HK1169456B - Method for coating micromechanical parts with high tribological performances for application in mechanical systems - Google Patents

Description

Method for coating micromechanical components with high friction properties for mechanical systems

Technical Field

The invention relates to a method for coating micromechanical components of a micromechanical system, in particular a timepiece movement, in order to reduce surface roughness and increase friction properties. The invention also relates to a corresponding micromechanical element for a micromechanical system, in particular a timepiece movement. The invention can be used in micromechanical timepiece movements, in particular for implementing escape wheels and pallets and other friction-related elements.

Background

The technical demand for micromechanical components is growing. In addition to maximum precision, there is a need to provide maximum energy efficiency in the mechanical system, long life and complete rejection of lubricant as much as possible.

In recent years, many documents have been published that deal with this problem. The method described achieves some tasks but does not provide a comprehensive solution, mainly due to the limitations of the materials used.

Micromechanical elements manufactured by machining (punching or form-cutting) present two main drawbacks. First, they are either expensive or economically profitable only for mass production, since investment is required for expensive production tools. Second, these processes reach their technical limits at a precision level of +/-5 microns.

Therefore, several alternative approaches have been discussed in the literature. One of the most promising ideas concerns the etching of micromechanical components from silicon wafers, which achieve the highest precision, even exceeding the results of mechanical machining so far. The tolerance can be reduced to the sub-micron range, but this is at the expense of lifetime: practical results show that the mechanical strength and wear of these parts are not satisfactory without the use of lubricants. One solution to this problem is disclosed in european patent EP 1904901: the strength and lifetime can be increased by treating the surface of the micromechanical component with oxygen, but no final solution has been achieved.

The friction performance can be enhanced by using special oils in the mechanical system, however this is at the cost of the need to run the system dry.

Classical machined parts made of steel achieve the longest lifetime, but these systems have limits in terms of high precision and must be additionally lubricated.

Another problem with lubrication systems is that frequent maintenance cycles are necessary, wherein the cartridge must be cleaned and re-lubricated. Therefore, the operation cycle is limited and additional costs are incurred. These maintenance cycles are necessary because the oils used age and lose their properties over time.

Several approaches have been taken to achieve all of these requirements in one system.

In EP 0732635B1 a method is described in which micromechanical parts are etched from a silicon wafer and then coated with a diamond film. The surface roughness of the diamond film obtained by the method is more than 400 nanometers. Thus, if the diamond coated parts are used in sliding contact applications, these films need to be subsequently polished.

EP 1233314 discloses a mechanical clockwork assembly for a timepiece, having a mechanical escapement with an escape wheel and a hinge, wherein the functional elements of the escape wheel are at least partially coated on their running surface with a DLC (diamond like carbon) coating. DLC has a high sp2 content (30-100%) and is amorphous carbon with insufficient hardness for effective wear protection applications.

EP 1622826 discloses a micromechanical element comprising a first surface and a second surface substantially perpendicular to each other, wherein the first and/or the second surface at least partly consists of diamond.

US 5308661 discloses a method for pretreating a carbon coated substrate to provide a uniform high density of nucleation sites thereon for subsequent deposition of a continuous diamond film without the need for applying a bias voltage to the substrate.

EP 1182274a1 discloses a method for post-treating diamond coatings, wherein a coarse-grained (micrometer range) diamond coating is deposited on a machining tool and subsequently treated by a plasma method. The goal of the post-treatment is to degrade the top layer of the sp 3-hybridized diamond coating into sp 2-hybridized carbon types. It is desirable to fill the "surface depression" between the coarse particles of the protruding surface to achieve a flatter surface. The result of the process is a film with coarse sp3 diamond on top of which is a top layer of sp2 hybridized amorphous carbon of several hundred nanometers. The top layer is relatively soft and will be worn away quickly in applications involving high friction.

All the solutions described above can solve only part of the problems of providing micromechanical elements characterized by a coefficient of friction lower than 0.05, thus preventing large-scale manufacturing, which is still highly desirable in the horological industry, for example.

In particular, when using diamond coated silicon, the above described solution creates the following problems: due to the microcrystalline structure of the diamond coating, the diamond coated micromechanical elements often exhibit a high initial coefficient of friction. This high friction coefficient severely limits the efficiency of the micromechanical system during the first hours of its lifetime.

