Chapter 6
Casting Alloys
I. Introduction
There are two types of casting alloys. The
alloys that are not to be veneered with porcelain are sometimes called casting alloys.
However, since alloys to be covered with porcelain are also cast, a better terminology for
these alloys is bare-metal alloys. Alloys that
are to be covered with porcelain are called
metal- ceramic alloys or more commonly
porcelain-fused-to-metal (PFM) alloys.
Table 1
Weight Conversions
Converts to
1 troy pound
12 troy ounces
1 troy ounce
20 penny weights
1 DWT
1.55 grams (g)
1 avoirdupois pound
16 avoirdupois ounces
1 avoirdupois pound
14.58 troy ounces
Weights and measures. Casting alloys
are sold by the pennyweight. One pennyweight is 1/20 of a troy ounce. The troy ounce
was first used for weighing gold, silver, and
precious stones at medieval fairs held at the
French city of Troyes. Table 1 summarizes
conversions between the troy ounce and both
avoirdupois and metric weights.
Measures of Gold Content. Two units of
measure are commonly used to report the gold
content of alloys. They are the carat and
fineness. Carat (also spelled Karat) measurement gives the number of parts pure gold in
24 parts of a gold-containing alloy. For example, a 24 K (24 carat) alloy is 100% gold. An
18 K alloy contains 18 out of 24 parts gold or
75% gold.
Fineness gives the parts pure gold in 1000
parts of alloy. An 1000 Fine alloy is 100%
gold. A 750 Fine alloy is 750/1000 or 75%
Revised 11/9/09
gold. Fineness is sometimes reported in decimal form; 0.750 Fine also refers to a 75% gold
alloy.
Applications. Alloys are used to make of
long list of dental prostheses including: inlays,
onlays, crowns, bridges, resin-bonded bridges,
periodontal splints, and post and cores. Fewer
inlays, onlays, and 3/4 crowns are being placed
today than in the past. The ease and durability of amalgam restorations has resulted in
fewer cast inlays. Many large restorations are
converted to full crowns. High speed handpieces have made it as easy (or easier) to prepare a tooth to receive a crown than to receive
a large inlay or onlay. The esthetic concerns
that made the 3/4 crown a popular prosthesis
can now be lessened by a porcelain veneered
crown.
Properties. There are two major requirements for bare-metal prostheses: 1) strength
and 2) corrosion resistance. As will be seen in
the next section, there are many alloys that
are sufficiently strong. As will be seen in the
section following the next section, there are
fewer alloys that are sufficiently corrosion
resistant.
II. Strength
Materials for crowns and bridges must have
high tensile and shear strengths. Crowns and
abutments replace partially destroyed tooth
structure, and consequently must be able to
withstand the same forces of mastication that
the tooth is able to withstand. If occlusion
relationships are to remain constant, the span
between abutments must resist bending.
Many materials (ceramics, plastics, porcelains and etc.) have sufficiently high compressive strengths for these requirements. Unfortunately, most lack sufficient tensile and shear
strengths to function as crowns and (especially) as bridges. The atoms in these materials
1
Casting Alloys
Oral Biomaterials - Chapter 6
are usually bonded together by either covalent
or ionic bonds. When an aggregate of atoms is
compressed, bonds are not threatened - atoms
are merely forced closer together. However,
when these materials are sheared or pulled in
tension, bonds are broken and since the bonds
are highly directional, new bonds are not readily formed. In metals, when bonds are broken
due to tensile or shear stresses, new bonds
readily form because the bonding in metals is
omnidirectional.
III. Corrosion Resistance
Chemical Corrosion. Metals are susceptible to two types of corrosion: chemical corrosion and electrochemical corrosion. In chemical corrosion, the metal chemically attacked by
its environment. For example, iron rusts in
air and silver is discolored by sulfur in the air.
Discoloration chemical corrosion is often called
tarnish.
In the free state, most metals are covered
by a thin layer of oxide. Surface oxides on
some can protect the underlying metal from
corrosion. To be protective, the oxide layer
must be adherent, non-porous, and continuous.
Metal oxides often form with a volume that
are greater than the metal that is consumed to
produce the oxide, in which case the oxide may
flake off the surface and/or crumble. Such an
oxide will not be protective. If, instead, the
oxide that has nearly the same volume as the
metal that is consumed, it is more likely to
form a continuous layer that remains attached
to the underlying metal. When an alloy forms
a protective metal oxide it is said to have passivated. The formation of such a layer is
called passivation. An alloy that exhibits low
corrosion rates because of a protective
oxide layer is said to be passive.
Most metals will form oxides when exposed
to oxygen. Only two metals, copper and gold,
are normally found in nature as pure metals.
Both are very resistant to oxidation by the
2
atmosphere. However, only gold is actively
resistant to oxidation in saliva.
Electrochemical Corrosion
When a metal is placed in an electrolytic
solution such as water or saliva, it may oxidize. That is, metal ions enter solution and an
excess of electrons is left on the metal.
M → M n+ + n e ( 1 )
Dissolution eventually stops when an equilibrium between the electrolyte and the metal
is obtained. Two factors are important in stopping the dissolution: (1) excess electrons in
the metal prevent the formation of metallic
ions and, (2) saturation of the solution with
metallic ions means there is no room for more
ions.
