Platinum/palladium alloy

Abstract

This invention relates to a platinum alloy for use in fine jewelry as an alternative to traditional grades of platinum or white gold. This alloy affords a much whiter color and a higher level of hardness to maintain the surface over an extended period of time that a piece of jewelry may be worn (sometimes many years). Platinum alloys have many desirable working properties, which this alloy equals or surpasses.

Description

This invention relates to a Platinum/Palladium alloy for use in fine jewelry which has better properties than either ingredient alone. The use of this alloy will allow the artist a whiter and more malleable material for the construction of such items as rings and other precious metal mounts. The alloy is also stronger than the individual metals, allowing smaller, more delicate settings to be made without brittleness or breakage, which is necessary to insure a good mount for expensive gems such as diamonds.

BACKGROUND OF THE INVENTION

In the jewelry trade the main competitor to platinum is white gold. Gold is not only the name of a precious metal; it is also its color. In order to comply with Quality Mark Standards and Trade Descriptions Acts in various parts of the world, there are strict limits on what may be described as standard platinum. The British Hallmarking Act, for instance, defines hallmarkable platinum as not less than 95% of the element platinum with no negative tolerance. In some countries, a small proportion of the ‘platinum’ may actually consist of platinum group metals. Thus, “standard platinum” can vary in its properties, including color.

Recently there has been increased interest in defining what qualifies as white gold and a code of practice is under international discussion. Several white metals may bleach the ‘yellow’ color from gold but the resulting alloy must also meet Quality Mark Standards. It is therefore generally impractical to make an acceptably white gold above 18 carat, while it is easier at 14 carat and easier still at 9 carat. 18 carat white gold is probably the strongest competitor to platinum and so the color of white gold is of considerable interest. Some of the 18 carat white golds have at least a slight yellow tinge. The code of practice relies on a color analyzer under standardized conditions and calibrated against a standard white tile surface to determine the spectrum of white color light components. For white gold, they are then converted to a ‘yellowness index’; a high value being evidently yellow and a low or near zero value being neutral white. The values obtained also depend on the reflectance of the surface which, in turn, depends on texture.

Another important factor in the competition of white gold is the hardness value. The harder the material, the easier it is to polish and the better the polish holds with wear. One skilled in the art would realize that often a harder material will prove more difficult to work. Hard materials tend to be brittle and will break or shatter when bent or flexed. Therefore, a platinum alloy having a high tensile strength, while maintaining a reasonable working level of flexibility would be highly desirable. Flexibility is reported as the percent elongation to break. Typically, for a material to be able to fold in half (double back on itself) repeatedly, a percent elongation to break of 50% or greater is needed.

SUMMARY OF THE INVENTION

The platinum alloy of this invention appears to the casual observer to be no different than commercial grades of platinum, yet is stronger and more easily workable material and provides a higher level of whiteness than other commercial platinum grades.

DETAILED DESCRIPTION OF THE INVENTION

The alloy of this invention provides a combination of the 45 to 55 platinum and 55 to 45 percent (respectively) palladium which provides a synergy, affording a stronger, whiter material than either of its two components. In an embodiment of this invention the ratio of platinum to palladium will be 50 to 50.5 percent platinum and 49.5 to 50 percent palladium. In another embodiment the ratio of platinum to palladium will be 50.5 to 49.5, respectively.

In one embodiment the alloy will have an ASTM Y1 (yellow) index of less than 12 (units) while having a red/green value of less than 0.2 and a blue yellow index of less than 6. All measurements were obtained using standard color measurements and were calibrated for surface conditions which can affect the color measurement. A measurement of less than 19 for the ASTM Y1 yellow index would, if used for white gold, put them in the top grade. The lower the yellow index value, the whiter the material.

Another embodiment of this invention is the penetration hardness as compared to other platinum alloys. Typical 100% platinum has a value of 55 VPN (Vickers Pyramid Number, also known as DPH, Diamond Pyramid Hardness.) An annealed platinum can be as low as 37. The alloy of this invention has a VPN of greater than 125. In another embodiment the VPN of this alloy is greater than 150.

