EP3304216B1 - Thermocompensated horology resonator and method for producing such a resonator - Google Patents

Description

Technical area

The invention relates to a method of manufacturing a spiral spring.

State of the art

The time base of a timepiece uses a resonator whose oscillations must be maintained. A resonator with a given resonant frequency most often performs this function. It is known, in particular, resonators such as the pendulum (which involves gravity), the quartz (piezoelectricity), the tuning fork (vibrating blades) or the return springs of more diverse forms, depending on whether they are designed to oscillate over large or small amplitudes.

In particular, in most mechanical watches, the regulating member consists of the balance wheel/hairspring assembly. The hairspring is in this case fixed by one end to the balance shaft and by the other end to a bridge in which the balance shaft pivots. In this way, the spring contracts and relaxes alternately around its center during the oscillations of the balance wheel. Since its creation, a few hundred years ago, and until today, the spiral springs are mainly made from a metallic blade of rectangular section wound on itself in the form of an Archimedean spiral.

Current metal spiral springs are mainly made from FeNiCr (commonly called elinvar) or NbZr, which the latter having in addition to the first a reduced magnetic susceptibility. The choice of these materials is mainly dictated by the need to have a resonator whose mechanical and geometric properties vary as little as possible during temperature changes to which the watch may be exposed, i.e. a range of up to sixty degrees, more specifically within a range of 8 to 38°C for watches certified to meet the criteria governing “chronometer” certification.

These resonators are difficult to manufacture. It is a question of guaranteeing the control and the repeatability of the complex metallurgical manufacturing processes having a definitive impact on the intrinsic mechanical properties of the hairspring. Slight variations in the chemical composition of the alloy, heat treatments, work hardening rate of the material during its transformation from an ingot to a fine wire, or even geometric inaccuracies in the wires produced are the cause of large variations in the mechanical properties of the hairsprings produced. Retouching operations of the hairspring during adjustment of the watch are carried out in order to partially overcome these drawbacks.

In addition, the high amplitude of the balance wheel and a low maintenance torque of its oscillations favored the development of a spiral-shaped spring to equip the regulating organ of mechanical watches. However, this geometric shape has drawbacks, such as the action of gravity. Indeed, the slight deformations due to the own weight of the hairspring can induce defects in the concentric development of the hairspring around the axis of the balance wheel and therefore affect the precision of the watch.

On the other hand, certain metal alloys which are widely used in the manufacture of hairsprings, such as those based on FeNiCr, are sensitive to the action of a magnetic field.

An alternative to metal resonators has been proposed based on silicon. This material has a number of advantages, such as stable mechanical properties, insensitivity to magnetic fields, microfabrication processes (DRIE in particular) allowing high precision and reproducibility of geometries, low density, therefore reduced disturbances due to gravity, and a high quality factor. The patent JPH06117470 (Yokogawa, 1992 ) mentions in particular a hairspring made of such a material.

The negative thermoelastic coefficient of the modulus of elasticity of this material is however too great to guarantee the frequency precision of such a resonator. Research on the subject has led to the addition of a layer of a material whose thermoelastic coefficient is opposite to that of silicon. Thus, Berry and Pritchet (IBM Technical Disclosure Bulletin #1237, Vol. 14, No. 4, 1971 ) have shown that amorphous silica (SiO ₂ ) responds advantageously to this condition. That's what Shen et al. (Sensors & Actuators A 95, 2001 ) have checked, exposing the calculations making it possible to optimize the thickness of this layer for a given thickness of the bar of a monocrystalline or polycrystalline silicon resonator, indicating that the same approach can be used for any other material of resonator.

The document EP1422436 discloses a spiral spring resulting from the cutting of a {001} silicon plate. The aim of this invention is to overcome the drawbacks described above, proposing a hairspring whose sensitivity to variations in temperature and to magnetic fields is minimized. In addition, the precision manufacturing technology (Ion Etching) proposed for the shaping of these hairsprings, combined with modeling and compensation of the anisotropy due to the crystalline orientation of the material, makes it possible to reduce the retouching of the regulating organ and to improve the reproducibility of the process.

