CHROMIUM

FIELD OFTHE INVENTION

[0001] This invention concerns a noble alloy suitable for dental purposes that contains cobalt and chromium in addition to ruthenium, and is non-magnetic.

BACKGROUND OFTHE 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. Typically, preferred alloy compositions have been tied to the cost of the alloy components. To this end, the economic advantage of the cheaper base metals such as 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). Exemplary US patents describing such dental alloys include:

up to 2 wt.% ruthenium, the balance cobalt.

Prasad 6,756,012 Describes a cobalt chromium alloy that may contain up to

20 wt.% platinum or palladium, up to 10 wt.% gold and up to 6 wt.% ruthenium

Cascone 7,794,652 Describes a cobalt chromium alloys that contains at least

15 wt.% ruthenium, at least 4-0 wt.% cobalt and from 5 to 15 wt.% gallium

[0004-] 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. However, recently it has been discovered that many of these alloys have magnetic properties. In the past, such magnetic properties have not posed a significant problem. However, the growing importance of magnetically based imaging techniques has put the issue of dental appliances made from strongly ferromagnetic materials in the spotlight, because the presence of such strongly ferromagnetic materials in a patient can interfere with obtaining clear images from such techniques.

[0005] One of the first attempts to describe a true ^" non-magnetic ^"' dental alloy family was recently submitted by the assignee of the present application, The Argen Corporation (U.S. Pat. Pub. No.2008/0232998). Unfortunately, the alloys disclosed in this publication require the use of palladium, which is a relatively expensive noble metal. Accordingly, it would be desirable to find a similarly non-magnetic family of noble dental alloys comprising the less expensive noble materials, such as, for example, ruthenium.

SUMMARY OFTHE INVENTION

[0006] Thus, there is provided in the practice of this invention according to a presently preferred embodiment, a workable noble alloy that is non-magnetic and can be used in dental applications. The noble alloy according to the invention comprises: (i) at least 25 wt.% ruthenium; (ii) between 15-35% chromium; (iii) thermal and mechanical property modifiers selected from the group of Ga, Ge, Si, B, In, Sn, Al and Rare Earths metals in amounts sufficient to result in an alloy that is non-magnetic and having a Lower, processable Liquidus temperature (preferably below 1 600 C° , and more preferably below 14-50 C°) ; (iv) and the balance cobalt.

[0007] In one such embodiment, the non-magnetic cobalt based dental alloy includes:

• at least 25 wt.% of Ru;

• from 1 5 to 35 wt.% Cr;

• from 30 to 50 wt.% Co; and

• at least 5 wt.% Ga;

• wherein where the concentration of Ga is from 5 to 1 0 wt.%, the alloy further contains a sufficient concentration of up to 5 wt.% of at least one modifier selected from the group consisting Ge, Fe, B, In, Sn and Re such that the liquidus temperature of the alloy is below 1600 C° ;

• wherein where the concentration of Ga is greater than 1 0 wt.%, the concentration of Co is at least 35 wt.%, the concentration of Cr is less than 25 wt.%, and the alloy further contains B in a concentration of up to 1 wt.%; and

• wherein the alloy is non-magnetic.

[0008] In one such embodiment, the concentration of Ga is less than 10 wt.% and the concentration of Ru is from 25 to 45 wt.%, and the ratio of Co to Ga is 4 to 1 or less.

[0009] In another such embodiment, the concentration of Ga is less than 1 0 wt.%, and the alloy further contains at least 1 wt.% Re.

[001 0] In still another such embodiment, the concentration of Ga is less than 10 wt.% and the total concentration of Ga and Re is at least 10 wt.%.

[001 1 ] In yet another such embodiment, the at least one modifier material is selected from the group consisting of up to 3 wt.% silicon, up to 1 wt.% boron, up to 3 wt.% aluminum, up to 3 wt.% germanium, and up to 1 wt.% cerium. [0012] In still yet another such embodiment, the alloy further comprises less than 5 wt.% of at least one trace additive selected from the group consisting of copper, nickel and iron.

[0013] In still yet another such embodiment, where the concentration of Ga is from 5 to 10 wt.%, the ratio of Co to Ga is greater than 4- to 1.

