Mainspring for the mainspring barrel of a clock movement with extended power reserve

Abstract

Mainspring (10) for clockwork mainspring barrel, which has a composite structure comprising a first portion (11) made of a carrier material, which is at least partially covered by a second portion (12) made of diamond, characterized in that the mainspring (10) consists predominantly of diamond volume with respect to its total volume.

Description

[0001] The present invention relates to the field of watches and, more particularly, to a mainspring 1 intended to be incorporated into a barrel 2 of a watch movement in order to collect and return the energy required for the running and power reserve of the movement.

[0002] A watch mechanism of the known type comprises a winding system, a movement and a dial for displaying the hour and minute, under which there is a barrel 2 forming a cylindrical case in which a mainspring 1 is housed, which represents the mechanical energy supply of the watch.

[0003] As in Fig. As shown in Figure 1A, the mainspring 1 is wound spirally around itself within the cylindrical walls 3 of the barrel drum and around a spring shaft 4 which rotates independently of the barrel 2.

[0004] The rotation of the spring shaft 4 is firmly connected to a ratchet wheel, which forms part of the winding system. The teeth of the ratchet wheel interact with a locking system to form a backstop, ensuring that the winding mechanism rotates the spring shaft 4 only in the direction that allows the winding and tensioning of the mainspring 1.

[0005] The barrel 2 has a base plate, cylindrical walls forming the barrel drum 3, and a generally flat cover. The barrel 2 has a toothing on its periphery that drives the wheels of the actual clockwork (not shown).

[0006] Fig. 1A, Fig. 1B and Fig.2 illustrate a mainspring 1 of a well-known type in the tensioned and untensioned positions inside a barrel, as well as in the untensioned position outside the barrel. The mainspring 1 is formed from a steel strip wound spirally around the spring shaft 4, to which it is attached by an inner end 6 provided with a notch. The other, outer end 7 of the mainspring 1 is rigidly connected to the barrel drum 3 (attached by a hook or, preferably, by a clamp 8). The barrel 2 is generally equipped with a system that limits the tensioning force, for example, a ratchet device (not shown - limiting the number of tensioning turns) or a disengageable device such as an elastic return blade 9 of the mainspring to prevent it from breaking.

[0007] The disadvantage of watches with fully mechanical movements of the existing type is that their power reserve is limited to one or several days. Traditionally, the metal bands of mainsprings are designed to provide an autonomous power reserve of 36 to 40 hours, thus providing a power reserve of 12 to 18 hours. (By convention, the power reserve is defined as the power reserve minus 24 hours.)

[0008] Now, certain collector's watches are equipped with a power reserve of 58 or 60 hours, that is, two and a half days of autonomy and a maximum power reserve of one and a half days.

[0009] In watchmaking, it has always been a constant goal to increase the running time (i.e. the autonomy of the movement or running time without winding) and consequently the power reserve of the movement (i.e. the said autonomy minus 24 hours) without increasing the space required by the barrel spring.

[0010] It is obvious that the duration of the walk is essentially determined by the maximum amount of elastic energy, W max , which the mainspring of the barrel can accumulate.

[0011] The upper limit, W max , the energy that can be accumulated by an ordinary metal strip of a tension spring of limited dimensions is obviously determined by the occurrence of fracture phenomena and / or excessive plastic deformations in certain areas of this strip.

[0012] The risks of breakage are concentrated in the inner end portion 6 of the spiral winding of the band 5 of the tension spring 1, which has a minimum radius of curvature ρ. This leads to an increase in the diameter of the spring shaft 4 and, more precisely, its corresponding collar radius Rb.

[0013] The risks of plastic deformation are concentrated at the other, outer end 7 of the tension spring 1 and on the outer side X of the blade of the band 5, i.e. the areas most exposed to tensile stresses.

[0014] The need to store a maximum of energy in a barrel mainspring of limited dimensions (due to the design and miniaturization of the watch case) is so great that it leads to the usual bands 5 of mainsprings 1 of a barrel 2 being allowed to operate, in the event of overload, close to the elastic limits and without a safety margin, and even beyond them, in the range of permanent deformation, which is unusual in mechanics.

[0015] The permanent deformation is such that, after being inserted into the barrel 2 with the aid of a spring winder and several daily cycles of use (tension / relaxation), spring 1 achieves a shape that no longer corresponds at all to its initial form (one says, "the spring has given way"). Above all, the fatigued spring has lost elastic force. This reduces the escapement drive and the amplitude of the balance oscillations. However, an excessively low balance amplitude is detrimental to the isochronism of the oscillations and, consequently, also detrimental to the chronometric properties of the movement.

[0016] To prevent these phenomena, the metal strip 5 of a tension spring 1 is initially given a profile with windings that have been excessively unwound by winding. As shown in Fig.2, the turns of the metal strip 5 have an initial curvature whose radius ρ increases at an accelerated rate; the radius of curvature ρ goes to infinity (right section, marked X), then the radius ρ reverses (curvature in the opposite direction) and finally decreases to produce an inverted spiral winding in the shape of the letter S, as shown schematically in Fig. 2, or a semi-inverted one in the shape of half the letter S (not shown). The optimal profile (L) in the plane transverse to the coils of a relaxed tension spring 1 of the conventional type is therefore not a profile that is uniformly curved into a regular spiral.

[0017] In order to maximize their storable energy, the fact remains that existing types of metallic tension springs operate at or beyond the maximum plastic deformation stress, which has the disadvantage of limiting their lifetime to a few hundred or about a thousand cycles.

[0018] Theoretical modeling of mainsprings and comparison with practical experiments have made it possible to express the general characteristics of a barrel spring. Reference can be made to Leopold Defossez's "Theorie générale de l'horlogerie" [General Theory of Watchmaking], Volume 1, Chapter IV, "The Driving Force," or to the collection "Techniques de construction horlogère pour l'ingénieur" [Techniques de construction horlogère pour l'ingénieur], folder "Mechanics, Theory," Chapter 2, "Energy," edited by Michel Vermot and published by the Centre de Compétences en Technologie et Design Horlogers.

[0019] In particular, it shows that: - the elastic moment of force emitted by a tension spring band is given by the formula: M=E(h⋅e3 / 12⋅L)⋅α where E is the elasticity coefficient of the material (Young's modulus), expressed in units of force per unit of cross-section (in Newton per square meter or in Pascal and its multiples: N / mm 2 or MPa); h is the height of the winding of the tension spring coils; e is the thickness of the strip; L is the length of the tape and α is the cumulative angle of rotation of the turns of the spring coil, which is the sum of the bending angles α i which corresponds to turns No. i = 1 to N; - the maximum amount of elastic energy W max , which can accumulate a spring band made of a material with the elastic coefficient (Young's modulus) E and an elastic limit stress σ max (in N / mm2 ) depends only on its volume V and these elasticity parameters σ max and E from: Wmax=(e⋅h⋅L / 6)⋅(σmax2 / E)=(V / 6)⋅(σmax2 / E).

[0020] The modeling shows that in order to achieve maximum unwinding, the spring must occupy a volume of V = e·h·L, which optimally corresponds to about half of the volume Vo available in the barrel, namely: V=e⋅h⋅L=Vopt≈Vo / 2.

[0021] The aim of the present invention is to increase the running time and the power reserve of a mechanical watch movement and to realize a barrel mainspring having increased running energy compared to the existing type of mainsprings consisting of a metal strip, without increasing the dimensions of the barrel and without reducing the maximum elastic moment (M0).

[0022] The first object of the invention is therefore to achieve the manufacture of a mainspring from materials offering an increased amount of elastic energy and that this maximum amount of energy corresponds to the energy required to run a watch movement for several days.