However, the frequency drift of resonators made of monocrystalline silicon may require complex corrections depending on the application. This comes from the anisotropic character of physical quantities such as the Young's modulus and the coefficient of thermal expansion of this material.

The object of the invention described in the document EP2590325 is to overcome at least in part the drawback mentioned above. This document discloses a thermocompensated resonator comprising a ceramic spring whose surface is coated with at least a second material whose CTE (thermoelastic coefficient) has the opposite sign to the

CTE of the material used for the spring core. The document EP2597536A1 describes the fabrication of a quartz glass balance spring, part of which is exposed to a femtosecond LASER.

The use of coatings in watch resonators compromises their mechanical robustness. The coatings deposited on the springs must withstand very high stresses. Typically these stresses are concentrated close to the coating-substrate interface. The repetition of the deformations during the concentric development of the spring (winding and unwinding) can therefore result in the breakage of the coating or in its delamination, this leading to a significant change in the frequency of oscillation.

Finally, we mention the patent EP1958031 which reports thermal compensation by modifying the thermoelastic properties of a so-called photopatternable glass following exposure to a UV source. This technique has several limitations inherent in the choice of material and process. The material is on the one hand limited to a particular category of so-called “photostructurable” glasses, which are glasses doped in particular with a photoactive element such as cerium and with a nucleating agent such as silver. When the photoactive element is exposed to ultraviolet radiation, it is photoionized, giving rise to an electron. These electrons will therefore be trapped subsequently by the nucleating agents, which will allow the formation of nucleation centers or clusters at the origin of the process of formation of nano or microcrystals in the vitreous matrix. The process described above is very dependent on the temperature, which is why substantial heat treatments (T > 500°C) are required in order to complete (freeze?) the process of photostructuring of this type of material.

On the other hand, the photostructuring process described in the patent EP1958031 requires the use of masks in order to hide some areas of the photopatternable material from exposure to UV radiation.

It is therefore not possible by this process to carry out local modifications of the photopatternable material without the part of the outer layer of the substrate located between the modified zones and the light source also being modified. Finally, this process is limited to UV wavelengths of light and the use of specific tools.

Brief summary of the invention

In the context of the present invention, it is proposed to overcome these drawbacks by proposing a method of manufacturing the resonator, according to claim 1.

In one embodiment, the irradiation process may involve multiphoton absorption in the vitreous material.

The modified portion is adjusted so that its physical properties compensate for the physical properties of the rest of the vitreous material (unmodified portion). In this way, the resonator is thermocompensated, and its stiffness can also be adjusted.

The resonator thus obtained is non-magnetic and thermocompensated while avoiding the use of coatings.

The material also comprises a locally modified portion so that said modified portion has a second stiffness which differs from the first stiffness of the unmodified material.

The method includes adjusting the frequency of the hairspring. This adjustment can be made before assembling the hairspring with a balance wheel.

The frequency of the hairspring is adjusted so as to pair the hairspring with the balance wheel.

Brief description of figures

• les figures 1 à 6 représentent une vue en coupe d'une lame d'un ressort-spiral selon des exemples utiles pour comprendre l'invention; les figures 5 et 6 indiquent des modes de réalisation selon la présente invention;
• la figure 7 montre une vue de dessus d'un ressort-spiral 1;
• la figure 8 montre un détail d'une section du ressort-spiral de la figure 7;
• la figure 9 montre un wafer en matériau vitreux dans lequel est fabriquée une pluralité de ressorts-spiraux, selon un mode de réalisation;
• la figure 10 est une vue en coupe du wafer de la figure 9 pendant un procédé d'irradiation, selon un mode de réalisation; et
• la figure 11 illustre la vue en coupe de la figure 9 après une étape d'usinage, selon un mode de réalisation.

