Note: Descriptions are shown in the official language in which they were submitted.
~7988
Background of the Invention
Dental restorations such as crowns or artificial teeth
have traditionally been made by firing porcelain on a cast
body of precious metal such as an alloy of gold. The physical
properties of these precious alloys are well understood in
dentistry, and the alloys bond properly with porcelain and are
compatible for use in the mouth. Gold alloys are easy to melt
and cast, are sufficiently ductile to permit burnishing of
casting margins in the finishing of dental restorations, and
can be polished to a high luster to resist plaque formation.
Alloys of precious metals, however, are relatively heavy,
and have increased in cost to such an extent that substitute
materials have been sought in recent years.
It is now known that certain stainless alloys of non-
precious metals can be used in dentistry, and examples of
specific nickel alloys and processing techni~ues are set forth
in United States Patents 3,716,418, 3,727,299, 3,749,570 and
3,761,728. These nickel alloys have a higher modulus of
elasticity than that of precious metal alloys, contributing
to better sag resistance of the ceramo-metal structure after
repeated firings in a furnace.
The higher strength of nickel alloys enables use of
thinner castings which minimize reduction of the natural
tooth structure in preparation for installation of the
restoration. Nickel-alloy restorations are also light in
weight and low in thermal conductivity, and these features
provide greater comfort to a patient with a sensitive or
deeply involved tooth. These alloys bond satisfactorily to
porcelain, and further have the important economic advantage
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l of being significantly lower in cast than gold or other
precious-metal alloys.
There are various shortcomings of known nickel alloys.
For example, these alloys are difficult to finish and polish,
thereby requiring more dental laboratory time as compared
to precious-metal alloys. Bond strengths of nickel alloys
to dental porcelains are sensitive to the necessary repeated
firings in laboratory manipulation and preparation, and this
factor can affect clinical performance of the porcelain and
nickel-alloy system. It is desirable, therefore, for a
nickel alloy to have a high porcelain-metal bond strength
to be compatible with the laboratory processing involved in
making a restoration. Another problem is that slags of known
nickel alloys tend to adhere to clay crucibles used to melt
the alloy prior to casting. It takes time and effort to
grind or chip off these tenacious slags to avoid possible
contamination of other alloys during subsequent casting.
It is believed that these and other problems arise
from the use in known alloys of elements such as beryllium,
tin, silicon, gallium and boron which are added for improved
melting and casting performance. In contrast to precious-
alloy ingots which melt into a pool with little or no slag,
prior-art nickel-alloy-ingots tend to form into an individual
molten mass covered by a thick slag when melted by a torch.
This problem is at least partially controlled by use of the
aforementioned elements, but not without incurring other
problems.
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For example, beryllium poses a health risk if not
1 carefully hand~cd duling alloy processing. Alloys containing
significant amounts of silicon and g~llium tend to be brittle,
and have an as-cast elongation of only about 2% due to the
formation of intermetallic compounds. Alloys of this type
must be heat~treated at 1800F for about 30 minutes, ~ollowed
by slow cooling in air, to provide sufficient ductility for
margin burnishing, and the increased labor cost arising from
this processing tends to cancel the reduced cost of a non-
precious alloy. Some other alloys exhibit satisfactory
ductility (over 5% elongation as cast), but microscopic
carbides and intermetallics in the alloy result in more
difficult and ~ime-consuming shaping and polishing as
compared to castings of precious alloys.
The new alloy of this invention overcomes these dis-
advantages of known nickel alloys, while maintaining theadvantages of these materials as described above.
Summary of the Invention
This invention relates to the use of non-precious alloys
characterized by high nickel-chrome content, and by the
inclusion of molybd4num, columbium plus tantalum, at least
one element selected from the rare-earth family, and other
lesser components to provide thermal-expansion characteristics
closely matched to commercially available dental porcelains,
excellent bonding characteristics to these porcelains, good
2 ductility, and easy ~haping and polishing characteristics.
~147988
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Thus the present invention provides a non-precious
metal base for a dental restoration, comprising:
a body of a stainless metal alloy configured for
intraoral installation, the alloy comprising about:
58 to 68 percent nickel, 18 to 23 percent chromium,
6 to 10 percent molybdenum, 1 to 4 percent columbium plus
tantalum, and 0.01 to 5 percent of at least one rare-earth
element selected from the group consisting of lanthanum, cerium,
praseodymium, neodymium, samarium, gadolinium, and dysprosium,
all by weight.
D~scri tion of the Preferred Emobdiments
P
The non-precious dental alloy of one aspect of
this invention has the following elemental components (percen-
tages are by weight):
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1 Element Acceptable Range (%)
Nickel 58 - 68
Chromium 18 - 23
Molybdenum 6 - 10
Columbium plus
5 tantalum 1 - 4
One or more rare-earth
elements 0.01 - 5
Iron 0.02 - 2
Silicon 0.01 - 0.5
10 Manganese 0.01 - 0.4
Titanium 0.01 - 0.2
Aluminum 0.01 - 1.0
Carbon 0.01 - 0.1
The relatively high chromium content of the alloy,
and the use of molybdenum,provide satisfactory corrosion
resistance when the alloy is exposed to mouth fluids. The
ranges specified for other elemental components are important
to insure proper formation of carbide in the alloy (tantalum,
columbium, titanium and chromium), for precipitation of the
gamma-prime phase (aluminum and titanium), and for solid-
solution hardening (molybedenum), these factors all contributing
to strength and desired ductility of the final cast alloy.