[0014] In still yet another such embodiment, the alloy is a composition selected from the group consisting of Co4oCr27.5Ru25Ga7.5, Co3sCr3oRu25Ga7, Co4iCr25Ru25GasGei, Co35Cr25Ru3oGaio, Co4oCr25Ru25Ga5Re5, and Co37.5Cr3oRu25Ga7Bo.5. The suffixes represent weight % of the elements in the alloys and not atomic fractions.

[0015] In still yet another such embodiment, the alloy has a thermal expansion coefficient within the range of from about 9 to about 18 x 10A

[0016] In another embodiment of the present invention, the concentration of Ga is greater than 10 wt.%, and the concentration of B is from 0.15 to 0.55 wt.%.

[0017] In still yet another embodiment, the concentration of Ga is from 10 to 11.5 wt.%, the concentration of B is from 0.15 to 0.55 wt.%, the concentration of Co is at least 37 wt.%, and the concentration of Cr is from 22 to 25 wt.%.

[0018] In still yet another such embodiment, the alloy composition is selected from the group consisting of Co40Cr24.35Ru25Ga10.5B0.15, Co40Cr23.35Ru25Gan.5B0.15,

Co40Cr23.95Ru25Ga10.5B0.55, Co37.5Cr22.95Ru25Ga11.5B0.35, and Co4oCr23.5Ru25GaiiBo.5. The suffixes in these alloys represent weight % of the elements in the alloys and not the atomic fractions.

[0019] In still yet another embodiment of the present invention, the concentration of Ga is from 5 to 10 wt.%, the concentration of Co is at least 35 wt.%, and the concentration of Cr is from 25 to 30 wt.%.

[0020] In still yet another embodiment, the alloy composition further comprises up to 10 wt.% of an additive selected from the group consisting of W, Ta, Nb, Re, Mo and V..

[0021] In another aspect, the invention is directed to a dental product formed using the alloys described above. [0022] In yet another aspect, the invention is directed to a method of manufacturing a dental product formed using the alloys described above and using a technique selected from casting, molding, milling or laser sintering.

BRIEF DESCRIPTION OFTHE DRAWINGS

[0023] The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:

[0024] Fig. 1 provides a data graph showing the magnetic properties of an exemplary alloy in accordance with an embodiment of the invention;

[0025] Fig. 2 provides a data graph showing the magnetic properties of other exemplary alloys in accordance with an embodiment of the invention;

[0026] Fig. 3 provides a data graph showing the magnetic properties of other exemplary alloys in accordance with an embodiment of the invention; and

[0027] Fig. 4- provides a data graph showing the magnetic properties of other exemplary alloys in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OFTHE INVENTION

[0028] 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 consisting of ruthenium, platinum, palladium, iridium, osmium, rhodium and gold. The alloy provided herein is noble, but is considered to be a cobalt-chromium based alloy since a high proportion of these base metals are in the alloy. It should be noted that unless otherwise indicated all percentages herein are by weight.

[0029] The choice of ruthenium as a major additive has both metallurgical and economic benefits. For example, consider the approximate price/troy ounce of gold and the platinum group metals as of December 2011: Rhodium $ 1 500

Platinum $ U30

Gold $ 1 600

Iridium $ 1 050

Ruthenium $ 1 20

Palladium $660

Ruthenium is one of the lower cost metals relative to the other platinum group metals so there is an economic advantage to maximize the content of ruthenium in place of gold and the other platinum group elements.

[0030] From a metallurgical perspective, ruthenium may substitute for other materials in cobalt, nickel and iron based alloys, such as for example molybdenum, tungsten, and, to a certain extent, chromium. Ruthenium 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. Both ruthenium and chromium protect the alloy from corrosion and oxidation. The ruthenium enobles the alloy, due to its non-reactive nature as opposed to the chromium that forms an oxide to protect the alloy from adverse reactions.

[0031 ] As discussed in the background, alloys incorporating elevated ruthenium concentrations have been proposed in the past, such as, for example, U.S. Patent No. 7, 794-, 652, which describes a family of highly processable cobalt-chromium based ruthenium containing noble dental alloys. While this patent focuses on maximizing the processability of these ruthenium containing cobalt-chromium alloys, no guidance is provided on how to ensure that the alloys are not strongly- ferromagnetic. Table 1 , below, provides a number of exemplary alloys meeting the requirements of the prior art that were found to have strong magnetic properties.

