Abstract:
Method for coating micromechanical components of a micromechanical system, in particular a watch movement, comprising:
       providing a substrate component to be coated;   providing said component with a diamond coating;   wherein said diamond coating conductivity is increased in order to reduce dust attraction by the coated component when used in said micromechanical system.
 
Corresponding micromechanical components and systems are also provided.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to micromechanical parts with high tribological performances (low friction and wear) for application in mechanical systems and namely a method for coating micromechanical components of a micromechanical system, in particular a watch, for preventing the agglomeration of particles (debris and/or dust) on the functional areas of micromechanical components. The invention also relates to a micromechanical component for a micromechanical system, in particular a watch movement coated with said method. The invention can be used in micromechanical watch movements, in particular for the realisation of escapement wheels and pallets and other friction-relevant components. 
       BACKGROUND TO THE INVENTION 
       [0002]    The technical demands relating to micromechanical components are growing: beside highest precision, the components need to provide highest energy efficiency in the mechanical system, long lifetime and complete abdication of lubricants insofar as possible. 
         [0003]    Many documents related to such characteristics have been published. The approaches presented generally fulfil some of the tasks without being able to deliver a complete solution mainly due to the restrictions on the materials used. 
         [0004]    Micromechanical components produced by mechanical machining (die cutting or shape cutting) exhibit two major disadvantages: firstly, they are either high priced or economically profitable only in mass production because investments are required for expensive production tools. Secondly, these processes did reach their technical limits with the accuracy lying by +/−5 micrometers. 
         [0005]    Several alternative approaches have been discussed in literature; the most promising idea being the etching of micromechanical parts from silicon wafers achieving highest precision exceeding the results of the machining techniques by far. In practical results, it was shown that the required mechanical strength as well as coefficient of friction of some micromechanical components very often exceed the known components characteristics. For instance, in the watch industry, this causes problems in the handling of the parts and in the regularity of the performance and reliability of watches on short and long terms. 
         [0006]    A solution to the problem has been demonstrated in EP 1 904 901 in which it is disclosed that by treating the surface of the micromechanical parts with oxygen the strength and the lifetime could be extended. 
         [0007]    The energy efficiency could be enhanced by using special oils in mechanical systems, however at the expense of the demand for a dry running system. 
         [0008]    Classical machined parts made of steel achieve the longest lifetime, but these systems have to be lubricated to perform their function. 
         [0009]    A further problem of the lubricated systems is the necessity for frequent service intervals, where the movements have to be cleaned and re-lubricated. Numerous approaches to fulfil all these requirements with one system have been undertaken: 
         [0010]    EP0732635B1 describes an approach where the micromechanical parts are etched from a silicon wafer and then coated with diamond films. The diamond films obtained via this method have a surface roughness higher than 400 nm. These films require a subsequent polishing if the diamond-coated parts are used in sliding contact applications. 
         [0011]    EP1233314 discloses a mechanical clockwork assembly for watches having a mechanical escapement with an escapement wheel and an anchor wherein the functional elements of the escapement wheel are at least partially coated on their operating surfaces with a DLC (diamond like carbon) coating. DLC has a high sp 2  content (ranging from 30 to 80%) and is amorphous carbon which hardness could not be sufficient for effective wear protection applications. 
         [0012]    EP1622826 discloses a micromechanical component comprising a first surface and a second surface, which are substantially perpendicular to each other wherein the first and/or the second surface consist at least partially of diamond. 
         [0013]    U.S. Pat. No. 5,308,661 discloses a process for the pre-treatment of a carbon-coated substrate to provide a uniform high density of nucleation sites thereon for the subsequent deposition of a continuous diamond film without the application of a bias voltage to the substrate. 