[0023] A further object of the invention is to find means for producing such a barrel mainspring with a special shape of the spring winding, which in particular has a curvature profile of the windings in its plane, if possible by a reliable, reproducible and cheaper process.

[0024] Among other objectives, it would also be appropriate to eliminate, i.e. avoid or at least reduce, the disadvantages of known strip springs of the state of the art: - the mutual friction of the coils inside the spring; - the disadvantages of lubrication; - the occurrence of plastic deformations that limit the spring energy; - the loss of elastic moment over time; - the delicate work of hardening, tempering or stress relieving the steel strips; - the surface finishing of the strip spring leaves; and - to provide a barrel mainspring that allows its conditions of use to be improved with safety margins in order to offer an increased service life.

[0025] The first problem is therefore to find a material capable of storing more or at least equivalent energy compared to existing materials such as special steels and, above all, special alloys such as non-ferrous metal alloys containing cobalt, nickel, chromium or molybdenum.

[0026] To this end, the idea is to create a barrel mainspring made of diamond, specifically a portion deposited by polycrystalline diamond. However, a new problem arises: the currently available processes are unable to produce a diamond blank with dimensions compatible with the amount of energy a barrel mainspring must store, or in a shape suitable for the winding of a barrel spring with a spiral-curved profile.

[0027] To achieve these goals, a barrel mainspring with a composite structure is developed. The mainspring consists of a first portion, which forms a supporting lamella and constitutes the core or framework of the spring. This portion is preferably made of crystalline silicon or metal. The surface of the first portion is directly covered, entirely or at least partially, by a second portion made of diamond.

[0028] The proposal to use diamond as a material for the components of a watch movement is known from the prior art. For example, WO 2004 / 029733 A2 proposes manufacturing parts of a watch movement exclusively from diamond. EP 0 732 635 A1 proposes a method for producing micromechanical parts, in particular rigid parts of a watch movement, in which the parts are cut from a plate of a first crystalline material (monocrystalline or polycrystalline silicon or its oxide or nitride, molybdenum, germanium, or aluminum oxide) by anisotropic vertical etching with a plasma-excited gas, and the areas of the plate that may come into contact with other components are coated with a thin layer of carbon crystallized as diamond.

[0029] US 2002 / 0191493 A1 describes a barrel with a mainspring made of a tough and elastic material, consisting of an alloy containing chromium, cobalt, and nickel, and whose surface is at least partially coated with a thin film that may have mechanical properties and compositions different from the material. The thin film is harder than the material and can consist of a thin layer of DLC (diamond-like carbon), monocrystalline, or polycrystalline diamond. Document CH 701 722 B1 also describes a mainspring with a main body made of metal coated with an additional film of diamond-like carbon (DLC). The thickness of the spring is typically specified as approximately 0.1 mm, and the thickness of the additional film is specified as approximately 1 to 5 µm, so that the main body of the spring is significantly thicker than the additional film.

[0030] In particular, with the aim of creating a spring that can withstand much higher maximum elastic stress, it is proposed that the mainspring consists predominantly of diamond volume in terms of its total volume and that the second part preferably forms a thick layer of gear mass made of polycrystalline diamond.

[0031] According to a first advantageous embodiment, the support material is silicon, preferably crystalline or polycrystalline silicon, or possibly germanium or, more generally, a semiconducting support material, in particular a high-temperature (HT) or refractory semimetal (i.e., withstanding temperatures greater than T1 = 500-600°C or, preferably, greater than T2 > 800°C) such as the elements of column IVB and the adjacent columns of the conventional periodic table of elements, called Mendeleev's table (IVB = C, Si, Ge, ..., i.e., carbon, silicon, germanium, etc.).

[0032] Alternatively, in other advantageous embodiments, the carrier material is a HT or high-melting metallic material, in particular titanium or a titanium-based alloy or advantageously an Invar ®-type, i.e. an alloy based on iron and nickel. More generally, the metallic support material can be chosen from the families of metals and metallic alloys based predominantly on iron (Fe) and / or nickel (Ni) and / or cobalt (Co) and / or chromium (Cr) and / or titanium (Ti) and / or tungsten (W) and / or manganese (Mn) and / or carbon (C) and / or silicon (Si), possibly containing oligoelements (<1%), and including in particular Fe-Ni-based alloys, Fe-Cr-based alloys, Fe-Ni-Cr and Fe-Ni-Co alloys, or even Fe-Co alloys. In simplified embodiments, the support can be made of steel, in particular a steel chosen from hard or semi-hard steels, special unalloyed or low-alloyed steels, and / or hardenable or stainless steels.

[0033] Advantageously, according to the present invention, such a tension spring structure makes it possible to have a metal or silicon framework forming a blank of predefined shape for the spring winding and having a cross-section and therefore a considerable external (longitudinal) surface, from which the external diamond layer is deposited and grown, in particular by chemical vapor deposition (CVD) or more precisely by plasma deposition, thus avoiding diamond cutting and ultimately obtaining a tension spring with a composite structure having a considerable cross-section and a considerable volume of diamond, such a diamond shell absorbing the essential part of the bending and tensile stresses of the spring with a much higher maximum elastic stress σ max than the usual steels.

[0034] The invention is realized with a mainspring for a clockwork barrel according to claim 1.

[0035] The invention also extends to a watch according to claim 18, which comprises such a mainspring.

[0036] The invention also provides a manufacturing method for such a tension spring according to claim 19.

[0037] Further objects, features and advantages of the invention will become apparent from the following detailed description of embodiments of the invention, given by way of non-limiting example with reference to the accompanying drawing figures.

[0038] In the attached plates of the drawing: - place Fig. 1A and Fig. 1B shows a mainspring consisting of a steel strip in the tensioned and untensioned position in a barrel according to the prior art; - shows Fig.2 an optimal, inverted S-shaped configuration of the coils of a tension spring according to the state of the art in the relaxed position; - show Fig. 3A and Fig. 3B schematically shows the internal stresses of a tension spring in radial and transverse sectional view of the metal strip windings according to the prior art; - illustrate Fig. 4A and Fig. 4B shows the prior art deposition and cutting operations of a diamond plate to produce a miniature watch part such as a balance spring; - is Fig. 5 a diagram of elasticity parameters of a conventional spring steel (stress σ against strain ε and elastic modulus E); - is Fig. 6 is a diagram of the elasticity of unusual materials such as diamond and silicon implemented in a tension spring according to the invention; - is Fig.7 is a schematic radial section of a tension spring coil with a composite structure according to a first embodiment of the invention; - is Fig. 8A is a scaled radial section of a composite tension spring coil according to another embodiment of the invention; - is Fig. 8B is a scaled cross-section of the composite structure of the coil of a tension spring manufactured according to the invention; - is Fig. 9 is a diagram of the moment of force as a function of the running time, as provided by a normal steel spring and a mainspring according to the invention; - show Fig. 10A to 10E schematically show the steps of the manufacturing method of a tension spring with a composite structure according to the invention; - is Fig. 11 is a detailed diagram in radial section for the first part of the composite structure of a tension spring produced according to the invention; and - show Fig. 12A and Fig.12B schematically shows further steps of the manufacturing process for the barrel mainspring according to the invention.

[0039] The first problem was therefore to find a material capable of storing increased or at least equivalent energy compared to existing materials (called “spring steels”) such as special steels and special alloys, in particular non-ferrous alloys containing cobalt, nickel, chromium or molybdenum.

