NOBLE ALLOY

FIELD OF THE INVENTION

[0001] This invention concerns a noble alloy containing at least 25 wt.% gold and/or platinum group elements, from 15 wt.% to 30 wt.% chromium, at least 5 wt.% molybdenum/tungsten content and a balance of nickel.

BACKGROUND OF THE INVENTION

[0002] Dental alloys employed in the porcelain-fused-to-metal processing technique may be classified into several groups-, gold based; palladium based; cobalt based and nickel based. The cost of the alloy is dependent upon the commodity prices of the alloy components. For example, as of October 2008, the costs of the major components of such alloys were: gold $852 per Troy ounce, palladium $2.05 per Troy ounce; cobalt $1 per Troy ounce; and nickel $0.3 per Troy ounce. The economic advantage of the base metals cobalt and nickel is obvious, but the functional characteristics of the base metal alloys do not compare with those of the gold or palladium based dental products. In general, the base metal alloys are more difficult to cast, grind and bond to porcelain. [0003] There have been numerous attempts to improve the functional characteristics of cobalt and nickel alloys through the addition of gold and the platinum group metals (the platinum group metals comprise platinum, palladium, rhodium, iridium, osmium and ruthenium). [0004] Exemplary US patents describing such dental alloys include:

[0005] In each case, some improvement in the functional characteristics of the base metal alloy is achieved through the addition of gold and the platinum group metals. This invention expands the effort to improve the base metal based alloys through the judicious use of palladium additions in higher amounts than used in previous alloys.

SUMMARY OF THE INVENTION

[0006] Thus, there is provided in practice of this invention according to a presently preferred embodiment, a workable noble alloy that can be used in dental applications comprising from 15 to 30 wt.% chromium, at least 25 wt.% palladium, at least 5 wt.% molybdenum and/or tungsten, and where the principal balance of the alloy, absent small contributions from additives, is nickel.

[0007] In another embodiment of the invention the alloy may include up to 1.5 wt.% silicon.

[0008] In still another embodiment of the invention the alloy may include up to 10 wt.% of metal selected from the group consisting of niobium, tantalum and rhenium.

[0009] In yet another embodiment of the invention up to 5 wt.% of iron or cobalt- may be included in the composition.

[0010] In still yet another embodiment of the invention the alloy composition comprises 25 wt.% palladium; 25 wt.% chromium; 0.5 wt.% silicon; 12 wt.% of one of

either molybdenum and/or tungsten; 6 wt.% of at least one material selected from the group of niobium, tantalum and rheniu,m; and a balance of nickel.

[0011] In still yet another embodiment the invention is directed to a dental product formed using the alloy described above.

[0012] In still yet another embodiment the invention is directed to a method of manufacturing a dental product using a technique selected from casting, molding, milling or laser sintering.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The description and claims of the current invention will be more fully understood with reference to the following data table, which presents exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:

[0014] FIG. 1 provides a table providing exemplary alloy compositions in accordance with the current invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] A noble alloy (as defined by the American Dental Association] is considered to be one with at least 25 wt.% noble metal content, the noble metals include ruthenium, platinum, palladium, iridium, osmium, rhodium and gold. The alloy provided herein is noble, but is considered to be a nickel based alloy since a high proportion of this base metal is in the alloy. The alloy has more than 25 wt.% palladium, at least 15 wt.% chromium, at least 5 wt.% molybdenum and/or tungsten, and a balance of nickel. It should be noted that unless otherwise indicated all percentages herein are by weight. [0016] The choice of palladium has both metallurgical and economic benefits. For example, consider the price of gold and the platinum group metals as of October 2008:

Rhodium $3,960

Platinum $995

Gold $852

Iridium $455

Ruthenium $290

Palladium $205

Palladium has a lower cost relative to the other platinum group metals so there is an economic advantage to maximize the palladium content in place of gold and the other platinum group elements.

[0017] From a metallurgical perspective, palladium usually substitutes for other materials in nickel based alloys, such as for example molybdenum and chromium.

Palladium acts as an alloy strengthener, is a thermal expansion adjuster for the alloys

(to better match thermal expansion of dental porcelains) and reduces the alloy's oxidation rate. However, thus far a suitable nickel based dental alloy having such a high content of palladium has not been proposed. The current invention identifies a nickel based alloy having high palladium content suitable for dental applications that defies conventional formulations by maintaining a high percentage of molybdenum and/or tungsten in the material.

[0018] Both palladium and chromium protect the alloy from corrosion and oxidation.

The palladium apparently enobles the alloy, as opposed to the chromium that forms an oxide to protect the alloy from adverse reactions.

[0019] The invention also incorporates the addition of at least 5 wt.% molybdenum and/or tungsten to further improve the oxidation resistance of the alloy and to adjust the thermal expansion properties of the alloy in order to make them compatible with porcelains.

