CH719203A2 - Functional set of micromechanics with a tribological coating. - Google Patents

Abstract

The present invention relates to a functional assembly (1) of micromechanics comprising at least a first part (2) with a first functional surface (2a) intended to come into frictional contact with a second functional surface (3a), said second functional surface belonging , either to said first part (2) or to at least one second part (3) constituting with said first part (2) said functional assembly (1), said functional assembly (1) being characterized in that the first functional surface ( 2a) and the second functional surface (3a) are formed of a first layer (9a) comprising ultrananocrystalline, nanocrystalline or microcrystalline diamond, said first layer (9a) being surmounted by a second layer (9b) comprising S atoms and F. It also relates to the diamond functionalization process.

Description

Technical field of the invention

The present invention relates to a functional micromechanical assembly comprising at least one part provided with surfaces subjected to contact by friction requiring a tribological coating. The micromechanical functional assembly relates more specifically to a pair of watchmaking micromechanical parts cooperating mechanically with each other, such as an escape wheel and pallets of an anchor.

Technology background

[0002] At the time of the appearance of technologies for deposition of diamond by chemical means in the gaseous phase (diamond CVD), many hopes had been expressed concerning tribology, thanks to the intrinsic hardness of diamond. Unfortunately, dry friction between two diamond surfaces is not tribologically viable because it leads to adhesion between the watch parts in contact. This is due to the creation of bonds between the carbon atoms (C-C) of the two partner materials of the Diamond vs. Diamond couple. To minimize this adhesive effect, the pair of Diamond vs. Diamond materials must be in the presence of an environment with a relative humidity (%RH) at least equal to 50%. This threshold value of 50% RH allows the supply of micro-droplets of water (H2O) in the contact surface. This then decreases the tangential displacement force FT of a surface relative to another surface. These microdroplets therefore play the role of local lubricants promoting the relative movement of the watch parts in contact. Below this threshold value, the adhesion phenomenon is exacerbated, with contact instabilities occurring over short stress times.

[0003] Thus, under the conditions of contact existing in motion (low normal force and high tangential force), the overall coefficient of friction, denoted CoF (CoefficientOfFriction), of the Diamond vs. Diamond couple can be less than 0.2 as illustrated in the figure 1. Unfortunately, these environmental conditions do not guarantee the tribological stability of the pair of materials beyond 5 minutes of testing. Indeed, friction instabilities appear from 3.5 minutes of testing.

[0004] The challenge therefore resides in the fact of using this Diamond/Diamond pair in a micromechanical functional assembly while guaranteeing stable tribological behavior over a longer period without lubricant.

Summary of the invention

The object of the invention is to overcome the aforementioned disadvantages by modifying the tribological properties of the diamond layer. Indeed, as explained previously, the atmospheres of hydrogen, oxygen or humidity are, in use over the long term, difficult to maintain. The object of the invention is to replace the passivation OH or H atomic bond caused by the media mentioned above by other molecules. These extreme surface modifications intended to change the tribological properties are carried out, according to the invention, by functionalizing the diamond layer. More specifically, according to the invention, the diamond layer is functionalized with a sulfur and fluorine compound.

Thus, the present invention relates to a functional micromechanical assembly comprising at least a first part with a first functional surface intended to come into frictional contact with a second functional surface, said second functional surface belonging either to said first part or at least one second part constituting with said first part said functional assembly, said functional assembly being characterized in that the first functional surface and the second functional surface are formed of a first layer comprising ultrananocrystalline, nanocrystalline or microcrystalline diamond, said first layer being surmounted by a second layer comprising S and F atoms. It will be understood that the first layer can be material with the substrate constituting the first and second parts or distinct from the substrate.

[0007] An improvement in the tribological behavior of these functional surfaces has been observed during tests. This improvement could be attributed to the S allowing to preserve itself from the appearance of bonds (C-C) when the relative humidity is no longer sufficient to play this role. Thus, it is possible to provide for operation of a mobile horological system, such as a Swiss lever escapement without lubrication of the pallet/escape-wheel contact, with performances at least equivalent to those of the standard references.

This improvement in tribological behavior is more particularly observed when the two functional surfaces in contact are coated with a tribological layer of the same composition.