[0040] Fig. 5 and Fig. 6 show elasticity diagrams for different materials, where each curve W indicates the magnitude of the stresses σ applied to a test specimen of the material to obtain a strain ε (ε = ΔL / L), where the linear slope of the curve corresponds to the elastic (or Young's) modulus E. Fig.Figure 5 shows a curve of elasticity W0 for a conventional material such as spring steel used to manufacture a metal strip for tension springs of the known type. Fig. Figure 6 shows the elasticity curves W1 and W2 for unusual materials such as diamond W2 and silicon W1, which are used to form the composite structure of a tension spring according to the invention.

[0041] At first glance, it appears that diamond has an increased Young's modulus E2 (about five or six times the elastic modulus or elastic constant E0 of steel), which would be inadvisable for the manufacture of a barrel spring (see the above-cited TCH collection, Chapter 2.1.5 "The search for an increased value of E proves completely useless [...], not only useless, but harmful"). The diagram at least shows that diamond can withstand a higher upper load limit σ2 than other materials and that diamond's elasticity range W2 is extended (increased maximum elastic energy).

[0042] It has been considered to make watch parts, and in particular hairsprings, from diamond.

[0043] Document WO 2004 / 029733A2 describes in general terms the use of diamond for the manufacture of mechanical watch parts, although the application relates, in a limited way, to the manufacture of a hairspring made of artificial diamond.

[0044] Today, polycrystalline diamond plates can be manufactured by deposition and epitaxial growth of microcrystals of diamond (on a substrate).

[0045] Unfortunately, the production (growth and extraction) of a thick plate of diamond is excessively difficult, slow and expensive.

[0046] Fig. 4A and Fig.4B schematically illustrate the steps of manufacturing a diamond spiral spring according to the prior art. The manufacturing steps consist of obtaining such a diamond plate Q by epitaxial growth and then cutting or grinding concentric spirals from the obtained raw diamond plate Q.

[0047] While the planar dimensions P of the workpiece made of deposited diamond can reach a size of less than one centimeter or a maximum of several centimeters, its thickness f is limited to a few tens of micrometers.

[0048] Furthermore, the deep cutting or grinding steps of diamond, for which plasma structuring techniques are used to carve out the shape of a workpiece, are extremely difficult and expensive. They are limited to tiny surfaces.

[0049] Consequently, the steps of deposition and growth of polycrystalline diamond plates, as well as the subsequent processes of plasma etching and cutting of the diamond, produce only tiny workpieces of insignificant volume and microscopic thickness, not exceeding a tenth of a millimetre (the volume not even exceeding one cubic millimetre).

[0050] Thus, these techniques of manufacturing a monolithic diamond plate followed by sculpting or chiseling a workpiece from the raw diamond plate only allow the realization of tiny workpieces that are not intended to store any driving energy and / or, above all, not to absorb any mechanical energy, such as a balance spring (in English hairspring: literally hair spring).

[0051] These techniques are unsuitable for the realization of tension springs, which have to store energy in the order of a tenth of a joule up to a few joules and whose volume (V) must be appropriate to store a maximum of energy (W max = V / 6·σ max 2 / E), so their dimensions must well exceed one cubic millimeter (typically by more than an order of magnitude). The volume V0 of a barrel naturally varies depending on the type, surface area, and flatness of a watch, but generally remains on the order of the surface area of ​​one square centimeter and the volume of about 100 cubic millimeters. The height h of the barrel springs is generally on the order of one millimeter. The thickness e is on the order of a tenth of a millimeter. In particular, however, as far as the length of a mainspring is concerned, the length L of the coils of the extended spring can easily reach one or more decimetres. How should one inscribe such a length of spring on a plate whose dimensions are millimeters (or centimeters at most)?

[0052] The invention makes it possible to solve this problem of manufacturing a barrel mainspring by starting from a material that allows to store increased running energy and to produce a barrel mainspring from materials such as diamond.

[0053] Fig. Figure 7 shows that, according to the invention, a barrel mainspring 10 is obtained which allows to store an increased running energy by starting from a first portion 11 of the spring made of a support material other than diamond, this first portion 11 being advantageously able to assume a three-dimensional shape corresponding to a framework or a rough mold of the final shape of the spring, and then this first portion 11 is completely or partially covered by a second portion 12 made of diamond.

[0054] The support material 11 can be selected from materials that can withstand high temperatures (of the order of 800 to 900 °C) on which diamond is deposited by chemical vapor deposition (CVD) or, more precisely, in a plasma (CVD plasma).

[0055] The carrier material is a body, particularly a metal or semi-metal, that can withstand such temperatures. The carrier material can be a metal or alloy, a semi-metal, or even a chemical compound (e.g., silicon carbide).

[0056] According to an advantageous alternative embodiment, the support portion is a strip or a blank of a leaf spring made of metal or a metal alloy that withstands high temperatures, such as temperatures greater than T1 = 600 °C or at least higher than T1 = 500-600 °C, preferably higher than T2 = 800 °C. The metallic material forming the support portion is preferably selected from metals and alloys having a low coefficient of thermal expansion.

[0057] Diamond has a thermal expansion coefficient, α Dia , of about one micrometer per meter and degree at ambient temperature (exactly between α Dia = 0.9·10 -6 / K and 1.1·10 -6 / K at T0 = 25 °C). More precisely, the thermal expansion coefficient of diamond, α Dia , with the temperature in the range of T0 to T2 = 800 °C and remains at values ​​in the order of αDia=1 to 4⋅10−6 / K.

[0058] Such a coefficient of expansion is particularly low. When diamond is deposited on a metallic support, there is a risk of a cohesion problem between the metallic support and the outer diamond layer due to the expansion and contraction stresses when returning to the ambient temperature of T0 after deposition of the diamond layer at a high temperature of T1 / T2. To produce a composite spring according to the invention having an outer portion made of diamond deposited on a metallic support portion, it is preferable to select metallic materials whose thermal expansion coefficient is close to the very low expansion coefficient of diamond, α. Dia, as close as possible to the temperature of the metal, while still withstanding high temperatures of T1 or T2. The metallic material is preferably selected from the families of composite materials of the following materials: - primarily the Invar ® materials, which are iron alloys of nickel and are also called nickel steels; for example, one can choose an Invar such as the alloy Fe36Ni (64% iron and 36% nickel), Invar N42 (58% iron, 42% nickel), Invar N93 and all other Fe-Ni alloys in any ratio, and in particular the Fe-Ni-Cr and Fe-Ni-Co alloys, as well as the Fe-Cr alloys and also the Fe-Co alloys; their coefficient of thermal expansion is approximately one to two micrometers per meter and degree at ambient temperature: αInv=1.5 to 2⋅10−6 / K at T0=25 °C; - secondarily titanium or titanium-based alloys and tungsten or tungsten-based alloys; - in simplified embodiments, alternatively, the first portion of support material may be formed from a steel sheet blank, in particular from a steel chosen from the families of hard or semi-hard steels, hardenable or stainless steels and special steels considered to be unalloyed or low-alloyed (content of non-ferrous elements and in particular manganese of less than 1% or less than 5%).

[0059] Thus, in such embodiments, the first or support portion is preferably formed from a metallic material selected from the families of metals and alloys offering good mechanical properties in terms of flexibility and resistance to expansion, good heat resistance (melting point above T1 = 500 to 600 °C or even above T2 = 800 °C), and a low coefficient of expansion α to avoid delamination caused by shear stress at the diamond / metal interface caused by differential stretching of the metallic support substrate and the outer diamond layer when the temperature returns to ambient after deposition. It is preferable to choose a metallic material having a coefficient of expansion close to that of diamond and equally low.The subsequent deposition of diamond by CVD in the gas or plasma phase takes place at temperatures compatible with the sublimation / condensation of carbonaceous particles under the action of a plasma flame, ie temperatures of approximately T1 = 500 to 600 °C or at T2 = 800 °C or even higher.

