BALANCE SPRING AND METHOD FOR MANUFACTURING THE SAME

Abstract

A balance spring (1) intended to be fitted to a timepiece balance having fixed inertia, the balance spring (1) being formed of a core (10) having lateral faces (10c) connecting an upper face (10a) to a lower face (10b), the balance spring (1) including on one of the lateral faces (10) in one portion of the outer coil (5), a coating formed of one or more layers, the coating including two layers with a first electrically conductive layer (12) coated with a second outer layer (13) made of a ceramic, or a combined layer (13), made of an electrically conductive ceramic. Also a method of manufacturing this balance spring.

Claims

1. A balance spring (1) intended to be fitted to a timepiece balance having fixed inertia, the balance spring (1) being formed of a core (10) having lateral faces (10c) connecting an upper face (10a) to a lower face (10b), said balance spring (1) comprising on one of the lateral faces (10) in one portion of the outer coil (5), a coating formed of one or more layers, said coating being characterized in that it includes: two layers with a first electrically conductive layer (12) coated with a second outer layer (13) made of a ceramic, or a combined layer (13), made of an electrically conductive ceramic.

2. The balance spring (1) according to claim 1, characterized in that said second outer layer (13) is made of a ceramic based on carbides, nitrides, borides or oxides.

3. The balance spring (1) according to claim 2, characterized in that said second outer layer (13) is made of a ceramic based on silicon carbide.

4. The balance spring (1) according to claim 1, characterized in that the combined layer (13) is made of a ceramic based on possibly doped oxides or based on borides.

5. The balance spring (1) according to claim 4, characterized in that the combined layer (13) is made of a fluoride-doped tin dioxide, an indium and tin oxide, a zinc oxide possibly doped with aluminium, or a titanium boride.

6. The balance spring (1) according to claim 1, characterized in that said portion extends over the outer coil (5) on an arc of a circle comprised between 10 and 60, and preferably between 30 and 40.

7. The balance spring (1) according to claim 1, characterized in that the second outer layer (13) or the combined layer (13) has a thickness comprised between 5 and 100 nm and preferably between 20 and 50 nm.

8. The balance spring (1) according to claim 1, characterized in that the coating further includes a layer (11) underlying the first layer (12) or underlying the combined layer (13), said underlying layer (11) compensating for variations in the thermoelastic coefficient of the core (10) with temperature.

9. The balance spring (1) according to claim 8, characterized in that the core (10) is made of silicon and said underlying layer (11) is made of silicon dioxide.

10. The balance spring (1) according to claim 1, characterized in that the first layer (12) is a metal layer.

11. The balance spring (1) according to claim 10, characterized in that the first layer (12) is made of a metal chosen from among gold, platinum, rhodium, palladium, tantalum, chromium and vanadium.

12. The balance spring (1) according to claim 1, characterized in that each of the lateral faces (10c) includes in said portion of the outer coil (5) a first layer (12) and a second outer layer (13) or the combined layer (13).

13. The balance spring (1) according to claim 12, characterized in that each of the lateral faces (10c) includes in said portion of the outer coil (5), the layer (11) underlying the first layer (23) or underlying the combined layer (13).

14. A balance spring (1) intended to be fitted to a timepiece balance having fixed inertia, said balance spring (1) being formed of a silicon core (10) having lateral faces (10c) connecting an upper face (10a) to a lower face (10b), said balance spring (1) being coated on at least one of the lateral faces (10c) in a portion of its outer coil (5) with a ceramic layer (13) with the exception of a silicon oxide, said layer (13) being directly deposited on the silicon core (10) and having a thickness comprised between 5 and 100 nm and, preferably, between 20 and 50 nm, said balance spring (1) comprising, apart from said portion, on all or part of the lateral, lower or upper faces, a silicon oxide layer in order to compensate for variations in the thermoelastic coefficient of the core with temperature, coated with a metal layer (12).

15. The balance spring (1) according to claim 14, characterized in that the layer (13) that coats at least one of the lateral faces (10c) in the portion of outer coil (5) is a nitride-based, carbide-based, boride-based or oxide-based ceramic.

16. Timepiece comprising a balance having fixed inertia and an index assembly comprising two index pins (9), characterized in that the timepiece includes the balance spring (1) according to claim 1 with said portion disposed facing said two pins (9).

17. A method for manufacturing a balance spring (1) intended to be fitted to a timepiece balance having fixed inertia, said method including the steps of: a) providing a balance spring (1) having a core (10) with lateral faces (10c) connecting an upper face (10b) to a lower face (10a); b) depositing a first electrically conductive layer (12) or a so-called combined layer (13) made of an electrically conductive ceramic (13) on at least one of the lateral faces (10c) of the core (10) on at least one portion of an outer coil (5) of the balance spring (1), c) when step b) consists in depositing the first layer (12), depositing a second ceramic layer (13) on the first layer (12).

18. The manufacturing method according to claim 17, characterized in that step b) is preceded by a step consisting in depositing a layer (11) underlying the first layer (12) or underlying the combined layer (13) in order to compensate for variations in the thermoelastic coefficient of the core (10) with temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1A is a top view of a balance spring according to the invention mounted on its balance. FIG. 1B is a cross-section along axis A-A of the outer coil of the balance spring of FIG. 1A.