[0032] The current invention addresses the issue of minimizing the magnetic properties of ruthenium-containing cobalt-chromium alloys by carefully balancing the concentrations of the base metals and through the judicious use of select additives. It should be understood that though truly non-magnetic, or paramagnetic alloys are preferred, in the following description of this invention the term ^" nonmagnetic alloy ^" refers to an alloy that is either paramagnetic or only weakly ferromagnetic, as shown in the attached data graphs.

[0033] The current invention is based upon the discovery that by carefully balancing the relative proportions of cobalt, gallium and modifiers, it is possible to control the magnetic properties of ruthenium containing noble cobalt chromium alloys and obtain processable alloys with non-magnetic properties. The challenge to the metallurgist in composing such an alloy is in finding the balance between processability on the one hand and non-magnetism on the other. As described above in the Background, previously it was thought that ruthenium containing cobalt- chromium alloys required a minimum concentration of gallium and cobalt to ensure that the processing characteristics of the alloys (such as liquidus temperature) would be such as to allow them to be used in the construction of dental appliances. However, it has now been discovered that when such alloys have a gallium content above about 10 wt.% they may exhibit strong ferromagnetism upon slow cooling. As discussed above, such strong ferromagnetism is an undesirable feature for dental prostheses and appliances. Reducing the gallium content below 1 0 wt.% eliminates this problem, but also lowers the thermal expansion coefficient of the alloy. This may restrict the alloy ^' s thermal compatibility with some popular dental porcelain brands.

[0034] The alloys of the current invention address both factors— processability and magnetic properties-- by judiciously manipulating the content of cobalt, chromium, gallium, and certain additives. In particular, there are two relevant compositional regimes for such ruthenium containing cobalt-chromium alloys: high gallium content alloys (greater than 10 wt.%) ; and low gallium content alloys (from 5 to 1 0 wt.% gallium). In the high gallium content alloys, it is necessary to include boron and carefully control the content of cobalt in the alloy to ensure that the alloy retains its non-magnetic properties. In the low gallium alloys, the cobalt content may be reduced and boron eliminated, but in turn it is important to monitor the ratio of cobalt to gallium, and, in some alloys to include sufficient concentrations of certain specific additives to ensure that the processability of these alloys is maintained.

[0035] More particularly, alloys within the scope of this invention comprise:

• at least 25 wt.% of ruthenium with only minor substitutions of the other noble alloys - platinum, palladium, iridium, osmium, rhodium and gold;

• between 1 5-35 wt.% chromium ;

• from above 30 to 50 wt.% cobalt;

• at least 5 wt.% gallium;

• wherein where the concentration of Ga is from 5 to 1 0 wt.%, preferably a ratio of Co to Ga of at least 4- to 1 , and the alloy further contains up to 5 wt.% of at least one modifier selected from the group consisting Ge, Fe, B, In, Sn and Re where the concentration of the modifier is selected such that the liquidus temperature of the alloy is below 1 600 C° ;

• wherein where the concentration of Ga is greater than 1 0 wt.%, the concentration of Co is at least 35 wt.%, the concentration of Cr is less than 25 wt.%, and the alloy further contains B in a concentration of up to 1 wt.%; and

• wherein where the concentration of Ru is greater than 25 wt.% then the Co to Ga ratio is less than 4- to 1 .

Low Gallium Content Non-Magnetic Alloys

[0036] In some embodiments of the invention, the ruthenium containing cobalt- chromium dental alloy has a gallium content of between 5 and 1 0 wt.%. In such embodiments, the alloy is a cobalt-chromium based alloy comprising at least 25 wt.% ruthenium; a sufficient amount of a modifier selected from Ge, Fe, B, I n, Sn and Re to ensure the material is sufficiently processable; chromium in the range of 15 to 35 wt.%, and preferably 25 to 35 wt.% and more preferably at least 30 wt. % cobalt. Preferably the ratio of cobalt to gallium is greater than 4 to 1 .

