Classifications

• • G—PHYSICS
  • G04—HOROLOGY
  • G04B—MECHANICALLY-DRIVEN CLOCKS OR WATCHES; MECHANICAL PARTS OF CLOCKS OR WATCHES IN GENERAL; TIME PIECES USING THE POSITION OF THE SUN, MOON OR STARS
  • G04B17/00—Mechanisms for stabilising frequency
  • G04B17/20—Compensation of mechanisms for stabilising frequency
  • G04B17/22—Compensation of mechanisms for stabilising frequency for the effect of variations of temperature
• • G—PHYSICS
  • G04—HOROLOGY
  • G04B—MECHANICALLY-DRIVEN CLOCKS OR WATCHES; MECHANICAL PARTS OF CLOCKS OR WATCHES IN GENERAL; TIME PIECES USING THE POSITION OF THE SUN, MOON OR STARS
  • G04B17/00—Mechanisms for stabilising frequency
  • G04B17/04—Oscillators acting by spring tension
  • G04B17/06—Oscillators with hairsprings, e.g. balance
  • G04B17/066—Manufacture of the spiral spring

Definitions

• a ceramic thermocompensated spiral spring comprising the silicon element in its composition and method of adjustment
• the present invention relates to a thermocompensated spiral spring intended to equip a resonator balance-spiral movement
• the invention also relates to a sprung balance resonator comprising the spiral spring and a balance and a method of adjusting the spiral spring.
• the regulating organ of mechanical watches is
• the movements of mechanical watches is an elastic metal blade of rectangular section wound on itself in the form of an Archimedean spiral and comprising from 12 to 15 revolutions.
• the sprung balance oscillates around its equilibrium position (or dead point). When the pendulum leaves this position, it arms the hairspring. This creates a return torque which, when the balance is released, returns it to its equilibrium position. As he has acquired a certain speed, therefore a kinetic energy, he exceeds his dead point until the opposite pair of the hairspring stops him and forces him to turn in the other direction. Thus, the spiral regulates the period of oscillation of the balance.
• the accuracy of the mechanical watches therefore depends on the stability of the fundamental natural frequency f 0 of the resonator formed of the balance-spiral.
• the fundamental natural frequency f 0 is related to the variations Af of frequency via the step M of the oscillating assembly according to equation 1: in which 86400 represents the number of oscillations performed in 24 hours at a frequency of 1 Hz.
• k s the stiffness of the spiral spring
• J B the moment of inertia of the balance relative to its axis of rotation.
• the moment of inertia of the pendulum can be expressed as:
• J B mr B 2 (3)
• m the mass of the pendulum and r B is the radius of the pendulum.
• Af / fo 1/2 (Ak s / k s - Ai B / h)
• Af / f 0 1/2 ( s + 3a s - 2a B ) (5)
• Ak s / k s is the variation of the stiffness of the spiral spring with respect to its nominal stiffness and Ai B I is the variation of the inertia of the balance with respect to its nominal inertia, which makes it possible to introduce for the disturbances thermals
• Ps the linear thermoelastic coefficient of the spiral spring, as the coefficient of linear expansion of the spiral spring
• a B the linear expansion coefficient of the balance.
• stiffness k s of a spiral spring must be as constant as possible, regardless of, in particular, the
• thermoelastic coefficient Ps of the spiral is obtained by adjusting the thermoelastic coefficient Ps of the spiral as a function of the coefficients of thermal expansion of the spiral a and the pendulum a B , according to the relation 5.
• EP1422436 discloses a spiral spring cut into a plate ⁇ 001 ⁇ of monocrystalline silicon.
• the hairspring comprises a layer of SiO 2 , having a thermoelastic coefficient opposite to that of silicon and formed around the outer surface of the hairspring, in order to minimize the thermal drift of the balance-hairspring assembly.
• the silicon dioxide layer also makes it possible to improve the mechanical properties of the silicon substrate.