         [0014]    EP1182274A1 discloses a method for the post-treatment of diamond coatings where a coarse grained (micrometer regime) diamond coating is deposited on machining tools and subsequently treated by means of plasma processes. The aim of this post-treatment is the degradation of the top layers of the sp 3 -hybridised diamond coating into sp 2 -hybridised carbon species. The expected effect is a filling of the “surface valleys” between the coarse grains protruding from the surface to achieve a more flat surface. The result of such method is a film having a coarse grain sp a  diamond on top of which is a top layer of several hundred nanometres of sp 2  hybridised amorphous carbon. The top layer is relatively soft and wears off quickly in applications involving high friction forces. 
         [0015]    U.S. Pat. No. 2,996,763 discloses a radioactive diamond material, i.e. diamond material containing a surface coating of a radioactive metal to avoid the diamond material retaining a charge of static electricity. This document mentions the problem of dust attraction and agglomeration on the diamond surface, when diamond is used as bearing material in timepieces. This solution may prove efficient. However, nowadays the use of a radioactive element in a timepiece could be not be envisaged. 
         [0016]    US 2008/0145675 discloses an anti-static surface treatment for a plastic film or similar material, where an additive that produces the anti-static properties is included in the surface. This document discloses additives in the form of siliconised top layer. This type of top layer does not resist to wear. Alternatively the additive can be graphite incorporated in the surface layer. However, this anti-static surface cannot be used in mechanical systems because the siliconised surface does not exhibit the necessary mechanical properties. Covering the diamond layer with such a siliconised layer could bury the diamond excellent mechanical properties. 
         [0017]    U.S. Pat. No. 6,080,445 discloses a method for forming films over an insulating material wherein an underlayer film having electrical conductivity is formed on the surface of the insulating material constituting a substrate, and a hard carbon film is formed over the underlayer film so that the surface electrical resistance value of the hard carbon film can be controlled so as not to cause the surface thereof to be charged with static electricity by varying an electrical resistance value of the underlayer film. In such a method, the charges once evacuated through the thin conductive layer can not go elsewhere and accumulate in the conductive layer. 
         [0018]    The above-described solutions can only partially solve the problem of preventing the retention of charges of static electricity satisfactorily. In particular, several deficiencies restrain the use in large-scale production, which is demanded in the watch industry for instance. 
         [0019]    Indeed, the use of diamond-coated silicon micromechanical parts raises two major problems. First, as the diamond coatings are originally electrically insulating, electrostatic charges quickly build up upon use, due to friction and contact between the micromechanical parts. This charging effect leads to attraction of particles, which have a strong effect especially in unlubricated contacts. Moreover, fabrication of micromechanical parts for the watch industry can not practically be achieved in clean rooms or at least not be cost effective namely this means that the after sale service of watches equipped with such parts would also have to be carried out in clean room conditions which is unrealistic. Preventing any penetration of small particles of the environment (dust, skin-scurf, clothes, etc.) is therefore from a practical point of view not conceivable. 
         [0020]    Another problem in micromechanical parts is based on the formation of a water thin film on their surface under ambient conditions. In particular, this has been described for CVD-diamond films in the following documents: Petrini, D. and Larsson, K., J. Phys. Chem. C 111 (2007) 795—“A theoretical approach to the energetic stability and geometry of hydrogen and oxygen terminated diamond (100) surfaces”, Larsson K. New Diamond Front Carbon Technol 2005; 15:229e45 “Surface properties of diamond under atmospheric conditions: a quantum mechanical approach”. The above publications are incorporated herein by reference. Such water film fosters the collection of dust particles on the surfaces leading to the generation of dust aggregates, which may cause the failure of the watch (blocking of the watch movement) when these diamond-coated silicon micromechanical parts are used in a watch movement. 
       SUMMARY OF THE INVENTION 
       [0021]    A general aim of the invention is therefore to provide a method and a device enabling elimination or reduction of particle (debris/dust) attraction in a micromechanical system. 
         [0022]    A further aim of the invention is to provide a process allowing providing micromechanical components for a micromechanical system, in particular a watch movement, avoiding particle (dust/debris) attraction and accumulation. 