[0060] By using a support portion made of metal or metal alloy having a low coefficient of thermal expansion as close as possible to that of diamond, it is advantageous to obtain a spring composite structure according to the invention having good manufacturability, good cohesion and good mechanical behavior upon return to ambient temperature at the end of manufacture.

[0061] In the alternative, other advantageous embodiment, the carrier portion is a ribbon or a three-dimensional sheet made of a HT or high-melting semiconductor or semi-metal material that allows deep cutting or etching operations, in particular DRIE (deep reactive ion etching) and litho-etching or chemical ablation processes of semiconductor materials (silicon or germanium or even carbon). Silicon has an expansion coefficient of approximately αSi=2 to 3⋅10−6 / K at T0=25 °C, which is particularly close to the expansion coefficient of diamond.

[0062] Preferably, the carrier material is a material that can be formed into the three-dimensional shape of a spring leaf.

[0063] According to the invention, however, the carrier material is primarily a material that can be formed into a spirally curved spring winding before diamond deposition.

[0064] Preferably, a material is selected from which a preform of the tension spring winding can be machined with an accuracy that is compatible with the dimensions and accuracy of the desired spring.

[0065] The carrier material is preferably silicon, in particular crystalline silicon and more precisely single-crystalline silicon.

[0066] Interestingly, silicon can easily be etched and cut into a three-dimensional shape from semiconductor wafers of macroscopic dimensions, and can reach dimensions amply sufficient for a watch or clockwork spring. Other materials, and in particular a semimetal such as germanium, could be suitable.

[0067] Crystalline silicon also has interesting elastic parameters, as shown in the elasticity diagram of Fig.6. Silicon has a high yield strength ε1, higher than the elastic strain limits ε2 of diamond, making it a soft and ductile material without suffering permanent plastic deformation or fracturing.

[0068] Advantageously, the silicon of the first portion 11 of the support can thus withstand all deformations under stress of the outer portion 12 made of diamond by remaining in its elastic range (without fracture).

[0069] In the diagram of Fig. However, Figure 6 indicates that silicon can withstand maximum stresses σ1 that are much lower than the maximum stresses σ2 of diamond.

[0070] However, since the silicon represents the first portion 11 arranged in the core of the composite structure 10 or on the inner side 11.1 of the windings 10 of the barrel mainspring according to the invention, it is subjected to much less high (tensile) stresses than the outer portion 12 made of diamond or even to stresses of almost zero (neutral fibers in the central axis NN of the windings), which corresponds to its suitability and its elastic limits.

[0071] In the scheme of Fig.In Figure 7, which shows a non-scale sectional view of a spring coil 10, the first portion 11 of support material appears in the center and concentric with the second portion 12 of diamond. The first portion 11 preferably has a rectangular cross-section h'·t, as schematically shown in this cross-sectional view (radially with respect to the coils). In the longitudinal direction, the first or support portion preferably forms a bend or lamella with a curved profile that follows a specific spiral winding of the tension spring, as described below.

[0072] The lamella of rectangular cross-section made of carrier material, which forms the first part, has a height h' (greater than one or two tenths of a millimeter) and a thickness t (greater than about ten micrometers, possibly several micrometers).

[0073] Advantageously, the volume and surface area offered by the first portion 11 provide a three-dimensional support for the deposition and growth of the diamond, so that an outer portion 12 of diamond can be obtained which offers a cross-sectional area and volume sufficient to withstand the stresses and energy absorbed by the spring.

[0074] In addition, the first or support portion 11 provides a rough shape or starting point of three-dimensional shape for the deposition of diamond, so that after the deposition of the diamond, a finished shape of the composite diamond tension spring can presumably be achieved, thereby avoiding later operations of cutting or grinding the diamond, which are extremely difficult.

[0075] Preferably, the diamond layer 12 of thickness d covers the entire surface of the first or carrier portion 11 and forms a shell 12 around the core of carrier material. The diamond shell 12 absorbs the significant part of the mechanical stresses and protects the carrier material 11. The embodiment of Fig. According to Figure 7, the first portion 11 has a substantially rectangular (possibly square) cross-section, and the diamond layer 12 of thickness completely covers the four transverse sides 11.1, 11.2, 11.3 and 11.4 of the first or support portion 11.

[0076] Alternatively, the layer 12 of thickness of diamond may cover only certain sides of the first or support portion 11, in particular the inner side 11.1 and the outer side 11.2 of the coils of the tension spring 10, which absorb the essential part of the forces F and bending, tensile and possibly compressive stresses in the volume of the tension spring 10, as in Fig. 8B.

[0077] According to another alternative (not shown), the layer of diamond 12 thick may also cover only one side of the first or supporting portion, in particular all or part of the outer surface 11.2 of the coils of the tension spring 10, which absorbs the majority of the spring's work and tensile stresses (the outer surface of conventional springs is the most exposed and most subject to plastic deformation, fracturing, and subsequent cracking). The composite structure of the tension spring 10 according to the invention is thus formed by a first lamina of supporting material adhered to an outer lamina 12 formed by depositing a thick layer of polycrystalline diamond on the outer surface 11.2 of the first lamina 11.

[0078] Thus, the first portion 11 forms a support which allows to obtain a second portion 12 of diamond, the effective values ​​of which can be considerably developed in terms of cross-section and volume.

[0079] The composite structure then makes it possible to achieve a considerable effective diamond cross-section in the coils of the tension spring according to the invention. In fact, the diamond cross-sectional area A reaches values ​​of approximately A=2⋅(H⋅d+t⋅d)(or one-sided at least A=h'⋅d) WHERE d is the thickness of the deposited layer of crystalline diamond, H is the (external) height of the composite structure of the spring coils, including the thickness d of diamond at the end edges (H = h' + 2·d); t is the thickness (horizontal or radial dimension) of the first portion of carrier material and h' is the height of the first part (vertical).

[0080] The composite structure according to the invention makes it possible to realize a composite diamond spring that achieves macroscopic dimensions compatible with the amounts of mechanical energy that a barrel mainspring must store, as shown below.

[0081] Fig. 8A shows a scaled example of the barrel mainspring 10 with a composite structure according to the invention.

[0082] The following design examples of the tension spring are given for guidance.

[0083] The height h' of the first portion 11 of carrier material can reach a value in the order of magnitude of one tenth of a millimeter up to a few millimeters, rarely more than one centimeter, typically between 0.5 and 1.5 mm.

[0084] The thickness d of the diamond deposit 12 is generally greater than five micrometers and can typically reach values ​​between 10 µm and 60 µm.

[0085] The thickness t of the first portion 11, measured perpendicular to the windings (in a plane transverse to the winding), generally has values ​​in the order of magnitude of a few micrometers up to approximately 100 micrometers. Preferably, the thickness t is chosen to be in the same order of magnitude as the thickness d of the diamond layer, between 10 µm and 60 µm.

[0086] The barrel mainspring 10 obtained according to the invention thus consists predominantly of diamond 12, wherein the proportion of diamond in the spring volume varies between 17 and 99%, typically between 33 and 67% and preferably over 50% (proportion of diamond 12 in the volume predominantly, based on the total volume of the mainspring 10, and effective diamond area predominantly, based on the total cross-sectional area of ​​a winding).

[0087] The following description provides some examples of the implementation and results of using mainsprings that correspond to normal barrels.

[0088] Such realizations of tension springs 10 are intended for a barrel 2 whose dimensions are as follows: Height of the barrel: Hb = 0.7 mm Radius of the barrel drum: Rt = 5.25 mm Collar radius: Rb = 1.2 mm Consequently, the volume available in the barrel is: V0 = 58 mm 3 ; the optimal volume of the spring is: V opt = 0,55 V = 32 mm 3 .