• thermoelastic coefficient of silicon is strongly influenced by the temperature and compensation of this effect is necessary for its use in horological applications. Indeed, the thermoelastic coefficient of silicon is of the order of -60 ⁇ 10 -6 / ° C and the thermal drift of a spiral spring made of silicon is thus about 2 minutes / day, for a temperature variation of 23 ° C +/- 1 5 ° C. This makes it incompatible with watchmaking requirements which are of the order of 0.6 seconds / day / ° C in the temperature range between 8 ° C and 38 ° C.
• EP2590325 discloses a spiral spring whose ceramic body borosilicate glasses type or silicon carbide is coated with a layer of Si0 2 , so that the resonator thus formed has a frequency variation of almost zero in temperature function.
• the SiO 2 coating ensures an almost independent temperature on the Young's modulus of the material of the resonator body.
• the invention relates to the selection of ceramic materials comprising the silicon element in their formulation for horological applications.
• the invention relates to a spiral spring intended to equip a balance spring-balance resonator or other precision instrument, the spiral spring comprising a core made of a ceramic material containing the silicon element in its formulation and comprising a section, the core having a first stiffness and a first thermoelastic coefficient; and a silicon dioxide coating of thickness and at least partially covering the core, the
• thermoelastic of opposite sign to the first thermoelastic coefficient; the section of the core and the thickness of the coating being adjustable
• thermoelastic coefficient of the spiral spring as a function of the first thermoelastic coefficient and the second thermoelastic coefficient
• stiffness of the spiral spring as a function of the first stiffness and the second stiffness
• the invention also relates to a balance-balance oscillator comprising the spiral spring having a coefficient of linear expansion of the spiral spring, and a balance having a coefficient of linear expansion of the balance; the section of the core and the thickness of the coating being adjusted so that the combination of the second thermoelastic coefficient and the first thermoelastic coefficient results in a thermoelastic coefficient of the spiral spring compensating a value corresponding to the difference between three times the coefficient linear expansion of the spiral spring and twice the linear expansion coefficient of the balance; the section of the core and the thickness of the coating being also adjusted so that the combination of the first stiffness and the second stiffness gives a stiffness of the spiral spring making it possible to obtain the fundamental fundamental frequency of the balance-wheel resonator; spiral.
• a method of adjusting the spiral spring comprising:
• the silicon dioxide coating having a predetermined thickness on at least a portion of the core so as to obtain a predetermined value of the second thermoelastic coefficient
• the section of the core is adjusted before the step of forming a silicon dioxide coating.
• the spiral spring and the sprung balance resonator of the invention has an invariance of the expansion and elasticity properties in a defined range of temperatures, according to the COSC (Swiss Official Chronometer Testing), between 8 ° C and 38 ° C. Such a resonator is also insensitive to external magnetic fields.
• the adjusting method allows to adjust the section of the core and the thickness of the coating independently so as to obtain a desired value of the thermoelastic coefficient of the spiral spring and a desired value of the stiffness of the spiral spring.
• the ceramic material containing the silicon element in its formulation is advantageous in horological applications because of its mechanical properties of use and, in particular, its toughness much higher than that of silicon. All of the expected properties are supported by the prior performance of aging tests under controlled temperature and atmosphere.
• Figure 1 shows a top view of a spiral spring according to the invention
• Figure 2 shows a right cross sectional view (Figure 2a) and longitudinal ( Figure 2b) of the spiral spring comprising a core and a coating, according to one embodiment
• FIG. 3 shows an example of a thermocompensated spiral spring comprising a ferrule and a stud, according to the invention
• FIG. 4 shows micrographs of a section of the spiral spring comprising a coating of silicon dioxide, according to a first (FIG. 4a) and second (FIG. 4b) embodiment;
• Figs. 5 and 6 show micrographs of a sectional view of the spiral spring comprising a silicon dioxide coating in a magnified view of x 5000 (Fig. 5) at an enlargement of x 18,000 (Fig. 6);
• Figure 7 is a micrograph of a section of the spiral spring showing good adhesion between the core and the silicon dioxide coating.
• Figure 1 shows a top view of a spiral spring 1 and Figures 2a and 2b show a longitudinal and transverse sectional view of the spiral spring 1 according to the invention.
