Patent Grant
7794652
This invention concerns an iron, cobalt and/or nickel based dental alloy containing at least 25% gold and/or platinum group elements with the principal portion of these noble metals being ruthenium.
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 December 2004, the cost of the major components of such alloys are: gold $442 per Troy ounce, palladium $183 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.
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:
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 ruthenium additions in higher amounts than used in previous alloys.
Thus, there is provided in practice of this invention according to a presently preferred embodiment, a dental alloy comprising from 15 to 30 wt % chromium and at least 25 wt % of metal selected from the group of ruthenium, platinum, palladium, iridium, osmium, rhodium and gold, provided that the major proportion of this group or over 15 wt %, whichever is greater, is ruthenium. The principal balance of the alloy is cobalt, nickel and/or iron.
A noble dental alloy is considered to be one with at least 25% noble metal content, the noble metals comprising ruthenium, platinum, palladium, iridium, osmium, rhodium and gold. The alloy provided herein is noble, but is considered to be a cobalt, nickel and/or iron base alloy since a high proportion of these base metals are in the alloy. The alloy has more than 15% ruthenium (all percentages herein are by weight).
The choice of ruthenium has both metallurgical and economic benefits.
Considering the price of gold and the platinum group metals as of December 2004:
Ruthenium is by far the lowest costing element of the group so that is an economic advantage to maximize the ruthenium additions in place of gold and the other platinum group elements. The main reason for the low cost is that ruthenium is a by-product of producing the other platinum group elements but does not have many industrial uses.
From a metallurgical perspective, ruthenium substitutes for molybdenum, tungsten and to a certain extent chromium in both cobalt and nickel based alloys, and even iron based alloys which are, surprisingly, suitable for dental prosthesis applications. Ruthenium acts as an alloy strengthener, lowers the thermal expansion of the alloy (to better match thermal expansion of dental porcelains) and reduces the alloy's oxidation rate.
Alloys suitable for practice of this invention comprise at least 25% of metal selected from the group consisting of ruthenium, platinum, palladium, iridium, osmium, rhodium and gold, provided that the major proportion of this group or over 15%, whichever is greater, is ruthenium; from 15 to 30% chromium; up to 15% gallium; up to 5% silicon; up to 1% boron; up to 5% of metal selected from the group consisting of niobium, tantalum and rhenium; and a balance of metal selected from the group consisting of iron, cobalt and nickel. Mixtures of cobalt, nickel and iron, for example, may be used since from a metallurgical perspective they may be considered equivalents when the ruthenium content is so high. The lower chromium alloys are preferred since easier to cast and they produce less slag upon melting.
Thus, the alloy is a iron- nickel- and/or cobalt-chromium base alloy with a principal addition of ruthenium of more than 15%, and preferably at least 25%. If the ruthenium content of the alloy is less than 25%, other noble metals to bring the total to at least 25% may be used, provided that most of the noble metal is ruthenium. The amount of noble metal in the alloy may be more than 25% (provided that most of the noble metal is ruthenium) but there is no economic advantage to using higher amounts of costly material.
Both ruthenium and chromium protect the alloy from corrosion and oxidation. All of the iron alloys tested passed the ISO corrosion and cytotoxicity tests. This is surprising since iron is so easily dissolved in an acidic environment. The ruthenium apparently enobles the alloy, as opposed to the chromium that forms an oxide to protect the alloy from adverse reactions.
This alloy may be modified by addition of up to about 10% gallium, up to about 5% silicon or up to about 1% boron (or appropriate combinations of these elements) to enhance the casting characteristics of the alloy. Gallium, for example, lowers the melting temperature of the alloy so that it can be melted with a natural gas- or propane-oxygen torch commonly used in dental laboratories.
Up to about 5% of niobium, tantalum and/or rhenium may 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.
An example of an alloy of this invention with ruthenium as the only noble metal comprises cobalt 40%, chromium 25%, ruthenium 25% and gallium 10%. This alloy is readily cast and processed using standard dental laboratory equipment and materials.
