EP2546371B1 - 18-carat grey gold - Google Patents

Description

Technical area

The present invention relates to an 18-carat white gold having a white color comparable to that of rhodium-plated gold and having good polishing ability for applications in jewelery, watchmaking, jewelery, or for writing instruments.

State of the art

18ct white gold alloys (also known as white gold) usually consist of gold, copper and elements that have the power to whiten gold. These whitening elements are often found in group VIII of the periodic table of the elements, such as Ni, Pd, Co and Fe. The whitening effect of certain metals on gold has been the subject of studies since the 1970s. as described in reference 1: " Improvement of 18 carat white gold alloys ", GP O'Connor, Gold Bull 11, 2 (1978) ).

In the current market, gray gold alloys are distinguished into two categories: palladium gold and nickel gold. Nickel brings an interesting color and is economically advantageous, but its allergenic effect has almost proscribed luxury applications (jewelry, watchmaking).

To whiten nickel-free gold, different tracks are explored with their disadvantages. For example, the platinum content can be increased. This solution is expensive and does not get a pretty white gold. Another solution is to rhodium the surface of the final product. In this case, a white color is well obtained but as it is a surface treatment, the colored layer is sensitive to wear due to friction and the white color is likely to be deteriorated with time.

Since 2003, the Applicant has studied various metallurgical systems in order to develop an 18-carat gray gold. The alloys studied are based on the Au-Pd system with usual additive elements such as Ag, Ga, Zn, Sn, Fe and Rh, but without nickel and without copper. This is a classic composition for a jewelery alloy. In this first study, the whitest alloy obtained consists of 75% Au, 21% Pd, 2% Ag and 2% Rh. Table 1 gives the colorimetric measurements on the L * a * b * scale, according to the CIE 1976 model, of different compositions of 18cts gray alloys on the market. The value of the Yellowness Index (YI), or yellow index, is also specified. This parameter, calculated from the three coordinates L * a * b *, is commonly used to characterize the white color of an alloy (What is a white gold? Progress on the issues! Dr. Christopher W. Corti, Santa Fe Symposium Proceedings, May 2005 ). On the basis of this parameter, the World Gold Council has defined several grades of gray gold: Non-White (YI>32); Off-White (24.5 <YI <32); Standard-White (19 <YI <24.5); Premium-White (YI <19). Table 1 Alloy composition L * at* b * YI Market situation Au750, Ni155, Cu40, Zn55 85 0.4 5.9 12.2 Market product Au750, Ni50, Pd50, Ag110, Cu40 78.2 0.7 5.9 14.1 Market product Au750, Pd120, Ag40, Cu90 80 2.8 8.2 20.6 Current product of the market Au750, Pd120, Ag40, Cu90; rhodium 90 1.0 2.2 5.6 Most common product on the market Au750, Pd210, In35, Ga5 79 1.6 6.1 15.2 Market product Au750, Pd210, Ag20, Rh20 79.4 1.4 5.2 13.1 CM prototype 2003, too expensive

The Applicant has also studied the development of a gray gold in the Au-In system. In particular, it has studied the feasibility of industrial exploitation of the Au75In25 alloy as an alloy for jewelery. According to the Au-In phase diagram, Au75In25 is in the zone (Ψ) which is an intermetallic phase (Au3In2). This The composition gives the alloy an extraordinarily white color, but its intermetallic structure gives it a very great fragility. This alloy is unfortunately practically unusable. The addition of elements like Sn, Al, Zn, Cr and Nb has also been considered in order to obtain 18-carat gold ternary alloys based on the Au-In system. It has been shown that the addition of an element comprising one of: Sn, Al, Zn, Cr and Nb, however, gives rise to either a loss of color or an inhomogeneous structure, and does not sufficiently improve the ductility of the alloy.

The whitening power of chromium on gold is a known feature which is cited in a number of publications such as reference 1, reference 2: " Mechanical properties of Au-Fe-Cr white gold, T. Suzuki, Y. Kaneko, S. Hashimoto, J. Japan Inst Metals, 67, No. 11 (2003) ); and reference 3: " White gold alloys for investment casting ", M. Poliero, Gold Technology, No. 31 (2001) ). From this observation, the plaintiff chose to direct her research towards the Au-Cr system.

The use of Cr for bleaching gold (combined with Ag, Cu, Ni and Fe) has, for example, been known from the patent application. DE10027605 . This document describes contents of Cr not exceeding 14% by weight.

The patent application WO2009 / 092920 also cites the use of Cr to whiten gold. In particular, it discloses the combination of gold with a refractory metal of columns IVB, VB and VIB of the periodic table (including Cr), without specifying the contents of refractory element.