• the spiral spring 1 comprises a core 2 formed in a ceramic material containing the silicon element in its composition (hereinafter ceramic material) and a coating of silicon dioxide 4 at least partially covering the outer surface 3 of the core 2.
• ceramic material containing the silicon element in its composition
• the term "core” is used to describe a central part, or the body, of the spiral spring.
• the coating 4 corresponds to a layer deposited superficially on the core, or body.
• the core has a helical shape and comprises at least one coil of rectangular section of thickness w and height h.
• the spiral spring 1 can be seen as being formed of a composite structure of "sandwich" type consisting of a central part, the core 2, and the coating 4 (see Figure 2b).
• the core 2 made of ceramic material has a first thermoelastic coefficient ⁇ ⁇ and a first stiffness k A.
• the SiO 2 coating has a second thermoelastic coefficient R of opposite sign to the first thermoelastic coefficient ⁇ ⁇ , and a second stiffness k R.
• the most common ceramic materials include aluminas (AI 2 OB), aluminum nitrides (AIN), beryllium oxide (BeO), quartz, silicon nitride ( Si 3 N 4 ), silicon oxynitride and aluminum (SiAlON).
• the ceramic material comprises a silicon nitride, a silicon carbide, or a silicon oxynitride. More particularly, the ceramic material may comprise one or a combination of the compounds: silicon nitride (Si 3 N 4 ), SiC or silicon and aluminum oxynitride (SiAlON), which comprise the silicon element in their composition. composition.
• the ceramic material comprises at least one of the following composite structures: Si 3 N 4 -SiC, Si 3 N 4 -TiCN, Si 3 N 4 - SiAlON, Si 3 N 4 -AlN, Si 3 N 4 -Al 2 O 3, Si 3 N 4 -ZrO 2, SiC-SiAlON, Si-SiC, SiC-Si 3 N 4 - Si 2 N 2 O or SiAlON-TiN, or a composite comprising at least one of these compounds.
• the ceramic material may also comprise a fiber-type composite such as SiC fibers dispersed in a ceramic matrix (SiC for example) of SiC (SiC-SiC composite), or a composite of acicular structure (example ⁇ Si 3 N 4 ) in a matrix of equiaxed structure (for example a Si 3 N 4 ) (composite Si 3 N 4 - Si 3 N 4 ).
• the ceramic material comprises at least one of the following composite structures: Si 3 N 4 -SiAlON or a-Si 3 N 4 - -Si 3 N 4 .
• Table I shows density values, open porosity, Young's modulus, maximum bending stress, Weibull modulus, toughness and thermal conductivity for Si3N4, SiC and SiAlON.
• Si 3 N 4 supplied by HC STARCK CERAMICS under the reference SSN Star Ceram TM N700, or by UMICORE under the reference FRIALIT HP79; the SiC provided by ESK CERAMICS for SiC, under the reference EKASIC TM F SiC 100; and SiAlON by society
• the ceramic material has good properties both at room temperature and at high temperature. Such ceramic materials are conventionally used as materials constituting engines, bearings, gas turbine elements, in particular in because of their good thermal resistance, their low thermal expansion, their good mechanical properties and their good resistance to corrosion. We can also mention their use in the semiconductor industry, for example for silicon nitride masks.
• the ceramic material is advantageous for horological applications since it has a low density and a linear expansion coefficient of the same order of magnitude as that of silicon. It also has a Young's modulus which is twice or even triple that of silicon, a flexural strength, a toughness much greater than that of silicon, and insensitivity to magnetic fields.
• the monolithic ceramic material is also advantageous because of its refractory properties and good resistance to dry and wet corrosion.
• the development of the ceramic material can be carried out using a sintering process or any other suitable method.
• the core 2 in the ceramic material can be machined from any block in the ceramic material so as to obtain a thickness (for example 1 50 ⁇ ) corresponding substantially to the desired height of the spiral spring 1.
• Preliminary machining of plates from industrial blocks can be performed by cutting, grinding, lapping and then mechanical or chemical polishing.
