Single-phase alloy of gold and tungsten

Abstract

A single-phase alloy is formed, as weight percentages, of N % of gold, M % of tungsten, with N+M=100, M8 and N60. Also disclosed is a process for preparing such an alloy use of such an alloy and decorative sheets made from such an alloy.

Claims

1. A homogeneous single-phase alloy comprising, in weight %, N % of gold, M % of Tungsten, with N+M=100, M8 and N60.

2. The homogeneous single-phase alloy according to claim 1, wherein 92N75.

3. The homogeneous single-phase alloy according to claim 2, wherein 8M25.

4. The homogeneous single-phase alloy according to claim 3, wherein the alloy has a density greater than 19.2 g/cm.sup.3.

5. The homogeneous single-phase alloy according to claim 1, wherein the alloy has a face-centered cubic crystallographic structure.

6. The homogeneous single-phase alloy according to claim 1, wherein the alloy is a decorative layer.

7. The homogeneous single-phase alloy according to claim 1, wherein the alloy is a decorative sheet.

8. The homogeneous single-phase alloy according to claim 1, wherein the alloy is a support layer of a pure gold layer.

9. A method for preparing a homogeneous single-phase alloy comprising, in weight %, N % of gold, M % of Tungsten, with N+M=100, M8 and N60, using physical vapor deposition of gold and Tungsten, and condensing vapors of the two metals on a substrate.

10. The method for preparing the homogeneous single-phase alloy according to claim 9, further comprising pre-coating the substrate with a sacrificial layer.

11. The method for preparing the homogeneous single-phase alloy according to claim 9, further comprising at least one additional step of vacuum deposition of a pure gold layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood from reading the following description relating to non-limiting exemplary embodiments wherein:

(2) FIG. 1 is a schematic view of a facility for forming a white gold layer according to the invention;

(3) FIG. 2 is a top view of a target example for implementing the invention;

(4) FIG. 3 illustrates an example of a X-ray reflectometry spectrum obtained on a thin layer of Au75W25 alloy; and

(5) FIG. 4 illustrates an example of X-ray diffraction spectrum obtained on a Au75W25 alloy sheet.

DETAILED DESCRIPTION

(6) The invention relates to a single-phase alloy of gold and Tungsten, which has a crystal structure homogeneous, stable and responding, in particular for alloys with a proportion of 75% of gold and 25% of Tungsten, with the qualification of white gold predicting, according to ASTM Method D1925, a value YI: D1925<19 (YI: yellowness index) considered as good white or premium, and integrated with the Grade 1

(7) Method for Preparing

(8) The alloy made of gold (Au) and tungsten (W) is obtained in the form of a thin film by Physical Vapor Deposition (PVD). The single-phase alloy has a tungsten mass concentration of 25% with a homogeneous distribution of tungsten atoms in gold. The resulting alloy has a density equal to that of pure gold, is biocompatible by the biocompatibility of the two elements, is harmless to the environment and easy to process.

(9) This alloy with a composition Au75W25 exhibits a Grade 1 white gold colour, which not only does not require any additional rhodium-plating but could substitute for this rhodium-plating phase to obtain an object only comprising gold alloys. All these properties make this alloy attractive for use as a decorative layer.

(10) On the other hand, the gold being a highly inert metal, it is difficult to stick it to surfaces. A getter material (Ta, Cr, Va, Ti) is thus often used in order to deposit a primer layer prior to the deposit of the gold layer. The AuW alloy may be used as a support for a pure gold layer that we would like to decrease the cost of starting from raw material while maintaining the density of pure gold.

(11) The gold-tungsten alloy according to this invention is obtained by mixing the vapors of each of the metals and condensing them on a substrate. There are several PVD techniques such as thermal evaporation, sputtering, pulsed laser deposition. The disclosed example is based sputtering technique on the diode mode with a magnetron reactor and an argon (Ar) glow discharge. Other sputtering modes may be used as well as other PVD techniques (thermal evaporation, for example).

(12) The facility shown in FIG. 1 comprises a generally vacuum enclosure (1). The substrate (2) is mounted on its substrate holder (3) and faces the gold-tungsten target (4). This latter is installed on a magnetron reactor (5) comprising an annular magnet surrounding a cylindrical central magnet, of reverse polarity, which ensures the concomitant spraying of gold and tungsten. The plasma confining walls (6) limit its dispersion while the removable cover (shutter) (7) makes it possible to control the thickness of the decorative layer by controlling its opening time.