The high tensile strength and good workability coupled with the exceptional whiteness of this alloy makes it perfectly suitable for use in jewelry applications. Ring and other fine jewelry pieces need the high strength in order to maintain their surface finish over extended wearing. One skilled in the art would recognize all the possible jewelry applications this alloy would be suitable for, but rings, earrings, bracelets and necklaces are among some of the options.

Experimental:

Color Analysis of Platinum:

The tests were performed on a spectrophotometer; model CE-XTH. The results are in accordance with ASTM D1925 Yellowness Index. While reflectance is not strictly a percentage of the incoming total light energy reflected by the specimen it is a close reflection, a qualitative measure of how well the surface has been prepared. As long as the value exceeds 75, certainly 80, the color value results are deemed valid.

TABLE 1
	Reflectance	ASTM	green−ve/	blue−ve/
Sample	(Units ???)	Yellow	red+ve	yellow+ve
Pt/5% Cu	83.4	4.26	−0.53	2.3
50.5 Pt/49.5 Pd	82.0	11.9	0.15	5.3
Pt 100%	80.0	13.4	0.27	5.8

As can be seen from the results in table 1, the experimental alloy prepared from 50.5/49.5 Pt/Pd (respectively) gave an unexpected whiteness of greater than pure platinum.

Hardness of Platinum Alloys:

Tensile strength is measured in tons/square inch (TSI) in the USA and in exactly relatable System International (SI) units elsewhere. The tensile strength of an alloy is the fracture load divided by the original cross section of the same type of test piece as described for % elongation.

Thermal diffusivity in this case is calculated from the physical properties of platinum and palladium. It is a measure of the ability to disperse heat through a cooler body of the alloy when heat is applied locally at a point or on one side, such as in casting or welding, where the temperature gradient is changing during the process. Thermal conductivity is the rate of transmission of heat down a constant steady temperature gradient, so it is an important factor but must be modified by the specific heat and the density of the alloy. So, thermal diffusivity is thermal conductivity divided by the product of density and specific heat. The important point is that platinum alloys have a thermal diffusivity around 0.24 whereas the corresponding values for fine gold and fine silver are 1.2 and 1.7 respectively. This means the heat ‘stays close to where you put it’ with platinum but migrates five to seven times faster with gold and silver. This allows platinum alloys to maintain a high hardness and retain most of that cold work hardness even after laser welding which is precisely located and not due to broad mass heating (which would result in annealing and lowering of the tensile strength).

The Vickers Hardness Machine uses a pyramid diamond as the penetrator. A standard load, usually 30 Kg, is applied to a standard square based pyramid diamond bearing on the material surface for 15 seconds. The diagonal of the indent is measured under a low power microscope. The hardness number is determined as the load/area of impression in kg/mm² but there are tables to convert from diagonal measurement to VPN.)

TABLE 2
	Hardness
	Penetration	Tensile	% Elongation	Thermal
Sample	(VPN)	Strength (TSI)	To break	Diffusivity
Pt	37-55	9	40	0.25
Pd	80-100	10	40	0.24
50.5Pt/	152	13.5	26	0.22
49.5Pd

As can be seen from the results shown in table 2 the alloy of this invention provides for a much improved tensile strength and hardness while maintaining its other beneficial properties (such as whiteness)

Claims

1. A platinum alloy consisting essentially of 45 to 55 percent platinum and 55 to 45 percent palladium in which the ASTM yellow index is less than 12 and a penetration hardness is greater than 125 VPN.

2. The alloy of claim 1 wherein the platinum is 50 to 50.5 percent and the palladium is at 50 to 49.5 percent.

3. The alloy of claim 2 wherein the penetration hardness is greater than 150 VPN.

4. The alloy of claim 2 which additionally has an elongation to break of greater than 20 percent.

5. The alloy of claim 2 which has a tensile strength of greater than 12 TSI.

6. An article of jewelry made from the alloy of claim 1.

7. A platinum alloy containing 50.5 percent platinum and 49.5 percent palladium.