EP3548972B1 - Method for optimising the tribological properties of a timepiece component - Google Patents

Description

Technical area

The present invention relates to the field of watchmaking. It relates, more particularly, to a watch component intended to undergo dynamic friction and having improved tribological properties, as well as a method for optimizing the tribological properties of such a watch component.

State of the art

In recent developments in watchmaking, new materials and microfabrication techniques have opened up new avenues of development. In particular, the use of silicon to make watch components intended to undergo dynamic friction has made it possible to produce parts with complex shapes, integrating several functions in a single part.

This material also has the advantage of having interesting tribological properties, which suggested that it would be possible to have two silicon parts rub against each other, without having to resort to lubrication. Indeed, the interaction between moving parts, which is necessary for the operation of a timepiece mechanism, induces friction which is detrimental to the efficiency of the mechanism and which wears out the parts. It is therefore necessary to lubricate the parts of the components which undergo this friction. However, the oils traditionally used age and degrade, posing problems of maintenance of the movement, in particular.

To date, the use of silicon without oiling has made it possible to operate watch movements or certain parts of mechanisms, but without fulfilling all the specifications allowing operation over a long period, compatible with daily use.

Research carried out recently has attempted to optimize the lubricants used, or to propose structures making it possible to improve the diffusion of oil at the level of the friction zones of silicon parts. Other research has also aimed to develop layers of materials, intended to coat the friction zones, in order to propose a so-called dry lubrication, that is to say not using a liquid lubricant, but allowing to improve the tribological properties of a component, in the areas thus coated.

By improved tribological properties is meant a reduction in the coefficient of friction, a reduction in wear with respect to a given stress or to given conditions (surface condition, hygrometry, temperature, topology, etc.).

The purpose of the present invention is to propose an alternative to the methods proposed above, making it possible to obtain a watch component intended to undergo dynamic friction, which has improved tribological properties, in particular by the implementation of the ion implantation of certain elements. under specific conditions.

Indeed, ion implantation on a particular type of watchmaking component made of silicon has already been proposed (see for example the documents CH708067 , CH709628 and CH699780 ), in order to modify the thermoelastic properties of a spiral spring for a regulating member, and not to improve its tribological properties. However, these documents state only little information as to the implementation parameters as well as certain resulting characteristics of the part. Indeed, in the case of this very particular application, it is a question of varying the volumetric ratios between the implanted zones and those not implanted, and this is the concentration, expressed in ions/cm ³ (or in at/cm ³ ) which takes precedence. In particular, it seems that the parameters set out in the document CH709628 did not promote, or even detracted in some cases, the tribological properties of the spiral spring. As part of a spiral spring, such adverse effects on the tribological properties are not of the slightest importance, since such components do not experience any dynamic friction during their operation, and therefore do not suggest a solution to the aforementioned problem. Furthermore, ion implantation on other types of watch components has also been carried out. For example, the document WO 2009/043391 describes a barrel spring for a mechanical watch, said barrel spring being made of a base material and comprising under at least part of its surface ions implanted in an associated implantation zone being made harder and more rigid than said material base, in order to increase the bending rigidity and thus the energy storage capacity of said barrel spring.

Disclosure of Invention

More specifically, the invention relates to a method for optimizing the tribological properties of a watch component intended to undergo dynamic friction during its normal operation, produced on the basis of a substrate comprising a layer of silicon whose crystallographic structure comprises at least one of the monocrystalline, polycrystalline or amorphous forms, said substrate being reinforced by a protective layer produced on its surface during an operation for protecting the substrate, said method comprising a step of ion implantation of at least one ion, by ion bombardment on the surface of said component.

Brief description of the drawings

• la figure 1 illustre les frottements mesurés avec un composant horloger implanté ioniquement à différentes doses, en fonction de la distance (en m) parcourue au contact du composant par une sphère en rubis,
• la figure 2 illustre l'évolution du coefficient de frottement moyen mesuré dans les conditions de la figure 1 pour un composant horloger selon l'invention, en fonction de la dose d'implantation,
• les figures 3, 4, 5 et 6 représentent différentes possibilités d'implantation dans un substrat, et
• les figures 7, 8 et 9 proposent différentes configurations de substrat pouvant convenir à la mise en œuvre de l'invention.

