbon, has a
high elongation without much decrease in strength.
Elastic modulus: the higher the value, the more rigid the structure
can be expected to be, provided the dimensions of the casting are
the same in both instances. (There are those within the profession
who recommend the use of a well-designed, rigid appliance on the
basis that it gives the proper distribution of forces on the
supporting tissues when in service. With a greater elastic moduls
it is possible to design the restorations with slightly reduced
dimensions. (It is a well established fact that the elastic modulus
of the cast metal alloys is at least twice that of the dental gold
alloys).
The cast cobalt-chromium dental alloys show comparable
values for elastic modulus of about 228GN/m2 (33x106lb/inch2
),
whereas nickel-chromium alloys possess an elastic modulus of
38

about 198MPa (27x106
lb/inch2
), which is approximately double
the value of 90MPa (13x106
lb/inch2
) for type IV cast metal gold
alloys.
MICROSTRUCTURE OF CAST BASE METAL ALLOYS
The microstructure of any substance is the basic parameter
that controls the properties. Cobalt-chromium or nickel-chromium
alloys microstructure changes by a slight alteration of
manipulative conditions. The microstructure of cobalt-chromium
alloys in these condition consists of an elastomeric matrix
composed of a solid solution of cobalt and chromium in a cored
dendritic structure. Many elements present in cast base metal
alloys, such as chromium, cobalt and molybdenum, are carbide
forming elements. Depending on the composition of a cast base
metal alloy and its manipulative condition, it may form more or
less of any given type of carbide.
Further more, the arrangements of these carbides may also
vary depending on the manipulative condition. The effect of the
microstructure on physical properties a commercial cobalt-
chromium alloy is illustrated in Figure. A one cane say that the
carbides are continuous along the grain boundaries. Such a
structure is obtained when the metal is cast as soon as it is
39

completely melted. In this condition the cast alloy possesses low
elongation values with a good and clean surface. Carbides that are
spherical and discontinuous like islands are shown in figure B.
Such a structure can be obtained if the alloy is heated about 100°C
about its normal melting temperature, and this results in casting
with good elongation values but with a very poor surface. The
surface is so poor that the casting cannot be used in dentistry.
Dark eutectoid areas which are lamellar in nature, are shown in
figure C. Such a structure is responsible for very low elongation
values but a good and clean casting.
Investment materials and casting operations:
The manner of casting relatively 1 stage partial denture
appliances differs in some details from the casting of simple
restorations such as single inlays or crowns, though in principle
the two operations are similar. In cast partial denture construction
a suitable cast of refractory material or investment serves as the
structure on which the wax pattern is formed, when the wax
pattern is completed on the refractory cast, both the wax pattern
and the cast are then invested in a appropriate investment
material. If gold alloys are to be used, the conventional gypsum-
bonded silica investment is acceptable, but only one cast nickel
40

chromium alloy (ticonium) has a sufficiently low melting
temperature to be cast into a specially formulated gypsum-silica
type of investment. For the higher melting base metals it is
necessary to use-------.
When casting any of the base metals in to molar designed to
accommodate the higher melting temperature of these onlays,
certain problems may be encountered that are less common when
casting lower melting alloys. One problem is that of trapping
gases in the mold during the casting process. To have sufficient
strength and resistance to thermal shock, some investments for the
cast base metal alloys lack sufficient porosity for the rapid escape
of gases from the mold cavity when the hot metal intern.
As a result, the gases may be trapped in the mold cavity and
produce voids and casting defects. Numerous methods have been
prepared to overcome such defects, such no venting is the surface
of the mold to permit rapid elimination of gases. The skillful
spruing at and venting of the mold, combined with complete
elimination of the wax residue and adequate heating of the metal,
tend to reduce this type of defective casting.
The melting of base metal alloys must be carefully
controlled to avoid inverse damage to the alloy during the melting
41

and casting process. Oxidation of the ingredient metals and
carbide or nitrite formation at the high temperature required to
melt these alloys demand procine control of the melting and
casting operation. Regardless of the method employed to melt the
alloy, it is possible to cause severe damage to the properties of the
casting if proper melting practices are not observed.
Other applications of cast base metal alloys:
It has been demonstrated that cast cobalt-chromium alloys
serve a useful purpose in appliances other than removable partial
denture restorations. In the surgical repair of bone fractures,
alloys of this type are used for bone plates, crown, various
fracture appliances and splints. Metallic obturation and oral
implants for a variety of purposes are formed from cast base metal
alloys. The use of the cobalt-chromium alloys for surgical
purposes in wall established, and these
-------------------------------------- periods of time without harmful
reactions. This favourable response of the tissues probably is
attributable is the low solubility and electrogalvanic action of the
alloy used, with the result that the metal is inert and produce no
inflammatory responses. The product known as surgical vitallium
is used extensively for this purposes. Cast titanium and its alloys
42

have recently been introduced as surgical implant materials
because of their excellent tissue compatibility.
Potential health hazards of nickel and beryllium
It is widely recognized beryllium is potentially toxic under
uncontrolled conditions. Lab-technicians may be exposed
occasionally or routinely to excessively high concentration of
beryllium and nickel dust and beryllium vapours.
The exposure to beryllium may result in acute and chronic
forms of beryllium disease (Physiologic). The responses may vary
from contact dermatitis to severe chemical pneumanitic which can
be fetal. However, the diagnosis of chronic beryllium disease is
difficult, since it often exhibits symptoms range from coughing,
chest pain and general weakness to pulmonary drifection and
requires the establishment of beryllium exposure.
The occupational health and safety administration specifies
that exposure to beryllium dust in an air should be limited to a
particular beryllium concentration of 2mg/m3
of air for a time
weighed, 8 hours day. In laboratory and clinical situations in a
grinding of beryllium containing alloys is of performed, adequate
43