If another metal, one which has a smaller
tendency to dissolve than the first, is immersed in the electrolyte and if the two are
connected by a conductor, dissolution of the
first metal will resume. The excess electrons
in the first metal are conducted to the second
where they combine with metal ions from the
solution. The dissolving piece of metal (electrode) is called the anode. The other electrode is the cathode. At the cathode, the electrons that are produced at the anode are consumed by reduction reactions. In electroplating, metal ions react with electrons and plate
out as the metal on the cathode. More commonly, electrons are consumed by reaction
with hydrogen ions or oxygen dissolved in the
solution (the electrolyte). There are many
possible reduction reactions. In acid solutions
(those containing H+ ions), a common reaction
is
2H + + 2e → 2H → H (gas) ( 2 ).
2
In saliva, an even more common reduction
reaction occurs
O2 + 2H2O + 4e → 4OH − ( 3 )
Revised 11/9/09
Casting Alloys
Oral Biomaterials - Chapter 6
When two different metals are placed in an
electrolyte and no external electrical connecoxidation
M
Mn+ + 2es
on
ctr
ele
anode
cathode
reduction
2H+ + 2e-
H2
O2 + 2H2O + 4e4OH-
ions
electrolyte
Fig. 1. Galvanic corrosion. The metal dissolves (oxidizes) at the anode.
Reduction reactions occur at the cathode.
tion is made, the metal pieces become charged.
Positive metal ions collect at the cathode and
an excess of electrons is left at the anode. As
a result of the movement of charge, an electrivoltage
Table 2 gives standard electrochemical
potentials for a list of metals. To measure
this standard potential one places a metal in
a 1M aqueous solution its own ions. This solu-
Table 2
Electrode potential
in water at 25oC.
(potential with respect to a hydrogen
electrode)
Metal
gold
platinum
palladium
silver
mercury
copper
tin
nickel
cobalt
iron
chromium
zinc
aluminum
Volts*
-1.498
-1.229
-0.987
-0.799
-0.401
-0.337
+0.136
+0.250
+0.277
+0.440
+0.744
+0.763
+1.662
* Adapted From Craig RG.
Restorative Dental Materials CV
Mosby Co, St. Louis, 1985.
25O C
Pt
H+ solution,
1.0 M
hydrogen gas
1 atm pressure
membrane
Fig. 2. A schematic of a standard hydrogran
electrode. The metal who standard
potential is to be measured is paired
(as anode or cathode) with this electrode.
cal potential (a voltage) develops between the
pieces of metal. When the circuit is completed,
the voltage will drive a corrosion current.
Revised 11/9/09
tion is separated from a standard “hydrogen”
electrode by a semipermeable membrane. The
latter allows the water to pass, but prevents
the passage of ions. The standard hydrogen
electrode is produced by placing a platinum
electrode in an aqueous solution of 1 M H+
ions. Hydrogen gas is bubbled into the solution. This hydrogen collects at the surface of
the platinum producing an electrode which act
as if it were hydrogen metal. The platinum
itself does not take part in the reaction. It
serves only as an inert surface upon to attract
the hydrogen bubbles.
The metal electrode and the hydrogen electrode are connected via a voltmeter. Since the
voltmeter has a very high resistance, very lit3
Casting Alloys
Oral Biomaterials - Chapter 6
Corrosion rate. Table 2 gives a rough
indication of which metal will dissolve, but we
need a bit more information to predict how
fast the anode will dissolve. Two factors that
affect corrosion rate are the corrosion current and the current density. The current
density depends on the surface area of the
anode. For a given current, the current density is high when the surface area is small.
Consequently, a tiny anode will dissolve faster
than an anode with a larger area.
The Standard Potentials in Table 2 are
measured in a cell that contains a voltmeter.
Since the voltmeter has a very high resistance,
very little current flows. The circuit is essentially “open.” If the voltmeter is removed, the
circuit is “closed” and current can flow. The
potential difference between the metals drives
dissolution of ions at the anode and reduction
of electrons at the cathode.
Fig. 3 illustrates the change in electrode
potential as the corrosion current at the anode
increases. Note that the decrease in potential
is linear with the log of the current. This
increase in potential is necessary to force ions
into the electrolyte. The electrode potential
also changes linearly with the log of the cur4
Electrode Potential (V)
The relative position of a metal on the list
in Table 2 is an indication of the metal’s relative tendency to dissolve: the more positive
(anodic) a metal is, the more likely it is to dissolve. Listed at the cathodic end of the list are
metals that are resistant to dissolution: gold,
platinum, palladium and silver. (Note that
iridium, osmium, ruthenium, and rhodium are
also noble, but large amounts of these elements are seldom found in dental casting
alloys.) Such metals are called noble metals.
Alloys consisting predominantly of these metals are called noble metal alloys.
- 0.7
- 0.6
- 0.5
- 0.4
Eo
- 0.3
io
- 0.2
10-6
10-5
10-4
10-3
10-2
10-1
2
Current Density (A/cm )
Fig. 3. The anode polarization curve for copper. As current density increases,
the potential decreases.
rent at cathodes, except that the potential at
the cathode increases. The change in electrode
potential when a current is present is called
polarization.
