ANTI-COORROSION PROTECTION FOR WATCH MOVEMENT MAGNETS, ESPECIALLY OF THE NEODYMIUM-IRON-BORON TYPE

Description

Scope of the invention

[0001] The invention relates to a method for protecting a neodymium-iron-boron watch magnet against corrosion, characterized in that a neodymium-iron-boron magnet is used, a surface preparation of said magnet is carried out, before subjecting it to an ion implantation treatment, to create a waterproof surface layer that acts as a barrier to oxidation with all surface bonds saturated by the implanted ions, to prevent corrosion of said magnet in humid environments, under the usual conditions of wearing watches.

[0002] The invention relates to the field of corrosion protection of watch parts, in particular watches, which may be subjected to very large variations in temperature and humidity under the conditions of use for which they are guaranteed, on the surface of the globe for watches for the general public, but also, for particular users, in the space and underwater domains. Background of the invention

[0003] Magnets are susceptible to corrosion, and it is difficult to protect them effectively.

[0004] However, the deposited layer must be inexpensive, requiring low application costs, good resistance to moisture corrosion, no reduction in magnetic properties, and sufficient mechanical strength. These requirements are still poorly addressed by known solutions, which involve depositing layers of Al₂O₃, SiO₂, epoxy, or nickel on the surface.

[0005] The prior art document FR2768551 describes a flat magnet with a coating layer designed to improve its magnetic properties. The document specifies the addition of additives to the coating to enhance mechanical properties or corrosion resistance. Summary of the invention

[0006] The invention aims to produce an anti-corrosion protective layer on watch magnets, and in particular on NdFeB magnets, by ion implantation.

[0007] For this purpose, the invention relates to a method of protecting watchmaking magnets against corrosion, according to claim 1 or 3 annexed. Detailed description of preferred embodiments

[0008] The invention relates to a method for protecting a neodymium-iron-boron watch magnet against corrosion, characterized in that a neodymium-iron-boron magnet is used, a surface preparation of said magnet is carried out, before subjecting it to an ion implantation treatment, to create a waterproof surface layer that acts as a barrier to oxidation with all surface bonds saturated by the implanted ions, to prevent corrosion of said magnet in humid environments, under the usual conditions of wearing watches.

[0009] Ion implantation technology makes it possible to create a barrier to natural oxidation by saturating the surface layer, particularly the oxide or nitride layer in the case of ion implantation treatment under oxygen or nitrogen respectively.

[0010] This saturation layer is created by accelerating multicharged ions from an O2, N2, or other plasma, with a potential difference between 10 kV and 40 kV. The ions thus become denser at the surface, at different depths (depending on the ion's charge), forming an oxidation barrier at the extreme surface, as all surface bonds are saturated by the implanted ions.

[0011] More particularly, for the implementation of the process according to the invention, a neodymium-iron-boron magnet is used, with 22% to 24% by mass of neodymium, 65% to 67% by mass of iron, 0.1% to 2% by mass of boron, the total being 100%, and a surface preparation of the magnet is carried out before subjecting it to an ion implantation treatment, to create a surface layer of watertight oxidation or nitriding, to prevent corrosion of the magnet in humid environments, under the usual conditions of wearing watches.

[0012] According to the invention as defined in the attached independent claim 1, an oxidation ion implantation treatment is carried out under oxygen, with a voltage of 10 kV to 40 kV.

[0013] In particular, an oxidation ion implantation treatment is performed under oxygen, with a voltage of 24 kV to 26 kV, a beam power of 5 mA to 7 mA, and a dose of 20. 10 16< to 30. 10 16< ions per square centimeter.

[0014] According to the invention as defined in the attached independent claim 3, an ion implantation nitriding treatment is carried out under nitrogen, with a voltage of 10 kV to 40 kV.

[0015] In particular, an ion implantation nitriding treatment is performed under nitrogen, with a voltage of 24 kV to 26 kV, a beam power of 5 mA to 7 mA, and a dose of 20. 10 16< to 30. 10 16< ions per square centimeter.

[0016] More specifically, the surface preparation of the magnet is carried out by sandblasting followed by rinsing with alcohol and air drying.

[0017] More specifically, sandblasting is carried out with alumina particles of a size of 200 mesh to 240 mesh, with a pressure of 1.4 to 1.8 bar, and a distance of 13 mm to 17 mm between the flow and the surface to be sandblasted.

[0018] More specifically, the process is applied to a magnet with a thickness less than or equal to 1.0 mm, and whose largest dimension is less than or equal to 8.0 mm.

[0019] More specifically, the process is applied to a magnet with a thickness less than or equal to 0.6 mm, and whose largest dimension is less than or equal to 5.0 mm.

