Patent Application
20230173788
The present invention relates to a micromechanical functional assembly comprising at least one part provided with surfaces subjected to frictional contact requiring a tribological coating. The micromechanical functional assembly relates more specifically to a pair of micromechanical horological parts, cooperating mechanically with one another such as an escapement wheel and pallets of an anchor.
When technologies were first developed for making diamonds by chemical vapour deposition (CVD diamond) there were high hopes for tribology, due to the intrinsic hardness of diamond. Unfortunately, dry friction between two diamond surface is not tribologically viable as it leads to adhesion between the contacting horological parts. This is due to the formation of bonds between the carbon atoms (C-C) of the two partner materials of the Diamond vs Diamond pair. To minimise this adhesive effect, the Diamond vs Diamond material pair has to be in the presence of an environment with a relative humidity (%RH) at least equal to 50%. This threshold value of 50%RH makes it possible to supply micro-droplets of water (H2O) in the contact surface. This reduces the tangential displacement force FT of one surface relative to another. These micro-droplets therefore act as local lubricants favouring the relative movement of the contacting horological parts. Below this threshold value, the phenomenon of adhesion is exacerbated, with contact instabilities occurring in short loads times.
Thus, in the contact conditions of movement (low normal force and high tangential force), the global coefficient of friction, CoF, of the Diamond vs Diamond pair can be lower than 0.2 as illustrated in
The challenge therefore relates to using this Diamond/Diamond pair in a micromechanical functional assembly while guaranteeing stable tribological behaviour without lubricant over a longer period of time.
The subject-matter of the invention is to overcome the aforementioned disadvantages by modifying the tribological properties of the diamond layer. Indeed, as explained above, the hydrogen, oxygen or humidity atmospheres are difficult to maintain with long-term use. The subject-matter of the invention is to replace the atomic OH or H passivation bond caused by the aforementioned environments with other molecules. These extreme surface modifications for changing the tribological properties are achieved, according to the invention, by functionalising the diamond layer. More specifically, according to the invention, the layer of diamond is functionalised with a sulphur and fluorine compound.
Thus, the present invention relates to a micromechanical functional assembly comprising at least one 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 to at least one second part constituting with said first part said functional assembly, said functional assembly being characterised in that the first functional surface and the second functional surface are formed by a first layer comprising ultrananocrystalline, nanocrystalline or microcrystalline diamond, said first layer being topped by a second layer including S and F atoms. It will be understood that the first layer can be made from the same material as the substrate constituting the first and second parts or different from the substrate.
An improvement in the tribological behaviour of these functional surfaces has been observed in tests. This improvement could be attributed to S which prevents the formation of (C-C) bonds when the relative humidity is no longer sufficient to perform this role. Thus, it is possible to provide functioning of a mobile horological system, such as a Swiss anchor escapement without lubrication of the pallet/escapement wheel contact, with performances that are at least equivalent to those of standard references.
This improvement in the tribological behaviour is observed more particularly when the two functional surfaces in contact are coated with a tribological layer of the same composition.
The present invention also relates to a method for functionalising diamond by reactive ion etching. Reactive ion etching is generally used for deep etching on silicon. It has been found by inventors that by using very low power reactive ion etching equipment, typically between 30 and 70 W, it is possible to synthesise this S and F compound on a part, such as a horology part which has small dimensions.
The aims, advantages and features will become apparent from the following figures:
The present invention relates to a functional assembly comprising at least one part subjected to friction on its surface or surfaces referred to as functional or contact surfaces. The functional assembly according to the invention can include a single part with two functional surfaces intended to be in frictional contact. For example, in the field of horology, this may consist of a barrel spring formed by a blade with one face of the spring intended to be in contact with another face of the spring. Alternatively, the functional assembly can include at least two parts with each part respectively including a functional surface intended to be subjected to friction with a functional surface of another part. For example, in the field of horology, the functional assembly 1 can include a first part 2 which is a pallet 4 of an anchor 5 and a second part 3 which is an escapement wheel 6 as shown in
As depicted in
According to the invention, the surfaces intended to be brought into contact are each covered with the layer of S- and F-functionalised diamond. The layer 9 including the first layer of diamond 9a and the second layer 9b of S and F has an average thickness of between 800 nm and 1200 nm, preferably between 900 and 1200 nm, this being MCD, NCD or even UNCD diamond.
For example, the substrates can be selected from a group of materials including ceramics, silicon, oxidised silicon, nitrided silicon, carburised silicon and steels. It is also possible that the substrate and the diamond layer form a single solid material. According to a preferred embodiment, the substrate is silicon with a layer of microcrystalline diamond functionalised with S and F.
The method of functionalising the diamond layer is as follows. In advance, the diamond layer is deposited by chemical vapour deposition (CVD) or by the hot filament technique if the substrate is not solid diamond. Then, the sulphur and fluorine are deposited in the form of a second ultra-fine layer on the diamond layer by a very low power reactive ion etching method (RIE) in order to achieve deposition rather than etching. The equipment 10 for functionalising the diamond with the plasma reactor 11 is outlined in
The parameters of the method are as follows:
The second functionalised layer obtained in this way has a very low thickness, in the order of several nanometres, even with a reaction time of in the order of one hour.
Tests were carried out to evaluate the tribological behaviour of a functional assembly according to the invention.
Escapement wheels made of Si as well as pallet lifts made of Si were coated with microcrystalline diamond functionalised with S and F and more specifically SF6. The tests were therefore carried out with a functionalised diamond/functionalised diamond pair and compared with a standard anchor with a lubricated steel on ruby contact. FEMTO-torque tests were carried out to measure the performance of a Swiss anchor escapement mounted on a work plate. The torque applied was 16 µN.m. The measurement of the amplitude of the balance wheel (with 3 arms) is shown in
In parallel, tribological tests were also carried out with a ball/plane tribometer with a ball of 2 mm diameter for a distance of 25 metres and 2500 metres. The 25 metre tests were performed with a ball/plane pair each having an Si substrate with a layer of SF6 (SF6//SF6) functionalised diamond and with a comparative ball/plane pair where the ball is an Si substrate coated with microcrystalline diamond without functionalisation and where the plane is an Si substrate coated with microcrystalline diamond functionalised according to the invention (MCD//SF6). The purpose of these tests is to highlight the advantage of functionalising the two contact surfaces. 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 friction coefficient as a function of distance is shown in
Thus, the FEMTO-torque tests and the tribological tests confirm the very good behaviour in use of the functionalised diamond on functionalised diamond pair.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21212556.1 | Dec 2021 | EP | regional |