Fig. 4 shows what happens when the circuit
between the anode and cathode is closed. The
potential at the anode decreases and potential
at the cathode decreases until each half-cell
electrode potential
tle current flows (as if the circuit were “open”).
This enables the voltmeter to record the open
circuit potential between the two electrodes.
cathodic
reduction
reaction
corrosion
rate
anodic
dissolution
mixed
corrosion
potential
current density
Fig. 4. In a galvanic cell, the electrode potentials at the anode and cathode change
until a they share a single mixed corrosion potential. The current density
at this potential determines the corrosion rate.
Revised 11/9/09
Casting Alloys
Oral Biomaterials - Chapter 6
shares a single potential. The current at this
potential determines the corrosion rate.
Note in Fig. 4 that if the slopes of the anodic and cathodic polarization lines do not
change, then the corrosion current will depend
on to relative potential difference between the
standard potentials. When the potential difference is large, the current density at the
mixed potential will be large.
electrode potential
concentration
polarization
i0
i1
anodic
dissolution
current density
Fig. 5. Concentration polarization due to a
consumption of oxygen at the cathode changes the corrosion current
density from an initial value of io at
to to a value of i1 and t1.
If the slopes of the anodic and polarization
lines change, the corrosion rate will change.
Fig. 5 gives an example of such a change in
slope. For example, if the cathodic reaction
consumes oxygen (see eq. 3), the reaction at
the cathode will slow down. The current dentisty decreases with time as the oxygen cocentration in the electrolyte decreases. This is a
function of the concentrationis called concentration polarization.
Effects of electrolyte - saliva. Table 2
gives a rough indication of how well metals
will perform in the oral environment. Generally, metals that are listed highly cathodic in
the Table are noble in the mouth. For examRevised 11/9/09
ple, gold, platinum, palladium and their alloys
resist corrosion in the mouth. However, unalloyed silver corrodes intraorally. Furthermore,
corrosion resistance depends on the electrolyte. In the mouth, the electrolyte is saliva.
Corrosion resistance depends on the concentration of salts and organics within the saliva
and on its pH. Changes in electrolyte chemistry will change the slopes of the polarization
lines at the anode or cathode similar to that
illustrated for changes in oxygen concentration
in Fig. 5. The bottom line is that an alloy that
preforms well in one patient may exhibit considerable corrosion in another.
Galvanic corrosion in the mouth. Galvanic corrosion can occur when dissimilar electrodes are placed in electrical contact. Such
electrical contact can occur when crowns and
bridges made of dissimilar metals come into
contact occlusally or are in close interproximal
contact. A corrosion current may also arise
when saliva produces an intermittent electrical connection between prostheses made of two
dissimilar metals.
Galvanic corrosion between prostheses
made of different noble metal alloys is rarely a
problem. However, galvanic corrosion
between an amalgam restoration and a noble
alloy prosthesis is possible. The amalgam is
anodic with respect to noble metal alloys and
will tend to dissolve.
Types of galvanic corrosion. It should
be obvious that galvanic corrosion occurs when
metals of different composition are in electrical
contact. Pairs of such electrodes are called
composition cells. The anode and cathode
necessary for galvanic corrosion may both be
present in a single crown or bridge. When the
prosthesis is made from a two-phase alloy,
there may be a sufficiently large electropotential difference between the two phases for corrosion to occur. A galvanic cell may be set up
with the less noble (anodic) phase dissolving.
5
Casting Alloys
Oral Biomaterials - Chapter 6
To prevent this, many manufacturers favor
single phase alloys for use in dental castings.
Galvanic corrosion can also occur even
when two sites in the same prosthesis have
the same composition. This can happen if the
concentration of the electrolyte is different at
the anode and cathode (for example, the oxygen at the electrolyte is depleted, as in Fig. 5).
Pairs of anodes and cathodes created by differences in electrolyte composition are called
concentration cells. The marginal interface
between tooth and an amalgam restoration is
an example of a site that can easily become
oxygen depleted, thereby, giving rise to a concentration cell.
Another cause of galvanic corrosion coring. When a liquid alloy made up of high
and low melting elements cools, the liquid that
solidifies first tends to be rich in the higher
melting point elements. As cooling continues,
the remaining liquid will solidify - this solid
will be rich in the lower melting phase. Grain
edges will be rich in the lower melting phase.
Grain centers will be rich in the high melting
ele ments. There will be increasing concentrations of lower melting elements as the grain
boundary is approached. This change in concentration from grain centers to grain boundaries is called coring.
moderate temperature. These treatments will
allow atoms to diffuse and even out the concentration gradients within each grain. As
has been mentioned earlier, any heat treatment to change the properties of an alloy is
called annealing. The treatments for eliminating coring are called homogenization
anneals. The process is called homogenization.
Base Metal Alloys. Metals toward the
anodic (positive) end of Table 1 are called base
metals. Such metals may be present in minor
concentrations in noble casting alloys but in
general are not suitable for use in the mouth.
Nevertheless, alloys based on cobalt-chromium
and nickel-chromium can be successfully used
in the mouth. Such alloys contain between 9
and 20% chromium. The chromium reacts
with oxygen from the atmosphere to form a
tenacious oxide layer which protects the
underlying metal from oxidation. Similar
technology gives stainless steel its ability to
resist corrosion. Ni-Cr alloys and stainless
steels are examples of alloys that are passive
to corrosion.