[0020] More specifically, the process is applied to a magnet with a through hole whose largest dimension is between 0.2 mm and 3.0 mm.

[0021] In a non-limiting watchmaking application, a toroidal or cylindrical magnet made of NdFeB, with an outer diameter of approximately 0.9 mm to 5.0 mm, a through orifice with an inner diameter of approximately 0.21 mm to 3.0 mm, and a thickness of approximately 0.15 mm to 0.55 mm, is subjected to ion implantation treatment to create a waterproof oxidation or nitriding layer, which prevents corrosion of the magnet in humid environments.

[0022] Magnets treated with this technique are visually inspected before and after an accelerated aging test (7 days at 60 degrees Celsius and 90% relative humidity).

[0023] The process proves highly effective for a neodymium-iron-boron magnet, with 23% by mass of neodymium, 66% by mass of iron, and 1% by mass of boron. Visual inspection of magnets of the same type, but without treatment, shows red corrosion typical of NdFeB material. However, when the oxygen ion implantation treatment according to the invention is carried out, this corrosion does not occur.

[0024] Oxygen ion implantation treatment creates an oxidation barrier on the magnet's surface. The surface becomes in a state where the corrosion rate is significantly slowed by the presence of an artificial passive surface state, compared to what it would be in the absence of this oxide layer.

[0025] Ion implantation nitriding, on the other hand, allows nitrogen to be incorporated into the surface of the magnet. The nitrogen reacts with the iron, diffusing onto the surface layers of the magnet. A layer of iron nitrides is created on the surface, which prevents oxygen from penetrating the interior of the parts.

[0026] The results thus show that protection by ion implantation of oxygen or nitrogen makes it possible to protect the NdFeB material against corrosion (60° Celsius and 90% relative humidity for 7 days). The two layers, obtained with these two variants of the process according to the invention, each provide a uniform, stable, and dense protective layer, which ensures effective protection against corrosion.

[0027] The invention thus offers significant advantages: good corrosion resistance; innovative and green technology, as it requires low gas consumption and generates little or no emissions; no risk of delamination, since the treatment is carried out in the material itself, and not by coating; repeatable process and easy to develop; creation of a protective layer with negligible magnetic loss; creation of a protective layer with suitable mechanical resistance.

Claims

1. A method for protecting a horology magnet against corrosion, characterised in that a magnet is provided, that said magnet is surface-treated before being exposed to an ion implantation treatment, in order to create an impervious surface layer acting as a barrier to oxidation, with all surface bonds saturated by the implanted ions, to prevent corrosion of said magnet in a humid environment, under normal conditions for wearing watches, characterised in that the ion implantation treatment is an oxidation ion implantation treatment, in oxygen, with a voltage of from 10 kV to 40 kV.

2. The method according to claim 1, characterised in that a said oxidising ion implantation treatment is carried out, in oxygen, with a voltage of from 24 kV to 26 kV, a beam power of from 5 mA to 7 mA, and a dose of from 20 × 1016 to 30 × 1016 ions per square centimetre.

3. A method for protecting a horology magnet against corrosion, characterised in that a magnet is provided, that said magnet is surface-treated before being exposed to an ion implantation treatment, in order to create an impervious surface layer acting as a barrier to oxidation, with all surface bonds saturated by the implanted ions, to prevent corrosion of said magnet in a humid environment, under normal conditions for wearing watches, characterised in that the ion implantation treatment is an oxidation ion implantation treatment, in nitrogen, with a voltage of from 10 kV to 40 kV.

4. The method according to claim 3, characterised in that a said nitriding ion implantation treatment is carried out, in nitrogen, with a voltage of from 24 kV to 26 kV, a beam power of from 5 mA to 7 mA, and a dose of from 20 × 1016 to 30 × 1016 ions per square centimetre.

5. The method according to any of claims 1 to 4, characterised in that said surface treatment is carried out on said magnet by sandblasting followed by rinsing with alcohol and air drying.

6. The method according to claim 5, characterised in that said sandblasting is carried out using alumina particles with a particle size of from 200 mesh to 240 mesh, at a pressure of from 1.4 to 1.8 bar, and with a distance of from 13 mm to 17 mm between the flux and the surface to be sandblasted.

7. The method according to any of claims 1 to 6, characterised in that said method is applied to a magnet with a thickness of less than or equal to 1.0 mm and a largest dimension of less than or equal to 8.0 mm.

8. The method according to claim 7, characterised in that said method is applied to a magnet with a thickness of less than or equal to 0.6 mm and a largest dimension of less than or equal to 5.0 mm.

9. The method according to claim 7 or 8, characterised in that said method is applied to a magnet that is perforated with a through-hole in which the largest dimension is comprised between 0.2 mm and 3.0 mm.