[0089] With such barrel dimensions, a band 5 of a mainspring 1 made of conventional spring steel has, in comparison, optimal dimensions with the following values: Strip thickness: e = 95 µm Band height: h = 700 µm Length L of the strap to achieve maximum unwinding and maximum running time: L = 480 mm.

[0090] This mainspring 1 made of conventional steel band provides the movement with dynamic parameters as follows: Maximum elastic moment: M0 = 3,5·10 -3 N·m (genau 348 g·mm) Elastic moment, 24 h: M 24 = 2,9·10 -3 N·m (Newtonmeter) Rotation speed in 24 hours: Nb = 4.4 turns Consequently, torque loss in 24 hours: ΔM 0-24 / M0 = (M0 - M 24 ) / M0 = 17 %.

[0091] For the movement in question, this band 5 of the mainspring 1 provides a maximum running time of 58 hours after N = 10.5 revolutions (power reserve 34 h).

[0092] The calculations for the inventive realisation of mainsprings with a composite structure intended for such a barrel are carried out while maintaining the height H and, in general, with the aim of preserving the maximum elastic moment M0 (the torsional moment applied to the movement is limited in order to avoid bounce phenomena, which are detrimental to the isochronism of the movement and harmful to the lifespan of the escapement parts).

[0093] Recall that the maximum elastic moment M0 of a spring is proportional to the square or cube of its thickness e. More precisely, M0 is proportional to the modulus E and to the cube of the thickness e, but M0 is also proportional to the maximum stress limit σ and to the square of the thickness e, as indicated by the following classical formulation: M0=E⋅e3⋅h12⋅L⋅αmax=e2⋅h6⋅σmax.

[0094] Diamond has a modulus of elasticity E2 and a limit of elasticity σ2 that are considerably higher than those of conventional spring steels (on the order of two to ten times, typically five or six times the values). This clearly shows that the thickness of the diamond d / 2d and the total thickness T of the coils of the mainspring 10 with the composite structure according to the invention can be reduced compared to the thickness e of the steel strip 5 of the conventional mainspring 1 (by approximately one-quarter to more than half, typically by one-third). Since the cross-section (S = H T) of the coils of the composite diamond spring according to the invention takes up less space than the cross-section (R = h e) of the strip 5 made of conventional spring steel, one can therefore consider increasing the length L' and the number of turns Nb of the coils in the mainsprings designed according to the invention, thus extending the running time of the movement.

[0095] The aim is therefore to produce tension springs 10 for such a barrel 2 according to the invention, the windings of which must ultimately achieve external dimensions which are set at a total height H of 700 micrometers and a total thickness T reduced by one third, i.e. at a total thickness T of 62 micrometers.

[0096] In the following examples for the realization of tension springs according to the invention, the second portion 12 is formed from a diamond layer thickness d that varies between 10 micrometers and 25 micrometers. The first or support portion 11 is formed by a bend or sheet of rectangular cross-section (h'·t), whose thickness t varies in a complementary manner between 42 micrometers and 12 micrometers.

[0097] For such a barrel, the examples of tension springs 10 with a composite structure realized according to the invention, with the same maximum elastic moment M0 = 0.5 N·m and the same number of Nb = 4.4 revolutions per 24 hours, can therefore take the following dimensions: ◯ With a diamond thickness d = 10 µm reaches the maximum unwinding length of the spring L' = 600 mm and provides a maximum running time of D' = 74 hours after N = 13.5 revolutions. ◯ With a diamond thickness d = 15 µm reaches the maximum unwinding length of the spring L' = 700 mm and provides a maximum running time of D' = 86 hours after N = 15.7 revolutions. ◯ With a diamond thickness of d = 20 µm reaches the maximum winding length of the spring L' = 722 mm and provides a maximum running time of D' = 92 hours after N = 16.7 revolutions. ◯ With a diamond thickness of d = 25 µm reaches the maximum winding length of the spring L' = 736 mm and provides a maximum running time of D' = 94 hours after N = 17 revolutions (power reserve 70 hours).

[0098] Thanks to the invention, a mainspring 10 with a composite structure comprising a second portion 12 made of diamond can thus provide a running time significantly greater than that of a strip 5 of the mainspring 1 made of conventional steel. Under the same conditions, a mainspring made of diamond according to the invention can achieve a running time that is one-third, one-half, two-thirds longer, even almost double, and possibly even more than double the running time of a strip 5 of the mainspring 1 made of conventional steel.

[0099] It should be noted that the gain in power reserve is advantageously greater the greater the relative diamond thickness d / T and the proportion of diamond in the composite structure of the mainspring according to the invention.

[0100] In the four previous realizations of the tension spring, according to this orientation example of the dimensions, the diamond occupies a surface portion (A = 2 d H + 2 dt) in relation to the total cross-section of a winding (S = H T) which is one sixth (17%), one third (33%), half (50%) and two thirds (67%) above the total cross-sectional area of ​​the winding (first portion + second portion).

[0101] Advantageously, a mainspring 10 according to the invention with a composite structure 11 / 12 therefore requires thicknesses 2d of diamond and T of coils that are much smaller than the thickness e of a conventional steel strip 5 of the mainspring under equivalent conditions in order to achieve the same value of the maximum elastic moment M0 (so that the barrel applies the same maximum torsional moment to the movement and avoids bouncing) while maintaining the same barrel and the same movement.

[0102] Such a refinement of the diamond-composed windings compared to the spring steel band can, according to the invention, optionally allow: - increase the length L', the number of turns Nb and the running time D of the mainspring with composite structure, taking into account that its volume V must be half the fixed volume V0 of the barrel (V = V0 / 2); and / or - to reduce the overall space required V = T·H·L' of the winding of the mainspring with composite structure compared to existing mainsprings and consequently the space required V0 of the barrel, and thus also the dimensions of the movement and the watch itself.

[0103] Fig. Figure 9 shows the elastic moment M plots of the preceding mainsprings, all under the same conditions (same standard barrel, same movement), against time, allowing their comparison. The elastic moment M is equivalent to the torsional moment applied by the barrel to the movement. The solid curve represents the torque delivered by the conventional steel strip mainspring according to the previous basic comparison example during its running time D. The dashed curve shows the torque delivered by the composite mainspring according to the last example during its running time D'.

[0104] It is clearly evident that the tension spring according to the invention initially (fully tensioned) has the same value of the maximum moment M0 (which had been limited upwards to avoid bouncing), the same moment M 24 at 24 hours and therefore at the end of a day J the essentially same moment loss ΔM 0-24 / M0. Thanks to the reduction in thickness T and the increase in length L' of the spring made possible by the invention, the unwinding α' (total tension angle) and the running time D' of the mainspring with a composite structure are considerably increased compared to the running time D of the mainspring made of conventional steel, assuming the same operating conditions (same dimension of the standard barrel, same movement).

[0105] In general, when designing a watch movement, the parameters of the barrel mainspring are determined as follows: M0, the value of the maximum elastic moment when the spring is tensioned, is determined (taking into account the moment of force required for the movement and gear train ratios); ΔM = (M0 - M 24 ) / M0, the change of the elastic moment within 24 hours, is limited depending on the tolerated range for the change of the vibrations. For a composite spring with a composite structure according to the invention, the maximum elastic moment is: M0=σ26[T2⋅H+E1−E2E2⋅t3⋅h'T]. For the loss of momentum within 24 hours, ΔM / M0=(M0−M24) / M0≈24D'⋅π⋅T⋅NbL'⋅Eσmax.