local exhaust ventilation safeguards should be employed, since all
forms of beryllium are toxic and the body cannot ------- beryllium.
It is also believed that beryllium localization (i.e.
movement of Be ions to the surface) enriches surface is the point
that beryllium makes upto 30% of the composition of the surface
layer. Beryllium release from the surface is enhanced by presence.
POTENTIAL HAZARDS OF NICKEL TO PATIENTS
In certain non-dental industraial applications and
subexperimental conditions, nickel and its compounds have been
implicated as potential carcinogens and as sensitizing agents.
Of great concern in dental patients is the intra oral exposure
to nickel, especially for patients with known allergy to this
elements, Dermatitis resulting from contact with nickel solutions
was reported as early as 1989. the incidence of allergic sensitivity
to nickel has been found to be 5 to 10 times higher for females
than for males with 5% to 8% of females showing sensitivity.
Many cases of respiratory organ concerns have been
documented in studies of workers involved in the plating,
refining, grinding and polishing of nickel and nickel alloys.
Because of the concerns over the carcinogenic potential of nickel,
44

the National institutes for occupational safety and health has
recommended a standard to limit employee exposure to inorganic
nickel in the work place to 15mg/m3
. It appears that the potential
carcinogenic risks of nickel are less likely to affect dental
patients.
To minimize exposing of metallic dust to patients and
dentists during intra oral metal grinding operations, a high-speed
evacuation system should be used. Patients should be informed of
the potential allergic effects of nickel exposures an a thorough
medical history should be taken to try to determine the patient
may be allergic to nickel.
Intra oral tissues are more resistant to symptoms of
sensitivity. However intra oral exposure to allergies can be
manifested in locations remote from dental restorations. The
systems of the sensitivity range from urticania, pruritis,
xerostomia, eczema or vesicular eruptions.
Etching of base metal alloys:
When it was first introduced micromechanical retentions of
etched metal resin-bonded retainers (Maryland bridges) was
obtained by electronically etching the base metal alloys. More
45

recently, chemical etchants have been marketed also less
expensive and more convenient for etching the metal substraction.
The intagets surface of the resin are to be binded to etched enamel
are treated with acid gels or liquids for a short period of time.
However reports on the comparative bond strength between
electrolytically etched and chemically etched surface on
conflicting. More study is needed to determine the relative value
of chemical etchants substitutes for conventional electrolytic
etching.
METAL CERAMIC ALLOYS
General Features:
The chief objection of the use of dental porcelain as a
restorative material is its low tensile and shear strength. Although
porcelain can resist compressive stresses with reasonable success,
substructure design does not permit shapes in which compressive
stress is the principal force.
A method by which this disadvantage can be minimized is
to fuse the porcelain directly to a cast alloy substructure made to
fit the prepared tooth. If a strong bond is attained between the
46

porcelain veneer and the metal, the porcelain veneer is reinforced.
Thus, brittle fracture can be avoided, or at least minimized.
The original metal ceramic alloy contained 88 percent gold
and were much too soft for stress-bearing restorations such as
fixed partial dentures. Since there was no evidence of a chemical
bond between these alloys and dental porcelain, mechanical
retention and undercuts were used to prevent detachments of the
ceramic veneer. By adding less than 1 percent oxide forming
elements such as iron, indium and tin to this high gold content
alloy, the porcelain-metal bond strength was improved by a factor
of 3 iron also increases the proportional limit and strength of the
alloy.
This 1 percent addition of base metals to the gold,
palladium, and platinum alloy was all that was necessary to
produce a slight oxide film on the surface of the substructure to
achieve a porcelain metal bond strength. This new type of alloy
with small amounts of base metals added, became the standard for
the metal ceramic restoration. In response to economic pressures,
other gold-and palladium-base metal ceramic alloys emerged. In
time, base metals were also developed for this same purpose.
47

Properties:
The clinical success of a metal ceramic restoration is
dependent in large measures on the ability of the underlying alloy
substructure to resist the potentially destructure masticatory
stresses. (Therefore it is imperative that the metal ceramic
restoration be use of variable casting conditions. Therefore, these
alloys are generally considered to be technique sensitive. One
reason for this sensitivity is that almost all elements in these
alloys such as chromium, silicon, molybdenum, cobalt and nickel
react with carbon to form carbides depending on the mold and
alloy-casting temperature, cooling rate, and other technical
variables, carbides of any one of these elements may form. The
formation of different carbides naturally changes the properties of
the alloys. As a result, careful control of manipulative variables in
the casting operations is necessary.
ANSI/ADA Specificaiton No. 14
According to this specification, the total weight of
chromium, cobalt and nickel should be not less than 85% or no
less than 20% chromium. Alloys having other compositions may
also be accepted by the ADA provides that the alloys comply
satisfactory with requirements for toxicity hypersensitivity and
48

corrosion. Composition to the nearest 0.5% shall be marked on the
package plus the presence and percentage of hazardous elements
and precautioning recommendations for processing the materials.
The specification also recommend minimum values for
elongation, yield strength and elastic modulus.
An important feature of this specification is that it has more
a standardized method of testing available, which has in turn,
made possible comparisons of results from one investigation to
another.
49

CONTENTS
 Introduction
 Historical Perspective on Dental Casting Alloys
 Properties of Noble Metal Alloys
 Classification of Dental Casting Alloys
 Alloys for All Metal & Resin Veneer Restorations
 High Noble Alloys for Metal Ceramic Restorations
 Base metal Alloys for Dental Castings
 Composition of Base metal Alloys for Small castings
 Effect of Alloy Constituents
 Handling Hazards and Precautions
 Summary & Conclusion
50

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