The patent application JPH0657356 discloses a decorative gold alloy containing, by weight, 75% Au, 15 to 24% of at least one of Fe and Cr, and 1 to 10% of Ni. The alloy is subjected to a treatment at a temperature between 250 and 500 ° C with an oxidizing flame so as to obtain a color ranging from yellow, red, purple, blue, green to gray.

If the whitening power of Cr has already been demonstrated, its use is limited because of its partial miscibility with gold. Indeed, if the limit of solubility of Cr in Au is exceeded, the alloy becomes inhomogeneous. The Cr precipitates present in the material deteriorate the polishing ability of the alloy, which then becomes unusable for an application in jewelery / watchmaking. The difficulty of developing a gray gold at Cr then consists in introducing additional alloying elements making it possible to increase the solubility of Cr in gold, without altering the white color.

Brief summary of the invention

An object of the present invention is to provide an 18-carat gray gold alloy free from the limitations of known gray gold.

Another object of the invention is to obtain an alloy of 18 carat gold of white color, in the mass, comparable to that of a gold rhodium, and having a good surface quality after a mirror polishing.

According to the invention, these objects are achieved in particular by means of a gray gold alloy comprising by weight: essentially 75% of gold; between 13% and 17% of Cr; between 5% and 10% of Pd; and between 1% and 5% Fe.

In one embodiment, the alloy comprises by weight 75% Au, 17% Cr, and 8% Pd. This alloy is characterized by a color L * a * b * having the component L * of about 82, the component a * of about 0.56 and the component b * of about 3.7.

In yet another embodiment, the Cr composition is between 13% and 16%; the composition of the Pd is between 5% and 8%; and the composition of the Fe is between 1% and 4%.

In yet another embodiment, the alloy comprises by weight essentially 75% Au, essentially 15% Cr, essentially 7% Pd, and essentially 3% Fe. This alloy is characterized by a color The color L * a * b * with a component L * of about 82, a component a * of about 0.45 and a component b * of about 3.0 .

The present disclosure also relates to a timepiece, jewelery, jewelery or writing instrument, comprising at least one component made with the alloy of the invention.

This solution has the particular advantage over the prior art of obtaining an 18-carat gold alloy making it possible to produce compositions combining a color close to rhodium-plated gold and a polishing ability that makes it possible to produce jewelery pieces. watchmaking, jewelery, or writing instruments.

Example (s) of embodiment of the invention

The good surface quality is directly related to the homogeneity of the alloy. If the alloy has a multiphase structure, inhomogeneities will appear on the surface of the mirror-polish.

In one embodiment, an 18-carat white gold alloy comprising by weight: at least 75% gold; between 13% and 23% of Cr; between 2% and 12% of at least one element selected from: Pd, Fe, Ru, Pt, Co, Ga, Ge, Mn, V, Ni. In a variant, the composition of Cr may be between 15% and 20% and the composition of said at least one element chosen from: Pd, Fe, Ru, Pt, Co, Ga, Ge, Mn, V, Ni is between 5% and 10%.

In another embodiment, the alloy comprises between 2% and 12% of Pd and at most 5% of at least one element chosen from Fe, Ru, Pt, Co, Ga, Ge and Mn. In this case, the composition of the Cr can be between 13% and 18%. The composition of the Pd can be between 5% and 10%.

Example (s) of embodiment of the invention

The good surface quality is directly related to the homogeneity of the alloy. If the alloy has a multiphase structure, inhomogeneities will appear on the surface of the mirror-polish.

In one embodiment, an 18-carat white gold alloy comprising by weight: at least 75% gold; between 13% and 23% of Cr; between 2% and 12% of at least one element selected from: Pd, Fe, Ru, Pt, Co, Ga, Ge, Mn, V, Ni. In a variant, the composition of Cr may be between 15% and 20% and the composition of said at least one element chosen from: Pd, Fe, Ru, Pt, Co, Ga, Ge, Mn, V, Ni is between 5% and 10%.

In another embodiment, the alloy comprises between 2% and 12% of Pd and at most 5% of at least one element chosen from Fe, Ru, Pt, Co, Ga, Ge and Mn. In this case, the composition of the Cr can be between 13% and 18%. The composition of the Pd can be between 5% and 10%.

In yet another embodiment, the alloy further comprises at most 1% of an element chosen from refining elements: Ir, Rh, Re, W, Mo, among the neutral elements: Nb, Ta, Ni, V , Ti, Zn, and among the deoxidizing elements: Zr, Si and B. Here, the expression "neutral elements" means elements that have no significant effect on the main characteristics of the alloy as described herein. above and including these neutral elements.