[0106] Thus, for an initially given maximum elastic moment M0, given the ratio between the elastic limits σ2 of diamond and those σ0 of conventional steels, a barrel mainspring composed of diamond according to the invention can have a total diamond thickness 2d that is significantly less than the thickness e of a conventional strip subject to equivalent conditions. Relative to this reference thickness e of a spring steel, the diamond thickness 2d can be reduced by a quarter to a half or even further (2d < e / 2); typically, the diamond thickness 2d is about a third smaller than the thickness e of conventional spring steel.

[0107] Consequently, the total thickness T of the coils of the tension spring according to the invention is reduced, and since in an optimal way a tension spring has a volume V opt of about half of the available volume V0 of the barrel: V=T⋅H⋅L'=Vopt≈0.5⋅V0, The length L' of the spring can be increased by one-third to double or more than double as a result of an extension; typically, the length L' of the diamond composite mainspring can be increased by half compared to the length L of the mainspring strip made of equivalent conventional steel.

[0108] Reducing the thickness T and increasing the length L' of the barrel mainspring according to the invention opens at least two ways of improving the properties of watch movements.

[0109] Firstly, increasing the length L' of the mainspring according to the invention allows an increase in the number of revolutions of the barrel (Nb) and consequently in the number of running hours D of the movement, while all other parameters of the movement and the dimensions of the barrel remain unchanged.

[0110] Thus, the invention makes it possible to increase the running time D in the same proportion and, in particular, to achieve a running time that is one third to twice or even more extended, typically half as long.

[0111] Secondly, reducing the thickness T (as well as increasing the running time D) of the mainspring according to the invention can reduce the elastic moment loss in 24 hours: ΔM / M0=(M0−M24) / M0=24D'⋅π⋅T⋅NbL'⋅E2σ2.

[0112] However, an increase in the length L' is generally associated with a corresponding increase in the number of coils or the number of turns of the barrel spring's unwinding Nb. Nevertheless, a reduction in the diamond thickness 2d and therefore in the thickness T, as well as an increase in the running time D', could lead to a reduction in the elastic moment loss ΔM. 0-24 / M0 in 24 hours.

[0113] The loss of torque over 24 hours determines the weakening of the escapement drives and the variation of the balance amplitude. Advantageously, reducing the thickness T of the coils of the diamond mainspring, combined with increasing the running time D' and increasing the length L' of the coils of the barrel mainspring according to the invention, allows the amplitude of the balance oscillations to be preserved and thus improves the chronometric accuracy of the watch.

[0114] For its fastening, the tension spring according to the invention is preferably suspended in the corresponding spring barrel with hooks which are formed at the ends of the tension spring 10 and are blocked in notches of the spring barrel 2.

[0115] More specifically, the mainspring has bent ends on the outside and inside, with these two bent ends being secured in a conventional manner in two notches located on the barrel's arbor and drum, respectively. The bent ends are formed or preformed during the etching and cutting of the first portion of a silicon wafer, as explained below.

[0116] Moreover, the invention advantageously allows the first portion 11 or the support lamella to be given a three-dimensional shape at the beginning for winding the coils of the desired tension spring, which has a spiral curvature profile or an inverted shape.

[0117] A profile (L) of the winding curvature of the spring 1 such as that of Fig. 2 develops in the winding plane (horizontal plane perpendicular to the spring) on ​​a two-dimensional surface, and the shape of the spring extends in space in three dimensions.

[0118] In order to obtain a first portion 11 of support material having a three-dimensional initial shape with a curvature profile in two dimensions, the invention advantageously provides for the shape of the first portion 11 to be carved out of a plate of the support material by etching processes, and in particular the processes known as litho-etching, as described hereinafter.

[0119] As already indicated, the carrier material is preferably silicon.

[0120] Silicon offers particularly advantageous properties for realizing the first portion 11, which forms the support structure for a barrel mainspring 10 according to the invention.

[0121] An interesting property of silicon is that it has a high melting point above 1400 °C, making it compatible with the high temperatures (in the range of 800 to 1000 °C) to which a support is subjected on which a deposit of polycrystalline diamond is grown by a chemical vapor deposition (CVD) process.

[0122] In general, the etching steps make it possible to easily and inexpensively obtain, in a crystalline silicon substrate wafer, a first or support portion 11 that represents a rough form of the final shape of the barrel mainspring 10 according to the invention and already has a three-dimensional preform specific to the winding of the mainspring according to the invention. In particular, it is envisaged to cut out a first portion with a spirally curved profile or an inverted shape.

[0123] The invention thus provides a method for producing a barrel mainspring 10 having a composite structure formed by a first portion 11 made of a support material, advantageously crystalline silicon, at least partially covered by a second portion 12 of polycrystalline diamond, the first portion 11 being obtained by deep vertical etching steps (the depth is considered here as the height h') of a plate of support material, preferably a silicon substrate, with a planar preform profile with the curvature of the mainspring winding, followed by steps of deposition and growth of polycrystalline diamond from carbon (or carbon compounds) by chemical vapor deposition (CVD) or plasma phase (CVD plasma) to produce the second portion 12 of diamond.which forms a thick layer on all or part of the surface of the first portion 11.,

[0124] Fig. 10A to 10E schematically show the important steps of such a method for producing the barrel mainspring according to the invention.

[0125] At the beginning of production, a silicon wafer 100 is provided, consisting of a thick layer 101 of crystalline silicon (particularly single-crystal silicon) deposited or bonded onto a base 102 called the substrate, generally with a layer 103 of silicon oxide sandwiched between them. Such wafers 100 of crystalline silicon substrate are commonly available from silicon suppliers and melters for integrated circuit manufacturing needs.

[0126] The height h' (vertical dimension, perpendicular to the horizontal plate) of the useful layer 101 of crystalline silicon can be selected. In this case, it is appropriate to choose a height h' corresponding to the desired height H of the windings of the barrel mainsprings 10. The height h' generally ranges from approximately 100 micrometers to a few millimeters, typically, for example, h' = 500 µm or h' = 700 µm or h' = 1 mm or h' = 1.2 mm.

[0127] In order to cut out one or more patterns of the first portion 11 from silicon, which have the shape of a spring blank, masking and essentially vertical deep etching steps (height h') are carried out in the silicon wafer 100.

[0128] As shown schematically in Fig. 10A and Fig.As shown in Figure 10B, a masking layer 104 is deposited on the surface of the silicon wafer. The masking layer is, in particular, a layer 104 of photoresist. Numerous known masking and etching methods exist in the field of etching semiconductor materials, and other equivalent methods may be used. The masking layer 104 is exposed through a ridge pattern, and the photoresist layer 104 is developed and then treated to form the etching mask 105. By developing and treating the exposed resin, the resin layer 104 is removed at the open areas of the mask, and one or more ridges 105 of masking resin are introduced, depending on the pattern used.

[0129] According to the invention, the ribs 105 have a profile in the plane of the wafer 100 (horizontal plane transverse to the winding of the planned spring), the profile of each rib 105 being wound following a spiral curvature or inverted shape.

[0130] A shape of the winding with a spiral profile like the one in Fig. 10D is advantageous because of the space requirements on the silicon wafer surface and allows the accommodation of a plurality of windings on the wafer as well as the extraction of a number of first portions for tension springs from a single wafer.

[0131] Fig. Figure 10B shows the ribs 105 (contacts) formed by a spirally rolled rib, seen in radial section with respect to the winding.

[0132] The following step, which is shown schematically in Fig.10C, is to perform a substantially vertical etching through the openings of the resin etching mask 105.

[0133] Deep reactive ion etching (DRIE) is preferably performed using a plasma (HF)-excited (ionized) gas stream or an ion plasma stream. This etching process can achieve an etch depth corresponding to the desired height h' of the windings.