It is known that surfaces with a roughness in excess of a few hundred nanometers do not directly achieve a low coefficient of friction. In addition, the use of coarse diamond films in mechanical systems requires very smooth counterparts. In this case, the rough diamond film will wear into its counterpart, causing the system to wear and tear very quickly.

In principle, a special case is conceivable in which different roughness modules are brought to special conditions so as to produce a low coefficient of friction. However, the pressure on each individual particle will be too high, causing the particles to fracture and/or interlock. As a result, the mechanical system will quickly lose its performance, end up with a high coefficient of friction and thus clog the system. After the coating breaks, the entire system will collapse and/or damage the entire timepiece.

The reasons why the solutions proposed to polish and smooth the surface of micromechanical elements after diamond coating have failed are the high cost, the low efficiency and the following fundamental technical reasons: the most important functional surface is the side of the micromechanical part, which is where mechanical polishing cannot reach when mounted in a wafer. Polishing after removal of the component from the wafer is not easy and not economical, since the micromechanical components are large and small. The technical solution of plasma etching diamond-coated wafers containing micromechanical components also fails, due to the non-uniformity of the plasma polishing, in particular on the sides of said components, which are the most important areas (see above).

Methods using small crystal sizes (hundreds of nanometers) suffer from similar problems in the smaller dimensions. For example, plasma etching the sides is not feasible because this method mainly affects the grain boundaries and etches the surface in an anisotropic manner.

In addition, the anisotropy of the etching process may result from several parameters. The etching efficiency strongly depends on the crystal orientation of the diamond crystals. Since diamond films grown on non-diamond substrates (silicon in most cases) exhibit mixed crystal orientations and thus etching is non-uniform, it is even possible to increase the surface roughness of the diamond rather than reduce it.

Disclosure of Invention

It is therefore a general object of the present invention to provide a method which allows to provide micromechanical components for micromechanical systems, in particular timepiece movements, with a long life and high friction properties.

Another object of the invention is to provide a method which allows to provide a micromechanical element for a micromechanical system, in particular a timepiece movement, which reduces the maintenance cycle.

It is a further object of the present invention to provide a micromechanical element for a micromechanical system, in particular a timepiece movement, which has improved friction properties, reduced wear and friction, and the like.

These aims are achieved thanks to a method for coating micromechanical components of a micromechanical system, in particular a timepiece movement, and to a micromechanical component for a micromechanical system, as defined in the claims.

Accordingly, a method is provided for coating micromechanical elements of a micromechanical system (in particular a timepiece movement), comprising:

providing a substrate element to be coated;

providing the element with a diamond coating;

wherein the diamond coating is provided by CVD (chemical vapour deposition) in a reaction chamber; in chemical vapor deposition, a change in carbon content is controlled within the reaction chamber during the final part of the growth process, thereby providing a change in sp2/sp3 carbon bonds near the surface.

Due to the sp 2-hybrid component, a lubricating effect is achieved, resulting in a further reduction of the friction coefficient. At the same time, the reduced surface roughness also leads to an increase in the friction properties. It is important to note that a separate surface layer of sp 2-hybridized carbon on top of a diamond film or diamond-like carbon (DLC) film does not provide the same result, since such layer or film will wear out rapidly due to the low hardness and will therefore be removed within a few cycles of the mechanical system. In addition, the adhesion of the sp 2-hybrid layer to the sp 3-hybrid matrix was better when the sp 2-hybrid carbon content was gradually changed, as shown in fig. 3.

In an advantageous embodiment, the change is an increase in the carbon content in the diamond layer. This increase is obtained by increasing the proportion of carbon-containing reactant gas (e.g. methane).

In another embodiment, the increase in carbon content is obtained by adding additional carbon-containing gas (e.g., acetylene).

In yet another embodiment, the increase in carbon content is obtained by increasing the temperature and/or pressure within the process chamber.

In yet another embodiment, the increase in carbon content is obtained by adding gaseous nitrogen to the reaction chamber.