IV. Classification of Alloys
In 1984 the American Dental Association
released the classification system for dental
casting alloys given in Table 3.
It should be emphasized that coring occurs
These classifications were not intended to
even in solid solution (single phase) alloys.
group dental alloys according to performance.
Since the noble elements have high
melting points, less corrosion resistTable 3
ant elements tend to concentrate
ADA Classification System for Casting Alloys
near the grain boundaries. In cases
Requirements
of severe coring, the concentration Classification
gradient can be so large that galHigh noble
noble metal content > 60 wt. %
vanic corrosion cells can arise with
gold > 40 wt. %
the anodic (less noble) regions near
Noble
noble metal content > 25 wt. %
grain boundaries dissolving.
noble metal content < 25 wt. %
Coring can be minimized by slow Predominantly Base
cooling the alloy or, after fast cool- * For the purpose of this classification, only Au, Pt, and Pd are noble metals.
ing, by reheating the alloy at a
6
Revised 11/9/09
Casting Alloys
Oral Biomaterials - Chapter 6
Its major purpose is to classify alloys (roughly)
according to cost. This is useful for the insurance industry, but will not help the clinician
select an alloy.
groupings of alloys with similar compositions
and clinical performance.
The ADA Specification for Casting Alloys
In the preceding, alloys are classified by
their composition. Another way to classify
In the ADA’s classification system, only
three metals are considered noble: gold, platinum, and palladium. The gold requirement
in the "high noble" classification is a holdover
resulting from the profession's high esteem for
gold-based alloys. Nevertheless, it should be
emphasized that from the standpoint of intraoral corrosion resistance platinum and palladium are "as good as gold".
Table 4
Types of Bar e Metal Alloys
High Noble Alloys
High-gold alloys
“Low Gold” High Noble Alloys
Noble Alloys
The terms "precious", "semiprecious", and
"nonprecious" also have been used to describe
dental casting alloys. These terms refer to the
monetary value of an alloy. However, monetary value may not indicate alloy suitability
for intraoral use. For example, silver is usually considered a "precious" metal, but does not
resist corrosion in the mouth. The term "semiprecious" may also mislead. It is usually used
to designate alloys that contain some "precious" metals; but there is no gen erally agreed
upon minimum percentage of "precious" metals that such alloys must contain. The term
"nonprecious" is not so abused, since it's connotation gives little advertising advantage.
Low Gold Alloys
Silver-Palladium Alloys
Predominantly Base Metal All oys
Silver-Indium Alloys
Nickel-Ch rom ium Alloys
alloys is by their mechanical properties. The
ADA standard for casting alloys uses the latter
method. Table 5 presents the yield strength
and minimum elongation requirements for
alloys classified by SCDP Specification No. 5
for dental casting alloys. The yield strength is
the stress at which the alloy will bend and
stay bent after the stress is removed. The
elongation is the percent elongation an alloy
can sustain without breaking. Alloys that
have low yield strengths are considered soft.
"Nonprecious" means an alloy consisting
predominantly of base metals. As will be discussed below, some base alloys are more successful in the mouth than
Table 5
poorly designed "precious"
Requir ements of Alloys Meeting ADA SCDP Specification No. 5
alloys.
In the following, dental
casting alloys will be discussed using the ADA classification given in Table 3.
However, each category will
be subdivided as shown in
Table 4. This Table gives
Revised 11/9/09
Yield Strength
(M Pa)
(0.1% offset)
Type
M inimum
Elongation (%)
annealed
hardened
annealed
hardened
I
140 max
none
18
none
II
140 - 200
none
18
none
III
200 - 340
none
12
none
IV
≥340
500
10
2
7
Casting Alloys
Oral Biomaterials - Chapter 6
Alloys with higher yield strengths are considered medium, hard, or extra hard. The hardness of an alloy is determined by its composition and heat treatment. Heat treatment is
the heating of an alloy to a specified temperature and holding it there for a period of time to
make change its properties. When a heat
treatment produces a stronger alloy, the alloy
is described as having been hardened. When a
heat treatment reduces the strength on an
alloy, the alloy is described as having been
annealed.
Up until 1989, specification No. 5 also
required that acceptable alloys contain at least
75 wt. % noble metals. The current specification includes no composition requirement. In
addition to the mechanical property requirements shown in Table 5, Specification No. 5
requires tests for toxicity and tarnish resistance.
Uses of the ADA Alloys. The four types
of alloy vary in hardness from soft (Type I) to
extra hard (Type IV). Type I alloys are designed for situations where only minimum
strength and wear resistance is required.
These alloys are burnishable. A small inlay is
an example of a restoration which could be
done with a Type I alloy.
Type II alloys are used for restorations that
have to withstand moderate stress. Typical
examples include onlays, three-quarter
crowns, and larger inlays. Like Type I alloys,
these alloys can be burnished.
Type III alloys are used for restorations
where relatively high strength is an advantage. Typical uses would be for multi-unit
fixed prostheses of all types and for crowns,
especially where a portion of the coping is
thin. These alloys can be hardened and softened by appropriate heating and cooling regimens. In the softened conditioned, burnishing
is possible.
8
Type IV alloys are the hardest of the ADA
alloys. They are recommended for removable
partial dentures. Today Type IV gold alloys
are seldom used, since Co-Cr alloys are a satisfactory and cheaper alternative.
