CH720338A2 - Watch spring based on silicon and its manufacturing process - Google Patents

Abstract

The watch spring according to the invention is a mainspring, a hammer spring, a lever spring, a rocker spring, a rake spring, a pawl spring, a jumper spring or a pull-tab spring. It comprises an elastic core (30) based on silicon covered with a metallic coating (31). The metal coating (31) is present at least on one or portions of the elastic core (30) where the stresses are the highest when the watch spring is deformed in normal use, in order to increase the mechanical resistance of the watch spring .

Description

[0001] The present invention relates to a watch spring based on silicon, in particular for a wristwatch or pocket watch.

[0002] Silicon is a material very appreciated in mechanical watchmaking for its advantageous properties, in particular its low density, its high resistance to corrosion, its non-magnetic nature and its ability to be machined by micro-manufacturing techniques. It is thus used to manufacture spiral springs, balance wheels, flexible guided oscillators, escapement levers and escapement wheels.

[0003] Silicon nevertheless has the disadvantage of being a fragile material (material without plastic deformation range). Silicon components can easily break when handled for assembly in a movement or in the event of shock to the watch. In addition, the etching method generally used for its machining, deep reactive ion etching known as DRIE, reduces the mechanical resistance of silicon by leaving sharp edges and creating flatness defects in the form of waves (called “scalloping” in English). ), as well as defects in the crystal mesh, on the sides of the part. To alleviate these problems, silicon watch components can be reinforced with a coating of silicon oxide with a thickness much greater than that of the native oxide, as described in patent application WO 2007/000271. According to the teaching of patent application EP 2277822, this silicon oxide coating can then be eliminated without significantly affecting the mechanical resistance.

[0004] In the case of springs, the mechanical strength must be sufficient not only to allow their handling but also so that the component can deform elastically without breaking during its operation in order to perform its function. For a spiral spring intended to equip a balance wheel or for the flexible guidance of an oscillator without pivots, the operating constraints are relatively low, of the order of a few hundred MPa at most, so that the mechanical resistance provided by the silicon oxide layer may be sufficient. On the other hand, for other springs such as mainsprings, notably barrel springs, or certain hammer or rocker springs, the stresses experienced during their operation are much higher, of the order of a few GPa.

[0005] Methods allowing the production of silicon watch springs capable of withstanding high stresses have been proposed by the applicant in patent applications WO 2019/202378 and WO 2021/186332. The process according to application WO 2019/202378 comprises at least one thermal oxidation and annealing. That according to application WO 2021/186332 includes several thermal oxidations. These operations are carried out at high temperature. However, at high temperatures, stresses can appear in the silicon and deform it, especially if the silicon is polycrystalline. It is therefore appropriate to limit the number of such operations, which prevents the mechanical resistance of the springs from being increased beyond a certain threshold. In addition, particular structures or decorations on the surface of the silicon may be incompatible with high temperatures.

[0006] The present invention aims to propose a new approach for improving the mechanical resistance of watch springs, which can complement known mechanical reinforcement solutions without significantly increasing the thermal balance of the complete manufacturing process.

[0007] For this purpose, a watch spring is proposed, namely a mainspring, a hammer spring, a lever spring, a rocker spring, a rake spring, a pawl spring, a jump spring or a pull spring, comprising an elastic core based on silicon covered with a metallic coating, the metallic coating being present at least on one or portions of the elastic core where the stresses are the highest when the watch spring is deformed in normal use, in order to increase the mechanical resistance of the watch spring.

[0008] Metallic coatings have the advantage of being able to be formed at low temperature, for example by PVD or ALD. They therefore do not affect the heat balance of manufacturing.

[0009] It is known in the state of the art to cover a silicon watch component with a metallic coating. However, the function of the metal coating in the state of the art is not to increase the mechanical resistance but to make the operation of a thermocompensated hairspring insensitive to climatic variations (CH 707225), to prevent the turns from a spiral to be attracted towards the cock and to stick to it (EP 1837722) or to allow the fixing of the silicon component on an axis by welding or brazing (EP 0732635, EP3309624 or WO 2020/148626). Additionally, the components affected are not mainsprings, hammer springs, lever springs, rocker springs, rake springs, pawl springs, jumper springs, or pull-tab springs.