[0134] By such etching, the layer 101 of crystalline silicon is excavated in the vertical direction h', leaving an intact island 11 of silicon in the shape of a bend or spirally rolled wall under each rib 105, while the wall has flanks that provide good verticality.

[0135] The etching can be stopped by a layer 103 of silicon oxide present between the useful silicon layer 101 and the substrate 102 of the wafer 100 to be removed.

[0136] Each silicon carrier portion 11 thus obtained according to the invention is detached from the substrate 102.

[0137] Such a portion 11 of silicon forms a lamella of substantially rectangular cross-section, but a profile extending along a spiral curvature, as in Fig. 10D, or wound along an inverted shape. This spirally wound silicon lamella, which forms the first portion 11 of the composite structure according to the invention, is then subjected to the steps of depositing and growing a thick layer 12 of polycrystalline diamond.

[0138] The first silicon portion 11 can be subjected to a surface treatment prior to the deposition steps to promote diamond nucleation, i.e., the formation of diamond nuclei. In particular, a micro- or nanocrystalline diamond coating can be deposited for nucleation, then starting the growth of polycrystalline diamond according to the teachings of document US Pat. No. 6,037,240 A.

[0139] The surface 11.1 - 11.2 - 11.3 - 11.4 of the crystalline and especially single-crystalline silicon is preferably pickled and freed of all traces of oxidation. In cases where a layer of silicon oxide (SiO2) has formed, it is removed, for example, by pickling in an acid bath or by plasma treatment in a chamber containing 100% gaseous hydrogen H2 or ionized hydrogen H + at low pressure, according to the teachings of US 6 037 240 A or document WO 94 / 08076 A1.

[0140] As previously stated, the first or support portion may alternatively consist of a metal strip or sheet onto which a thick layer of polycrystalline diamond is deposited to form the second or outer diamond portion. It is preferred that the metallic support material of the first portion has a low coefficient of thermal expansion close to that of diamond to ensure good adhesion of the spring to the composite structure according to the invention.The metallic material can be chosen in particular from the families of metals and alloys consisting of invars (iron and nickel alloys or nickel steels), Fe-Ni and in particular Fe-Ni-Cr or Fe-Ni-Co-based alloys, Fe-Cr and Fe-Co-based alloys, alloys based on iron as a majority and elements such as C, Si, Ni, Co, Mn, Cr, Va, Ti, Sc, W (plus <1% of oligoelements), hard or semi-hard steels, hardenable or stainless steels and / or special steels called unalloyed or low-alloyed, titanium, tungsten and their alloys.Such a metal support component, consisting of a strip or sheet which (depending on the elastic properties of the outer layer: elastic modulus and elastic limit) is advantageously wound in spirals or wound in the shape of a letter S inverted or in the shape of half a letter S half inverted, can be obtained in a conventional manner by rolling and spring winding operations and, if necessary, hardening.

[0141] Diamond deposition is carried out using processes collectively known as chemical vapor deposition (CVD). It involves the deposition of polycrystalline diamond starting from carbon, or more precisely from carbon compounds (e.g., acetylene, acetone, fluorocarbon or other carbon-containing radicals, methane and its hydrocarbon radicals CH4, CH2**, CH***, C****), which are generally introduced in gaseous form and then ionized in the form of a plasma stream, emitted by a superheated filament, or excited by radio frequencies (microwaves). The ionized and superheated carbon deposits on the relatively cold surface (800-1000°C to 1400°C) of each first portion 11 of the crystalline silicon support, crystallizing in the form of polycrystalline diamond, particularly as microcrystals or nanocrystals.

[0142] It has been observed that the maximum stress limit σ2' of the artificial diamond deposit is higher and tends towards the maximum limit of the (natural or synthetic) single-crystal diamond, the smaller the crystals are.

[0143] In this specification, the term ‘polycrystalline diamond’ includes the deposition of microcrystalline diamond, which is composed of crystals in the micrometre range, and nanocrystalline diamond, which is composed of crystals in the nanometre range.

[0144] The outer portion of the spring according to the invention can advantageously be composed of microcrystalline diamond or, preferably, of nanocrystalline diamond, even of crystals with a size below one nanometer; the diamond resists increased elastic limit stresses σ2' the more the smaller the crystals are.

[0145] Further details on the steps of depositing and growing a diamond layer of thickness 12 by CVD can be found in patent document EP 0 732 635 A1, filed in the name of CSEM, to which reference may be made.

[0146] How Fig. 10D and Fig. 10E, the deposit 12 of polycrystalline diamond forms on the entire surface 11.1 to 11.4 of the first portion 11 and grows regularly until a thick outer diamond layer 12 is formed, which envelops the first portion 11.

[0147] Advantageously, such a process enables the formation of an outer layer 12 or diamond gangue of uniform thickness, easily reaching a thickness of more than five or ten micrometers and preferably of more than 20 or 30 µm.

[0148] Fig.However, Figure 11 shows in detail the view of a cross-section through a first or silicon portion 11 as it results from the etching step ( Fig.10C), where it can be seen that the lateral flanks 11.1 and 11.2 of the first portion 11 are not strictly parallel, but form a shallow angle δ with the vertical, which has been exaggerated in this representation. The angle δ, called the 'undercut angle' and inherent in the etching process, is very shallow, on the order of 1 to 2°, and often even smaller. However, on a first portion 11 with a height h' of the order of 0.5 to 1.5 mm and an initial thickness t of 30 µm at the level of the rib 105, it leads to a final thickness t' that has been reduced by half or has disappeared completely (triangular or obliquely cut cross-section).In order to avoid such a defect, it has been proposed, according to an alternative to the method according to the invention, to carry out two practically perpendicular etchings starting from the two opposite sides of a wafer 100 of the silicon substrate through two etching masks 104 and 114 which are symmetrical with respect to the central plane 0-0 of the wafer, as shown schematically in . Fig. 12A and Fig. 12B. The support portion 11' thus obtained has a transverse thickness t which decreases slightly towards the central plane 103, but remains acceptable over the height h1 + h2 of this first portion 11.

[0149] Such a process makes it possible, in particular, to use conventional silicon wafers formed from a layer 101 of height h1 of silicon deposited on a silicon substrate 102 (which in turn has a height h2 of the order of about 100 micrometers to a few millimeters), the layer 101 and the substrate 102 being separated or not by an intermediate layer of silicon oxide.

[0150] According to one such variant of the manufacturing method of a barrel mainspring 10 according to the invention, two masking layers 104 and 114 are deposited on the two opposite sides of a wafer 100 of the substrate 102, which has at least one layer 101 of crystalline silicon (with a height of more than approximately 100 micrometers). Two etching masks are cut from the masking layers 104 and 114, comprising at least one pair of ribs 105 / 115 that are substantially symmetrical to a center plane 103 of the wafer, and at least one practically perpendicular etch is performed over the entire height of the wafer 100 of the silicon substrate through the two opposite masks, preferably by deep reactive ion etching.

[0151] The following steps consist of extracting at least one first portion 11 of silicon thus obtained, then depositing an outer, thick layer 12 of polycrystalline diamond on at least part of the surface of such first portion 11 by chemical vapor deposition (CVD) of carbon or carbon compounds or by plasma decomposition of carbon-containing gases crystallizing into polycrystalline diamond in a high-frequency field (CVD plasma), and then allowing it to grow.

[0152] Such diamond deposition and growth steps advantageously make the total thickness T of the final tension spring 10 more regular.

