Method for plasma polishing a timepiece or piece of jewellery

Abstract

The present invention relates to a method of plasma polishing of a watch or jewelry item comprising the following steps: - in an electrochemical cell, an electrolyte solution is brought into contact with a watch or jewelry item, comprising two metal parts assembled by a platinum alloy braze, the two metal parts being made of a gold, platinum or palladium alloy, - an electrical voltage is applied in the electrochemical cell so as to generate a plasma membrane on a surface of the braze, the watch or jewelry item corresponding to the anode.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a method for plasma polishing a timepiece or jewelry item comprising at least two pieces of gold of at least 750% or platinum of at least 800%, which are assembled together by brazing. The invention also relates to a brazing composition, as well as to a timepiece or jewelry item having a brazing material whose material comprises such a composition. TECHNOLOGICAL BACKGROUND

[0002] Electropolishing, or electrochemical polishing, is a well-known technique used to remove material from the surface of a metal part.

[0003] This type of process allows for homogeneous material removal by anodic dissolution, in order to reduce the roughness of the treated surface. This gives the resulting surface a shine. The main advantage of this treatment compared to automated tribofinishing treatments and manual polishing is that the parts are not subject to thermal or mechanical stress. Parts with complex geometries and very hard metals can also be treated more easily than with the methods mentioned above.

[0004] Electropolishing a part usually requires a current generator (direct or pulsed), an electrolyte, a cathode (negatively polarized) and an anode (positively polarized).

[0005] The cathode is the seat of reduction reactions, while the anode is the seat of oxidation reactions.

[0006] The most well-known electropolishing solutions are those dedicated to steels and copper metals. They are composed of a mixture of acids at different concentrations and sometimes organic additives.

[0007] Solutions for precious metals usually contain large amounts of complexing agents, such as thiocyanate salts or thiourea.

[0008] Electropolishing involves a region where the metal to be treated dissolves preferentially, called the electropolishing region or electropolishing plateau, range or step. It is in this region that a viscous film forms on the surface of the anode, which allows the part to be polished.

[0009] Electropolishing allows for macro-smoothing and micro-smoothing of the surface, depending on the nature of the electrolyte used. Macro-smoothing is defined as the removal of roughness and defects greater than 1 µm in height, and micro-smoothing as the removal of roughness and defects less than 1 µm.

[0010] Electropolishing does, however, have some disadvantages.

[0011] Indeed, the electrolyte used is often harmful to health or the environment. In addition, the degree of gloss of the treated surface is sometimes insufficient depending on the type of parts they are intended for. Some micro-roughness may also still be visible after treatment.

[0012] Plasma polishing, or “Plasma Electrolytic Polishing” in English (acronym PEP), can at least partially resolve these drawbacks.

[0013] This type of process makes it possible to smooth the surface of a metal part to be treated, by generating a plasma at a surface of the part.

[0014] Plasma polishing involves placing the metal workpiece as an anode in an electrochemical cell, the nature of the electrolyte of which has been previously determined. A voltage, usually between 100 V and 1000 V, is applied across the electrochemical cell to generate a plasma membrane on the surface of the metal workpiece to be treated, which acts to remove material from the surface of the workpiece to be polished. The plasma membrane is maintained for a period effective to bring about a satisfactory reduction in the roughness of the workpiece.

[0015] Plasma metal polishing has several advantages over electrochemical polishing. It uses electrolytes that are less hazardous to health and less harmful to the environment, particularly without cyanide derivatives. It also allows access to the entire exterior surface of the part, regardless of the complexity of its geometry, and leads to efficient and consistent roughness reduction.

[0016] However, in the case where the part to be treated consists of two metal elements or parts joined by brazing, the brazing is likely to deteriorate during the plasma polishing process. More specifically, this process can lead to uneven material removal between the metal elements and the brazing, particularly when the materials of the metal elements and the brazing are different, whether in their nature or in their contents. Brazing consists of joining two metal parts by melting a filler metal whose melting point is lower than that of the metal parts.

[0017] The heterogeneity of the treatment leads to a piece whose visual appearance is not satisfactory, which is particularly problematic in the luxury industry when the piece is an item of jewelry or watchmaking whose visual qualities must be remarkable and meet very strict specifications. BRIEF DESCRIPTION OF THE INVENTION

[0018] An aim of the invention is to provide a method for plasma polishing a watch or jewelry item, making it possible to overcome the aforementioned drawbacks (alteration of the solder and unsatisfactory visual appearance).