DETAILED DESCRIPTION

[0027] The present invention relates to a balance spring more specifically intended to be fitted to a balance having fixed inertia. In a known manner, as represented in FIG. 1A, balance spring 1 has an inner end 2 via which it is attached to a collet 3 fitted on a staff 4 of balance 8. Particularly in the case of a balance spring made from silicon, quartz or ceramic, the collet is made in one piece with the balance spring. Balance spring 1 includes an outer coil 5 which ends in an end 6 attached to a balance spring stud 7. The stud is integral with a balance cock (not represented) secured to the movement plate. Before the point of attachment to the stud, the outer coil passes between the index pins 9. In this particular area, which will be referred to as the contact area, the two lateral faces of the balance spring respectively abut against one pin and then against the other pin with each vibration of the balance. In some configurations of the balance/balance spring assembly, it is also possible for only one of the lateral faces to abut against one of the pins. Consequently, according to the invention, at least the lateral face(s) of the balance spring in this contact area are coated with a layer of hard ceramic material which protects the underlying material from wear during contact between the balance spring and the pin.

[0028] The ceramic layer is preferably made of a carbide, such as, for example, SiC. It may also be made of a nitride such as, for example, Si.sub.3N.sub.4, of an oxide such as, for example, ZrO.sub.2 Al.sub.2O.sub.3, TiO.sub.2 or of a boride such as TiB.sub.2. This layer has a thickness comprised between 5 and 100 nm and, preferably, between 20 and 50 nm.

[0029] According to a preferred variant, the underlying material is a constituent material of a layer having an antistatic function and acting as a moisture barrier. Preferably, this layer is metal and formed of a stainless and non-magnetic metal such as gold, platinum, rhodium, palladium, tantalum, chromium, vanadium, etc. Typically, this layer has a thickness less than or equal to 100 nm.

[0030] According to another variant, the wear-resistant ceramic layer is also electrically conductive and moisture-resistant. In which case, the aforecited layer having an antistatic function and acting as a moisture barrier is no longer required. In this variant, the conductive ceramic layer can be made of an oxide such as SnO.sub.2:F, which is a fluoride-doped tin oxide, of an indium tin oxide (ITO), ZnO, which is a zinc oxide sometimes doped with aluminium (ZnO:Al), etc. It may also be a boride such as TiB.sub.2 which is conductive.

[0031] Preferably, this layer having an antistatic and moisture barrier function, whether it is the aforecited metal layer or the aforecited conductive ceramic layer, itself coats a layer, referred to as the temperature compensation layer, which has the function of compensating for variations in the thermoelastic coefficient of the core with temperature. For example, this temperature compensation layer is formed of silicon oxide (SiO.sub.2). Its thickness is adapted to take account of the effect of the conductive and wear-resistant layers on the thermal behaviour of the balance spring.

[0032] The balance spring 1 represented in detail in FIG. 1B for the preferred variant thus has a core 10 made of silicon, quartz or, generally, ceramic. This core typically has a quadrilateral shape with an upper face 10a connected to a lower face 10b by two lateral faces 10c. This core 10 is preferably wholly or partly coated with temperature compensation layer 11. In the case of a silicon core, one or more of the core faces is thus coated with a layer 11 formed of SiO.sub.2. The core is thus coated on one or more of its faces with metal layer 12 having an antistatic and moisture barrier function. Then, at least one or both lateral faces 10c of core 10, which is pre-coated with layers 11 and 12, is coated with the hard ceramic layer 13 in the area where contact occurs during use with the pin or pins. The present invention does not preclude the hard layer extending beyond the contact area. Thus, the hard layer extends over at least one portion of outer coil 5 in proximity to the end thereof. Advantageously, the portion extends on an arc of a circle a comprised between 10 and 60 and, preferably, between 30 and 40 (FIG. 1A). Said portion extends on an arc of a circle sufficient to ensure that, for any arrangement of the pins within the balance/balance spring assembly, the lateral face(s) intended to be in the contact area are properly coated with the hard layer.

[0033] The balance spring is made by the manufacturing method which comprises the following successive steps, described, by way of example, for a silicon balance spring. The balance spring with its silicon core can be obtained from a silicon wafer (wafer process). In a known manner, it is possible, for example, to perform wet chemical etching, plasma dry etching or reactive ion etching (RIE) using masks suitable for the desired balance spring contour. The silicon dioxide temperature compensation layer is obtained by thermal oxidation of one or more of the core faces. Then, if the anti-wear ceramic layer is not conductive, the metal layer is deposited on one or more of the core faces. The conductive layer is deposited by means of various known processes, such as sputtering, physical vapour deposition, ion implantation or electrolytic deposition. Finally, the ceramic layer according to the invention is deposited by PVD, CVD (chemical vapour deposition) ALD (atomic layer deposition) etc. on the lateral face(s) of the outer coil portion. It will be noted that the present invention does not preclude the ceramic layer also being deposited on the upper and lower faces on this portion.

[0034] Further, the present invention does not preclude the conductive or non-conductive ceramic layer being directly deposited on one or both of the lateral faces of the silicon core in the contact area. Said core is thus devoid of the SiO.sub.2 temperature compensation layer and of the metal layer in the contact layer but provided with these layers over all or part of the core outside this area. The hard layer is thus a nitride-based, carbide-based or oxide-based ceramic layer with the exception in this latter case of SiO.sub.2, intended to protect the silicon core from wear in the contact area.

KEY

[0035] (1) Balance spring [0036] (2) Inner end [0037] (3) Collet [0038] (4) Shaft [0039] (5) Outer coil [0040] (6) Inner end [0041] (7) Balance spring stud [0042] (8) Balance [0043] (9) Pin [0044] (10) Core [0045] a. Upper face [0046] b. Lower face [0047] c. Lateral face [0048] (11) Underlying layer, also referred to as the temperature compensation layer [0049] (12) First layer also referred to as the metal layer [0050] (13) Second layer, also referred to as the ceramic layer, or combined layer