[0153] In general, an advantageous feature of the invention consists in that a first portion 11 forming a support is presented vertically (here, by convention, in the direction of the height h'), thereby allowing a deposit of a thick diamond layer 12 to grow horizontally, corresponding to the thickness (T; d / t / d) of the windings of the mainspring (ie in the radial directions), over the entire height h' of the first supporting portion 11, instead of attempting, as is conventional, to grow a diamond deposit vertically while attempting to achieve the desired height of the windings of the barrel mainspring.

[0154] As in Fig. 10E, the outer portion 12 made of polycrystalline diamond advantageously has smoothed or rounded edges with a radius of curvature r without sharp edges.

[0155] More generally, the barrel mainspring 10 according to the invention has a diamond outer surface, which offers a low surface roughness (Ra < 1 µm) and a low coefficient of friction. Furthermore, the roughness can be reduced by CVD formation of nanocrystals or by plasma polishing, which further mitigates all sources of friction.

[0156] As an alternative to polishing or the deposition of nanocrystals, it is possible to add a dry-lubricating external coating, in particular a coating made of Teflon, deposited for example by CVD, or of molybdenum disulfide, which adheres well to the surface crystals of the diamond portion.

[0157] Advantageously, the mainspring 10 obtained according to the invention makes it possible to virtually eliminate the friction previously present between a conventional mainspring and the barrel 2. Furthermore, the barrel mainspring 10 according to the invention practically no longer exhibits any internal friction between its coils as they unwind. This point is particularly favorable for the regularity of the torque delivered to the movement and thus for the regularity of the balance amplitude, and it makes it possible to eliminate the lubrication problem (see the previously cited work by L. Defossez, "Theorie générale de l'horlogerie," Chapter IV, "Drive Force." Fig., Factors influencing spring fatigue). According to the invention, the tension spring 10 with an outer layer of 12 mm thickness made of diamond no longer requires lubrication, nor any grinding or rounding of edges (nor, of course, any annealing, hardening, tempering, or heat treatment as with metal strips made of conventional spring steel).

[0158] An important advantage of the inventive diamond mainspring 10 is that it loses virtually no elastic force over the course of its operation, since diamond is not subject to plastic deformation. Thus, the elastic moment and the maximum energy accumulated by the inventive barrel mainspring remain constant over time and no longer exhibit fatigue problems, which contributes to ensuring the running time and power reserve of the movement.

[0159] In addition to the absence of deformation, the hardness of diamond allows it to resist wear and tear, so that the tension springs 10 composed of diamond according to the invention promise to have a long service life, diamond being famous for its long service life.

Claims

[1] Mainspring (10) for a watch movement barrel, comprising a composite structure comprising a first portion (11) made of a support material, which is at least partially covered by a second portion (12) made of diamond, characterized by that the mainspring (10) consists predominantly of diamond volume in terms of its total volume. [2] Barrel mainspring according to claim 1, characterized by that the carrier material of the first portion (11) is a semimetal, in particular carbon, silicon or germanium. [3] Barrel mainspring according to claim 1 or 2, characterized by that the carrier material of the first portion is crystalline silicon. [4] Barrel mainspring according to claim 1, characterized by that the first or carrier part (11) consists of metallic material. [5] Barrel mainspring according to claim 1 or 4, characterized byin that the first portion (11) consists of a support material chosen from the families of metals and metal-based alloys consisting of one or more of the elements Fe, Ni, Cr, Co, Cu, Mn, V, Ti, Sc, W, C, Si, the families of metallic materials comprising in particular Fe-Ni-based alloys, Fe-Cr-based alloys, Fe-Co-based alloys, Fe-Ni-Cr or Fe-Ni-Co-based alloys, hard or semi-hard steels, hardenable or stainless steels, special steels classified as unalloyed or low-alloyed, as well as titanium and / or titanium alloys, tungsten or tungsten alloys. [6] Barrel mainspring according to one of claims 1 to 5, characterized by that the first portion (11) consists of a carrier material which has an expansion coefficient similar to that of diamond. [7] Barrel mainspring according to one of claims 1 to 6, characterized by that the second portion (12) is formed from at least one layer (12.2) of polycrystalline diamond with a thickness of more than ten micrometers. [8] Barrel mainspring according to one of claims 1 to 7, characterized by that the first portion (11) is arranged in the core of the second portion (12) made of diamond. [9] Barrel mainspring according to one of claims 1 to 8, characterized by that the first portion (11) and the second portion (12) have substantially concentric, rectangular cross-sections (h'·t / H·T). [10] Barrel mainspring according to one of claims 1 to 9, characterized by that the second portion (12) made of diamond has a thickness (d / 2d) which is more than one tenth of the thickness (t) of the first portion (11). [11] Barrel mainspring according to one of claims 1 to 10, characterized by that the second portion (12.2) covers at least one flank (11.2) of the first portion (11). [12] Barrel mainspring according to one of claims 1 to 11, characterized by that the second portion (12) has a thickness (d) of diamond perpendicular to the surface (11.1 to 11.4) of the first portion (11) which is more than one sixth or one third of the thickness (t) of the first portion (11). [13] Barrel mainspring according to one of claims 1 to 12, characterized by that each winding of the tension spring has a cross-sectional area (A = h'·d to A = 2d·H + 2d·t) of diamond which is larger than the cross-sectional area (h'·t) of the first portion (11). [14] Barrel mainspring according to one of claims 1 to 13, characterized by that it has a thickness (T) of the turns which is more than about ten micrometers and a height (H) of the turns which is more than about one hundred micrometers. [15] Barrel mainspring according to one of claims 1 to 14, characterized by that the relaxed tension spring (10) has a spiral shape or an inverted shape. [16] Barrel mainspring according to one of claims 1 to 15, characterized by that the second part (12) is in direct contact with the surface (11.1 to 11.4) of the first part (11). [17] Barrel mainspring according to one of claims 1 to 15, characterized by that an intermediate layer for nucleation or as a crystallization nucleus is inserted between the first portion (11) and the second portion (12) made of diamond. [6] o'clock, characterized by that it comprises at least one barrel mainspring (10) according to one of the preceding claims for operating a clockwork. [19] A method for manufacturing a barrel mainspring (10) having a composite structure comprising a first portion (11) made of a support material, which is at least partially covered by a second portion (12) made of diamond, the method comprising steps consisting in: - to present a plate (100) consisting of carrier material (101); - to carry out an etching (106) substantially perpendicularly (h') in the carrier material (101) through an etching mask (105); - extracting at least a first portion (11) obtained from carrier material; - depositing a second portion (12) of diamond on each first portion (11); and wherein the tension spring (10) consists predominantly of diamond volume with respect to its total volume. [20] A method according to the preceding claim, wherein the plate of support material is a plate (100) comprising at least one layer (101) of crystalline silicon. [21] A method according to the preceding claim, wherein the surface of each first portion (11) of silicon is protected from any oxidation or, where appropriate, such a silicon oxide layer is pickled away. [22] A method according to any one of claims 19 to 21, wherein the etching (106) is carried out by substantially perpendicular, deep reactive ion etching at the height (h') of the plate (100). [23] A method according to any one of claims 19 to 22, wherein the etching mask comprises at least one rib (105) having a winding profile parallel to the plane of the plate forming a spiral or inverted curvature. [24] A method according to any one of claims 19 to 23, wherein the second portion (12) is formed by depositing at least one layer (12.2) of polycrystalline diamond to a thickness (d) exceeding about ten micrometers. [25] Method according to claim 24, wherein the deposition (12) of diamond on the first portion (11) takes place perpendicular to the turns (10) and / or to the height (h') of the first portion (11). [26] A method according to any one of claims 19 to 25, wherein two substantially vertical etches are performed through two masks (104, 114) for symmetric etching (105, 115) arranged on two opposite sides of a substrate plate, the etches being performed over the entire height (h1 + h2) of the substrate plate (100).

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