[0019] The invention aims in particular to provide such a plasma polishing method for treating a watch or jewelry item comprising two metal parts assembled by brazing, leading to a homogeneous removal of material from the metal parts and the brazing. The method thus aims to avoid structurally and visually damaging the brazing.

[0020] To this end, the invention proposes a method for plasma polishing a watch or jewelry item comprising the following steps: in an electrochemical cell, an electrolyte solution is brought into contact with a timepiece or jewelry item, comprising two metal parts assembled by a platinum alloy brazing having a melting temperature of less than 1000°C, the two metal parts being made of a gold alloy comprising at least 583 ‰ of gold (advantageously at least 750 ‰ of gold), or of a platinum alloy comprising at least 500% of platinum (advantageously at least 950% of platinum), or of a palladium alloy comprising at least 500% of palladium (advantageously at least 950% of palladium), an electrical voltage is applied in the electrochemical cell so as to generate a plasma membrane on at least one surface of the brazing, the timepiece or jewelry item corresponding to the anode.

[0021] Since precious metals are expensive, controlling material removal makes it possible to reduce or at least control the manufacturing costs of the watch or jewelry item.

[0022] Thus, the plasma polishing process according to the invention is particularly suitable for the luxury industry by improving the aesthetics of the brazing while limiting material loss. Watch or jewelry item

[0023] As already indicated, the watch or jewelry item comprises two metal parts assembled by a platinum alloy brazing.

[0024] The metal parts joined by soldering are made of gold alloy or platinum alloy.

[0025] When the alloy is gold-based (at least 750% by mass of gold or at least 18 carats), it may, for example, contain at least 917% by mass of gold (22 carats). This may include yellow gold, white gold, pink gold or red gold.

[0026] When the alloy is platinum-based (at least 500 ‰ by mass of platinum), it advantageously comprises at least 800 ‰ by mass of platinum, more advantageously at least 950% by mass of platinum, for example 953% by mass of platinum.

[0027] When the alloy is palladium-based (at least 500 ‰ by mass of palladium), it advantageously comprises at least 800% by mass of palladium, more advantageously at least 950% by mass of palladium, for example 953% by mass of palladium. According to a particular embodiment, the two metal parts joined by brazing comprise the same alloy (gold or platinum or palladium), at least at the brazing level.

[0028] According to another particular embodiment, one metal part is made of gold alloy and the other metal part is made of gold alloy of a different color and / or different carat weight, at least at the level of the brazing.

[0029] According to another particular embodiment, one metal part is made of gold alloy and the other metal part is made of platinum or palladium alloy, at least at the solder. According to another particular embodiment, one metal part is made of platinum alloy and the other metal part is made of palladium alloy, at least at the solder.

[0030] The watch or jewelry item may include precious stones (diamonds, emeralds, rubies, sapphires).

[0031] By way of non-limiting examples, the watch or jewelry item may be a pendant, a ring, earrings, a necklace, a bracelet, etc. Solder

[0032] As already mentioned, brazing is an operation of joining two metal parts by melting a filler alloy, without melting the alloys of the metal parts. The melting temperature of the filler alloy (braze) is lower than that of the alloys of the metal parts being joined, so that the alloy of the metal parts and the filler alloy (braze) differ at least in the nature or in the contents of the metals.

[0033] Platinum alloy brazing is able to resist structural degradation when used in the plasma polishing process.

[0034] Generally speaking, the platinum alloy used to form the solder has a lower melting temperature than the alloys of the two metal parts, at least at the solder joint. For this reason, this alloy constituting the solder can have a eutectic composition, that is to say a platinum-based composition whose behavior, during its fusion, corresponds to that of a pure body.

[0035] In the process according to the invention, the platinum alloy (braze) has a melting temperature of less than 1000°C, advantageously less than or equal to 800°C. It is advantageously greater than or equal to 100°C, more advantageously greater than 400°C. These temperature ranges allow industrial optimization, while avoiding degradation of the metal parts during brazing.

[0036] Advantageously, the platinum alloy (braze) is an alloy comprising at least 500‰ by mass of platinum, for example at least 800‰.

[0037] The platinum alloy (braze) is advantageously an alloy comprising at least 500 ‰ by mass of platinum (more advantageously at least 800 ‰) and at least one alloying element chosen from: 10 to 50% by mass of silicon, 20 to 110% by mass of copper, advantageously 70 to 110%, 2 to 220% by mass of germanium, or 2 to 200% when the alloy comprises at least 800% by mass of platinum, 50 to 220% by mass of gallium, or 50 to 200% when the alloy comprises at least 800% by mass of platinum, 50 to 150% by mass of silver, 2 to 170% by mass of tin, the sum of the platinum and alloying element contents being equal to 1000 ‰.