[0010] The present invention further proposes a timepiece, in particular a watch, more particularly a wristwatch or a pocket watch, comprising a watch spring as defined above, as well as a method of manufacturing the spring watchmaker.

[0011] Other characteristics and advantages of the present invention will appear on reading the following detailed description made with reference to the appended drawings in which: – Figure 1 is a top view of a barrel spring according to the invention , the barrel spring being shown in the relaxed state, before its introduction into the barrel; – Figure 2 is a top view of a hammer spring according to the invention; – Figure 3 is a schematic cross-sectional view of a leaf of a watch spring according to the invention; – Figure 4 is a graph showing by points and boxplots the breaking stress values obtained in four different cases.

[0012] In the context of the present invention, the expression “metallic coating” means a coating made of one or more metals. The term “metal” means a metal itself or a metal-based alloy.

[0013] The watch spring according to the invention is any one of: – a driving spring, that is to say a spring intended to store mechanical energy during its winding and to gradually restore it to power the operation of a cog or other watch mechanism; it is typically a barrel spring; – a hammer spring, that is to say (i) a return spring intended, when released, to suddenly move a hammer so that it hits, for example, a reset cam (in a chronograph) or a gong (in a striking mechanism), or (ii) a counter-spring intended to act against the return spring to enable the hammer, for example, to move away from a gong immediately after striking it to that the timbre can vibrate freely; – a lever spring, that is to say a return spring intended to act on a lever, for example to bring it back against a stop or to make it cooperate with a partner movable member, for example a toothed wheel or a star; – a rocker spring, that is to say a return spring intended to act on a rocker, for example to bring it back against a stop, to make it cooperate with a partner movable member, for example a toothed wheel or a star, or to make a sliding pinion carried by the rocker cooperate with a toothed member; – a rake spring, that is to say a return spring intended to act on a rake, for example to bring it back against a stop or to make a feeler that it carries cooperate with a cam, for example a cam snail in a striking mechanism; – a pawl spring, that is to say a return spring intended to act on a pawl, for example to bring it back into a space between the teeth of a toothed member, for example a barrel ratchet, after displacement of the toothed member in its authorized direction of rotation; – a jump spring, that is to say a spring intended to position a movable member, for example a wheel, a star, a zipper or a shuttle, for example by engaging in a tooth gap of the member movable or cooperating with a pin carried by the movable member; – a pull spring, that is to say a spring intended to act on a pull to keep it engaged with a winding stem and against which we can act to decouple the pull from the winding stem in order to allow the removal of the winding stem; the zipper spring is generally part of the same part as the zipper jumper spring which angularly positions the zipper.

[0014] These springs, and more particularly the first two, can undergo very high stresses during their normal operation, greater than 2.5 GPa or even greater than 3 GPa. Figures 1 and 2 respectively show a mainspring 10 (here a spring intended to be wound around a barrel shaft in a barrel drum) and a hammer spring 20 whose end 21 is intended to act on a pin carried by a hammer in order to activate the latter to reset a chronograph counter to zero. The maximum stresses are generally near the embedding point(s), namely, in the case of the hammer spring 20, the point P of junction between an elastic blade 22 of the spring and a rigid base 23 of the spring, the rigid base 23 used to fix the spring on a support such as a watch movement plate.

[0015] With reference to Figure 3, and in general, the watch spring according to the invention comprises, in addition to one or more possible rigid parts, an elastic core 30 based on silicon covered with a metallic coating 31 of thickness e. The elastic core 30 and the metal covering 31 together form an elastic leaf of the spring, for example the leaf 22 illustrated in Figure 2, the metal coating 31 having the function of increasing the mechanical resistance of this leaf.

The elastic core 30 is for example made of monocrystalline silicon, whatever its crystalline orientation, of polycrystalline silicon or of amorphous silicon. In all cases, silicon can be doped or not. The elastic core 30 can be obtained by etching, typically by deep reactive ion etching (DRIE), of a substrate. Instead of silicon itself, the elastic core 30 can be made from a composite material comprising thick layers of silicon separated by one or more thin intermediate layers of silicon dioxide, by etching in a silicon-on-insulator substrate (SOI substrate ). The elastic core 30 can also consist of a silicon core covered with a layer of silicon dioxide.