• the machining itself can be performed using a wet or dry etching process.
• the machining can be performed using a reactive ion etching process such as the DRIE process (Deep
• the DRIE process promotes deep engraving and good precision on engraved forms. It also promotes the formation of vertical walls on the soul 2 thus etched.
• a pulsed laser beam with a diameter of 10 microns and 30 microns can be used.
• the pulse trains can be composed of 2 to 10 laser pulses separated by 10 to 50 ns.
• the thickness t R of the coating 4 can be adjusted to obtain a desired value of the thermoelastic coefficient of the spiral spring s .
• the thermoelastic coefficient of the spiral spring s depends on the combination of the first thermoelastic coefficient ⁇ ⁇ and the second thermoelastic coefficient p R and can therefore be modified by modifying the thickness t R of the coating 4.
• the section of the core 2 can be adjusted to obtain a desired value of the stiffness of the spiral spring k s .
• the stiffness of the spiral spring k s is determined by a combination of the stiffness k of the core A and the stiffness of the coating R k.
• the thickness t R of the coating 4 can be adjusted independently so as to independently modify the value of the stiffness of the spiral spring k s and the value of the thermoelastic coefficient of the spiral spring s .
• the thickness of the coating 4 will be between 0.1 ⁇ and 10 ⁇ , and preferably between 1 ⁇ and 6 ⁇ , or even more preferably between 2 ⁇ and 5 ⁇ .
• the invention also concerns the adjustment of the spiral spring 1 so as to adjust the stiffness k s of the spiral spring 1 and the minimization of the variations of the expansion and elasticity properties of the spiral spring 1 so as to minimize the thermal variations. spiral spring 1.
• a method of adjusting the spring 1 comprises: adjusting the section of the core 2 so as to obtain a predetermined value of the first stiffness k A ;
• the silicon dioxide coating 4 having a predetermined thickness t R on at least a portion of the core 2 so as to obtain a predetermined value of the second thermoelastic coefficient
• the section of the core 2 is adjusted before the step of forming a silicon dioxide coating.
• the adjustment of the section of the core 2 can be achieved by removal of material at the periphery of the core.
• the removal of the material on the core 2 formed of the ceramic material is achieved by means of an isotropic etching of the core 2.
• the removal of the material can be achieved by an attack in a hot solution of phosphoric acid with or without nitric acid and water, to adjust the thickness of the silicon nitride core.
• the predetermined value of the first stiffness k A corresponds to the value that must have the first stiffness k A of the core 2 (without the
• the stiffness k s of the spiral spring 1 corresponds to a combination of the first stiffness k A and the second stiffness k R.
• the predetermined value of the first stiffness k A can be calculated as a function of the second stiffness k R , which depends on the thickness t R of the coating 4 and the desired value of the stiffness k s of the spiral spring 1.
• the calculation of the predetermined value of the first stiffness k A can be performed using numerical simulations using finite elements as a function of the stiffness of the spiral spring k s desired.
• the method comprises a step of measuring a first measured stiffness k Am , and a step of comparing the first measured stiffness k Am with respect to a value
• the quantity of material to be removed for adjusting the section of the core 2 so as to obtain the predetermined value of the first stiffness k A can then be determined from the difference between the first measured stiffness k Am and the predetermined value of the first stiffness k A.
• the relationship between the first stiffness k A and the amount of material to be removed is given by equation 4 wherein the stiffness of the spiral spring is replaced by s k I k A first stiffness of the core 2 and where E is the modulus of Young of the soul, w, h and L respectively the thickness, the height and the length of the soul 2.
• the measurement of the first stiffness k A can be performed alternately with the step of adjusting the section of the core 2.
• the measurement of the first stiffness k Am can be performed simultaneously with the step of adjusting the section of the core 2.
• the adjustment of the section of the core 2 comprises a material removal corresponding to a thickness of about 0.1 ⁇ at 3 ⁇ at the periphery of the core 2.
• the formation of the silicon dioxide coating 4 is carried out at least on a portion of the core 2.
