Pd-Ag-Ga DENTAL ALLOYS WITH INTERNAL OXIDATION CHARACTERISTICS
A Pd—Ag alloy system that utilizes internal oxidation by including a base metal addition of only Ga is provided. The Pd—Ag—Ga alloy is particularly well-suited for use in porcelain-fused-to-metal (PFM) applications because the alloy does not form a dark oxide upon heating. The high oxygen potential of the material ensures the development of a tenacious chemical bond with the porcelain when used in PFM applications. Moreover, the addition of gallium lowers the liquidus temperature of the alloy to improve castability and strengthens the alloy significantly.
This invention relates generally to improved palladium-silver alloys; and more particularly to a palladium-silver-gallium dental alloy with internal oxidation characteristics.
BACKGROUND OF THE INVENTIONCompositional analyses of palladium-silver dental alloys intended for porcelain-fused-to-metal (PFM) applications reveal that all commercial alloys currently available utilize various combinations of base metals, such as, In, Sn, Ga, Zn and Mn to create materials which are easily cast, have high mechanical strengths, have suitable coefficients of thermal expansion (CTE) and can create a proper bond with porcelains. However, all of these conventional alloys suffer from a serious limitation, namely, that they tend to develop dark oxides during porcelain firing. These dark oxides are difficult to mask and can negatively effect the overall esthetics of the porcelain, particularly in the thin sections of the porcelain.
One method of addressing this aesthetic limitation is to use palladium-silver alloys that do not include any base metals in their formulations. While using such compositions resolves the problem of dark oxide formation, it introduces new limitations. Specifically, these palladium-silver alloys possess inferior mechanical properties, are potentially prone to distortion during fabrication and in service, have higher liquidus temperatures, and require an additional step in their processing (the application of a bonding agent prior to the porcelain bakes) in order to develop a bond with the porcelain.
Accordingly, there is a need for a palladium-silver alloy that includes a base metal, possesses oxidation characteristics similar to those of its base metal free counterparts, is strong enough to resist the forces involved in dental articulation, can be easily cast, and improves processing efficiency by not requiring a bonding agent application step.
BRIEF SUMMARY OF THE INVENTIONThe present invention provides a palladium-silver alloy suitable for PFM dental applications that prevents the formation of dark oxides through a process of internal oxidation of the alloy during the firing of the porcelain. According to a presently preferred embodiment, the Pd—Ag alloy of the invention utilizes internal oxidation by including a base metal of only gallium.
In another embodiment of the current invention, the Pd—Ag—Ga alloy may further include small quantities of at least one grain refiner such as Ru, Ir, and Re. In one such embodiment the grain refiner may have a concentration of less than 3 wt %
In still another embodiment, the Pd—Ag—Ga alloy has a concentration of Ga of from 0.1 to 3 wt %. In another such embodiment the alloy has a concentration of Pd of from 45 to 90 wt %. In yet another such embodiment the alloy has a concentration of Ag of from 10 to 55 wt %.
In yet another embodiment, the alloy may also contain one or both of Au and Pt. In such an embodiment, the concentration of the Au or Pt (or combination) may be up to 10 wt %.
In still yet another embodiment, the alloy has a composition of Pd51.2Ag46.5Ga1.5Ir0.4Ru0.4.
In still yet another embodiment, the invention is directed to a dental appliance formed using the alloy of the current invention.
The description will be more fully understood with reference to the following figure, which is presented as an exemplary embodiment of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:
The current invention is directed to a Pd—Ag—Ga alloy substantially free of additional base metals. The current invention utilizes the concept of internal oxidation to prevent the formation of dark oxides when porcelain is fired on the alloy such that the oxide color of the alloy matches that achieved when using palladium-silver alloys free of base metals.
Internal oxidation is the process by which oxygen diffuses into an alloy and causes subsurface formation of oxides of one or more alloying elements. The partial pressure of oxygen in a porcelain firing oven is typically at or below 0.2 atm (in air), and can be substantially higher near the metal/porcelain interface (>0.5 atm. For internal oxidation without external scaling, the oxygen diffusion into the alloy must be greater than the solute diffusion out of the alloy. The inventors of the present invention have discovered that it is possible to take advantage of this process of internal oxidation to produce an improved palladium-silver alloy for use in dental applications, and particularly porcelain-fused-to-metal applications. It has been surprisingly discovered that the judicious use of gallium alone as the base metal alloying element instills internal oxidation characteristics to a Pd—Ag alloy. The diffusion rate for such an alloy is well-suited for effecting internal oxidation during the short thermal treatments involved in multiple firing cycles and at the pressures used in porcelain firing ovens (i.e., at or below 0.2 atm).
In accordance with the current invention, Pd may be provided in a concentration range of from about 45 to 90 wt %, Ag may be provided in a concentration range of from about 10 to 55 wt %, and Ga may be provided in a concentration range of from about 0.1 to 3 wt %. In addition to these required materials, the alloy may also include optional materials to improve some properties. For example, the alloy may include concentrations of other noble metals, such as, for example, Au or Pt in a concentration of up to 10 wt %. In addition, the alloy may optionally include small quantities of grain refining elements, such as, for example, Ru, Ir, and Re in a concentration of up to 3 wt %. Table 1, below provides a summary of the relative concentrations of elements, apart from contaminants that may be included in the alloys in accordance with the current invention.