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which:

• the figures 1 to 6 represent a sectional view of a blade of a hairspring according to examples useful for understanding the invention; the figures 5 and 6 indicate embodiments according to the present invention;
• the figure 7 shows a top view of a spiral spring 1;
• the figure 8 shows a detail of a section of the hairspring of the figure 7 ;
• the figure 9 shows a glassy material wafer in which a plurality of spiral springs are fabricated, according to one embodiment;
• the figure 10 is a sectional view of the wafer of the figure 9 during an irradiation process, according to one embodiment; and
• the figure 11 illustrates the sectional view of the figure 9 after a machining step, according to one embodiment.

Example(s) of embodiment of the invention

According to one embodiment, a balance-spring intended to equip a resonator of the balance-spring type of a mechanical watch is made of a vitreous material. Here, the expression "vitreous material" includes in particular pseudo-amorphous (that is to say non-crystalline) materials exhibiting the glass transition phenomenon. The glass transition is a reversible phenomenon of transition between the hard form and the "molten" or rubbery form of an amorphous material. The temperature of The glass transition of an amorphous material is always lower than the melting point of its crystalline form. The term “vitreous material” also means a material that is partially vitreous (therefore at least partially amorphous).

The vitreous material can comprise any glass as defined above. Preferably, the vitreous material has a low coefficient of thermal expansion, that is to say less than 1 ppm/°C. For example, the vitreous material is based on silica (amorphous αSiO ₂ ) having a coefficient of thermal expansion of around 0.5 ppm/°C. For example, the vitreous material can comprise a pure amorphous silica, a borosilicate, an aluminosilicate, or a silica-based glass with a controlled level of impurities.

The spiral spring 1 comprises a vitreous material having a first β ₁ . The vitreous material is locally modified so as to produce a modified portion 3 directly from the vitreous material, the modified portion 3 of the vitreous material having a second CTE β ₂ different from the first CTE β ₁ . In other words, the modified portion comprises said vitreous material having undergone, locally, structural modifications. In this way, the hairspring 1 is characterized by an effective CTE β _eff which is defined by the combination of the first CTE β ₁ and the second CTE β ₂ . The presence of the modified portion makes it possible to minimize the thermal drift of the resonator formed by the hairspring and the balance wheel.

The vitreous material also has a first coefficient of thermal expansion α ₁ and the modified portion has a second coefficient of thermal expansion α ₂ which may be different from the first coefficient of thermal expansion α ₁ . The vitreous material also has a first Young's modulus E ₁ and the modified portion has a second Young's modulus E ₂ which may be different from the first Young's modulus E ₁ . The hairspring 1 is therefore also characterized by an effective coefficient of thermal expansion α _eff which is defined by the combination of the first and second coefficients of thermal expansion α ₁ , α ₂ and by an effective Young's modulus E _eff which is defined by the combination of the first and second Young's modulus E ₁ , E ₂ .

The figures 1 to 6 represent a section of the blade of a hairspring 1 comprising a matrix in an at least partially vitreous material 2 and a modified portion 3. Said modified portion may comprise a central modified zone 3 ( figures 1 and 4 ), two surface modified areas 3 ( figures 2 and 3 ) according to examples useful for understanding the invention. comprises A plurality of modified zones 3 are distributed in the matrix of vitreous material ( figures 5 and 6 ) according to the present invention. Without being limiting to the variants illustrated, the exposed zones 3 may have varied geometries and distributions, an asymmetry of these zones also being able to be chosen to overcome different compressive and tensile stresses with respect to the neutral fiber of the blade. These different geometries can be, if necessary, combined on portions distributed along the blade to respect the isotropy of the material or to meet other needs. The exposed zones 3 can be located anywhere on the blade, preferably without contact with the surface of the latter.