         [0023]    A further aim of the invention is to provide a process allowing providing micromechanical components for a micromechanical system, in particular a watch movement, avoiding the formation of a water film on the surface of the components. 
         [0024]    Still another aim of the invention is to provide micromechanical components for a micromechanical system, in particular a watch movement, avoiding particle (dust/debris) attraction and accumulation. 
         [0025]    Yet another aim of the invention is to provide micromechanical components for a micromechanical system, in particular a watch movement, avoiding the formation of a water film on the surface of the components. 
         [0026]    These aims are achieved thanks to a process for coating micromechanical components of a micromechanical system, in particular a watch movement, and a micromechanical component for a micromechanical system, as defined in the claims. 
         [0027]    There is accordingly provided a method for coating micromechanical components of a micromechanical system, in particular a watch movement, comprising:
       providing a substrate component to be coated;   providing said component with a diamond coating;
 
wherein said diamond coating conductivity is increased in order to reduce particle (dust/debris) attraction by the coated component when used in said micromechanical system.
       
 
         [0030]    Such a process enables increasing the electrical conductivity of the diamond films thereby decreasing the resulting resistivity of the nanocrystalline diamond film down to a few Ohm·cm, but preferentially down to 10 −2  Ohm·cm. Electrostatic charging effect is reduced or eliminated and particle (dust/debris) attraction is preferably totally eliminated. 
         [0031]    The component having a diamond coating, advantageously produced by a CVD process may have a thickness between a few nanometres and several micrometer (preferably 3 or more nanometres, and most preferably around 10 nanometres). 
         [0032]    The diamond coating is preferably provided to have the following properties:
       diamond grain size below 30 nm, preferably below 10 nm;   a surface roughness below 30 nm, preferably below 10 nm;   a coefficient of friction below 0.1, preferably below 0.05 and most preferably below 0.03;   electrical resistivity of at most few OHM·cm, preferably 10 −2  OHM·cm or below;   a hydrophobic or lipophobic or ultraphobic surface termination.       
 
         [0038]    In a preferred embodiment, the diamond coating conductivity is increased in order to change the coating polarity from hydrophilic to hydrophobic, thus avoiding water film accumulation on the components. 
         [0039]    Diamond coating is advantageously provided by CVD and during CVD deposition, electronic conductivity of said coating is increased by doping the diamond film. Such process is reliable, allowing the production at a reasonable cost of a large number of components if required. 
         [0040]    Several variants are possible for doping the diamond film, such as adding a dopant in gaseous, liquid, solid and/or vapour form during the deposition process. In such a case, the dopant is one material or a combination of the materials selected from the group consisting of boron, phosphorus, nitrogen, sulphur, arsenic, and aluminium. In other variants, doping is achieved by means of ion implantation, ion bombardment or sputtering technique. Doping may also be achieved by depositing adsorbates. In further variants, doping is achieved by deuteration and post-hydrogenation, by lattice distortion. (e.g. via radiation), by terminating the nanocrystalline diamond surface with an element or material rendering the surface hydrophobic. In the latter variant, a preferred example of used element is fluorine, without being restricted to this element. Other atoms and molecules adsorbed/attached/bound to diamond surface can induce the same effect, resulting again in strong hydrophobicity (surface unfavourable for water adhesion) or lipophobicity (surface unfavourable for oil adhesion). Preferred examples are: wax, oils, without being restricted to them. In still a further variant, termination is advantageous provided using an element selected from the list consisting of hydrogen, halogens, metals, electrical conductive minerals/organic molecules or proteins. 
         [0041]    The invention also provides a micromechanical component for a micromechanical system, in particular a watch movement, wherein said component is obtained with a method as previously described. 
         [0042]    Such component can be used to eliminate the particle (debris/dust) attraction in any other macroscopic or microscopic mechanisms where at least one component can be coated with conductive diamond to prevent dust attraction. 