As was mentioned in the introduction, procedures requiring Type I and II alloys are
rarely performed today. Only the Type III
alloys saw wide use in the 1990s.
V. High Noble Alloys (noble metal content
> 60 wt. %; gold > 40 wt. %)
High-gold alloys
All four types of alloy (I, II, III, and IV) can
be made with high noble compositions. If
enough gold is used, these alloys will have a
yellow color, a characteristic that is valued by
patients and some clinicians. Typical compositions of three Type III high-gold alloys are
given in Table 6 (alloys ‘A', ‘B', and ‘C').
Hardening (strengthening) of alloys.
When gold or any other elemental metal is
alloyed, it becomes stronger and harder. To
minimize the hardening, one minimizes the
amount of the second and third elements that
are added to the gold. The effects of alloying
gold are especially dramatic. Pure gold is very
soft. It can be elongated into long wires or
beat into paper thin sheets. However, when
even small amounts of silver, platinum or palladium are added to gold, they harden it significantly. This type of hardening is an example of the solid solution hardening that was
discussed in Chapter 5.
Order hardening. Note in Table 5 that
ADA SCDP Specification No. 5 specifies yield
strength and minimum elongation for Types I,
II, and III alloys only in the annealed (e.g.,
softened) condition. Type IV alloys, on the
other hand, are specified for both annealed
and hardened conditions. The hardness of
gold-based Type III and IV alloys (and some
Revised 11/9/09
Casting Alloys
Oral Biomaterials - Chapter 6
dered structure, con siderable energy must be
supplied to move a dislocation. Consequently,
the yield strength (and hardness) of an
ordered alloy will be much higher than that of
the same alloy in the disordered condition.
gold
copper
disordered
ordered
Fig. 6. The face-centered-cubic 50 at. % copper-gold lattice. In the disordered
state, copper and gold atoms are distributed randomly on the cubic crystal lattice, In the ordered state, copper and gold atoms segregate to
alternate planes parallel to one of
the cube faces. The lattice contracts
normal to these planes, producing as
face-centered-tetragonal lattice.
other casting alloys) can be controlled by the
way they are heated and cooled. By appropriate heat treatment an alloy can be sof tened
sufficiently to permit margins to be closed by
burnishing. After burnishing, the alloy can
be then given another heat treatment to make
it strong enough to resist occlusal forces.
The addition of copper to gold alloys makes
hardening possible. When gold-copper alloys
containing 50 at. % copper (24.5 wt. % Cu) are
heated at 300oC, the copper and gold atoms
assume an ordered arrangement on the facecentered-cubic lattice. Copper atoms and gold
atoms occupy alternating planes parallel to a
cube face (see Fig. 6).
Mechanism of order hardening. Dislocations have great difficulty moving in
ordered crystals. Movement of dislocations
causes one part of the crystal to slip with
respect to another. In ordered alloys, such slip
will produce local disorder. Since the ordered
structure has a lower energy than the disorRevised 11/9/09
Ordering strengthens an alloy by making
dislocation movement difficult. In Au-Cu
alloys there is another mechanism associated
with ordering that contributes to the strengthening. When atoms of copper and gold move
to the ordered arrangement, the crystal
changes from cubic to tetragonal (see Fig. 6).
Two axes of the crystal remain equal, but the
axis at right angles to the plane on which the
copper and gold atoms are alternately segregated contracts. Within a given crystal, copper and gold may segregate to any one of the
three cube faces. Consequently, the tetragonal
contraction in adjacent areas of the crystal
may lie in different directions, leading to
strains and to sharp discontinuities at the
boundaries between such areas. These boundaries are also a barrier to dislocation movement and are another source of hardening in
ordered Au-Cu alloys.
By fast cooling gold-copper alloys from high
temperatures, they can be disordered. In the
disordered condition this alloy is like most
other solid solution alloys - copper and gold
atoms can occupy any position in the crystal.
That is, there is no preference for particular
positions on the crystal lattice.
Softening gold-based alloys by fast
cooling. When heated above 450oC, gold-copper alloys become disordered (random) solid
solutions. Since the ordering reaction requires
time to allow the gold and copper atoms to diffuse to preferred positions, the disordered condition can be frozen in by fast cooling the alloy
from high temperature. The fast-cooling traps
the atoms in the high temperature disordered
condition. Once an alloy is frozen at room
temperature into the disordered state, it will
9
Casting Alloys
Oral Biomaterials - Chapter 6
remain in that state. Although the ordered
lattice is the equilibrium state at room temperature, solid state diffusion at room temperature is so slow that the disordered lattice will
remain disordered for centuries. Thus, fast
cooling (quenching) will produce a soft (disordered or "annealed") alloy. Note that the softened alloy has not undergone a traditional
heat treatment involving holding at a higher
temperature for a period of time. The fast
cooled alloy is, nevertheless, called an "annealed" alloy. It is the fast cooled condition
that is the "annealed" condition sited in Table
5.
Hardening gold-based alloys. Below
450oC Au-Cu alloys are ordered; above this
temperature they are disordered. There are
two methods for hardening these alloys. First,
the disordered alloy can be hardened by heating just below 450oC for a period of time (e.g.,
at 350oC for one hour). Second, the alloy can
be allowed to harden during initial cooling by
slow cooling through the 450 to 250oC range.