The configuration of the modified portion 3 can also vary along the hairspring 1. In particular, this variation can include the geometric configuration of the modified portion 3 which varies along the hairspring 1, but also a variation of the value of the second CTE β ₂ , and possibly also of the second coefficient of thermal expansion α ₂ and of the second Young's modulus E ₂ , in the various modified portions 3, along the hairspring 1. The modified portion 3 can be arranged so as to extend in the longitudinal direction of the hairspring 1 in a continuous manner (such as continuous fibers) or in a discontinuous manner. In the same way, elongated geometries of the modified portion 3 can be oriented in any direction, preferably in the longitudinal direction of the hairspring 1.

In order to take advantage of the high precision and repeatability of the microfabrication processes and of the proposed thermocompensation process, the geometry of the hairspring 1 may advantageously incorporate attachment means at its two ends, as they are known to man. trade, with rigid mounting means or, preferably, elastics. For various identification needs (pairing, etc.), identification references can be micro-engraved on the hairspring 1.

The thermal drift of the resonator relates to the relative variations in the frequency of oscillation of the regulating organ following changes in temperature in the range of 8 to 38°C. The relative frequency variations with temperature depend mainly on the effective coefficient of thermal expansion α _eff and on the effective CTE β _eff of the hairspring 1.

où K est la raideur du ressort-spiral et l le moment d'inertie du balancier. Ce dernier est décrit par l'équation (2): $$I={\mathit{mr}}^2$$

où m est la masse du balancier et r est le rayon de giration du balancier, et K est décrit par l'équation (3): $$K=\frac{E_{\mathit{eff}}{\mathit{he}}^3}{12L}$$

où E_eff est le module de Young effectif du ressort-spiral, h la hauteur ressort-spiral, e l'épaisseur ressort-spiral et L la longueur ressort-spiral.In the particular case of the resonator, the oscillation frequency can be written as: $$f=\frac{1}{2}\sqrt{\frac{K}{I}}$$

where K is the stiffness of the hairspring and l the moment of inertia of the balance. The latter is described by equation (2): $$I={\mathit{<figure-callout\; id="2"\; label="mr"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">mr</figure-callout>}}^2$$

where m is the mass of the pendulum and r is the radius of gyration of the pendulum, and K is described by equation (3): $$K=\frac{E_{\mathit{delete}}{\mathit{Hey}}^3}{12L}$$

where E _eff is the effective Young's modulus of the hairspring-hairspring, h the height of the hairspring-hairspring, e the thickness of the hairspring-hairspring and L the length of the hairspring-hairspring.

où A est une constante, β_eff le coefficient thermoélastique effectif du ressort-spiral, α_eff le coefficient d'expansion thermique effectif du ressort-spiral, αb le coefficient d'expansion thermique du balancier.As a first approximation, the relative variation of frequency as a function of temperature can be formulated as: $$\frac{\mathit{\Delta f}}{f\left(T_0\right)}=\mathrm{HAS}\left(\frac{{\beta }_{\mathit{delete}}}{2}+\frac{3}{2}{\alpha }_{\mathit{delete}}-{\alpha }_b\right)\mathit{\Delta T}$$

where A is a constant, β _eff the effective thermoelastic coefficient of the hairspring, α _eff the effective thermal expansion coefficient of the hairspring, α b the thermal expansion coefficient of the balance wheel.

If one seeks to minimize the thermal drift of the frequency of the resonator, it would be necessary to satisfy the following relation: $$\left(\frac{{\beta }_{\mathit{delete}}}{2}+\frac{3}{2}{\alpha }_{\mathit{delete}}\right)={\alpha }_b$$

As the CTE of most metals is very negative, around 1000 ppm/C, and the coefficient of thermal expansion is rather around 10 ppm/C, complex alloys such as Nivarox CT ^® or Parachrom ^® had to be developed in order to satisfy the previous equation.