         [0043]    Such mechanical components, produced for micromechanical systems are long-time stable, allow high energy efficiency, prevent the collection of dust or other impurities at local areas on movements or micromechanical parts, allow the running of the mechanical system under dry (unlubricated) conditions, thus eliminating the need of regular maintenance. 
         [0044]    The bearings of the micromechanical system are advantageously coated with such electrically conductive nanocrystalline diamond coating. Variants in which the bearings of the micromechanical system and the glues used are electrically conductive to allow electrical charges to be evacuated are also provided. 
         [0045]    The invention further provides a micromechanical system; in particular a watch movement, comprising a component obtained with a method as previously described, and further comprising at least one silicon base component, wherein said silicon base material is electrically conductive. 
         [0046]    In other variants, the micromechanical system may comprise other components made of a base material selected from the following list comprising silicon, silicon-oxide, silicon carbide, silicon-nitride, ceramics, nitrides, metals, cemented carbide, graphite, carbon-based materials, composites, glass or combinations thereof without being restricted to these materials. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]    The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
           [0048]      FIG. 1  is a diagram showing the average coefficient of friction of a nanocrystalline diamond film sliding on a nanocrystalline diamond film without lubrication as a function of the diamond grain size; 
           [0049]      FIGS. 2   a  and  2   b  are schematic representations of the surface structure of a diamond film deposited according to the method of the invention for different grain sizes, resulting in a low roughness, and therefore a low coefficient of friction, when the grain size is the smallest; and 
           [0050]      FIGS. 3   a  and  3   b  illustrates how the hydrophobic diamond surface obtained after fluorination prevents the accumulation of dust on the whole surface. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]    The invention presented is based on micromechanical components coated with NCD (nanocrystalline diamond) films of thicknesses between tens of nanometers and several micrometers. The crystals or grains have a size of a few nanometers, preferably less than 10 nanometers, as shown in  FIG. 1 . These diamond films comprise a coefficient of friction of less than 0.1, preferably less than 0.05, in special cases 0.03 or less. Such nanocrystalline diamond films are produced by a CVD (chemical vapour deposition) process. In an advantageous CVD process (discloses in AT399726B incorporated herein by reference) a carbon containing gas species (for example methane) is thermally activated and is deposited on a substrate as diamond (sp 3 -hybridised carbon), as graphite (sp 2 -hybridised carbon) as well as carbon hydrates or other carbon species (mixtures of sp 2 - and sp 3 -hybridised carbon). To obtain a pure diamond layer it is necessary to use a second gas: for instance hydrogen. The hydrogen gas (H 2 ) is also thermally activated resulting in mono-atomic hydrogen, where, as an important process step, the efficiency of the activation process is extremely high (more than 50%), preferably more than 75% and achieving in special cases 90% or more. Such a method is described in AT399726B which is incorporated herein by reference. 
         [0052]    With this process, the nanocrystalline diamond coatings  1  ( FIG. 2 ) can be deposited on silicon, with diamond crystal sizes of less than 8 nanometers having a surface roughness of less than 10 nanometers for instance as shown in  FIG. 2   b.    
         [0053]    The solution of the described dust-problem due to electrostatic charging effect can be achieved by increasing the electrical conductivity of the diamond films thereby decreasing the resulting resistivity of the nanocrystalline diamond film down to a few Ohm·cm, but preferentially down to 10 −2  Ohm·cm. 