Other details of the invention will appear more clearly on reading the following description, made with reference to the appended drawings in which:

• the figure 1 illustrates the friction measured with an ion implanted watch component at different doses, depending on the distance (in m) traveled in contact with the component by a ruby sphere,
• the figure 2 illustrates the evolution of the average coefficient of friction measured under the conditions of the figure 1 for a watch component according to the invention, as a function of the implantation dose,
• the figures 3, 4, 5 and 6 represent different possibilities of implantation in a substrate, and
• the figures 7, 8 and 9 propose different substrate configurations that may be suitable for the implementation of the invention.

Embodiment of the invention

Recent research carried out by the applicant on the improvement of the tribological properties of watchmaking components intended to undergo dynamic friction, particularly components made from a substrate comprising a layer of silicon, has made it possible to show that ion implantation makes it possible to obtain promising results. The components concerned are intended to dynamically interact with another component in order to transmit a kinematic function to each other. This is done by dynamic contact between the elements, that is to say by a contact which varies during at least part of the time of the interaction and which does not always remain static. In other words, this transmission involves the generation of a relative displacement between the two components at their points of contact, this contact generating reaction forces and thus dynamic friction at these places. A toothed wheel meshing with another therefore undergoes dynamic friction at the level of the contact surfaces of its teeth, which is also the case between certain components of the escapement, in particular an anchor, an escape wheel or the part of the escapement. a frictional resonator, such as a platter peg, which experiences dynamic friction where it interacts with neighboring components. On the other hand, a spiral spring being integral on the one hand with its peak and on the other hand with the balance shaft, it does not undergo any dynamic friction in the sense of the invention and is therefore excluded from its scope since no relative displacement is only present at the points of contact between the spring and the components with which it interacts.

In the present application, the expression "silicon layer" encompasses substrates for which, within the layer, the silicon does not form atomic bond with other atoms. The different crystallographic natures of silicon (monocrystalline, polycrystalline and amorphous, alone or in combination) are included in the scope of the invention and dopings or possible impurities present in the substrate are also possible and included in the scope of the claims. A single layer of silicon, forming a silicon core, is also included in the scope of protection.

Such a result was unexpected, because the fragility of silicon-based substrates makes it necessary to use a protective layer on the substrates, in order to be able to handle and use them. Thus, whether the substrates are made of monocrystalline silicon, polycrystalline silicon or amorphous silicon, in the field of watchmaking, these substrates always undergo a protection operation during which a protective layer is produced at the surface of the substrate, by modifying the substrate. These protective layers can be an oxide layer obtained by oxidation of the substrate or a nitride layer obtained by nitriding of the substrate. It is also possible to envisage that the protective layer be obtained by deposition.

As mentioned, this protective layer is obtained by an effective protective operation and a natural oxidation layer is not a protective layer within the meaning of the present application, because it does not result from a protective operation as as such, carried out in an effective and positive manner.

By way of example, a protective layer made of SiO ₂ obtained by an oxidation step measures at least 50 nm in thickness, which cannot be obtained with natural oxidation.

Thus, the presence of a protective layer, with the consequence that the silicon is not directly outside the component, in contact with the other horological components with which it can interact, makes it difficult to anticipate the effects of an ion implantation in the substrate, more generally on the surface of the component.

The figures 7, 8 and 9 thus represent different substrate and protective layer configurations that can be implemented within the scope of the invention. To the figure 7 , the substrate takes the form of a single layer of silicon 10, covered with a protective layer 12. At the figure 8 , the substrate takes the form of two layers of silicon 10, separated and covered with a protective layer 12. There could thus be an alternation of several layers of silicon. Finally, at the figure 9 , the substrate comprises a core 14 covered with a layer of silicon 10 whose thickness is such that its tribological properties are equivalent to those of a silicon substrate. The protection layer 12 covers the silicon layer. Preferably, the thickness of the silicon layer is greater than the depth of ion implantation, so that the ions do not penetrate into the core.