In the case of the use of monocrystalline silicon (CTE ~ -60 ppm/C), thermal compensation is obtained thanks to the growth of a layer of amorphous SiO ₂ around the core of the spring-spring. Amorphous SiO ₂ is one of the rare materials with a positive CTE, around 200 ppm/C. The thickness of the SiO ₂ layer for the thermal compensation of the hairspring is predicted according to the dimensions of the blade of the hairspring as described in the document EP1422436 . The CTE of the hairspring is in this case an effective CTE β _eff comprising the contribution of the monocrystalline silicon core of the hairspring and the contribution of the outer layer of amorphous SiO ₂ . This effective CTE β _eff satisfies equation (6) provided that the α b is known and that the α of the hairspring does not differ significantly from the α of the web: $${\beta }_{\mathit{delete}}=2{\alpha }_b-3\alpha$$

In the present invention, said structural modifications of the vitreous material result in a second CTE β ₂ of the locally modified vitreous material (modified portion) which differs from the first CTE β ₁ of the unmodified vitreous material. The hairspring therefore has an effective CTE β _eff which differs from that which it would have in the absence of the modified portion (which would then correspond to the first CTE β ₁ ). Equation (6) can therefore be satisfied for the resonator using the contributions of the first CTE β ₁ and of the second CTE β ₂ to the effective CTE β _eff .

An advantage of the proposed solution is that it is not necessary to add or grow a different material to that forming the hairspring to modify the effective CTE β _eff of the hairspring. In addition, the second CTE β ₂ can be modulated in a controlled manner.

• d'usiner une pièce d'un matériau vitreux ayant un premier CTE β₁ afin de former le résonateur; et
• de modifier localement le matériau vitreux de manière à former ladite portion modifiée 3 ayant un second CTE β₂ différent du premier CTE β₁.

According to the invention, the process for manufacturing the resonator comprises the steps:

• machining a piece of vitreous material having a first CTE β ₁ to form the resonator; and
• to locally modify the vitreous material so as to form said modified portion 3 having a second CTE β ₂ different from the first CTE β ₁ .

The machining step can be carried out by a chemical etching process, by a physical etching process or by a combination of the two processes.

The step of forming said modified portion 3 can be performed before, during or after the machining step. In the case where the formation of the modified portion 3 is carried out before the machining step, the method can also comprise another step of forming the modified portion, after the machining step.

The step of locally modifying the vitreous material includes laser treatment.

In a variant, the laser is operated in a non-ablative mode. That is to say that we do not remove material in the zone where the laser is focused.

The laser uses ultrashort pulse durations, that is to say pulse durations comprised between a few femtoseconds to a few nanoseconds, and preferably between a few femtoseconds to a few picoseconds.

Ultrashort laser pulse durations induce structural changes resulting from complex nonlinear phenomena which also give rise to a local modification of the CTE of the modulus of elasticity as well as of the modulus of elasticity itself.

In particular, the use of pulse durations between a few femtoseconds and a few picoseconds promotes radiation-matter interaction mechanisms based on multiphoton absorption.

Pulse durations comprised between a few femtoseconds and a few nanoseconds can be obtained with several types of lasers of very diverse wavelengths such as, for example, and preferably, a Ti:Sapphire laser (650 to 1100 nm), a laser Yb (1030 nm) or even a mid-infrared laser (mid infrared, 1050 nm).

Depending on the combination of laser operating parameters, including pulse duration, power, and repetition rate, the precise nature of the laser-matter interaction will differ. However, insofar as a structural modification can take place, the first CTE β ₁ of the vitreous matrix will be modified, at least locally so as to obtain the modified portion 3 having the second CTE β ₂ . The same reasoning also applies to obtain the modified portion 3 having the second coefficient of thermal expansion α ₂ different from the first coefficient of thermal expansion α ₁ and having a second Young's modulus E ₂ different from the first Young's modulus E ₁ .

Such a laser treatment would make it possible to modify the effective CTE β _eff of the hairspring 1, to modify the effective coefficient of thermal expansion α _eff and/or to adjust the value of the effective Young's modulus E _eff of said spring 1. Combined with the possibility of working on a "case by case" basis, the fine adjustment of the effective Young's modulus E _eff of the spring 1 makes it possible to adjust its stiffness (and therefore the frequency of the resonator) without needing to modify either the height or the thickness of the vibrating body (1). This facilitates, for example, hairspring-balance pairing operations at the same time as allowing a significant increase in the output of these operations.