A specific example of a Pd—Ag—Ga alloy is provided in Table 2, below. For comparison purposes, a base metal free, Pd—Ag alloy (X0901A3) is included to highlight the similarity of the clean oxide color obtainable by both the prior art Pd—Ag alloy and the Pd—Ag—Ga alloy of the current invention.
Beyond this aesthetic improvement, the ability of the Pd—Ag—Ga alloy in accordance with the invention to internally oxidize leads to improved physical properties. For example, one of the advantages of using the Pd—Ag—Ga alloy of the invention is that, because of the high oxygen potential of the material, a clean or uncolored oxide forms under the porcelain, which ensures the development of a tenacious chemical bond with the porcelain when used in PFM applications. Moreover, gallium (due to its thermal and electronic properties) on a relatively smaller weight fraction basis also lowers the liquidus temperature to improve castability and strengthens the alloy significantly. The data in Table 2 demonstrates the improvements in thermal and mechanical properties of the alloys of the current invention, which again are of particular importance when using the alloys for PFM applications.
One of the principal considerations in obtaining these improved properties is that the Pd—Ag alloy of the current invention must be substantially free of other base metals. It will be understood that the term “substantially free of other base metals” does not exclude the presence of impurities and other minor additives having concentrations of less than 1 wt % or less each.
In addition, it is important to note that prolonged heat treatment times at high temperatures and under low pressures can force even those base elements well-suited for internal oxidation up to the surface and form an external oxide on the bare metal surfaces. Accordingly, forming temperatures, pressures, and treatment times must be controlled to ensure that the partial pressure of oxygen during porcelain firing remains within the range such that oxygen diffusion into the alloy must be greater than the solute diffusion out of the alloy.
From the information above, it is evident that the Pd—Ag—Ga alloy of the current invention displays significant improvement in physical properties while having the transparent oxide characteristics of the base metal free alloys.
Although the above discussion has focused on the composition of the dental alloy, it should be understood that the current invention is also directed to dental appliances formed using the Pd—Ag—Ga dental alloy of the current invention. In particular, the alloy of the current invention is well-suited for use in PFM applications where the dental appliance is formed by the firing of porcelain on a substrate formed of the alloy of the invention.
Doctrine of EquivalentsThose 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 overall composition 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 the inclusion of additional non-base materials and impurities within the indicated limits would not affect the improved properties of the Pd—Ag—Ga alloy alloy of the current invention nor render the alloy unsuitable for its 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.
Claims
1. A dental alloy capable of undergoing internal oxidation upon heating, consisting essentially of Pd, Ag and Ga, wherein the alloy is substantially free of any additional base metal.
2. The dental alloy of claim 1, having a composition of:
- Pd45-90Ag10-55Ga0.1-3
3. The dental alloy of claim 1, wherein the alloy further includes one or more additional base metals at a concentration no greater than 1 wt %.
4. The dental alloy of claim 1, wherein the alloy further includes at least one grain refiner.
5. The dental alloy of claim 4, wherein the at least one grain refiner is selected from the group consisting of Ru, Ir and Re.
6. The dental alloy of claim 4, wherein the at least one grain refiner has a concentration of from about 0 to 3 wt %.
7. The dental alloy of claim 1, wherein the alloy further includes at least one additional noble metal selected from the group consisting of Au and Pt.
8. The dental alloy of claim 7, wherein the additional noble metal has a concentration of up to 10 wt %.
9. The dental alloy of claim 5, wherein the composition has a molecular formulation of Pd51.2Ag46.5Ga1.5Ir0.4RU0.4.
10. A dental appliance comprising a dental body formed at least partially from an alloy material capable of undergoing internal oxidation upon heating, the alloy consisting essentially of Pd, Ag and Ga, wherein the alloy is substantially free of any additional base metal.
11. The dental appliance of claim 10, wherein the alloy further includes one or more base metals at a concentration no greater than 1 wt %.
12. The dental appliance of claim 10, wherein the alloy has the following composition:
- Pd45-90Ag10-55Ga0.1-3
13. The dental appliance of claim 10, wherein the alloy further includes at least one grain refiner.
14. The dental appliance of claim 13, wherein the at least one grain refiner is selected from the group consisting of Ru, Ir and Re.
15. The dental appliance of claim 13, wherein the at least one grain refiner has a concentration of from about 0 to 3 wt %.
16. The dental appliance of claim 14, wherein the alloy has a molecular formulation of Pd51.2Ag46.5Ga1.5Ir0.4Ru0.4.
17. The dental appliance of claim 10, wherein the alloy further includes at least one additional noble metal selected from the group consisting of Au and Pt.
18. The dental appliance of claim 17, wherein the additional noble metal has a concentration of up to about 10 wt %.
19. The dental appliance of claim 10, wherein the appliance is a formed by fusing porcelain to the alloy.
20. The dental applicant of claim 19, wherein the porcelain fusing occurs in an oxygen containing environment, and where the fusing conditions are controlled such that the rate of diffusion of the oxygen into the alloy is greater than the rate of diffusion of a solute out of the alloy.
Type: Application
Filed: Apr 20, 2009
Publication Date: Oct 21, 2010
Inventors: Paul J. Cascone (Del Mar, CA), Arun Prasad (Cheshire, CT)
Application Number: 12/426,490
International Classification: C22C 5/04 (20060101); C22C 5/06 (20060101);