The metal coating 31 is formed for example by PVD (physical vapor deposition), including vacuum evaporation or cathode sputtering, or by CVD (chemical vapor deposition), including ALD (atomic layer deposition). or MVD (molecular vapor deposition). The thickness e of the metal coating 31 is preferably at least 20 nm, preferably at least 25 nm, preferably at least 30 nm, preferably at least 35 nm, preferably at least 35 nm. least 40 nm, preferably at least 45 nm, preferably at least 50 nm, preferably at least 55 nm, preferably at least 60 nm, preferably at least 65 nm, preferably at least 70 nm, preferably at least 75 nm, and is preferably at most 10 µm, preferably at most 5 µm, preferably at most 1 µm, preferably at at most 500 nm, preferably at most 400 nm, preferably at most 300 nm, preferably at most 200 nm, preferably at most 100 nm, preferably at most 90 nm, any of the aforementioned lower limits can be combined with any of the aforementioned upper limits to define a range of thicknesses for the metal coating 31 of the watch spring according to the invention.

The metal coating 31 may comprise one or more metal layers, for example one or more layers each made of one of the following metals: platinum, gold, copper, titanium, chromium, tantalum, nickel, aluminum, molybdenum, tungsten , zinc, palladium, silver, tin, lead, iridium, bismuth, alloy of one or more of these metals. A thin layer, for example of titanium, deposited on the elastic core 30 can constitute an adhesion layer improving the adhesion of a thicker layer, for example of platinum, deposited on the thin layer.

The metal coating 31 can completely cover the surface of the elastic core 30, or can partially cover it. In the second case, the metal coating 31 is formed at least on one or more portions of the elastic core 30 where the stresses are the highest when the watch spring is deformed in normal use (that is to say in the conditions of use for which it is intended). Typically, this or these maximum stress portions are junction portions of the elastic core 30 to one or more rigid parts of the watch spring. In the example of Figure 2, the elastic core comprises a single portion of maximum stresses, located in the area of point P and ensuring the junction of the elastic core with the rigid base 23. In the case of springs whose cross section of the elastic core varies significantly, the maximum stress portion(s) can be moved away from the connection point(s) and be for example located in the middle of the elastic core.

Preferably, the metal coating 31 completely surrounds the elastic core 30 at the level of the maximum stress portion(s), as illustrated in Figure 3. The metal coating 31 can nevertheless be present only on the sides 32 , or on one of the sides 32, of the elastic core 30 at the level of this or these portions of maximum stress.

[0021] A localized deposition of the metal coating 31 on the elastic core 30, in particular a localized deposition only on the maximum stress portion(s), can be obtained in a manner known per se by means of a mask or a stencil. Such localized deposition is useful when the metallic coating 31 is not desired in other locations for aesthetic (color) or tribological reasons, for example.

[0022] In a variant, the metal coating 31 can cover the entire spring, including the rigid part(s).

The metallic coating 31 can be deposited immediately after etching the elastic core 30 in the substrate or after one or more treatments applied to the elastic core 30.

[0024] In addition to the metal coating 31, the watch spring according to the invention may comprise one or more additional coatings, for example an exterior coating or an intermediate coating. Parylene can be the material for one of these coatings.

[0025] In order to test the invention, bending specimens were etched by DRIE in the same silicon substrate, then these specimens were subjected to two oxidation-deoxidation sequences according to the principle described in the application for patent WO 2021/186332, namely a first thermal oxidation followed by deoxidation to remove a thickness of 1 µm over the entire surface of the test pieces, then a second thermal oxidation followed by deoxidation to remove an additional thickness of 1 µm over the entire surface of the test pieces. the entire surface of the test pieces. This treatment made it possible to smooth the surfaces, in other words to reduce their defects, including those of the crystal mesh due to DRIE etching, and therefore to already improve the mechanical resistance of the specimens. After this treatment, four batches were made: – batch 1: specimens having undergone only this smoothing treatment; – lot 2: test pieces having undergone, in addition to the smoothing treatment, an ALD treatment to cover them with a layer of platinum 50 nm thick; – lot 3: test pieces having undergone, in addition to the smoothing treatment, an ALD treatment to cover them with a layer of platinum 80 nm thick; – lot 4: test pieces having undergone, in addition to the smoothing treatment, a PVD treatment to cover them with a 7 nm thick titanium bonding layer and to cover the titanium layer with a second layer, in platinum, 80 nm thick.