More preferably, the 18-carat white gold alloy comprises by weight: at least 75% Au, essentially 17% Cr, and essentially 8% Pd.

The whitening effect of Pd is widely used in conventional 18K gray golds. Also, Cr having a significant solubility in Pd, this element has been introduced as a secondary addition element in different proportions in order to improve the homogeneity of the material. The use of this element makes it possible, in fact, to maintain high levels of Cr without altering the white color. The polishing ability has been improved but is not optimal because Cr precipitates are sometimes present from the casting. In this Au-Cr-Pd system, the most favorable composition contains essentially 17% Cr and essentially 8% Pd (see Table 2).

The Au-Cr-Pd-Fe system has also been studied. In this system, Fe has indeed a high solubility in Cr and Pd at high temperature. In addition, it is a known element for bleaching gold (see references 2 and 3). The substitution of a few mass percents of Cr in favor of Fe has considerably improved the homogeneity of the material and thus the polishing ability, while maintaining a very interesting color.

In another embodiment, the 18-carat white gold alloy comprises by weight: at least 75% gold; between 13% and 17% of Cr; between 5% and 10% of Pd; and between 1% and 5% Fe. Preferably, the Cr composition is between 13% and 16%; and the composition of the Pd is between 5% and 8%. More preferably, the gold alloy comprises by weight: at least 75% Au, essentially 15% Cr, and essentially 7% Pd and essentially 3% Fe. This latter alloy has the best compromise between color and polishing ability. .

Examples of conventional alloy compositions and the alloy according to the invention are given in Table 2. In particular, Table 2 reports results of measurements on the L * a * b * scale according to the CIE 1976 model. and the Yellowness Index (YI), or yellow index, measured according to ASTM D-1925 for each of these alloys, as well as observations on the quality of the polishing. In particular, Table 2 reports colorimetric measurements and observations on the polishing quality of four conventional gray gold alloys (Nos. 1 to 4) and seven examples of gold alloys according to the invention (Nos. 5 to 11). The compositions are expressed in% by weight. For example, the measurements have shown in particular that the alloy comprises by weight: at least 75% of gold; between 13% and 23% of Cr; between 2% and 12% of at least one element selected from: Pd, Fe, Ru, Pt, Co, Ga, Ge, Mn, V, Ni, is characterized by a yellow index being less than 10. a yellow index of less than 10 for the other alloys of the invention. Here, the color is in the mass of the alloy and can not be damaged by surface wear. Table 2 Compositions of alloys (% by weight) The at* b * YI Ability to polish and various observations 1 Au75.0, Ni15.5, Cu4.0, Zn5.5 85 0.1 5.9 12.2 Market product, with nickel 2 Au75.0, Pd12.0, Ag4.0, Cu9.0 80 2.8 8.2 20.6 Market product, not white enough 3 Au75.0, Pd12.0, Ag4.0, Cu9.0; rhodium 90 1.0 2.2 5.6 Market product, with rhodium 4 Au75.0, In25.0 85.7 0.13 2.58 5.9 Good polishing ability, very fragile 5 Au75.0, Cr20.0, Pd5.0 80.9 0.35 3.4 8.1 Hard to polish 6 Au75.0, Cr17.0, Pd8.0 81.8 0.56 3.7 8.9 Polishing ability dependent on casting 7 Au75.0, Cr17.0, Ni8.0 81 0.25 3.8 8.9 Polishing ability dependent on casting 8 Au75.0, Cr17.0, Pd5.0, Fe3.0 82.5 0.27 2.7 6.4 Hard to polish 9 Au75.0, Cr15.0, Pd7.0, Fe3.0 82 0.45 3.0 7.2 Good polishing ability 10 Au75.0, Cr15.0, Ni7.0, Fe3.0 80.7 0.30 4.2 9.8 Good polishing ability 11 Au75.0, Cr13.0, Pd7.0, Fe5.0 83.5 0.70 4.1 9.7 Good polishing ability

For the alloy comprising by weight: at least 75% Au, essentially 17% Cr, and essentially 8% Pd, a color L * a * b * is measured with the L * component of about 82, the component a * d about 0.56 and the b * component about 3.7.

For the alloy comprising by weight: at least 75% Au, essentially 15% Cr, essentially 7% Pd, and essentially 3% Fe, a color L * a * b * is measured with the L * component of about 82%. , the component a * of about 0.45 and the component b * of about 3.0.