Thus, the invention relates to a method for optimizing the tribological properties of a watch component intended to undergo dynamic friction, produced on the basis of a substrate comprising a layer of silicon whose crystallographic structure comprises at least one of the forms monocrystalline, polycrystalline or amorphous, said substrate being reinforced by a protective layer produced on its surface during a substrate protection operation, said method comprising a step of ion implantation of at least one ion, by ion bombardment the surface of said component, in which the implantation step is carried out with C ions, at a dose greater than 5.10 ¹⁷ ions/cm ² and/or with N ions, at a dose greater than 1.10 ¹⁶ ions/cm ² .

Advantageously, the ion-implanted zone 16 is shared between the substrate, illustrated here in the form of a single layer of silicon 10, and the protective layer 12, the substrate having an implantation (in number of ions) greater than the protective layer. The figure 4 has such a configuration. It is observed, more specifically, that the ions present in the protective layer are few, or even very few in number and that they are essentially concentrated in a precise zone of the substrate. In the case of an ion implantation carried out by ion bombardment (for example, ion beam, MEWA - Metal Vapor Vacuum Arc), the curve of density of the ions according to the depth of the component forms a narrow Gaussian showing this concentration in a precise region. In the case of an ion implantation carried out under a plasma atmosphere (for example, Plasma Immersion Ion Implantation), the distribution will be done with a concentration of ions with a decreasing tendency according to the depth reached. Depending on the substrates to be treated and the complexity of the geometry, one or the other of these general methods may be preferable, or even a combination of the two.

It cannot be excluded that, with an implantation under sufficient energy, it is possible to have ions only in the substrate (silicon layer 10) and not in the protective layer 12 ( picture 3 ). More commonly, the implanted zone is located at least in the substrate.

The reverse is also possible, that is to say that the implanted zone 16 is located only in the protective layer 12 ( figure 5 ). This is particularly the case with a protective layer having a greater thickness than the implantation depth obtained with a given energy. More commonly, the implanted zone is located at least in the protective layer.

Thus, the ion-implanted zone can be located either in the protection layer only, or in the substrate only or shared between the protection layer and the substrate, that is to say in the region of the interface between the layer protection and the substrate.

In particular because of structural or crystallographic modifications, or modifications of the interactions between the substrate and the protective layer, due to ion implantation, the tribological properties of such an implanted component are improved.

In particular, an improvement has been observed in the tribological properties of watch components having a zone implanted ionically mainly, or even only with C ions, implanted with a dose greater than 5× ^{10 17} ions/cm ² , preferably greater than 1×10 ¹⁸ ions/cm ² .

Thus, the figure 1 shows the friction coefficient of a substrate on a ruby sphere, rolling on the substrate over a distance of 100m (abscissa). The first centimeters of the curve correspond to a break-in and the establishment of regular conditions, and are not significant. Curve A is the reference curve, obtained for a substrate without ion implantation. Curve B is obtained for an identical substrate, implanted ionically with a dose of C ions of 2.10 ¹⁷ ions/cm ² , with an energy of 600 keV. Curve C is obtained for an identical substrate, implanted ionically with a dose of C ions of 1.10 ¹⁸ ions/cm ² , with an energy of 600 keV. It can be seen that the coefficient of friction of the sample represented on curve B has a higher coefficient of friction than the reference sample, whereas the ion implantation of curve C makes it possible to reduce this coefficient of friction over almost all the length of the path traveled by the sphere.

The figure 2 shows the average coefficients of friction measured under conditions similar to those of the figure 1 , apart from the energy which in this case is 35keV, depending on the implantation dose, and on different substrates with a layer of SiO ₂ of different thickness, which explains the results which are not directly comparable. The effects obtained with a dose of 5.10 ¹⁷ ions/cm ² are already interesting. The values of the dashed curve are extrapolations. On the other hand, no measurement between the non-implanted substrate (dose = 0.10 ¹⁶ ions/cm ² ) and a dose of 25.10 ¹⁶ ions/cm ² was carried out, and the results on another sample giving nonlinear results (deterioration before improvement), no extrapolation can be proposed.

Other tests have made it possible to show that an implantation carried out mainly, or even only with N ions, with a dose > 1.10 ¹⁶ ions/cm ² , also made it possible to improve the tribological properties of a component produced from a silicon-based substrate, in particular its coefficient of friction.

Just as with carbon, the nitrogen ions can be implanted in the protective layer, in the substrate, or in the region of the interface between the substrate and the protective layer, mainly in the substrate, or even only in the substrate if the energy is sufficient.