The figure 7 shows a top view of a spiral spring 1 . The spiral spring 1 comprises an inner terminal curve 4 and an outer terminal curve 5. A detail of a section 6 of the spiral spring 1 is shown in figure 8 . In section 6, the modified portion 3 comprises a portion 3' extending in the longitudinal direction of the hairspring 1 continuously and discontinuously.

The modification of the effective Young's modulus E _eff in a specific portion of the balance-spring 1, such as for example in the terminal curve of the balance-spring, makes it possible to optimize the concentricity of its deployment with respect to the axis of the balance wheel. It is also possible to compensate for subsidence effects due to gravity or other isochronism defects.

The structure of a solid material exhibiting the glass transition phenomenon (in other words, its specific volume or its density) can be fixed according to the thermal cycle (heating-cooling ramp) to which it is subjected. It is therefore possible, depending on the thermal cycle, to freeze the structure of a material exhibiting the glass transition phenomenon, either in a particular vitreous state or even in a crystalline state. In the particular case of a glass, it is possible to freeze its structure by subjecting it to a thermal cycle which does not include passing through the liquid state of the material. That said, by remaining in the solid vitreous zone of the material, it is possible to change its structure by subjecting it to a particular thermal cycle.

Silica-based glasses become denser following an increase in their fictitious temperature. The fictitious temperature of a glass is the temperature at which its structure (atomic arrangement) is fixed. The fictitious temperature depends on the rate of cooling during a thermal cycle. For an ordinary glass, the higher is its fictitious temperature (the faster it is cooled), the greater its specific volume. In the particular case of silica, an opposite trend is observed. Indeed, at higher fictitious temperature one will find smaller specific volumes.

The local modification of the vitreous material by a focused laser treatment using ultrashort laser pulse durations makes it possible to obtain a modified portion thanks to phenomena of a thermal nature such as those described above. The phenomena of a thermal nature leading to a local change in the structure of the vitreous material can be modulated, in particular by playing with the repetition rate of the laser.

It is also known that multiphoton absorption in the vitreous material can lead to phenomena of a more complex nature than phenomena of a thermal nature, for example photoionization and even the creation of nano empty spaces. Indeed local modifications of the density of a vitreous matrix based on silica resulting from the interaction with a laser with ultrashort pulse duration have been reported and this without these nonlinear phenomena being completely understood.

In one embodiment, the modified portion 3 has a density which differs from the density of the vitreous material. The different density of the modified portion 3 can comprise the creation of a crystalline polymorph of an amorphous silica (such as for example alpha or beta quartz, stishovite, tridymite, chalcedony or even cristobalite), or the formation of metallic clusters according to the presence or absence of impurities or the formation of a densified region following a local increment of the fictitious temperature of the silica, or the formation of structurally modified zones due to any absorption mechanism nonlinear.

Advantageously, the laser is focused at the nano or micrometric scale. Such a laser allows the generation of the modified portion whose CTE of the modulus of elasticity and the stiffness can be modified, and this in proportions which are not necessarily constant or linear.

The formation of the modified portion having a second CTE β ₂ different from the first CTE β ₁ makes it possible to adjust, case by case, the value of the effective CTE β _eff of the resonator.

The method of the invention offers the advantage of being able to precisely and individually adjust the thermomechanical properties of each resonator with the aim of fine-tuning the temperature behavior of the oscillator.

The figure 9 shows a wafer 7 of vitreous material in which a plurality of spiral springs 1 are made. figure 10 shows a sectional view of the wafer of the figure 9 during the local modification step making it possible to form the modified portion 3, for example, by irradiation with a focused laser 8. As suggested in the figure 10 , the laser beam 8 can move, for example in x, y, z, so as to carry out the local modification according to a more or less complex shape, here an "annular" shape in the mass of the body of the hairspring 1. In the figure 10 the parts indicated by the number "9" correspond to the vitreous material which is machined in the machining step. The figure 11 illustrates the sectional view of the figure 9 after the machining step during which the parts 9 have been eliminated by the machining releasing the spiral springs 1 comprising the modified portion 3.