[0038] The platinum alloy (braze) used has a composition advantageously chosen from the following compositions:at least 800 ‰ of platinum (for example at least 850 ‰), between 10 and 50 ‰ of silicon, between 70 and 110 %o of copper, between 10 and 40 %o of germanium, for example of type Pt 850-851 Si 23-24 Cu 99-100 Ge 26-27 , at least 800 ‰ of platinum (for example at least 845 ‰), between 10 and 50 ‰ of silicon, between 90 and 120 of germanium, for example of type Pt 849-850 Si 43-44 Ge 107-108 , at least 800 ‰ of platinum (for example at least 810 ‰), between 10 and 50 ‰ of silicon, between 70 and 110 ‰ of copper, between 2 and 30 of germanium, between 50 and 100 of gallium, for example of type Pt 814 - 815 Si 12-14 Cu 76-77 Ge 15-16 Ga 80-81 , at least 950 ‰ of platinum (for example at least 960 ‰), between 10 and 50 ‰ of silicon, for example of type Pt 960-962 Si 38-40 , at least 800 ‰ of platinum (for example at least 805 ‰), between 100 and 200 of gallium, for example of type Pt 807-808 Ga 192-193 , at least 800 ‰ of platinum (for example at least 820 ‰), between 100 and 200 of germanium,for example of type Pt 823-824 Ge 176-177 , at least 800 ‰ of platinum (for example at least 804 ‰), between 50 and 150 ‰ of gallium, between 50 and 150 of silver, for example of type Pt 804-806 Ga 94-96 Ag 99-101 , at least 800 ‰ of platinum (for example at least 850 ‰), between 10 and 50 %o of silicon, between 50 and 150 %o of silver, for example of type Pt 849-851 Si 29-31 Ag 119-121 at least 840 ‰ of platinum (for example at least 849 ‰), between 140 and 160 ‰ of tin, for example of type Pt 849-851 Sn 149-151. ,

[0039] In addition to the possible presence of impurities (less than 1%o), the sum of the contents in thousandths (‰) of the metals in the platinum alloy (braze) is equal to 1000 thousandths (1000%o).

[0040] The present invention also relates to this solder. Electrolyte

[0041] The electrolyte is advantageously an aqueous solution of salts or organic compounds. For example, it may be an aqueous solution comprising a compound selected from ammonium sulfate, ammonium halides such as ammonium fluoride or ammonium chloride, citric acid, urea, glucose, sodium saccharin, ammonium nitrate and mixtures thereof.

[0042] The composition of the electrolyte can be adapted according to the nature of the alloy of the metal parts assembled by a platinum alloy brazing. For example, the electrolyte can be an aqueous solution comprising: ammonium sulfate, citric acid, urea, glucose and sodium saccharin, especially when the metal parts are made of platinum alloy, or gold alloy, a mixture of ammonium trichloride and ammonium chloride, a mixture of ammonium sulfate, EDTA (ethylenediaminetetraacetic acid), ammonium citrate and sodium tartrate (or sodium oxalate), a mixture of ammonium chloride and ammonium fluoride, a mixture of ammonium fluoride and ammonia, a mixture of ammonium sulfate, ammonium fluoride, ammonia and citric acid, , especially when the metal parts are made of gold alloy.

[0043] Advantageously, the electrolyte comprises ammonium sulfate and ammonium fluoride when the metal parts are made of gold alloy.

[0044] Advantageously, the electrolyte comprises ammonium chloride and ammonium nitrate when the metal parts are made of platinum alloy or palladium alloy.

[0045] The electrolyte may also include media, such as beads. Plasma polishing conditions

[0046] In the method according to the invention, the watch or jewelry item corresponds to the anode of the electrochemical cell while the cathode is made of conventional metal (for example titanium).

[0047] The generation of a plasma is achieved by applying an electrical voltage between the anode and the cathode, advantageously between 100 and 1000 V, more advantageously between 300 and 400 V, for example between 320 and 350 V.

[0048] According to a particular embodiment, the electrical voltage is lowered after immersion of the watch or jewelry item in the electrolyte.

[0049] The duration of the polishing treatment (application of electrical voltage) is advantageously between 1 minute and 120 minutes, more advantageously between 5 minutes and 120 minutes, more advantageously between 30 minutes and 60 minutes.

[0050] During polishing, the electrolyte is at a temperature advantageously between 50 and 95°C, more advantageously between 60 and 95°C, even more advantageously between 55 and 80°C, for example between 60 and 70°C.

[0051] The electrolyte can be heated before or after applying the electrical voltage.