The present invention also relates to the process for functionalizing diamond by reactive ion etching. Reactive ion etching is generally used for deep etching on silicon. It has been observed by the inventors that by using reactive ion etching equipment at very low power, typically between 30 and 70 W, it is possible to synthesize this compound of S and F on a part, such as a part of watchmaking which has small dimensions.

Brief description of figures

[0010] The aims, advantages and characteristics will appear in the light of the following figures: - figure 1 represents the curve of the global coefficient of friction (CoF(global)) as a function of time, of the couple microcrystalline diamond/non-lubricated microcrystalline diamond according to the prior art, for a relative humidity rate of 30%. - Figure 2 schematically shows in section a portion of two parts of the functional assembly according to the invention. - Figure 3 partially shows a functional assembly comprising two parts, namely an escape wheel and a pallet pallet with contact surfaces functionalized according to the invention. – Figure 4 shows an electron microscopy image of the morphology of the second layer of S and F, with a rod structure. – Figures 5a and 5b represent respectively the tribological results for a distance of 25m, for a functional assembly comprising two parts. In FIG. 5a, by way of comparison not covered by the invention, one part is coated on its functional surface with microcrystalline diamond MCD and the other part with microcrystalline diamond functionalized with SF6. In FIG. 5b, according to the invention, the two parts are coated on their functional surface with microcrystalline diamond functionalized with SF6. – figure 6 represents the tribological results for the couple of figure 5b over a longer distance of 2500m. – Figures 7a and 7b represent the amplitude of the balance respectively for a reference escapement wheel/lever pair and an escapement wheel/lever pair according to the invention with a layer of microcrystalline diamond functionalized with SF6. – Figure 8 is a schematic view of the equipment used for the functionalization of diamond.

Detailed description of the invention

The present invention relates to a functional assembly comprising at least one part subjected on its or its so-called functional or contact surfaces to friction. The functional assembly according to the invention may comprise a single piece with two functional surfaces intended to be in contact by friction. For example, in the watchmaking field, it may be a barrel spring formed of a blade with one face of the spring intended to be in contact with another face of the spring. As a variant, the functional assembly can comprise at least two parts with each part respectively comprising a functional surface intended to be subjected to friction with a functional surface of another part. For example, in the watchmaking field, the functional assembly 1 can comprise a first part 2 which is a pallet 4 of an anchor 5 and a second part 3 which is an escape wheel 6 as represented in figure 3. More precisely, the pallet 4 has a rest plane A and an impulse plane B which cooperate with the rest planes C and impulse D of the tooth 7 of the escape wheel 6. These planes A, B, C, D are highly stressed functional surfaces subject to high levels of friction and/or contact which may require a tribological layer according to the invention to reduce friction. In another horological application, the first part can be an axis of a mobile and the second part a bearing. According to another application in this field, the first and second parts can be gear wheel teeth.

As shown schematically in Figure 2, the functional assembly 1 comprises the first part 2 and the second part 3 formed of a substrate 8 with at least at their functional surface 2a, 3a the tribological layer 9. This layer 9 consists of a first layer of diamond 9a which may be ultrananocrystalline diamond (UNCD, according to the English name Ultrananocrystalline Diamond), nanocrystalline (NCD, according to the English name Nanocrystalline Diamond) or microcrystalline (MCD, d 'after the English name Microcrystalline Diamond). According to the invention, this first layer 9a is functionalized with a sulfur compound, and more specifically S and F, which forms a second layer 9b on the first layer 9a. Advantageously, the first layer 9a is functionalized with SF6. Other gases, although more dangerous and of more limited use, such as SF2 and SF4, are possible. Generally, the functionalized S and F on the surface of the diamond layer are in the form of rods as shown in Figure 4. The second layer of S and F is generally of non-constant thickness and of nanometric size with typically a average thickness between 2 and 50 nm, more specifically between 5 and 10 nm. The techniques suitable for visualizing and chemically analyzing the second layer of S and F are, for example, X-ray photoelectron spectrometry (XPS) or TOF-SIMS type mass spectrometry (Time Of Flight Secondary Ion Mass Spectrometry). It will be noted that since the layer is extremely thin, it may prove difficult to precisely identify the compound present. We will therefore speak of a layer comprising S and F.