         [0054]    A sufficient conductivity of the diamond layer and the change of its polarity (hydrophilic/hydrophobic) can be achieved using the following methods or processes:
       Doping of NCD films by incorporation of atoms or molecules: during the diamond growth a dopant in gaseous, liquid, solid and/or vapour form is added. For instance, boron-doped diamond films (p-type) or phosphorous-doped diamond films (n-type) are used to increase electrical conductivity. Further noted dopants are: nitrogen, sulphur, silicon, arsenic etc. without being limited to these elements.   Boron doping (p-type): This process is achieved during the growth of diamond using a solid, liquid or gaseous source of boron. The substitution of a part (up to 0.1%) of the carbon atoms from diamond by boron atoms leads to the generation of holes (lack of electrons). The ionisation energy of boron is 0.37 eV and thus very low. This allows the generation of a sufficient quantity of the free charge carriers. The availability of free charge carriers (in this case holes) increases the electrical conductivity of the diamond film. The electrostatic charges are thus evacuated to the bulk of the diamond film. The attraction of particle (debris/dust) is therefore considerably reduced or totally eliminated. The higher the boron concentration in diamond, the higher is the diamond conductivity, in the range of 10 16 -10 21  cm −3 . The resistivity can decrease down to 10 −2  Ohm·cm.   Phosphorus doping (n-type): The phosphorus incorporation is achieved during the growth of diamond from a solid, liquid or gaseous source of phosphorus or phosphorus compounds such as TBP (tertiarybutylphosphine) and TMP (trimethylphosphate). The substitution of a part of carbon atoms from diamond by phosphorus atoms leads to an excess of electrons. The ionisation energy of phosphorus is 0.6 eV and thus relatively low, and frees a certain part of the provided electrons. The availability of free charges carriers (electrons in this case) increases the electrical conductivity of the diamond film. The electrostatic charges are evacuated to the bulk of the diamond film. The attraction of particle (debris/dust) is thus considerably reduced or totally eliminated. The higher the phosphorus concentration (in the range of 10 16 -10 21  cm −3 ) in diamond, the higher is the diamond conductivity.   Doping by other elements: a doping process can also be achieved by incorporating into diamond other elements such as sulphur, aluminium, nitrogen, and other elements or a combination of several elements.       
 
         [0059]    In a first variant, doping is performed by means of ion implantation, ion bombardment or sputtering techniques: an ion beam is focused on a diamond film so that the ions are incorporated inside the diamond lattice (penetration depth depending on the dosage/energy of the ion beam). 
         [0060]    Some carbon atoms are replaced by the incorporated ions via bombardment (boron atoms for instance). The doping process and the behaviour of the conductivity are similar to the mechanism described above in connection with the doping of the diamond during growth. Other elements (phosphorus, sulphur, nitrogen, metals, etc) can also be implanted in diamond giving rise to an increase of its conductivity. Ion bombardment or sputtering of metals will destroy the near-surface lattice leading thus to increasing the diamond conductivity. 
         [0061]    In a second variant, doping is performed by depositing adsorbates: depositing or adsorbing of a metal (silver, copper, gold, etc) or any other element/molecule on a part or the entire diamond surface might change the diamond band structure at the interface, thus increasing the conductivity for example as disclosed in Ref. Doping semiconductor nanocrystals, Steven C. Erwin et al. Nature 436, 91-94 (7 Jul. 2005) which is incorporated herein by reference. 
         [0062]    In a third variant, doping is performed by Deuteration and post-hydrogenation: after the growth of a boron-doped diamond, it is exposed to deuterium plasma and/or hydrogen plasma at a temperature lower than 500° C. This leads to the formation of Boron-2deuterium (BD 2 ) or Boron-2hydrogen (BH 2 ) complexes. The diamond film treated in this way exhibits n-type conductivity (generation of free electrons). For example as disclosed in “Shallow donors with high n-type electrical conductivity in homoepitaxial deuterated boron-doped diamond layers”, Zéphirin Teukam et al. Nature Materials 2, 482-486 (2003), which is incorporated herein by reference. 
         [0063]    In a fourth variant, doping is performed by post-treatment: including a plasma or laser treatment, also using gases changing the surface of diamond and/or being incorporated into its structure. 
         [0064]    In a fifth variant, doping is performed by Lattice distortion (annealing in air or under controlled atmosphere, UV illumination, x-rays, ion implantation, etc.) to increase the diamond conductivity. 