• the coating 4 may cover all the faces 3 of the core 2, or only some faces 3 of the 2.
• the coating 4 may cover only the three free faces of the core 2 but not the integral side to the substrate.
• the thickness t R of the coating 4 is determined so as to obtain a predetermined value of the second thermoelastic coefficient R of the coating 4.
• Oxidizing agents may include oxygen and / or water vapor (wet thermo-oxidation). Oxidizing agents may also comprise, for example, example and without being exhaustive, ozone, oxygen-nitrogen mixtures, or oxygen-helium.
• the growth of the silicon oxide layer can also be carried out by low temperature plasma oxidation (between 300 ° C. to 600 ° C. and preferably between 400 ° C. and 500 ° C.) using a oxygen plasma.
• the core 2 can be placed in the anode position so as to avoid spray effects in the oxide layer.
• the core 2 can be brought into contact with an oxygen plasma generated by a radio-frequency source, or by a microwave source, both positioned a few centimeters from the core 2.
• the surface of soul 2 is
• a cathode is located several tens of centimeters from the core to be oxidized.
• the silicon oxide layer can also be produced by plasma-enhanced chemical vapor deposition (PECVD) with a thickness varying between 0.2 and 10 microns and preferably between 2 and 5 microns. .
• PECVD plasma-enhanced chemical vapor deposition
• partially covalent silicon-based ceramics comprising a silicon nitride, carbide or oxynitride promote the continuity of the structures at the interface between the ceramic material and the SiO 2 layer.
• composition and the structure of the coating 4 of silicon dioxide depend on the method of elaboration of the monolithic ceramic material.
• the ceramic material comprises silicon nitride produced by hot-phase isostatic pressure-sintering (HIP-SN) or by chemical vapor deposition (CVD) technology, both processes being carried out without addition of material , the coating 4 essentially contains amorphous SiO 2 without disturbing the texture of the ceramic material.
• the coating 4 comprises compounds dispersed in the silica coating and the Si 2 N 2 O compound at the interface between the ceramic and the silica coating (for example the compound Y 2 Si 2 O 7 in the case of addition of Y 2 0 3 ).
• the oxidation reaction forming the coating 4 can be expressed by equations 6 and 7:
• ceramic is further modified by different mechanisms of cationic diffusion of the elements of the additions.
• the presence of the silicon element in the ceramic material substrate constituting the core 2 of the spiral spring 1 allows good adhesion of the coating 4 to the ceramic substrate. This good adhesion is due to a continuity at the atomic scale between the substrate and the coating 4 in a zone of accommodation (also known as the "terrace region") of some atomic distances from the surface of the substrate.
• the ceramic material containing the silicon element in its composition preferably has a resistivity which is typically very high (> 10 12 ⁇ . ⁇ ) and therefore can be considered as a dielectric material.
• the silica coating 4 is made on such a dielectric ceramic material, the silicic layer grows from the extreme surface of the substrate, ensuring good adhesion of the coating to the substrate, and eliminating the diffusion of oxygen to the substrate. inside of the core 2, as found during the oxidation operations of the silica.
• FIGS. 4a and 4b relate to micrographs obtained by scanning electron microscopy, showing a sectional view of the spiral spring 1 comprising the core 2 in the ceramic material and the silicon dioxide coating 4 formed by the thermal oxidation process under air at 1200 ° C. for two hours (FIG. 4a), and by the plasma oxidation method at low temperature using an oxygen plasma (Figure 4b), under conditions favoring passive oxidation of the ceramic material.
• a coating protecting the coating 4 during the metallographic cutting operation is also visible in Figures 4a and 4b.
• FIGS. 5 and 6 show micrographs, obtained by scanning electron microscopy, of a section of the spiral spring comprising the silicon dioxide coating 4 in an enlarged view of ⁇ 5000 (FIG. 5), at a magnification of magnification x 18000 ( Figure 6).
• the coating 4 is formed by a low temperature plasma oxidation process.
• FIG. 7 shows another micrograph, also obtained by scanning electron microscopy, of a section of the spiral spring in which good adhesion between the core and the silicon dioxide coating can be seen, even in areas with tearing off. granular (such a zone is represented in FIG. 7 by number 8).