The breaking stress results for each of these batches are presented in the graph of Figure 4, where the rectangular boxes represent 50% of the parts, the two vertical segments on either side of each box represent each 25% of the coins, the horizontal line inside each box represents the median value and the dot represents the average value. It can be seen that the deposition of a metallic layer further improves the mechanical strength, and that the lowest breaking stress in batches 3 and 4 is particularly high, being well above 3 GPa.

[0027] The improvement in mechanical strength obtained by the present invention makes it possible to reduce the length/bulk of watch springs or to increase their energy storage capacity. The thermal balance of the spring manufacturing process is very little affected by the deposition of the metal coating, since the PVD and ALD treatments, for example, are carried out at temperatures lower than 100°C, or even lower than 80°C, at compare to temperatures of 800°C to 1200°C for thermal oxidation. The invention is particularly advantageous in the case of polycrystalline silicon, which is less resistant to high temperatures than other forms of silicon.

The watch spring according to the invention can be a part in its own right or be in one piece with the component (hammer, lever, rocker, etc.) on which it acts or even be in one piece with another spring having a different function or acting on another component.

Claims (14)

1. Watch spring, namely mainspring (10), hammer spring (20), lever spring, rocker spring, rake spring, pawl spring, jumper spring or pull spring, comprising an elastic core (30) based on silicon covered with a metallic coating (31), the metallic coating (31) being present at least on one or portions of the elastic core (30) where the stresses are the highest when the watch spring is deformed in normal use, in order to increase the mechanical resistance of the watch spring. 2. Watch spring according to claim 1, characterized in that the or each said portion where the stresses are the highest is a junction portion of the elastic core (30) to a rigid part (23) of the watch spring. 3. Watch spring according to claim 1 or 2, characterized in that the metal coating (31) is present at least on one side (32) of the elastic core (30) at the level of said portion(s) where the stresses are the highest. 4. Watch spring according to one of claims 1 to 3, characterized in that the metal coating (31) completely surrounds the elastic core (30) at least at the level of said portion(s) where the stresses are the highest . 5. Watch spring according to one of claims 1 to 4, characterized in that the metal coating (31) is present over the entire surface of the elastic core (30). 6. Watch spring according to one of claims 1 to 4, characterized in that the metal coating (31) is present only on the said portion(s) of the elastic core (30) where the stresses are the highest. 7. Watch spring according to one of claims 1 to 6, characterized in that the thickness (e) of the metal coating (31) is at least 20 nm, preferably at least 25 nm, preferably d at least 30 nm, preferably at least 35 nm, preferably at least 40 nm, preferably at least 45 nm, preferably at least 50 nm, preferably at least 55 nm , preferably at least 60 nm, preferably at least 65 nm, preferably at least 70 nm, preferably at least 75 nm. 8. Watch spring according to one of claims 1 to 7, characterized in that the metallic coating (31) comprises at least one of the following metals: platinum, gold, copper, titanium, chromium, tantalum, nickel, aluminum, molybdenum, tungsten, zinc, palladium, silver, tin, lead, iridium, bismuth, alloy of one or more of these metals. 9. Watch spring according to one of claims 1 to 8, characterized in that the elastic core (30) comprises polycrystalline silicon. 10. Timepiece comprising a watch spring according to one of claims 1 to 9. 11. Timepiece according to claim 10, characterized in that the watch spring receives in normal operation stresses greater than 2.5 GPa, preferably greater than 3 GPa. 12. Method for manufacturing a watch spring, namely a mainspring (10), a hammer spring (20), a lever spring, a rocker spring, a spring rake, a pawl spring, a jumper spring or a pull spring, comprising a step consisting of producing an elastic core (30) based on silicon and a step consisting of covering the elastic core ( 30) of a metallic coating (31) so that the metallic coating (31) is present at least on one or portions of the elastic core (30) where the stresses will be the highest when the watch spring is deformed in normal use, in order to increase the mechanical resistance of the watch spring. 13. Method according to claim 12, characterized in that the step consisting of covering the elastic core (30) with a metallic coating (31) comprises an ALD operation. 14. Method according to claim 12, characterized in that the step consisting of covering the elastic core (30) with a metallic coating (31) comprises a PVD operation.