(13) This technique implements a plasma to extract the material from a target of the desired material in the form of vapor. This vapor diffuses into the plasma and condenses on the substrate to form a thin film. The gold and tungsten being immiscible at the thermodynamic equilibrium, it is not possible to directly spray a target of the desired single-phase AuW alloy, which cannot be produced by conventional physical metallurgy pathways.

(14) The target is thus made of a pure gold disk (8), the diameter of which substantially corresponds to the diameter of the magnetron reactor (5), and on which pure tungsten pieces (7) are arranged according to the diagram of FIG. 2. The pure tungsten pieces (7) are distributed so as to form a crown, the radius of which substantially corresponds to the median radius of the annular magnet of the magnetron.

(15) The total surface of the tungsten pieces (7) is selected based on the parameters of the plasma discharge used to obtain an Aul-xWx alloy (x being the tungsten atomic concentration). The relationship between the tungsten surface and the atomic concentration of W is as follows:

(16) $x\left(\%\mathrm{at}.W\right)=\frac{R_W{S}_W}{R_W{S}_W+R_{\mathrm{Au}}\left(S_T-S_W\right)}$

(17) with R.sub.W=tungsten sputtering efficiency at the discharge voltage

(18) S.sub.w=tungsten surface

(19) R.sub.Au=sputtering efficiency of the gold at the discharge voltage

(20) S.sub.T=total surface of the target

(21) Once the thin layer of Au-w has been obtained, its density is measured by X-ray reflectometry and in all cases, it is greater than 19.2 g/cm.sup.3. The homogeneity of the alloy is characterized by electronic microscopies (scanning and/or transmission) associated with chemical analysis techniques of the EDS (Energy Dispersive Spectroscopy) or WDS (Wavelength Dispersive Spectroscopy) type. These analyses show a single chemical phase, without the presence of tungsten clusters, precipitates or aggregates.

(22) The crystallinity of the alloy is characterized by x-ray diffraction. For all compositions, the spectrum obtained has two main peaks close to the positions relative to the planes (111) and (222) of gold. The AuW thin layers are polycrystalline with a fiber texture (111) which means that the grains of the alloy Au.sub.75W.sub.25 have planes (111) parallel to the surface and a random orientation of these planes (111) in the plane of the thin layer. These plans (111) correspond to the dense planes of a cubic structure of the face-centered type, which is the initial structure of pure gold.

(23) The color of the resulting thin layers is measured by spectrometry with a C-illuminant, an observation angle of 2 taking into account specular and ultraviolet components. In this configuration, the measured colors on the thin layers of the alloy AU.sub.75W.sub.25 (or 18 carats) have coordinates La*b* such as L>75, 2<a*<2 and b*<10 or indexes YI<19.

(24) The substrate is a glass disc on which is deposited a layer of photosensitive resin, the solvent of which is acetone. The substrate is introduced into the sputtering reactor in which the target shown in FIG. 2 is placed.

(25) The target is a pure gold disk (8) and has a diameter of 75 mm and the sum of the tungsten surface (9) is 976 mm.sup.2. The plasma reactor is operated at a pressure of 2.5.10.sup.1 Pa of argon with a distance of 12 cm between the target and the substrate. Once the thin layer is deposited on the substrate, the latter is placed in acetone in order to dissolve the photoresist layer and thereby recover the sheet detached from the glass substrate.

(26) Generally, the disclosed method provides for the deposit of the ad-hoc surface tungsten pieces on a gold target. A variant consists in reversing the position of each metal (tungsten target and gold pieces), taking into account the spraying efficiencies of the two metals. This variant then comprises preparing the homogeneous and single-phase gold-tungsten alloy from a tungsten target on which gold pieces are deposited. The cost of the method is reduced by the significantly lower proportion of gold which is used.

(27) The sheet is analyzed by X-ray reflectometry and diffraction. FIG. 3 illustrates an example of a X-ray reflectometry spectrum obtained on a thin layer of alloy Au75W25. FIG. 4 illustrates an example of X-ray diffraction spectrum obtained on a sheet of alloy Au75W25.