[0037] As discussed above, in some of these embodiments of the invention the addition of a sufficient amount of the modifier/s selected from the group of Ge, Fe, B, In, Sn and Re is necessary to obtain thermal expansion properties suitable for the construction of dental appliances. In particular, the combination of high noble content and low gallium concentration in alloys of this type can make it difficult for the alloys to be melted and shaped in the absence of very high temperatures. The addition of appropriate additives in accordance with the present invention lowers the melting temperature of the alloys so that they can be melted with a natural gas- or propane-oxygen torch commonly used in dental laboratories. Accordingly, in such embodiments sufficient modifier is added to reduce the liquidus temperature sufficiently low to allow for the use of inductive heating to shape the material (less than 1600 C°), and preferably below 14-50 C° .

[0038] Accordingly, in these embodiments of the invention (between 5 and 10 wt.%, gallium), the total amount of such additives is at least 5 wt.%, and preferably between 5 to 1 5 wt.%, with least 5 wt.% of the additive being gallium. In such alloys, as discussed, other substitutions may be made and other additives may be included in amounts sufficient to lower the liquidus temperature preferably below at least 1600 C° . Examples of such substitutions and/or modifier additions include increasing the ruthenium content in place of cobalt, or adding one of the modifiers listed from the group of Ge, Fe, B, In, Sn and Re as a substitute in place of or in addition to gallium to ensure a suitable thermal expansion coefficient.

[0039] Table 2, below, provides a listing of non-magnetic chromium-cobalt- ruthenium-gallium alloys according to the invention having between 5 and 1 0 wt.% gallium, and preferably with cobalt to gallium ratios of greater than 4- to 1 , that also demonstrate a liquidus temperature sufficiently low to ensure that the material is workable in dental applications (less than 1600 C° , and preferably less than 1450 C°). As is shown, a Lower thermal expansion may be obtained by the judicious use of the additives Listed. In particular, as shown in Table 1, by adding additives such as rhenium, germanium, iron or boron (Alloys 17 to 27, Table 2 below). (The magnetic behavior of alloy 22 is shown as an example of the non-magnetic behavior of this group of compositions in Fig. 1.) The linear relationship between B and H in the data plot demonstrates that the material has no magnetic response. The suffixes represent weight % of the elements in the alloys and not atomic fraction.

[0040] To demonstrate the criticality of the compositional Limitations, other experiments were conducted to show the Loss of non-magnetic properties if these compositionaL Limitations are vioLated. As discussed with regard to TabLes 1 and 2, above, the importance of the cobaLt and gaLLium ratio was demonstrated by studying the prior art aLLoys, however, it was aLso discovered that there are important Lower (30 wt.%) and upper (50 wt.%) Limits to the amount of cobaLt in the materiaL and a Lower Limit on the amount of gaLLium (greater than 5 wt.%) in the aLLoy, as shown in aLLoys 28 to 34-, beLow, a LL of which showed strongLy ferromagnetic properties during testing.

High Gallium Content Non-Magnetic Alloys

[004-1] As discussed above, in other embodiments of the invention, ruthenium containing cobaLt-chromium aLLoys with high gaLLium concentrations (in excess of 10 wt.%) that show Low ferromagnetism may be obtained. As can be appreciated with reference to Figs.2 to 4-, by adding boron, and by judiciousLy controLLing the reLative concentrations of cobaLt, it is possibLe to obtain compositions that are either paramagnetic, or that show onLy weak or no ferromagnetism.

[004-2] Fig. 2 provides data in connection with a range of aLLoys that were tested showing a fuLL range of magnetic behavior--from strongLy ferromagnetic to paramagnetic. Fig.3 shows those aLLoys (those with Low cobaLt concentrations) that exhibited a strong magnetic response (note the typical ^' S ^' curve in these data pLots, and for a few samples, a hysteresis effect that are indicative of a strong ferromagnetic behavior.) However, note that as the cobaLt concentration increases and the boron is added the compositions Lose their ferromagnetic behavior and become more paramagnetic (i.e. the relation between B and H in the data plots is linear). Finally, Fig. 4- shows those alloys, according to the invention, that were weakly ferromagnetic or paramagnetic. (The change in the y-axis scale should be noted.) These alloys are of particular value since the materials also exhibit the required thermal properties to bond to popular porcelains, yet show little or no ferromagnetic behavior.