• the thickness of the silicon oxide layer can be estimated using parameters such as the oxidation time; the degree of hygrometry and the temperature.
• the method comprises a step of reducing the thickness of the coating 4. This step in which a fraction of the thickness of the coating 4 is removed by etching, allows to adjust more finely the stiffness k s of the spiral spring 1. This step which is performed after the step of forming the silicon dioxide coating 4 on the core 2 also makes it possible to make a fine adjustment of the predetermined value of the second thermoelastic coefficient R.
• obtaining the predetermined value of the first stiffness k A can be achieved in a single step.
• the adjustment of the core section by the removal of the material is typically accomplished with a first step of growing an oxide layer on the core and a second step of attack of the oxide layer.
• the growth of the oxide layer is largely at the expense of the silicon substrate, typically in a portion corresponding to about 44% of the total thickness of the layer.
• This two-step adjustment process is necessary to control with sufficient accuracy the removal of silicon.
• the removal of the material of the core 2 of the invention in ceramic material can be achieved by etching in an isotropic and controlled manner. Consequently, the section of the core 2 can be adjusted before the step of forming a silicon dioxide coating 4.
• the invention also relates to a spring balance resonator (not shown) for a watch movement or other precision instrument comprising the spiral spring 1 cooperating with a balance.
• the value of the stiffness k s of the spiral spring 1 is determined so as to obtain a setpoint in its tolerance for the fundamental natural frequency f 0 of the balance-spring resonator (see Equation 2).
• the value of the stiffness k s of the spiral spring 1 is determined by the section of the core 2 and the thickness t R of the coating 4.
• the fundamental natural frequency f 0 of the balance-spring resonator is typically between 2 Hz and 20 Hz, or between 2 Hz and 5 Hz.
• the predetermined value of the thermoelastic coefficient of the spiral spring p s can also be adjusted so as to compensate for the term (3a s - 2a B ) of equation 5.
• the core 2 of ceramic material containing the silicon element typically has a first negative thermoelastic coefficient ⁇ ⁇ which must be partially compensated by the silica coating 4 having a second positive thermoelastic coefficient R of approximately 140 ⁇ 10 -6 / ° C.
• the combination of the first thermoelastic coefficient ⁇ ⁇ and the second thermoelastic coefficient R should result in a predetermined value of the thermoelastic coefficient s of the spiral spring 1 around + 18 ⁇ 10 -6 / ° C.
• the predetermined value of the thermoelastic coefficient s of the spiral spring 1 can be obtained by adjusting the section of the core 2 and the thickness t R of the coating 4.
• the compensation of the term (3a s - 2a B ) of equation 5 by the thermoelastic coefficient of the spiral spring s makes it possible to minimize the thermal drift of the balance-spring resonator, and therefore the variations of the instantaneous diurnal step of a watch comprising such a resonator.
• the balance-spring resonator may exhibit an invariance of the expansion and elasticity properties of the spiral spring 1 in a defined range of temperatures, according to the COSC, between 8 ° C and 38 ° C. Such a resonator is also insensitive to external magnetic fields.
• Figure 3 shows an example of a spiral spring 1
• thermocompensated ceramic made according to the method of the invention with a shell 5 and a piton 6 (the ferrule and the piton are made
• the present invention is also applicable to other types of resonators capable of regulating a mechanical watch movement, such as in particular a tuning fork-shaped resonator. Notations used in the text and figures
• thermoelastic coefficient of the thermocompensated spiral spring A variation of the frequency of a balance-spring resonator

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• 2015
  • 2015-01-27 CN CN201580006293.7A patent/CN106104393A/zh active Pending
  • 2015-01-27 WO PCT/EP2015/051618 patent/WO2015113973A1/fr active Application Filing
  • 2015-01-27 EP EP15701046.3A patent/EP3100120A1/de not_active Withdrawn
  • 2015-01-27 EP EP21194853.4A patent/EP3958066A1/de active Pending

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