In another advantageous embodiment, the increase in carbon content is obtained by replacing hydrogen (by up to 100% of the hydrogen content) with argon (or another element such as nitrogen).

In a further advantageous embodiment, the increase in carbon content is obtained by adding at least one rare gas, i.e. neon, helium, krypton or xenon.

In one variant, the increase in carbon content is obtained by lattice distortion.

In another variant, the increase in carbon content is obtained by post-treatment (for example plasma or laser treatment).

In one variant, the post-treatment consists essentially of terminating the diamond surface, wherein the termination is achieved using an element selected from the group consisting of: hydrogen, halogens, metals, conductive minerals/organic molecules or proteins.

In another variation, post-treatment consists essentially of adding a metal-containing compound on top of the diamond to reduce the tack.

In an advantageous variant, after the CVD step, the grain size is reduced (which enables the surface roughness to be reduced).

The invention also provides a micromechanical element for a micromechanical system, in particular a timepiece movement, obtained by the aforementioned method. In an advantageous embodiment, the surface layer has a gradually increasing sp2 hybridized carbon content.

Such mechanical elements are produced for micromechanical systems which allow them to operate in dry (unlubricated) conditions, characterized by high friction properties (extremely low friction coefficient, reduced wear, etc.), which are stable for long periods and characterized by high energy efficiency.

Drawings

The above and other objects, features, aspects and advantages of the present invention will become apparent from the following detailed description of embodiments thereof, given by way of illustration and not of limitation, with reference to the accompanying drawings, wherein:

FIG. 1 is a graph showing the average coefficient of friction of a nanocrystalline diamond film (sliding on the nanocrystalline diamond film without lubrication) as a function of the grain size of the diamond;

figures 2a and 2b schematically show the surface structure of a diamond film deposited according to the method of the invention for different grain sizes, which results in a low roughness and thus a low initial friction coefficient when the grain size is the smallest;

fig. 3 illustrates an example of a local sp2 distribution function;

FIGS. 4 and 5 are X-ray diffraction and atomic force microscopy measurements, respectively, of a film obtained by the method disclosed in AT 399726; and

FIG. 6 shows the evolution of the sp3/sp2 content as a function of the thickness of the deposited diamond film.

Detailed Description

The invention is based on micromechanical components coated with nanocrystalline diamond films with a thickness of a few nanometers to a few micrometers. The size of the crystals/grains is a few nanometers, preferably less than 10 nanometers. The coefficient of friction of these diamond films is less than 0.1, preferably less than 0.05, in particular less than or equal to 0.03 (FIG. 1). The nanocrystalline diamond film is produced by CVD (chemical vapor deposition). In a particular chemical vapor deposition process (disclosed in AT 399726B, which is incorporated herein by reference), a carbon-containing gaseous species, such as methane, is thermally activated and deposited on a substrate as diamond (sp 3-hybridized carbon), graphite (sp 2-hybridized carbon), and sugars or other carbon species (a mixture of sp 2-and sp 3-hybridized carbon). To obtain a pure diamond layer, a second gas must be used: and (3) hydrogen. Hydrogen (H)₂) Are also thermally activated, leading to the formation of monatomic hydrogen, wherein, as an important process step, the efficiency of the activation process is extremely high (over 50%), preferably over 75% and in some cases up to 90% or more. Such a method is disclosed in AT 399726B.

In this way, a nanocrystalline diamond coating 1 (fig. 2a and 2 b) can be deposited on silicon, the crystal size of the diamond being less than 8 nm and the surface roughness being less than 10 nm, as shown, for example, in fig. 2 b.

X-ray diffraction and atomic force microscopy measurements of films obtained by the method disclosed in AT 399726B are shown in fig. 4 and 5.

The main aspect of the invention relates to a nanocrystalline diamond coating, wherein during the growth of the diamond film the method is controlled to achieve an increase in the sp 2-hybridized carbon content 2 in the sp 3-hybridized layer matrix near the surface of the substrate 4, as shown in fig. 3. The right part of fig. 3 shows the evolution 5 of the sp2 content at the grain boundaries of the UNCD (Ultra Nano crystal Diamond) coating 3.

The following methods are demonstrated to result in the formation of an increasing sp2 enrichment, which includes the idea of the present invention, but the present invention is not limited to these methods.