According to the invention, the surfaces intended to be brought into contact are each covered with the layer of diamond functionalized with S and with F. The layer 9 including the first layer of diamond 9a and the second layer 9b of S and of F has an average thickness of between 800 nm and 1200 nm, preferably between 900 and 1200 nm, and whether it is MCD, NCD or even UNCD diamond.

[0014] For example, the substrates may be chosen from all materials including ceramics, silicon, oxidized silicon, nitrided silicon, carburized silicon and steels. It is also possible for the substrate and the diamond layer to form a single solid material. According to a preferred embodiment, the substrate is made of silicon with a layer of microcrystalline diamond functionalized with S and F.

[0015] The method for functionalizing the diamond layer is as follows. Beforehand, the diamond layer is deposited by chemical vapor phase (CVD) or by the hot filament technique, if the substrate is not solid diamond. Next, the sulfur and fluorine are deposited as a second ultra-thin layer on top of the diamond layer by a very low power Reactive Ion Etching (RIE) process to achieve deposition rather than an engraving. The equipment 10 for the functionalization of the diamond with the plasma reactor 11 is schematized in FIG. 8. It can for example be of the "capacitive coupling" type.

[0016] The process parameters are as follows: – Reactive gas (12) comprising S and F such as SF6, SF4 or SF2, with a flow rate of between 3 and 20 sccm (for Standard Cubic Centimeter per Minute), i.e. cm<3> min<-1>, preferably between 5 and 10 sccm, – Radio frequency (RF) power between 30 and 70W, preferably between 40 and 60W, – Pressure in the reactor between 30 and 150 µbar, preferably between 80 and 120 µbar, – Reaction time between 20 and 120 minutes, preferably between 30 and 70 minutes.

The second functionalized layer thus obtained has a very small thickness, of the order of a few nanometers, even with a reaction time of the order of an hour.

[0018] Tests have been carried out to evaluate the tribological behavior of a functional assembly according to the invention.

[0019] Si escape wheels and Si pallet lifts were coated with microcrystalline diamond functionalized with S and F and more specifically with SF6. The tests are therefore carried out with a couple of functionalized diamond on functionalized diamond and compared with a standard lever with a lubricated steel contact on a ruby. FEMTO-torque tests were carried out to measure the efficiency of a Swiss lever escapement mounted on a work plate. The torque applied was 16 μN.m. The measurement of the amplitude of the balance (with 3 arms) is represented in FIGS. 7a and 7b respectively for the standard anchor and the anchor treated according to the invention. For the standard anchor, the average amplitude is 274° for one hour of testing. For the anchor treated according to the invention, the average amplitude is 256° for one hour of testing, ie 18° below that of the standard anchor. Apart from the temporary failure at 700s, it can be seen that the regularity of the amplitude is very good, even better than on the standard reference version.

[0020] In parallel, tribological tests were also carried out with a ball/plane tribometer with a ball 2 mm in diameter for a distance of 25 meters and 2500 meters. The tests over 25 meters were carried out with a ball/plane pair each having a Si substrate with a layer of diamond functionalized with SF6 (SF6//SF6) and with a comparative ball/plane pair where the ball is an Si substrate coated with microcrystalline diamond without functionalization and where the plane is an Si substrate coated with functionalized microcrystalline diamond according to the invention (MCD//SF6). The purpose of these tests is to highlight the advantage of functionalizing the two surfaces in contact. The tests were carried out with a normal force of 10 mN, a sliding speed of 10 mm/s and an amplitude of 4 mm. The dynamic coefficient of friction as a function of distance is represented in FIGS. 5a and 5b respectively for the comparative torque and the torque according to the invention. It can be seen for the comparative couple that the average coefficient of friction is greater than 0.1. It is unstable with many peaks. On the other hand, for the couple according to the invention, the average coefficient of friction is less than 0.1 and stable. On this same pair according to the invention, a longer test over a distance of 2500 meters was carried out. The result is presented in Figure 6. The average coefficient of friction is low and identical to that of the short test of 25 meters. After a short break-in period, the coefficient of friction is stable over the entire test period.

[0021] Thus, the FEMTO-torque tests and the tribological tests confirm the very good behavior in use of the couple functionalized diamond on functionalized diamond.