The figure 9 also shows parts 7a, 7b, 7c of the wafer 7 for which the hairsprings 1 have undergone a different irradiation treatment depending on the part 7a, 7b, 7c of the wafer in which the hairsprings 1 are located. Consequently, the spiral springs 1 in one of the different parts 7a, 7b, 7c may have a modified portion 3 whose physical properties differ from those of the modified portion 3 of the spiral springs 1 in the other parts 7a, 7b, 7c of the wafer 7.

The step of locally modifying the vitreous material can therefore be carried out locally not only on the scale of the body used in deformation (spring-spiral) 1 but also on the scale of the wafer 7.

The method also has the advantage of being applicable to a vast category of glasses, in particular those said to be “non-photostructurable”. Consequently, the process of transformation of the vitreous material takes place directly without the need for additional steps such as the heat treatments required in the case of so-called “photostructurable” glasses. In addition, the use of multiphoton absorption mechanisms makes it possible to use a wide range of wavelengths ranging from infrared to ultraviolet. Also, such a method makes it possible to locally modify the thermoelastic properties of the vitreous material without any restriction as to the location of the modified portion in the volume of the vitreous material.

The material is locally modified (using a laser, preferably of the femtosecond type) so as to produce a modified portion 3 having a second Young's modulus E ₂ which differs from a first Young's modulus E ₁ of the unmodified portion of the material . In this way, it is possible to adjust a value of the effective Young's modulus E _eff of the hairspring 1, defined by the combination of the first and second Young's modulus E ₁ , E ₂ .

The stiffness of the hairspring 1 depends on its Young's modulus (its rigidity) but also on the ratio of its section to its length. It can however be assumed that the ratio of the section to the length of the hairspring will not be changed by the local modification. In other words, the material has a first stiffness K ₁ and the modified portion has a second stiffness K ₂ which may be different from the first stiffness K ₁ . The adjustment of the effective Young's modulus E _eff of the balance-spring 1 therefore makes it possible to adjust a value of the effective stiffness K _eff of the balance-spring 1, without needing to modify either the height or the thickness of the balance-spring 1. The effective stiffness K _eff of the hairspring 1 is defined by the combination of the first and the second stiffness K ₁ , K ₂ .

The local modification of the stiffness of the hairspring 1 can be used to adjust the frequency of the hairspring 1, for example before it is assembled with a balance wheel. This facilitates, for example, balance-spring pairing operations, or even pairing of the regulating member as a whole with respect to the movement, at the same time as allowing a significant increase in the efficiency of this operation.

The local modification of the Young's modulus and of the stiffness can be applied to the outer terminal curve 5 and/or to the curve inner terminal 4 of spiral spring 1, as shown in figure 7 . The modified portion 3 may comprise a portion 3' extending in the longitudinal direction of the hairspring 1 continuously and/or discontinuously.

The modification of the effective Young's modulus E _eff and/or of the effective stiffness K _eff in a specific portion of the hairspring 1, such as for example in the terminal curve of the hairspring, makes it possible to optimize the concentricity of its deployment with respect to the axis of the balance. It is also possible to compensate for subsidence effects due to gravity or other isochronism defects.

Still according to one embodiment, the material is locally modified so as to adjust the internal stresses in the modified portion 3.

In the case where the local modification is carried out using laser treatment, preferably using a laser using ultrashort pulse durations, that is to say pulse durations between a few femtoseconds to a few nanoseconds, and preferably between a few femtoseconds to a few picoseconds.

