DK2546371T3

DK2546371T3 - 18 carat gray gold

Info

Publication number: DK2546371T3
Application number: DK12176002.9T
Authority: DK
Inventors: Niklaus Baltzer; Thierry Copponnex; Guo Kangping Huang; Laurence Simonet
Original assignee: Cendres + Métaux Sa
Priority date: 2011-07-12
Filing date: 2012-07-11
Publication date: 2017-04-03
Also published as: PT2546371T; ES2616542T3; EP2546371A1; EP2546371B1

Description

18-carat grey gold

Description

Technical domain [0001] The present invention concerns an 18-carat grey gold having a white colour comparable to that of rhodium-plated gold and being well suited for polishing for applications in watchmaking, jewellery and gems or for writing implements.

State of the art [0002] 18-carat grey gold alloys (also called white gold) are usually composed of gold, copper and elements that have the power to whiten gold. These whitening elements are often found in group VIII of the periodical table of elements, such as Ni, Pd, Co and Fe. The whitening effect of certain metals on gold was the object of studies from the 1970ies onwards, as described in reference 1: "Improvement of 18 carat white gold alloys", G.P. O'Connor, Gold, Bull. 11, 2 (1978).

[0003] On the current market, grey gold alloys can be distinguished in two categories: palladium gold and nickel gold. Nickel contributes an interesting colour and is economically advantageous, but its allergenic effect has virtually banned it from luxury applications (jewellery, watchmaking).

[0004] In order to whiten gold without nickel, various options are explored, with their drawbacks. It is for example possible to increase the contents of platinum elements. This solution is expensive and does not enable a sufficiently white gold to be obtained. Another solution consists in rhodium-plating the surface of the final product In this case, a white colour is well achieved but as it is a superficial treatment, the coloured layer is sensitive to wear and tear due to friction and the white colour is likely to deteriorate over time.

[0005] From 2003, the applicant has explored different metallurgical systems with the aim of developing an 18-carat grey gold. The studied alloys 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. It is a classical composition for a jewellery alloy. In this first study, the whitest alloy obtained was composed of 75%Au, 21%Pd, 2%Ag and 2%Rh. Table 1 provides colorimetric measurements on the L*a*b* scale according to the 1976 C.I.E. model, of different 18-carat grey alloy compositions available on the market. The Yellowness Index (YI) value is also indicated. This parameter, calculated from the three L*a*b* coordinates, is commonly used for characterizing the white colour 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 grey gold: non-white (YI>32); off-white (24.5>YI>32); standard-white (19>YI>24.5); premium-white (YI>19).

Table 1

[0006] The applicant has also studied the development of a grey gold in the Au-In system. In particular, the feasibility of an industrial exploitation of the Au75In25 alloy as alloy for jewellery was explored. According to the phase diagram of the Au-In system, Au75In25 is in the zone (Ψ) which is an intermetallic phase (Au3In2). This composition gives the alloy an extraordinarily white colour, but its intermetallic structure gives it a very great fragility. This alloy is unfortunately practically unusable. The adjunction of elements such as Sn, Al, Zn, Cr and Nb was also considered in order to obtain 18-carat ternary gold alloys based on the Au-In system. It has been shown that the adjunction of an element comprising one among: Sn, Al, Zn, Cr and Nb however generates either a loss of colour or a non-homogenous structure, and does not sufficiently improve the alloy's ductility.

[0007] The whitening power of chromium on gold is a known characteristic that has been cited in a certain 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, Vol. 67, No. 11 (2003), and the reference 3: "White gold alloys for investment casting", M. Poliero, Gold Technology, No. 31 (2001). On the basis of this observation, the applicant has chosen to orient its research towards the Au-Cr system.

[0008] The use of Cr for whitening gold (combined with Ag, Cu, Ni and Fe) is known notably from patent application DE10027605. This document describes Cr contents not exceeding 14% by weight.

[0009] Patent application W02009/092920 also cites the use of Cr for whitening gold. In particular, it discloses the combination of gold to a refractory metal from the columns IVB, VB and VIB of the periodic classification (including Cr), without indicating the refractory element contents.

[0010] Patent application JPH0657356 describes a decorative gold alloy containing, by weight, 75% Au, 15 to 24% of at least one of Fe and Cr, and 1 to 10% Ni. The alloy is subjected to a treatment at a temperature comprised between 250 and 500°C with an oxidizing flame so as to obtain a colour ranging from yellow, red, purple, blue, green to grey.

[0011] Although the whitening power of Cr has already been demonstrated, its use is limited due to its partial miscibility with gold. Indeed, if the solubility limit of Cr in Au is exceeded, the alloy becomes inhomogeneous. The Cr precipitates present in the material deteriorate the suitability for polishing of the alloy, which then becomes unusable for an application in jewellery/watchmaking. The difficulty in developing a grey gold with Cr thus consists in introducing additional alloy elements enabling the solubility of Cr in gold to be increased, without alteration to the white colour.