In an advantageous variant, it is possible to implant C ions and N ions in a combined manner, either alternately or simultaneously, depending on the type of installation used.

It is possible, preferably, to use ion-implanted components, as proposed above, to produce components subjected to repeated or significant shocks or friction, in particular the components of the escapement, in particular an anchor, a steering wheel anchor or part of a resonator subject to friction, such as a platter peg.

The invention also relates to the process for implanting the components described above, also defining a method for optimizing the tribological properties of a horological component as mentioned above, produced on the basis of a substrate comprising a layer of silicon 10 whose crystallographic structure comprises at least one of the monocrystalline, polycrystalline or amorphous forms, said substrate being reinforced by a protective layer 12 produced on its surface during a substrate protection operation. According to the invention, the method comprises a step of ion implantation of at least one ion, by ion bombardment on the surface of said component.

Thus, the implantation step can be carried out with C ions, at a dose greater than 5.10 ¹⁷ ions/cm ² , preferably greater than 1.10 ¹⁸ ions/cm ² and with an energy greater than 600keV, or with ions N, at a dose greater than 1.10 ¹⁶ ions/cm ² and with an energy greater than 600keV.

The implantation is carried out on the components when, after etching, they have been detached from the base wafer in which they are etched. Indeed, the areas of the components that it is relevant to treat at the tribological level, are the side faces which, in general in watchmaking, are those which are exposed to friction. Thus, at the wafer level, not only these side faces are not yet defined and therefore not accessible either to be able to carry out the protection operation, fundamental in the present application. For reasons of handling convenience, it is nevertheless possible for the components to remain attached to the wafer or to a part of the wafer, at least at one point, to allow batch handling of the components, the components being able to be easily separated from the support. formed by the wafer, for example by breaking the junction between the wafer and the component, after the latter has undergone protection and ion implantation operations.

It will be noted that, on the side of the lateral faces of the components, depending on the crystallographic structure of the wafer, there will be non-homogeneous flanks at the crystallographic level. For example, with a wafer 111, the lateral flanks of the components can pass at any place of the crystallographic lattice. Despite this, and despite the fact that the ion implantation is directional and that the ion bombardment can have a variable angle of incidence on the side faces of the component, a tribological improvement is obtained which is averaged over the entire side surface. of the component and which is significant. Consequently, the method proposed in the present application can be applied with substrates of any crystallographic orientation, given that, at the level of the lateral surfaces, the part can assume all crystallographic orientations.

Thus, a new way is proposed in the improvement of the tribological properties of watchmaking components. The skilled person can, on the basis of the teaching of the present application, consider playing on the implantation parameters in terms of dose and energy, or even treatment duration, but without departing from the scope of the present invention defined by the claims.

Those skilled in the art could even consider having several implanted zones 16, at different depths, typically two, separated by a non-implanted or weakly implanted zone, independently of the layer (of protection or of silicon). Such a structure can in particular be obtained by carrying out two implantation steps with different parameters, in particular at the level of the implantation energy ( figure 6 ).

Claims (5)

1. Method for optimizing the tribological properties of a timepiece component intended to undergo dynamic friction, realised on the basis of a substrate comprising a layer of silicon (10) of which the crystallographic structure comprises at least one of the monocrystalline, polycrystalline or amorphous forms, said substrate being reinforced by a protection layer (12) realised on its surface during a substrate protection operation, said method comprising a step of ion implantation of at least one ion, by ion bombardment on the surface of said component,
   wherein the implantation step is performed with C ions, with a dose greater than 5.10¹⁷ ions/cm² and/or with N ions, with a does greater than 1.10¹⁶ ions/cm².
2. Method according to Claim 1, characterized in that the dose of C ions is greater than 1.10¹⁸ ions/cm².
3. Method according to one of Claims 1 and 2, characterized in that it is realised on formed components, at least partially detached from a base wafer, to allow treatment of the lateral faces of said components.
4. Method according to one of the preceding claims, characterized in that said ions are implanted with an energy greater than 600 keV.
5. Method according to one of the preceding claims, characterized in that said protection layer is realised in SiO₂ obtained by an oxidation step and measuring at least 50 nm thickness.

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