Slowing the cooling rate allows the atoms sufficient time for solid state diffusion to produce
an ordered crystal structure.
An annealing heat treatment. It is also
possible soften an ordered Au-Cu alloy by heat
treating at temperatures above 450oC (e.g.,
one hour at 550oC). This softened alloy is also
called an "annealed" alloy.
Having spent time describing order hardening, one must admit that such hardening is
used infrequently. Dental casting alloys are
usually used in their annealed condition.
Roles of Elements in Gold-based Alloys.
Gold is primarily responsible for the corrosion resistance of gold-based alloys. If sufficient gold is present, the alloy will be soft and
burnishable. Silver produces solid solution
hardening and reduces the melting range.
Large amounts of silver can be added to gold
10
without "whitening" the alloy (e.g., turning the
alloy grayish). Some cooling conditions may
produce silver-rich regions that may corrode.
Platinum and Palladium. Like gold,
platinum and palladium are noble elements,
which are inherently corrosion resistant.
Because they melt at high temperatures, both
palladium and platinum will increase the
melting temperature of gold alloys. As mentioned in the preceding, they also contribute to
the solid solution hardening of the alloy.
When palladium is added to alloys that also
contain silver, it reduces the tendency of silver
to tarnish. More than 12% of either platinum
or palladium will give the alloy a silver or as
its described in the literature a "whitish" color.
Since the color yellow indicates value to many
patients and clinicians as well, manufacturers
attempt to maintain the yellowish color. It is
important to note that corrosion resistance is
not necessarily decreased by the whitish color
of high palladium or platinum alloys.
Copper. As discussed in the preceding,
copper forms ordered alloys with gold. In
alloys containing substantial palladium or
platinum, there is also the possibility of forming ordered Cu-Pt and Cu-Pd phases. These
ordered phases make it possible to heat treat
alloys to produce "soft" or "hard" prostheses.
In alloys containing substantial amounts of
silver, copper reacts with silver to form a
eutectic. Recall that eutectics are low fusing
alloys. Consequently, copper and silver in a
gold alloy will lower the fusion temperature of
the alloy.
Copper also "reddens" the alloy - meaning
that it lends an orange-yellow tinge to the
alloy. Finally, copper is the primary element
which reacts to form an oxide "crust" on the
liquid alloy during melting and is the primary
reactant which produces the surface oxides on
the casting.
Revised 11/9/09
Casting Alloys
Oral Biomaterials - Chapter 6
Base Metals. Base metals such as zinc or
indium are often added in concentrations of
less than two percent. These elements react
with oxygen in the melt, thereby, producing
castings that contain fewer pores.
Grain Size Refinement. It has been
found that very small amounts (0.005% or 50
ppm) of the high melting elements iridium and
ruthenium will produce very small grains in
gold alloys. Although the mechanism of grain
size refinement is not well understood, the
small grain size increases the yield strength
and ductility of high noble alloys.
"Low Gold" High Noble Alloys
There are a small number of
alloys that contain more than 40%
gold and enough platinum and
palladium so that the total content of noble metals exceeds 60%.
Such alloys are rarely marketed:
1) "enough" platinum or palladium will raise the melting temperature excessively and 2) platinum
( $963 / Troy oz - 1/06) and palladium ( $253 / Troy oz) will raise
the costs of the alloy excessively.
Class
Au
Pt
Pd
Ag
Cu
Zn/In
A
71
1
---
13
8.5
?
B
74.5
---
3.5
11
?
?
C
77
1
1.0
14
7.5
0.5
D
58
---
0.35
27
9.6
1.0
E
47
---
6.5
37
8.0
1.0
F
42
---
2.0
26
20.9
2.0
G
40
---
4.0
47
7.5
1.5
High Gold
Low Gold
Low Gold Alloys
Revised 11/9/09
Cost Comparisons. If the charge to the
patient for a crown is $900, how much of this
charge is for gold and other precious metals?
The cost of elements other than gold ($1108 /
Troy oz - 11/09), platinum ($1360 / Troy oz 11/09), and palladium ($333 / Troy oz - 11/09)
can be neglected. The base metals and silver
are much less costly. For example, silver is
only $17.70 / troy oz. For gold at $1108 / troy
oz, the gold in a typical dental crown (volume
Table 6
Typical Compositions (wt. %) for
High & Low Gold Alloys
VI. Noble Alloys (noble metal
content > 25 wt. %)
All That Glitters. Until 1977,
the price of gold was maintained
at an artificially low level
($35/Troy oz) by the U.S. government. In 1977, the dollar was
taken off the gold standard and
the price of gold began to rise. It
reached a peak of over $800/troy
oz in 1980. For a while the price
has since "stabilized" in the range
of $300-600. Recently, the price of
gold has soared to $1108 / Troy oz
(11/09). At this price, one can see
why there has been incentive to develop alloys
containing less gold and other noble metals.
low gold alloys are one response to this perception.