Brief summary of the invention [0012] One aim of the present invention is to propose an 18-carat grey gold alloy free from the limitations of the known grey gold.

[0013] Another aim of the invention is to obtain an 18-carat gold alloy of a white colour, throughout, comparable to that of a rhodium-plated gold, and having a good surface quality after mirror polishing.

[0014] According to the invention, these aims are achieved notably by means of a grey gold alloy comprising by weight: essentially 75% gold, between 13% and 17% of Cr; between 5% and 10% of Pd; and between 1% and 5% of Fe.

[0015] In one embodiment, the alloy comprises by weight 75% Au, 17%Cr and 8%Pd. This alloy is 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.

[0016] In yet another embodiment, 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%.

[0017] 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 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.

[0018] The present disclosure also concerns an item of watchmaking, jewellery, gems or a writing implement comprising at least one component made with the alloy of the invention.

[0019] This solution has notably the advantage over the prior art to obtain an 18-carat gold alloy making it possible to combine a colour close to rhodium-plated gold and a suitability for polishing for the manufacture of items of jewellery, watchmaking, gems or writing implements.

Example(s) of embodiments of the invention [0020] Good surface quality is directly linked to the homogeneity of the alloy. If the alloy exhibits a multi-phased structure, inhomogeneities will appear at the mirror-polished surface.

[0021] In one embodiment, an 18-carat grey gold alloy comprises by weight: at least 75% gold; between 13% and 23% of Cr; between 2% and 12% of at least one element chosen from among: Pd, Fe, Ru, Pt, Co, Ga, Ge, Μη, V, Ni. In a variant, the composition of the Cr can be comprised between 15% and 20% and the composition of said at least one element chosen from among: Pd, Fe, Ru, Pt, Co, Ga, Ge, Μη, V, Ni is comprised between 5% and 10%.

[0022] In another embodiment, the alloy comprises between 2% and 12% of Pd and at most 5% of at least one element chosen from among: Fe, Ru, Pt, Co, Ga, Ge and Mn. In this case, the composition of the Cr can be comprised between 13% and 18%. The composition of the Pd can be comprised between 5% and 10%.

Example(s) of embodiments of the invention [0023] Good surface quality is directly linked to the homogeneity of the alloy. If the alloy exhibits a multi-phased structure, inhomogeneities will appear at the mirror-polished surface.

[0024] In one embodiment, an 18-carat grey gold alloy comprising by weight: at least 75% gold; between 13% and 23% of Cr; between 2% and 12% of at least one element chosen from among: Pd, Fe, Ru, Pt, Co, Ga, Ge, Μη, V, Ni. In a variant, the composition of the Cr can be comprised between 15% and 20% and the composition of said at least one element chosen from among: Pd, Fe,

Ru, Pt, Co, Ga, Ge, Μη, V, Ni is comprised between 5% and 10%.

[0025] In another embodiment, the alloy comprises between 2% and 12% of Pd and at most 5% of at least one element chosen from among: Fe, Ru, Pt, Co, Ga, Ge and Mn. In this case, the composition of the Cr can be comprised between 13% and 18%. The composition of the Pd can be comprised between 5% and 10%.

[0026] In yet another embodiment, the alloy further comprises at most 1% of an element chosen from among the 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. The expression "neutral elements" here means elements that have no noticeable effect on the main characteristics of the alloy as described here above and comprising these neutral elements.

[0027] In another preferred embodiment, the 18-carat grey gold alloy comprises by weight: at least 75%Au, essentially 17%Cr, and essentially 8%Pd.

[0028] The whitening effect of the Pd is widely used in conventional 18-carat grey gold. Also, as Cr is greatly soluble in the Pd, this element has been introduced as secondary adjunction element in various proportions in order to improve the matter's homogeneity. Use of this element in fact allows high Cr contents to be maintained without alteration to the white colour. The suitability for polishing has been increased but is not optimal because Cr precipitates are sometimes present since casting. In this Au-Cr-Pd system, the most favourable composition contains essentially 17% Cr and essentially 8% Pd (see table 2).

[0029] The Au-Cr-Pd-Fe system was also examined. In this system, the Fe is in fact highly soluble in the Cr and Pd at high temperature. Furthermore, it is an element known for whitening gold (see references 2 and 3). The substitution of some mass percentages of Cr in favour of Fe has made it possible to improve notably the homogeneity of the matter and thus the suitability for polishing, whilst maintaining a very interesting colour.

[0030] In another embodiment, the 18-carat grey gold alloy comprises by weight: at least 75% gold; between 13% and 17% Cr; between 5% and 10% Pd; and between 1% and 5% Fe. In a preferred manner, the composition of the Cr is comprised between 13% and 16%; and the composition of the Pd is comprised between 5% and 8%. In another preferred manner, the gold alloy comprises by weight: at least 75%Au, essentially 15%Cr, and essentially 7%Pd and essentially 3%Fe. This latter alloy constitutes the best compromise between colour and suitability for polishing.