Legend

[0022] (1) Functional assembly (2) First part a) First contact surface, also called functional surface (3) Second part a) Second contact surface, also called functional surface (4) Pallet A. Rest plane B Impulse plane (5) Anchor (6) Escape wheel (7) Tooth C. Rest plane D. Impulse plane (8) Substrate (9) Tribological layer a) First layer comprising diamond b) Second layer comprising S and F (10) Equipment for functionalization (11) Reactor (12) Gas (13) Plasma (14) Electrodes

Claims (17)

1. Functional assembly (1) of micromechanics comprising at least a first part (2) formed of a first substrate (8) surmounted by a first functional surface (2a) intended to come into frictional contact with a second functional surface ( 3a), said second functional surface belonging either to said first part (2) or to at least one second part (3) formed of a second substrate surmounted by said second functional surface (3a), the second part (3) constituting with said first part (2) said functional assembly (1), said functional assembly (1) being characterized in that the first functional surface (2a) and the second functional surface (3a) are formed of a first layer (9a) which is either integral with the first substrate (8) and the second substrate (8) or distinct from the first substrate (8) and the second substrate (8), the first layer (9a) comprising ultrananocrystalline, nanocrystalline or microcrystalline diamond and being surmounted by a second layer (9b) comprising S and F atoms. 2. Functional assembly (1) according to the preceding claim, characterized in that the second layer (9b) comprises SF6. 3. Functional assembly (1) according to one of the preceding claims, characterized in that the second layer (9b) is formed of rods. 4. Functional assembly (1) according to one of the preceding claims, characterized in that the second layer (9b) has an average thickness of between 2 and 50 nm, preferably between 5 and 10 nm. 5. Functional assembly (1) according to one of the preceding claims, characterized in that the first part (2) is a pallet (4) and in that the second part (3) is an escapement wheel (6) . 6. Functional assembly (1) according to one of claims 1 to 4, characterized in that the first part (2) is an axis of a mobile and in that the second part (3) is a bearing. 7. Functional assembly (1) according to one of claims 1 to 4, characterized in that the first part (2) and the second part (3) are gear wheel teeth. 8. Functional assembly (1) according to one of claims 1 to 4, characterized in that said second functional surface belongs to said first part (2) and in that the first part (2) is a barrel spring formed of a blade and in that a front face of said blade forms said first functional surface and in that the rear face of said blade forms said second functional surface. 9. Functional assembly (1) according to one of the preceding claims, characterized in that the first substrate (8) and the second substrate (8) are chosen from ceramics, silicon, oxidized silicon, nitrided silicon, carburized silicon and steels, when said first layer (9a) is distinct from the first substrate (8) and from the second substrate (8). 10. Process for functionalizing ultrananocrystalline, nanocrystalline or microcrystalline diamond, comprising the following steps: a) Provision of at least a first substrate (8) coated with a first layer (9a) of said ultrananocrystalline, nanocrystalline or microcrystalline diamond or provision of at least a first substrate (8) made of ultrananocrystalline, nanocrystalline diamond or microcrystalline, b) Functionalization of said first substrate (8) of step a) in equipment (10) for etching by reactive ions provided with a reactor (11) and electrodes (14), the functionalization being carried out with a power radiofrequency between 30 and 70W, with as reactive gas (12) a compound comprising S and F. 11. Method according to the preceding claim, characterized in that the compound is SF6. 12. Method according to claim 10 or 11, characterized in that the radiofrequency power is between 40 and 60W. 13. Method according to one of claims 10 to 12, characterized in that the voltage between the electrodes (14) is between 130 and 170V, preferably between 140 and 155V. 14. Method according to one of claims 10 to 13, characterized in that the pressure in the reactor (11) is between 30 and 150 µbar, preferably between 80 and 120 µbar. 15. Method according to one of claims 10 to 14, characterized in that the flow rate of the reactive gas (12) in the reactor (11) is between 3 and 20 sccm, preferably between 5 and 10 sccm. 16. Method according to one of claims 10 to 15, characterized in that the functionalization time of the first substrate (8) in the reactor (11) is between 20 and 120 minutes, preferably between 30 and 70 minutes. 17. Method according to one of claims 10 to 16, characterized in that the equipment (10) is provided with a capacitive plasma coupling.