Controlled increase of methane or carbon-containing gas concentration or addition of additional carbon-containing gas at the end of the growth process: during the final stages of diamond growth in chemical vapour deposition processes, gradual control of the addition of a carbon-containing reactant gas (e.g. methane) or the addition of an additional carbon-containing gas (e.g. acetylene) changes the ratio of sp2/sp3 within the diamond matrix or diamond host material, resulting in, for example, a highest volume or other local distribution function being obtained at the surface, respectively.

Deposition parameter variation: the deposition of nanocrystalline diamond with a maximum sp3 content exceeding 97% (detection limit) according to the above (AT 399726B) method is carried out under an optimum set of parameters including the pressure of the vacuum system, the temperature of the filament, the temperature of the substrate, the flow of carbon-containing gas, the flow of hydrogen and the distance between the filament and the substrate. The ratio sp2/sp3 may additionally be influenced as a result of an increase or decrease in substrate temperature and/or pressure. This change should be effected at the end of the growth process to achieve incremental sp2 enrichment near the surface.

Nitrogen addition method: during the growth of the diamond, a certain amount of gaseous nitrogen is introduced into the reaction chamber. The secondary nucleation process (the generation of new diamond particles rather than the growth of those already formed) is enhanced, which results in a reduction in particle size down to only a few nanometers. Smaller particles make the roughness of the coating lower and additionally increase the amount of sp2 hybridized carbon. Another approach also expands the sp2 content of diamond, which is the incorporation of sp2 particles in an sp3 diamond matrix.

Argon addition method: increasing the concentration of methane to very high levels and/or replacing up to 100% of the hydrogen with argon or other elements such as nitrogen during the growth of diamond can also achieve the same effect.

Adding other elements: any type of gas may be used, such as noble gases: neon, helium, krypton or xenon, without limitation.

And (3) post-treatment: including plasma or laser treatment, possibly in combination with gases that can alter the surface of the diamond and/or its internal structure.

Lattice distortion (annealing in air or under a controlled atmosphere, UV irradiation, X-rays, ion implantation, etc.)

In combination with or instead of the above solutions, the performance of UNCD (super nanocrystalline diamond) coatings for mechanical systems can be further enhanced by post-treatments such as:

terminating the surface of the diamond (saturating the dangling bonds) with hydrogen, oxygen, fluorine, molecules, oil, wax, or the like

The addition of a metal-containing compound on top of the diamond reduces the adhesion or deposit deposits.

In addition to the above solutions, the particle size can also be further reduced to reduce the surface roughness. The smaller the particles, the smoother surface can be achieved (see fig. 2), so that the friction properties (coefficient of friction, wear, etc.) can be further enhanced.

In addition to the above techniques, the tribological properties can also be improved by appropriate nano-structuring of the surface of the friction element. The structuring can be achieved by structuring the substrate or by structuring the diamond coating itself. In the case of structured substrates, it is preferred to use diamond coatings with very small particles to allow accurate replication of the structured surface.

These methods may be implemented independently or in combination to achieve the desired sp3/sp2 ratio.

Table 1 below shows an example of the evolution of the sp3/sp2 content as a function of the thickness of the deposited diamond film for micromechanical components having a low coefficient of friction, suitable for use in a timepiece movement.

TABLE 1

Film thickness [ mu m]	sp3Content [% ]]	sp2 content [% ]]
			0.1	100	0
0.2	100	0
			0.3	100	0
0.4	100	0
			0.5	100	0
0.6	100	0
			0.7	100	0
0.8	100	0
			0.9	100	0
1	100	0
			1.1	100	0
1.2	100	0
			1.3	100	0
1.4	100	0
			1.5	100	0
1.6	100	0
			1.7	100	0
1.8	100	0
			1.9	100	0
2	100	0
			2.1	100	0
2.2	100	0
			2.3	100	0
2.4	100	0
			2.5	100	0
2.6	95	5
			2.7	90	10
2.8	85	15
			Film thickness [ mu m]]	sp3 content [% ]]	sp2 content [% ]]
2.9	80	20
			3	75	25
3.1	70	30
			3.2	65	35
3.3	60	40
			3.4	55	45
3.5	50	50

The invention can be used in micromechanical timepiece movements, in particular for implementing escape wheels and pallets and other friction-related systems.