[0043] Of importance in these embodiments of the invention is the inclusion of sufficient cobalt, the judicious use of boron, and the careful titration of chromium to render the alloy non-magnetic. With decreasing gallium, as shown in the embodiments above, it is possible to decrease the cobalt concentration and increase the concentration of chromium. As the gallium concentration increases, however, it is necessary to add boron, decrease the concentration of chromium and increase the concentration of cobalt. Thus, by adjusting the compositional contributions of chromium, cobalt and boron, the alloys of the invention can be made paramagnetic or ferromagnetic, even at high gallium content, which has never before been demonstrated.

[0044] Thus, in these embodiments, the alloys of the present invention include cobalt-chromium based alloys comprising at least 25 wt.% of ruthenium; greater than 1 0 wt.% gallium, and preferably from between 10.5 to 1 1 .5 wt.% gallium; a nonzero concentration of up to 1 wt.% boron, and preferably from 0.1 5 to 0.55 wt.% boron; chromium in the range of 20 to 25 wt.%, and preferably 22 to 25 wt.%; and a balance of Co, where the cobalt content is above 35 wt. %.

[0045] Table 4, below, provides a summary listing of the chromium-cobalt- ruthenium-gallium alloys studied. In these alloys, non-magnetic properties were obtained by the judicious use of boron and chromium. I n particular, as shown in Table 2 by including a small amount of boron, and decreasing the chromium concentration, it is possible to obtain alloys with high gallium concentrations (greater than 1 0 wt.%) that consistently demonstrate non-magnetic properties. The suffixes represent weight % of the elements in the alloys and not atomic fractions. Table 4: High Gallium Content Non-Magnetic Alloys

Alloy No. Formulation Magnetic Property

35 C035Cr29.35RU25GaiO.5Bo.15 Ferromagnetic

36 C040Cr23.5Ru25Ga11Bo.15 Paramagnetic

37 C040Cr24.35Ru25Ga10.5Bo.15 Paramagnetic

38 C035Cr28.35RU25Gai1.5Bo.15 Ferromagnetic

39 CO40Cr23.35RU25Gail.5B0.15 Paramagnetic

40 co40cr24RU25Ga10.85B0.15 Paramagnetic

41 co37.5cr22.95RU25Ga11.5B0.35 Paramagnetic

42 C033.5Cr3oRU25GailBo.5 Ferromagnetic

43 C04oCr23.5RU25GailBo.5 Paramagnetic

44 co35cr28.95RU25Ga10.5B0.55 Ferromagnetic

45 co40cr23.95RU25Ga10.5B0.55 Paramagnetic

46 co35cr27.95RU25Ga11.5B0.55 Ferromagnetic

47 co40cr22.95RU25Ga11.5B0.55 Ferromagnetic

General Compositional Considerations

[0046] It should be noted that in the above embodiments the amount of noble metal in the alloy may be greater than 25 wt.%, up to as much as 45 wt.% (as shown in Table 5, below) and still maintain the non-magnetic properties described herein, but there is no economic advantage to using these higher concentrations of the costly material. In addition, although the alloys of the invention comprise at least 25 wt.% ruthenium, it will be understood that additions or substitutions of other noble metals selected from the list of ruthenium, platinum, palladium, iridium, osmium, rhodium and gold may be made without fundamentally changing the nature of the material.

[0047] In addition to the main components as described above, the alloys of the present invention may also contain trace concentrations of other additives to improve specific properties. For example, up to about 10% of molybdenum, silicon, vanadium, tungsten, niobium, tantalum and/or the Rare Earths (or appropriate combinations of these elements) to further adjust coefficient of thermal expansion (CTE), to enhance the casting characteristics of the alloy and for grain refinement. Finer grain castings are more readily ground to a smooth finish suitable for covering with dental ceramics. However, it should be understood that these additives are not essential to the practice of the current invention.

[0048] 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, small concentrations (less than 5 wt.%) of other materials may also be added or be found in the alloy as impurities without affecting the properties of the overall composition, some of these include, for example, Al, Si, V, W, Ta, N b, Re, Mo and the Rare Earths.

[0049] 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. I n this regard, the alloys of the present invention allow for the use of a number of conventional shaping techniques, such as, casting and molding. Moreover, the alloys of the 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 alloys of the present invention.

[0050] Specifically, despite their high hardness value, the alloys may be ground using traditional dental laboratory grinding media making them 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.

DOCTRI N E OF EQU IVALENTS

[0051 ] 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.