A. Harmony Line Medium, Williams Gold RefiningCo.,Buffalo,
NY
B. Firmilay, Jelenko Dental Health Products, Armonk, NY
C. Modulay, Williams Gold Refining Co., Buffalo1
D. Rajah, Jelenko Dental Health Products, Armonk, NY2
E. Midas, Jelenko Dental Health Products, Armonk, NY2
F. Forticast, Jelenko Dental Health Products, Armonk, NY1
G. Minigold, Williams Gold Refining Co., Buffalo, NY1
1
2
Composition from Sturdevant et al. Dent Mater
1987,3:347-352.
Composition from Bessing & Bergman Acta Odontol Scand
1986,44:101-112.
11
Casting Alloys
Oral Biomaterials - Chapter 6
= 0.08 cm3) is worth $34.34 in a 78% gold alloy
and $16.70 in a 46% gold alloy. The only other
metal precious metal that these alloys are likely to contain is palladium. For a typical palladium addition of, say 6%, the cost of each of
these alloys will be increased by about $1.00.
In either case, the metal cost is a small fraction of the final charge to the patient.
Composition. Table 6 gives the compositions of several low and high-gold alloys. Note
that the low gold alloys generally contain
enough gold to be classified as ADA high noble
alloys, but are classified instead as "noble"
because their noble metal content is less than
60 wt. %. Although low gold alloys may contain between 40 and 70% gold, a large number
of the most successful alloys of this type contain gold in the 40-50% range. The silver content in these alloys is more than double that of
Type III high-gold alloy; low gold alloys usually contain anywhere between 20 and 40% silver. The other major component of these
alloys is copper. Typical alloys usually contain
from 8 to 12% of this element.
Type III high-gold alloy (78% Au, Pd and Pt).
Note that the properties of these two alloys
are not very different. Both can be hardened.
When hardened, the low gold alloy reaches a
Vickers hardness of over 200. A number of
workers have related these mechanical properties to alloy burnishability. For example,
Moon et al. (J Prosthet Dent 1976:36:404-408.)
define a burnishability index:
burnishability =
hardness
ductility
When the hardness is high and its ductility
(as measured by total percent elongation) low,
this index will be large. Large burnishability
indices mean that an alloy cannot be burnished. That is, the alloy is too hard to be
penetrated and too brittle to flow.
Low gold alloys are found to have a wide
range of burnishabilities. Some, like the alloy
in Table 7, have burnishabilities very close to
that of the Type III high-gold alloys. Others,
however, are much less burnishable than the
higher gold alloys.
Mechanical Properties. Table 7 compares the mechanical properties of a Type III
low gold alloy (52% noble Au, Pd or Pt) and a
Hardenability. As in the high-gold alloys,
copper and gold form ordered solid solutions in
these alloys. If the alloys contain as much as 20% palladiTable 7
um, an ordered Cu-Pd solid
Properties of High-Gold alloys and Low-Gold Alloys
solution forms. Heating at
temperatures between 300 and
Property
High Gold
Lo w Gold
500o C will cause ordering to
Hardness
125
140
take place. However, quench(Vickers)
ing the alloy after casting will
Yield
220
241
prevent the ordering of both
Strength ( MPa)
Au-Cu and Pd-Cu regions.
Elongation
(%)
from:
12
15.5
12.8
Bertolotti. J Calif Dent Assoc 1983,11(8):37-43. The AD A
gold is B-2, J.M. Ney Co. The low gold is Midas, Jelenko
Dental Health Products.
Tarnish Resistance.
Laboratory measurements
suggest that many of the low
gold alloys might tarnish in
the mouth. Especially suspect
are those alloys that contain
Revised 11/9/09
Casting Alloys
Oral Biomaterials - Chapter 6
Silver and palladium form solid solution
alloys (single phase alloys) at all compositions.
However, alloy additions such as copper, zinc
and indium will form two-phase alloys. These
alloys are added to the Ag-Pd for strengthening and to lower the fusion temperature.
(Here the goal is lower the fusion temperature
enough so that gypsum-bonded investments
can be used.) Table 8 gives the compositions
of several Ag-Pd alloys.
relatively large amounts of silver and small
amounts of palladium. However, in an eightyear study of over 264 crowns made with fifteen low gold alloys, no difference was detected in the tarnish of the low gold alloy crowns
and crowns made with an Type III high-gold
alloy (Sturdevant JR et al. Dent Mater
1987:3:347-352). Few of the restorations
showed more than slight tarnish.
Silver-Palladium Alloys
Role of copper. Copper is an example of
an element that will lower the fusion temperature. Unlike pure Ag-Pd alloys, Ag-Pd-Cu
alloys can be hardened. In quenched Ag-PdCu alloys, ordered Cu-Pd regions will form
when these alloys are heated below 400oC.
The precipitation of other phases from the
supersaturated solid solution may also play a
role in hardening some alloys.
According to the ADA's classification (Table
3), these alloys are "noble" by virtue of the
more than 25% palladium they contain. The
palladium is added to the silver to change the
normally corrosion prone silver to an alloy
that is corrosion resistant in the mouth. If too
much palladium is added, the fusion temperature of the alloy is raised significantly.
Table 8
Typical Compositions (wt. %)
of Ag-Pd Bare Metal Alloys
Class
Alloy
Ag
A
58.5
B
Pd
Au
Cu
In
Zn
25
---
14.5
---
2.3
55.4
24.5
5.3
13.5
---
---
C
68
26
---
---
3.0
---
D
54
40
---
---
5.0
2.3
Ag-Cu-P d
Ag-P d-Cu-Au
Au-P d-In
A.