[0031] Examples of conventional alloy compositions and of the alloy according to the invention are mentioned in table 2. In particular, table 2 reports the results of measurements on the L*a*b* scale according to the 1976 C.I.E. model and the Yellowness Index (YI), measured according to standard ASTM D-1925 for each of these alloys, as well as observations on the quality of polishing. More particularly, table 2 reports colorimetric measurements and observations on the quality of polishing of four conventional grey gold alloys (numbers 1 to 4) and seven examples of gold alloys according to the invention (numbers 5 to 11). The compositions are expressed in % by weight. For example, measurements have shown notably that the alloy comprising by weight: at least 75% gold; between 13% and 23% Cr; between 2% and 12% of at least one element chosen from among: Pd, Fe, Ru, Pt, Co, Ga, Ge, Μη, V, Ni, is characterized by a yellowness index being lower than 10. A yellowness index below 10 is also obtained for the other alloys of the invention. Here, the coloration is in the alloy's mass and cannot be deteriorated by surface wear and tear.

Table 2

[0032] For the alloy comprising by weight: at least 75%Au, essentially 17%Cr, and essentially 8%Pd, a colour L*a*b* is measured with the component L* of approx. 82, the component a* of approx. 0.56 and the component b* of approx. 3.7.

[0033] For the alloy comprising by weight: at least 75%Au, essentially 15%Cr, essentially 7%Pd and essentially 3%Fe, a colour L*a*b* is measured with the component L* of approx. 82, the component a* of approx. 0.45 and the component b* of approx. 3.0.

[0034] Among the alloys tested, the compositions AuCrl7Pd8 (at least 75%Au, essentially 17%Cr and essentially 8%Pd) and AuCrl5Pd7Fe3 (at least 75%Au, essentially 15%Cr, essentially 7%Pd and essentially 3%Fe) are the most interesting. The composition containing Fe (AuCrl5Pd7Fe3) however shows the most promising characteristics for an application in the luxury sector (jewellery, watchmaking) as it combines both a very white colour with a good suitability for polishing.

[0035] Results of corrosion tests according to the dental standard (ISO 10271-2001) were performed on seven alloys to assess the rate of release (in μg/cm2x7d) of the ions of the sensitive elements such as Ni and Cr (table 3). For a dental application, the standard ISO 10271-2001 limits the total rate of release to 200μg/cm2x7d. According to these assays, the two alloys AuCrl7Pd8 and AuCrl5Pd7Fe exhibit a very good resistance to corrosion.

Table 3

[0036] Table 4 provides a summary of our research into 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 among: Pd, Fe, Ru, Pt, Co,

Ga, Ge, Μη, V, Ni.

Table 4

[0037] Studying the ternary system Au-Cr-Pd has allowed 18-carat alloys having a white colour comparable to that of rhodium-plated gold to be achieved. The best composition corresponds to: at least 75%Au, essentially 17%Cr and essentially 8%Pd.

[0038] The quaternary Au-Cr-Pd-Fe system has allowed 18-carat alloys with even more attractive properties in terms of colour, suitability for polishing, ductility and price to be achieved. The best composition corresponds to: at least 75%Au, essentially 15%Cr, essentially 7%Pd and essentially 3%Fe.

[0039] The Cr is introduced as main whitening element. The Pd and the Fe are introduced in order to improve the solubility of the chromium in gold, without alteration of the white colour that is sought. Only the combination of the three alloy elements Cr, Pd and Fe made it possible to achieve compositions combining a colour close to rhodium-plated gold and a suitability for polishing that allow items of watchmaking, jewellery, gems or writing implements to be made.

Claims

A gray gold alloy comprising, by weight: at least 75% Au; and between 13% and 17% Cr; characterized in that it further comprises between 5% and 10% Pd; and between 1% and 5% Fe; so that the alloy is characterized by a Yellowness Index of less than 10.

The alloy of claim 1, wherein the composition of Cr is between 13% and 16%; the composition of Pd is between 5% and 8%; and the composition of Fe is between 1% and 4%.

An alloy according to any one of the preceding claims 1 or 2, comprising by weight: at least 75% Au, substantially 15% Cr, substantially 7% Pd and substantially 3% Fe.

An alloy according to any one of the preceding claims 1 or 2, comprising by weight: at least 75% Au, substantially 17% Cr and substantially 8% Pd.

An alloy according to claim 4, characterized by a color L * a * b * having the component L * of approx. 82, the component a * of ca. 0.56 and the component b * of approx. 3.7.

An alloy according to claim 3, characterized by a color L * a * b *, with a component L * of approx. 82, a component a * of ca. 0.45 and a component b * of approx. 3.0.

An alloy according to any one of claims 1 to 6, used for applications in watchmaking, jewelery, jewelery or writing tools.

A part for watchmaking, jewelry, jewelery or writing utensils, comprising at least one component made with the alloy according to one of claims 1 to 6.