The physical properties of this exemplary alloy are:
The thermal expansion of the alloy was found to be 13.9×10−6/° C. at 600° C. Several popular dental porcelains are successfully bonded to the alloy. and both single crowns and bridgework are readily fabricated with no special handling requirements beyond usual dental laboratory procedures.
This alloy was successfully processed in a pilot production batch of 15 pounds using standard foundry processing techniques for cobalt alloys. Biological testing found the alloy to be non-cytotoxic.
The ruthenium content of about 25 wt % eliminated the need for any addition of molybdenum or tungsten.
The gallium addition lowered the melting range so that the alloy was able to be cast with a gas-oxygen torch. Alternatively, small silicon and boron additions can also improve the alloy's ability to be cast. If the alloy is to be cast by induction heating then the melting range can be higher, reducing the need for gallium.
Despite the high hardness value the alloy was able to be ground using traditional dental laboratory grinding media. The alloy is especially suited for use with newer CAD/CAM applications where no casting is required. A block of the alloy is ground into the final prosthesis shape.
Other exemplary compositions made in practice of this invention include:
A sinterable noble alloy may also be prepared comprising at least 25% noble metal including from 25 to 30% ruthenium, 5 to 10% chromium, 10 to 15% gallium and a balance selected from the group consisting of iron, nickel and cobalt, preferably primarily or entirely iron, and preferably without nickel. Such an alloy may also be used as a casting or as a blank for machining a dental prosthesis.
By reducing the chromium content below 10%, and preferably to about 7%, formation of surface oxides or slag that would interfere with sintering can be avoided. Appreciable amounts of readily oxidizable metals with stable high melting oxides, such as aluminum, should also be avoided.
Up to 5% silicon and/or up to 1% boron may be substituted for part of the gallium for lowering melting point of the alloy. Higher amounts of ruthenium than 30% may also be suitable with sufficient additions of gallium or other metals for maintaining an acceptable melting point.
The alloy may also comprise up to 5% of a metal selected from the group consisting of niobium, tantalum and rhenium for grain refining.
A suitable alloy without nickel may comprise more than 15% ruthenium, from 15 to 30% chromium and a balance of metal selected from the group consisting of cobalt and iron, and substantially free of aluminum.
This application claims the benefit of the filing date of U.S. Provisional Application 60/639,359, filed Dec. 27, 2004.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4038074 | Davitz | Jul 1977 | A |
| 4253869 | Prosen | Mar 1981 | A |
| 4255190 | Prosen | Mar 1981 | A |
| 4382909 | Zwingmann | May 1983 | A |
| 4387072 | Schaffer | Jun 1983 | A |
| 4459263 | Prasad | Jul 1984 | A |
| 6554920 | Jackson et al. | Apr 2003 | B1 |
| 6613275 | Vuilleme | Sep 2003 | B1 |
| 6656420 | Prasad et al. | Dec 2003 | B2 |
| 6756012 | Prasad | Jun 2004 | B2 |
| 20020041820 | Prasad | Apr 2002 | A1 |
| 20050158693 | Prasad et al. | Jul 2005 | A1 |
| Number | Date | Country |
|---|---|---|
| 1104195 | Apr 1961 | DE |
| 101 36 997 | Feb 2003 | DE |
| 0 347 614 | Dec 1989 | EP |
| 2015889 | Apr 1970 | FR |
| 2 733 416 | Oct 1996 | FR |
| 2 750 858 | Jan 1998 | FR |
| 2 750 867 | Jan 1998 | FR |
| 2750858 | Jan 1998 | FR |
| 2750867 | Jan 1998 | FR |
| 58-110633 | Jul 1983 | JP |
| 58-110633 | Jul 1983 | JP |
| 60-135550 | Jul 1985 | JP |
| WO 03011231 | Feb 2003 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20060147334 A1 | Jul 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60639359 | Dec 2004 | US |