The detailed description set forth above with reference to the drawings is intended to be illustrative, not limiting, of the invention. There are numerous alternatives, which fall within the scope of the appended claims. For example, the invention may also be used to enhance the tribological properties of other macro or micro machines (at least one of which/a part may be coated with diamond). Examples of applications are micro-electromechanical systems (MEMS), nano-electromechanical systems (NEMS), electric motors (in particular micro-motors), pumps (in particular micro-pumps), vacuum systems, static and/or dynamic systems such as engines, etc. without departing from the invention.

The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps. The fact that the dependent claims define respective additional features does not exclude combinations of additional features, which correspond to combinations of the dependent claims.

Claims (17)

1. Method for coating micromechanical components of a micromechanical system, comprising:

-providing a substrate (4) element to be coated;

-providing the element with a diamond coating (1) of a given final thickness;

wherein:

-the diamond coating (1) is provided in a reaction chamber by chemical vapour deposition;

wherein only sp3 hybridized carbon is deposited on the substrate at an initial stage of the chemical vapor deposition process, having a first thickness;

and in a second phase of the chemical vapour deposition process, providing a controlled change in carbon content within the reaction chamber, thereby providing a change in sp2/sp3 carbon (2) bonds near the surface, having a second thickness, wherein the change in sp2/sp3 carbon (2) bonds refers to a gradual increase in sp2 hybridized carbon content towards the outer surface.

2. Method for coating micromechanical components of a micromechanical system according to claim 1, wherein the second thickness is of the order of 1 micrometer.

3. Method for coating micromechanical components of a micromechanical system according to claim 1 or 2, wherein said change is an increase in the carbon content within the reaction chamber.

4. Method for coating micromechanical components of a micromechanical system according to claim 3, wherein said increase in carbon content is obtained by increasing the proportion of reactant gases containing carbon.

5. Method for coating micromechanical components of a micromechanical system according to claim 3, wherein said increase in carbon content is obtained by adding additional carbon-containing gas.

6. Method for coating micromechanical components of a micromechanical system according to claim 3, wherein said increase in carbon content is obtained by increasing the temperature and/or the pressure inside the process reaction chamber.

7. Method for coating micromechanical components of a micromechanical system according to claim 3, wherein said increase in carbon content is obtained by adding gaseous nitrogen to the reaction chamber.

8. Method for coating micromechanical components of a micromechanical system according to claim 3, wherein said increase in carbon content is obtained by replacing hydrogen with argon.

9. Method for coating micromechanical components of a micromechanical system according to claim 3, wherein said increase in carbon content is obtained by adding at least one rare gas.

10. Method for coating micromechanical components of a micromechanical system according to claim 3, wherein said increase in carbon content is obtained by lattice distortion.

11. Method for coating micromechanical components of a micromechanical system according to claim 3, further comprising a post-treatment that essentially comprises terminating the diamond surface, wherein said termination is achieved using a substance selected from the group consisting of: hydrogen, halogens, metals, conductive minerals/organic molecules or proteins.

12. Method for coating micromechanical components of a micromechanical system according to claim 1, wherein said micromechanical system is a timepiece movement.

13. Micromechanical component for a micromechanical system, wherein said component is obtained according to the method of claim 3.

14. Micromechanical component for a micromechanical system according to claim 13, wherein the micromechanical system is a clock movement.

15. Micromechanical component for a micromechanical system, said component defining a substrate for a diamond outer surface layer, wherein said diamond surface layer comprises a first portion of a first thickness contacting said substrate, said first portion consisting of sp 3-hybridized carbon; and a second portion of a second thickness distal to the substrate, the second portion consisting of a mixture of sp2 hybridized carbon and sp3 hybridized carbon, wherein the second portion comprises an increasing sp2 hybridized carbon content.

16. Micromechanical element for a micromechanical system according to claim 15, wherein the second thickness is of the order of 1 micrometer.

17. Micromechanical component for a micromechanical system according to claim 15, wherein the micromechanical system is a clock movement.