Elektra, Williams Gold Refining Co., Buffalo, N Y1
B.
Hvitstop , K. A. Rasmussen , Hamar, Norway. 2
C.
Albacast, J.F. Jelenko & Co., Armonk, New York 3
D.
Alba V, Heraeus Dental Products, Queens Village, New York 3
Compositions from:
1
Naylor & Young. Non-gold Base Dental Casting Alloys: Vol. I,
USAF School of Aerospace Medicine, 1984.
2
Niemi L, Hero H. J Dent Res 1984:63:149-154.
3
Bessing C, Bergman M. Acta O dontol Scand 1986:44:101-112.
Revised 11/9/09
Grain boundary phases. In copper-containing AgPd alloys, copper-rich phases form at grain boundaries.
Similarly, in alloys enriched
in zinc or indium, phases
rich in these elements are
found at grain boundaries.
The significance of these
grain boundary phases is
that, because of their base
metal content, they are
potential sites for corrosion.
Castability. The results
of several studies and word
of mouth from laboratory
technicians suggests that
there may be a problem with
casting Ag-Pd alloys. A
recent study measured the
sharpness of margins of
crowns cast with Type III
high gold, Type III low gold,
and Type III silver-palladi13
Casting Alloys
Oral Biomaterials - Chapter 6
um alloys (Bessing C. Acta Odontol Scand
1986:44:165-172). No difference was found
between the sharpness of margins of the highgold alloy and two low gold alloys. However,
margins of castings made with either of two
Ag-Pd alloys were significantly less sharp. It
appears that using current casting techniques
that these alloys are less castable than either
high or low gold alloys.
conversion between pennyweights and
grams.
5.
correct conversions between percent gold
in a gold alloy and both fineness and the
number of carats.
6.
elements which when added to high-gold
alloys produce solid solution hardening.
7.
the elements responsible for order hardening of high-gold alloys.
8.
cooling and/or heating schedules that will
either harden or soften Type III high- gold
alloys that contain copper.
9.
the three elements that in the ADA
Classification are considered to be noble
elements.
VII. Predominantly Base Metal Alloys
Silver-Indium Alloys
These white alloys contain no noble elements. 25% indium is added to silver. Indium
forms a surface oxide that protects this alloy
from corrosion. This alloy has two disadvantages: 1) low ductility, making castings difficult to burnish and 2) low fusing temperature,
which means that a low fusing solder must be
used. Recall that low fusing solders contain
large percentages of base metals and, consequently, tarnish and corrode intraorally.
Nickel-Chromium Alloys
Nickel-chromium alloys are usually used as
ceramic-metal alloys. They can also be used
for bare metal restorations. These alloys are
discussed in the chapter on ceramic-metal
alloys.
VIII. Behavioral Objectives
You will be able to select from a list of
choices:
1.
2.
3.
4.
14
reasons why inlays and 3/4 crowns are
infrequently used in dentistry today.
characteristics of an alloy which is passive
to tarnish and corrosion.
two mechanical properties that are used in
ANSI/ADA Specification No. 5 for dental
casting alloys.
correct conversions of troy ounces to pennyweights and (approximately) the correct
10. the noble element content and the gold
content of the alloys classified as "high
noble", "noble" and "predominantly base"
by the American Dental Association.
11. the approximate weight percent gold, copper, palladium and silver in high gold and
low gold alloys.
12. two elements, which when added to gold
alloys in minute quantities, decrease in
grain size of gold alloys.
13. correct statements about the possibility of
order hardening silver-palladium binary
alloys.
14. the three metals regarded as noble metals
in the ADA classification of casting alloys.
15. reactions that occur at cathodes and
anodes during galvanic corrosion.
16. dental applications for each of the alloy
types classified in ADA SCDP Specification No. 5.
17. correct statements about how yield
strength and minimum percent elongation
Revised 11/9/09
Casting Alloys
Oral Biomaterials - Chapter 6
vary from Type I to Type IV alloys in ADA
SCDP Specifi-cation No. 5.
18. the effect of copper and palladium additions on the color of gold alloys.
19. correct statements about the in vivo tarnish resistance of low gold casting alloys
20. the function of indium in silver-indium
casting alloys.
21. the percentage of palladium in silver-palladium casting alloys
22. correct statements about how alloy hardness and ductility affect burnishability.
23. two characteristics of an oxide that will
passivate an alloy.
24. correct statements about the relative yield
strengths and minimum elongations that
alloys must meet to be classified as Type
I, II, III, or IV by the ADA SCDP Specification No. 5 for dental casting alloys.
high-gold alloy
homogenization
low-gold alloy
metal-ceramic alloy
noble alloy
nonprecious alloy
ordered alloy
order hardening
passivated
passivation
passive
polarization
precious alloy
predominantly noble alloy
tarnish
Type III casting alloy
solid solution hardening
You will be able to select the correct definition
or description of the following terms, or, given
the definition or description, select the correct
term.
annealing
anode
base metal
burnishability index
carat
cathode
chemical corrosion
composition cell
concentration cell
coring
corrosion current
electrochemical corrosion
fineness
galvanic corrosion
hardening
heat treatment
Revised 11/9/09
15