The use of such lasers allows the precise control of very localized modifications of the material by the nonlinear methods of absorption of the energy of the laser. The modifications thus induced in the modified portion of the material can be of a continuous nature, formation of (nano) self-organized structures, and/or the formation of nanovoids. These modifications can induce different types of anisotropy in the modified portion of the material, which can have important effects on the chemical, optical, thermal and/or mechanical properties. In particular, the change in the volume of the material resulting from the exposure induces stresses around the zones exposed by laser, and thus modifications of the internal stresses, which can be compressive or tensile stresses, in the modified portion. The induced stress depends mainly on two parameters: the polarization and the energy per pulsation.

The local modification can be carried out in such a way as not to influence the thermocompensation of the modified portion of the material, or at least not in a linear manner. Indeed, the effective Young's modulus and the effective CTE can be controlled by the total quantity of modified zones while the residual stresses can be controlled by the location of the affected zones with respect to the component produced.

Reference numbers used in the figures

1

spiral spring

2

glassy material matrix

3

modified portion

4

interior terminal curve

5

outer terminal curve

6

hairspring section

7

wafer

7a

portion of the wafer

8

laser radiation

9

machined parts of the wafer

α1

first coefficient of thermal expansion

α2

second coefficient of thermal expansion

αeff

effective coefficient of thermal expansion

β1

first thermoelastic coefficient

β2

second thermoelastic coefficient

βeff

effective thermoelastic coefficient

ETC

thermoelastic coefficient

E1

Young's first modulus

E2

second Young's modulus

Eeff

effective Young's modulus

K1

first stiffness

K2

second stiffness

Keff

effective stiffness

Claims (12)

1. Method of manufacturing a coil spring of a balance-spring assembly of a timepiece, the coil spring including a body used in deformation (1), the body (1) being made of a glassy material having a first thermoelastic coefficient CTE (β1) and a first stiffness (K₁), the method comprising:

   • machining a piece of said glassy material to form the body used in deformation (1) of the coil spring; and

   • locally modifying said glassy material so as to form a modified portion (3) having a second CTE (β2) different from said first CTE (β1) so that the coil spring is thermocompensated;

   characterized in that said local modification is performed by laser irradiation using ultrashort pulses, and in that the modified portion (3) is distributed in the matrix of the glassy material and is modified so as to have a second stiffness (K2) which differs from the first stiffness (K₁), and in that the modification of the second stiffness (K2) of the modified portion (3) allows the frequency of the coil spring (1) to be adjusted so as to match the coil spring (1) with the balance wheel.
2. Method according to claim 1, wherein said local modification is performed in a direct manner, without the need for additional steps.
3. Method according to claim 1 or 2, wherein the laser is a nanoscale or microscale focused laser.
4. Method according to one of claims 1 to 3, wherein the laser uses ultrashort pulses comprised between a few femtoseconds and a few nanoseconds or between a few femtoseconds and a few picoseconds.
5. Method according to one of claims 1 to 4, wherein the machining can be performed by a chemical etching process, by a physical etching process, or by a combination of both processes.
6. Method according to one of claims 1 to 5, wherein the frequency of the coil spring (1) is adjusted prior to its assembly with a balance wheel.
7. Method according to one of claims 1 to 6, wherein the frequency of the coil spring (1) is adjusted so as to match the regulating organ in the movement.
8. Method according to one of claims 1 to 7, wherein the modified portion (3) comprises an amorphous silica which has been modified by creating a crystalline polymorph.
9. Method of claim 8, wherein the crystalline polymorph comprises one of: alpha quartz, beta quartz, stishovite, tridymite, chalcedony or cristobalite.
10. Method of one of claims 1 to 9, wherein said modified portion (3) has internal stresses that differ from those of the unmodified portion of said material.
11. Method according to one of claims 1 to 10, wherein the modified portion (3) comprises a portion (3') extending in the longitudinal direction of the coil spring (1) continuously and/or discontinuously.
12. Method according to one of claims 1 to 11, wherein said modified portion (3) is located in the outer end curve (5) and/or in the inner end curve (4) of the coil spring (1).

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