[0020] Alloys suitable for practice of this invention comprise at least 25 wt.% palladium; from 15 to 30 wt.% chromium; at least 5% molybdenum and/or tungsten; up

to 1.5 wt. % silicon; optionally up to 10 wt.% of metal selected from the group consisting of niobium, tantalum and rhenium; and a balance of nickel.

[0021] Although pure nickel alloys are discussed above, it should be understood that mixtures of cobalt, nickel and iron may be used since from a metallurgical perspective they may be considered equivalents when the palladium content is as high as proposed in the current invention.

[0022] Although chromium may be present in the alloys in concentrations as high as

30 wt.%, lower concentrations are preferred since these materials are easier to cast and they produce less slag upon melting.

[0023] Thus, in a preferred embodiment, the alloy is a nickel-base alloy with a principal addition of palladium of more than 25 wt.%, and preferably approximately 25 wt.%; from 15 to 30 wt.% chromium, and preferably 25 wt.%; and at least 5 wt.% molybdenum and/or tungsten, and preferably 12 wt.%. The amount of palladium in the alloy may be more than 25 wt.%, up to as much as 45 wt.%, but there is no economic advantage to using these higher concentrations of the costly material.

[0024] , Optionally, the alloy may also contain up to about 1.5 wt.% silicon, and up to about 10% of niobium, tantalum and/or rhenium may also be added to the alloy for grain refinement. Finer grain castings are more readily ground to a smooth finish suitable for covering with dental ceramics. Interestingly, iron alloys are easier to grind and finish than similar alloys with cobalt.

[0025] In light of the above discussion, an example of an alloy of this invention with palladium as the only noble metal may be summarized as follows:

Niobium + Tantalum + Rhenium O 10 if present

[0026] In addition to the main components, the alloys of the current invention may also contain concentrations of other additives to improve specific properties. For example, small concentrations (up to -5 wt.%] of gallium, boron, aluminum, germanium and cerium can serve to deoxidize, lower the melting range, and improve the castability of the alloys. Specifically, the addition of gallium can lower the melting range of the alloy so that the material can be cast with a gas-oxygen torch. Alternatively, small boron additions can also be used to improve the alloy's thermal expansion and castability. However, it should be understood that these additives are not essential to the practice of the current invention. For example, if the alloy is to be cast by induction heating, then the melting range can be higher eliminating the need for any of these additives.

[0027] A number of exemplary alloy compositions in accordance with the current invention were prepared and their properties tested. These materials, and their physical properties, are listed in the table provided in FIG. 1.

[0028] It is appreciated that the above compositions are suitable for use with dental appliances, but are not to be considered exclusive. Those of skill in the art will be aware that some of the materials can be substituted or additional materials may be added without altering the key properties of the alloys of the current invention. For example, it is well known that small amounts of palladium can be substituted with copper, nickel and iron. Alternatively, small concentrations (less than 5 wt.%) of these materials may also be added or be found in the alloy as impurities without affecting the properties of the overall composition.

[0029] Although the above description has focused on a range of compositions for alloys of the current invention suitable for use in, for example, dental applications, the invention is also directed to dental products made from the alloys and to methods of

manufacturing dental products from the alloys. In general, such methods will include the steps of providing an alloy having a composition in accordance with the above description and then shaping the dental product with that alloy using any suitable means. In this regard, the alloy of the instant invention allows for the use of a number of conventional shaping techniques, such as, casting and molding. Moreover, the alloys of the current invention also allow for the use of more recent advances in shaping technologies, such as, for example, CAD/CAM milling and selective laser sintering. It should be understood that any of these techniques or a combination thereof may be used with the alloy of the current invention.

[0030] Specifically, despite the high hardness value the alloy may be ground using traditional dental laboratory grinding media making it especially suited for use with newer CAD/CAM and powder metallurgical applications where no casting is required. In one such technique, substrates or final restorations can be milled from blocks made from these alloys. As powders, these alloys can be used either to create three dimensional performs utilizing appropriate binders and then be sintered, or can be directly sintered/melted such as for example, with a laser, to create substrate or final restoratives. Exemplary disclosures of such processes can be found, for example, in U.S. Patent Nos. 7,084,370 and 6,994,549, the disclosures of which are incorporated herein by reference. It should be understood that while some prior art laser sintering techniques specify a specific range of useable alloy particulate sizes, the alloys of the current invention are contemplated for use in laser sintering techniques over all possible particulate size ranges.

[0031] Those skilled in the art will appreciate that the foregoing examples and descriptions of various preferred embodiments of the present invention are merely illustrative of the invention as a whole, and that variations in the relative composition of the various components of the present invention may be made within the spirit and scope of the invention. For example, it will be clear to one skilled in the art that typical

impurities and/or additives may be included in the compositions discussed above that would not affect the improved properties of the alloys of the current invention nor render the alloys unsuitable for their intended purpose. Accordingly, the present invention is not limited to the specific embodiments described herein but, rather, is defined by the scope of the appended claims.