Of the alloys tested, compositions AuCr17Pd8 (at least 75% Au, essentially 17% Cr, and essentially 8% Pd) and AuCr15Pd7Fe3 (at least 75% Au, essentially 15% Cr, essentially 7% Pd, and essentially 3% Fe ) are the most interesting. The composition containing Fe (AuCr15Pd7Fe3), however, has the most promising characteristics for an application in the field of luxury (jewelery, watchmaking) because it combines both a very white color and good polishing ability.

Corrosion test results according to the dental standard (ISO 10271-2001) were performed on seven alloys to evaluate the release rate (in μg / cm ² x7d) of the ions of sensitive elements such as Ni and Cr (Table 3). For a dental application, the ISO 10271-2001 standard limits the total release rate to 200 μg / cm ² x 7d. According to these tests, the two alloys AuCr17Pd8 and AuCr15Pd7Fe have a very good resistance to corrosion. Table 3 Alloy μg / cm ² x7d AuCr17Pd8 0.22 AuCr15Pd7Fe3 0.44 AuCr14Pd6Fe5 0.63 AuNi25 6.08 AuNi20Pd5 10.00 AuNi20Fe5 468.08 AuPd21Fe3Ge 54.93

Table 4 gives a summary of our studies in the Au-Cr-X and Au-Cr-Pd-Fe systems as well as the characteristics of the two preferred alloys. Here, X corresponds to at least one element chosen from: Pd, Fe, Ru, Pt, Co, Ga, Ge, Mn, V, Ni. Table 4 System Ternary: Au-Cr-X Quaternary: Au-Cr-Pd-Fe Variations of compositions 13-23wt% Cr, 2-12wt% X 13-17wt% Cr, 5-10wt% Pd, 1-5wt% Fe Favorite compositions 15-20wt% Cr, 5-10wt% X 13-16wt% Cr, 5-8wt% Pd, 1-5wt% Fe Best compositions and their characteristics Composition, wt% 75% Au, 17% Cr, 8% Pd 75% Au, 15% Cr, 7% Pd, 3% Fe Color, L * / a * / b *, YI 82 / 0.56 / 3.7,8.9 83 / 0.45 / 3.0, 7.2 Corrosion, μg / cm ² x7d 0.22 0.44 Hardness, HV 400HV 270HV Polishing ability Yes yes, good Possible applications Casting jewelry Semi-products watchmaking, jewelery Additive manufacturing processes (Selective Laser Melting, ...) watchmaking, jewelery

The study of the Au-Cr-Pd ternary system has made it possible to develop 18-carat alloys with a white color comparable to that of rhodium-plated gold. The best composition corresponds to: at least 75% Au, essentially 17% Cr and essentially 8% Pd.

The Au-Cr-Pd-Fe quaternary system made 18cts alloys with even more attractive properties in terms of color, polishing ability, ductility and price. The best composition corresponds to: at least: 75% Au, essentially 15% Cr, essentially 7% Pd and essentially 3% Fe.

Cr is introduced as the main bleaching element. Pd and Fe are introduced to improve the solubility of chromium in gold, without altering the desired white color. Only the combination of the three elements of alloy Cr, Pd and Fe has made it possible to develop compositions combining a color close to rhodium-plated golds and a polishing ability allowing the production of pieces of jewelery, timepieces, jewelery, or jewelry. writing instruments.

Claims (8)

1. Grey gold alloy comprising by weight: at least 75% Au; and between 13% and 17% of Cr; characterized in that it further comprises between 5% and 10% of Pd; and between 1 % and 5% of Fe; so that the alloy is characterized by a yellowness index of less than 10.
2. Alloy according to claim 1, wherein the composition of the Cr is comprised between 13% and 16%; the composition of the Pd is comprised between 5% and 8%; and the composition of the Fe is comprised between 1 % and 4%.
3. Alloy according to claim 1 or 2, comprising by weight: at least 75% Au, essentially 15% Cr, essentially 7% Pd and essentially 3% Fe.
4. Alloy according to one of the claims 1 or 2, comprising by weight: at least 75% Au, essentially 17% Cr and essentially 8% Pd.
5. Alloy according to claim 4, characterized by a colour L*a*b* having the component L* of approximately 82, the component a* of approximately 0.56 and the component b* of approximately 3.7.
6. Alloy according to claim 3, characterized by a colour L*a*b* having a component L* of approximately 82, a component a* of approximately 0.45 and a component b* of approximately 3.0.
7. Alloy according to one of the claims from 1 to 6, used for applications in watchmaking, jewellery and gems or writing implements.
8. Item of watchmaking, jewellery, gems or writing implements comprising at least one component made with the alloy according to one of the claims from 1 to 6.