Nickel, chromium and molybdenum are the primary determinants
of the thermal-expansion properties of the alloy, though the
other components play some role in this characteristic. One
or more rare-earth elements (defined as those ele~ents with
atomic numbers from 57 through 71 in the periodic chart of
elements3 and the use of aluminum, contribute to the ease of
shaping and polishing the alloy, and provide good melting and
casting characteristics. Beryllium and tin are avoided in
formulating the alloy.
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124'':RRP ~1479~8
1 The componcnts are ~lloyed by induction melting under
argon, and rare-earth elements are the last addition to the
melt. The molten alloy is cast into small slugs or pellets
for convenient remelting when the alloy is subsequently cast
into a dental prosthesis.
Conventional techniques are used to make a finished
dental restoration with the alloy. A ceramic ~old is prepared
using the usual lost-wax or burnout-plastic methods. The
alloy is then melted (a torch fed with propane at 10 psi
and oxygen at 20 psi is used to achieve a 2360 - 2450F
melting range of the alloy) and poured in the mold which is
mounted in a centrifugal casting machine. After cooling, the
mold is broken away and the casting is cleaned, trimmed,
polished and finished in preparation for application of
porcelain by the usual firing techniques.
Polishing of the alloy is done with conventional
equipment such as a Shofu Brownie and Greenie rubber wheel.
The casting is brought to a high luster with an Abbott-
Robinson brush (used with polishing compound) and Black's
felt wheels impregnated with tin oxide.
The alloys of this invention are particularly well
matched to the thermal properties of commercially available
dental porcelains such as distributed by Vita Zahnfabrik
under the trademark VMR-68, distributed by Dentsply
International, Inc. under the trademark BIOBOND and
those distributed by Ceramco Division of Johnson & Johnson.
These above-named dental porcelains generally form a strong
bond to the metal alloy casting of the present invention.
3~
12 l:~RP ~ ~ ~7988
1 The alloys are also useful in making removable dental
appliances such as orthodontic retainers. The relative
softness of the alloys avoids surface damage to natural
teeth over which the appliance is fitted, and the alloys
are sufficiently ductile to permit hand shaping for interim
or final alignment of the appliance. Utility of the invention
is thus not limited to appliances on which porcelain is fired.
Strength, elongation and modulus of elasticity are
tested by using an Instron tensile instrument. Vickers
hardness is obtained by testing specimens of the alloy with
a microhardness tester with diamond indenter. Thermal
expansion coefficients are measured with a dilatometer.
These tests and instruments are well known to those skilled
in the art.
Typical properties of the alloy of this invention as
cast are as follows:
~ltimate tensile strength 75,000 psi
Yield strength (0.2~ offset) 54,000 psi
Modulus of elasticity 27 X lO psi
Elongation 8 percent
Vickers hardness 200
Thermal expansion coefficient 14 -6 -l
The following examples further illustrate the invention
and some of the tests which have been made in evaluatin~ the
invention, and are not intended to be limiting. The figures
shown are element percentages by weight.
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12~ :R~P 1147988
1 Example 1 Example 2 Example 3
Nickel 63.06 60.54 62.80
Chromium 21.60 20.74 21.76
Molybdenum 8.40 8.06 8.45
Dysprosium 1.00 5.00 0
Neodymium 0 0 1.00
Columbium plus
Tantalum 3.80 3.64 3.85
Iron 1.25 1.20 1.25
Silicon 0.35 0.33 0.34
Manganese 0.28 0.27 0.27
Aluminum 0.10 0.10 0.12
Titanium 0.10 0.07 0.10
Carbon 0.06 0.05 0.06
Alloys of Examples 1 through 3 melt similarly to precious alloys,
and form only very thin layers of oxide which cover the molten
alloy pool. These alloys are easy to shape and to polish, and
exhibitgood ductility forburnishing the margins of castings.
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1 Ex~lml)lc 4 Exam~lc 5 Example 6
Nickel 60.62 62.37 64.18
Chromium 21.12 20.87 21.22
Molybdenum 8.12 8.26 8.34
Samarium 5.00 0 0
Praseodymium 0 3.00 0
Gadolinium 0 0 0.50
Columbium plus
Tantalum 3.11 3.42 3.71
Iron ~.23 1.24 1.20
Silicon 0.32 0.32 0.30
Manganese 0.25 0.28 0.29
Aluminum 0.10 0.11 0.13
Titanium 0.08 0.07 0.09
Carbon 0.05 0.06 0.04
Alloys of these examples are melted easlly and are ductile.
These alloys are not shaped and polished as easily as alloys
of Examples 1 through 3.
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1 E~mple 7 Example 8 Example 9
Nickel 60.01 62.99 62.46
Chromium 21.00 21.60 21.63
Molybdenum 8.20 8.40 8.40
Cerium 2.50 0.50 0.50
Lanthanum 1.50 0.50 0.50
Neodymium 0.70 0
Praseodymium 0.30 0 0
Tin 0 0 0.60
10 Columbium plus
Tantalum 3.72 3.80 3.80
Iron 1.23 1.25 1.25
Silicon 0.32 0.35 0.35
Manqanese 0.26 0.28 0.28
Aluminum 0.12 0.17 0.07
Titanium 0.09 0.10 0.10
Carbon 0.05 0.06 0.06
Alloys of Example 7 through 9 are ductile. Alloys of Example
7 and 8 are very easy to shape and polish. The alloy of
Example 9 which contains tin is difficult to shape and polish.
Alloys of Example 7 through 9 when melted form a molten mass
covered by a somewhat thicker oxide as compared to alloys
of Example 1 through 3.
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