Patent Application
20080070192
There are several properties exhibited by the alloy(s) of the present invention that make it suitable for porcelain fused to metal (PFM) applications. The alloy is grey in color with an oxide coating for bonding porcelain to the oxidized cast alloy substrate. The alloy has mechanical properties for cast prostheses and for the support of the porcelain and is readily polished to a bright sheen. The alloy is based on a portion of the palladium-cobalt binary system wherein palladium is about 20 to 90 wt % and cobalt is about 10 to 80 wt % to obtain a coefficient of thermal expansion (CTE) in the range of about 14.0 to 15.3. To the base Pd/Co alloy is added up to about 20 wt % of the following metals: aluminum, boron, chromium, gallium, lithium, rhenium, ruthenium, silicon, tantalum, titanium, tungsten or combinations thereof, to improve physical, chemical, mechanical and handling properties. The alloy of the invention has a solidus high enough that no fusion occurs during firing of normal porcelains and a coefficient (CTE) in a range that has been demonstrated to be compatible with porcelains.
The alloy of the invention can be readily cast by normal dental procedures, and can be recast using normal dental laboratory procedures. The cast alloy unit can be ground and polished to a high shine. The alloy has a light oxide color that does not affect the apparent color of the porcelain layer and the oxide does not increase during the firing of the porcelain. When heated to the porcelain firing temperature, a thin, continuous, tenacious oxide is formed, which enters into a bond with the porcelain. The alloy has strength that withstands loads in excess of those that would cause pain to the patient.
The alloy of the present invention meets the aesthetic needs while using a palladium-cobalt base. That is, the alloy system reproduces the normal coloration of natural dentition. The enamel layer of healthy natural dentition is quite translucent and porcelain can be made with similar translucency. The translucency of enamel allows the color of healthy dentine to be seen. This color normally has a yellowish tint. With the porcelain alloy combination, a layer of oxide must be present to form a bond with the porcelain. While high gold alloys may provide a yellowish background for the porcelain other metals they are cost prohibitive and alloys such as nickel, cobalt, palladium, etc., provide a gray background. For proper bonding, the alloying elements form an oxide on the cast metal surface. This dark gray to black colored oxide layer, can affect the apparent color of the porcelain veneering layer. The alloy system of the present invention includes elements added to regulate the amount and color of the oxide layer selected from the group including, but not limited to aluminum, boron, chromium, and/or silicon.
The mechanical properties of the alloy follow ANSI/ADA specification #38 and ISO standard IS9693 which require yield strength of at least 250 MPa for the alloy. To attain such strength, significant amounts of alloying elements selected from the group comprising, but not limited to, chromium, silicon, tantalum, titanium, and/or tungsten may be added to the alloy formulation.
The above mentioned standards do not require minimum or maximum values for coefficient of thermal expansion (CTE); however physical properties are required including the CTE value for both porcelain and alloy. The alloy of the invention includes elements added to regulate the grain size selected from the group including, but not limited to, chromium, gallium, tantalum, titanium, tungsten, rhenium and/or ruthenium.
Elements added to regulate oxidation during melting and casting includes but is not limited to, aluminum, boron, lithium, silicon. Also, heat transfer rate must be taken into consideration. When cooling from the porcelain firing temperature, shrinkage of both porcelain and alloy take place and the alloy, which cools faster, shrinks faster and thus puts tensile forces on the porcelain to metal bond. If this disparity of shrinkage is too much, the porcelain will no longer be bonded to the alloy when the composite reaches room temperature. It is readily understood that the solidus of the alloy must be sufficiently higher than the firing temperature of the porcelain so that the alloy is not even partially melted during firing.
Concerning the bonding of the porcelain to the alloy of the invention, it does not occur between porcelain and metal, it occurs between porcelain and the metal oxide layer formed when the alloy is heated prior to and during the firing of the porcelain. If the oxide is not adherent to the alloy, it can be simply removed by the porcelain. Some of the bond is simply mechanical but the primary bonding takes place as a mutual solution of metal oxide in porcelain and vice versa. If the oxide is not soluble in the porcelain and/or vice versa, no bond takes place.
While the invention has been described in detail, the following examples are for the purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention which is defined by the following claims.
For successful use of the alloys of the invention with porcelains in contemporary use, the CTE should be in the range of about 14.0 to about 15.3. When two metals comprise the base of an alloy, it would be expected that the CTE of such an alloy be somewhere between the CTE's of the single metals. It has been determined that this does not hold necessarily true for alloys of palladium and cobalt. Whereas Pd has a CTE of 12.5 and Co 11.75, the alloys of the invention comprising Pd/Co have higher values as shown in the following examples:
The minimum solidus temperature of the alloys of the invention is to determined to be about 1025° C. in order that the alloy does not start to melt during the firing of the porcelain on its surface.
This application claims the benefit under 35 USC 119 (e) of provisional application No. 10/844,672, filed Sep. 15, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60844672 | Sep 2006 | US |