[0052] Applying an electrical voltage to the electrochemical cell generates a plasma membrane on the surface of the solder, but also on the metal parts. More precisely, a viscous film forms on the surface of the anode and causes polishing by dissolution in a homogeneous manner.

[0053] If the polishing conditions are not respected, the viscous film does not form and the granular appearance may even be accentuated due to possible attack on the grain boundaries of the metal parts.

[0054] In other words, the plasma polishing process according to the invention makes it possible to optimize and control the removal of material over the entire part subjected to polishing.

[0055] After immersing the watch or jewelry item (anode) and the cathode in the electrolyte bath, applying an electrical voltage between the anode and the cathode generates an electric current that results in complex reactions. Many reaction hypotheses exist, including, at the anode, the following oxidation reactions: 2H 2 O (1) ⇄ 4H +< (aq) + O 2 (g) + 4e -< M (s) ⇄ M n+< (aq) + ne -<

[0056] In these equations, M represents a metal (metal parts and solder). The metal oxidizes, losing n electrons denoted e -< , and becomes positively charged with a corresponding charge n+. In doing so, the initially solid metal passes into solution in ionic form, leading to a removal of material from the surface of the metal parts and the solder. In addition, a release of oxygen O 2 (g) is observed in the form of gas.

[0057] The cathode can involve the corresponding reduction reactions: 2H +< (aq) + 2e -< ⇄ H 2 (g) M n+< (aq) + ne -< ⇄ M (s)

[0058] Thus, the metal ions M n+< (aq) in solution pass into solid form M (s) by being deposited on the surface of the cathode. In addition, a release of hydrogen H 2 (g) is observed in the form of gas.

[0059] The polishing conditions and the nature of the gold and platinum alloys used allow for uniform polishing across the entire surface of the metal parts and the solder. The solder is also not degraded and also has an excellent visual appearance, comparable to that of metal parts.

[0060] In general, the polishing process according to the invention makes it possible to reduce the surface roughness to a Ra of 0.22 µm or even less.

[0061] Thus, advantageously, the watch or jewelry item, having been subjected to the plasma polishing process according to the invention, has a surface roughness of less than 0.5 µm, more advantageously less than 0.3 µm, for example between 0.01 µm and 0.25 µm.

[0062] The present invention also relates to a timepiece or jewelry item comprising two metal parts assembled by a platinum alloy brazing according to the invention, the two metal parts being made of a gold alloy comprising at least 583 ‰ of gold or a platinum alloy comprising at least 500% of platinum, or a palladium alloy comprising at least 500% of palladium, this watch or jewelry item having a surface roughness of less than 0.5 µm, more advantageously less than 0.3 µm.

[0063] The invention and the advantages arising therefrom will become more apparent from the following examples given to illustrate the invention and not in a limiting manner. EXAMPLES OF CARRYING OUT THE INVENTION

[0064] An article by assembling two pieces of platinum alloy (Pt 953 Cu 47 ), by brazing using a platinum alloy (Pt 807-808 Ga 192-193 ).

[0065] This article was then subjected to a plasma polishing process in an electrochemical cell comprising an electrolyte (aqueous solution of ammonium sulfate and ammonium chloride).

[0066] For this, a voltage of 365 V is applied between the anode (article in Pt 953 Cu 47 ) and the cathode (titanium) so as to generate a plasma membrane on the surface of the article.

[0067] This voltage is applied for a period of 30 minutes while the temperature of the electrolyte is 65°C.

[0068] The polished article has a homogeneous appearance and a surface roughness of between 0.01 µm and 0.3 µm. On the other hand, an identical article having undergone conventional electropolishing has a certain heterogeneity making its visual appearance unsatisfactory.

[0069] Plasma polishing thus provides an alternative to manual polishing.

[0070] The advantages of plasma polishing include its ease of implementation and low cost, while allowing industrial optimization of polishing, which is not possible for manual polishing.

Claims

1. A method of plasma polishing a timepiece or jewelry item comprising the following steps: - in an electrochemical cell, an electrolyte solution is brought into contact with a timepiece or jewelry item, comprising two metal parts joined by a platinum alloy braze having a melting point of less than 1000°C, the two metal parts being made of a gold alloy comprising at least 583 ‰ of gold or a platinum alloy comprising at least 500% platinum, or a palladium alloy comprising at least 500% palladium, - an electrical voltage is applied in the electrochemical cell so as to generate a plasma membrane on at least one surface of the braze, the timepiece or jewelry item corresponding to the anode.