         [0065]    Surface conductivity may also be increased by termination, i.e. saturation of the dangling bonds (ligants) of the surface atoms of the diamond lattice. For example, hydrogen-termination of the diamond surface generates a surface conductivity, as described in “F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, Phys. Rev. Lett. 85, 3472 2000”. Additional alternatives of possible terminations are: halogens, metals, electrically conductive mineral/organic molecules and proteins. 
         [0066]    In an advantageous configuration, the generation of the above mentioned problematic water film on the diamond surface can be prevented by terminating the nanocrystalline diamond surface with an element or material rendering the surface hydrophobic. The main example of such an element is fluorine, without being restricted to this element. Example of fluorination treatment is shown in  FIGS. 3   a  and  3   b  illustrating H-terminated, with an average angle of 70° ( FIG. 3   a ) and F-terminated, with an average angle of 105° ( FIG. 3   b ) hydrophobic diamond surfaces  1 . The F-terminated surface provides a substantially long-time stable solution. Other atoms and molecules either adsorbed or attached or bound to diamond surface can induce the same effect, resulting again in strong hydrophobicity (surface unfavourable for water adhesion) or lipophobicity (surface unfavourable for oil adhesion). Examples are: wax, oils, without being restricted to them. According to the invention, the termination can be achieved by covalently bonded adsorbates or by binding via H-bonds or Van-der-Waals forces without being restricted to them. 
         [0067]    Examples of methods to terminate nanocrystalline diamond films are wet chemical treatment, plasma processes, thermal processes, electrochemical processes (reduction, oxidation, electrolysis, etc), illumination in special atmospheres (UV, IR, etc.), spin-coating of adsorbates, sputtering of adsorbates, dip-coating of adsorbates, ink-jet with adsorbates, electro-spraying of adsorbates. Other techniques and processes may also be used. 
         [0068]    By means of well-defined nano-structuring processes (lithography, etching of the NOD-surface), a so-called lotus-effect defined hereinafter can be obtained and thus the attachment of dust and the water film at the surface is prevented. Such nano-structures can be previously processed in the substrate itself (e.g. silicon). The nanometric scale of the diamond crystals and the low film thickness would be sufficient to maintain the geometry of the nanostructures and therefore maintain the lotus effect. The lotus effect refers to the very high water repellency (superhydrophobicity) exhibited by the leaves of the lotus flower, as described in Lafuma, A.; Quere, D. (2003). “Superhydrophobic states”.  Nature Materials  2: 457-460. Dirt particles are swept away by water droplets due to complex nanoscopic architecture of the surface which enables minimization of adhesion. This publication is incorporated herein by reference. 
         [0069]    Combination of the methods described above: it is advantageous to use electrical conductive silicon as a substrate for all parts according to this invention in order to achieve the transport of electrical charges/current from the working surfaces. According to the invention, the conductive nanocrystalline diamond films are also be deposited on the bearing surfaces. Preferable substrates are silicon, silicon-carbide or silicon-nitride as bearing material without being restricted to these materials. Further substrate materials for bearings coated according to this invention are e.g.: ruby, tungsten carbide, ceramics, glass, etc. 
         [0070]    When the parts are not fabricated from one silicon piece but glued together from different parts, conductive glues are preferably used. 
         [0071]    The above detailed description with reference to the drawings illustrates rather than limit the invention. There are numerous alternatives, which fall within the scope of the appended claims. For instance, the invention can also be used to eliminate dust attraction in others macroscopic or microscopic mechanisms where at least one part/piece can be coated with conductive diamond to prevent dust attraction. Application examples are Micro-Electro-Mechanical Systems (MEMS), Nano-Electro-Mechanical Systems (NEMS), Electrical motors, in particular micro-motors, pumps, in particular micro-pumps, vacuum systems, static and/or kinetic systems such as engines, etc., without departing from the invention. 
         [0072]    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 mere fact that respective dependent claims define respective additional features, does not exclude a combination of additional features, which corresponds to a combination of dependent claims.