2. Plasma polishing method according to claim 1, characterized in thatthe platinum alloy of the brazing has a melting temperature advantageously less than or equal to 1000°C, more advantageously less than or equal to 800°C.

3. A plasma polishing method according to claim 1 or claim 2, characterized in that the platinum alloy of the solder comprises at least 800% by mass of platinum.

4. Plasma polishing method according to any one of the preceding claims, characterized in that the platinum alloy of the brazing is an alloy comprising at least 800% by mass of platinum and at least one alloying element chosen from: - 10 to 50% by mass of silicon, - 20 to 110% by mass of copper, - 2 to 200% by mass of germanium, - 50 to 200% by mass of gallium, - 50 to 150% by mass of silver, - 2 to 170% by mass of tin, the sum of the contents of the metals being equal to 1000 ‰.

5. Plasma polishing method according to any one of the preceding claims, characterized in thatthe platinum alloy of the brazing is an alloy chosen from the group consisting of: - Pt 850-851 If 23-24 Cu 99-100 Ge 26-27 , - Pt 849-850 If 43-44 Ge 107-108 , - Pt 814-815 If 12-14 Cu 76-77 Ge 15-16 Ga 80-81 , - Pt 960-962 If 38-40 , - Pt 807-808 Ga 192-193 , - Pt 823-824 Ge 176-177 , - Pt 804-806 Ga 94-96 Ag 99-101 , - Pt 849-851 If 29-31 Ag 119-121 , - Pt 849-851 Sn 149-151 , the sum of the metal contents being equal to 1000 ‰.

6. Plasma polishing method according to any one of the preceding claims, characterized in that the electrolyte is an aqueous solution comprising a compound selected from ammonium sulfate, ammonium halides such as ammonium fluoride or ammonium chloride, citric acid, urea, glucose, sodium saccharin, ammonium nitrate and mixtures thereof.

7. Plasma polishing method according to any one of the preceding claims, characterized in that the applied electrical voltage is between 100 and 1000 V, advantageously between 300 and 400 V.

8. Plasma polishing method according to any one of the preceding claims, characterized in that the electrical voltage is lowered after immersing the watch or jewelry item in the electrolyte.

9. Plasma polishing method according to any one of the preceding claims, characterized in that the electrical voltage is applied for a period of between 1 and 120 minutes, advantageously between 30 and 60 minutes.

10. Plasma polishing method according to any one of the preceding claims, characterized in that , during polishing, the electrolyte is at a temperature between 50 and 95°C, advantageously between 60 and 70°C.

11. Plasma polishing method according to any one of the preceding claims, characterized in that , during polishing, the electrolyte is at a temperature between 50 and 95 °C, advantageously between 55 and 80 °C, the electrical voltage is applied for a period between 5 minutes and 120 minutes, the electrical voltage being between 300 and 400 V.

12. Platinum alloy brazing, capable of resisting structural degradation when used in a plasma polishing process, comprising at least 500 ‰ by mass of platinum and one or more metals chosen from: silver, silicon, gallium, germanium, copper in which the brazing has a melting temperature of less than 1000°C, preferably less than 800°C.

13. Metal brazing according to claim 12, characterized in thatit comprises at least 800 ‰ by mass of platinum and at least one alloying element chosen from: - 10 to 50 % by mass of silicon, - 20 to 110 % by mass of copper, - 2 to 200 % by mass of germanium, - 50 to 200 % by mass of gallium, - 50 to 150 % by mass of silver, - 2 to 170 % by mass of tin, the sum of the contents of the metals being equal to 1000 ‰.

14. Metal brazing according to claim 12 or 13, characterized in that it presents a composition chosen from the following compositions: - Pt 850-851 If 23-24 Cu 99-100 Ge 26-27 , - Pt 849-850 If 43-44 Ge 107-108 , - Pt 814-815 If 12-14 Cu 76-77 Ge 15-16 Ga 80-81 , - Pt 960-962 If 38-40 , - Pt 807-808 Ga 192-193 , - Pt 823-824 Ge 176-177 , - Pt 804-806 Ga 94-96 Ag 99-101 , - Pt 849-851 If 29-31 Ag 119-121 , - Pt 849-851 Sn 149-151 , the sum of the metal contents being equal to 1000 ‰.

15. A timepiece or jewelry item comprising two metal parts assembled by a platinum alloy brazing according to one of claims 12 to 14, the two metal parts being made of a gold alloy comprising at least 583 ‰ of gold or a platinum alloy comprising at least 500% of platinum, or a palladium alloy comprising at least 500% of palladium, this timepiece or jewelry item having a surface roughness of less than 0.5 µm, more advantageously less than 0.3 µm.

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